Sulfite-based advanced oxidation and reduction processes for water treatment

Sulfite has attracted increasing attention as a precursor to produce highly reactive species to eliminate pollutants from water due to the merits of abundant sources, low cost and low eco-toxicity. The systems can be cataloged into advanced oxidation processes (AOPs) and advanced reduction processes (ARPs) according to the redox characteristics of reactive species produced by sulfite activation, thus broadening the scope of application. Herein, this critical review provides a fundamental aspect of sulfite-based AOPs/ARPs and their state-of-the-art developments in water purification. Oxidative species can be generated via sulfite activation using homogenous or heterogeneous transition metals (e.g., zero-valent metals, metal oxides, metal sulfides and supported metals), and the corresponding activation mechanisms are discussed in detail. More importantly, sulfite can be activated by UV irradiation to produce reductive species (including hydrated electrons, hydrogen radicals and sulfite radicals), with emphasis on their chemical properties, reaction mechanisms and improved strategies. In addition, the key factors affecting the removal performance of sulfite-based AOPs/ARPs are also highlighted, such as dissolved oxygen, pH, sulfite concentration and water constituents. The reactive species produced are identified in quantitative and qualitative ways, and their roles in destructing pollutants are analyzed. Furthermore, key knowledge gaps are identified and future research directions are proposed to address the application challenges. This review article aims to advance our understanding and consequent applications of sulfite-based AOPs/ARPs in water treatment.