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REMEDIATION SECTION
In situ Chemical Oxidation for Recalcitrant Chemicals
INTRODUCTION
In situ Chemical Oxidation (ISCO) is being utilized increasingly for the remediation of recalcitrant chemicals in soil and ground water. Ground water remediation using ISCO involves injecting oxidants into the plume, allowing the oxidants to react with the contaminants to produce harmless substances such as carbon dioxide, water and inorganic chlorides. Specific oxidants used in the ISCO process may be sodium and potassium permanganate, hydrogen peroxide (Fenton’s reagent), and ozone. ISCO offers advantages over pump and treat and other technologies as it does not generate large volumes of waste material that must be treated and/or disposed and is implemented over a shorter time period. Each reactant has unique benefits and some challenges that are described below.
PERMANGANATE
Permanganate degrades many volatile organic compounds (VOCs) into byproducts through oxidation. Permanganate has an affinity for oxidizing organic compounds containing carbon-carbon double bonds (e.g. chlorinated solvents). The permanganate ion is strongly attracted to the electrons in the double bond and works to destabilize the bond to form unstable oxygen compounds. Under normal subsurface conditions, the carbon-carbon double bond is broken spontaneously and the unstable intermediates are converted to carbon dioxide. However, the permanganate can react with naturally-occurring organic carbon in the subsurface, which will reduce the amount of permanganate available to oxidize contaminants. Appropriate testing can be performed to determine the amount of naturally occurring organic carbon present in the soil to calculate the total amount of permanganate necessary to treat the impacted area.
FENTON’S REAGENT (Hydrogen Peroxide)
Fenton’s reagent is actually the hydroxyl radical (OH-) that is generated during the decomposition of hydrogen peroxide, in the presence of iron salts. Hydrogen peroxide is a strong oxidizing agent that can be used to degrade chlorinated compounds, stabilize some metals and degrade organic compounds found in gasoline. Hydrogen peroxide however, will react with any oxidizable material present in the soil and is toxic to some bacteria at high concentrations. Peroxide can also mobilize metals, and if the water is hard, magnesium and/or calcium phosphates can precipitate which can effectively plug the infiltration gallery. Care must be taken when using this chemical to stabilize the compound in the environment it is being injected to ensure that the decomposition of the peroxide is maximized to enhance contaminant degradation.
OZONE
Ozone (O3) is a very strong oxidizing agent that is highly unstable and extremely reactive. Ozone is used to break down organic compounds and recalcitrant compounds without chemical degradation. Dosage can be controlled to emphasize chemical oxidation, or at lower doses both chemical oxidation and oxygen enhanced biodegradation. Ozone however, has a half life less than thirty minutes and cannot be delivered very far to oxidize large plumes. Oxidizing ground water can also result in the precipitation of iron and manganese oxides which can clog an aquifer.
LIMITATIONS
Oxidants by their nature require special handling. Regulatory issues associated with ISCO will involve reporting or permitting through various state and federal programs associated with underground injection control (UIC) and air quality.
SUMMARY
ISCO is another tool in the arsenal for treating soil and ground water at sites where organic chemicals have been released. These technologies are not effective in all situations but can provide a low-cost remedial alternative in the proper circumstances. For more information, please contact William Reid or Michael Woodruff at (513) 489-2255 or by e-mail at whr@paynefirm.com or mlw@paynefirm.com. You may visit our web site at www.paynefirm.com.
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