User:Snehakrishna/ISCO

In situ chemical oxidation (ISCO), a form of advanced oxidation processes and advanced oxidation technology, is an environmental remediation technique used for soil and/or groundwater remediation to reduce the concentrations of targeted environmental contaminants to acceptable levels. ISCO is accomplished by injecting or otherwise introducing strong chemical oxidizers directly into the contaminated medium (soil or groundwater) 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.

Chemical oxidation is one half of a redox reaction, which results in the loss of electrons. One of the reactants in the reaction becomes oxidized, or loses electrons, while the other reactant becomes reduced, or gains electrons. In ISCO, oxidizing compounds, compounds that give electrons away to other compounds in a reaction, are used to change to change the contaminants into harmless compounds. 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(trichloroethene and tetrachloroethene), and gasoline-related compounds (benzene, toluene, ethylbenzene, MTBE, and xylenes) by ISCO is possible. Some other contaminants can be made less toxic through chemical oxidation.

A wide range of ground water contaminants react either moderately or highly with the ISCO method, and ISCO can also be used in a variety of different situations (ex. unsaturated vs saturated ground, above ground or underground, etc), so it is a popular method to use.

History

Fenton's reagent (hydrogen peroxide catalyzed with iron) and potassium permanganate are the oxidants that have been used the longest, and are now used the most widely. The systems of using hydrogen peroxide were based on the work of H.J.H. Fenton, who used iron salts to catalyze the compound. 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.

As the industry shifted its focus to the remediation of chlorinated solvents, hydrogen peroxide was found to be effective in both the hydrocarbon industry and the chlorinated solvent industry. Scientists also found that permanganate could be used on chlorinated solvents. Permanganate based ISCO started to be used in the late 1990's. At this time, ISCO had also become and established remedial technology.

The development of sodium persulfate for ISCO began in the late 1990's because of the limitations in using peroxide or permanganate as oxidants. Peroxide has such a short life that it can not be transported properly. Permanganate only treats chlorinated solvents with double bonds and is easily used up by organic material in soil. Persulfate is more stable, treats a wider range of contaminants, and is not used up by soil organics as easily.

Agents of Oxidization

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

Permanganate

Permanganate is used in groundwater remediation in the form of potassium permanganate (KMnO
₄) and sodium permanganate (NaMnO
₄). 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 (PCE), trichloroethylene (TCE), and vinyl chloride (VC). 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 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. MnO⁻
   ₄ + 8H⁺
   + 5^-
   _e → Mn²⁺
   + 4H
   ₂O — (for pH < 3.5)
2. MnO⁻
   ₄ + 2H
   ₂O + 3^-
   _e → MnO
   ₂(S) + 4OH⁻
   — (for pH 3.5 to 12)
3. MnO⁻
   ₄ + ^-
   _e → MnO²⁻
   ₄ — (for pH > 12)

The typical reaction that occurs under common environmental conditions is equation 2. This reaction forms a solid product, MnO
₂.

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 clean up 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 MnO
₂, 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 MnO
₂ from the contaminated area.

Fenton's Reagent

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 [ferric] iron (Iron III). At a low pH, it forms hydroxyl radicals(·OH) that oxidize contaminants such as chlorinated solvents, fuel oils, and BTEX. 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.

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· + H
   ₂O
   ₂ → Fe(III) + HO^·
   ₂ + H⁺
   
4. HO· + Fe(II) → Fe(III) + OH⁻
   
5. Fe(III) + HO^·
   ₂ → Fe(II) + O
   ₂H⁺
   
6. Fe(II) + HO^·
   ₂ + H⁺
   → Fe(III) + H
   ₂O
   ₂
7. HO^·
   ₂ + HO^·
   ₂ → H
   ₂O
   ₂ + O
   ₂

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 Fe²⁺
as a catalyst. The overall end result of the process can be described by the following reaction:

H
₂O
₂ + contaminant → H
₂O + CO
₂ + O
₂

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.

