Novel aliphatic polyamide membrane with high mono-/divalent ion selectivity, excellent Ca2+, Mg2+ rejection, and improved antifouling properties

Monomer design and reconstruction is typically a preferred route to tune the inner structure and screen the performance of polyamide nanofilms for their efficiency and ease of scaling up in industrial production. Herein, two kinds of novel acyl chloride monomers, 1,2,3,4-cyclobutane tetracarboxylic acid chloride (BTC) and 1,2,4,5-cyclohexanetetracarboxylic acid chloride (HTeC), have been designed and synthesized. The BTC and HTeC monomers can rapidly react with amine molecule at interface to form an aliphatic polyamide nanofilm that is denser than that of TMC based polyamide membrane. The resulting mean effective pore size of the BTC and HTeC polyamide nanofilms is 0.184 nm and 0.197 nm, which is lower than that of the TMC nanofilm at 0.238 nm. Desalination experiments revealed that the aliphatic polyamide membrane shows more than a 98% rejection rate for CaCl2, MgCl2, and MgSO4 and a water flux of 84.4 kg m(-2) h(-1) MPa-1 (for 2000 ppm MgSO4). Moreover, the ion selectivity of Na+/Mg2+ and Na+/Ca2+ of the BTC membrane is as high as 126 and 31.5, respectively. These are much higher than those of the related TMC and commercial nanofiltration membranes. Fouling experiments indicate that the flux decline rate (FDR) of the aliphatic polyamide membrane is only 38%, whereas the FDR of the full-aromatic polyamide membrane is 60%. Further investigations confirmed that surface roughness is the main factor affecting the fouling behaviors of polyamide membranes. Our results demonstrate that BTC and HTeC monomers are unique potential materials in the fabrication of nanofiltration membranes used for water treatment such as water softening and ion sieving.