Internal Electric Field Facilitates Facet-Dependent Photocatalytic Cl– Utilization on BiOCl in High-Salinity Wastewater for Ammonium Removal

High Cl^- concentration in saline wastewater (e.g., landfill leachate) limits wastewater purification. Catalytic Cl^- conversion into reactive chlorine species (RCS) arises as a sustainable strategy, making the salinity profitable for efficient wastewater treatment. Herein, aiming to reveal the structure-property relationship in Cl^- utilization, bismuth oxychloride (BiOCl) photocatalysts with coexposed {001} and {110} facets are synthesized. With an increasing {001} ratio, the RCS production efficiency increases from 75.64 to 96.89 μg L^-1 min^-1. Mechanism investigation demonstrates the fast release of lattice Cl^- as an RCS and the compensation of ambient Cl^-. Correlation analysis between the internal electric field (IEF, parallel to [001]) and normalized efficiency on {110} (k_RCS/S_{110}, perpendicular to [001]) displays a coefficient of 0.86, validating that the promoted carrier dynamics eventually affects Cl^- conversion on the open layered structure. The BiOCl photocatalyst is well behaved in ammonium (NH₄⁺-N) degradation ranging from 20 to 800 mg N L^-1 with different chlorinity (3-12 g L^-1 NaCl). The sustainable Cl^- conversion into RCS also realizes 85.4% of NH₄⁺-N removal in the treatment of realistic landfill leachate (662 mg of N L^-1 NH₄⁺-N). The structure-property relationship provides insights into the design of efficient catalysts for environment remediation using ambient Cl^-.