Fabrication Of Reverse Osmosis Membranes With High Boric Acid Rejection Performance Enhanced By Ultraviolet-initiated Polymerization

Polyamide（PA）reverse osmosis membranes suffer from the unsatisfactory rejection performance of boric acid（H₃BO₃）.To solve this problem,taking the ultraviolet-initiated polymerization as the enhancing tool,in this work,the fabrication of reverse osmosis membranes with high boric acid rejection performance was carried out by reinforcing the denseness of reverse osmosis membranes and regulating the interaction between membrane materials and H₃BO₃.2-Hydroxy-3-（methyl（2,3,4,5,6-pentahydroxyhexyl）amino）propylmethacrylate（HAPM）with ultraviolet-initiated polymerization activity was selected as the aqueous additive,and then the hydroxyl-rich HAPM polymerization product was introduced into the PA separation layer of the reverse osmosis membrane uniformly and stably by the ultraviolet-enhanced interfacial polymerization process,due to the excellent dispersion of HAPM within the interfacial polymerization reaction zone.This method improved the hydrophilicity of the surface of the reverse osmosis membrane and avoided the blockage of the water channels within the membrane caused by the excessive presence of polymerization product in the local area of the membrane.By the dipole-dipole interaction and hydrogen bonding interaction between the hydroxyl-rich material and H₃BO₃,as well as reinforcing the denseness of the separation layer,this method could slow down the H₃BO₃ transfer rate within the reverse osmosis membrane.At the test condition of the simulated seawater desalination（aqueous solution containing 32000 mg/L Na Cl and 5 mg/L boron,5.50 MPa,25°C）,the H₃BO₃rejection of the prepared membrane was increased from 83.60%to 91.13%,Na Cl rejection was increased from 99.02%to 99.16%,and water permeation was increased from 40.91 L m^-2 h^-1 to 43.08 L m^-2 h^-1.Both 2-hydroxyethyl acrylate（HEA）and acrylamide（AA）,which possess the ultraviolet-initiated polymerization activity,were selected as the aqueous additives,and the hydroxyl-rich HEA-AA polymerization product was introduced into the PA separation layer of the reverse osmosis membrane by the ultraviolet-enhanced interfacial polymerization process.The nucleophilic substitution reaction between the primary amine groups in the HEA-AA polymerization product and the acyl chloride groups in the PA material achieved the stable chemical bonding between the two materials within the separation layer.At the test condition of the simulated seawater desalination,the H₃BO₃ rejection of the prepared membrane was increased from 84.42%to 92.80%,Na Cl rejection was increased from 98.95%to 99.21%,and water permeation was increased from 42.83 L·m^-2·h^-1 to 45.10 L·m^-2·h^-1.Besides,the prepared reverse osmosis membrane with the high H₃BO₃ rejection performance exhibited the excellent structural stability during the experiment where the high-concentration chlorine solution was used to attack the structure of the separation layer of the reverse osmosis membrane.2-Sulfoethyl methacrylate（SMA）with the ultraviolet-initiated polymerization activity was selected as the aqueous additive,therefore the SMA polymerization product was successfully enriched in the bulk of the PA separation layer during the ultraviolet-enhanced interfacial polymerization process because SMA containing sulfonic acid group was difficult to diffuse deep into the interfacial polymerization reaction zone in the organic phase.This method could slow down the H₃BO₃ transfer rate within the reverse osmosis membrane by the strong intermolecular interaction between the sulfonic acid group and H₃BO₃,as well as reinforcing the denseness of the separation layer,and thus upgraded the bulk of the PA separation layer.Besides,after the interfacial polymerization process,hexyl methacrylate（HMA）with ultraviolet-initiated polymerization activity was added to the n-hexane solvent used to rinse the reverse osmosis membrane surface,and the HMA containing non-polar alkyl chains was introduced onto the surface of the PA separation layer by the ultraviolet grafting modification process.This method significantly weakened the polarity of the surface of the separation layer,and inhibited the enrichment of H₃BO₃ containing polar groups in the feed water on the surface of the reverse osmosis membrane,thereby upgrading the surface of the separation layer.By combining the above methods,the reverse osmosis membrane with upgraded bulk and surface was prepared,and the boric acid rejection was increased from 83.82%to 92.47%,Na Cl rejection was increased from 98.89%to 99.19%,water permeation was slightly decreased from42.67 L·m^-2·h^-1 to 40.95 L·m^-2·h^-1 at the test condition of the simulated seawater desalination.