Grantee Research Project Results
Final Report: Development of Electrochemical Techniques for the Detection/Quantification of Mercury using Boron-Doped Diamond Electrodes
EPA Grant Number: R829410E02Title: Development of Electrochemical Techniques for the Detection/Quantification of Mercury using Boron-Doped Diamond Electrodes
Investigators: Seehra, Mohindar S.
Institution: West Virginia University
EPA Project Officer: Chung, Serena
Project Period: October 1, 2001 through September 30, 2003
Project Amount: $274,928
RFA: EPSCoR (Experimental Program to Stimulate Competitive Research) (2001) RFA Text | Recipients Lists
Research Category: EPSCoR (The Experimental Program to Stimulate Competitive Research)
Objective:
The objective of this research project was to develop electrochemical techniques using boron-doped diamond (BDD) electrodes to detect and quantify mercury in solutions in the ppb range. The long-range aim of this technology, when fully developed, is rapid onsite quantification of mercury emissions from coal-fired power plants and in aquatic systems. Our task in this project was to develop calibration curves for mercury detection in the ppb range in laboratory-prepared solutions and then use these calibration curves for quantifying mercury in unknowns such as impinger solutions obtained from the flue gas of the coal-fired research power plant of the National Energy Technology Laboratory (NETL), in Pittsburgh, Pennsylvania. For validation, these results were compared with mercury estimations in the same solutions obtained with cold vapor atomic absorption spectrometry (CVAAS).
Summary/Accomplishments (Outputs/Outcomes):
The summary of the experimental results presented below were carried out independently by two research teams: The Manivannan-Seehra team working with graduate student Latha Ramakrishnan in the Physics Department, and Professor Ronald Smart in the Chemistry Department working with graduate student Carol Babyak. Based on this research, Ms. Ramakrishnan has completed and successfully defended her M.S thesis and Ms. Babyak has completed her Ph.D. dissertation. A summary of the major findings follows.
Differential pulse voltammetry (DPV) experiments were performed by the Manivannan-Seehra team in the nitrate (KNO 3), thiocyanate (KSCN), and chloride (KCl) media to develop calibration curves for the quantification of mercury ions at the ppb levels, using both the stationary and rotating disk electrode (RDE) of BDD. The plots of the peak differential currents (after subtracting the background) against the mercury concentration show a linear variation. To avoid the formation of calomel in the chloride medium, 3 ppm of gold standard was codeposited on the BDD electrode during DPV detection.
It was found that the RDE technique provided higher sensitivity than the stationary electrode. Using the calibration curves and the standard addition method, mercury in the 0.005-50 ppb range has been quantified using the RDE technique in real samples (KCl impinger solutions) prepared from flue gas released by a pilot-scale coal-fired combustion facility. In Figure 1, we show a comparison plot of the mercury concentrations determined by the RDE-DPV technique and those determined by the CVAAS technique for the same samples. A good one-to-one correspondence observed between the two techniques provides assurance of the accuracy of the method developed in this project. Finally, a portable instrument also has been used for the efficient detection of mercury. These studies have demonstrated that BDD mounted in an RDE system together with gold codeposition is able to detect mercury with high sensitivity.
Figure 1. Comparison Plot of the Concentrations Determined by the Standard Addition RDE-DPV and CVAA Methods. The solid line represents a one-to-one correlation between the two methods.
The potential of this technique for rapid onsite monitoring of mercury using a portable instrument will be investigated in future work. One unresolved issue is that the calibration curves, although linear, are not entirely reproducible for successive measurements using the same electrode. Although the standard addition method used here for quantifying mercury is not affected by this issue, a solution for this problem and a complete reproducibility of the calibration curves will allow a more rapid determination. This problem will be the focus of our future research.
Results from the Smart-Babyak team on the effects of electrochemical cleaning and acid-washing on repeated measurements of the mercury stripping current indicated that precipitation of a mercurous or mercuric salt probably occurred on the BDD surface and affected subsequent measurements. We also continued our efforts to improve the detection of mercury by using ultrasound during the deposition step. It was observed that free-standing polished BDD electrodes could withstand very high-intensity ultrasound, but that the response of the electrode toward mercury had permanently changed after this exposure to ultrasound.
We have consistently observed that the electrochemical detection of mercury is more difficult compared to other elements such as cadmium, lead, and copper. Therefore, we set out to gain a more fundamental understanding of the electrochemical behavior of mercury during the deposition and stripping steps in anodic stripping voltammetry (ASV). The effects of electrochemical cleaning and acid-washing on the detection of mercury were systematically investigated.
