Lithium ion-regulated monomer reactive sites heighten selectivity of polyamide nanofiltration membranes

Abstract

Interfacial polymerization (IP) is an ultrafast process due to the highly reactive sites of the monomers (e.g., piperazine (PIP) with amine groups), leading to uncontrollable formation of polyamide (PA) films. To manipulate the IP reaction precisely for well-designed PA properties, we applied a lithium (Li⁺) salt to control the reactivity of PIP and further adjust the characteristics of PA layer. The interaction between Li⁺ and PIP was investigated by systematic characterizations to reveal its impacts on IP and PA formation. The Li⁺-PIP interaction weakened the reactivity and diffusion of PIP, leading to an optimized PA film (NF-0.2) with halved film thickness and a more uniform nodule structure as a result of the competing effects between PIP diffusion and PA formation. This PA membrane shows nearly doubled water permeance and greatly enhanced Na₂SO₄ rejection to 99.8 %. We further demonstrate that the NF-0.2 membrane exhibited superior NaCl/Na₂SO₄ selectivity of over 1700 and improved micropollutants rejection. This Li ⁺ -regulated IP (Li-IP) strategy provides fundamental insights into the design and regulation of high-performance PA membranes.