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Selective membrane complexation and uranium isotopes analysis in tap water and seawater samples Nikolaos Georgios Kallithrakas-Kontos, Dimitrios C. Xarchoulakos, Paraskevi E. Boultadaki, Constantinos Potiriadis, and Konstantina Kehagia Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b05115 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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Analytical Chemistry

Selective membrane complexation and uranium isotopes analysis in tap water and seawater samples Nikolaos G. Kallithrakas-Kontos*1, Dimitrios C. Xarchoulakos1,2, Paraskevi Boultadaki1, Constantinos Potiriadis2, Konstantina Kehagia2 1

Technical University of Crete, School of Mineral Resources Engineering, Analytical and Environmental Chemistry Laboratory, 73100 Chania-Greece, *E-mail: [email protected] Fax:+30 2821037841 2

Greek Atomic Energy Commission, Department of Environmental Radioactivity Monitoring, 15310 Athens-Greece

ABSTRACT: The complexation of 238U and 234U in tap water and seawater after the use of a selective membrane was examined. At a first step many ligands were used for original membranes preparation and their yield in uranium analysis were evaluated by X-Ray Fluorescence, in order to select the ligand with the highest uranium selectivity in water samples. At the second step the new prepared membrane was used for uranium analysis by alpha spectrometry. Various factors were tested for a more effective uranium binding such as membrane’s active surface, water sample volume, equilibration time and stirring during the process. After membrane complexation, uranium was separated by anion exchange and electroplated onto stainless steel plates in order to prepare suitable alpha ray sources; these sources were measured by alpha spectrometry and gave high chemical uranium recoveries and very good energy resolution spectra. The method can successfully been applied even for relatively small sample volumes and seawater samples.

Uranium is a primordial radioactive element that is widely distributed at the terrestrial environment, it is toxic and it is present in various concentrations depending on the system. Uranium isotopes have been used as tracers in various hydrological studies1,2 and they contribute to the annual public radiation exposure among other sources and water consumption3,4. In addition, uranium is used as a raw material in nuclear technology and new approaches relative to its determination are always of great interest. The previous reasons make uranium analysis and its activity concentration estimation essential. Uranium is present in water systems in various oxidative states. In oxidizing waters uranium is present in a predominant more soluble uranyl hexavalent form, while in reducing waters it is present in the tetravalent form. It has the tendency to form hydroxides in fresh water systems and mainly carbonates in ocean water. Solubility and as a consequence the mobility of uranium in water systems is primarily determined by the pH and redox conditions of the system and by its affinity for particle reactivity and complexing formation5. Although potable water samples with volumes of the order of 0.5 to 1 liter are enough to achieve efficient uranium analysis, major issues are faced by laboratories in analyzing seawater. A primary issue in seawater analysis is that larger amounts of collected samples are required, such as of the order of 8L6, depending on the desired detection limit and the chemical recovery. Additionally, seawater is a complicated matrix and hence the chemical analysis process is usually tough comprising low recoveries in many cases due to losses during sample preparation and separation stages. Consequently there is the necessity for the development of new methods which do not require large sample volumes and provide high yields and good energy resolution.

Membrane complexation is a quantitative and efficient technique which has been initially developed for the selective collection of specific elements before x-ray analysis7,8. Trace elements can be effectively removed from a solution by complexing reactions between the analyzed elements and a suitable ligand firmly bounded in the membrane matrix. They give the possibility of significant very low detection limits, speciation analysis and interference avoidance. In the present work selective complexing membranes were applied for the first time for uranium analysis in water samples by alpha spectrometry. Although uranium determination had been previously performed by x-ray fluorescence (XRF) with a detection limit of 0.8 ng/mL9 this concentration is not satisfactory in most cases of environmental water samples. Additionally, seawater samples present an extra difficulty due to sodium chloride matrix. Τhe high salt content makes difficult to prepare an ultra-thin source that is necessary for the α-spectroscopy measurements. For this reason numerous complexing membranes were prepared and tested in uranium doped water samples by XRF, as this method is quite faster than alpha spectrometry. Subsequently the best membrane that was determined by the XRF procedure was selected and used for uranium analysis by alpha spectrometry. As this technique is applied for the first time in radionuclide analysis, it could be a promising alternative for many other type of radioanalytical determinations.

