University of Strathclyde1, Vanderbilt University2
Introduction
Contaminated soils represent significant threats to water resources throughout the world. Smouldering is a recently developed process that can be used to destroy 99.9+% of hydrocarbon-based contaminants in place in heavily contaminated soils (1-2). The use of smouldering can remove contamination from soil rapidly, but the high temperatures change the soil surfaces and how it interacts with water (3). The aims of this research are to examine the effects of water re-infiltration into the treatment area and determine the mobility of trace organic contaminants and potentially toxic element (PTE) co-contaminants that remain in the water table after remediation.
Methods/Materials
Two field-obtained materials were used in this research: an acidic loamy sand from the Scottish Highlands and made ground from an industrial site in the USA. A 3kg sample of the Scottish soil was air-dried, sieved to a size fraction <2.36mm, and wet with deionised water to 10% moisture content prior to contamination with 80,000 mg/kg of coal tar and remediation with smouldering. A further 3kg sample was air-dried, sieved to a size fraction <2.36mm, wet with deionised water to 10% moisture content, spiked with cadmium, copper, lead, mercury, nickel, and zinc to a level of 500mg/kg/element, and allowed to age for 30 days before contamination with coal tar and remediation with smouldering. Samples obtained from the industrial site included coal tar contaminated material from a backfilled former tar lagoon and uncontaminated material obtained adjacent to the lagoon. The contaminated sample was treated with smouldering remediation.
Six materials were tested for PTE and trace organic contaminants: soil before contamination and remediation; soil after contamination and remediation; soil spiked with additional PTE contaminants; spiked soil after contamination and remediation; made ground without coal tar; and made ground after remediation. Acid digestion with glacial acetic acid was used to establish the total PTE load in each material. Induction coupled plasma optical emission spectroscopy (ICP-OES) was used to measure 22 elements. Solvent extraction and analysis by gas chromatography-mass spectroscopy (GC-MS) as well as two-dimensional gas chromatography with time of flight mass spectrometry (GCxGC-TOFMS) was used to establish the levels of organic contaminants in each sample.
PTE leaching potential was established with two tests. USEPA Method 1313 was used to determine the pH-dependent availability in all six materials over the pH range of 2-13. USEPA Method 1314 was used to determine liquid to solid partitioning during continuous column experiments. LeachXS software (ECN, Petten, Netherlands) was used to analyse the leaching data and simulate leaching in a range of field-relevant conditions.
Results and Discussion
Smouldering reduced hydrocarbon contaminant levels by 99.9+% in all materials, a level that would be considered completely cleaned of hydrocarbon contaminants. Based on the hydrocarbon levels, the post-remediation materials would be considered safe for reuse according to standards in the USA, UK, Canada, and elsewhere(2); however, the presence of other contaminants may affect this assessment.
Chemical profiling by GCxGC-TOFMS determined that traces of polycyclic and heterocyclic compounds remained in the materials after remediation. PTE content was reduced by smouldering, though some content remained in the soil after remediation.
Laboratory tests involving smouldering exposed soils to peak temperatures of 1000-1100°C for several minutes and a total time above 400°C of approximately 45min. Mineralogy and other properties of the materials were affected by these temperatures and changes were noted in the quartz and clay minerals (3,4). In addition to removing contaminants, smouldering also removed the soil organic matter. All of these changes affected PTE availability and mobility from the materials in the leaching tests. Shifts in arsenic and vanadium corresponded with the destruction of organic matter. Effects on other elements such as cadmium and lead were more complex. LeachXS simulations explored how field conditions affected contaminant mobility in the subsurface from all six materials and the effects of re-infiltration of water into the affected area. Simulations were used to establish the effects of substantial hydrocarbon remediation on the shallow aquifer surrounding the site.
Smouldering remediation can deliver rapid reductions in soil contaminant levels in unsaturated and fully water-saturated soils. Dissolved contaminant in the surrounding aquifer outside the treatment zone will not be affected by smouldering. Water moving through the treated area will encounter different soil conditions including mineralogy that will affect availability of PTEs and other co-contaminants. Further remediation measures or monitored natural attenuation may be necessary to fully restore the aquifer.
Conclusion
Aggressive remediation processes such as smouldering have the potential to deliver rapid and substantial remediation of contaminated soils. Remediation and changes in soil properties and mineralogy during remediation affect trace contaminant and PTE levels and availability, subsequently affecting aquifer restoration after remediation.
(1) Switzer, C, Pironi, P, Gerhard, JI, Rein, G, Torero,JL (2009) Self-sustainingsmouldering combustion: a novel remediation process for non-aqueous-phase liquids in porous media, Environ Sci and Technol 43 5871–5877
(2) Switzer, C, Pironi, P, Gerhard, JI, Rein, G, Torero,JL (2014) Volumetric scale-up of smouldering remediation of contaminated materials, J Hazard Mater 268 51-60.
(3) Zihms, SG, Switzer, C, Irvine, J, Karstunen, M (2013) Effects of high temperature processes on physical properties of silica sand, Eng Geol 164 139-145.
(4) Zihms, SG, Switzer, C, Karstunen, M, Tarantino A (2013) Understanding the effects of high temperature processes on the engineering properties of soils, in: Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris, France.