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Environmental fate & pathways

Adsorption / desorption

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Description of key information

OECD 121/EU Method C.19: Koc > 4.27 x 10E5, logKoc > 5.63
OECD 106: The higher the organic carbon content, the higher the mobility of the test substance (supporting information)

Key value for chemical safety assessment

Koc at 20 °C:
427 000

Additional information

Two reliable data sources are available regarding soil adsorption of 1,3,4-Thiadiazolidine-2,5-dithione, reaction product with hydrogen peroxide and tert-nonanethiol. Fox (2012) utilized the High Performance Liquid Chromatography (HPLC) according to OECD Guideline 121 / EU Method C.19. Based on the intrinsic substance characteristics, the HPLC method seems to be the most appropriate one. The test substance interacts with the stationary phase of the cyanopropyl reverse phase HPLC column containing lipophilic and polar moieties. The dual composition of the stationary phase, having polar and non-polar sites allows for interaction of polar and non-polar groups of the test substance in a similar way as it is the case for organic matter in soil or sewage sludge matrices. In particular for polar substances, the pH has a significant influence on sorption behaviour. The test substance, however, does not contain any relevant dissociation constants, thus the experiment was carried out at approx. neutral pH in a non-ionized form of the test substance. 12 reference standard solutions, each prepared in methanol, where used (i.e. Acetanilide, Phenol, Atrazine, Isoproturon, Triadimenol, Linuron, Naphthalene, Endosulfan-diol, Fenthion, Phenanthrene, and DDT). Also the test substance was diluted to 100 mL with methanol before analysis. The Koc was determined to be > 4.27 x 10E5 with a corresponding logKoc of > 5.63. In accordance to the classification scheme of McCall et al. (1981), the test substance is therefore considered to be environmentally immobile.

Supporting data is given by an experiment conducted with the test substance using the batch equilibrium method according to OECD Guideline 106 (Rocchio, 1991). Three different soils (all silt or clay) were used with the following characteristics: Soil 1 with 28.4 meq/100 g CEC (cation exchange capacity), 0.82 % TOC (total organic carbon) and a pH of 7.4. Soil 2 possessed 46.2 meq/100 g CEC, 10.2 % TOC and a pH of 7.3. The last one (Soil 3) has a CEC of 24.6 meq/100 g, 8.6 % TOC and pH of 6.4. A preliminary adsorption test was performed using a test substance solution at a concentration of 0.626 mg/L prior to the definitive test. The definitive test used 5 test concentrations, 0.26, 0.60, 0.99, 1.58 and 3.04 mg/L at solution:soil ratio of 5:1 for 50 h equilibration time to further define the adsorption coefficient. All data generated in the definitive test were evaluated using the Freundlich equation and adsorption isotherms were plotted. The results indicated that the adsorption characteristics were similar among the three soil types tested. The adsorption constants ranged from 3.89 to 20.80 (Kd: soil 1: 3.89, soil 2: 4.63, soil 3: 20.80, respectively). The adsorption constants based upon the organic carbon content ranged from 45 to 473 (Koc: soil 1: 473, soil 2: 45, soil 3: 242, respectively). These values indicate that the test substance is 'mobile' in the soil with the highest organic carbon content and 'moderately mobile' in the other 2 soils which had lower organic carbon contents.

Based on the more appropriate method used by Fox (2012), 1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol is environmentally immobile with a Koc > 4.27 x 10E5 and a corresponding logKoc > 5.63. However, the supporting information (Rocchio, 1991) indicates that the higher the organic carbon content of the soil, the higher will be mobility of the test substance.

[LogKoc: 5.63]