Poly(3,4-ethylenedioxythiophene)–Poly(styrenesulfonate)-Coated

Publication Date (Web): August 9, 2018 ... Uranium (U) and plutonium (Pu) contents in nuclear materials must be maintained to a definite level in orde...
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Cite This: Anal. Chem. 2018, 90, 10187−10195

Poly(3,4-ethylenedioxythiophene)−Poly(styrenesulfonate)-Coated Glassy-Carbon Electrode for Simultaneous Voltammetric Determination of Uranium and Plutonium in Fast-Breeder-TestReactor Fuel Rahul Agarwal,*,†,‡ Manoj K. Sharma,†,‡ Kavitha Jayachandran,†,‡ Jayashree S. Gamare,‡ Donald M. Noronha,‡ and Kaiprath V. Lohithakshan‡ Anal. Chem. 2018.90:10187-10195. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/06/18. For personal use only.

†

Homi Bhabha National Institute, Mumbai 400 094, India Fuel Chemistry Division, Bhabha Atomic Research Centre (BARC), Trombay, Mumbai 400 085, India

‡

S Supporting Information *

ABSTRACT: Uranium (U) and plutonium (Pu) contents in nuclear materials must be maintained to a definite level in order to get the desired performance of the fuel inside the reactor. Therefore, high accuracy and precision is an essential criterion for the determination of U and Pu. We already reported the voltammetric determination of Pu in the presence of U in fast-breeder-test-reactor (FBTR) fuel samples, but interfacial, coupled chemical reactions between U(IV) and Pu(IV) enhance the peak-current density of U(VI) reduction and thus make voltammetry unsuitable for the quantitative determination of U in the presence of Pu. Thus, developing a voltammetric method for the simultaneous determination of U and Pu is highly challenging. Herein, we report the simultaneous voltammetric determination of U and Pu in 1 M sulfuric acid (H2SO4) on a poly(3,4-ethylenedioxythiophene) (PEDOT)−poly(styrenesulfonate) (PSS)-modified glassy-carbon (GC) electrode (PEDOT−PSS/GC). The modified electrode shows enhanced performance compared with bare GC electrodes. The peak-current density for U(VI) reduction is enhanced in the presence of Pu(IV), but it attains saturation when [Pu]/[U] in solution is maintained ≥2. Hence, under these circumstances, the variation of Pu concentration no longer influences the U(VI)-reduction peak, and thus the quantitative determination of U in the presence of Pu is possible. No interference is observed from commonly encountered impurities present in FBTR fuel samples. This method shows accuracy and precision comparable to those of the biamperometry method. High robustness, fast analysis, simultaneous determination, reduced radiation exposure to the analyst, and ease of recovery of U and Pu from analytical waste makes it a suitable candidate to substitute the presently applied biamperometry method.

F

A large number of destructive and nondestructive analytical techniques6−21 are available in the arsenal of an analyst for determination of U and Pu in nuclear fuel samples, but redox titrimetric methods offer several advantages over other techniques if cost, time, precision, accuracy, space, and the analyst’s effort are all taken into account. Most of the methods involve expensive equipment, are time-consuming, and are not capable of giving results with accuracy and precision better than 1%.22 Redox titrimetry is a low-cost technique with high precision and accuracy, and biamperometry is routinely employed to determine U and Pu contents in nuclear fuel samples.17,20 Because the availability of the sample is large, methods are standardized in milligram scales. The modified

or the development of fast breeder reactors (FBR) in India, a 13.2 MWe fast breeder test reactor (FBTR) was constructed at Kalpakkam, having been commissioned in October 1985.1 It is a forerunner to the second stage of the Indian three-stage nuclear-energy program.2 The experience gained in the operation of the FBTR has enabled Indian nuclear scientists and engineers to face the challenge of designing and building the 500 MWe prototype fast breeder reactor (PFBR) at Kalpakkam.3 The FBTR not only provides experience in handling large quantities of liquid sodium, but it is also a test bed for developing fuels and structural materials for other reactors. Indigenously developed mixed uranium− plutonium carbide is used as driver fuel in FBTR.4 On this ground, the fissile content (U and Pu) must be maintained at a definite level to get the desired performance of the fuel inside the reactor. Thus, as far as nuclear fuel samples are considered, very high accuracy and precision on the order of 0.2% are essential criteria.5 © 2018 American Chemical Society

Received: February 15, 2018 Accepted: August 9, 2018 Published: August 9, 2018 10187

