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Comparison of Analytical Methods for the Determination of Uranium in Seawater using Inductively Coupled Plasma Mass Spectrometry Jordana Wood, Gary A. Gill, Li-Jung Kuo, Jonathan E. Strivens, and Key-Young Choe Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03680 • Publication Date (Web): 03 Feb 2016 Downloaded from http://pubs.acs.org on February 7, 2016

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Industrial & Engineering Chemistry Research

Comparison of Analytical Methods for the Determination of Uranium in Seawater using Inductively Coupled Plasma Mass Spectrometry Jordana R. Wood, Gary A. Gill, Li-Jung Kuo, Jonathan E. Strivens, and Key-Young Choe Marine Sciences Laboratory, Pacific Northwest National Laboratory, Sequim, WA 98382, USA

Abstract Trace element determinations in seawater by inductively coupled plasma mass spectrometry are analytically challenging due to the typically very low concentrations of the trace elements and the potential interference of the salt matrix. In this study, we did a comparison for uranium analysis using inductively coupled plasma mass spectrometry (ICP-MS) of Sequim Bay seawater samples and three seawater certified reference materials (SLEW-3, CASS-5 and NASS-6) using eight different analytical approaches. The methods evaluated include: direct analysis, Fe/Pd reductive precipitation, off-line preconcentration using the actinide specific resin, UTEVA, standard addition calibration, online automated dilution using an external calibration with and without matrix matching, and online automated pre-concentration using the seaFast preconcentration resin. The two methods which produced the most accurate results were the method of standard addition calibration and offline preconcentration using the UTEVA resin, recovering uranium from a Sequim Bay seawater sample at 101 ± 1.2% and 98 ± 2.7%, respectively. The on-line preconcentration method and the automated dilution with matrixmatched calibration method also performed very well. The two least effective methods were the direct analysis and the Fe/Pd reductive precipitation method using sodium borohydride.

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1. INTRODUCTION The Fuel Resources Program at the U.S. Department of Energy’s Office of Nuclear Energy is developing adsorbent technology to extract uranium from seawater with the goal to provide a sustainable and economically viable supply of uranium fuel for nuclear reactors into the next century.1, 2 The Pacific Northwest National Laboratory (PNNL)’s Marine Science Laboratory (MSL) is involved in performance testing of the uranium adsorbents that are being developed by program participants. Testing involves allowing natural seawater to contact the adsorbent material for periods of time between a few days and 10 weeks and then determining the amount of uranium that the adsorbents have retained over time3-7. A critical component of the testing process is the determination of ambient levels of uranium in the seawater that comes into contact with the adsorbents to verify the seawater uranium concentrations the adsorbents are exposed to and to help assess adsorbent performance. Because of the low concentrations of uranium in ambient seawater (~3.3 µg/L), sensitive analytical techniques, like Inductively Coupled Plasma Mass Spectrometry (ICP-MS) are necessary to reliably and reproducibly detect any small changes in the concentration of uranium in the exposure systems. The major challenge for analysis of uranium in seawater by ICP-MS is eliminating interferences that can result from the high salt matrix. Direct analysis of seawater can result in salt deposition on the sampler and skimmer cones, causing narrowing of the openings in the cones. Salt build-up results in sensitivity decreases and loss of stability in the instrument over a long analytical run. The salt matrix also contributes to loss in sensitivity due to the high levels of major salt ions in the seawater, causing ionization suppression. A number of approaches have been used to reduce matrix effects of high salt solutions, like seawater, for analysis by ICPMS. Dilution of samples to reduce the salt matrix effect is a


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common practice, but dilution also results in a loss of sensitivity and does not always eliminate the signal suppression. For elements like uranium, that are already at low ppb levels in seawater, significant dilution to reduce matrix effects can push detection limits to unacceptable levels. The direct determination of trace elements in seawater, including uranium and vanadium, has been reported using the sample dilution approach on an ICP-MS equipped with a reaction/collision cell8, 9. However, some analytes were still affected by ion interferences from the salt matrix. Another common approach to eliminate the high salt matrix is use of a chelating ion exchange resin to pre-concentrate the elements of interest from the seawater matrix, using either on-line or off-line approaches, prior to determination by ICP-MS. 10-18 A minor limitation to this approach is that the chelating resin will be efficient for many trace elements, but not for all elements of potential interest. Moreover, while this approach has been well documented for the analysis of transition metals and rare earths, there has been less effort to describe the approach for the analysis of uranium in seawater. Nicolai et al.12 used chelation ion chromatography, using Metpac-CC1 resin, coupled to an ICP-MS for the analysis of trace metals including uranium in seawater. There was an issue with the carbon matrix being magnified due to the reagents used in the process, which formed a deposit on the cones possibly altering the ion extraction. Rožmarić et al.19 describe a chromatographic separation method for the off-line extraction of uranium from seawater using the UTEVA® resin (Eichrom technologies), followed by spectrophotometric and ICP-MS detection. Another technique used for the isolation of trace elements from seawater, prior to analysis by ICPMS, is co-precipitation with magnesium hydroxide. 18, 20-26 While this method has been applied to several trace elements (e.g. Fe, Mn, Cr, Pb, Cd and Zn), we could not find



