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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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A discussion of biodegradation in sediment is presented.

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Muir et al.(1989). The tri-m-cresyl phosphate tested was 14C-labelled mixed with purified non-labelled tri-m-cresyl phosphate. The sediment samples used in the study were collected from a eutrophic farm pond and the Red River, Winnipeg (both samples were from agricultural areas remote from industry). The pond sediment consisted of 75 per cent clay, 24 per cent silt and one per cent sand and had an organic carbon content of 3.7 per cent and a pH of 7.6 and the river sediment consisted of 48 per cent clay, seven per cent sand and 43 per cent silt and had an organic carbon content of 2.3 per cent and a pH of 7.7.

 

The aerobic sediment experiments were carried out using loosely capped flasks (static test) or in respirometer flasks with air flowing through the system (1-2 ml/minute). The sediments incubated under anaerobic conditions (in respirometer flasks under a nitrogen flow (1-2 ml/minute)) were amended with one per cent by weight of microcrystalline cellulose to provide an additional source of carbon. The degradation experiments were carried out using around 10 g (dry weight) of sediment in dechlorinated water (sediment:water ratios of either 1:10 (static test) or 1:20 (respirometer flask)). Each sediment sample was pre-incubated for 21 days at the intended experimental temperature prior to the addition of the test substance. The concentration tested was either 0.1 mg/l (static test) or 0.05 mg/l (respirometer flasks) and the substance was added as 0.1 ml of a solution in acetone. All experiments were carried out in duplicate for up to 64 days and sterile controls were also run to investigate the abiotic degradation of tri-m-cresyl phosphate under the conditions used. The aerobic experiments were incubated with a 16:8 hours light:dark photoperiod (using low intensity light) whereas the anaerobic experiments were incubated in darkness. The microbial biomass present in the test systems was between 9×106 to 32×106 colony forming units (CFU)/g in the experiments with river sediments. The microbial biomass present in the aerobic pond respirometer sediments was found to decline from 42×106 CFU/g to 0.3 CFU/g over the 64-day period. The total microbial biomass (aerobic and facultative anaerobic heterotrophs) present in the N2-purged respirometer experiments was 5.3×106 CFU/g after 3 to 8 days and 24×106 after 30 to 40 days, but the number of strict anaerobes present was around eight to 40 times less, and so the incubations were not strictly anaerobic.

 

The results show extensive degradation of tri-m-cresyl phosphate in the study. Initially, most of the tri-m-cresyl phosphate added to the system adsorbed onto the sediment phase but by the end of the experiment the amount of extractable radioactivity associated with the sediment phase had decreased substantially. Detailed analysis of the sediment extracts indicated that the major portion of the extractable radioactivity was as unchanged tri-m-cresyl phosphate, with low levels of degradation products, including di-m-cresyl phosphate, also present.

 

Biodegradation of tri-m-cresyl phosphate therefore proceeded rapidly in river and pond sediments, at temperatures (10 and 25°C) and redox conditions typical of aquatic environments in North America and Europe. About 5-fold slower degradation was observed at 2°C which suggests that phosphate esters will persist in sediments during winter months in northern latitudes. The results confirm previous studies indicating that increasing alkyl substitution on the phenyl groups or at the phosphate ester linkage leads to decreasing biodegradability.

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