Hydroxyl radical and non-hydroxyl radical pathways for trichloroethylene and perchloroethylene degradation in catalyzed HO propagation systems.

Catalyzed HO propagations (CHP) is characterized by the most robust reactivity of any of the in situ chemical oxidation (ISCO) technologies because it generates the strong oxidant hydroxyl radical along with nucleophiles + reductants, such as superoxide radical. The most common groundwater contaminants, trichloroethylene (TCE) and perchloroethylene (PCE), were used as model contaminants in evaluating the effect of hydrogen peroxide (HO) dosage on contaminant destruction kinetics. Both TCE and PCE degradation rates increased with HO dosages up to 0.1 M, and then decreased with higher HO dosages. Parallel reactions conducted with the addition of the hydroxyl radical scavenger 2-propanol and the hydroxyl radical-specific probe nitrobenzene confirmed that hydroxyl radical is primarily responsible for TCE and PCE degradation; however, 5-20% of their degradation was attributed to a non-hydroxyl radical mechanism. Reactions conducted with the superoxide probe tetranitromethane showed that superoxide generation rates increased with increasing HO doses. These results were confirmed by electron spin resonance spectroscopy. Therefore, the non-hydroxyl radical pathway for TCE and PCE degradation at HO ≥0.5 M was likely via nucleophilic attack by superoxide. The results of this research demonstrate that contaminants present in the aqueous phase that are reactive with hydroxyl radical require only low doses of HO (≤0.1 M), but subsurface systems contaminated with species not reactive with hydroxyl radical (e.g., carbon tetrachloride) require HO concentrations ≥0.5 M.