Biodegradation of petroleum hydrocarbons in a weathered, unsaturated soil is inhibited by peroxide oxidants.

Field-weathered crude oil-containing soils have a residual concentration of hydrocarbons with complex chemical structure, low solubility, and high viscosity, often poorly amenable to microbial degradation. Hydrogen peroxide (H2O2)-based oxidation can generate oxygenated compounds that are smaller and/or more soluble and thus increase petroleum hydrocarbon biodegradability. In this study, we assessed the efficacy of H2O2-based oxidation under unsaturated soil conditions to promote biodegradation in a field-contaminated and weathered soil containing high concentrations of total petroleum hydrocarbons (25200 mg TPH kg-1) and total organic carbon (80900 mg TOC kg-1). Microcosms amended with three doses of 48 g H2O2 kg-1 soil (unactivated or Fe2+-activated) or 24 g sodium percarbonate kg-1 soil and nutrients did not show substantial TPH changes during the experiment. However, 7.6-41.8% of the TOC concentration was removed. Furthermore, production of DOC was enhanced and highest in the microcosms with oxidants, with approximately 20-40-fold DOC increase by the end of incubation. In the absence of oxidants, biostimulation led to > 50% TPH removal in 42 days. Oxidants limited TPH biodegradation by diminishing the viable concentration of microorganisms, altering the composition of the soil microbial communities, and/or creating inhibitory conditions in soil. Study's findings underscore the importance of soil characteristics and petroleum hydrocarbon properties and inform on potential limitations of combined H2O2 oxidation and biodegradation in weathered soils.

Pilot-Scale Pyrolytic Remediation of Crude-Oil-Contaminated Soil in a Continuously-Fed Reactor: Treatment Intensity Trade-Offs.

Pyrolytic treatment offers the potential for the rapid remediation of contaminated soils. However, soil fertility restoration can be highly variable, underscoring the need to understand how treatment conditions affect soil detoxification and the ability to support plant growth. We report here the first pilot-scale study of pyrolytic remediation of crude-oil-contaminated soil using a continuously fed rotary kiln reactor. Treatment at 420 °C with only 15 min of residence time resulted in high removal efficiencies for both total petroleum hydrocarbons (TPH) (99.9%) and polycyclic aromatic hydrocarbons (PAHs) (94.5%) and restored fertility to clean soil levels (i.e., Lactuca sativa biomass dry weight yield after 21 days increased from 3.0 ± 0.3 mg for contaminated soil to 8.8 ± 1.1 mg for treated soil, which is similar to 9.0 ± 0.7 mg for uncontaminated soil). Viability assays with a human bronchial epithelial cell line showed that pyrolytic treatment effectively achieved detoxification of contaminated soil extracts. As expected, TPH and PAH removal efficiencies increased with increasing treatment intensity (i.e., higher temperatures and longer residence times). However, higher treatment intensities decreased soil fertility, suggesting that there is an optimal system-specific intensity for fertility restoration. Overall, this study highlights trade-offs between pyrolytic treatment intensity, hydrocarbon removal efficiency, and fertility restoration while informing the design, optimization, and operation of large-scale pyrolytic systems to efficiently remediate crude-oil-contaminated soils.

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.