Electrothermal heating in direct contact membrane distillation via PTFE membrane coated by tea polyphenols and carbon nanotube for enhanced conductivity and long-term stability

Membrane Distillation (MD) is one of the technologies which exhibits great potential in the desalination and zero liquid discharge (ZLD) area but is currently limited by the heat loss in hydraulic circulation and continuous heating requirements for feed solution. In this study, tea polymer (TP) was adopted to modify the traditional carbon nanotube (CNT) polytetrafluoroethylene (PTFE) membrane as a potential candidate to improve heating efficiency through electrothermal heating. Compared with the CNT membrane, CNT-TP membrane exhibited better electrical conductivity, thermal efficiency and less hydrophilicity, which was also reflected by the mechanisms of the binding between TP and CNT on the membrane surface through ATR-FTIR, XP,S and SEM analysis. The conducting membranes were applied in electrothermal heating of the desalination by direct contact membrane distillation, and membrane performance with the application of different AC voltages was compared. It was found that for the CNT-TP membrane, the permeate flux at AC 10 V, 20 V and 30 V was 28.8%, 43.7% and 45.2% higher than that of the CNT membrane, respectively. Simultaneously, the increase in permeate flux was accompanied by an improvement in thermal efficiency of over 10%. During another long-time operation of 180 h, CNT-TP membranes exhibited long-term stability relative to the CNT membrane. We believe this electrically CNT-TP membrane has a promising potential in the electrothermal heating area, paving the way for smart and low-energy localized heating MD systems.