Mechanistic Insights into Buffer and pH Effects in Peroxymonosulfate Activation by a Cobalt-Based Heterogeneous Catalyst

Heterogeneous advanced oxidation processes (AOPs) have shown great promise for degrading organic pollutants in natural waters and industrial wastewaters. However, although there is an extensive body of literature focused on catalyst optimization, the effects of reaction pH and water composition on heterogeneous catalytic performance have been understudied. More importantly, there are inconclusive reports of how pH buffer selection, particularly for bicarbonate and phosphate conditions, alters catalyst performance, making comparisons of novel materials across studies difficult. To better understand the impact of pH control on peroxymonosulfate (PMS) activation and phenol oxidation, we systematically evaluate reaction performance, oxidation mechanism, and catalyst stability in a pH-controlled baseline system without pH buffer in comparison to bicarbonate, phosphate, and borate buffer conditions. In a cobalt Prussian blue analogue/PMS system, we demonstrate that (i) bicarbonate has a complex interaction in PMS activation, facilitating the production of peroxymonocarbonate ions, (ii) phosphate accelerates the production of sulfate radical species, accelerating phenol decay and increasing catalyst metal leaching, and (iii) borate controls reaction pH with minimal impact on the oxidation mechanism and catalyst stability when compared to pH adjustments without buffer. These conclusions provide the foundation for the development of standardized AOP testing conditions and emphasize the need for early testing of AOP materials in realistic water matrices.