Heavy Metal Removal from Aqueous Media Using Lignocellulosic Waste Material (Corn Stalk)
Abstract
Chromium showed peak adsorption at pH 2 (67%), consistent with electrostatic attraction of chromate anions to protonated surface sites, while the four cationic metals performed best at pH 6. Unlike many lignocellulosic biosorbents, equilibrium data for all five metals including Cr(VI) conformed to the Langmuir isotherm model (R² = 0.957–0.998), indicating predominantly monolayer adsorption on a comparatively homogeneous set of binding sites across the tubular, lignocellulosic COS matrix. The PseudoSecond-Order kinetic model provided an excellent fit for all metals (R² = 0.997–0.999), with calculated equilibrium capacities closely matching experimental values, implicating chemisorption as the dominant rate-controlling mechanism. Thermodynamic analysis yielded negative ΔG° (−0.414 to −0.731 kJ/mol), negative ΔH°, and positive ΔS° values for all five metals, confirming that adsorption is spontaneous, mildly exothermic, and entropically driven. Scanning electron micrographs showed the characteristic rough tubular pre-adsorption texture becoming densely aggregated after metal loading, while FTIR spectroscopy identified hydroxyl, carboxylic, thiol, and aliphatic C–H groups as primary binding sites, with post-adsorption shifts in the O–H stretching region confirming metal–ligand complexation. These findings establish COS as a high-performing, and readily available biosorbent for metal water remediation.