Preconcentration of f-Elements from Aqueous Solution Utilizing a

Jan 27, 2011 - ABSTRACT: An evaluation using paraffin oil based, Acheson 38 carbon paste electrodes modified with R-hydroxyisobutyric acid (HIBA) to ...
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Preconcentration of f-Elements from Aqueous Solution Utilizing a Modified Carbon Paste Electrode Paul D. Schumacher, Kelly A. Fitzgerald, James O. Schenk, and Sue B. Clark* Department of Chemistry, Washington State University, Pullman, Washington 99164, United States

bS Supporting Information ABSTRACT: An evaluation using paraffin oil based, Acheson 38 carbon paste electrodes modified with R-hydroxyisobutyric acid (HIBA) to preconcentrate f-elements cathodically is described. The modified paste was made by directly mixing solid HIBA into the carbon paste. A chemically reversible cyclic voltammogram for HIBA was observed on this modified carbon paste, which was found to be a non-Nerstian, single electron transfer process. Lanthanides (less promethium) were found to accumulate onto the electrode surface during a 30 s electrodeposition step at -0.4 V vs Ag/AgCl from 0.1 M LiCl. The elements were then stripped off into a 2% HNO3 solution by an oxidative step at þ0.8 V vs Ag/AgCl; quantitative removal from the electrode was confirmed by ICPMS. Ultratrace solutions with initial concentrations down to 5 parts per quadrillion (ppq) were preconcentrated in 5 min above our instrumental limit of detection (LOD) of around 1 ppt for lanthanides.

ince its first reporting in 1958 by Ralph N. Adams,1 carbon paste (CP) electrodes have become widely used in electrochemical research.2 Many factors have led to their popularity: low ohmic resistance,3 large potential window,2 and ease of modification,4 to name a few. A quick review of recent literature indicates that CP or modified carbon paste (MCP) electrodes are applicable to aqueous5,6 and nonaqueous7,8 matrices in determination of organic9,10 and inorganic 11,12 elements and compounds. Ease of modification is one of the most valuable features of CP electrodes.2 This is due to the well-developed surface of CP, which has a high adsorptivity for substances.13 Modification of CP electrodes can be achieved through a multitude of methods. A few of these methods are chemical pretreatment where the carbon is soaked in the modifier and then evaporated to dryness before being prepared as an electrode;14 in situ modification where the modifier adsorbs to the surface of plain CP electrode, thus allowing for determination of analyte in the solution;15 dissolution in the binding liquid which is typically achieved through the use of an ion-exchange resin;16 or direct mixing of dry modifiers into the paste through mechanical means which is the most frequently used method.13 It has been known since the 19500 s that separation, preconcentration, and purification of f-elements could be achieved through formation of mercury amalgams.17,18 Early techniques used mercury pools as the cathode, and more recent work has focused on thin mercury films to detect or preconcentrate f-elements.19 Due to the inherent toxicity of mercury and problems with associated hazardous waste disposal, a stronger focus has been on developing mercury-free techniques of which CP and MCP electrodes are particularly well-suited to perform.20,21

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While applications of CP and MCP electrodes are numerous, relatively little is reported in the area of f-elements. Li et al.22 developed a novel MCP electrode with alizarin used as the complexant modifier. The MCP electrode responded well for the middle to heavy lanthanides with a limit of detection (LOD) of 10-10 M for Ho3þ in an acetate buffer. Linear sweep voltammetry (-0.2 to þ0.8 V vs SCE) was applied after a 60-120 s preconcentration period at -0.2 V. A linear, concentrationdependent signal was obtained for the range of 10-10 to 10-7 M, and concentrations of the heavier lanthanides in a dissolved, cast iron sample were quantitatively determined using this electrode. Li’s subsequent work23 focused on Ce3þ using the same electrode and similar solution conditions described previously. After optimization, the alizarin MCP electrode exhibited a LOD for Ce3þ of 10-9 M and a linear response range of 10-9 to 10-7 M. Ganjali et al.24 developed an ion selective electrode (ISE) for Ho3þ utilizing a MCP electrode containing multiwalled carbon nanotubes, nanosilica, and the ionophore N0 -(2-hydroxybenzylidene)furan-2-carbohydrazide in addition to graphite. The MCP electrode had a detection limit of 10-8 M for Ho3þ and a linear response range from 10-7 to 10-2 M. Additionally, a single conditioned electrode showed a reproducible stable response to standard solutions for up to 2 months. Continuing the work of Ganjali et al., Norouzi et al.25developed an Er3þ ISE with the same basic components in the MCP electrode except the ionophore was changed to N0 -(2-hydroxy-1,2-diphenylethylidene) benzohydrazide. The response of the electrode to Er3þ was Received: October 29, 2010 Accepted: January 4, 2011 Published: January 27, 2011 1388

