Polymer based ion-sensor for the selective recognition of UO22+ and Th4+ ions

REACTIVE & FUNCTIONAL POLYMERS

Reactive & Functional Polymers 66 (2006) 1452–1461

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Polymer based ion-sensor for the selective recognition 4+ ions of UO2þ 2 and Th M. Akhila Maheswari, M.S. Subramanian

*

Department of Chemistry, Indian Institute of Technology, Chennai 600 036, India Received 14 December 2005; received in revised form 4 April 2006; accepted 25 April 2006 Available online 5 June 2006

Abstract 4+ A polymer based ion-sensor, selective for concentrating UO2þ ions is developed based on the use of a novel 2 and Th multi-donor ionophore, termed EHCOP (4,4-Bis-[bis-(2-ethyl-hexyl)-carbamoyl]-2-oxo-butyl}-phosphinic acid). The fabrication involves the stepwise chemical grafting of Amberlite XAD-16 resin beads with phosphinic acid and branched long alkyl chain amide moiety, to the desired EHCOP ligand. The grafted polymer exhibits phenomenal behavior in sensing both Th4+ and UO2þ 2 from acidic and neutral conditions, with greater decontamination factor values. Moreover, in this sensor the only possible co-extractable lanthanides ions can also be selectively eliminated with 2 M HCl. The concentrated ions can be sequentially recovered by gradient elution, by first stripping off Th4+ ions with 2 M HCl–NaCl mixture, followed by 0.5 M (NH4)2CO3 for pure UO2þ fraction. The polymer’s lowest sensing limit for UO2þ was 5 lg L1 and 2 2 10 lg L1 for Th4+. As the EHCOP moiety exhibits dual extraction property, it was successful in recovering uranium from seawater samples, with an RSD value of >3.9%. Based on the metal sorption capacity studies, the sensor material can be utilized both as an analytical and preparative column depending on the application. The response time involved for trace ion targeting was statically observed to be within 10 min and by dynamic mode with a flow rate of 14–16 mL min1. The resin can be recycled as much as 15 cycles without any significant change in their sorption capacity. The chemical grafting was characterized by 13C CPMAS NMR, FT-NIR-FIR, and CHNP analyzer and the metal ion sensing was estimated by UV–vis and fluorescence spectroscopy.  2006 Elsevier B.V. All rights reserved.

Keywords: Grafted polymer; Ion-sensor; Preconcentration and uranium recovery

1. Introduction In the recent years an increasing demand for environmental monitoring has stimulated analytical chemists in their efforts to develop small, reliable, * Corresponding author. Tel.: +91 44 2257 4211; fax: +91 44 2257 4202. E-mail address: mssu@rediffmail.com (M.S. Subramanian).

sensitive, selective, and fast responding detection systems. This has directed in the launch of many research projects in the development of chemical sensors for their implementation in environmental monitoring especially heavy metal ions [1,2]. In this context, a ‘sensor’ is described as a device/material, which is capable of responding to the presence of a heavy metal ion in a reversible and continuous manner. Sensors which produce an irreversible response

1381-5148/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2006.04.007

M.A. Maheswari, M.S. Subramanian / Reactive & Functional Polymers 66 (2006) 1452–1461

are referred to as ‘probes’ [3]. Over the last two decades, developing chemosensors for nuclear applications has been undertaken based on their scientific importance. Sensing of actinides and lanthanides ion and their elimination from nuclear storage/discharge is of great public concern besides economical prospective. The world health organization (WHO) has established a maximum tolerable daily intake (TDI) value for uranium as 0.6 lg kg1 body weight per day [4]. Moreover, their limited resource availability has also driven us for reprocessing programme for which efficient preconcentration methods are required for mineral conservation. Besides these, preconcentrating uranium and thorium from lanthanide and other acidic matrices is important in evaluating the performance of various nuclear processes [5]. It has also been estimated that seawater contains about 3–5 lg L1 of uranium besides its original mineral content [6]. Conventional methods like solvent extraction, co-precipitation, etc., using organo-phosphorous and amides-based ligands are still employed at the cost of environmental safety due to ineffective alternatives [7–9]. Recently, membrane materials have been researched as sensors and preconcentrators in this area. But these materials get ruptured or deteriorated under high acidities and temperature [10,11]. Currently, polymer based ion-sensors for heavy metal ions are being tried with extraction chromatographic (EXC) techniques using dynamically coated extracting agents onto a solid support. However, the life spans of these stationary phases are relatively lesser creating bias in data reproducibility. Besides this, many are subjected to limited pH workability due to poor acidic property of the extracting reagents. Hence chemical sensor developed for environmental monitoring are under active development but at the moment are of only limited values. At present, most of the work done is focused on academic problems. What we really need is a sensor, which does not have to be miniaturized, but is capable of working reliably under severe conditions. The newly fabricated ion-sensor possesses unique features in terms its grafted ionophore, in sensing Th4+ and UO2þ 2 with selectivity and capacity factor from both high acidities and saline conditions. The novelty has been achieved in grafting a synergetic combination of phosphinic acid and N,N,N 0 ,N 0 -tetrakis(2-ethylhexyl)malonamide groups, based on the principle concept of solvent extraction. The targeting mechanism involved in acidic media is the

