Sustainable magnesium doped nanofibrillated cellulose/bentonite composite for enhanced phosphate removal: process optimization and mechanistic insights

Abstract

Excessive accumulation of phosphate anions in aquatic systems accelerates ecological degradation and threatens both environmental quality and public health. In this study, we developed magnesium-doped nanofibrillated cellulose/bentonite (Mg@NFC/BN) composite through a facile in situ modification, yielding a cellulose–clay hybrid with enhanced anion affinity. The optimized composite exhibited maximum phosphate adsorption capacity of 19.2 mg g⁻¹ and removal efficiency of 88.6%. The adsorption process attained equilibrium within 90 min, following pseudo-second-order kinetics and conforming to Langmuir isotherm behaviour, while thermodynamic analysis indicated a spontaneous and exothermic mechanism. The synthesized adsorbent remained effective across a wide pH range, with strong selectivity even in the presence of competing anions. Structural and surface analyses through point of zero charge (pH_PZC), XRD, FTIR, FESEM, EDX and XPS confirmed the successful synthesis of composite and incorporation of Mg, which strengthened electrostatic interactions and promoted inner-sphere ligand exchange with phosphate ions. Alkaline-assisted desorption enabled up to 90% phosphate release, and the adsorbent retained over 75% of its initial capacity even after seven cycles, demonstrating excellent stability and recyclability for nutrient recovery. Beyond water treatment, the phosphate-laden composites improved soil water retention, highlighting their potential for sustainable agricultural applications. Overall, this work expands the functional versatility of cellulose–clay composites, offering an environmentally benign strategy for integrated wastewater remediation and resource recovery.