Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 328–332

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Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

journal homepage: www.elsevier .com/locate /saa

Application of hollow cylindrical wheat stem for electromembraneextraction of thorium in water samples

http://dx.doi.org/10.1016/j.saa.2014.08.1031386-1425/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +98 542 2232961.E-mail address: m_khajeh@uoz.ac.ir (M. Khajeh).

Mostafa Khajeh a,⇑, Stig Pedersen-Bjergaard b, Afsaneh Barkhordar a, Mousa Bohlooli c

a Department of Chemistry, University of Zabol, P.O. Box 98615-538, Zabol, Iranb School of Pharmacy, University of Oslo, Oslo, Norwayc Department of Biology, University of Zabol, P.O. Box 98615-538, Zabol, Iran

h i g h l i g h t s

� Thorium is a radio element whichoften uses as fuel for nuclear reactors.� The electromembrane extraction was

done for cleanup of the watersamples.� Wheat stem was used in the

electromembrane extraction insteadof hollow fiber.� Determination of the thorium was

successfully performed in the watersamples.

g r a p h i c a l a b s t r a c t

Schematic diagram of EME setup.

a r t i c l e i n f o

Article history:Received 4 July 2014Received in revised form 7 August 2014Accepted 24 August 2014Available online 3 September 2014

Keywords:ThoriumElectromembrane extractionWheat stemWater samples

a b s t r a c t

In this study, wheat stem was used for electromembrane extraction (EME) for the first time. The EMEtechnique involved the use of a wheat stem whose channel was filled with 3 M HCl, immersed in10 mL of an aqueous sample solution. Thorium migrated from aqueous samples, through a thin layerof 1-octanol and 5%v/v Di-(2-ethylhexyl) phosphate (DEHP) immobilized in the pores of a porous stem,and into an acceptor phase solution present inside the lumen of the stem. The pH of donor and acceptorphases, extraction time, voltage, and stirring speed were optimized. At the optimum conditions, anenrichment factor of 50 and a limit of detection of 0.29 ng mL�1 was obtained for thorium. The developedprocedure was then applied to the extraction and determination of thorium in water samples and in ref-erence material.

� 2014 Elsevier B.V. All rights reserved.

Introduction

Thorium is a radio element which is often used as a fuel fornuclear reactors besides its use in industrial applications. Thoriumoccurs in solutions exclusively in the positive tetravalent oxidation

state. The main sources of it in nature are plants, sand, soil, rocksand water. Normally, very little amounts of thorium from rivers,oceans, and lakes are accumulated into fish or seafood. However,near an uncontrolled hazardous waste site thorium may have acutetoxicological influences for human. The continuous exposure tothorium may cause an increased chance of developing cancer ofthe pancreas, lung or bone, and changes in genetic material ofbody cells [1–5]. Thus, determination of thorium is a significant

M. Khajeh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 328–332 329

analytical process to monitor and control its concentrations. Spec-trophotometric methods are widely used because of their simplic-ity, time efficiency, low costs and wide applications [6–8].

In 2006, Pedersen-Bjergaard et al. introduced a novel microex-traction technique which was called electromembrane extraction(EME) [9]. This technique has been demonstrated for extractionof several acidic or basic compounds [10–12]. In this technique, ahollow fiber made of porous polypropylene where the porous wallis impregnated with organic solvent, is used as a supported liquidmembrane (SLM). An aqueous solution is filled inside the lumen ofthe fiber serving as the acceptor solution, and the fiber is placedinto the (aqueous) sample. The driving force in EME is an appliedelectrical potential over the SLM. One of the electrodes is placedin the acceptor solution inside the lumen of the fiber, while theother electrode is located in the sample solution [9]. Charged com-pounds in the solution are extracted across the SLM towards theelectrode of opposite polarity in the acceptor solution. Basheeret al. [11] have studied extraction of lead ions by electromembraneisolation. Hosseiny Davarani et al. [13] reported a selective electro-membrane extraction of uranium (VI) prior to its fluorometricdetermination in water samples. Kuban et al. [14] studied electro-membrane extraction of heavy metal cations followed by capillaryelectrophoresis.

