Chemical Oxidation

In situ Chemical Oxidation (CHEMOX or ISCO)

In situ chemical oxidation (ISCO) has emerged as a cost-effective and viable remediation technology for the treatment of VOCs (e.g TCE) and petroleum-based contaminants (BTEX) in groundwater, soils, and sediments. The most common oxidants ARS has experience with are permanganate and persulfate. Both have been widely used in the past few years since they are relatively inexpensive and have a long history of successful uses.

Chemical Oxidation - Potassium Permanganate & Sodium Permanganate

Chemical OxidationPermanganate is commercially available in both the potassium and sodium salt. Potassium permanganate is solid, purple crystal that is used as a 1 to 7% solution. Sodium permanganate is a dark purple solution that is used as a 40 to 10% solution.

Permanganate is a relatively mild oxidant and its reaction products are essentially benign. Permanganate rapidly converts a wide range of chlorinated alkenes to carbon dioxide, water, and chloride ions. The permanganate is reduced to insoluble manganese dioxide during the reaction.

Permanganate oxidation involves a direct electron transfer unlike other oxidants, such as persulfate and hydrogen peroxide that uses a free radical process. Permanganate has a unique affinity for oxidizing organic compounds containing carbon-carbon double bonds, aldehyde groups or hydroxyl groups. As an electrophile, the permanganate ion is strongly attracted to the electrons in carbon-carbon double bonds found in chlorinated alkenes, borrowing electron density from these bonds to form a bridged, unstable oxygen compound known as the cyclic hypomanganate ester. This intermediate product further reacts by a number of mechanisms including hydroxylation, hydrolysis or cleavage. Under most naturally occurring subsurface pH and temperature conditions, the carbon-carbon double bond of alkenes is broken spontaneously and the unstable intermediates are converted to carbon dioxide through either hydrolysis or further oxidation by the permanganate ion.


From Siegrist et al., �In Site Chemical Oxidation Using Permanganate�, Battelle Press, 2001

The oxidation reaction for KMnO4 with a chlorinated ethene can be written as follows:

The by-products of the reaction shown above are reaction end-points. Carbon dioxide (CO2) exists naturally in subsurface from biological processes and bicarbonate partitioning in the ground water. Manganese dioxide (MnO2) is a natural mineral already found in the soils in many parts of the country. If the precipitation of manganese dioxide in the soils is excessive, it can reduce the permeability of the soil, thus limiting injection of the aqueous oxidant. Although the manganese dioxide is insoluble in ground water, manganate (Mn+4) may be reduced to dissolved divalent manganese (Mn+2) under low pH or redox conditions. Therefore, elevated concentrations of dissolved manganese may develop in the immediate treatment area. The chloride ion (Cl-) released by the oxidation reaction may be converted into chloride salts (i.e., KCl, HCl, etc.) or chlorine gas (Cl2) due to the high redox conditions. Chlorine gas reacts quickly with ground water and pore water to form hypochlorous acid (HOCl). This hypochlorous acid may react with methane to form trace concentrations of chloromethanes in the ground water immediately after treatment. However, this phenomenon is typically short lived as the subsurface conditions are converted from an anoxic state to an oxidized state.

Permanganate can also be used to treat organic compounds that contain hydroxy functional groups such as primary and secondary alcohols, as well as some organic acids such as phenol. These oxidation reactions occur best at higher pH values where hydrogen abstraction creates a negative charge on the oxygen atom. The permanganate is attracted to the negative charge, resulting in an oxidation reaction that converts the compound into an aldehyde, ketone or carboxylic acid. Saturated aldehydes, methyl ketones and aliphatic carboxylic acids can be further oxidized by permanganate, but incomplete oxidation may occur with more complex oxygenated hydrocarbons.

Chemical Oxidation: Persulfate

Sodium persulfate is a yellow crystal that is typical used as a 10 to 40 % solution. It is a highly reactive oxidant, but more stable than hydrogen peroxide or ozone, and can persist in the subsurface for weeks. When properly activated, sodium persulfate can destroy many organic contaminants. Sodium persulfate disassociates in water to form sodium and persulfate ions. In the presence of heat, ferrous iron and/or basic conditions, the persulfate is converted to two (2) sulfate free radicals as illustrated in Equation 1.

The conversion of the persulfate to sulfate free radicals results in the production of electrical energy and sulfate radicals or a single electron radical. Free radicals act as strong oxidizing agents and are known to oxidize many VOCs and petroleum-based contaminants. Some sites have sufficient background conditions that allow the persulfate to oxidize the targeted VOCs present in the soils and groundwater without the addition of a catalyst.

Soil Oxidant Demand (SOD)

The important design parameter to consider when applying ISCO is the background soil oxidant demand (SOD) of the treatment zone. When estimating the dosage of oxidant reagent required to treat the target contaminant, it is critical that all potential oxidant demands including reduced soil minerals and natural organic matter (NOM) are considered. If site contaminant levels suggest the presence of dense non-aqueous phase liquids (DNAPL), then the oxidant demand must be conservatively adjusted to account for the fact that the actual contaminant levels may be 10 to 100 times higher after the chemical injections. Without consideration of all these potential oxidant demands in the application design process, the required amount of oxidant delivered will be underestimated and ISCO will fail. On all ISCO projects ARS performs a site specific treatability test to determine the background SOD for the In situ environment at the site.

Liquid Atomization Injection of Oxidants

When applying chemical oxidants in the field, ARS uses a proprietary Liquid Atomization Injection method to emplace the materials within the subsurface. ARS exclusively offers this Liquid Atomization Injection process. When integrated with the patented Pneumatic Fracturing process, more effective emplacement of the chemical oxidants and the subsequent treatment of the contaminants occur. Additionally a greatly injection expanded radius of influence minimizes the number of application points necessary to treat an area and facilitate a higher material injection rate. With our commercial equipment and Liquid Atomization Injection process, ARS can inject chemical oxidants at dosages well above the normal solubilities.

During the injection process, the oxidant in liquid or slurry form is introduced into a gas stream prior to being injected into the formation. The energy and flow volume of the gas as the carrier fluid "atomize" the liquid into a mist or aerosols that are dispersed into the soil matrix. This technique distributes the oxidants at a faster rate and more uniformly into the targeted zones. The atomized injection technique can significantly minimize the potential for soil plugging, since the chemical oxidant is rapidly delivered to the targeted locations before significant reaction of the oxidant can occur.

To the right is a simplified process schematic of our Liquid Atomization Injection method.