Microwave photo-oxidation with diverse oxidants for Congo red degradation: effect of oxidants, degradation pathway and economic analysis

The present study investigated the efficacy of microwave photo-oxidation (MWPO) process with two oxidants i.e. persulphate (PS) and hydrogen peroxide (H2O2) for degradation of Congo red (CR). The result indicated a CR degradation efficiency of 98% and 96.8% with PS and H2O2, respectively, in 30?min of reaction with corresponding PS dosage of 50?mg/L and H2O2 dosage of 180?mg/L. The COD removal efficiency with the two oxidants were 97.7% and 94.9%, respectively. Higher dosages of oxidant and CR reduced the efficiency of the process in both the cases due to self-quenching. Effect of pH and initial CR concentration on CR removal efficiency also has been studied. Degradation of CR followed pseudo-first-order kinetics with a removal rate constant of 0.12/min and 0.09/min, respectively, with PS and H2O2. The main mechanism of CR degradation was cleavage of the benzene?benzene bond, cleavage of benzene?N bond and hydroxylation. Economic analysis of the MWPO process indicated an energy consumption of 18.3?kWh/g of CR removal and 18.4?kWh/g of COD removal. The process was effective in the rapid degradation and mineralization of high concentration of CR within 30?min.