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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

OECD 301D: 0 % in 28 days based on oxygen consumption 

Key value for chemical safety assessment

Biodegradation in water:
under test conditions no biodegradation observed

Additional information

Methylcyclohexane combines a series of substance properties which are likely to be indicative of presenting technical difficulties for performance of ready biodegradation testing, i.e. low water solubility (14 mg/L), high volatility (Henry´s Law constant of > 34000 Pa m3/mol (25 °C) and high biological oxygen demand (3.42 mg O2 per mg test item). In addition, the substance is assumed to be toxic for microorganisms considering the acute aquatic toxicity of this substance. A comprehensive testing program was initiated in order to overcome these technical difficulties and to judge on the best suited method and experimental conditions.

Three standard biodegradation test designs were considered. An OECD 301F test (Manometric Respirometry) revealed no biodegradation at 10 and 100 mg/L test substance concentration. Furthermore several experiments using the test design of sealed vessels (OECD 310) failed as well. For both these test designs it could not be safely excluded that Methylcyclohexane accumulated in air during testing and was thus not bioavailable. Thus an OECD 301D with analytical test material verification was considered appropriate (Simon, 2015; key information for biodegradation). In this study, test solutions were incubated in closed flasks at 20 °C ± 1 °C for 28 days. The rate of degradation was monitored by measuring the decrease of oxygen in the medium over a 28-d period. The amount of oxygen taken up by the microbial population during biodegradation of the test item at a concentration of nominal 2 - 3 mg/L (ThOD = 6.8 – 10.3 mg/L), corrected for uptake by the blank inoculum run in parallel, was expressed as a percentage of the ThOD (theoretical oxygen demand). In order to check the procedure, sodium benzoate was used as a degradable reference item at a concentration of 2 mg/L, along with a toxicity control at 2 - 3 mg/L Methylcyclohexane and 2 mg/L sodium benzoate. To verify test item concentrations in the test vessels, six vessels of the test item assays and six vessels from the toxicity control assays were sampled at test start and test end. The mean test item concentration at test start in the test assays and the toxicity control assays was found to be 2.45 mg/L and 3.03 mg/L, respectively. At test end, mean test item concentration in the test assays and the toxicity control assays was found to be 1.71 mg/L and 2.42 mg/L, respectively. Recovery rate at test end corresponds to 70 % and 78 %, respectively. The biodegradation of Methylcyclohexane in the static test was found to be 0 % with a standard deviation of 1.7 % after 28 days, based on biochemical oxygen demand. Thus, no biodegradation within a 10-day-window could be obtained. The degradation of the reference substance sodium benzoate had reached 119 % within the first 14 days. The difference of extremes of replicate values of the removal of the test item at the end of the test is less than 20%. Therefore, the test can be considered as valid. No inhibitory effects of the test item were observed (more than 25 % degradation occurred within 14 days) in the toxicity control. Therefore, Methylcyclohexane must be considered as not readily biodegradable under the chosen test conditions.

It should be noted, that due to the high volatility of Methylcyclohexane the target compartment for environmental distribution will be air. Even if released to a sewage treatment plant, immediate losses via evaporation can be expected. As an approximation, if the Henry´s Law constant is greater than 100 Pa m3/mol, more than 50% of the substance could be lost from the water phase-in 3-4 hours (Mackay, 1992). Considering the Henry´s Law Constant of Methylcyclohexane (> 34000 Pa m3/mol) the evaporation should occur even more rapidly. Consequently, abiotic degradation processes in air may represent a more important and relevant environmental sink for this substance.