Mechanism and Kinetics of Electrochemical Activation of Sulfite Process for Emerging Contaminants Removal

Abstract

The increasing diversity and occurrence of emerging contaminants in aquatic environments present significant challenges for water treatment. This study developed an electrochemical activation of (bi)sulfite (EC/S(IV)) process capable of simultaneous generation of reductive and oxidative species, enabling the removal of chemically diverse emerging contaminants. Comprehensive experimental investigations, kinetic modeling and theoretical calculations were performed to investigate the reaction mechanism, kinetics and process optimization of EC/S(IV) process, with BrO3- and carbamazepine (CBZ) selected as representative high-valence oxyanion and emerging organic contaminant (EOC). Fe-based catalysts were coupled with EC/S(IV) process to further enhance cost-effectiveness.
The mechanism and kinetics of S(IV) oxidation vary across different regions, leading to the dominance of different radicals. In kinetics-controlled and mix-controlled regions, S(IV) is electro-oxidized via two successive electrons transfer, with HSO3•/SO3•- governing BrO3- reduction. In diffusion-controlled region, in-situ generated O2 captures HSO3•/SO3•- to initiate oxysulfur species conversions, along with rapid S(IV) oxidation, where SO4•- serves as the primary oxidant for CBZ. Furthermore, EC/S(IV) process achieves 99% removal of BrO3- and EOCs within 20 min, driven by the synergistic effects of BrO3- reduction on EOCs oxidation. HSO3• produced during BrO3- reduction reacts with O2 to form multiple reactive species (SO4•-, HO•, Br• and Br2•-), significantly increasing the oxidation rates of EOCs while effectively suppressing brominated by-products formation.
Coupling of Fe ions with EC/S(IV) (EC-Fe/S(IV)) significantly boosts reactive species production and energy efficiency. Electro-generated O2 accelerates Fe ions conversion and reactive species generation, while protons released during S(IV) activation mitigates Fe ions precipitation and improves the complexation between Fe ions and S(IV).
To overcome Fe ions leakage of EC-Fe/S(IV) process, a g-C3N4 supported Fe single-atom catalyst (Fe-C3N4) was synthesized and integrated with EC/S(IV) process (EC-Fe-C3N4/S(IV)). Leveraging the strong synergy between Fe-C3N4 and electrochemistry, this system achieves over 95% removal of emerging contaminants within 20 min at Fe loading of 1.08%, with low energy consumption and minimized environmental impacts. Electric field enhances S(IV) adsorption, accelerates the electron transfer of structural Fe(III)-S(IV) complex for HSO3•/SO3•- generation and evokes the high catalytic activity of Fe-C3N4.
This work establishes a comprehensive mechanistic and kinetic framework for EC/S(IV) process, offering scalable and economical strategies for advanced electrochemical
treatment systems targeting complex water pollution scenarios.

Date of Award	24 Jun 2026
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zhiguo YUAN (Supervisor), Yan Zhang (External Supervisor) & Guohua CHEN (External Co-Supervisor)