Spectroelectrochemical Sensing Based on Multimode Selectivity

Publication Date (Web): November 27, 2002 ... related opera tions have accumulated at the Hanford Site in underground waste tanks since the mid-1940s...
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Environ. Sci. Technol. 2003, 37, 123-130

Spectroelectrochemical Sensing Based on Multimode Selectivity Simultaneously Achievable in a Single Device. 11. Design and Evaluation of a Small Portable Sensor for the Determination of Ferrocyanide in Hanford Waste Samples MICHAEL L. STEGEMILLER,† W I L L I A M R . H E I N E M A N , * ,† C A R L J . S E L I S K A R , * ,† THOMAS H. RIDGWAY,† S A M U E L A . B R Y A N , * ,‡ T I M H U B L E R , ‡ A N D RICHARD L. SELL‡ Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172, and Pacific Northwest National Laboratory, Battelle Boulevard, P.O. Box 999, MS P7-25, Richland, Washington 99352

A portable spectroelectrochemical sensor has been designed, evaluated, and demonstrated on a complex sample of radioactive waste. The sensor consisted of a black delrin sample compartment with a total internal sample volume of 800 µL, attached to an indium tin oxide coated glass multiple internal reflection optical element. Detection was by total internal reflection of light from a blue light emitting diode source. After a 10 min uptake for each standard, the sensor showed a linear response in absorbance change for 5 × 10-5 to 5 × 10-3 M ferrocyanide with electrochemical modulation by scanning at 20 mV/s from -0.30 V to +0.55 V vs a Ag/AgCl reference electrode. Due to the complex nature of Hanford radioactive tank waste samples containing ferrocyanide, a standard addition method was developed for analysis. The spectroelectrochemical sensor determined a concentration of 9.2 mM ferrocyanide for U-Plant-2 simulant solution containing 9.38 mM ferrocyanide that was prepared according to Hanford process flowsheets. A radioactive tank waste sample from Hanford Tank 241-C-112 was determined to be 1.0 mM in ferrocyanide using the spectroelectrochemical sensor. A value for the ferrocyanide concentration in the sample of 0.61 mM was determined by FTIR spectroscopy.

Introduction The remediation of over 300 underground nuclear waste storage tanks and related disposal cribs and trenches at U.S. DOE sites, together with the associated needs to characterize and monitor the chemical compositions of the contained waste, present a major scientific challenge (1-6). The * Corresponding author phone: (513)556-9210; fax: (513)556-9239; e-mail: [email protected]. † University of Cincinnati. ‡ Pacific Northwest National Laboratory. 10.1021/es020601l CCC: $25.00 Published on Web 11/27/2002

