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Electromembrane extraction and spectrophotometric determination of As(V) in water samples
Food Chemistry 212 (2016) 65–71
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Analytical Methods
Electromembrane extraction and spectrophotometric determination of As(V) in water samples Mohammad Ali Kamyabi ⇑, Ali Aghaei Department of Chemistry, Faculty of Science, University of Zanjan, 45371-38791 Zanjan, Iran
a r t i c l e
i n f o
Article history: Received 16 November 2015 Received in revised form 17 May 2016 Accepted 23 May 2016 Available online 24 May 2016 Keywords: As(V) Electromembrane extraction Molybdenum blue complex Water samples
a b s t r a c t In this study, for the first time electromembrane extraction (EME) was used as a highly efficient sample pre-treatment method for the UV–VIS spectrophotometric determination of As(V) in water samples. The influences of experimental parameters during EME were investigated and optimized using one-variableat-a-time methodology as follows: organic solvent: 1-octanol + 2.5% (V/V) di-(2-ethylhexyl) phosphate, applied voltage: 70 V, extraction time: 15 min, pH of acceptor: 13, stirring rate: 750 rpm. The method allowed the determination of As(V) in the range of 5–300 ng mL1. The relative standard deviation was found to be within the range of 3.4–7.6%. The limit of detection, corresponding to a signal to noise ratio of three, was 1.5 ng mL1. The proposed method was finally applied to the determination of As(V) in water samples and relative recoveries ranging from 95 to 102% were obtained. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Arsenic is an important environmental element because of its high toxicity at the level of parts per billion. It enters drinking water supplies from natural deposits in the earth or from agricultural and industrial practices. Industrial arsenic is mainly used as a wood preservative, but arsenic is also used in dyes, paints, drugs, soaps, and semi-conductors. Agricultural applications, mining, and smelting are other sources which may release arsenic in the environment. The contamination of drinking water and groundwater with arsenic has been reported in various regions of the world especially in developing countries (Meharg, 2005). Arsenic can occur in the environment in several oxidation states (3, 0, +3 and +5) but in natural waters is mostly found as inorganic forms of trivalent arsenite (As(III)) and pentavalent arsenate (As(V)) which the first form, As(III), is more toxic (Jain & Ali, 2000). On the other hand inorganic arsenic forms are more toxic than organic forms which may however occur where waters are significantly impacted by industrial pollution (Hughes, 2002; Jain & Ali, 2000). Redox potential and pH are the most important factors controlling arsenic speciation. Under oxidising conditions, H2AsO 4 is dominant at low pH (less than about pH 6.9), whilst at higher pH, HAsO2 4 becomes dominant (H3AsO4 and AsO3 4 may be present in extremely acidic and alkaline conditions respectively). Under reducing conditions at pH less than about pH 9.2, the uncharged arsenite ⇑ Corresponding author. E-mail address: [email protected] (M.A. Kamyabi). http://dx.doi.org/10.1016/j.foodchem.2016.05.139 0308-8146/Ó 2016 Elsevier Ltd. All rights reserved.
species H3AsO3 will predominate. As a result, As(V) is the predominant form of inorganic arsenic especially in surface water (Smedley & Kinniburgh, 2001). It was previously proved that in excessive amounts, arsenic causes gastrointestinal damage and cardiac damage. There are some evidences that arsenic is also carcinogenic (Abernathy, Thomas, & Calderon, 2003; Zackheim, 1968). According to World Health Organization (WHO) guideline on drinking water quality, the maximum permitted concentration of arsenic in drinking water is 10 lg L1(WHO, 2011) Therefore, simple, rapid, highly sensitive, and accurate methods required for the determination of trace amounts of arsenic especially in environmental samples. Various analytical methods have been used for arsenic determination, including titrimetry (Rao, Sarojini, & Gandikota, 1972), chemiluminescence (Li & Lee, 2005), atomic absorption spectrometry (Welz & Melcher, 1985), inductively coupled plasma atomic emission spectrometry (ICP-OES) (De Oliveira, McLaren, & Berman, 1983), inductively coupled plasma mass spectrometry (ICP-MS) (Feng, Chen, Tian, & Narasaki, 1998), and spectrophotometry (Hu, Lu, & Jing, 2012; Lenoble, Deluchat, Serpaud, & Bollinger, 2003; Morita & Kaneko, 2006; Revanasiddappa, Dayananda, & Kumar, 2007). Recently, Ma et al. reviewed the papers published since 2005 on techniques for the measurement of arsenic in water samples (Ma, Sengupta, Yuan, & Dasgupta, 2014). Except spectrophotometry, most of the other methods used in determination of arsenic require trained staff, expensive experimental setup and running cost. Therefore, many spectrophotometric methods have been developed as an alternative for the
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determination of arsenic instead of conventional techniques. Method of Molybdenum blue is one of the most popular spectrophotometry methods in determination of arsenic (DeSasa & Rogers, 1954). It is based on the reduction of the yellow molybdeoarsenic acid to intensely absorbing blue color which subsequently can be measured spectrophotometrically at 840 nm. It should be noted that among different forms of arsenic, only As (V) can form molybdenum blue. Hence, any other forms of arsenic such as As(III) must be first converted to As(V) in order to determine using molybdenum blue method. Although the method of molybdenum blue is poorer than that of atomic absorption in sensitivity and rapidity, it is characterized by a higher accuracy and precision. On the other hand it suffers from poor selectivity because of strong interference of Phosphate ion and different metal ions, such as copper, nickel, cobalt and zinc. Electromembrane extraction (EME) is a new microextraction method in which an electrical voltage is applied to enhance the transport of charged species across a hollow fiber membrane (Pedersen-Bjergaard & Rasmussen, 2006). Although electromembrane extraction has been mainly used for charged organic samples such as basic drugs, it was generalized to metal ions for the first time in 2008 (Basheer, Tan, & Lee, 2008). Some other successful applications of this method have been reported for determination of inorganic anions and heavy metals, so far (Chanthasakda, Nitiyanontakit, & Varanusupakul, 2016; Davarani, Moazami, Keshtkar, Banitaba, & Nojavan, 2013; Khajeh, Pedersen-Bjergaard, ˇ , Strieglerová, Gebauer, & Barkhordar, & Bohlooli, 2015; Kubán Bocˇek, 2011; Safari, Nojavan, Davarani, & Morteza-Najarian, 2013; Tan, Basheer, Ng, & Lee, 2012). According to our knowledge, to date, no electromembrane extraction has been reported for determination of arsenic. In this study, we aim to improve the selectivity of the conventional Molybdenum blue method by using Electromembrane extraction as a very efficient sample clean up and pre-concentration method and also to lower the limit of detection of the method down to a level that covers the WHO recommended value of 10 lg L1 arsenic in drinking water. We successfully applied the proposed method for determination of As(V) in different environmental samples. 2. Experimental 2.1. Chemicals Analytical grade standard solution of 1000 mg L–1 H3AsO4, 1octanol (C8H18O), ammonium heptamolybdate ((NH4)6Mo7O244H2O), sodium metabisulfite (Na2S2O5), potassium iodide (KI), iodine (I2), hydrazine sulphate (H6N2O4S) and sulfuric acid (H2SO4) were purchased from Merck (Darmstadt, Germany). Di(2-ethylhexyl) phosphate (DEHP) was purchased from Fluka (Buchs, Switzerland). weak base anion-exchange resin, Indion GS 3000 (type 1), was purchased from ion exchange India ltd (Delhi, India) with an effective particle size of 0.5–0.65 mm. All chemicals were analytical grade and were used as supplied without further purification. HPLC-grade water was purchased from Merck and was used to prepare all solutions.
