Final Report: Hybrid Electrochemical-Piezoelectric Sensor for RCRA Metals in Groundwater: Detection of Hexavalent ChromiumEPA Contract Number: 68D99077
Title: Hybrid Electrochemical-Piezoelectric Sensor for RCRA Metals in Groundwater: Detection of Hexavalent Chromium
Investigators: Andle, Jeffrey C.
Small Business: BIODE Inc.
EPA Contact: Manager, SBIR Program
Project Period: September 1, 1999 through September 1, 2001
Project Amount: $224,155
RFA: Small Business Innovation Research (SBIR) - Phase II (1999) Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , SBIR - Monitoring , Small Business Innovation Research (SBIR)
Purpose of the Research
The research was performed to demonstrate the feasibility of directly detecting hexavalent chromium using a hybrid sensor technology combining piezoelectric mass detection and electrochemical processes. Prior work has resulted in a similar detection capability for ionic mercury. The present effort seeks to extend the technique to chromate and dichromate detection.
Description of the Research
The research consisted of three major topics. These were the creation of small, yet accurate electrochemical circuitry that was compatible with the piezoelectric sensor; the study of electrode modification procedures; and the demonstration of chromate detection that was distinguishable from mercury and other cationic metals.
Task I effort was directed at compact and accurate current measurement technology. Several circuits were built and evaluated.
Task II effort was performed under subcontract by Marquette University. It explored the use of pyridine to chemically modify gold electrodes as a means of enhancing the selectivity and sensitivity of the sensor to chromate.
Task III effort explored the sensor response to chromate using existing SHAPM sensor technology in conjunction with electrochemical deposition and stripping cycles. Both pure electrochemical and hybrid measurements were made.
Task I effort indicates that it is possible to accurately measure current
over a limited range; however, that controlling small currents is much more
straightforward. Several Phase I concepts were evaluated and met with limited
success. The task resulted in a robust circuit topology that will be explored in
Task II effort demonstrated 200 ppb detection of chromate using electrochemical quartz crystal microbalance (EQCM) technology with pyridine modified gold electrode material. This task indicates that modified electrodes will allow improved selectivity and sensitivity to chromate.
Task III effort underscored the complexity of chromate electrochemistry. When redox reactions which convert Cr(VI) to Cr(III) and back are employed as the detection mechanism, the resulting reduction and oxidization peaks are difficult to predict. A separate reaction, involving only Cr(VI) ions and the redox of the chromate ion as a whole, appears to have been identified using the hybrid sensor.
Chromate was detected at 111 ppb using the Phase I hybrid sensor.
BIODE has been asked to supply chromate sensors to General Atomics for placement at Tinker AFB as soon as possible. Other air field and nuclear weapons production facility locations also have a critical requirement for on-site chromate detection. Industrial hygiene requirements exist in several industries. Finally, drinking water safety monitors are required to verify that groundwater contamination does not lead to drinking water contamination.
This innovative sensor technology expands on the recently-introduced electrochemical quartz crystal microbalance (EQCM) in three major areas. First, superior electrochemical processes are employed. Second, more sensitive piezoelectric sensors are employed. Third, electrode modification is performed to improve selectivity and sensitivity.
If successful, this effort will result in the only detector for hexavalent chromium that does not require chemical pretreatment.
Phase I effort resulted in the demonstration of metals detection for Cr(VI) with a distinctly different sensor response than the response to Hg(II) or Cu(II). Preliminary data indicates the ability to measure 111 ng/ml of chromate with a response signature which is readily distinguished from all positive metal ions (e.g. Cr(III), Hg(II) and Cu(II)). A typical response of the hybrid sensor for chromate is shown in Figure 1. The chromate deposited at 0.25 V in an cathodic direction and stripped at 0.0 V in an anodic direction, indicating that a negatively charged species is responsible for the response. The concentration of the chromate was 111 ppb, which induced a fractional frequency shift of approximately 0.75 ppm (120 Hz). Based on the existing stability of the sensor, detection limits of 20 ppb are readily possible. Further improvements are proposed to provide < ppb detection limits.
Figure 1. Hybrid sensor response to 111 ppb of CrO4-2 is shown. The chromate deposited at 0.25 V and stripped at 0.0V. The data is not temperature corrected; a simple process which would eliminate much of the 0.25 ppm frequency fluctuations away from the redox potentials. A single scan is shown.
Figure 2. Mercury (II) frequency vs. voltage plot for 10 ppb. Mercury is characterized by slow deposition at potentials more negative than the stripping potential. Deposition occurs in the anodic direction and stripping occurs in the cathodic direction, as expected of a typical cation.
By contrast, mercury deposits slowly at all potentials more negative than the stripping potential. Stripping occurs at a slightly negative potential, between -0.2 and 0V in our present instrument. This is consistent with a positive ionic species. Figure 2 shows typical data for ionic mercury at 10 ppb concentrations in 0.1M KCl, 1% HNO3. The sensor is thus seen to detect parts-per-billion levels of Hg(II) and Cr(VI) with distinguishable waveforms. The sensor exhibits no interference to nitrate, chloride or potassium ions.
Other sensor data was taken with the EQCM to validate the electrode modification technique. Figure 3 shows the frequency shift incurred by a pyridinium-containing polymer-coated QCM due to the addition of the indicated quantity of chromate ion at a constant potential of +0.2V with respect to Ag/AgCl. The absolute frequency shifts may be divided by 9 to obtain parts-per-million (ppm) fractional frequency shifts. The frequency axis thus spans 20 ppm of frequency shift over a 1.3 ppm range of ionic concentration. Even with the deficiencies of the initial electrochemistry cell, the stability was approximately 1 ppm of frequency shift (65 ppb of chromate).
Figure 3. Frequency decreases (negative frequency change) of pyridinium-containing polymer-coated EQCM exposed to different concentrations of Cr(VI).
Phase I resulted in two major accomplishments: (1) the demonstration of a hybrid electrochemical piezoelectric sensor with sensitivity to chromate substantially better than 100 ppb (estimated detection limit for Phase I prototype is 20 ppb) and unique chromate detection features, and (2) a chemical modification procedure for gold electrodes which dramatically (and electrochemically reversibly) enhances the partition coefficient of chromate from solution to the surface at depositing potentials.