(84a) Pyrolysis of Soils Contaminated with Heavy Hydrocarbons: Reaction Mechanisms and Morphology of the Produced Char

Authors: 
Zygourakis, K., Rice University
Masiello, C. A., Rice University
Vidonish, J. E., Rice University
Soil contamination by petroleum and other heavy hydrocarbons is a major global environmental problem. Although offshore oil rigs and tankers are responsible for occasional large-volume spills, 98% of oil spills occur on land, with an average of 70 spills per day (more than 25,000 per year) reported to the U.S. EPA. Without adequate response, the effects of major spills could last decades. Whereas numerous remediation approaches have been proposed, most existing technologies have severe drawbacks. For example, in situ bioremediation of sites impacted by petroleum release can take years, particularly when recalcitrant species such as high molecular weight hydrocarbons are involved. Thermal technologies, on the other hand, can remediate sites quickly and efficiently (hours to months), often removing over 99% of a wide range of hydrocarbon fractions. However, current thermal technologies often destroy key soil constituents and severely degrade soil properties such as organic carbon content, water retention, stability and microbial activity. Thus, treated soils become unsuitable for reuse since they cannot support vegetative growth or even provide erosional stability. We have recently reported[1] that pyrolytic treatment at 400-450°C of soils contaminated with heavy petroleum crudes reduced total petroleum hydrocarbons (TPH) to levels well below regulatory standards (<0.1% by weight). A fraction of the original hydrocarbons remained in the treated soil in the form of a highly inert carbonaceous material (char). Standard EPA tests revealed that the post-pyrolysis levels of toxic polycyclic aromatic hydrocarbons (PAHs) were well below applicable standards. More importantly, plant growth studies showed significantly higher (by 80-900%) biomass production of Arabidopsis thaliana and Lactuca sativa (Simpson black-seeded lettuce) in pyrolyzed soils than in either contaminated or incinerated soils. And because it is carried out at lower temperatures, pyrolysis requires less energy than other thermal methods (like incineration) that can achieve similar TPH reduction levels. Overall, these results suggest that soil pyrolysis could be a viable thermal treatment to quickly remediate soils impacted by weathered oil while improving soil fertility, potentially enhancing revegetation.

 We present here results from a follow-up study aimed at elucidating the fundamental mechanisms of the pyrolysis process and the morphology of the produced char.

 The dispersion of the char in the pyrolytically treated soils will determine to a large extent its ultimate fate in the environment and modulate its interactions with water and the regenerated soil microbiome. Optical microscopy images of soils treated with pyrolysis and incineration led us to hypothesize that the produced char is distributed as a very thin film coating the soil particles. To test our â??char filmâ? hypothesis, we carried out controlled oxidation of pyrolyzed soils followed by surface analysis using XPS. We exposed treated soil to oxygen and reacted it in the TGA for one hour at temperatures between 350 and 450°C in order to progressively burn off the char film. Weight loss measurements and the XPS results confirmed our char film theory. As we burnt the film away, the percentage of C-C bonds on the surface of soil particles decreased and the amount of Si increased (our soils had high quartz content). On the other hand, the relative amounts of C and Si detected remained constant when uncontaminated â??backgroundâ? soil was exposed to the same treatment of controlled oxidation. Surface analysis by XPS detected the same relative amounts of C and Si when uncontaminated soil and pyrolyzed soil were oxidized at 450°C. This treatment â??burnt awayâ? the char film that covered the particles of pyrolyzed soil.

To further differentiate our process from existing thermal desorption technologies, we employed thermogravimetry with online evolved gas analysis (TG-MS) and differential scanning calorimetry (DSC) to gain a better understanding of the fundamental pyrolysis mechanisms. Our results show that as mixtures of soil and petroleum crudes are heated in an anoxic atmosphere, we first observe desorption of the lighter hydrocarbons at temperatures between 100 and 350°C. At temperatures between 350 and 450°C, however, thermal cracking and condensation reactions dominate and convert the heavier hydrocarbons (resins and asphaltenes) into char (C:H atomic ratio approximately equal to 1) and light volatiles. If the temperature rises above 550°C, however, key soil constituents (like carbonates) start to decompose and release CO2 that becomes the dominant peak in the time-resolved MS spectra. Carbonate decomposition can explain the significant increase of the pH and the accompanying loss of fertility we observed when we incinerated contaminated soils at 650°C. MS data also show that H2O evolves between 300 and 500°C, indicating dehydroxylation of clay constituents of soil. All these observations have significant implications for secondary reactions that may occur when pyrolytic treatment of contaminated soils is carried in commercial reactors.

 Knowing the temperatures at which decomposition of soil constituents occurs will allow us to determine the least damaging thermal treatment to reach desired cleanup levels. The clearest lesson that can be gleaned from our studies is that the lowest effective treatment temperature should be used to minimize soil decomposition. While allowing for effective treatment of nearly all hydrocarbons, the high temperatures utilized by incineration and other thermal remediation technologies can cause extensive soil damage, such as decomposition of clays, carbonates, and organic content. This will physically alter the soil and change its geochemical, biological, and fertility properties.




[1] Vidonish, J.E., et al., â??Pyrolytic treatment and fertility enhancement of soils contaminated with heavy hydrocarbons.â? Environmental Science & Technology, 50 (5), 2498â??2506 (2016).