Chemical Oxidation
Insitu Chemical Oxidation (CHEMOX or ISCO)
Insitu chemical oxidation (ISCO) has emerged as a cost-effective and viable insitu remediation technology for the treatment of VOCs in groundwater, soils, and sediments. The most common oxidants ARS has experience with are potassium permanganate and hydrogen peroxide. Both have been widely used in the past few years since they are relatively inexpensive and have a long history of successful uses in the wastewater treatment industry.
Soil Oxidant Demand (SOD)
The key 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 insitu environment at the site.
Chemical Oxidation - Potassium Permanganate & Sodium Permanganate
Potassium permanganate is a relatively mild oxidant solution 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 converted to insoluble manganese dioxide during the reaction occurring over the span of a few days.
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 hypomanganate diester. 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. There are two forms of permanganate, KMnO4 and NaMnO4. The oxidation reaction for KMnO4 with a chlorinated ethene can be written as follows:
C2HYClX + 2KMnO4 > 2CO2 + 2K+ + YH+ +2MnO2 + XCl-
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.
Liquid Atomization Injection of Oxidants
When applying chemical oxidants in the field, ARS uses a proprietary Liquid Atomization Injection (LAI) method to emplace the materials within the subsurface. This LAI process is exclusively offered by ARS. When integrated with the patented Pneumatic Fracturing process, more effective emplacement of the chemical oxidants and the subsequent treatment of the contaminants occurs. 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 solubility of 5-6%.
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.
The following is a simplified process schematic of our Liquid Atomization Injection method.
Permanganate with Persulfate Chemical Oxidation
Alternate oxidation approach for treating VOCs in soil and groundwater is to use persulfate in combination with permanganate, by either injecting them individually, sequentially or as a mixture. Sodium persulfate disassociates in water to form sodium and persulfate ions. In the presence of heat, ferrous iron or acidic conditions, the persulfate is converted to two (2) sulfate ions as illustrated in Equation 1.
(1) S2O8- + e- > 2 SO4-
The conversion of the persulfate to sulfate 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. Many sites have sufficient background conditions which allow the persulfate to oxidize the targeted VOCs present in the soils and groundwater without the addition of a catalyst.
At some sites, the total soil oxidant demand within the targeted treatment zone may be too high for persulfate addition alone, because of regulatory issues relating to the secondary drinking water standard for sulfate (250 ppm). In these instances, permanganate may be injected after the persulfate. The addition of persulfate (and subsequently formed sulfates) can generally lower the pH of subsurface environments, which is beneficial, since the electrical energy provided at the lower pH will increase the oxidation potential of the permanganate. Therefore, by adding the persulfate first, the oxidation power of the permanganate in the subsurface will be increased as identified in Equation 2 and Equation 3.
(2) (3.5<pH<7) MnO4- + 4H+ + 3e- > MnO2 + 2 H2O E0 = 1.70 V
(3) (7.0<pH<12) MnO4- +2H2O + 3e- > MnO2 + 2 H2O E0 = 0.59 V
Studies have shown that the reduction of permanganate in the subsurface produces generally insoluble manganese dioxide. The formation of these precipitates can result in soil plugging, rendering localized zones adjacent to the point of delivery significantly less permeability. These localized zones of reduced permeability around the point of delivery can significantly minimize the effective distribution of the chemical oxidant within the subsurface. Conventional delivery techniques which solely rely on hydraulic pumping or slower delivery rates to deliver the permanganate into the subsurface can have the potential of causing manganese dioxide precipitation close to the delivery points, resulting in localized soil plugging.
Chemical Oxidation Using Fenton's reagent
Fenton's reagent is the reaction of ferrous iron and hydrogen peroxide(H2O2) to produce hydroxyl radicals. Hydroxyl radicals are powerful oxidants. Fenton's chemistry in a laboratory setting is well understood and has been studied for more than 100 years. Applying Fenton's chemistry in the field to treat soil and groundwater contamination has challenges because of varying site geochemistry and delivery issues.
Chemical Oxidation using H2O2 in the presence of native or supplemental ferrous iron produces Fenton's reagent, which yields free radicals (OH°) that can rapidly degrade a variety of organic compounds.
However, the application of peroxide to soil and ground water systems involves a variety of competing reactions as follows:
H2O2 + Fe+2 > OH- + Fe+3 + OH°
H2O2 + Fe+3 > HO2° + H- + Fe+2
OH° + Fe+2 > OH- + Fe+3
HO2° + Fe+3 > O2 + H- + Fe+2
H2O2 + OH° > H2O + HO2°
RH + OH° > H2O + R°
Hydrogen peroxide also can auto-decompose in aqueous solutions with accelerated rates upon contact with mineral surfaces as well as carbonate and bicarbonate.
2 H2O2 > 2 H2O + O2
The simplified stoichiometric reaction for peroxide degradation of TCE is shown as:
3H2O2 + C2HCl3 > 2CO2 + 2H2O + 3HCl
Chemical oxidation using Fenton's Reagent is most effective under very acidic pH (e.g., pH 2 to 4) and becomes ineffective under moderate to strongly alkaline conditions and/or where free radical scavengers are present (e.g., CO3-2). The reaction is strongly exothermic and can evolve substantial gas and heat. The oxidative reactions are extremely rapid and follow first-order kinetics.
The complexity of reactions that occur when hydrogen peroxide is added to a soil and groundwater environment make it difficult to describe the reaction kinetics explicitly.
In the field, application of Fenton's Reagent is performed in sequential steps. Step one usually comprises a pH adjustment. An acid (phosphorous) is injected to temporarily reduce the pH of the soil and groundwater to below 4. This step may also serve to bring naturally occurring ferrous iron into solution. By using the LAI process to inject the acid, relatively uniform pH adjustment occurs within the subsurface.
The second step involves the injection of the H202. Utilizing H202 at a concentration ranging from 10% to 17% by mass, the reagent is emplaced within the subsurface. Careful delineation of buried utilities, structures and existing monitoring wells is essential for achieving a safe and effective result. Since H202 reacts very quickly, the critical aspect of having the chemical in contact with the target organic is accomplished using liquid atomized injection.
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Pneumatic Fracturing