Effects of ozonation on soil organic matter of contaminated soil containing residual oil

Ozonation has been shown to be a feasible method for removal of organic pollutants resistant to biodegradation in contaminated soils. However, little is known about the impact of ozonation on soil organic matter (SOM). This study was conducted to investigate the change in SOM characteristics and the consequent change in sequestration of contaminants in soil during ozonation. Contaminated soil containing residual oil at 27.0 mg/g was ozonated in laboratory-scale columns for 2, 4, 6, 10, and 15 h. The SOM in these soils was fractionated into dissolved organic matter (DOM), humic aids (HA), fulvic acids (FA), and humin (HM). SOM and its fractions were characterized by content measurement, elemental analysis, and/or spectroscopy analysis. Residual oils binding in different SOM fractions were extracted and identified. Phenanthrene sorption isotherms were obtained for the soil and humin-mineral fraction (HMF) of soil. Ozonation for 15 h reduced the SOM content by 45.2% and caused a change in SOM fractions. HM decreased during the 15 h of ozonation while DOM, HA, and FA increased in the first 10, 4, and 4 h of ozonation, respectively, but then decreased. Ozonation reduced the residual oil content in soil and changed its binding state in the SOM fractions. HMF was the primary region in which residual oil was bound (similar to 80%) in unozonated soil, and the percentage of residual oil binding in HMF decreased during the first 6 h but increased during 6-15 h of ozonation. An oxidation-desorption-oxidation interaction mechanism among SOM, residual oil, and ozone was proposed based on these results. Moreover, ozonation weakened the sorption of phenanthrene in the soil and HMF, which may have been caused by the decrease of SOM and residual oil contents and increase of polar oxidation intermediates. The contribution of sorption onto HMF to overall sorption in the soil also showed a decrease-increase trend during ozonation. Ozonation reduced both residual oil and SOM and caused consequent changes in binding state of residual oil in SOM fractions and sorption properties for hydrophobic organic compounds in ozonated soils. The decreased content of HM and reduced contribution of HM to the overall sequestration of organic compounds in soil treated with certain levels of ozone would improve the bioavailability of residual oil after ozonation.