Wheat is the main crop all over the world. Every year largeamount of wheat stems is produced. Usually, they are burned inthe field that causes serious atmospheric contamination and wasteof resource. Wheat has erect, hollow cylindrical stems called cul-ms. It is made up of a series of hollow cylindrical internodes joinedtogether by means of solid joints or nodes. In this study, wheatstem has been utilized as tubes instead of hollow fiber for theextraction of thorium from water samples for the first time. Tothe best of our knowledge, no report has yet appeared on EME ofthorium.

The aims of this study is to use of culm for EME of thorium fromwater samples. Finally, UV–Vis spectrophotometry was employedfor analysis of the acceptor solutions.

Materials and methods

Reagents and samples

1-Octanol was purchased from Merck (Darmstadt, Germany).Di-(2-ethylhexyl) phosphate (DEHP) was purchased from Fluka(Buchs, Switzerland). Arsenazo III was obtained from Lobachemie(Mumbai, India). Reagent grade Th(NO3)4 5H2O and nitrate or chlo-ride salts of other cations were obtained from Merck. A stock stan-dard solution of thorium (1000 mg L�1) was prepared in doubledistilled water. Dilutions of stock solutions in water were used tooptimize the experimental conditions.

Fig. 1. SEM image of culm.

In this study, wheat stem has been utilized as tubes instead ofhollow fiber for the extraction of thorium from water samples.Wheat stem was obtained from a local source (Zobol, Iran).Thereby, the extraction process was carried out using a wheat stemcalls culm. The representative scanning electron microscopy (SEM)image of culm is shown in Fig. 1. Culm has an internal diameter ofapproximately 1.5 mm (±0.15 mm), wall thickness of 160 lm(±20 lm) and 234 nm (±40 nm) wall pores size. EME was alsoaccomplished with polypropylene hollow fibers (PPQ3/2 polypro-pylene hollow fiber from Membrana (Wuppertal, Germany) with1.2 mm internal diameter, 200 lm wall thickness, and 0.2 lm poresize) for comparison.

Equipment for EME

The equipment used for the extraction method is shown inFig. 2. The electrodes used were platinum wires with diametersof 0.2 mm obtained from Pars Pelatine (Tehran, Iran). The elec-trodes were coupled to a power supply model 8760T3 with a pro-grammable voltage in the range of 0–600 V from Paya Pajoohesh

Fig. 2. A schematic illustration of (a) the extraction process and (b) diagram of EMEsetup.

40

60

80

100

ER%

Fig. 3. Effect of SLM composition on extraction recovery. Extraction conditionswere: voltage of 100 V; extraction time of 30 min; stirring rate of 700 rpm.

50

60

70

80

90

100

40 60 80 100 120 140

ER%

Voltage (V)

Fig. 4. Effect of voltage on extraction recovery. Other extraction conditions were:extraction time of 30 min; stirring rate of 700 rpm.

330 M. Khajeh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 328–332

Pars (Tehran, Iran). During the extraction, the EME unit was stirredby a heater–magnetic stirrer model 3001 from Heidolph (Kelheim,Germany).

Apparatus

The measurements were carried out with a UV–Vis (UV-2100RAY Leigh, Beijing, China) by monitoring the absorbance at maxi-mum absorbance (660 nm). All experiments were performed intriplicate and the mean values were used for optimization. ThepH was determined with a model 630 Metrohm pH meter with(Metrohm, Herisau, Switzerland) combined glass-calomelelectrode.

