Impact of humic constituents on microbial dechlorination of polychlorinated dioxins

Abstract

We recently reported on the ability of a microbial consortium eluted from dioxin-contaminated Passaic River sediments to dechlorinate polychlorinated dibenzo-p-dioxins (PCDDs) through peridechlorination of 2,3,7,8-substituted hepta- to penta-CDDs resulting in the transient production of 2,3,7,8-tetra-CDD and through perilateral dechlorination pathway of non-2,3,7,8-substituted congeners. The dechlorination of PCDDs under combined reductive activities of the same microbial community and model humic constituents (MHCs) (resorcinol, 3,4-dihydroxybenzoic acid [3,4-DHBA], and catechol) was investigated. Model humic constituents alone caused reductive dechlorination of PCDDs, resulting in the production of hepta-, hexa-, and tetra-CDD congeners. Whereas abiotic MHC-stimulated dechlorination ceased at the tetra-CDD group of congeners, microbial activity caused further dechlorination to tri-, di-, and mono-CDDs. Combined with the microbial community, MHCs effected the appearance of a new profile of tetra through mono-CDD congeners that was distinguishable from either microbially or MHC-stimulated processes alone. An important overall trend could be discerned: the peridechlorination of PCDDs was promoted by MHCs, resulting in both a relative and an absolute (except for 3,4-DHBA) increase of 2,3,7,8-tetra-CDD relative to either microbially or MHC-stimulated dechlorination. This study indicates that, aside from direct deposition, the increase of 2,3,7,8-tetra-CDD concentrations in the environment may result from combined biotic and abiotic PCDD dechlorination.