Remediation of trichloroethylene-contaminated soils by star technology using vegetable oil smoldering
Introduction
Chlorinated volatile organic compounds (CVOCs), such as, trichloroethylene (TCE) and tetrachloroethylene (PCE), are frequently encountered soil and groundwater contaminants [1], [2]. These compounds are often present as nonaqueous phase liquids (NAPLs) forming a source zone for long term groundwater contamination [3]. As known or suspected carcinogens [4], many CVOCs are high priority pollutants for clean-up.
TCE (C2HCl3), with a density of 1.46 g/ml at 20 °C [5], is often found below the watertable. It exhibits low solubility (1450 mg/l), low boiling point (86.7 °C), and high vapour pressure (9700 Pa at 25 °C) [6]. TCE concentrations between 103 and 106 μg/L in groundwater are typical at contaminated sites [7], while 5 μg/L is the typical regulatory limit [1]. Thus, a small amount of TCE DNAPL can result in contaminated groundwater for decades [8], [9]. Below the watertable, TCE can undergo anaerobic dechlorination under favorable geochemical and microbiological conditions [10], [11], [12]; biodegradation half-life values are typically from six months to one year, but rates are highly dependent on site conditions and engineering intervention [13], [14].
A remediation approach based upon smoldering NAPLs in soils was recently introduced [15], [16]. Smoldering is a flameless form of combustion in which the exothermic oxidation reaction occurs on the surface of the fuel in a porous medium [17]. This reaction can be self-sustaining in the presence of sufficient fuel and oxygen (e.g., charcoal in a barbeque). Most of the studies on smoldering combustion consider porous solid fuels in the context of material synthesis [18] and fire safety [19], [20], [21], [22], [23], [24]. In situ combustion has been studied by the petroleum industry for enhanced oil recovery [25], [26]. In addition, smoldering of peat deposits has been studied due to environmental concerns [27], [28].
The first proof that a liquid distributed within an inert porous solid could be smouldered was provided by Pironi et al. [29]. Application of the process for the remediation of NAPLs within soils was first proposed by Switzer et al. [16]. NAPL smoldering was initiated by injecting air following the preheating of a local region of the soil with a one-time, short-duration energy input. The establishment and propagation of a self-sustaining smoldering front suggested that this process may have utility in subsurface remediation [16]. The process was demonstrated to be robust over a wide range of operating conditions [15]. Subsequently, it was demonstrated that smouldering NAPL could be scaled up 1000-fold from laboratory conditions [30]. Recently, several successful pilot field trials of in situ STAR have been completed beneath a former chemical manufacturing facility contaminated by coal tar [31]. These field tests demonstrated that a short, in-well ignition event (several hours) generated a self-sustaining smoldering reaction lasting more than 10 days that propagated outwards to remediate the soil within a 3.5 m radius of influence. More than 4000 kg of coal tar were destroyed in the pilot tests conducted below the water table, revealing that groundwater is not a barrier to in situ STAR.
All of the published research and field trials on remediation using smoldering have treated heavy, complex compounds such as coal tar and crude oil (i.e., non-volatile). The high volatility of CVOC NAPLs is expected to be a barrier to self-sustaining smouldering. Switzer et al. [16]; however, showed that a smoldering reaction could be initiated in TCE NAPL mixed with vegetable oil (75%:25% mass ratio of TCE: oil) with a single proof-of-concept bench test.
Subsurface injection of vegetable oils is a well-established practice to support in situ anaerobic biodegradation [32], [33], [34], [35]. There is evidence that injected vegetable oil partitions into TCE NAPL in soil due to their mutual miscibility [36]. This work postulates that vegetable oil could provide a supplemental fuel for self-sustaining smoldering to remove volatile NAPLs from the subsurface. Volatilizing and oxidizing CVOCs in situ via smoldering could provide numerous cost, energy, carbon footprint, and time savings relative to existing remediation techniques; these are discussed more fully in Section 4.