 2003 American Chemical Society

chemical complexity, harsh radiological environment, and limited tank access further complicate present well-established laboratory-based analytical techniques employed to analyze the wastes. Various radioactive wastes from defense related operations have accumulated at the Hanford Site in underground waste tanks since the mid-1940s. During the 1950s, additional tank storage space was required, so Hanford Site engineers developed procedures to obtain this additional storage volume without requiring the construction of additional tanks. In one procedure, Na4Fe(CN)6, K4Fe(CN)6, and NiSO4 were used to precipitate radio-cesium, in the form of Cs2NiFe(CN)6 from tank mixtures containing NO3- and NO2salts. After precipitation of other radioactive constituents, the supernatant liquids were pumped to disposal cribs and trenches, thereby providing additional tank storage volume. When some of the tanks were subsequently found to be leaking, pumpable liquids were removed from these tanks, leaving behind a wet sludge containing ferrocyanide precipitates (7). While implementing this process, approximately 140 metric tons of ferrocyanide was added. Since ferrocyanide/nitrate mixtures are potentially explosive in the dry state at elevated temperatures, a ferrocyanide remote sensor was needed for the characterization of tank waste. This spectroelectrochemical sensor was developed to address that need. In a recent report (8), the safety issue of ferrocyanide waste storage at the Hanford Site was analyzed and a conclusion was reached that ferrocyanide had decomposed sufficiently to alleviate the safety concern. Nonetheless existing tank waste containing ferrocyanide serves as a good test of the ability of the sensor to analyze such a complex sample of environmental interest. In tanks that originally contained ferrocyanide, residual ferrocyanide was estimated to be in the range 0.1-15 wt % (14). The sensor is potentially useful for monitoring ferrocyanide levels in tank waste streams during the vitrification process prior to final storage of the waste. The new sensor concept combines electrochemistry, optical spectroscopy, and selective partitioning into a single device. Our group has been developing a functional spectroelectrochemical sensor for several years based on this concept (9-12). The sensor in its simplest form consists of an optically transparent electrode that is coated with a chemically selective film. The sensor functions according to the following steps: 1) preconcentration of the analyte by the chemically selective film on the electrode, 2) electrochemical oxidation or reduction of the analyte, and 3) detection of the oxidized or reduced species using optical spectroscopy. Ferrocyanide is an ideal candidate for spectroelectrochemical sensing: it can be preconcentrated into an anion-selective film; it can be reversibly electrochemically oxidized to ferricyanide, and the ferricyanide species absorbs light with a maximum at 420 nm (ferrocyanide does not absorb light at this wavelength). While we have successfully demonstrated the utility of the spectroelectrochemical sensor concept on model analytes, implementation of a sensor for measuring actual samples (e.g., radioactive wastes) has not been previously demonstrated. A few critical factors that must be incorporated into the sensor design include considerations of the sample size, chemical interferences, and stability of the sensor’s film to the chemical and radioactive environment associated with the waste samples at Hanford. A study of the potential chemical interferences that might be encountered in Hanford tank wastes has already been described by our group. These studies showed that the major interferences in the deterVOL. 37, NO. 1, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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mination of ferrocyanide by the spectroelectrochemical sensor arise from competition for ion-exchange sites in the PDMDAAC-SiO2 film by the other anions in the sample. As the concentration of other anions increases, the concentration of ferrocyanide in the film decreases, causing a decrease in absorbance change for ferrocyanide. The edta complexes of Fe2+ and Fe3+ pose a minor interference. They partition in the film and are electroactive. However, they undergo only a partial electromodulation because the E0′ is more negative than ferro/ferricyanide and ∆A for the complex is considerably less than for ferro/ferricyanide (13). In comparison with our earlier spectroelectrochemical sensor designs, several improvements were made. These include the addition of a fixed optical angle system, a reduction in the overall size and a reduction in the sample volume required for analysis (11). The fixed angle system simplified the optical configuration. The reduction in sample volume amounted to an order of magnitude. This paper describes the development and testing of a portable spectroelectrochemical sensor as well as the analysis of an actual waste sample containing ferrocyanide from the Hanford Site.

TABLE 1. Approximate Composition of the U-Plant-2 Hanford Waste Simulant Sludge and Solution, pH ) 9.5

species Na+ NO3NO2SO42SO32PO43NH4+ Ca2+ Ni2+ Fe3+ Fe(CN)64en edta H2O a

composition of U-Plant-2 sludge, wt % 8.3 13.2 3.3 4.6 0.2 2.5 0.1 0.9 0.4 1.3 2.06a 65

composition of U-Plant-2 simulant dissolved in elixir, wt % 0.78 1.23 0.30 0.42 0.02 0.23 0.01 0.08 0.04 0.12 0.192a 4.54 4.54 87

molar 0.35 0.21 0.07 0.05 0.00 0.03 0.01 0.02 0.01 0.02 0.00938a 0.78 0.16

Ferrocyanide concentration is measured value (see text).