Ammonium molybdate solution was prepared by dissolving 1.0 g of the solid in 10 mL water and 90 mL of 3.0 M sulphuric acid. Hydrazinium sulphate solution was prepared by dissolving 0.15 g hydrazinium sulphate in 100 mL water. Molybdenum reagent was prepared by mixing same volumes of ammonium molybdate solution and hydrazinium sulphate solution just before running the experiment. 2.3. Apparatus A UV–VIS spectrophotometer,(HACH-LANGE, model DR-2800, Germany), with a micro-volume glass cell (500 lL) and a macrovolume glass cell (3500 lL) both with a path length of 1.0 cm was used for absorbance measurements at 840 nm. The pH measurements were made with bench top pH meter, (WTW, model Inolab 730, Germany). 2.3.1. Equipments for EME The equipment used for the EME of As(V) is presented by Fig. 1. A 10 mL glass vial with an internal diameter of 2.0 cm and a height of 4.5 cm was used as the extraction vessel. A PP Q3/2 polypropylene hollow fiber (Membrana, Wuppertal, Germany) with an internal diameter of 0.60 mm, 200 lm wall thickness, and 0.2 lm pores was used to immobilize the supported liquid membrane (SLM) and holding the acceptor phase. The platinum electrodes with diameters of 0.25 mm, obtained from Pars Pelatine (Tehran,Iran), coupled to a power supply model Hy-30002E with a programmable voltage in the range of 0–300 V and with a current output in the range of 0–2 A from Hyelec (Zhejiang, China). During the extraction, the vessel was stirred by a magnetic hot plate stirrer from VELP Scientifica (Milan, Italy). 2.4. EME procedure To a 50 mL sample solution, 1 mL of 0.1 mol L–1 Bis-Tris buffer solution (pH 7.3) was added and the mixture was passed through an ion exchange resin column at a flow rate of 2 mL min1 in order to remove the phosphate ion which may be present in sample solution. (Indion GS 3000 (type 1)). Five milliliters of the sample solution pretreated as described above was transferred into the sample vial. A short piece of polypropylene hollow fiber (3.5 cm) dipped in 1-octanol with 2.5% (v/v) DEHP for 10 s and then the excess amount of solvent was removed with a medical wipe. 10 lL of a 100 mM sodium hydroxide solution, as acceptor solution, was injected into the lumen of the hollow fiber using a microsyringe.
2.2. Working solutions and reagents Working solutions of As(V) were daily prepared by diluting appropriate volumes of the standard solution of arsenic. Iodine reagent was prepared by dissolving 0.25 g of iodine in 100 mL water containing 0.4 g potassium iodide. Sodium metabisulfite solution was freshly prepared by dissolving 0.5 g of the solid reagent in 10 mL water. Sodium hydrogen carbonate solution was prepared by dissolving 4.2 g of the solid in 100 mL water.
Fig. 1. Equipments used for electromembrane extraction of As(V).
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One of the electrodes, the anode, was placed inside the lumen of the fiber and the lower end of the hollow fiber was sealed with small piece of aluminum foil (thickness of 280 lm). The other electrode, cathode, was directly placed inside the sample solution. The electrodes were then coupled to the power supply and the extraction vessel was placed on a magnetic stirrer with a stirring rate of 700 rpm. A voltage of 70 V was turned on and extraction was performed for 15 min. Under the applied voltage, the As(V) migrated from sample solution (donor phase) into acceptor phases through the SLM. After the extraction was completed, the acceptor solution was collected by a microsyringe and transferred directly to a microvial for further analysis by UV–VIS spectrophotometery.
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known amount of standard which was spiked into the real sample, respectively. 3. Results and discussion 3.1. Optimization of EME Different experimental variables may affect efficiency of EME, including extraction time, organic solvent, pH of sample solution, acceptor solution pH, stirring rate and applied voltage. The effect of each variable was investigated using one-variable-at-a-time methodology. All the experiments were performed in triplicate, and the mean of absorbance values was used to evaluate the influence of those variables on the efficiency of EME.
2.5. Spectrophotometric procedure The acceptor solution inside the microvial first neutralized with 10 lL of a 100 mM HCl solution. Then, it was mixed with 10 lL of iodine reagent, 5 lL of sodium hydrogen carbonate solution, 50 lL of molybdenum reagent, 5 lL of sodium metabisulfite solution, respectively. The mixture was heated at 95 °C for 10 min, cooled to laboratory temperature and diluted to 100 lL and 1000 lL using deionized water regarding initial concentration of As(V) in sample solution. Finally, the absorbance of the solution was measured at 840 nm against the reagent blank. It should be noted that the maximum concentration of As(V) that can be determined using molybdenum blue method is 1000 ng mL1 (DeSasa & Rogers, 1954). Since the final extract will be about 30-fold concentrated after EME, it must be diluted before spectrophotometery method. Hence, the sample solutions containing less than 30 ng mL1 As (V) were diluted to 100 lL and the samples containing higher concentration of arsenic (more than 30 ng mL1) diluted to 1000 lL as mentioned above. 2.6. Calculation of enrichment factor and extraction recovery and relative recovery The enrichment factor (EF) was defined as the ratio of the final concentration of As(V) in the acceptor phase (Cf,a) and its initial concentration (Ci,s) in the sample solution
EF ¼
C f ;a : C i;s
ð1Þ
where Cf,a was calculated from a calibration graph obtained by direct determination of standard solutions of arsenic (100– 900 ng mL1). Extraction recovery (ER) was defined as the number of moles of As(V) which extracted to the acceptor phase (nf,a) divided by the number of moles of As(V) originally found in the sample solution (ni,s).
nf ;a C f ;a V f ;a ER ¼ 100 ¼ 100 ni;s C t;s V t;s ER ¼
V f ;a EF 100: V i;s
ð2Þ
ð3Þ
where Vf,a and Vi,s are the volumes of acceptor phase and sample solution, respectively. Relative recovery (RR) was calculated by the following equation:
RR ¼
C found C real 100: C added
ð4Þ
where Cfound, Creal and Cadded are the concentrations of As(V) after addition of known amount of standard into the real sample, the concentration of As(V) in real sample, and the concentration of
3.1.1. Selection of organic solvent (membrane composition) Previous findings show that the chemical nature of organic solvent has a significant effect on transfer of analyte into the acceptor solution. Among different organic solvents used in electromembrane extractions of metal ions, the best results have been achieved for SLM consisting of alcohols (Chanthasakda et al., ˇ et al., 2011). Moreover, it was previously shown that 2016; Kubán addition of ion-pair reagents such as DEHP to SLM composition has a significant effect on efficiency of EME. For cationic analytes the transport mechanism is based on proton/analyte ion exchange between the carrier and the donor solution and subsequently on analyte ion/proton exchange between the carrier/analyte complex and the acceptor solution. For anionic compounds it seems that by addition of DEHP, the electrical resistance of the liquid membrane is decreased and hence the current level will be increased. As a ˇ et al., result, the extraction recovery can be improved (Kubán 2011). Therefore, 1-octanol and different mixtures of 1-octanol and DEHP were investigated as composition of SLM in this study. A solution containing 250 ng mL1 of As(V) was used for the optimization of the SLM composition according to the absorption intensity obtained for each composition. The initial estimate for extraction time and applied voltage was 10 min and 50 V, respectively during the optimization of SLM composition. As indicated by Fig. 2 the absorbance was increased by increasing the concentration of DEHP up to 2.5% v/v in 1-octanol and decreased for the concentrations of DEHP higher than 2.5%v/v. As it was expected the electric current during EME was increased by increasing DEHP concentration. Fig. 3 shows the dependency of electric current on DEHP concentration. Therefore, 1-octanol with 2.5%v/v DEHP was selected as the optimized composition of SLM and was used in all subsequent measurements. At this SLM composition, the electric current in the EME system was around 180 lA at an applied voltage of 50 V. 3.1.2. Effect of extraction time Different extraction time from 5 to 20 min was selected to determine the optimum value of extraction time and the results are shown in Fig. 4A. The absorbance was increased as the extraction time increased up to 15 min and gradually decreased for longer extraction times. The decrease in absorbance is probably due to passive back diffusion of target ions into SLM and donor solution, which has been previously reported (Basheer et al., 2008). Hence, an extraction time of 15 min was selected for following steps. 3.1.3. Effect of applied voltage Applied voltage is one of the most important factors that should be taken into account. In order to determine the best applied voltage, absorbance values were recorded as a function of voltage for voltages in the range of 20–100 V. The absorbance increased lin-
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Fig. 2. Effect of DEHP concentration on EME of As(V). EME conditions: liquid membrane, 1-octanol + 1DEHP; stirring rate, 750 rpm; voltage, 50 V; acceptor, 100 mM NaOH; donor, 250 ng mL1 As(V) in DI water; extraction time, 10 min.
early by increasing applied voltage up to 70 V and then started to decline by increasing the voltage up to 100 V. (see Fig. 4B). Some authors proposed that Joule heating is the main reason for low efficiency of EME systems over an optimized voltage. Moreover; no absorbance was observed when the extraction performed at 0 V. The optimum extraction voltage (70 V) was used in all subsequent EME experiments.
