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nity Institution

c-Wchns-m

Red uc

Reductive Dechlorination in 3 Continuous

Flow Electrolysis Cell

By
Sanjay Syal

A Thesis

Submitted to
MICHIGAN STATE UNIVERSITY
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE 0F
MASTER OF SCIENCE

Department of Civil & Environmental Engineering
1992

Halogcnate
and wastes.
containing L'

logcnan'on W

RCdUCthC dc
halogenated s
don. The flVe

Tofithylcnc, (3‘

 

Parallel path w‘
NS, and a pal]
Ulation of the a;
Chemicals as we
leading to comp
hl’dmgcnonSI-S \
[med Were mm

for 311(ch5 as co

 

2—} "

I
I

AQZ - l//

g

Abstract

Reductive Dechlorination in 3 Continuous

Flow Electrolysis Cell

Halogenated organics constitute one of the largest single classes of hazardous chemicals
and wastes. Electrochemical reduction may be a suitable method to treat wastewaters
containing halogenated organics. This research addresses the principals of reductive deha-

logenation with a focus on electrochemical processes.

Reductive dechlorination by electrolysis converts highly halogenated molecules into less
halogenated species that are more susceptible to either chemical oxidation or biodegrada-
tion. The five target chemicals chosen for this study are (1)perchloroethy1ene, (2) trichlo-
roethylene, (3) carbon tetrachloride, (4) chloroform and 1,1,1-trichloroethane. Two
parallel pathways were observed for the degradation of these target chemicals: hydrogenol-
ysis, and a pathway leading to the formation of completely dechlorinated products. Manip-
ulation of the applied p0tential permitted control over the degradation rates of target
chemicals as well as on the product distribution. At lower applied voltages, the pathway
leading to complete dechlorinatiOn was dominant whereas at higher reference voltages, the
hydrogenolysis was more important. The overall degradation rates of the target chemicals
tested were increased by increasing the applied voltage. The operational costs were higher

for alkenes as compared to those for alkanes.

Dedicated to

Dr. Craig S. Criddle
and

my parents

 

 

I would Iii
major advi

and cspcci.

lwouid als

suggestion.

I WOUid ilk:

their consta

This resean
EXCCUchc

Chemical C

ACKNOWLEDGEMENTS

I would like to express my appreciation and gratitude to Professor Craig S. Criddle, my
major advisor, for his constant support, encouragement, patience, guidance and friendship,

and especially invaluable assistance in the preparation of this thesis.

I would also like to acknowledge Dr. Simon Davies and Dr. Susan Masten for their
suggestions and guidance.

I would like to express my deepest gratitude to my parents, Inderjeet and N irrnala Syal, for

their constant love and support.

This research was financially supported in part, by the State of Michigan Research
Excellence Fund administered through the Center for Microbial Ecology at Michigan
State University and, in part, by an Environmental Enhancement Grant from the Dow
Chemical Company.

iv

 

Acknov
List of 1
List of 'l
CHAPT
l.

 

M;

 

Table of Contents

Page
Acknowledgements ............................................................................................................. iv
List of Figures .................................................................................................................... vii
List of Tables ...................................................................................................................... xii

CHAPTER ............................................................................................................................
1. INTRODUCTION ................................................................................................ 1
Background ............................................................................................... 1
Electrochemical reduction ......................................................................... 3
Dehalogenation chemistry ......................................................................... 3
Pathways of reductive transformation ....................................................... 6
Perchloroethylene (PCB) and Trichloroethylene (TCE) .................. 6
Carbon tetrachloride ......................................................................... 9
Chloroform. .................................................................................... 13
1,1,1-trichloroethane...._. ................................................................. 13
Hypothesis ............................................................................................... 16
2. ELECTROCHEMICAL REACTOR THEORY .................................................. 17
Fundamental Concepts ............................................................................ l7

Simplified analysis of plug—flow model of electro—chemical

reactors .................................................................................................... 19
Estimation of mass transfer coeflicient ................................................... 23
3. MATERIALS & MATHODS .............................................................................. 28
Experimental setup .................................................................................. 28

General experimental procedure ............................................................. 31
Assembly of the electrolysis cell .............................................. 31

. Analytical methods .................................................................. 31
Electrolysis experiment. ........................................................... 34

Tracer Studies ........................................................................... 37

Efficiencies of electron transfer ................................................ 37
RESULTS & DISCUSSION ............................................................................... 40
Tetrachloroethylene ................................................................................. 40
Trichloroethylene .................................................................................... 56
Carbonterachloride .................................................................................. 64
Chloroforrn ............................................................................................. 72
1,1,1-trichloroethane ............................................................................... 80
CONCLUSIONS ................................................................................................ 89
RECOMMENDATIONS FOR FUTURE WORK .............................................. 90
REFERENCES ................................................................................................... 92
APPENDICES
Appendix A QA/QC protocol ..................................................... 97
Appendix B Modelling of electrochemical plug flow
reactor ..................................................................... 99
Appendix C Calibration constants for volatile
organics and chloride measurements .................... 104
Appendix D Statistical analysis.......... ...................................... 107

vi

 

 

LIST 0

Figure}
1.1 Red
1.2 Pati
1.3 Sug

1.4 Knc

1.5 Pati

cont

2.1 Van
2.2 lllus
2.3 Con
3.1 Elcc

LIST OF FIGURES

Figure Number Page
1.1 Reductive Dehalogenation by electrochemical cell ....................................................... 5
1.2 Pathways for transformation of PCB to VC ................................................................... 7

1.3 Suggested anaerobic dechlorination pathways for PCB and TCE
transformation ................................................................................................................ 8
1.4 Known biotic and abiotic transformations of CI‘ ......................................................... 10

1.5 Pathways for the transformations of TCA under methanogenic

conditions ..................................................................................................................... 14
2.1 Variation of current with voltage drop .......................................................................... 18
2.2 Illustration of plug-flow model ..................................................................................... 20
2.3 Concentration profile along y-axis ............................................................................... 24
3.1 Electrochemical plug flow reactor for reductive dehalogenation ................................. 29
3.2. Effluent sampling technique ........................................................................................ 36
4.1 PCB degradation at reference voltage of -1.8V. ........................................................... 41
4.2 Chlorine balance for PCE degradation at reference voltage of -1.8V. .......................... 41
4.3 Carbon balance for PCE degradation at reference voltage of -1.8V. ............................ 41
4.4 PCB degradation at reference voltage of -1.9V ............................................................ 42
4.5 Chlorine balance for PCE degradation at reference voltage of -l.9V. .......................... 42
4.6 Carbon balance for PCE degradation at reference voltage of -l.9V. ............................ 42
4.7 PCB degradation at reference voltage of -2.0V. ........................................................... 43
4.8 Chlorine balance for PCE degradation at reference voltage of -2.0V. .......................... 43
4.9 Carbon balance for PCE degradation at reference voltage of -2.0V. ............................ 43
4.10 PCE degradation at reference voltage of -2.1V. ......................................................... 44
4.11 Chlorine balance for PCE degradation at reference voltage of -2.1V. ........................ 44
4.12 Carbon balance for PCE degradation at reference voltage of -2.1V. .......................... 44

vii

4.13 Parallel pathways for PCE degradation using 3.2 g cathode

and 5.8 g anode fibres. .............................................................................................. 45
4.14 PCE degradation at reference voltage of -1.8V. ......................................................... 46
4.15 Chlorine balance for PCE degradation at reference voltage of -1.8V. ........................ 46
4.16 Carbon balance for PCE degradation at reference voltage of -1.8V ........................... 46
4.17 PCE degradation at reference voltage of -18V ................ 47
4.18 Chlorine balance for PCE degradation at reference voltage of -l.8V ......................... 47
4.19 Carbon balance for PCE degradation at reference voltage of -l.8V. .......................... 47
4.20 PCE degradation at reference voltage of -1.9V. ......................................................... 48
4.21 Chlorine balance for PCE degradation at reference voltage of -1.9V. ........................ 48
4.22 Carbon balance for PCE degradation at reference voltage of -1.9V. .......................... 48
4.23 PCE degradation at reference voltage of -2.0V. ......................................................... 49
4.24 Chlorine balance for PCB degradation at reference voltage of -2.0V. ........................ 49
4.25 Carbon balance for PCE degradation at reference voltage of -2.0V. .......................... 49
4.26 PCE degradation at reference voltage of -2.1V. ......................................................... 50
4.27 Chlorine balance for PCE degradation at reference voltage of -2.1V. ........................ 50
4.28 Carbon balance for PCE degradation at reference voltage of -2.1V. .......................... 50
4.29 Parallel pathways for PCE degradation using 5.5 g cathode

and 5.8 g anode fibres. .............................................................................................. 51
4.29a Sorption and degradation area ................................................................................. 54
4.30 TCE degradation at reference voltage of -1.8V. ......................................................... 57
4.31 Chlorine balance for TCE degradation at reference voltage of - 1.8V ......................... 57
4.32 Carbon balance for TCE degadation at reference voltage of -l.8V. .......................... 57
4.33 TCE degradation at reference voltage of -1.9V. ......................................................... 58
4.34 Chlorine balance for TCE degradation at reference voltage of - 1.9V. ........................ 58
4.35 Carbon balance for TCE degradation at reference voltage of -1.9V. .......................... 58
4.36 TCE degradation at reference voltage of -1.9V. ......................................................... 59

viii

4.37 Chlorine balance for TCE degradation at reference voltage of - 1.9V ......................... 59

4.38 Carbon balance for TCE degradation at reference voltage of -1.9V. .......................... 59
4.39 TCE degradation at reference voltage of -2.0V. ......................................................... 60
4.40 Chlorine balance for TCE degradation at reference voltage of -2.0V. ........................ 60
4.41 Carbon balance for TCE degradation at reference voltage of -2.0V. .......................... 60
4.42 TCE degradation at reference voltage of -2.1V. .......... A ............................................... 61
4.43 Chlorine balance for TCE degradation at reference voltage of -2.1V. ........................ 61
4.44 Carbon balance for TCE degradation at reference voltage of -2.1V. .......................... 61
4.45 Parallel pathways for TCE degradation using 5.5 g cathode

and 5.8 g anode fibres ................................................................................................ 62
4.46a Hydrogenolysis pathway of CI‘ degradation ............................................................ 64
4.46 CI‘ degradation at reference voltage of -1.0V. ............................................................ 65
4.47 Chlorine balance for CI‘ degradation at reference voltage of -1 .0V. .......................... 65
4.48 Carbon balance for CI‘ degradation at reference voltage of - 1 .0V. ............................ 65
4.49 CI‘ degradation at reference voltage of -1.0V. ............................................................ 66
4.50 Chlorine balance for CI‘ degradation at reference voltage of -1.0V. .......................... 66
4.51 Carbon balance for CI‘ degradation at reference voltage of -1.0V. ............................ 66
4.52 CT degradation at reference voltage of -1.4V. ............................................................ 67
4.53 Chlorine balance for CI‘ degradation at reference voltage of -1.4V. .......................... 67
4.54 Carbon balance for CI‘ degradation at reference voltage of - 1.4V. ............................ 67
4.55 CI‘ degradation at reference voltage of -1.5V. ............................................................ 68
4.56 Chlorine balance for CI‘ degradation at rerence voltage of -1.5V. ............................. 68
4.57 Carbon balance for CI‘ degradation at reference voltage of - 1 .5V. ............................ 68
4.58 CI“ degradation at reference voltage of -1.8V. ............................................................ 69
4.59 Chlorine balance for CI‘ degradation at reference voltage of -1.8V. .......................... 69
4.60 Carbon balance for CI‘ degradation at reference voltage of -1.8V. ............................ 69

ix

 

 

461Pm

 

am
462CF’
463CM1
464Cmi
465CF¢
466Chk
467Chfl
468CFc
469Chm
470Cmb
471CFd
4.72 Chlo
4J3Chfi)
4.74 c}: d
4.75 Chlo
4.76 Carb
477Pmal
and

4.61 Parallel pathways for CT degradation using 5.5 g cathode

and 5.8 g anode fibres ................................................................................................ 70
4.62 CF degradation at reference voltage of -l.0V. ............................................................ 73
4.63 Chlorine balance at reference voltage of -1.0V. ......................................................... 73
4.64 Carbon balance for CF degradation at reference voltage of -1.0V. ............................. 73
4.65 CF degradation at reference voltage of -1.4V. ............................................................ 74
4.66 Chlorine balance for CF degradation at reference voltage of -l.4V ........................... 74
4.67 Carbon balance for CF degradation at reference voltage of - 1 .4V. ............................. 74
4.68 CF degradation at reference voltage of -l.5V. ............................................................ 75
4.69 Chlorine balance for CF degradation at reference voltage of -1.5V. .......................... 75
4.70 Carbon balance for CF degradation at reference voltage of ~1.5V. ............................. 75
4.71 CF degradation at reference voltage of -1.8V. ............................................................ 76
4.72 Chlorine balance for CF degradation at reference voltage of -1.8V. .......................... 76
4.73 Carbon balance for CF degradation at reference voltage of -1.8V ............................. 76
4.74 CF degradation at reference voltage of -1.8V ............................................................. 77
4.75 Chlorine balance for CF degradation at reference voltage of -1.8V. .......................... 77
4.76 Carbon balance for CF degradation at reference voltage of -1.8V .............................. 77
4.77 Parallel pathways for CI' degradation using 5.5 g cathode

and 5.8 g anode fibres ................................................................................................ 78
4.78 TCA degradation at reference voltage of -l.0V. ......................................................... 81
4.79 Chlorine balance at reference voltage of -1.0V. ......................................................... 81
4.80 Carbon balance for TCA degradation at reference voltage of -1.0V. ......................... 81
4.81 TCA degradation at reference voltage of - 1.4V. ......................................................... 82
4.82 Chlorine balance for TCA degradation at reference voltage of -1.4V. ....................... 82
4.83 Carbon balance for TCA degradation at reference voltage of -1.4V. ......................... 82
4.84 TCA degradation at reference voltage of -1.5V .......................................................... 83
4.85 Chlorine balance for TCA degradation at reference voltage of -1.5V. ....................... 83

 

4.86

4.87

4.88 1
4.89 t
4.90 '1
4.91 C
4.92 C
4.93 P

6.1 Rec
B.l Ma
D.l Stat
D.1 Star

4.86 Carbon balance for TCA degradation at reference voltage of -1.5V. ......................... 83

4.87 TCA degradation at reference voltage of -1.5V. ......................................................... 84
4.88 Chlorine balance for TCA degradation at reference voltage of -1.5V ........................ 84
4.89 Carbon balance for TCA degradation at reference voltage of -l.5V. ......................... 84
4.90 TCA degradation at reference voltage of -1.8V. ......................................................... 85
4.91 Chlorine balance for TCA degradation at reference voltage of -1.8V. ....................... 85
4.92 Carbon balance for TCA degradation at reference voltage of -1.8V. ......................... 85
4.93 Parallel pathways for TCA degradation using 5.5 g cathode

and 5.8 g anode fibres ................................................................................................ 86
6.1 Recommended reactor configuration using chloride exchange beeds .......................... 91
3.1 Mass balance on a small element of electrochemical plug flow reactor .................... 101
D.l Statistics (reference voltage Vs applied voltage) ...................................................... 108
DJ Statistics (reference voltage Vs current) ................................................................... 109

xi

 

 

LIST

1.1T]
3.10

LIST OF TABLES

 

Page

1.1 Transformations of halogenated aliphatic compounds ................................................... 4
3.1 Concentration ranges for the stock solutions for the

secondary dilutions ................................................ i ...................................................... 3 3
3.2 List of experiments conducted ..................................................................................... 35
3.3 Assumed values of fma, ............... 38
4.1 PCB degradation using 3.2 g cathode fibres and 5.8 g anode fibres ............................. 45
4.2 PCB degradation using 5.5 g cathode fibres and 5.8 g anode fibres ............................. 51
4.3 Emciency and energy calculations for PCE degradation ............................................. 53
4.4 Sorption studies for PCE .................................................... 54
4.5 TCE degradation using 5.5 g cathode and 5.8 g anode fibres ....................................... 62
4.6 Efficiency and energy calculations for TCE degradation ............................................. 63
4.7 CI‘ degradation using 5.5 g cathode and 5.8 g anode fibres ......................................... 70
4.8 Efficiency and energy calculations for CT degradation .................................... ' ............ 71
4.9 CF degradation rates using 5.5 g cathode and 5.8 g anode fibres ................................. 78
4.10 Efficiency and energy calculations for CF degradation .............................................. 79
4.11 TCA degradation rates using 5.5 g cathode and 5.8 g anode fibers ............................ 86
4.12 Efliciency and energy calculation for TCA degradation ............................................ 87
4.13 Degradation and production rates information ........................................................... 88

xii

 

Backgr

Chlorin

ally in q
x 106 to:
degreasi

finpnnxn
POHuuun
Uiar inter
Inform ar

chlorinan‘

Halogenat
Significant
Percent of
Gals (0053:
C2 Chlorjna
methane, c1
Although. 11
detected, the

carcinogenic

Chapter 1

Introduction

Background

Chlorinated compounds derived from C1 and C2 hydrocarbons are manufactured industri-
ally in quantities exceeding a total annual world production of 36 x 106 tons. More than 4
x 106 tons are used either as aerosol propellants or solvents in dry cleaning and metal
degreasing (Giger & Moluar-Kubica, 1988). Owing to their wide spread usage and
improper storage and disposal, these compounds have become ubiquitous environmental
pollutants and have been identified in the atmosphere and aquatic environment. Of partic-
ular interest is the occurrence of these compounds in finished drinking waters, where chlo-
roform and other nihalomethanes are found to occur due to the impact of water

chlorination (Rook, 1974, Beller et.al., 1974, Glaze et.al., 1975, Trussell, 1978).

