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Adsorption / desorption

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

adsorption / desorption: screening

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

No data

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

other: US EPA Pesticide Assessment Guidelines, Subdivision N, § 163-1

Deviations:

no

Principles of method if other than guideline:

Not applicable

GLP compliance:

yes (incl. QA statement)

Type of method:

batch equilibrium method

Media:

other: water/ soil/ sediment

Specific details on test material used for the study:

Details on properties of test surrogate or analogue material (migrated information):
Not applicable

Radiolabelling:

yes

Test temperature:

25 +/- 1°C

Details on study design: HPLC method:

Not applicable

Analytical monitoring:

yes

Details on sampling:

No data

Details on matrix:

See Table 10.1.2/2 Classification, textural class/content and physico-chemical properties of the soils used as adsorbents

test system:
Known quantities of soil/sediment samples were brought into contact with known volumes of aqueous solutions of test substance at known
concentrations in Pyrex screw-cap glass test tubes. The soil/sediment suspensions were shaken in darkness on a platform shaker in an
environmental chamber set at 25 ± 1°C. At each sampling interval the soil/sediment suspensions were centrifuged at 2000 rpm for
10 minutes. The supernatants were removed by decanting, and their volumes were measured and recorded. LSC analyses were
performed on triplicate aliquots. For the desorption phase, an aliquot of fresh 0.01 M CaCl2 solution, equal to the volume removed
after the adsorption phase, was added back to each test tube containing the pellet. Samples were return back to the shaker.
LSC analyses were performed the same way.

Details on test conditions:

In the definitive test, determination of the Freundlich adsorption and desorption isotherms were performed with four aqueous test
solutions of test substance at nominal concentrations of 1, 10, 100 and 1000 µg/ml (expressed as anhydrous active substance).
A primary stock solution of non-radiolabelled THPS was prepared by weighing 51.2 mg of THPS (expressed as anhydrous active
substance) into a 50 ml class A volumetric flask and diluting to volume with 0.01 M CaCl2. The primary stock solution had a
concentration of 1024 µg/ml. Two stock solutions with concentrations of 10.2 and 102 µg/ml were prepared by serial dilutions of the
primary stock solution. A 1 µl aliquot of 14C-THPS as received was added to the non-radiolabelled stock solutions with concentrations
of 1024, 102 and 10.2 µg/ml. A 1 µg/ml dose solution was prepared by diluting a 1 µl aliquot of the 14C-THPS as received with 45 ml
of 0.01 M CaCl2. Triplicate 100 µl aliquots were taken from each dose solution and analysed by LSC to determine the radioactivity
concentration. The 1000 µg/ml dose solution was analysed by 31P-NMR.
The definitive test was performed with a soil:water ratio of 1:2 and an equilibrium time of 2 hours for the adsorption and desorption
stages.

Preliminary study:
To determine equilibrium time for sorption of the test substance to the soil/sediment matrices, duplicate samples of 1.00 (dry weight)
of each soil/sediment type were weighed into test tubes. To each tube, a 10 ml aliquot of a 250.3 µg/ml aqueous test solution was added. The test tubes were capped and placed on the shaker in the environmental chamber in the dark. Two replicates of each of the
soil/sediment types were sampled at 0, 1, 2, 4 and 6 hours and the supernatants analysed by LSC on triplicate 1.00 ml aliquots.
Triplicate blank samples containing 10 ml of the aqueous 14C-test solution in each test tube with no soil or sediment were included
to determine if adsorption of THPS to the test containers would occur.
At each sampling interval, an aliquot of fresh 0.01 M CaCl2 solution, equal to the volume removed after the adsorption phase,
was added back to each test tube containing the pellet. The samples were returned back to the shaker for approximately 20 hours.
After the desorption period, the samples were centrifuged and the supernatants analysed by LSC on triplicate 1.00 ml aliquots.
From the percent adsorption results of the above test, soil to water ratios of 1:3 and 1:2 were selected. Duplicate samples of 10.00 g
(dry weight) of each soil/sediment type were weighed into test tubes. One sample received 30 ml of a 260 µg/ml aqueous test solution,
and the other sample received 20 ml of the same test solution. The aqueous phases were sampled following 1, 2, 3 and 4 hours of
equilibration. Supernatants were analysed by LSC on triplicate 100 µl aliquots.
The stability of the test substance was not assessed during the preliminary study. The stability of the test substance was assessed
in the definitive study, in the 1000 µg/ml adsorption phases of each soil/sediment type, by 31P-NMR.

