Hydrogen Peroxide (H2O2)—Hydrogen peroxide has been used alone or in combination with other chemicals (such as using ferrous iron as a catalyst to generate free radicals through Fenton's chemistry) or with ultraviolet (UV) light. When used alone, hydrogen peroxide is typically injected as a concentrated solution (35-50%), which decomposes violently when contacting groundwater, generating heat and high volumes of gas. When using Fenton's chemistry, the pH of the treatment area is typically maintained at acidic (pH < 4) conditions and a more dilute hydrogen peroxide solution is used. Hydrogen peroxide and iron catalysts are typically injected separately, such as through specific ports in an injection lance, or through injection wells, because free radicals tend to react rapidly and can dissociate if generated prior to injection. Excess hydrogen peroxide that is not used in degrading organic compounds will rapidly degrade to water and oxygen.77
Ozone (O3)—Ozone is a highly reactive chemical that has been used to treat organic compounds in ex situ groundwater and drinking water treatment systems.78 It can also be used to treat MTBE in an ozone-air sparging system. This system injects ozone through tubing to a microporous sparge point designed to generate very small bubbles ("microbubbles," approximately 50 pm in diameter), which have a high surface-to-volume ratio. Organic contaminants in groundwater, such as MTBE, volatilize into ozone bubbles and are oxidized. Ozone that is not consumed degrades to oxygen. Ozone is often used in combination with other oxidants, such as hydrogen peroxide, to enhance oxidation through the generation of free radicals. If bromine is present in the treatment area, bromate generation, which can occur during ozonation, is typically monitored during treatment.
Permanganate (MnO-)—Permanganate is often employed in the form of solid or a solution of potassium or sodium permanganate for groundwater treatment.79-81 It has a smaller oxidizing potential than ozone and hydrogen peroxide using Fenton's chemistry, resulting in the relatively slower oxidation of MTBE and other oxygenates. However, permanganate has a longer half-life compared with the stronger oxidants, and persists in the environment for a longer time. The end product of permanganate oxidation is manganese dioxide, which, depending on the groundwater pH, can precipitate into the formation. Excessive precipitation may reduce soil permeability.
Other Oxidants—Combinations of the above oxidants and other oxidants such as persulfate compounds are also being used to treat MTBE and other oxygenates. These and other combinations and other oxidants are being developed to maximize the generation of highly oxidizing free radicals, increase oxidant persistence, or otherwise enhance in situ oxidation.
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