Insights into the multiple roles of Ca2+ ion in peroxymonosulfate activation over Mn2O3 for organic pollutants degradation

The development of efficient and eco-friendly catalysts for peroxymonosulfate (PMS) activation is highly desired for pollutants removal in wastewater. Recently calcium ion (Ca2+) has been recognized to promote the catalytic performance of Mn oxides by enhancing the oxidizing power of PMS. However, the impact of pollutants types and influence of solution pH on the catalyst-oxidant interaction towards Ca2+ promotion have not been considered until now. This study investigated the effect of Ca2+ for pollutant degradation by Mn2O3 activated PMS system, with various factors considered in detail such as the dose of Ca2+, pKa of organic pollutants, and solution pH. To deepen the understanding of the formation of Mn2O3/PMS complex in the presence of Ca2+, XPS, ATR-FTIR, radical scavenging, EPR, as well as PMS decomposition experiments, were analyzed. The binding of Ca2+ on the surface of Mn2O3 efficiently promoted PMS decomposition and accelerated the redox cycles of Mn(III)/Mn(IV) with a faster rate for the production of HO• and 1O2. Ca2+ also enriched the pollutants with negative charges around the catalyst to speed up the degradation process. This work points out a novel thought on the improvement of the PMS activation system with nonreducible metal ions.