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Ionic liquid based solvent micro-extraction of Ag and Cd from saline and hyper-saline waters
Chemical Engineering Journal 308 (2017) 649–655
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Ionic liquid based solvent micro-extraction of Ag and Cd from saline and hyper-saline waters Belén Herce-Sesa, José A. López-López ⇑, Juan J. Pinto, Carlos Moreno Department of Analytical Chemistry, Faculty of Marine and Environmental Sciences, University of Cádiz, Puerto Real, 11510 Cádiz, Spain
h i g h l i g h t s Aliquat
Ò
336 SBME is suitable for extraction of Ag and Cd in saline waters.
Extraction of Ag and Cd is enhanced by salinity of the samples. Dissolved organic matter of samples does not affect extraction of Ag and Cd.
a r t i c l e
i n f o
Article history: Received 18 July 2016 Received in revised form 8 September 2016 Accepted 18 September 2016 Available online 19 September 2016 Keywords: Silver Cadmium Seawater Hyper-saline waters Ionic liquid Solvent bar extraction
a b s t r a c t Cadmium (Cd) and silver (Ag) are naturally occurring metals in saline natural waters, which may present toxic effects even at trace level. Membrane technology has been widely applied for their extraction, including hollow fiber supported liquid membranes. However, their application to saline waters is limited. In this work, a hollow fiber liquid micro-extraction (HFLPME) system, with a configuration of 2 phase solvent bar micro-extraction (2SBME), using the ionic liquid N-methyl-N,N,N-trioctylammonium chloride (AliquatÒ 336), dissolved in kerosene as extractant is proposed to overcome the limitations of existing HFLPME of Ag and Cd in saline waters. The use of an ionic liquid solution in the 2SBME leaded to higher stability of the organic solution in the fiber. The effect of chemical variables on the extraction was evaluated. Extraction of Cd and Ag with AliquatÒ 336 was enhanced by Cl in the sample, but it was independent of the concentration of organic matter. Extraction yield varied in the range 65–80% for Ag, and 45–95% for Cd, depending on the salinity of samples. The highest extraction was obtained in seawater samples for 75% AliquatÒ 336 dissolved in kerosene with 18% dodecan-1-ol, after 45 min, and 800 rpm stirring rate in the sample. Efficacy of the proposed system when applied to real samples was 88.10 ± 4.14% for Cd, and 61.47 ± 3.00% for Ag in seawater, and 92.73 ± 5.37% for Cd and 64.23 ± 2.85% for Ag in a hyper-saline lagoon (70 g L1 NaCl). In conclusion, the proposed methodology allowed a miniaturization of Ag and Cd extraction in short times, requiring lower amount of reagents and solvents, less energy as well as reducing operational cost and wastes if compared with existing liquid membrane based methods. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Silver (Ag) and cadmium (Cd) are metals that can appear as pollutants in naturals waters, which are considered as priority substances for European legislation [1,2]. Ag is considered to be toxic for aquatic organisms even at trace level, and Cd is extremely poisoning and carcinogenic [3,4]. Both metals may appear in urban, mining and industrial effluents [5,6]. Additionally, silver has been massively used during the last decade as a bactericide with application in hospital painting, medicine, textile industry, as well as ⇑ Corresponding author. E-mail address: [email protected] (J.A. López-López). http://dx.doi.org/10.1016/j.cej.2016.09.095 1385-8947/Ó 2016 Elsevier B.V. All rights reserved.
personal care and home care products [7,8]. For this reason, it can be considered as an emergent pollutant. Development of extraction systems for these metals is a main worry for environmentalists and policy makers because in some cases they appear at excessive level even in drinking waters [9,10]. These metals are precipitated by Cl ion but in saline waters they can be remobilized in the form of their chlorinated AgCl(n1) and CdCl(n2) n n complexes, increasing their bioavailability [3,4]. Among the existing methodologies for extraction of metals from natural water samples, membrane based technologies can be highlighted [11]. In particular liquid membrane extraction of Cd and Ag has been reported for their removal and also with analytical purposes [12,13]. However, they require significant amounts of
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solvents and reagents. According to the principles of Green Chemistry, an important effort has been done in the last years for miniaturization and automation of extraction processes towards economical and environmental sustainability [14]. In this sense, liquid micro-extraction has gained increasing interest not only due to the miniaturization of the process, but also by the versatility of the chemical systems [15]. Different configurations can be found for the liquid microextraction of metals: dispersive liquid-liquid micro-extraction, single drop micro-extraction, and hollow fiber liquid phase micro-extraction [16]. In particular hollow fiber liquid phase micro-extraction (HFLPME) presents several advantages over the other micro-extraction based methods due to the use of a support for the chemical systems [17]. In general, they offer higher stability of the organic solutions, they can be automated, and the recovery of the final solution enriched in the metal is simplified. HFLPME for Ag and Cd has been used in a three-phase configuration [18,19]. It is based in the principles of supported liquid membranes, where an organic solvent containing a carrier impregnates the pores in the wall of the polymeric fiber. This way, the metal is transported from the sample into the acceptor aqueous solution – inside the fiber- through the fiber pores [15]. Despite the potential of HFLPME, there are still some aspects associated to the stability of the chemical system and the need of a support for the fibers that must be improved [14]. In this regard, the use of room temperature ionic liquids (RTILs) has recently been published to improve the performance of HFLPME [20]. RTILs are organic salts which are liquid at a temperature below 100 °C. They offer higher thermal stability and viscosity than traditional solvents, improving their stability in hollow fiber systems, offering a better environmental profile [21]. However, extraction of metals using ionic liquids (ILs) is limited by a lower diffusion of the metal due to their high viscosity, so they can be combined with traditional solvents [22]. Additionally, some ILs present functional groups that make them suitable to play the double role of solvent and carrier. These liquids are known as task specific ionic liquids (TSILs), and can be used for the extraction of metals using a twophase HFLPME configuration (2HFLPME) [23]. In 2HFLPME, the internal part of the fiber and the pores in the fiber wall are impregnated by the organic solution [17]. Recently, the use of HFLPME in the configuration of a solvent extraction bar has been reported to overcome the drawbacks associated with the need of a support for the fibers. For solvent bar extraction, the ends of a piece of fiber are sealed and the fiber is left free in the sample, reducing experimental cost and simplifying the process [24]. A wide variety of reagents can be found for the extraction of metals. In the case of Cd and Ag, extractions with diethylhexylphosphoric acid (DEHPA) and tri-isobutyl phosphine sulphur (Cyanex 471XÒ) are well documented [24,25]. However, the extraction using those carriers is severely affected by the presence of a saline matrix, in particular in marine systems [24]. This can be particularly important in the case of hyper-saline waters such as effluents from desalination plants, or marshlands waters. In this work the ionic liquid N-methyl-N,N,N-trioctylammonium chloride (AliquatÒ 336) has been selected for the extraction of both metals. Additionally to the advantages of ionic liquids over traditional solvents, the use of AliquatÒ 336 allows the extraction of Ag and Cd from saline solutions [26]. Extraction of Cd and Ag by AliquatÒ 336 has been described as an ionic exchange of Cl ion from Aliquat 336Ò by CdCl(n2) and AgCl(n1) complexes from the n n sample [27]. In the case of saline solutions as seawater samples, Cd can be found mainly as CdCl 3 [27]; while silver is in the form 2 of AgCl 2 and AgCl3 [28] The aim of this work is the development and application of an AliquatÒ 336 based solvent bar for the micro-extraction of Ag
