Spray-coated porous photothermal layer on PTFE membranes for robust anti-scaling and solar membrane distillation

This paper presents a novel spray-coating procedure for the deposition of a photothermal carbon black (CB) coating, embedded in poly(vinyl alcohol) (PVA), onto polytetrafluoroethylene (PTFE) membranes for solar membrane distillation (MD) in complex, scale-inducing waters. To maintain water flux after coating, lithium nitrate (LiNO3) was added as a pore former, which resulted in a porous PVA-CB coating after elution. The performance of the PTFE/PVA-CB membranes was evaluated for a range of PVA, CB, and LiNO3 concentrations using a synthetic brine solution. Results showed that adding the hydrophilic PVA-CB increased the scaling resistance of the membrane compared to bare PTFE. Increasing the PVA loading on the membrane resulted in a decrease in water flux that could be mitigated by the addition of a pore former; however, at high (5 %) LiNO3 concentration, the high porosity increased the scaling propensity of the PTFE/PVA-CB membrane. The optimal coating condition (1.5 % PVA, 0.75 % CB, 1 % LiNO3 applied at a wet coating thickness of 55 μm) was stable over a range of operating conditions and able to use simulated sunlight as a driving force for MD desalination. Therefore, the membranes developed in this work show promising performance for off-grid solar desalination of high scaling potential waters.