Interpreting Interactions between Ozone and Residual Petroleum Hydrocarbons in Soil.

We evaluated how gas-phase O3 interacts with residual petroleum hydrocarbons in soil. Total petroleum hydrocarbons (TPH) were 18 ± 0.6 g/kg soil, and TPH carbon constituted ∼40% of the dichloromethane-extractable carbon (DeOC) in the soil. At the benchmark dose of 3.4 kg O3/kg initial TPH, TPH carbon was reduced by nearly 6 gC/kg soil (40%), which was accompanied by an increase of about 4 gC/kg soil in dissolved organic carbon (DOC) and a 4-fold increase in 5-day biochemical oxygen demand (BOD5). Disrupting gas channeling in the soil improved mass transport of O3 to TPH bound to soil and increased TPH removal. Ozonation resulted in two measurable alterations of the composition of the organic carbon. First, part of DeOC was converted to DOC (∼4.1 gC/kg soil), 75% of which was not extractable by dichloromethane. Second, the DeOC containing saturates, aromatics, resins, and asphaltenes (SARA), was partially oxidized, resulting in a decline in saturates and aromatics, but increases in resins and asphaltenes. Ozone attack on resins, asphaltenes, and soil organic matter led to the production of NO3-, SO42-, and PO43-. The results illuminate the mechanisms by which ozone gas interacted with the weathered petroleum residuals in soil to generate soluble and biodegradable products.