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Figure 1. Structure of the ligand R-hydroxyisobutyric acid.

similar to that demonstrated by Ganjali et al. with a LOD of 10-8 M and a linear range of 10-7 to 10-2 M. Extending our previous work on preconcentrating trace lanthanides utilizing a thin mercury film electrode in aqueous solution,19 we describe here our initial studies on the rapid preconcentration of the rare earth elements (REEs) using a MCP electrode. The ligand of choice for the MCP electrode was Rhydroxyisobutyric acid (HIBA) shown in Figure 1. HIBA was first reported for trivalent f-element separations by Choppin and Silva in 1956,26 and it remains a premier reagent for trivalent felement separation.27 In this study, a large redox signal is observed with the HIBA modified carbon paste (HIBA-CP), and ultratrace levels of lanthanides were quickly and effectively preconcentrated on this electrode, as validated by subsequent analysis using ICPMS.

’ EXPERIMENTAL SECTION Reagents. Reagent grade graphite, LiCl, paraffin oil, and

HIBA were used as received, from Fisher Scientific, (Waltham, Massachusetts, USA, www.fishersci.com). For the multielement analysis, a stock solution containing 10 ppm of analytes (Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Th) in 2% HNO3 was purchased from High-Purity Standards, (Charleston, South Carolina, USA, www.highpuritystandards. com). This solution was used as received. Procedures. Electrode Fabrication. Procedures for preparation of the CP electrode were modified from Adams.28 In brief, 5 g of Acheson 38 grade graphite was mechanically mixed with 3 mL of paraffin oil using a glass mortar and pestle to produce a thick, uniform paste. To prepare the HIBA-CP electrode, 5 mmol of solid HIBA was added to 3 mL of paraffin oil and mixed until the slurry was homogeneous. Then, five grams of carbon were added and mixed to form the paste. Procedures for smoothing and renewing the electrode followed those of Adams.28 Electrochemical Procedures. Electrochemical measurements were made using a Model 100B potentiostat, 15 mL Teflon electrochemical cell, and 3 mm CP Teflon electrode body purchased from Bioanalytical Systems Inc. (West Lafayette, Indiana, USA, www. basinc.com). The 15 mL Teflon electrochemical cell was constructed and setup similarly to Schumacher et al.19 The reference and auxiliary electrodes were Ag/AgCl and Pt wire, respectively. Solutions within the electrochemical cell were purged with purified Ar prior to conducting an experiment. Cyclic voltammetric (CV) experiments were typically scanned at a rate of 100 mV/s starting at þ0.8 to -0.4 V. Double potential step chronoamperometric (DPSC) and chronocoulometric (CC) experiments were stepped from þ0.8 to -0.4 V and back to þ0.8 V vs Ag/AgCl. The potential was pulsed for 1, 3, or 5 s for each potential step with a prepulse holding time of 1 s. The background electrolyte was 0.1 M LiCl that was pH adjusted using 2% HNO3. Preconcentration and

Figure 2. (Panel A) a CV scan showing the electrochemical response of a CP electrode in 0.1 M LiCl. (Panel B) a HIBA-CP, solid line, electrode response in the same 0.1 M LiCl solution. For comparison, the CP electrode response is plotted in Panel B as the dashed line. Scan rate was 100 mV/s; pH 3.5; and potential reported vs Ag/AgCl. Scan begins and ends at 0.8 V for both voltammograms.

stripping experiments followed the procedures outlined in Wang29,30 with modifications. After a deposition step of 30 s, the CP or HIBA-CP electrode was removed from the cell, wiped with a Kim-Wipe on the insulating shroud, and transferred to a separate vial containing 2 mL of 2% HNO3. A stripping step from -0.4 to þ0.8 V vs Ag/AgCl for 30 s was performed, and the solution was analyzed by ICPMS. A typical experiment involving conditioning the CP or HIBA-CP electrode, preconcentration of the trivalent f-element, and stripping into 2% HNO3 required approximately 5 min. The cell was cleaned between experiments following the procedures of Schumacher et al.19 ICPMS Procedures. ICPMS measurements were performed on an Agilent 7500 ICPMS utilizing an internal indium and rare earth standard and scanned in the positive mode. Prior to analyzing any samples, the instrument was calibrated with a set of prepared standards in 2% HNO3 and plain 2% HNO3 was used as the blank to correct for background. Analyses of electroanalytical and ICPMS data were performed using GraphPad Prism version 5.02 for Windows, (GraphPad Software, San Diego, California, USA, www.graphpad.com).

