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.

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.

Remediation of heavy hydrocarbon impacted soil using biopolymer and polystyrene foam beads.

A green chemistry solution is presented for the remediation of heavy hydrocarbon impacted soils. The two-phase recovery system relies on a plant-based biopolymer, which releases hydrocarbons from soil, and polystyrene foam beads, which recover them from solids and water. The efficiency of the process was demonstrated by comparisons with control experiments, where water, biopolymer, or beads alone yielded total petroleum hydrocarbon (TPH) reductions of 25%, 52%, and 58%, respectively, compared to 94% when 1.25 mL of 1% biopolymer and 15 mg beads per gram of soil were agitated for 30 min. Reductions in TPH content were substantial regardless of soil fraction, with removals of 97%, 91%, and 75% from sand, silt, and clay size fractions, respectively. Additionally, treatment efficiency was independent of carbon number, C13 to C43, as demonstrated by reductions in both diesel fuel (C13-C28) and residual-range organics (C25-C43) of ∼90%. Compared to other published polymer- and surfactant-based treatment methods, this system requires less mobilizing agent, sorbent, and mixing time. The remediation process is both efficient and sustainable because the biopolymer is re-useable and sourced from renewable crops and polystyrene beads are obtained from recycled materials.

Impacts of moisture content during ozonation of soils containing residual petroleum.

We tested the effect of soil moisture content on the efficiency of gas-phase ozonation for two types of soils containing residual petroleum. For the first soil (BM2), having a total petroleum hydrocarbons (TPH) concentration of 18,000mg/kg soil, a moisture content of 5% benefited oxidation, giving the highest efficiency of ozonation for TPH removal and for producing soluble and biodegradable products. In contrast, higher moisture content hindered O3 from oxidizing reactive materials in the second soil (BM3), which had a higher TPH concentration, 33,000mg/kg soil. This trend was documented by less TPH removal, less generation of soluble and biodegradable organic products, and a carbon balance that showed retarded carbon oxidation. An unexpected phenomenon was smoldering during ozonation of air-dried (<1% moisture) BM3, which did not occur with the same moisture conditions for BM2. BM3 smoldered was due to its higher TPH content, low heat buffering, and more release of volatiles with low self-ignition points. Smoldering did not occur for ≥ 5% water content, as it suppressed the temperature increase needed to volatilize the organics that initiated smoldering. The findings underscore the importance of controlling water content during ozonation to optimize the effectiveness of ozonation and prevent smoldering.

Effect of simulated rhizodeposition on the relative abundance of polynuclear aromatic hydrocarbon catabolic genes in a contaminated soil.

Microcosms were used to investigate whether soil exposure to mulberry root extracts (rich in phenolic compounds) select for bacteria that degrade polynuclear aromatic hydrocarbons (PAHs). Unlike previous studies with freshly spiked soil, the present experiments were conducted with soils aged for 518 d with [14C]phenanthrene to decrease bioavailability and avoid exaggerating the selective pressure exerted by PAHs relative to the rhizosphere effect. Microcosms simulating contaminated planted soil were exposed to carbon at 20 mg/L/week of mulberry root extract for 211 d to simulate rhizodeposition. Contaminated bulk soils microcosms were amended with a C-free mineral medium to discern the effect of rhizodeposition. Uncontaminated soil controls also were exposed to similar dose regimes. Real-time quantitative polymerase chain reaction was used to enumerate total bacteria and PAH degraders harboring the genes nahAc (coding for naphthalene dioxygenase), todC1 (coding for toluene/benzene/chlorobenzene dioxygenase), bmoA (coding for hydroxylating monooxygenases), and dmpN (coding for phenol hydroxylase). Exposure to root extracts enhanced the growth of total bacteria and PAH degraders in both contaminated and uncontaminated rhizosphere microcosms. The relative abundance of PAH-degrader gene copies (as a fraction of the total bacteria) was similar for different treatments, suggesting that the root extracts did not select for PAH degraders. Overall, these results suggest that rhizodeposition from phenolic releasers contributes to the fortuitous (but not selective) proliferation of PAH degraders, which may enhance phytoremediation.

Review of quantitative surveys of the length and stability of MTBE, TBA, and benzene plumes in groundwater at UST sites.

Quantitative information regarding the length and stability condition of groundwater plumes of benzene, methyl tert-butyl ether (MTBE), and tert-butyl alcohol (TBA) has been compiled from thousands of underground storage tank (UST) sites in the United States where gasoline fuel releases have occurred. This paper presents a review and summary of 13 published scientific surveys, of which 10 address benzene and/or MTBE plumes only, and 3 address benzene, MTBE, and TBA plumes. These data show the observed lengths of benzene and MTBE plumes to be relatively consistent among various regions and hydrogeologic settings, with median lengths at a delineation limit of 10 µg/L falling into relatively narrow ranges from 101 to 185 feet for benzene and 110 to 178 feet for MTBE. The observed statistical distributions of MTBE and benzene plumes show the two plume types to be of comparable lengths, with 90th percentile MTBE plume lengths moderately exceeding benzene plume lengths by 16% at a 10-µg/L delineation limit (400 feet vs. 345 feet) and 25% at a 5-µg/L delineation limit (530 feet vs. 425 feet). Stability analyses for benzene and MTBE plumes found 94 and 93% of these plumes, respectively, to be in a nonexpanding condition, and over 91% of individual monitoring wells to exhibit nonincreasing concentration trends. Three published studies addressing TBA found TBA plumes to be of comparable length to MTBE and benzene plumes, with 86% of wells in one study showing nonincreasing concentration trends.

Effect of root-derived substrates on the expression of nah-lux genes in Pseudomonas fluorescens HK44: implications for PAH biodegradation in the rhizosphere.

The bioluminescent reporter strain Pseudomonas fluorescens HK44 with a nah-lux fusion, was used to investigate the effect of root material (from hybrid poplars, willow, kou, milo, Osage orange, mulberry, and switch grass) and potential root-derived substrates (e.g., sugars, carboxylic acids, amino acids, and phenolics) on the expression of nahG, one of the genes responsible for naphthalene dioxygenase transcription. Whereas nahG was induced by some phenolic substrates that could be released by plants (i.e., salicylate, methyl salicylate, and acetyl salicylate), no induction by root extracts was observed. Rather, increasing root extract concentrations (50 to 275 mg L(-1) as total organic carbon) inhibited nahG expression in assays with cells concurrently exposed to naphthalene. Root extracts also decreased nahG expression at the individual cell level during naphthalene degradation assays. However, treatments with root extracts exhibited significantly higher microbial growth and overall bioluminescence, indicating a higher level of nahG expression by the resulting larger microbial population. This generally resulted in faster naphthalene degradation rates, suggesting that plant-promoted proliferation of competent genotypes could compensate for the interference that labile substrates exert on the expression of genes that code for the degradation of polynuclear aromatic hydrocarbons (PAHs). This could explain the faster PAH degradation commonly reported in planted than in unplanted soils.