Experimental Section Chemicals and Materials. The following chemicals were used: tetraethyl orthosilicate (TEOS, Aldrich), poly(dimethyldiallylammonium chloride) (PDMDAAC, 20 wt % aqueous solution, Polysciences), potassium ferricyanide and potassium ferrocyanide (Aldrich), and potassium nitrate (Fisher Scientific). All reagents were used without further purification. All reagent solutions were made by dissolving the appropriate amounts of chemicals in 0.1 M potassium nitrate solution (prepared with deionized water from a Barnstead water purification system). Indium tin oxide (ITO) coated glass (Corning 1737F and 7059, 11-50 Ω/sq, 150 nm thick film on 1.14 mm glass, Thin Film Devices) was cut into 10 mm × 45 mm slides, scrubbed with Alconox, and rinsed thoroughly with deionized water. Finally, the cleaned slides were rinsed with methanol and equilibrated with air for 24 h before use. The materials used to construct the sensor were: black delrin, Zebra Strips (FujiPoly), copper foil, Luer-lock fittings, poly(vinyl) chloride (PVC), 9 mm diameter f/1 lenses (Edmund Scientific, #45081), conductive tubing (FujiPoly), Tygon tubing, Pt wire, 9 mm O-rings, silicone sheeting (0.005” thick, Specialty Manufacturing, Inc.) and various interference and band-pass filters (ESCO Products, S224037, S224012, S914500). Preparation of Chemically Selective Films. Silica sols were prepared according to a previously reported protocol (9-11). TEOS (4.0 mL), deionized water (2.0 mL), and 0.1 M HCl (100 µL) were combined in a sealed 15 mL vial and stirred at room temperature for 3 h. Polyelectrolyte solutions were used either directly from a commercial stock solution or diluted with deionized water as needed. Incorporation of PDMDAAC into the silica sol was performed by mechanically blending the polyelectrolyte solution with the silica sol just prior to spin-coating. The final mixture was then spin-coated onto the surface of ITO coated slides with a Headway photoresist spinner operated at 3000 rpm for 30 s. Before spin-coating, the slide was masked with four pieces of tape, two across the ends of the slide and two along the edges, leaving a film-coated area approximately 5 mm × 30 mm. The areas of the ITO slide not covered by selective film material were used for the optical interface and for establishing electrical contact with the working electrode. All filmcoated slides were dried overnight at room temperature before use. Following this procedure, an average film thickness of about 900 nm was achieved. Simulated Hanford Waste Preparation and Evaluation. Simulants were prepared to mimic tank waste conditions at Hanford. In the case of those wastes containing ferrocyanide, Bryan et al. (14) have reported a procedure for dissolving 124

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ferrocyanide containing sludges with an elixir composed of aqueous ethylenediamine (en) and ethylenediaminetetraacetic acid (edta) to produce solutions from simulated and actual Hanford wastes. A simulated Hanford tank waste containing ferrocyanide was prepared for sensor testing, according to flowsheets developed by Scheele, Burger and co-workers (15-17) and compiled by Jeppson and Wong (18). The resulting sludge (designated U-Plant-2) was prepared on a laboratory scale mimicking the actual waste flowsheets and was chemically identical (excluding radioactive components) to actual waste produced from the radio-cesium scavenging campaign within the Hanford Site Uranium Reclamation Plant. The U-Plant-2 simulated ferrocyanide waste was dissolved using the en/edta elixir and analyzed by IR spectroscopy and found to contain 9.38 (( 0.06) mM ferrocyanide (as Fe(CN)64-). The composition of the U-Plant-2 simulant sludge and solution, along with the approximate concentrations of the other components coprecipitated from the U-Plant flowsheet are given in Table 1. The determination of the ferrocyanide content was performed according to the procedure by Bryan et al. (14) using a Nicolet 510P FTIR spectrometer equipped with an Axiom Analytical ATR tunnel cell with a cubic zirconia cylindrical reflectance element. Actual Hanford Waste Preparation and Evaluation. Actual Hanford tank waste samples containing ferrocyanide taken from Tank 241-C-112 were handled and prepared within the Radiochemical Processing Laboratory, a Category II nuclear facility operated by the Pacific Northwest National Laboratory. Hanford tank waste samples contain high concentrations of radionuclides generated from plutonium production, and processing required adequate shielding and contamination controls designed for high dose-rate gamma, beta, and alpha emitting samples. Within the shielded analytical facility, a composite from Hanford Tank 241-C-112 core samples (7.33 g) was dissolved in 76.67 g en/edta elixir solution. Prior to release from the shielded facility, the sample solution was passed through an ion-exchange column to remove radio-cesium, the principal gamma emitting radionuclide in this sample. A portion of the tank waste elixir solution (10.20 g) was passed through a cation exchange column containing 13.3 g wet resin, BioRad AG50W-X8 100-200 mesh. The resin was prepared by presoaking in 2 M NaOH for 24 h, followed by deionized water washings until neutral pH was obtained. The tank waste sample was washed through the column with three column volumes of fresh en/edta elixir solution (approximately 30

