Selective Micellar Extraction of Ultratrace Levels of Uranium in

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Selective micellar extraction of ultratrace levels of uranium in aqueous samples by task specific ionic liquid followed by its detection employing TXRF spectrometry Abhijit Saha, Kaushik Sanyal, Neetika Rawat, Sadhan Bijoy Deb, Manoj Kumar Saxena, and Bhupendra Singh Tomar Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b02427 • Publication Date (Web): 04 Sep 2017 Downloaded from http://pubs.acs.org on September 5, 2017

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Analytical Chemistry

Selective micellar extraction of ultratrace levels of uranium in aqueous samples by task specific ionic liquid followed by its detection employing TXRF spectrometry Abhijit Sahaa,b*, Kaushik Sanyala,c, Neetika Rawatb, Sadhan Bijoy Debb, Manoj Kumar Saxenab,* and Bhupendra Singh Tomarb a

b

Homi Bhabha National Institute, Mumbai 400 094, India

Radioanalytical Chemistry Division, cFuel Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India *Address correspondence to [email protected] (Abhijit Saha) and [email protected] (Manoj Kumar Saxena)

Abstract A

task

specific

ionic

liquid

(TSIL) bearing

phosphoramidate

propyl(diphenylphosphoramidate)trimethylammonium was synthesized and characterized by 1H NMR,

13

group

viz.,

N-

bis(trifluoromethanesulfonyl)imide,

C NMR,

31

P NMR, IR spectroscopy,

elemental (C H N S) analysis and electrospray ionization mass spectrometry (ESI-MS). Using this TSIL a cloud point extraction (CPE) or micelle mediated extraction (MME) procedure was developed for preconcentration of uranium (U) in environmental aqueous samples. Total reflection X-ray fluorescence (TXRF) spectrometry was utilized to determine the concentration of U in the preconcentrated samples. In order to understand the mechanism of the CPE procedure, complexation study of the TSIL with U was carried out by isothermal calorimetric titration (ICT), liquid-liquid extraction, 31P NMR, IR spectroscopy and ESI-MS. The developed analytical technique resulted in quantitative extraction efficiency (EE) of (99.0±0.5)% and preconcentration factor (PF) of 99 for U. The linear dynamic range (LDR)

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and method detection limit (MDL) of the procedure were found to be 0.1-1000 ng mL-1 and 0.02 ng mL-1 respectively. The CPE procedure was found to tolerate a higher concentration of commonly available interfering cations and anions, especially the lanthanides. The developed analytical method was validated by determining the concentration of U in a certified reference material (CRM) viz., NIST SRM 1640a natural water, which was found to be in good agreement at 95% confidence limit with the certified value. The method was successfully applied to the U determination in three natural water samples with ≤ 4% of relative standard deviation (RSD, 1σ).

Keywords: Task specific ionic liquid; Uranium; Cloud point extraction; Total Reflection Xray Fluorescence

Introduction Uranium (U) is one of the most abundant naturally occurring actinides in the earth’s crust.1-3 In addition to it’s naturally occurring isotopes viz., (99.274%), some of its anthropogenic isotopes viz.,

234 232

U (0.005%),

235

U (0.720%) and

238

U

U, 233U and 236U are being released in

the environment due to men made nuclear activities.2-4 U has appreciable amount of solubility in water and the most stable chemical form is uranyl ion (UO22+).5 The water soluble compounds of U are both chemically and radiologically toxic even at ultratrace levels and therefore bioavailable U pose greater risk to human health.2-5 World Health Organization (WHO) and Environmental Protection Agency (EPA) have set a limit of 30 ng mL-1 for U, as the maximum permissible limit in drinking water.6,7 Detection of ultra trace levels of toxic heavy metals in contaminated environmental samples can prevent their damaging effects of pollution in the very early stages. Precise determination