Procedure

A 3.0 cm piece of culm was cut out and then, it was treated with5 mL HCl (3.0 M) and 5 mL methanol. One side of the culm wasconnected to a medical needle tip as a guiding tube and the otherside was closed by natural node. The culm was immersed inorganic solutions for 10 s and then 20.0 lL HCl (3.0 M) as acceptorsolution was poured into the lumen of the culm in which the tho-rium ions were trapped in the form of Th4+ ions [15]. A piece ofplatinum wire as the electrode (negative electrode) was placed intothe culm. For adjusted of pH of the donor phase to 0.5 a concen-trated HCl was used. Because Th(OH)4 [15] is formed in the solu-tion at pHs higher than 3.5, thereby; the acidification of thesample is required. After that, 10 mL of the sample solution wastransferred to a glass vial. A piece a platinum wire was used as apositive electrode. Under the applying the voltage (90 V), thoriumions migrated from the sample solution towards the cathode incontact with the acceptor phase. The sample was agitated at700 rpm for 30.0 min to facilitate extraction. After 30.0 min, thevoltage was turned off; the acceptor solution was collected andthen diluted with 0.9 mL of 3.0 M arsenazo III. Finally, the preparedsolution was analysed by UV–Vis spectrophotometry.

Enrichment factor and extraction recovery

The enrichment factor (EF) was defined as follow:

EF ¼ Ca; finalCs; initial

ð1Þ

where Ca,final is the final concentration in the acceptor phase, Cs,initial

is the initial concentration of analyte in the sample solution. Theextraction recovery (ER%) was defined as the percentage of thenumber of mole of analyte originally present in the sample (ns,initial),which was extracted to the acceptor phase (na,final). ER% is definedas follow:

ER% ¼ na; finalns; initial

� 100 ¼ Ca; finalVa

Cs; initialVs� 100 ð2Þ

ER% ¼ Va

Vs

� �EF� 100 ð3Þ

where Va and Vs represent the volumes of acceptor phase and sam-ple solution, respectively.

Results and discussion

Optimization of procedure

Effect of solventAccording to earlier findings, the chemical nature of the SLM

is highly critical for the success of electromembrane extraction[13,16]. 1-octanol alone or in combination with DEHP was

investigated as supported liquid membrane in this work. The ideaof using 1-octanol and DEHP was resulting from the fact that thereare many researches on the application of them for extraction ofdifferent target compounds [13,16]. It has been found that additionof hydrophobic ion-par reagents to SLM would enable better phasetransfer and electrokinetic migration of analyte [16]. Additional ofDEHP showed good enhancement in extraction efficiency of ana-lyte. A solution containing 2.0 mg L�1 of thorium was used forthe optimization of the SLM composition according to the extrac-tion recovery. The extraction time and voltage were 30 min and100 V, respectively. The results showed (Fig. 3) that a mixture of1-octanol and 5%v/v DEHP provided the highest extraction recov-ery, while, 15%v/v DEHP caused a decrease in thorium extractabil-ity. Usually, a decrease of target compound extractability withaddition of ion pair reagents may be because of a decrease in theelectrical resistance of the SLM and an increase in the current leveland bubble formation, or because of strong interaction of the ion-pair [9,16].

Effect of extraction voltageA series of experiments with different extraction voltages of

between 50 and 120 V were conducted to determine the mostappropriate voltage. The results are summarized in Fig. 4. The high-est extraction recovery was obtained at 90 V at 30 min extractiontime. Beyond this voltage, a decrease in the extraction recoverywas observed. Some authors have suggested that Joule heating isthe main limiting factor that lowers the enrichment factor andthe extraction recovery of EME systems above the optimal voltage[11].

60

70

80

90

100

0 10 20 30 40 50

ER%

�me (min)

Fig. 5. Effect of extraction time on extraction recovery. Other extraction conditionswere: applied voltage of 90 V; stirring rate of 700 rpm.

M. Khajeh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 328–332 331

Effect of extraction timeElectromembrane extraction is an equilibrium distribution pro-

cess [11,17]. To achieve equilibrium, extraction was performed forbetween 10 and 40 min. As seen from Fig. 5, the amount of thoriumextracted by EME increased with increasing extraction time from10 to 30 min and was decreased after that. These results showedthat EME attained equilibrium at 30 min. The decrease extractionrecovery after 30 min was perhaps due to the saturation of the ana-lyte in the acceptor phase. This phenomenon resulted in back-dif-fusion based on passive transport of the analyte from the acceptorsolution to the sample solution; this observation has previouslybeen reported [11]. Based on this result, 30 min was selected asthe optimum extraction time.

