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THE THERMODYNAMICSOF THE. -
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ABSTRACT

THE THERMODYNAMICS OF THE REMOVAL
OF HEAVY METALS FROM WASTEWATER
AND SLUDGE

BY

James D. Scott

The presence of heavy metals in sewage sludge is a
major constraint to land application as a method of dispo-
sal. To eliminate this problem by removing the metals from
the sludge or from the influent prior to treatment, it is
necessary to know the magnitude of the energy required for
removal.

To determine this energy requirement, a sampling
survey was conducted to obtain metals concentrations in a
domestic wastewater and sludge, and an attempt was made to
apply the principles of Gibb's free energy to the metal
compounds determined to be present in the wastewater and
sludge through a literature review.

It was found that very few metals compounds have
been identified as actual components of the wastewater or
sludge systems, but many others were given as possible or

probable components. Such gross assumptions must be made to

James D. Scott

apply the principles of free energy to the complex chemical
system which exists in the wastewater or sludge that the
system can no longer approximate reality. Therefore this
method of determining the energy requirement for metals
removal is not feasible. Any further attempt to determine
the amount of energy required should be made experimentally

on a bench or pilot plant scale.

THE THERMODYNAMICS OF THE REMOVAL
OF HEAVY METALS FROM WASTEWATER

AND SLUDGE

BY

0
i
. x \

James Df Scott

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE
Department of Civil and Sanitary Engineering

1976

ACKNOWLEDGMENTS

I would like to thank the Institute of Water
Research for the research grant which enable me to carry
out this project. Special thanks is extended to Dr. M. L.
Davis for his assistance and advice and to Rio Sorrel for
his assistance with the lab work. In addition I would like
to extend my appreciation to the Water Quality Division of
the Michigan Department of Natural Resources, to the
personnel at the East Lansing Wastewater Treatment Plant,
and to all the other people who so willingly gave their

assistance.

ii

TABLE OF CONTENTS

Page
LIST OF TABLES . . . . . . . . . . . . . iv
INTRODUCT ION O O O O O O O O O O O O O O 1
LITERATURE REVIEW . . . . . . . . . . . . 5
THEORY O O O O O O O O O O O O O O O O 1 0
METHODS O O O O O O O O O O O 0 O O O 1 4
Sampling Survey Design. . . . . . . . . . 14
Sampling Procedures. . . . . . . . . . . 17
Sample Preparation . . . . . . . . . . . 18
Atomic Absorption Analysis . . . . . . . . l9
Thermodynamic Analysis. . . . . . . . . . 20
RESULTS . . . . . . . . . . . . . . . 23
DISCUSSION . . . . . . . . . . . . . . 26
Metals Analysis . . . . . . . . . . . . 26
Metal-Ligand Systems . . . . . . . . . . 28
Energy Considerations . . . . . . . . . . 30
CONCLUSIONS AND RECOMMENDATIONS . . . . . . . 31
APPENDIXES
Appendix A - Tables of Metals Compounds . . . . 32
Appendix B - Calculations of Minimum Energy
Requirements. . . . . . . . . 38
LIST OF REFERENCES. . . . . . . . . . . . 40

iii

LIST OF TABLES

Table . Page
1. Heavy metals removal by sedimentation. . . . 5

2. Heavy metals removal by activated sludge treat-

ment followed by sedimentation . . . . . 6
3. 02, d, and n for several metals. . . . . . 15
4. Sampling dates for heavy metals. . . . . . 16
5. Supplementary sampling survey dates . . . . l7

6. Heavy metals concentrations in the influent
of the East Lansing Wastewater Treatment
Plant. I O O O O O O O I O O O O 23

7. Heavy metals concentrations in the sludge
of the East Lansing Wastewater Treatment

Plant. 0 O O O O O O 0 O O O O O 24
8. Chemical reactions and minimum energy

requirements . . . . . . . . . . . 25
9. Heavy metal contaminants in ferric chloride. . 27

Al. Organic-metal complexes which may occur in
wastewater or sludge. . . . . . . . . 32

A2. Inorganic-metal complexes which could be
present in wastewater or sludge . . . . . 35

iv

INTRODUCTION

Most methods of treatment of municipal wastewater
result in the production of large amounts of sewage sludge.
The ultimate disposal of this sludge is a major problem.

A measure of the enormity of this problem can be found in
the results of a 1969 inventory of sludge disposal prac-
tices in Michigan which indicated that over 200,000 tons of
wastewater sludge are generated each year in this state.1

Several alternatives exist for ultimate disposal
under the general headings of ocean disposal and land dis-
posal. In most cases, ocean disposal is impractical, thus
limiting one to the alternatives of land disposal which
include landfilling, dumping, and spreading on the soil.2
Of these alternatives spreading on the soil as a fertilizer
or soil conditioner is probably the most attractive, since
it offers the Opportunity to recycle the nutrients in the
sludge.