This method, however, is very exothermic. Over the course of the reaction, the water starts beating up and, in some cases, can boil water out of the soil. The chemicals used in the process are detrimental to microbes and rapidly degrade the soil, so Fenton's reagent must be used with extreme caution. The hydroxyl radicals also react with many other substances other than the targeted contaminant, so it is not as effective in soils containing carbonate, bicarbonate, and organic matter.

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 (S
₂O²⁻
₈) but is generally called a persulfate ion by scientists in the field of environmental engineering. More specifically, sodium persulfate is used because it has 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 SO⁻
₄ is usually derived from the persulfate because sulfate radicals can degrade a wider range of contaminants at a faster pace(about 1,000-100,000 times) 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 substance called 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 (-NH2), hydroxyl (-OH), and alkoxy (-OR) groups. Conversely, the sulfate radical does not react as much in compounds that contain electron attracting groups like nitro (-NO2) and carbonyl (C=O) 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 (it must be turned into the sulfate radial) to be effective in the decontamination. The catalyst that is most commonly used is ferrous iron (Iron II). When ferrous iron and persulfate ions are mixed together, they produce ferric iron (iron III) 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 (ZVI) 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.

Ozone

While oxygen is a very strong oxidant, it's elemental form O
₂ is not very soluble in water. This posses a problem in ground water remediation, because the chemical must be able to mix with water to remove the contaminant. Fortunately, ozone (O
₃) is about 12 times more soluble than O
₂ and, although it is still comparably insoluble, it is a strong oxidant.

The unique part of ozone oxidation is its in-situ application. Because, unlike other oxidants used in ISCO, it is a gas, it needs to be injected into the contamination site from the bottom rather than the top. Tubes are built into the ground to transport the ozone to its starting place; the bubbles then rise to the surface. Whatever volatile substances are left over are sucked up by a vacuum pump. Because the bubbles travel more vertically than horizontally, close placement of ozone injection wells is needed for uniform distribution.

The biggest advantage in using ozone in ISCO is that ozone does not leave any residual chemical like persulfate leaves SO²⁻
₄ or permanganate leaves MnO
₂. The processes involved with ozonation (treating water with ozone) only leave behind O
₂. Ozone can also react with many of the important environmental contaminants. In addition, because ozone is a gas, adding ozone to the bottom of the contaminant pool forces the ozone to rise up through the contaminants and react. Because of this property, ozone can also be delivered more quickly. Also, in theory, H
₂O
₂ co-injected with ozone will result in -OH ions, which are very strong oxidants.

However, ozone has many properties that pose problems. Ozone reacts with a variety of contaminants, but the problem is that it also reacts quickly with many other substances such as minerals, organic matter, etc. that are not the targeted substances. Again, it is not very soluble and stays in gas form in the water, which makes ozone prone to nonuniform distribution and rising up to the top of contamination site by the shortest routes rather than traveling through the entire material. In addition, ozone must be generated, and that requires a huge amount of energy.

Implementation

In-situ application of ozone

Application of ozone during in-situ chemical oxidation

The primary delivery mechanism for ISCO is through perforated, hollow metal rods hammered into the ground by "direct-push" drilling methods or by injecting the oxidant into wells installed using hollow stem auger, rotary drilling methods. One advantage of injection wells is that they can be used for multiple applications of the oxidant material, while direct push injection techniques are generally quicker and less expensive. Injection wells for ozone are typically constructed of a 1-2" stainless-steel screen set in sand pack, grouted to the surface using a combination of cement and bentoine clay. Often, a field pilot study must be performed to determine injection parameters and well spacing.

Oxidants such as Permanganate and Fenton's Reagent are delivered as water-based solutions. These substances are injected into the aquifer and then allowed to propagate by gravity and water current. As contaminants are encountered, the substances oxidize them and purify the water. Ozone is delivered (sparged) as a gas in either a dry air or oxygen carrier gas. Specialized equipment is required for in-situ oxidation via ozone gas injection. The ozone has to be pumped into the groundwater from the bottom of the aquifer because the ozone gas is less dense than the water. As the ozone travels through the aquifer against gravity, it reacts with contaminants along the way. However, there are some specific methods of oxidant delivery including injection probes, hydraulic fracturing, soil mixing, vertical wells, horizontal wells, and treatment walls.

