Reduced graphene oxide-based composite membranes for in-situ catalytic oxidation of sulfamethoxazole operated in membrane filtration

As a novel technology to address the micro-pollution issue of water environments, graphene-based composite membranes were employed to operate an in-situ carbonaceous catalytic oxidation for the degradation of sulfamethoxazole (SMX), which is frequently detected in the water bodies due to its widespread use and antibacterial characteristic. The carbon mats were fabricated by assembling reduced graphene oxide (rGO) sheets with multi-walled carbon nanotubes (CNTs) at a mass ratio of 3:1 on nylon support membranes via vacuum filtration. In this study, the reduction degree of rGO was considered as a critical factor with respect to changing the structural and chemical properties of the carbon mats, further, this correlation was investigated comprehensively. It led to an improvement in the water permeability of membranes, and enhancement of the SMX removal by persulfate (PS) activation. The results indicated that partial reduction causes the carbon mats, namely rGO3h/CNTs, to adopt an optimal carbon to oxygen atomic ratio of 2.9. For in-situ catalytic oxidation, a stable SMX degradation above 77% and a relatively low transmembrane pressure (TMP) of 0.42 bar were obtained in the single-pass through rGO3h/CNTs composite membranes, however, those membranes without hydrothermal treatment exhibited a 3.5-time lower SMX removal efficiency and a 3.2-time higher TMP at the same flowrate. In addition, the pH of solution pH had a significant influences on the SMX removal by PS self-oxidation, adsorptive filtration and in-situ catalytic oxidation, which revealed that the degradation of organic compounds in the ternary system can be attributed to the free radicals generated from PS activation and to the reactive complexes on carbonaceous surface. The results obtained in this study confirm the potential of in-situ carbonaceous catalytic oxidation via membrane filtration for practical applications in water purification.