Halogenated aliphatic compounds are also prevalent groundwater contaminants and are a
significant component of hazardous waste and landfill leachates. Approximately, 15-28
percent of all groundwater supplies in the United States contain synthetic organic chemi-
cals (Gosset et. al., 1985). Among the most frequently detected contaminants are C1 and
Oz chlorinated hydrocarbons such as perchloroethylene, trichloroethylene, 1,1,1 -trichlo-
roethane, chloroform and methylene chloride (Westrick et. al., 1984, Roberts et.al., 1982).
Although, in most instances extremely low concentrations of these contaminants are
detected, there is a major concern that chronic exposure to chlorinated solvents proposes a

carcinogenic or mutagenic hazard (Love & Eilers, 1982, Infante & Tsongas, 1982).

A wide spectrum of technologies are available for the treatment of halogenated wastes

such as concentration techniques, chemical oxidation and reduction, and biological treat-

ment. Chemical oxidation, although very effective, has the disadvantages of the cost of
chemical addition or the generation of UV radiation or ozone. Highly halogenated com-
pounds are also diflicult to degrade using oxidation (Vogel et. al., 1987). Incineration
recently has immerged as an effective method for the destruction of halogenated wastes,
but it suffers from the disadvantages of costs and social acceptance and the need to meet
rigorous air quality constraints (Theordore & Reynolds, 1987). Concentration techniques
such as air stripping, activated carbon and membrane separation are widely used. These
processes do not destroy the contaminants and generally generate concentrated side
streams that require further treatment (Schmal et. al., 1987). Chemical reduction using
hydrogen at high temperatures or other reducing agents have also been described in litera-
ture (Chang & Bozzelli, 1986, Mortland & Boyd, 1989).

Interest has been growing in the biological processes because they ofl'er the prospect of
converting the contaminants into harmless products rather than transferring them from one
part of the environment to the other. The potential of degradation of trace contaminants
when bacteria are attached as biofilms to solid surfaces in soils, natural water bodies, or
engineered systems (Bouwer & McCarty, 1985). Aerobic degradation of polychlorinated
aliphatics and ethanes have not shown promising results (Fogel et. al., 1986, Fathepure &
Boyd, 1988). Thus a number of researchers(Criddle, 1989, Fathepure & Boyd, 1988,
Feedman & Gossett, 1989 and many others) are currently studying the anaerobic degrada-

tion of halogenated contaminants in either in situ or above ground treatment systems.

Researchers in electrochemistry have compiled extensive information on the reductive
dehalogenation, but most of this work has been conducted in non-aqueous environments.
In this work, electrolysis experiments are conducted in water to demostrate that reductive

dehalogenation of some halogenated aliphatics occurs via competing pathways. ‘

 

 

 

 

Electroche

EleCtrochet
removal of
merit of dil
techniques
063565 have
ComPOUnd:

(CNS PaIC]

Eiscuoche
die er al., ‘
factors f0,
‘9 invesfié
Wag Studjg

E“alocgenat
DehWOg.

Generally transformations of halogenated aliphatic compounds can be divided into two
general classes (Vogel et. al., 1987) as shown in Table 1.1.
1. Reactions that require external electron transfer (oxidation and reduction).

2. Reactions that do not require electron transfer substitution and hydrohalogenation.
Electrochemical Reduction

Electrochemical deposition has been established as a widely used technology for the
removal of medium or low concentration of metals from waste waters. By contrast, treat-
ment of dilute process waste waters containing halogenated organics using electrolysis
techniques has received much less attention (Schmal et al., 1986). Electrochemical pro—
cesses have been commerciallydeveloped for the degradation of halogenated organic
compounds (PCB ’s, aromatics) by Mazur et. a1. (U .S. Patent No. 4,702,804) and Habeeb
(U .8. Patent No. 4,585,533).

Electrochemical reduction of chloroform and 1,1,1- trichloroethane was studied (by Crid-
dle et al., 1991) in a batch type electrochemical cell in the absence of many complicating
factors found in biotic systems. Later, a detailed study was conducted by Rajayya (1992)
to investigate the reductive dehalogenation of a number of halogenated aliphatics. These
two studies have successfully demonstrated the transformations of these chemicals to less

halogenated or completely dechlorinated species.
Dehalogenation Electrochemistry
The overall reaction of cathodic reduction for halogenated aliphatic can be written as

R-Cl + H+ + 2e‘ —> R-H + Cl'

 

 

 

 

Table .

(Fran

 

4

‘ Table 1.1 Transformations of halogenated aliphatic compounds
(From Vogel et al.1987).

 

Reactions Examples

 

I. Substitution
a sotvolysis. hydrotysis

—> ‘
RX 4» H20 FIG-i HX CH3CHZCHZBI +HZO —‘> CHSCHZCHZOH + H8!
b. other nucleopnitic reacrrons
RX-o-N--"> RN+X- CH3CH28r+HS-—->CH30HZSH+Br-

 

II. Oxidation

a a("trycrroxytation
| I
-<':- x + H209 -;.:-x + 2H++ 2e- a-tacaaz, Hag—b CHBCCIZOH + 2H" + 25'
H OH

in. epoxidation

\ /x \ /O\IX + _ + __
IG-C\+ HZO-F ,c- CC 2H + 29 CHOICCtz+ Hzo—DCHCDCGZ, 2H 4» 2e

 

Ill. Reduction
a. hydrogenotysis
RX + H”+ 29‘» RH . X" 0314+ H+ * 29"» CHClg' C‘-
b. dihato-eiiminatton
I 1

—c- _ _ \. / 2x‘ _ _
IX E + 29 ->/-.=c\+ ceraccn.3 4 2e —>ccrzccrz+ 2oz
c. coupling
28X+2e"—> R-R +2x‘ 2c:t:r,,.2e’—>c:013ccr3 4201'

 

IV. Dehydrohatogenation

ZCHZ + FBI

x-O "'

' \ /
‘0'» C'C+HX CCICH —>CC1
111 / \ 3 3

 

 

 

 

The electrochemical cell offers a means of supplying electrons for this reaction as shown
in Figure 1.1. (Griddle, 1989). In electrolysis, reduction and oxidation can be physically
separated as electrons produced that are at the anode can be sent to the reducible target
compound at the cathode. Electrolytic reduction can be achieved at high rates because a
small voltage increase at the cathode can create powerful overpotentials for electron trans-
fer. One of the major problems in this process is that the protons compete with the target '
chemicals in accepting electrons at the cathode and thus producing hydrogen gas. Also,

 

 

1‘:
Energy
Sourcet e’

E» ‘

;= \‘
electron :' ax
donor

moaned ,
cannon
donor R-

 

 

EDOCO CIIDOUO

Figure 1.1 Reductive Dehalogenation by electrochemical cell.

the efiiciency of electron transfer and the nature of products formed have great impact on

cost effectiveness.

 

 

 

Path

Perchlt

A sum
of the i
Numer
1977,]
contan
cis and
lowing
WET an.
under a
remov;

IllOllths

Pathways of Reductive 'Il'ansformation

Perchloroethylene (PCB) and Trichloroethylene (TCE)

A survey of US. EPA Office of Drinking water has revealed that PCB and TCE were two
of the five most frequently found compounds in the groundwater (Westrick et. al.,1984).
Numerous incidents of well contamination have been well documented (Giger et. al.,
1977, Petura, 1981, Seraglitz et.al.,-1978). Biological transformation of PCB and TCE in a
contaminated site was reported by Parsons et. a1. (1984). In this study, the appearance of
cis and trans 1,2-dichloroethylene and the depletion of PCB and TCE were observed fol-
lowing incubation in microcosms containing muck from an aquifer recharge basin. Bou-
wer and McCarty (1985) studied the PCB and TCE transformations with biofilms grown
under aerobic conditions but the results were not very encouraging. Less than 20 percent
removal efliciency for these compounds were observed with a retention time of 2-3

months using acetate as the primary substrate.

The potential for biotransformation of halogenated organics has been investigated by a
number of researchers recently. The use of continuous flow fixed film bioreactors for PCB
and TCE transformation has been studied under anaerobic conditions (Bouwer et. al.
1983, Bouwer et. al., 1988). Further study has revealed that PCE is degraded under meth-
anogenic conditions to form TCE, DCE, vinyl chloride (VC) and carbon dioxide (Vogel &
McCarty, 1985). These studies clearly demonstrated that TCE and VC were the major
intermediates in PCE transformation and suggested that the potential existed for complete

mineralization of PCB to C02 in soil and aquifer systems.

Based on the above studies a general agreement exists that under anaerobic conditions

PCE transformation proceeds by sequential reductive dechlorination to TCE, DCE and

VC. Each chlorine atom is replaced by hydrogen to give one of three possible DCE iso—
mers. l,1-DCE is the least significant intermediate; several studies have reported that cis-
1,2-DCE predominated over trans-1,2-DCE (Parsons et. al., 1984, Parsons & Lage, 1985,
Kleopfer et. a1. 1985). Electrolytic reduction of PCB in a highly reducing environment has
also indicated the production of TCE, t-DCE and products of complete dechlorination
(Rajayya, 1992). Vinyl chloride was also observed as a daughter product of TCE in a
microsm Study involving biotransformation of methanogenic aquifer material (Wilson et.
al.). Barrio-Lage et. al.(1986) have constructed the following pathway for the transforma-
tion of PCB to VC as shown in Frgure 1.2. Other studies have indicated anaerobic dechlo-
rination of PCB to produce carbon dioxide or chloroethanes as shown in figme 1.3
(Fathepure et al., 1988).

a ’1‘ O‘H‘ n /

[c.c\ ‘4 "1’
er a m c:’ ‘et \9 \

Ct °”’ no; u
c:21/¢./."i “.2.“

 

xr\I-.
)1

Figure 1.2 Pathways for transformation of PCB to VC.

 

 

 

Figure l.

Transforr

C814

i 1.]

”ME 3,

Figure 1.3 Suggested anaerobic dechlorination pathways for PCB and TCE
Transformation (From Fathepure et al., 1988)

 

 

(sacrum,

 

 

TCE 1

RD no RD

CHgCClg CHCICHCI CHCICHCI

 

 

 

 

 

r,r.ocr. i crane; coll-DCE

 

 

 

 

an an + u2
an

crrzcrrcr

 

vc

 

 

crrzcrr2 cn3crr2cr

 

 

surname a CA

i

Goa l

 

 

 

 

RD represents reductive dechlorination.

 

1
|

 

 

 

Carbor

Figure
& Met
Reacti
Althor
ICSCIII
agree

romet

Reac

OUCI
Caltc

Reac
This ;
dichlt

SOme

‘0' of a

Carbon Tetrachloride (CI‘)

Figure 1.4 provides a synthesis of various pathways for the transformation of CT (Criddle
& McCarty, 1991). The various steps in this Figure are described below.

Reaction 1.

Although there is a possibility that CT may undergo direct hydrolysis to form C02, most
researchers (Wade & Castro,l973, Koster & Asmus, 1971, Bakac &Espenson, 1986)
agree that the first step in the CI' reduction is the one electron reduction to form trichlo-

romethyl radical and a chloride ion.

CCI4 + e" —)"CC13 +Cl'
Reaction 2.
One of the suggested pathways(Fowler,1969) is the dimerization of trichloromethyl radi-
cal to produce hexachloroethane

2(ccr3) —> c206

Reaction 3.
This pathway results from the sequential dechlorination of CI‘ to chloroform and finally to
dichloromethane. The hydrogenolysis reaction of CI‘ to CF can be written as a two step

process. First is the removal of chloride as given earlier in Step 1 and the second step is

‘1 CCI3 + H+ + e' —) CHCI3

Some researchers (Kubik and Anders,1981, Egil et al., 1988) have examined the possibil-
ity of above reaction as being a two step process such that

'CCI3 + e’ —> CCl3'

CCI3' + H+ —-) CHCI3

 

 

 

Figurr
1991)
C:
- cs
J
*The

 

10

Figure 1.4 Known biotic and abiotic transformations of CI‘.(From Criddle & McCarty,
1991).*

CD
bywoiysis ?

 

carbon tetrachloride - CT

 

a. Cl Cl
6) ‘ ‘L
or _ cr rlc-c or

 

C1 Ci

R = R 2 i—CI \ . - X%r hexachloroethane
c": 1 2 — " °\
V
4 C' 02 or
c, 949 G) <9 ,
C
cell bound I ‘
”DEC! 'c ‘ 6le Cl H
Cl \ ’Cl chloroform - CF

C
A + -
211ml 0-0;} H * 2 e strong
2HCI - _ reductant
2am y 7 cr

HCOOH

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Cl
formic acid 9' \ Cl ,
c :0 \
mic—n CI I I ‘
monoxide . H H
phosgene

 

 

 

"2° dlchloromethane
2 HCI

carbon dioxide
*‘I'he products are shown in boxes.

 

 

 

1987.

Mole
(Saw

The p
hydro

to Pro

11

The other possible hydrogenation reaction (by Galli & McCarty, 1989, Vogel et al.,
1987) is

cool, + R-H —> crrcr3 + R'

Chloroforrn production can be avoided either by creating more reducing conditions result-
ing in the production of dichloromethane or by creating oxidizing conditions to produce
002.
Reaction 4.
Molecular oxygen rapidly adds to the trichloromethyl radical to give a peroxy radical
(Sawyer & Roberts, 1983) by the reaction

‘CC13 + 02—) CCI3OO'

The peroxy radical can be converted to C02 by two pathways. In the first pathway takes
hydrogen from an organic compound to produce trichlorohydroperoxide which hydrolyzes

to produce H202 and phosgene (Asmus et al., 1985).

ccrsoo-+ R-H -+ ccr3oorr + R
C0300“ '1' H20 '9 O=CCI2 + H202 + BC]

In the second pathway, two peroxy radicals combine releasing molecular oxygen and
alkoxy radicals (Monig et al., 1983).
2(CC13OU) —) 2CC130'+ Oz

The alkoxy radical can degrade to form phosgene (Bahnemann et al., 1987)

ccr3o- -> o=ccr2 + Cl-

 

 

 

A ’4?

 

Phosget

COaan

b

Reactit
Dichlor
1950;A

Dichlo

lion (A

DlChlo

12

OT
CCI3O‘+ H+ + e' —) CC13OH
CCI3OH —') O=CC12 +HC|

Phosgene produced by all the above mentioned reactions can be further degraded to form
002 and hydrochloric acid.

O=CC|2 + H20 —) C0; + 2HCl
Reaction 5.
Dichlorocarbene (CClz) is formed from chloroform under basic conditions, (Hine,
1950;Andres, 1982;Krimse,1971; March 1985).

CHCI3 + OH' —-) CCl3' 4» H20
CCl3' ->:CC12 + Cl‘
Dichlorocarbene is also formed by a two electron reduction of CT, as shown in the reac-
tion (Anders,1982)
CC14 + 2e' —-):CC12 + 2Cl'

Dichlorocarbene hydrolyzes to give CO and [or formic acid(March, 1985).

:CClz + H20 -) CO + 2HCl
:CClz + 2 H20 —) HCOOH + 2HCI
In the batch type electrolysis studies (Criddle, 1990, Rajayya, 1992) have observed two
pathways: hydrogenolysis of CT, and an unknown pathway resulting in production of
completely dechlorinated products.
Reaction 6.

CT can be incorporated into cell material by the addition to double bonds via pathway 6

 

 

 

 

Addifion

leXide (

 

19131'Tri

It

13

(March,1985).

Chloroform (CF)

The products of chloroform degradation difl‘er from those of CT and high yields of formic
acid are observed in one electron transfer of CF at a pH 7 (Amus et. al., 1985). The signif-
icant difference is a result of the existence of C-H bond in chloroform. In low oxygen sys-
tem various steps of CF degradation can proceed as follows:

CHCI3 + e' —) CHClz + Cl'

CHClz + 02 -—) CHCIzOO
2CHCIZOO —) 2CHCIZO + O;
CHCLZO —) HCl + Cl-C=O
CI-C=O —) C1 + CO

Addition of Oz in last two reactions would ultimately lead to the production of carbon

dioxide (Asmus et al., 1985).

l,l,l-'Irichloroethane (TCA)

Figure 1.5 illustrates the different pathways possible for the transformation of TCA (Vogel
et al., 1987), which can undergo two abiotic transformations as well as reductive dehaloge-

nation by anaerobic microorganisms.