Definitive test:
In the definitive test, the Freundlich adsorption and desorption isotherms were performed.
Adsorption procedure:
A 2 g (dry weight) portion of each soil/sediment was weighed into 10 test tubes. A 4 ml aliquot of each dosing solution was added to
two replicate test tubes of each soil/sediment type. The soil/sediment suspensions were shaken for 2 hours, then centrifuged and the
supernatants analysed for radioactivity on triplicate 100 µl aliquots. The adsorption phase of the 1000 µg/ml concentration from each
soil/sediment type was analysed by 31P-NMR, and 10 µl of this adsorption phase was also analysed by TLC.
Desorption procedure:
An aliquot of fresh 0.01 M CaCl2 solution was added to each sample test tube containing the pellet corresponding to the volume
removed after the adsorption phase. The soil/sediment suspensions were shaken for 2 hours, then centrifuged and the supernatants
analysed for radioactivity on triplicate 100 µl aliquots. The radioactive residues in the pellets after the centrifugation step were
determined by combustion of triplicate aliquots of soil/sediment followed by LSC analysis.
A second isotherm test was performed for the sandy loam soil because of low radioactivity mass balance observed in the first study.
Loss of 14C-material was suspected to be due to microbial degradation. Two tests were conducted using the above procedure.
In one case the soil and water were not autoclaved, and in the other case the soil and water were autoclaved for 40 minutes at
121°C and 1034 hPa.

Sample No.:

#1

Duration:

2 h

Temp.:

25 °C

Sample no.:

#1

Duration:

2 h

Computational methods:

The adsorption and desorption properties of the test substance were characterised by the Freundlich equation:
ln (x/m) = ln (K) + 1/n ln (Ce)
with:
x = µg of test substance adsorbed at equilibrium
m = mass of soil in grams
Ce = equilibrium solution concentration (µg/ml)
K = Ka (adsorption coefficient) or Kd (desorption coefficient)
1/n = empirical exponent

The constants Ka, Kd and n were determined for each soil/sediment type from the slope (1/n) and the y-intercept (ln Ka or ln Kd) of the
corresponding line equation.

Key result

Type:

Koc

Value:

153 L/kg

Remarks on result:

other: Mean adsorption Koc.

Sample No.:

#1

Type:

Koc

Value:

210 L/kg

Remarks on result:

other: Agricultural sand

Sample No.:

#2

Type:

Koc

Value:

126 L/kg

Remarks on result:

other: Silt loam

Sample No.:

#3

Value:

72 L/kg

Remarks on result:

other: Pond sediment

Sample No.:

#4

Value:

146 L/kg

Remarks on result:

other: Sandy loam 1

Sample No.:

#5

Value:

266 L/kg

Remarks on result:

other: Sandy loam 2

Sample No.:

#6

Value:

81 L/kg

Remarks on result:

other: Sandy loam 3

Sample No.:

#7

Value:

167 L/kg

Remarks on result:

other: Marine sediment

Details on results (HPLC method):

Not applicable

Adsorption and desorption constants:

Aqueous solutions of 14C-THPS diluted in cold Tolcide PS75 were  equilibrated with five soil/sediment types, i.e. agricultural sand, silt  loam, pond sediment, sandy loam and marine sediment, and the adsorption  and desoprtion coefficients were determined according to US EPA Pesticide  Assessment Guidelines, Subdivision N, § 163-1. Liquid Scintillation  Counting (LSC) analysis was employed to measure the test material  concentrations in the aqueous phases. The 14C-activity remaining adsorbed  to the soil was determined by combustion/LSC analysis. Preliminary studies were performed to determine the equilibration time,  the soil:water ratio and the possible adsorption of the test substance to  the test containers. The isotherm test was conducted at 25 ± 1 °C, in the dark, with four test  substance concentration levels (1, 10, 100 and 1000 µg/ml, nominal  concentrations expressed as anhydrous active substance). The chemical  purity of the 1000 µg/ml test solution was determined by 31P-NMR. For  each soil/sediment type, the 1000 µg/ml adsorption phase was  characterised by 31P-NMR. The Freundlich equations were calculated for  each soil/sediment type. The isotherm test for sandy loam was repeated because of a loss of  14C-material possibly due to microbial degradation. Two tests were  concurrently conducted: in one case the soil and water were not  autoclaved, and in the other case the soil and water were autoclaved.

