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Environmental fate & pathways

Adsorption / desorption

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adsorption / desorption, other
Type of information:
experimental study
Adequacy of study:
key study
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
no guideline followed
Principles of method if other than guideline:
Measuring the sorption and desorption in soils, under aerobic conditions, using 14C labeled MDA.
GLP compliance:
Type of method:
batch equilibrium method
Test temperature:
Details on test conditions:
Details on soil characteristics:
Soil type: Washington silt loam
21% sand
60% silt
19% clay
Cation exchange capacity: 13.6 meq/100g
Ca: 7.59 meq/100g
Mg: 1.26 meq/100g
K: 0.68 meq/100g
pH: 5
Combustible content: 5.7 %
Field capacity at 1/3 Bar: 23.05%
TOC: 1.3%

Soil type: Freehold sandy loam
53% sand
40% silt
7% clay
Cation exchange capacity: 17.7 meq/100g
Ca: 7.22 meq/100g
Mg: 2.14 meq/100g
K: 0.5 meq/100g
pH: 5.8
Combustible content: 6.88 %
Field capacity at 1/3 Bar: 31.46%
TOC: 1.6%

Aerobic Isotherms
Test soils (silt loam and sandy loam) were sieved to pass a 2 mm mesh sieve, then were autoclaved three times over three days, 1 hour each time. The autoclaved soils were weighed into sterilized, acid-washed 250-mL Erlenmeyer flasks. Into each flask were dispensed: 1) a known (variable) volume of non-radioactive stock aqueous MDA solution, 2) a (constant) known volume of aqueous C-14 labeled MDA solution, and 3) a volume of 0.1M CaCl2 equal to 1/10 the final solution phase volume. Enough deionized water was then added to give the desired total volume of solution phase and soil/water ratio. The final concentration of CaCl2 in all tests was 0.01M; six concentration levels of non-radioactive compound were set up in duplicate. In addition, duplicate flasks were set up with the C-14 and non-labeled compound but without soil, and two additional sets of duplicate flasks at an intermediate concentration were set up to be shaken for 3 days and 7 days, then used for desorption testing.

The flasks were stoppered with rubber stoppers and shaken for 8 hours, 3 days, or 7 days on a New Brunswick Scientific controlled environment incubator-shaker at 200 RPM and 23°C.

After shaking, the flask contents were transferred to screw-cap polypropylene centrifuge tubes and centrifuged at 2000 RPM and the supernatant solution were filtered through 0.2micrometer nylon syringe filters. The filtrates (5 mL) were added to 10 mL of Beckman Ready Gel for liquid scintillation counting (LSC) of the C-14.

Preliminary sorption tests were run with a constant concentration of compound over a 48-hour time period with samples analyzed at 4, 8, 24 and 48 hours to establish the proper isotherm equilibration time for MDA and the two soil types. The known initial specific activity and total concentration in each test flask were used to calculate the equilibrium concentration of compound after contact with the soil, by analysis of the filtrates for C-14 activity. The microgram compound sorbed per g. soil was then calculated based on a dry soil weight basis, using % moisture determinations made on the test soils. Plots were constructed of log microgram compound sorbed/g soil vs. log equilibrium compound concentration (Ce), using linear regression fit. These plots were then used to compute the Freundlich distribution coefficient (Kd) from the equation:

log (microgram/g sorbed) = log(Kd)+(1/n)log(Ce)
where n = constant
Log Kd = Y-intercept
(1/n) = slope of the plot

Calculations of the Freundlich distribution coefficient normalized to the % of organic carbon in the soil (Koc) were made with the equation:
Koc = [K/(% C in soil)] X 100%

Estimates of the Kd value for samples equilibrated 7 days were made by substituting the log (microgram/g sorbed after 7 days) and log (Ce after 7 days) values into the above equation and calculating the log Kd. This method assumed that the 1/n value (slope) was the same for the 8-hr and 7-day tests. The estimated Koc for 7 days was calculated as given above.