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complexation of neutral metal species through phosphoryl and amide carbonyl oxygen. In low acidic and neutral conditions, the P–OH group acts as a cation exchanger for these ions. For the effective function of this grafted material in aqueous media, the grafted hydrophilic P–OH group facilitates greater ion accessibility to the recognition sites. Based on the relative stability constants of the formed Mn+–EHCOP complex (where Mn+ cor4+ 3+ 3+ responds to UO2þ 2 , Th , La , and Nd ), sequential elution of metal ions from the EXC column can also be achieved. 2. Experimental 2.1. Instrumentation A Bruker-Avance 400 model nuclear magnetic resonance (NMR) spectrometer was used for recording the 13C cross polarized magic angle spin (CPMAS) and 31P solid state NMR spectra (7.5 kHz, Magnetic field – 9.01 Tesla) of the grafted polymer. A Perkin–Elmer spectrum one model Fourier transform-infra red (FT-IR) spectrometer was used to monitor the IR spectra during each stage of the polymer grafting (potassium bromide pellets). An Elementar Vario EL model CHNPS elemental analyzer was also employed for monitoring each stage of the grafting process on quantification scale. A Bruker IFS 66V model Far IR Spectrometer was used as sensing tool for metal ion complexation with the anchored ligand moiety. A Perkin–Elmer TGA7 model thermal analyzer was employed to estimate the polymer’s compatibility in aqueous media by measuring its water regaining capacity. A Jasco V530 model UV–vis spectrophotometer was used in 3+ 3+ the estimation of Th4+, UO2þ ions 2 , La , and Nd by their intrinsic optical properties with chromophores. Fluorescence spectra for ultra trace (lg L1) UO2þ 2 ions was conducted on a Hitachi F-4500 fluorescence spectrophotometer at an excitation wavelength of 266 nm and emission wavelength of 515 nm, with 2.5 mm as slit width. A Varian SpectrAA-20 model flame atomic absorption spectrometer was used in the estimation of foreign ions during interference studies. An Orbitek DL model mechanical shaker with 200 rpm (rotations per minute) was used for static equilibration studies. A Ravel HiTech S-50 model peristaltic pump was interfaced between the packed column and the sample reservoir to ensure constant flow rate during dynamic studies.

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2.2. Chemicals and reagents The standard metal ion stock solutions were prepared by dissolving exact amounts of UO2(NO3)2Æ6H2O and Th(NO3)4Æ5H2O (Fluka Chemicals) in 0.01 M HNO3 and were subsequently standardized by complexometric titrations using 0.1 M EDTA. La2O3 and Nd2O3 (Indian rare earths) salts were dissolved in concentrated HNO3 and evaporated to dryness (thrice) under IR lamp and the stocks were prepared in 0.01 M HNO3 and were also standardized by EDTA titration. All the chemicals and reagents required for the grafting process were of AR grade, which were purchased from Lancaster and E-Merck. AXAD-16 resin beads (20–50 mesh, surface area 825 m2 g1, bead size 0.3–1.2 mm) were purchased from Fluka Chemicals. The resin beads were purified from basic and acidic impurities by first washing with 2 M HCl followed by 2 M NaOH. The beads were washed with distilled water until the washings showed neutral pH and finally with ethanol (to remove monomer impurities) and dried in vacuum.