and Cd from different water samples matrices including hypersaline waters. The proposed system constitutes a low-cost and green methodology that uses only a few micro-litres of reagent with high efficacy and stability, which minimizes operational cost, manpower requirements, and loss of organic solution into the sample. The conditions in the sample and organic solution have been optimized prior application to the extraction of Ag and Cd from saline and hyper-saline water samples. 2. Material and methods 2.1. Reagents and solutions All reagents were analytical-reagent grade unless otherwise stated. Kerosene (97.5%), tri-octylmethylammonium chloride (AliquatÒ 336) (99%) and dodecan-1-ol (97%) were purchased from Fluka (Buchs, Switzerland). Sodium hydroxide (98%), hydrochloric acid (35%), and sodium chloride (99.5%) were obtained from Panreac (Barcelona, Spain). Sodium salt of humic acids was purchased from Aldrich (Steinheim, Germany). Aqueous solutions of silver and cadmium were prepared from a 1000 mg L1 standard solution obtained from Merck (Darmstadt, Germany). Pure water obtained by a Millipore Quantum Ultrapure water supplier (Millipore, USA) was used exclusively. Acetylene for atomic spectrometry was obtained from Air Liquid (Madrid, Spain). Acceptor solutions of AliquatÒ 336 were prepared using kerosene and dodecan-1-ol as solvents. 2.2. Apparatus Polypropylene Accurel PP S6/2 hollow fibers of 0.2 lm pore size and 1800 lm internal diameter were used in this study (Membrana, Germany). Samples were stirred using an IKA-Big Squid magnetic stirrer (Ika-Werke, Germany). The pH of water samples was measured using a Jenway 4330 conductivity & pH meter (Jenway, UK). Chloride ion concentration was measured using a pH & ion-meter GLP 22+ (Crison, Spain). Analytical signal of silver and cadmium was registered using a continuum source atomic absorption spectrometer model ContrAA 700 (Analytik Jena, Germany), using flame atomizer (FAAS) with a Xenon lamp as continuous source of radiation. Concentration of silver and cadmium in the sample before and after extraction was measured by flame atomic absorption spectroscopy. Silver measurements were carried out at a wavelength of 328.1 nm and for Cd a wavelength of 228 nm was used. The spectrometer detector was setup between pixels 98–102. Concentration of dissolved organic carbon in the samples before and after extraction was measured using a total carbon analyzer Analytik Jena multi N/C 3100 (Analytik Jena, Germany). 2.3. Procedure Solvent bars were made up using 15 cm of capillary fiber as follows (Fig. 1): – One end of the fiber was sealed using a hot tip. – The fiber lumen and the pores were filled with the acceptor solution using a syringe until the organic solution dropped through the fiber pores. – Finally, the other end of the fiber was sealed and the solvent bar was rinsed with ultrapure water prior it was left free into the sample. – After extraction, the solvent bars were discarded and Ag and Cd concentration remaining in the sample was measured by FAAS.
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Fig. 1. Schematic procedure for 2SBME setup.
Each sample was extracted by triplicate. In all cases a sample volume of 35 mL was used. In the case of real samples they were collected without filtering and they were spiked with 1 mg L1 of Ag and Cd. Samples were divided in three aliquots, which were treated independently. 2.4. Optimization of Ag and Cd extraction
3. Results and discussion
½M0 ½Mt 100 ½M0
ð1Þ
Additionally, leaching of IL into the samples is a key factor regarding the applicability of ILs in extraction processes. For this reason, leaching was also considered during optimization. This is a measure of the stability of the organic solution in the fiber system during extraction when other variables are modified. For each parameter: pH, extraction time and stirring rate in the sample, leaching was evaluated using constant amount of the ionic liquid
3.1. Optimization of Ag and Cd extraction 3.1.1. Effect of the sample pH First, the effect of the sample pH on extraction efficacy for Cd and Ag was evaluated (Fig. 2). In general, the AliquatÒ 336 solvent bar presented higher extraction yield for Cd than for Ag. In the case of Cd, the best extraction was obtained at pH = 10, while at slightly acidic pH, results showed poorer efficacy and reproducibility. This enhancement of extraction at high pH, could be related with the 2 formation of HCdO 2 and CdO2 in the presence of excess of OH in the sample, which could be transported by exchange with Cl from AliquatÒ 336 simultaneously with CdCl 3 [29]. Regarding Ag, extraction was clearly higher at basic pH, as it can be seen in Fig. 2; however in the case of Ag hydroxide remobilization due to an excess of OH does not take place [29]. Regarding the effect of the sample pH on leaching, it decreased with the increasing pH. Although AliquatÒ 336 has been widely used for extraction of metals, solubility of AliquatÒ 336 has only
100
Extracon efficacy (%)
In order to evaluate the different factors affecting the efficacy of AliquatÒ 336 solvent bar extraction of Ag and Cd, physic-chemical conditions in the sample and the organic solution were studied. Samples for optimization studies contained 1 mg L1 of Ag and Cd. During optimization, synthetic samples contained 30 g L1 NaCl and AliquatÒ 336 dissolved in kerosene was used as acceptor solution. Dodecan-1-ol was used as an additive to the organic solution to facilitate solubility of AliquatÒ 336 in kerosene. Optimized chemical variables were sample pH and AliquatÒ 336 concentration in the organic solution. Moreover, stirring speed in the sample during extraction and time of extraction were also optimized. Initial conditions of operation were pH = 2, AliquatÒ 336 10% w/v, 800 rpm stirring speed, and 30 min of extraction time. In the cases that AliquatÒ 336 concentration was higher than its solubility in kerosene, dodecan-1-ol was added as to facilitate its solubility in the organic solution. Finally, the effect of different Cl concentrations and dissolved organic matter in the form of humic acids (HA), measured as dissolved organic carbon on the efficacy of extraction was evaluated. Extraction efficacy was measured as the percentage of metal extracted from the sample as represented in Eq. (1), where [M]t is the concentration remaining in the sample after extraction time and [M]0 is the initial concentration of metal in the sample. [M]0 was measured in an non-extracted aliquot of sample to avoid overestimation of extraction efficacy.
