Oxidation of Nonaqueous Phase Trichloroethylene with Permanganate in Epikarst

Abstract

In situ chemical oxidation (ISCO) is one of the effective technologies used for source zone remediation of dense nonaqueous phase liquids (DNAPLs) such as chlorinated solvents in the subsurface environments. In karst systems, DNAPL source zones reside in epikarst where the contaminant is generally trapped in the soil or at the carbonate bedrock contact. The efficiency of oxidation of residual trichloroethylene (TCE) masses trapped in such environments by potassium permanganate was characterized via a series of batch, column, and flow cell experiments. The performance of oxidation reaction was evaluated by considering numerous factors, such as soil oxidant demand, mineralogy, heterogeneity and kinetics of dissolution, and oxidation processes. Batch experiments showed that at the low permanganate concentrations (2-10 mM), the rate of TCE oxidation by permanganate (0.86–5.1 × 10⁻³ s⁻¹) was in a range similar to the rate of soil oxidant consumption (0.55–2.1 × 10⁻³ s⁻¹); however, at the high permanganate concentrations (40–120 mM), this rate was about two orders of magnitude higher than the rate of soil oxidant consumption (2.4–2.9 × 10⁻⁴ s⁻¹). In addition, at the high oxidant concentrations, the oxidation kinetics of pure phase TCE (1.1–1.5 × 10⁻³ s⁻¹) was limited by the dissolution kinetics (1.4–1.6 × 10⁻³ s⁻¹). Column and flow-cell experiments showed that significant fraction of residual TCE masses trapped in the soil and at the bedrock contact were oxidized by ISCO technology using permanganate, however the efficiency of ISCO was limited by the rate-limited kinetics of TCE dissolution and the desorption. The results of flow-cell experiments also indicated that instead of continuous permanganate flushing of the TCE source zones, periodic permanganate injections providing sufficient time and oxidant dosage would enhance the performance of ISCO in epikarst environment much efficiently.