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?
In situ chemical oxidation uses liquid or gaseous oxidants that are pumped into injection 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 agents available for in situ chemical oxidation that are used to treat a wide range of contaminants.
The persulfate anion (S2O82-) is one of 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 chemicals of concern (COCs). Typical persulfate activation methods include heat activation, a reaction with a reduced transition metal such as ferrous iron, and autodecomposition of persulfate under alkaline conditions.
For in situ chemical oxidation, 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
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 in situ chemical oxidation.
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 in situ chemical oxidation is localized to the immediate radius of influence of ozone injection.
What compounds are treated?
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.
When to use In situ chemical oxidation?
ISCO works on a broad range of chemical compounds. The benefit to ISCO over several other technologies is the short time frame for implementation and the destruction of the COCs in situ. Generally, in situ chemical oxidation costs are dominated by chemical purchases, with typically lower operation and maintenance costs compared to other treatment options. 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, in situ chemical oxidation 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?
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 or if additional applications are needed.
Not all oxidants treat all contaminants – What is the contaminant?
Requires significant understanding site geology:
- Variable geology may result in preferential treatment of higher-permeable soils, or poor treatment of low-permeability soils
Bench-scale tests are used to select oxidant and determine loading:
- Oxidant will attack contaminants and non-target compounds (e.g. organic material, minerals, metals). Understanding Non-Target Demand critical to successful application
- Primary costs associated with chemicals (oxidant and activator)
How many applications are required to meet the total oxidant demand?
- Direct push
- Vertical injection well network
- Horizontal injection wells
- Recirculation to avoid “pushing” aqueous phase contamination
Assumes a Radius of Influence (ROI) based on pilot testing, previous remedial efforts, or assumed based on experience/geology