Step (a): O Cl Cl

O

Dry ether

+ 2 HN

N

0˚C 2.1 equiv. TEA

O

N O

Step (b): (

)n

(i) Anhy.AlC l3 / PCl3 (ii) OH / H2O

(

HO

(ii) Et3N / Cl PH O

Cl

+

(iii) H / H2O (

HO

O

P O

Cl O

)n

(i) N,N,N',N'-2-ethylhexylmalonamide NaH / dry DMF (ii) H+ / H2O

)n

(i) Me3SiCl / Et3N / Dry DCE

+

(iii) H / H2O

(

)n

O HO

P O

N OO

N

Fig. 1. Scheme for polymer grafting: Step (a): Synthesis of N,N,N 0 ,N 0 -tetrakis(2-ethylhexyl)malonamide. Step (b): Chemical modification of AXAD-16.

ether medium at 5 to 0 C). The product was purified by vacuum distilled and spectroscopically characterized prior to usage.

2.3. Grafting procedure

2.4. Methodology for metal ion sensing and preconcentration

The first stage of grating involves three steps, (a) AXAD-16 beads (5 g) were refluxed for 4 h with phosphorus trichloride (20 mL) and anhydrous AlCl3 (2.5 g) in petroleum ether, (b) the beads were further refluxed with aqueous NaOH for 3 h, and (c) acid treatment using dilute HCl, to obtain the phosphinic acid resin. The grafted beads were water washed to neutral pH and vacuum dried resin. The second stage is the protection of P–OH groups by treating with chlorotrimethylsilane in dry DCE medium for 1 h at 50 C, prior to reaction with 1,3-dichloroacetone in the presence of triethylamine. The third stage involves the anchoring of N,N,N 0 ,N 0 -tetrakis(2-ethylhexyl)malonamide, TEHMA (7.5 mL) pretreated with sodium hydride (NaH) in dry dimethyl formamide (DMF), for 48 h at 80C. The chemically modified resin beads were deprotected washed with water and acetone, filtered and vacuum dried to arrive at the final desired grafted polymer. The various stages involved in polymer grafting and the synthesis of TEHMA are depicted in Fig. 1. TEHMA was synthesis by reacting 2-ethylhexylamine with malonyl chloride in the presence of triethyl amine in the molar ratios of 2:1:2, in dry

2.4.1. Static method Analyte ions of specific concentrations are equilibrated batch-wise with 50 mg of the grafted beads, in a well-capped reagent bottles for a definite time period. The extent of metal ion sensing was then estimated spectrophotometrically after desorbing from the polymer template with 15 mL of optimum stripping agents. Arsenazo-III (2,7-bis(2-arsonophenylazo)chromotropic acid), a chromogenic reagent selective for specific actinides and lanthanides ions was used for estimation at 655 nm. The selectivity among the actinides and lanthanides are achieved with the reaction conditions where, UO2þ in 7 M 2 HNO3 and La3+ and Nd3+ at pH 3 [12]. For Th4+, Thoron (1-(o-Arsonophenylazo)-2-naphthol3,6-disulphonic acid disodium salt), was used as the selective chromophore at 545 nm, in 0.5 M HCl medium [13]. The degree of metal ion response is normally denoted in terms of distribution ratios (D, mL g1) values, as it also involves preconcentration. The experimental parameters required to understand the basic extractive behaviors and sensing levels of the developed grafted polymer were studied and the results are itemized in Table 1.

M.A. Maheswari, M.S. Subramanian / Reactive & Functional Polymers 66 (2006) 1452–1461 Table 1 Optimized experimental parameters for the separation of UO2þ 2 and Th4+ Experimental parameters

UO2þ 2

Th4+

pH range t1/2 (min)

5.0–6.5 3.0

2.0–3.0 3.3

Metal sorption capacity (mmol g1) (i) At optimum pH (ii) At 2 M HNO3 (iii) At 2 M HCl

1.59 0.98 0.87

1.41 0.71 0.10

Eluting agent Maximum flow rate (mLmin1) Average % recovery LOQ (lg L1) Sample breakthrough volume (mL) Enrichment factor (2 M HNO3)

0.5 M (NH4)2CO3 2 M HCl+2MNaCl 16

14

99.0 5 5000

98.9 10 4000

333

267

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of water. The sample solution was pumped from the reservoir to the packed column by means of a peristaltic pump under optimized flow rates. The preconcentrated metal ions were desorbed by xpassing optimum volumes of suitable eluant and ions analyzed. Trace concentrations of UO2þ 2 extracted from synthetic and real samples were estimated by steady state spectrofluorimetric method, using 1 M H3PO4 [14]. The various column parameters required for reproducible and quantitative sensing are listed in Table 1, with their optimized values. 3. Results and discussion

2.4.2. Dynamic method One gram of grafted beads was slurry packed (bed volume – 8.5 mL) in an EXC column of dimension (16 · 0.4 cm). The packed column was initially buffered and then washed with 15 mL bed volumes

Fig. 2a.