’ RESULTS AND DISCUSSION Figure 2A shows the electrochemical response for a CP electrode to 0.1 M LiCl at pH 3.5. While this voltammogram is only the response to background electrolyte, a series of scans were performed in various solutions containing HIBA and/or f-elements to determine if CP exhibited any electrochemical response. In these 1389

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Figure 3. Family of scans for a single HIBA-CP electrode in 0.1 M LiCl, pH 3.5. Scan rate was varied from 1 to 500 mV/s. The ratio of ic/ia was 1.12 ( 0.03, suggesting chemical reversibility. Wave shape departed from a strict Nernstian diffusional process, > 59/n mV, indicating electrochemical charge transfer irreversibility.

cases, the CP electrode showed no electrochemical response to dissolved f-elements (Figure S-1, Supporting Information). Figure 2B shows the electrochemical response for a HIBA-CP electrode to 0.1 M LiCl at pH 3.5; the pKa for HIBA is 3.7.31 For this voltammogram, the scan rate was 100 mV/s and started at þ0.8 V. A large reduction and oxidation signal was observed for the HIBA-CP electrode. Each new HIBA-CP electrode typically required two to three conditioning scans to achieve a stable (exceeding three hours) electrochemical response. Variability of peak intensity between electrodes was less than 10%, which falls within the range expected for MCP electrodes.29 Additionally, the cathodic and anodic peaks varied less than 5% between electrodes for a given scan rate. This demonstrates that, once conditioned, the HIBA-CP electrode yields a stable and reproducible response. Watanabe et al.32 reported that f-elements will adsorb to carbonaceous material in acidic environments. Our result for the CP electrode is not in disagreement with Watanabe et al. as the contact time for their study was much longer (3 to 4 h)32 than the time period used in this study (1 to 2 min). Figure 3 is a series of CVs for a single HIBA-CP electrode in 0.1 M LiCl where the scan rate was varied from 1 to 500 mV/s. The measured ratio of cathodic (ic) and anodic (ia) peak intensities was constant across scan rates at 1.12 ( 0.03 (n = 3), suggesting the electrochemical response from the HIBA-CP is chemically reversible. The plots of scan rate, υ, versus ip and υ1/2 versus ip were inconclusive as to whether the observed results represent an adsorption or diffusion phenomenon (Figure S-2, Supporting Information). This result is not surprising given the range of υ used. The shapes of the voltammograms depart from those predicted for a strict Nernstian (reversible), diffusional process; e.g., the difference of anodic and cathodic peak potentials is greater than 230/n mV instead of 59/n mV. Additionally, the difference in peak potentials increases with increasing scan rate. To further evaluate diffusion vs adsorption, chronoamperometric analysis of the voltammetric wave shapes were conducted using the Cottrell equation pffiffiffiffi nFAC D ð1Þ i ¼ pffiffiffiffiffi πt where i = current (amps), n = number of electrons, F = Faraday constant (96 485 C/mol), A = area of the electrode (cm2),

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Figure 4. Background subtracted CV for 0.1 M HIBA on a 3 mm Pt working electrode. Background electrolyte was 0.1 M LiCl, pH 3.5, and scan rate was 100 mV/s. Potential is reported vs Ag/AgCl. Nernstian and faradaic comparison to 0.1 mM K3[FeCN6] on the same electrode indicates an electron transfer of 1.2 ( 0.2 electrons. The anodic peak circa -0.85 V is the result of H2 adsorption on Pt.28,33