The value of these nutrients as a fertilizer has

been known for some time. In fact, raw sludge in the form

of night soil (a mixture of feces and urine undiluted with

water) has been returned to farm land as an important source
of fertilizer since ancient times.3

Several constraints must be considered in determin-
ing the feasibility of large scale sludge disposal as a
fertilizer on agricultural land. These include climate,
location, and sludge properties.2 Under the heading of
sludge properties, one finds the major constraints of
nitrogen (N), heavy metals, and pathogens.4

Even though we can manage land application sites to
control short- and long-term hazards from N, organic matter
and pathogens in the sludge, the toxic heavy metals (i.e.
Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Zn) will accumulate and/or
persist, thus becoming the long-term environmental hazard.5
This accumulation of heavy metals can seriously harm both
plants and animals and even result in fatalities. A good
example of such a situation is the chronic itai—itai disease
found in Japan as the result of consuming food and drink
which was highly contaminated with cadmium (.005%). This.
disease results in a loss of much of the calcium from the
bone tissue and an accumulation of cadmium in the liver and
kidney.6

Other metals such as zinc and copper are essential
Inicronutrients in small concentrations. These metals can
accumulate to levels toxic to plants thus stunting or

inhibiting growth. Because of the problems which are

associated with it, elimination or control of heavy metals

accumulation is the major problem which must be addressed
before land spreading can be widely practiced.

The sludge contains almost all of the metal ions
that are discharged to sewers or extracted from plumbing.7
The easiest way to limit metals in sludge is to control the
amount of metals which may be discharged into sewers.

Source control with limits on discharges of toxic substances
is practiced by cities that use their sludge for agricul-
tural purposes. However, even the tightest source control
is unlikely to reduce the metal content much below the
median value.8

In a study done in New York City, it was found that
about a third of the copper and zinc in the wastewater comes
from domestic plumbing from the corrosion of brass fix-
tures.7 Elimination of plumbing as a source of metals could
be accomplished by using all plastic plumbing. But even
such a step would not completely eliminate the problem of
metals, because the human body excretes zinc.7

Similarly, much of the other metals in wastewater
will come from domestic sources. Because of the large
number of these sources, it would be virtually impossible
to control them as one controls industrial sources. This
fact seems to leave only two alternatives:

1. the metals of concern might be prohibited from
sale for consumer use, or
2. the metals might be removed at the sewage treatment

facility.

Due to the difficulty of limiting heavy metals
through legislative control, it seems more desirable to
try to remove the metals from the sludge before land appli-
cation. Alternatively, the metals might be removed more
easily from the wastewater. Several techniques are availa-
ble for removing the metals from either the sludge or the
wastewater. These include adsorption, ion exchange, reverse

9 10 The choice

osmosis, electrodialysis and electrOplating.
of one of these alternatives should be based on a comparison
of their energy requirements as well as economic and
technologic requirements.

The efficacy of this approach and any individual
metal removal (efficiency of removal may be process depen-
dent) is fundamentally based on the minimum thermodynamic
energy requirement to separate the metal from the waste
water and/or its complexes. This, in turn requires a
knowledge of the chemical form in which the metal occurs in
the sludge or wastewater. This report reflects the initial
attempts to define the minimum thermodynamic energy

requirement based on available knowledge of chemical forms

of metals in wastewater.

LITERATURE REVIEW

To determine whether or not metals can be more
easily removed from wastewater or from sludge, it is
necessary to know if the metals in the raw wastewater are
in solution or if they are adsorbed on or complexed with
the suspended solids. Stonesll-l7 (1955-60) has done some
work in this area with sewage from the Daveyhulme Sewage
Works in Manchester, England. He found that sedimentation
of raw sewage resulted in significant reductions of heavy

metals concentrations. Results of his sedimentation

experiments are given in Table l.

*
Table l.--Heavy metals removal by sedimentation.

 

 

Metal % Removal
Chromium 28
COpper- 45
Iron 67
Lead 40
Nickel 20
Zinc 40

*-detention time of 12 hours

These results indicate that 40 pecent or more of the
copper, iron, lead and zinc are probably adsorbed on or
complexed with the suspended solids. Thus, for these metals,
it might be impractical to try to remove them from the
wastewater.

Stonesll-17 also experimented with the removal of
metals by activated sludge. The results of these experi-
ments are listed in Table 2. These tests show that most of
the heavy metals tested are removed from the influent and
deposited in the sludge.

Table 2.--Heavy metals removal by activated sludge
treatment followed by sedimentation.

 

 

Metals % Removal
Chromium 67
COpper 80*
Iron 80
Lead 90*
Nickel 30*
Zinc 60*

 

*-Removal is based on the amount of metal present
in settled sewage.

