Efficient and durable iridium-doped SnO2 anode for reactive chlorine species-mediated urine wastewater treatment

Abstract

The prevalence of nitrogen pollutants in urine wastewater has become a pressing environmental issue. Among various methods for addressing this concern, electrochemical oxidation processes that generate reactive chlorine species (RCS) from chloride ions commonly found in urine wastewater stand out as a promising approach. Despite the exploration of diverse anode materials, existing options still suffer from low selectivity in RCS generation, poor stability, and considerable energy consumption. In this study, we synthesized iridium-doped tin oxide (Ir-SnO2) as an efficient, durable, and energy-conserving anode and systematically evaluated its electrochemical activity for RCS-mediated removal of nitrogen pollutants in urine wastewater. The Ir-SnO2 anode outperformed counterparts such as IrO2, SnO2, and Sb-SnO2, demonstrating its superior electrochemical performance characterized by high current density, low charge transfer resistance, and substantial active surface area. It also achieved exceptional current efficiency of 91.7 % and energy efficiency of 7.79 mmol/Wh for RCS generation at 30 mA cm−2 in diluted 0.1 M chloride solution. Computational analysis based on density functional theory additionally supported experimental results, revealing that Ir doping to SnO2 enhances chlorine adsorption through electronic structure modification, significantly lowering the overpotential and improving electrocatalytic activity for chlorine evolution reactions. In the electrolysis of synthetic urine wastewater at 30 mA cm−2 for 5 h, the Ir-SnO2 anode demonstrated remarkable efficacy in degrading ammonia nitrogen (NH3-N), total nitrogen (TN), and chemical oxygen demand (COD) with efficiencies of 100 %, 97.8 %, and 89.1 %, respectively. These results were comparable to those achieved by IrO2 (dimensional stable anode, DSA) and exceeded the performance of the boron-doped diamond (BDD) anode. Moreover, Ir-SnO2 exhibited the lowest energy consumption compared to benchmark DSA and BDD anodes in the treatment of urine wastewater. In stability assessments, furthermore, the Ir-SnO2 electrode maintained stable anodic cell potential for 100 h of continuous electrolysis in a 0.1 M NaCl electrolyte at 30 mA cm−2 and remained functional over five cycles without any signs of deactivation. This study provides valuable insights into the development of a suitable anode for energy-efficient urine wastewater treatment.