Fig. 3. Effect of DEHP concentration on electric current during EME of As(V). EME conditions was as that of in Fig. 2.
3.1.4. Effect of pH in the donor and acceptor solutions The pH of donor solution determines the existing form of the arsenic species in the solution. Therefore, pH of donor solution plays a decisive role in EME of As(V). The pka values of arsenic acid are 2.19, 6.94 and 11.5 respectively. Therefore, in strongly acidic solution, it exists as arsenic acid (H3AsO4), in weakly acidic condition exists as dihydrogen arsenate ion (H2AsO 4 ), in weakly basic condition exists as hydrogen arsenate ion (HAsO2 4 ) and in strongly basic condition exists as arsenate ion (AsO3 4 ). It is obvious that all
Fig. 4. Effect of Different working variables: extraction time (A), applied voltage (B), pH of acceptor (C) and stirring rate (D) on EME efficiency of As(V). EME conditions: liquid membrane, 1-octanol + 2.5% DEHP; donor, 250 ng mL1 As(V) in DI water.
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As(V) species are in ionic form in neutral water. Therefore, deionized water was selected as donor solution to facilitate the sample preparation in following measurements. To determine the effects of acceptor solution pH on EME efficiency, different pH values of the acceptor solutions were investigated using 100 mM and 1000 mM NaOH solutions. The pH adjustment was performed with a 1 M HNO3 solution. The absorbance values were recorded as a function of pH values of acceptor solution to find the optimum pH value. As indicated by Fig. 4.C the absorbance increased by increasing the pH of acceptor solution up to 12 and then leveled off between pH values of 13 and 14. Thus, pH of 13 was selected as optimum pH value and 100 mM NaOH was utilized as the acceptor solution during the rest of this study.
3.1.5. Effect of stirring speed The main effect of stirring is to reduce the thickness of the boundary layer at the interface between the sample solution and the SLM where mass transfer is only promoted by diffusion. To investigate the effect of this factor, the absorbance values were recorded as a function of stirring rate. The obtained results are illustrated in Fig. 4D. As can been seen, by increasing the stirring rate from 300 to 700 rpm, the absorbance increased but at higher stirring rates (700–1200 rpm) the absorbance decreased. Accordingly, the stirring rate of 700 rpm was selected as the optimum value for the subsequent experiments. The decrease in absorbance at higher rates (700–1200 rpm) was perhaps due to inhibition of passive diffusion of the analyte from the bulk solution into the SLM because of bubble formation (Pedersen-Bjergaard & Rasmussen, 2008).
3.1.6. Effect of interfering ions To investigate the influence of excipient ions on the EME of arsenic, various ions commonly found in natural waters such as 3 Mg2+, Ca2+, CO2 3 , PO4 and also those ions which strongly interfere in determination of arsenic by the molybdenum blue method such as Cu2+, Ni2+, Zn2+ and Co2+ were added to the sample solution with concentration ratios of 1:1, 1:10, 1:100 and 1:1000 As(V) to studied interferents. A deviation greater than ±10% from the optimized signals was considered as the criterion for interference. No significant interference observed for Mg2+, Ca2+, Cu2+, Co2+, Ni2+, Zn2+ and CO2 3 up to a concentration ratio of 1:100. This could be expected according to the cationic nature of the metal ions, which results in a migration in the opposite direction in an electric field. On the other hand, a great positive deviation of the original signal observed in the case of PO3 4 .This can be attributed to the fact that phosphate can react with ammonium molybdate in the same manner as arsenate (and with about the same sensitivity). Therefore, a sample pretreatment is needed to eliminate the effect of phosphate before running the proposed method in natural waters which usually contains phosphate. The best method of phosphate removal to our knowledge is using an ion exchange resin column. According to a previously published method to a 50 mL sample solution, 1 mL of 0.1 mol L– 1 Bis–Tris buffer solution (pH 7.3) was added. Then the mixture was passed through an ion exchange resin column (Indion GS 3000 (type 1)) to eliminate the phosphate (Morita & Kaneko, 2006). This solution was used in subsequent steps of the proposed method. The results showed that the interference of phosphate completely removed. Hence, in natural waters or water samples containing high concentration of phosphate, it is necessary to pass the sample solution first through an ion exchange resin column and then follow the proposed method.
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3.2. Calibration and analytical figures of merit Under optimized conditions, the absorbance values for various concentrations of As(V) after being extracted by means of EME method were recorded and plotted as shown in Fig. 4. The linear relationships were observed between the absorbance values with the concentration of arsenic over two different intervals in the range of 5–30 ng mL1 (Fig. 5A) and 30–300 ng mL1 (Fig. 5B).The extraction recovery (ER%) and enrichment factor (EF) were investigated in deionized water under optimized conditions of EME using three standard solution of arsenic (10, 20 and 50 ng mL1) .The enrichment factor was obtained with the EME technique after the final dilution step (according to Eq. (1)), and it was measured between 30 and 32.5 which correspond to extraction recoveries from 60 to 65%. The repeatability of the method was obtained by applying five replicate measurements for the mentioned standard solutions of arsenic with the same electrode for each concentration. The relative standard deviations (RSDs) were found to be within the range of 3.4–7.6%. LOD for EME-treated sample was found to be 1.5 ng mL1. It was estimated based on repetitive dilution of a standard sample solution with 10 ng mL1 of As(V); analytical signal for EME of a blank sample was always subtracted. Concentration of As(V) corresponding to signal-to-noise ratio of 3 was considered as LOD. In order to evaluate the resulting LOD of this method, the LOD of molybdenum blue method investigated alone and without any pretreatment. The obtained LOD of 50 ng mL1 showed that the EME pre-treatment has improved the LOD by about 1.5 orders of magnitude. Note, however, that even lower LODs can be achieved for trace metal analysis with other analytical techniques such as GF-AAS, ICP-OES, ICP-MS, albeit using expensive and complex instrumentation. However, a comparison of the proposed method with previously reported methods for the determination of As(V) (Table 1) showed that the proposed method along with its simplicity and easy operation has a low limit of detection, an acceptable linear dynamic range and better selectivity with an important emphasis on the enrichment factor as well as low cost of analysis that make this method more efficient for determination of arsenic. 3.3. Application to real samples The developed EME method was applied for the determination of As(V) in tap water, underground water, bovine gelatin powder and their spiked samples. Gelatin powder was obtained from Faravari Darooi Gelatin Halal Factory (Qazvin, Iran). All samples first passed through an ion exchange resin column then were sampled into the glass vial for EME. For gelatin sample it was first dissolved
Fig. 5. Calibration curves for determination of As(V) in the range of 5–30 ng mL1 (A) and 30–300 ng mL1 (B) in the optimum conditions.
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Table 1 Comparison of the proposed method with other analytical methods applied for the determination of As(V).
