(192c) Pyrolytic Treatment of Soils Contaminated with Polyaromatic Hydrocarbons (PAH): Kinetics and Reactor Design

Background and Motivation

The main objective of the Federal Superfund program, administered by the U.S. Environmental Protection Agency, is to study and remediate sites contaminated with hazardous substances. Cost-effective and reliable remediation of Superfund sites impacted by polyaromatic hydrocarbons (PAHs) and other persistent hazardous pollutants is of critical importance to mitigate exposure and protect public health. Soils and sediments contaminated with persistent PAH priority pollutants represent a long-term source of contaminant release into surface water, groundwater, edible biota and air exposure pathways. Such exposure pathways may be initiated by wind-blown PAH-contaminated dust, ingestion of contaminated sediments by benthic organisms that introduce PAHs into the food chain, or resuspension of contaminated sediments into the water column with subsequent ingestion by fish. Therefore, remediation of soils and sediments contaminated by PAHs and other recalcitrant lipophilic organic pollutants is often a Superfund site management priority.

Current remediation methods for soils contaminated with PAHs are either relatively slow or have unintended consequences in the form of soil damage and high-energy usage. Even worse, some processes such as aerobic bioremediation can activate these toxic hydrocarbons and transform them to more noxious PAH derivatives [1]. Thus, there is still a pressing need for more efficient and sustainable remediation of soils contaminated with PAHs and other recalcitrant lipophilic organic pollutants.

Over the past several years, our group has developed a novel pyrolytic method for ex-situ remediation of soils contaminated with heavy petroleum hydrocarbons that contain significant amounts of PAHs. This approach can rapidly and reliably remove total petroleum hydrocarbons (TPH) and PAHs with lower energy requirements and better post-treatment soil fertility than other ex-situ thermal remediation approaches [2-4]. For example, pyrolytic treatment of contaminated soils at 420^oC with only 15-min residence time in a continuous reactor reduced TPH by 99.9%, lowered the total PAH concentration by almost 95%, and restored soil fertility to 98% of the clean soil level [4]. These results clearly suggest that pyrolysis has the potential for improved ecosystem restoration following remediation compared to traditional thermal technologies.

However, our studies also revealed potential tradeoffs between pyrolytic treatment intensity, soil detoxification efficacy and soil fertility restoration. Specifically, treatment at 470^oC for 15- or 30-min reduced soil fertility to 51% and 39% of the clean soil level, which was only marginally higher that the fertility of the contaminated soil. The existence of an optimal treatment intensity for some contaminated soils underscores the need for a deeper understanding of the pyrolysis kinetics and the development of models that will allow us to tackle the multi-objective optimization problem of designing reactors that can:

• Detoxify the contaminated soil by removing PAHs and other pollutants;
• Restore the fertility of the treated soil; and
• Minimize the energy requirements.

Reactor Modeling and the Effect of Operating Conditions

Using the Distributed Activation Energy (DAE) kinetic model for the pyrolysis of petroleum-contaminated soils we have recently developed [5], we simulated the operation of a non-isothermal rotary kiln reactor that may be used for ex situ treatment of soils and sediments contaminated with PAHs. To preserve anoxic conditions, the reactor was externally heated while a stream of nitrogen gas flowed through the rotary kiln to sweep the desorbing hydrocarbons and pyrolysis products. A finite stage or mixing cell model with separate solid and gas phases was used to describe the operation of the pyrolysis reactor. Both co-current and countercurrent solid and gas flows were considered.

Model predictions were compared to experimental data obtained by treating two contaminated soils in a pilot-scale continuous rotary kiln reactor [4]. The two soils were contaminated with heavy petroleum crudes with total petroleum hydrocarbon content (TPH) of 14,000 and 18,000 mg/kg. The soils also contained significant amounts of the 16 EPA-regulated PAHs: 6,739 and 17,797 µg/kg respectively. The hydrocarbons were modeled with two DAE pseudo-components that react with activation energies distributed according to Gaussian probability density functions with means and standard deviations obtained from thermogravimetric experiments as shown in [5]. A third DAE pseudo-component was introduced to model the PAH fraction. The overall TPH and PAH reductions predicted by the model agreed very well with the experimental data for all the conditions considered: Pyrolysis temperatures of 370, 420 and 470^oC; reactor residence times of 15, 30 and 60 min; and solid flow rates of 7, 14, and 25 lb/h.

More importantly, however, our model was able to explain the trade-offs observed in our earlier study between pyrolysis treatment intensity, soil detoxification and soil fertility restoration [4]. The clay components of the soil used in [4] undergo significant dehydration between 350 and 450^oC, a range that overlaps with that of pyrolysis reactions. As contaminated soil moves through the isothermal reactor zone kept at 420^oC, the reaction rate for the heavy hydrocarbon fraction is fast enough to achieve almost complete conversion within 15 or 30 min. But, the same conditions are not severe enough to completely and irreversibly dehydrate the clay components of the soil. When the soil is treated at 470^oC, on the other hand, the clay components are almost completely dehydrated, causing irreversible soil damage and loss of fertility.

We will finally present results that extend our theoretical models to commercial-scale kiln reactors that may be used for ex-situ pyrolytic treatment of soils and sediments contaminated with PAHs and other recalcitrant lipophilic organic pollutants. Our focus here will be on providing guidelines for estimating the optimal operating conditions (pyrolysis temperature and residence time) for contaminated soils with various soil properties and pollutant content.

References

1. Chibwe, L.; Geier, M. C.; Nakamura, J.; Tanguay, R. L.; Aitken, M. D.; Simonich, S. L.,. Environmental Science & Technology 2015, 49, (23), 13889-98.
2. Vidonish, J. E., Zygourakis, K., Masiello, C. A., Gao, X., Mathieu, J., Alvarez, P. J.J., Environmental Science & Technology 2016, 50, (5), 2498-2506.
3. Vidonish, J. E., Alvarez, P. J. J., Zygourakis, K., Industrial & Engineering Chemistry Research 2018, 57, (10), 3489-3500.
4. Song, W.; Vidonish, J. E.; Kamath, R.; Yu, P. F.; Chu, C.; Moorthy, B.; Gao, B. Y.; Zygourakis, K.; Alvarez, P. J. J., Environmental Science & Technology 2019, 53, (4), 2045-2053.
5. Gao, Y., Zygourakis, K., Industrial & Engineering Chemistry Research, 2019, 58, 10829-10843.