Structure Design And Regulation Of The Selective Layer For Reverse Osmosis Membranes With High Performance

Aromatic polyamide（PA）reverse osmosis（RO）membranes prepared by interfacial polymerization（IP）are the benchmark in the RO membrane market,and the development of high permselectivity PA-RO membranes is of great significance.In the current study,existing technologies offer limited enhancement of membrane performance and a trade-off exists between permeability and selectivity;the feasibility of industrial implementation of membrane preparation process is low.To address these problems,new methods for the design and regulation of membrane selective layer structure are developed in this paper to achieve facile and efficient preparation of RO membranes with high flux,high salt rejection and high boron rejection.The high flux RO membranes were prepared by the“IP-UV graft polymerization coupling”interfacial polymerization+（IP+）method using m-phenylenediamine（MPD）and homophthaloyl chloride（TMC）as reaction monomers and highly soluble ionic liquid 1-Allyl-3-methylimidazolium chloride as the aqueous phase additive.The IP+method integrates the formation and evolution of the selective layer by MPD diffusion inhibition,primary PA layer formation,UV-induced graft polymerization and thermal-treatment reconstruction.As a result,the thickness,roughness and chargeability of the selective layer was reduced and the hydrophilicity of the selective layer was increased.Under the standard brackish water test conditions,the flux of optimal membrane prepared via IP+method increased substantially from 47.8±1.9 L·m^-2·h^-1 to 96.5±3.2L·m^-2·h^-1（101.9%improvement）,while the salt rejection decreased slightly from 98.85±0.19%to 98.35±0.15%.The membrane permselectivity was significantly higher than the virgin and commercial membranes prepared by conventional IP.The high salt rejection RO membranes were prepared by introducing a novel additive,Tween 80,into the organic phase to regulate the formation and stacking of PA molecules in the selective layer.A hydroxyl group on Tween 80 can react with an acyl chloride group on trimesoyl chloride（TMC）to form a new product（TW-TMC）.TW-TMC is arranged as a surfactant on the surface of the organic phase and can promote the enrichment of TMC at the phase interface.During the IP process,TW-TMC prompted the reaction between TMC and MPD to increase the thickness and proportion of cross-linked part of selective layer,and reacted with MPD to fill the low selective regions in the selective layer.Under the standard seawater test conditions,the optimal membrane prepared by introducing Tween 80 into organic phase showed a high salt rejection of 99.61±0.06%,a high selectivity（water/salt permeability ratio）of（1.29±0.07）×10⁴,and a high flux of 40.3±0.7 L·m^-2·h^-1.The selectivity was improved by138.9%,while the permeability was reduced by 23.2%.The effect of the ester chain length of Tween on the membrane structure and performance was investigated by introducing Tween series additives into the organic phase,and high boron rejection RO membranes were prepared.The increase in the ester chain length of Tween could reduce the interfacial tension between the organic and aqueous phases,thus further promoting the reaction between TMC and MPD and improving the PA production and the compactness of selective layer;the introduction of long ester chains could reduce the membrane hydrophilicity and charge.The improved compactness and hydrophobicity of the selective layer substantially increased the boron rejection of membrane and slightly reduced the flux of membrane.The desalination and boron removal performance of the membranes were investigated under different pressures and feed concentrations,which provided a basis for the application of the high boron rejection membranes.Under the standard seawater test conditions,the flux,salt rejection and boron rejection of membrane prepared with Tween 20,which has the lowest ester chain length,reached 43.2±1.0 L·m^-2·h^-1,99.51±0.03%and 89.8±0.4%,respectively.While the flux,salt rejection and boron rejection of membrane prepared with Tween 85,which has the highest ester chain length,reached 35.8±0.7L·m^-2·h^-1,99.70±0.02%and 92.5±0.3%,respectively.