Calcined coal gangue-derived Mg- and Ca-based layered double hydroxides for Cr(VI) soil remediation: Experiments and...

Layered double hydroxides (LDHs), including magnesium-aluminum-type (Mg-C-LDH) and calcium-aluminum-type (Ca-C-LDH), were synthesized from coal gangue and calcined at 500 °C to form layered double oxides (LDOs). Both materials exhibited effective Cr(VI) immobilization in contaminated soils. Under optimal conditions, magnesium-based material achieved a maximum immobilization efficiency of 95.64 %, whereas calcium-based material reached 48.65 %. Magnesium-based material showed superior performance due to electrostatic adsorption, anion exchange, and partial reduction of Cr(VI) to Cr(III). In contrast, calcium-based material immobilized Cr(VI) mainly through Ca²⁺ release and precipitation with carbonate and chromate species, forming surface-associated products without significant reduction contribution. To further identify controlling factors, four machine learning models-k-Nearest Neighbors, Random Forest, Backpropagation Neural Network, and eXtreme Gradient Boosting -were established. XGBoost exhibited the highest predictive accuracy, with an R² of 0.999 and a root mean square error of 0.001. Shapley Additive Explanations analysis indicated that reaction time and initial Cr(VI) concentration were the dominant drivers in both systems. In the Mg-C-LDO system, pH played a notable role, whereas temperature and dosage exerted greater influence in the Ca-C-LDO system. Correlation heatmaps further revealed variable interactions, suggesting that distinct factor couplings governed immobilization efficiency and mechanistic pathways. Overall, Mg-C-LDO demonstrated stronger immobilization capacity than Ca-C-LDO and highlights the potential for high-value utilization of coal gangue while integrating mechanistic insights with interpretable machine learning to provide predictive guidance for Cr(VI) remediation in soils.