Recovery of test material:

No data

Concentration of test substance at end of adsorption equilibration period:

No data

Concentration of test substance at end of desorption equilibration period:

No data

Transformation products:

not measured

Details on results (Batch equilibrium method):

No data

Statistics:

No data

From the preliminary studies, an equilibration time of 2 hours was  determined and a soil:water ratio of 1:2 was selected for the definitive  study. 

The test substance did not adsorb to the test containers.
In the definitive study, the mean percent of compound adsorbed to the  test soils/sediments was 17.5, 34.6, 35.9, 29.3, 45.1, 27.1 and 17.2  for  

agricultural sand, silt loam, pond sediment, sandy loam, sandy loam  (repeat, non-autoclaved), sandy loam (repeat, autoclaved) and marine  

sediment respectively. The mean 14C-mass balance was 97.4 %, 91.1 %, 93.2  %, 84.0 %, 80.8 %, 104 % and 98.2 % for agricultural sand, 

silt loam,  pond sediment, sandy loam, sandy loam (repeat, non-autoclaved), sandy  loam (repeat, autoclaved) and marine sediment respectively. 

The  Freundlich adsorption and desorption isotherm equations showed a high  degree of linear correlation.
In the 1000 µg/ml asdorption phases, THPS was observed at 60.5 %, 33.2 %,  18.4 %, 69.8 %, 41.7 %, 100 % and 82.3 % for agricultural sand, 

silt  loam, pond sediment, sandy loam, sandy loam (repeat, non-autoclaved),  sandy loam (repeat, autoclaved) and marine sediment respectively, 

and the  major degradate THPO was observed at 39.5 %, 66.8 %, 39.1 %, 30.2 %, 58.3  % and 17.7 % for agricultural sand, silt loam, pond 

sediment, sandy loam,  sandy loam (repeat, non-autoclaved), and marine sediment respectively.
The test substance was not stable in the test matrix of soil/sediment and  water to accurately determine the purity at initiation, and degradation  

of the test substance occurred during the 2 hour equilibration period.  Since the potential for adsorption of the test substance to the  soil/sediment 

matrix is low, and this degradation will occur in the  environment, the adsorption/desorption values determined represent an  environmentally 

relevant estimate for mobility of the test substance in  the environment.

Koc(adsorption) :
Agricultural sand (210), Silt loam (126), Pond sediment (72), Sandy loam  (146), Sandy loan non autoclaved (266), Sandy loam autoclaved (81),  

Marine sediment (167).

Koc(desorption) :
Agricultural sand (960), Silt loam (149), Pond sediment (159), Sandy loam  (240), Sandy loan non autoclaved (223), Sandy loam autoclaved (210),  

Marine sediment (735).

The mean adsorption Koc is 153 +/- 69.2.

THPO (Tris(hydroxymethyl) phosphine oxide, CAS No 1067-12-5) was the  major degradate observed.

Validity criteria fulfilled:

yes

Conclusions:

THPS is likely to have medium to high mobility in various soil and sediment matrices, with a mean Koc at 153 L/kg.

Executive summary:

Aqueous solutions of¹⁴C-THPS diluted in cold Tolcide PS75 were equilibrated with five soil/sediment types, i.e. agricultural sand, silt loam, pond sediment, sandy loam and marine sediment, and the adsorption and desoprtion coefficients were determined according to US EPA Pesticide Assessment Guidelines, Subdivision N, § 163-1. Liquid Scintillation Counting (LSC) analysis was employed to measure the test material concentrations in the aqueous phases. The ¹⁴C-activity remaining adsorbed to the soil was determined by combustion/LSC analysis.

Preliminary studies were performed to determine the equilibration time, the soil:water ratio and the possible adsorption of the test substance to the test containers. The isotherm test was conducted at 25 ± 1 °C, in the dark, with four test substance concentration levels (1, 10, 100 and 1000 µg/ml, nominal concentrations expressed as anhydrous active substance). The chemical purity of the 1000 µg/ml test solution was determined by³¹P-NMR. For each soil/sediment type, the 1000 µg/ml adsorption phase was characterised by³¹P-NMR. The Freundlich equations were calculated for each soil/sediment type.

