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The persulfate anion (S2O8^2-) is one of the most powerful compounds of the peroxygen family used for ISCO and is also commonly used for water and wastewater treatment.  The persulfate molecule can be activated by several methods to form the sulfate free radical (SO4^–·), a powerful oxidant (2.6 V) capable of reacting with many chemicals of concern (COCs). Typical persulfate activation methods include heat activation, a reaction with a reduced transition metal such as ferrous iron, auto decomposition of persulfate under alkaline conditions, and the addition of a simple carbohydrate.  The alkaline activation method is based on the auto-decomposition of persulfate under alkaline conditions, likely causing the persulfate molecule to break at the –O-O- bond, resulting in the formation of two (2) sulfate radicals.  Additional propagation reactions are possible after this initial step, including the formation of the hydroxyl radical or hydrogen peroxide.

Potassium and sodium permanganate has been used to oxidize organic chemicals in drinking water and wastewater treatment, including removal of iron (Fe) and manganese (Mn), phenols, and more recently, ISCO of petroleum related compounds and chlorinated solvents related to industrial solvents.  During in situ applications, a permanganate solution is delivered to the subsurface to contact and react with target chemicals, which are either completely oxidized to CO2 or converted into innocuous compounds.

Permanganate is an effective oxidizing agent (1.7 V) when it contacts aromatic hydrocarbons (with the exception of benzene) and simple PAHs.  Permanganate is more commonly used for and reacts rapidly with the double carbon bonds found in chlorinated ethenes.  It will also oxidize ketones and alcohols.  Permanganate has slower reaction kinetics (i.e., is more stable) compared to other oxidants.

Fenton’s reaction, in which the decomposition of a solution of dilute hydrogen peroxide (H2O2) is catalyzed by excess iron (II), resulting in near-stoichiometric generation of hydroxyl radicals (OH·), is the basis for ISCO using catalyzed hydrogen peroxide.

With a standard reduction potential of 2.8 volts (V), the hydroxyl radical reacts with most contaminants of concern rapidly (at near diffusion-controlled rates).  ISCO applications of catalyzed hydrogen peroxide differs from the classic Fenton’s reagent by using higher concentrations of peroxide and varying the type of catalyst (i.e., iron (III), iron chelates or iron oxyhydroxide minerals).  These modifications result in the formation of additional transient oxygen species such as superoxide anion and hydroperoxide.

Superoxide anion (O2•–) is a reductant and a weak nucleophile that has been found to be reactive with compounds such as carbon tetrachloride and 1,1,1-trichloroethane.  Hydroperoxide (HO2^–) is a reductant and a strong nucleophile capable of degrading problematic compounds such as trinitrobenzene.  The combination of hydroxyl radicals, superoxide, and hydroperoxide anions can oxidize reduced compounds and reduce oxidized compounds, increasing the likelihood of degradation of recalcitrant contaminants. Catalyzed hydrogen peroxide reactions that generate all three (3) transient oxygen species have the potential to provide a complete treatment matrix for ISCO.

Ozone has been used for drinking water disinfection and treatment for over a century. Ozone exists as a gas at standard temperatures and pressures, is colorless and has a distinctive smell at concentrations as low as 0.02 parts per million by volume (ppmv). Ozone has a low solubility (570 mg/L) when compared with liquid oxidants such as sodium persulfate (556,000 mg/L).

Ozone is a relatively strong oxidant at 2.07 V. Therefore, when used for ISCO, it is capable of directly oxidizing many organic and inorganic compounds in groundwater.  Under high pH conditions, exposure to ultraviolet light, or the addition of hydrogen peroxide, ozone can form hydroxyl free radicals.

Ozone is not a very stable compound and disassociates quickly in groundwater, generating oxygen and potentially hydroxyl free radicals. Overall, ozone reaction kinetics are quick, and therefore treatment via ISCO is localized to the immediate radius of influence of ozone injection.

ISCO can treat a wide variety of COCs in the aqueous phase. The selection of the appropriate oxidant for a given site and COC along with appropriate dosing, activation, and distribution is crucial to success of an ISCO application.

ISCO works on a broad range of chemical compounds. The benefit to using ISCO compared to several other technologies is the short time frame for implementation and the destruction of the COCs in situ.  ISCO costs are dominated by chemical purchases, with typically lower operation and maintenance costs compared to other treatment options.  Large contaminant masses (i.e., soil impacts or non-aqeous phase liquids) can result in significant chemical purchases, and therefore result in other technologies being more cost effective.  ISCO can be inefficient when dealing with lower surface area pooled non-aqeous phase liquids due to reduced oxidant contaminant contact.

ISCO works quickly relative to most other remedial technologies. Oxidants can persist in the subsurface for days to months, after which post application monitoring can determine if the site remedial goals have been met or if additional applications are needed.