LIBRARY

Michigan State
University

This is to certify that the

thesis entitled

Effect of High Concentrations of Initial Surface P
on the Langmuir Adsorption Isotherm

presented by

Daniel C. O'Neill

has been accepted towards fulfillment
of the requirements for

M.S. Crop & Soil Science

degree in

 

 

gay/Jag

Major professor

 

Date 11-11-77

 

0-7639

EFFECT OF HIGH CONCENTRATIONS OF INITIAL SURFACE P

ON THE LANGMUIR ABSORPTION ISOTHERM

By

Daniel C. O'Neill

A THESIS
Submitted to
Michigan State University

in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE
Department of Crop and Soil Sciences

1977

ABSTRACT

Effect of High Concentrations of Initial Surface P
on the Langmuir Adsorption Isotherm

By
Daniel C. O'Neill

The effect of high levels of applied P on the adsorption maxima
(b) and k values as predicted by the Langmuir adsorption isotherm was
examined for seven Michigan soil types. The use of P extracted by Bray
P1, NHAF, and Na2B407 as well as isotopically exchangeable P to estimate
the initial level of surface adsorbed P was examined.

The b value along with the R value were affected by the soil type
and initial level of freshly applied P. In one case the change in R
value could not be corrected by any method used in this study.

Bray P corrected b and k values for high and low P concentrations

1
had correlations with 32P corrected b and k values significant at the

one percent level.

Bray P1 extracted P correlated at the one percent level of signifi-

cance with surface P estimated by 32F isotopic exchange.

Acknowledgments

 

The author wishes to thank Dr. Boyd G. Ellis and Dr. James E. Hook
for their help in writing and interpreting these results.
Special appreciation is expressed to Dr. Domy Adriano for the

opportunity to achieve this.

11

TABLE

List of Tables . . . . . . .
List of Figures . . . . . . .
1) Introduction . . . . . . .
2) Materials and Methods.

3) Results and Discussions

4) Summary and Conclusions .
5) Literature Cited . . . . .

6) Appendix . . . . . . . . .

OF CONTENTS

1'”

iv

. l7

. 18

. l9

LIST OF TABLES

 

Table

1. Description and properties of soils

2. Correlation between 32F corrected b
extractant corrected b and K values

3. Change in b value by correction for
adsorbed P O O O O O O O O O O O O O

4. Influence of P level and extractant

and K values . . . . . . . . . . .

iv

used in this study. 4

and K values and

o o o o o o o o o 0 8-11
estimated surface

. . . . . . . . . 12
on correction of b

O O O O O O I O O O 15

LIST OF FIGURES

FIGURE PAGE

1. Correlation Between Bray P1 and 32F Surface P. 13

INTRODUCTION

The Langmuir adsorption isotherm was originally proposed to des-
cribe adsorption of gases on solid surfaces. More recently it has been
expanded to describe adsorption of ions from solutions onto solid
surfaces. In the study of adsorption of ions by soils numerous investi-
gators have made use of this isotherm to explain many different soil
properties. A few which pertain to the study of P are: prediction of P
movgnent in soils which have been heavily fertilized (Susuki, 1963),
evaluation of P requirement of soils (Fox, 1970), mathematical model for
P movement in soils (Fox, 1970), mathematical model for P movement in
soils (Shah et al., 1975) and mechanism of P sorption.(Muljadi, 1966).

In the study of P adsorption by soils, Olsen and Watanake (1957)

used the Langmuir equation in the following form:

C C + 1 (1)

x/m b kb

 

 

 

Where: x/m - amount of P adsorbed per weight of soil (mg P/lOO g soil)

0‘
I

the adsorption.maximum (mg P/lOO g soil)

the equilibrium P concentration (moles/liter)

O
I

k = a constant related to the bonding energy of the adsorbent
for the adsorbate (liter/mole)

A plot of __§L__ versus C should yield a straight line with a slope
of l/b and an iiéZrcept of l/kb. Deviations from the theoretical
straight line (equation 1) with its single value for maximum adsorption
(b) and its single value for the constant (k) have frequently been
observed for soils and soil minerals. At higher equilibrium solution
concentrations (C) the observed values of __g___ versus C fall below the

x/m
line defined by the low C values. This type of deviation can be

resolved into two intersecting straight lines suggesting that two dif-
ferent reactions control the adsorption of P on soils (Taylor, 1977).
The two lines result in two pairs of b and k values.

