Highly permeable acid-resistant nanofiltration membranes for metallurgical spent acid treatment: Structure–performance relationships and selective separation mechanisms

Acid-resistant nanofiltration (NF) membranes hold great promise for the treatment of metallurgical waste acids; however, their practical deployment is often constrained by limited water permeance. In this study, we systematically investigate a highly permeable acid-stable NF membrane (ASNF), focusing on its structure–property relationships, separation mechanisms, and operational suitability. The ASNF membrane achieves a high water permeance of 9.80 L m⁻² h⁻¹ bar⁻¹, attributed to its moderate pore size, thin selective layer, strong hydrophilicity, and highly porous finger-like support structure. Notably, its high positive surface charge (~10 mV at neutral pH) combined with the moderate pore size enables an exceptional rejection of magnesium chloride (>98 %), effectively overcoming the typical trade-off between ion rejection and permeance in acid-resistant NF membranes. ASNF outperforms commercial membranes and those reported in the literature in both permeance and cation selectivity. It also shows excellent chemical stability in 20 % sulfuric, hydrochloric, and phosphoric acid, with negligible loss in performance after 30 days of exposure. In hydrochloric acid systems, enhanced electrostatic repulsion and co-ion competition lead to superior cation selectivity, whereas sulfate ions may reduce cation rejection due to electrostatic attraction. Salt rejection is influenced by pH-dependent ion speciation and electrostatic interactions, as well as by co-ion competition and charge shielding at varying salt concentrations. The membrane also exhibits strong antifouling resistance and scalability. An 1812-type ASNF module demonstrated high ion selectivity under low-pressure operation in lithium slag leachate treatment, underscoring its potential for industrial applications.