Abstract

The toxic effects of pollutants on the mineralization of 2 mug/l [U-14C] 4-chlorophenol and benzoate were studied in microcosms with methanogenic sediment from the Rhine river. In contrast with studies using a high substrate concentration no lag time was observed and the half-lives for 4-chlorophenol and benzoate were 1.6 and 0.55 hours, respectively. The effect of increasing additions of benzene, chloroform, 1,2-dichloroethane, pentachlorophenol and zinc on each mineralization reaction was measured. Toxicity data were fitted with a logistic dose-effect curve. The IC10 is defined as the concentration of a toxicant inhibiting the mineralization rate for 10%. The IC10 concentration of benzene, chloroform, 1,2-dichloroethane, pentachlorophenol and zinc on the benzoate mineralization was 150, 0.04, 71, 6 and 842 mg/kg sediment d.w. respectively. This latter value includes the background concentration of 800 mg Zn/kg sediment. The mineralization of 4-chlorophenol and benzoate showed similarities in the sensitivity for these toxicants. 4-Chlorophenol can be degraded via benzoate which might explain the similarities in sensitivity of both mineralization reactions. Chloroform proved to be extremely toxic to anaerobic mineralization reactions, probably due to the formation of very toxic and reactive intermediates formed during the slow anaerobic degradation of the chloroform in anaerobic sediments. Sediment quality criteria derived solely from standard toxicity tests using aerobic organisms, may lead to complete inhibition of several important microbial processes in anaerobic sediments.