Kinetic and mechanistic insights into the degradation of clofibric acid in saline wastewater by Co(2+)/PMS process: a modeling and theoretical study

Recently, the degradation of non-chlorinated organic pollutants in saline pharmaceutical wastewater by SO(4)˙(−)-based advanced oxidation processes (AOPs) has received widespread attention. However, little is known about the oxidation of chlorinated compounds in SO(4)˙(−)-based AOPs. This study chose clofibric acid (CA) as a chlorinated pollutant model; the oxidation kinetics and mechanistic pathway were explored in the Co(2+)/peroxymonosulfate (PMS) system. Notably, a high removal efficiency (81.0%) but low mineralization rate (9.15%) of CA within 120 min were observed at pH 3.0 during Co(2+)/PMS treatment. Exogenic Cl(−) had a dual effect (inhibitory then promoting) on CA degradation. Several undesirable chlorinated by-products were formed in the Co(2+)/PMS system. This demonstrated endogenic chlorine and exogenic Cl(−) both reacted with SO(4)˙(−) to generate chlorine radicals, which participated in the dechlorination and rechlorination of CA and its by-products. Furthermore, SO(4)˙(−) was the dominant species responsible for CA degradation at low Cl(−) concentrations (≤1 mM), whereas Cl(2)˙(−) was the predominant radical at [Cl(−)](0) > 1 mM. A possible degradation pathway of CA was proposed. Our findings suggested that chlorinated compounds in highly saline pharmaceutical wastewater will be more resistant and deserve more attention.