Chitosan nanofiltration membranes with gradient cross-linking and improved mechanical performance for the removal of divalent salts and heavy metal ions

Chitosan (CTS) and 1,3,5-triglycidyl isocyanurate (TGIC) were gradiently cross-linked on the polyethersulfone ultrafiltration membrane (PES) to prepare a new type of positively charged composite nanofiltration membrane. The influence of preparing conditions on the membrane performance, such as permeability, mechanical strength, membrane morphology, and nanostructures was systematically discussed. The rejection performance to MgCl2, Na2SO4, MgSO4, and NaCl revealed that the prepared membrane was positively charged. The membrane prepared under the optimal conditions showed a molecular weight cut-off of 1500 Da. Finally, rejections to heavy metal ions were determined, and excellent removal rate was found. Among the nanofiltration membranes prepared from chitosan, CTS/TGIC-NF membranes prepared in the current work exhibited superior permeability and mechanical strength to the membranes shown in literature, indicating that our membranes can be used to reduce water hardness and produce portable water. [Display omitted] •1.High permeability due to gradient cross-linking.•High selectivity to divalent cations and heavy metal ions.•The excellent mechanical properties.