Galvanic oxidation processes (GOPs): An effective direct electron transfer approach for organic contaminant oxidation

Abstract

The activation of peroxymonosulfate (PMS) for organic contaminant oxidation usually relies on the formation of reactive oxygen species (ROSs). However, the ubiquitous anions and natural organic matter can easily scavenge ROSs and/or PMS, resulting in lower efficiencies and/or the formation of toxic byproducts. Relying on the unique long-distance electron transfer property, the recently developed Galvanic Oxidation Process (GOP) successfully achieved bisphenol A (BPA) degradation when BPA and PMS were physically separated in two reactors. In this study, we systematically investigated the performance of GOP at different PMS or BPA concentrations, pH, and ionic strength (IS) in both PMS and BPA solutions. The kinetic modeling employing the Langmuir-Hinshelwood model at different BPA concentrations suggested that although BPA and PMS were physically separated, the oxidation of the adsorbed BPA and reduction of the adsorbed PMS still followed a similar mechanism to that in traditional heterogeneous catalytic processes. The anions in the target water showed little impact on BPA degradation; higher IS enhanced the solution conductivity but inhibited BPA and electrode interactions, resulting in increased and then decrease BPA degradation rate. The electrodes presented high stability with a rate increase of 12% after 13 times of uses, and their hydration significantly facilitated BPA degradation but reduced the current by decreasing the potential difference between the anode and cathode. The graphite sheet itself without catalyst coating was also capable of shuttling electrons, while the use of a graphite fiber anode increased the BPA degradation by near 100% because of the larger surface area. The developed continuous stirred-tank reactor coupled with GOP (CSTR-GOP) achieved stable BPA degradation in less than 35 min and its scaling up is promising for future applications.