With over 10 years of chemical oxidation experience, including industry leading design engineers, ARS uses a wide range of materials/chemicals when designing remediation options involving chemical oxidation. ARS' Design Engineers will select the chemical and an activator as needed based on site specific factors, including: the target contaminants, lithology, and hydrogeology. Equally as important to the proper chemical solutions, is the injection protocol that will ensure the contact between the chemical oxidant and the contaminant. ARS' real field experience and proprietary injection techniques are the reason we achieve successful remediation designs on nearly every project we are involved with.
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 safe and successful use.
Permanganate 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 and rapidly converts a wide range of chlorinated alkenes (R-C=C-R) 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. It 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.
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. Although the manganese dioxide is insoluble in ground water, another potential by-product, manganate (MnO42-) 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 typically used as a 10 to 20 % 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. When injected into an aquifer, persulfate salts dissociate in groundwater to form persulfate anions (S2O82-) that serve as a powerful oxidant. More notably, the addition of a activator dramatically increases the oxidatve strength of the persulfate through the formation of sulfate free radicals (SO4-) as represented in Equations 1 through 4 below. Free radicals are molecular fragments that have an unpaired electron causing them to be highly reactive as strong oxidizing agents and are known to rapidy oxidize many COCs including BTEX, PAHs, TPH.
Sulfate Radical Generation and Reactions:
RH represents organic compound; R represent oxidized organic compound Soil oxidant demand and metals contribute to oxidant consumption.
Note: Free radical chemistry is not necessarily stoichiometric or straightforward.
Ethyl benzene Destruction (general equation):
One mole of persulfate is required to destroy one mole of ethylbenzene
Toluene Destruction (general equation):
One mole of persulfate is required to destroy one mole of toluene
Changes in subsurface geochemistry associated with sodium persulfate activation chemistry consist of either a localized temporary elevated pH (alkaline activation), a temporary localized drop in pH (metal activation) and a temporary accumulation of sulfate in and around the treatment zone. Contaminant treatment times with activated sodium persulfate are on the order of weeks rather than months. If, however, a significant amount of absorbed mass exists with the soil matrix, longer treatment times may be required to treat partioning of contaminats from the absorbed to the dissolved phase.
One persulfate product used by ARS engineers, is a product called Klozur-CR™. Klozur CR is a single, formulated product consisting of high pH - activated Klozur Persulfate and PermeOx®Plus, engineered calcium peroxide, combining chemical oxidation with biostimulation to treat contaminant source zones and down-gradient plumes. As a result, the product can be applied to either source areas using initially chemical oxidation and then promoting biological activate on the remaining contaminant in the source area and in the down gradient plumes. When injected into an aquifer, persulfate salts dissociate in groundwater to form persulfate anions (S2O82-) which serve as a powerful oxidant. More notably, the addition of an activator (pH modifier -PermeOx®-plus) dramatically increases the oxidative strength of the persulfate through the formation of sulfate free radicals (SO4-)
Biological activity is promoted by the generation of oxygen in the subsurface were it is typically limiting. Oxygen is generated by the following reaction:
In the presence of oxygen, the petroleum-based contaminants are degraded. Depending on the site specific conditions, PermeOx®-Plus can generate oxygen for 3 to 6 months after applied.
Soil Oxidant Demand (SOD)An 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 OxidantsWhen 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 a 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 (ROI) 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.