Humin as an Electron Mediator for Microbial Reductive Dehalogenation

We report that humins extracted as the solid fractions from paddy soils or sediment are involved in extracellular electron transfer, coupled with microbial reductive dehalogenation of pentachlorophenol (PCP), by serving as both electron acceptor and electron donor. In our system, humin is requisite for the dechlorination of PCP, and this activity cannot be maintained when humin is replaced with soluble humic substances or related compounds, including 0.1 M NaOH-extracted humic acid from soil, Aldrich humic acid, and anthraquinone-2,6-disulfonate. The function of humins is stable against treatments with H2O2 (30%, 30 min), HCl (0.1 M, 48 h), NH2OH·HCl (0.1 M, 48 h), NaBH4 (0.1 M, 15 h), and heat (121 °C, 30 min). Cyclic voltammograms indicated that humin harbors redox-active moieties, and electron spin resonance suggested that quinone moieties within humin are the redox-active centers. Fourier-transform infrared and nuclear magnetic resonance analyses verified the presence of the aryl carbonyl carbon group in humin. Although the proportion of redox-active carbon is very small, the potential electron-mediating ability is not negligible. The finding that humin, in solid form, is redox active has important implications for in situ bioremediation, given the wide distribution of humin and the diversity and ubiquity of humic substance-utilizing microorganisms.