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of such low analyte concentration in complex environmental matrices by sophisticated analytical techniques like atomic absorption spectroscopy (AAS),8-11 atomic emission spectroscopy (AES),12,13 mass spectrometry (MS),14,15 X-ray fluorescence spectroscopy (XRF)16 etc. require preconcentration of the analyte. Although a good number of preconcentration and separation techniques like liquid-liquid extraction (LLE),10,15 solid phase extraction (SPE),10-14,16 ion-exchange chromatography (IEC)10 have been developed in the past but cloud point extraction (CPE) or micelle mediated extraction (MME)2-4,17-28 has emerged as the most simple and eco-friendly preconcentration procedure in the last ten years. CPE is essentially based on the extraction of metal ion into the dispersed micelle phase of non-ionic polyoxyethylene surfactants followed by temperature driven phase separation and aggregation of micelles i.e., separation into bulk aqueous and surfactant rich phase (SRP).2,3 Use of specific extractant and maintaining the optimum solution conditions enable selective extraction of metal ions into the hydrophobic core of the micelle. In the last two decades room temperature ionic liquids (RTILs) have emerged as the more eco-friendly alternative to the common volatile organic solvents used in organic synthesis, catalysis, electrochemistry and extraction.29,30 However partitioning of the metal ions from aqueous phase to the ionic liquid (IL) phase is limited due to the lack of coordinating groups in IL.31 Nowadays scientists are developing tailored RTILs with complexing abilities towards the metal ions leading to improvement in the partition coefficient values.32-34 Such ILs are commonly known as task specific ILs (TSILs). To the best of our knowledge till now no reports are available on CPE of metal ion employing any TSIL. Only Gao et al.36 had reported improvements in U(VI) extraction efficiency (EE) and selectivity in CPE by employing some hydrophobic ILs together with trioctylphosphine oxide. In their work they could successfully demonstrate the increase in EE of U(VI) from 45% to 80%. However in this report the methodology is valid only up to equimolar concentrations of lanthanum. Hence



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in order to achieve better percentage EE of U(VI) in presence of higher amount of foreign metal ions, we have synthesized and characterized a TSIL bearing phosphoramidate (phamd) group and used it for the CPE of U(VI) in aqueous samples. Total reflection X-ray fluorescence (TXRF) spectrometry37 was utilized for the first time to quantify uranium directly in SRP and to evaluate the percentage recovery of U(VI) by CPE. Various CPE conditions were optimized to obtain the maximum percentage recovery. The developed methodology was found to quantify precisely ng mL-1 levels of U in presence µg to mg mL-1 levels of various foreign ions, especially the lanthanides. The higher selectivity of the TSIL towards the U(VI) ion in CPE was successfully explained by studies using isothermal calorimetry, liquid-liquid extraction,

31

P NMR, IR spectroscopy and electrospray ionization

mass spectrometry (ESI-MS). Oxidative pyrolysis of the SRP phase before its analysis by TXRF showed improvement in the method detection limit (MDL). The proposed analytical methodology was validated by analyzing a certified reference material (CRM) viz., NIST SRM 1640a natural water. Three real water samples from various sources, in India, were analyzed by the developed technique. Experimental Materials and instruments Diphenylchlorophosphate (97%), 3-dimethylamino-1-propylamine (99%),

triethyleamine

(Et3N, 99+%), methyl iodide (CH3I, 99.5%), diethylether (Et2O, 99%, anhydrous) were procured from AlfaAesar. Et2O was dried by keeping it in contact with 3Å molecular sieves (20% mass/volume loading) for 72 h in a desicator.38 N-lithiotrifluoromethanesulfonimide (LiNTf2, 99.95%), butyltrimethylammonium bis(trifluoromethylsulfonyl)imideacetonitrile ([Me3NBu][NTf2], 99%), acetonitrile (ACN, 99.8%), and 3K30 laboratory centrifuge were procured from Sigma-Aldrich. MilliQ water (18MΩ.cm) was used throughout the


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experiment. High purity UO2(NO3)2.6H2O (99.9%) was procured from Indian Rare Earth (IRE), India to prepare 1000 µg mL-1 stock solution in 1% (v/v) nitric acid (HNO3) medium. The metal ion stock solution was standardized by the well known biamperometric titration.39 This solution was diluted accordingly to carry out the CPE procedure. Other reagents used in the development of CPE system were: HNO3 (Merck) and sodium hydroxide (NaOH, Merck), potassium nitrate (KNO3, Alfa Aesar), Triton X-114 (TTX-114, Sigma-Aldrich) and sodiumdodecyl sulphate (SDS; Sigma-Aldrich). 10 mL conical graduated glass centrifuge tubes with stopper from Borosil (India) were used. The 1H and 13C NMR spectroscopic data were recorded in CD3COCD3 solvent with 500 MHz (1H NMR: 500 MHz,