FIGURE 1. Diagram of the portable spectroelectrochemical sensor. Section A: Optical Element. Section B: Sample Chamber. Section C: Base Component. The inset is a bottom view of Section B. L1 and L2 are a two lens focusing system. F1 is a filter. OT1 is for electrical contact while I is an inlet hole for solution flow. P1 and P2 are prisms. OTE is the optically transparent electrode made of indium tin oxide on glass. PVC1 and PVC2 are elements of the fixed angle system. Inset: OC indicates the outer chambers in Section B. OT1 and I are the same as in the side view of sensor unit. mL). The column washings were collected and combined resulting in a total mass of 41.60 g of pretreated ion exchange solution. The dose-rate from the resulting ion exchange solution was sufficiently low to allow transfer from the shielded facility to a radiochemically controlled fume-hood enabling hand manipulation. Although the gamma emitting components (primarily in the form of 137Cs) were removed by cation exchange, the solution still contained a significant concentration of beta emitters, primarily 90Sr. Precautions for contact dose to personnel were still required. The concentration of ferrocyanide (analyzed as Fe(CN)64-) within the pretreated tank waste solution was independently determined by a FTIR procedure (14) as 0.61 ( 0.04 mM (0.0124 wt %). Based on the dilution of the sample due to dissolution and ion exchange treatment, the concentration of ferrocyanide in the original Tank 241-C-112 sample was 0.58 wt % (measured as Fe(CN)64-), which is in good agreement with literature values for the ferrocyanide content within this tank (there were no other cyanide containing species or free cyanide ion detected in the tank samples) (14). The pretreated Hanford tank ferrocyanide waste solution was used directly for the spectroelectrochemical sensor testing.

Results and Discussion Sensor Description: General. The three key sections of the sensor unit are shown in cross-section in Figure 1. Section A contains the optical elements, Section B contains the sample and electrical contact chambers, and Section C is the base. These three sections combine (as indicated with vertical arrows) to make a leak-free sensor containing all optical and electrochemical components. Section A was made of black delrin. The three holes in the top of the piece serve as ports for the reference electrode and two auxiliary electrodes. Each end of Section A was bored at 33° from horizontal with two holes (6 mm and 9.5 mm) bored concentrically as shown. This allowed lenses, L1 and L2, and filter, F1, to rest on the ledge created between the larger and smaller diameter holes. To achieve optical coupling and light detection two cylindrical pieces, made of PVC, were made to fit into the 9.5 mm holes. PVC1 was used to hold the optical fiber with respect to the lens couple. PVC2 was used to hold

