KMnO₄-modified coal-based humic acid residue for Cd²⁺ removal: preparation, adsorption performance and mechanism

Currently, cadmium pollution poses a serious threat to human health and the ecological environment. In recent years, coal-based humic acid residue (HAS) has attracted widespread attention due to its adsorption properties. In this study, HAS was used as raw material to prepare KMnO₄ modified coal-based humic acid residue (K-HAS). Through adsorption experiments and characterization analyses (SEM, XPS, XRD, and FTIR), the adsorption process and mechanism of K-HAS toward Cd²⁺ in water were investigated. Through field trials, the effects of different K-HAS treatments on the physicochemical properties and cadmium content of cadmium-contaminated soil were investigated. The results indicate that the adsorption of Cd²⁺ by K-HAS follows quasi-second-order kinetics and the Langmuir adsorption model, with a maximum adsorption capacity of 121.61 ± 1.0665 mg·g^-1. The adsorption process is an exothermic, spontaneous reaction dominated by chemical adsorption; After five adsorption-desorption cycles, the removal efficiency of K-HAS for Cd²⁺ remained above 65%, demonstrating stable cyclic adsorption performance; the site energy distribution (SED) indicates that Cd²⁺ preferentially occupies high-energy adsorption sites on K-HAS before occupying low-energy sites, and increased temperature promotes the reaction; Field trials indicated that, compared with the control group, K-HAS treatment significantly reduced cadmium content in corn seeds (p < 0.05) by 10.67-20.60% and significantly reduced cadmium content in soil (p < 0.05) by 15.1-23.47%; the optimal application rate of K-HAS was 0.4 kg/m². In summary, K-HAS can serve as an effective adsorbent for treating cadmium pollution in water and soil.