DOI: 10.1021/acs.analchem.8b00769 Anal. Chem. 2018, 90, 10187−10195

Article

Analytical Chemistry Drummond and Grant18 method is used in our laboratory to determine Pu (SI). The Davies and Gray method19 is wellknown for U determination, but the quantitative recovery of Pu from phosphoric acid (H3PO4) is very difficult, so this method was modified in our laboratory for U determination, as reported elsewhere (SI).17 Although biamperometry is the most preferred redox titrimetric method for the quantitative determination of U and Pu in nuclear fuel in our laboratories, it too suffers from a major disadvantage. Recovery of precious Pu and U from analytical waste is a cumbersome process, because it requires the separation of various metallic impurities (Fe, Cr, Ti, Ag, K, etc.) present in the redox titrants added during the analysis. Also, biamperometry requires separate solutions for the determination of U and Pu in the same sample. Our group is actively involved in the development of alternate electroanalytical methods for the simultaneous determination of U and Pu in a single aliquot solution with the same accuracy and precision as those of biamperometry but without any metallic impurities in the generated analytical waste. This will reduce the number of steps in the recovery process and ease the management of analytical nuclear waste. Controlled potential coulometry (CPC) is another wellestablished technique for the precise and accurate determination of U and Pu, and it generates analytical waste with no metallic impurities.22−25 Two different working electrodes (a Hg-pool electrode for U and a Pt electrode for Pu) are required for the determination of U and Pu (SI). Furthermore, Pt gets passivated as a result of the formation of platinum oxide on the surface at higher anodic potentials,26,27 and Hg is toxic and very difficult to handle in radioactive fume hoods and glove boxes. The simultaneous determination of U and Pu by coulometry on single working electrode (graphite) has also been reported.28 Although coulometry is an absolute electroanalytical method; its application is limited because of its long analysis time. Voltammetry is another precise and accurate electroanalytical technique that has the additional advantages of being simple and fast and, like CPC, does not require the addition of chemical reagents. Our group is involved in the development of fast and simple voltammetric methods to determine U and Pu contents in nuclear fuel samples. There are only two reports in the literature for the simultaneous determination of U and Pu by voltammetry, and both were published by our lab.29,30 Gupta et al. reported the use of single-wall-carbon-nanotube-modified gold electrodes for the simultaneous determination of U and Pu,29 but Guin et al. found some inconsistencies in the U(VI)-reduction-peak current in the presence of Pu(IV) on an electrochemically reduced graphene oxide modified glassy-carbon electrode. The inconsistency in the U(VI)-reduction-peak current is explained in terms of the coupled chemical reaction between U(IV) and Pu(IV), and Guin et al. concluded that the simultaneous determination of U and Pu is not feasible by voltammetric methods.30 Thus, developing a voltammetric method for the simultaneous determination of U and Pu in 1 M H2SO4 is extremely challenging because of the following reasons: (i) Coupled chemical reactions between U(IV) and Pu(IV) at the electrode−solution interface result in inconsistencies in the U(VI)-reduction peak. (ii) The conventionally used electrodes (Pt, Au, and GC) cannot be used for this purpose. Pt and Au result in H2 evolution in the negative-potential region where U(VI) is reduced and also get passivated as a result of the formation of surface oxide films in the positive-potential region. The U(VI)-reduction peak is very broad on GC

electrodes. Hg is toxic and very difficult to handle in a radioactive fume hood. Hence, the modification of the electrode material is important. (iii) The accuracy and precision of the method should match those of the wellestablished biamperometry method. (iv) Finally, previous reports on this topic that contradict each other are available in the literature. Conducting polymers (CPs) are extensively exploited in developing chemically modified electrodes (CMEs) for a wide range of analytical applications.31,32 Among different CPs used, PEDOT emerges as an efficient electrode material for sensors,33 solar cells,34 supercapacitors,35 electrocatalysts,36 transistor,36,37 and visual displays38 because of its very high electrical conductivity, robustness, low cost, low band gap, processability, and enhanced performance. 39 Although PEDOT is water insoluble, its composite PEDOT−PSS forms dispersion in water,40 having the additional advantages of aqueous processability and transparency in the visible region.41 Recently, we explored the aqueous electrochemistry of the Pu(IV)/Pu(III) redox couple in 1 M H2SO4 on a PEDOT−PSS/GC electrode.42 In that publication, we observed that the surface of the PEDOT−PSS film on the GC electrode possesses randomly distributed spherical and hemispherical particles along with pores or pits of varying radii, which results in a large surface area and leads to enhancement of the oxidation and reduction current of the Pu(IV)/Pu(III) redox couple. However, simultaneous voltammetric determination of U and Pu in nuclear fuel samples is yet unexplored on PEDOT−PSS/GC electrodes. In this paper, we first study the basic electrochemistry of uranium in 1 M H2SO4; then, the modified electrode is exploited for simultaneous voltammetric determination of U and Pu in FBTR fuel samples, and the results are compared with the well-established biamperometry method. The present methodology overcomes the above stated problems for the simultaneous voltammetric determination of U and Pu. Moreover, it produces clean analytical waste from which the recovery of U and Pu is very simple.

