Availability of organic chemicals for biodegradation in settled bottom sediments

Biodegradation rates for dodecyltrimethylammonium chloride (TMAC), a quaternary ammonium compound, and phenol were measured in settled sediments to determine if adsorbed chemicals were directly available for biodegradation by sediment-associated bacteria. In settled sediment cores, biodegradation rates for TMAC, which is charged at environmental pH, was a function of the amount of unadsorbed chemical; adsorbed material was not directly degraded by the sediment-associated bacteria. However, the rate of biodegradation of adsorbed phenol, a relatively hydrophobic and neutral chemical, was apparently a function of the total concentration of material present, suggesting that at least a fraction of the adsorbed material was directly degraded. These results indicate that chemical structure and, possibly, the mechanism of adsorption may influence biodegradation in sediments. Studies on TMAC biodegradation in completely mixed sediment/water slurries (up to 10 g/liter sediment) showed that TMAC biodegradation in slurries differed from that in settled sediments. Biodegradation in slurries was a function of the total amount of material present, both adsorbed and unadsorbed. These results suggest that biodegradation in settled sediments may be influenced by high concentrations of sediment present and/or the lack of mixing. Thus mixed, low-sediment-level slurries may not be realistic surrogates for modeling biodegradation processes in settled bottom sediments.