Abstract

1. Testing Benzene’s and Bicarbonate’s Effect on Potassium Permanganate Oxidation of TCE Kelly L. Pennell, PE--ARCADIS Sharon A. Jones, PE, PhD--Rose-Hulman Institute of Technology Anamaria Witaszczyk--Rose-Hulman Institute of Technology Abstract Recently, published literature has proposed several compounds are possible “scavengers” of potassium permanganate during in-situ oxidation. The majority of these scavengers occur naturally in the subsurface. However, contaminants, such as petroleum hydrocarbons, that are present in the subsurface may also act as scavengers. In order to adequately design an in-situ application process using potassium permanganate, an understanding of possible scavengers is essential. Notably, two compounds, benzene and bicarbonate, have been suggested as potential scavengers during the oxidation process even though their interference with the oxidation process is not intuitive. To investigate the potential for benzene and bicarbonate to act as scavengers during potassium permanganate oxidation, a series of laboratory batch tests were performed. [Top right: Shows reactivity of the benzene ring.] Electrostatic potential-Benzene (Blue indicates more negative charge; red more positive charge) Case studies have reported the decrease in benzene concentrations during in-situ oxidation applications, suggesting benzene consumed some or all of the oxidant. Theoretically, chemical oxidation of benzene is difficult due to delocalization effects of the carbon-double bonds within benzene’s aromatic ring structure (shown above). While research shows that aromatic ring compounds can be oxidized, if given adequate time and a strong enough oxidizing agent, such conditions can not usually be achieved during field applications. Background The primary mechanisms for permanganate oxidation have been studied from a physiochemical approach for over fifty years. In general, oxidation by permanganate can be attributed to manganese ions from the valence states ranging from VII to II, hydroxyl radicals and oxygenated species. In addition, oxidation by permanganate in acidic and alkaline environments occurs according to different mechanisms. Alkaline permanganate oxidation is the most common for the oxidation of organic compounds. One mechanism for alkaline permanganate oxidation is by hydrogen-atom abstraction. In this scenario, hydroxyl radicals remove the hydrogen atom from organic compounds. Hydroxyl radicals can be formed by the reaction of permanganate in water. These molecules have be shown to react readily with bicarbonate. [Bottom right: Depiction of the hydroxyl radical formation and reaction with bicarbonate.] Aside from the reaction mechanisms and pH ranges by which permanganate oxidizes, the order of the oxidation rate is also important. The reaction of the permanganate and the organic compound is first order during the initial phases of oxidation. However, second order reactions are often seen after the initial stages of oxidation. Therefore, for the purpose of this poster, permanganate oxidation order of reaction is considered pseudo-first order. Possible mechanism for alkaline permanganate oxidation (hydrogen abstraction) MnO4- + H2O  MnO4-2 + OH + H+ Bicarbonate as scavenger HCO3– + OH  CO3– + H2O The bicarbonate ion has been shown to react with hydroxyl radicals--a key to many oxidation reactions. Although oxidation by potassium permanganate can occur via several pathways, research has shown potassium permanganate can react with water to form hydroxyl radicals.

2. Trial TCE Rate Constants (k min-1) Test Molar Ratio KMnO4:TCE HCO3- mg/L Time (min) Test 1 Test 2 Test 3 1 0.0208 0.0179 0.0271 Trial 1 Trial 2 Trial 3 2 0.0527 0.0512 0.0547 1 5.5 13.8 27.6 0 90 3 0.0794 0.055 0.0711 2 5.5 13.8 27.6 350 90 3 5.5 13.8 27.6 1350 90 Testing Benzene’s and Bicarbonate’s Effect on Potassium Permanganate Oxidation of TCE Summary Reactor Cell Figures of Results Overall, the results of the tests and experiments suggest that TCE was effectively oxidized by potassium permanganate, and that benzene did not appear to scavenge potassium permanganate. Reductions and fluctuations in benzene concentrations due to potassium permanganate did not appear to be mathematically describable. In addition, since benzene concentrations only decreased slightly, one can conclude that permanganate oxidation of benzene is not feasible and has little effect on potassium permanganate oxidation of TCE. This finding confirms potassium permanganate's inability to effectively oxidize benzene. Therefore, when designing a full-scale oxidation application, benzene’s effect as a potential scavenger can be ignored. As an example, Figure 1 shows the graph of benzene concentration versus time for Test 1 Trial 1. In addition, the claims made in literature that the effectiveness of chemical oxidation of TCE was decreased when the waste solutions contained bicarbonate was shown to be incorrect for solutions where the bicarbonate concentration does not exceed 1350 mg/L and the oxidant was potassium permanganate. Figures 2 and 3 depict the decrease of TCE with respect to time for solutions without and with bicarbonate, respectively. The TCE rate constants for these data sets were similar, regardless of the presence, or absence, of bicarbonate. This finding suggests that the oxidation pathway for TCE using potassium permanganate does not involve the formation of the hydroxyl radical ion. Test Summary References Gates, Dianne D., Robert, L. Siegrist, and Steven R. Cline. Chemical Oxidation of Volatile and Semi-Volatile Organic Compounds in Soil. Oak Ridge National Laboratory. 1995. Glaze, William H. and Joon-Wun Kang. “Advanced Oxidation Processes for Treating Groundwater Contaminated with TCE and PCE: Laboratory Studies.” Journal of American Water Works Association. 90(July 1998): 57-63. Ladbury, J.W. and C.F. Cullis. “Kinetics and Mechanism of Oxidation by Permanganate.” Chem. Rev. 58:403 (1958). Ohio State University. Progress Report 2000, Permanganate Treatment of DNAPLs in Reactive Barriers and Source Zones. January 2000. United States Department of Energy. Office of Environmental Management and Office of Science and Technology. Innovative Technology Summary Report, In-Situ Chemical Oxidation Using Potassium Permanganate. September 1999. United States Environmental Protection Agency. Office of Emergency and Remedial Response and Office of Research and Development. Engineering Bulletin: Chemical Oxidation Treatment. October 1991. • Summary of Results • Benzene concentrations did not exhibit an overall decline, indicating that benzene was not acting as a scavenger of potassium --Figure 1. • TCE concentrations decreased according to first-order decay during all tests--Figures 2 and 3. • The presence of bicarbonate did not affect the overall rate constants for TCE oxidation.