EXPERIMENTAL SECTION Membrane solution preparation. At the first step membranes were prepared in their liquid form by mixing tetrahydrofuran solvent (THF, Carlo Erba 41245200C), membrane matrix containing suitable ingredients and complexing reagents in a similar way that we have previously applied for trace element analysis8. Two types of membrane matrixes and many complexing reagents were tested. The anion membrane was composed as following: Polyvinyl chloride (PVC, Aldrich 81387) 46%, Tricaprylylmethylammonium chloride

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anion carrier (Aliquat-336, 205613 Aldrich) 31%, complexing reagent 17% and dibutylphthalate plasticizer (36736 Riedel de Haen) 6%. The cation membrane was prepared as following: PVC 48%, dibutylphthalate plasticizer 28%, complexing reagent 18%, 5,5′-Dithiobis (2-nitrobenzoic acid) ionophore (DTNB Fluka 43760) 6 %. Both membrane matrixes were diluted in 20 mL THF. The solid membrane was prepared by spreading 1-8 mL of membrane solution on the bottom of glass beakers or crystallization dishes, afterwards it was left to dry in the ambient temperature overnight. Oven or IR drying did not cause any difference in the membrane properties.

Complexing reagents for membrane preparation. Taking into account their chemical properties as well as our previous work9 the following 19 complexing reagents were tested for membrane preparation: 2-Hydroxy-1-(1’-hydroxy4-sulfonaphthyl-1-azo)-naphthalin-3-carboxylic acid (Merck Art 4595), 1-(3-Dimethylaminopropyl)-3-ethyl carbodiimide polymer bound (Aldrich 424331), Sodium 1-(1Hydroxynaphthylazo)-6-nitro-2-naphthol-4-sulfonate (SigmaAldrich 858390), 4-(2-pyridylazo) resorcinol (Lancaster 9667), 2,7-Bis(2-arsonophenylazo) chromotropic acid (Fluka 11090), 4-(2-Thiazolylazo) resorcinol (Aldrich 127345], Trans-1,2-diaminocyclohexane-Ν,Ν,Ν',Ν'-Tetraacetic acid (Aldrich 31,994-5), Quinaldic Acid (Aldrich 16,066), Xylenol Orange Sodium Salt (Merck 1.08677.0001), o-dianisidin (Fluka 33430), Ammonium pyrrolidinedithiocarbamate (APDC) (Fluka 09935], Mercury Ionophore I (Fluka 39075), Sodium dibenzyldithiocarbamate (Fluka 71485), 1-(2pyridylazo)-2-naphthol (Aldrich 46,088-5), Triethylenetetramine-N,N,N',N'',N'',N''' hexaacetic acid (Fluka 90471), 1Nitroso-2-Naphthol (Fluka 73910), 2-Aminobenzothiazol (Fluka 08090), 2-Mercaptopyrimidine (Fluka 63851), 8Hydroxyquinoline (Aldrich 25,256-5). As alpha spectrometry is a time consuming technique, Energy Dispersive X-Ray Fluorescence (EDXRF) was used as a membrane selection technique. 50 µL from each membrane solution were deposited on a thin Prolene® thin film that was affixed with a nitrile O-rings in a plastic XRF Chemplex® sample cup as it has been previously described8. Liquid membrane solutions were left to dry and to prepare a thin film on the Prolene support. Chemplex cups with these thin films were immersed in Uranium water solutions of 50 µg/kg prepared by using Uranium standard (Aldrich 20,762-4). Membranes were left for 24 hours in the solutions, and then were taken out, rinsed with high purity water and they were left to dry. EDXRF Membrane analysis. Analysis was performed by a Spectro-Xepos EDXRF Spectrometer, X-Lab Pro 4.0 software, palladium (Pd) anode, 50 W, 50 kV. Measurements were performed in helium atmosphere, three excitation modes: a) Compton scattering on secondary molybdenum target (Energy=35 keV, current=1 mA). b) X-ray beam polarized by Barkla scattering on aluminum oxide (Energy=49.2 keV, current=0.7 mA) and c) High intensity Bragg reflection in highly oriented pyrolytic graphite (HOPG) crystal (Energy=17.5 keV, current=1.5 mA). A silicon drift detector with Peltier cooling with an 8 µm Moxtek Dura-Be window was used (resolution of 160 eV at 5.9 keV). The irradiation time was 300 s for every irradiation mode (15 min per membrane sample).

