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Environmental fate & pathways

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In a ready biodegradability study (OECD 301B) only 24% biodegradation to carbon dioxide was observed after 29 days. Based on this data, Dinoseb is defined as not readily biodegradable.

Photo degradation data generated by Saxena (1987) and Blair (1986) show that Dinoseb can be expected to photo-degrade in water. At low concentrations, (5-10mg/L) a half-life of 12-18 days was determined. Hydrolysis of the substance will not be significant.

Photo degradation studies in soil (Hazelton, 1986), (Saxena, 1989), showed the half-life of Dinoseb to drastically decrease when exposed to natural sunlight, suggesting that photolytic degradation from soil surface may be an important degradation pathway. The half-life determined by the soil photolysis studies ranged between 0.6 and 6.2 days.

Dinoseb was used for many years as an herbicide at very high rates. The Food and Agriculture Organisation (FAO) reports thatDinoseb is of low persistence, regardless of the form (phenol or salt), reporting field half-lives for both types of Dinoseb in the range 5 to 31 days.

Stevenset al. (1990) [Appl Environ Microbiol.1990 January; 56(1): 133-139] examined the ability of native microorganisms in various Idaho soils to degrade Dinoseb, in addition to physical and chemical soil characteristics which might affect the biodegradation process. Biodegradation rates were higher in silt-loam soils than in loamy-sand soils. Bacterial numbers were the best predictors of biodegradation rates, accounting for 53% of the variability between soils. Soil nitrate-N inhibited Dinoseb biodegradation and accounted for 39% of the variability. Sorption of Dinoseb to soil surfaces also appeared to influence biodegradation rates. It was shown that biodegradation of Dinoseb can occur in soils and is not dependent on long term acclimation to the substance.

 

Extensive metabolism and mineralisation of14C-phenyl Dinoseb occurred by isolated soil microorganism cultures (Hammill and Crawford 1996). Rapid degradation of Dinoseb was observed via co-metabolism in the presence of a fermentable carbon source. The formed anaerobic degradation products were incubated with aerobic soil bacteria and shown to further mineralise to carbon dioxide.

 

Based on the available data, Dinoseb can be considered to be ‘inherently biodegradable – not fulfilling criteria’. Extensive metabolism and mineralisation of Dinoseb by isolated soil microorganism cultures has been observed, therefore it is likely that Dinoseb will completely be degraded in an STP, particularly as the concentration would be expected to be low. Exposure of marine waters and sediments are unlikely, however it is considered that the half-lives for these compartments would not differ significantly from those demonstrated in freshwater systems.

It is therefore concluded that Dinoseb does not satisfy the persistence criteria (not P or vP).