13

3.1. Characterization of TEHMA and the grafted EHCOP polymer The 1H-NMR spectra of TEHMA in CDCl3, showed a sharp singlet at 5.6 (2H, s), which corresponds to the active methylene protons. The branched long-chain group showed resonance signals at 3.3–3.1 ppm (8H, m), 2.7 ppm (4H, m), 1.4 ppm (32H, m) and 0.9 ppm (24H, m). In the 13 C-NMR spectra, a singlet at 54.9 ppm corresponds to the methylene carbon sandwiched

C-CPMAS NMR spectra of AXAD-16-EHCOP grafted polymer.

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between the two amide carbonyl groups, as a result of strong deshielding effect. The branched longchain alkyl carbons appeared between 35 ppm and 20 ppm and the two carbonyl carbons appeared at 166.1 ppm and 164.3 ppm. The IR spectra of TEHMA showed a prominent band at 1653.7 cm1 corresponding to amide carbonyl stretching vibration. The purity of the product was also tested by CHN elemental analysis, with an experiment data of %C 76.2, %H 12.9, and %N 5.0, with respect to the theoretical value of %C 76.4, %H 12.7, and %N 5.1. The 13C-CPMAS NMR spectra of the grafted EHCOP resin shows a broad resonance signal at 199.0 ppm corresponding to the carbonyl groups, as shown in Fig. 2a. Also, the resonance signals for the aliphatic side chains were enhanced and

Fig. 2b.

shifted from 34.5 ppm to 41.0 ppm by the longchain alkyl groups. The resonance signal for the lateral –CH3group was observed at 13.1 ppm. The corresponding spectra for the non-functionalized AXAD-16 polymer, is shown in Fig. 2b. The presence of P=O moiety in the grafted polymer was inferred from 31P solid-state NMR spectra, whose resonance signal was observed as a doublet at 12.9 ppm and 14.9 ppm. The FT-NIR spectra for each stage of grafting were confirmed by their corresponding stretching frequencies, which are tabulated in Table 2. The CHNPS elemental analysis data observed for each stage of grafting are listed in Table 2. On comparing the experimental data with the theoretical value, the extent of ligand grafting to the polymeric support was estimated to be >92%.

13

C-CPMAS NMR spectra of non-functionalized AXAD-16 polymer.

Table 2 Characterization during various stages of grafting process Grafting stage

FT-NIR spectral data (cm1)

CHNPS elemental data

AXAD-16 AXAD-16-PHOH

–C@C– (1610.2) P–H (2314.1), P–OH (1044.5), P@O (1157.5), O–H (3435.5) O–H (3440.6), P–OH (1042.9), C@O (1705.3), CH2–Cl (689.7) C@O (1659.3 b), P–OH (1043.8), P@O (1150.1), O–H (3440.2)

C 91.74 (92.31): H 7.86 (7.69) C 57.41 (57.45): H 5.21 (5.42): P 18.31 (18.27)

AXAD-16-chlorophos AXAD-16-phos-mal

C 52.69 (51.48): H 4.87 (4.67): P 12.10 (11.87) C 73.11 (71.20): H 10.44 (10.56): N 1.43 (1.79) P 7.69(7.96)

The values given in parenthesis under CHNPS elemental data are the corresponding theoretical values, b-broad.

M.A. Maheswari, M.S. Subramanian / Reactive & Functional Polymers 66 (2006) 1452–1461

