Pollution Engineering - April 2009 - (Page 25) Stacking the court One of the novel components of the team’s approach was the sequential application of different oxidants to achieve the CTLs. Plume management strategies typically employ ISCO in concert with other methods, such as engineered or natural bioremediation, extraction or air sparge/soil vapor extraction. However, most of these secondary technologies require longer periods of time to achieve cleanup goals (months or years). When time is a pressing issue, an alternative approach is to couple different ISCO technologies to more rapidly achieve low cleanup standards. Each general chemical compound such as catalyzed hydrogen peroxide (CHP), activated persulfate, or permanganate, has advantages and disadvantages. But in certain cases different oxidants can be applied together or in succession to rapidly reduce contaminant concentrations, as was shown in this application. CHP includes a range of chemical reactions responsible for degradation of organic compounds such as PCE. When catalyzed by a transition metal such as iron, H2O2 generates powerful radicals such as the hydroxyl radical and superoxide radical. However, the chemical also has disadvantages. One is the short lifetime of the oxidant in the subsurface. In most cases, hydrogen peroxide will persist for several days to perhaps a week in the subsurface. As a result, residual oxidant may not persist long enough to address contaminants slowly diffusing out of fine-grained (silt or clay) aquifer matrices, and the oxidant itself does not survive long enough to diffuse into those matrices. Long-term diffusion of contaminants from fine-grained aquifer matrices may pose a rebound problem, and prevent achievement or maintenance of the CTLs. APRIL2009 www.pollutionengineering.com 25 http://www.pollutionengineering.com
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