Metals are capable of forming complexes with a large
number of organic and inorganic ligands which are present in
wastewaters and sludges. Though few specific compounds have
been isolated, many species which could complex have been

18

identified. Cheng states that when the pH is lower than

the precipitation level for a given metal concentration,

interactions take place between metal ions and biological
material to form metal-organic matter complexes.» The

organic material for these complexes generally comes from
one of two sources, natural degradation products of plant

19

and animal tissues or chemicals applied by man. The

natural degradation products include polypeptides, amino

20 21

acids, polysaccharides, proteins, DNA, RNA, humic and

22 3

fulvic acids. Ethylenediaminetetraacetic acid (EDTA),2
nitrilotriacetate (NTA), and sodium citrate20 are strong
chelating agents which are included in the classification
of organic matter from chemicals applied by man along with
pesticides and a large group of synthetic macrocyclic com-

19

pounds. The functional groups of the organic matter which

are involved in chelating the metals are considered to be
the carboxyl, hydroxyl, and amino groups.18
Some specific metal-organic complexes have also
been identified. These include some metallo-phosphate and
metallo-phenolic complexes and some metal-amino acid and

metal-organic acid chelates.18

A study in Great Britain
identified eight amino acids which complexed with copper
to give CuL+ and CuL2, where L is the amino acid ligand.24
These organic-metal complexes are listed in Table A1 in the
appendix.

In addition to the organic-metal complexes, there
are many inorganic complexes which may exist in the waste-

water and sludge. For example, the suspended and colloidal

particles in natural waters may consist of metals in the

form of their hydroxides, oxides, silicates, sulfides, or as

19

other compounds. Significant fractions of divalent and

higher-valence metal ions are also complexed with carbonate,

sulfate, and phosphate ligands.25

Other important inorganic
complexing ligands include chloride, fluoride, and bicar-
bonate species.19 In digested sludge, the heavy metals

26 and carbonates.27

exist mainly as insoluble sulfides
Metal-sulfites, -phosphates, -bicarbonates, -oxides, and
-hydroxides might also be present in a digester.28 Since
these compounds, with the exception of chlorides, fluorides
and silicates exist in both natural waters and in the
reducing environment of sludge digesters, it is possible to
assume that they could exist in a wastewater or raw sludge.
Chlorides should also exist in wastewaters as a result of
the use of chlorine bleaches and in sludges as a result of
ferric chloride addition.

Some specific inorganic metal compounds which could
be found in wastewaters or sludges have been determined.
The iron phosphate compound vivianite (Fe3(PO4)2' 8 H20)
was identified in the dried secondary sludge of the

29

Milwaukee Jones Island Sewage Treatment Plant. It has

also been found that when iron is added for phosphorus
removal, the stoichiometric amount required (1.8 mg/l Fe
per mg/l P which implies FePO4 formation) must be supple-

mented by at least 10 mg/l of iron for hydroxide forma-

30

tion. If anaerobic conditions are allowed to exist, iron

30

sulfide may be formed. Cadmium carbonate (CdCO3) and

zinc hydroxide (Zn(OH)2) should be the least soluble pure
compounds of cadmium and zinc under commonly occurring con-

ditions of pH and carbonate concentration in natural aqueous

systems.31 However, if sulfide is present (i.e. from

anaerobic conditions), cadmium and zinc should be precipi-

tated as CdS and ZnS.31 Copper removal by sewage at a pH

level of 7.6 is due, at least in part, to the precipitation

18

of c0pper hydroxides. Particulate copper includes such

insoluble copper compounds as the oxide and sulfide.32
A list of possible inorganic-metal compounds and
complexes is given in Table A2 in the appendix. Soluble

compounds might be more readily found in the wastewater

and insoluble compounds will be more common in the sludge.

THEORY

There are two state functions which are commonly
called "free energy." These are the Gibb's free energy
denoted by the letter G and the Helmholtz free energy

35 Helmholtz free energy is a function of

denoted by A.
volume (V) and temperature (T), and Gibb's free energy is
a function of pressure (P) and temperature. These facts
make the function G the better choice for thermodynamic

35 which is

conditions at constant pressure and temperature,
generally what is experienced in a wastewater treatment
plant (P= 1 atm.).

The function G = H - TS where

G

Gibb's free energy,

H enthalpy,

T = temperature, OK, and

entropy.35

S
The difference in free energy of the reactants and
the products of a chemical reaction, represented by dc
(or AG for a finite change) is the maximum net work that the
system is capable of performing at constant temperature and

pressure.35 Natural reactions always tend to move toward

10

ll

5 in the

lower energy and/or higher disorder (entrOpy)3
product. Therefore the free energy of the metal-complex
products must be lower than that of the reactants in all of
the reactions which occur in sewage. To reverse these
reactions and return the metals to the free state (e.g.,
Fe(OH)3+ Fe), it is necessary to supply energy to the sys-
tem. The minimum energy required to do this, provided that
the reactions are reversible, equals the change in free
energy from the reactants to the products of the naturally
occurring reactions. Since the reactions in the wastewater
are natural reactions, they must of necessity be irreversi-
ble.