In discussing the use of the Langmuir adsorption isotherm to study
P adsorption by soils, Olsen and watanabe (1957) pointed out that the
intercepts of a plot of __g___versus C increased as the initial level of
surface P increased. The:::ore, soil containing appreciable surface P
initially gave erroneous k values. A correction was made by adding the
amount of initial surface P (determined by a separate analysis) to the
x/m value of the P adsorbed from the equilibrium solution. This assumed
that the initial surface P was bonded by the same mechanism as P ad-
sorbed from an added solution of KHZPO4 (Olsen and watanabe, 1957).

The method generally recognized as a measure of initial surface P
is isotopic dilution with a solution containing a known concentration of
32P. This principle was apparently first applied to soils by McAuliffe
et a1. (1948). If surface 31F is the original exchanging ion on the
soil surface to which 32P has been added in a water solution, the ex-
change with chemically identical ions can be described at equilibrium of
the equation:

surface 32P . surface 31P (2)

solution 32? solution 31P

 

 

Since solution 31? can be experimentally determined as the ratio of
activities between solid and solution, the surface 31? can be calcu-

lated.

Isotopic dilution was used by Olsen and Watanabe (1957) to correct
for initial surface P. Although this appears to be an accurate method
of estimating surface P, it is expensive and time consuming. A pro-
cedure based upon a simple extraction would eliminate the need for
radioactive precautions and special instrumentation and would facilitate
the use of the Langmuir adsorption isotherm. The extractant which would
be a practical alternative to 32P should be easy to use and should yield
an extract from which P can be easily measured. The P value obtained
should be the same as the surface P estimated by 32P or give a highly
correlated value which could be related with linear regression to 32P. A
preliminary study (appendix) suggested that three extract solutions
would have the greatest potential. These are 0.03 N NH F-0.025N HCl

4

(Bray P1)(Doll, 1972), neutral NH4F(Bray, 1941) and Na2B407.

Materials and Methods

 

Soil Samples

 

Seven soil types common to Michigan were selected for this study
(Table 1). Soil samples were collected from the south-end of Michigan
State University campus as part of a soil characterization for the
Institute of water Research. Each sample was air dried, ground to pass
a 10 mesh screen and stored in a cardboard container. Because these
soils were low in surface P two hundred grams were separated from each
soil and ammended with 10 mg P as Ca(H4P04)2 with enough water to bring
each soil to field capacity. This quantity of P was 50 mg P/kg soil and
ranged from 25 to 78 percent of the b value for individual soils with an

average of 47 percent. This set of soils was used to determine the

 

 

 

Table 1. Description and prOperties of soils used in this study.
Series Class Drainage* Depth Texture** Horizon
cm
Spinks Hapludalf W 5 - 30 1 f s A
30 - 119 1 f s Bgl
Riddles Typic W 5 - 30 f s 1 A
Hapludalf 48 - 127 s c 1 BP
Hillsdale Typic MW 5 - 35 f s 1 A
Hapludalf 35 - 91 c s 1 BP
Miami Typic _ WP 0 - 25 l A
Hapludalf 25 - 88 c 1 BP
Brookston Argiaquoll PD 5 - 27 1 A
35 - 71 c 1 BEZt
Celina Typic MW-SPD 0 - 33 l A
Hapludalf 58 - 101 c 1 BP
Conover Ochraqyalf SPD 0 - 25 1 A
35 - 104 c 1 BP
* W - Well
MW - Moderately Well
PD - Poorly Drained
SPD = Somewhat Poorly
** 1 f s - Loamy fine sand
c 1 - Clay loam

effect of large quantities of initial P on the Langmuir adsorption
isotherm. The soils were covered with polyethelene to prevent evapor-

ation and yet allow passage of O incubated for one week at room

2’

temperature, then air dried and ground to pass a 10 mesh screen.

