Introduction

Permanganate

Figure 1. Stopped-flow method.	Figure 2. Static method.

Figure 3. Summary of second-order rate constants (k") for compounds in all classes. (Click figure to enlarge.)

The size of the data points represents the temperature at which the data were collected; most of the temperatures for the literature values vary between 16 °C to 35 °C, the exception being that the k" reported for benzene was determined at 70 °C. The pH for the literature values of k" ranges from 4.6 to 8.0. Compounds labeled in red text are too insoluble for the method we used. Experimental conditions for the data points from this study: 25 °C, pH 7, and phosphate buffer concentrations between 50 mM and 100 mM. Data on which the figure is based can be found in the ISCOKIN database.

We found that the oxidation reactions of most contaminants are first-order in contaminant concentration (second-order overall) by varying initial concentrations of contaminant. The second-order rate constants obtained with our method are in good agreement with previously reported values of permanganate oxidation rate data. The rate constants obtained from this study are compared with literature values (all of which can be accessed from the ISCOKIN database) and organized by chemical family in Figure 3 (from http://dx.doi.org/10.1021/es051330s).

Figure 3 is a summary of second-order rate constants (k") for compounds in all classes. The size of the data points represent the temperature at which the data were collected; most of the temperatures for the literature values vary between 16 °C to 35 °C, the exception being that the k" reported for benzene was determined at 70 °C. The pH for the literature values of k" ranges from 4.6 to 8.0. Compounds labeled in red text are too insoluble for the method we used. Experimental conditions for the data points from this study: 25 °C, pH 7, and phosphate buffer concentrations between 50 mM and 100 mM. Data on which the figure is based can be found in the ISCOKIN database.

The attractiveness of k" is that it provides a general basis for design calculations because it can be used to calculate kobs (and, from this, half-lives and necessary contact times) for any dose (or range of doses) of oxidant. However, the values of k" shown in Figure 3 were obtained at temperatures primarily near 25 °C and ranging from 20 °C to 30 °C, whereas groundwater temperature is typically around 10 °C. Values of k" can easily be adjusted for small differences in temperature if the Arrhenius equation applies and appropriate activation energies are available. We did not vary temperature in our work (all experiments were done at 25 °C) because it would have limited the scope of COCs that we were able to address. However, the temperature dependence for reactions of permanganate with BTEX compounds and chlorinated ethenes has been studied previously and the data were found to fit the Arrhenius model with activation energies ranging from 5.8 kcal/mol for trans-DCE to 15.8 kcal/mol for toluene (1, 2). These activation energies translate to correction factors, for a 10 °C change in temperature, of 1.4 for trans-DCE and 2.5 for toluene, which are small compared to the ~5 orders of magnitude difference in k" between trans-DCE and toluene. Given these considerations, and the fact that activation energies are not available for the majority of COCs, an approximate correction factor of 2 per 10 °C would seem appropriate for preliminary characterizations of the reactivities of COCs with permanganate. We have used this correction factor to adjust individual values of k" to 10 °C, and then averaged the resulting data for each COC to obtain a representative value of k" at 10 °C.

Figure 4. Half-lives for selected contaminants calculated for the range of permanganate concentrations used in ISCO. Half-lives are based on average values of k" adjusted to 10 °C.

Using the representative value at 10 °C we can calculate kobs and the corresponding half-lives for each COC over the range of permanganate concentrations that are commonly used in field applications of ISCO: 100 to 40,000 mg/L. The results of this analysis are summarized in Figure 4, with COCs sorted from most reactive to least reactive. Figure 4 suggests several generalizations that may be of practical significance. First, the range of permanganate concentrations that can be used in ISCO results in a wide range of accessible half-lives for any particular COC. This range is as great or greater than the variation in half-lives among many COCs due to differences in k". Second, some COCs (roughly the top third in Figure 4) react so rapidly with permanganate that the kinetics of contaminant disappearance will not be a limiting factor in almost any design scenario. Conversely, a few contaminants (including benzene in Figure 1.4 and the chlorinated methanes and ethanes, which are not shown) react so slowly that effective remediation with permanganate will be hard to achieve under most circumstances.

1. Rudakov, E. S.; Lobachev, V. L. The first step of oxidation of alkylbenzenes by permanganates in acidic aqueous solutions. Russian Chemical Bulletin 2000, 49, 761-777.
2. Huang, K. C.; Hoag, G. E.; Chheda, P.; Woody, B. A.; Dobbs, G. M. Oxidation of chlorinated ethenes by potassium permanganate: A kinetics study. Journal of Hazardous Materials 2001, 87, 155-169.
3. Siegrist, R. L.; Urynowicz, M. A.; West, O. A.; Crimi, M. L.; Lowe, K., S. Principles and Practices of In Situ Chemical Oxidation Using Permanganate; Battelle Press: Columbus, OH, 2001.

Peroxide

A zero-headspace reactor was chosen to help regulate the pressure increase that arises from production of gases during the Fenton's reaction. So far, we have used the reactor to study the degradation kinetics of trichloroethene (TCE) and 1,1-dichloroethene (1,1-DCE). The data for TCE and 1,1-DCE fit a kinetic model that assumes one reactive species (presumably hydroxyl radical) with a steady-state concentration that is suppressed by high concentrations of reactants. Globally fitting these data gives first-order rate constants that are consistent with literature values for chlorinated solvents.

In addition, Professor David Waite's laboratory (University of New South Wales, Australia) has developed and thoroughly tested a fully mechanistic/kinetic model for the Fenton system. The Fenton's reaction data collected for TCE and the DCEs were provided to this group for analysis and are expected to provide interesting results. This analysis is in progress.

Persulfate

Publications

1. Waldemer, R. H.; Tratnyek, P. G. Kinetics of contaminant degradation by permanganate. Environ. Sci. Technol. 2006, 40, 1055-1061. http://dx.doi.org/10.1021/es051330s.
2. Waldemer, R. H.; Tratnyek, P. G. The efficient determination of rate constants for oxidations by permanganate. In Proceedings of the Fourth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, 24-27 May 2004, Monterey, CA; Battelle Press: Columbus, OH, 2004; Paper 2A-09.
3. Waldemer, R. H. Determination of the Rate of Contaminant Oxidations by Permanganate: Implications for In Situ Chemical Oxidation (ISCO). Thesis, OGI School of Science and Engineering, Oregon Health and Science University, 2004.

Credits