13

C NMR: 125

MHz) spectrometers. The 1H and 13C chemical shifts in ppm (δ scale) were measured relative to CH3COCH3 (3.22 ppm) and CD3COCD3 (205.75 ppm) respectively, as internal standard. The

31

P NMR spectroscopic data was recorded in CD3SOCD3 solvent with 400 MHz

spectrometer and the chemical shift was measured relative to external standard 85% H3PO4. The IR spectrum was recorded on a Bruker ALPHA Platinum-ATR spectrophotometer. Mass spectrum was recorded using a microTOF (Bruker Daltonic) electrospray ionization mass spectrometer (ESI-MS). Elemental analysis (C H N S) was carried out by Eurovector – EA 3000 using combustion followed by GC detection method. The zeta potential (ζ) values were determined by Zitasizer Nano Z zeta potential analyzer. The calorimetric experiments were carried out with an isothermal titration calorimeter system (Nanocalorimeter TAM-III, Thermometric AB). The preconcentrated samples were analyzed by TX-2000 ITAL structures TXRF spectrometer, having Mo Kα as excitation source. The tube voltage and tube current applied during the TXRF measurement were 40 kV and 30 mA respectively. A Si (Li) detector having resolution of 139 eV (at 5.59 keV) was used to detect and measure X-rays. Quartz was used as a sample support for TXRF measurements on which a few microlitre of the sample was deposited and dried. The complete details of this spectrometer is given



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elsewhere.40 For TXRF measurements, a live time of 1000s was used. The U concentration in the supernatant was determined by using ICP- quadrupole MS (VG PlasmaQuad PQ2 Turbo Plus).

Synthesis of TSIL The complete synthesis route of the TSIL is given in the Supporting Information (Section S 2.1.). The characterization of the synthesized TSIL was carried out by 1H NMR (given as Supporting Information, Figure S1), 13C NMR (given as Supporting Information, Figure S2), 31

P NMR (given as Supporting Information, Figure S3a), IR spectrum (given as Supporting

Information, Figure S4a), C H N S elemental analysis (given as Supporting Information, Table S1) and ESI-MS (given as Supporting Information, Figure S6).

Cloud point extraction procedure and TXRF analysis Details of the CPE procedure developed for ultratrace levels of U determination are given as Supporting Information (Section S 2.2.). The concentration of the analyte was determined by using the equation: cA = (IA/SiA) x ci. Here cA is the concentration of analyte A, IA is the analyte peak area, SiA is sensitivity of the analyte with respect to internal standard and ci is the concentration of the internal standard.41

Results and discussions Complexation study of TSIL with uranyl ion The

reaction

scheme

of

propyl(diphenylphosphoramidate)trimethylammonium

synthesizing

N-

bis(trifluoromethanesulfonyl)imide

i.e., [phamdNMe3][NTf2] is shown in Figure 1. The liquid-liquid extraction studies of UO22+ from HNO3 medium by TSIL in [Me3NBu][NTf2] were carried out to generate the log-log plot of distribution ratio of UO22+ (DU) and the concentration of the HNO3 in the aqueous phase. This plot (given as Supporting Information, Figure S6) results in two different straight