1 2 3 4 a b
Detection
Method
Detection limit (ng mL1)
Linear range (ng mL1)
RR (%)
Matrix
References
UV/VIS UV/VIS
Colorimetric Colorimetric
4 76
Up to 300 90–900
Water Water & soil
Morita and Kaneko (2006) Revanasiddappa et al. (2007)
UV/VIS
Colorimetric
8
100–800
Water
Hu et al. (2012)
UV/VIS UV/VIS
SLME1 CPE2
27 1.1
200–2000 4–450
96–103 99.5– 100 88– 127% – 97–102
Hylton and Mitra (2008) Gürkan, Kır, and Altunay (2015)
HG-AFSa ETAASb
DLLME3 DSLLME4
1.2 0.02
– 0.08–2.0
92–102 96–99
Water Water & beverage Fruit juice Water
UV/VIS
EME
1.5
5–300
95–102
Water
Lai, Chen, and Chen (2016) (Asadollahzadeh, Tavakoli, Torab-Mostaedi, Hosseini, & Hemmati, 2014) This work
SLME: Supported liquid membrane extraction. CPE: Cloud point extraction. DLLME: Dispersive liquid–liquid microextraction. DSLLME: Dispersive-solidification liquid–liquid microextraction. HG-AFS: Hydride generation-Atomic fluorescence spectrometry. ETAAS: Electrothermal atomic absorption spectrometry.
Table 2 Quantitative results of analyses of As(V) in real samples. Sample
As(V) concentration (ng mL1)
Relative recovery
Added
Found
(%)
Tap water 1
– 10 50
– 9.7 50.5
– 97 101
Tap water 2
– 10 50
– 10.2 49.1
– 102 98
Under ground water
– 10 50
18.6 28 66.5
– 98 97
Gelatin powder
– 10 50
– 9.6 47.5
– 96 95
in deionized water at 60 °C then treated by ion exchange resin. The EME analysis was performed according to the obtained optimum extraction conditions. No As(V) was detected in the tap water and gelatin powder samples. The concentration of As(V) in underground water sample was determined as 18.6 ng mL1. Subsequently, the real samples were spiked with known concentration of As(V) and recoveries were calculated according to Eq. (3). Quantitative results of analyses of As(V) in real samples are summarized in Table 2. These results show that the recoveries are not significantly affected by the matrix effect in real samples. 4. Conclusion In the present work, for the first time, EME was combined with molybdenum blue method for the spectrophotometric determination of As(V). The proposed method has been successfully applied for the determination of arsenic in tap and underground water. This new analytical approach has advantages of both EME and spectrophotometric determination including a good selectivity and cleanup, low cost, simplicity and easy operation. In addition, a high sample throughput is attained since the whole analytical process, including sample preparation and determination, is performed in about 30 min. However, the repeatability of the method described here is not as well as that of traditional spectrophotometric method and this is the most important limitation of the pro-
posed method. One reason to this limitation is the occurrence of electrolysis at both electrodes which will result in bubble formation around the fiber. The bubble formation can make the system unstable which can affect the repeatability of the method, accordingly. Nevertheless, the proposed method can be a good choice for determination of arsenic in environmental analysis. Acknowledgment The authors wish to express their gratitude to the University of Zanjan Research Council for support of this work. References Abernathy, C. O., Thomas, D. J., & Calderon, R. L. (2003). Health effects and risk assessment of arsenic. The Journal of Nutrition, 133(5), 1536S–1538S. Asadollahzadeh, M., Tavakoli, H., Torab-Mostaedi, M., Hosseini, G., & Hemmati, A. (2014). Response surface methodology based on central composite design as a chemometric tool for optimization of dispersive-solidification liquid–liquid microextraction for speciation of inorganic arsenic in environmental water samples. Talanta, 123, 25–31. Basheer, C., Tan, S. H., & Lee, H. K. (2008). Extraction of lead ions by electromembrane isolation. Journal of Chromatography A, 1213(1), 14–18. Chanthasakda, N., Nitiyanontakit, S., & Varanusupakul, P. (2016). Electro-enhanced hollow fiber membrane liquid phase microextraction of Cr (VI) oxoanions in drinking water samples. Talanta, 148, 680–685. Davarani, S. S. H., Moazami, H. R., Keshtkar, A. R., Banitaba, M. H., & Nojavan, S. (2013). A selective electromembrane extraction of uranium (VI) prior to its fluorometric determination in water. Analytica Chimica Acta, 783, 74–79. De Oliveira, E., McLaren, J., & Berman, S. (1983). Simultaneous determination of arsenic, antimony, and selenium in marine samples by inductively coupled plasma atomic emission spectrometry. Analytical Chemistry, 55(13), 2047–2050. DeSasa, M., & Rogers, L. (1954). Spectrophotometric determination of arsenic, phosphorus, and silicon in presence of each other. Analytical Chemistry, 26(8), 1381–1383. Feng, Y.-L., Chen, H.-Y., Tian, L.-C., & Narasaki, H. (1998). Off-line separation and determination of inorganic arsenic species in natural water by high resolution inductively coupled plasma mass spectrometry with hydride generation combined with reaction of arsenic (V) and L-cysteine. Analytica Chimica Acta, 375(1), 167–175. Gürkan, R., Kır, U., & Altunay, N. (2015). Development of a simple, sensitive and inexpensive ion-pairing cloud point extraction approach for the determination of trace inorganic arsenic species in spring water, beverage and rice samples by UV–Vis spectrophotometry. Food Chemistry, 180, 32–41. Hu, S., Lu, J., & Jing, C. (2012). A novel colorimetric method for field arsenic speciation analysis. Journal of Environmental Sciences, 24(7), 1341–1346. Hughes, M. F. (2002). Arsenic toxicity and potential mechanisms of action. Toxicology Letters, 133(1), 1–16. Hylton, K., & Mitra, S. (2008). A microfluidic hollow fiber membrane extractor for arsenic(V) detection. Analytica Chimica Acta, 607(1), 45–49. Jain, C., & Ali, I. (2000). Arsenic: Occurrence, toxicity and speciation techniques. Water Research, 34(17), 4304–4312.
M.A. Kamyabi, A. Aghaei / Food Chemistry 212 (2016) 65–71 Khajeh, M., Pedersen-Bjergaard, S., Barkhordar, A., & Bohlooli, M. (2015). Application of hollow cylindrical wheat stem for electromembrane extraction of thorium in water samples. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 137, 328–332. ˇ , P., Strieglerová, L., Gebauer, P., & Bocˇek, P. (2011). Electromembrane Kubán extraction of heavy metal cations followed by capillary electrophoresis with capacitively coupled contactless conductivity detection. Electrophoresis, 32(9), 1025–1032. Lai, G., Chen, G., & Chen, T. (2016). Speciation of AsIII and AsV in fruit juices by dispersive liquid–liquid microextraction and hydride generation-atomic fluorescence spectrometry. Food Chemistry, 190, 158–163. Lenoble, V., Deluchat, V., Serpaud, B., & Bollinger, J.-C. (2003). Arsenite oxidation and arsenate determination by the molybdene blue method. Talanta, 61(3), 267–276. Li, M., & Lee, S. H. (2005). Determination of As (III) and As (V) ions by chemiluminescence method. Microchemical Journal, 80(2), 237–240. Ma, J., Sengupta, M. K., Yuan, D., & Dasgupta, P. K. (2014). Speciation and detection of arsenic in aqueous samples: A review of recent progress in non-atomic spectrometric methods. Analytica Chimica Acta, 831, 1–23. Meharg, A. A. (2005). Venomous Earth: How Arsenic Caused the World’s Worst Mass Poisoning. . Morita, K., & Kaneko, E. (2006). Spectrophotometric determination of arsenic in water samples based on micro particle formation of ethyl violetmolybdoarsenate. Analytical Sciences, 22(8), 1085–1089. Pedersen-Bjergaard, S., & Rasmussen, K. E. (2006). Electrokinetic migration across artificial liquid membranes: new concept for rapid sample preparation of biological fluids. Journal of Chromatography A, 1109(2), 183–190.