3.2. Static equilibration studies 3.2.1. Analyte extraction from high mineral acid media The role played by solution acidity during metal ion decontamination was studied in the normal range of HNO3 and HCl acidity, employed during nuclear reprocessing and digestion of environmental samples. To study the extractability of the lantha4+ nide ions along with the actinides UO2þ 2 and Th 3+ 3+ by the polymer, La and Nd ions were selected and studied. For this study, the individual metal 4+ 3+ 3+ ion solutions (UO2þ 2 , Th , La , and Nd ) 1 (40 mL, 10 lg mL ) were individually equilibrated with 50 mg resin beads for 1 h at 200 rpm. Then after, the aqueous phase concentrations were analyzed spectrophotometrically and the corresponding analyte distribution ratios were plotted as a function of HNO3 and HCl concentrations, as shown in Fig. 3. From the figure, it is evident that the synthesized polymer material exhibits greater sensitivity and extracting ability (D  104) for UO2þ 2 ions and Th4+ (D > 103) at 3 M HNO3 and HCl media, which is attributed to the synergistic extractive behavior exhibited by the grafted polymer. But poor extraction profiles are observed for the investigated lanthanide ions (D < 200). This extraction profile proves the active involvement of the incorporated TEHMA moiety that has selective steric hindrance  towards neutral La3+ and Nd3+ (NO 3 &Cl ) species, thereby provoking the selectivity for the studied actinide ions over the lanthanides.

10000

1000 -1

D (mL g )

The polymer’s sensing ability and its rapidity for metal ion extraction, in aqueous media was determined by its water regaining capacity. This was performed by equilibrating 0.1 g of the grafted resin beads with distilled water for 2 h and then compared with the non-grafted AXAD-16 beads. The equilibrated resin beads were filtered, air-dried and subjected to TGA measurements. At 110 C, the observed weight loss due to percolated water molecules in the polymer pores was 17.5% for the grafted resin, with respect to 1.1% for AXAD-16 resins. This is due to the presence of the grafted phosphinic acid group, which enhances the hydrophilic character of the polymeric resin. Hence, the hydrophilic site will increase the accessibility of the metal ions to the chelating recognition sites by providing better surface contact with the aqueous phase, thereby enhancing the rapidity of metal ion preconcentration.

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HNO3 HCl U Th Nd La

100

1

2

3

4

5

6

-1

Con. of HNO3 & HCl (mol L )

Fig. 3. Extraction profile in mineral acid matrix.

The decreasing sensitivity and extractability in high acidity medium, is due to the formation of non-extractable metal anionic complexes. This is more predominating in HCl medium, due to the existence of stable metal anionic chloro complexes. and Since, the resin is more selective for UO2þ 2 Th4+ ions, further studies are restricted to these 4+ ions. The complexation of UO2þ through 2 and Th phosphoryl and carbonyl oxygen’s was monitored by Far-IR spectral measurements, with a broad spectral band between 230 and 210 cm1. The sorption capacity values for these ions at 3 M HNO3 and HCl media, by batch equilibrating 0.05 g resin with metal ion solution (50 mL, 300 lg mL1) for 6 h and the observed metal sorption capacity values are listed in Table 1. 3.2.2. Extractability in low acidities and near neutral conditions The metal extractive behavior of the chelating ion-exchange resin under wide range of solution pH (1–6.5) was studied in terms of maximum metal sorption capacity. For this study, 0.05 g of the resin beads were equilibrated with a solution of excess concentration (50 mL, 500 lg mL1) of UO2þ and 2 Th4+ for 6 h. The sorption capacity values were plotted as a function of solution pH, as shown in Fig. 4. The maximum sorption capacity was observed at pHs 6–6.5 for UO2þ and pH 2.0 for 2 Th4+. Therefore, separation of UO2þ from Th4+ 2 can easily achieved at sample pH 6.5. It is also evident from Table 1 that the sorption capacity values observed at low acidic media were greater than the values observed at 3 M acidities. This is due to

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1

0 1

2

3

4

5

6

7

sample pH

Fig. 4. Effect of sample pH on analyte sorption.

parallel participation of both chelating and ionexchange site, under these conditions. However, with increasing solution acidity, the ion-exchange mechanism diminishes, besides the formation of non-extractable anionic metal complexes, resulting to a net decrease in the D values. But still, the observed sorption capacity values at 3 M acidities are superior when compared to the acidic extractants explored earlier. 3.2.3. Sequential elution profile The sequential elution methodology leading to the separation of the sorbed metal ions was investigated. To optimize this, 0.05 g of the resin beads were equilibrated with metal ion solutions (40 mL, 10 lg mL1) in both 3 M HNO3 and optimum pH media. From the knowledge gained from the uptake profile in acid matrices, the sorbed metal ions in the resin phase were eluted with mobile phases like HCl, HCl + NaCl mixtures and (NH4)2CO3 solutions. As expected, the sensor material exhibited different elution behaviors for the sorbed metal ions, where lanthanides showed a quantitative elution using 2 M HCl, Th4+ with 2 M HCL+2 M NaCl and UO2þ 2 ions only using 0.5 M (NH4)2CO3. Thus, the complete separation of lantha4+ nides, UO2þ can be achieved. 2 and Th 3.2.4. Sensor’s ion response as a function of time The time frame required for target ion sensing and preconcentration was studied at different time lapses and their exchange kinetics was expressed in terms of fractional attainment of equilibrium data (F), using the following expression,

