What is in situ chemical oxidation?
In situ chemical oxidation (ISCO) generally uses an injection well network to introduce oxidizing agents to the subsurface to mineralize target compounds.
How does it work?
The oxidants are generally pumped into injections wells that are installed to maximize contact with both soil and groundwater impacts. As the oxidant requires contact with the compounds to be treated, the injection wells are spaced so that the solution spreads throughout the full treatment volume. For mostly dissolved plumes, or to control the potential migration towards sensitive receptors, a re-circulation strategy can be implemented, where impacted groundwater is extracted, treated above grade with an oxidant and then re-injected. There are a variety of oxidizing agents available that are used to treat a wide range of contaminants.
The persulfate anion (S2O82-) is the most powerful compounds of the peroxygen family used for in situ chemical oxidation 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 reaction with many COCs. Persulfate activation methods include heat activation, a reaction with a reduced transition metal such as ferrous iron, and autodecomposition of persulfate under slightly acidic or alkaline conditions.
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.
Permanganate has been used for over 50 years to oxidize organic chemicals in drinking water and wastewater treatment, including removal of iron (Fe) and manganese (Mn), phenols, and more recently chlorinated hydrocarbons related to industrial solvents. Permanganate has been used for the in situ chemical oxidation of petroleum related compounds and chlorinated solvents in the subsurface for over 10 years. 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.
Generally permanganate has slower reaction kinetics (i.e., is more stable) than other oxidants.
Catalyzed Hydrogen Peroxide
CHP is based on the standard 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·):
With a standard reduction potential of 2.8 volts (V), the hydroxyl radical reacts with most contaminants of concern at near diffusion-controlled rates. In situ chemical oxidation applications of CHP 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 mineralization of recalcitrant contaminants. CHP 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.
Ozone is a relatively strong oxidant at 2.07 V. Therefore, when used for in situ chemical oxidation 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, hydroxyl free radicals can be generated from ozone.
Ozone is not a very stable compound, and disassociates quickly in groundwater, generating oxygen and possibly hydroxyl free radicals. Overall, ozone reaction kinetics are relatively quick, and therefore treatment is localized to the immediate radius of influence of ozone injection.
When to use ISCO
In situ chemical oxidation works on a broad range of chemical compounds. Generally, ISCO costs are dominated by chemical purchases. Large contaminant masses in the form of soil impacts or non-aqeous phase liquids can result in significant chemical purchases, and therefore result in other technologies being more cost effective. Additionally, ISCO can be inefficient when dealing with lower surface area pooled non-aqeous phase liquids due to reduced oxidant contaminant contact.
How long will it take?
In situ chemical oxidation (ISCO) works relatively fast 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.