Concomitant Electro-Fenton Processes in Iron-Based Electrocoagulation Systems for Sulfanilamide Degradation: Roles of Ca²⁺in Fe(II)/Fe(III) Complexation and Electron Transfer

Oxidation mediated by concomitant electro-Fenton (EF) processes in iron-based electrocoagulation (iron-EC) systems plays a crucial role in degrading recalcitrant organic pollutants but is poorly noticed in complex water systems. This study investigated the impact of Ca²⁺on the performance of an iron-EC system based on degradation of sulfanilamide (SA). The concomitant EF reactions in the iron-EC system occurred under pH-neutral conditions in the presence of humic acids (HAs) complexing with the Fe²⁺/Fe³⁺couple. Our results showed a significantly improved performance of EF processes in the presence of 4 mM Ca²⁺compared with that in its absence, indicated by the SA degradation rate of up to 64%, accompanied by 50% higher Fe²⁺and a 4.7 times higher ratio of Fe²⁺/Fe³⁺. A suitable Ca²⁺concentration facilitated the cycle of Fe(II)/Fe(III) via inducing changes in solution pH and accelerating electron transfer from HAs to Fe(II)/Fe(III)-complex based on formation of a tribasic complex of Ca(II)/Fe(II)/Fe(III)-HAs, as evidenced by X-ray photoelectron spectroscopy (XPS) results. This study not only offers new insight into the role of Ca²⁺in the EC process but also suggests an eco-friendly strategy to make water treatment more sustainable.