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Pedersen-Bjergaard, S., & Rasmussen, K. E. (2008). Liquid-phase microextraction with porous hollow fibers, a miniaturized and highly flexible format for liquid– liquid extraction. Journal of Chromatography A, 1184(1), 132–142. Rao, G. G., Sarojini, B., & Gandikota, M. (1972). Titrimetric determination of arsenic (III) with potassium dichromate, using redox indicators. Talanta, 19(1), 71–74. Revanasiddappa, H., Dayananda, B., & Kumar, T. (2007). A sensitive spectrophotometric method for the determination of arsenic in environmental samples. Environmental Chemistry Letters, 5(3), 151–155. Safari, M., Nojavan, S., Davarani, S. S. H., & Morteza-Najarian, A. (2013). Speciation of chromium in environmental samples by dual electromembrane extraction system followed by high performance liquid chromatography. Analytica Chimica Acta, 789, 58–64. Smedley, P. L., & Kinniburgh, D. G. (2001). Source and Behavior of Arsenic in Natural Waters. United Nations Synthesis Report on Arsenic in Drinking Water (pp. 1–61). Geneva, Switzerland: World Health Organization. http://www. who. int/ water_sanitation_health/dwq/arsenicun1. pdf. Tan, T. Y., Basheer, C., Ng, K. P., & Lee, H. K. (2012). Electro membrane extraction of biological anions with ion chromatographic analysis. Analytica Chimica Acta, 739, 31–36. Welz, B., & Melcher, M. (1985). Decomposition of marine biological tissues for determination of arsenic, selenium, and mercury using hydride-generation and cold-vapor atomic absorption spectrometries. Analytical Chemistry, 57(2), 427–431. WHO (2011). Arsenic in drinking-water. Background Document for Development of WHO Guidelines for Drinking-Water Quality. . Zackheim, H. S. (1968). Carcinogenicity of arsenic. Archives of Dermatology, 98(6), 682.
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Analytical Methods
Electromembrane extraction and spectrophotometric determination of As(V) in water samples Mohammad Ali Kamyabi ⇑, Ali Aghaei Department of Chemistry, Faculty of Science, University of Zanjan, 45371-38791 Zanjan, Iran
a r t i c l e
i n f o
Article history: Received 16 November 2015 Received in revised form 17 May 2016 Accepted 23 May 2016 Available online 24 May 2016 Keywords: As(V) Electromembrane extraction Molybdenum blue complex Water samples
a b s t r a c t In this study, for the first time electromembrane extraction (EME) was used as a highly efficient sample pre-treatment method for the UV–VIS spectrophotometric determination of As(V) in water samples. The influences of experimental parameters during EME were investigated and optimized using one-variableat-a-time methodology as follows: organic solvent: 1-octanol + 2.5% (V/V) di-(2-ethylhexyl) phosphate, applied voltage: 70 V, extraction time: 15 min, pH of acceptor: 13, stirring rate: 750 rpm. The method allowed the determination of As(V) in the range of 5–300 ng mL1. The relative standard deviation was found to be within the range of 3.4–7.6%. The limit of detection, corresponding to a signal to noise ratio of three, was 1.5 ng mL1. The proposed method was finally applied to the determination of As(V) in water samples and relative recoveries ranging from 95 to 102% were obtained. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Arsenic is an important environmental element because of its high toxicity at the level of parts per billion. It enters drinking water supplies from natural deposits in the earth or from agricultural and industrial practices. Industrial arsenic is mainly used as a wood preservative, but arsenic is also used in dyes, paints, drugs, soaps, and semi-conductors. Agricultural applications, mining, and smelting are other sources which may release arsenic in the environment. The contamination of drinking water and groundwater with arsenic has been reported in various regions of the world especially in developing countries (Meharg, 2005). Arsenic can occur in the environment in several oxidation states (3, 0, +3 and +5) but in natural waters is mostly found as inorganic forms of trivalent arsenite (As(III)) and pentavalent arsenate (As(V)) which the first form, As(III), is more toxic (Jain & Ali, 2000). On the other hand inorganic arsenic forms are more toxic than organic forms which may however occur where waters are significantly impacted by industrial pollution (Hughes, 2002; Jain & Ali, 2000). Redox potential and pH are the most important factors controlling arsenic speciation. Under oxidising conditions, H2AsO 4 is dominant at low pH (less than about pH 6.9), whilst at higher pH, HAsO2 4 becomes dominant (H3AsO4 and AsO3 4 may be present in extremely acidic and alkaline conditions respectively). Under reducing conditions at pH less than about pH 9.2, the uncharged arsenite ⇑ Corresponding author. E-mail address: [email protected] (M.A. Kamyabi). http://dx.doi.org/10.1016/j.foodchem.2016.05.139 0308-8146/Ó 2016 Elsevier Ltd. All rights reserved.
species H3AsO3 will predominate. As a result, As(V) is the predominant form of inorganic arsenic especially in surface water (Smedley & Kinniburgh, 2001). It was previously proved that in excessive amounts, arsenic causes gastrointestinal damage and cardiac damage. There are some evidences that arsenic is also carcinogenic (Abernathy, Thomas, & Calderon, 2003; Zackheim, 1968). According to World Health Organization (WHO) guideline on drinking water quality, the maximum permitted concentration of arsenic in drinking water is 10 lg L1(WHO, 2011) Therefore, simple, rapid, highly sensitive, and accurate methods required for the determination of trace amounts of arsenic especially in environmental samples. Various analytical methods have been used for arsenic determination, including titrimetry (Rao, Sarojini, & Gandikota, 1972), chemiluminescence (Li & Lee, 2005), atomic absorption spectrometry (Welz & Melcher, 1985), inductively coupled plasma atomic emission spectrometry (ICP-OES) (De Oliveira, McLaren, & Berman, 1983), inductively coupled plasma mass spectrometry (ICP-MS) (Feng, Chen, Tian, & Narasaki, 1998), and spectrophotometry (Hu, Lu, & Jing, 2012; Lenoble, Deluchat, Serpaud, & Bollinger, 2003; Morita & Kaneko, 2006; Revanasiddappa, Dayananda, & Kumar, 2007). Recently, Ma et al. reviewed the papers published since 2005 on techniques for the measurement of arsenic in water samples (Ma, Sengupta, Yuan, & Dasgupta, 2014). Except spectrophotometry, most of the other methods used in determination of arsenic require trained staff, expensive experimental setup and running cost. Therefore, many spectrophotometric methods have been developed as an alternative for the
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determination of arsenic instead of conventional techniques. Method of Molybdenum blue is one of the most popular spectrophotometry methods in determination of arsenic (DeSasa & Rogers, 1954). It is based on the reduction of the yellow molybdeoarsenic acid to intensely absorbing blue color which subsequently can be measured spectrophotometrically at 840 nm. It should be noted that among different forms of arsenic, only As (V) can form molybdenum blue. Hence, any other forms of arsenic such as As(III) must be first converted to As(V) in order to determine using molybdenum blue method. Although the method of molybdenum blue is poorer than that of atomic absorption in sensitivity and rapidity, it is characterized by a higher accuracy and precision. On the other hand it suffers from poor selectivity because of strong interference of Phosphate ion and different metal ions, such as copper, nickel, cobalt and zinc. Electromembrane extraction (EME) is a new microextraction method in which an electrical voltage is applied to enhance the transport of charged species across a hollow fiber membrane (Pedersen-Bjergaard & Rasmussen, 2006). Although electromembrane extraction has been mainly used for charged organic samples such as basic drugs, it was generalized to metal ions for the first time in 2008 (Basheer, Tan, & Lee, 2008). Some other successful applications of this method have been reported for determination of inorganic anions and heavy metals, so far (Chanthasakda, Nitiyanontakit, & Varanusupakul, 2016; Davarani, Moazami, Keshtkar, Banitaba, & Nojavan, 2013; Khajeh, Pedersen-Bjergaard, ˇ , Strieglerová, Gebauer, & Barkhordar, & Bohlooli, 2015; Kubán Bocˇek, 2011; Safari, Nojavan, Davarani, & Morteza-Najarian, 2013; Tan, Basheer, Ng, & Lee, 2012). According to our knowledge, to date, no electromembrane extraction has been reported for determination of arsenic. In this study, we aim to improve the selectivity of the conventional Molybdenum blue method by using Electromembrane extraction as a very efficient sample clean up and pre-concentration method and also to lower the limit of detection of the method down to a level that covers the WHO recommended value of 10 lg L1 arsenic in drinking water. We successfully applied the proposed method for determination of As(V) in different environmental samples. 2. Experimental 2.1. Chemicals Analytical grade standard solution of 1000 mg L–1 H3AsO4, 1octanol (C8H18O), ammonium heptamolybdate ((NH4)6Mo7O244H2O), sodium metabisulfite (Na2S2O5), potassium iodide (KI), iodine (I2), hydrazine sulphate (H6N2O4S) and sulfuric acid (H2SO4) were purchased from Merck (Darmstadt, Germany). Di(2-ethylhexyl) phosphate (DEHP) was purchased from Fluka (Buchs, Switzerland). weak base anion-exchange resin, Indion GS 3000 (type 1), was purchased from ion exchange India ltd (Delhi, India) with an effective particle size of 0.5–0.65 mm. All chemicals were analytical grade and were used as supplied without further purification. HPLC-grade water was purchased from Merck and was used to prepare all solutions.
