Comprehensive Performance Assessment and Mechanistic Insights into Zero-Valent Iron/Ferrous Hydroxide Complex Composites for Optimized Se(IV) Sequestration in Aqueous Media

Abstract ⁷⁹Se is one of the key fission products in spent fuel, with a half - life of 6.5×10⁴ a. Owing to its exceptionally high mobility and extremely low solubility, this isotope can effortlessly permeate into groundwater, resulting in widespread contamination. This type of pollution poses an extremely grave threat to both ecosystems and human well-being. Therefore, the long-term safe fixation and disposal of ⁷⁹Se has become a challenging and cutting-edge topic in research. In this study, Ferrous Hydroxide Complex (FHC) was employed as a carrier to explore its effectiveness in removing selenium (IV). Zero-valent iron/Ferrous Hydroxide Complex (Fe⁰/FHC) was synthesized using simple and cost-effective methods, and their performance in selenium (IV) removal was evaluated. The findings demonstrated that, under specific conditions-pH 7.0, a 40-min adsorption period, 35% Fe⁰ content, and a ratio of solid to liquid that was 0.20 g L⁻ ¹, the Fe⁰/FHC composite achieved a Se(IV) adsorption capacity of 201.62 mg g⁻ ¹. Kinetic analysis showed that adsorption behavior matches the pseudo-second-order model, suggesting that the chemical process was the dominant adsorption mechanism. Isothermal adsorption modeling further showed that the Langmuir model provided a better fit, suggesting that the adsorption predominantly occurred through monolayer coverage. Thermodynamic investigations indicated that the adsorption of Se(IV) by Fe⁰/FHC was an exothermic reaction. Further analysis via FT-IR and XPS showed that Fe⁰/FHC's removal of Se(IV) involves electrostatic adsorption, complexation, and reduction precipitation processes. Overall, the Fe⁰/FHC composite offered distinct benefits, including ease of synthesis, a large specific surface area, and exceptional adsorption capacity. These characteristics endowed Fe⁰/FHC with great potential in treating selenium-containing wastewater. Consequently, as a new adsorption material, it held broad application prospects in this field.