Degradation by and toxicity to bacteria of chlorinated phenols and benzenes, and hexachlorocyclohexane isomers

Abstract

Mixed cultures degrading chlorinated benzenes, chlorinated phenols, or hexachlorocyclohexane (HCH) as the sole source of carbon and energy were obtained by enrichment from contaminated soil samples. Cultures which metabolized 3-chlorophenol (3-CP), 2,3-dichlorophenol (2,3-DCP), or 2,6-dichlorophenol (2,6-DCP) were able to utilize several other chlorinated compounds as substrates, whereas cultures enriched with 1,2,4,5-tetrachlorobenzene (1,2,4,5-TeCB), α-HCH, or γ-HCH did not metabolize most of the other chlorinated congeners tested. Chloride release and growth rates with all four chlorinated phenols decreased with increasing initial substrate concentrations within the range of 30–250 μmol liter⁻¹. Maximum chloride release was 3.8 mg liter⁻¹ corresponding to 35 μmol liter⁻¹ trichlorophenol within 7 weeks. In contrast, the rate of metabolism of the nonphenolic compounds 1,2,4,5-TeCB, α-HCH, or γ-HCH increased with increasing substrate concentrations. Initial concentrations of 750 μmol liter⁻¹ α-HCH or 1,2,4,5-TeCB were completely dechlorinated within 2 weeks. Because aqueous solubility and bioavailability of the chlorophenolic compounds is much higher than that of the nonphenolic compounds, it is suggested that the high bioavailability of the chlorophenolic compounds is the reason for the high toxicity of these substrates to the degrading cultures. In contrast, the low aqueous solubilities of the chlorinated benzenes and HCH-isomers caused consistently low concentrations in the medium, which were high enough to induce degradation but too low to damage the bacterial cells.