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Introduction Permanganate Peroxide Persulfate Publications IscoKin Credits

[alt] Introduction

This page provides an overview of research on contaminant remediation with several oxidants (mainly permanganate, Fenton's reagent, and persulfate) that are commonly used for in situ chemical oxidation (ISCO) applications. Assessing the feasibility of ISCO requires accurate kinetic data for oxidation of the contaminants to be treated. While the kinetics of permanganate reaction with most chlorinated ethylenes have been well characterized, few kinetic data are available for the oxidation of other classes of environmental contaminants (BTEX, oxygenates, etc). With respect to Fenton's reaction, this problem is especially complicated, because treatments ascribed to Fenton's reaction involve a variety of reactive species (mainly hydroxyl radical, but probably also others) whose concentrations are rarely determined and whose reactivity with contaminants varies greatly. Persulfate has similar complications to Fenton's reaction in that it also has to be activated in order to obtain degradation rates that have significance for field applications. The active species is likely sulfate radical however like Fenton's reaction, the radicals generated after activation can participate in a wide variety of chain reactions that produce a variety of potentially active species, including hydroxyl radical.


[alt] Permanganate

Figure 1.1 Figure 1.2
Figure 1. Stopped-flow method. Figure 2. Static method.
We developed an efficient protocol for measuring oxidation rates of many contaminants by permanganate using time-resolved spectrometry at an absorbance maximum of permanganate (525 nm). Two mixing methods were used: the static method, in which reactants were added (premixed) directly into a cuvette and the stop-flow method, which used a syringe pump to ensure quick mixing of the reactants. The stopped-flow method allowed us to measure rate constants for fast reactions (order of seconds). The two methods have been validated by comparing values for the second-order rate constant of TCE (the contaminant with the most literature data available on its reaction with permanganate). The colloidal manganese dioxide that formed from permanganate oxidation gave Beer's law behavior, and a procedure was developed to account for its absorbance. Further discussion of this topic can be found in publications 1-3 listed in the publication section.
[alt]
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.

[alt]
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.

References
  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.

[alt] 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.


[alt] Persulfate

Coming soon...


[alt] 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.

[alt] Credits

The work summarized above was performed by Rachel Waldemer (permanganate and persulfate) and Dr. Jaimie Powell (Fenton), under the supervision of Dr. Paul Tratnyek. The work was supported by the DOD's Strategic Environmental Research and Development Program (SERDP) as part of an initiative started in 2002. The project (CU-1289) was a collaboration with Eric Hood (Geosyntec) and Neal Thomson (U. Waterloo). Funding for an earlier seed project on the byproducts of ISCO of chlorinated solvents came from OHSU's Superfund Basic Research Center.

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