Extraction efficacy ð%Þ ¼
in the fiber system. In the case of pH, this study was also used to evaluate possible effect of the sample pH on AliquatÒ 336 solubility in aqueous solution. Leaching was measured as the difference between the concentration of dissolved organic carbon in the sample after and before extraction.
Cd 80
Ag
60 40 20 0 0
2
4
6 pH
8
10
12
Fig. 2. Effect of pH in the sample on extraction efficacy for Ag and Cd. NaCl 30 g L1 in the sample, Aliquat 10% as acceptor solution, stirring rate 800 rpm, time of extraction 30 min. Ag r, Cd j.
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been reported in 2 M HCl samples as 80 mg L1 [30]. Results show that the sample pH has an effect on AliquatÒ 336 solubility in aqueous solution as it is observed in a decrease of leaching of IL into the sample. Fig. 3a shows that the lowest leaching is obtained at pH 10; however, for further experiments pH = 8 was selected to simulate pH conditions of most saline natural waters.
3.1.2. Aliquat Ò 336 concentration in the acceptor solution In the case of the effect of AliquatÒ 336 concentration on the extraction efficacy, a similar behaviour was observed for both metals (Fig. 4). Cd extraction was stabilized at 25% AliquatÒ 336, reaching a maximum 88.32 ± 2.52% extraction. For Ag, extraction was enhanced with increasing concentration of AliquatÒ 336 until it was stable at 75% AliquatÒ 336, offering 81.95 ± 2.25% extraction. This suggests that equilibrium between AliquatÒ 336 and Cd is reached at lower extractant concentration than in the case of Ag. The transport mechanism of chlorinated complexes of metals using AliquatÒ 336 has been recently described by Bhatluri et al. [27]. Transport takes place by exchange of Cl from AliquatÒ 336 in the organic solution with the negatively charged chlorinated complexes from the sample. On the one hand, predominant Ag chloro2 complexes in seawater conditions are AgCl 2 (46.9%) and AgCl3 (47.7%) [28], requiring a molar ratio 1:1 in the case of AgCl and 2 1:2 in the case of AgCl2 3 . On the other hand, Cd can be found as CdCl 3 in seawater [27], which is extracted at a ratio 1:1 (CdCl3 : Cl), explaining that the highest extraction for Cd was reached at lower AliquatÒ 336 concentration. In this case, leaching was increased at higher ionic liquid concentration (Fig. 3b). First, higher amount of ionic liquid leaded to an increase of available AliquatÒ 336. Second, for higher AliquatÒ 336 concentrations, dodecan-1-ol was added to kerosene in the
a
100 Extracon efficacy (%)
652
40 20
0
20
40 60 80 Aliquat 336® (%)
100
120
Fig. 4. Effect of AliquatÒ 336 concentration in the acceptor solution on extraction efficacy for Ag and Cd. Conditions in the sample: pH = 8 and NaCl 30 g L1, stirring rate 800 rpm, time of extraction 30 min. Ag r, Cd j.
organic solution, to permit AliquatÒ 336 solubility, increasing the polarity of the organic solution. Taking into account the results obtained for both metals in terms of extraction and leaching, 75% AliquatÒ 336 was used for further studies. 3.1.3. Effect of stirring speed and time of extraction Regarding hydrodynamic conditions, stirring speed and extraction time were evaluated. On the one hand, Fig. 5a shows that the proposed system allows the fast extraction of Cd and Ag after 5 min. Moreover, quantitative extraction for Cd was obtained after 45 min. However, in the case of Ag, extraction efficacy was stabilized between 45 and 60 min, showing 94.87 ± 1.35% extraction in 60 min. This trend is similar to that observed in the case of
b
Aliquat 336 ®
25
10
20
DOC (mg L-1)
DOC (mg L-1)
60
0
pH
12
Cd Ag
80
8 6 4
15 10 5
2
0
0 0
2
4
6
8
10
0
12
25
Extracon me
c
20
50
75
100
Aliquat 336®(%)
pH
d
14
Srring speed
DOC (mg L-1)
DOC (mg L-1)
12 15 10
10 8 6 4
5
2 0
0 0
15
30 Time (min)
45
60
0
300
600
900
Srring speed (rpm)
Fig. 3. Effect of different experimental conditions on leaching of organic solution into the sample during extraction. a) Sample pH, b) concentration of AliquatÒ 336, c) extraction time, d) stirring speed.
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120
Cd Extracon efficacy (%)
Extracon effiacy (%)
120 100
Ag
80 60 40 20
Cd
100 80
Ag
60 40 20
0
0 0
10
20
30 40 Time (min)
50
60
70
300
500
700 900 Srring speed (rpm)
1100
1300
Fig. 5. Effect of a) time of extraction and b) stirring speed Ag r and Cd j on extraction efficacy.