In this case, irreversibility does not imply that
it is impossible to force a process in the reverse
direction, but rather that such a reversal cannot be
achieved3§imply by changing parameters by infinitesimal
amounts.

This implies that more energy must be supplied than is
indicated by the change in free energy.

Several means exist for determining the free
energies of compounds. These methods include determination
from standard molar free energies of formation, from the
electromotive force (a). and from the chemical potential (u).

Since only the differences in energy are defined by
thermodynamics, it is possible to assign a value of zero
free energy of formation to certain reference states by

. 35
convention.

Standard molar free energies of formation of
compounds (and of elements in less stable modifications) are

then their free energies of formation at standard conditions

12

referenced to the most stable form of the elements.35 These
free energies are tabulated in several chemical publications.
Because free energy is a function of state, values of AG for
different chemical reactions may be combined in the same way
that the chemical equations of the reactions considered may

35 This

be combined into the equations of new reactions.
method is the simplest and thus most desirable of the three
methods.

The chemical potential ui of each species i has the

property that the free energy of the mixture is the sum of

the “i multiplied by the respective number of moles, ni,
G = i ”i ui.35 For dilute solutions, as in sewage or

sludge, concentrations are less than .01 molar. In this
range the chemical potential is ”i = u; + RT 1n Ci where Ci
is the concentration in moles per liter of the species 1
and u; is the chemical potential of the species i in a
solution at standard conditions and may be set equal to
GE (T) the standard molar free energy of formation for the
species i.35
The third method of determining free energy is

through the use of the electromotive force. This is related

to the change in free energy by the formula AG° = ~nFe° where

n is the number of moles of electrons involved,
F is the faraday, and

8° is the standard electromotive force (emf) of
the given reactions.

13

The values of 5° are also tabulated in chemical
publications (as are AG° values). The values of 8° are
given for half-cell reactions which may be combined to give

the 2° of the full reaction. This is done using the formula

6 = e -e 35
cell cathode anode'

standard conditions can be obtained through use of the

Nernst equation, 8 = 8° - g; ln Q where Q is the mass action

35

The emf (s) at other than

quotient for the reaction.
In this analysis the standard free energy method

will be used whenever the data is available. Since the free

energy is a function of the state, the paths of the reactions

are irrelevant to the energy requirement.

METHODS

Sampling Survey Design

 

A sampling survey was conducted on the raw influent
and the sludge of the East Lansing Wastewater Treatment
Plant to determine the concentrations of heavy metals in
the wastewater. The sampling survey was set up to give a
good statistical representation of the mean concentration
of the heavy metals. The number of samples to be taken was

determined through the use of the statistical formula

1.96102/n = (x - “o’ = d,35'37 where
“o = population mean
Y = sample mean
n = number of samples
02 = variance
d = (X-uo).

Rearranging this formula gives

n = 3.84 oz/dz.

Variances were approximated from data on raw

influent metals concentrations obtained at Grand Rapids

38

Michigan from July to November 1972. Grand Rapids, like

15

East Lansing, treats a waste stream that is largely from
domestic sources. Values of d, the difference between the
sample and population means, were assumed to permit calcu-
lation of the number of samples required for the initial
survey. The assumed values of 02, d, and n are listed in
Table 3 for each of the metals on which Grand Rapids data
were available. While it is recognized that this method of

determining n is far from ideal, it was decided that the

method used was much better than assuming both 02 and d.

Table 3.--02, d, and n for several metals.

 

 

Metal 02* d** n***
Chromium .067 .10 26
Copper .008 .05 13
Iron .068 .10 27
Nickel .072 .10 28
Zinc 3.32 .60 36

 

*-determined from Grand Rapids, Michigan data
**-assumed

***-rounded to the next higher integer

Using the values of n from Table 3 as a minimum
sampling requirement, it was decided that the original
sample would include 42 sampling dates over a period of 6
months. This would enable one sample to be taken on each

day of the week in each month. With these constraints, a

16

random number table was consulted to find the sampling dates.

The dates listed in Table 4 were selected for sampling.

Table 4.--Sampling dates for heavy metals.

 

 

December January February March April May

2 10 1 2 5 2
4 16 4 6 8 S
15 20 9 10 13 18
19 21 12 19 14 20
27 22 17 25 16 24
28 25 20 28 17 28
31 26 28 29 25 29

 

Samples of raw wastewater (plant influent) and
sludge were obtained from the East Lansing Wastewater Treat-
ment Plant following this schedule.

Approximately half of the originally scheduled
samples were not taken. No samples were taken in December
due to problems getting the sampler to work prOperly. As a
result of a flood on April 18, 1975 which overturned the
sampler and rendered the electrical components inoperable,
it was not possible to take any of the samples scheduled
after April 17. The April 17 sample was contaminated so it
was not analyzed.

