Electrokinetic remediation of petroleum hydrocarbon contaminated soil (I)
Introduction
Total petroleum hydrocarbons (TPH) and its derivatives are persistent organic contaminants in soil that are difficult to remediate (Huang et al., 2005, Li et al., 2010). As TPH contains hazardous chemicals including benzene, toluene, ethylbenzene, xylene, naphthalene, etc, contaminated soils can be hazardous to the health of plants, animals, and humans (Liebeg and Cutright, 1999, Ting et al., 1999, Vasudevan and Rajaram, 2001). Remediating persistent organic contaminants from soils is a slow and expensive process and the high molecular weight fractions (C35–40) of TPH are extremely hard to remediate (EPA, 2000, McNicoll and Baweja, 1995). Though TPH can be biodegraded by the TPH degrading bacteria in the soils, their ability to mineralize the TPH contaminants decreases if the TPH concentration is too high in the soil (Pollard et al., 1994).
Subsurface soil contamination by TPH is difficult to remediate, due to the heterogeneity of soil properties, low TPH bioavailability, and the possibility of TPH leaching into groundwater from the contaminant plume or point. To solve the remediation difficulty, TPH-contaminated subsurface soils have been remediated with various conventional technologies. These include chemical, physical or biological processes (Moliterni et al., 2012). They commonly involve transporting contaminated soils to off-site treatment facilities and/or disposing of contaminated soil to landfills, or by incineration, and are generally ex-situ. Consequently, such ex-situ remediation is expensive, inefficient and unsustainable (Moliterni et al., 2012, Varjani, 2017). In-situ chemical oxidation is thus preferable. However, its application is limited, such as by pumping oxidant, in particular for subsurface soils with dense clay, due to the limited radius of influence of the delivery of oxidant to target contaminant areas.
Electrokinetic (EK) remediation is the application of small electrical currents between the anodes and cathodes in soil to remediate contaminants (Korolev et al., 2008, Mena Ramirez et al., 2015, Reddy and Cameselle, 2009). EK is an in-situ “green” technology that is economical, non-disruptive and the most viable option where all other conventional remediation technologies might have failed to clean up dense clayey vadose zone contaminated soil (Maturi and Reddy, 2006, Park et al., 2009, Reddy and Saichek, 2003, Saini et al., 2019, Schnarr et al., 1998, Wang et al., 2007).
EK acts by electrochemical transport phenomena; (i) electromigration — movement of ions in the electric field, (ii) electroosmosis — bulk movement of fluid, (iii) electrophoresis — movement of charged, dissolved or suspended particles in pore fluid, (iv) electrochemically-induced reactions within the soil, (v) redox reactions occurring on the electrode surfaces, such as electrolysis of water (Gill et al., 2014, Guo et al., 2014), as suggested below, (vi) diffusion and convection should also be taken into account. The efficiency of EK depends upon the nature of the substances present in the soil and their specific properties (Gill et al., 2014).
Electrolysis of water occurs at the surface of electrodes (Virkutyte et al., 2002): Hydrogen (H) and hydroxyl (OH) ions are thus generated during electrolysis, and move towards electrodes of opposite charge, generating acidic and basic fronts, which might lead to heterogeneous remediation (Acar et al., 1993, Gill et al., 2014).
In principle, an applied electric field increases the contaminant bioavailability by increasing the movement of target contaminants (Guo et al., 2014). The electromigration is specifically difficult for TPH compounds since they are uncharged (Jackman et al., 2001, Wick et al., 2004). On the contrary, an electrical field applied to a soil can increase electroosmosis and electrophoresis of TPH entrenched in the subsurface (Reddy and Saichek, 2003, Schnarr et al., 1998). Moreover, EK can help in generating mass flux in regions resistant to advective transport, no matter via diffusion or convection (Jones et al., 2011). In general, EK can help remediate immobile and persistent organic contaminants in soil, including TPH (Rahner et al., 2002, Röhrs et al., 2002).
Key factors influencing EK efficiency include current density, electric field or voltage (Haidar et al., 2013, Jin and Fallgren, 2010), and other physio-chemical factors of soil, such as soil pH and moisture content (Acar et al., 1995). Another one is, when current is applied to a soil, heat is produced due to the Joule heating phenomenon. This could have an impact on the remediation efficiency as well.
The aim of this study is to investigate and validate EK remediation of TPH-contaminated subsurface soil. The study determined remediation of TPH fractions at different depth, which mimics a subsurface soil environment. Spatial contour maps were created of the TPH reduction to gain insights on the remediation mechanisms in the subsurface. The research highlights the factors governing EK remediation and suggests improvements to EK technology to increase efficiency.
Section snippets
Soils
Soil was collected from a TPH-contaminated site at BHP (BHP Group Limited) in Perth, Western Australia, which consists three wastewater evaporation ponds used to store oily wastewater and runoff generated from vehicle wash-down bays, refuelling areas and mechanical workshops. Collection was carried out from one of the ponds from various depths. The soil was very wet and dense clay materials, excavated from the unsaturated subsurface zone. The soil was air-dried and then crushed, large stones or
Soil temperature, moisture content, pH, and EC
In general, the soil temperature remained stable and close to the ambient room temperature range of 22–24°C for the duration of the experiment, indicating there was little soil heating due to EK Joule heat. The minimal amounts of heating due to EK Joule heat that were measured were around the electrodes, where there was closest contact between the soil and the electrical current applied.
As the chamber was not covered at the top, loss of moisture can be attributed to evaporation and Joule
Conclusions
The results show that there was a 37% reduction in the concentration of C10–C16 chain compounds within a 7-day EK remediation period which was statistically significant in all the layers. The reductions in the TPH concentrations at three depths up to 24 cm, show that EK can be effective even at deeper layers of soil. There was little heat generated during EK and the temperature of the soil remained around the room temperature with only minor fluctuations around the electrodes. At the end of the
Declaration of Competing Interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Anish Saini, Dawit Nega Bekele, Sreenivasulu Chadalavada, Cheng Fang, Ravi Naidu
Acknowledgements
The authors would like to thank the University of Newcastle, Australia and CRC CARE, Australia for providing the necessary funding and equipment to carry out the research. We would also like to thank BHP, Australia for providing funding and soil samples to carry out the research.
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