The isotherm test for sandy loam was repeated because of a loss of¹⁴C-material possibly due to microbial degradation. Two tests were concurrently conducted: in one case the soil and water were not autoclaved, and in the other case the soil and water were autoclaved. According to the results of this study, the test substance has a medium to high mobility in various soil and sediment matrices.The mean K_oc is 153 L/kg ± 69.2.

Reference 2

Endpoint:

adsorption / desorption, other

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline followed

Principles of method if other than guideline:

Laboratory incubation studies were conducted to measure adsorption, movement, and trans¬formations of urea and hydrolyzed ammoniacal N in flooded soil columns. Thus the goal of this study was to determine urea adsorption over 48h in sterilised Crowley silt loam soil and urea diffusion, urea hydrolysis and subsequent NH+4-N diffusion over 30 days in sterilised and non-sterilised Crowley silt loam soil.

GLP compliance:

not specified

Type of method:

other: not known

Media:

soil

Specific details on test material used for the study:

standardized urea-N solutions (mg N/L)

Radiolabelling:

no

Analytical monitoring:

yes

Matrix no.:

#1

Matrix type:

silt loam

% Clay:

11

% Silt:

71

% Org. carbon:

0.7

pH:

5.8

CEC:

9.4 meq/100 g soil d.w.

Details on matrix:

The soil used in the incubation studies was a Crowley silt loam collected from the Rice Research Station, Crowley, LA. Soil properties included: pH of 5.8 (1:1, soil/water ratio), CEC of 9.4 cmolc/ kg of soil, 7.0 g total C/ kg, 0.8 g total N/kg, 11% clay, and 71% silt. Approximately 80 kg of soil was dried, ground, and sieved through a 2-mm mesh screen and then thoroughly mixed.

Key result

Type:

Koc

Value:

5.3 - 9.1 L/kg

Temp.:

30 °C

% Org. carbon:

0.7

Key result

Phase system:

solids-water in soil

Type:

Kp

Value:

0.037 - 0.064 L/kg

Temp.:

30 °C

% Org. carbon:

0.7

Urea adsorption by the soil increased with increasing concentration of added urea-N.

Distribution coefficients (Kp) ranged from 0.037 to 0.064 with an average value of about 5.1 corresponding to an adsorption coefficient of 5.2 to 9.1 L/Kg taking into account the fraction of organic carbon in this soil.

In a soil column with a volumetric water content of 0.5 and a bulk density of 1, the ratio of urea in solution to that sorbed on the soil would be 10. These data indicate that urea adsorption by the Crowley silt loam was low and of minor significance.

 

In addition, this study shows that diffusion of urea in sterilized soil columns and subsequent diffusion of produced NH4+ in nonsterile flooded soil columns proceeded at the same rate after urea was dissolved and added to the floodwater. Urea-hydrolysis rates were variable and increased with incubation time and followed first-order reaction kinetics.

Conclusions:

Urea adsorption by the soil increased with increasing concentration of added urea-N.
Distribution coefficients (Kp) ranged from 0.037 to 0.064 with an average value of about 5.1 corresponding to an adsorption coefficient (Koc) from 5.2 to 9.1 L/Kg taking into account the fraction of organic carbon in this soil.

Executive summary:

Laboratory incubation studies were conducted to measure adsorption, movement, and transformations of urea and hydrolyzed ammoniacal N in flooded soil columns. Thus the goal of this study was to determine urea adsorption over 48h in sterilised Crowley silt loam soil and urea diffusion, urea hydrolysis and subsequent NH+4-N diffusion over 30 days in sterilised and non-sterilised Crowley silt loam soil.

As a result, the distribution coefficients (Kp) ranged from 0.037 to 0.064 with an average value of about 5.1 corresponding to an adsorption coefficient (Koc) from 5.2 to 9.1 L/Kg taking into account the fraction of organic carbon in this soil and Urea adsorption by the soil increased with increasing concentration of added urea-N

These data indicate that urea adsorption by the Crowley silt loam was low and of minor significance.

Reference 3

Endpoint:

adsorption / desorption, other

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline followed

Principles of method if other than guideline:

The absorption of urea and a number of its derivatives by different soils was investigated using a slurry-type procedure.The objects of this investigation were to assess which factors in British soil are involved in adsorption and to study the relationship between the structure of substituted urea herbicides and their adsorption.

