Oxidants on the Job

Chemical oxidation is proving successful in degrading soil and groundwater contaminants

• By Jason Muessig
• Jun 01, 2006

The need for removal and destruction of contaminants of concern (COC), like petroleum hydrocarbons and chlorinated organics, in soil and groundwater has led to the development of a wide range of technologies for both in ground (in-situ) and above ground (ex-situ) treatment of these contaminants. Many of these technologies involve the use of peroxygens for either direct oxidation or enhanced bioremediation of these pollutants. The choice of the appropriate treatment approach depends upon a careful evaluation of the site and a good understanding of available technologies.

Oxidation
Oxidation potential is a good way to compare the oxidizing power of various peroxygens. However, there are many other parameters that affect performance in remediation.

Site evaluation is also crucial in properly selecting the appropriate peroxygen. Careful attention should be paid to the target contaminant, the hydrogeology of the site, its location, time frame available for remediation, and remediation goals.

Chemical oxidants, by definition, are not selective. Due to this intrinsic property, identification of areas with high contaminant concentrations results in more efficient oxidation because there is less natural background competition to the oxidizer. Hotspots or point-source areas are prime examples of where oxidation is most effective.

Intimate mixing of the chemical oxidant and the contaminant is also vital for efficient oxidation. In in-situ treatment, highly permeable soils that are loose or sandy are best in allowing migration of the oxidant to the source area. Tight or compact soils may require closer spacing of injection points or fracturing of the soil.

For ex-situ treatment, thorough mixing of the contaminated soil and peroxygen ensures highly efficient oxidation. Common forms of ex-situ treatment with peroxygens are pump-and-treat systems and furrow-based remediation. The industry is continuously developing new approaches to soil remediation that enhance the performance of oxidizing agents. Of note is the recent introduction of activated persulfate technology, enhanced by the addition of hydrogen peroxide, for the improved degradation of petrochemical and chlorinated compounds.

Another approach is the combined oxidation and bioremediation of petrochemical contaminants. This technology involves the preliminary oxidation of pollutants using a modified Fenton's technology, based on sodium carbonate peroxyhydrate or hydrogen peroxide, followed by bioremediation using calcium peroxide.

Enhanced Bioremediation
Enhanced bioremediation is a process in which organic contaminants found in soil and/or groundwater are degraded by indigenous or inoculated micro-organisms, transforming them to innocuous end products.

Peroxygens enhance bioremediation through the release of oxygen into the subsurface, to supplement the often rate-limiting oxygen needed by aerobic microorganisms.

Available oxygen is a measure of the total amount of active oxygen that can be produced from a particular chemical, usually expressed as a percentage by weight of the chemical. The rate of oxygen release is a function of the nature of the product as well as its formulation.

The choice of peroxygen for enhancing bioremediation depends upon the specific site needs. Sometimes a fast initial oxygen release coupled with a slow long-term release should be used.

Faster oxygen-releasing chemicals such as percarbonate or peroxide are used to immediately bring up the dissolved oxygen content of the soil. These products also have the potential to oxidize species that compete for oxygen demand, such as reduced nitrogen and sulfur, which helps prevent a relapse of the soil and water milieu into anaerobic conditions.

It is important to note that, contrary to initial belief, hydrogen peroxide does not sterilize the soil and its use typically results in an increased microbial population and activity a few weeks after treatment.

Slow oxygen-releasing compounds such as magnesium peroxide and calcium peroxide are usually the preferred choice in long-term bioremediation.

Product Properties
Although all peroxygen-based chemicals are safe when used properly, careful attention should be paid to the intrinsic properties of each chemical to ensure that the appropriate safety precautions are implemented.

Hydrogen Peroxide (H₂O₂)
H₂O₂ can be used alone, in conjunction with iron catalysts, or as a supplemental oxygen source to aid in microbiological treatment. The benefits of hydrogen peroxide include its strong oxidizing power, its ability to treat a wide range of pollutants, and its environmentally compatible byproducts -- water and oxygen.

Concentrated H₂O₂ (35 to 50 percent by weight) should be diluted prior to injection with clean water. Typical concentrations used for oxidation are between 10 and 15 percent. Concentrations used for enhanced bioremediation are between 0.5 and 6 percent.

Sodium Percarbonate (2Na₂CO_32O2)
Sodium percarbonate is a granular product containing approximately 27 to 30 percent H₂O₂ by weight. When dissolved in water, it produces a solution of soda ash and H₂O₂. An 8- to 10-percent solution of sodium percarbonate contains about 3 percent H₂O₂ and has an alkaline pH.

The main advantage of sodium percarbonate is its ease of handling when compared to liquid H₂O₂. This is of particular value in remediation projects in close proximity to residential areas, or in remote locations.

Sodium Persulfate (Na₂S₂O₈)
Recently the use of sodium persulfate, either alone or in combination with other chemicals, has increased. Sodium persulfate dissociates in water into persulfate anions, which are strong oxidants but are kinetically slow in destroying many organic contaminants.

Typically, persulfate is activated through ferrous-ion or heat, and most recently, diluted hydrogen peroxide. This leads to the production of sulfate-free radicals (SO4^¸-), which are highly reactive but short lived.

Magnesium Peroxide (MgO₂)
Magnesium peroxide is the first inorganic peroxide introduced for the enhanced bioremediation of many pollutants. Originally it was applied through the use of socks in wells, but the technology evolved to direct injection in a slurry form. Recently, magnesium peroxide has been largely replaced by calcium peroxide.

Calcium Peroxide (CaO₂)
In the past few years, calcium peroxide has become the preferred source of slow-release oxygen for assisted bioremediation because of its high concentration of active oxygen.

The rate of oxygen generation is influenced by the physical and chemical properties of the surrounding medium, as well as the inherent properties of the product itself.

Formulation -- This has a direct relationship to product stability and oxygen release profile. Commercial brands have different oxygen release rates, which affects performance and timing of site closure.

Particle size -- This plays a key role in remediation efficiency as it affects the product's ability to horizontally penetrate the soil matrix. Particles in the range of 10 to 15 microns will have a lower potential of clogging pores and a higher chance of deep horizontal penetration in the formation. A smaller particle size also produces a more homogeneous slurry.

Conclusion
The use of peroxygen technology in soil and groundwater remediation continues to evolve. New products such as sodium percarbonate have been introduced, and new technologies and patents are being developed to improve on existing chemistries. While chemical oxidation is proving to be of great value for the degradation of a wide array of pollutants, the challenge is to find better ways to bring the active species in close contact with the contaminant.

Enhanced bioremediation through peroxygens has quickly gained acceptance as an economical means of remediating low level pollutants. The introduction of calcium peroxide products in the past few years has led to better performance in soil remediation.

Oxidation and bioremediation often complement each other. A thorough site characterization, coupled with a good understanding of peroxygen chemistry, improves the potential of successfully remediating a site.

References

• WO 2005/012181 publ. Feb.10, 2005, Treatment of Environmental Contaminants.
• WO 2005/118170 publ. Dec. 15, 2005,, Combined Chemical Oxidation/Assisted Bioremediation of Contaminants.
• In-situ Oxidation Team "Technical and Regulatory Guidance for In Situ Chemical Oxidation of Contaminated Soil and Groundwater", Second Edition, Interstate Regulatory & Research Council, Jan 2005.

This article originally appeared in the 06/01/2006 issue of Environmental Protection.