In-Situ Chemical Oxidation (ISCO)

Chemical oxidation involves the use of concentrated oxidants to facilitate the chemical breakdown of hydrocarbons in the soil and groundwater. This chemical breakdown occurs as molecular bonds in organic compounds are “cleaved” and oxygen is inserted into the resulting fragments, producing end products of carbon dioxide, water, and harmless salts. The oxidation process is generally driven by the creation of aggressive oxidant radicals that react on contact. Site treatment works on contact and needs full oxidant contact for success.

Application methods include In-Situ Chemical Oxidation (ISCO) and soil blending. ISCO applications involve target liquid injection usually under mild pressure in order to “flush” the vadose zone and underlying saturated zone in order to desorb and oxidize target compounds. To reach target goals, desorption of contaminants from the soil matrix into the groundwater is required.

Eden professionals utilize a variety of oxidants and oxidant combinations based on target contaminants and site conditions. Oxidant selection and dosing is often determined by completion of a treatability study, performed in our in-house laboratory.

Common chemical oxidants include the following:

Activated Sodium Persulfate

Activated Sodium Persulfate Chemical oxidation using activated sodium persulfate offers treatment of a wide range of hydrocarbons. Direct oxidation of sodium persulfate produces the following reaction:

S2O8-2 + 2H+ + 2e- → 2HSO4

The standard oxidation–reduction potential for this reaction is 2.1 Volts (V). In order to increase the oxidation strength and effectiveness of the reaction, sodium persulfate can be induced to form sulfate radicals with a redox potential estimated to be 2.6 V, similar to that of the hydroxyl radical (2.7 V) generated during CHP oxidation. This reaction is as follows:

S2O8-2 + activator → SO4∙- + (SO4∙- or SO4 -2)

Current research indicates that persulfate activation with sulfate radical production is successful with the four following activators:

Chelated iron or natural occurring transition metals;
Hydrogen peroxide; • Alkaline (High pH) activation; and
Heat.

Each activator has been shown to be a suitable catalyst for the oxidation of different compounds. The most powerful activators are alkaline and heat activation working on the widest range of organic compounds. Alkaline activation has the added benefit of producing a non-corrosive high pH oxidant solution that is safe around metallic objects. Chelated iron activation is useful for chlorinated and non-chlorinated alkenes and other less recalcitrant compounds. Hydrogen peroxide is also a powerful activator for persulfate; however, due to its rapid decomposition, it may not generate sufficient sulfate radicals prior to degradation.

Advantages of activated persulfate oxidation include the less aggressive nature of the reaction, greater persistence of sulfate radicals in the subsurface as compared to hydroxyl radicals, lower SOD, and effectiveness on a wide variety of hydrocarbons. Potential disadvantages include the need for activation to facilitate the reaction, the generation of residual sulfates that may exceed secondary drinking water standard (250 mg/L), temporary adjustment of the groundwater pH, and cost considerations.

Fenton’s Reaction/Catalyzed Hydrogen Peroxide Oxidation

Chemical oxidation using Fenton’s reaction or CHP involves free radical generation and direct oxidation when hydrogen peroxide is combined with a metal (iron) catalyst. Ferrous salts or iron chelates are commonly used as catalysts. In some cases, naturally occurring iron in the soil may be used to catalyze the peroxide. When an iron catalyst is used in a pH range of 3-5, a “Fenton’s Reaction” is produced releasing hydroxyl radicals under strongly exothermic conditions. If this reaction is created without pH modification or by using a slow release iron catalyst or similar stabilizer, a “Modified Fenton’s” or CHP reaction is produced. The CHP reaction generally has a greater longevity in the subsurface due to slower reaction times.

The basic reaction for CHP or Fenton’s oxidation is as follows:

H₂O₂ + Fe⁺² → Fe⁺³ + OH^– + OH

The hydroxyl radicals serve as very powerful, effective, and nonspecific oxidizing agents. Many side reactions are common with hydroxyl oxidation along with significant gas production due to the rapid decomposition of hydrogen peroxide. The reaction is relatively fast acting, taking only hours or days for completion.

Advantages of using CHP include its high oxidation potential (2.7-2.8 electron volts), rapid reaction with efficiency in a short time frame, usefulness in destruction of NAPL, remediation of a wide variety of hydrocarbon compounds, and low cost potential. The primary limitation of CHP is limited contaminant contact and reduced radius of treatment often caused by rapid decomposition.

Sodium and Potassium Permanganate

Permanganate is an oxidizing agent with an affinity for oxidizing organic compounds containing carbon-carbon double bonds, aldehyde groups or hydroxyl groups. Permanganate oxidation is generally performed using one of two commercially available products:

Potassium permanganate – KMnO4
Sodium permanganate – NaMnO4.

The permanganate anion is useful for oxidation of a select number of compounds including chlorinated ethenes and some aromatics and polynuclear aromatic hydrocarbons. Typical oxidant byproducts include carbon dioxide (CO₂), manganese dioxide (MnO₂), and chloride salts. Advantages of using permanganate over other oxidants include the longevity of the permanganate ion in the subsurface (up to a year or longer at some sites), easier handling, as well as potential cost savings. Disadvantages include a lower oxidation strength (1.7 volts), higher SOD, and its limited use on select compounds.

Ozone

Ozone is a tri-atomic form of oxygen comprised of three oxygen atoms typically formed under the influence of ultraviolet radiation or an electrical arc (corona discharge). In nature, ozone is formed in the atmosphere from ultraviolet rays of the sun or the electrical discharge of lightning. In a typical ozone generator, ozone is formed by passing an oxygen stream across an electrical corona discharge within a reaction chamber. This process creates an ozone concentration of approximately 5% by weight in the process air stream, as shown in the formula below.

O₂ + Electric Current O₃

Ozone is a relatively unstable molecule with a short half-life of approximately two minutes when exposed to air as it degrades back to oxygen. In the aqueous phase, dissolved ozone has a half-life of approximately -20- minutes as it reacts with water to form hydroxyl-radicals or degrades back to oxygen. The relative oxidization potential of ozone is 1.77 volts. The short life span can make ozone an ideal oxidant for in-situ remediation of contaminants, as it reacts quickly in the subsurface and rapidly reverts back to oxygen.

Once ozone is introduced into the subsurface, the ozone gas can oxidize both organic and inorganic contaminants either directly with ozone molecules or through the generation of free-radical intermediates, such as hydroxyl radicals. Common contaminants oxidized by ozone gas include aromatics, PAHs, chlorinated alkenes, pesticides, aliphatic hydrocarbons, phenols, and chlorinated solvents.

Ozone has been most widely used for vadose zone treatment but has also been injected into the subsurface along with air sparging for remediation in the saturated zone. Limitations of in-situ ozone oxidation in subsurface applications include its high reactive rate, instability, short half-life, and lower oxidation potential as compared to some liquid oxidants. In addition, since ozone is an unstable gas, it must be produced on-site using a treatment system. Advantages include ease of delivery, potential cost reduction as compared to liquid oxidant purchase, and the production of an oxidant source to enable subsequent biodegradation.