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EXPERIMENTAL SECTION Caution! 239Pu (t1/2 = 2.4 × 104 years) is a radioactive isotope and an α-active radionuclide. Because of the radiation hazards caused by 239Pu, it should be handled in a radioactive fume hood with proper precautions, and the handler should be trained in the safe handling of radioactive material to avoid the health risks caused by radiation exposure. All the chemical reagents used were of analytical grade and used without any further purification. The sulfuric acid used in the experiments was of analytical grade, and PEDOT−PSS (1.3 wt % dispersion in H2O) was purchased from SigmaAldrich. Deionized water, purified by a Milli-Q water-purifier system from Millipore (18 MΩ cm), was used to prepare all solutions needed for electrochemical studies. Two separate standard stock solutions of U (2.2745 mg g−1) and Pu (8.7748 mg g−1) were prepared from the chemical-assay standards rubidium uranyl sulfate (Rb 2U(SO 4) 3) and potassium plutonium sulfate dihydrate (K4Pu(SO4)4·2H2O), respectively, in the Fuel Chemistry Division (FCD) of BARC by methods published elsewhere.43,44 The detailed procedure for the preparation of the chemical-assay standards are provided in the SI. FBTR Mark-I fuel samples were dissolved in our laboratory by a method published elsewhere.45 Then, the U and Pu contents in the sample solutions were determined by 10188

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H2SO4.47−49 The disproportionation is so fast that at the potential corresponding to eq 1, UVIO22+ is completely reduced to U(IV)4+ (eq 3). We do not observe any oxidation peak as these involve U−O-bond formation, which is slow with respect to the time frame of the potential-sweep rate of the CV. Thus, the UVIO22+/U(IV)4+ redox couple shows irreversible electrochemistry in 1 M H2SO4 irrespective of the scan rate.47−49 From Figure 1, it is obvious that, PEDOT−PSS shows enhanced performance as compared with that of the GC electrode. Hence, PEDOT−PSS enhances the U(VI)-reduction current, and this is due to the large surface area of PEDOT−PSS/GC.47 The well-defined peak with the higher peak-current density signifies the analytical importance of CV for the quantitative measurement of U(VI) in 1 M H2SO4.

biamperometry and voltammetry methods (discussed in this paper), and the results were compared. The electrochemical measurements were performed at room temperature (T = 298 K) using a CHI 760D electrochemical workstation in a conventional three-electrode cell, and the experimental details are provided in the SI. PEDOT−PSS (10 μL) was drop-casted on a GC electrode; the modified electrode was rinsed with ethanol to remove any excess PEDOT−PSS on the electrode surface and then kept in the open air for drying. These conditions for the modification of the electrode were optimized by the drop-casting method, published elsewhere.42 We calculated the electrochemically active surface areas (Ae) of GC and PEDOT−PSS/GC by examining their cyclic voltammograms (CVs) with 10 mM K3[Fe(CN)6] in 0.1 M KCl as the redox probe at a scan rate of 10 mV s−1 (Figure S1). The Ae values of GC and PEDOT− PSS/GC were calculated to be 0.0609 and 0.1104 cm2, respectively, using the Randles−Sevcik equation.46 The current densities (j) were calculated using these Ae values, as otherwise specified. The roughness factors (ρ = Ae/Ag) for GC and PEDOT−PSS/GC were calculated to be 0.86 and 1.56, respectively. The higher roughness of PEDOT−PSS/GC is attributable to the higher particle density of PEDOT−PSS over the GC electrode surface. Unless otherwise specified, all the CVs represented in this manuscript were blank-subtracted.