C = initial concentration of the analyte (mol/cm3), D = diffusion coefficient for the species (cm2/s), and t = time (s) was used to evaluate the waveforms. A plot of t-1/2 vs i deviated from linearity based on time of exposure to the analyte (Figure S-3, Supporting Information), suggesting other processes were either occurring at the surface of the electrode or impeding diffusion from the bulk solution to the electrode surface.33,34 A series of experiments were performed at different values of pH, and no pH effects were observed (Figure S-4, Supporting Information), suggesting that the observed phenomenon is occurring on the surface of the electrode and not a direct result of solution conditions. To determine the number of electrons transferred per mole of HIBA, a solution containing 0.1 M LiCl with 0.1 mM K3[FeCN6], which has a known 1 e- transfer, [Fe(CN)6]3- þ e- H[Fe(CN)6]4-, was analyzed by CV on a 3 mm Pt electrode to determine the ip and integrated voltammetric wave area in coulombs (Figure S-5, Supporting Information). A separate solution containing 0.1 M LiCl with 1 mM HIBA was analyzed by CV on the same Pt electrode (Figure 4). Comparison of the ratios of magnitude of ip and integrated voltammetric wave areas for HIBA to [FeCN6]3- indicated an electron transfer of 1.2 ( 0.2 electrons per mole of HIBA. This comparison is only valid if the diffusion coefficients (D) for [FeCN6]3-, 0.76  10-5 cm2/s28 and HIBA are similar. While no D for HIBA was found in the literature, lactic acid with a D of 0.84  10-5 cm2/s35 and isobutyric acid, D = 0.59  10-5 cm2/s,36 were found. Since ip values are proportional to D1/2, the differing values of D would fall within a 13% error. Kvaratskhelia and Kvaratskhelia37 examined the voltammetric responses of hydroxycarboxylic acids in aqueous solutions using various solid electrodes. Their E1/2 values of the observed waves on Pt in 0.1 M NaClO4 occurred in the range of -0.47 to -0.49 V vs a saturated calomel electrode. Our voltammetric response for HIBA is in agreement with their observed results. In a MCP electrode study involving complexes between rare earths and alizarin, Li et al.22 reported a 2 e- charge transfer irreversible process for alizarin that was not pH dependent. They point out that most electrode processes of organic compounds involve proton ion transfers; thus, a pH dependence is expected. However, in the case of alizarin, this was not observed. Our characterization of HIBA, a 1 e- irreversible process with no pH dependence, agrees with Li et al.22 1390

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Figure 5. Anson plot for CP and HIBA-CP electrode response to La3þ. Solid line represents the response of HIBA-CP to a mixture containing 0.1 M LiCl and 1  10-5 M La3þ at pH 3.5. Dashed line is CP response to an identical mixture of LiCl and La3þ. The CP line intersects on the xaxis, indicating no sorption phenomenon. The HIBA-CP intersection above the x-axis is indicative of a sorption phenomenon with total moles of lanthanum accumulated of 3.6 ( 0.7  10-12, n = 3.

Table 1. Electrochemical Response of HIBA-CP to Varying [La3þ] [La3þ]

measured current

1  10-3 M

6.2 ( 0.7  10-6 A

1  10-4 M

3.1 ( 0.3  10-6 A

1  10-5 M 1  10-6 M

2.7 ( 0.5  10-6 A 5.3 ( 0.9  10-7 A

1  10-7 M

1.1 ( 0.6  10-7 A

Figure 5 shows the resultant Anson plot38 for a 3 s chronocoulometric experiment using both a HIBA-CP and CP electrode in 0.1 M LiCl and 1  10-5 M La3þ . The reduction lines are plotted as t1/2 vs Q, and the oxidation lines are plotted as θ vs Q, where θ = [τ1/2 þ (t - τ)1/2 - t1/2] as defined by Anson.36 For HIBA-CP, the reduction and oxidation lines exhibit different slopes with an intersection above the x-axis. The CP reduction and oxidation lines have nearly identical absolute values of their slopes, and the lines intersect on the x-axis. One second and 5 s chronocoulometric experiments were also performed with nearly identical results to the 3 s experiment (data not shown). This range of time scales were chosen to minimize contributions from convective mass transport, which occurs at solid electrodes at time periods greater than 5 s.28 According to Anson, intersection above the x-axis represents the total coulombs of adsorbed reactant because this analysis of chronocoulometric data effectively removes any contribution due to double layer charging.36,39 Using the intersection value, 1.1 ( 0.3  10-6 C, with Faraday’s Law; the total number of moles of lanthanum accumulated equals 3.6 ( 0.7  10-12, n = 3. Table 1 shows the results of chronoamperometric experiments with a HIBA-CP electrode in varying solution concentrations of La3þ (10-7 to 10-3 M) with 0.1 M LiCl as the background electrolyte at pH 3.5. The data points were obtained by running 1 s chronoamperometric experiments (an experimentally convenient time interval) and measuring a background corrected i value at 500 ms. This time span was chosen because it excludes

