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Environmental fate & pathways

Adsorption / desorption

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Endpoint:
adsorption / desorption: screening
Type of information:
experimental study
Adequacy of study:
key study
Study period:
20 October 2005 - 22 March 2006
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: GLP/Guideline study

Data source

Reference
Reference Type:
study report
Title:
Unnamed
Year:
2006
Report Date:
2006

Materials and methods

Test guideline
Qualifier:
according to
Guideline:
OECD Guideline 106 (Adsorption - Desorption Using a Batch Equilibrium Method)
Deviations:
not specified
GLP compliance:
yes
Type of method:
batch equilibrium method
Media:
soil

Test material

Reference
Name:
Unnamed
Type:
Constituent
Test material form:
other: Not stated in report
Details on test material:
A sample of 14C-labelled RH-573 (lot number 1063.0009) was supplied by Rohm and Haas Research Laboratories, Spring House, PA The total radioactivity of the sample was 52.8 MBq. The radiochemical purity and the specific activity on the specifications dated 26 September 2005 was 96.9% and 48.5 mCi/g, equivalent to 1794 Bq/ug, respectively.

A sample of RH-573 (lot number 41814, PAIL1) was supplied by Rohm and Haas Research Laboratories, Spring House, PA. The purity stated on the specifications dated September 26, 2005 was 51.1%.
Specific details on test material used for the study:
Details on properties of test surrogate or analogue material (migrated information):
Not applicable
Radiolabelling:
yes

Study design

Test temperature:
Not stated in report but report states it follows OECD 106 which states temperature should be 20-25C.

Batch equilibrium or other method

Analytical monitoring:
yes
Details on sampling:
2, 4, 8, 24 and 48 hours.
Details on matrix:
Five soils were used to cover the range of pH described below. All soils were collected from the top 20 centimeters of agricultural land in the United Kingdom. Ordnance Survey Map references for the collection sites are as follows:

Sandy loam.................pH 6.2 ('Hyde Farm', SU 850 834)
Clay loam....................pH 6.5 ('Pickett Piece', SP 684 053)
Silty clay loam............pH 5.0 ('Wisborough Green', TQ 052 271)
Sand............................pH 4.7 ('Lillifield', SU 834 394)
Loam...........................pH 7.4 ('Kenny Hill', TL 673 801)

The soils were supplied air dried and had been passed through a 2 mm sieve and sterilised by gamma irradiation (Isotron Plc UK). The soils selected covered a pH range 4.7-7.4.
Details on test conditions:
Test vessel selection
The choice of test vessels was made based on work carried out during analytical validation. This data, not reported, showed the [14C]RH-573 did not sorb to centrifuge tubes to any appreciable extent. Therefore all tests were conducted in these vessels.

Soil:solution ratio
Two soils, the clay loam pH 6.5 and the loam pH 7.4 were selected for the soil:solution ratio determination. The loam pH 7.4 soil was selected as it had the largest organic matter. The clay loam pH 6.5 soil was selected as having the least range of particle size.

Kinetics test
Experiments were performed to determine the time required for the adsorption process to come to equilibrium and to determine the adsorption parameters of all five soils at nominal 5 mg 14C-RH-573/l 0.01 M CaCl2. For each of the soils and soil-less controls containing 20 ml of 5 mg/l of 14C-RH-573 in 0.01 M CaCl2, triplicate tubes were set up for each time point. For comparative soils without test substance, 20 ml of 0.01 M CaCl2 was added to single tubes.

20 g of soil............................................................20 ml of 5 mg/l test.....................................................20 ml of 0.01 M
.............................................................................material in 0.01 M CaCl2........................................................CaCl2
Sandy loam pH 6.2........................................................Yes....................................................................................No
Sandy loam pH 6.2.........................................................No...................................................................................Yes
Clay loam pH 6.5...........................................................Yes....................................................................................No
Clay loam pH 6.5............................................................No....................................................................................Yes
Silty clay loam pH 5.0....................................................Yes...................................................................................No
Silty clay loam pH 5.0....................................................No....................................................................................Yes
Sand pH 4.7.....................................................................Yes...................................................................................No
Sand pH 4.7.....................................................................No....................................................................................Yes
Loam pH 7.4...................................................................Yes....................................................................................No
Loam pH 7.4....................................................................No....................................................................................Yes
None................................................................................Yes....................................................................................No