Because of these problems, additional sampling was
required to obtain a good statistical representation of the

mean concentration of metals. A supplementary sampling

17

schedule was developed that would include 14 sampling dates
over a period of two months. This schedule was subject to
the same constraints as the initial plan and dates were

determined in the same manner as for the previous schedule.

The supplementary schedule is given in Table 5.

Table 5.--Supplementary sampling survey dates.

 

 

August September

6 7

9 10
11 ll

12 13

15 15

24 16

28 26

 

Sampling Procedures

The influent samples in both the original and
supplementary surveys were collected using ISCO samplers*,
model numbers 1390 and 1391, which took samples every hour
on the hour from 1:00 A.M. to midnight on each sampling
date. Each hourly sample was deposited in an individual
bottle. On the following day, approximately 150 milliliters
(m1) from each of the 24 bottles was put in a one gallon

plastic container to make a single 24 hour composite for

 

*Mention of a product name does not imply endorse-
ment or recommendation by the author.

18

each sampling date. The samples were not acidified for
preservation, but were refrigerated because it was origin-
ally planned that both dissolved metals and suspended metals
concentrations be determined. However, due to the limited
laboratory time available, the samples were analyzed only
for total metals.

All sludge samples were taken as grab samples from
the sludge line to the vacuum filters on the day following
the collection of each influent sample (immediately after
the influent samples were composited). A one quart grab
sample of sludge was obtained on each date. Like the raw
influent, the sludge was not acidified but was stored at

4° C. until the analysis could be completed.

Sample Preparation

 

All samples were prepared for total metals analysis.
The pH of each sample from the supplemental survey was
determined as soon as possible following collection.

The samples were prepared for analysis by atomic
absorption spectroscopy following the method for metals
soluble in hot, dilute hydrochloric and nitric acids as

presented by the 0.8. EPA.39’ 40

This procedure results in
a value which may be slightly less than the actual total
metals present.

Influent samples from the original survey were

prepared by adding 0.5 ml concentrated, redistilled nitric

acid (HNO3) and 5 ml of redistilled 1:1 hydrochloric acid

19

(HCl) to a 100 ml aliquot of each sample. The sample was
then heated on a hot plate until less than 50 m1 remained.
After this digestion, it was filtered through Whatman number
41 filter paper and the volume was adjusted to 50 ml.

Sludge samples from the original survey were pre-
pared in the same manner as were the influent samples.
However, the sludge was only digested for 15 minutes and
was pretreated by centrifugation at 3000 rpm for 20 minutes
in an International Centrifuge (size 2, model V, No. 6988D)
to prevent blinding of the filter paper. After decanting
and filtering the supernatant, each sludge sample was
adjusted to 100 ml.

Samples from the supplementary survey were prepared
using 400 ml aliquots of influent and 200 ml aliquots of
sludge. Four ml of 1:1 HNO3 and 10 ml of 1:1 HCl were
added to each aliquot. The samples were then reduced to
less than 100 m1 on a hot water bath, filtered through
Whatman number 40 filter paper and adjusted to 100 ml.

Centrifugation of the sludge samples prior to filtration was

not necessary probably due to a change in sludge properties.

Atomic Absorption Analysis

 

All samples from the original survey were analyzed
on a Jarrell-Ash 800 Atomic Absorption Spectrophotometer
(serial number 24071). The samples from the supplemental
survey were analyzed on an Instrumentation Laboratories

151 Atomic Absorption Spectrophotometer (serial number 1251).

20

All stock and standard solutions for the analyses were pre-

pared following the procedure of the EPA40

, with the
exception of manganese where manganese hydroxide (Mn(OH)2)
was used instead of manganese dioxide (MnOz).

All analyses were completed within four months of
the time of sampling. This long delay between collection
and analysis was due to difficulty in obtaining access to
the atomic absorption units. The shelf life of samples

prepared for atomic absorption is six months39

so analysis
was completed well within the allowable time limit. There
was a potential for the sample containers to introduce
either positive or negative errors in the measurement of
trace metals by contributing contaminants through leaching
or surface desorption and by depleting concentrations

through adsorption.39

The significance of these effects was
not investigated since this study is intended to determine
the order of magnitude of the minimum energy requirements
for metals removal. Consequently only order of magnitude
concentrations were considered to be of significance.

Therefore, it was assumed that these effects were unimpor-

tant .

Thermodynamic Analysis
Calculations were made to determine the change in
free energy for reactions that started with the metals
complexes in wastewater and sludge detailed in the litera—

ture review and ended in the free metal state. These

21

reactions to the free metal state are either unnatural
(positive change in free energy) or have a high activation
energy which prohibits the reaction from occurring.

In the case of those reactions which have a positive
change in free energy, an amount of energy equal to the
change in free energy is necessary to drive the reaction in
the desired direction. When the activation energy prevents
the desired reaction from occurring, that amount of energy
must be applied to the system to enable the reaction to
proceed. If the system is assumed to be a closed system,
the activation energy minus the change in free energy will
be returned to the system. Thus the net energy requirement
is again equal to the change in free energy. In an Open
system, this activation energy cannot be returned. The
change in free energy does not reflect the minimum energy
requirement. Therefore, the energy requirement (activation
energy) must be determined experimentally.