Injection Probes

Injection probes are used in areas where there is very low permeability. A small diameter probe (2 to 4 cm in diameter) is rotated or pushed into the ground while reagents are inserted into it at low pressure. The reagents travel down the core of the probe and exit out though small perforations along the sides of the probe which are located at certain intervals. The reagents travel away from the core by going into existing cracks and pores and create a "halo of reactivity" (from pg. 182 or Principles and Practices of In Situ Chemical Oxidation Using Permanganate). In order to optimize the amount of contaminant that is oxidized, the probes are set into the ground relatively close together, about .6-1.2 meters apart.

Hydraulic Fracturing

Hydraulic fracturing is the process of artificially creating fractures in a site that has low permeability and then filling the fractures with oxidants. First a hole is drilled into the ground, and then a forceful jet of water is used to create fractures. Course sand, which allows just enough permeability for oxidants to get though, is used to fill the fractures and prevent them from closing up, and after that, the oxidant is injected into the fracture.

Soil Mixing

Soil mixing can be used to deliver solid or liquid forms of oxidants to contaminated soil. For near surface contamination, standard construction equipment can be used. In order to apply this method in-situ and in deep soil, the oxidant must be pumped through a kelly bar (a piece of earth drilling equipment) to the place where the soil needs to be oxidized. The soil then has to be mixed by using mixing blades attached to the kelly bar.

Horizontal and Vertical Wells

Horizontal and vertical well networks are basically the use of long pipes that lead in and out of the contaminated aquifer or plume used to inject oxidants and extract the treated ground water. Vertical wells consist of one injection well and a number of extraction wells surrounding it. Horizontal well networks work similarly, but the pipe is slightly L-shaped at the bottom and injects oxidants and extracts treated groundwater horizontally. Horizontal wells are used especially when oxidants need to be delivered to thin layers of saturation.

Treatment Walls

Treatment walls are used to deliver oxidants to the end of a contaminant plume and can be used to prevent the migration of an oxidant. The walls usually consist of continuous trenches that are connected to a piping network into which oxidants can be injected into. Another version of this delivery system is the use of a disconnected series of vertical wells to inject the oxidant into the ground water. The factors that affect treatment wall application and performance are similar to the factors that effect the performance of permeable reactive barriers.

Case Studies

The ISCO technology has been tested many times in the field. The following are a few examples of studies that have been conducted to observe the effectiveness of ISCO.

Naval Air Station North Island

In January of 2007, the groundwater around the Naval Air Station North Island in San Diego County, California. This test treated a total of 60,000 gallons of groundwater and used about 22,646 pounds of sodium persulfate to do it. No catalysts were added to the persulfate, but there was a significant amount of contaminant reduction. The production of radical was concluded to be due to the elevated temperature of the groundwater (20°C-24°C). At the end of 19 days after the last injection of sodium persulfate, there was an overall TCE concentration reduction of greater than 90%.

Space Launch Complex 37

Space Launch Complex 37 supported the Saturn spacecraft launches from 1961-1971. Activities in the Complex included parts cleaning and engine flushing, which left two chlorinated volatile organic compound (CVOCs) source areas. The United Launch Allliance also used the area for launching the Delta IV launch vehicles prior to any remediation activities on the site. Maximum concentrations of CVOCs in the site were 9500 micro grams/Liter of cis 1,2-DCE and 7900 micro grams/Liter of vinyl chloride. Both sites were cleaned up with the use of ozone. An ozone injection grid was used that consisted of 116 stainless steel wells. After 16 months of ozone treatment, there was a contaminant mass reduction of 44% in one site and 70% in the other site.

