The use of ozone in the remediation of polycyclic aromatic hydrocarbon contaminated soil

Abstract

The potential of using ozone for the removal of phenanthrene from several different soils, both alone and in combination with biodegradation using a microbial inoculant (Pseudomonas alcaligenes PA-10), was examined. The greater the water content of the soil the less effective the ozone treatment, with air-dried soils showing the greatest removal of phenanthrene; while soils with higher levels of clay also reduced the effectiveness of the ozone treatments. However, at least a 50% reduction in phenanthrene levels was achieved in air-dried soil after an ozone treatment of 6 h at 20 ppm, with up to 85% removal of phenanthrene achieved in sandy soils. The biodegradation results indicate that P. alcaligenes PA-10 may be useful as an inoculant for the removal of PAHs from contaminated soils. Under the conditions used in our experiments, however, pre-ozonation did not enhance subsequent biodegradation of phenanthrene in the soils. Similar levels of phenanthrene removal occurred in both non-ozonated and ozonated Cruden Bay soil inoculated with P. alcaligenes PA-10. However, the biodegradation of phenanthrene in ozonated Boyndie soil was much slower. This may be due to the release of toxic products in this soil during ozonation.

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.