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Biodegradation in water: screening tests

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Description of key information

Chloromethane is readily biodegradable

Key value for chemical safety assessment

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Chloromethane originates from natural sources and consequently occurs in the oceans, soils and fresh water systems. Natural occurrence and the numerous articles describing the microbial degradation of chloromethane point to the ready biodegradability of chloromethane.

In order to assess the biotic degradation, a ready biodegradability test was performed which allows the biodegradability to be measured in an aerobic aqueous medium (van Ginkel, 2013). The ready biodegradability was determined in the Closed Bottle test performed according to OECD TG 301D, EU and ISO Test Guidelines, and in compliance with the OECD principles of Good Laboratory Practice. Chloromethane was biodegraded by 77 % at day 28 in the Closed Bottle test. Over 60 % biodegradation was achieved in a period of 6 days immediately following the attainment of 10 % biodegradation. Hence, this substance should be classified as readily biodegradable.

Furthermore, publication data were used to underline classification of chloromethane as readily biodegradable. A ready biodegradability test result informs about the following aspects of biodegradation ; a) ultimate (complete) biodegradation by microorganism capable of utilizing the test substance as sole carbon and energy source, b) rate of biodegradation by microorganisms growing on chloromethane and c) the number and occurrence of competent micro-organisms present in “unacclimated” ecosystems and biological treatment plants. Below all three aspects are dealt with to enable classification of chloromethane using evidence presented in peer reviewed scientific journals.

A)Ultimate (complete) biodegradation; Ultimate biodegradation of chloromethane has been proven with pure culture studies (biodegradation pathways). Methylobacterium sp, Pseudomonas sp and Hyphomicrobium sp utilize chloromethane as sole carbon and energy source (Hartmanset al., 1986; Vannelli et al., 1998; Freedmanet al., 2004). All threebacteriautilized chloromethane with stoichiometric release of chloride. The chlorine-carbon bond is either cleaved by methyl transferases and dehydrogenases (Vannelliet al., 1998) or a monooxygenase (Hartmanset al., 1986). Cleavage result in the formation of methanal which is channelled into the central metabolism of microorganisms i. e carbon and energy source. The proposed biodegradation pathway and the chlorine mass balances demonstrate that chloromethane is ultimately completely biodegraded by micro-organisms. Complete (ultimate) biodegradation in OECD 301 tests results in biodegradation percentages of ≥60.

B)Rate of biodegradation; The maximum growth rates of Methylobacterium sp, and Hyphomicrobium sp are 0.12 and 0.09 h-1, respectively. Painter and King (1983) used a model based on the Monod equation to interpret the biodegradation curves in ready biodegradability tests. According to this model growth rates of competent micro-organisms of 0.025 h-1or higher will result in a ready biodegradation of the test substance (degradation within 28 days). The biodegradation of chloromethane has been determined very comprehensively in seawater. The degradation rates ranged from 0.00 to 0.3 d-1with a mean of 0.07 day-1(Tokarczyket al., 2003). A half-life of 10 days (0.07 day-1) is below the default half-life of ready biodegradable substances (TGD). A mass-balance box model was used by Huang et al., (2000) to estimate the chloromethane degradation in a lake. In wintertime, the net sink from the atmosphere was2.6-3.2 pM d-1. In summer, the mass balance model indicates that there was a sink for chloromethane of 1.9-2.9 pM d-1. A half-life of <15 days was calculated using a chloromethane concentration of 70 pM in the lake (Huanget al., 2000). This half-life is also below the default half-life of ready biodegradable substances (TGD).

C)Number of competent micro-organisms (ubiquitousness); Chloromethane utilizing bacteria have been detected in a variety of pristine terrestrial, fresh water, and marine environments using enrichment cultures (McAnulla et al., 2001), These findings demonstrate the ubiquitousness of micro-organisms capable of degrading chloromethane. The isolates are also able to grow on methanal and formate (readily biodegradable substances) also indicating that chloromethane degrading micro-organisms occur in the environment. The presence of chloromethane utilizing bacteria has also been shown in various ecosystems (soil, sea, fresh water) by measuring the loss of chloromethane in environmental samples(Tokarczyk et al., 2003; Huang et al., 2000; Miller et al., 2004).

The wide distribution of chloromethane degrading micro-organisms, the very high growth rate of competent micro-organisms, and the evidence of ultimate degradation of chloromethane lead to the conclusion that chloromethane should be classified as readily biodegradable.