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To enhance hexavalent chromium (Cr(VI)) removal performance under acidic conditions, the nanofiltration (NF) membrane with enhanced negative charge was fabricated via introducing 2, 5-diaminobenzenesulfonic acid (DABSA) into polyamide layer. The control membrane (NF-P) was directly prepared from piperazine and 1, 3, 5-benzenetricarbonyltrichloride. Surface chemical compositions, morphology, surface charge, pore size, permeability and pH-dependent separation performance of the fabricated membranes were characterized. The membranes showed the similar water permeance (∼11.5 L m-2 h−1 bar−1) and Na 2 SO 4 rejections (∼98%) under neutral environments. The DABSA introduced NF membrane (NF-PD) was negatively charged in the pH range of 2.5–11, while the isoelectric point for NF-P was ∼pH 4.0. Cr(VI) removal ability was then evaluated under various concentrations and pH environments. The results indicated that NF-PD showed the better Cr(VI) rejection performance in all tested conditions than NF-P, especially under acidic environments (e.g., pH 4 and pH 5). Moreover, there was a fluctuation of the rejection with the increase of Cr(VI) concentration under acidic environments, which was mainly caused by the formation of Cr 2 O 7 2− species. The harmful Cr(VI) was efficiently removed by the NF membrane with enhanced negative charge under acidic environments, which indicated the wider application range of the NF membrane. [ABSTRACT FROM AUTHOR]

Positively-charged nanofiltration (NF) membranes have attracted more attention compared with traditional negative NF membranes. In this work, one-step self-assembly strategy was designed to prepare positively-charged NF separation layer by the close packing of poly(styrene- r -quaternized 2-(dimethylamino)ethyl methacrylate) (P(St- r -QDMA)) nanoparticles (NPs) with intramolecular crosslinking structure. Meanwhile, the pore size of NF membrane was tailored by regulating the aggregation of the NPs using different ions and annealing conditions. Herein, water was used as a dispersant to obtain P(St- r -QDMA) NPs dispersion, which is non-toxic and green. The pore size cut-off (PSCO) of the separation layer is well controlled in the range of 0.98–4.04 nm. The optimal membrane showed molecular weight cut-off (MWCO) of 380 Da (i.e., PSCO, 0.98 nm) with the isoelectric point of 9.4, which effectively improved the separation performance for divalent cations (97.2 % for MgCl 2 and 96.0 % for CaCl 2) and neutral small molecules (97.7 % for VB 12 and 92.5 % for PEG400). This work gives a novel and convenient method to construct positively-charged NF membrane with controlled pore sizes, which will be a promising candidate for water softening and molecular separation. [Display omitted] • Positively-charged nanofiltration membrane was prepared by nanoparticles assembly. • The pore size of membrane was tailored by ion concentration and annealing temperature. • It is a novel and convenient preparation method. • The membrane is promising in water softening and molecular separation. [ABSTRACT FROM AUTHOR]