Impact of humic constituents on microbial dechlorination of polychlorinated dioxins

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.