Ammonium molybdate solution was prepared by dissolving 1.0 g of the solid in 10 mL water and 90 mL of 3.0 M sulphuric acid. Hydrazinium sulphate solution was prepared by dissolving 0.15 g hydrazinium sulphate in 100 mL water. Molybdenum reagent was prepared by mixing same volumes of ammonium molybdate solution and hydrazinium sulphate solution just before running the experiment. 2.3. Apparatus A UV–VIS spectrophotometer,(HACH-LANGE, model DR-2800, Germany), with a micro-volume glass cell (500 lL) and a macrovolume glass cell (3500 lL) both with a path length of 1.0 cm was used for absorbance measurements at 840 nm. The pH measurements were made with bench top pH meter, (WTW, model Inolab 730, Germany). 2.3.1. Equipments for EME The equipment used for the EME of As(V) is presented by Fig. 1. A 10 mL glass vial with an internal diameter of 2.0 cm and a height of 4.5 cm was used as the extraction vessel. A PP Q3/2 polypropylene hollow fiber (Membrana, Wuppertal, Germany) with an internal diameter of 0.60 mm, 200 lm wall thickness, and 0.2 lm pores was used to immobilize the supported liquid membrane (SLM) and holding the acceptor phase. The platinum electrodes with diameters of 0.25 mm, obtained from Pars Pelatine (Tehran,Iran), coupled to a power supply model Hy-30002E with a programmable voltage in the range of 0–300 V and with a current output in the range of 0–2 A from Hyelec (Zhejiang, China). During the extraction, the vessel was stirred by a magnetic hot plate stirrer from VELP Scientifica (Milan, Italy). 2.4. EME procedure To a 50 mL sample solution, 1 mL of 0.1 mol L–1 Bis-Tris buffer solution (pH 7.3) was added and the mixture was passed through an ion exchange resin column at a flow rate of 2 mL min1 in order to remove the phosphate ion which may be present in sample solution. (Indion GS 3000 (type 1)). Five milliliters of the sample solution pretreated as described above was transferred into the sample vial. A short piece of polypropylene hollow fiber (3.5 cm) dipped in 1-octanol with 2.5% (v/v) DEHP for 10 s and then the excess amount of solvent was removed with a medical wipe. 10 lL of a 100 mM sodium hydroxide solution, as acceptor solution, was injected into the lumen of the hollow fiber using a microsyringe.
2.2. Working solutions and reagents Working solutions of As(V) were daily prepared by diluting appropriate volumes of the standard solution of arsenic. Iodine reagent was prepared by dissolving 0.25 g of iodine in 100 mL water containing 0.4 g potassium iodide. Sodium metabisulfite solution was freshly prepared by dissolving 0.5 g of the solid reagent in 10 mL water. Sodium hydrogen carbonate solution was prepared by dissolving 4.2 g of the solid in 100 mL water.
Fig. 1. Equipments used for electromembrane extraction of As(V).
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One of the electrodes, the anode, was placed inside the lumen of the fiber and the lower end of the hollow fiber was sealed with small piece of aluminum foil (thickness of 280 lm). The other electrode, cathode, was directly placed inside the sample solution. The electrodes were then coupled to the power supply and the extraction vessel was placed on a magnetic stirrer with a stirring rate of 700 rpm. A voltage of 70 V was turned on and extraction was performed for 15 min. Under the applied voltage, the As(V) migrated from sample solution (donor phase) into acceptor phases through the SLM. After the extraction was completed, the acceptor solution was collected by a microsyringe and transferred directly to a microvial for further analysis by UV–VIS spectrophotometery.
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known amount of standard which was spiked into the real sample, respectively. 3. Results and discussion 3.1. Optimization of EME Different experimental variables may affect efficiency of EME, including extraction time, organic solvent, pH of sample solution, acceptor solution pH, stirring rate and applied voltage. The effect of each variable was investigated using one-variable-at-a-time methodology. All the experiments were performed in triplicate, and the mean of absorbance values was used to evaluate the influence of those variables on the efficiency of EME.
2.5. Spectrophotometric procedure The acceptor solution inside the microvial first neutralized with 10 lL of a 100 mM HCl solution. Then, it was mixed with 10 lL of iodine reagent, 5 lL of sodium hydrogen carbonate solution, 50 lL of molybdenum reagent, 5 lL of sodium metabisulfite solution, respectively. The mixture was heated at 95 °C for 10 min, cooled to laboratory temperature and diluted to 100 lL and 1000 lL using deionized water regarding initial concentration of As(V) in sample solution. Finally, the absorbance of the solution was measured at 840 nm against the reagent blank. It should be noted that the maximum concentration of As(V) that can be determined using molybdenum blue method is 1000 ng mL1 (DeSasa & Rogers, 1954). Since the final extract will be about 30-fold concentrated after EME, it must be diluted before spectrophotometery method. Hence, the sample solutions containing less than 30 ng mL1 As (V) were diluted to 100 lL and the samples containing higher concentration of arsenic (more than 30 ng mL1) diluted to 1000 lL as mentioned above. 2.6. Calculation of enrichment factor and extraction recovery and relative recovery The enrichment factor (EF) was defined as the ratio of the final concentration of As(V) in the acceptor phase (Cf,a) and its initial concentration (Ci,s) in the sample solution
EF ¼
C f ;a : C i;s
ð1Þ
where Cf,a was calculated from a calibration graph obtained by direct determination of standard solutions of arsenic (100– 900 ng mL1). Extraction recovery (ER) was defined as the number of moles of As(V) which extracted to the acceptor phase (nf,a) divided by the number of moles of As(V) originally found in the sample solution (ni,s).
