Ozone Reactions in Groundwater Treatment

A number of organic compounds have ended up in the groundwater due to spills. Motor fuels and their associated constituents such as MTBE and various solvents used in a wide variety of commercial and industrial applications especially chlorinated solvents and 1,4 dioxane. These compounds have a high degree of toxicity and there is a requirement to reduce the toxicity by some type of transformation to a less toxic compound. Oxidation in general and ozone based oxidation has been found to be effective.

Molecular ozone directly reacts with both organic and inorganic materials. It is a selective and slow acting oxidant in this form. It readily reacts with aromatic compounds and certain inorganics such as Fe, Mn and H2S which can be found in ground water. Ozone in also in equilibrium with the hydroxyl radical which is a non-selective and rapid oxidant. This compound is a short lived meaning that it rapidly self decomposes.

General Ozone Oxidation and Related Chemical Reactions

Direct Oxidation
O3 + C2HCl3 + H2O -> 2 CO2 + 3 H+ + 3 Cl- (1)
OH Formation
O3 + H2O -> O2 + 2•OH (Slow) (2)

2 O3 + 3 H2O2 -> 4 O2 + 2•OH + 2 H2O (Fast) (3)

O3 + hν -> O2 + O (ozone exposed to UV photons)
O + H2O -> 2OH•(continuous of reaction to hydroxyl radical)

The equilibrium can be shifted to produce more hydroxyl radicals by a variety of means including increasing pH to alkaline conditions, adding hydrogen peroxide or exposing the ozone to UV light at 254 nm. Even in situations where the intent is to use molecular ozone water or subsurface soli conditions could push the equilibrium towards hydroxyl radicals. As a result, it is possible to have a mixed oxidant environment.

It is important to note that ozone demand in ground water treatment can be effected by both dissolved and undissolved species. ISCO application of ozone can see ozone demand from the subsurface soils and the materials contained therein. In pump and treat applications, non-anthropogenic dissolved species, notably minerals such as Fe and Mn can create demand for ozone, as well any oxidant that might be used.

With the above ideas in mind, molecular ozone reacts rapidly with olefins and aromatic compounds. Increasing chlorine substitution reduces the reaction rate. This means for these compounds the application of ozone must be done over a long time period. So, PCE does not react well with ozone, but DCE and vinyl chloride do react well with molecular ozone. Reaction rates with benzene are slow by the substitution of functional groups increases ozone’s rate of reaction, e.g. phenols or chlorophenols. Aliphatic alcohols, aldehydes, ketones and carboxylic acids generally do not react well with ozone. Under certain conditions, NH3 and amines can react with ozone.

In cases where molecular ozone is not effective, shifting the equilibrium toward the hydroxyl radical can speed up the reactions. Ozone has been used in ISCO to treat PAH, BTEX, MTBE, DCE, pyrene, and phenanthrene. In conjunction with hydroxyl radicals a broader range of compounds has been treated, e.g. TCE, PCE and 1.4 dioxane. Not all of these reactions can be easily conducted In-Situ, but may require Pump and Treat.