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43535-AC5
Understanding Water and Organic Compound Oxidation at Boron Doped Diamond Film Electrodes

James Farrell, University of Arizona

Introduction

The objective of this research was to investigate the effectiveness of boron doped diamond film (BDD) electrodes for removing contaminants from wastewaters.  Work thus far has investigated oxidation of perfluorooctyl sulfonate (PFOS) and reduction of trichloroethylene (TCE). 

Oxidation

PFOS was selected for investigating the effectiveness of BDD electrodes for contaminant oxidation because it is widely found in the environment and because perfluorinated compounds are the most difficult to oxidize of any class of organic compounds.  The recalcitrance of PFOS to oxidation by advanced oxidation processes has been demonstrated in two studies attempting to oxidize PFOS using hydrogen peroxide with either Fenton's reagent, ultraviolet light or ozone ([1], [2]).  In neither of theses studies was any oxidation of PFOS observed. 

Figure 1 shows PFOS and total organic carbon (TOC) concentrations as a function of treatment time in a BDD flow-through reactor.  Flow-cell reaction rates for PFOS and TOC were first order with respect to concentration, with a destruction half-life of ~7.5 minutes.  Throughout the course of the experiments, no organic reaction products were detected and sulfate and fluoride concentrations were equivalent to 1 sulfate and 17 fluoride ions released per PFOS molecule degraded.  This indicates that PFOS was completely mineralized in one interaction with the electrode surface. 

Reduction

Experiments investigating the mechanisms responsible for TCE reduction at BDD electrodes suggests that TCE chemically adsorbs at specific surface sites.  Figure 2 shows the reaction rates for TCE as a function of the electrode potential.  Reduction of TCE resulted in the stoichiometric production of acetate with no detectable intermediate products.  This is very unusual since reduction of TCE by metal electrodes has been found to produce ethene and ethane ([3], [4]).  TCE reduction at BDD cathodes was accompanied by a decline in the solution pH value, with an average of 4 moles of H+ produced per mole of TCE reacted.  The absence of any intermediate chlorinated products suggests that TCE dechlorination occurred via a mechanism involving chemisorbed intermediates.

Density functional theory (DFT) simulations were used to investigate possible TCE reaction mechanisms with deprotonated hydroxyl (–C-O-), carbonyl (–C=O) and carbon radical (–C*) sites on the BDD surface.  Figure 3 illustrates the initial structure, the transition state, and the final product that resulted when a TCE molecule approached the diamond surface along the intrinsic reaction coordinate at a –C-O- site.  Complex formation of TCE with the diamond cluster resulted in the loss of one chloride ion and the formation of a bond between a carbon atom in TCE and an oxygen atom on the cluster surface.  The overall reaction energy at this site was -42 kJ/mol with an activation energy of 25 kJ/mol at a potential of -0.61 V/SHE.  A much higher activation energy of 120 kJ/mol was calculated for TCE reacting at the –C=O on this same cluster.  The low Ea of 25 kJ/mol is close to the experimental value of 22 kJ/mol ([5]) measured at -1.25 V, and indicates that there is sufficient thermal energy at room temperature for TCE complex formation at the –C-O- site.  The formation of chemisorbed intermediates may explain the absence of any intermediate reaction products in the solution. 

Conclusions

This research demonstrated that PFOS can be readily oxidized at BDD electrodes.  This is significant because all other methods tested have not been able to oxidize PFOS in aqueous solutions.  This research was also the first to demonstrate that chemisorption reactions are likely involved in organic compound reduction at BDD electrodes.  This finding opens up new possibilities for using BDD electrodes in the electrosynthesis of organic compounds.

References

[1].  Moriwaki, H.; Takagi, Y.; Tanaka, M.; Tsuruho, K.; Okitsu, K.; Maeda, Y., “Sonochemical decomposition of perfluorooctane sulfonate and perfluorooctanoic acid”, Environ. Sci. Technol. 2005, 39, 3388-3392.
[2]. Schroder, F.; Meesters, R., “Stability of fluorinated surfactants in advanced oxidation processes: A follow up of degradation products using flow injection–mass spectrometry, liquid chromatography–mass spectrometry and liquid chromatography–multiple stage mass spectrometry”, J. Chromatogr. A. 2005, 1082, 110-119.
[3]. Li, T.; Farrell, J., “Electrochemical investigation of the rate limiting mechanisms for trichloroethylene and carbon tetrachloride reduction at iron surfaces,” Environ. Sci. Technol., 2001, 35, 3560-3565.

[4]. Li, T.; Farrell, J., “Reductive dechlorination of trichloroethylene and carbon tetrachloride using iron and palladized iron cathodes,” Environ. Sci. Technol., 2000, 34, 173-179.

[5]. Mishra, D.; Farrell, J. “Reductive destruction of trichloroethylene using boron-doped Diamond electrodes,” American Water Works Association 2007 Annual Conference, Toronto, Canada, June 24 -28, 2007.

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