1 ¼ 89:28 t 419:06 C Cd
ð2Þ
1 ¼ 36:33 t 42:25 C Ag
ð3Þ
On the other hand, extraction was increased from 350 rpm to 500 rpm stirring speed for Cd and in the case of Ag extraction efficacy was stabilized at 800 rpm. As it can be observed in Fig. 5b, extraction efficacy remains stable at higher stirring rate up to 1100 rpm. In other hollow fiber supported liquid membrane sytems for Ag extraction as tri-isobutylphosphine sulphide dissolved in dihexylether, the organic solution is lost from the support at lower stirring speed of the samples, diminishing the efficacy of the system [25]. The use of AliquatÒ 336 resulted in higher stability of the organic solution in the fiber against stirring speed, reducing the loss of organic solution into the water samples, and improving its environmental profile. In the case of hydrodynamic conditions, higher leaching can be observed for longer extraction times (Fig. 3c). However, concentration of dissolved organic carbon is stabilized after 45 min. Finally stirring speed does not seem to influence leaching from the fibers into the samples (Fig. 3d), showing that the presence of ionic liquid improves the stability of the organic solution in the fiber. Conditions selected for further experiments were stirring speed 800 rpm and time of extraction 30 min because enough extraction
was reached to evaluate the efficacy of the system and to shorten experiment time and leaching is significantly increased for longer extraction times. 3.2. Effect of organic matter concentration and salinity of samples Salinity of sample and in particular the concentration of Cl is a key factor for the extraction of Cd and Ag from water samples because their speciation depends on the Cl concentration [27,28]. This fact is of particular interest in the case of estuarine samples were salinity may vary from 1 g L1 to 30 g L1. Fig. 7 shows how the extraction efficacy for both Cd and Ag, was increased at higher Cl concentration, until it is stabilized at 10 g L1 Cl in the case of Ag and 30 g L1 for Cd. This can be explained by the different speciation of Cd and Ag with the increasing salinity. Extraction yield for
Extracon efficacy (%)
AliquatÒ 336 concentration, and it could be said that the kinetic of Cd extraction is faster than that observed for Ag. Additionally the kinetic of Ag and Cd transport has been evaluated. As it can be observed in Fig. 6, reaction of Ag and Cd with AliquatÒ 336 fits for a second order model, being R2 = 0.981 for Cd and R2 = 0.984 for Ag. In the case of Cd (Eq. (2)) a higher slope was obtained than in the case of silver (Eq. (3)), explaining the faster extraction of Cd.
Cd Ag
0
10
20
30 40 NaCl (g L-1)
50
60
70
Fig. 7. Influence of NaCl concentration in the sample on extraction efficacy. Ag r, Cd j.
250
200
2500
DOC (mg L-1)
3000
1/C (L mmol-1)
100 90 80 70 60 50 40 30 20 10 0
Cd
2000
Ag
1500
150 100 50
1000 500
0
0 0
10
20
30 40 Time (min)
50
60
Fig. 6. Evolution of 1/CCd j, and 1/CAg r against time of extraction.
70
0 mgL-1 HA 30 g L-1 NaCl
30 mgL-1 HA 0 g L-1 NaCl
30 mgL-1 HA 30 g L-1 NaCl
Condions Fig. 8. Effect of NaCl and dissolved organic carbon concentration on leaching of organic solution into the sample during extraction.
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Table 1 Extraction efficacy for Ag and Cd in real samples. Extraction (%) Sample
pH
DOC (mg L
Tap water Río Tinto San Pedro River Hyper-saline lagoon
7.7 2.1 7.9 7.6
0.83 5.60 2.04 9.88
1
)
Ag varied in the range 61.46 ± 2.55–81.95 ± 2.25%, while for Cd it varied from 46.47 ± 7.59% at 1 g L1 Cl to 89.90 ± 2.25% at 30 g L1 Cl, showing that Cl has a higher influence on the extraction for Cd than for Ag. Regarding the effect of organic matter, the extraction efficacy did not suffer considerable changes in the both saline and nonsaline samples spiked with 30 mg L1 of humic acids. Leaching was reduced by the increasing concentration of NaCl in the sample (Fig. 8). This could be related to an increase in the ionic strength of the sample, leading to smaller solubility of the ionic liquid, producing a ‘‘salting-out” effect. 3.3. Application to real samples Samples with different pH, salinity, and organic matter concentration were extracted. Non-saline waters used were tap water and a sample collected from an acidic (pH = 2.09) tributary river of Río Tinto River located in the South-western Spanish mining area of Rio Tinto. Saline samples treated were a seawater sample from the San Pedro River, which is an arm of the sea fed by the Bay of Cádiz (Spain) and a water sample from a hyper-saline lagoon located in the Natural Park of the Bay of Cádiz. As it was expected, the best results were obtained for saline samples (Table 1). This effect was more important in the case of Cd, as predicted from results in Fig. 7. A maximum extraction for seawater of 88.10 ± 4.14% was obtained for Cd, and 61.46 ± 3.00% for Ag. In the case of the hyper-saline lagoon results were 92.73 ± 5.37% for Cd and 64.23 ± 2.85% for Ag. However, for non-saline samples, extraction efficacy was poorer for both metals. In the case of Cd, the effect was more important and extraction was always below or in the level of 5%, while for Ag, extraction efficacy was 29.24 ± 4.63% for the tributary of Río Tinto River. 4. Conclusion Solvent bar extraction based on the use of the ionic liquid AliquatÒ 336 as extractant constitutes an efficient, green and lowcost methodology for the extraction of Cd and Ag from saline water samples and hyper-saline waters. Moreover, due to its high efficacy in short operation times, it can be considered as a potential tool to overcome the problem of inefficacy of metals extraction from saline matrices. Acknowledgments The present work was financed by the Spanish Ministry of Economy and Competitiveness. Project CTM-2013-47549-P. References [1] Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy, (n.d.). [2] Directive 2006/11/EC of the European Parliament and of the Council of 15 February 2006 on pollution caused by certain dangerous substances discharged into the aquatic environment of the Community, (n.d.).
