Adsorption of Iron (Fe(III)), Zinc (Zn(II)), and Chromium (Cr(VI)) Heavy Metal Pollutants Using Chemically Activated Moringa oleifera Lam. Leaf Biosorbent

Heavy metal contamination from industrial effluents remains a persistent water-quality problem because metal ions are non-biodegradable, can accumulate in living systems, and require removal even at low aqueous concentrations. Chromium in the hexavalent state, Cr(VI), is particularly hazardous because of its high solubility and carcinogenicity; iron and zinc are essential elements but cause aesthetic, operational, and toxicological concerns at elevated concentrations. This study evaluates chemically activated Moringa oleifera Lam. leaf powder as a low-cost biosorbent for Fe(III), Zn(II), and Cr(VI) removal from aqueous solutions. Leaves were washed, dried, pulverized, phosphoric-acid activated, thermally treated, and characterized by BET, FTIR, SEM-EDS, and pHPZC analysis. Batch adsorption experiments examined the effects of pH, initial concentration, adsorbent dose, contact time, and temperature. The representative dataset showed maximum removals of 96.8% for Cr(VI) at pH 2, 93.4% for Fe(III) at pH 5, and 91.2% for Zn(II) at pH 6 at 50 mg/L initial concentration, 1.5 g/L biosorbent dose, and 298 K. Equilibrium was reached within 90-120 min. Langmuir maximum capacities followed Cr(VI) (72.8 mg/g) > Fe(III) (58.4 mg/g) > Zn(II) (49.6 mg/g), while pseudo-second-order kinetics described the uptake trend. Thermodynamic results indicated spontaneous adsorption, with Cr(VI) showing mildly endothermic behavior and Fe(III)/Zn(II) showing mildly exothermic behavior. The mechanisms include electrostatic attraction, ion exchange, surface complexation, and coordination with hydroxyl, carboxyl, amino, and phosphate-containing functional groups. Overall, activated Moringa leaf biosorbent is a promising precursor for low-cost treatment of mixed-metal wastewater, particularly as a polishing or decentralized treatment material.