The Construction Of A High-efficiency Forward Osmosis System For Oil-water Separation

Forward osmosis(FO)process is driven by the osmotic pressure difference between the semipermeable membrane,and exhibits unique advantages of the membrane process including a low membrane fouling propensity and a high water permeability.The major components of the highly efficient FO process are a highly permeable-selective FO membrane and an efficient draw solution(DS)that can be regenerated cost-effectively.The efficient FO process has been reported to treat some feed solutions(FS)containing macromolecules or particles using the ultrafiltration-like(UF-like)FO membranes and macromolecule polyelectrolytes as DS.This process has advantages of high water flux,low membrane fouling and low operational cost.However,this UF-like FO membrane process still needs to be further optimized by improving the perm-selectivity of the UF-like membranes and developing the cost-effective DS regeneration method.Moreover,the long-term stability of the UF-like FO membrane process in practical applications needs to be investigated.Herein,this work aims to fabricate a high-performance UF-like FO membrane and to develop the efficient regeneration method for polyelectrolytes of DS.The application of UF-like FO membrane process for oil-water separation has been investigated.(1)The carbon nanotubes-based porous composite(PES-MWCNTs)membrane was fabricated by filtering polydopamine modified multiwalled carbon nanotube(PDA-modified MWCNTs)onto the polyethersulfone(PES)microfiltration membrane.Its oil-water separation performances in both UF and FO processes were evaluated.Results show that the permeable-selective property of the PES-MWCNTs membrane could be tailored easily by adjusting the loadings of PDA-modified MWCNTs.When the deposition amount of PDA-modified MWCNTs suspension was 2.92μg/cm~2,the PES-MWCNTs-3 membrane exhibited superior perm-selectivity with the water flux of 31.20LMH and the reverse salt flux of 0.14 mmol MH under the optimal test conditions,when0.50 wt.%poly(sodium 4-styrene-sulfonate)(PSS)was used as a draw solution.The achieved flux of the PES-MWCNTs-3 membrane was higher than the reported flux of the UF-like FO membranes.Importantly,separation experiments demonstrate that the optimized PES-MWCNTs membrane showed a negligible membrane fouling behavior in the FO process to treat oil-water emulsion,as a comparison with that when operated in the UF process.(2)PNIPAM was synthesized by the reversible addition-fragmentation chain transfer(RAFT)polymerization,and used as a thermo-responsive DS to drive the FO process of PES-MWCNTs membrane for oil-water separation.Results show that the PES-MWCNTs-3 membrane achieved the water flux of 27.49 LMH and the reverse salt flux of 0.055 mmol MH when 2.00 wt.%PNIPAM draw solution was selected as a DS in this FO process.The ultrafiltration process under a low-pressure of 0.04 bar was developed to concentrate the PNIPAM draw solution for regeneration,and the initial water flux reached to about 25 LMH at 32℃,which is the lower critical solution temperature(LCST)of PNIPAM.In conclusion,this work developed a highly efficient FO process to treat oil-water emulsion.The pore size of porous composite FO membrane was tailored through the loading of MWCNTs to match the size of feed solutes and draw solutes.Meanwhile,the ultrafiltration process was developed as a cost-effective and efficient regeneration process of the PNIPAM draw solution.Our work provides insights on the development of the high-performance UF-like FO membranes and the efficient FO process for the treatment of feed solutions containing large molecules or particles.