1
NaCl (g L 0.08 0.03 37.82 70.39
)
Ag
Cd
9.14 ± 1.17 29.24 ± 4.63 61.46 ± 3.00 64.23 ± 2.85
1.79 ± 1.08 5.09 ± 2.58 88.10 ± 4.14 92.73 ± 5.37
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Mater. 176 (2010) 481–486, http://dx.doi.org/10.1016/j.jhazmat.2009.11.054. [23] L. Fischer, T. Falta, G. Koellensperger, A. Stojanovic, D. Kogelnig, M. Galanski, R. Krachler, B.K. Keppler, S. Hann, Ionic liquids for extraction of metals and metal containing compounds from communal and industrial waste water, Water Res. 45 (2011) 4601–4614, http://dx.doi.org/10.1016/j.watres.2011.06.011.
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Ionic liquid based solvent micro-extraction of Ag and Cd from saline and hyper-saline waters Belén Herce-Sesa, José A. López-López ⇑, Juan J. Pinto, Carlos Moreno Department of Analytical Chemistry, Faculty of Marine and Environmental Sciences, University of Cádiz, Puerto Real, 11510 Cádiz, Spain
h i g h l i g h t s Aliquat
Ò
336 SBME is suitable for extraction of Ag and Cd in saline waters.
Extraction of Ag and Cd is enhanced by salinity of the samples. Dissolved organic matter of samples does not affect extraction of Ag and Cd.
a r t i c l e
i n f o
Article history: Received 18 July 2016 Received in revised form 8 September 2016 Accepted 18 September 2016 Available online 19 September 2016 Keywords: Silver Cadmium Seawater Hyper-saline waters Ionic liquid Solvent bar extraction
a b s t r a c t Cadmium (Cd) and silver (Ag) are naturally occurring metals in saline natural waters, which may present toxic effects even at trace level. Membrane technology has been widely applied for their extraction, including hollow fiber supported liquid membranes. However, their application to saline waters is limited. In this work, a hollow fiber liquid micro-extraction (HFLPME) system, with a configuration of 2 phase solvent bar micro-extraction (2SBME), using the ionic liquid N-methyl-N,N,N-trioctylammonium chloride (AliquatÒ 336), dissolved in kerosene as extractant is proposed to overcome the limitations of existing HFLPME of Ag and Cd in saline waters. The use of an ionic liquid solution in the 2SBME leaded to higher stability of the organic solution in the fiber. The effect of chemical variables on the extraction was evaluated. Extraction of Cd and Ag with AliquatÒ 336 was enhanced by Cl in the sample, but it was independent of the concentration of organic matter. Extraction yield varied in the range 65–80% for Ag, and 45–95% for Cd, depending on the salinity of samples. The highest extraction was obtained in seawater samples for 75% AliquatÒ 336 dissolved in kerosene with 18% dodecan-1-ol, after 45 min, and 800 rpm stirring rate in the sample. Efficacy of the proposed system when applied to real samples was 88.10 ± 4.14% for Cd, and 61.47 ± 3.00% for Ag in seawater, and 92.73 ± 5.37% for Cd and 64.23 ± 2.85% for Ag in a hyper-saline lagoon (70 g L1 NaCl). In conclusion, the proposed methodology allowed a miniaturization of Ag and Cd extraction in short times, requiring lower amount of reagents and solvents, less energy as well as reducing operational cost and wastes if compared with existing liquid membrane based methods. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Silver (Ag) and cadmium (Cd) are metals that can appear as pollutants in naturals waters, which are considered as priority substances for European legislation [1,2]. Ag is considered to be toxic for aquatic organisms even at trace level, and Cd is extremely poisoning and carcinogenic [3,4]. Both metals may appear in urban, mining and industrial effluents [5,6]. Additionally, silver has been massively used during the last decade as a bactericide with application in hospital painting, medicine, textile industry, as well as ⇑ Corresponding author. E-mail address: [email protected] (J.A. López-López). http://dx.doi.org/10.1016/j.cej.2016.09.095 1385-8947/Ó 2016 Elsevier B.V. All rights reserved.
personal care and home care products [7,8]. For this reason, it can be considered as an emergent pollutant. Development of extraction systems for these metals is a main worry for environmentalists and policy makers because in some cases they appear at excessive level even in drinking waters [9,10]. These metals are precipitated by Cl ion but in saline waters they can be remobilized in the form of their chlorinated AgCl(n1) and CdCl(n2) n n complexes, increasing their bioavailability [3,4]. Among the existing methodologies for extraction of metals from natural water samples, membrane based technologies can be highlighted [11]. In particular liquid membrane extraction of Cd and Ag has been reported for their removal and also with analytical purposes [12,13]. However, they require significant amounts of
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solvents and reagents. According to the principles of Green Chemistry, an important effort has been done in the last years for miniaturization and automation of extraction processes towards economical and environmental sustainability [14]. In this sense, liquid micro-extraction has gained increasing interest not only due to the miniaturization of the process, but also by the versatility of the chemical systems [15]. Different configurations can be found for the liquid microextraction of metals: dispersive liquid-liquid micro-extraction, single drop micro-extraction, and hollow fiber liquid phase micro-extraction [16]. In particular hollow fiber liquid phase micro-extraction (HFLPME) presents several advantages over the other micro-extraction based methods due to the use of a support for the chemical systems [17]. In general, they offer higher stability of the organic solutions, they can be automated, and the recovery of the final solution enriched in the metal is simplified. HFLPME for Ag and Cd has been used in a three-phase configuration [18,19]. It is based in the principles of supported liquid membranes, where an organic solvent containing a carrier impregnates the pores in the wall of the polymeric