ISCO Process

ISCO is a technology in which an oxidant, and other amendments as necessary, is introduced into contaminated media to react with site contaminants such as MTBE, other fuel oxygenates, and other organic compounds, converting them to innocuous products, such as carbon dioxide and water. Typically, hydrogen peroxide (H^^), ozone (O3), or permanganate (MnO-) oxidants have been used to treat MTBE in soil and groundwater. Persulfate (S2O-) compounds have also been used as chemical oxidants for treating MTBE. All of these chemicals react, either directly or through the generation of highly reactive free radicals, such as OH and H^, or SO-^, with organic compounds such as MTBE to break hydrocarbon bonds and form degradation products such as alcohols, carbon dioxide, and water. In some applications, different oxidants may be used in combination, such as H2O2/O3, or in conjunction with catalysts, such as H2O2 in the presence of ferrous iron (Fenton's chemistry or reagent), to enhance oxidation through the generation of free radicals. Depending on site conditions, oxidants may be introduced to the contaminated area using a variety of engineered approaches, including groundwater well injection, groundwater well recirculation, lance injection (jetting), PRBs, deep soil mixing, or soil fracturing.75

24.8.2 Effect of the Properties of MTBE and Other Oxygenates on Treatment

As with other organic and some inorganic contaminants, MTBE and other oxygenates are susceptible to degradation through oxidation reactions. If a sufficient amount and strength of oxidant and enough time are provided, all of the ether- and alcohol-based fuel oxygenates can be mineralized to carbon dioxide and water. For example, the following equations show the stoichiometric mineralization of some of the common oxygenates through oxidation using hydrogen peroxide:

ETBE, TAME, DIPE:

C6HwO + 18H2O2

6CO2 + 25H2O

MTBE:

C5H12O + 15H2O2

5CO2 + 21H2O

TBA:

C4H1()O + 12H2O2

4CO2 + 17H2O

Ethanol:

C2H6O+6H2O2

2CO2 + 9H2O

Methanol:

CH3O + 5/2H2O2

CO2 + 4H2O

Analogous equations can be derived for mineralization using other oxidants. However, the oxidation of MTBE or other oxygenates to carbon and water is a multistep, multipath process in which each step has different equilibrium and kinetic factors that govern the extent and rate that each reaction can take place. Not all oxidants have proven successful in the mineralization of MTBE, leaving by-products such as tert-butyl formate (TBF) and TBA. The full spectrum of possible reaction intermediates and governing criteria have not been determined for MTBE and the other oxygenates. However, in general, the greater the number of carbon atoms in the oxygenate, the greater the stoichiometric proportion of oxidant that will be required (under the same conditions) to fully oxidize it. For example, based on the previous equations, the complete mineralization of 1 lb of ETBE, MTBE, TBA, ethanol, and methanol would require 6.0, 5.7, 5.5, 4.4, and 1.22 kg (2.7 lb) of hydrogen peroxide, respectively.76

While the above comparison (or similar comparisons for other oxidants) of the amount of oxi-dant required for different oxidants may hold under controlled laboratory conditions, the actual amount and type of oxidant that is necessary for the treatment of MTBE or other oxygenates at a given site will depend on numerous factors beyond the amount of contaminant present including

1. The amount and types of other contaminants (such as other petroleum constituents) that will also consume oxidant.

2. The chemical composition of the soil and groundwater, specifically the amount of natural organic matter (NOM) and other reduced species, such as iron (II) or manganese (II); often analyzed as the chemical oxygen demand (COD) of the soil, or the soil oxidant demand.

3. The pH, alkalinity, and temperature of the treatment area; these conditions will affect equilibrium and kinetic constants defining the extent and rate that each oxidation step can take place.

4. The potential for biodegradation of site contaminants or oxidation products.

5. Hydraulic and geologic parameters, such as hydraulic conductivity, hydraulic gradient, and permeability, that will affect the migration and dissolution of the oxidant once it is introduced to the subsurface.

Because these factors can vary from site to site, typically, field analyses of these parameters and bench- and pilot-scale studies are conducted to determine the type and amount of oxidant required for a specific application.

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