(63c) Chelate-Modified Fenton Reaction for the Destruction of Trichloroethylene in Two-Phase Systems

Trichloroethylene (TCE) is a volatile, halogenated organic compound that was once a widely used degreasing agent. Large plumes of TCE have been reported in various groundwater systems. Because concentrations of TCE can be above the solubility limit in water (~1000 ppm), a majority of the TCE is often present in the form of a dense, non-aqueous phase liquid (DNAPL) particularly at high depth. Several current remediation techniques successfully treat the dissolved TCE, but are unable to treat both aqueous and non-aqueous phases, eventually resulting in a rebound of TCE in the groundwater. A procedure to destroy the DNAPL efficiently and cost effectively is required. One such way is through oxidative destruction.

The most common oxidative destruction technique used is the Standard Fenton reaction, in which Fe(II) reacts with hydrogen peroxide to form Fe(III), a hydroxide ion, and a hydroxyl radical. This hydroxyl radical then reacts with TCE to form carbon dioxide and organic acids. The Standard Fenton reaction requires highly corrosive reaction conditions, and thus has limitations for in situ applications: it requires a low pH and hydroxyl radicals are rapidly consumed by hydroxyl scavengers found in the subsurface.

These problems are alleviated through the Chelate-Modified Fenton reaction, which includes the addition of a nontoxic chelate (L) such as citrate or gluconic acid. The chelate allows the reaction to take place at near neutral pH and control hydrogen peroxide consumption by binding to Fe(II), forming an FeL complex. This increases the H2O2:Fe(II) ratio greatly, therefore accelerating the rate of superoxide radical anion formation, aiding in DNAPL destruction. The chelate also binds to Fe(III), preventing its precipitation as ferric hydroxide and thus prevents problems associated with injection well plugging. It is also possible to generate both H2O2 and chelate (gluconic acid) through enzymatic reactions. Experimentation has shown that increasing the chelate:Fe(II) molar ratio reduces the amount of hydrogen peroxide used to degrade TCE. Our experimental results have also shown that > 90% dechlorination (through chloride ion and TCE analysis) can be achieved. This project is funded by DOE-KRCEE and by NIEHS.