nf ;a C f ;a V f ;a ER ¼ 100 ¼ 100 ni;s C t;s V t;s ER ¼
V f ;a EF 100: V i;s
ð2Þ
ð3Þ
where Vf,a and Vi,s are the volumes of acceptor phase and sample solution, respectively. Relative recovery (RR) was calculated by the following equation:
RR ¼
C found C real 100: C added
ð4Þ
where Cfound, Creal and Cadded are the concentrations of As(V) after addition of known amount of standard into the real sample, the concentration of As(V) in real sample, and the concentration of
3.1.1. Selection of organic solvent (membrane composition) Previous findings show that the chemical nature of organic solvent has a significant effect on transfer of analyte into the acceptor solution. Among different organic solvents used in electromembrane extractions of metal ions, the best results have been achieved for SLM consisting of alcohols (Chanthasakda et al., ˇ et al., 2011). Moreover, it was previously shown that 2016; Kubán addition of ion-pair reagents such as DEHP to SLM composition has a significant effect on efficiency of EME. For cationic analytes the transport mechanism is based on proton/analyte ion exchange between the carrier and the donor solution and subsequently on analyte ion/proton exchange between the carrier/analyte complex and the acceptor solution. For anionic compounds it seems that by addition of DEHP, the electrical resistance of the liquid membrane is decreased and hence the current level will be increased. As a ˇ et al., result, the extraction recovery can be improved (Kubán 2011). Therefore, 1-octanol and different mixtures of 1-octanol and DEHP were investigated as composition of SLM in this study. A solution containing 250 ng mL1 of As(V) was used for the optimization of the SLM composition according to the absorption intensity obtained for each composition. The initial estimate for extraction time and applied voltage was 10 min and 50 V, respectively during the optimization of SLM composition. As indicated by Fig. 2 the absorbance was increased by increasing the concentration of DEHP up to 2.5% v/v in 1-octanol and decreased for the concentrations of DEHP higher than 2.5%v/v. As it was expected the electric current during EME was increased by increasing DEHP concentration. Fig. 3 shows the dependency of electric current on DEHP concentration. Therefore, 1-octanol with 2.5%v/v DEHP was selected as the optimized composition of SLM and was used in all subsequent measurements. At this SLM composition, the electric current in the EME system was around 180 lA at an applied voltage of 50 V. 3.1.2. Effect of extraction time Different extraction time from 5 to 20 min was selected to determine the optimum value of extraction time and the results are shown in Fig. 4A. The absorbance was increased as the extraction time increased up to 15 min and gradually decreased for longer extraction times. The decrease in absorbance is probably due to passive back diffusion of target ions into SLM and donor solution, which has been previously reported (Basheer et al., 2008). Hence, an extraction time of 15 min was selected for following steps. 3.1.3. Effect of applied voltage Applied voltage is one of the most important factors that should be taken into account. In order to determine the best applied voltage, absorbance values were recorded as a function of voltage for voltages in the range of 20–100 V. The absorbance increased lin-
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Fig. 2. Effect of DEHP concentration on EME of As(V). EME conditions: liquid membrane, 1-octanol + 1DEHP; stirring rate, 750 rpm; voltage, 50 V; acceptor, 100 mM NaOH; donor, 250 ng mL1 As(V) in DI water; extraction time, 10 min.
early by increasing applied voltage up to 70 V and then started to decline by increasing the voltage up to 100 V. (see Fig. 4B). Some authors proposed that Joule heating is the main reason for low efficiency of EME systems over an optimized voltage. Moreover; no absorbance was observed when the extraction performed at 0 V. The optimum extraction voltage (70 V) was used in all subsequent EME experiments.
Fig. 3. Effect of DEHP concentration on electric current during EME of As(V). EME conditions was as that of in Fig. 2.
3.1.4. Effect of pH in the donor and acceptor solutions The pH of donor solution determines the existing form of the arsenic species in the solution. Therefore, pH of donor solution plays a decisive role in EME of As(V). The pka values of arsenic acid are 2.19, 6.94 and 11.5 respectively. Therefore, in strongly acidic solution, it exists as arsenic acid (H3AsO4), in weakly acidic condition exists as dihydrogen arsenate ion (H2AsO 4 ), in weakly basic condition exists as hydrogen arsenate ion (HAsO2 4 ) and in strongly basic condition exists as arsenate ion (AsO3 4 ). It is obvious that all
Fig. 4. Effect of Different working variables: extraction time (A), applied voltage (B), pH of acceptor (C) and stirring rate (D) on EME efficiency of As(V). EME conditions: liquid membrane, 1-octanol + 2.5% DEHP; donor, 250 ng mL1 As(V) in DI water.
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As(V) species are in ionic form in neutral water. Therefore, deionized water was selected as donor solution to facilitate the sample preparation in following measurements. To determine the effects of acceptor solution pH on EME efficiency, different pH values of the acceptor solutions were investigated using 100 mM and 1000 mM NaOH solutions. The pH adjustment was performed with a 1 M HNO3 solution. The absorbance values were recorded as a function of pH values of acceptor solution to find the optimum pH value. As indicated by Fig. 4.C the absorbance increased by increasing the pH of acceptor solution up to 12 and then leveled off between pH values of 13 and 14. Thus, pH of 13 was selected as optimum pH value and 100 mM NaOH was utilized as the acceptor solution during the rest of this study.
3.1.5. Effect of stirring speed The main effect of stirring is to reduce the thickness of the boundary layer at the interface between the sample solution and the SLM where mass transfer is only promoted by diffusion. To investigate the effect of this factor, the absorbance values were recorded as a function of stirring rate. The obtained results are illustrated in Fig. 4D. As can been seen, by increasing the stirring rate from 300 to 700 rpm, the absorbance increased but at higher stirring rates (700–1200 rpm) the absorbance decreased. Accordingly, the stirring rate of 700 rpm was selected as the optimum value for the subsequent experiments. The decrease in absorbance at higher rates (700–1200 rpm) was perhaps due to inhibition of passive diffusion of the analyte from the bulk solution into the SLM because of bubble formation (Pedersen-Bjergaard & Rasmussen, 2008).
3.1.6. Effect of interfering ions To investigate the influence of excipient ions on the EME of arsenic, various ions commonly found in natural waters such as 3 Mg2+, Ca2+, CO2 3 , PO4 and also those ions which strongly interfere in determination of arsenic by the molybdenum blue method such as Cu2+, Ni2+, Zn2+ and Co2+ were added to the sample solution with concentration ratios of 1:1, 1:10, 1:100 and 1:1000 As(V) to studied interferents. A deviation greater than ±10% from the optimized signals was considered as the criterion for interference. No significant interference observed for Mg2+, Ca2+, Cu2+, Co2+, Ni2+, Zn2+ and CO2 3 up to a concentration ratio of 1:100. This could be expected according to the cationic nature of the metal ions, which results in a migration in the opposite direction in an electric field. On the other hand, a great positive deviation of the original signal observed in the case of PO3 4 .This can be attributed to the fact that phosphate can react with ammonium molybdate in the same manner as arsenate (and with about the same sensitivity). Therefore, a sample pretreatment is needed to eliminate the effect of phosphate before running the proposed method in natural waters which usually contains phosphate. The best method of phosphate removal to our knowledge is using an ion exchange resin column. According to a previously published method to a 50 mL sample solution, 1 mL of 0.1 mol L– 1 Bis–Tris buffer solution (pH 7.3) was added. Then the mixture was passed through an ion exchange resin column (Indion GS 3000 (type 1)) to eliminate the phosphate (Morita & Kaneko, 2006). This solution was used in subsequent steps of the proposed method. The results showed that the interference of phosphate completely removed. Hence, in natural waters or water samples containing high concentration of phosphate, it is necessary to pass the sample solution first through an ion exchange resin column and then follow the proposed method.