-Dehydrohalogenation is one possible abiotic process in which 1,1 dichloroethene is pro-
duced, which can further be transformed into to vinyl chloride by reductive dechlorination

under methanogenic conditions.

 

CH;

{1

.I

l4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

crraccr3
TCA i
A
P)
A
C3290: ‘ ca3crrcr,
1.1-DCE— 1.1-DC A 7
RD RD
y Y
crrzcrrm crracrrzcr
1C CA '
B
Y Y
cn3crr=orr ca3coorr
ETHANOL ACETIC
- ' ACID
B 13
i r
C02 C03 C02

 

 

 

 

 

 

Figure 1.5 Pathways for the transformation of TCA under methanogenic conditions. A
indicates abiodc conditions; B indicates biotic conditions; an indicates reducrive dechlori-

nation. (From Vogel et al.1987)

 

It

 

 

 

-The or

chemie

- Under
roethant
ired by
field ob

(Parson

15

-The other abiotic process is hydrolysis that produces acetic acid. Acetic acid is fairly inert
chemically, but can be mineralized rapidly by microorganisms.

- Under the biological transformation route 1,1,1 trichloroethane is reduced to 1,1 dichlo-
roethane and then transformed abiotically by hydrolysis to ethanol. which can be. mineral-
ized by microorganisms. The complex pathways shown in figure 1.5 are consistent with the
field observations of products consistently found in groundwaters contaminated with TCA

(Parson et al. 1984, Parson & Lage, 1985).

 

 

The un

to trans.
This res
rates of
dehalog
applied
triehlor
bane. '1
applied
process

techno]

16

Hypothesis

The underlying hypothesis of this work is that electrolysis can be used as an efl'ective tool
to transform halogenated aliphatics via competing pathways in a continuous flow system.
This research adds to the current understanding of factors efl‘ecting these pathways and the
rates of reductive dehalogenation of simple halogenated aliphatics. The rates of reductive
dehalogenation, and the distribution of products can be by manipulated by varying the
applied voltage. Five compounds chosen for this study are: perehloroethylene(PCE),
trichloroethyleneCTCE), carbon tetrachloride (CT), chloroform (CF) and 1,1,1-trichloroet-
hane. The product distribution for each of these compounds is evaluated at four difierent
applied voltages. Reactor configurations and factors efi‘ectin g the cost and efficiency of the
process are also investigated to explore electrolysis as a possible new treatment

technology.

17

Chapter 2
Electrochemical Reactor Theory

Fundamental Concepts

An electrochemical reaction is a heterogenous chemical process involving the transfer of
the charge to or from an electrode, generally a metal, carbon or semiconductor. The charge
transfer may be a cathodic process in which an otherwise stable species are reduced by

transfer of electrons from an electrode, as shown in the reactions below:

2H20 + 26- = H2 + 20H-

CU2+ + 28- : Cu

Conversely, the charge transfer may be an anodic process involving removal of electrons,

from otherwise stable species, to the electrode. Example of such reaction would be:

CH3OH + H20 — 66— = 002 + 6H+

2H20 — 48- = 02 + 4H+

Electrolysis is only possible in a cell which contains both an anode and a cathode, and in
order to avoid the accumulation of net positive or not negative charges, the amount of
reduction at the cathode is equal to the amount of oxidation at the anode. Electric current
flow between an anode and a cathode placed in an electrolyte is the combined result of
electrode processes, charge transport and mass transport brought about by the imposition

of an electric field across the electrodes. The relative strength of individual components in

ll

 

 

derot

nanue

voltag:
extrem
region

VD,du
flow in
mama
BC rep
uxnpo
leconr
ftpresc

aquem

18

the total current flow depends essentially upon the strength of the electric field, if the
nature and the composition of the electrolyte is known. In figure 2.1, a typical current-
voltage drop relationship is shown. At the voltage drops below VD (curve portion OA),
extremely small current flows through the reactor due to a transient capacitive effect. This
region is not of much interesr in practical electrolysis. As the voltage is increased beyond
VD, there is a sharp increase in the current due to the electrolytic polarization. The current
flow in this region is due to the kinetics of the electrode processes involved. The ohmic »
resistance of the electrolyte plays a much lesser role than the kinetic resistance. The portion
BC represents the mixed control region where kinetic, convective and diffusive
components are all significant contributors to the overall transport process. The last two
become increasingly important as V is increased. The rising portion DB of the curve
represents the on set of a new electrochemical process such as the electrolysis of water in

aqueous solution.

f Limiting Current

 

 

 

C D / E
[L __ __________ _ /
B
l
+ 1
Current :
| l
l
l
A , i
/
l
/ [W 1/ VB .
0 Voltage Drop betwwen cathode & anode

Figure 2.1 The variation of elecuic current in an electrochemical reactor with voltage drop

across the anode and the cathode.

 

Inrt
isrej

thee

react
eieet
hrco
agnt
ofCu
to(3u
fall in
vohag
hegcy

I ' A“
srgrm

19

In region CD, convective and diffusive components become fully predominant. This range
is represented by the limiting current IL, which is the largest current that can flow between
the electrodes for a particular set of electrode reactions. In this region, the kinetic rates are
so rapid that the concentration at the surface of the electrode approaches zero and the
reacrion rate is solely determine by the mass transfer. In most of the industrial
electrochemical processes, the practitioner encounters only regions AC or AB of figure 2.1.
In contrast, there are special situations where processes at one particular electrode will
significantly determine the conditions for the reactor design. For example, in the deposition
of Cu from aqueous Cu(NO3)2 solution, the cathode process is the discharge of eupric ions
to Cu and anodic process results in the evolution of oxygen gas. The anode reaction will
fall in region AB of the curve, but the cathode reaction can occur in region CD, if the
voltage drop across electrodes is properly set. Then, the overall ionic transport process will
be governed by the limiting current at the cathode. Hence cathodic process will be a

significant factor in the process analysis and would determine the overall reactor behavior.
Simplified analysis of plug-flow model of electrochemical reactors

Consider two electrodes, an anode and a cathode separated by a certain distance, as shown
in Figure 2.2. Electrolyte is flowing parallel to the electrodes at a constant volumetric flow
rate Q and a current (of density i) is applied between the electrodes in a direction normal to
the electrolyte flow. In the small section of the reactor shown in Figure 2.2, the mass

balance equation at the steady state is written as

. AA
QC: - QCr-l-Ar: : 2r‘_ (2'1)
2F

20

 

——->l ~-—->-
CX T Cx-r-Ax
ran

 

 

 

x ® x+Ax

Figure 2.2 Illustration of plug-flow model.

In equation 2.2 cx is the average concentration of an ionic species at an axial distance x.
Considering A" as a small increment in spatial position x, the Taylor expansion can be

written for average exit concentration as

If Ax is very small, the squared and higher order terms can be neglected. Substituting

equation 2.2 in 2.1 we get,

dc
dz:

AA _. an
2F —tz 2F

 

Q01: - Q [or + Ax] = iI (3.3)
Where a is the electrode area per unit electrode length. Upon simplifying on the left hand

side and dropping the multiplying factor Ax on borh sides, we obtain the fundamental PFM

21

equation,

 

The overall material balance of the PFM is obtained by integrating equation (2.4).

 

C: a L .
‘/Cr dc: _zFQ/o ads: (25)

where L is the electrode length, c1 and 02 are the average inlet and outlet concentrations of

the reacting ion. Substituting the value of current given by the equation
L
I = '1d 2.6
a [0 z :r ( )

into equation (2.5), a very simple expression is obtained relating influent and effluent

concentrations.

_1_
zFQ

C2=C1'—

Equation (2.7) does not require any information about the variation of the current density
along the electrodes or the knowledge of the velocity profile between the electrodes. The
importance of the PFM is that it will allow the estimation of the electrode length, hence
electrode area, required for a specific conversion if variation of the current density with

axial distance is known.

The current density can be expressed in terms of concentration driving force and apparent

22

mass transfer coefficient as

ir:ZFkr(C—C6) (.

[\J
00
V

where CC is the x axis dependent concentration of the reacting ionic species at the electrode.

Combining equations 2.4 and 2.8, following expression is obtained.

C2 dc . a L
= —— krd 2o
fa, c — cc Q/o a: ( 9)

 

The average mass transfer coefficient can be written as

k 1Lid ‘210
m—Z/O :3 ()

Substituting this in equation (2.9), the length of the electrode can be written as,

 

L__9_/c’ dc (2.11)

akm C] C — Cc

Under limiting conditions when I=IL, we can assume ce=0, hence the above equation can

be integrated to give a simpler form:

Q C 1
Lmtn. = — e — ‘ '
C”cmlog C? (2.12)
where Lmin. is the minimum electrode length required for this reaction, according to the
plug-flow model. Since the actual current flowing through the reactor will be smaller than

IL, the actual length of the reactor would be greater than Lmin.

23

Estimation of Mass transfer Coefficient

The mass transfer coefficient km is a cornerstone of the rational design and analysis of
electrochemical reactors. There are three methods for the computation of the mass transfer
coefficient.

1. Determination of km via experimental boundary layer thickness and diffusivity data.

2. Approximation of km with either data obtained elsewhere by researchers or theoretical
relationships in the literature related to a similar system.

3. Sometimes, mass transport data of acceptable accuracy does not even' exist for a
particular mass transport- geometric configuration. In such a case, heat transport data is
employed assuming sufficient similarity between heat and mass transfer phenomena.

(Fahidy, 1985).
These methods for estimation of mass transfer coefficient are discussed below.
1. Determination of kIn using diffusivity data

For a simple preliminary approach, the total current flowing through an electrolysis cell can
be considered to be made up of two components: the first is due to the diffusion of the

active ions as described by Frck’s First Law; and the second is due to the migration of ions
in an electric field. Let y be the distance normal to the electrode surface, d is the diffusivity
of the electrode and t is the transference number of active ions. Then, ti is the portion of the

current I carried by migration of the active ion, and the total current can be written as

I = zFDlAc (dc/ax)y=o + :1

24

The current density i is given by

, zFDl
r = — (dc/dx)y=o
1— t

Assuming i1 -—> 0: Ce—)0, the concentration derivative at the electrode surface may be

replaced by the ratio Cal AN, where AN is defined by the projection of the intersection in
Figure 2.3. Consequently, expression

. ZFDI Co:
1L " .—
1- I AN

 

relates the limiting current density to the length of the Nemst boundary layer AN which is

the average value of the boundary layer varying along the electrode. In the presence of

other ions whose migration is faster than the migration of active ions, t approaches zero

and the above expression is reduced to

 

iL: ZFDl—C:.
7m
C
A
T C... Xle

 

* 3’
Distance from electrode perpendicular to flow

Figure 2.3 Concentration profile along y-axis.

25

The final expression for AN is

O:

The value of AN can be determined experimentally by observing the concentration from
electrode surface perpendicular to the direction of flow. The mass transfer coefiicient can

then be defined as
Km = DIAN

2. Estimation of mass transfer coefficient using empirical relationships

Although, the above mentioned method for the computation of km is most reliable yet the
electrochemical engineer very often does not have the time and means necessary for such
experimentation. Thus, there is a strong need to estimate the value of mass transfer
coefficient using empirical relationships presented in the literature for systems that are
similar to the system of interest. In this case, it is very important to exercise proper
engineering judgement in estimating the values of mass transfer coefficients.

Most of the mass transfer data in the literature has conveniently been expressed in terms of
dimensionless relationships relating to Sherwood number (Sh), Reynold’s Number (Re),
and Schmidt Number (Sc). These dimensionless numbers can be defined as

Sh = iLL/zFDc , where km = (d/L) Sh
Where L = length of the reactor, D is the axial diffusivity.

R6=VOUV

and

26

Se = V/D.
For a flat plate electrodes following relation holds good (Fahidy, 1985)
Sh = 0.0678 Rel/2 Sal/3

For a porous packed bed electrodes the empirical relation (Fahidy, 1985) between these

dimensionless variables is given by

Sh = [(1-c)0'5/(1-e)] Rel/2 Sc1/3 for 0.25<e<0.5
40< (Rel 1 -e) <4000
A number of empirical relations have been given (Schmal at el., 1985) to represent the mass
transport phenomenon for various electrode materials:

For a single platinum fibre (dia 10 ltm), the following relation holds good

Sh = 6.1 Re0'352 for 0.02<Re<0.15

Measurements on carbon felts (dia 25 pm), have been published by Kinoshita and

Leach(1983) as
For 0.25 cm thick felt Sh = 1.29 Rho-72 for 0.01<Re<0.25
For 0.175 cm thick felt Sh = 1.01 Room for 0.01<Re<0.25

The mass transfer correlation for a single fiber electrode of 8 “m diameter is given by

Schmal at el.(1985) as
Sh = 6.1 Re°352 for 0.02<Re<0.15

—-——

 

27

3. Estimation of km using heat transfer Data
As mentioned earlier, the process and equations in the heat transfer are compatible with
those in mass transfer. Simmonsson (1984) has given an empirical relationship between Sh,

Re, and Se numbers for heat transfer for a single wire:

Sh = 0.9 Re0'38 Scan for 0.1< Re < 50
For dilute aqueous solutions, Sc=1400 resulting in

Sh = 5.1 Re°°385 For 0.1< Re < 50

Based upon several mass transfer studies, Schmal at el.(1985) have also concluded that
carbon fibres have higher mass transfer coefficients and are very suitable for the treatment
of aqueous waste. They also concluded that for scaling up purposes, the mass transfer
properties of the fibres can be directly calculated from the relation for a single fibre and the
total external surface area of the fibres in the felt.

Some of the modelling aspects of an electrochemical reactor are discussed in Appendix B.

Chapter 3
Materials & Methods

Experimental Setup

An electrochemical plug flow reactor was constructed to Study the reductive dehalogination
of contaminated waters as shown in Figure 3.1. The reactor consists of two plexi glass anode
and cathode compartments that can be tightly held together by twelve screws provided
along their circumference. Each compartment has an inlet and outlet port and a series of
sampling ports along its length. A bell is provided at the top of each compartment to capture
gases produced during electrolysis. The two compartments are separated by a proton per-
meable Nafion 117 membrane (Dupont Co., Wilmington, DE). The membrane is sand-
wiched between the two compartments, and the entire unit is screwed together. Each
compartment has an O-ring along its cross section to prevent any leakage between the
plexi glass and the membrane. Graphite fibers (Thomell VMA Material, Amco Perfor-
mence Products, Greenville, SC) were used as cathode material for the following reasons,

as given by Schmal et a1. (1987).

Electrodes with high specific surface area have higher volumetric removal efficiencies.
Graphite fibers are well suited due to their high specific surface area.

-Graphite fibers are manufactured in large quantities for the production of strong, light
weighted reinforced plastics and are readily available and cheap.

-Graphite fiber electrodes are less prone to clogging by small particles in the waste streams
than many other possible electrode materials.

-Graphite fibers have a relatively high over potential for hydrogen evolution which is favor-

able for efliciency of the dehalogenation reaction.

29

.533 5 5553.23 03.262 .3 8:53 och—8.3.9 o... 8.. 5.88 So: wan .au_Eo._uo=uo_m fin 9.53m

‘|

   

I];

N 95a 2:232.

    

 

2.35:2.

    

 

Eggs—:8

  
 

_ O

       

be?“
838 >w

    

   

9:2.
09:53

 
      

     

=0.—

  
    

   
  

 

Ezczaa .5228
:53
3.2. \g cummawoe
COS—10m M::Q—=mm [I

  

.50“ £3.02:

         

Stan
eummuwue 3.5a
SEQ—cw

ucflenEu—z
cocaz

09:95.0.—

       

~=Te~ + :3
:z Alb + +2 + .m
.X + «To + xx
”3233. EuEzmnEoQ coo—=8

< :9. EesEe
.3. + +2.. + ”0‘19: N
”:OEUNUE .=05_thEOu DCOCN

30

Graphite fibers were also used for the anode because the current required for reductive
dehalogenation was found to be limited by the anode surface area when anode materials
with less specific surface area were used. To prevent this limitation, anode surface area had
to exceed that of the cathode. A regulated power supply source (Lambda Model LLS-6008)
was used to apply a constant voltage between the anode and the cathode. The power supply
source was programmed for a constant voltage, and this voltage was periodically adjusted
by reprogramming the supply source to maintain a constant potential between the cathode
and a reference electrode. A reference electrode (Orion Model 90-02 Double Junction Ref-
erence Electrode with Ag/AgCl with a 1M NaZSO4 filling solution) was placed in the cath-
ode compartment, and the potential difference between the tip of the reference electrode

and the point X on the cathode electrode were constantly monitored with a multimeter.

In order to achieve a constant concentration of the contaminant in the influent line, two sep-
arate flow lines were combined using a three way valve. One of these two lines delivered
the degassed buffer solution (with pH of 7.0 using 0.1M NaHzPO4 + 0.06 M NaOH solu-
tions) from a reservoir at a constant flow rate of 2.3 mL/min using a peristaltic pump. The
second flow line provided a saturated solution of the contaminant at a flow rate varying
from 0.05—0.1 mL/min depending upon the influent concentration desired after combining
the two flow lines. Flushing of the anode compartment was accomplished by pumping at a

constant rate of 2.7 mL/min.

