Boosting non-radical oxidation in peroxydisulfate activation with carbonaceous catalytic membranes by coupling structural defects and nitrogen doping sites

Abstract

Non-radical oxidation is a critical pathway of peroxydisulfate (PDS) activation for organic degradation with metal-free carbonaceous nanomaterial. In this study, reduced graphene oxide (rGO) sheets with carbon to oxygen atomic ratios of 2.9 and 6.6 (rGO2.9 and rGO6.6) were composited with pristine or nitrogen-doped carbon nanotubes (CNTs or N-CNTs) to fabricate rGO2.9/CNTs, rGO2.9/N-CNTs, rGO6.6/CNTs, and rGO6.6/N-CNTs catalytic membranes. These membranes were employed to perform in-situ catalytic oxidation to remove N4-acetyl-sulfamethoxazole (NSMX) as the target pollutant in continuous filtration mode. The results showed that rGO6.6/N-CNTs coupled structural defects and nitrogen (N)-doping sites to obtain superior water flux and efficient NSMX removal via PDS activation. After 50 h of continuous filtration experiments, rGO6.6/N-CNTs exhibited NSMX removal rates (rNSMX) of 16.3 mg.m−2.h−1 at a carbon loading dosage of 8 g.m−2, which was 3.1-, 2.2-, and 1.4-times higher than the values for rGO6.6/CNTs, rGO2.9/N-CNTs, and rGO2.9/CNTs, respectively. N-doping sites took precedence over structural defects in activating PDS and enhanced the catalytic stability of carbon mats. In addition, surface chemical characterization and radical quenching experiments verified that the singlet oxygen (1O2) and surface-active species dominated in NSMX degradation as non-radical pathways. Although carbon loading, PDS dosage, initial pH, inorganic ions, and natural organic matter (NOM) influenced in-situ catalytic oxidation, both higher rNSMX and lower transmembrane pressures were achieved with rGO6.6/N-CNTs in real water matrices as compared to rGO6.6/CNTs. Further rational tailoring of carbonaceous catalytic membranes should focus on improving the catalytic stability and anti-fouling performance in advanced water treatment.