Preparation of reduced graphene oxide-carbon nanotubes membranes for conductive heating membrane distillation treatment of humic acid

Membrane distillation (MD) has attracted extensive attention as a technology to solve water scarcity in the fields of desalination. Surface-heated membrane distillation techniques that have emerged in recent years have improved flux due to the elimination of temperature polarization, but still suffer from membrane fouling. The use of electrochemical techniques for in-situ cleaning of membrane fouling in MD is expected to be an effective solution. Herein, electroactive membranes were prepared by loading mixtures of reduced graphene oxide (rGO) and carbon nanotube (CNT) with four mass ratios on polytetrafluoroethylene (PTFE) hydrophobic membranes. A series of analytical methods, such as scanning electron microscopy, atomic force microscopy, pore size analysis, water contact angle, Raman spectroscopy, membrane surface zeta potential, and cyclic voltammetry curve, were employed to characterize the properties of the rGO-CNT membranes. The best separation performance and conductivity were achieved at a 2:1 mass ratio of rGO to CNTs (R2 membrane). Then, the rGO-CNT membranes were applied to a conductive heating vacuum membrane distillation system to treat humic acid (HA). Compared with the PTFE membrane, the four rGO-CNT membranes showed a significant improvement in the rejection of HA. Using the rGO-CNT membranes as the cathode and the titanium as the anode, after a certain potential was applied, the flux decreases slowly. The fluxes of all rGO-CNT membranes were maintained over 9.5 kg/(m2·h) for 10 h of continuous operation. An increase in potential helps to increase change by alleviating membrane fouling. The highest flux and HA rejection reached 13 kg/(m2·h) and 99.8 %, respectively, with R2 membrane and −5 V potential applied. The mechanism of antifouling included electrostatic repulsion and electrocatalytic oxidation.