Removal of Chlorinated Organic Compounds by Advanced Reduction Technology Based on a UV/Sulfite System: Status and Prospects

Chlorinated organic compounds (COCs) are persistent contaminants that are difficult to remove by conventional oxidation and biological treatment because of the stability of C-Cl bonds. UV/sulfite advanced reduction processes have emerged as promising technologies for reductive dechlorination because sulfite photolysis can generate hydrated electrons, hydrogen atoms, and sulfite anion radicals. However, the roles of these reactive species are strongly dependent on pollutant structure, pH, dissolved oxygen, and water-matrix constituents, making the degradation mechanisms and practical applicability difficult to generalize. This review critically summarizes the formation, detection, and mechanistic roles of reactive species in UV/sulfite systems for COC degradation. Particular attention is given to structure-dependent dechlorination pathways, the dual effects of sulfite dosage and pH, the inhibitory roles of oxygen and electron-accepting matrix components, and the toxicity evolution of transformation products. Current evidence indicates that high parent-compound removal does not necessarily imply detoxification, because partially dechlorinated or ring-opening intermediates may retain or increase predicted acute toxicity. Based on the above analysis, this paper further clarifies the inherent limitations of the UV/sulfite system and defines its applicable scenarios and boundary conditions. Future research should move beyond single-pollutant systems toward multi-contaminant matrices, quantitative species contribution analysis, experimentally validated toxicity assessment, and reactor-scale evaluation. This review provides a mechanistic framework for assessing when UV/sulfite treatment is chemically favorable, environmentally safe, and practically feasible for COC-contaminated water.