the detector (photodiode) in place with respect to the decoupling prism. Filter, F1, limited light to the detector. Section B was also manufactured out of black delrin. There were three holes in the top of this piece that aligned with the three holes in Section A. On the side of Section B hole I, as shown in Figure 1, functioned as the inlet for solution to flow into the sample chamber of Section B. A hole on the opposite side near the top (not shown) functioned as the outlet for solution flow. OT1 allowed for connection to the working electrode. An underside view of Section B can be seen in the inset of Figure 1 showing the three chambers in Section B. In the center chamber the solution made contact with the electrodes and it had the following dimensions: 20 mm × 4 mm × 10 mm and an internal volume of 800 µL. The two outer chambers, OC, were for electrical contact to the working electrode, i.e., the optically transparent electrode made with ITO. Section C is the base component of the sensor and it was made of two flat pieces. The top piece was made of black delrin, while the bottom piece was made of aluminum. The black delrin presented a soft surface for the OTE to rest on, while the aluminum provides the necessary stiffness to the base section. The OTE and prisms rested on the base piece as shown. On top of this, the liquid sample chamber allowed the solution to be in contact with the OTE that was coated with a selective film. The electrochemical cell part of the sensor consisted of two platinum auxiliary electrode wires, a Ag/AgCl reference electrode (3 M KCl, Cypress Systems, Inc.) and an ITO coated glass substrate (OTE). Contact with the working electrode was made through Zebra Strips, covered with copper foil and placed in each of the two outside chambers (OC) in Section B. Conducting tubing was placed at the top of each outside chamber in Section B. A 22 gauge wire was placed between the conductive tubing and the top of a copper covered Zebra Strip and extended out through the hole at OT1 and an identical hole on the opposite side (see inset, Figure 1). The 22 gauge wire was compressed between the conductive tubing and the copper covered zebra strip. When assembled, this compression ensured good contact with the working electrode. The light source was a Panasonic blue LED (λmax ) 460 nm). Light from the LED was butt-coupled to a 600 µm stepVOL. 37, NO. 1, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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index optical fiber (Fiberguide Superguide G). To accomplish this a hole was bored on center into the LED plastic lens allowing the optical fiber to be epoxied into place next to the emitter. The optical interface consisted of the fiber-coupled LED and the optical fiber that was held on the optical axis of a two lens system (L1 and L2) that weakly focused light on the launching prism (P1). The light was prism-coupled into the ITO OTE using a Schott SF6 coupling prism (P1, Karl Lambrecht, Chicago, IL). A high-viscosity refractive index standard fluid (Cargill, n ) 1.517) was used to span the gap between the prism and OTE. A second SF6 prism (P2) decoupled the light from the ITO OTE. A UV-enhanced photodiode (D, Digi-Key, DB-V107) detected the light after it passed through filter F1. The photodiode signal was then amplified and fed into a National Instruments A/D, D/A card (PCI-MIO-16E-4) for further processing. Optical Design Considerations. The primary design consideration was to determine the fixed prism coupling angle for the light path. The coupling angle was chosen to maximize the number of internal reflections in the sample liquid contact area, while minimizing light loss at the output end of the slide. An angle of 12° to the prism normal was chosen to allow a maximum of four internal reflections with minimal light loss. Portability, disposability, and cost were determining factors in the light source selection. A blue LED was chosen as a suitable light source for this sensor design. In the case of this sensor, the output of the blue LED does not fall totally within the absorbance spectrum of ferricyanide. Therefore, attempts were made to remedy this situation by utilizing different optical filters such that the spectrum of the input light fell entirely within the absorbance spectrum of ferricyanide. Figure 2a, curve D, shows the raw output of the LED as a function of wavelength along with the output when intercepted by various filters (curves A-C). Superimposed on this figure is the absorbance spectrum of ferricyanide. The best light source for sensor absorbance measurements would have a narrow wavelength output with its maximum at the absorbance maximum of ferricyanide (420 nm). As shown in Figure 2a, the unfiltered output of the LED chosen does not have these properties. Figure 2b shows the sensor absorbance values for a ferrocyanide loaded sensor recorded with the unfiltered and variously filtered LED light sources as the potential was cycled through the ferri/ferrocyanide couple (see also refs 9-12). The final choice of filter A gave the highest value of sensor absorbance. However, since the absorbance spectrum of ferricyanide changes significantly over the intensity profile of the A-filtered LED light source, some deviations from the Lambert-Beer law were expected. Electrochemical Design Considerations. Bubble-free filling of the cell was a concern in the design of the sensor. Bubbles contribute to inconsistencies in cell volume and could also prove to be detrimental to the electrochemical measurements. A bubble within the sample chamber could cause an unanticipated potential to be applied to the working electrode. Filling the solution chamber from the bottom of the sample chamber, with an outlet at the top minimized the chance of bubble formation. The reference electrode is placed in a position that is between the auxiliary electrodes and the working electrode to ensure the accuracy of the applied potentials (19). The two separate chambers allowed electrical contact with the ITO surface without contacting the sample solution. Separation was maintained through a simple gasket placed between the sample piece and the OTE. When the sensor was assembled, not only did compression maintain the seal of the gasket but also compression on the wire from the conducting tubing on top and the Zebra Strip on the bottom allowed for a tight connection of the 22 gauge wire to the working electrode surface. Identical chambers on each side 126