The chemistry of smoldering combustion is complex. Smoldering is characterized by both pyrolysis (endothermic thermal degradation of the fuel to form a carbon-rich char) and oxidation (exothermic reaction between the char and oxygen) reactions. Self-sustaining smoldering is; however, necessarily dominated by oxidation as its exothermic nature provides the energy required for the reaction to propagate [27]. The study of the chemical reactions of smoldering is not a mature topic and in general only simple, qualitative reaction frameworks are reported; even for the most studied fuels (e.g., polyurethane foam), quantitative (stoichiometric) chemical reactions are not known [28]. Assuming that the oxidative and non-oxidative thermal decomposition products reported in the literature for TCE incineration are relevant to this study, the potential chemical by-products associated with TCE decomposition are summarized [37], [38]:
PyrolysisC2HCl3 (TCE) → C2Cl2 (DCA) + C2Cl3 (Chlorinated vinyl radical) + C2Cl4 (PCE) + HCl
OxidationC2HCl3 (TCE) + O2 → CO + CO2 + Cl2 + COCl (Carbonyl chloride) + COCl2 (Phosgene) + C2Cl4 (PCE
The mode of combustion (flaming or smoldering) will play a significant role in determining the products of combustion and the stoichiometry, therefore, the above reactions are presented only as a basis for likely products. It is noted that these products include some of concern such as phosgene, a toxic gas [37], [38].
The objective of this study was to explore the conditions necessary to treat TCE NAPL-contaminated soil by smoldering combustion using vegetable oil as a supplemental fuel. Bench-scale experiments were conducted to provide a proof-of concept and to evaluate the sensitivity of the process to oil type and mass ratio of oil to TCE. Experiments were also conducted to assess if the oil could be delivered by injection of pure oil and as an oil emulsion. The smoldering characteristics, rate of remediation, and resulting concentrations of key compounds in sand and vapors were quantified to assess the fate of TCE and vegetable oil. This represents the first evaluation of the smoldering of vegetable oil and the first consideration of treating volatile NAPLs by smoldering.
Section snippets
Materials and methodology
Number 12 silica sand (Bell & Mackenzie Co., Ltd., mean grain diameter = 0.88 mm, coefficient of uniformity = 1.6) was employed for all experiments. TCE (commercial ACS grade, Alfa Aesar) was mixed manually with commercially available vegetable oil and the sand until homogeneous (precautions taken to minimize volatilization); the organic liquids were observed to coalesce into a single NAPL. In each experiment, a quartz column (Quartz Scientific Inc., 280 mm high × 138 mm internal diameter) was
Base cases
The temperature-time profiles for the base case (experiment 1) illustrates that, upon initiating air flow, a temperature spike was observed that represents the onset of smoldering combustion (Fig. 2). After the heater was turned off (t = 44 min), the reaction was self-sustaining as evidenced by the succession of nearly constant peak temperatures. The figure illustrates that the smoldering front required 20 min to propagate the length of the column and, following the reaction’s natural extinction
Discussion on environmental relevance
Enhanced in situ bioremediation (EISB) of CVOC NAPL source zones has significant potential but it is slow, requiring on the order of years to decades [42]. More rapid treatment can be achieved with standard thermal remediation techniques, such as, in situ thermal desorption and electrical resistance heating. However, these require continual energy input since they heat the entire site to above 100 °C, a process which requires several months of electricity injection [43] leading to substantial
Conclusions
This study demonstrated, for the first time, the ability for volatile NAPLs to be removed from soil via self-sustaining smoldering combustion of vegetable oil. While some destruction occurred, the majority of TCE mass was volatilized. The study further demonstrated that emulsified vegetable oil could equally be used instead of neat vegetable oil. In addition, it was demonstrated that the oil could be injected into previously contaminated soil. It is expected that vegetable oil or EVO could be
Acknowledgments
This research was supported by the Natural Science and Engineering Research Council (Canada) and Ontario Ministry of Research and Innovation. Smoldering for remediation is patented by University of Edinburgh (International PCT Filing PCT/GB2006/004591, Granted Patents US 8,132,987 B2, AU 2006323431 B9, JP4 934832, CA 2,632,710, and PRC ZL20068005254.X) and Geosyntec Consultants Ltd. (International PCT Filing PCT/US12/35248) and employed by University of Western Ontario under a research license.
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Present Address: Institute for Infrastructure & Environment, University of Edinburgh, Edinburgh, EH9 3JL, UK.