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Environmental fate & pathways

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The biodegradability of dibutyl diphenyl phosphate was tested in different primary and ultimate biodegradation assays.

 

There are different relevant and reliable studies available. The most recent study is regarded as the most relevant and most reliable study, however as one of the available studies displays negative results while all other tests present positive results these studies will be evaluated in a Weight of Evidence approach.

 

The most recently performed biodegradability study is a ready biodegradability study performed according to the OECD 301F guideline (Manometric Respiration test) (Currenta, 2012).

During the 28-day test period, domestic activated sludge was exposed to an initial concentration of 100 mg/L DBPP. Oxygen consumption was determined via determination of BOD. The percentage of degradation observed was 0, 19.7, 52 and 60.9% after respectively 7, 14, 21 and 28 days of exposure. The results of this study lead to the conclusion that the substance can be regarded as readily biodegradable, without reaching the 10-day window.

 

Another study also evaluated ready biodegradability using another method. In that study, the Biochemical Oxygen Demand (BOD) and the residual test substance according to the OECD guideline 301C (Modified MITI test by Mitsubishi, 2009). Within 28 days, the biodegradability rate amounted to only 11% based on the BOD measurements and 9% based on the residual amount of test substance. The degradability of each component of the test substance was calculated by each peak on the LC/MS chromatogram for measurement of the residual test substance amount. Degradability of t tri-butyl phosphate (TBP) was 0%, that of dibutyl phenyl phosphate (DBPP) was 0-4% and that of butyl diphenyl phospohate (BDPP) was over 98%. Under the conditions used in this test, limited to no degradation was observed.

 

The oldest, ultimate biodegradability study (Saeger and Tucker, 1973) evaluated the biodegradability of many different phosphate esters among which DBPP using the Thompson-Duthie Carbon Dioxide Evaluation test in which CO2 production is monitored. In this test activated sludge from a domestic sewage treatment plant was used. This test yielded an average of 36, 80 and 85% degradation after respectively 7, 28 and 48 days.

 

The same authors (Saeger and Tucker, 1973) also evaluated the test substance in a SCAS (Semicontinuous activated sludge) test and a RDA (River Die Away) assay (two supporting studies). In the SCAS test, biodegradation was clearly observed with higher levels of biodegradability at lower loading rates. In this test, 95% and approximately 52% degradation was observed at 3 and 13 mg/L loading rate respectively. Clearly a higher loading rate influences the degradation rate of DBPP. The results of the RDA test were in agreement with the SCAS test as the half life of the substance was determined to be 3-4 days.

In conclusion, based on these arguments the substance can be regarded as readily biodegradable.

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