Here, [MR]t/[MR]eq are the ratios of metal ion concentration in the resin phase at time ‘t’ to that at equilibrium. For this, 0.05 g of sensor beads was equilibrated under different time intervals in 10 lg mL1 of individual metal ion solutions maintained at 3 M HNO3. After equilibration, the residual metal ion concentration in the aqueous phase (1F) was analyzed and plotted as a function of time (Fig. 5). From the plot, it is evident that complete metal ion extraction was possible within 8–10 min of equilibration. At this stage, it is important to specify that the use of macroreticular porous polymer with large internal surface area was one of reasons for such faster exchange rates, besides the enhanced site accessibility by invoking dual and bifunctional mechanism. 3.2.5. Tolerance to co-existing electrolytes As NaNO3 and NaCl form the major matrix constituents during spent fuel dissolution and repro4+ cessing, the sensing profile for UO2þ 2 and Th , in their presence was studied. For this study, an increasing NaNO3 and NaCl concentration containing 10 lg mL1 (40 mL) metal ion solutions in 2 M acidity, was batch equilibrated with 50 mg of resin beads. It was observed that with increasing NaNO3content, the extraction efficiency was signif4+ icantly enhanced for UO2þ ions, due to 2 and Th the salting out effect associated with NO 3 anions.

0

10

U(VI) Th(VI)

-2

1-F

U(VI) Th(IV)

-1

Metal sorption capacities (mmol g )

F ¼ ½M R t =½M R eq

10

-4

10

0

3

6

9

12

15

Time (minutes)

Fig. 5. Phase exchange kinetics in 3 M HNO3 conditions.

M.A. Maheswari, M.S. Subramanian / Reactive & Functional Polymers 66 (2006) 1452–1461

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and Th4+, respectively (Table 1). These for UO2þ 2 results well corroborate with the batch kinetics experiments on the enhanced chelating site accessibility. Similarly, the mobile phase (eluant) flow rate was fixed at 0.5 mL min1 for the quantitative recovery of metal ions.

However, in NaCl medium, a negative trend was observed owing to the formation of more stable, non-extractable metal chloro anionic complexes in both cases. 3.2.6. Tolerance to foreign anions and cations It is a well known fact that foreign ions compete for the active sites along with the target ions and sometimes even resist the sensing process. Hence, the tolerance limit values for such species are normally recommended to determine ion-selectivity. This is defined as the maximum diverse ion concentration up to which 0% loss in the analyte signal can be observed. For this study, 1.25 lg mL1 concentrations of UO2þ and Th4+ in 40 mL volume 2 was equilibrated with 0.05 g resin, in the presence of individual matrix components. From the data tabulated in Table 3, it is clear that the grafted poly4+ mer is highly selective in sensing UO2þ ion 2 and Th from high saline matrices. It was also interesting to observe that <2% sorption was observed for transition metal ions and a D value of <200 for lanthanide ions. However, the co-extracted lanthanides could be separated from the actinides with 2 M HCl. Similar studies performed under pH conditions also showed encouraging results, with diverse ions that are normally present in environmental samples.

3.3.2. Sample breakthrough volume The grafted polymer’s ability in preconcentrating trace UO2þ and Th4+ ions from large sample 2 volumes was studied in terms of sample breakthrough volume. To study this parameter, various volumes (0.5–6 L) of individual actinide concentration (20 lg L1) were passed through the packed column in 3 M HNO3, at a flow rate of 13– 14 mL min1. The column was washed with 5 bed volumes of 3 M HNO3 and the selectively desorbed UO2þ and Th4+ ions were analyzed with 2 post-column reagents like Arsenazo-III and Thoron, respectively. A dynamic sample breakthrough 4+ of 5 L and 4 L were obtained for UO2þ 2 and Th , respectively, with an enrichment factor of 333 and 267 (Table 1). 3.3.3. Sequential elution profiles To investigate the sequential elution capability of the material, 1000 mL of metal ion mixture (UO2þ 2 , Th4+, La3+, and Nd3+) of 0.5 lg mL1 each in 2 M HNO3 medium through the packed extraction column. As observed from the static experiments, the co-extracted lanthanides were quantitatively eluted in the early stage using 2 M HCl, followed by the Th4+ elution using 2 M HCL + 2M NaCl mixture and finally, UO2þ could be leached with 0.5 M 2 (NH4)2CO3. This elution behavior of the polymer material is of great significance during practical applications of extracting pure fractions of the analyte material.