31

General Experimental Procedure

The following sections describe the methodology followed in a typical experiment.
Assembly of the electrolysis cell

The pattern in which the fibers are packed in the two compartments had a significant efi‘ect
on the dehalogenation and the voltage requirements. Under similar experimental condi-
tions, a change in orientation of fibers resulted in Significant variation in applied potentials
(to maintain the same reference voltage) and also resulted in different degradation rates for
the target chemicals. Thus, a standard pattern was adopted to pack both electrodes in all
experiments. The graphite fibers are available in sheets of layered fibrous material. Pieces
of standard size 6.0” x 0.75” were cut from these sheets stacked parallel to one another and
weighed to obtain the required electrode weight. The weighed electrode material was
soaked in water for 2 minutes and then squeezed by hand to remove most of the water. The
graphite fibers were then packed in the cathode or anode compartment so that the flat sur-
face of the layers was parallel to the membrane. Care was taken to ensure that the layered
pattern of the fibers was not disturbed upon packing and sealing the electrode cell. To com-
plete cell assembly, the membrane was placed between the two compartments, and the two
compartments were tightly screwed together by the twelve screws that looked the mem-

brane into position. The rest of the connections were made as shown in Figure 3.1.

Analytical Methods

After applying a voltage, efiluent chloride concentration and volatile organics were con-
stantly monitored. Volatile organics were also monitored before applying voltage in order
to obtain the breakthrough curve. Chloride measurements were made with a chloride elec-
trode (Orion Research, Inc. Cambridge, MA) connected to an Ion Analyzer (Orion Model
801 , Cambridge, MA). For calibration, a 100 ppm standard NaCl solution and various dilu-

tions were prepared to cover the range of expected effluent chloride concentrations. To

32

monitor effluent chloride concentrations, SmL samples were collected regularly and
assayed. The chloride concentration in the effluent was also determined before applying a
potential to determine the background chloride. This background chloride was subtracted
from each observed chloride reading to obtain a corrected chloride concentration attribut-
able to dechlorination reactions within the reactor. The effluent concentration of bromide
(used in the tracer studies) was monitored using a bromide electrode (Orion Model 94—35).
The calibration and measurement procedure was similar to the one adopted for chloride

measurement.

A standard procedure was adopted to prepare the standards and secondary dilutions for the
analysis of volatile organics. A 6-mL bottle weighed dry to the nearest milligram was filled
with methanol and tightly capped with a Teflon Mininert valve (Alltech Associates, Inc.,
Deerfield, IL). After 15 minutes, the vial was reweighed to the nearest milligram. Care was
taken that the weight remained constant and there was no evaporation of methanol from the
vial. Using a 50—111. glass syringe, pure compound was injected into the methanol solution.
The vial was weighed again and the pure compound was added to the methanol solution till
the desired stock concentration of 20-30 mg/mL was achieved. Standards for all the com-
pounds used for analysis in one experiment were prepared in the same serum bottle. Fresh
standards were prepared for every experiment, and secondary dilutions were prepared
immediately before GC analysis.

For secondary dilutions, a 20 mL vial containing 5 mL degassed bufier solution was spiked
with 1-6 “L of methanol standard and quickly capped A series of dilutions were prepared
to cover the complete range of desired concentrations(0-30 mg/L). When the effluent con-
centrations were less than 4 ppm, a second set of standard solution was prepared in metha-
nol using the same procedure indicated above, but over a lower concentration range. The

ranges of standard stock solution concentrations and secondary dilution concentrations for

Table 3.1 Concentration ranges for the stock solutions and secondary dilutions.

 

 

 

 

 

 

 

 

 

 

 

Target Volatile Stock Solutions Secondary Dilutions
Chemical Products (cone. in mg/mL) (cone. in mg/L)
Analyzed
low range high range low range high range
PCB PCB 05 0-30 0-4.8 036
TCE 02 0-20 02.4 024
TCE T CE 0.30 036
CT 0-4 030 0-4.8 036
CF 04 0-15 04.8 0-18
CT
MC 0-10 012
CM 0-10 0-12
CF CF 0-4 0-30 04.8 036
MC 0-4 0—10 0-12
TCA TCA 0—4 0-35 0-4.8 040
DCA 0-30 0-36
CA 0-10 0-12
Nomenclature: PCE Perchloroethylene or tetrachloroethylene
TCE Trichloroethylene
CT Carbon tetrachloride
CF Chloroforrn
MC Methylene chloride
CM Chloro methane
TCA ‘Iiichloroethane
DCA Dichloroethane
CA Chloroetlrane

34

various target chemicals and their products are given in table 3.1.

Samples were analyzed for volatile organics on a Perkin Elmer Auto System GC connected
to a Auto Sampler (Hewlett Packard Model 19395A). The Flame Ionization Detector was
used for the analysis. The samples were placed in the auto sampler and the standards were
placed between them. The auto sampler was programmed for an equilibration time of 90

minutes, a bath temperature of 90°C, and a valve temperature of 95°C.

Electrolysis Experiments
A list of experiments and the typical conditions under which these experiments were con-
ducted as given in Table 3.2. For all contaminant studies the following protocol was

adopted.

Initially both compartments were filled with the degassed buffer solution. Peristaltic pump

1 was started, and the desired flow was established. Buffer was pumped through the cathode
compartment for 15 minutes so that most of the air in the fibers was flushed out. The syringe
pump was then started, marking the initiation of timing for each experiment.

The syringe pump was loaded with two 140-mL syringes containing 280 mL of degassed

buffer solution, and was operated at a flow rate of 005-0.] mL/min giving the desired influ-

ent concentrations of various target contaminants.

Samples were collected fi'om the efiluent port at regular intervals. In a typical sampling pro-
cedure, the bottom of port A was plugged by a glass stopper. The tip of a 5 ml glass syringe
was lowered into the T joint and a 5 mL sample was collected as shown in Figure 3.2. This
sample was transferred to a 20-mL vial and quickly capped. Samples were collected every
20-30 minutes until the influent concentration of the contaminant equaled the cflluent con-

centration in the cathode compartment. This stage(breakthrough) was reached when all the

35

Table 3.2 List of experiments conducted

  

Target Reference ‘Weight of Weight of Cathode
Chemical Voltage“ Cathode Anode

(a) (g)

 

 

 

 

1 3.0 5.8
2 3.0 5.8
3 3.0 5.8
4 3.0 5.8
5 5.5 5.8
6 5.5 5.8
7 5.5 5.8
8 5.5 5.8
9 5.5 5.8
10 5.8
11 5.8
12 5.8
13 5.8
14 5.8
15
16
17
18
19
25
26
27
28
29

 

* Versus Ag/AgCl reference electrode with NaSO4 filling solution

36

   
  

5 ml glass syringe

 

4— reactor wall

sample

cathode

compartment
glass stopper

 

effluent port A

 

  

Figure 3.2. Effluent sampling.

degassed buffer solution in the cathode compartment was completely replaced by the con-
taminated water, and there was no more sorption of the contaminant either to the fibers or

to the walls of the reactor. Generally it took 3-3.5 hours to achieve breakthrough.

Once breakthrough was achieved, peristaltic pump 2 was started, and a flow of

2.7 mL/min was maintained. Most of the air in the anode compartment was flushed out by
pumping the compartment with degassed buffer solution for 15 min, and the power supply
was switched on. The applied voltage was gradually increased until the reference voltage
was equal to the desired set point. A constant reference voltage (between the cathode and
the reference electrode) was maintained by manually adjusting the applied voltage. Sam-
ples were taken every 10-20 minutes. The electrolysis experiment was stopped when the

chloride concentration remained constant for 40-60 minutes, indicating steady state condi-

tions

37

Tracer Studies

Tracer studies are conducted to determine the difl‘usion characteristics of the reactor. After
completing electrolysis of a test solution, the contaminated water in the cathode compart-
ment was drained and filled with fresh degassed buffer solution. The cathode compartment
was pumped with degassed buffer solution for 15 minutes to remove trapped air in the com-
partment. A standard solution of NaBr was then pumped at a flow rate of 2.3 mIJmin. The
effluent bromide concentrations were constantly monitored and the experiment was

stopped when the efiluent bromide concentration was constant for 20—30 minutes.

Efficiencies of electron transfer

The generic expression for hydrogenolysis of the target chemicals can be written as

CaleCIC + f e' + f H+ —> CaHb+f_d Clc,d + d HCl

The current (in Amps.) required for this hydrogenolysis reaction can be calculated by the

 

 

 

equation
i = $3,135,?" CaHbC'c x fmoles of e’ x 96.48103coulomb
hydrogenolysis time (in sec) mole of C,,III,C1c mole of e‘
degraded

The efficiency of hydrogenolysis is calculated as

__ lhydrogenolysis X 100
n hydrogenolysis " i

 

total

Let fmax and fmin represent the maximum and minimum number of electrons that can be
transferred] mole of target chemical degraded to achieve complete dechlorination. In addi-
tion to calculating the efficiency of hydrogenolysis, the maximum and minimum possible

efficiencies ‘1th wand 11mm”) are also calculated by the above equations utilizing

38

{m and fm respectively instead of f. Here 11min and Tina“ represent the minimum and
maximum efficiency of the pathway leading to complete dechlorination. To find the overall
range of efficiency of the complete reaction, nm(m)and nmmxmax) are calculated by

using the following equations

‘1 toml(min) = “hydrogenolysis +11 min

Tl mama) = 1] hydrogenolysis +nmax

Table 3.3 lists the values of fmax used for calculating “(maro- The calculations for maxi-

mum efficiency are based upon the assumption that the maximum number of electrons

Table .3.3 assumed values of fmax

 

 

 

 

 

Chemical fmax
PCE 8
TCE 6
CT 8
CF 6
MC 4
CM 2

 

used for dechlorination is equal to twice the number of chlorine atoms present in the target
chemical (hydrogenolysis). In order to calculate "(111111), it is assumed that at least 1.0
moles of electrons are used [mole of target chemical degraded to achieve complete degra-
dation. This is not a correct assumption as complete dechlorination can be achieved via

hydrolysis without any transference of elecnons, but in such case no current will be

39

required for the complete dechlorination pathway. Thus in order to chose a reasonable

minimum value, fmin is assumed to be 1.

The results of each experiment were reported as the compounds degraded or produced
(represented in micro moles as chloride) plotted against the number of pore volumes
exchanged. The number of pore volumes exchanged were obtained by dividing the time
elapsed since the beginning of the experiment for each sample by the effective reactor

retention time as observed by the tracer studies.

Chapter 4.

Results & Discussion

The results of electrolytic degradation of the target chemicals (PCE, TCE, CT, CF and
TCA) are represented in this chapter. Each degradation curve is accompanied by mass bal-
ance curves for the chlorine and carbon. All chloride measurements were corrected for
background chloride present in the influent Chlorine mass balances were obtained from the
concentrations of volatile reactants, volatile products and aqueous free chloride in the efliu-
ent. Carbon balance curves were constructed using the concentrations of volatile reactants
and products. Due to hydrogen evolution in the reactor, stripping of volatile organics was
also observed. A good carbon balance could not be obtained because the eflluent was not
analyzed for the dissolved organic carbon in the aqueous phase. This resulted in an uniden-
tified sink term in the carbon balance curves. Degradation rates of various target chemicals
and the production rates of the volatile products and free chlorides are also calculated. As
described in Chapter 3, efficiency calculations and cost analyses were tabulated for each
experiment conducted. Patterns of degradation are compared with those of Rajayya (1992)
as summarized in Table 4.13. Calculations were preformed to estimate the amount of PCB
sorbed to the fibres. The amounts of other sorbed chemicals can be calculated using the

same approach as the one used for PCE.

Tetrachloroethylene

The results of PCB degradation, including the mass balance curves for all nine experi-
ments conducted, are shown in figures 4.1-4.12 and figures 4.14-4.28. In all these experi-
ments, two parallel pathways were observed for the PCB degradation: hydrogenolysis of
PCB to TCE, and degradation of PCB to completely dechlorinated products. The degrada-
tion rates of PCB, and the production rates of TCE and free chloride are shown in Tables

4.1 and 4.2. Figures 4.13 and 4.29 Show the percentages of PCB degradation contributing

4O

41

 

500 r l l

 

 

 

 

 

Q) E I I l
E
L 1 '4
2
‘5 400- -
U)
o q
E 500- -
o - : fl.
is
8 2004 __
.2
E .
Chloride
.5 100.1 Bromide(trocer) . a . . _
g owe-«mud. .,
8 TCE
O 1 l r ‘ l r r I
0 1 2 3 4 5 6 7 8

# of pore volumes exchanged

PCE DEGRADATION AT REF. VOLTAGE OF —1.8V

FIGURE 4.1

 

 

 

$00 4 j , Y
D ’\/
2 ‘00
§
:2
u
«n 300
O
in
2
o 2004
E
I
2
9 100
E

0' I I T

3 9 5 5 7 ll

 

I of pore volumes exchanged
CHLORINE BALANCE FOR PCE DEGRADATION AT -—1 8V

HGURE 4.2

 

micro—moles as carbon
§

 

 

 

.1 3 i e i 0
§ of pore volume! exchanged
CARBON BALANCE FOR PCE DEGRADATION AI -l,8V

FIGURE 4.3

42

 

Cone
ES
0 0
L J
\
t
I
I. .
.-
._I
0
m
L

 

r8 7004 l T ' I I r I r

'c y
o

E 600-1 .1
o 'i
8 500-1 _
a ‘ 1
‘5 4.00.. -
E s I 4
c', 5004 _
L.

.2 ‘

E 200- .- --

C J . -" " Chloride "
._.. BI'OMIGOUM) .I "

 

 

 

4;
01
0)
N.
m

# of pore volumes exchanged

PCE DEGRADATION AT REF. VOLTAGE OF —1.9V

FIGURE 4.4

 

Micro—motes as Chloride

 

 

 

.1 l E E i 0
I of pore volume: exchanged
Cl-ILORINE BALANCE FOR PCE DEGRADATION AT -l.9v

FIGURE 4.5

 

 

 

8 \
e 1204 ‘ 4
C
U
. tool \ 4
o
'2‘ so- 1
° 1
IE so
0
§ ‘°*
E

zo-I

O r f Y ‘

.I 4 5 I 7 II

 

I of pore volume: exchanged
CARBON BAUNCE FOR PCE DEGRADATION AT -I.9V

HGURE 4.6

43

 

   
  

 

 

 

q, 500 ‘T r“ l l
E .
L
O
E 500-J _
U
m -I
° 400-1 ..
tn
2 I 1
o 0
IE 300* Chloride "
O l ..I . I I -<
8
-- 200-1 . u
E .
C Bromide(Trdcer) . '1
d 100-1 . Md. /.———‘ TCE .—
c . _ .
8
0 l r
0 i 2 i l S 6 7 a

# of pore volumes exchanged

PCE DEGRADATION AT REF. VOLTAGE OF -2.0V

FIGURE 4.7

Micro-moles o: chloride
r 3 E
micro-molds as carbon
8 8 8
JL L L

Im-I

 

 

 

 

 

 

.1 i 3 s i 11 ' .1 i i 3. 5 11
Id! pore volume: uehmqed Iotpenvolumu exchanged
CHLORINE BALANCE FOR PCE DEGRADATION AT -2.UV CARBON BALANCE FOR POE DEGRADATION AT -2.0V

FIGURE 4.8 FIGURE 4.9

 

44

 

 

 

 

 

(D I 1
:9 700- -
L.
.9. * '1
‘5 600- , a —-
g ' 1

500- -
U)
2 ..
0
IE 400': "'
9. 300- chlonde_
.2 .l .- fl... D-I
E 200- " -
.E q .i' .
. mam) ." TCE
2 100'” ff“... [W -
8 ‘ -' '

O l I l l l l r l
0 1 2 3 4 5 6 7 8 9

# of pore volumes exchanged

POE DEGRADATION AT REF..VOLTAGE OF —2.1V

 

 

 

 

 

 

 

 

FIGURE 4.1a
800-1{—- r v a ‘a K ' - ‘ 1
0 700-1 1. c M" .
§ 500.1 \_\ /~.—-——°—-—-" j g Ice-1 \ 4
5 500-1 / 4 :1 1204
a j 1 : TOO-4 W
E ‘°° % 00-1
{5’ “1 J E m
g zoo-T ‘ g 004
E 'm‘: 4 204
o :1 5 5 i 11 a” i A 6 11
{upomvoomooxchonqoa '“mmuw
momma am: FOR PCE oeomomou AT -2.1v cm ammo: roe PCE DEGRADATION AT -2-W

FIGURE 4.1 T ncuRE 4,12

45

Table 4.1 PCB degradation using 3.2 g cathode fibres and 5.8 g anode fibres.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Expt. Ref. PCE Degradation TCE Producrion Chloride production
No. Voltage Rate( 11 moles/min Rate( 11 moles/min Rate ( [.1 moles/min
auhlmidpl 41211112111191 We)
1 -1.8 1.65 0.73 0.91
2 -1.9 A 2.51 0.48 2.03
3 -2.0 3.54 1.01 2.51
4 -2.1 4.72 1.51 2.83
PCE
Expt. Ref.
No. Voltage
1 -1.8 44.2 % 55.7 % 0.1 %
2 -l.9 19.1 % . 80.9 % 0.0 %
3 -2.0 28.5 % 71.6 % 0.0 %
4 -2.1 31.3 % 59.9 % 8.8 %
Hydrogenolysis Complete Sampling
Dechlorination Errors

Figure 4.13 Parallel Pathways for the PCB degradation using 3. 2 g cathode and 5. 8 g
anode fibres.

