Highly efficient total nitrogen and simultaneous total organic carbon removal for urine based on the photoelectrochemical cycle reaction of chlorine and hydroxyl radicals

Urine is a major nitrogen and COD source and threatens the aqueous environment, leading to eutrophication and energy consumption. However, the conventional technologies encounter the bottleneck due to the high load of nitrogen and organics in urine. Here we propose an innovative idea for complete transformation of urea-nitrogen to N-2 and simultaneous degradation of organics to CO2 with the assistance of chlorine and hydroxyls radicals (Cl-center dot and HO center dot) in photoelectrochemical cell system. This idea reveals circulation process of these two radicals: photoexcited holes from WO3 anode oxidize chloride and hydroxyl ions to Cl-center dot and HO center dot radicals under illumination, these radicals convert urea-nitrogen to N-2 or slight nitrate, and then slight nitrate is selectively reduced to N-2 on a novel Pd/Au modified porous metallic sheet. TN analysis of a simulated urea solution of 30 mg L-1 demonstrated that N-2 and CO2 are the primary oxidation products of the urea, and the urea-N was completely stripped (99.45%) in 90 min. Meanwhile, with the detection of EPR spectrum, organics can be transformed to CO2 by OH center dot in this cyclic system. Significantly, the feasibility of actual urine treatment was also investigated, indicating great denitrification efficiency (99.37%) and TOC removal efficiency (50.91%), respectively. This work sheds light upon a new economical, efficient, and environment-friendly means of urine sewage treatment. (C) 2018 Published by Elsevier Ltd.