Electrically-conductive, Joule-heated pervaporation membranes for desalination: Investigating energy-saving, self-cleaning and anti-swelling trifecta

Abstract

Desalination pervaporation (PV) process can be a promising technique to address global water-energy challenges. However, the energy-intensive nature of the process, which typically requires heating of the entire bulk feed, has been a significant challenge. Here, we reveal a novel self-heated PV membrane utilizing carbon nanostructures (CNS)-alginate (CaAlg) membrane Joule heater that merges interfacial heating of a feed solution and conventional PV separation. Low voltage is supplied to the electrically conductive alginate membrane to initiate resistive heating, which directly heats the solution at the feed/membrane interface. The developed self-heated membranes produce a high flux of 36 L m−2 h−1 at 50 % CNS loading while supplying a voltage of 5 V to the membrane to treat 1000 ppm NaCl solution. A flux of 24 L m−2 h−1 is obtained for the same membrane and the same conditions while treating hypersaline feed of 100,000 ppm. The self-heated PV membrane eliminates the preheating of the entire feed solution while enhancing permeate flux, resulting in a significantly reduced energy requirement. To produce a flux of 10 L m−2 h−1, electrothermally-heated PV membrane achieved exceptionally higher Gain output ratio (GOR) of 2.19 compared to the conventionally-heated (0.37). Besides their unique self-heating capability, the developed membranes are resistant to swelling and showed impressive antifouling/cleaning characteristics. Joule-heated CaAlg/CNS membranes showed a reduction in swelling degree from 150 % (Pristine CaAlg) to 50 % after 2-h exposure to water. Whether electrolysis (in-situ or cleaning cycles) or joule-heating cleaning is used, CaAlg/CNS membranes exhibited immaculate capabilities to self-clean surface foulants and maintain steady flux with only 3–8 % flux decline compared to pristine membranes that suffers 45–47 % flux decline.