Enhanced bioremediation of oil contaminated soil by graded modified Fenton oxidation

Graded modified Fenton's (MF) oxidation is a strategy in which H₂O₂ is added intermittently to prevent a sharp temperature increase and undesired soil sterilization at soil circumneutral pH versus adding the same amount of H₂O₂ continuously. The primary objective of the present study was to investigate whether a mild MF pre-oxidation such as a stepwise addition of H₂O₂ can prevent sterilization and achieve a maximum degradation of tank oil in soil. Optimization experiments of graded MF oxidation were conducted using citric acid, oxalic acid and SOLV-X as iron chelators under different frequencies of H₂O₂ addition. The results indicated that the activity order of iron chelates decreased as: citric acid (51%) > SOLV-X (44%) > oxalic acid (9%), and citric acid was found to be an optimized iron chelating agent of graded MF oxidation. Three-time addition of H₂O₂ was found to be favorable and economical due to decreasing total petroleum hydrocarbon removal from three time addition (51%) to five time addition (59%). Biological experiments were conducted after graded MF oxidation of tank oil completed under optimum conditions mentioned above. After graded oxidation, substantially higher increase (31%) in microbial activity was observed with excessive H₂O₂ (1470 mmol/L, the mol ratio of H₂O₂:Fe²⁺ was 210:1) than that of non-oxidized soil. Removal efficiency of tank oil was up to 93% after four weeks. Especially, the oil fraction (C₁₀-C₄₀) became more biodagradable after graded MF oxidation than its absence. Therefore, graded MF oxidation is a mild pretreatment to achieve an effective bioremediation of oil contaminated soil.