The use of ozone in the remediation of polycyclic aromatic hydrocarbon contaminated soil
Introduction
Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants that originate from both natural and anthropogenic sources. Natural sources include volcanic eruptions as well as forest and prairie fires. However, anthropogenic sources have now become the major route of entry of PAHs into the environment. Sources include combustion of fossil fuels, coal gasification and liquefaction, coke production, oil and diesel spills, waste incineration and motor vehicle emissions (Blumer, 1976, Cerniglia, 1992, Harvey, 1997). PAHs tend to persist in the environment due to their hydrophobic nature and low water solubility and become rapidly associated with sediments (Cerniglia, 1992). Concerning their carcinogenic and mutagenic potential, The United States Environmental Protection Agency (EPA) has listed 16 PAHs as priority pollutants (Keith and Telliard, 1979).
Due to their toxicity and widespread distribution there is considerable interest in the remediation of PAH contaminated sites. Biodegradation of low-molecular weight PAHs by both bacteria and fungi has been well documented (Cerniglia, 1984, Cerniglia, 1992, Cerniglia, 1997), but high-molecular weight PAHs are more recalcitrant. In recent years there has been interest in using chemical techniques to overcome some of the problems associated with bioremediation. Chemical techniques can offer a rapid and aggressive alternative that is not as sensitive to the type and concentration of contaminant (Kim and Choi, 2002). They may also offer an alternative treatment to overcome some of the limitations of bioremediation in colder climates where microbial activity in the soil is often lower (Goi and Trapido, 2004). One such chemical technique is the use of ozone. Ozone is a highly reactive and powerful oxidant that has been used in the chemical industry as an oxidising agent and is also used extensively in the treatment of drinking water (Bailey, 1978, Camel and Bermond, 1998, Rositono et al., 2001). There has been considerable interest in using ozone to remediate contaminated soils, especially sites containing low or non-volatile organic compounds that are not removed by conventional soil venting. Ozone can be used in either the gas or aqueous phase (Choi et al., 2001) and in the gaseous phase can be pumped through soil in a similar manner to that used in soil venting (Hsu and Masten, 1997). Another benefit of using ozone is that after a short period of time ozone that has not reacted reverts back to atmospheric oxygen and therefore no toxic residues of the oxidant remain in the soil. Ozone has been successfully used in a number of field studies for the remediation of chlorinated compounds, with overall costs involved being reported to be competitive when compared to other soil remediation techniques (Masten and Davies, 1997). In addition ozone has also been reported to be useful for the transformation of PAHs in soil (Masten and Davies, 1997, Ottinger et al., 1999, Choi et al., 2001, Goi and Trapido, 2004). Of particular interest is the potential of using ozone to transform PAHs into intermediates that are more soluble in the aqueous phase (Kornmuller and Wiesmann, 2003). Bioremediation studies suggest that PAHs are only biodegradable when dissolved in the aqueous phase and that microorganisms are not capable of readily transforming PAHs that are sorbed onto solid surfaces (Luthy et al., 1994, Bosma et al., 1997). Intermediates that are more soluble would therefore be more available to microbes for biodegradation. This may lead to the potential use of a combined chemical and biological treatment for PAH contaminated soils.
PAHs vary considerably in their reactivity towards ozone, being more reactive than benzene but less reactive than olefins (Bailey, 1982). With respect to phenanthrene the 9, 10 bond in the molecule has a lower bond-localisation energy than that of any other bonds and it is at this position that ozone reacts directly with phenanthrene. This leads to the addition of the ozone molecule across this bond to form a primary ozonide, which is subsequently transformed to biphenyl compounds (Bailey, 1982). Oxidation of PAHs by ozone can also take place by indirect radical reactions, with ozone decomposing to OH radicals, which are powerful non-specific oxidising agents (Yao and Masten, 1992, Choi et al., 2002, Kornmuller and Wiesmann, 2003).
This study set out to further understand both the potential and limitations of using ozone as a remediation method for PAH contaminated soil, using phenanthrene as a model PAH together with Pseudomonas alcaligenes PA-10, which is known to utilise this PAH as a sole source of carbon and energy (Gordon and Dobson, 2001, Alemayehu et al., 2004). To further understand the limitations of ozone remediation of soil we investigated the effects of the water content and the physical properties of soils on removal of phenanthrene and also examined the effect of ozone concentration and duration of exposure on phenanthrene removal. The microbial inoculant was used in an attempt to examine the potential of using a combined ozonation/biodegradation approach to remove phenanthrene from spiked soil samples.
Section snippets
Chemicals
Phenanthrene (>98% purity) was supplied by Acros Organics (Fisher Scientific, UK). LB Broth were purchased from Sigma-Aldrich, UK and where required media was solidified with 1.5% LAB M agar no. 1. Solvents were supplied by Fisher Scientific and acetonitrile was HPLC grade. Other chemicals were supplied by Sigma-Aldrich, UK or VWR International.
Soils
Cockle Park soil was obtained from the Cockle Park Farm at the University of Newcastle-upon-Tyne, UK. All other soils were obtained from the University
Effect of the water content of the soils on ozonation of phenanthrene
Firstly the effect of the soil water content on the efficiency of PAH removal by ozone was investigated. Both a sandy soil, Boyndie, and a clay soil, Cruden Bay, were spiked with 200 mg kg−1 phenanthrene and sterile water was added to give either a 50% or a 20% water-holding capacity. Air dried soil samples were also analysed. The soil samples were exposed to ozone at 20 ppm for 6 h, with control samples exposed to air. As Fig. 1 indicates, the greater the water content of the soil the less
Conclusion
In conclusion, we have investigated the feasibility and limitations of using ozone as a remediation method for PAH contaminated soil. Ozone has promising potential for in situ remediation, either alone or in combination with biodegradation. We have demonstrated that significant phenanthrene removal can be achieved in unsaturated sandy soil using ozone. Good levels of phenanthrene removal were also achieved in both soils inoculated with P. alcaligenes PA-10, indicating the potential use of this
Acknowledgements
This work was supported by a Marie Curie Ph.D. Training Fellowship at the University of Newcastle-upon-Tyne. MOM is in receipt of an ERTDI Doctoral Scholarship from the Irish Environmental Protection Agency. We would also like to thank Professor Ken Killham at the University of Aberdeen for supplying soil samples and provision of soil characteristics.
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2021, Science of the Total EnvironmentCitation Excerpt :Ozone can be injected in the deep soil layers both in liquid and gaseous phases (O’Mahony et al., 2006; Lim et al., 2002; Choi et al., 2001) through horizontal or vertical wells (Rivas, 2006), ensuring its efficacy in different site-specific conditions. In particular, ozone gas presents a lot of advantages in comparison with other oxidants (Gómez-Alvarez et al., 2012), especially thanks to its capability to diffuse quickly in unsaturated porous media, thus ensuring high removal efficiencies (Liang et al., 2009; Luster-Teasley et al., 2009; Yu et al., 2007; O’Mahony et al., 2006; Pierpoint et al., 2003; Lee and Kim, 2002). In addition, because of its short half-life, ozone decays rapidly, and transforms into oxygen by means of an exothermic decomposition (Oyama, 2000) reducing the accumulation of toxic fractions, favoring subsequent biodegradation of residuals and the reuse of treated soils (Chen et al., 2016b; O’Mahony et al., 2006).