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lines having different slopes, thereby indicating two different extraction mechanisms of UO22+ by TSIL in [Me3NBu][NTf2] phase at higher and lower HNO3 concentrations. At higher acidity (1.0-5.0 mol L-1) the extraction occurs via solvation or ion-pair mechanism whereas at lower acidity (0.1-1x10-6 mol L-1) it follows a cation exchange mechanism.42,43 However in lower acidity a plateau was observed below 1x10-4 mol L-1 HNO3 concentration which is a typical behaviour of cation exchange extraction mechanism. In order to confirm the complexation mechanism of TSIL with UO22+ ion, studies were carried out at lower (0.01 mol L-1) and higher (2.0 mol L-1) HNO3 concentration using isothermal calorimetric titration (ICT). The calculations regarding ICT experiments are given elsewhere.44 The plot of heat flow in terms of power vs time is shown in given as Supporting Information (Figure S7). Using the data from this graph, a plot of heat of reaction per mole of UO22+ ion (hvi) vs. number of TSIL bound per UO22+ ion (navg) was obtained (given as Supporting Information, Figure S8). The resulting plot shows a linear relationship between them up to navg equals to 2. This reveals that under these experimental conditions the TSIL forms only 1:2 UO22+-TSIL complex. Beyond navg value of 2 no increase in hvi value was observed and the plot becomes parallel to the x-axis which confirms that maximum two ligands can bond to each UO22+ ion. From this figure (Figure S8) it is clear that though both the reactions are exothermic but higher amount of heat i.e., - 75.7±0.3 kJ mole-1 is released at low acidic medium compared to that at high acidic medium i.e., - 38.7±0.2 kJ mole-1. In order to explain the release of excess heat at lower acidity (0.01 mol L-1) equal volumes of 0.02 mol L-1 of TSIL in ACN was stirred overnight at room temperature (24±1 oC) with 0.01 mol L-1 of UO22+ in 0.02 mol L-1 of HNO3. The solution was then dried under reduced pressure and the viscous liquid residue was used to record the

31

P NMR (given as Supporting Information, Figure S3b), IR (given as

Supporting Information, Figure S4b, c) and ESI-MS spectra (given as Supporting Information, Figure S9). It can be seen from

31

P NMR spectra (Figure S3) the singlet at -



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11.887 ppm of pure TSIL undergoes a downfield shift to 0.761 ppm after complexing with UO22+ ion. This observation confirms the delocalization of electrons over the P=O group and hence the bonding of the TSIL to UO22+ is via O-atom of P=O group. The IR spectra of the complex reveals that the P=O stretching vibration of TSIL (Figure S4) shifts from 1230 cm-1 to 1132 cm-1 on coordination with UO22+ ion. The decrease in the P=O stretching frequency by 98 cm-1 also confirms the strong bonding of TSIL to UO22+ via O-atom of P=O group.45 The presence of a strong peak at 927 cm-1 corresponds to the U=O stretching frequency of UO22+ ion. The P-O-C vibration at 1053 cm-1 does not change even after metal coordination. The broad stretching frequency at 3420 cm-1 (Figure S4) indicates the presence of coordinated H2O molecules in the UO22+-TSIL complex. Hence the excess heat released in the ICT experiment of UO22+ with TSIL at low acidity is being attributed to the interaction of NTf2- ion of the TSIL with the hydrated UO22+ ions through H-bonding. It was previously reported that NTf2- ion cannot interact directly with the UO22+ ion in the inner coordination sphere while it interacts with hydrated UO22+ through H-bonding forming an outer-sphere complex.36 The ion exchange extraction mechanism of UO22+ by TSIL from low HNO3 medium indicates no involvement of NO3- ion in the inner-sphere complexation and hence the remaining four coordination sites of UO22+ (coordination number is six) should be bonded with H2O molecules. This statement is supported by the ICT experimental results, where (75.7- 38.7) or 37.0 kJ mole-1 of excess heat was observed at lower acidity and is explained as follows. According to the literature each H-bonding in water releases about -9.80 kJ mole-1 of heat.46 Now if we consider four H-bonds in each UO22+-TSIL complex then the excess heat released at low acidity would be

(4 x 9.80) or 39.2 kJ mole-1 and this value is in

approximation with the excess heat observed in ICT experiments. Based on these studies carried out it can be proposed that the complex between UO22+ and TSIL will be [(UO2)(OH2)4(phamdNMe3)2(NTf2)2]2+ in low acidic medium. The schematic representation


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of the bonding pattern in the complex is shown in Figure 2. The ESI-MS spectrum of the UO22+-TSIL complex revealed mass peaks at m/z 800.2289, 440.2245, 260.2200 which corresponds to the complex [(UO2)(OH2)4(phamdNMe3)2(NTf2)2]2+ (exact mass 1600.2526 amu) and its fragments as indicated in Figure S9. Bühl et al.47 had previously demonstrated that in solution phase, water is a better ligand compared to ACN where the UO22+ ion remain surrounded by five water molecules in the primary coordination sphere and the ACN molecules modified the second coordination shell via (UO2)OH2---NCMe hydrogen bonds. The difference in extraction mechanisms at lower and higher HNO3 concentrations clearly indicates that the primary coordination shell of UO22+ ion in both these medium is responsible for different complexation. Since the primary coordination shell of UO22+ remains the same ([UO2(H2O)5]2+) in water and water/ACN mixed solvent, both maintained at lower acidity, it can therefore be concluded that the complexation study in water/ACN mixed solvent could easily be extrapolated to the CPE extraction conditions.47