Effect of pH of donor and acceptor phaseThe pH values of the donor and acceptor phases can determine

the ion balance in the system. It has been shown that the total ionicconcentration on the donor phase to that on the acceptor phaseimpresses the flux over the membrane [14]. To investigate theeffect of theses parameters, pH of the donor phase was changedin the range of 0.5–3.0, and 0.1–3 M HCl was used as the acceptorphase. Maximum amounts of thorium was extracted when pH ofthe donor phase was adjusted at 0.5, and 3.0 M HCl was used asacceptor phase. The acidification of the samples is required duoto at pH 3.5 or lower, thorium ions were trapped in the form ofTh4+ ions [14]. 3.0 M HCl also provided high selectivity for theinteraction of thorium with arsenazo III [17].

Table 2Comparison of the present procedure with other related methods for determination ofthorium.

Effect of sample stirringThe volume ratio of donor to acceptor phase in EME has varied

between several tens to several hundreds in earlier EME work, andto ensure constant contact of analyte ions in the donor solutionwith the SLM surface, stirring/or agitation is usually performed[14]. In this study, stirring of the donor solution was examinedbetween 100 and 1000 rpm at 30 min extraction time. Obviously,stirring is essential for proper mixing of the donor phase, and anincrease in extraction recovery was observed in the range of100–700 rpm. At stirring rates above 700 rpm recoveries remainedconstant. Based on this result, 700 rpm was selected as theoptimum stirring rate.

Table 1Comparison of EME with culm and with polypropylene membrane (n = 6).

Membrane ER% RSD%

Polypropylene 93.2 2.9Culm 1 92.7 2.7Culm 2 91.8 2.1Culm 3 93.1 2.6

Comparison of EME with culm and polypropylene hollow fibersUnder optimized conditions (as discussed above), EME using

Three culm was compared with EME based on conventional hollowfibers made of polypropylene (Table 1). As seen from the data, bothextraction recoveries and relative standard deviations based on sixreplicate experiments between three culm and hollow fiber werecomparable. Therefore, culm could well be applied as tubes insteadof hollow fiber for the extraction of thorium from water samples.This experiment further supported that EME can be performedusing culm as the support for the organic solvent. The compres-sions of three culms showed the extraction efficiency and relativestandard deviations are comparable.

Effect of coexisting ionsThe interferences can mostly be attributed to the preconcentra-

tion step. In order to demonstrate the selectivity of the developedEME technique for the determination of thorium, the effect of alkaliand alkaline earth metals and several heavy metals that are com-mon elements in environmental water samples have been investi-gated. Interferences studies were performed in the presence ofconstant concentration of thorium (0.02 mg L�1) and different con-centration of foreign species. Among the interfering ions tested, Li+,Na+, K+, Mg2+, Ba2+ and Ca2+ (1 mg L�1; Ion/Th (w/w%) = 50) werefound not to interfere with the thorium determination. The resultsindicated that the alkaline ions had no significant effect on extrac-tion of thorium at the studied concentration. Other interfering ionstested, Pb2+, Ni2+, Cd2+, Fe3+, Zn2+, Zr4+, Ce4+, UO2

2+ and Cu2+

(0.02 mg L�1; Ion/Th (w/w%) = 1) were found not to interfere withthe thorium determination.