fiber. This way, the metal is transported from the sample into the acceptor aqueous solution – inside the fiber- through the fiber pores [15]. Despite the potential of HFLPME, there are still some aspects associated to the stability of the chemical system and the need of a support for the fibers that must be improved [14]. In this regard, the use of room temperature ionic liquids (RTILs) has recently been published to improve the performance of HFLPME [20]. RTILs are organic salts which are liquid at a temperature below 100 °C. They offer higher thermal stability and viscosity than traditional solvents, improving their stability in hollow fiber systems, offering a better environmental profile [21]. However, extraction of metals using ionic liquids (ILs) is limited by a lower diffusion of the metal due to their high viscosity, so they can be combined with traditional solvents [22]. Additionally, some ILs present functional groups that make them suitable to play the double role of solvent and carrier. These liquids are known as task specific ionic liquids (TSILs), and can be used for the extraction of metals using a twophase HFLPME configuration (2HFLPME) [23]. In 2HFLPME, the internal part of the fiber and the pores in the fiber wall are impregnated by the organic solution [17]. Recently, the use of HFLPME in the configuration of a solvent extraction bar has been reported to overcome the drawbacks associated with the need of a support for the fibers. For solvent bar extraction, the ends of a piece of fiber are sealed and the fiber is left free in the sample, reducing experimental cost and simplifying the process [24]. A wide variety of reagents can be found for the extraction of metals. In the case of Cd and Ag, extractions with diethylhexylphosphoric acid (DEHPA) and tri-isobutyl phosphine sulphur (Cyanex 471XÒ) are well documented [24,25]. However, the extraction using those carriers is severely affected by the presence of a saline matrix, in particular in marine systems [24]. This can be particularly important in the case of hyper-saline waters such as effluents from desalination plants, or marshlands waters. In this work the ionic liquid N-methyl-N,N,N-trioctylammonium chloride (AliquatÒ 336) has been selected for the extraction of both metals. Additionally to the advantages of ionic liquids over traditional solvents, the use of AliquatÒ 336 allows the extraction of Ag and Cd from saline solutions [26]. Extraction of Cd and Ag by AliquatÒ 336 has been described as an ionic exchange of Cl ion from Aliquat 336Ò by CdCl(n2) and AgCl(n1) complexes from the n n sample [27]. In the case of saline solutions as seawater samples, Cd can be found mainly as CdCl 3 [27]; while silver is in the form 2 of AgCl 2 and AgCl3 [28] The aim of this work is the development and application of an AliquatÒ 336 based solvent bar for the micro-extraction of Ag
and Cd from different water samples matrices including hypersaline waters. The proposed system constitutes a low-cost and green methodology that uses only a few micro-litres of reagent with high efficacy and stability, which minimizes operational cost, manpower requirements, and loss of organic solution into the sample. The conditions in the sample and organic solution have been optimized prior application to the extraction of Ag and Cd from saline and hyper-saline water samples. 2. Material and methods 2.1. Reagents and solutions All reagents were analytical-reagent grade unless otherwise stated. Kerosene (97.5%), tri-octylmethylammonium chloride (AliquatÒ 336) (99%) and dodecan-1-ol (97%) were purchased from Fluka (Buchs, Switzerland). Sodium hydroxide (98%), hydrochloric acid (35%), and sodium chloride (99.5%) were obtained from Panreac (Barcelona, Spain). Sodium salt of humic acids was purchased from Aldrich (Steinheim, Germany). Aqueous solutions of silver and cadmium were prepared from a 1000 mg L1 standard solution obtained from Merck (Darmstadt, Germany). Pure water obtained by a Millipore Quantum Ultrapure water supplier (Millipore, USA) was used exclusively. Acetylene for atomic spectrometry was obtained from Air Liquid (Madrid, Spain). Acceptor solutions of AliquatÒ 336 were prepared using kerosene and dodecan-1-ol as solvents. 2.2. Apparatus Polypropylene Accurel PP S6/2 hollow fibers of 0.2 lm pore size and 1800 lm internal diameter were used in this study (Membrana, Germany). Samples were stirred using an IKA-Big Squid magnetic stirrer (Ika-Werke, Germany). The pH of water samples was measured using a Jenway 4330 conductivity & pH meter (Jenway, UK). Chloride ion concentration was measured using a pH & ion-meter GLP 22+ (Crison, Spain). Analytical signal of silver and cadmium was registered using a continuum source atomic absorption spectrometer model ContrAA 700 (Analytik Jena, Germany), using flame atomizer (FAAS) with a Xenon lamp as continuous source of radiation. Concentration of silver and cadmium in the sample before and after extraction was measured by flame atomic absorption spectroscopy. Silver measurements were carried out at a wavelength of 328.1 nm and for Cd a wavelength of 228 nm was used. The spectrometer detector was setup between pixels 98–102. Concentration of dissolved organic carbon in the samples before and after extraction was measured using a total carbon analyzer Analytik Jena multi N/C 3100 (Analytik Jena, Germany). 2.3. Procedure Solvent bars were made up using 15 cm of capillary fiber as follows (Fig. 1): – One end of the fiber was sealed using a hot tip. – The fiber lumen and the pores were filled with the acceptor solution using a syringe until the organic solution dropped through the fiber pores. – Finally, the other end of the fiber was sealed and the solvent bar was rinsed with ultrapure water prior it was left free into the sample. – After extraction, the solvent bars were discarded and Ag and Cd concentration remaining in the sample was measured by FAAS.
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Fig. 1. Schematic procedure for 2SBME setup.