Optimization of CPE parameters The various conditions of CPE of U by TSIL were optimized to get maximum percentage EE, percentage recovery and preconcentration factor (PF) of U. The optimized CPE conditions of the aqueous phase before coacervation of micelles were obtained through cross-optimization process of all parameters and are presented in Table 1. The equations for determining these factors are given elsewhere.3 All experiments were carried out by taking 10 ng mL-1 of U in the aqueous phase. The concentration of TSIL was varied from 0.05 to 0.5 mmol L-1 to study its effect on the recovery of U (Figure 3). From this figure it can be seen that optimized recovery of ≥ 98% was obtained at and above 0.3 mmol L-1 of TSIL and therefore 0.4 mmol L-1 concentration of TSIL was fixed for further experiments. The unique complexation between the UO22+ and TSIL in low acidic medium due to the non requirement of counter ion



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and also the induced ionic environment of the micelle in presence of TSIL results in optimum recovery of U. It was reported that by increasing the hydrophobicity of the cationic part of the IL, a part of the cation will penetrate into the micelle attracting more NTf2- to penetrate into the micelle.36 The zeta potential (ζ) measurement of the aggregates formed by 5 mmol L-1 TTX-114 in absence and presence of the TSIL was carried out at 10oC to confirm the presence of NTf2- in the micelles. The ζ values of the 4 mmol L-1 TTX-114 solution was found to be - 2±1 mV whereas the value in presence of TSIL (0.4 mmol L-1) was found to be - 42±4 mV. These results confirm the penetration of the NTf2- in the micelles and their distribution in the micellar corona. Since the polar groups of the ligands protrude towards the hydrophilic part of the surfactant,48 it can be inferred that the interaction between the metal ions and the ligands take place in the micellar corona, as shown in Figure 4.

The effect of SDS concentration on the recovery of U was also studied. By maintaining all the other parameters of CPE under optimized conditions, it was found that in absence of SDS the maximum recovery of U was ~ 10%, as shown in Figure 5. Such low recoveries could be explained on the basis of high hydrophobicity of the TSIL, which would not allow the extraction of UO22+ ion through cation exchange mechanism. The addition of small quantity of SDS was found to drastically increase the recovery of UO22+ (Figure 5). Since the added concentration of SDS was much below its critical micelle concentration (CMC: 8.5 mmol L-1 at 298K), it can be expected that they could exist either as monomers or as mixed micelles with TTX-114. The addition of SDS thus increases the recovery of U by allowing the cation exchange mechanism through its hydrophilic counterpart i.e., Na+ ion. Additionally, the electrostatic attraction between the anionic part of SDS and the cationic UO22+-TSIL complex (Figure 4) allows the extraction of the metal complex into the hydrophobic micellar core as a neutral moiety. Optimized recovery of ≥ 98% was obtained in the range 0.1-0.2 mmol L-1 of


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SDS and beyond 0.2 mmol L-1 SDS concentration, the recovery of U was found to decrease drastically (Figure 5). At higher SDS concentration the greater electrostatic repulsion between the same charge moieties increases the cloud point temperature (CPT) and this could be the reason for decrease in recovery values as suggested by Gu et al.49 Therefore the concentration of SDS was fixed at 0.12 mmol L-1.

The pH of the solution is known to have the profound effect on the recovery of the metal ion by CPE.3 The recovery of U by CPE was studied in the pH range of 1-10 and the results are represented in Figure 6. It was previously reported in the literature the pH affects the critical micelle concentration (CMC) and size of the micelles of non-ionic surfactant.50 The CMC of TTX-114 was known to be 0.22-0.24 mmol L-1 in its 5% aqueous solution of pH 6-8. The CMC was reported to increase with lowering the pH of the solution. As a consequnce higher concentration of surfactant will be needed for optimimum recovery at lower pH compared to higher pH region. On the other hand the hydrated volume of the micelle was reported to increase with lowering the pH due to increase in H-bonds between water molecules and ‘O’ atoms of amphiphilic polyethylene glycol chain of the TTX-114. Therefore it can be expected that the S=O group of NTf2- and the P=O group of the TSIL are largely involved in Hboinding with those water molecules at low pH conditions. In addition to these two factors, the competition between H+ and UO22+ to get extracted through the cation exchange mechanism are expected to result in very low recovery of U in the pH region of 1-2. With increase in pH the CMC, the hydrated volume of micelles and the competition between H+ and UO22+ to get extracted decreases which results in higher recovery of U beyond pH 2. Maximum recovery was found in the pH range of 5.5-7.5. Further decrease in recoveries of U was observed beyond pH 7.5. The decrease in recoveries at higher pH region could be due to



