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CHP ISCO

Catalyzed H2O2 Propagations

CHP stands for catalyzed H2O2 propagations or, more colloquially, catalyzed hydrogen peroxide. An older term for CHP is modified Fenton’s reagent, because it is a variation on the classic Fenton’s reagent, which is dilute hydrogen peroxide added slowly to a solution of iron (II). In the classic Fenton’s reaction, iron (II) mediates the decomposition of hydrogen peroxide to form hydroxyl radical (OH•). The generation of hydroxyl radical in this system is highly efficient, because the dilute nature of the hydrogen peroxide prevents side reactions from occurring. Hydroxyl radical is a strong, relatively nonspecific oxidant that reacts rapidly with a wide range of compounds, even those with a high degree of halogenation. However, it does not react with completely halogenated compounds, such as carbon tetrachloride or perfluorooctanoic acid (PFOA), because their oxidation state is to high to be further oxidized. In addition, hydroxyl radical does not react with contaminants that are sorbed to soil or present in nonaqueous phase liquids (NAPLs).

Adding dilute hydrogen peroxide drop by drop is obviously impractical for ISCO applications, so the classic Fenton’s reagent was modified by using higher concentrations of hydrogen peroxide (usually 2–12%) and often other catalysts as well, such as iron (III) instead of iron (II), iron chelates, or naturally occurring metal oxide minerals in the subsurface. The high concentration of hydrogen peroxide and the variety of catalysts allow a complex series of propagation reactions to occur—hence the name “catalyzed H2O2 propagations.” These propagation reactions generate a number of other reactive oxygen species, including hydroperoxide anion (HO2) and superoxide (O2). Hydroperoxide is a nucleophile and superoxide is a reductant and a nucleophile. Unlike hydroxyl radical these species can react with highly oxidized contaminants such as carbon tetrachloride and PFOA. More importantly, we have shown that superoxide generated in CHP systems can disrupt NAPLs and actively desorb contaminants that are sorbed to soil, allowing sorbed and NAPL contaminants to then be destroyed in the aqueous phase. The generation of superoxide along with hydroxyl radical makes CHP systems a sort of “universal treatment matrix” that can treat almost any organic contaminant in the subsurface.

For an in-depth description of CHP chemistry and CHP ISCO, see our 2005 review article: Watts, R. J., Teel, A. L., 2005. Chemistry of modified Fenton’s reagent (catalyzed H2O2 propagations—CHP) for in situ soil and groundwater remediation., J. Environ. Eng., 131(4), 612–622. 

For more details on non-hydroxyl radical reactivity in CHP systems, see:

Reduction of highly oxidized compounds:
Mitchell, S. M., Ahmad, M., Teel, A. L., Watts, R. J. 2014. Degradation of perfluorooctanoic acid (PFOA) by reactive species generated through catalyzed H2O2 propagation reactions. Environ. Sci. Technol. Lett., 1(1), 117–121.

Smith, B. A., Teel, A. L., Watts, R. J., 2004. Identification of the reactive oxygen species responsible for carbon tetrachloride degradation in modified Fenton’s systems. Environ. Sci. Technol., 38(20), 5465–5469.

DNAPL dissolution/destruction:
Smith, B. A., Teel, A. L., Watts, R. J., 2009. Destruction of trichloroethylene and perchloroethylene DNAPLs by catalyzed H2O2 propagations (CHP—modified Fenton’s reagent). J. Environ. Eng., 135(7), 535–543.

Smith, B. A., Teel, A. L., Watts, R. J., 2006. Mechanism for the destruction of carbon tetrachloride and chloroform DNAPLs by modified Fenton’s reagent. J. Contam. Hydrol. 85(3–4), 229–246.

Desorption of sorbed contaminants:
Corbin III, J. F., Teel, A. L., Allen–King, R. M., Watts, R. J., 2007. Reactive oxygen species responsible for the enhanced desorption of dodecane in modified Fenton’s systems. Water Environ. Res. 79(1), 37–42.

 

What is ISCO?

In situ chemical oxidation, or ISCO, is one of the more rapid methods for treating subsurface contamination. Sites with contaminated soil and groundwater are a serious concern in the U.S. and around the world; tens of thousands of such sites exist, often the result of improper or illegal disposal of hazardous wastes such as chlorinated dry cleaning solvents, gasoline, dioxins, or PCBs.

In situ means in place; ISCO treatments are injected into the ground through wells to destroy contaminants within the subsurface, without the need to excavate contaminated soil or pump out contaminated groundwater for treatment. Chemical oxidation refers to the type of chemical reaction that is common to all ISCO processes; it can be defined as a loss of electrons or a gain of oxygen. Oxidation transforms organic contaminants into other, usually less hazardous compounds, including water and carbon dioxide. The four most common ISCO technologies are permanganate, ozone, activated persulfate, and CHP, which stands for catalyzed H2O2 propagations or catalyzed hydrogen peroxide and was formerly known as modified Fenton’s reaction. ISCO treatment is usually more rapid than alternative processes such as pump-and-treat, bioremediation, natural attenuation, air sparging, or soil vapor extraction.

Our research has shown that in addition to chemical oxidations, activated persulfate and CHP ISCO processes also include chemical reductions and nucleophilic substitutions among their mechanisms, which broadens the range of contaminants they are able to treat. These ISCO processes have also been shown to treat difficult or recalcitrant contaminants, including highly oxidized compounds, sorbed contaminants, and contaminants found in dense nonaqueous phase liquid (DNAPL) pools.

For a more detailed overview of the four major ISCO technologies, please see our review article:
Watts, R. J., Teel, A. L., 2006. Treatment of contaminated soils and groundwater using in situ chemical oxidation. Pract. Period. Haz. Waste Manag., 10(1), 2–9

and the recent book:

In Situ Chemical Oxidation for Groundwater Remediation (SERDP/ESTCP Environmental Remediation Technology Volume 3). R.L. Siegrist, M. Crimi, and T.E. Simpkin, Eds. Springer, 2011.