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.

Automated summary

This study investigated the impact of Ca2+ on the performance of an iron-based electrocoagulation system and found that an appropriate Ca2+ concentration significantly improved the efficiency of the concomitant electro-Fenton reactions, leading to enhanced degradation of organic pollutants.