Fluoroalkylsilane grafted FeOOH nanorods impregnated PVDF-co-HFP membranes with enhanced wetting and fouling resistance for direct contact membrane distillation

Abstract

Direct contact membrane distillation (DCMD) is a versatile thermal-driven membrane separation technique that can be used for seawater distillation and water recovery. Initially, the central composite design was used to optimize the concentration/molecular weight of pore former and operating feed/permeate flow rates. Further, a dual modification strategy was used to fabricate robust hydrophobic polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP) membranes- (i) incorporation of fluoroalkylsilane (FAS) grafted FeOOH nanorods (F-g-FeOOH) into the membrane and (ii) surface coating of the membrane with FAS. The membrane with 0.015 wt% FeOOH nanorods (M2) displayed the highest water contact angle (132 ± 1.6ᵒ) with more than 99.98% salt rejection for 10,000 ppm NaCl solution (ΔT= 60 ℃ & feed/permeate flow rate = 20 L/h). The M2 membrane exhibited stable performance, and no pore wetting could be observed even when the feed contained polyhydric alcohol (5 wt% propylene glycol) or surfactant (80 ppm sodium dodecyl sulfate). The M2 membrane also displayed a stable permeate flux of 3.5 L/m2h and a total organic carbon rejection > 96% for up to 12 h for a saline feed (10,000 ppm NaCl) containing hexadecane/water emulsion (100 ppm oil). The prepared membranes were also determined to be suitable for long-term seawater desalination and exhibited a stable permeate flux of 10 L/m2h for 60 h with permeate conductivity < 5 µS/cm. The facile fabrication strategy holds the potential to be further explored to prepare membranes with excellent anti-wetting and anti-fouling properties.