For each soil type, triplicate PTFE centrifuge tubes were prepared by adding 20 g of soil and 20 ml of 5 mg/l 14C-RH-573 in 0.01 M CaCl2. A single control tube was prepared for each soil type by adding 20 g of soil plus 20 ml of 0.01 M CaCl2 (without 14C-RH-573). The centrifuge tubes were rolled on a Multimix Major rolling machine equipped with a transverse rocking action, at approximately 80-120 rpm. After 2, 4, 8, 24, and 48 hours, the tubes were centrifuged for 10 minutes at 4C at a speed of 7500 rpm (10395 g, 101,944 m/s2). Triplicate 30 ul aliquots of the supernatant were removed for LSC analysis and a 10 ul aliquot was removed for radio Thin Layer Chromatography (radio TLC). Fresh 0.01 M CaCl2 solution (100 ul) was then added to each tube to replace that removed for analysis. These tubes were returned to the Multimix major after each time interval.

Desorption test
For each of the five soils, triplicate tubes were prepared as described for the kinetics test. To each PTFE centrifuge tube 19 ml 0.01 M CaCl2 solution and 20 g soil were added. Tubes were then rolled for 15 hours to equilibrate. Following the equilibrium period, 1 ml of a nominal 100 mg/l 14C-RH-573 in 0.01 M CaCl2 solution was added to each tube. Tubes were rolled on a Multimix major for 2 hours, which was equivalent to the adsorption equilibration time. The tubes were then centrifuged for 10 minutes at 4C at a speed of 7500 rpm (10395 g, 101,944 m/s2) and the supernatant removed. The volume of supernatant was determined prior to analysis by LSC. The volumen of supernatant removed from each tube was replaced with an equivalent volume of fresh 0.01 M CaCl2 solution to start the desorption test. Tubes were replaced on the Multimix major. After 2, 4, 24 and 48 hours, the tubes were centrifuged for 10 minutes at 4C at a speed of 7500 rpm (10395 g, 101,944 m/s2). triplicate 30 ul aliquots of the supernatant were removed for LSC analysis. Fresh 0.01 M CaCl2 solution (90 ul) was then added to each tube to replace that removed for analysis.

Isotherm test
This test is only required if the soil kinetics and desorption tests show the test substance is stable in the test system, removal of the test substance from solution due to adsorption to soil is greater than 25% and that less than 75% of the adsorbed test substance is subsequently desorbed. The test was performed using each soil for which these criteria applied. All five soils were tested.

As [14C]RH-573 was stable in the soil-less controls and the test substance concentrations remaining in solution were all above the detection limit, it was possible to carry out the isotherm test for all five soils. The results of the Kinetics test show that equilibration was achieved within 2 hours with little difference between the 2 and 24 hour data. A 16-hour equilibraiton period was used.

A new concentrate, nominally 100 mg/l was prepared as previously described. A 20 mg/l concentration was prepared by diluting 6 ml of the 100 mg/l with 24 ml 0.01 M CaCl2. In a similar manner dosing solutions of 4, 0.8 and 0.16 mg/l were prepared.

Aliquots (1 ml) of hte 100, 20, 4, 0.8 and 0.16 mg/l concentrates were mixed with 19 ml 0.01 M CaCl2 to give nominal test concentrations of 5, 1,0.2, 0.04 and 0.008 mg/l, respectively. Aliquots of each of the 5 test concentrations were placed, in triplicate, into centrifuge tubes along with 20 g of one of the 5 soil types (75 tubes total). These tubes, together with triplicate tubes of each test solution without soil (soil-less controls: 15 tubes total) and three tubes containing 0.01 M CaCl2 and 20 g of soil (i.e., system blanks without test substance), were then rolled for 2 hours. All tubes were then centrifuged for 10 minutes at 4C at a speed of 7500 rpm (10395 g, 101,944 m/s2) and the supernatant removed and radioassayed as described above. The soil was allowed to air dry in the tube prior to combustion and radioassay.