Soil Analysis
Initial P was extracted with 0.05 N_NH4F (2 g soil in 50 m1), 0.03fi

Na2B407 (2 g soil in 50 ml) and 0.03‘11NH4

40 ml) by shaking on a rotary shaker at 200 r.p.m. for five minutes.

F - 0.025 §_HC1 (5 g soil in

Clear extract was obtained by use of Darco charcoal and filtration

through Whatman No. 42 filter paper.

Isotopic Dilution

 

One gram of soil was equilibrated with 99 m1 of H at 24° C by

2
shaking on a rotary shaker at 200 r.p.m. for one hour. To this was
added 1 m1 of a solution containing a known amount of 32F. Samples were
then shaken for 24 hrs., centrifuged and an aliquot was taken for anal-

ysis of 32P and 31P in solution. Radioactive 32P was counted by

Cerenkov radiation by the method by White and Ellis (1968).

 

Langmuir Adsorption Isotherm

Phosphorus adsorption isotherms were measured on duplicate soil
samples by equilibrating 5 g. of soil (ground to pass a 18 mesh screen)
for 24 hrs. at 24° C with 50 m1 of 0, 2, 4, 6, 8, 10, 12, 15, 20, 30 ppm
P in 0.01 M CaClz. At the end of 24 hrs. the samples were centrifuged
and an aliquot taken for analysis. The quantity of P in solution.was

subtracted from the amount added and the difference was regarded as the

quantity adsorbed.

P Analysis

 

Phosphorus in all extracts and equilibrium solutions was analyzed

by ammonium molybdate ascorbic acid method (Murphy and Riley, 1962).

Results and Discussion
The b and k values for low P soils were affected relatively little
by any method used for the correction of initial surface P (Table 2).
There was a trend for larger b and k values when corrected by either

2P or Bray P1 estimate. The need for a correction became evident when

the k values for high P concentration were compared with the k values

for low P concentration increased. In only two of these deviations did

32

Bray P or P fail to correct the k value back to the initial k values,

1

(low P) where no correction was made. The k vlaues of the

Spinks soil A horizon decreased by a factor of eight and no method used
was able to correct back to the k value (Low P). The k value for the A
horizon of the Celina soil also decreased by a factor of eight. Both

Bray P and 32P estimate of surface P partially corrected this.

1
Isotopic dilution with 32P was considered the standard method of

estimating surface P and all resultant values were referenced to this.
Linear correlations were calculated for b values and for k values
between those corrected by 32P and those corrected by the extractants

(Table 3). Bray P corrected b values had the highest correlation with

1
32? corrected b values both for all soils and high level P soils. Sodium

borate corrected b values correlated nearly as well. For correcting k

32

values Bray P correlated better with the k values corrected by P

1
than the other extractant.

Fig. 1 shows the relationship between Bray P1 extracted P and
surface adsorbed P measured by 32P. The r2 was significant at the one

percent level but did not account for 25 percent of the variation. The

slope indicated almost a one to one agreement between Bray P1 and 32P

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12

32F corrected b and k values

 

 

 

 

 

 

 

Table 3. Correlation between
and extractant corrected b and k values.
b k

Extractant A11 Soils High P soils All 80118 High P Soils

____________ r2*_ - - - _ _ _ - - - - - _
Bray P 0.95 0.94 0.98 0.91
NH4F 0.91 0.86 0.90 0.84
Ha2H4O7 0.93 0.94 0.95 0.72
None 0.89 0.92 0.75 0.73

 

Any r2 greater than 0.5 is significant at the 1% level.