Each sample was extracted by triplicate. In all cases a sample volume of 35 mL was used. In the case of real samples they were collected without filtering and they were spiked with 1 mg L1 of Ag and Cd. Samples were divided in three aliquots, which were treated independently. 2.4. Optimization of Ag and Cd extraction
3. Results and discussion
½M0 ½Mt 100 ½M0
ð1Þ
Additionally, leaching of IL into the samples is a key factor regarding the applicability of ILs in extraction processes. For this reason, leaching was also considered during optimization. This is a measure of the stability of the organic solution in the fiber system during extraction when other variables are modified. For each parameter: pH, extraction time and stirring rate in the sample, leaching was evaluated using constant amount of the ionic liquid
3.1. Optimization of Ag and Cd extraction 3.1.1. Effect of the sample pH First, the effect of the sample pH on extraction efficacy for Cd and Ag was evaluated (Fig. 2). In general, the AliquatÒ 336 solvent bar presented higher extraction yield for Cd than for Ag. In the case of Cd, the best extraction was obtained at pH = 10, while at slightly acidic pH, results showed poorer efficacy and reproducibility. This enhancement of extraction at high pH, could be related with the 2 formation of HCdO 2 and CdO2 in the presence of excess of OH in the sample, which could be transported by exchange with Cl from AliquatÒ 336 simultaneously with CdCl 3 [29]. Regarding Ag, extraction was clearly higher at basic pH, as it can be seen in Fig. 2; however in the case of Ag hydroxide remobilization due to an excess of OH does not take place [29]. Regarding the effect of the sample pH on leaching, it decreased with the increasing pH. Although AliquatÒ 336 has been widely used for extraction of metals, solubility of AliquatÒ 336 has only
100
Extracon efficacy (%)
In order to evaluate the different factors affecting the efficacy of AliquatÒ 336 solvent bar extraction of Ag and Cd, physic-chemical conditions in the sample and the organic solution were studied. Samples for optimization studies contained 1 mg L1 of Ag and Cd. During optimization, synthetic samples contained 30 g L1 NaCl and AliquatÒ 336 dissolved in kerosene was used as acceptor solution. Dodecan-1-ol was used as an additive to the organic solution to facilitate solubility of AliquatÒ 336 in kerosene. Optimized chemical variables were sample pH and AliquatÒ 336 concentration in the organic solution. Moreover, stirring speed in the sample during extraction and time of extraction were also optimized. Initial conditions of operation were pH = 2, AliquatÒ 336 10% w/v, 800 rpm stirring speed, and 30 min of extraction time. In the cases that AliquatÒ 336 concentration was higher than its solubility in kerosene, dodecan-1-ol was added as to facilitate its solubility in the organic solution. Finally, the effect of different Cl concentrations and dissolved organic matter in the form of humic acids (HA), measured as dissolved organic carbon on the efficacy of extraction was evaluated. Extraction efficacy was measured as the percentage of metal extracted from the sample as represented in Eq. (1), where [M]t is the concentration remaining in the sample after extraction time and [M]0 is the initial concentration of metal in the sample. [M]0 was measured in an non-extracted aliquot of sample to avoid overestimation of extraction efficacy.
Extraction efficacy ð%Þ ¼
in the fiber system. In the case of pH, this study was also used to evaluate possible effect of the sample pH on AliquatÒ 336 solubility in aqueous solution. Leaching was measured as the difference between the concentration of dissolved organic carbon in the sample after and before extraction.
Cd 80
Ag
60 40 20 0 0
2
4
6 pH
8
10
12
Fig. 2. Effect of pH in the sample on extraction efficacy for Ag and Cd. NaCl 30 g L1 in the sample, Aliquat 10% as acceptor solution, stirring rate 800 rpm, time of extraction 30 min. Ag r, Cd j.
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been reported in 2 M HCl samples as 80 mg L1 [30]. Results show that the sample pH has an effect on AliquatÒ 336 solubility in aqueous solution as it is observed in a decrease of leaching of IL into the sample. Fig. 3a shows that the lowest leaching is obtained at pH 10; however, for further experiments pH = 8 was selected to simulate pH conditions of most saline natural waters.
3.1.2. Aliquat Ò 336 concentration in the acceptor solution In the case of the effect of AliquatÒ 336 concentration on the extraction efficacy, a similar behaviour was observed for both metals (Fig. 4). Cd extraction was stabilized at 25% AliquatÒ 336, reaching a maximum 88.32 ± 2.52% extraction. For Ag, extraction was enhanced with increasing concentration of AliquatÒ 336 until it was stable at 75% AliquatÒ 336, offering 81.95 ± 2.25% extraction. This suggests that equilibrium between AliquatÒ 336 and Cd is reached at lower extractant concentration than in the case of Ag. The transport mechanism of chlorinated complexes of metals using AliquatÒ 336 has been recently described by Bhatluri et al. [27]. Transport takes place by exchange of Cl from AliquatÒ 336 in the organic solution with the negatively charged chlorinated complexes from the sample. On the one hand, predominant Ag chloro2 complexes in seawater conditions are AgCl 2 (46.9%) and AgCl3 (47.7%) [28], requiring a molar ratio 1:1 in the case of AgCl and 2 1:2 in the case of AgCl2 3 . On the other hand, Cd can be found as CdCl 3 in seawater [27], which is extracted at a ratio 1:1 (CdCl3 : Cl), explaining that the highest extraction for Cd was reached at lower AliquatÒ 336 concentration. In this case, leaching was increased at higher ionic liquid concentration (Fig. 3b). First, higher amount of ionic liquid leaded to an increase of available AliquatÒ 336. Second, for higher AliquatÒ 336 concentrations, dodecan-1-ol was added to kerosene in the
a
100 Extracon efficacy (%)
652
40 20
0
20
40 60 80 Aliquat 336® (%)
100
120
Fig. 4. Effect of AliquatÒ 336 concentration in the acceptor solution on extraction efficacy for Ag and Cd. Conditions in the sample: pH = 8 and NaCl 30 g L1, stirring rate 800 rpm, time of extraction 30 min. Ag r, Cd j.
organic solution, to permit AliquatÒ 336 solubility, increasing the polarity of the organic solution. Taking into account the results obtained for both metals in terms of extraction and leaching, 75% AliquatÒ 336 was used for further studies. 3.1.3. Effect of stirring speed and time of extraction Regarding hydrodynamic conditions, stirring speed and extraction time were evaluated. On the one hand, Fig. 5a shows that the proposed system allows the fast extraction of Cd and Ag after 5 min. Moreover, quantitative extraction for Cd was obtained after 45 min. However, in the case of Ag, extraction efficacy was stabilized between 45 and 60 min, showing 94.87 ± 1.35% extraction in 60 min. This trend is similar to that observed in the case of
b
Aliquat 336 ®
25
10
20
DOC (mg L-1)
DOC (mg L-1)
60
0
pH
12
Cd Ag
80
8 6 4
15 10 5
2
0
0 0
2
4
6
8
10
0
12
25
Extracon me
c
20
50
75
100
Aliquat 336®(%)
pH
d
14
Srring speed
DOC (mg L-1)
DOC (mg L-1)
12 15 10
10 8 6 4
5
2 0
0 0
15
30 Time (min)
45
60
0
300
600
900
Srring speed (rpm)
Fig. 3. Effect of different experimental conditions on leaching of organic solution into the sample during extraction. a) Sample pH, b) concentration of AliquatÒ 336, c) extraction time, d) stirring speed.