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any references which specifically described use of this pre-concentration and isolation method for uranium from seawater. A technique for the analysis and preconcentration of trace elements used successfully at the Marine Science Lab (MSL) for a number of trace elements is an off-line reductive precipitation and pre-concentration technique that employs a Fe-Pd borohydride reduction and precipitation step.27, 28 Because this method does not rely on the interaction of an element with a chelation site, it has broad elemental capability as a preconcentration tool for seawater analysis, with the obvious exception of Fe and Pd. How well this approach worked for preconcentrating uranium from seawater prior to analysis was not known. This paper describes a comparison of several analytical approaches for the quantitative analysis of uranium in seawater by ICP-MS. The analytical approaches tested include direct analysis, offline preconcentration via Fe-Pd reductive precipitation, offline preconcentration using the UTEVA® resin (Eichrom Technologies), sample dilution with and without matrix matching, the method of standard addition calibration, and an on-line pre-concentration technique using a chelating ion-exchange resin. The aim of this study is to provide the information needed to choose a preferable analytical method for the analysis of uranium in seawater. 2. MATERIALS AND METHODS 2.1. Materials

All chemicals used were trace metal grade or Optima grade (Fisher Scientific). Sodium borohydride and ammonium pyrrolidinedithiocarbamate (APDC) were purchased from SigmaAldrich. The iron and palladium standards were obtained from High-Purity™ Standards. UTEVA® resin and columns parts were purchased from Eichrom Technologies. The estuarine, 4|Page ACS Paragon Plus Environment

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nearshore seawater and seawater certified reference materials (CRMs) SLEW-3, CASS-5 and NASS-6, respectively, were obtained from the National Research Council of Canada. SLEW-3 is estuarine water from San Francisco Bay with a salinity of 15; CASS-5 is nearshore water obtained from Halifax Harbour at a depth of 12m and has a salinity of 33.5; and NASS-6 seawater was obtained from Sandy Cove, Nova Scotia and has a salinity of 33.5. Raw seawater was collected from the dock of the Marine Science Laboratory (MSL) located in Sequim, WA. The raw seawater was filtered using a 0.45 µm Durapore filter (EMD Millipore) into a 2 L acid-cleaned Teflon bottle followed by acidification (0.2%, v/v) with Optima grade HNO3. Ultra-pure water was produced by first passing a low ion and trace element content ground water source through a reverse osmosis process, followed by passage through mixed-bed ion exchange columns. Water quality was maintained at a level above 17.5 MΩ of resistivity. 2.2. ICP-MS Analysis. The analysis of dissolved uranium in seawater was performed using either a Perkin-Elmer ELAN® DRC-e or a Thermo Scientific™ iCap™ Q ICP-MS equipped with an Elemental Scientific seaFAST S2 sample introduction system which incorporated on-line pre-concentration. The ICP-MS instrumental operating parameters are given in Table 1. The iCapQ was run in either standard mode (STD) or in kinetic energy discrimination mode (KED). KED is a technique of using a single gas (helium) as the collision cell gas to simultaneously remove or reduce the polyatomic species in complex matrices for more accurate quantification. The high chloride content of seawater produces many polyatomic species that result in interferences with the determination of the lower mass elements. Fortunately, uranium is not directly affected by polyatomic interferences which simplify its determination by ICPMS. Rhenium (Re) was used



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as the internal standard during analysis of uranium on the Thermo iCapQ for all analysis that required an internal standard. Bismuth (Bi) was used as the internal standard in the method of the Fe-Pd reductive precipitation on the Perkin Elmer DRCe.

