Innovative hydrophobic membranes for membrane distillation and membrane crystallization

Hydrophobic membranes, especially fluoropolymer membranes are applied in membrane operations such as membrane distillation and membrane crystallization, where hydrophobic porous membranes act as a physical barrier separating two phases. Due to their hydrophobic nature, only gaseous molecules are allowed to pass through the membrane and are collected on the permeate side, while the aqueous solution cannot penetrate. However, the solvents used for preparing these membranes are traditionally toxic, such as N-methyl-2-pyrroli-done (NMP), N,N-dimethylacetamide (DMA), N,N- dimethylformamide (DMF), etc, which are toxic to human beings as well as the environment. Replacing these toxic solvents with greener one is one of the research directions for many researchers. These greener solvents include cyrene, 2-methyl-5-ox- opentanoate (polarclean), ! − #$%&'()*+%(,-, acetyl tributyl citrate (ATBC), triethyl phosphate (TEP), dimethyl isosorbide (DMI), etc. In this direction, different preparation methods of these fluoropolymer membrane, such as non-solvent-induced phase separation (NIPS), thermally-induced phase separation (TIPS), vapor-induced phase separation (VIPS) methods, etc., were investigated on different membranes by using green solvents, and finally applied in membrane distillation (MD) process. In particular, hydrophobic fluoropolymer poly(ethylene chlorotrifluoroethylene) (ECTFE) and poly(vinylidene fluoride) (PVDF) membrane are prepared with diethyl adipate (DEA) and DMI as green solvent for membrane distillation, respectively. All the prepared membranes were characterized by SEM, pore size, contact angle, porosity, thickness, etc. Suitable membranes were selected and applied in MD process. The recovery ability of these membranes in MD were also investigated in 0.6 M NaCl solution. Moreover, the membrane crystallization (MCr) process was also investigated by using the hydrophobic PP membrane for the mixed salt solutions, simulating the industrial RO brine, to recover the valuable salts. All the work shows the good application of the hydrophobic membranes.