46

 

 

 

 

Q) I T I T I
33 300-1 _
:3 275-1 / POE _
Mb.“
0 250-I Min-u ..
m
D 225.1 I'll ,
3 2004 c onde _
0 175-1 0"“ ° ° ° ..
E . .
A 1 50" ,’ Bremideflraoer) T
5 125.1 unmet-unle- ° _
.9. / o TCE
E 100- ..
Bro-0
.E 75'J 6° A] "
L5 50"I o ’ J 1
g 25- o/ _
Q 0 1
I
o 1 2 3 i 5 E "I a

# of pore volumes exchanged

RCE DEGRADATION AT THE REF. VOLTAGE OF —l.8V

 

 

 

 

 

 

 

FIGURE 4.14

013' r IOOVi
5 z
- O
5 soc-I 8 ° .\
= W e. .
3 \ \r, 3
3 3 1°-
;1 ZOO-I CI .
2 2
I I
:3, 1004 g
g E 204

O 0 v . ‘

o s e 7 11 4 5 e 7 0

l at pore volume. exchanged
CHLORINE MOE FOR PCE DEGRADATION AT -1.8V

noun: 4.15

 

 

I of pore volumee exchanged
CARBON BMANCE FOR PCE DEGRADATION AT -I.8V

FIGURE 4.16

47

 

700 r r
6004
5004

400%

 

200- Chloride- "' " _
. .-

in micro—moles as chloride

BromideCl' racer)

-' TCE-
. Ollie-lunar W
0" l r 1 T.. r r
O 1 2 3 4 5 6 7
# of pore volumes exchanged

PCE DEGRADATION AT REF. VOLTAGE OF —1.8V

Conc.

 

 

FIGURE 4.17

 

 

 

 

 

 

 

700.....- r . . 150..__..s A —~ —1—»---- <1- —— ,_..--_
1 I N j
g 500* 4 g 120 I \\\ I
u u \
" IW‘I \_‘ I‘
a 900‘ < O V’ »—-o
c 1 3 0°
3 3004 4 o
E E 1104
1 I g)
3 200-4 4 § ‘0‘
E ‘°°‘ ‘ E 204
c I Y I c 1 v 1’
.1 6 5 I .l 4 5 6 7 II
I at pore volumes exchanged l at pore volumee exchanged
CHLORINE BALANCE FOR PCE DEGRADAIION AT -1.av CARBON BALANCE FOR 9c: DEGRADATION AT -1.8v

FIGURE 4.18 FIGURE 4.19

 

48

 

  

 

 

 

Q) 700 ' I ' I I I I I I
.‘2 . .
L
2 600-1 ..
.C
U . .1
8 500-1 a
3 .
'5 400- _
.E
8 300* Chloridé
.2 4 _ .
E 200-1 -
.E « .
0' 100- (n, —
é . “flu. "'

0+ ' I ' I ' I I I I I

0 1 2 :5 4 5 6 7 8

pore volumes exchan ed

of
PCE DEGRADATION AT REF. VOLTAG OF —l.9\/

FIGURE 4.20

 

 

 

 

 

 

 

 

I80 f T 799 v T I
1
use-I \ . 0 600-1
3 160-1 '\ 1 g
a I 4
‘ ‘ - 500-4
3 120-1 . 5 l V ‘
. 1
TE ”'4 " § “4 '1
:9 eo-I 1‘: 200-1 4
2 ‘04 2 I
E E too-l
ZO-l I
o A 5 e i 11 ' 41 5 3 7 11
{melumee exchanged lotporevolmneeexchanqea
CARNN BALANCE FOR PCE DEGRADATION AT -I.9V CHLORINE BALANCE FOR PCE DEGRADATION AT -1.9V

FIGURE 4.21 FIGURE 412

in micro—moles as chloride

Conc.

i of p
RCE DEGRAD TION

micro-moles as chlonde

7004

700

49

 

600g

500%

400- ’

300%
2004
1001

0

  

chloride "

 

 

0

 

ore volumes exchan ed

AT REF. VOLTAG OF —2.0V

FIGURE 4.23

 

 

 

4|

5 3 5
I at pore volumee exchanged

 

CHLORINE BALANCE FOR PCE DEGRADATION AT -2.0V

FIGURE 4.24

 

1604 \
1001 7.2
1204 k,“

micro-moles as carbon

 

 

 

41 i i 5 11
I of pore volumes exchanged
CARBON BALANCE FOR PCE DEGRADATION AT -2.0V

FIGURE 4. 25

50

 

   
   

II/- I J q
/'I -I

~
e ‘0' "
I

Chloride

  

 

 

a) I I
E 700-1

L.

2 1

'5 500-1

(II '1

° 500-1

(0

2 .

° -1

I; 400‘

8 300-1

.9. I

E 2004

.E .

C

o It

0 o . .

 

# of pore volumes exchanged

RCE DEGRADATION AT REEVOLTAGE OF —2.IV

FIGURE 4.26

 

micro-moles as chloride
é

 

 

 

I at pare valumee exchanged
CHLORINE BALANCE FOR PCE DEGRADATION AT -2.1V

FIGURE 4.27

 

micro-melee as carbon

 

 

 

41 s e 7 e 11
I at pore volume: exchanged
CARBON BALANCE FOR PCE DEGRADATION AT -2.1V

FIGURE 4.28

51

Table 4.2 PCB degradation using 5.5 g cathode fibres and 5.8 g anode fibres.

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4. 29 Parallel Pathways for the PCB degradation using5.5 g cathode and 5.8 g

anode fibres.

 

 

 

Expt. Ref. PCB Degradation TCE Production Chloride production
No. Voltage Rate (It moles/min Rate (ll moles/min. Rate( 1.1. moles/min
115001011012) Jamming) anhlmidpl
5 -1.8 2.85 0.82 1.80
6 -1.8 3.03 0.56 2.32
7 -1.9 4.68 1.97 2.44
8 2.0 5.47 2.59 2.65
9 -2. 1 7.44 4.28 2.65
PCE
Expt. Ref.
No. Voltage
5 -l.8 V 28.3 % 63.7 % 8.0 %
6 ~13 V 18.5 % 76.6 % 4.9 %
7 -l.9 V 47.3 % 48.4 % 4.3 %
8 -2.0 V 43.6 % 51.5 % 4.9 %
9 .2.1v v 57.5 % v 35.7 % T 6.8 %
Hydrogenolysis Complete Sampling
Dechlorination Errors

 

52

to various pathways at different reference voltages. Table 4.3 tabulates the complete range
of efficiencies of electron transfer for all nine experiments. This table also gives the
energy consumed in degrading one mole of PCB and provides a preliminary cost analysis.

Based upon the above results, following observations are made:

1. An increase in reference voltage resulted in an increase in the degradation rate of PCB.
Also, an increase in the cathode surface area from 3.2 g to 5.5 g not only resulted in an

increase in- PCB degradation rate, but also reduced the energy consumption considerably.

2. At lower voltages, the pathway leading to complete dechlorination was dominant. At
higher reference voltages, the hydrogenolysis of PCB began to dominate and resulted in
higher production of TCE. These results are consistent with those obtained by Rajayya
(Table 4.13). An increase in hydrogenolysis rates at higher voltages was consistently
observed in all experiments except for Bxpt. 1. This may be because Bxpt. l was not con-
ducted under the same anode pumping conditions as Bxpts. 2-9.

3. Experiments 5-9 (with 5.5 g cathode fibres) provided evidence that energy requirements/
mole of PCB degraded are significantly less at reference voltage of -1.8 V using 5.5 g cath-
ode fibres. Thus in order to reduce operating costs, a reference voltage of -1.8 V is recom-
mended for the experimental conditions indicated. An additional advantage of operation at
this voltages is the fact that the pathway resulting in complete dechlorination was predom-
inant. On the other hand, these advantages should be weighed against the higher capital cost
of a large reactor because PCB degradation rates increase nearly three fold at a reference

voltage of -2.1 V thus reducing the size of the reactor required. At least two possibilities

exist for the additional treatment of the eflluent from this reactor: (l)biological treatment

or (2) additional electrolytic treatment. Electrolysis of TCE to produce completely dechlo-

rinated products is discussed in the following section. Economics will be the major factor

.mom can sewage—e Bede Eon magma 038 2a meets—=28 Eocm .Bamcconxo 338 3305
Ho: 82. 300.8%— oo_ «o 52352.8 Bonus 5 55 5:28 33:35:00 Ba 05 “gob 9 :2 $523 58.585
8% 583 we whom .3 3638—0 .8 8: 320888 05 .5: com—Eu 55.5.93 we 2.: 82.50 05 .33 33:28 mm sou ..

 

53

 

 

:3 «4.2 new as com «to :8 85 a: Z- 5
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.5332»? RE .8 26:23.8 «559.33.. .335 ES .3535- Qv «Bah.

54

 

 

 

 

 

 

Figure 4.29a Sorption and degradation area. in a generalized curve.

Table 4.4 Sorption study for PCE using 5.5 g fibers in the cathode chamber.

 

 

 

 

 

 

Bxpt. Ref. Calculated sorption Calculated degradation
No. Voltage (mg) (mg)

6 -1.8V 23.3 8.42

7 -l.9V 25.7 7.16

8 -2.0V 26.7 11.7

9 -2.1V 24.1 14.0

 

 

55

in selecting one option over another.

4. Amount of PCB sorbed to the fibres can be calculated by the left hand side hached area
representing the sorption as shown in Figure 4.29a. Similarly the the amount of PCB
degraded can be estimated utilizing the right hand side hached area. There is a possibility
that PCE might be desorbed after applying the potential. In that case, the amount of degra-
dation shown‘in Table 4.4 truely represents the total degraded plus desorbed PCB. The phe-
nomenon of sorption and desorption (after applying the voltage) is further complicated by
the fact that TCE produced might also be undergoing sorption-desorption.

5. From Table 4.1 , it is indicated that the TCE production rates were higher at a reference
voltage of -l.8 V than at the reference voltage of -l.9V. This seems to be in contradiction
to the observation made by the remaining eight experiments that hydrogenolysis path is
predominant at higher voltages. This is perhaphs due to the reason that Experiment 1(at
Ref. Voltage of '- 1 .8V) was conducted without any pumping of the anode chamber and thus

' the experimental conditions for Experiment 1. were not the same as those for the other

experiments.

56

Trichloroethylene

Of all the chemicals studied, TCE was the most resistant to reduction. As indicated in fig-
ures 4.30 -4.44, degradation of TCE resulted in the production of completely dechlorinated
products. As shown in Table 4.5 and Figure 4.45, the effluent chloride accounted for 90-
95% of the initial chloride present in TCE. Hydrogenolysis was net observed, even at
higher reference voltages. This contrasts with the results of Rajayya (Table 4.13) in which
production of t-DCB, ethylene and trace amounts of VC were observed. A possible expla-
nation for this discrepancy is that the batch type experiments of Raj ayya were conducted in
more reducing environment (Ref. voltage of -1.2 and -l.4 V instead of Ref. Voltage of -
0.8V to - 1.0V used for Other chemicals) thus making the hydrogenolysis pathway was more
significant. Another possible explanation is that TCE might have been transformed to eth-
ylene and was not detected in the effluent samples as FID was used to analyze the volatile

organics.

As shown in Table 4.6, the efficiencies of electron transfer were also low as compared with
those of PCB (Table 4.3). The energy requirements/ mole of TCE degraded were lowest at
reference voltage of -2.1 V. This is due to the increased TCE degradation rate with
increased reference potential. Very little degradation was observed at a reference voltage of
-1.8 V. The operating costs for TCB treatment were nearly twice as much as that of PCB at
corresponding voltages. In spite of the high operating costs, there is a distinct advantages
of electrolytic reduction of TCE over other target chemicals degraded:

-TCB degradation resulted in the formation of completely dechlorinated products. No fur-
ther treatment would be necessary if the dechlorinated products are shown to be non toxic.
- Operation of reactor at higher reference voltage of -2.1 resulted in higher TCE degrada-
tion rates and lower operating costs when compared with the reactor operation at lower ref-

erence voltages.

57

 

 

 

 

Q) 700 l l T F I
E J
L .1
o . / 1
g 5004 / “a...“ TCE _

J / due-H
8 . .
3 400~ / 1
IE ‘ / ‘
0 300-4 -
L
.2 i
E 200-4 _.
.E « .
- 100- ,
8 / amid-(Tram) Chlorlde
8 ‘ nun—undo _ u I I '

0
0 I 2 3'5 4— 5 5

# of pore volumes exchanged

TCE DEGRADATION AT REF. VOLTAGE OF —1.8V

 

 

 

 

 

 

 

 

 

 

FIGURE 4.30
799, - 9 T
., m4 *— 1
E ‘ g 2m4 \f—
2 saw 2'
U 4 U 7.0%
3 5004 I 3
n ‘ 0 m4
g 5004 g
E . E
Io ZOO-I g “H
E E»
E too-I E 604
0 I f Y
.I o 5 s o J I s u
l of pore volume: exchanged I a! pore volume: exchanged
CHLORINE BALANCE FOR TCE DEGRADATION Al -1 8V CARBON BALANCE FOR TCE DEGRADATION AT —l.8V

FIGURE 4.31 FIGURE 4.32

58

 

 

 

 

 

CD 700 I I 1 r f
P.
L.
2 600'- —4
.C
U 4 .,
Menu-e
g SOD-I ‘ufl -
8 " .
‘5 400-1 -
.E
o 300‘ "‘
L
Q .I
E 200-4 .1
.E . Chloride
. Bromideflrocer) I . ._
0 100-4 cube-mend. .-
C 4 e -«
8 -- '
O r I r fnT— I
D I 2 3 4- 5 5

# of pore volumes exchanged

TCE DEGRADATION AT REF. VOLTAGE OF —1.9V
FIGURE 4.33

 

 

 

 

 

 

 

 

I w ‘ 1

§ 5004 . E
5 3 ICC-I 4
8 400+ 3
a 3 ml
3 3004 3
.6 E J
0 200-4 cl: “H
9 z,
E roux E “H

v .I A 5 ll 0 J 4 ? 6

I of pore volumee exchanged I of pore volumes Ilchonch
CHLORINE BALANCE FOR TCE DEGRADATION Al -l.9V CARBON BALANCE FOR TCE DEGRADATION Al -1 9V

FIGURE 4.34 ncuag 4,35

 

 

 

 

 

 

q, 700 I I l I I I
:9 1
8 'l
E 600% -4
U 4 4
g 5004 -
8 'l
'3 4004 a
l5 - * I
9. 3004 -I
.2 . .
E ZOO-I / -
.E + Emmam) Chloride
6 100-4 Odie-«Ind. . . I l ' ..I
c: /
8 4 f; e ' I

O T I r I I - r I

D I 2 3 4- 5 6 7

# of pore volumes exchanged

TCE DEGRADATION AT REF. VOLTAGE OF —l.9V

 

 

 

 

 

 

 

 

FIGURE 4.36
700 r V v v: v Y Y
I

g “4 W § zm4 \
2 b
5 sow I 3 tee-l
a w-I d a
r s w .
o 3004 a
s E I
I I 00-4
2 1°“ 2
2 .2
E I004 E 404

c .l I 5' I I ° J z 3 z I

I at pore volume- exchenqed I of pore volumee exchanged
CHI-m MOE FOR TCE DEGRADATION A! -1 9v moon BAIANCE ron rcr 050mm" AT -I.9V

FIGURE 4-37 FIGURE 4.38

60 D

 

 

 

 

 

 

micro-moles oe chloride
E

 

ID 800 r I I I I V
:9 q
_‘c_3 700- ‘
J: .
U ..
a) 600-

o ..

B SOD-d ‘
o “ _
E

l ..
O ..
L.

.2 I I I.

