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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Based on the available information, potassium isoamyl xanthate is considered inherently biodegradable. Xanthates have been found to be biodegradable in ready biodegradability test and in inherent biodegradability test indicating that the substance is not persistent. However, based on the knowledge of the degradation at the relevant environmental conditions, the degradation cannot be considered to be rapid. This is based on the reported half-lives and the fact that biodegradation is not expected as an important process in the tailings ponds. The substance is not fully mineralized but rapidly degraded to less degradable degradation products and this has to be considered in the assessment. In addition, some of the degradation products are more toxic to aquatic organisms (e.g hydrogen sulphide) than the parent substance. 

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable

Additional information

Purity of the target substance (potassium isoamyl xanthate; PIAX) is 84 %. It contains also 11% of another analogue xanthate potassium amyl xanthate (PAX) as an impurity. Therefore, the read-across data on PAX was used in the weight of evidence approach to evaluate the biodegradation of the target substance. In addition, as the xanhates have a similar degradation behavior, data on other analogue substances were used for this endpoint as no valid results were available for PIAX. The read-across justifications are presented in Annex I of the CSR.

Ready biodegradability of the aqueous analogue and impurity n-amyl xanthate (PAX) was screened in a study conducted according to ISO 7827, corresponding to OECD Guideline 301 A (new version, Ready biodegradability: DOC die away test). Initial test concentration was 30 mg/L, pH 7.2 and temperature 25oC. 74 % of PAX was degraded in 8 days. The determined half-life was 3.94 days, and reaction rate k 0.1758 1/day (Chen et al.1988). The study was considered as reliable with restrictions and used part of the weight-of-evidence approach for hazard assessment and classification and labeling of the substance.

Chen et al.(1988) also reported carbon disulphide, monothiocarbonate and dixanthogen to accumulate in the test system. The findings suggest slower biodegradation rate for PIAX having a branched alkyl chain.

PAX was also found biodegradable in 5 days by sewage sludge under laboratory conditions at levels of 1 and 10 mg/L in an older study of Read and Manser (1973). No inhibitory effects on microbes were found at above 25 mg/L concentrations. The findings are consistent with the results of Chen et al.(1988).

Biodegradability of another read-across substance potassium isobutyl xanthate (PIBX) was assessed in a modified Zahn-Wellens test (OECD 302 B). 98 % of the test substance was degraded in 12 days. As 75 % of the substance was degraded in 5 days, the result suggest that PIBX is inherently biodegradable (Hoecst 1987).

Carbon disulphide, monothiocarbonate and dixanthogen were found to accumulate in the test system of the standard biodegradability screening tests reviewed.

Both 3-methylbutan-1-ol and pentan-1-ol are readily biodegradable. The studies of Langley (1970) and Gerhold & Malaney (1966) is included as surrogates of available published data..