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Environmental fate & pathways

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DIDP is readily biodegradable (74% biodegradation in 28 days), and does meet the 10-day window based on results from a standard OECD ready biodegradation test guideline (EMBSI, 2009). DIDP also biodegraded to 67.0% in 28 days and was readily biodegradable but did not meet the 10 -day window (EBSI, 1995). The principle transformation products would be mono-isodecyl phthalate (MIDP) and isodecyl alcohol (IDA) (Stapleset al., 1997b). Data are available to assess the potential biodegradability of these two transformation products. Biodegradation data are available for a mono ester that is a mixture containing approximately equal amounts of monoesters with normal octyl (n-C8) and normal decyl (n-C10) side chains. Results show that show that this mixture is readily biodegradable (Scholz, 2003) and data also show that IDA is readily biodegradable, but does not meet the 10-day window (Exxon Biomedical Sciences, Inc., 1997a). The n-C8/n-C10 mono ester biodegraded to 94% after 28 days, while IDA biodegraded to 71% after 28 days.

Studies are not available to assess the biodegradability of DIDP under simulated conditions (i.e., wastewater treatment). However, there are data for di-n-decyl phthalate (CAS No. 84-77-5; DnDP), an analog to DIDP, using treated wastewater that suggest DIDP would demonstrate a high extent of biodegradation under wastewater conditions (Furtmann, 1993). DnDP biodegraded 82% after 7 days based on the disappearance of the parent compound from the test system. The initial DnDP concentration was 7.8 μg/l and the DT50 was 1 day.

The elimination of DIDP in a sewage treatment plant (STP) through biodegradation and distribution, as reported by the European Commission (2003), was estimated using the SIMPLETREAT model. The model calculated that 91.9% of DIDP would be eliminated in a STP, which is consistent with the high loss reported by Furtmann (1993). The measured data for DnDP and the modeled data suggest that DIDP will be largely eliminated in a STP.

Studies are not available to assess the biodegradability of DIDP in sediment. Although there are no data specifically for the diester, there are biodegradation data for the monoester of DIDP (mono-isodecyl phthalate, MIDP) that showed an average half-life of 25 hours in marine sediments under aerobic conditions based on results from two studies (Otton et al., 2008). Research suggests that the formation of the monoester occurs as the first step in the biotic degradation of DIDP (Staples et al., 1997b). Because this step does not appear to be rate limiting, as evidenced by the high extent of biodegradation demonstrated by DIDP in a ready test, the aerobic degradation of the diester in sediment is expected to occur at a similar rate.

Studies are not available to assess the biodegradability of DIDP specifically in soil. However, data for an analog substance di-isononyl phthalate (CAS No. 68515-48-0; DINP) can be used to estimate the loss rate of DIDP in soil. DINP exhibited a half-life in soil of approximately 51 days, based on the loss of parent substance (ExxonMobil Biomedical Sciences, Inc., 2009). These data were developed in an earthworm toxicity test conducted in soil in which the concentration of DINP was monitored over a 56-day period. During that period DINP concentration decreased from 982 to 441 mg/kg soil (wet weight). Because DIDP and DINP exhibited similar extents of biodegradation in ready biodegradability tests, 67 and 71% respectively, they would be expected to biodegrade in soil at similar rates and to similar extents.

DINP is appropriate to use as an analog to DIDP on the principle that substances of similar structure have similar properties. The two substances are structurally similar; each is a diester containing two alkyl chains differing in length by one carbon, C9 for DINP and C10 for DIDP. The similarities in physical structure result in similar physico-chemical properties as well. DINP and DIDP have similar water solubilities (0.61 and 0.17 ug/L), and partition coefficients (both 8.8), and demonstrate similar results in ready biodegradibility tests and environmental toxicity tests. Given that both substances would be expected to act similarly, and do act similarly it is reasonable to use DINP as an analog for DIDP.