13

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14

although 32? estimated surface P is slightly greater than Bray P by an

1
amount of 0.24. This relationship between surface P and Bray P is

1
similar to results obtained by Doll (l972)1. No meaningful relationship
between NHaF extracted P or Na28407 extracted P and surface adsorbed P
estimated by 32P was obtained.

A four-way analysis of variance of levels of initial P, soils type,
soil depth and extracts was conducted for the parameter b. It revealed
that there was a significant interaction (.052) for the b values with P
level, soil series, and depth. The ability of a soil to adsorb P has
been shown to be dependent on soil texture and horizon (Ellis, 1973).
Sandy soils adsorb less P than heavy textured soils and the A horizon
adsorbs less P than the B horizon in soils. In this study the type of
soil was shown to have an interaction with depth significant at the .05%
level. But the soil also had an interaction with the level of P which
was significant at the .05% level. Changes in the b value would be
expected because theoretically the slope (and therefore b) should not be
affected by the initial P status of the soil.

The average of the mean b values for low and high levels of P gives
the impression that the b values decrease as the initial level of sur-
face P increases (Table 4). But since the analysis of variance showed a
three way significant interaction this may not be a valid conclusion.

Influence of initial P level and extractant on correction of b

values showed that eight of the 14 soils decreased in b value for an

 

1Doll, E. C. 1972. Unpublished Report.

15

Table 4. Change in b value by correction for estimated
surface adsorbed P.

 

Extractant Used to Estimate Surface P
32

 

P Level None Na2B407 Bray P1 (NH4F . P Average
b (mg/100 g)*

Low 11.5 11.9 12.7 11.7 12.9 12.1

High 10.2 11.1 13.8 11.6 13.1 11.9

 

* Each value reported is a mean of 14 soil samples.

16

increase in initial surface P when no correction was made and three
remained the same. For the Bray P1 corrected b values six were un-
changed and six increased. For NH4F corrected b values none were the
same and three increased. Sodium borate correction resulted in six
unchanged and five decreased.

The decrease in predicted maxima with increasing P in the uncor-
rected isotherms and the increase in b values with increasing P in the
Bray P1 corrected isotherms can be accounted for if something has‘
occurred in the P amended soils which altered adsorption sites or
rendered than unavailable. The maxima predicted for uncorrected soils
would then show a decrease. Precipitation of AlPO4 (without creation of
new sites) or crystallization (eg. FePO4) could account for this. Bray
P1 extraction could, however, measure either of these as adsorption
sites when the correction is made.

The large number of NH F and Na B O corrected b values which were

4 2 4 7

unchanged is evidence that AlPO compounds may be involved, because

4

these two extracts remove P from A1 fractions.

17
Summary and Conclusions

The purpose of this study was to examine several extraction tech-
niques which could be used in place of 32F when correcting the Langmuir
adsorption isotherm for initial surface P. ,Based upon regression anal—

ysis and analysis of variance, Bray P can readily be used in place of

1
32F. The use of the isotherm would be greatly facilitated, since this
extraction is routinely carried out on soils in the humid region.
Ammonium fluoride appeared to be quite capable of correcting the
adsorption isotherm, but did not correlate with 32F correction as well

as Bray P , nor did it show any type of agreement with 32F exchangeable

1
P.
Analysis of variance shows that correction of the isotherm is de-
pendent upon the initial concentration of surface P. The data obtained
indicates that the correction was necessary as the concentration of
initial P increased but when soils with low initial P were corrected by

32? or Bray P no change in the isotherm values occurs.

1
Two important points which should be examined in the future are the
deviation in k values which could not be accounted for by any method

used in this study for correcting initial P and the fact that the maxima

did decrease as the initial surface P increased.

10.

ll.

12.

13.

18
LITERATURE CITED

Bray, R. B., and L. T. Kurtz. 1945. Determination of total,
organic, and available forms of phosphorus in soils. Soil

Sci. 59:39-45.

Dickman, S. R. and R. H. Bray. 1941. Colorimetrics determination
of phosphate. Ind. Eng. Chem. Anal. Ed. 12:665-668.

