Sulfide-driven nitrous oxide recovery during the mixotrophic denitrification process

Abstract

We propose a novel sulfide-driven process to recover N2O during the traditional denitrification process. The optimum initial sulfide concentration was 120 mg/L, and the N2O percentage in the gaseous products (N2O+N2) was up to 82.9%. Moreover, sulfide involved in denitrification processes could substitute for organic carbon as an electron donor, e.g., 1 g sulfide was equivalent to 0.5-2 g COD when sulfide was oxidized to sulfur and sulfate. The accumulation of N2O was mainly due to the inhibiting effect of sulfide on nitrous oxide reductase (N2OR), which was induced by the supply insufficiency of electrons from cytochrome c (cyt c) to N2OR. When the initial sulfide concentration was 120 mg/L, the N2OR activity was only 36.8% of its original level. According to the results of cyclic voltammetry, circular dichroism spectra and fluorescence spectra, significant changes in the conformations and protein structures of cyt c were caused by sulfide, and cyt c completely lost its electron transport capacity. This study provides a new concept for N2O recovery driven by sulfide in the denitrification process. In addition, the findings regarding the mechanism of the inhibition of N2OR activity have important implications both for reducing emissions of N2O and recovering N2O in the sulfide-driven denitrification process.