Table 1. ICP-MS Instrument Operating Parameters Parameter RF power (W) Coolant airflow (L/min) Auxiliary airflow (L/min) Carrier airflow (L/min) Nebulizer Spray chamber Detector mode Dwell times (s) Sweeps

Thermo iCapQ 1550 14 0.8 1.05 Micro flow buffered cyclonic pulse and analog 0.01-0.04 25

Perkin Elmer DRCe 1400 15 1.2 1.01 Meinhard buffered cyclonic pulse 0.2 10

During each analytical run, quality control (QC) requirements were applied to assure a high quality data set was generated. Each calibration had a correlation coefficient of at least 0.995. After calibration an (initial) calibration verification sample was run, with the criteria that the recovery must be ±10%. A calibration blank (typically 1% high purity HNO3) was run immediately before the sample run, with the criteria that it must be < 3 times the method detection limit (MDL). If the concentration of the blank was greater than 3 times the MDL, the concentration of the samples must be greater than 10 times the blank concentration. The calibration verification and calibration blanks were repeated after every ten samples. The (continuing) calibration verification was required to recover within ±15%, and the (continuing) calibration blank, value was required to be < 3 times the MDL. If the concentration in the blank was greater than 3 times the MDL, the concentration of the samples must be greater than 10


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times the blank concentration. Further QC requirements were applied to CRM’s, sample duplicates and matrix spikes. The requirements for both CRMs and matrix spikes was ±25% of the certified value or the amount spiked. A matrix spike pair was prepared and analyzed in all analytical methods except for direct analysis. A known amount of U was spiked on top of the seawater and then analyzed for the recovery of added U.

2.3. Experimental Approach. To evaluate the various methods, three procedural blanks, ten replicate determinations of Sequim Bay Seawater, two matrix spikes (using Sequim Bay Seawater), and three seawater CRM’s were analyzed. The reproducibility and recovery associated with these methods was compared as a means to evaluate the different analytical approaches described below.

2.3.1. Direct analysis without dilution or matrix matching. Seawater samples were analyzed directly with no dilution or matrix matching of the standards. This analytical approach was considered the least likely to be accurate or reproducible as no attempt was made to correct for or reduce the high salt matrix of seawater. However, it provides a baseline with which to evaluate the other methods.

2.3.2. Fe/Pd reductive precipitation pre-concentration using sodium borohydride. Precipitation is one of the most commonly used techniques in the analysis of trace metals. This analytical method for seawater evaluated is based on the reductive precipitation preconcentration techniques using Fe and Pd. 27-29 Using borohydride as the reductive mechanism in an alkali solution results in a high reductive potential and the reduction of a wide range of



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trace metals is highly favored. The major cations Ca, Mg, Na and K are not reduced and therefore are eliminated from the precipitate. With the major cations excluded from the preconcentrated sample, ionization changes commonly seen in the plasma are significantly reduced or eliminated. A major strength of this preconcentration method is its ability to preconcentrate a wide number of trace elements from seawater simultaneously, with the obvious exception of Fe and Pd. The following procedure has been optimized for the simultaneous preconcentration of several trace elements from seawater, including: Al, As, Be, Cd, Co, Cr, Cu, Mn, Ni, Pb, Sb, Se, Ag, Th, Tl, V, and Zn. Forty mL seawater samples were pre-concentrated by sodium borohydride reductive precipitation by adding 0.5 mL of a 500 mg/L Fe:Pd mixture (1:1 solution; final concentration 6.25 mg/L of Fe and Pd). Ammonium hydroxide (0.25 mL) was added to adjust the pH to approximately 8.5. After mixing, 0.5 mL of a 5% (w/v) sodium borohydride solution and 0.25 mL of a 2% (w/v) ammonium pyrrolidine-dithio-carbamate (APDC) solution were added into the samples for reductive precipitation and to complex trace elements and facilitate the interaction with the Fe/Pd. The samples were then briefly shaken, the caps were loosened, and the samples were left to sit overnight. After a minimum of 15 hours, the samples were centrifuged at 3500 rpm for 30 minutes, and the overlying water was carefully decanted off the precipitate. The centrifuge step was then repeated. The precipitate was dissolved with 0.1 mL of concentrated high-purity nitric acid (OptimaTM grade, Fisher Scientific) by heating at 80°C for 20 minutes and then diluted to a suitable volume (~5 mL) for analysis with high purity de-ionized (DI) water. This scheme produced a sample pre-concentration of approximately 8-fold, depending on the exact starting and final solution volumes.