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FIGURE 2. (a) Overlay of ferricyanide absorbance spectrum and the emission spectrum of the light source with various filters. (Esco filters: A -450 nm interference filter; B - Schott BG12; C - Schott BG37, D is a scan with no filter). (b) Absorbance vs time plots during cyclic voltammograms with a PDMDAAC-SiO2 film equilibrated with 0.4 mM ferrocyanide. Scans are from -0.40 V to +0.55 V vs Ag/AgCl at 20 mV/s. Sensor absorbance measurements were calculated as log Io/I, where Io is the initial light intensity and I is the amount measured during the experiment. allowed for connection across the sample chamber and minimization of the resistance across the working electrode. Sensor Analytical Performance. When compared to previous prototypes (9-13), the portable sensor contained a smaller film area (0.8 cm2 versus 8.4 cm2), employed a shorter effective path length, and used a different light source. The anion selective film (PDMDAAC-SiO2) is critical to the performance of this sensor. As the potential is scanned, the ferrocyanide initially present in the film is converted to ferricyanide. This change is monitored optically and displayed as the increase in absorbance. On the reverse scan, ferricyanide is converted back to ferrocyanide, which can be seen by the decrease in absorbance (see refs 9-12 for additional details). Figure 3a shows a greater than 10-fold increase in optical signal using the PDMDAAC-SiO2 film relative to the bare OTE, a 4-fold enhancement of the electrochemical signal can be seen in Figure 3b. The enhancement of the optical signal is substantially larger than the enhancement of the electrochemical signal. This difference is attributed to the involvement of the diffusion coefficient in the electrochemical

FIGURE 4. Single scan (0.4 mM ferrocyanide) from -0.3 V to +0.55 V vs Ag/AgCl, 20 mV/s. CV was started after a 60 min uptake of ferrocyanide by the PDMDAAC-SiO2 film. Trace (a) is from the portable sensor with 4 reflections, trace (b) is from the larger prototype sensor with 8 reflections.

FIGURE 3. Enhancement of the optical and electrochemical signal by the presence of PDMDAAC-SiO2. (a) Sensor absorbance vs time and (b) cyclic voltammograms of a bare OTE and an OTE modified with PDMDAAC-SiO2. The measurements were begun at 5 min postinjection of 1.0 mM ferrocyanide solution. The potential was scanned from -0.30 V to +0.55 V vs Ag/AgCl at 20 mV/s. signal but not in the optical signal. Peak current is proportional to D1/2C, where D is the diffusion coefficient and C is the concentration of the species being electrolyzed. Partitioning of ferrocyanide into the film causes an increase in Cfilm compared to Csolution, which leads to the expected increase in peak current that accompanies preconcentration. However, the diffusion coefficient of ferrocyanide in a film of PDMDAAC, Dfilm, is considerably smaller than Dsolution. Thus, some of the enhancement gained by Cfilm > Csolution is lost by Dfilm < Dsolution. By comparison, the plateau value of ∆A, which corresponds to quantitative electrolysis within the entire optical cell as defined by the evanescent wave (an extension of the electric field component of the propagating light (20)), is affected by Cfilm but not by Dfilm. Only the time to reach the plateau is affected. The consequence is that the optical signal benefits more from preconcentration into the film than does the electrochemical signal. Once the spectroelectrochemical measurements had been demonstrated, it was necessary to characterize any differences in performance between the initial prototype (9-13) and the portable sensor. Figure 4 shows the spectroelectrochemical response from both the initial prototype (large cell) and the small portable sensor after a 60 min uptake of ferrocyanide into the PDMDAAC-SiO2 film. The current responses (not shown) and optical responses of these two