2UVIO2 2 + + 2e− = 2UVO2+

(1)

2UVO2+ + 4H+ = UVIO2 2 + + U(IV)4 + + 2H 2O

(2)

2UVIO2 2 + + 4H+ + 2e− = 2U(IV)4 + + 2H 2O

(3)

VI

2+

Figure S2a represents the CVs of U O2 in 1 M H2SO4 at different scan rates on the PEDOT−PSS/GC electrode. The peak-current density systematically increases with increases in the scan rate. It is well-known that the peak-current density is directly proportional to the square root of the scan rate and the scan rate for diffusion-controlled and adsorption-controlled reactions, respectively. Hence, the slope of the plot of ln(-jp) vs ln(ν) is 0.5 and 1.0 for diffusion controlled and adsorption controlled reactions, respectively.47 The peak-current density varies linearly with the square root of the scan rate (Figure S2b), and the slope of the plot of ln(−jp) versus ln(ν) is found to be 0.38 (Figure S2c). This shows that the electrochemical behavior of the UVIO22+/U(IV)4+ redox couple is diffusion controlled on the PEDOT−PSS/GC electrode. We have already explored the electrochemical behavior of the Pu(IV)/ Pu(III) redox couple in 1 M H2SO4 on the PEDOT−PSS/GC electrode, and the results are published elsewhere.42 Cyclic-Voltammetric Response in a U and Pu Mixed Solution. The applicability of the PEDOT−PSS/GC electrode for the simultaneous voltammetric determination of U and Pu is explored by CV studies in a mixture solution. Figure 2 represents CVs in a mixture solution of U (0.2106 mg g−1) and Pu (0.8684 mg g−1) in 1 M H2SO4 at a scan rate of 50 mV s−1 on the GC (Figure 2i) and PEDOT−PSS/GC (Figure 2ii) electrodes. The GC electrode shows a cathodic peak at 0.116 V (Ecp), which represents Pu(IV) reduction to Pu(III), and an anodic peak at 0.700 V (Eap), representing Pu(III) oxidation to Pu(IV), with a peak-potential separation (ΔE = Eap − Ecp) of 0.584 V. The U-reduction peak is not observed because of H2 evolution at more negative potentials (Figure 1i). Thus, the GC electrode cannot be used for the simultaneous voltammetric determination of U and Pu. Contrary to that, the PEDOT−PSS/GC electrode shows two cathodic and one anodic peak (Figure 2ii). The cathodic peak at Ecp = 0.581 V and the anodic peak at Eap = 0.507 V represent the Pu(IV)/Pu(III) redox couple, having a ΔEp of 0.074 V, and the second cathodic peak at −0.158 V represents UVIO22+ reduction to U(IV)4+. Thus, the overpotential needed for electrochemical studies of both Pu(IV)/Pu(III) and UVIO22+/ U(IV)4+ redox couples is significantly lowered on PEDOT− PSS/GC. PEDOT−PSS/GC leads to enhancements in the peak-current densities of both U and Pu in mixed solution, and the enhancements are attributable to the large surface area of

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RESULTS AND DISCUSSION Cyclic-Voltammetric Response in a Pure Uranium Solution. Figure 1 represents the CVs in 2.2745 mg g−1

Figure 1. CVs of 10 mM (or 2.2745 mg g−1) U(VI) in 1 M H2SO4 on (i) GC and (ii) PEDOT−PSS/GC at a scan rate of 50 mV s−1.

UVIO22+ in 1 M H2SO4 at a scan rate of 50 mV s−1 on the (i) GC and (ii) PEDOT−PSS/GC electrodes. The initial scan direction is negative, and the hold time at the initial potential is 10 s. No sharp U(VI)-reduction peak is observed on the bare GC electrode (Figure 1i) as hydrogen-gas evolution starts at negative potentials; this proves that the GC electrode is unsuitable and necessitates modification of GC surface to study the aqueous electrochemistry of U(VI) in 1 M H2SO4.47 However, under similar circumstances, the PEDOT−PSS/GC electrode (Figure 1ii) shows a well-defined cathodic peak at −0.158 V, which represents the reduction of UVIO22+ to U(IV)4+. At first, UVIO22+ is electrochemically reduced to UVO2+ (eq 1), which in turn is chemically disproportionated into UVIO22+ and U(IV)4+ (eq 2) at a very fast rate in 1 M 10189

