High-performance antifouling photothermal membranes for enhanced membrane distillation crystallization

Membrane Distillation-Crystallization (MDCr) technology has the potential to address the simultaneous need for water production and raw materials recovery from aqueous hypersaline solutions. However, MDCr implementation at a large scale is hindered by energy-intensive bulk feed heating and membrane fouling. Here, we developed a photothermal self-heating membrane, prepared by spray-coating a hydrophobic polyvinylidene fluoride (PVDF) membrane with graphene oxide nanoparticles (GO), exhibiting superior sunlight-to-heat conversion efficiency and antifouling properties. Under optimal conditions (PVDF-GO5), a membrane surface temperature of 81.4°C was reached in just 2 min under 1-sun irradiation (1000 W/m2), achieving a thermal efficiency of 75%, which represents a 115% increase compared to the unmodified PVDF membrane, which had a thermal efficiency of only 35%. The membrane also exhibited an evaporation flux of 1.10 L m−2 h−1, more than double that of the pristine membrane (0.51 L m−2 h−1). This performance facilitated the extraction of NaCl crystals from seawater and 5 M NaCl brine: cubic crystals with a dispersion of 36% around the mean crystal size were obtained. The fouling stability of photothermal membranes was studied by evaluating the role of membrane surface free energy components in the fouling process. Interestingly, the biofilm formation was reduced up to 60% for E. coli and 90% for S. aureus, with protein fouling also significantly lowered compared to the uncoated membrane. These findings demonstrate the feasibility of GO-based nanomaterials to address both temperature polarization and fouling issues in MDCr operation, bringing it closer to large-scale implementation.