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Sampling and pre-treatment. A tap water sample of 15L was collected from the public water network in AthensGreece, acidified at a pH value of 2 and divided into the appropriate sub-samples to assure the homogeneity of the process. The samples were then stored at polyethylene bottles. Respectively, a similar procedure was used after the collection of a single seawater sample. Alpha spectrometry analysis. The separation of uranium is necessary, in order to avoid overlapping of peaks by other radionuclides. Since many natural radionuclides emits alpha radiation in various energies, peak overlapping may be a challenging issue on spectral deconvolution. In case of uranium isotopes analysis overlapping might take place between 232U which was used as an internal tracer and emits alpha particles at Ea= 5.320 MeV (68.1%) and 228Th which emits alpha particles at Ea= 5.340 MeV (27.2%). 228Th could be either naturally occurring or might interfere the analysis as a decay product of 232U. 210Po which is a pure alpha emitter with alpha energy Ea= 5.304 MeV, could also be an interfering nuclide at 232U peak area. Similar peak overlapping could happen between 234U which has Ea=4.775 MeV and 226Ra with Ea=4.784 MeV. Furthermore, energy overlapping could occur between 230Th (Ea1=4.687 MeV, Ea2= 4.620 MeV) and 234U which emits alpha particles also at Ea= 4.722 MeV as well as between 230Th and 226Ra which emits alpha particles also at Ea=4.601 MeV. Water samples where spiked with a known amount of 232U tracer in order to calculate the chemical recovery at the end of the process and then were added at the glassware where the membranes had already been deposited and dried. In cases where stirring was applied, magnets were placed at the bottom of the glassware after membranes deposition and water addition. After the uranium complexation process the membranes were dissolved in concentrated nitic acid and wet ashed into ceramic crucibles at hotplates, after the consecutive addition of concentrated nitric and hydrochloric acid; the residues were dry-ashed at 600OC. The dry ashed residues were diluted in hydrochloric acid solution and uranium was separated from its daughter nuclides and impurities via anion exchange10. Source preparation and counting. The source preparation (i.e. preparation of a target suitable for alpha spectrometry) is an essential analytical step in radiochemistry, in order to produce thin sources and to achieve good energy peak resolution alpha spectra. Electrodeposition is the most efficient method; if the deposition is not appropriate the quality of the spectra deteriorates due to energy losses of the alpha particles inside the source matrix. At the present study the sources were prepared via electrodeposition onto stainless steel plates of 25 mm diameter and 1 mm thickness, in an ammonium sulfate/H2SO4 electrolyte. A fully integrated and automated Canberra Alpha-analyst spectroscopic system consisting of twelve passivated implanted planar silicon (PIPSi) detectors with 600 mm2 active area, was finally used for the sources measurements. Spectra were analyzed by Alpha Analyst Genie 2000 software. A standard 239 Pu, 241Am and 244Cm source was used for energy and efficiency calibration. Uranium analysis in tap water was


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performed according to the ISO13166:2014 (Water quality— Uranium isotopes—Test method using alpha spectrometry). Uranium analysis in seawater samples was validated by the use of IAEA-381 Irish seawater certified reference material.

RESULTS AND DISCUSSION Complexing reagent selection. The results for the examined complexing reagents are presented in Table 1. The membrane uranium selection ability for nineteen different ligands was examined and evaluated according to the uranium Lα x-ray yield (13.61 keV). The best results were taken with 3Hydroxy-4-(2-hydroxy-4-sulfo-1-naphthylazo)-2naphthalenecarboxylic acid (L1). As the next two complexing reagents (L2, L3) gave less than a half of the L1 yield, L1 was selected as a complexing reagent for membrane preparation for uranium analysis. Alpha spectrometry analysis. The first parameter which was examined in uranium complexation, was the surface area of the membrane that is in contact with the water samples. Beakers having different bottom diameters, as well as crystallization dishes of 22 cm of diameter were tested. The used membrane volumes were selected in order to give a similar membrane surface density in all cases. The results are presented in the Figure 1; it was observed that increasing the contact surface increases the chemical recovery to a maximum of 64% for a membrane surface of about 380 cm2, membrane-water equilibration time 1 day and water sample volume 1L. Similar observations have also been noticed for seawater. It was also observed that when increasing the equilibration time (between membrane and water samples) from 1 day to 3 days the chemical recovery was slightly increased, to a maximum value of 68%.

Chemical recovery (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 60 50 40 30 20 10 0 50

150

250

Membrane's surface

350

450

(cm2)

Figure 1. Uranium chemical recovery as a function of contact surface in tap water.

