Denitrification of Nitrate in the Presence of Biochar and Zero-Valent Iron.

Denitrification of nitrate (NO₃^-) by mixed cultures in the presence of zero-valent iron (Fe(0)) and biochar was investigated through a series of batch experiments. It was hypothesized that biochar may provide microbes with porous structures to enhance the anaerobic biotransformation of nitrate in the presence of Fe(0) due to facilitating electron transfer. Compared to anaerobic transformation of nitrate by microbes with Fe(0), the introduction of biochar significantly enhance the anaerobic denitrification by microbes with Fe(0). Graphite also promoted the anaerobic microbial transformation of nitrate with Fe(0), indicating that electron-conducting graphene moieties may account for the enhancement. Dosage amount (0.2-1.0 g) and pyrolysis temperature of biochar (400-900^oC) did not significantly affect the enhancement of denitrification under the given conditions. Instead, increasing dosage of mixed culture (10-30 mL) markedly accelerated the denitrification in the presence of Fe(0) and biochar. Analysis of NH₄⁺ and total nitrogen indicated that a product of microbial denitrification with Fe(0) and biochar was mostly N₂ and that the formation of NH₄⁺ was not dominant. Our results suggest that nitrate may be effectively denitrified by microbes with Fe(0) and biochar in natural and engineered systems.