Enhanced low-temperature denitrification by microbial consortium using solid-phase humin

Reducing the use of liquid organic carbon electron donors during biostimulation of heterotrophic denitrification is critical for sustainable groundwater remediation. Solid-phase humin isolated from natural sources can provide a cost-effective alternative to classical electron donors. In this study, the low-temperature denitrification capacity of an acetate-fed microbial community was enhanced using humin at 20 ?C and 10 ?C. These enhancements were not caused by faster acetate consumption and greater bacterial growth with the addition of humin. Estimation of the electron balance and first-order kinetics suggested that the enhancement in denitrification occurred mainly after acetate exhaustion. Humin may therefore have acted as an additional electron donor for the denitrifying microbial community, with the reduced quinone group in humin potentially responsible for electron donation. The addition of humin increased the richness and diversity of the denitrifying microbial community, in which Dechloromonas spp. played a critical role. Given the prevalence of humin and denitrifiers using humic substances, our results have important implications in the bioremediation of nitrate-contaminated groundwater using less liquid organic carbon electron donors.