Impact of Chemical Oxidation on Soil Organic Carbon Dynamics and Sequestration in Crude Oil-Contaminated Soils Using H₂O₂ Systems

Chemical oxidation using hydrogen peroxide (H₂O₂) and the combined H₂O₂/sodium persulfate ( Na₂S₂O ₈) system is widely applied for in situ remediation of petroleum-contaminated soils; however, its collateral impact on native soil organic carbon (SOC) dynamics and sequestration potential remains poorly understood in tropical African contexts. This laboratory study evaluated these impacts at three artisanal crude oil-contaminated farmland sites , Ele-Iga (CS1), Ula- Ugbobi (CS2), and Ilo (CS3) , in Idu-Ekpeye , Ahoada West Local Government Area, Rivers State, Nigeria, relative to an uncontaminated reference at Ugbobi Community Secondary School. Soils were subjected to H₂O₂ alone (H) and combined H₂O₂/ Na₂S₂O ₈ (HP) treatments over 28 days, with sampling at Days 0, 7, 14, and 28. Baseline total petroleum hydrocarbon (TPH) concentrations ranged from 15,920 to 22,380 mg/kg, substantially exceeding the 5,000 mg/kg intervention threshold. Both treatments achieved meaningful TPH removal (H: 36.06–36.48%; HP: 54.78–56.38%), but simultaneously produced significant SOC losses of 34.1–36.6% (H) and 49.4–57.0% (HP). The HP treatment preferentially destroyed humic acid carbon (HA-C), collapsing the humification ratio to below unity at all sites (minimum 0.62 at CS2) and reducing the carbon management index (CMI) to 19.18–29.44, representing reductions of 70.6–80.8% below the uncontaminated reference. Pearson correlation analysis confirmed strong negative cross-site relationships between SOC and TPH removal (r = −0.974, p < 0.01) and between CMI and TPH removal (r = −0.981, p < 0.01). These findings establish a fundamental remediation–sequestration trade-off and recommend integrated chemo-bioremediation, post-treatment liming, and biochar amendment to reconcile hydrocarbon removal with soil carbon sequestration restoration in Niger Delta contaminated soils.