Paired photoelectrochemical system for total nitrogen removal via engineered active sites in spaced TiO2 nanotube platform

This study presents a paired photoelectrochemical (PEC) system using dual photoelectrodes designed for comprehensive total nitrogen (TN) removal by converting NO₃^⁻ into non-toxic N₂ gas. Utilizing spaced TiO₂ nanotube arrays (STNA) optimized for precise metallic site engineering, bimetallic Pd-Cu and RuO₂ catalysts were tailored onto the STNA for the photocathode and photoanode, respectively. This configuration enabled the selective reduction of NO₃^⁻ to NH₃ at the photocathode, while the NH₃ was simultaneously oxidized to N₂ via reactive chlorine species (RCS) generated under light illumination. The paired PEC system achieved a TN removal efficiency of over 99.2 % over eight cycles, totaling 32 h of continuous operation without metal leaching and by-products formation. Even in complex real wastewater matrices, the paired photoelectrode system maintained a TN removal efficiency of 96.3 %, demonstrating consistent performance in practical applications. Furthermore, the system exhibited a specific electric energy consumption (SEEC) approximately 7 times lower than that of conventional electrochemical (EC) systems, indicating improvements in cost-effectiveness and energy efficiency. This study establishes a foundation for advancing PEC systems in nitrogen removal and provides insights into the application of tailored metallic site engineering on photoelectrodes for environmental applications.