Advanced oxidative degradation of acetaminophen by carbon catalysts: Radical vs non-radical pathways

Two representative carbon catalysts, granular activated carbon (GAC) and carbon nanotube (CNT) were selected for the degradation of acetaminophen (ACT) by persulfate (PS) activation. In this study, the materials characterization study indicated that the surface functional groups and surface area of the GAC were more abundant and larger, respectively, than the CNT. Whereas the contribution of sp² and CO groups of the CNT was superior to the GAC; the structural defect levels and surface charging characteristics (pH_PZC) were similar in both. The mass-based pseudo-first-order reaction rate constant of the CNT was 1.5 m^-1 g^-1, which was 50 times higher than that of the GAC (0.03 m^-1 g^-1). Radical and non-radical pathways were evaluated for catalytic reactions by the GAC/PS and CNT/PS systems, respectively. Experimental results using scavenger tests, linear voltammetry, and electron spin resonance (ESR) showed that the radical pathway was dominant in the GAC/PS system, whereas the non-radical pathway was dominant in the CNT/PS systems. To confirm this, the influence of affecting factors such as initial ACT concentration, the dosage of oxidant (PS), and initial pH were also investigated and compared for the two systems. The results showed that the catalytic activity of the GAC/PS system was highly dependent on initial concentrations of ACT and PS, while these were less influential in the CNT/PS system. The removal efficiency of ACT was not affected under a pH of 3-7 in both systems. Reusability experiments were conducted five times, and both the CNT/PS and GAC/PS systems demonstrated that the removal rate of ACT did not notably decrease with the number of experiment repetitions. This means that the application of a carbon catalyst to treat pharmaceutical contaminants in wastewater is effective.