Synthesis of chemically regenerable magnetic bio-nanocomposite adsorbents derived from agricultural waste for sustainable remediation of Pb(II), Cd(II), and Cr(VI) in contaminated water systems in the Niger delta

Abstract

Heavy metal contamination poses a critical threat to water security in oil-producing regions, particularly the Niger Delta where Pb(II), Cd(II), and Cr(VI) levels exceed World Health Organization guidelines by up to 500-fold. This study addresses this environmental challenge through the development and optimization of chemically regenerable magnetic bio-nanocomposite adsorbents synthesized from agricultural waste materials including rice husk, coconut shell, and plantain pseudostem. The integrated ternary nanocomposite, comprising nano-cellulose, nano-lignin, and Fe₃O₄ nanoparticles at an optimized 65:20:15 ratio, was systematically characterized using FTIR, XRD, SEM, TEM, BET, VSM, TGA, XPS, zeta potential, and EDX analysis. The optimized adsorbent demonstrated exceptional removal with maximum Langmuir monolayer capacities (Qm) of 385 mg/g for Pb(II), 312 mg/g for Cd(II), and 348 mg/g for Cr(VI) at 35 °C, and experimental equilibrium capacities (qe) of 95.5 mg/g, 77.2 mg/g, and 86.1 mg/g respectively at optimized batch conditions, surpassing commercial activated carbon. Kinetic modeling confirmed chemisorption as the dominant mechanism through pseudo-second-order kinetics (R² > 0.99) and Elovich model validation. Equilibrium data fitted the Langmuir isotherm, while thermodynamic parameters indicated spontaneous, endothermic adsorption. The nanocomposite exhibited excellent regenerability, maintaining > 75% capacity after ten 0.1 M HCl regeneration cycles (with > 80% retention maintained for the first 8 cycles), with magnetic separation enabling 98.5% recovery. Techno-economic and life cycle assessments confirmed economic viability and reduced environmental impact compared to conventional adsorbents. These findings demonstrate a sustainable, cost-effective approach for heavy metal remediation in resource-limited settings while advancing circular economy principles through agricultural waste valorization. Novel magnetic bio-nanocomposite adsorbents synthesized from rice husk, coconut shell, and plantain pseudostem agricultural waste. Maximum Langmuir monolayer adsorption capacities (Qm at 35 °C): 385 mg/g (Pb(II)), 312 mg/g (Cd(II)), and 348 mg/g (Cr(VI)), with experimental equilibrium capacities (qe) of 95.5, 77.2, and 86.1 mg/g respectively at optimized batch conditions. Chemical regeneration enabled 10 consecutive adsorption-desorption cycles with > 75% efficiency retention (75.8% for CSB-NC at cycle 10, with > 80% maintained for the first 8 cycles). RSM-CCD optimization achieved 96.2% Pb(II), 88.5% Cd(II), and 91.8% Cr(VI) removal simultaneously under validated global optimum conditions (pH 5.9, dosage 0.95 g/L, 24 °C). Low-cost production at $12.5/kg with 65% lower carbon footprint than commercial activated carbon.