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2.3.3 Offline pre-concentration using UTEVA® resin The UTEVA resin has been employed for the isolation of actinide series elements from a variety of environmental matrices. The resin is often used when isotopic information is sought and where significant reduction in other matrix elements is needed for good isotopic characterization30, 31. The resin has been applied for the isolation and preconcentration of uranium from aqueous solutions, including seawater32-36. In most cases, the resin is used as an off-line isolation and preconcentration tool with subsequent analysis by ICP-MS or, more recently, by multi-collector ICPMS30, 31, 37. For this assessment, 1.5 mL of UTEVA® resin (Eichrom Technologies), suspended in water, was pipetted into 2 mL polypropylene columns fitted with a frit. A second frit was placed on the top of the resin, and then the column was back flushed with high purity DI water. The columns were prepared for use by passing 5 mL of 3 M HNO3, followed by two rinses with 5 mL of DI water. Immediately before loading the sample, 5 mL of 3 M HNO3 was passed through the column to condition it. Five mL of concentrated HNO3 was added to 45 mL of Sequim Bay seawater or the CRMs. The samples were then passed through the columns by gravity feed, at a flow-rate of approximately 1.5 mL/min. Another 5 mL of 3 M HNO3 was passed through the column as a rinse. Uranium was eluted from the column using 10 mL of 0.1 M HCl into 15 mL Teflon vials. The samples were placed on a hot plated and evaporated to dryness. One mL of concentrated HNO3 was added to the vials and evaporated to dryness. The samples were brought up in 5 mL of 2% (v/v) HNO3. The seawater samples were diluted 10-fold for analysis based on expected values, the associated blanks were run direct. 2.3.4. Method of Standard Addition Calibration.



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Using standard (external) calibration approaches in dilute solution, when attempting to analyze high background matrices like seawater, often results in biased results.38 An alternative to an external calibration is the use of a standard addition calibration when complex matrices are analyzed for certain analytes. Addition calibration is a variant of the standard additions method and is often used when all samples have a similar matrix. Instrumental calibration curves were prepared in Sequim Bay seawater that was diluted 20-fold with high purity DI water and then spiked at 4 different concentration levels (e.g. 0.05, 0.1, 0.15 and 0.2 µg/L), along with a 2% nitric acid blank in diluted seawater. The seawater samples were then analyzed at 20-fold dilution prepared using high purity DI water and then quantified using the methods of additions calibration curve. The ICP-MS software measured the response from samples to which a spike has been added. The calibration curve was prepared based on intensity of each spiked element against its concentration. Using the slope of the calibration curve, the software can determine the unspiked concentration of U in the unknown. 2.3.5. Automated 15-fold on-line dilution Sequim Bay seawater was analyzed with fifteen-fold automated on-line dilution using the seaFAST S2 sample introduction system (Elemental Scientific, Inc.). A dilution factor of fifteen was chosen based on sufficient uranium intensity counts to achieve a reliable quantification balanced against the dilution necessary to reduce interferences caused by the salt matrix. An external calibration with standards prepared with and without matrix to examine the effect of matrix-matching on the accuracy of uranium quantification. Standards and blanks were matrix matched by preparing them in a 3% sodium chloride solution that were diluted from seaBlank, an ultrapure sodium chloride solution available from Elemental Scientific (10-11% NaCl).

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2.3.6. On-line Pre-concentration and Analysis On-line pre-concentration of U was conducted using the seaFAST S2 automated sample introduction system equipped with a seaFAST PFA chelation column packed with iminodiacetic acid chelating ion exchange resin (ESI, Seawater Concentrator Column CF-N-0200). The chelation column binds the uranium and allows seawater matrix ions in high concentration to pass through the column. The seaFAST S2 system was used with a 2 mL sample loop. High purity water was used to push the sample from the loop where it was combined with an ammonium-acetate buffer (pH 6.0) then loaded on the pre-concentration column; the seawater matrix rinsed through. After this pre-concentration step, the sample was then eluted from the column (back flushed) with 10% HNO3 into the nebulizer for analysis by ICP-MS. The preconcentration step was used for the analysis of U in both standard mode (STD) and kinetic energy dispersion mode (KED).