3.3. Column extraction and separation of ions 3.3.1. Effect of sample and eluant flow rate The influence of sample flow rate on the quantitative extraction of metal ions was studied by passing the actinide solutions (2000 mL, 0.5 lg mL1) in 3 M HNO3 medium through the extraction column. The flow rates were varied from 10 to 25 mL min1 using a peristaltic pump. It was surprising to observe that even at a flow rate of 16 mL min1 and 14 mL min1 quantitative sorption was achieved

Table 3 Limits of tolerance for diverse ions and electrolytes in low acidic conditions Metal ionsa

U(VI) Th(IV)

Tolerance limits for electrolytes (mol g1 resin) NaCl

KNO3

Na2SO4

Na3PO4

NaF

CH3COO

Ca2+

Mg2+

0.99 0.91

0.79 0.82

0.48 0.52

0.21 0.18

0.24 0.17

0.35 0.49

0.40 0.52

0.52 0.64

Tolerance limits for interfering metal ions (mmol g1 resin)

U(VI) Th(IV) a

Mn(II)

Co(II)

Cu(II)

Fe(III)

Ni(II)

Zn(II)

Pb(II)

Cd(II)

Zr(IV)

Bi(III)

0.19 0.26

0.26 0.27

0.34 0.34

0.14 0.19

0.37 0.29

0.35 0.34

0.25 0.24

0.12 0.17

0.10 0.13

0.41 0.26

4+ Amount of UO2þ – 50 lg (RSD < 3.6% for triplicate measurements). 2 , Th

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Table 4 Extraction of UO2þ 2 from seawater Sample

Source of the sample, year of sampling

Method

1 Concentration of UO2þ 2 (lg L )

RSDa (%)

Seawater - 1

Mahabalipuram, India September 2004 Chennai, India August 2004 Besant Nagar, Chennai, India October 2004

Direct SA Direct SA Direct SA

5.88 ± 0.53 5.76 ± 0.54 5.57 ± 0.51 5.51 ± 0.53 5.49 ± 0.52 5.40 ± 0.52

3.6 3.8 3.7 3.9 3.8 3.9

Seawater - 2 Seawater - 3 a

Data obtained on triplicate measurements.

3.3.4. Limit of quantification (LOQ): sensitivity studies The polymer’s sensitivity is normally reciprocated in terms of LOQ values, which defines the lowest level of ion-sensing that can be achieved with the developed method. This was determined individually 4+ by passing 5–25 lg L1 of UO2þ ions in 2 and Th 3 M HNO3 through the preconditioned resin bed. The sorbed metal ions were desorbed with 10 mL of appropriate eluants at a flow rate of 0.5 mL min1. Ultra trace UO2þ 2 ions were estimated spectrofluorimetrically and Th4+ ions by thoron method. The LOQ values were found to be 5 lg L1 1 for UO2þ for Th4+ ions (Table 1). 2 and 10 lg L 3.3.5. Column bed reusability The reusability of the resin matrix was studied by passing 1000 lg L1 metal ion solution in 3 M HNO3, through the preconditioned resin bed. The reproducibility in analyte recovery was checked using the same resin bed column up to 15 cycles and the values obtained were within 3.7% RSD. This reflects the durability and reusability nature of the grafted polymer even under extreme experimental conditions. 4. Applications From the optimized data obtained from both static and dynamic studies, the grafted polymer was tested with various synthetic and real samples. 4+ 4.1. Separation of UO2þ from simulated 2 and Th acidic mixture

