(46d) Pyrolytic Remediation of Oil-Contaminated Soils: Reaction Mechanisms and Treated Soil Fertility

Thermal treatment technologies are generally effective for the remediation of soils contaminated with heavy hydrocarbons, with removal commonly over 99%. However, these technologies frequently employ high temperatures and oxidative conditions that may damage soil integrity, ultimately impairing soil fertility. As a result, contaminated soils that have been remediated with thermal techniques often cannot sustain vegetation and can only be used as backfill in construction projects or sent to landfills. Thus, full ecosystem restoration of petroleum-contaminated sites is hard to achieve with current thermal remediation processes.

We recently proposed the use of pyrolysis as a thermal remediation option that can restore the agricultural value of contaminated soils and facilitate ecosystem recovery [1]. In this earlier study, we pyrolyzed at 420°C two soils contaminated with petroleum crudes and total petroleum hydrocarbon (TPH) contents of 16,000 mg/kg and 19,000 mg/kg. TPH quantitates the fraction of petroleum hydrocarbons that are solvent-extractable from samples of contaminated or treated soils. Pyrolysis was effective in reducing the TPH of treated soils to levels well below applicable regulatory standards (usually <0.1% by weight). More importantly, plants grown in pyrolyzed soils (Lactuca sativa, Arabidopsis thaliana) also showed better germination and growth metrics than in untreated or incinerated soils. These encouraging results suggest that pyrolysis may be a more sustainable option to many current thermal treatment technologies [2]. However, the rational development and implementation of pyrolytic soil remediation require mechanistic understanding of the formation, spatial distribution and chemical composition of the carbonaceous compound or “char” left behind after pyrolytic treatment.

This study elucidates the fundamental mechanisms of contaminated soil pyrolysis. Using thermogravimetry and evolved gas analysis (EGA) with mass spectroscopy (TG-MS) and infrared analysis (TG-IR), we characterize the stages of the pyrolysis process, compare our observations to the literature results, and quantify the effect of thermal treatment on soil minerals. Our results show that pyrolytic soil remediation involves two distinct processes. As the contaminated soil is heated to the final processing temperature, desorption of the lighter hydrocarbons is the dominant process for temperatures between 150 and 350°C. Thermal cracking reactions start, however, when the temperature rises above this level the mass spectrometer detects hydrogen and methane in the exit gas stream when the treatment temperature is between 400 and 550°C. Since methane is one of the main products of asphaltene pyrolysis and hydrogen is a key product of the initiation and propagation reactions of hydrocarbon pyrolysis, we can conclude that pyrolysis reactions dominate in this temperature range. Alkane fragments are also detected in the same temperature range since low molecular weight hydrocarbons are also produced during the pyrolysis of asphaltenes. Fragments corresponding to unsaturated (olefins) or cyclic hydrocarbons are also observed between 400 and 500°C, providing additional evidence that pyrolysis reactions (like β-scission) take place in this temperature range.

The final product of pyrolysis is a carbonaceous solid material (char) that remains in the treated soil. Elemental analysis shows that the H/C ratio of the residual char decreases with increasing pyrolysis temperature and approaches typical values measured for petroleum coke. XPS analysis on samples that were oxidized at progressively higher temperatures reveals that the char forms a thin layer coating the particles of treated soils. A series of separate TG-IR experiments were used to confirm this hypothesis.

Another important finding is that thermal soil remediation processes can trigger significant transformations of soil minerals like clay dehydration or carbonate decomposition that can significantly affect the fertility of treated soils. Our work indicates that thermal remediation processes should avoid exposing the contaminated soil to temperatures greater than 550°C to avoid the possible decomposition of carbonates that will lead to significant increases of soil pH and loss fertility. This and similar findings from the literature emphasize the importance of carefully designing remediation projects to strike a balance between achieving sufficient contaminant reduction and avoiding high temperatures that can lead to severe deterioration of soil function or unnecessarily high energy costs.

We will finally present data from a series of experiments with a continuous pilot-scale reactor aimed at determining the optimal processing conditions (residence time, pyrolysis temperature etc.) that minimize the energy costs of pyrolytic remediation. In this continuous reactor, TPH reductions to levels well below 0.1 wt% can be achieved with temperatures below 400°C, residence times as short as 15-30 min and energy costs that are competitive with those of existing thermal technologies. We believe that the pilot study results will facilitate regulatory acceptance of this novel technology, and ultimately enable deploying a thermal remediation technology that offers the potential for cost-effective contaminated soil reuse and ecosystem recovery.

[1] Vidonish, J. E.; Zygourakis, K.; Masiello, C. A.; Gao, X.; Mathieu, J.; Alvarez, P. J., Pyrolytic treatment and fertility enhancement of soils contaminated with heavy hydrocarbons. Environmental Sci. Technol. 2016, 50(5), 2498-2506.

[2] Vidonish, J. E.; Zygourakis, K.; Masiello, C. A.; Sabadell, G.; Alvarez, P. J., Thermal Treatment of Hydrocarbon-Impacted Soils: A Review of Technology Innovation for Sustainable Remediation. Engineering 2016, 2(4), 426-437.