Biodegradation of alpha- and beta-hexachlorocyclohexane in a soil slurry under different redox conditions

Abstract

Aerobic conditions proved to be best for the microbial conversion of alpha-hexachlorocyclohexane (alpha-HCH) in a soil slurry. The dry soil contained 400 mg of alpha-HCH per kg. This xenobiotic compound was mineralized within about 18 days at an initial rate of 23 mg/kg of soil per day by the mixed native microbial population of the soil. The only intermediate that was detected during breakdown was pentachlorocyclohexene, which was detected at very small concentrations. Alpha-HCH was also bioconverted under methanogenic conditions. However, a rather long acclimation period (about 30 days) was necessary before degradation started, at a rate of 13 mg/kg of soil per day. Mass balance calculations showed that about 85% of the initial alpha-HCH that was present was converted to monochlorobenzene, 3,5-dichlorophenol, and a trichlorophenol isomer, possibly 2,4,5-trichlorophenol. Under both denitrifying and sulfate-reducing conditions, no significant bioconversion of alpha-HCH was observed. The beta isomer of HCH was recalcitrant at all of the four redox conditions studied. We propose that the specific spatial chloride arrangement of the beta isomer is responsible for its stability. The results reported here with complex soil slurry systems showed that alpha-HCH is, in contrast to the existing data in the literature, best degraded biologically in the presence of oxygen.

Genetic manipulations of microorganisms for the degradation of hexachlorocyclohexane

“…As early as 1974, Melnikov [19] summarized the lindane residue data from soils of USA, England and Japan and found residues to vary from 0.04 to 0.26 mg/kg. Similar reports were then documented from other developed countries, such as the Netherlands [20,21], Sweden [22] and Japan [23]. Residues of HCH were also detected in soils in various parts of India, ranging from 0.125 ppm to 0.5 ppm [24,25].…”

“…The HCH isomers were considered to be highly persistent in aerobic environments, but they undergo rapid degradation in predominantly anaerobic ecosystem. However, aerobic mineralization of HCH isomers has been recently reported in a soil slurry and by microorganisms isolated from the soil [21,[80][81][82].…”

“…In P. paucimobilis UT26, gamma-HCH was converted by two steps of dehydrochlorination to the chemically unstable intermediate 1,3,4,6-tetrachloro-l,4cyclohexadiene (1,4-TCDN) which in turn is converted to 2,5-dichloro-2,5-cyclohexadiene-l,4-diol (2,5-DDOL) by two steps of hydrolytic dehalogenation via the chemically unstable intermediate 2,4,5trichloro-2,5-cyclohexadiene-l-ol (2,4,5-DNOL) [83]. Studies were conducted on the biodegradation of alpha-HCH and beta-HCH in a soil slurry under different redox conditions: aerobic, methogenic, denitrifying and sulfate-reducing and it was observed that aerobic conditions proved to be the best for the microbial conversion of alpha-HCH [21]. This alpha-HCH was mineralized within about 18 days, at an initial rate of 23 mg/kg of soil per day, by the mixed microbial populations in the soil.…”

Reaction of the soil microflora after contamination with chlorinated aromatic compounds and HCH

“…It is noticeable that in both experimental approaches (soil and liquid culture), even the higher chlorinated compounds were dehalogenated only under aerobic but not under anaerobic conditions. In studies published before 1980, the anaerobic metabolism of HCH in soil and cultures was described [13,14] whereas investigations of the last decade revealed higher degradation rates under aerobic conditions [15][16][17]. For chlorinated aromatic compounds, their aerobic metabolism was discovered first and their anaerobic dehalogenation was not investigated until the last few years [18].…”

Rhizoremediation of lindane by root‐colonizing Sphingomonas

“…Natural attenuation of HCH is attributed to microbial activity and as a result, bioremediation is considered a potential strategy for the long-term in situ attenuation of HCH contamination. Support for this strategy comes from studies suggesting that HCH degradation occurs in a variety of soil types, including aerobic and anaerobic ones (MacCrae et al, 1969;Tu, 1976;Jagnow et al, 1977;Doelman et al, 1985;Bachmann et al, 1988), and that the organisms responsible for HCH degradation are enriched with repeated applications of lindane (Wada et al, 1989;Nagata et al, 1999). However, direct application of Sphingomonas sp.…”