Nebraska Ordnance Plant

The Nebraska Ordnance Plant is located near Mead, Nebraska was a military facility in World War II that produced bombs, rockets, and shells. In the process of creating these products, high explosives like 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) were used; to reduce the plant's workers' chemical exposure to these materials, RDX and TNT residues that collected on the floor were washed away with water routinely. The water went outside and into unlined ditches. The soil in the plant consequently became contaminated with RDX and TNT. Use of trichloroethene (TCE) to degrease pipelines further contaminated the area. The contaminants over the course of fifty years has also entered the groundwater in that area.

In order to stop the spread of the contaminated groundwater, an elaborate system of 11 extraction wells has been placed to contain the plumes. This method treats the water with granular activated carbon. This field was chosen as a testing site to see how effectively permanganate could remove explosive contaminants. On the field, two injection wells were placed so that a curtain of permanganate existed between them through which the contaminant plume would flow through. The results of the oxidation was a temporary decrease of the contaminant in the wells by 70-80%, but permanganate was not evenly distributed through the curtain. However, the test showed that permanganate was an effective tool to use to remove explosive contaminants from groundwater.

Performance Application

The effectiveness of the oxidation is contingent on the site lithology, the residence time of the oxidant, the amount of oxidant used, the presence of oxidizing materials other than the targeted contaminant, the degree of effective contact between the oxidant and the contaminant(s), and the kinetics of the oxidation reaction between the oxidant and contaminant.

The soil and groundwater are tested both before and after oxidant application to verify the effectiveness of the process. Monitoring of gases given off during oxidation can also help determine if contaminants are being destroyed. Elevated levels of CO₂ is an indicator of oxidation.

Safety and Hazards

The four main types of oxidants that are used in ISCO—Fenton's reagent, ozone, permanganate, and persulfate—are all strong oxidizing agents and pose serious hazards to the people who are working with them. For worker safety, site that are using ozone as the oxidant must test ozone levels in the air periodically because ozone has adverse respiratory effects. All oxidants must be stored properly so that they do not decompose and workers must ensure that they do not have skin contact with any of the oxidants.

Some ISCO compounds can react aggressively with organic contaminants and must be used with care on the site. Fenton's reagent in particular is highly exothermic and can cause unwanted effects on microbial life in the aquifer if it is not used carefully.

Future of ISCO

Although it is a relatively mature technology, there is a lot that can be done to improve ISCO technology. As of now, not enough is known about the processes that occur in ISCO to apply it to every contamination field immediately. For example, right now, ISCO is mostly applied alone, but there is potential for combining ISCO with other technologies like In Situ Chemical Reduction (ISCR).Because ISCO is not effective at treating contaminant plumes, we could have ISCO treat the source of the contaminant while In Situ Chemical Reduction (ISCR) treats the plume.

ISCO's implementation technology also needs to be enhanced. Right now, ISCO just depends on injecting an oxidant into the site of contamination and just letting the natural movement of the water at the site drive the spread of the oxidant. This method is not very effective because many of the oxidants, especially the stronger ones, are not very stable and react almost as soon as they are injected into the contaminated site, and not necessarily with the targeted substance. This problem can be fixed by creating oxidants that are more stable (so that they will spread out more and decontaminate water that might not otherwise be decontaminated) and specifically targeted to a couple of contaminants (so that the oxidant will not unnecessarily oxidize naturally occurring substances). Doing these two things will really help improve the effectiveness of ISCO. The delivery systems could also be improved to allow a greater spread to sent the oxidant to the optimal locations for counteracting the contaminant.

External Links

Additional information on this topic may be found at the following sites:

1. In Situ Chemical Oxidation for Groundwater Remediation--Book
2. Principles and Practices of In Situ Chemical Oxidation: A Short Course
3. Reaction and Transport Processes Controlling In Situ Chemical Oxidation of DNAPLs
4. Technology Practices Manual
5. US EPA Clean-Up Information
6. Oregon Health and Science University Rate Constant Database

There are also a variety of companies available for implementing this technology including:

1. Carus Corporation