Study On Fabrication And Water Softening Performance Of New Nanofiltration Membranes

With the rapid development of society,the quantity and quality of human demand for water continues to increase.The scaling and corrosion problems caused by minerals in hard water seriously affect domestic water supply and industrial application.Therefore,water softening nanofiltration membrane with high selectivity,high permeability and pollution resistance is an ideal choice for high efficiency water purification.In this paper,the anti-fouling and water softening properties of polyamide nanofiltration membranes were optimized by using molecular structure to control surface charge,surface morphology,pore size and active layer thickness.Two new acyl chloride monomers,1,2,3,4-cyclobutane tetracarboxylic chloride（BTC）and 1,2,4,5-cyclohexane tetracarboxylic chloride（HTeC）,were designed and synthesized.Aliphatic polyamide nanofiltration membranes were formed by interfacial polymerization of BTC and HTeC monomers with tri（2-aminoethyl）amine（TAEA）respectively,which were more compact than TAEA-TMC membranes.The average effective pore sizes of TAEA-BTC and TAEA-HTeC composite nanofiltration membranes were 0.1837 and 0.1965 nm,which were lower than the average effective pore sizes of TAEA-TMC membranes of 0.2377 nm.Desalination experiments showed that the retention of CaCl₂,MgCl₂ and MgSO₄ by TAEA-BTC and TAEA-HTe C aliphatic polyamide membranes exceeded 98%.In addition,the ionic selectivity of Na⁺/Mg²⁺ and Na⁺/Ca²⁺ in TAEA-BTC membranes is 126 and 31.5,respectively,which is much higher than that of TMC-based polyamide membranes and commercial nanofiltration membranes.The PEI-BTC positive charge nanofiltration membranes prepared by interfacial polymerization of BTC and PEI with molecular weight 800 Da showed excellent desalination rate(R_MgCl2:97.46±1.22%)and water flux（156.85±1.34 kg/m²/h）compared with the reported literature.Even in the high concentration of mixed salt,it still has excellent water softening properties.TEPA-HTeC aliphatic polyamide nanofiltration membrane prepared by HTeC monomer instead of commercialized TMC interfacial polymerization with TEPA.TEPA-HTe C aliphatic polyamide nanofiltration membrane not only has excellent water softening properties(water flux:114.6±1.20 kgm^-2 h^-1,MgCl₂ rejection rate:97.26±0.22%,Na⁺/Mg²⁺selectivity:90.39),but also has smooth surface and strong hydrophilicity.The anti-fouling experiments showed that the flux decline rate（FDR）of TEPA-HTe C polyamide nanofiltration membrane was only10.11%,the flux recovery rate（FRR）was 100%,the flux decline rate（FDR）of TAEA-HTeC aliphatic polyamide membrane was 38%,and the total aromatic polyamide membrane was 60%.Further investigations confirmed that surface roughness is the main factor to affect the fouling properties of polyamide membranes.Our results demonstrate that BTC and HTe C monomers are unique potential materials in the fabrication of nanofiltration membrane used for water softening.