In situ chemical oxidation


In situ chemical oxidation is an environmental remediation technique proposed for soil and/or groundwater remediation to lower the concentrations of targeted environmental contaminants to acceptable levels. ISCO would be accomplished by introducing strong chemical oxidizers into the contaminated medium to destroy chemical contaminants in place. It can be used to remediate a variety of organic compounds, including some that are resistant to natural degradation. The in situ in ISCO is just Latin for "in place", signifying that ISCO is a chemical oxidation reaction that occurs at the site of the contamination.
The remediation of certain organic substances such as chlorinated solvents, and gasoline-related compounds by ISCO is effective. Some other contaminants can be made less toxic through chemical oxidation.
A wide range of ground water contaminants respond well to ISCO, so it is a popular method to use.

History

Fenton's reagent and potassium permanganate are the oxidants that have been used the longest and remain used widely. Hydrogen peroxide was first used in 1985 to treat a formaldehyde spill at Monsanto's Indian Orchard Plant in Springfield, Massachusetts. At this site, a 10% solution of hydrogen peroxide was injected into a formaldehyde plume. Fenton's reagent was initially used to treat hydrocarbon sites where benzene, toluene, and ethylbenzene were present.
Hydrogen peroxide is effective for chlorinated solvents as is permanganate, which also became an established remedial agent.
Sodium persulfate was introduced for ISCO in the late 1990s because of the limitations in using peroxide or permanganate as oxidants. The specificity of permanganate and consumption by non-target organic material in soil are examples of these limitations. Persulfate is more stable, treats a wider range of contaminants, and is not consumed by soil organics as easily.

Agents of Oxidization

Common oxidants used in this process are permanganate, Fenton's Reagent, persulfate, and ozone. Other oxidants can be used, but these four are the most commonly used.

Permanganate

is used in groundwater remediation in the form of potassium permanganate and sodium permanganate. Both compounds have the same oxidizing capabilities and limitations and react similarly to contaminants. The biggest difference between the two chemicals is that potassium permanganate is less soluble than sodium permanganate.
Potassium permanganate is a crystalline solid that is typically dissolved in water before application to the contaminated site. Unfortunately, the solubility of potassium permanganate is dependent on temperature. Because the temperature in the aquifer is usually less than the temperature in the area that the solution is mixed, the potassium permanganate becomes a solid material again. This solid material then does not react with the contaminants. Over time, the permanganate will become soluble again, but the process takes a long time. This compound has been shown to oxidize many different contaminants but is notable for oxidizing chlorinated solvents such as perchloroethylene, trichloroethylene, and vinyl chloride. However, potassium permanganate is unable to efficiently oxidize diesel, gasoline, or BTEX.
Sodium permanganate is more expensive than potassium permanganate, but because sodium permanganate is more soluble than potassium permanganate, it can be applied to the site of contamination at a much higher concentration. This shortens the time required for the contaminant to be oxidized. Sodium permanganate is also useful in that it can be used in places where the potassium ion cannot be used. Another advantage that sodium permanganate has over potassium permanganate is that sodium permanganate, due to its high solubility, can be transported above ground as a liquid, decreasing the risk of exposure to granules or skin contact with the substance.
The primary redox reactions for permanganate are given by the equations:
  1. + 8 + 5e− → + 4 —
  2. + 2 + 3e− → + 4 —
  3. + e− → —
The typical reaction that occurs under common environmental conditions is equation 2. This reaction forms a solid product,.
The advantage of using permanganate in ISCO is that it reacts comparatively slowly in the subsurface which allows the compound to move further into the contaminated space and oxidize more contaminants. Permanganate can also help with the cleanup of materials that are not very permeable. In addition, because both sodium permanganate and potassium permanganate solutions have a density greater than water's density, permanganate can travel through the contaminated area through density-driven diffusion.
The use of permanganate creates the byproduct, which is naturally present in the soil and is therefore a safe byproduct. Unfortunately, several studies have shown that this byproduct seems to cement sand particles together forming rock-like material that has very low permeability. As the rock-like materials build up, it blocks the permanganate from getting to the rest of the contaminant and lowers the efficiency of the permanganate. This can be prevented by extracting the from the contaminated area.