The residual volume of supernatant in the tubes originally dosed with 5 mg/l was measured. For each concentration, the weight of test substance adsorbed to the soil (X), the mass of dy soil (m) and the concentration of [14C]RH-573 equivalents in solution (Ce) were detemined. Graphs of (X/m) against Ce were plotted for each soil.

Mass balance test
The triplicate tubes, from the nominal 1 and 5 mg/l [14C]RH-573 solutions for the clay loam pH 6.5 and the loam pH 7.4 soils, from the isotherm tests, were used to detmine mass balance. After centrifugation the residual supernatant volume in the tubes was removed. To each tube 20 ml of acetone, was added and the tubes were then rolled for 1 hour. Following centrifugation (7500 rpm for 10 min at 4C the acetone was removed, volume measured, and aliquots analysed by LSC. One additional methanol extraction of the soil was carried out in an identical manner on each tube. The mass balance was calculated to be the total amount of [14C]RH-573 equivalents recovered in the 0.01 M CaCL2 from the adsorption and desorption stages plus that in the acetone extracts plus that recovered from aliquots of the soil that were combusted and radioassayed.


Computational methods:
% Adsorption = 100 x [(Co-Ce)Co]

Kd = (X/m)/Ce

Koc = Kd x (100/% organic carbon)

Results and discussion

Adsorption coefficientopen allclose all
Type:
Kd
Value:
0.1
% Org. carbon:
1.3
Remarks on result:
other: Sandy loam (Specific temperature not stated)
Type:
Kd
Value:
0.27
% Org. carbon:
3.9
Remarks on result:
other: Clay loam (Specific temperature not stated)
Type:
Kd
Value:
0.14
% Org. carbon:
2.1
Remarks on result:
other: Silty clay loam (Specific temperature not stated)
Type:
Kd
Value:
0.03
% Org. carbon:
0.3
Remarks on result:
other: Sand (Specific temperature not stated)
Type:
Kd
Value:
1.07
% Org. carbon:
16.8
Remarks on result:
other: Loam (Specific temperature not stated)

Results: Batch equilibrium or other method

Adsorption and desorption constants:
See Table 1.
Recovery of test material:
In the mass balance studies, total recovery was measured in two soils where measurable amounts of radioactivity remained in the soil. The total recovery for both soils was 99.92 +/- 12.90%.
Concentration of test substance at end of adsorption equilibration period:
The data indicate that [14C]RH-573 was adsorbed to all five of the soils tested and that equilibrium was attained in 2 hours.

Radio TLC analysis of the supernatants for the soils from all time points showed that the the equilibration time of 2 h the solutions were stable. There was little if any degradation of parent in the gamma irradiated soil after a 48 hour exposure except for the Loan pH 7.4 soil where RH-573 had declined to 72.6% of the applied activity.

From the Kd values, the adsorption distribution coefficient corrected for soil organic content, Koc was then calculated. These data show that adsorption of [14C]RH-573 to the sand pH 4.7 soil was the least with <2%. Adsorption to the sandy loam pH 6.2 and the silty clay loam pH 5.0 soils were similar 7.5-13% and 15-17% respectively. The clay loam pH 6.5 and the loam pH 7.4 adsorbed most with 23-26% and 42-49%, respectively. The Kd values ranged from <0.019 to 1.1 and the Koc from <6.3 to 12.

One test vessel, the silty clay loam pH 5.0 replicate A was suspected of being double dosed. These results have been omitted from all calculations.
Concentration of test substance at end of desorption equilibration period:
Equilibration for the desorption was achieved within 2 hours for all 5 soils. After a 2 hour desorption period, the desorption parameters were determined. Desorption of [14C]RH-573 was found to be similar for all soils. The percent desorption for the sand pH 4.7 was 5.8-43% and 29-38% for the sandy loam pH 6.2 soil. The clay loam pH 6.5 desorbed 6.1-24%, the silty clay loam pH 5.0 2.9-17% and the loam pH 7.4 12-17%.
Transformation products:
not specified
Details on results (Batch equilibrium method):
Test vessel selection
PTFE was chosen as the preferred material for the test vessel as RH-573 showed no significant adsorption to these tubes.

