Dual-functional acyl chloride monomer for interfacial polymerization: Toward enhanced water softening and antifouling performance

Abstract

Here, a strategy to control surface negative charge, pore size and active layer thickness by manipulating the polyamide (PA) layer structure is proposed to fabricate an advanced water softening nanofiltration (NF) membrane. Based on this strategy, a dual-functional acyl dichloride monomer containing a protected amino group, 5-sulfinyl amino isophthaloyl dichloride (NSO), is designed as organic phase alternative. Because the protected amine group of NSO has higher hydrolytic activity and is prone to de-protect and form free amino group, it has important impact on the interfacial polymerization (IP) process and formed polyamide (PA) structure. To elucidate the role of the protected amino group in regulating PA structure, the desalination performances and structural characteristics of the three PA membranes prepared by NSO monomer and its structurally similar monomers (isophthalyl chloride (IPC) and trimesoyl chloride (TMC)) are systematically investigated. Compared with TMC-based NF membrane, the NSO-based membrane shows a smoother and lower carboxyl group density surface, decreased pore size and a thinner PA rejection layer (~30 nm). As a result, the novel NF membrane exhibits outstanding water softening performance in terms of ultrahigh divalent salts (Na2SO4, MgSO4, MgCl2 and CaCl2) rejections (above 98%) and preferable high water flux (138.25 ± 6.32 LMH), together with superior antifouling property. More importantly, its water softening capability remain stable even under the mixed salt solution including abundant SO42−, which far outperforms the typical poly(piperazine-amide)-based and state-of-the-art commercial NF membranes. This work further establishes the structure-property relationship and enables the rational design of high performance of NF membrane at the molecular level.