E , ' d
.5 . Chloride 4
d .-
C ' .l
8 ‘ -

0 3 I I
o T E 3 I 5 6 7

# of pore volumes exchanged

TCE DEGRADATION AT REF. VOLTAGE OF --2.0V

FIGURE 4.39

 

 

3 B i ‘r
I a! pore voiumee "changed

CHLORINE BALANCE FOR TCE DEGRADATION AT -2.0V

FIGURE 4.40

 

2‘04

2““

tea-l

I204

uncle-melee as cotbon

 

 

 

3 5 5 'r
I 0! pore voiumee "changed

CARBON BAIANCE FOR TCE DEGRADATION AT -2.DV

FIGURE 4.41

 

micro-moles a: chloride

61

 

1

l

 

LLLIJJllllJ

' Chloride

. in micro—moles as chloride

4 BmmldeCfracer)
1 00 4 a m d &

 

4141

 

 

 

Conc
O
L\

I I I

o 1 2 3 1i 5 6

\a'

# of pore volumes exchanged

TCE DEGRADATION AT REF. VOLTAGE OF —2.‘IV

 

 

 

 

 

 

 

 

FIGURE 4.42
EDD4 1 Y '\_..—-—-—-—- 4 2'“
7004 § zoo-I
6004 § 200.4
. I
5004‘ : 1.0-I 1
2
“m g IzoI
300-4 ‘I,
2004 .§ “H
1004 g E 00-4
° I ' v 0 I T r
J A 5 O I .I 4 S C "
I III pore vqumee exchanged I 0! pore vqumee exchanged
CHLORINE BAUNCE FOR TCE DEGRADATION AT -2.IV CARBON BALANCE FOR TCE DEGRADATION AT -2.IV
FIGURE £43 FIGURE 4.44

Table 4.5 TCE degradation using 5.5 g cathode and 5.8 g anode fibres.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Bxpt. Ref. TCE Degradation Rate Chloride Production Rate
No. Voltage Rate( It moles/min Rate( It moles/min
asshlondcl W)
10 -1.8 0.56 0.53
l l - 1.9 1.33 1.22
12 -1.9 1.41 1.30
13 -2.0 2.92 2.68
14 -2.1 4.25 4.09
| TCE |
Expt. Ref.
No. Voltage
10 -l.8 V 0.0 % . 95.0 % 5.0 %
ll -l.9 V 0.0 % 92.7 % 7.3 %
12 -l.9 V 0.0 % 94.8 % 5.2 %
13 -2.o 0.0 % ’ 91.8 % 8.2 %
14 -2.1 v 0.0 % Y 96.4 % v 3.6 %
Hydrogenolysis Complete Sampling
Dechlorination Errors

Figure 4.45 Parallel Pathways for the TCE degradation using5.5 g cathode and 5.8 g
anode fibres.

.mOp. a8 nausea—woo EEO 80a mega? 038 0.3 macaw—=28 38cm .68 3:98
3205 so: 8% .80 .83 o2 .«o cougeoocoo 83E: 5 5m? noun—8 38:33:00 mob 05 30.: 3 3 $523 .bBEoBm
d 0083 he 28m .3 3638—0 mo 8: 38058 05 .5: vow—20 givod a .«o 8E Ego 05 55 0228—8 mm .80 ..

 

63

 

 

Om.m 8.9m wed mod wed mad cod mod 86 — .NI .3
mg? cnfim nmé and nmé th 86 ¢Wd de QNI m—
NNfi ovfim "Wu Ned “Wu Nvd 86 mvd mm.m a. — I N"
nod 2.. _ m OWN mvd chm mvd 86 mvd ON.M a. m I :
wvé 86m :.A aNd :3 and 86 . Omd 8d w. — I o—
nce—Em EVEN/Wm“ 383 A3
own“ samwwoamwfi 353: 352: 5...: 5...: 2a.... “Mama Mama saw» awn

 

 

 

 

 

 

 

 

 

 

 

 

doze—.83.. HOP 3.. 20:23.8 «5523—00.. 3.55 was maxefim e... 033—.

64

Carbon Tetrachloride

The results of CI‘ degradation were very interesting and encouraging. Both hydrogenolysis
and the pathway leading to completely dechlorinated products were observed as shown in
Figures 4.46 -4.60. Figure 4.46a represents the observed hydrogenolysis pathway for the
five experiments conducted to study CI‘ degradation.

step 1. step 2. step 3.

CT CF MC "F

 

Step 4.

l Methane

Figure 4.46a

 

At reference voltage of -1.0 V, only steps 1&2 were observed, but at reference voltage of -
1.4 & -1.5 V, the hydrogenolysis pathway was pushed to step 3 resulting in the formation
of CM. At reference voltage of -1.8 V, there is a strong indication of methane production
although the methane was not quantified in the efliuent. The idea of methane production is
reinforced by the fact that there is a sudden increase in chloride production at reference
voltage of -1.8V as shown in Figure 4.61a and Table 4.7. Another interesting observation
is the fact that no CM detected in the effluent at the reference voltage of -1.8. A possible
explanation is that Step 4 occurs faster than Step 3 such that all chloromethane produced is
quickly transformed to methane which was not detected on the GC using FID detector. The
results are consistant with those obtained by Rajayya(l992). As shown in Table 4.13 large

dechlorination is observed at the higher refrence voltage of -1.1V.

Due to high CI‘ degradation rates even at low reference voltages, energy requirements are

very low compared with those of PCB and TCE. As shown in Table 4.8, the operation of

65

 

 

 

 

q) 700 I I I I I 1 I
E

‘6
E 500% CT -I
U " Mb“ ‘I
g; SOD-I “We -
(n . ‘4
g 4004 Orland? -
E « -

c', 3004 " -
‘9 . CF

E 2004 ..
.E BramideCl’racer)
. 100* .mv. _
é / CT ‘
“--_’-_._.—.-.-.-.-.
0 0 I I I I "T" " I I MQI
O 1 2 3 4 5 6 7 8

# of pore volumes exchanged

CT DEGRADATION AT REF. VOLTAGE OF -1.0V

 

 

 

 

 

 

 

 

 

FIGURE 4.46
790 T :x
"x I/K ; Afi

”N V
g g '.°_I \.\
3 e
2 m4 ‘ o \

U
U l I
3 m I ‘ g 1204
5 JOB-I J g
g 'E 004 I
6 2004 2
5 2
r «H
E too-I E
o ' Y I c r r r
‘I 5 e 7 II 0 s e 7 II
I 0' Dore volumes exchanged I of pore We. exchanged
I I

CHLORINE BALANCE FOR CT DEGRADATION AT -I 0v CARNN BALANCE FOR CT DEGRADATION AT -I.OV

FIGURE 4.47 neon; 4.43

mIcro-moles OS Chlond!

in micro-moles as chloride

Conc.

66

 

 

 

 

 

500 I I I I I
7001 _
. CT ‘
600‘ :1“; -.
. +.
5004 J
400i. - a
I ChlorI_de 1
3004 . _
‘ ' CF
2001 4
1 00 q EmmideUraseJ) CT d
J / . u - 4* JI
0 I I I T g :{f I MO
0 I 2 3 4 6 7

# of pore volumes exchanged

CT DEGRADATION AT REF. VOLTAGE OF -I.OV

FIGURE 4.49

 

rooI
500+
sooI

4004

2004

I004

 

0

r" F\

 

IOOJ

carbon

nu

”I

micro - moles as

604

 

 

 

I

I 0! pore votumee exchanged
CHLORINE awe: FOR cr orcrumnou AT -I.OV

Y
S

O

 

 

V 0

FIGURE 4.50

i

r
6

FIGURE 4.51

I a! pore volume exchanged
CARBON BALANCE FOR CT DEGRADATION AT -1 0V

67

 

 

 

 

 

 

500 , I
I
m soc-I _
2
O
'E 400 - -
O
8
'g 300 -I . e a a _
E I .Chloride
LI 200 -I _
5? ~ Bromide(Tracer) .
1 00'I . lube-melee . D CF "c-
~ cr
/ CM ‘
0 l l I I— " I I CT
0 I 2 3 4 5 6 7

# of pore volumes exchanged

CT DEGRADATION AT REF. VOLTAGE OF —1.4V

 

 

 

 

 

FIGURE 4.52
700 200

a ”0" _.
E so‘H\/.- \ g loo-I
U U
3 “I 3 "°*\_/
3 l :
2 3°“ 2 004
:0 zoo-I '0
.2 "
E IN-l 1 E ‘04

I I I

'0 3 I " 0“ r i "

 

I 0! pore volumes exchanged
CHLORINE BALANCE FOR CT DEGRADATION AT -I.4V

FIGURE 4.53

 

 

 

I 0! pore volumes exchanged
cmaon'aumcr ran 01 momma AT -I.4v

FIGURE 4.54-

68

 

700 I I
600-:
500;
400-
300-

200-

in micro-moles as chloride

100-

 

Conc.
O

 

 

 

# of pore volumes exchanged

CT DEGRADATION AT REF. VOLTAGE OF -I.5V

 

 

 

 

 

 

 

 

FIGURE 4.55
700 Y 7 20:
I
I. 0004 .
,3 W i E "M ‘
2 500% 4 ~ M
‘ a
U U
8 400-4 3 120-4 4
3 z
" -l
E 300 g III-I .
' I
S ”M 2 I
.2
E 1004 E ‘0‘ -I
"I 5 I 'r 0“ I I ‘r

I III pore volumes exchanged
CHLORINE BALANCE FOR CT DEGRADATION AT -l.5V

FIGURE 4.56

 

I III pore m exchanged
CARBON we: ran c1 oemmmu AT -I.sv

FIGURE 4.57

69

 

  

 

1
Chloride

 

 

 

 

 

700 I I
.8 J
': 500 -l
2
g.
o 500 -1
0‘)
0 l
m 400 -l
2
0 l
IE 300 -l
o -4
L.
.9 200 -l
E . Bramideflracer)
.S 1 oo .1 - an"... . e
g ‘/
o 0 4
O

 

 

m-
\I

4 5

# of pore volumes exchanged

CT DEGRADATION AT REF. VOLTAGE OF -l.8V

 

 

 

 

FIGURE 4.58
70:: 202 f
I \ t ‘1‘
e 0004 HT « c
5 D
g 5004 a .
U
U I J
3 0004 3 m’ ‘
s .... a ‘
° 004 <
f l :5
O 2N4 e 4
.§ 5" eo-l 1
5 100-1 E
l
o T T a Y r
0 s g ‘r 0 s g 'r

 

 

 

I at pare valomee exchanged
CHLORINE BALANCE FOR CT DEGRADATION AT -1.8V

FIGURE 4.59

 

 

I a! pore volunee exchanged
CARBON. awe: FOR C? DEGRADATION A! -I.BV

FIGURE 4.60

Table 4.7 CT degradation using 5.5 g cathode and 5.8 g anode fibres.

7O

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Bxpt. Ref. CT CF MC CM Chloride
Voltage Degradation Production Production Production Production
Rate Rate Rate Rate Rate
(u. moles/min. (u moles/min (umoles/ ' (p moles/min (p. moles/min
as chloride) as chloride) as chloride] as chloride) as chloride)
15 -1.0 6.49 2.39 0.29 0.00 3.98
16 -1.0 6.91 2.81 0.37 0.00 3.72
17 -1-4 7.44 0.86 1.32 0.47 4.12
18 -1.5 9-11 0.37 2.12 0.83 4.93
19 -1-3 14-14 0-00 3.19 0.00 10.10
| CT |
Expt. Ref.
No. Voltage
15 -l.0 V 40.3 % 61.5 % 0.0 %
16 -l.0 V 46.2 % 53.9 % 0.0 %
l7 -l.4 V 35.73 % 55.4 % 8.87 %
18 -l.5 V 36.5 % 54.2 % 9.34 %
19 .1.8 v I 22.6 % v 71.4 % V 6.0 %
Hydrogenolysis Complete . Stripping
Dechlormatton

Figure 4.61 Parallel Pathways for the CT degradation using5.5 g cathode and 5.8 g
anode fibres.

HO com cote—8300 880 80» 988.80 038 0.8 Sana—=23 mucoam 886530 .9800
08:08 8: 0.0% 8.60 8.5 9: .8 88880050 80:8: :0 .23 cog—om 020888.50 EU 05 808 9 :2 .8883 588005
0% 0033 «a 88m .3 36800—0 .«o 00.: 3808800 05 8.: vow—.20 .56556 a .«o 008 80.8.0 05 .23 020828 .3 $00 ...

 

71

 

 

m8 m? 8.2 8m «02 3.0. was 03 Ea 2- 2
Rd 03 on... was 23 03 #2 03 m3 3- M:
Ed a; 2.8 3:. on: 38 n: 8.0 02 E- t
8.0 Re 8.8 2.2 on? 2.0 can 35 2.” 3- 2
8.0 So 33 8.2 8.3 3.: Re 3... SN 3. 2
£38

223 8283 a e e s 0 see . as

822 Dec 28. 20:5 005:5 one; .02
.3on E08850 .3005 A5589: 8. 5.59:. 5...: is: 802: 80:85 00:83 «0M amxm

 

 

 

 

 

 

 

 

 

 

 

 

80:389.. .5 00.. mecca—50.8 8082890.. 90:0 use 3:305”. a... 020,—.

72

reactor at -1.0 V will result in low operating costs but this factor should be weighed against
the advantages of higher dechlorination rates and higher degradation rates for reactor oper-

ation at a reference voltage of -l.8V.

Chloroforrn

Figures 4. 4.62 to 4.76 represent the results of CF degradation at various reference voltages.
The products obtained are very consistent with those indicated by the CT degradation in the
previous section as well as those obtained by Rajayya (Table 4.13). With an increase in ref-
erence potential, the hydrogenolysis path becomes more important over the dechlorination
pathway, as indicated by Table 4.9.

Operation of reactor at lower operating voltages between -1.0 and -1.4 V is strongly recom-

mended due to low operating costs and higher dechlorination rates as shown in Table 4.10.

73

 

 

 

 

0 600 I a . I I r .
E .
E3
3 soc-I ..
U
m 1
° 4004 ..
(n
2
O .
T 3004 -
O
0‘
.- 2004 -
E
C J
“t 1004 adv-waned. ChIorIde
é . _. e. u 4
O 0 Jan"!
I l l l l I I
o 1 2 3 4 s 6 7 8

 

# of pore volumes exchanged

CF DEGRADATION AT REF. VOLTAGE OF —I.OV

 

 

 

 

FIGURE 4.62
0 m v if I _: 200 ' 1 y
.2 .
a 4
8 4 I g ISO-I P‘s-—
. m1 -1 U ‘
-:- . 3
5 Jon-I . g 100
3 . I a
E zoo. E ‘
E ‘ I.
.5 .§ 5‘"
2 100-1 E J
8 e A
0 i 5 i 1 v“ g 3 I 0

 

Iolparevalumeeexchmged

 

CHLOIIlNE macs FOR er 050mm AT -1.0v

FIGURE 4.63

 

 

I 0! pore volumes exchanged
CARBON BALANCE AT REF. voltage of -1.0V

FIGURE 4.64

 

74

 

in micro-moles as chloride

Conc.

 

 

 

 

600

I l I I

I 2 3 4- 5 6 7 8

r I I

# of pore volumes exchanged

CF DEGRADATION AT REF. VOLTAGE OF —1.4V
nouns 4.65

 

0004

300%

2004

Ioo-T

Conc m mucm—molcs os cnlom):

 

 

0
II

 

r

6 7 I!

5 a! core vommes exchanged

CHLORINE BALANCE FOR CF DEGRADATION AT - I .4V

FIGURE 4.66

 

203 v v v
C
O
O
U
a I
o
3 IW-T ‘
O
E
c':
g 30-4
E
9 r r v
6 5 C 7 II

 

 

 

I a! pore volume exchanged
CARBON BALANCE AT REF. voltage of -I.4V

FIGURE 4.67

 

in micro—moles as chloride

Conc.

Conc. in micro-moles as chloride

75

 

 

 

 

600 l I I r I I I
. ,1 .
SOD-l CF -
.. ”hue“
«Chm .
400+ _
I .
BOO-l Chloride
200-4 _
J eon-i ‘
100+ . “d. .-
. MO
o-l ,
0 1 i :5 i s E 5 a

# of pore volumes exchanged

CF DEGRADATION AT REF. VOLTAGE OF -—1.5V

 

 

 

 

 

[clearevalumeeexchangea

moan: MOE FOR CF DEGRADATDN AT -I.5V

FIGURE 4.69

 

FIGURE 4.68
soc «. . fi 200
\\ A \ r f
5004 c 1
a
“4 4 E ‘W
I J 3 l
3M4 ' 1N4
.... g
A 3 :04
too-l I .9
l 1 E
c a s I 5 II n

 

 

 

‘ r I i
7

‘l 5 C
I a! pore «have. exchanged
CARBON BALANCE AT REF. voltage of -I.5V

FIGURE 4.70

76

 

 

 

 

Q) 600 I I I I I I I
E
‘5
E 500-l CF -
U
(I) ‘ ’ b
“It”
0 400-1 -
(D
g . Chloride
E 300-4 _
' l
0 cl
‘6
-- ZOO-l -
E MC
: «I M -‘
. 1004 a “it .-
2 I .
O
O A
0 I I f I ‘ fi T r '
O 1 2 3 4- 5 6 7 8

# of pore volumes exchanged

CF DEGRADATION AT

REF. “/OLTAGE OF —l.8V

FIGURE 4.71

 

soul

Jon-l 4
2004

Im'l

Gone. in micro-moles as chloride

 

 

 

ll s i 5 u
I a! pore valumee excuangea
cm ammo: row or oecmmnou AT -I.BV

FIGURE 4.72

200

 

1304

Im-l

SO-l I

micro-moles as carbon

 

 

 

4 5 i i a
I at pare volumes exchanged
CARBON BALANCE AT REF. voltage of -I.8V

FIGURE 4.73

77

 

 

 

.8 I I1 j I
g rune-2*.ou

3 500- CF -
o

8 I

w 400- Chloride
2 .l
0

IE 3004 _
e 'l
o

-- 200-4 ~
8 MC
c d I
. IOO-I / _
O m

g .I -Hdh .l
0 0

I I I I r I
0 1 2 3 4 5 6 7 8

 

# of pore volumes exchanged

CF DEGRADATION AT REF. VOLTAGE OF —I.8V

 

 

 

 

 

FIGURE 4.74
% sou . ‘
c
.
3 «m g I504 .
g a
.5 W a ‘ .
2 loo-l
.9 'l
E oooI g
S. 1004 ' I
g I 5 50-1 _‘
o .
o ._. . E
‘ 3 3 5 II ‘
I 0' we volumee exchanged n
CHLORM: sumo: ron cr DEGRADAIION AI -1 av '0 I I I 1.

