Evaluation of reverse osmosis and nanofiltration membranes performance in the permeation of organic solvents

The aim of this study was to evaluate the influence of different solvents and time conditionings in the permeation of n-hexane and oil/n-hexane solutions through several reverse osmosis (ORAK - Osmonics, BW30 - Dow Filmtec) and nanofiltration (NF270 - Dow Filmtec, NP030 - Microdyn Nadir) polymeric membranes. The study of the influence of membrane conditioning on the n-hexane flux was carried out using different solvents (n-hexane, ethanol, n-propanol, iso-propanol and butanol) as pretreating agents in different time intervals. Membrane performance was tested by measuring n-hexane fluxes and rejections of dissolved castor oil. The conditioning with ethanol increased n-hexane fluxes in the polyamide membranes ORAK, NF270 and BW30, while the polyethersulfone membrane NP030 did not present any increase in permeability. An increase in contact angle after membrane conditioning was detected, as well as a reduction in free surface energy. This suggests that pretreatment steps alter the surface hydrophilic character. Salt rejections for all the membranes decreased after the pretreatments and permeation with n-hexane for 8 h. These rejections dropped from 87-99% to 72-77% for membranes NF270, NP030 and BW30; and from 99% to 88% for ORAK, which showed better stability to solvent exposure. For castor oil, the highest retention was 60% for reverse osmosis membrane ORAK. The results obtained in this work with commercial RO and NF membranes, normally used in aqueous solutions operations, indicate that most of these membranes do not present adequate stability towards non-aqueous solvents.