Dually Charged Acid Stable Tight Polyamine Composite Nanofiltration Membranes with Low Molecular Weight Amine Molecules for MgCl2 Rejection

Construction of tight acid-resistant nanofiltration membranes (ARNFMs) with high performance is attracting more and more attention due to the high demand for resourceful treatment of acidic wastewater. In this work, we designed and prepared high-performance polyamine composite ARNFMs with low molecular weight cutoff (MWCO) for MgCl2 rejection based on the differences in diffusion rates between low molecular weight amine molecules (LA-molecules) and polyethylenimine (PEI, Mw = 1800 Da). The LA-molecules reacted with cyanuric chloride (CC) before PEI and the formed primary layer hindered the contact between PEI and CC to some extent. As a result, a thinner separation layer of the polyamine composite ARNFM was constructed due to the introduction of LA-molecules, which endowed the membrane with a much higher water permeability (up to 4.1 times) compared with the pristine polyamine ARNFM. Moreover, the effects of the primary layer structure on the polyamine composited performance and structure were further investigated by utilizing the different reactivity of four LA-molecules (piperazine (PIP), m-phenylenediamine (MPD), diethylene triamine (DETA), and PEI (Mw = 600 Da)) with CC. The triamine structure and short molecular chain of DETA helped to form a just right primary layer with appropriate defects and hence assisted the fabrication of a tight polyamine composite ARNFM. The optimized PEI/DETA2:8-CC composite ARNFM exhibited a quite high MgCl2 rejection rate of 99.1% due to the nondefective separation layer, low MWCO (∼144 Da), and the narrow pore size distribution. In addition, all the polyamine composite ARNFMs showed good acid stabilities in 3 wt % HCl for 72 h at a high temperature of 50 °C.