Study on the adsorption performance of coal gangue-loaded nano-FeOOH for the removal of Pb²⁺, Cu²⁺, and Cd²⁺ from acid mine drainage

In view of the serious pollution caused by heavy metals such as Pb²⁺, Cu²⁺ and Cd²⁺ present in acid mine drainage (AMD) and the difficulty in their treatment, based on the stable mineral skeleton structure of coal gangue and the good adsorption of nano-FeOOH, coal gangue-loaded nano-FeOOH (nFeOOH-CG) was successfully prepared <i>via</i> chemical precipitation. The effects of nFeOOH-CG on the removal of Pb²⁺, Cu²⁺ and Cd²⁺ from AMD were systematically investigated under different adsorbent dosage, initial pH, reaction time and initial concentration conditions. The adsorption mechanism of nFeOOH-CG was revealed using the adsorption isotherm, adsorption kinetics, adsorption thermodynamic model, SEM-EDS, XRD, TEM, FTIR spectroscopy and BET characterization. Results indicated that the rate of Pb²⁺, Cu²⁺ and Cd²⁺ removal by nFeOOH-CG reached 96.85%, 88.38% and 73.1%, respectively, under the conditions of 5 g per L dosage; pH 4; 150 min reaction time and 100 mg per L initial concentration of Pb²⁺, Cu²⁺ and Cd²⁺, which were significantly better than those of unmodified coal gangue. The process of Pb²⁺, Cu²⁺ and Cd²⁺ adsorption by nFeOOH-CG conforms to the Langmuir and pseudo-second-order kinetic models, indicating that the adsorption mechanism mainly involves monolayer and chemical adsorption. The adsorption of Pb²⁺ and Cu²⁺ by nFeOOH-CG is a spontaneous, endothermic and entropy-increasing process, while the adsorption of Cd²⁺ is a non-spontaneous, endothermic and entropy-increasing process. Characterization analysis showed that the specific surface area of nFeOOH-CG was 103.68 m² g^-1, which was 13.22 times higher than that of coal gangue. The loading of nFeOOH significantly increased the specific surface area and surface active sites of coal gangue. The adsorption mechanism of nFeOOH-CG on Pb²⁺, Cu²⁺ and Cd²⁺ was mainly attributed to the increased specific surface area, surface complexation, electrostatic attraction and ion exchange. This study provides a theoretical basis and technical reference for the efficient resource utilization of coal gangue and the environmental remediation of AMD.