Mechanistic and Kinetic Insights into Time-Dependent Fenton Oxidation Pathways for Polycyclic Aromatic Hydrocarbon Degradation in Crude Oil–Contaminated Soils

Abstract

Despite the widespread application of Fenton oxidation for hydrocarbon remediation, the time-dependent kinetic transition between matrix-limited and radical-dominated degradation regimes in crude oil–contaminated soils remains insufficiently resolved. This study systematically investigated long-term (60-day) PAH degradation under controlled batch Fenton oxidation using Fe²⁺-activated H₂O₂ at concentrations of 5–40 ppm. The degradation process was evaluated relative to natural attenuation. GC–MS quantification was integrated with pseudo-first-order kinetic modeling to elucidate the degradation dynamics. Natural attenuation produced moderate PAH reduction (≈42% in crude oil matrix and 85.7% in contaminated soil), reflecting matrix-governed constraints on bioavailability and oxidative transformation. Fenton treatment significantly enhanced degradation in a concentration-dependent manner, achieving >99.4% removal at 40 ppm. Apparent rate constants remained comparable at 5–30 ppm (0.0290–0.0317 day⁻¹) but increased markedly at 40 ppm (0.0853 day⁻¹; R² = 0.970; p < 0.05), indicating a statistically significant kinetic regime shift. This transition suggests reduced scavenging dominance and improved hydroxyl radical availability rather than simple linear oxidant acceleration. Degradation behavior was consistent with established hydroxyl radical–mediated pathways involving aromatic hydroxylation and progressive ring destabilization. By quantitatively identifying an oxidant threshold beyond which radical control supersedes matrix limitation, this study advances predictive understanding of dose-dependent Fenton reactivity in heterogeneous soil systems and provides mechanistically informed guidance for optimizing PAH remediation strategies.