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GeoInsighter Fall 1999 Newsletter
Volume 4 Number 4

Inside Oxide - In Situ Chemical Oxidation

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Over the last five to ten years, there has been a significant shift in approach to remediation of environmental contamination. Historically, remediation focused on physical removal of the source, typically by excavation, coupled with pumping and treating contaminated ground water. These approaches were generally destructive and costly and posed certain potential liabilities and risks while in progress. In the case of ground water treatment, pumping and treating was found to be relatively inefficient for more dilute impacts and to pose a serious risk of potentially increasing the scale of impact when dense non-aqueous phase liquids (DNAPLs) were present.

These limitations spurred a considerable level of interest in and development of in situ remedial techniques for both sources and impacted ground water. Soil vapor extraction was one of the first in situ methods to gain acceptance and relatively widespread application to sources. Building on this experience, dual phase extraction was developed to enhance the removal of contaminants from both soil and ground water (see article in the Summer 1999 issue of the GeoInsighter). The removal of soil gas inevitably involved the introduction of replacement air, and the continuing contact with oxygen was found to enhance naturally occurring in situ biodegradation, leading to increasing understanding and development of this method and spurring interest in other potential in situ methods involving chemical reactions. 

Direct chemical oxidation of contaminants is one of the newer techniques now being applied to remediation of impacted sites and is built upon considerable experience with its use for treatment of organic contaminants in waste waters. Application of this method involves delivering chemical oxidizing agents to the contaminated media in a manner that results in complete oxidation of the contaminants to carbon dioxide or other innocuous compounds. The oxidizing agents typically used include hydrogen peroxide, potassium permanganate (KMnO4), or ozone. These materials are aggressive oxidizers capable of destroying a wide range of organic compounds in soil and ground water. Because of the aggressive oxidation behavior, contaminant mass is typically reduced almost immediately upon contact with these agents. Significant cost savings are achieved by delivering the chemical oxidation treatment to the contaminants and achieving permanent destruction instead of moving large volumes of soil and ground water to achieve physical transfer of the contaminants from one location or medium to another. 

Initial interest in this method focused on treatment of soil sources. Experience in the field indicates that the chemical reactions are sufficiently aggressive to oxidize contaminants absorbed to a wide range of soil types, including silt and clay. In the last several years, use of this method has expanded to include successful treatment of impacted ground water, particularly ground water containing relatively mobile chlorinated and aromatic hydrocarbons, such as tetrachloroethene (PCE), trichloroethene (TCE), benzene, toluene, ethylbenzene, and xylenes. 

Results achieved with chemical oxidation in a variety of hydrogeologic settings and for a range of contaminants have been impressive. The more highly chlorinated volatile organic compounds (VOCs), e.g., PCE and TCE, which are typically slower to degrade, have been reduced by 80 to 100 percent in initial applications to both soil and ground water over periods of several days to several weeks, depending upon the initial size of the source area or plume. Similar reductions have been achieved for aromatic VOCs and polynuclear aromatic hydrocarbons (common components of petroleum products), chlorinated phenols used in wood preservatives, and dioxins.

There are some key factors to consider in evaluating the use of chemical oxidation for a given situation. To ensure efficient application of oxidant by injection into the subsurface, it is important to accurately identify both the lateral and vertical extent of the area to be treated. Given the aggressive nature of the oxidants typically used, natural organic content in soil will consume some of the oxidative capacity of the reagent added. Accordingly, the amount of oxidant required will exceed that expected based solely upon contaminant mass, and multiple applications may be required to complete remediation. If large amounts of natural organic matter are present, the use of oxidation may be economically impractical. The presence of high concentrations of other reduced species (e.g., metals) in the soil can exert a similar effect. 

The permeability of the soil can significantly affect the ability to deliver oxidants to the subsurface through injection. Use of hydrogen peroxide and KMnO4 can result in formation of particles that reduce the permeability of soil, potentially affecting ground water flow and the ability to introduce the reagent as remediation progresses. More permeable soil will facilitate injection and result in treatment of a larger volume around a given injection point. These considerations also suggest that smaller, more concentrated source areas and ground water plumes are more effectively treated using this method than large, more diffuse source areas or plumes. 

Hydrogen peroxide decomposes into water and oxygen relatively easily, which may result in pressure buildup and VOC off?gassing under suitable conditions. Hydrogen chloride can also be produced by oxidation of chlorinated compounds, which may become an air quality issue depending upon the initial contaminant concentration. These considerations suggest that application to sources or plumes beneath buildings will have to be carefully managed with ventilation controls. On the positive side, the additional oxygen released can stimulate biodegradation activity. 

In summary, chemical oxidation can be a rapid, cost-effective remedial technique for the right conditions, i.e., smaller, more concentrated sources and plumes in more permeable hydrogeologic settings without excessive amounts of natural organic matter. In applying this approach, a choice is made to trade a higher level of engineering design to achieve greater efficiency of treatment for a lesser level of design and the lesser efficiency of historical remedial methods (e.g., excavation and ground water pump and treat). A thorough understanding of site conditions that may bear on the factors, particularly oxidant delivery, influencing the performance of chemical oxidation is, therefore, critical to selection and application of this approach.

John A. Gilbert
jagilbert@geoinc.com

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