Exploiting dual-halide mediated synergy to enhance the energy efficiency of the UVA/chlorine process for sustainable micropollutant abatement in bromide-containing water.

The transition from mercury-vapor lamps to UV-LEDs in advanced oxidation processes (AOPs) is hindered by a critical energy bottleneck. While UVC-LEDs suffer from low wall-plug efficiency (3-10%), highly efficient UVA-LEDs (> 70%) are limited by the poor molar absorptivity of free chlorine. This study presents a "waste-treats-waste" strategy to bridge this gap by exploiting ambient bromide (Br⁻) in water as a photochemical catalyst for the UVA/chlorine process. We demonstrated that simulated ambient Br⁻ (10 μM, a typical concentration in Br⁻-impacted water) increased pseudo-first-order rate constants for ibuprofen, carbamazepine, and ornidazole by 2.8-3.9-fold through establishment of a UVA/chlorine-bromine system. A comprehensive 205-reaction kinetic model was established, validated, and employed to elucidate the dual-halide mediated synergistic mechanism for the enhancement: (1) rapid conversion of chlorine to bromine by Br⁻; (2) elevated hydroxyl radical (HO•) yields enabled by bromine's higher UVA absorptivity (ε365 = 22 M⁻1 cm⁻1 at pH 7.0) than chlorine (5 M⁻1 cm⁻1 at pH 7.0), despite lower quantum yields; (3) conversion of bromine-derived radicals into reactive chlorine species (RCS) via chloride (Cl⁻), increasing Cl• and Cl2•⁻ concentrations up to 3.6-fold; and (4) direct oxidation of electron-rich compounds by reactive bromine species (RBS). Experiments in authentic surface water confirmed that the synergy remains effective despite the scavenging effects of natural organic matter and alkalinity. This study provides a mechanistic framework for leveraging native halides to unlock the potential of energy-efficient UVA light sources for sustainable water treatment, directly supporting water-energy nexus objectives.