GLP compliance:

no

Type of method:

other:

Media:

soil

Radiolabelling:

no

Analytical monitoring:

yes

Details on matrix:

Eleven soils were used in the investigation. Eight soils were obtained from N.A.A.S. Experimental Husbandly farms, two from private farms and one from the farm of the A.R.C. Weed Research Organization.
However, Experiments with N,N- dimethylurea (diuron), were carried out on all eleven soils, both unoxidized and oxidized, but experiments with the other ureas were confined to unoxidized soils 1-6.

Sample No.:

#1

Type:

Koc

Value:

7.9 L/kg

Matrix:

Soil N°1

% Org. carbon:

36.5

Key result

Sample No.:

#1

Phase system:

solids-water in soil

Type:

Kp

Value:

2.9 L/kg

Matrix:

Soil N°1

% Org. carbon:

36.5

Phase system:

solids-water in soil

Type:

Kp

Value:

0 L/kg

Matrix:

Soil N°2, 3, 4, 5 and 6

% Org. carbon:

>= 1.72 - <= 12

The results were assessed with the aid of the empirical Freundlirh adsorption isotherm

As results:

- N-aryl and N-aryl substituents play a part in the adsorption of substituted ureas by soils. When both are present the contribution to adsorption of each substituent is reduced, possibly for steric reasons.

- Increasing chain length in alkyl substituents or chloro and chlorophenoxy substitution in aryl substituents increases adsorption.

- No close relationship was apparent between adsorption of substituted ureas and their water solubility.

- Organic matter seems to be the principal site of adsorption.

- Only a part of the total soil surface appears to be available for adsorption.

Conclusions:

It appears that urea adsorption is low. Distribution coefficients (Kp) ranged from 0 to 2.9 L/Kg in six different soils. This corresponds to an adsorption coefficient (Koc) from 0 to 7.9 L/Kg taking into account the fraction of organic carbon of their soil.

Executive summary:

The absorption of urea and a number of its derivatives by different soils was investigated using a slurry-type procedure.

This study shows increasing chain length in the alkyl substituents and chloro and chlorophenoxy substitution in the aryl substituents increased adsorption. There was no relationship between adsorption and water solubility. It appears that urea adsorption is low since distribution coefficients (Kp) ranged from 0 to 2.9 L/Kg in six different soils. This corresponds to an adsorption coefficient (Koc) from 0 to 7.9 L/Kg taking into account the fraction of organic carbon of their soil.

Description of key information

As no data is available on the Reaction mass THPS-urea monomer itself, the highest adsorption coefficient of the both components of the reaction mass is used as key value i.e 153 L/kg.

Key value for chemical safety assessment

Koc at 20 °C:

153

Additional information

No data is available on the Reaction mass THPS-urea monomer itself.

However, reliable data are available on both component of the Reaction mass THPS-urea monomer i.e. the THPS and the urea.

 

One study of reliability 1 is available to assess the soil/sediment adsorption/desorption of the THPS. This study was performed on the THPS in solution. In this study (Heim, 1998), the adsorption and desorption coefficients on seven soil/sediment types (i.e. one agricultural sand, one silt loam, one pond sediment, three sandy loam and one marine sediment) were determined according to US EPA Pesticide Assessment Guidelines, Subdivision N 163-1.The mean value of the adsorption coefficients of the seven soil/sediment types and based on their organic carbon is 153 +/- 69.2 L/kg. Therefore, THPS has a medium to high mobility in various soil and sediment matrices. And the major degradable product observed was THPO (Tris(hydroxymethyl) phosphine oxide, CAS No 1067-12-5).

 

Two peer reviewed publication are available to assess the adsorption/desorption of the urea. The first study (Hongprayoon et al 1991), the partition coefficient was determined on silt loam soil. The distribution coefficients (Kp) ranged from 0.037 to 0.064 with an average value of about 5.1 corresponding to an adsorption coefficient (Koc) from 5.2 to 9.1 L/Kg taking into account the fraction of organic carbon in this soil.

In the second study (Hance et al 1965), the absorption of urea and a number of its derivatives by different soils was investigated using a slurry-type procedure. It appears that urea adsorption is low since distribution coefficients (Kp) ranged from 0 to 2.9 L/Kg in six different soils. This corresponds to an adsorption coefficient (Koc) from 0 to 7.9 L/Kg taking into account the fraction of organic carbon of their soil.

 

The reaction product of the THPS and urea will not have an adsorption/desorption higher than these both components. Therefore the highest adsorption coefficient is used as key value i.e 153 L/kg.