Synergistic enhancement of uranium(Ⅵ) immobilization by humin—iron hydroxide composites: Performance and mechanisms

Composite systems of soil organic matter and iron hydroxides hold significant potential for the remediation of heavy metal pollution. However, the synergistic immobilization mechanisms of U(Ⅵ) by humin (HM)—iron hydroxide composites remain insufficiently understood. In this study, goethite (Ge)—HM and magnetite (Mag)—HM composites with varying C-to-Fe molar ratios were synthesized via co-precipitation, and their adsorption behavior and underlying mechanisms towards U(Ⅵ) in aqueous solutions were systematically investigated. The results showed that under conditions of a pH value of 5.0 and temperature of 298 K, the maximum U(Ⅵ) adsorption capacities of Ge—HM and Mag—HM reached 60.56 mg/g and 64.12 mg/g, respectively, significantly surpassing those of pure Ge (39.6 mg/g), Mag (22.5 mg/g), and HM (20.2 mg/g). Kinetic studies revealed that the adsorption processes for both Ge—HM and Mag—HM conformed to the pseudo-second-order kinetic model, indicating chemical reaction control. Isothermal adsorption results were well described by the Langmuir model, suggesting monolayer adsorption on the composite surfaces. Crucially, characterization results elucidated distinct reaction mechanisms. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses revealed that U(Ⅵ) primarily complexed with oxygen-containing functional groups, such as carboxyl and hydroxyl groups, on both composite surfaces, leading to stable immobilization. For Mag—HM, a notable reduction of U(Ⅵ) to U(Ⅳ) was observed, with XPS U 4f spectra showing characteristic U(Ⅳ) peaks, indicating a significant reductive immobilization pathway facilitated by the presence of both Mag and HM. In contrast, for the Ge—HM composite with a C-to-Fe ratio of 0.5, the complete consumption of Fe(Ⅱ) after U(Ⅵ) adsorption, with its proportion in total Fe decreasing from 53.7% to zero, strongly suggested that Fe(Ⅱ) played a key role in electron transfer during the immobilization process. This study elucidates the synergistic enhancement mechanisms of HM—iron hydroxide composites in U(Ⅵ) removal, providing new insights for developing green and efficient materials for the remediation of heavy metal pollution.