To compare removal costs with other treatment costs
on a treatment plant scale, it was assumed that only one
complex (or compound) of each metal exists in the wastewater
or sludge. This assumption was made to eliminate problems
of interactions and interferences between the different
compounds and the subsequent complex calculations. A simple
calculation can then be performed with the free energy of
each reaction in kilocalories per mole (kcal/mole) and the
concentration of the metal in milligrams per liter (mg/l) to

get a minimum energy in kilowatt-hours per million gallons

22

(kWh/MG). The magnitude of the minimum energy requirements
will determine the feasibility of any attempts to remove the

heavy metals.

RESULTS

Results of the heavy metals study on influent and
sludge from the East Lansing Wastewater Treatment Plant are
given in Tables 6 and 7. Included in these tables are the
number of samples analyzed, the mean of the samples, and the
standard deviation.

Table 6.--Heavy metals concentrations in the influent of
the East Lansing Wastewater Treatment Plant.

 

Metal Number of Mean Concentration Standard Deviation

 

Samples (mg/l)

(n) (if) (s)
Cadmium 28 .055 .031
Chromium 21 .066 .073
C0pper 21 .107 .048
Iron 34 1.11 .21
Lead 14 .321 .018
Manganese 14 .093 .013
Nickel 28 .151 .077
Zinc 35 .343 .114

 

Calculations were made for several of the general

forms of inorganic metal compounds assumed to be found in

23

24

Table 7.--Heavy metals concentrations in the sludge of the
East Lansing Wastewater Treatment Plant.

 

Metal Number of Mean Concentration Standard Deviation

 

Samples (mg/1)

(n) (i) (s)
Cadmium_ 29 .177 .061
Chromium 28 13.8 10.6
Copper 27 11.4 1.8
Iron 29 52.6 5.4
Lead 29 8.14 1.92
Manganese 12 1.93 .65
Nickel 29 2.32 .46
Zinc 27 35.95 8.90

 

sludge. The results of these calculations for the hydro-
xide, carbonate, and sulfide forms of c0pper and iron are
listed with the chemical reactions used in the calculations
in Table 8. The calculations are given in Appendix B. Due
to the questionable validity of the results as explained in
the discussion, calculations were only completed on the

compounds listed above .

25

Table 8.--Chemical reactions and minimum energy require-

ments.

 

Reaction

.Energy Requirement
(kcal/mole) (kWh/MG)

 

CUS + H2 -+ Cu + H S

2
CuCO3 + H2 -+ Cu + H2C03
**
Cu++(aq) + H2 -+ Cu + 2H+
*
Cu(OH)2

**

Fe(OH)3 + 3/2 H2 -+ Fe + 3H20

Fe(OH)3 + 3/2 H2 -+ Fe + 33+ +30H'

Fe(OH)2 + H2 -+ Fe + 2H 0

2
FeS + H2 —+ Fe + H28
FeS2 + 2H2 -+ Fe + ZHZS
FeCO3 + H2 -+ Fe + H2C03

5.16 .248
31.49 1.51
-15.53 -.745
-4.07 -1.27
53.2 16.64

2.19 .685
16.78 5.25
26.76 8.37

68.75 21.5

 

*-Free energy of formation not available

**-Negative free energy means reaction is in
direction written. Such reactions might not occur due to

high activation energies.

DISCUSSION

Metals Analysis
Analysis of the samples taken from the East Lansing
Wastewater Treatment Plant showed that all 8 metals (Cd, Cr,
Cu, Fe, Pb, Mn, Ni, Zn) were present in the influent in
amounts significantly larger than the detection limits for

atomic absorption.39

All of the metals except cadmium were
found in the sludge in concentrations 1 to 2 orders of
magnitude greater than in the influent. The standard
deviations listed in Tables 6 and 7 in the results indicate
a high degree of variability in the data for most of the
samples.

There are several possible reasons for this varia-
bility in the influent. First, the actual concentrations
of heavy metals may vary on a day to day and seasonal basis
with variations in the amounts and types of flows (i.e.
lower concentrations in rainwater since some segments of the
sewer system are combined). Other possible causes of

variation are differences between the two atomic absorp-

tion units used and differences in the hollow cathode

26

27

lamps and the standard metals solutions. A final source of
variability is the fluctuation of the digital readout system
on the atomic absorption unit.

All of the above reasons for variability of the
influent hold for the sludge. The sludge also presents
several other possibilities. Most important of these is
the variation in the feed rate of ferric chloride used in
the treatment process, and the large amount of heavy metal
contaminants present in the ferric chloride as shown in
Table 9. The fact that sludge samples were taken as grab
samples, often after the sludge had spent some time in hold-
ing tanks where metals could concentrate, must also be taken

into consideration.

