In-Situ Chemical Oxidation

In-Situ Chemical Oxidation (ISCO), also known as “chemical oxidation “, is an in situ remedial technology that reduces concentrations of volatile constituents adsorbed to soils in the saturated zone. In-situ chemical oxidation involves the introduction of a chemical oxidant into the subsurface for the purpose of transforming ground-water or soil contaminants into less harmful chemical species. There are several different forms of oxidants that have been used for ISCO;  the four most commonly used oxidants: permanganate (MnO4-), hydrogen peroxide (H2O2) and iron (Fe) (Fenton-driven, or H2O2-derived oxi­dation), persulfate (S2O82-), and ozone (O3). The type and physical form of the oxidant indicates the general materials handling and injection requirements. The persistence of the oxidant in the subsurface is important since this affects the contact time for advec­tive and diffusive transport and ultimately the delivery of oxidant to targeted zones in the subsurface.

Application

This technology has been proven effective in reducing concentrations of volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs). In-Situ Chemical Oxidation (ISCO) is generally more cost effective when applied in the source area of contamination.

Site-specific conditions and parameters, in conjunction with oxidant-specific characteristics, must be carefully considered to determine whether ISCO is a viable technology for deployment relative to other candidate technologies, and to determine which oxidant is most appropriate. These issues and the advantages and disadvantages should be considered. The breadth of ground-water contaminants amenable to transformation via various oxidants is large. That is, many environmental contaminants react at moderately high rates with these oxidants. Therefore, a wide range of contaminant classes are amenable to chemical oxidative treatment.  Mixtures of contaminants may require treatment trains involving the sequential applica­tion of technologies to accomplish the treatment objec­tive. Chemical oxidation can be deployed under a variety of applications, i.e., in either the unsaturated or saturated zones, or possibly above-ground, and under a variety of hydrogeologic environments. There are potential advantages and disadvantages of ISCO that should be assessed when con­sidering the deployment of this technology.

Operation Principles

In-situ Chemical Oxidation (ISCO) is a process in which the oxidation state of a substance is increased. In general, the oxidant is reduced by accepting electrons released from the transformation (oxidation) of target and non-target reactive species. Oxidation of non-target species, including reduced inorganics in the subsurface, also involves the loss of electrons; however, the main target during ISCO involves organic chemicals. Oxidation of organic compounds may include oxygen addition, hydrogen abstraction (removal), and/or withdrawal of electrons with or without the withdrawal of protons. The main objective of chemical oxidation is to transform undesirable chemical species into species that are harmless or nonobjectionable.

For example, oxidation of trichloroethylene (TCE) and perchloroethylene (PCE) may produce reaction byproducts that include dichloroacetaldehyde and dichloroacetic acid, compounds with lower toxicity. Similarly, oxidation of phenolic compounds may produce an assortment of carboxylic acids (Huling et al., 1998) that are nontoxic. Oxidation of these byproducts to CO2 and H2O could be accomplished through additional oxidative treatment and expense, but may not be practical for economic purposes. These reaction byproducts may also serve as microbial substrate for natural attenuation processes.

In oxidative treatment systems, numerous reactions could potentially occur, including acid/base reactions, adsorption/desorption, dissolution, hydrolysis, ion exchange, oxidation/reduction, precipitation, etc. In environmental systems there is a wide array of reactants and conditions that influence reaction rates and pathways that vary from site to site. Often, numerous reactions are required to achieve innocuous end products, and many of the reaction intermediates are never identified.

Some of the factors that determine the effectiveness of ISCO are:

  • permeability of the soil,
  • soil structure and stratification,
  • soil moisture, and
  • metals in soils
  • depth to groundwater.

 

System Design

Bench-scale treatability studies and pilot tests can be useful to gain insight on the feasibility of contaminant oxidation prior to field-scale applications. In complex, heterogeneous sys­tems it is difficult to predict specific reactions, oxidation efficiency, oxidation byproducts, or whether any of the potential limitations apply. The methods and materials of bench-scale treatability studies may vary based on the oxidant used and the objectives. It is important to recog­nize the physical differences between bench- and field-scale systems. The use of bench-scale treatability results from simplified systems to design field-scale ISCO sys­tems must be heavily scrutinized.

 

Physical and chemical characteristics of the subsurface environment (hydrogeology, geology, geochemistry) vary from site to site and impact the fate and transport of the injected oxidant and reagents. Site characterization is critical to the feasibility assessment of ISCO and in the planning and design of pilot- and full-scale ISCO sys­tems.

 

 

Advantages and Disadvantages

Advantages

Disadvantages

Applicable to a wide range of contaminants

Concentration reductions greater than about 90% are   difficult to achieve using this technology alone, rebound is common.

Contaminants are destroyed in-situ

Effectiveness less certain when applied to sites with   low-permeability soil or stratified soils. Requires enhancement.

Short treatment times (usually 2 months to 6 months under   optimal conditions).

Oxidant delivery problems due to reactive transport and   aquifer heterogeneities.

 

Cost competitive: $40-60/ton of contaminated soil.

Contaminant mixtures may require treatment trains.

 

Potentially enhances post-oxidation microbial activity and   natural attenuation.

 

Limitations for application at heavily contaminated sites.

 

References

U.S.Environmental Protection Agency (EPA). 1991b. Guide for Treatability Studies Under CERCLA: Soil Vapor Extraction.Washington,DC: Office of Emergency and Remedial Response. EPA/540/2-91/019A.

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