Environment-friendly fluoride-free membranes from plasma-activated hydrophilic PES and alkylsilanes applied in MD: grafting optimization, surface properties, and performance

Abstract

Because of the certain limitations of traditional MD (membrane distillation) membranes and environmental and cost factors of perfluoroalkyl silanes, this study modified the surface of the PES membrane for applications in MD using fluoride-free agents. Oxygen plasma-activated PES membranes generate reactive hydroxyl groups, and these functional groups then react with hydrolyzed chloroalkylsilanes to attach alkyl chains to the membrane surface via Si–O bridging. The plasma treatment and grafting reaction processes were optimized by the Box–Behnken design with the response function of water contact angles (WCA). The effect of membrane aging after plasma treatments with air atmosphere or oxygen atmosphere on the obtained WCAs was also investigated. In addition, the number of grafting reactions also significantly affected WCAs and permeate fluxes in MD operations. The coated membranes were characterized by methods such as WCA measurements, ATR-FTIR, XPS/ESCA, SEM, EDS, and AFM. MD performance using these modified membranes was performed with 4 wt% NaCl solution and actual wastewater from fish ponds. The results showed that the optimal conditions for plasma treatment and grafting reactions were a plasma power of 95 W, grafting reaction temperature of 70 °C, and alkylsilane concentration of 13.20 wt%. Membrane aging after plasma treatment in an oxygen atmosphere gives better performance than the air atmosphere. Two times grafting reactions resulted in higher WCA but permeate flux in MD was about half lower than that of the one-time grafting reaction. The highest WCA achieved for modified membranes is about 124°, which is very significant compared to the WCA of pristine PES membrane (0°). The longer the alkyl chain of the alkylsilanes, the better for the obtained WCAs but is not always good for performance in MD. The MD performance of modified membranes is comparable to commercial membranes such as PVDF or PTFE/PP in terms of permeate fluxes, rejections of salts, and COD and BOD removal.