 

 

Table 9.--Heavy metal contaminants in ferric chloride.42 *
Metal Concentration
(mg/ 1)
Cadmium <1
Chromium 100
COpper 450
Lead 150
Nickel 130
Zinc 370

 

*-Determined from a single grab sample from the
East Lansing Wastewater Treatment Plant.

28

Metal-Ligand Systems

Both the influent and sludge are very complex
systems containing, in addition to the metals, large numbers
of organic and inorganic ligands. A measure of this com-
plexity is given by the listings of possible metals com-
pounds in Appendix A.

In order to perform any calculations on energy
requirements, it is necessary to know the exact composition
of the system to be treated. This means that the kinds and
amounts of all free metals, free ligands, and metal-ligand
complexes must be determined. The enormity of such an
undertaking far exceeds the scope of this study.

It was hoped that a search of the literature would
provide a sufficient amount of data to allow some prelimi—
nary calculations to be made. However, as related in the
literature review, only a very small number of compounds
have been positively identified.

An even greater problem exists in determining
metal-ligand reactions when one tries to treat the system
to remove the metals. The ideal situation would result in
all the desired reactions proceeding simultaneously with no
interferences. In actuality this is impossible since some
reactions will take place preferentially over others and in
so doing cause other reactions to take place.

A partial solution to the composition of the system
problem is a computer program outlined in Stumm and

Morgan.43 This program develOped by Morel and Morgan will

29

give the concentrations of complexes in a single system
containing 9 metals and 9 ligands. The data required for
this proqram includes the concentrations of the 9 metals and
the 9 ligands in the system. Other pertinent data includes
stability constants for all of the metal-ligand equilibrium
species which can exist in the system. To aid in the calcu-
1ations of the equilibrium concentrations, pH, temperature,
and ionic strength of the system must be held constant.

However, there are some problems which might be
associated with use of this program. It was designed to
model natural water systems, so the effect of greater
concentrations and a larger number of complexes in the
influent or sludge is unknown. The necessary constants of
temperature, pH, and ionic strength are not found in the
real system. Thus the model might not be a good approxima-
tion.

Even if all of these problems are successfully
compared, another problem then appears. The free energies
which were used in the calculations were standard free
energies, meaning that they are accurate only for the
standard state of 1 atmosphere, 2980 K., and an effective

concentration of l mole/liter.35

This requires such gross
assumptions that the end result cannot possibly approximate

reality.

30

Energy Considerations

If one solves all of the aforementioned problems, a
minimum thermodynamic energy requirement can be found. This
energy is the thermodynamic energy difference between the
initial and final states of the system (or reaction).
Unfortunately the application of this amount of energy to
the system might not result in the desired reactions because
these reactions might require an activation energy greater
than the difference between the two steady states. In a
closed system, the initial expense of activation energy
would be returned as the reaction proceeded. However, in
the open system with which one is forced to deal, this
activation energy will be lost as heat or in some other
form.

A further increase in the amount of energy required
will result from the inefficiencies of transfer of energy
from one form to another to produce the reactions. This
could develop into a serious problem if much energy is lost
in this transfer as it will probably be lost in the form
of heat, thus raising the temperature of the system to
undesirable levels in addition to greatly increasing the

energy requirements.

CONCLUS IONS AND RECOMMENDATIONS

Due to all of the problems related in the preceding
section, the obvious conclusion is that the method proposed
in this study as a means to determine the minimum energy
required to remove heavy metals from influent or sludge is
not feasible.

The first step which must be taken to eventually
reach this goal is to determine what metals compounds
actually exist in the influent and sludge. Each metal
should be considered individually, and all of its compounds
and complexes should be determined. It might then be
possible for an experienced chemist to work out the thermo-
dynamics of the system. However, because of the many
possible complications, I believe the final energy require-
ment must be determined experimentally by picking several
processes and determining which process is most energy

efficient on a pilot plant scale.

31

APPENDICE S

APPENDIX A

APPENDIX A

TABLES OF METALS COMPOUNDS

Table A1.--Organic-metal complexes which may occur in
wastewater or sludge.

 

 

Metal Compound Reference
Cadmium
CdNTA- 25
CdCit- 25
CdHCit- 25
Copper
CuNTA- 25
CuCit- 25
CuCit24- 25
Cu2c1t22' 25
CuZCit+ 25
CuHCit 25
Cu(OH)Cit2- 25
Cu2(OH)2Cit24- 25
Cu2(OH)Cit 25
B-alamino-Cu(II) ion 18
*Cu(II)L+ 24
*CuL2 24
Cu(II) ascorbate 18
Diphenol Oxidase 18

*L = leucine, valine, alanine, serine, glutamic
acid, aspartic acid, tyrosine, histidine.

