Enhanced nitrate removal in an Fe0-driven autotrophic denitrification system using hydrogen-rich water

Abstract

Autotrophic denitrification can be driven using zero valent iron (ZVI) as an electron donor. Balancing the competitive reactions of abiotic nitrate removal and biotic nitrate removal by H₂ is the key problem for iron-assisted autotrophic denitrification. Hence, H₂ production by ZVI corrosion plays an important role in the iron-assisted nitrate reduction process, which determines the nitrate reduction rate and the end product. However, few studies concentrate on the effect of soluble H₂ on ZVI corrosion and the performance of iron-assisted autotrophic denitrification. A hydrogen-rich solution is obtained by dual-chamber MFCs, which is usually directly discarded. The present study aimed to demonstrate that the dissolved hydrogen can be feasibly applied in iron-assisted autotrophic denitrification to accelerate nitrate reduction during ZVI corrosion. It was observed that a continuous dissolved hydrogen supply via electrolysis promoted and stabilized the performance of iron-assisted autotrophic denitrification. The average nitrate removal was 47.3% ± 0.2% in ZVI reactors with dissolved hydrogen-rich water (DH reactors), and 30.2% ± 0.3% in ZVI reactors without dissolved hydrogen-rich water (control reactors). Moreover, the concentration of nitrite was 0.04 mg L⁻¹ in the DH reactors compared with 0.50 mg L⁻¹ in the control reactors. No other intermediates (e.g. N₂O) were found in both the autotrophic denitrification reactors. Finally, the hydrogen-rich water enriched the denitrifying bacteria and increased the abundance of specific functional genes, resulting in the promotion of hydrophobic denitrification during the iron-assisted nitrate removal.