Dickman, S. R. and R. H. Bray. 1941. Replacement of adsorbed
phosphate from kaolinite by fluoride. Soil Sci. 52:263-275.

Fox, R. L. and E. J. Kamprath. 1970. Phosphate sorption isotherms

for evaluating the phosphate requirement of soils. Soil Sci. Soc.
Amer. Proc. 34:903-907.

Harter, Robert D. 1969. Phosphorus adsorption sites in soils.
Soil Sci. Soc. Amer. Proc. 33:630-631.

Muljadi, D., A. M. Posner and J. P. Quirk. 1966. The mechanism
of phosphate adsorption by kaolinite, gibbsite and pseudoboeh-
mite. Jour. of Soil Sci. 17:212—229.

Murphy, J. and J. P. Riley. 1962. A modified single soulution
method for determination of phosphate in natural waters. Anal.
Chem. Acts. 27:31-36.

Olsen, S. R. 1952. Measurement of surface phosphate on hydroxyl-
apatite and phosphate rock with radio-phosphorus. J. Phys. Chem.
56:630-632.

Olsen, S. R. and F. S. watanabe. 1957. A method to determine a
phosphorus adsorption maximum of soils as measured by the Langmuir
isotherm. Soil Sci. Soc. Amer. Proc. 21:144-149.

Parfitt, R. L. , R. J. Atkinson and R. S. Smart. 1975. The
mechanism of phosphate fixation by iron oxides. Soil Sci. Soc.
Amer. Proc. 39:837-841.

Shah, D. B., G. A. Coulman, L. T. NOvak and B. G. Ellis. 1975.
A mathematical model for phosphorus movement in soils. Jour. of

Suzuki, A., K. Lawton and E. C. Doll. 1963. Phosphorus uptake
and soil tests as related to forms of phosphorus in some Michigan
soils. Soil Sci. Soc. Amer. Proc. 27:401-403.

White, R. P. and B. G. Ellis. 1968. Routine counting of 32P

in colored solutions from dry ashed plant samples utilizing
Cerenkov radiation. Soil Sci. Soc. Amer. Proc. 32:740-741.

APPENDIX

19

APPENDIX

A number of anions which have been reported in the literature to be

capable of replacing P from soils were examined to select the few that

might be most likely to correlate with surface P as determined by 32P

isotopic dilution with 32P. A preliminary experiment was conducted on
four soils considered to be low in P and two known to be high in adsorb-

ed P. The results are given in Table 1A. Both KSCN and NH4OAc failed

to extract P from the low P soils and removed a much smaller portion of
P from the two high P soils (Wayland and Middleville). The other three
extracts removed large quantities of P from the two high P soils and

varying small quantities form the low P soils. Bray P1 removed the

largest quantities of P in all cases. From these data it was decided to

1, NHAF and Na2B4O7 for the thesis study. Both KSCN and

OAc were eliminated because of their inability to extract P from

use Bray P

““4

soils which contained greater than 10 ppm P as Bray P1 extractable.

Table 1A.

20

Soil P extracted using various extracting solutions.

 

Soil Type or

 

 

 

Location Depth Bray P NH4F Na23407 KSCN NH4OAc
cm ppm P
Spinks 5 - 30 14.87 2.81 2.98 0.0 0.0
30 - 120 14.10 3.00 1.00 0.0 0.0
Hillsdale 5 - 30 17.78 1.61 1.13 0.0 0.0
35 - 90 4.35 0.93 1.81 0.0 0.0
Riddles 5 - 30 5.23 1.00 1.10 0.0 0.0
50 - 130 3.61 0.13 1.13 0.0 0.0
Miami 0 — 25 6.25 0.39 0.35 0.0 0.0
25 - 90 4.81 0.20 0.20 0.0 0.0
Wayland 0 - 15 68.77 30.00 25.00 13.44 10.00
Middleville 0 - 15 117.95 71.90 48.62 16.00 18.13