Fabrication and characterization of phosphorylated chitosan nanofiltration membranes with tunable surface charges and improved selectivities

Phosphorylated chitosan (PCS) with tailored amount of phosphate groups was synthesized, and novel composite nanofiltration membranes (NFMs) with tunable surface charges were prepared by coating PCS onto polyacrylonitrile (PAN) supporting layer and subsequently cross-linked by glutaraldehyde (GA). Chemical structures and compositions of PCS and NFMs, along with morphologies, surface charges, and performance of NFMs were represented by using FTIR, TG, ATR-IR, SEM-EDX, AFM, streaming potential analyzer, and cross-flow flat permeation test, respectively. The effect of different phosphorous abundance of PCS precursors on the surface charge and permselectivity of NFMs was investigated systematically. It was illustrated that the incorporation of phosphate group with high-substitute degree did enhance the surface charge and selectivity of NFM, respectively, while retaining its permeability. The resultant membrane showed a zeta potential of −77.6 mV at 1 mol·m−3 KCl electrolyte solution and pH 7.0, significantly more superior than that of the commercial DL membrane. For the feedwater (Na2SO4 + MgCl2, 1.0 g·L−1), the mass ratio of SO42−/Cl− and Mg2+/Na+ decreased from initial 0.5:1 in the feed to 1.58 × 10−2 and 0.254 in the permeate after filtration by the optimal NF membrane. Anionic dyes removal tests also confirmed the existence of negative charge characteristics from the prepared membranes, and pivotal role of the Donnan exclusion in separating performance. In addition, NFM4 exhibits good chemical stability in the long-term operation.