To study the practical usage of the grafted polymer, a 5 L synthetic mixture containing 100 lg L1 of UO2þ and Th4+ ions, mimicking a low level 2 nuclear spent fuel system (low level wastes – LLW) was prepared [14]. The extracted analytes were sequentially eluted from the packed column

in 3 M HNO3, with a recovery value of >99% and an RSD 3.8%, based on triplicate trials. LLW contain negligible amounts of short-lived radioactive isotopes (specific activity around 107 l Ci L1), which is tolerable under normal operating conditions [15]. Such low radiation levels cannot produce any adverse effects on the performance of the developed polymer even when it is subjected to prolonged usage. In order to investigate the radiolytic stability of the developed polymer, the metal sorption capacity was tested using the same packed material (batch method) after the above investigation. The polymer showed good repeatability in sorption capacity values, thus proving its high durability towards such low level radiations. 4.2. Recovery of UO2þ 2 from synthetic and real seawater A 5 L synthetic seawater sample spiked with 50 lg UO2þ 2 was prepared as per the standard literature composition [6]. The synthetic mixture was passed through the preconditioned resin bed at a flow rate of 12 mL min1 and the sorbed ions were eluted and estimated spectrofluorimetrically, with a recovery value of > 99.2% and an RSD value of 3.9%. Based on this, studies were further extended to real samples wherein, 5 L of the seawater samples was filtered through a membrane filter (0.45 mm) and was passed as such through the chromatographic column. The sorbed UO2þ 2 ions were eluted and analyzed spectrofluorimetrically and the results are tabulated in Table 4. The reliability of the analysis was further confirmed by standard addition method, with 20 lg of UO2þ 2 ions spiked to the real samples. 5. Conclusions The newly developed polymer based sensor material was successful in selectively sensing ultra trace

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concentrations of UO2þ and Th4+ from low level 2 acidic nuclear spent solutions. The recovery of uranium from seawater is a highlighting feature with this grafter polymer, as this process is under active research by nuclear chemist globally. The superior metal sorption capacity and high enrichment factor of the polymer ion-sensor has proved its utility for a wide range of commercial applications from trace analysis to large scale recycling process. With the sequential separation of analytes by gradient elution, high purity of UO2þ and Th4+ ions can be 2 recovered in quantitative amounts from complex fission matrix. The new material method is rapid in sensing metal ions, which is vital for any analytical procedure. References [1] R. Niessner, Chemical sensors for environmental analysis, Trends Anal. Chem. 10 (1991) 310–316. [2] J. Shi, Y. Zhu, X. Zhang, W.R.G. Baeyens, A.M. GarciaCampana, Recent developments in nanomaterial optical sensors, Trends Anal. Chem. 23 (2004) 351–360. [3] B. Kuswandi, Optical chemical sensors for the determination of heavy metal ions: a mini review, J. ILMU DASAR 1 (2000) 18–29.

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[4] A.P. Gilman, D.C. Villeneuve, V.E. Secours, A.P. Yagminas, B.L. Tracy, J.M. Quinn, V.E. Valli, R.J. Willes, M.A. Moss, Toxicol. Sci. 41 (1998) 117. [5] A. Suresh, C.V.S.B. Rao, R. Deivanayaki, T.G. Srinivasan, P.R.V. Rao, Solvent Extr. Ion Exc. 21 (2003) 449. [6] H. Whitefield, D. Jagnee, Marine Electrochemistry: A Practical Introduction, A Wiley-Interscience Publication, North Ireland, 1956, 35. [7] C. Musikas, Inorg. Chim. Acta 140 (1987) 197. [8] N. Condamines, C. Musikas, Solvent Extr. Ion Exc. 10 (1992) 69. [9] E.A. Mowafy, H.F. Aly, Solvent Extr. Ion Exc. 20 (2003) 177. [10] J.S. Fritz, Analytical Solid Phase Extraction, Wiley-VCH, New York, 1999, 39. [11] E.K. GeckelerAdvanced Functional Molecules and Polymers, vol. 4, Gordon and Breach Science publishers, Singapore, 2001, 323. [12] P.R. Vasudeva Rao, S.K. Patil, J. Radioanal. Chem. 42 (1978) 399. [13] A.N. Oscar, W.W. Bernard, A. Michael, Anal. Chem. 30 (1958) 1182. [14] S. Maji, K. Sundarajan, G. Hemamalini, K.S. Viswanathan, Fluorimetric Estimation of Uranium: Applications in Nuclear Technology IGC, Indra Gandhi Centre for Atomic Research, India, 2001, p. 228. [15] M. Mishra, Membrane processes in waste treatment, in: Proceedings of the National Symposium on Management of Radioactive and Toxic Waste, Kalpakkam, India (1993), p. 307.