24-Hr. Aerobic Soil Desorption Tests
The duplicate set of flasks which were shaken for 3 and 7 days were processed as described above for the 8-hr. isotherm flasks, to determine the equilibrium solution concentration (Ce) of compound and ug/g sorbed. Any remaining supernatant solution not taken for liquid scintillation counting was then removed from the centrifuge tube and replaced with sufficient 0.1M CaCl2 and deionized water to give the original solution phase volume and a 0.01M concentration of CaCl2. The soil pellet was resuspended with a vortex mixer and the soil suspension was returned to the test flask for shaking 24 hours at 23°C. After re-equilibration, the flask was processed as described above, and the C-14 in the filtered supernatant solution was counted with Ready Gel. The calculated microgram/g of desorbed compound was compared to the known microgram/g of compound present on the soil after the 3-day or 7-day contact period, to give the % of sorbed compound desorbable in a 24-hour period. In addition, distribution ratios (Rdesorb) were calculated as the ratio of the calculated microgram/g remaining on the soil divided by the equilibrium solution concentration (microgram/mL) of compound after 24 hr. of desorption:

Rdesorb = (microgram/g remaining on soil)/Ce
8 h
Initial conc. measured:
> 10 - < 20 other: microgram/ml
3 d
Initial conc. measured:
> 10 - < 20 other: microgram/ml
7 d
Initial conc. measured:
> 10 - < 20 other: microgram/ml
24 h
23 °C
7 041
Adsorption and desorption constants:
Calculated average sorption constant (Koc) under aerobic conditions

8 hours Koc = 4848 l/kg
7 days Koc = 7041 l/kg
Recovery of test material:
Calculated distribution ratios (Rdesorb) from 24h desorption tests conducted after 3 and 7 days of sorption time

Sandy loam
3 d: 136
7 d: 173

Silt loam
3 d: 58
7 d: 103
Transformation products:
not specified
Details on results (Batch equilibrium method):
MDA was rapidly sorbed in silt loam soil, and nearly complete by 4 hours. In this soil, 14.7% of the sorbed MDA was desorbable after 3 days of contact time and 8.9% after 7 days of contact.

The adsorption behaviour for both the sandy loam and silt loam soils was very similar and therefore the Koc figures are averaged.

Description of key information

Binding to soild soil phase and/or organic matter is possible.

Key value for chemical safety assessment

Koc at 20 °C:
7 041

Additional information

Experiments with radiolabelled 4,4'-MDA revealed that the substance forms covalent bonds with the organic fraction in soils. Initial sorption of MDA in silt loam was nearly completed by 4 hours under aerobic conditions. Under anaerobic conditions, sorption appeared to still be proceeding at 7 days. The KOC values were determined to 4848 after 8 hours and 7041 after 7 days for aerobic conditions (Cowen et al. 1996 c). The values for anaerobic conditions are 3828 and 10729 for 8 h and 7 d (Cowen et al. 1996 b), respectively. Furthermore, surface adsorption or ion exchange processes were found with minerals without organic content. It should be kept in mind that the term "KOC" generally describes the distribution of a substance between the pore water and the organic matter when the substance is physically bound; if chemisorption occurs the use of this term is not quite correct. In the case of MDA, measured Kd or Koc values may be lof little value as the sorption of MDA is dominated by an irreversible chemical reaction which is governed by chemical kinetics. (In sand soil: t1/2 (for removal)  = ca. 50 days; In loamy soil: initial t1/2 (for removal) = ca. < 0.1 days) (West et al. 2002)

MDA has been shown to irreversibly (i.e. covalently) bind with soils under normal aerobic condition. This binding is suppressed under low redox conditions (i.e. anaerobic). This behaviour is consistent with the literature where aromatic amino groups are known to chemically react with aldehyde and keto groups as well as double bonds of quinoid systems typically found in humic substances present in soils. Under low redox conditions such groups / quinoid systems would not be available for reaction with the amino moiety.

For further assessment, the highest Koc value measured under aerobic conditions was applied.