(390e) Chelate-Modified Hydroxyl Radical Reactions for Detoxification of Chlorinated Organics: Experimental Results and Model Development

Oxidative techniques utilizing free radicals have proven effective for the destruction of toxic organic compounds such as trichloroethylene (TCE) and polychlorinated biphenyls (PCBs). TCE is a groundwater contaminant commonly found in large pools known as dense non-aqueous phase liquids (DNAPLs). The most common oxidative destruction techniques used for groundwater remediation applications are modified forms of the Standard Fenton reaction, in which Fe(II) reacts with hydrogen peroxide to form Fe(III), a hydroxide ion, and a hydroxyl radical. This reaction takes place at low pH and cannot be performed effectively at near-neutral pH due to ferric hydroxide precipitation. This problem can be alleviated through the use of a non-toxic chelate (L), such as citrate or polyacrylic acid (PAA). The addition of a chelate allows the reaction to take place at near neutral pH and control hydrogen peroxide consumption. This chelate can be incorporated either into the aqueous phase in soluble form or in a polymer matrix.

In addition to experimental data, we have modeled the effects of the chelate:Fe ratio on contaminant degradation and hydrogen peroxide consumption. The chelate reduces the amount of Fe(II) available in the solution phase for reaction with hydrogen peroxide, affecting the production of hydroxyl radicals. The generation of these, as well as other, radicals determines the rate of degradation of TCE in the aqueous phase and the rate of disappearance of TCE DNAPLs (present in droplet form). Many remediation strategies have successfully degraded TCE present in the aqueous phase, only to experience a rebound from the dissolution of the DNAPL. In order to degrade the contaminants present in an efficient manner, it is necessary to develop a model to predict the degradation of contaminants dissolved in the aqueous and non-aqueous phases. This model is comprised of free radical concentrations and iron species distributions obtained by simultaneously solving equilibrium equations for Fe-chelate interactions and rate laws associated with pertinent hydroxyl radical reactions.

Support of this research has been provided by NSF-IGERT, NIEHS/SBRP, and DOE-KRCEE.