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PEDOT−PSS. The well-defined and sharp U- and Pu-redox peaks on PEDOT−PSS/GC in the CV (Figure 2) show its potential for the simultaneous voltammetric determination of U and Pu in fuel samples. Interfacial, Coupled Chemical Reaction between U(IV) and Pu(IV). The presence of variable oxidation states and chemical interactions between different redox states of U and Pu makes their aqueous electrochemistry extremely rich and complex. CV studies are done to examine the chemical interactions between U and Pu in a mixed solution. Figure 3a represents the CVs in a pure U solution and a mixed solution of U and Pu in 1 M H2SO4 at a scan rate of 50 mV s−1 on the PEDOT−PSS/GC electrode. The peak-current density of U(VI) reduction is almost two times higher in the mixed solution as compared with the corresponding peak-current density in the pure U solution, although the concentration of U is almost the same. Thus, the presence of Pu enhances the peak current for U(VI) reduction. However, there is no effect in the peak-current density of the Pu(IV)/Pu(III) redox couple in the presence of U, as similar values of peak-current densities for the corresponding redox couple are observed in the absence

Figure 2. CVs of a U and Pu mixture (0.2106 mg g−1 U(VI) and 0.8684 mg g−1 Pu(IV)) in 1 M H2SO4 on (i) GC and (ii) PEDOT− PSS/GC at a scan rate of 50 mV s−1.

Figure 3. CVs of (a) pure U(VI) (0.2134 mg g−1) and U and Pu mixture (0.2106 mg g−1 U(VI) and 0.8684 mg g−1 Pu(IV)); (b) pure Pu(IV) (0.8529 mg g−1) and U and Pu mixture (0.2106 mg g−1 U(VI) and 0.8684 mg g−1 Pu(IV)); (c) U and Pu mixture containing 0, 0.02, 0.05, 0.06, and 0.08 mg g−1 Pu(IV) (i−v, respectively) and a fixed concentration of U(VI) (0.213 mg g−1); and (d) U and Pu mixture containing 0, 0.004, 0.008, 0.012, 0.015, and 0.019 mg g−1 U(VI) (i−vi, respectively) and a fixed concentration of Pu(IV) (0.085 mg g−1) in 1 M H2SO4 on PEDOT− PSS/GC at a scan rate of 50 mV s−1. 10190

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Figure 4. (a) CVs of U and Pu mixtures having a fixed U(VI) concentration of 0.25 mg g−1 and varying Pu(IV) concentrations of 0.16, 0.25, 0.33, 0.57, 0.67, and 0.75 mg g−1 (i−vi, respectively) in 1 M H2SO4 on PEDOT−PSS/GC at a scan rate of 50 mV s−1. (b) Plot of the corresponding peak-current density of U(VI)/U(IV) reduction vs the [Pu]/[U] ratio.

of U (Figure 3b).30 Thus, the presence of U does not modify the electrochemical behavior of Pu in 1 M H2SO4 on the PEDOT−PSS/GC electrode.30 This is further verified by the CV of PEDOT−PSS/GC in a mixed solution with different concentrations of U and Pu. It is observed that as the Pu concentration increases and the concentration of U is kept constant in the U and Pu mixture, the peak-current density for U(VI) reduction increases after each successive addition of Pu, which is a characteristic of a catalytically coupled chemical reaction (EC′ mechanism, Figure 3c). Initially the mixed solution contained U(VI) and Pu(IV). During the cathodic scan of the CV, the product of the electrochemical reduction, U(IV), that formed at the electrode surface (eq 3) underwent a chemical reaction with Pu(IV), diffusing toward the electrode to regenerate the starting material at the electrode−solution interface (eq 4) and thus causing an enhancement in the current of U(VI) reduction in the presence of Pu. This reaction is referred as catalytic regeneration.50,51

