Graded modified Fenton's (MF) oxidation is a strategy in which H 2 O 2 is added intermittently to prevent a sharp temperature increase and undesired soil sterilization at soil circumneutral pH versus adding the same amount of H 2 O 2 continuously.The primary objective of the present study was to investigate whether a mild MF pre-oxidation such as a stepwise addition of H 2 O 2 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 H 2 O 2 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 H 2 O 2 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 H 2 O 2 (1470 mmol/L,the mol ratio of H 2 O 2:Fe 2+ 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 (C 10 -C 40 ) 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.
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