3. RESULTS and DISCUSSION All the analytical methods evaluated were very sensitive, having detection limits ≤ 0.001 µg U/L . The procedural blanks that accompanied the sample runs were below the detection limit for all the methods assessed. Because procedural blanks were ≤ 0.001 µg U/L, blank issues were not a concern for seawater samples or SRM’s which have uranium levels of approximately 1-3 µg/L. Ten replicate uranium measurements from of a common seawater sample obtained from Sequim Bay were determined by each of the analytical methods described above (Table 2). Determination of the uranium content of three seawater CRM’s (SLEW-3, CASS-5, and NASS6) by all the methods is given in Table 3. Uranium concentrations were normalized to a salinity



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of 35 for comparison against the predicted uranium seawater concentration at a salinity of 35 of 3.29 µg U/L. This uranium concentration in seawater at a salinity of 35 (3.29 µg U/L) is based on a 238-U concentration in seawater of 3.187 µg U/kg of seawater at a salinity of 3539, an isotopic abundance of 238-U of 99.2745% and an average density of surface seawater of 1.025 g/cm3. Table 2. Uranium concentration (mean ± SD) of ten replicate measurements of Sequim Bay seawater, coefficient of variance (CV), recovery of internal standard (IS), matrix spike (MS), and matrix spike duplicates (MSD) of eight analytical approaches.

Method

Uranium in Sequim Bay Seawater (ug/L)a

35 psu Seawater

3.29b

Direct Analysis

3.09±0.04

CV (%)

Sequim Bay Seawater (% Recovery)

Internal Standard (% Recovery)

MS and MSD (% Recovery)

1.6

93.9±1.2

21

-

77

64, 109

97.5

91, 99

-

99, 99

77

93, 92

90

101, 102

-

104, 105

-

105, 108

Fe/Pd Reductive 2.60±0.06 2.2 79.0±1.8 Precipitation UTEVA off-line 3.21±0.09 2.7 98±2.7 Preconcentration Method of Standard 3.32±0.04 1.1 101±1.2 Addition Calibration Automated Dilution 3.34±0.02 0.5 102±0.6 (not matrix matched) Automated Dilution 3.06±0.01 0.4 93.0±0.3 (matrix matched) Online Preconcentration, STD 3.37±0.03 0.8 102±0.9 mode Online Preconcentration, KED 3.46±0.04 1.1 105±1.2 mode - data not available a Uranium concentrations were normalized to a salinity of 35. b Based on the relationship between salinity and 238-U in seawater39


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Table 3. Uranium concentrations (µg/L) determined for the three seawater certified reference materials SLEW-3, CASS-5, and NASS-6, using different analytical approaches. The values in parentheses are the percent recoveries of uranium concentrations compared to the certified/reference values for the CRMs. Analytical Method SLEW-3 Certified Reference Value (µg/L) 1.80 Fe/Pd Reductive Precipitation 1.43 (79%) UTEVA off-line Preconcentration 1.48 (82%) Method of Standard Addition Calibration 1.72 (96%) Automated Dilution (not matrix matched) 1.47 (82%) Automated Dilution (matrix matched) 1.70 (94%) Online Pre-concentration, STD mode 1.73 (96%) Online Pre-concentration, KED mode 1.88 (104%)

CASS-5 3.18 1.02 (32%) -2.92 (92%) 2.60 (82%) 2.97 (93%) 2.92 (92%) 3.05 (96%)

NASS-6 3.00 0.68 (23%) 2.73 (91%) 3.00 (100%) 2.65 (88%) 3.00 (100%) 2.96 (99%) 3.07 (102%)

All of the methods evaluated did an acceptable job of quantifying the uranium content of Sequim Bay seawater except for the Fe/Pd reductive precipitation method, which had a uranium recovery of 79.0 ± 1.8 %. The quality control samples for the Fe/Pd reductive precipitation method also had the lowest recovery of the various methods (Tables 1 and 2). The MS and MSD duplicate recoveries for the Fe/Pd method varied significantly (64% and 109%, respectively). The Fe/Pd method also had the lowest uranium recovery with the CRM’s CASS-5 and NASS-6 yielding recoveries of 32% and 23%, respectively. The recovery for SLEW-3 (79%) was better (Table 2). The significant difference in recovery among the three CRMs with the Fe/Pd method may be related to the salinity of the CRM’s. SLEW-3 has a much lower salinity (15) than does CASS-5 or NASS-6 (both have a salinity of 33.5). Hence, the Fe/Pd method may not work as well for determining uranium in Sequim Bay seawater which has a salinity of approximately 30. While the Fe/Pd reductive precipitation method used to determine uranium in seawater in this assessment performed poorly, it may be possible to modify the procedure such that its 13 | P a g e ACS Paragon Plus Environment


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performance for uranium determinations in seawater will improve. However, an Fe/Pd reductive precipitation procedure optimized for uranium in seawater may not prove to also be optimal for use with other trace elements. Uranium concentrations of Sequim Bay seawater obtained from five out of the eight analytical approaches were within 5% of the predicted value (3.29 µg/L) for seawater with salinity 35 (Table 2). All the methods showed good reproducibility, with a coefficient of variation (CV) of

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