Biological removal of nitrate, a highly concerning contaminant, is limited when the aqueous environment lacks bioavailable electron donors. In this study, we demonstrated, for the first time, that bacteria can directly use the electrons originated from the photoelectrochemical process to carry out the denitrification. In such photoelectrotrophic denitrification (PEDeN) systems (denitrification biocathode coupling with TiO photoanode), nitrogen removal was verified solely relying on the illumination dosing without consuming additional chemical reductant or electric power. Under the UV illumination (30 mW·cm, wavelength at 380 ± 20 nm), nitrate reduction in PEDeN apparently followed the first-order kinetics with a constant of 0.13 ± 0.023 h. Nitrate was found to be almost completely converted to nitrogen gas at the end of batch test. Compared to the electrotrophic denitrification systems driven by organics (OEDeN, biocathode/acetate consuming bioanode) or electricity (EEDeN, biocathode/abiotic anode), the denitrification rate in PEDeN equaled that in OEDeN with a COD/N ratio of 9.0 or that in EEDeN with an applied voltage at 2.0 V. This study provides a sustainable technical approach for eliminating nitrate from water. PEDeN as a novel microbial metabolism may shed further light onto the role of sunlight played in the nitrogen cycling in certain semiconductive and conductive minerals-enriched aqueous environment.

Sulfur-based denitrification process has attracted increasing attentions because it does not rely on the external addition of organics and avoids the risk of COD exceeding the limit. Traditionally, limestone is commonly employed to maintain a neutral condition (SLAD process), but it may reduce the efficiency as the occupied zone by limestone cannot directly contribute to the denitrification. In this study, we propose a novel sulfurbased denitrification process by coupling with iron(II) carbonate ore (SICAD system). The ore was demonstrated to play roles as the buffer agent and additional electron donor. Moreover, the acid produced through sulfur driven denitrification was found to promote the Fe(II) leaching from the ore and likely extend the reaction zone from the surface to the liquid. As a result, more biomass was accumulated in the SICAD system compared with the controls (sulfur, iron(II) carbonate ore and SLAD systems). Owing to these synergistic effects of sulfur and iron(II) carbonate on denitrification, SICAD system showed much higher denitrification rate (up to 720.35 g•N/m 3 •d) and less accumulation of intermediates (NO 2 − and N 2 O) than the controls. Additionally, sulfate production in SICAD system was reduced. These findings offer great potential of SICAD system for practical use as a highly efficient postdenitrification process.

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