The volume of the sample was an additional determinant factor which could affect the complexing process; when reducing the sample volume from one liter to half a liter a similar chemical recovery was observed for both tap and seawater. This recovery was 68% for tap water and 70% for sea water respectively for 1 day of water-membrane contact. Stirring also was a very important factor, since after its application the chemical recoveries for tap water increased as much as 96%, at 380 cm2 of membrane surface for 1 day of contact. The corresponding maximum recovery for seawater was 93%. According to all previous observations this parameter combination was the optimum, in order to achieve the maximum chemical recoveries and the optimum analysis process is given in the Figure 2. The stirring magnet (in a low frequency stirring mode) did not cause any damages to the membrane, so the membrane exhibits very satisfactory mechanical properties. Chemical recovery values comparing to the progress of analyses are shown in Table 2.All the described analyses were performed at room temperature. To check the repeatability of the method, three iterations of the process have taken place in the case of the optimum chemical recovery conditions (parameters 9 and 12, Table 1). The calculated uranium isotopes activities at these conditions for tap water, were in average 2.8±0.3 mBq/L for 234U and 1.6±0.2 mBq/L for 238U. The corresponding average activities for the seawater were 45±3 mBq/L for 234U and 38±2 mBq/L for 238U. The progress of tap water analyses were monitored and compared as mentioned at previous paragraph by international certified analytical standards, as it has been similarly described in our previous works11,12. Additionally, seawater analysis was validated by the IAEA-381 Irish seawater certified reference material (Table. 3)13. The energy resolution corresponding to the optimum chemical recovery, expressed as the FWHM of the peaks in tap water analysis (parameters 9) was 28.3 keV for 232U, 10.5 keV for 234U and 13.8 keV for 238U. The respective value for seawater (parameters 12) were 32.8 keV for 232U, 26.2 keV for 234 U and 30.1 keV for 238U. A typical uranium alpha spectrum which obtained after the application of the present methodology is presented in the Figure 3. The effect of equilibration time was also examined. Contact times of 1h, 2h, 4h and 8h were tested in addition to the 24h of contact time which has already been presented. A gradual increase in chemical recovery was observed for both types of water samples (tap water and seawater). The results are shown in Figure 4; the rate of complexation was very fast in the first 4 hours and then it has significantly slowed down and stopped at about 8 hours due to water uranium exhaust.



Membrane complexation of uranium/380 cm2 membrane surface/ 0.5L of sample/ stirring

Wet ashing with c.HNO3/ c.HCl

Dry ashing at 600oC

Separation of uranium by anion exchange

Source preparation by electrodeposition on stainless steel plates

Counting by alpha spectrometry

Figure 2. Optimum uranium analysis process described at the present method.

140

234U

238U

120

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 80 232U

60 40 20 0 200

400

600

800

Channel Figure 3. A characteristic uranium spectrum after membrane complexation in seawater analysis.


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Analytical Chemistry Chemical recovery (%)

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120 90 60 30 0 0

2

4

6

8 10 12 14 16 18 20 22 24

t (h) Tap water

Seawater

Figure 4. Chemical recovery as a function of the complexation time in tap water and seawater.

CONCLUSIONS The use of selective complexing membranes followed by alpha spectrometry is an efficient method for uranium analysis. X-ray Fluorescence can be used at a first step as a screening method for ligand evaluation. The results can be concluded as following. (1) The best membrane was based on a PVC plus Aliquat-336 plus 2-Hydroxy-1-(1’-hydroxy-4-sulfonaphthyl-1-azo)-naphthalin-3-carboxylic acid system. (2) Determination of uranium ions is applicable either in tap and seawater samples. (3) There is a long-term membrane stability and very good adhesion. (4) A membrane surface of 250-400 cm2 and a complexation time of 8 hours were sufficient. (5) Uranium recoveries of 93-96% were achieved. (6) Very good energy resolution spectra were achieved. (7) Small sample volumes (0.5 L) were used and they were proved suitable even for seawater samples.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Phone: +30 2821037666. Fax: +30 2821037841

REFEREΝCES (1) Juanjuan, S.; Zhigang, Y., Bochao, X.; Wenhua, D.; Dong, X.; Xueyan, J. J. Environ. Radioact. 2014, 128, 38-46. (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)

