Nanofiltration membranes based on cellulose triacetate from millet husk

Abstract

This study reports the sustainable fabrication of nanofiltration (NF) membranes based on cellulose triacetate (CTA) derived from millet husk, an abundant and underutilized agricultural residue in Sahelian Africa. Cellulose was extracted, acetylated to a degree of substitution of 2.67, and processed into asymmetric membranes by non-solvent induced phase inversion using contrasting solvents (chloroform and dichloromethane) and glycerol as a benign pore-forming additive. Structural, electrokinetic, and separation analyses reveal that solvent choice governs phase-inversion kinetics, pore size distribution, and surface charge, thereby controlling the permeability–selectivity balance. Chloroform-based membranes exhibit macrovoid-rich morphologies with broader pore size distributions, delivering higher water permeability (9.7 L·m⁻²·h⁻¹·bar⁻¹) while maintaining high divalent-ion rejection (up to 95% for MgSO₄). Dichloromethane induces denser skins, narrower pore size distributions, and more negative surface charge, enhancing steric and electrostatic exclusion at the expense of flux. The combined steric and Donnan effects explain the selective rejection of multivalent ions over monovalent salts. This work demonstrates that agro-residue-derived CTA membranes can achieve performance comparable to commercial NF membranes while offering a low-carbon, bio-based alternative, providing a viable pathway toward sustainable water treatment technologies.