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N r ? the production of hydroxyl radicals (OH'). In addition, a chain reaction is initiated, causing the formation of new radicals. Hydrogen peroxide is one of the earliest chemical oxidants to be discovered and used in industry. It was discovered in the late 1700s and was commercialized in the early 1800s. Hydrogen peroxide works as a remedial treatment chemical in two ways: free radical production and direct chemical oxidation using hydrogen peroxide. A British Professor H.LH. Fenton (1893, 1894) described the exothermic and somewhat aggressive reaction of hydrogen peroxide with iron salts (ferrous sulfate). Fenton's chemistry or Fenton's reagent uses a transition metal catalyst or an acid to enhance the oxidation chemical reaction of hydrogen peroxide by producing the hydroxyl radical. The reaction of iron catalyzed peroxide oxidation is called a "Fenton's Chemistry" after its discoverer H.J.H. Fenton. The iron/peroxide combination is known as "Fenton's Reagent." Any deviation from the traditional low-concentration hydrogen peroxide/iron mixture is known as a "Modified Fenton's System" . A modified Fenton's reagent, with added chelating agents, is used at neutral pH. A common oxidizer used for in-situ applications is hydrogen peroxide (H202)- Hydrogen peroxide is the most common peroxide in commerce. Pure hydrogen peroxide and its aqueous solutions are clear liquids resembling water. Unlike water, the hydrogen peroxide has a slightly sharp and distinctive odor. Low concentrations of hydrogen 1 peroxide are sold in drug stores ( 1-3%) as a mild antiseptic, l When chemical oxidantH202 is injected at concentrations of 10 to 35% into the subsurface, it decomposes readily into reactive hydroxyl radicals (OH*) and water. The hydroxyl radical (OH•) in the subsurface can be used to rapidly mineralize hydrocarbon, solvent -and-othercontamimnts-to-wateraml-carbon-dioxide. This-reaction-is-enhance in the presence of iron. Iron is naturally occurring in soil and groundwater or can be added during the injection process, if needed. The reaction is based on the principle of Fenton's Chemistry where iron and hydrogen peroxide react to form hydroxyl radicals and other by-products as shown in equation below: The principal chemical reaction associated with the Fenton's process is provided below. Fe" + HO, 4 OH• + OH- + Fe" Fe" + HO, 4 Fe' + HOaJOZ- + H} Fe3+ + H02/O2- 4 Fe2+ + 02 + H+ (iron catalyst gets regenerated) OHO + H'O2 4 HO,/02- + H20 OH* + Fe2+ 4 Fe'+ + OH- 2