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GeoInsighter Spring
2004 Newsletter In-Situ Redux - Chemical Oxidation Revisited Return to the Newsletter Index The field of contaminated site remediation is no less susceptible to fads than are other areas of American life. We have recently seen a vastly increased effort on the part of remedial technology vendors to push in situ chemical oxidation as “THE” solution to a multitude of potential ills and thought that it was worth re-visiting the development and utility of this technology since it last appeared in this newsletter in 1999. As a quick review, this technique involves delivering oxidizing chemicals to contaminated media in a manner that results in complete oxidation of the contaminants to carbon dioxide or other innocuous compounds. The chemicals typically used 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 of these agents with contaminants and much more quickly than by other methods.
Fenton’s reagent is a particularly powerful, fast acting oxidant. It, too, therefore, reacts with a wide range of non-target materials reducing its efficiency. In addition, because of its strength and high reaction rate, it does not persist for long in the subsurface, requiring extensive injection systems to ensure adequate contact throughout the area to be treated. It also requires that the pH be lowered, at least temporarily, below 4 Standard Units and produces a considerable amount of heat and gas in a relatively violent reaction. Continuing development of this chemistry has led to identification of other catalyst systems that do not require lowering the pH and that react more mildly, improving persistence of the peroxide. Permanganates are milder oxidants and, therefore, persist longer in the subsurface than the more reactive ozone and Fenton’s reagents; however, they also react readily with natural organic matter and are not strong enough to oxidize all potential volatile organic compound targets, particularly benzene and chlorinated alkanes. Because of their intense purple color, permanganates can present a handling problem in preparing treatment solutions, staining a wide range of materials and equipment. Persulfate is the newest oxidant on the scene. Similar to hydrogen peroxide, it requires use of a catalyst to accomplish the required destruction, the most common of which are heat (above 35 to 40 degrees centigrade) or a transition metal (e.g., iron). These compounds are extremely soluble, allowing efficient delivery of relatively high mass loads of oxidant to the remedial target. The persulfate radical that is responsible for the oxidizing effect is nearly as strong as the hydroxyl radical generated by Fenton’s reagent, but is more persistent, making it more efficient to deliver. Importantly, persulfate is less readily reactive with natural organic material that may be present in soil. In addition to development and refinement of the oxidizing agents that are available, methods for their delivery to the target remedial area have also advanced. Initially, application of oxidizing agents was accomplished using larger diameter wells or lateral pipe galleries. These methods proved to be inefficient in reaching the entire impacted area to be remediated, particularly given the highly reactive nature of some of the early oxidizing agents. It is now possible to achieve much more effective distribution using a range of techniques that provide more rapid mass transfer throughout the impacted area. These techniques include geoprobe injection points, pneumatic fracturing, hydraulic fracturing, and jet grouting. In some circumstances, these methods will actually fluidize the soil matrix, vastly increasing the effectiveness of reagent mixing and contact with the target medium. Despite these advances, in situ chemical oxidation is not the panacea for all environmental ills. It is still critical to develop a thorough understanding of site conditions to ensure its utility. Chief among the considerations should be the level of soil oxidant demand (SOD), which is the degree to which naturally occurring materials in the matrix to be treated will compete with the target contamination for oxidizing reagent. In our experience, this factor is probably the primary determinant of success or failure of chemical oxidation at a particular site, followed closely by the ability to efficiently introduce the oxidizing to the subsurface. A high SOD can require the use of so much oxidizing agent that the method becomes cost-prohibitive relative to alternatives. If injection of oxidant is inhibited, the need to install many more injection points or resort to more aggressive injection methods can also raise costs to an unacceptable level, from either a construction or a reagent mass perspective. These considerations 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. A further consideration is the relative location of subsurface and surface structures and utilities at sites. Aggressive oxidizing agents can damage construction materials with which they come into contact, including piping, tanks, and concrete. Proximity to such materials in the area to be remediated is, therefore, a key consideration. Particular caution should be exercised at relatively small, congested sites (e.g., gas stations) to ensure that larger problems are not created in the effort to solve the initial ones. In summary, chemical oxidation can be a rapid, cost-effective remedial technique for the right conditions; however, careful analysis of site conditions (i.e., SOD, vertical and lateral extent of contamination) and the presence of subsurface structures that are at risk for damage by oxidant must be carefully analyzed to ensure its success. John A. Gilbert, P.E. Return to the Newsletter
Index
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In recent years, the
stable of potential oxidizing agents has expanded from the old standbys
of ozone, permanganate (sodium and potassium), and Fenton’s reagent
(i.e., hydrogen peroxide and ferrous iron) to include persulfate and
modified forms of Fenton’s reagent. Continuing experience with these
agents has clarified their relative advantages and disadvantages for
various applications. A central limitation of ozone is its lack of
specificity; it will react with a wide range of materials including
naturally occurring minerals and organic matter in addition to chemical
contaminants. Also, because it is a gas, it is difficult to efficiently
introduce it into the subsurface to bring it into contact with the
target contaminants. Recent advances in delivery systems have mitigated
the latter issue somewhat, but the lack of specificity remains a
consideration for sites with significant levels of natural organic
material. An advantage to ozone, however, is its ability to oxidize some
of the more complex, recalcitrant classes of organic compounds such as
pesticides, polychlorinated biphenyls, and polycyclic aromatic
hydrocarbons.