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

Abstract

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 systemspecific 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.

Kinetic Study of the Pyrolytic Treatment of Petroleum-Contaminated Soils

“…Cell viability assays revealed that pyrolytic treatment effectively detoxified the contaminated soil, indicating that pyrolysis does not have the residual toxicity drawback of some aerobic bioremediation methods. 12 At the same time, this study 24 revealed an interesting tradeoff. While TPH and PAH removal levels improved with increasing treatment intensity 25 (i.e., higher temperatures and longer reactor residence times), the fertility of treated soil first improved and then declined when the treatment intensity exceeded a soil-specific threshold.…”

“…In a recently published article, we reported results from the first pilot-scale study of pyrolytic soil treatment performed in a continuous rotating kiln reactor. 24 Pyrolysis at 420 °C with 15 min residence time achieved almost complete TPH removal (99.9%), removed almost 95% of the PAHs present in the contaminated soil, and restored the fertility of treated soil to almost clean soil levels. Cell viability assays revealed that pyrolytic treatment effectively detoxified the contaminated soil, indicating that pyrolysis does not have the residual toxicity drawback of some aerobic bioremediation methods.…”

“…After the clean soil was dried, homogenized, and sieved to remove large particles, it was blended with a heavy crude oil (21°API, Chevron ETC, Houston, TX) to prepare contaminated soil A with 3 wt % oil concentration. 24 Contaminated soil B was prepared by spiking an Arizona topsoil with oil sludge from a crude oil well site. 15 On the basis of X-ray diffraction, the composition of background soil B was 12% clays, 10% carbonates, 31% quartz, 20% K-spar, 26% plagioclase, and 1% pyrite.…”

“…However, a fertility "maximum" was not observed in our earlier studies, which used different background soils than the one used for our recent work. 24 This raises an obvious question: Which system properties or interactions are responsible for the observed trade-off between detoxification effectiveness and fertility restoration? At the very minimum, the existence of an optimal treatment intensity for some soils emphasizes the importance of accounting for differences in the composition of soils when designing a pyrolytic remediation process.…”

Thermochemical Recycling of Oily Sludge by Catalytic Pyrolysis: A Review

“…Hydrocarbons will generate free radicals due to thermal activation at around 400°C, and a series of free radical reactions will occur. On the one hand, it will proceed towards the cracking of small-molecule hydrocarbons while it proceed towards the condensation direction of coking and finally produce oil, water, noncondensable gas, and coke [ 19 ]. In comparison, the pyrolysis method has the advantages of faster processing speed, more thorough oily sludge treatment, and recyclable oil and gas and residues.…”