on the PEDOT−PSS/GC electrode. The cathodic-peakcurrent density for U(VI) reduction in the case of the pure U solution initially decreases and then remains almost constant for the consecutive CV cycles (Figure S4a).30,52,53 This is attributable to the decrease in the effective surface area of the electrode as a result of the deposition of insoluble uranium(IV)-sulfate species. In the case of the pure Pu solution, both the anodic- and cathodic-peak-current densities for the Pu(IV)/Pu(III) redox couple remain constant for the 25 CV cycles (Figure S4b). In the case of the mixed solution (Figure S4c), although the cathodic-peak-current density for U(VI) reduction is enhanced as a result of the catalytically coupled chemical reaction between U(IV) and Pu(IV), we observed constant peak-current densities for both the U(VI)/U(IV) and Pu(IV)/Pu(III) redox couples for the consecutive CV cycles. These results were confirmed by carrying out repetitive experiments; similar experiments were performed in FBTR fuel samples, and similar results are observed. This behavior is observed irrespective of the concentrations of U and Pu in the mixed solution. The cause of the constant peak-current density of U(VI) reduction in the presence of Pu(IV) is attributable to the fast kinetics of the coupled chemical reaction, and this reaction goes to completion at a faster rate than the time frame of the potential-sweep rate of the CV. The constant peak-current densities of both the U(VI)/ U(IV) and Pu(IV)/Pu(III) reactions over the consecutive CV cycles brings hope for the simultaneous determination of U and Pu, but the enhancement in the peak-current density for U(VI) reduction as a result of the chemical reaction between U(IV) and Pu(IV) makes voltammetry unsuitable for the determination of U in the presence of Pu. Thus, extended CV studies were done in mixed solutions to recheck the feasibility of the simultaneous determination of U and Pu. Figure 4 shows CVs in a mixed U and Pu solution at a scan rate of 50 mV s−1 on the PEDOT−PSS/GC electrode with a constant concentration of U and varying concentrations of Pu. The peak-current density for U(VI) reduction increases systematically with increases in the Pu concentration when the [Pu]/ [U] ratio is in the 0.69−1.3 range, but at higher [Pu]/[U] ratios (>2.0), the peak-current density reaches a steady state,

U(IV)4 + + 2Pu(IV)4 + + 2H 2O = UVIO2 2 + + 2Pu(III)3 + + 4H+

(4)

However, no enhancement in the peak-current density of the Pu(IV)/Pu(III) redox couple is observed by successive additions of U to a solution with a constant amount of Pu. A slight decrease in the peak-current density of the Pu(IV)/ Pu(III) redox couple is attributable to the decrease in the Pu content from the dilution by the addition of the U solution. This result further confirmed that the presence of U does not modify the electrochemical behavior of Pu. Pu(III) formed at the electrode surface diffuses toward the bulk solution, and hence excess Pu(III) (formed by a coupled chemical reaction) is not available at the electrode−solution interface during the positive potential scan. Hence, no enhancement in the Pu(III)−Pu(IV)-oxidation current is observed in the presence of U. A more detailed representation of the autocatalytic reduction of U(VI) in a mixed solution of U and Pu is shown in Figure S3. Figure S4 shows CVs for 25 continuous cycles in pure U (Figure S4a), pure Pu (Figure S4b), and a mixture of U and Pu (Figure S4c) in 1 M H2SO4 at a scan rate of 50 mV s−1 10191

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Figure 5. (a) CVs of PEDOT−PSS/GC in 1 M H2SO4 containing different concentrations of U(VI) and Pu(IV) at a scan rate of 50 mV s−1: (i) 0.0430 mg g−1 U and 0.1026 mg g−1 Pu, (ii) 0.0891 mg g−1 U and 0.2127 mg g−1 Pu, (iii) 0.1356 mg g−1 U and 0.3238 mg g−1 Pu, (iv) 0.1874 mg g−1 U and 0.4474 mg g−1 Pu, and (v) 0.2309 mg g−1 U and 0.5513 mg g−1 Pu. (b) Plot of the anodic-peak current of the Pu(IV)/Pu(III) reaction vs the Pu concentration. (c) Plot of the cathodic-peak current of the U(VI)/U(IV) reaction vs the U concentration.

Rb2U(SO4)3 and K4Pu(SO4)4 and were on the order of 0.5 and 0.2%, respectively (Tables S1 and S2). Some FBTR fuel samples were analyzed for their fissile contents by our method and counterchecked by the biamperometry method. The results are compared in Tables S3 and S4, and for simplicity, they are also represented as a bar graph in Figure 6. It is obvious from Tables S1−S4 that both of the methods give similar results, which proves the reliability of the present

