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Environmental fate & pathways

Adsorption / desorption

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In reliable GLP studies (using the batch equilibrium method), adsorption of trisodium EDDS to activated sewage sludge was considered minimal (Geukens, 1993c), whereas the test substance had a high affinity for soil (de Vries, 1993d). 

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In a GLP study, adsorption (Kd abs) and desorption (Kd des) coefficients were determined (using the batch equilibrium method) for incubations of two different concentrations (0.1 and 1.0 mg/L) of trisodium EDDS with activated domestic sewage sludge that had not been pre-exposed to the test substance (non-adapted). Incubations of the test substance at concentrations of 0.1 and 1.0 mg/L with non-adapted sludge yielded K abs values of 40 and 37, respectively. Corresponding Kd des values of 421 and 334 were determined using the sludge residues from these adsorption tests. A 14C-radiolabelled test substance tracer was used to assess partitioning between the aqueous and suspended solid phases of the sludge. A preliminary study determined the adsorption of trisodium EDDS to labware (i.e. glass test vessel and pipette tips) to be insignificant. From the Kd abs and Kd des values derived, it was concluded that adsorption of trisodium EDDS to activated sewage sludge was minimal (Geukens, 1993c).

A batch equilibrium screening study was conducted to assess the adsorption/desorption of trisodium EDDS with three different soils. The study methodology was based on OECD Guideline 106 (1981) available at the time, however the study has several deviations from the current version adopted in 2000. The test soils were - Soil I; Strong silty sand (pH 4.0, 6% clay, 1.4% organic matter); Soil II: Strong sandy loam (pH 7.5, 13.6% clay, 1.8% organic matter); Soil III; Weak sandy loam (pH 6.6, 12.1% clay, 1.1% organic matter). The test substance had high affinity for the test soils with adsorption (A) determined as 98%, 83% and 89% for soil I, II and III, respectively, with corresponding Kd values of 260, 28 and 44. Desorption (D) of the test substance from the soil was low with values determined as <4%, <3% and <4% for soil I, II and III, respectively. The amount of adsorbed material which was not desorbed (R) was determined to be >96%, >97% and >96% for Soil I, II and III, respectively. The corresponding K' values were calculated to be 2.6 x 102, 28 and 44 for Soil I, II and III, respectively, and Koc values of about 1.9 x 104, 1.6 x 103 and 4 x 103 [log koc ranging from 3.6 to 4.28]. Therefore, under the conditions of this test, it can be concluded that trisodium EDDS has a high affinity for soil (de Vries, 1993d).

[Data on trisodium EDDS is considered relevant to use for understanding the potential adsorption of EDDS acid to sludge and soil, and is acceptable for using as read-across information].