Superfund Research Program
In-situ Ozonation of PAHs - Feasibility, Pathways, and Byproduct Identification
While conventional methods are applicable for the remediation of many Superfund sites, clean up of some chemicals requires novel approaches. In-situ oxidation has been successfully applied for the clean-up of contaminants that:
- are not biodegradable and cannot be removed by biological treatment processes;
- are nonvolatile and cannot be removed by processes such as soil vapor extraction or air sparging;
- partition to the soil and are not easily removed by pump and treat technologies;
- pose a significant health risk and need to be removed from the soil expeditiously; or
- are found in complex oil solvent mixtures.
Oxidation of polycyclic aromatic hydrocarbons (PAHs) results in the formation of products that are more polar than the parent compound. This increases both biodegradability and aqueous solubility, thereby enhancing remediation rates. Because ozone is a strong, selective oxidant, scientists at Michigan State University (MSU) are conducting studies to assess the feasibility of in situ ozonation for remediation of PAH-contaminated Superfund sites. The MSU researchers are focusing their efforts on understanding the complex chemistry of in-situ ozonation with the goals of predicting the movement of ozone in soils; increasing our understanding of the mechanisms by which ozone reacts with PAHs; and determining if toxic byproducts are formed from the ozonation of these chemicals.
As a first step, they investigated the effect of soil moisture, soil organic matter, and soil texture on the transport of ozone through unsaturated soil. The MSU scientists determined that soil moisture can have a profound effect on ozone transport. Ozone dissolves in soil pore water and hydroxyl ions catalyze the decomposition of ozone, decreasing the amount of ozone travelling through the soil and that is available for oxidation of a target compound.
They examined the ozone demand of three types of soil to evaluate potential for ozone penetration through the soils. Ozone demand is the amount of ozone that can be consumed by the reactive portion of soil particles. It appears that soils exert a finite demand for ozone, beyond which ozone can be transported through the soil. The MSU team determined that in each soil type, after the initial ozone demand was met, there was minimal ozone degradation. Therefore, it should be possible to achieve ozone penetration to a considerable distance from an injection well in unsaturated soil, suggesting that in situ ozonation is a feasible means of site remediation. Engineering firms have conducted limited field tests of in situ ozonation of chlorinated solvents and their results support the MSU findings concerning ozone penetration through soil.
MSU scientists then studied the reactivity of pyrene and chrysene, both PAHs, with gaseous ozone in unsaturated soil columns. In addition, they examined the byproducts of the reaction of ozone with PAHs. This is particularly important because it is essential in the design of remediation strategies to ensure that the methods applied do not result in the formation of toxic compounds that are not removed by the remediation process. The researchers used gas chromatography-mass spectrometry to evaluate PAH degradation rates and tentatively identify ozonation byproducts. They used inhibition of gap junctional intercellular communication (GJIC) as an indicator of chemical toxicity. Intercellular communication through gap junctions provides cells with the crucial links to their neighbors - enabling individual cells to function communally in a multicellular organism.
They determined that PAHs are very reactive with ozone. The MSU team found that a stoichiometric ozone:contaminant ratio greater than one is needed to completely remove PAHs because some of the ozone is consumed in reactions with ozonation products prior to the elimination of the target compound.
By varying the dosage of ozone, the researchers were able to control the extent of the ozone-PAH reaction, enabling them to "take snapshots" at different phases of the ozonation process. This allowed them to tentatively identify the ozonation products and evaluate the relative concentrations of the target compound and its ozonation products at each phase of the ozonation process. They learned that the ozonation of pyrene and chrysene results in the formation of several intermediates that are more inhibitory to GJIC than the target compounds and that the byproducts formed initially are more toxic than those formed later in the ozonation process. The MSU researchers found that increasing ozone levels both reduces the toxicity of the byproducts and increases the polarity of the ozonation products (which increases biodegradability and aqueous solubility).
This work provides valuable, basic information required by those charged with the responsibility of remediation of Superfund sites. First, it is a relevant reminder that simply removing a target compound does not ensure removal of toxicity from a contaminated site. In addition, by identifying the byproducts of the ozonation of PAHs and the reaction pathway by which they are formed, the MSU scientists are establishing a foundation of knowledge for the development of treatment alternatives to minimize the formation of toxic byproducts.
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To learn more about this research, please refer to the following sources:
- Weis LM, Rummel AM, Masten S, Trosko JE, Upham BL. 1998. Bay or bay-like regions of polycyclic aromatic hydrocarbons were potent inhibitors of gap junction intercellular communication. Environ Health Perspect 106(1):17-22.
- Yao JJ, Huang Z, Masten S. 1998. The ozonation of pyrene: pathway and product identification. Water Res 32:3001-3012.
- Masten S, Davies S. 1997. Efficacy of in situ ozonation for the remediation of PAH contaminated soils. J Contam Hydrol 28:327-335.
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