and no further enhancement is observed despite further addition of Pu (Figure 4b). From Figure 4b, we can approximately conclude that for [Pu]/[U] > 2, there is no further enhancement in the cathodic-peak-current density of U(VI) reduction. Thus, if the [Pu]/[U] ratio is maintained >2, then changes in the Pu concentration would no longer influence the U(VI)-reduction-peak current, as the maximum peak-current density is attained because of the completion of eq 4, and no further enhancement is possible. The critical value of [Pu]/[U] being close to 2 is understandable on the basis of the stoichiometry of eq 4, as a single U(IV) requires two Pu(IV) for completion of the chemical reaction. Hence, the PEDOT−PSS/GC electrode can be used for the simultaneous determination of U and Pu by CV for FBTR fuel samples in which the ratio of [Pu]/[U] is always greater than 2 (Pu: 66 ± 1 wt %, U + Pu: ≥ 94 wt %) Determination of U and Pu in FBTR Fuel Samples. Next, we performed CV experiments for the simultaneous determination of U and Pu in FBTR fuel samples. Figure 5a shows the CVs of PEDOT−PSS/GC in 1 M H2SO4 containing different concentrations of U and Pu at a scan rate of 50 mV s−1 (for all solutions, [Pu]/[U] > 2). The peak-current densities for both U(VI) reduction and the Pu(IV)/Pu(III) redox couple increase linearly with increases in the concentrations of U and Pu, respectively. Figure 5b,c shows the plots of the peak-current densities of the Pu(IV)/Pu(III) and U(VI)/U(IV) reactions versus the Pu and U concentrations, respectively. It shows the high sensitivities of 100.35 μA (mg g−1)−1 for U and 58.06 μA (mg g−1)−1 for Pu. The detection limit is found to be 2.15 and 3.72 μg g−1 for U and Pu, respectively. The accuracy and precision of the present method were examined using the chemical-assay standards

Figure 6. Comparison of biamperometry and cyclic-voltammetry results for U and Pu contents in some FBTR fuel samples. 10192

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7

6

55

simultaneous determination of U and Pu, precision: 0.5%, accuracy: 1%, simple and fast

simultaneous determination of U and Pu

spectrophotometry (visible) spectrophotometry (ratio derivative)

TXRF

DPV CV PEDOT−PSS/GC PEDOT−PSS/GC

Pu determination accuracy: 0.5%; precision: 0.2%; simultaneous determination of U and Pu possible in spite of coupled chemical reaction between U(IV) and Pu(IV); no chemical impurities added; simple recovery of U and Pu from analyzed solution; cheap, reusable electrode material direct determination of U and Pu in solid state, dissolution of fuel samples avoided, fast analysis

SWV rGO/GC

observed chemical interference between U(IV) and Pu(IV), accuracy: 15%,, precision: 5.5% for Pu determination

precision: 2.6%, accuracy: 5%, poor accuracy and precision precision: ∼3%

42 this work

30

29 DPV SWCNT/Au

simultaneous determination of U and Pu

addition of chemical impurities, cumbersome recovery of U and Pu, U and Pu determined separately no interference of U(IV) and Pu(IV) reported, accuracy and precision not reported reports that the simultaneous voltammetric determination of U and Pu not possible accuracy and precision not reported applicable for samples having [Pu]/[U] ≥ 2 (FBTR Mark I fuel) accuracy: 0.2%; precision: 0.2%; applicable for (U, Pu)C, (U, Pu)O2, etc. biamperometry Pt wire

advantages method electrode

Table 1. Comparison of Different Methods for the Determination Uranium and Plutonium in Nuclear Fuel

