Waste-to-Resource Approach for Heavy Metal Removal from Industrial Effluents

Abstract

Heavy metal contamination in industrial wastewater represents a critical environmental challenge requiring cost-effective and sustainable remediation technologies. Red mud, an alkaline waste by-product generated during bauxite processing in the Bayer process, has emerged as a promising low-cost adsorbent for heavy metal removal due to its high specific surface area, abundant hydroxyl groups, and iron oxide content. This research investigates the equilibrium adsorption behavior of lead (Pb²⁺), cadmium (Cd²⁺), and chromium (Cr⁶⁺) onto chemically modified red mud through comprehensive isotherm modeling studies. The study employs four classical isotherm models—Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich—to characterize the adsorption mechanism and predict equilibrium adsorption capacity under varying operational conditions including initial metal concentration (10-200 mg/L), adsorbent dosage (1-10 g/L), pH (2-9), temperature (298-328 K), and contact time (5-180 minutes). Red mud was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and Brunauer-Emmett-Teller (BET) surface area analysis. Results demonstrate that acid-activated red mud exhibits maximum adsorption capacities of 89.3 mg/g for Pb²⁺, 67.5 mg/g for Cd²⁺, and 54.2 mg/g for Cr⁶⁺ at pH 5.0 and 298 K. The Langmuir isotherm provided the best fit for Pb²⁺ and Cd²⁺ adsorption (R² > 0.98), indicating monolayer adsorption on homogeneous surface sites, while Cr⁶⁺ adsorption followed the Freundlich model (R² = 0.96), suggesting multilayer heterogeneous adsorption. Thermodynamic parameters revealed that adsorption processes were spontaneous and exothermic in nature. This study contributes to the sustainable utilization of industrial waste materials for environmental remediation while providing comprehensive equilibrium modeling that enables prediction and optimization of heavy metal removal in large-scale wastewater treatment applications.