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Environmental fate & pathways

Biodegradation in soil

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Description of key information

Degradation of the test substance in four aerobic soils proceeded readily, with DT50values ranging from 2.7 to 18.6 days and with a geometric mean of 7.1 days at 20°C (BASF SE, 394788, 2015; BASF SE, 394798, 2015). The mean half-live was re-calculated to 12 °C according to equation R.16-9 in the respective REACH guidance. The resulting DT50 value was 13.4 days with the corresponding degradation rate of 0.0516 d-1, respectively. Four metabolites exceeding 10% of the total applied radioactivity (TAR) were

observed. These major metabolites were M440I002 (max 11.3% TAR), M440I003 (max 14.1% TAR), M440I024 (max 12.1% TAR), and M440I057 (max 36.5% TAR). No other soil metabolites exceeding 10% were formed. Mineralization was observed with levels of CO2reaching a maximum value of approximately 28.1% TAR after 120 days of incubation, demonstrating further degradation of the primary degradation products. Bound residues reached maximum levels of 51.0% TAR by 120 days. The major metabolites were not persistent and demonstrated some decline from peak levels in all soils by the end of the 120-day aerobic soil incubations. In some cases, calculation of metabolite DT50values was possible. The degradation DT50values (pathway fitting starting from parent) ranged from 0.3 to 5.5 days for M440I002, 9.3 to 36.1 days for M440I003, and 3.5-9.9 days for M440I057. For M440I024 only a simple dissipation DT50(fit from the peak onward) in a single soil was determined, and was 28.4 days

Key value for chemical safety assessment

Half-life in soil:
7.1 d
at the temperature of:
20 °C

Additional information

A key study (BASF SE, 394788, 2015) was conducted according to OPPTS 835.4100 (2008). The test substance degraded quickly in two soils when incubated under aerobic conditions at a soil moisture of 40% of the maximum water holding capacity and a temperature of 20°C. Best-fit DegT50(degradation DT50) values in soil LUFA 2.2 (pyranone label), soil LUFA 2.2 (pyranone/pyridine label), NJ soil (pyranone label) and NJ soil (pyranone/pyridine label) for the test substance were 6.4 days (FOMC), 6.1 days (FOMC), 11.3 days (FOMC) and 4.0 days (FOMC), respectively. The DegT90 values ranged from 50.5 to 115.8 days.

Degradation of the test substance in aerobic soil proceeds primarily via hydrolysis of the cyclopropanecarboxylic acid ester groups, via oxidation of the pyridine ring and via oxidation of the cyclohexanol ring formed following hydrolysis. Another degradation path for the test substance in aerobic soil comprises dehydration of the dihydropyran ring. The metabolites formed were not stable in soil and under went further degradation to be finally mineralized to CO2or incorporated into bound residues. Three metabolites reached concentrations >10% of the total applied radioactivity which were identified as M440I002, M440I003 and M440I024. DisT50 (dissipation DT50) values derived from SFO decline fits from the peak level forward were 20.5 and 28.4 days for M440I002, 45.2 and 55.5 days for M440I003 and 28.4 days for M440I024.

 Characterization of the non-extractable radioactive residues (NER) revealed that the predominant part of the radioactivity was tightly bound to the humins or incorporated into the fulvic acids. Since the test substance was not detectable in the NER after extraction with NaOH and treatment with acid, it is assumed that no significant amounts of the test substance will accumulate from the bound residues under alkaline or acidic conditions.

 

In addition the aerobic soil degradation of the test substance was investigated in a study (BASF SE, 394798, 2015) according to OPPTS 835.4100, 2008. The test substance degraded quickly in aerobic soil and several metabolites were formed. Metabolites M440I002 and M440I057 were observed at maximum levels of 10% TAR or higher while M440I003 reached maximum levels between 5 and 10% TAR. Metabolites M440I048, M440I049, M440I050, M440I051, M440I052 and M440I053 as well as further unidentified metabolites were observed at maximum levels of 5% TAR or lower. Final degradation resulted in non-extractable residues and substantial amounts of 14CO2. 

 Degradation DT50 (DegT50) values of 18.6 and 2.7 days and DegT90 values of 180.9 and 22.1 days were calculated according to the best fit kinetics for the test substance in LUFA 5M and Metz soils, respectively. For metabolites M440I002, M440I003 and M440I057 DegT50values between 0.3 and 9.9 days were calculated. 

Moreover, the anaerobic soil metabolism of the radiolabeled test substance was studied in a supporting study (BASF SE, 394789, 2015) according to OECD 307 (2002) in four soils. Two US and two German soils. The radiolabeled test substance (nicotinic or pyranone radiolabel) was applied to the soils at a target application rate of 50 g/ha (0.2 ppm). Samples were incubated aerobically for 14-29 days (approximately one aerobic half-life), then flooded with water, continually purged with nitrogen gas, and incubated another 120-121 days in the dark at 20 ± 2°C. Extensive degradation of the test substance was observed in the anaerobic soils and six major (>10% TAR) metabolites were formed. Metabolites M440I002, M440I003, and M440I057 began forming during the aerobic portion of the incubation, while M440I001, M440I046, and M440I047 appeared to form primarily or exclusively during the flooded portion of the incubation. Other unidentified minor products were also observed. Kinetic assessment gave degradation DT50values that ranged from 26-66 days for the test substance in all four soils. Dissipation DT50values (peak forward regression) for metabolites ranged from 5.2-188 days for M440I002 in all four soils, 58-85 days for M440I003 in three soils (NJ, L2, L5), and 66 days for M440I057 on one soil (CA).  Based on the results obtained, it may be concluded that the test substance can degrade under anaerobic soil conditions.