Multifunctional CaO-enabled persulfate activation for pentachlorophenol degradation in contaminated soil: Calcium-mediated transformation and oxidative enhancement

Pentachlorophenol (PCP) is a persistent chlorinated organic pollutant with high toxicity, strong environmental stability, and resistance to biodegradation. Although persulfate-based oxidation is promising for PCP remediation, its performance in contaminated soil is often limited by complex soil matrices and unclear activation mechanisms. In this study, CaO-activated persulfate was applied to PCP degradation in actual contaminated soil, with emphasis on synergistic alkaline–thermal activation and calcium-associated PCP transformation. CaO significantly enhanced PCP degradation, achieving 86.0% removal after 7 d, whereas persulfate alone showed limited oxidation capability. Mechanistic analysis showed that CaO promoted PCP degradation through Ca(OH) 2 -induced alkaline activation, hydration-induced transient thermal activation, and Ca 2 + -mediated formation of calcium-associated pentachlorophenolate species (PCP-Ca). PCP-Ca formation reduced the apparent degradation activation energy from 112.8 to 92.3 kJ mol −1 and increased the apparent pseudo-first-order rate constant by 62.7% at 30 °C, indicating enhanced oxidative susceptibility. SO 4 • - and •OH were identified as the main reactive species. LC-MS analysis suggested that PCP degradation proceeded through hydroxylation, dechlorination, and ring-cleavage pathways, generating intermediates with lower predicted toxicity. This study reveals the multifunctional role of CaO in persulfate activation and provides mechanistic insight into PCP degradation in contaminated soil.