Optimizing synthetic schwertmannite for novel high-performance stabilization of fluoride contaminated soils

Soil fluoride contamination is considered a severe environmental problem. Schwertmannite (Sch) has unique structural properties and is highly efficient in fluoride adsorption. The synthesizing conditions of Sch can significantly influence its structure and adsorption properties. However, the relationship between Sch synthesis conditions-structure-fluoride adsorption performance has not been investigated. In this paper, we synthesized the HSch with the highest adsorption capacity at 1.5 times SO₄²⁻ content, pH = 2.5, and 15 °C by modulating the synthesis conditions (67.96 mg/g). The synthesis pH (52 %) was the main factor affecting the adsorption performance of Sch by using machine learning (ML). In this study, HSch was firstly applied to both high (Soil-H) and low (Soil-L) concentration fluoride contaminated soils, and the short-term stabilization and long-term stabilization (freeze-thaw cycles, wetting-drying alternation) effect of HSch was assessed. HSch addition had the most pronounced effect on reducing fluoride leaching concentration. Stabilization time had the greatest impact on the pH of Soil-H (40 %), while the HSch addition had the greatest impact on the pH of Soil-L (45 %). Compared with freeze-thaw cycles, the increase in wetting-drying alternations had a more significant effect on the immobilization of fluoride ions in Soil-H and Soil-L by Sch (70 % and 74 %, respectively). Additionally, this study revealed that the fluoride adsorption by HSch is a chemisorption process characterized by monolayer adsorption. This process primarily involves the dissociation of surface-OH groups, releasing H⁺ ions. These H⁺ ions react with OH⁻ in Soil-H and Soil-L, lowering the soil pH. The reduced pH promotes the reaction between HF and SO₄²⁻, thereby enhancing fluoride immobilization. This study provides strategic insights into the application of Sch for the remediation of fluoride-contaminated soils.