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the formation of carbonate and/or bicarbonate complexes of U in aerobic conditions. Hence pH 6 was fixed for maximum analyte recovery.

KNO3 was added to the solution to maintain the ionic strength. KNO3 also acts as a saltingout agent and thereby forcing the metal-ligand complex formation inside the micelles. 0.1 mol L-1 concentration of KNO3 was found to be sufficient for maximum analyte recovery (see Supporting Information, Figure S10).

The low theoretical CPT of TTX-114 solution (CPT: 28oC) above its CMC (0.2 mmol L-1) encouraged us to use it as the non-ionic surfactant in the proposed CPE procedure. The CPT of the system was examined to be 26±1oC. The cloud point temperature (CPT) of the system (26±1 oC) was determined by increasing and decreasing the temperature by 1oC after every 10 min. In the process of heating (starting temperature was kept at 20±1 oC) the temperature was observed at which the transarent solution turns transluscent and vice versa in the process of cooling (starting temperature was kept at 30±1 oC). To enhance the complexation process the extraction temperature was kept at 4-5oC as the UO22+ - TSIL complexation is an exothermic process. The concentration of TTX-114 was optimized in such a way to yield maximum recovery of U and minimization of SRP volume for better preconcentration factor (PF). The concentration of TTX-114 was varied from 2.0-10 mmol L-1 (Figure 7) and maximum recovery was found beyond 4 mmol L-1. Therefore 5 mmol L-1 of TTX-114 was chosen as the optimum value for further experiments. In order to have minimum SRP volume the phase separations were carried out at various temperatures ranging from its CPT to 60oC. Optimal recoveries of U were observed every time but the minimum SRP volume of 0.1 mL was obtained at and above 50oC. Hence extraction of metal ion was carried out at 4-5oC with constant stirring whereas phase separation was carried out at 50oC.


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Optimization of TXRF spectroscopic procedure In the CPE procedure the SRP phase has small volume and its analysis by

analytical

techniques like inductively coupled plasma MS (ICP-MS), ICP-AES, flame AAS (FAAS) and UV-Visible spectroscopy etc. needs dilution, as the requirement of sample volume for these techniques are higher.2-4,22-27 On the other hand the TXRF method can be used for direct analysis of SRP phase as this technique requires very small amount of sample (as low as 510µL).51 Direct analysis of the SRP would result in improving the detection limit of the proposed methodology. The detection limit (DL) was calculated using the equation: DL = 3 x (√IB/IA) x cA.52 Here IB denotes the background area below the peak, IA is the analyte peak area and cA is the concentration of the analyte deposited on the quartz sample support. The method detection limit (MDL) of U by TXRF was found to be 1 ng mL-1 when 50 µL of the SRP phase along with 10 µL of 10 µg mL-1 Ga (internal standard) solution was deposited on quartz sample support and dried at 100ºC, as shown in Figure 8 (black line). From the same figure it can be seen that the scattered background is very large. This scattered background is due to the presence of significant amount of low atomic number constituent elements of surfactants and TSIL. This large background deteriorates the MDL of U. In order to minimize the scattered background it was necessary to remove the organic surfactant layer without affecting the metal ions. Therefore oxidative pyrolysis of the SRP was carried out by keeping the quartz sample support into a Muffle furnace at 650oC for 30 min. In this way all C and H atoms get oxidized to the CO2 and water vapour, leaving behind the metal ions on the quartz support. From Figure 8 it can be seen that after pyrolysis there is a drastic reduction of the background in the TXRF spectra (red line). The detection limit obtained after carrying out pyrolysis of the sample was found to be 0.02 ng mL-1.

