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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Test substance met criteria for readily biodegradable in OECD 301F test

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

The ready biodegradability of 2-(2-Phenoxyethoxy)-ethanol (DiEPh) was evaluated according to the OECD 301F Manometric Respirometry Test Guideline. This test employed biodegradation reaction mixtures containing a defined mineral medium that was inoculated with 30 mg/L (dry solids) activated sludge collected from the City of Midland Wastewater Treatment Plant (Midland, Michigan). Duplicate biodegradation Test Mixtures were prepared by adding DiEPh (49.2 mg/L) to the inoculated mineral medium. Duplicate Positive Control mixtures were similarly prepared containing aniline (99.9 mg/L) as reference substance. A single Toxicity Control mixture was prepared containing both DiEPh (48.7 mg/L) and aniline (98.9 mg/L) to determine whether DiEPh was toxic or inhibitory to the microbial inoculum. An Abiotic Control mixture, in which DiEPh (48.9 mg/L) was added to chemically-sterilized mineral medium (244 mg/L HgCl2), was used to determine the extent to which the substance was removed in the absence of microbial activity. These reaction mixtures were incubated in darkness, with continual stirring at approximately 150 rpm. Measurements of biological oxygen demand (BOD) and carbon dioxide evolution (mineralization) were performed at 6-hour intervals over the entire 28-day test period, using a MicroOxymax respirometer system. These measurements were corrected for oxygen consumed and carbon dioxide evolved in the Inoculum Blanks, which contained only the inoculated mineral medium. The percentage biodegradation based on BOD was corrected for oxygen consumption from nitrification.

Removal of dissolved organic carbon (DOC) and formation of nitrite and nitrate also were determined after 28 days. The onset of DiEPh biodegradation occurred after 3.1 days in the Test Mixtures, and biodegradation achieved the pass level of 60% theoretical oxygen demand (ThOD) consumption within 11.2 days total. The extent of DiEPh biodegradation based on BOD, CO2 evolution, and DOC removal reached 100.5 ± 1.1, 87.0 ± 0.3%, and 95.3 ± 0.6%, (mean ± 1 SD), respectively, at the end of the 28-day test. Thus, the biodegradation of DiEPh achieved both the pass level and 10-day window criteria for classification as readily biodegradable in the Manometric Respirometry Test. In addition, the recorded extents of CO2 evolution and DOC removal achieved the pass levels specified for similar ready biodegradability tests that are based on these measurements. In the Positive Control mixtures, the onset of biodegradation occurred after 4.1 days, and the 60% DO2 level was achieved after 6.6 total days, and thus confirmed the viability of the inoculum.

A Toxicity Control mixture, containing both aniline and the test substance, showed no evidence for inhibition of the microbial medium. DOC analysis showed no removal of the test substance in the Abiotic Control mixture containing the test substance and a chemical sterilant (HgCl2), which verified that the observed test substance degradation was due solely to biological activity. Other experimental parameters, such as reaction mixture pH and total oxygen consumption in the Inoculum Blanks, were maintained within their required ranges. According to the guideline, the biodegradation reactions should be incubated at a target temperature of 22 ± 2 °C, and maintained to within ±1 °C of the average temperature. The overall mean incubation temperature was 21.5 ± 0.5 °C (mean ± 1 SD, n = 220). The minimum and maximum recorded temperatures over the 28-day test were 20.8 and 23.4 °C, respectively. Slightly elevated incubation temperature of 23.4 ± 0.1 °C (mean ± 1 SD, n = 8) occurred around Day 7 of the study. Excluding the Day 7 elevation, the mean, minimum, and maximum incubation temperatures were 21.4, 20.8, and 22.0 °C, respectively.

The results of this test provide a conclusive and reliable basis for classification of diethylene glycol phenyl ether (DiEPh) as “readily biodegradable”, according to the OECD 301F: Manometric Respirometry Test.