Plasma-grafting surface modifications to enhance membrane hydrophobicity for brine membrane distillation

Abstract

In this study, a novel approach is presented to significantly enhance hydrophobicity and performance in direct contact membrane distillation (DCMD) against concentrated reverse osmosis (RO) brines by modifying commercially available polypropylene (PP), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE) membranes using RF-plasma/absorption-grafting treatments with controlled fluorination. Grafting with 1H,1H,2H,2H-perfluorodecyl-triethoxysilane (PDTS) in an anhydrous medium was employed. Morphological, chemical, and electrokinetic surface analyses revealed increased hydrophobic properties, with fluoropolymers achieving the limit of superhydrophilicity and maintaining the original morphology. Modified membranes demonstrated stable percent salt rejection and moderately higher permeate flux yields than commercial counterparts, with improvements of 14 % for PP, 25 % for PVDF, and 9 % for PTFE. The two-step RF-Plasma-O2 treatment and silane absorption-grafting technique effectively enhance surface hydrophobicity and anti-wetting effect of these different inert polymeric membranes, improving their performance for membrane distillation (MD) in all cases. The findings of this study have significant implications for the development and optimization of membrane distillation and related technologies, especially in areas with limited access to high-quality energy and in industries that deal with highly concentrated feeds such as RO brines.