FIGURE 4.75

 

 

 

I a! pore volume. exchanged
CARBON BALANCE AT REF. voltage of —1.8V

FIGURE 4.76

 

78

Table 4.9 CF degradation using 5.5 g cathode and 5.8 g anode fibres.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Exp. Ref.Voltage CF Degradation MC Production Chloride Production
N o. (Volts) Ratemmoles/min) Rate(umoles/min) Rate(umoles/min)
20 -1.0 1.13 0.0 0.94
21 -1.4 2.88 0.53 2.11
22 -1.5 3.84 0.82 2.69
23 -1.8 4.93 1.37 3.48
24 -1.8 5.07 1.45 3.19
I CF I
E t. Ref.
N11? Voltage
20 -l.0 v 0.0 % 88.5 % 11.5 %
21 .14 v 18.4 % 73.3 % 8.3 %
22 .15 v 21.4 % 70.0 % 8.6 %
23 .13 v 27.8 % 69.9 % 2.3 %
24 .13 v v 28.6 % V 62.9 % v 8.5 %
Hydrogenolysis Complete Stripping
Dechlorination '

Figure 4.77 Parallel Pathways for the CF degradation usingS 5 g cathode and 5.8 g
anode fibres.

mo you gouge—o .895 Ben 9:833 038 Be «nous—:38 Room .38 2296
02:2: so: meow .80 .83 2: mo cougcooeoo 80:5: 5 5m? cons—om Bap—Ego mo 05 gob o. 34 .mfimg 56535
a. .833 go Egon .3 3658—0 .8 3.: 33.0888 05 .5: 3893 5,356.0 a .«o 38 59:8 05 55 noun—=28 mm sou ..

 

 

79

 

o2. mam 43. “2 3m 33 one and 3A 3- x
48 So fie :2 saw Nod $5 «2 «on 3- mm
one 8.4 no.2 mom 8.: EN :8 c3 «.3 2- am
one 2.: 5.: Re 2.3 m3. m3 8... 42 E- a
as o2 Eé NS 32 N3 8... 35 SN S- cm
23%
8%» 338 383 5
sea Eommofim 2%.... o 5...... a: a... so: 3......wa mama 3% ...._.z

 

 

 

 

 

 

 

 

 

 

 

iota—cacao: mu 8.. 28:23.8 «55233.. 3.5:» .23 353:5 3... «Exp.

 

80

TCA

The results of TCA degradation are shown in Figures 4.78-4.93. It is clear from
Table 4.11 and Figure 4.94 that the efliuent chloride accounted for 90-95% of the
initial chloride present in TCA. Pathway leading to complete dechlorination is pre-
dominant at lower voltages. As the reference voltage was increased, hydrogenolysis
started dominating resulting in the production of DCA and CA. These results are
consistent with those obtained in previous electrolysis studies(Rajayyal992, Crid-
dle & McCarty 1991).

Efficiencies of electron transfer were high at lower voltages and there was a gradual
drop in efliciencies as the reference voltage was increased. Similar trend was
observed for the energy requirements and the cost. There are two distinct advan-
tages of operating at lower voltages:

-Production of completely dechlorinated products.

- Lower operational costs.

81

 

 

  
    

 

 

.8 QOOJ T T l
E 800_ Chloride q
% 7 ‘ III .4
00-
m o ' 1
° 600-4 ' d
m
0) ~ 4
'5 5004 ..
i 4 -
o 4004 -
o ‘ I
-— 3004 .
E . .
.E ZOO-I Bromidefl racer) J
g 1ooq/lbe-au-der '-
8 J -
0"I r m
0 I 2 3 4 5 6 7

# of pore volumes exchanged

TCA DEGRADATION AT REF. VOLTAGE OF —1.0V

FIGURE 4.78

 

qfi ---. .-
— r r c—a—q
'1 -
q

Gone. in micro-moles ae chloride
§ § E E i i i 5 §
1_

CD

 

 

 

T— l T

J 4 5 5
I oI pore volume: exchanged
CHLORINE BALANCE FOR TCA DEGRADATION AT -I.OV

FIGURE 4.79

 

 

150-4

ISO-4
1004

sol

Cone. in micro-melee as carbon

 

 

 

v .— ._.- T

.l I s s
I aI pore volume: exchanged

CARBON BALANCE FOR TCA DEGRADATION AT - 1 .0V

FIGURE 4.80

in micro-moles as chloride

Conc.

Cone. in micro-molee ax chloride

82

 

 

 

9004 . r 1 o~ I T
8004 a... TCA ‘-
. “a .t ..
700- at you +
6004 . A
4 Chloride .
500-4 . T T '1
I - 'l
400‘ - "
J - d
300-4 - ' DCA _
4 'l
200'“ BromideCl’racer) - T
I I ‘
1 00—4 fl"... a o. .' ..
0 fi I I F n r I " =
O 1 2 3 4 5 6 7

 

 

# of pore volumes exchanged

TCA DEGRADATION AT REF. VOLTAGE OF —1.4V

 

 

 

I of pore volume: exchanged

CHLORINE BALANCE FOR TCA DEGRADATION AT -I.4V

FIGURE 4.82

 

 

 

FIGURE 4.81
'—‘—*--r-- 1 , c J”
3
\/\_/—\ 8 2304
‘ a
-l . m-4
2
‘ ‘e’
150-I ,
é \\
g loo-I
.E
0' 5°
5
f f ' U 9 A T—' T
.‘l 4 5 I 'r .I 4 f 6

 

 

I of pore volume: exchanged
CARBON BALANCE FOR TCA DEGRADATION AT - I .4V

FIGURE 4.83

in micro—moles as chloride

Conc.

Gone. in micro—melee ae chloride
.. g g g g g g a a a

CHLORINE BALANCE FOR TCA DEGRADATION AT -—1.5V

900

83

 

800-
700;
6004
5003
400A
3004
2001

100-

 

”b
“d
I.”

TCA

Chloride

L+Jll

l o

LJJJLIJI

 

 

 

# of pore volumes exchanged

TCA DEGRADATION AT REF. VOLTAGE OF —1.5V

 

L Pg

L

 

W

17

 

 

lb

5 3 I
I of pore volume: exchanged

FIGURE 4.85

‘ i I! 5

L

Conc. in micro-melee ae carbon
§

FIGURE 4.84-

\I

 

§

 

 

 

I r T
I of pore volumes exchanged

CARBON BALANCE FOR TCA DEGRADATION AT -1.5V

FIGURE 4.86

 

84

 

 

900* TCA
BOOA

7OD-I rouge-nu
dlbufl

5°°'j Chloride
5004
.I

IIJJJJIIIILJ

400-4
4

in micro—moles as chloride
o J

ZOO-I Wmoer) '

l

1001
o r Cl-
0

 

I

Conc.

 

# of pore volumes exchanged

TCA DEGRADATION AT REF. VOLTAGE OF -1.5V

 

 

 

 

 

 

 

 

FIGURE 4.87

.3 ”M r . f c an
E!“ \\\///\TT/\ E’”
g m‘I 8 1504
e m4
.2
E ”J g zoo-4 -4
‘1’ 4aa-4 é 150-4
.2 -9
E ”I I E 100-4
.E M4 .S
2' 100 3 5°“
0 o
° = . A ° '-

.l 4 S e " '.l I 5 E ‘l

I oI pare volunee exchanged I of pore volumee exchanged

CHLORINE BALANCE FOR TCA DEGRADATION AT -1.5v CARBON We: ran m DEGRADATION A1’ -1,5v

FIGURE 4.88 FIGURE 4.69

85

 

 

 

 

 

:99) 900‘ l 1 F I T l .
L
2 800'" eel-nu TCA “
J: ‘ all-III "
0 700- 5.... _
8 4 ~
BOO-I -l
U)
.2 ‘ . «
o 500- .
E . Chlonde ‘
I 400— ... I .1
g . .- T
E 300T ." DCA A
"E 200‘ amateur) :
‘8’ 1oo-l fl... CA ..
0 d .
0 I r l - - 7
O 1 2 3 4 5 6 7

# of pore volumes exchanged

TCA DEGRADATION AT REF. VOLTAGE OF -I.8V

FIGURE 4.90

§
I
1

 

 

 

 

 

 

 

 

. m V Y I C l'
.I ISO-I -l
“loo-l .l
8 8
eon-I .l W .1
m g
% l ...I
3 «m I
«g Jon-l q Imd
.5 100-4 .1 .S
- d 504
g 1004 8
8 -.- ° =
.l 3 i T 'r .l 5 5 i '
I a! pore volumes exchanged I a! pore volume exchanged
cmmronmoecmnou A? -1.av CARBONBNMGEFORTCADEGRADATION At -1.sv

FIGURE 4.91 FIGURE 4.92

 

86

Table 4.11 TCA degradation using 5.5 g cathode and 5.8 g anode fibres.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Bxpt. Ref. TCA DCA CA Chloride
Voltage Degradation Production Production Production
Rate Rate Rate Rate .
u moles/min u moles/min “moles/min [.1 males/mm
as chloride as chloride as chloride as chloride
25 -1.0 _ 9.12 0.31 0.00 8.12
26 -l.0 9.90 2.88 0.00 6.07
27 -1.4 10.6 3.52 0.00 6,09
23 .-1.5 11.1 3.10 0.26 6.57
29 -1.8 12.8 3.72 1.13 6.30
I TCA I
Expt. Ref.
No. Voltage
25 -1.0 V 3.44% 89.0 % 7.5 %
26 -l.4 V 21.1% 51.3 % 9.6 %
27 -l.5 V 33.3 % 57.4 % 9.4 %
28 -l.5 V 30.3 % 59.3 % 11.4 %
29 -l.8 V Y 38.0 % v 52.6 % V 11.9 %
Hydrogenolysis Complete Stripping
Dechlorination

Figure 4.93 Parallel Pathways for the TCA degradation using5.5 g cathode and 5.8 g
anode fibres.

87

 

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Ed mm; 3: 9.6 Q: aim 86 Ed afim 3. MN
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88

Table 4.13 Degradation and production rate information (From Rajayya, 1992)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CT Degradation
Ref. CI‘ CF MC CM chloride
Voltage degradation production production production prod.
rate rate rate rate
(0 temoles/min) (1.1 moles/min) (p. moles/min) (p. moles/min) (1.1 moles
lmin)
-0.9V 1.26 0.28 0.03 0.00 0.57
-1.1V 3.47 0.59 0.31 0.05 2.36
CF Degradation
Ref. CF MC CM chloride
Voltage degradation production production production
rate rate rate
(11. tomoles/min) (p. moles/min) (1.1 moles/min) (u moles/min)
-0.9V 1.64 0.14 0.06 , 1.14
-1.1V 3.06 0.26 0.14 2.36
TCA degradation
Ref. TCA DCA CA chloride
Voltage degradation production production production
rate rate rate rate
(1.1 moles/min) (u moles/min) (jJ. moles/min) (ll moles/min)
-0.9V 2.19 0.05 0.03 1.58
-1.1V 2.44 0.16 0.03 1.96
PCE degradation
Ref. PCE TCE t-DCE chloride
Voltage degradation production production production
rate rate rate rate
(11 moles/min) (p. moles/min) (u moles/min) (p. moles/min)
-1.2V 1.25 0.42 0.02 0.73
-1.4V 2.78 0.92 0.06 1.53
TCE degradation
Ref. TCE degradation rate t-DCE production rate chloride production rate
Voltage (ll moles/min) (u moles/min) (1.1 moles/min)
-1.2V 0.42 0.00
-1.4V 1.39 0.06

 

 

 

 

 

 

 

 

 

 

Chapter 5

Conclusions

Parallel pathways exist for the degradation of halogenated aliphatics undergoing electroly-
sis: hydrogenolysis, which becomes predominant by increasing reduction potentials and
another pathway resulting in the formation of completely dechlorinated products. The sec-
ond pathway was predominant at lower applied voltages. The overall degradation rates of
the target chemicals tested were increased by increasing the applied voltage. The opera-

tional costs were higher for alkenes as compared to those for alkanes.

The choice of operating reference potential is very crucial from economic point of view and
hence the following factors should be carefully considered in selecting the optimum
applied voltage: By controlling the applied voltage, the desired products of electrolytic
reduction can be obtained while minimizing unwanted products. The operation of reactor
at lower reference voltages has the distinct advantages of lower operating cost and the for-
mation of highly dechlorinated products. The main disadvantage of operating the reactor at
lower voltages is the higher capital costs. An increase in voltage results in a significant
increase in degradation rates. Operation of reactor at higher voltages will reduce the size of

the reactor resulting in lower capital costs.

Last but not the least important design consideration is the possible need for additional
treatment of the effluent from the electrochemical reactor is to meet desired eflluent stan-
dards. Biological treatment and additional electrolysis are two of the many possible alter-

natives.

89

Chapter 6

Recommendations for Future Work

In considering the future research needs, first and foremost step should be the identification
of the products of complete dehalogenation. Some of these products have been identified
by Rajayya(1992), yet additional experiments are recommended to characterize the dis-
solved organic products. As mentioned in Chapter 3, the degradation rates of target chem-
icals at a fixed reference voltage should be cathode surface area limited and hence the anode
surface area has to exceed the anode surface area in the reactor. Further research is needed
to determine the optimum ratio of the weights of cathode and anode fibres to achieve max-
imum degradation. Another factor that will influence the degradation cosiderably is the
anode compartment pumping rate and its correlation with cathode pumping rate. Experi-
ments are recommended to optimize the anode pumping rate. A mixture of aliphatics

should also be studied to evaluate the performance of the reactor for such conditions.

Further research is needed to identify the most economical technology for the treatment of
effluent from the electrochemical reactor. As the products of electrolysis can be controlled
by changing applied voltage, a number of technologies can be considered to treat this efflu-
ent. Optimum reference and applied voltages should be obtained by combining the effects

of secondary treatment on electrolysis.

Experiments should also be performed to study the effects of pH, temperature, dissolved
oxygen and the nature of the electrolyte on the electrolytic reduction process.

Dissolved oxygen can result in altering the products of electrolysis and hence may result in
production of C02(Criddle & McCarty, 1991). Modification of cathode materials to
improve mass transfer properties should also be investigated. Use of certain cathode mate-

rials which are highly conductive and sarb the target chemicals should also be explored.

9O

 

91

Instead of using water as the proton donor in the anode chamber, a fluidized powdered iron
bed may result in a decrease in applied voltage because iron requires less energy (eV) than

water to produce protons.

One of the major contributor to the capital cost of the reactor the Nafion Membrane. In the
absence of a membrane, the reduced products at the cathode can get oxidized at the anode
to form the reactants again. A different configuration of reactor as shown in figure 6.1 is
recommended. In this new design chloride exchange resins can be used to eliminate the

D

 

effluent to secondary
treatment process

anode

 

chloride exchange resins

 

 

cathode

 

 

 

 

Influent

Figure 6.1 Recommended reactor configuration using chloride exchange beeds.

possibility of the reverse reaction at the anode. Experiments should be conducted to explore
the possibility of elimination of membrane by this new process.

 

References

Anders, M. W. 1982. Aliphatic halogenated hydrocarbons. Chapter 2 in: Wig
WW W. B. Jakoby. J. R. Bend. and J. Cald-

well.

Asmus, K.-D.‘, D. Bahnemann, K. Krischer, M Lal, and J. Moing. 1985. One-electron

induced degradation of halogenated methane and ethanes in oxygenated and anoxic aque-

ous solutions. Wm 3: 1-15.

Bakac, A. and J. H. Espenson, 1985. Kinetics and mechanism of the alkylnickel fomation
in one-electron reductions of alkyl halides and hydroperoxides by a macrocyclic nickel (1)

complex. W 108: 713-719.

Bario-Lage, G., F. Z. Parsons, R. S. Nassar, and P. A. Lorenzo. 1986. Sequential dehaloge-
nation of chlorinated ethenes. W. 20: 96-99.

Bouwer, E. J ., B. E. Rittmann and P. L. McCarty. 1981. Anaerobic degradation of haloge-
nated 1- and 2-carbon organic compounds._primp,_S_ci._Imhpgl. 15: 596-599.