Effect of interfering ions



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Development of any methodology for the preconcentration of a specific metal ion in environmental samples requires the requisite recovery studies in presence of common interfering ions. As these interfering ions are present in higher concentrations compared to the metal ion of interest, it can be expected that they can affect the CPE procedure either by altering the CPE parameters or by competing with the specific metal ion in the extraction process. The maximum tolerable concentration of each individual interfering ion was arrived at when the recovery of U was ≥ 95% in its presence. A list of the interfering ions with their maximum tolerable limits towards the recovery of U is represented in Table 2. These ions were tested individually along with 10 ng mL-1 of U in the solution. Even though the alkali metals are not expected to be extracted by the TSIL due to their low oxidation state, but it was necessary to carry out the recovery studies of U in their presence due to their high abundance in nature. On the other hand alkaline earth and transition metals are expected to compete with UO22+ during its extraction. However an optimal recovery of U in presence of large amount of these interfering ions was achieved. This could be due to the unique complexation pattern of the TSIL with UO22+ (Figure 4) which provides a soft template of supramolecular recognition36 in the micelles. At low acidic medium tri- and tetra-valent lanthanides are known to get co-extracted along with UO22+ by P=O based ligands.35 However in the present case the recovery of U by CPE is not affected in the presence of lanthanides with concentrations as high as 100 µg mL-1 (Table 2). This may be due to the fact that under CPE conditions lanthanides are expected to form bulky complexes due to their higher coordination numbers and therefore will not be extracted into the micelles due to steric hindrance. In the environmental aqueous samples, Zr(IV) and Th(IV) are expected to be present in small concentrations due to the low solubility of their compounds.53 Therefore the effect of their interference was evaluated with equimolar concentrations of U(VI), Zr(IV) or Th(IV) and the recovery of U was found to be unaffected. The method was also tested for the


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recovery of U in the presence of some common anions like carbonate (CO32-), phosphate (PO43-), sulphate (SO42-), chloride (Cl-) and bromide (Br-). It was observed that higher concentrations of these anions (Table 2) do not affect the quantitative recovery of U. It is well known that the natural water system contains a large number of organic acids. Therefore it was necessary to evaluate the CPE procedure in their presence. The CPE procedure was studied in presence of humic acid, the most abundant in nature, and the recovery of U was found unaffected even in presence of humic acid (in the form of its sodium salt) concentration as high as 100 µg mL-1. Analytical figures of merit The proposed TSIL assisted CPE procedure resulted in quantitative EE of (99.0±0.5)% and recovery of ≥ 98% for U. The PF of this method was found to be 99. Both the percentage EE and PF values were calculated by determining the U concentration in the supernatant using ICP-MS.3 The proposed method has a detection limit (MDL) of 0.02 ng mL-1 for U in aqueous samples and a linear dynamic range (LDR) of 0.1–1000 ng mL-1. The minimum value of LDR was calculated from the equation of limit of quantification (LOQ) i.e., 10 x (√IB/IA) x cA. The relative standard deviation (RSD, 1σ) of the method was found to be ≤ 4%. The analytical figures of merit of the proposed methodology were compared with the previously reported CPE procedures of U as shown in Supporting Information (Table S2). The MDL of the methodology was found to be better than previously reported works,4,22-28 except one report where more costly and highly sensitive ICP-MS technique2 was used. The proposed methodology was also compared with other preconcentration procedures (given as Supporting Information, Table S3). It can be seen from Table S3 that this offline pairing of CPE procedure with TXRF spectrometry provides much better MDL value of U compared to most of the other reported methodologies.16,54-60 The wide dynamic linear range also makes this technique superior over most of the reported ones.3,4,16,22-28,54-60 The developed CPE



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methodology showed good resilience to large concentrations of common interfering ions and to the best of authors’ knowledge is the first work to report such high tolerance limits for lanthanides.

Method validation and analysis of real sample analysis The proposed analytical technique was validated by determining the concentration of U in a CRM viz., NIST SRM 1640a natural water. The result is given in Table 3. The analyzed value shows a good agreement with the certified value at 95% confidence interval. The applicability of the developed analytical methodology was demonstrated by analyzing three natural water samples employing standard addition. These studies are also tabulated in Table 3. It can be seen from the table that the recovery of U in these samples was found to be ≥ 96%. In order to emphasis on the reproducibility

of the developed analytical methodology, the results of the replicate measurements of each of the above samples are shown in supporting information as Table S4. The results reported in

Table 3 are the average values of these replicate measurents and the standard deviation reported with each value was determined by error propagation.

