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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.