Bouwer, E. J. and P. L. McCarty. 1983 a. Transformations of 1- and 2- carbon halogenated
aliphatic organic compounds under methanogenic conditions. AW

45(4): 1286-1294.

Bouwer, E. J. and P. L. McCarty. 1985. Utilization rates of trace halogenated organic com-
pounds in acetate-grown biofilms. Bimmhngjimp; 27: 1564-1571.

92

 

93

Britten, E. C. and W. R. Reed. 1932. Chem. Apsg. 26: 5578-5583.

Criddle, C. S. and P. L. McCarty. 1991. Electrolytic model system for reductive dehaloge-
nation in aqueous environmentsmmgi, 25: 973-978.

Criddle, C. S. 1989. Reductive dehalogenation and electrolytic model systems. 2L1;
jljnesjs Dissenmgp, Stanford University.

Dean, L. (ed.). 1979. W. 12th edition. McGraw-Hill, New
York, NY. ‘

Egli, C., R. Scholtz, A. M. Cook, and T. Leisinger, 1987. Anaerobic dechlorination of tet-
rachloromethane and 1,2-dichloroethane to degradable products by pure cultures of Des-
ulfobacterium sp. and Methanobacterium sp. FEMS Mimqhiglggy Impers, 43; 257-261.

Fahidy, T.J. , W, Elsevier Science Publishers,
Amsterdam, The Netherlands.

Fathepure, B. 2., J. M. Tidjie, and S. A. Boyd. 1988. Anaerobic bacteria taht dechlorinate
perchlorethylene._AppLE_pximp__Mimhing. 53: 2671-2684.

Fathepure, B. Z. and S. A. Boyd. 1988. Dependence of Tetrachlorethylene dechlorination
on methanogenic substrate consumption by methanosarcina sp. strain DCM. MM-
;gn, Mimbigl, 54: 2976-2980.

Fogel, M. M., A. R. Taddeo and S. Fogel. 1986. Biodegradation of chlorinated ethenes by
a methane-utilizing mixed culture. W51: 720-726

 

94

Freedman, D. L., and J. M. Gossett. 1989. Biological reductive dechlorination of tetra-
chloroethylene tio ethylene under methanogenic conditions. App]. Enviggn, Mimhiol. 55:
2144-2148.

Galli, R. and P. L. McCarty, 1989b. Kinetics of biotransformation of 1,1,1-trichloroethane
by Clostridium sp. strain TCA IIB. Appl, Envimn. Mimo. 55 (4); 845-851.

Giger, W. and E. Molnar-Kubica. 1978. Tetrachloroethylene in contaminated ground and
drinking waters. Bpll, Eng/m n. QQHM°11,TQX1§, 15: 475-492

Gossett, J. M., 1985. Anaerobic degradation of C1 and C2 chlorinated hydrocarbons. Final
report ESL-TR85-88, Air Force Engineering and Services Center, Tyndall Air Force Base.

Halpcfl. 1- 1982. W 10: 465468-

I-Iine, J. 1950. Carbon dichloride as an intermediate in the basic hydrolysis of chloroform.
A mechanism for substitution reactions in saturated carbon atom. ,1. Am, Chem, 5m. 72:
2438-2445.

Infante, P. E and T. A. Tsongas. 1982. Mutagenic and oncogenic effects of chlo-
romethanes, chloroethanes, and halogenated analogs of vinyl chloride. Enggg’ n, Sci, Res,
25: 301-327.

Jeffers, P. M., L. M. Ward, L. M. Woytowitch, and N. L. Wolfe. 1989. Homogeneous

hydrolysis rate constants for selected chlorinated methanes, ethanes, ethenes, and pro-

panes. W. 23, (8):965-969.

95

Kinoshita, K and S. C. Leach. 1982. Mass transfer study of carbon felt, flow-through elec-
trode. W. 129(9): 1993-1997.

Kleopfer, R D., D. M. Easley, B. B. Haas, Jr. and T. G. Delhi. 1985. Anaerobic degrada-
tion of trichloroethylene in soil._E_n_v_ppp._S_cp_Iml_r_r_rgl. 19: 277-279.

Kubik, V. L. and M. W. Anders. 1981. Mechanism of the microsomal reduction of carbon
tetrachloride and halothane. MW; 34; 201-207.

 

Mazur, D. J. and N. L. Weinberg. 1987. Methods for electrochemical reductions of haloge-
nated organic compounds. U. S. Patent No. 4, 702, 804.

Monig, J., D. Bahenemann, K. -D. Asmus. 1983. One-electron reduction of CCl4 in oxy-
genated aqueous solutions: a CC1302-free radical mediated formation of Cl- and C02.

W. 45: 15-27.

Parsons, F. and G. B. Lage. 1985. Chlorinated organics in simulated groundwater environ-
ments. Wm, 1985:53-59.

Parsons, F., P. R. Wood and J. DeMareo. 1984. Transformations of Tetrachloroethene and
dichloroethene in microcosms and groundwater. WW3, Feb. 1984: 57-
59.

Rook, J. J. 1974. W. 23: 234-43.

Sawyer, T. S. and J. L. Roberts. 1983. Degradation of halogenated carbon compounds, U.
S. Patent # 4,410402. 10/18/83.

96

Schmal, 1)., J. vanErkel, A. M. c. P. deJong, and P. J. van Duin. 1937. Electrochemical
treatment of organohalogens in process waste water. In: Environmental Technology. Pro-

ceedinigs of the second European Conference on Environmental Technology. K. J. A.
deWaal and W. J. van DenBrink, ed., Martinus N ijhoff Pub., Dordrecht, Neth., R 284-293.

Vogel, T. M. and P. L. McCarty. 1987. J. Contaminant Hydrology- 1: 299-308.

Vogel, T. M., C. S. Criddle and P L. McCarty. 1985. Biouansformation of tetrachlorthyl-
one to trichloroethylene, dichloroethylene, vinyl chloride, and carbon dioxide under meth-
anogenic conditions. Appl. Environ. Microbiol. 49: 1080-1083.

Vogel, T. M. C. S. Criddle and R L. McCarty. 1987. Transformations of halogenated ali-
phatic compounds. Environ. Sci. Technol. 21: 722-736.

Wade, R. S. and C. E. Castro. 1973. Oxidation of iron (11) porphyrins by alkyl halides. J.
Amer. Chem, Soc. 95(1); 226-230.

Wilson, B. H., Smith, G. B. and Rees, J. F. 1986. Biotransformations of selected alkylben-
zenes and halogenated aliphatic hydrocarbons in methanogen aquifer material: A rnicro-
cosm study. Environ. Sci. Technol. 20: 997-1001.

Additional References

Greef, R., R.Peat, L.M. Peter, D.Pletchher, J. Robinson. 19851ntsrumental methods in
electrochemistry. EllisHorwood limited.

Pickett, David. J. 1977. Electrochemical reactor design. Amsterdam, New York, Elsevier
scientific publication Co., New York.

Milazzo, Giulio. 1963. Electrochemistry : theoretical principles and practical applications.

American Institute of Chemical Engineers. Electrochemical engineering applications.

 

Appendix A
QA/QC Protocol

Calibration and Standardization

* A six point calibration curve was generated for the analysis of volatile organics. The
concentrations of the standards always bracketed the concentrations quantified.
*A different set of standards were prepared for the analysis of volatile organics when the

effluent concentrations were less than 4 ppm.

 

* Fresh standards were prepared for each experiment and secondary dilutions were pre- i
pared immediately before GC analysis.

* Standards were placed between the samples in the Auto-Sampler in the GC analysis.

* A six point calibration curve was prepared for the chloride and bromide measurements.
Although calibration measurements were done in the beginning of each the experiment,
the standards were saved and the MV readings were again taken and compared with the
earlier measurements to ensure consistency of chloride probe with time.

* The same buffer solution was pumped through the cathode and anode chamber.

* Duplicate experiments were performed for at least one of the four reference voltages at
which experiments were performed for each target chemical.

* Same buffer solution was prepared in the preparation of standards for the volatile organ-
ics, chloride and bromide measurements.

* Rigorous sorption studies were conducted separately for each target chemical to estab—
lish the time in which the breakthrough was achieved in absence of electrolysis.

* Care was taken to ensure that the orientation of the fibres in both the chambers was the
same for every experiment. For all experiments related to one target chemical, pieces of

size 6.0" x 0.75” were cut from the same sheet of the carbon fibres (fibres are available in

97

98

large sheets of different thicknesses).

* Reference voltage was constantly monitored and applied voltage was adjusted after reg-
ular intervals (not exceeding 5 minutes) to keep reference voltage constant.

Membrane was soaked in 1% HNO3 solution after finishing experiments for each target
chemicals to remove any deposits that might reduce its proton permeability.

* Statistical analysis was done to estimate 95% confidence intervals of the applied voltage
and applied current w.r.t. reference voltage as shown in appdendix D.

Sampling

* Initial sampling intervals before electrolysis were 25-30 min but sampling interval was

 

.-a‘

reduced to 15-25 minutes after electrolysis was started. Sampling continued until] the
effluent chloride concentration was constant for 30-45 minutes.
* In taking samples for volatile analysis, care was taken to ensure that samples were not

taken close to the time when hydrogen bubbles escaped from the reactor outlet.

Record Keeping

* Al] chromatograms were saved.
* All results were entered in a bound note book in pen.
* Statistical analysis to estimate the regression coefficients was done by using PLOTIT

software package (Scientific Programming Enterprises, Haslett, MD.

Appendix B
Modelling of Electrochemical Plug Flow Reactor

Although detailed modelling of experimental system used was beyond the scope of this
thesis, some preliminary mathematical analysis was conducted. The results of this analysis
are presented in this appendix. Hopefully the approach adopted here for the mathematical
modelling of a plug flow electrochemical reactor will prove useful for the development of
appropriate expressions for the design and simulation of reactors for dehalogenation. A
solution technique is developed to determine the relationship between the influent and
effluent concentrations. Emphasis is also given to methods determining the mass transfer
coefficient at the cathode. The following assumptions were made in the mode] develop-
ment of the model .

1. Reaction is assumed to be mass transfer-limited rather than reaction-limited.

2.The aqueous solution used is assumed to be of high ionic strength.

3. The diffusion equation is developed for steady state condition so that sorption of con-
taminant to the electrode can be neglected.

4. All target chemicals are assumed to undergo first order degradation reaction. This
assumption is probably incorrect as the potential varies along the length of reactor, but is

adopted as a reasonable first approximation.
Determination of Diffusivity Constant

Estimation of the difiusivity constant D1 is closely related to the residence time of the
reactor. An experimental study can be performed to determine the diffusivity by measur-
ing the residence time of tracer. Assuming that the tracer is injected at time t=0, the con-
centrations of the tracer are determined at regular intervals at the outlet port. Let (ti, Ci) be

a set of tracer concentrations vs. time at the outlet. The sample variance is given by

99

 

100

 

 

.r
.0

For a closed plug flow system,

.C- InlI I...II‘ }-I

From this equation DL can be determined by trial & error analysis.

 

 

101

Mass Balance

x-axis

AreaA

 

___, y-axis

 

Figure B.3 Mass balance on a small element of electrochemical plug flow reactor.

In any small element of the reactor, the accumulation of the contaminant is a net result of

mass coming in, mass going out, and degradation.
[Accumulation] = [Mass In] - [Mass Out] - [Reaction]

[Accumulation] = (A 6x 2) 6C

[Mass In] = Q c, 6: - A DH «SC/8x)x 8:
[Mass 0m] = Q CM, - A r)H (SC/Saw, 8t
[Reaction] = (rc A 8x) St

The total shell balance equation can be written as
(A 8:: 2) SC = [Q c, 8: - A DH (SC/89x- Q CM, + A DH (5C/5x)x+5x - (re A 8x)] 5t

102

Dividing both sides of the equation by A 5x5t
e 505: = (01A) (c, - C,,5,,)/8x - DH [(5C/5x)x+5x - (SC/8:0,] /5x - r,

Clearly if Sat—>0, then 5t—)0 and thus
5C/5t = dC./dt

(c,- c,,5,)/6x = deX

[(5C/Sx)x+5xv- (SC/510;] /5x = -d2C/dx2

Hence the expression

dC/dt = (DH/8) (12(7ch2 - (v/8) dC/dx - we

 

Where
(DH/8) = 131,
(VIE) = V

The final equation is
det = I)L (PC/ax2 - V dC/dx - we

Assuming steady state, the equation reduces to

 

I)L dZC/crx2 - V dC/dx - We = o

 

Boundary Conditions
In order to solve this differential equation, two boundary conditions are needed. These

boundary conditions can be obtained as follows

103

Assuming C0 is the concentration of the contaminant entering the reactor at x=0. The con-

centration at the entrance is reduced by diffusion and is given by

 

 

[VCenW=VCO+DLdC/dx atx=0 Ce

is the first B.C.

Similarly at the outlet x=L

VCouua=VCe-DLdC/dx at x=l \

C e’ (Du-v) dC/dx
in this case two possibilities exit

1. <1de < 0, which implies that the concen-
tration at the exit is greater than the concen
tration at end point in the reactor.

2. dC/dx > 0, in this case concentration has

 

 

 

 

to pass through a minimum value somewhere /’ Co+ (D1, 7;) dC/dx
in the reactor before rising at the dis end. /
Co
It if quite obvious that none of the above possibilities is feasible and hence it is correct to
assume that
l dC/dx =0, at x-L I
which is the second B.C.

Using these two boundary conditions the differential equation can be solved.

 

Appendix C.

Calibration constants for volatile products and chloride analysis

ho: is the intercept of the linear calibration curve
b1: is the slope of the linear calibration curve
r2: is the coefficient of correlation of the linear calibration curve

.51 ra n 0118 '11
....... 1......
N0. Chemical
mu- “fl .2
l PCE PCE

-0.984 0.472 0.997
TCE -0.421 0.358 0.995
Chloride 241.7 -5771 0.998
PCE PCE 0.831 0.564 0.994
TCE . 0.240 0.421 0.996
Chloride 235.8 -56.34 0.998
PCE 5.3% -0.236 0.459 0.992
Chlofidc 0.132 0.439 0.995
240.9 -55.33 0.998
PCE PCE

mg 0.352 0.882 0.994
Chloride 0.048 0.687 0.997
225.5 -56.75 1.000
PCE PCE -0.780 1.235 0.997
TCE. -0231 0.632 0.996
Chlonde 254.9 -55.95 0.999
pCE pCE 2.151 0.776 0.992
TCE 0.800 0.564 0.994
Chloride 234.5 -58.28 1.000
PCE PCE 1.278 1.464 0.993
TCE -0.456 0.983 0.994
Chloride 243.9 -58.98 0.997
PCE para 025 0.99., 0.874 0.678 0.995
0.954 0.547 0.993
TCE 0.29 0.992 2422 _57 63 0998

Chloride - - -
0.46 0.993 1.437 0.897 0.991
PCE % 0.32 0.996 -0.125 0.792 0.995
Chloride 231.3 -55.73 0.997

 

 

 

 

105

Calibration _onstants

H...

Expt. Target Analytes
No. Chemical

 

 

 

 

TCE 0.892 1. 22 0. 994
Chloride 187. 6 -58. 53 0. 998
11 TCE TCE 1.259 0.889 0.992
Chloride 182.3 -55.44 0.997
12 TCE TCE -1.473 0.759 0.995
Chloride 177.6 -62.15 0.996
13 TCE TCE -1.629 0.658 0.991
Chloride 193.2 -60.22 0.998
14 TCE TCE 1.438 0.626 0.992
Chloride 211.0 -54.32 0.999
15 CT CI’ -0.279 0.435 0.995 2.102 1.127 0.990
CF -0.173 0.218 0.996 1.148 0.874 0.993
MC - l .293 0.795 0.994
CM -1.434 1 .674 0.991
Chloride 198.7 -56. 73 0.999
16 CT CI' 0.375 0.227 0.995 1.133 0.674 0.994
CF 0.673 0.216 0.995 1.189 0.885 0.985
MC 0.455 0.628 0.990
CM -1.936 1.199 0.989
Chloride 205.3 -55 .91 0.996
17 CT CI' -0.011 0.476 0.992 1.876 0.983 0.994
CF 0.453 0.589 0.995 1.342 1.167 0.994
MC -1. 248 0. 467 0.993
CM -2. 865 2. 934 0.991
Chloride 189. 2 -56. 49 0.999
18 CI‘ CT 0.468 0.593 ).996 -0.452 1.797 0.992
CF -0.349 0.461 0.992 1.456 1.820 0. 995
MC 2.348 0.893 0. 991
CM 3.484 2.010 0. 991
Chloride 215.9 -57.26 1.000
19 ‘ CT CI‘ 0.226 0.401 0.992 0.923 0.648 0.996
CF -0.986 0.276 0.990 -0.673 0. 597 0.992
MC -2.174 3 .245 0. 991
CM -l.639 3 .469 0. 989
Chloride 186.5 -55. 18 0. 995

 

106

Expt. Target Analytes Calibration Constants
No. Chemical
I...
m-
19 CF CF . .

20

  

21

22

23

 

 

 

 

 

 

Appendix D

STATISTICAL
ANALYSIS

 

108

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