| As an emerging membrane technology,forward osmosis(FO)has attracted a lot of attention in recent years,which leads to a bloom of studies focusing on the fabrication of high-performance FO membranes.The thin film composite(TFC)membrane which consists of a porous support layer and an active rejection layer is a competitive FO membrane configuration.In order to mitigate the adverse effects caused by internal concentration polarization(ICP)in FO process,most of prior research work concentrated on the fabrication of high performance TFC FO membranes and highlighted the tuning of support layer to mitigate ICP.It is believed that the hydrophilic substrate may benefit the membrane performance in FO process,however,it may significantly affect the formation of polyamide rejection layer.This study investigated the influence of substrate hydrophilicity on the formation of polyamide layer by fabricating TFC membrane from a relatively hydrophobic pristine polyacrylonitrile(PAN)and a super hydrophilic hydrolyzed PAN substrate with TFC membranes fabricated from PAN after heating treatment as control.It was found that the polyamide layer would delaminate from the hydrophilic support,leading to the failure of TFC membrane fabrication.It was hypothesized that the delamination was probably due to the existence of a very thin water layer located between the hydrophilic substrate and the PA layer during the fabrication process.Three kinds of polycations(PAH,PEI,PDADMAC)were used to tune the surface hydrophilicity of the hydrolyzed PAN substrate and then the modified substrates were used to fabricate TFC membrane.The effects of polyelectrolytes interlayer on the substrate and PA layer were investigated by comparing the morphology and performance of the TFC membranes fabricated.According to the results,after the polyelectrolytes modification,the surfaces of the substrates became more hydrophobic and no delamination was observed during the interfacial polymerization.The water permeability of PAH modified TFC membrane and PEI modified TFC membrane were 1.32 LMH,1.82 LMH respectively and the rejection of 20 mmol/L NaCl solution were 87.96% and 68.94%,which had an obvious improvement compared to the TFC membrane synthesized on a pristine PAN(1.24 LMH in water permeability and 61.17% in rejection).The PDADMAC modified TFC membrane whose substrate was relatively hydrophilic had a PA layer with low rejection.Based on the results of membrane characterization,polyelectrolytes had effectively tuned the hydrophilicity of the substrate which affected the diffusion of m-Phenylenediamine(MPD)solution and resulted in the change of PA layer morphology and membrane performance.The PAH modified TFC membrane and the TFC membrane fabricated on a pristine PAN were evaluated in FO process with an active layer facing draw solution(AL-DS)orientation using 2mol/L NaCl solution as draw solution and pure water as feed solution.The water flux of the modified TFC membrane increased about 45% the efficiency raised about 40% and the selectivity slightly increased compared to the TFC membrane synthesized on a pristine PAN substrate. |