distortion due to charging current and minimizes contributions due to convective mass transport.40 A new HIBA-CP electrode was used for each change in the concentration of La3þ, and each value represents the average of triplicate runs. Analysis of the data indicates a concentration dependent response which is approaching maxima. This is indicative of a sorption phenomenon and is in agreement with previous work preconcentrating trivalent felements with Hg films.19 Figure 6 shows representative results from the preconcentration experiments. The multielement standard used contained all the lanthanides, minus promethium, plus scandium, yttrium, and thorium. The four graphs represent the range of responses of the lanthanides and shows that the HIBA-CP electrode will preconcentrate above the limit of detection (LOD) for the ICPMS while the CP electrode does not preconcentrate under the same conditions as the HIBA-CP electrode. While Figure 6 represents the overall range, the other trivalent lanthanides were also measured and their plots can be found in Supporting Information (Figures S-5-7). The counts for all the lanthanides were totaled and applied to a calibration curve to determine total moles accumulated, 3.0 ( 0.6  10-12, n = 3. Comparing this number to the total moles adsorbed via the Anson plot in Figure 5, 3.6 ( 0.7  10-12, we find good agreement indicating that the HIBACP electrode accumulates individual lanthanides or a mixture with equal efficiency. Interestingly, for Sc, Ce, and Th, the HIBA-CP electrode did not preconcentrate above LOD. A possible explanation for the case of Sc is that, while in the same group as La, Sc responds in solution more as a d-element while Y, which does preconcentrate, responds more like an f-element.27 To gain some insight into the mechanism of HIBA-CP preconcentration, a comparison of stepwise formation constant (log K) values for HIBA in 0.1 M ionic strength from Martell and Smith31 and the total amount of f-element preconcentrated by the HIBA-CP electrode was conducted. Since HIBA exhibits a systematic increase in log K values across the series of lanthanides,41 one would expect that if HIBA-CP preconcentration capability was solely a function of HIBA in the electrode, then a similar trend would be observed. While in general, heavier lanthanides preconcentrated more readily than lighter lanthanides, no direct comparison could be made, indicating that more factors are involved in HIBA-CP preconcentration capability and further work is required to elucidate these factors. While this work has been performed in a neat solution, interferences are expected since HIBA complexes to some extent with most metal cations are present in solution. While many factors affect the strength of metal-ligand complexes, a good first approximation for determining potential interferences are thermodynamic stability constants. Nash and Jensen27 thoroughly discuss the solution chemistry aspects of metal-ligand complexes and provide an excellent justification for the use of stability constants for initial approximations. Surprisingly, while HIBA has been in use since its first reporting in 1956,26 relatively little critically reviewed thermodynamic stability constant data are available for metal cations other than the f-elements.31 While no stability constant data exists for a Liþ-HIBA complex, taking considerations of ionic charge, radius, and strength of ion-dipole interactions, we estimate that Liþ interactions with HIBA are minimal, resulting in little interference. In our case, Liþ was in 100 000-fold excess of trivalent f-elements and did not serve as a major interference. 1391

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Figure 6. ICPMS results for preconcentration of select f-elements. LOD was determined from seven blank runs þ 3 σ SD. Closed circles represent the results from calibration standards (1 ppb-1 ppt). Open triangles represent the preconcentration results of 5 ppq with the HIBA-CP electrode, n = 5. Open diamonds represent preconcentration results of 5 ppq with a CP electrode. Open squares are the results of 5 ppq solutions with no preconcentration. Errors bars are 1σ SD, most of which are within the dimensions of the symbols. Y-axis for all graphs are expressed in terms of counts per second.

’ CONCLUSIONS The aim of this work was to develop a simple, quick, and mercury-free technique for preconcentrating trace level trivalent f-elements from bulk aqueous solution that allows for follow-on separation and detection. As the data have shown, the technique is simple and quick with the capability of preconcentrating ultratrace levels, 5 ppq, of trivalent f-elements within 5 min. Trivalent f-elements will sorb to and can be removed from the surface of the HIBA-CP electrode as a mixture, thus allowing for follow-on separation techniques at the trace level. Further work on the optimal solution conditions and electrode parameters for the HIBA-CP electrode along with elucidating the mechanism by which trivalent f-elements sorb is still required to realize the full potential of this technique. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: 1-509-335-1411. Fax: 1-509335-8867.

’ ACKNOWLEDGMENT The authors would like to thank Dr. Charles Knaack of the Washington State University GeoAnalytical Lab for his assistance

with ICPMS experiments. The authors also thank the US Army for its funding for P.D.S. and the Academic Research Initiative of the Joint Domestic Nuclear Detection Office, Department of Homeland Security, and the National Science Foundation, for funding under Grant Numbers ECCS-0833548 and DN-077ARI-03302.

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