Preparation of Polymeric Aluminum Chloride-Loaded Porous Carbon and Evaluation of Its Pb²⁺ Immobilization Mechanisms in Soil

In recent years, the remediation of heavy metal-contaminated soils has attracted great attention worldwide. Previous research on the removal of toxic heavy metals from wastewater effluents through adsorption by typical solid wastes (e.g., fly ash and coal gangue) has mainly focused on the control of wastewater pollutants. In this study, a coal gangue (CG) by-product from Hancheng City was used as a raw material to prepare polymeric aluminum chloride-loaded coal gangue-based porous carbon (PAC-CGPC) by hydrothermal synthesis. This material was subsequently employed to assess its performance in mitigating Pb²⁺ in soils. In addition, the effects of the pore structure of the prepared material on the adsorption rates, adsorption mechanisms, and plant root uptakes of soil Pb²⁺ were investigated in this study. The raw CG and prepared PAC-CGPC materials exhibited specific surface areas of 1.8997 and 152.7892 m²/g, respectively. The results of adsorption kinetics and isotherms indicate that the adsorption of Pb²⁺ based on PAC-CGPC mainly follows a pseudo-second-order kinetic model, suggesting that chemisorption may be the dominant process. In addition, the adsorption isotherm results showed that the Freundlich model explained better the adsorption process of Pb²⁺, suggests that the adsorption sites of lead ions on APC-CGPC are not uniformly distributed and tend to be enriched in APC, and also shows the ion exchange between aluminum and lead ions. The thermodynamic model fitting results demonstrated the occurrence of spontaneous and exothermic PAC-CGPC-based adsorption of Pb²⁺, involving ion exchange and surface complexation. The effects of the PAC-CGPC addition on soybean plants were further explored through pot experiments. The results revealed substantial decreases in the Pb²⁺ contents in the soybean organs (roots, stems, and leaves) following the addition of the PAC-CGPC material at a dose of 3% compared with the control and raw CG groups. Furthermore, the addition of the PAC-CGPC material at a dose of 3% effectively reduced the bioavailable Pb²⁺ content in the soil by 82.11 and enhanced soybean growth by 15.3%. These findings demonstrated the inhibition effect of the PAC-CGPC material on the translocation of Pb²⁺ in the soybean seedlings. The modified CG adsorbent has highly pore structure and good hydrophilicity, making it prone to migration in unsaturated soils and, consequently, enhancing Pb²⁺ immobilization. This research provides theoretical support for the development of CG-based materials capable of immobilizing soil pollutants.