Fe(Ⅲ)/Fe(Ⅱ) cycle enables biological nitrate ammoniation at low C/N ratio for reactive nitrogen recovery

The recovery of reactive nitrogen from industrial wastewater by biological ammoniation is a promising strategy of sustainable wastewater treatment. However, the low C/N ratio of pickling wastewater restricts nitrate towards ammonium, resulting in nitrogen loss. Here, we have proven the feasibility of nitrate ammoniation utilizing the Fe(Ⅲ)/Fe(Ⅱ) cycle at low C/N ratios, where partial organic electrons were employed to reduce Fe(Ⅲ) and the regenerated Fe(Ⅱ) facilitated ammoniation. It achieved the nitrate reduction efficiency of 96 ± 2 % and ammoniation efficiency of 49 ± 2 % for 3 cycles. Iron cycle enhanced the DNRA, demonstrated by the upregulation of nrfA and the downregulation of nirK. Electroactive Geobacter spp. with DNRA capability was sustained at high abundance (∼23 %) by the electron exchange process with extracellular iron. These findings provided a novel approach to recover active N using the iron cycle from wastewater, which has broader implications for sustainable circular economy and the ecology of carbon, iron and nitrogen cycle.