Enhanced long-term oxidative remediation of calcium sulfate encapsulated KMnO₄ for phenol-contaminated soil

To develop a controlled-release material for in-situ chemical oxidation (ISCO) in soil remediation, KMnO₄ encapsulated by calcium sulfate (KMnO₄@CaSO₄) was prepared in this study via a rotary granulation process, which combines slow-release with enhanced oxidation capabilities. Characterization (XRD, FTIR, SEM-EDS) confirmed the successful physical encapsulation of KMnO₄ within a hydrated CaSO₄matrix without chemical reaction. The release lifetime of MnO₄^- from KMnO₄@CaSO₄ in the quartz sand column was approximately 17-fold longer than that of raw KMnO₄. And the release kinetics of MnO₄^- were well described by a Fickian diffusion model (R²>0.9). MnO₄^- was sustainably released through the interconnected pore network generated by Ca²⁺ dissolution from the hydrated CaSO₄ shell. In dynamic quartz sand column degradation tests, KMnO₄@CaSO₄ extended the complete phenol removal time 11-fold (60 h vs. 5 h) and increased the cumulative degradation by approximately 7-fold (54 mg vs. 6.7 mg), compared to raw KMnO₄. Meanwhile, Ca²⁺ dissolved from KMnO₄@CaSO₄ enhanced the oxidation of phenol by MnO₄^-. Density Functional Theory calculations confirmed that Ca²⁺ coordination with phenol weakens the phenolic O-H bond and reduces the reaction energy barrier from 17.94 to -16.03 kcal mol^-1. In soil column experiments, KMnO₄@CaSO₄ effectively suppressed the phenol rebound seen with raw KMnO₄ and demonstrated sustained oxidative activity by maintaining phenol concentrations below the detection limit for over 56 h. After treatment, the soil properties (pH, organic matter, enzyme activities) recovered to uncontaminated baselines, and some bacteria (e.g., Pseudomonadota and Bacillota) were enriched.