Conclusion A new TSIL was synthesized, characterized and employed for the first time in the quantitative CPE of U followed by its quantification using TXRF spectrometry. The quantitative EE and high PF for U could be successfully employed for the extraction of this metal from sources like sea water (on average contains 3 ng mL-1 of U). A novel approach of oxidative pyrolysis of the SRP before TXRF spectroscopic analysis was found to improve the detection limit of the methodology. The offline coupling of CPE with TXRF spectrometry results in achieving better detection limit for U. The proposed methodology is eco-friendly in nature as it utilizes micromolar to millimolar concentrations of surfactants and extracting


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agent and can be used for accurate and precise determination of ultratrace amounts of U in aqueous samples.

Acknowledgement The authors are thankful to Shri. Bal Govind Vats, FCD, BARC and Dr. Raghunath Chowdhury, BOD, BARC for their support and cooperation in this work.

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Table 1 Optimized conditions of CPE procedure for total 10 mL aqueous phase before coacervation Parameters Sample volume pH [TTX-114] [SDS] [TSIL] [KNO3] Textraction textraction Tphase separation tphase separation Centrifugation

Optimized conditions 7.5 6.0 5.0 0.12 0.4 0.1 4-5 30 50 30 4000

Units mL --mmol L-1 mmol L-1 mmol L-1 mol L-1 o C min o C min rpm



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Table 2 Tolerance of the method to common interfering metal ions (experiments carried out with 10 ng mL-1 of UO22+) Metal ion (Mn+)a,b

Tolerance limit (µg mL-1) Na+, K+ 3000 + Li 2000 Ca2+ 500 2+ 2+ 2+ 200 Mg , Sr , Ba 3+ Al 100 3+ 3+ Cr , Fe 50 Mn2+, Co2+ 200 Hg2+ 100 2+ Pb 100 La3+, Ce4+, Eu3+, Tb3+ 100 Cl-, Br3000 232CO3 , PO4 , SO4 1500 a,b Cations were prepared using their chloride or nitrate salts. Anions were prepared using their sodium or potassium salts.


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Table 3 Analysis of a CRM and real samples viz., sea water from Mumbai coast, India; well water from West Bengal, India and Spring water from Sikkim, India (n = 5) Sample

NIST SRM 1640a Sea water

Certified or added U conc. (ng mL-1)

Analyzed U conc. (ng mL-1)

Recovery (%)

25.35±0.27a

25.5±0.6

100±4.0

2.7±0.1 5.0 7.5±0.3 96±6 50.0 53.0±1.0 101±2 Well BDL water 5.0 4.8±0.2 96±4 50.0 49.4±0.5 99±1 Spring BDL water 5.0 4.9±0.2 98±4 50.0 49.0±0.5 98±1 a Certified value; BDL = Below detection limit



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Figure caption Figure 1 Schematic representation of the complete synthesis route of TSIL. Figure 2 Schematic representation of the bonding pattern of the complex between UO22+ and TSIL. Figure 3 Effect of TSIL concentration on the recoveries of 10 ng mL-1 of U. Other parameters were kept constant as presented in Table 1. Figure 4 Proposed mechanism of selected CPE of UO22+ by TSIL. Figure 5 Effect of SDS concentration on the recoveries of 10 ng mL-1 of U. Other parameters were kept constant as presented in Table 1. Figure 6 Effect of pH on the recoveries of 10 ng mL-1 of U. Other parameters were kept constant as presented in Table 1. Figure 7 Effect of TTX-114 concentration on the recoveries of 10 ng mL-1 of U. Other parameters were kept constant as presented in Table 1. Figure 8 TXRF spectra of 50 µL SRP phase deposited on quartz sample support before the pyrolysis (black) and after the pyrolysis (red).


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Figure 1



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Figure 2


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Figure 3



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Figure 4


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Figure 5



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Figure 6


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Figure 7



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Figure 8


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Graphical abstract


Selective Micellar Extraction of Ultratrace Levels of Uranium in

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