Preparation of acid-resistant nanofiltration membrane with dually charged separation layer for enhanced salts removal

•An acid-stable dually charged membrane is obtained by two-layer reverse interfacial polymerization.•Poly(amide-sulfonamide) and polyurea are introduced to enhance the acid resistance and retention ability.•Environmentally friendly additives improve the dually charged structures of separation layer.•Salt-induced permeability enhancement effect of the polyurea separation layer is clarified.•The dually charged nanofiltration membrane has desirable long-term stability in heavy metal removal.