Advances in non-photo sulfite-based advanced reduction processes: Key mechanism, system activation, and future perspectives.

Refractory organic pollutants (e.g., halogenated organics, nitro compounds, bromate) that are poorly degraded by conventional water treatment methods pose persistent risks to aquatic ecosystems and human health. Non-photo sulfite-based advanced reduction processes (ARPs) have emerged as a promising treatment technology for refractory pollutants. This review systematically summarizes the state of the art in this field, with an emphasis on the key mechanisms and system activation. The generation pathways, chain reaction networks, and sulfur speciation dependence of core reductive species including hydrated electrons (eaq-) and sulfite radicals (SO3•-) are discussed. Major activation systems are investigated. Homogeneous metal ions (e.g., Fe(II), Al(III)) offer well-defined mechanisms and fast kinetics but suffer from metal precipitation and dissolved oxygen (DO) sensitivity. Heterogeneous metal/metal compounds enable catalyst recyclability yet face stability and metal leaching challenges. Carbon-based materials provide nanoconfinement-enhanced options but are limited by high cost and membrane fouling. Electrochemical activation avoids external chemicals and operates over a wide pH range, yet incurs high energy consumption and potential toxic byproducts. For the effects of operating parameters, pH determines S(IV) speciation and catalyst surface charge, thereby influencing the dominant reactive species. DO quenches radicals at near-diffusion-limited rates. Coexisting matrix components (NOM, HCO3-/CO32-, Cl-, Br-) compete for reductive species and reduce treatment efficiency. Temperature accelerates kinetics but may also promote side reactions. Future efforts should focus on developing stable and poison-resistant heterogeneous catalysts, designing coupled ARP-AOP processes, scaling up continuous-flow reactors, and implementing full-chain "removal-intermediates-toxicity" assessment. Overall, this review emphasizes the unique advantages of non-photo operation that are often overlooked in conventional ARP research and provides a practical guideline for scaling up non-photo sulfite-based ARPs.