limitations

ref

approach. The interfacial, catalytically coupled chemical reaction between U(IV), produced at the electrode−solution interface via the reduction of U(VI), and Pu(IV) enhances the current of U(VI) reduction and rather facilitates the simultaneous voltammetric determination of lower quantities of uranium in the presence of large quantities of plutonium, as in the case of the FBTR samples. Thus, the present method opens new doors in the nuclear industry for the determination of U and Pu in nuclear fuel samples and is the most probable substitute for the well-established biamperometry method. A detailed comparison of present approach with earlier reports is shown in Table 1. Repeatability and Reproducibility of the Cyclic Voltammetric Response and Stability of the PEDOT− PSS/GC electrode. The repeatability of the results obtained from the present method was checked by recording 10 repetitive CVs of a U and Pu mixture in 1 M H2SO4 on the PEDOT−PSS/GC electrode at a scan rate of 50 mV s−1 (Figure S5). Precise peak currents were obtained with relative standard deviations (RSDs) of 1.6% for U(VI)/U(IV) reduction, 1.27% for Pu(IV)/Pu(III) reduction, and 0.46% for Pu(IV)/Pu(III) oxidation. The reproducibility of the results was verified by recording the CVs in the same solution for 5 consecutive days (Figure S6). Precise peak currents were obtained with RSDs of 3.27% for U(VI)/U(IV) reduction, 2.21% for Pu(IV)/Pu(III) reduction, and 2.69% for Pu(IV)/ Pu(III) oxidation. This also proves the stability of the PEDOT−PSS/GC electrode for analyzing FBTR fuel samples: no degradation in the electrochemical response is observed. This proves that one single modified electrode can be used for a long period of time for FBTR-fuel-sample analysis. It was examined for 3 month duration and is still is in use. However, the RSD values of the peak currents of U and Pu for five consecutive days show that fresh calibration plots should be made before the sample analysis with the previously used electrode. Interference Study. The commonly encountered impurities in FBTR fuel samples and their maximum permissible concentrations are shown in Table S5.54 To find out the effect of foreign impurities on the determination of U and Pu by the present methodology, cyclic-voltammetric studies were done in the presence of the most frequently encountered impurities. Figures S7 and S8 show the CVs of PEDOT−PSS/GC in the presence and absence of CoII, NiII, MnII, ZnII, AlIII, NaI, SmIII, BIII, CdII, CeIV, and RuIII in 1 M H2SO4 at a scan rate of 50 mV s−1 in the same electrochemical window used for the determination of U and Pu. Because the cyclic-voltammetric responses of PEDOT−PSS/GC remained unaffected in the presence of these elements, it can be concluded that the present methodology gives accurate quantitative results of U and Pu in the presence of large quantities of Co, Ni, Mn, Zn, Al, Na, Sm, B, Cd, and Ce. The CV response of PEDOT− PSS/GC was marginally enhanced in the presence of Ru. PbII is insoluble in 1 M H2SO4 and hence creates no interference. FeIII, II, and CuII were found to be most interfering ions, which was due to the nearness of their formal electrode potential (E′0) with those of the Pu(IV)/Pu(III) redox couple (Fe and I) and the U(VI)/U(IV) redox couple (Cu). This was proved by recording the CVs of a U and Pu mixture, Fe, Cu, and KI in 1 M H2SO4 on PEDOT−PSS/GC at a scan rate of 50 mV s−1 (Figure S9). At low concentrations, Fe and Cu (16.6 μM each) do not interfere in the analysis of U and Pu mixtures, as identical CV responses are obtained in their absence (Figure

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Krishnan, FCD, BARC, for providing chemical-assay standards of U and Pu. R.A. thanks to Dr. S. K. Guin, FCD, BARC, for scientific discussion.

S10a). However, if they are present in comparable concentrations to U and Pu, then the presently described methodology leads to overestimated results (Figure S10b) as they provides enhancement of the peak-current densities of both the Pu(IV)/Pu(III) and U(VI)/U(IV) redox couples. At low concentrations, KI and RuIII (83 μM each) do not interfere in the analysis of U and Pu, as observed from Figure S11. However, the total concentration of impurities is always maintained below 3000 ppm in FBTR fuel samples (Table S5). Thus, this methodology can provide accurate results in the presence of commonly encountered impurities at the permissible limit.

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CONCLUSIONS PEDOT−PSS enhances the peak-current densities for U(VI) reduction and Pu(IV)/Pu(III) redox in 1 M H2SO4 as compared with GC. Although well-separated and sharp U(VI)reduction- and Pu(IV)/Pu(III)-redox peaks are observed in the CVs of mixed solutions, coupled chemical reactions between U(IV) and Pu(IV) make voltammetry unsuitable for their simultaneous determination. However, if [Pu]/[U] is maintained >2 in solution, then the simultaneous determination of U and Pu in FBTR fuel samples by voltammetric methods is no longer a problem. High accuracy and precision for the determination of U and Pu in FBTR fuel samples are obtained with the present method, which makes it extremely suitable to substitute the presently applied biamperometry method on the grounds of the simultaneous determination, decreased radiation exposure, and the fact that the analytical waste is free from chemical impurities.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.8b00769. Determination of Pu and U, preparation of chemicalassay standards, electrochemically active surface area calculation using CV, characterization of FBTR fuel samples by biamperometry and CV, and specifications for MC pellets of FBTR (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +91 22 2559 0642. Fax: +91 22 2550 5151. E-mail: [email protected] or [email protected]. ORCID

Rahul Agarwal: 0000-0003-1785-6959 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Our institute is a government-funded research institute, so financial support is from BARC, Government of India. The authors wish to thank Dr. S. Kannan and Dr. Renu Agarwal, FCD, BARC, for their constant support and encouragement in this work. The authors also wish to acknowledge Dr. K. 10194

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