Ammonia oxidation by in-situ chloride electrolysis in etching wastewater of semiconductor manufacturing using RuSnOx/Ti electrode: effect of plating mode and metal ratio

The indirect chloride-mediated ammonia oxidation encounters challenges in maintaining the effectiveness of metal oxide anodes when treating wastewaters with complex compositions. This study aims to develop a highly stable anode with RuO2-SnO2 coatings for treating an etching effluent from semiconductor manufacturing, which majorly contains NH3 and organic compounds. The RuSnOx/Ti electrode was synthesized using wet impregnation and calcination processes. The metal oxide configuration on Ti plate substrate was tuned by varying the step-dipping process in RuCl3 and SnCl4 baths. A 10-day continuous-flow electrolysis was conducted for studying the ammonia removal and chlorine yield under variable conditions, including detention, pH, current density, and initial ammonia and chloride concentrations. In the RuSnOx coatings, the configuration comprising RuO2 nanorods as the surface layer and an intermediate layer of SnO2 crystallites (by plating Ru3+ for three times to cover one Sn4+ layer, denoted as the Ru3Sn/Ti electrode) exhibited the best durability for acid washing, along with relatively high Faradaic efficiency and low energy consumption. To further improve the treatability of real wastewater (NH3-N = 634mgL-1, chemical oxygen demand (COD) = 6700mgL-1, Cl- = 2,000mgL-1, pH 11), the duel-cell electrolyzers were constructed in series under a current density of 30mAcm-2 and 45min detention. Ultimately, removals of NH3 and COD reached 95.8% and 76.3%, respectively, with successful limitation of chloramine formation. Environmental Implication Ammonia nitrogen exhibits negative impacts on the human health, and residual nitrogen level in surface water has to be brought down to prevent eutrophication, and lower its eco-toxicity as well. The durability of electrochemical process is enhanced by tuning the crystallography characteristics of Ru-Sn oxide catalysts. We found that the coating configuration with RuO2 nanorods shielding SnO2 as the sublayer could withstand long-term operation and acid washing. The research provides a feasible method for remediating industrial effluents containing high levels of ammonia and organic compounds simultaneously.