Tissot, F.L.H.; Dauphas, N. Geochim. Cosmochim. Acta. 2015, 167, 113-143. Rozmaric, M.; Rogic, M.; Benedik, L.; Strok, M. Sci. Total. Environ. 2012, 437, 53-60. United Nations Scientific Committee on the Effects of Atomic Radiation-UNSCEAR. vol. I., 2000. Chopin, G.R. J. Radioanal. Nucl. Chem. 2008, 273,695-703. Maxwell, S.L.; Culligan. B.K.; Hutchison. J.B.; Utsey, R.C.; McAlister, D.R. J. Radioanal. Nucl. Chem. 2014, 300, 1175-1189. Hatzistavros, V.; Kallithrakas-Kontos, Ν. Anal. Chem. 2011, 83, 3386-3391. Hatzistavros, V.; Kallithrakas-Kontos, Ν. Anal. Chim. Acta 2014, 809, 25-29. Hatzistavros, V.; Koulouridakis, P.; Kallithrakas-Kontos, Ν. Anal. Sci. 2005, 21, 823-826. Eaton A.D.; Clesceri, L.S.; Greenberg, A.E. Public Health Association. 1995, ISBN 0-87553-223-3. Xarchoulakos, D.C.; Kehagia, K.; Kallithrakas-Kontos, N.; Potiriadis, C. J. Radioanal. Nucl. Chem. 2017, 312, 285-292. Kehagia, K.; Koukouliou, V.; Bratakos, S.; Seferlis, S.; Tzoumerkas, F.; Potiriadis, C. Desalination. 2007, 213, 98-103. Povinec, P.P.; Badie, C.; Baeza. A. et.al. J. Radioanal. Nucl. Chem. 2002, 251, 369-374.



Code

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Uranium La yield (counts/s)

Complexing reagent

L1

3-Hydroxy-4-(2-hydroxy-4-sulfo-1-naphthylazo)-2-naphthalenecarboxylic acid

3.58

L2

1-(3-Dimethylaminopropyl)-3-ethyl carbodiimide polymer bound

1.59

L3

Sodium 1-(1-Hydroxynaphthylazo)-6-nitro-2-naphthol-4-sulfonate

1.26

L4

4-(2-pyridylazo) resorcinol

1.14

L5

2,7-Bis(2-arsonophenylazo) chromotropic acid

0.43

L6

4-(2-Thiazolylazo) resorcinol

0.37

L7

Ν,Ν,Ν',Ν' -Tetraacetic acid

0.26

L8

Quinaldic Acid 98 %

0.23

L9

Xylenol Orange Sodium Salt

0.17

L10

3,3′-Dimethoxybenzidine

0.14

L11

Ammonium pyrrolidinedithiocarbamate

0.00

L12

Mercury Ionophore I

0.00

L13

Sodium dibenzyldithiocarbamate

0.00

L14

1-(2-pyridylazo)-2-naphthol

0.00

L15

Triethylenetriamine-N,N,N',N'',N'',N''' hexaacetic acid

0.00

L16

1-Nitroso-2-Naphthol

0.00

L17

2- Aminobenzothiazol

0.00

L18

2- Mercaptopyrimidine

0.00

L19

8-Hydroxyquinoline

0.00

Table 1. The examined complexing reagents and the coresponding La x-ray yield of the membrane EDXRF analysis.


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Parameters

Description

C.R. (%)

Tap water 1

1mL membrane/1L of sample/ 79 cm2 membrane surface

16±1

2

1mL membrane/1L of sample/123 cm2 membrane surface

19±1

3


37±2

2

4

2mL membrane/1L of sample/201 cm membrane surface

53±2

5


64±3

6

8mL membrane/1L of sample/ 380 cm2 membrane surface/ 3days of contact

68±3

2

7

8mL membrane/0.5 L of sample/ 380 cm membrane surface

68±3

8

2mL membrane/1L of sample /201 cm2/stirring

85±3

9

8mL membrane/0.5 L of sample/380 cm2/stirring

96±5

10

2mL membrane/1L of sample/201 cm2

Seawater 67±3 2

11

8mL membrane/0.5 L of sample/380 cm membrane surface

70±3

12

8mL membrane/0.5 L of sample/380 cm2 membrane surface/stirring

93±6

Table 2. Examined parameters and the corresponding chemical recoveries (C.R. %). Except parmeters 6 where the contact time was 3 days, in all other cases the contact time was 1 day. Parameters 9 and 12 refer the average C.R. after three iterations of the present methodology.


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IAEA 381- Information massic activities (Bq/kg)

Determined activities (Bq/kg)

234

0.051±0.006

0.044±0.012

238

0.042±0.004

0.041±0.012

Radionuclide U U

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Table 3. Validation of uranium analysis in seawater. All values are presented in 95% confidence level.


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