Fenton's Reagent

is basically a mixture of ferrous iron salts as a catalyst and hydrogen peroxide. A similar sort of reaction can be made by mixing hydrogen peroxide with iron. When the peroxide is catalyzed by soluble iron it forms hydroxyl radicals that oxidize contaminants such as chlorinated solvents, fuel oils, and BTEX. Traditional Fenton's reagent usually requires a significant pH reduction of the soils and groundwater in the treatment zone to allow for the introduction and distribution of aqueous iron as iron will oxidize and precipitate at a pH greater than 3.5. Unfortunately, the contaminated groundwater that needs to be treated has a pH level that is at or near neutral. Due to this, there are controversies on whether ISCO using Fenton's reagent is really a Fenton reaction. Instead, scientists call these reactions Fenton-like.
However, some ISCO vendors successfully apply pH neutral Fenton's reagent by chelating the iron which keeps the iron in solution and mitigates the need for acidifying the treatment zone.
The Fenton chemistry is complex and has many steps, including the following:
  1. Fe2+ + H2O2 → Fe3+ + OH· + OH−
  2. Fe3+ + H2O2 → Fe2+ + OOH· + H+
  3. HO· + → Fe + +
  4. HO· + Fe → Fe +
  5. Fe + → Fe +
  6. Fe + + → Fe +
  7. + → +
These reactions do not occur step by step but simultaneously.
When applied to In Situ Chemical Oxidation, the collective reaction results in the degradation of contaminants in the presence of as a catalyst. The overall end result of the process can be described by the following reaction:
Advantages of this method include that the hydroxyl radicals are very strong oxidants and react very rapidly with contaminants and impurities in the ground water. Moreover, the chemicals needed for this process are inexpensive and abundant.
Traditional Fenton's reagent applications can be very exothermic when significant iron, manganese or contaminant are present in an injection zone. Over the course of the reaction, the groundwater heats up and, in some cases, reagent and vapors can surface out of the soil. Stabilizing the peroxide can significantly increase the residence time and distribution of the reagent while reducing the potential for excessive temperatures by effectively isolating the peroxide from naturally occurring divalent transition metals in the treatment zone. However, NAPL contaminant concentrations can still result in rapid oxidation reactions with an associated temperature increase and more potential for surfacing even with reagent stabilization. The hydroxyl radicals can be scavenged by carbonate, bicarbonate, and naturally occurring organic matter in addition to the targeted contaminant, so it important to evaluate a site's soil matrix and apply additional reagent when these soil components are present in significant abundance.

Persulfate

Persulfate is a newer oxidant used in ISCO technology. The persulfate compound that is used in groundwater remediation is in the form of peroxodisulfate or peroxydisulfate but is generally called a persulfate ion by scientists in the field of environmental engineering. More specifically, sodium persulfate is used because it has the highest water solubility and its reaction with contaminants leaves least harmful side products. Although sodium persulfate by itself can degrade many environmental contaminants, the sulfate radical is usually derived from the persulfate because sulfate radicals can degrade a wider range of contaminants at a faster pace than the persulfate ion. Various agents, such as heat, ultraviolet light, high pH, hydrogen peroxide, and transition metals, are used to activate persulfate ions and generate sulfate radicals.
The sulfate radical is an electrophile, a compound that is attracted to electrons and that reacts by accepting an electron pair in order to bond to a nucleophile. Therefore the performance of sulfate radicals is enhanced in an area where there are many electron donating organic compounds. The sulfate radical reacts with the organic compounds to form an organic radical cation. Examples of electron donating groups present in organic compounds are the amino, hydroxyl, and alkoxy groups. Conversely, the sulfate radical does not react as much in compounds that contain electron attracting groups like nitro and carbonyl and also in the presence of substances containing chlorine atoms. Also, as the number of ether bonds increases, the reaction rates decrease.
When applied in the field, persulfate must first be activated to be effective in the decontamination. The catalyst that is most commonly used is ferrous iron. When ferrous iron and persulfate ions are mixed together, they produce ferric iron and two types of sulfate radicals, one with a charge of −1 and the other with a charge of −2. New research has shown that Zero Valent Iron can also be used with persulfate with success. The persulfate and the iron are not mixed beforehand, but are injected into the area of contamination together. The persulfate and iron react underground to produce the sulfate radicals. The rate of contaminant destruction increases as the temperature of the surroundings increases.
The advantage of using persulfate is that persulfate is much more stable than either hydrogen peroxide or ozone above the surface and it does not react quickly by nature. This means fewer transportation limitations, it can be injected into the site of contamination at high concentrations, and can be transported through porous media by density driven diffusion. The disadvantage is that this is an emerging field of technology and there are only a few reports of testing it in the field and more research needs to be done with it. Additionally, each mole of persulfate creates one mole of oxidizer. These radicals have low atomic weights while the persulfate molecule has a high atomic weight. Therefore, the value for expense is low compared to some other oxidizing reagents.