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3.2. Calibration and analytical figures of merit Under optimized conditions, the absorbance values for various concentrations of As(V) after being extracted by means of EME method were recorded and plotted as shown in Fig. 4. The linear relationships were observed between the absorbance values with the concentration of arsenic over two different intervals in the range of 5–30 ng mL1 (Fig. 5A) and 30–300 ng mL1 (Fig. 5B).The extraction recovery (ER%) and enrichment factor (EF) were investigated in deionized water under optimized conditions of EME using three standard solution of arsenic (10, 20 and 50 ng mL1) .The enrichment factor was obtained with the EME technique after the final dilution step (according to Eq. (1)), and it was measured between 30 and 32.5 which correspond to extraction recoveries from 60 to 65%. The repeatability of the method was obtained by applying five replicate measurements for the mentioned standard solutions of arsenic with the same electrode for each concentration. The relative standard deviations (RSDs) were found to be within the range of 3.4–7.6%. LOD for EME-treated sample was found to be 1.5 ng mL1. It was estimated based on repetitive dilution of a standard sample solution with 10 ng mL1 of As(V); analytical signal for EME of a blank sample was always subtracted. Concentration of As(V) corresponding to signal-to-noise ratio of 3 was considered as LOD. In order to evaluate the resulting LOD of this method, the LOD of molybdenum blue method investigated alone and without any pretreatment. The obtained LOD of 50 ng mL1 showed that the EME pre-treatment has improved the LOD by about 1.5 orders of magnitude. Note, however, that even lower LODs can be achieved for trace metal analysis with other analytical techniques such as GF-AAS, ICP-OES, ICP-MS, albeit using expensive and complex instrumentation. However, a comparison of the proposed method with previously reported methods for the determination of As(V) (Table 1) showed that the proposed method along with its simplicity and easy operation has a low limit of detection, an acceptable linear dynamic range and better selectivity with an important emphasis on the enrichment factor as well as low cost of analysis that make this method more efficient for determination of arsenic. 3.3. Application to real samples The developed EME method was applied for the determination of As(V) in tap water, underground water, bovine gelatin powder and their spiked samples. Gelatin powder was obtained from Faravari Darooi Gelatin Halal Factory (Qazvin, Iran). All samples first passed through an ion exchange resin column then were sampled into the glass vial for EME. For gelatin sample it was first dissolved
Fig. 5. Calibration curves for determination of As(V) in the range of 5–30 ng mL1 (A) and 30–300 ng mL1 (B) in the optimum conditions.
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M.A. Kamyabi, A. Aghaei / Food Chemistry 212 (2016) 65–71
Table 1 Comparison of the proposed method with other analytical methods applied for the determination of As(V).
1 2 3 4 a b
Detection
Method
Detection limit (ng mL1)
Linear range (ng mL1)
RR (%)
Matrix
References
UV/VIS UV/VIS
Colorimetric Colorimetric
4 76
Up to 300 90–900
Water Water & soil
Morita and Kaneko (2006) Revanasiddappa et al. (2007)
UV/VIS
Colorimetric
8
100–800
Water
Hu et al. (2012)
UV/VIS UV/VIS
SLME1 CPE2
27 1.1
200–2000 4–450
96–103 99.5– 100 88– 127% – 97–102
Hylton and Mitra (2008) Gürkan, Kır, and Altunay (2015)
HG-AFSa ETAASb
DLLME3 DSLLME4
1.2 0.02
– 0.08–2.0
92–102 96–99
Water Water & beverage Fruit juice Water
UV/VIS
EME
1.5
5–300
95–102
Water
Lai, Chen, and Chen (2016) (Asadollahzadeh, Tavakoli, Torab-Mostaedi, Hosseini, & Hemmati, 2014) This work
SLME: Supported liquid membrane extraction. CPE: Cloud point extraction. DLLME: Dispersive liquid–liquid microextraction. DSLLME: Dispersive-solidification liquid–liquid microextraction. HG-AFS: Hydride generation-Atomic fluorescence spectrometry. ETAAS: Electrothermal atomic absorption spectrometry.
Table 2 Quantitative results of analyses of As(V) in real samples. Sample
As(V) concentration (ng mL1)
Relative recovery
Added
Found
(%)
Tap water 1
– 10 50
– 9.7 50.5
– 97 101
Tap water 2
– 10 50
– 10.2 49.1
– 102 98
Under ground water
– 10 50
18.6 28 66.5
– 98 97
Gelatin powder
– 10 50
– 9.6 47.5
– 96 95
in deionized water at 60 °C then treated by ion exchange resin. The EME analysis was performed according to the obtained optimum extraction conditions. No As(V) was detected in the tap water and gelatin powder samples. The concentration of As(V) in underground water sample was determined as 18.6 ng mL1. Subsequently, the real samples were spiked with known concentration of As(V) and recoveries were calculated according to Eq. (3). Quantitative results of analyses of As(V) in real samples are summarized in Table 2. These results show that the recoveries are not significantly affected by the matrix effect in real samples. 4. Conclusion In the present work, for the first time, EME was combined with molybdenum blue method for the spectrophotometric determination of As(V). The proposed method has been successfully applied for the determination of arsenic in tap and underground water. This new analytical approach has advantages of both EME and spectrophotometric determination including a good selectivity and cleanup, low cost, simplicity and easy operation. In addition, a high sample throughput is attained since the whole analytical process, including sample preparation and determination, is performed in about 30 min. However, the repeatability of the method described here is not as well as that of traditional spectrophotometric method and this is the most important limitation of the pro-
posed method. One reason to this limitation is the occurrence of electrolysis at both electrodes which will result in bubble formation around the fiber. The bubble formation can make the system unstable which can affect the repeatability of the method, accordingly. Nevertheless, the proposed method can be a good choice for determination of arsenic in environmental analysis. Acknowledgment The authors wish to express their gratitude to the University of Zanjan Research Council for support of this work. References Abernathy, C. O., Thomas, D. J., & Calderon, R. L. (2003). Health effects and risk assessment of arsenic. The Journal of Nutrition, 133(5), 1536S–1538S. Asadollahzadeh, M., Tavakoli, H., Torab-Mostaedi, M., Hosseini, G., & Hemmati, A. (2014). Response surface methodology based on central composite design as a chemometric tool for optimization of dispersive-solidification liquid–liquid microextraction for speciation of inorganic arsenic in environmental water samples. Talanta, 123, 25–31. Basheer, C., Tan, S. H., & Lee, H. K. (2008). Extraction of lead ions by electromembrane isolation. Journal of Chromatography A, 1213(1), 14–18. Chanthasakda, N., Nitiyanontakit, S., & Varanusupakul, P. (2016). Electro-enhanced hollow fiber membrane liquid phase microextraction of Cr (VI) oxoanions in drinking water samples. Talanta, 148, 680–685. Davarani, S. S. H., Moazami, H. R., Keshtkar, A. R., Banitaba, M. H., & Nojavan, S. (2013). A selective electromembrane extraction of uranium (VI) prior to its fluorometric determination in water. Analytica Chimica Acta, 783, 74–79. De Oliveira, E., McLaren, J., & Berman, S. (1983). Simultaneous determination of arsenic, antimony, and selenium in marine samples by inductively coupled plasma atomic emission spectrometry. Analytical Chemistry, 55(13), 2047–2050. DeSasa, M., & Rogers, L. (1954). Spectrophotometric determination of arsenic, phosphorus, and silicon in presence of each other. Analytical Chemistry, 26(8), 1381–1383. Feng, Y.-L., Chen, H.-Y., Tian, L.-C., & Narasaki, H. (1998). Off-line separation and determination of inorganic arsenic species in natural water by high resolution inductively coupled plasma mass spectrometry with hydride generation combined with reaction of arsenic (V) and L-cysteine. Analytica Chimica Acta, 375(1), 167–175. Gürkan, R., Kır, U., & Altunay, N. (2015). Development of a simple, sensitive and inexpensive ion-pairing cloud point extraction approach for the determination of trace inorganic arsenic species in spring water, beverage and rice samples by UV–Vis spectrophotometry. Food Chemistry, 180, 32–41. Hu, S., Lu, J., & Jing, C. (2012). A novel colorimetric method for field arsenic speciation analysis. Journal of Environmental Sciences, 24(7), 1341–1346. Hughes, M. F. (2002). Arsenic toxicity and potential mechanisms of action. Toxicology Letters, 133(1), 1–16. Hylton, K., & Mitra, S. (2008). A microfluidic hollow fiber membrane extractor for arsenic(V) detection. Analytica Chimica Acta, 607(1), 45–49. Jain, C., & Ali, I. (2000). Arsenic: Occurrence, toxicity and speciation techniques. Water Research, 34(17), 4304–4312.
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