Evaluation of method performanceTo validate the developed technique, linearity, coefficient of

determination, detection limit, and enrichment factor using spikedsolution of sample were tested under the optimum conditions ofthis method. Linearity for thorium was obtained in the concentra-tion range of 0.01–2.0 lg mL�1, with the coefficient of determina-tion (r2) = 0.9993. The LOD obtained from CLOD = 3 (Sd)blank/m was0.29 ng mL�1 where Sd is the standard deviation of eight consecu-tive measurements of the blank and m is the slope of calibrationcurve. The enrichment factor was obtained with the EME techniquebefore the final dilution step (according to Eq. (1)), and it was mea-sured to 50. The relative standard deviation (RSD%) of five replicatedeterminations was less than 3.9% (n = 5, C = 0.05 mg L�1), andindicated that this method has good precision for the analysis oftrace thorium in water samples.

The investigation of figure of merit (Table 2) indicated that theLOD of this procedure (0.29 lg L�1) was lower than the reportedLODs for other methods such as solid phase extraction with UV–Vis detection (0.43 lg L�1) [18], mesoporous silica with inductivelycoupled plasma optical emission spectrometry detection(0.3 lg L�1) [19] and ion imprinted polymers with inductively cou-pled plasma optical emission spectrometry detection (0.51 lg L�1)[20]. There is a report based on solid phase extraction-ICP–MS [20]with LOD of 0.0045 lg L�1 that it is lower than the LOD of this

Method Apparatus LOD (lg L�1) Reference

Solid phase extraction UV–Vis 0.43 [18]Mesoporous silica ICP–OESa 0.3 [19]Ion imprinted polymer ICP–OES 0.51 [20]Solid phase extraction ICP–MSb 0.0045 [21]Electromembrane extraction UV–Vis 0.29 This method

a Inductively coupled plasma optical emission spectrometry.b Inductively coupled plasma-mass spectrometry.

Table 3Determination of the thorium in water samples (n = 3).

Sample Thorium content (lg L�1) ER%

Added Found (±RSD%)a

Sample 1 20 19.1 (±2.4) 95.5100 92.5 (±3.5) 92.5200 187.5 (±2.9) 93.8

Sample 2 20 19.3 (±2.1) 96.5100 89.8 (±2.7) 89.8200 181.5 (±3.8) 90.8

Sample 3 20 18.8 (±3.1) 94.0100 91.5(±5.2) 91.5200 185.7(±3.5) 92.9

Waste water 20 19.2 (±2.9) 96.0100 93.4(±3.1) 93.4200 185.8(±3.2) 93.0

a Relative standard deviation.

Table 4Thorium(IV) level of reference material.

Certified value (lg g�1) Measured value (lg g�1)a

13.0 12.56 ± 2.80

a Mean of three replicate extractions.

332 M. Khajeh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 328–332

method. Usually, it seems that ICP–MS technique provide lowestLODs for determination of thorium. However, the culm-EME-UV–Vis method is very simple and inexpensive in comparison withICP–MS. Additionally, the optimized method in this work had ahigher enrichment factor (EF = 50) than the ICP–MS-based work(EF = 30) [21].

Real sample analysisThe applicability of this method for separation and preconcen-

tration of thorium was tested using three water samples. Thesewater samples were collected from Chah-e Nimeh (Zabol, Iran).In these real samples thorium was not detected, thereby; spikedwith various concentration of Th (Table 3). The pHs of the real sam-ples was adjusted to 0.5 and 10 mL of the solutions without filtra-tion were transferred to the glass vial and EME was carried out asdescribed. Table 3 shows that the results of each real samples. Thevalidation of this procedure was carried out by the analysis ofMontana I Soil certified reference material (CRM) (NIST-SRM2710). 0.2 g of it was digested according to a published method[21] and diluted to 100 mL with deionized water. The observedand certified values for reference material were given in Table 4.The statistical comparison by t-test showed no significant differ-ence between the certificated value and the experimental result.

Conclusion

In this study, for the first time the wheat stem (culm) was usedinstead of hollow fibers in EME for the determination of thorium inwater samples. This procedure provided efficient sample cleanup

and was combined directly with a UV–Vis detection method forthe determination of thorium in water samples. Also, the investiga-tion of figure of merit indicated that the LOD of this procedure(0.29 lg L�1) was lower than the reported LODs for other methods.

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