A typical experiment began with repeated measurements of the mercury stripping current. It was observed that the current increased with each repetition, which is likely the result of an insoluble mercurous or mercuric salt that formed during the stripping step, remained on the BDD surface, and behaved as a nucleation center for the subsequent measurements. The formation of precipitates during the stripping step in ASV has been suggested by others, and a criterion has been developed to predict if precipitation will occur.
When the BDD electrode was cleaned electrochemically prior to each run, the current decreased. This indicated that nucleation centers were either removed or altered by electrochemical cleaning. To distinguish between these two possibilities, the BDD electrode was removed, washed with acid, and returned to the solution. The stripping current for the subsequent measurement increased, indicating that washing had removed nucleation centers that had been altered by electrochemical cleaning.
This observation should be considered when analyzing real samples. To decrease the possibility of precipitation near the electrode surface, the concentration of stripped mercuric ions near the electrode surface should be kept low during the stripping step. This can be accomplished by using shorter deposition times or adding a complexing agent for the stripped mercuric ions.
As stated in our last progress report, two competing processes occur when ultrasound is applied during the deposition step in ASV:
- mass transport of the analyte to the electrode surface increases,
- and the deposited analyte is eroded off the electrode surface.
The second effect was much more dramatic for mercury compared to other elements tested, such as cadmium and lead. This may indicate poor adhesion between the deposited mercury and the BDD electrode, or may be a result of the volatile nature of elemental mercury.
The effect of high-intensity ultrasound on free-standing, polished BDD electrodes was investigated. An ultrasound intensity of 1,000 W/cm2, which is much higher than any intensity reported in the literature, was applied during the deposition step. Detection of 100 ppb Hg2+ was achieved in 1 minute under these extreme ultrasound conditions. In addition, two stripping peaks located at 0 and +500 mV were observed for mercury. The peak at +500 mV had never been observed using a 1-minute deposition time until ultrasound was applied during the deposition step. This peak is linear with mercury concentration.
Because short deposition times are desired to prevent precipitation of mercurous or mercuric salts on the electrode surface, the use of ultrasound during the deposition step should be considered. Exposure to this high-intensity ultrasound permanently altered the electrode surface. After exposure to ultrasound, two stripping peaks were observed for mercury. In addition, it is likely that the BDD surface was eroded as a result of exposure to ultrasound.
Conclusions:
Future work should focus on minimizing the deposition time in ASV to prevent precipitation of mercurous or mercuric salts on the electrode surface. Ultrasound may be used during the deposition step, but the optimum intensity, which results in shorter deposition times while preserving the electrode surface, should be determined. The free-standing, polished BDD electrodes (Windsor Scientific, Ltd.) used in these experiments are recommended for the detection of mercury in the presence of ultrasound.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 15 publications | 5 publications in selected types | All 4 journal articles |
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Type | Citation | ||
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Babyak C, Smart RB. Electrochemical detection of trace concentrations of cadmium and lead with a boron-doped diamond electrode: effect of KCl and KNO 3 electrolytes, interferences and measurements in river water. Electroanalysis 2004;16(3):175-182. |
R829410E02 (Final) |
not available |
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Manivannan A, Seehra MS, Fujishima A. Detection of mercury at the ppb level in solution using boron-doped diamond electrodes. Fuel Processing Technology 2004;85(6-7):513-519. |
R829410E02 (Final) |
not available |
Supplemental Keywords:
RFA, Scientific Discipline, Air, Waste, Ecology, Engineering, Chemistry, & Physics, Incineration/Combustion, electrochemical technology, mercury , boron doped diamoond electrodes, mercury, mercury emissions, air pollution, air sampling, mercury monitoring, mercury absorbtion, combustion kinetics, combustion flue gases, combustion gases, air qualityRelevant Websites:
mercury, detection, electrochemical, monitoring, portable instrument, electrode, water, innovative technology, analytical, air, waste, engineering, chemistry, physics, incineration, combustion, air pollution, air quality, air sampling, boron doped diamond, BDD, electrodes, combustion flue gases, combustion gases, combustion kinetics, electrochemical technology, mercury abatement technology, mercury emissions water, analytical innovative technology, boron doped diamond electrodes, mercury absorption, mercury emissions,
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.