FIGURE 5. Five consecutive runs with 0.5 mM ferrocyanide; the film was regenerated between runs. The PDMDAAC-SiO2 film was equilibrated for 5 min in ferrocyanide solution before each run. The CV was scanned from -0.3 V to +0.55 V vs Ag/AgCl at 20 mV/s. systems were quite similar. The portable spectroelectrochemical sensor was angle-tuned to have approximately half the number of internal reflections compared to the larger prototype design. With fewer reflections the portable sensor had a shorter effective path length and a smaller change in absorbance. A sensor absorbance maximum of 0.04 and 0.09 (Figure 4) for the portable and prototype sensors agreed with expectations. Variations in the PDMDAAC-SiO2 film might explain why this change was not exactly a factor of 2. Development of a Standard Addition Method. The complex nature of the Hanford simulant solution and the actual waste tank sample required the use of a standard addition procedure. Prior to developing this method it was necessary to determine the linear response region of the portable sensor. To determine the linear region a calibration curve was prepared. A single PDMDAAC-SiO2 coated OTE was utilized to generate this calibration curve. Consequently, it was necessary to reestablish initial conditions prior to each sample run by extracting all ferrocyanide from the film. The reproducibility of the regeneration procedure developed is shown in Figure 5. A sample of 0.4 mM ferrocyanide was injected into the sensor and allowed to uptake into the PDMDAAC-SiO2 film for 5 min. After 5 min the potential was VOL. 37, NO. 1, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Calibration plot of ∆A vs concentration after a10 min uptake of ferrocyanide. The potential was scanned from -0.30 V to +0.55 V vs Ag/AgCl at 20 mV/s. scanned from -0.3 V to +0.55 V at 20 mV/s. After scanning, the sensor was flushed with 5 mL of 1.0 M KNO3 and the last 800 µL of this volume was left in the cell for 5 min. This procedure was executed twice. Finally, 20 mL of 0.1 M KNO3 was flushed through the cell and the last 800 µL of this volume left for a period of 10 min to reestablish initial conditions before the next sample was run. This was the procedure followed for the five consecutive sample measurements and the results are shown in Figure 5. The average sensor absorbance change, ∆A, for the 5 runs was 0.019 units. Since analyte quantification is based on ∆A rather than absolute absorbance, the small residual absorbance after regeneration had a negligible effect on the results. A plot, not shown, of the current response for each run showed no difference between scans. No degradation of the chemically selective film was observed during these experiments. This same procedure was followed for each sample tested in the generation of the calibration curve shown in Figure 6. A 10 min uptake at each concentration was allowed prior to obtaining a single scan measurement. The sensor showed a good linear region between 5 × 10-5 M and 1 × 10-3 M ferrocyanide. Above 1 × 10-3 M ferrocyanide, a 10 min uptake resulted in saturation of the film capacity and a plateau of the calibration curve. A series of tests performed on the simulated waste indicated that the PDMDAAC-SiO2 film began to degrade at about 45 min after beginning constant scanning of the electrode potential. This degradation was indicated by the formation of a second peak in the voltammogram consistent with a bare electrode scan of ferrocyanide. Consequently, efforts directed at determination of ferrocyanide were focused on limited scanning of the potential in the shortest amount of time. Once the linear region for the sensor had been established, a procedure for the determination of ferrocyanide in the simulant solution by standard addition was developed. Attempts were made to establish a standard calibration curve with known concentrations of ferrocyanide in a spiked “blank” simulant solution. However, the film could not withstand the cleaning conditions once exposed to these solutions. The high concentrations of edta and en in the simulant and the high ionic strength of the cleaning solution contributed to an increase in the degradation rate of the film. Again, this was marked by a loss in the optical signal and the appearance of a second peak in the voltammogram consistent with a ferrocyanide on a bare electrode. To reduce the effects of the edta and en, dilution was required. Therefore 128

