Electrochemical Oxidation of Phenol Using Boron-Doped Diamond Electrodes
The cleanup of liquids or slurries that contain >1% total organic carbon (TOC), defined as non-wastewater hazardous waste (1), offer particular difficulties for on-site remediation efforts. The electrolytic destruction of organic wastes shows real promise for remediation of a wide variety of organic materials in aqueous waste streams (2). However, the electrochemical oxidation of organic waste is limited with the traditional electrode materials, platinum, ruthenium dioxide, lead dioxide and tin dioxide. The specific limitations arise from low reaction rates and efficiencies, corrosion of the electrodes and fouling and poisoning of the active electrode surfaces. Using phenol as a test compound, the work reported here demonstrates that boron-doped diamond electrodes prepared by chemical vapor deposition (cvd) are not susceptible to these limitations and can oxidize phenol completely to CO2 (3).
The oxidation of phenol was chosen as a test reaction because it is one of the most difficult organic molecules to oxidize electrochemically. It is well known for its rapid fouling of the electrode surface due to formation of a blocking polymer layer produced by the polymerization of the phenoxy radicals generated in the initial stages of the reaction (4,5). This results in termination of the reaction in minutes. This communication compares previous work using Ti mesh substrates with approximately 10 μm of boron doped diamond to results obtained using a parallel plate flow cell (3).
The working electrode is a Nb plate (8.5 cm x 9 cm) coated with approximately 1 μm of boron doped diamond via microwave plasma enhanced chemical vapor deposition. A two step deposition process was utilized to achieve uniform nucleation and good film adhesion. The films were characterized with SEM and Raman to establish the quality and stability of the cvd-carbon films.
The oxidation of phenol was studied with cyclic voltammetry in a solution of 0.1M H2SO4 containing 0.16 mM phenol in a beaker cell with a Pt counter electrode and a reversible hydrogen reference electrode. Experiments were also run with a parallel flow cell in which 5 liters of 10 mM phenol in 0.1M H2SO4 was circulated through the cell at 100 ml/sec and the total organic carbon (TOC) was monitored as a function of time and cell current.
Results and Discussion
In our previous experiments phenol has been oxidized on boron-doped diamond electrodes without any reduction of the current arising from fouling or poisoning of the electrode even for concentrations of phenol which would put it in the category of non-wastewater hazardous waste. The total carbon in solution was reduced at high efficiency (80-90 %) from ~1% TOC to <0.1% TOC with no observable decrease in decomposition rate. This means that the reacted phenol was converted completely to CO2. These results are extraordinary in that they are preliminary experiments on small surface area electrodes and the experiment has not been optimized. In light of these results, it should also be possible to treat a 10 % TOC solution as readily as a 1 % TOC solution. We will report recent efforts that explore this possibility with more concentrated phenol solutions using a parallel plate flow cell and compare degradation rates to our previous work.
Support for this work from the Naval Research Laboratory is gratefully acknowledged.
1. DOE Waste Treatability Group Guidance, US Department of Energy, DOE/LLW-217, (January, 1993).
2. E. Plattner and Ch. Comninellis in Process Technologies in Water Treatment, ed. S. Stucki, Plenum, New York p. 205 (1988).
3. P. L. Hagans, P. M. Natishan, B.R. Stoner,and W.E. O’Grady, J. Electrochem. Soc. 148, E298-E301 (2001).
4. M. Gattrell and D.W. Kirk, J. Electrochem. Soc., 140, 1534 (1993).
5. Ch. Comninellis and C. Pulgarin, J. Appl. Electrochem, 21, 703 (1991).