32

33

Table A1.--Continued

 

 

Metal Compound Reference
Iron
FeNTA 25
Fe(OH)NTA- 25
(Fe(OH)NTA)22- 25
Fe(OH)2NTA2- 25
Fe(II)NTA— 25
FeCit 25
FeHCit+ 25
Fe(OH)Cit- 25
Fe(OH)2Cit2- 25
Fe2(OH)2Cit22- 25
Fe(II)Cit- 25
Fe(II)HCit 25
Fe(III)Sa1icylate 18
Pyrocatechin complex 18
Fe(OC6H5)3 18
Lead
PbNTA- 25
PbCit- 25
PbHCit 25
Manganese
MnNTA' 25
MnCit- 25

MnHCit 25

34

Table Al.--Continued

 

 

Metal Compound Reference
Nickel

NiNTA- 25

NiCit- 25

NiHCit 25
Zinc

Zn(II)cysteine 18

ZnNTA- 25

Zn(NTA)24- 25

ZnCit- 25

ZnHCit 25

Heavy metals in general (M = metal)

Meadenosine monophosphate
M-adenosine diphosphate
M-adenosine triphosphate

M-tartrate

18
18
18
18

 

35

Table A2.--Inorganic-meta1 complexes which could be
present in wastewater or sludge.

 

 

Metal Complex Solubility42 Reference
Cadmium
Cdc1+ - 19
CdCl2 s 19
CdOHCl - l9
CdCO3 i 31
Cd(CN)2 s 31
CdO i 31
Cd022- - 19
Cd(NO3)2 s 18
Cd3(PO4)2 i 31
Cd(OH)+ - 19
Cd(OH)2 s s 19
Hodoz' - 19
CdS s s 31
CdSO4 s 19
CdNH32+ - 19
Cd(NH3) 22+ - 19
Cd(NH3)32+ - 19
Chromium
Cr042- - 33
HCrO4- - 33

Cr(OH)3 - 34

36

Table A2.--Continued

 

42

 

Metal Complex Solubility Reference
Copper
CuCN i 33
Cu(CN)2 i 33
CuCO3 s 18
Cu(OH)2 i 18
Cu2(H20)102+ - 32
CuZO i 32
CuO i 32
CuS s s 32
Iron
Fe(OH)3 i 30
FePO4 vss 30
F63(PO4)2.8H20 i 29
FeS s s 30
Lead
Pbc1+ - 19
PbCl2 s 19
9503+ - 19
Pb(OH)2 s s 19
Pb(oa)3' - 19

Pb3(PO4)2 i

 

 

 

 

37
Table A2.--Continued
Metal Complex Solubility42 Reference
Zinc
ZnCl2 s 19
2:101+ - 19
ZnOH+ - 19
Zn(OH)2 -vss l9
zn(on)3' - 19
zn(0H) 42- " 19
ZnO s s 31
ZnS s s 31
ZnSO4 s 19
i= insoluble
s= soluble

s s= slightly soluble

vss= very slightly soluble

APPENDIX B

APPENDIX B

CALCULATIONS OF MINIMUM
ENERGY REQUIREMENTS

Fe(OH)3 + 3/2 H -+ Fe‘+ 3H+ + 30H“

2
AGO = (o + 3(-37.6)) - ((-l66 + 0) = 53.2 kcal/mole

Fe(OH)2 + H -+ Fe + 2H 0

2 2
AG0 = (o + 2(-56.69)) - {-115.57 + 0) = 2.19 kcal/mole

FeS + H2 -+ Fe + H23
AGO = (o + -6.54) - (~23.32 + 0) = 16.78 kcal/mole

ac° = (o + 2(-6.54)) - (-39.84 + 0) = 26.76 kcal/mole

FeCO3 + H2 -+ Fe + H2C03
0

AG = (O + -92.31) - (-161.06 + O) 68.75 kcal/mole

Fe(OH)3 + 3/2 H2 -+ Fe + 3H20

AGO = (o + 3(-56.69)) - (-166 + o)

-4.07 kcal/mole

CuCO3 + H2 -+ Cu + H CO

2 3
AGO = (o + -92.31) - (-123.8 + 0) = 31.49 kcal/mole

38

39

Cu++(aq) + H -+ Cu + 2H+

2

16° = (o + 0) - (15.53 + 0) = -15.53

C118 4- H2 -+ Cu + H28

16° = (o + -6.54) - (~11.7 + 0) = 5.16 kcal/mole

To convert kcal/mole to kwh/MG
__ kcal/mole x 1000 cal/kcal x ___mg/l M x l g/1000 mg
x 1 mole M/ __g x 4.186 joules/cal x 2.778 x 10-7kwh/jou1e

6

x .2642 1/gal x 10 gal/MG = __kwh/MG

The blanks in the above equation are filled as follows:
1. minimum energy requirement in kcal/mole,
2. concentration of metal (M) in mg/l, and

3. atomic weight of the metal (1 mole) in grams.

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MICHIGQN STATE UNIV. LIB

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