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Environmental fate & pathways

Biodegradation in soil

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Description of key information

Aerobic Soil Metabolism: Stable; half-life of 5.9 years (American Cyanamid Company, 34927, 1988)

Anaerobic Soil Metabolism: Stable (American Cyanamid Company, 1990)

Key value for chemical safety assessment

Half-life in soil:
5.9 yr
at the temperature of:
25 °C

Additional information

Key study: Aerobic Soil Metabolism (American Cyanamid Company, 34927, 1988)

The aerobic soil metabolism of the test substance was evaluated in a laboratory test equivalent to OPPTS 835.4100. 14C/13C-labeled test substance was added to Princeton sandy loam soil (soil moisture 75% of filed capacity) at a concentration of 1.5 ppm. Samples were maintained at 25 °C in the dark for up to 365 days.

Volatiles and/or CO2 were collected in traps. At 365 days, 88% of the applied radioactivity remained as parent test substance. An apparent half-life of 5.9 years was calculated for the test substance under aerobic test conditions in sandy loam soil. Five unknown degradates were observed intermittently during the study. None accounted for greater than 1.3% of the initial applied dose (IDA). Unextractable residues averaged 4% of the applied radioactivity during the study and accounted for 5% at 365 days. Cumulative 14CO2 accounted for up to 7% of the applied radioactivity after 365 days. 14C-labeled test substance accountability at day 365 was 91.5% of IDA while the average for the study to date was 101% of IDA.

In conclusion, the test substance is essentially stable to degradation in soil maintained under aerobic conditions.

 

Supporting study: Aerobic Soil Metabolism (American Cyanamid Company, ENV 98-029, 1999)

A supporting aerobic soil metabolism study, conducted over a period of 121 days in sandy loam soil from New Jersey, is available. The study was performed using 14C-labeled test substance with the radiolabel in the 6-carbon of the pyridine ring. The test compound was applied to the soil at an initial application rate of 0.22 ppm. The soil was maintained aerobically in the dark at 25±1 °C for up to 4 months at 75% of 1/3 bar moisture. Because the 121-day study period was short compared to the persistence of 14C-labeled test substance, there was insufficient time for the full pattern of formation and decline of products to develop.

Material balance ranged from approximately 100% to 97%. Approximately 0.1-0.2% of the radioactivity was accounted as unextracted residues at 0-time and this value increased to approximately 6-6.1% after 4 months of incubation. After 4 months of incubation, there was approximately 5.6% of the total applied radioactivity has been evolved as 14CO2 indicating that the test substance is mineralized in soil. There were no detectable levels of organic volatiles in the ethylene glycol traps. Approximately 26% of the test substance was degraded after 121 days of aerobic incubation. The first-order degradation half-life was calculated to be 313 days. The rate of degradation of the test substance decreased after 1 month of incubation and appeared to be a biphasic pattern with transition point at approximately 28 days. Using the first month degradation data only, the half-life of the test substance in soil was calculated to be 117 days. The half-life in the second phase was 438 days.

A structurally similar transformation product, 2 -(4 -isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)-3-hydroxy pyridine, was present at approximately 6.4% of parent radioactivity recovered at the beginning of the study. Two additional minor transformation products, 2-[(1-carbamoyl-1,2-dimethylpropyl)carbamoyl] nicotinic acid and 2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)-3-carboxymethyl pyridine, were each detected at 2.1 and 2.3% of the applied dose.

 

Supporting study: Anaerobic Soil Metabolism (American Cyanamid Company, PD-M Volume 20-15, 1983)

The anaerobic soil metabolism was evaluated in a laboratory test equivalent to OPPTS 835.4200. Sandy loam was treated with a 50:50 mixture of 14C and 13C-carboxy-labeled test substance at 1 Ib ae/acre (1 ppm) and maintained under anaerobic conditions, after aging aerobically for one month. The test substance was stable in sandy loam soil and thus no half-life was calculated. The total radioactivity recovered at one and two months was 109.4% and 107.6%, respectively. The radioactivity in the water extract was three-fold that in the soil after 2 months.

In conclusion, C14-labeled test substance in Princeton sandy loam was not metabolised under the anaerobic conditions studied.