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FIGURE 7. Standard addition plot showing the detection of ferrocyanide in U-Plant-2 simulant solution. The standard solution contained 10 mM ferrocyanide in 0.1 M KNO3. All other conditions were as noted in legend for Figure 6. standard addition proved to be an excellent method for analysis of both the simulated waste and actual waste samples. The standard used for this procedure was a 1.0 mM ferrocyanide solution in 0.1 M KNO3. Each testing solution consisted of 0.5 mL of the simulant with the appropriate addition of the known standard and brought to the total volume (25 mL) with 0.1 M KNO3 (dilution of original sample by a factor of 50). The choice of these volumes allowed for the concentration ranges to fall within the linear range previously determined for this sensor. The same film was exposed to each sample for 10 min prior to scanning the potential and measuring the absorbance. After each sample was analyzed, a plot of the volume of standard added versus sensor absorbance was generated. By calculating the slope and intercept, determination of the concentration of ferrocyanide within the initial simulant solution could be obtained. Analysis of Simulant and Tank Waste Samples. The standard addition plot for determination of ferrocyanide in the U-Plant-2 simulant solution by the standard addition method is shown in Figure 7. A value of 9.2 mM ferrocyanide was determined. Analysis of the simulant using an FTIR method (14) determined the concentration to be 9.38 (( 0.06) mM ferrocyanide (as Fe(CN)64-). The analysis of a sample from waste tank 241-C-112 was conducted following the same procedure used for the U-Plant-2 simulant solution. Figure 8 shows the cyclic voltammograms(a), the absorbance measurements (b) and the standard addition plot for the analysis (c). This figure illustrates how the optical signal quantitatively follows the electrochemical signal. Figure 8a shows voltammograms that are characteristic of the film behavior seen with ideal solutions. Despite the harsh conditions associated radiological waste, the sensor behaved as expected. Ferrocyanide was incorporated into the anion selective film and the electrochemical change associated with the conversion of ferrocyanide to ferricyanide produced a measurable optical change that was indicative of the concentration of ferrocyanide in the sample from the waste tank. The maximum absorbance change associated with each solution can be determined from Figure 8b. A final plot of volume added vs the maximum absorbance change associated with each sample solution provided the slope and intercept that was used to calculate the concentration of ferrocyanide in the original sample from tank 241-C-112. A concentration of 1.0 mM ferrocyanide was determined with a single analysis. Restrictions on the use of more sample prevented multiple measurements by the sensor, and so the precision is unknown. The value measured

a low signal-to-noise ratio at this concentration. In addition, the FTIR method depends on the exclusive assignment of any υCN intensity to ferrocyanide rather than other similar solution species. In this regard, the spectroelectrochemical method may be more specific to ferrocyanide but further studies would be required to clarify this point. For these reasons, we conclude that the agreement between our result and that reported by Bryan et al. is reasonable. Further testing and refinement in the sensor design and method may help to bring the two procedures into better agreement. Several improvements could be made to the system to allow for a better detection limit. One such improvement would be to use a blue laser diode at 405 nm to decrease the wide bandwidth, increase the value of  at the measuring wavelength, and minimize the variation of  associated with an LED. This may help reduce the contributions from optical interferences that may have contributed to the higher result for the spectroelectrochemical sensor in comparison to the standard FTIR analysis for the tank sample. A laser diode would also permit the use of waveguides that are inherently more sensitive than a multiple internal reflection device. However, many of the improvements will come with associated increases in cost. Future modifications in portable sensors will require that these increased costs be weighed against the potential gains in order to make a suitable long-term testing apparatus that utilizes spectroscopic, electrochemical, and chemical modes of selectivity. A single determination on a tank waste sample agreed sufficiently well with results from an accepted laboratory method to warrant further development of the sensor for this application if monitoring of ferrocyanide is required.

Acknowledgments Financial support provided by the Environmental Management Sciences Program of the Department of Energy, Office of Environmental Management (Grant DE-FG07-96ER62311) is gratefully acknowledged. We thank William Brauntz at the University of Cincinnati for machining the sensor, Susan Ross (University of Cincinnati) for her input into the initial design of the sensor, and Jay Johnson for his helpful discussions.

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FIGURE 8. Analysis of radioactive waste from Hanford Tank 241C-112. (a) Cyclic voltammograms of solutions were taken after 10 min equilibration and scanned from -0.30 V to +0.55 V vs Ag/AgCl (3M KCl) at 20 mV/s. (b) The optical response driven by the electrochemical change of ferrocyanide to ferricyanide. (c) A standard addition plot determining the concentration of ferrocyanide in the radioactive waste sample. The standard solution contained 10 mM ferrocyanide in 0.1 M KNO3. (14) by a FTIR technique (absorbance of CN stretching vibration) was 0.61 ( 0.04 mM. It is important to note that we had only one chance to measure the ferrocyanide concentration and were unable to make a statistically meaningful measurement. Both of these methods suffer from

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Received for review February 15, 2002. Revised manuscript received October 7, 2002. Accepted October 14, 2002. ES020601L