Soil:Solution ratio test
The clay loam pH 6.5 and the loam pH 7.4 soils were selected for the soil solution ratio determination. The results of the analysis at 22 hours demonstrated thee was no adsorption in either soil at a soil:solution ratio of 1:25 as essentially all of the 14C radioactivity remained in solution. At a soil:solution ratio of 1:5 only the loam pH 7.4 showed any significant adsorption. Both soils showed adsorption at a soil:solution ratio of 1:1. Therefore, a 1:1 ratio was selected for all further tests. Solutions were subjected to radio-TLC analysis to check that the test substance was not degrading significantly during the soil contact period. The percent RH-573 for the loam pH 7.4 (68.9) was significantly less than the control (96.3%) and the other soils (80.2% -94.9%).
Statistics:
No data.

Any other information on results incl. tables

Isotherm determination

An isotherm test was performed for all five soils employing five RH-573 concentrations: 5, 1, 0.2, 0.4 and 0.08 mg 14C Rh-573/l 0.01 M CaCl2. The results from this test demonstrate that there was a linear relationship, from which the slope of the line of each graph represents the adsorption distribution coefficient, Kd (Table 1).

Table 1

    Soil        Derived Parameters
 Type  pH  Kd  Koc  Goodness of fit, R2
 Sandy loam  6.2  0.10  7.7  0.89
 Clay loam  6.5  0.27  6.9  0.99
 Silty clay loam  5.0  0.14  6.7  0.97
 Sand  4.7  0.03  10  0.72
 Loam  7.4  1.07  6.4  0.50

Of the five different soils used in this study only two soils, the clay loam pH 6.5 and the loam pH 7.4, adsorbed greater than 25%. The sandy loam pH 6.2 desorbed 29 -38% with all other soils desorbing in the range 2.9 -43%. The Kd values for all soils, 0.03 -1.07, and the Koc values for all soils, 6.4 -10 were similar.

These results indicate that RH-573 will be mobile in the soils tested here. According to the US EPA, RH-573 is in the very high mobility class.

Mass Balance

The mass balance for two soils, clay loam pH 6.5 and loam pH 7.4 was performed in triplicate at two RH-573 concentration, 1 mg/l and 5 mg/l. Recovery of the applied radioactivity ranged form 90 -126%. For the clay loam pH 6.5 soil, 90 -99% of the applied radioactivity was recovered, of which 50% remained in solution. Similarly for the loam pH 7.4, recovery of the applied radioactivity amounted to 93 -126%, of which 30% remained in solution. The average recovery for the clay loam pH 6.5 was 92.66 +/-3.44% and for the loam pH 7.4, 107.17 +/- 15.11%. The total recovery for both soils was 99.92 +/- 12.90.

Mass balances were not carried out for the remaining three soils as there was insufficient radioactivity remaining on the soils to be measured. Two replicates of the loam pH 7.4 soils had recoveries of 125 -126%. These recoveries were not investigated further.

Applicant's summary and conclusion

Validity criteria fulfilled:
yes
Conclusions:
The empirically derived Kd values for RH-573 were 0.03 -1.07. The calculated Koc values were in the range 6.4 -10. This indicates that the test substance will be relatively mobile in soils of the type tested in this study.
Executive summary:

The adsorption and desorption potential of RH-573 to 5 different soils was examined.

    Soil        Derived Parameters
 Type  pH  Kd  Koc  Goodness of fit, R2
 Sandy loam  6.2  0.10  7.7  0.89
 Clay loam  6.5  0.27  6.9  0.99
 Silty clay loam  5.0  0.14  6.7  0.97
 Sand  4.7  0.03  10  0.72
 Loam  7.4  1.07  6.4  0.50

The empirically derived Kdvalues for RH-573 were 0.03 -1.07. The calculated Kocvalues were in the range 6.4 -10. This indicates that the test substance will be relatively mobile in soils of the type tested in this study.