Analyzing multi-current step approaches in electrocatalytic nitrate reduction for wastewater treatment through response surface methodology and techno-economic analysis

Abstract

Electrocatalytic nitrate reduction (NO<inf>3</inf>RR) offers a dual solution by converting nitrate into ammonia (NH<inf>3</inf>), realizing the conversion from pollution to the valuable resource. However, existing NO<inf>3</inf>RR technologies struggle with low nitrate concentrations in typical real-world wastewater, where sluggish kinetics and competing hydrogen evolution hinder efficiency. To address these limitations, we introduce a multi-current step approach based on tandem electrolysis mechanism that systematically decouples and optimizes the two key reaction steps, significantly enhancing ammonia production under ultra-low nitrate conditions. However, optimizing step − specific parameters required a systematic approach beyond trial-and-error experimentation. Herein, using Response Surface Methodology (RSM) with the Box-Behnken design, we systematically evaluated the interplay between current density and time ratio across two reaction steps, optimizing key responses regarding ammonia production performances and energy consumption. Experiments conducted on a scalable flow reactor (active area of 20 cm2) demonstrated that RSM-optimized conditions achieved an ammonia yield of 0.225 g h−1 and the reduced energy consumption to 30.99 kWh kg−1. Techno-economic analysis revealed a competitive ammonia price lower than the market value of $1.15 kg−1 NH<inf>3</inf>, supported by favorable net present value projections for industrial-scale deployment. Sensitivity analysis highlighted energy efficiency and catalyst lifetime as critical economic drivers. This work bridges the gap between laboratory-scale achievements and real-world wastewater treatment, offering a sustainable pathway for green ammonia synthesis and reactive nitrogen management.