pH-dependent generation of radical and nonradical species for sulfamethoxazole degradation in different carbon/persulfate systems

The impacts of pH on purification efficiency can be phenomenal in advanced oxidation processes (AOPs), because solution pH affects persulfate (PS) activation processes. However, consensus has not been reached on the regimes of pH-regulated oxidation in persulfate-based AOPs (PS-AOPs). Particularly, the impacts of pH on carbon-catalyzed generation of radical and nonradical species remain unclear. In this work, we evaluated three typical carbonaceous materials including pyrolytic carbon (PC), activated carbon (AC), and carbon nanotube (CNT) to activate PS for sulfamethoxazole (SMX) degradation within a pH range from 4 to 9. The experiment revealed pH-dependent SMX removal in PC/PS, AC/PS, and CNT/PS, and the kinetics followed an order of pH 4 > pH 7 > pH 9. Solution pH simultaneously affected SMX adsorption and degradation, but the latter was more profound. Chemical quenching experiment, electrochemical measurement, kinetics calculation, and ATR-FTIR tests collectively revealed that high pH was not favorable for both radical and nonradical oxidation. In the PC/PS system, increased pH decreased the amount of phenolic -OH on PC surface, thereby restraining the generation of SO4•- and •OH due to the lack of electron donors. For AC/PS system, elevated pH hindered the interactions between AC and PS, thus suppressing the formation of surface-bound radicals. CNT/PS initiated an electron-transfer pathway, and increased pH reduced the oxidation potential of surface CNT-PS* complex, which was not favorable for nonradical oxidation of adsorbed pollutants. Therefore, outcomes of this work will advance the current knowledge on the intrinsic impacts of pH in PS-AOPs catalyzed by carbonaceous materials for wastewater purification.