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120
Cd Extracon efficacy (%)
Extracon effiacy (%)
120 100
Ag
80 60 40 20
Cd
100 80
Ag
60 40 20
0
0 0
10
20
30 40 Time (min)
50
60
70
300
500
700 900 Srring speed (rpm)
1100
1300
Fig. 5. Effect of a) time of extraction and b) stirring speed Ag r and Cd j on extraction efficacy.
1 ¼ 89:28 t 419:06 C Cd
ð2Þ
1 ¼ 36:33 t 42:25 C Ag
ð3Þ
On the other hand, extraction was increased from 350 rpm to 500 rpm stirring speed for Cd and in the case of Ag extraction efficacy was stabilized at 800 rpm. As it can be observed in Fig. 5b, extraction efficacy remains stable at higher stirring rate up to 1100 rpm. In other hollow fiber supported liquid membrane sytems for Ag extraction as tri-isobutylphosphine sulphide dissolved in dihexylether, the organic solution is lost from the support at lower stirring speed of the samples, diminishing the efficacy of the system [25]. The use of AliquatÒ 336 resulted in higher stability of the organic solution in the fiber against stirring speed, reducing the loss of organic solution into the water samples, and improving its environmental profile. In the case of hydrodynamic conditions, higher leaching can be observed for longer extraction times (Fig. 3c). However, concentration of dissolved organic carbon is stabilized after 45 min. Finally stirring speed does not seem to influence leaching from the fibers into the samples (Fig. 3d), showing that the presence of ionic liquid improves the stability of the organic solution in the fiber. Conditions selected for further experiments were stirring speed 800 rpm and time of extraction 30 min because enough extraction
was reached to evaluate the efficacy of the system and to shorten experiment time and leaching is significantly increased for longer extraction times. 3.2. Effect of organic matter concentration and salinity of samples Salinity of sample and in particular the concentration of Cl is a key factor for the extraction of Cd and Ag from water samples because their speciation depends on the Cl concentration [27,28]. This fact is of particular interest in the case of estuarine samples were salinity may vary from 1 g L1 to 30 g L1. Fig. 7 shows how the extraction efficacy for both Cd and Ag, was increased at higher Cl concentration, until it is stabilized at 10 g L1 Cl in the case of Ag and 30 g L1 for Cd. This can be explained by the different speciation of Cd and Ag with the increasing salinity. Extraction yield for
Extracon efficacy (%)
AliquatÒ 336 concentration, and it could be said that the kinetic of Cd extraction is faster than that observed for Ag. Additionally the kinetic of Ag and Cd transport has been evaluated. As it can be observed in Fig. 6, reaction of Ag and Cd with AliquatÒ 336 fits for a second order model, being R2 = 0.981 for Cd and R2 = 0.984 for Ag. In the case of Cd (Eq. (2)) a higher slope was obtained than in the case of silver (Eq. (3)), explaining the faster extraction of Cd.
Cd Ag
0
10
20
30 40 NaCl (g L-1)
50
60
70
Fig. 7. Influence of NaCl concentration in the sample on extraction efficacy. Ag r, Cd j.
250
200
2500
DOC (mg L-1)
3000
1/C (L mmol-1)
100 90 80 70 60 50 40 30 20 10 0
Cd
2000
Ag
1500
150 100 50
1000 500
0
0 0
10
20
30 40 Time (min)
50
60
Fig. 6. Evolution of 1/CCd j, and 1/CAg r against time of extraction.
70
0 mgL-1 HA 30 g L-1 NaCl
30 mgL-1 HA 0 g L-1 NaCl
30 mgL-1 HA 30 g L-1 NaCl
Condions Fig. 8. Effect of NaCl and dissolved organic carbon concentration on leaching of organic solution into the sample during extraction.
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Table 1 Extraction efficacy for Ag and Cd in real samples. Extraction (%) Sample
pH
DOC (mg L
Tap water Río Tinto San Pedro River Hyper-saline lagoon
7.7 2.1 7.9 7.6
0.83 5.60 2.04 9.88
1
)
Ag varied in the range 61.46 ± 2.55–81.95 ± 2.25%, while for Cd it varied from 46.47 ± 7.59% at 1 g L1 Cl to 89.90 ± 2.25% at 30 g L1 Cl, showing that Cl has a higher influence on the extraction for Cd than for Ag. Regarding the effect of organic matter, the extraction efficacy did not suffer considerable changes in the both saline and nonsaline samples spiked with 30 mg L1 of humic acids. Leaching was reduced by the increasing concentration of NaCl in the sample (Fig. 8). This could be related to an increase in the ionic strength of the sample, leading to smaller solubility of the ionic liquid, producing a ‘‘salting-out” effect. 3.3. Application to real samples Samples with different pH, salinity, and organic matter concentration were extracted. Non-saline waters used were tap water and a sample collected from an acidic (pH = 2.09) tributary river of Río Tinto River located in the South-western Spanish mining area of Rio Tinto. Saline samples treated were a seawater sample from the San Pedro River, which is an arm of the sea fed by the Bay of Cádiz (Spain) and a water sample from a hyper-saline lagoon located in the Natural Park of the Bay of Cádiz. As it was expected, the best results were obtained for saline samples (Table 1). This effect was more important in the case of Cd, as predicted from results in Fig. 7. A maximum extraction for seawater of 88.10 ± 4.14% was obtained for Cd, and 61.46 ± 3.00% for Ag. In the case of the hyper-saline lagoon results were 92.73 ± 5.37% for Cd and 64.23 ± 2.85% for Ag. However, for non-saline samples, extraction efficacy was poorer for both metals. In the case of Cd, the effect was more important and extraction was always below or in the level of 5%, while for Ag, extraction efficacy was 29.24 ± 4.63% for the tributary of Río Tinto River. 4. Conclusion Solvent bar extraction based on the use of the ionic liquid AliquatÒ 336 as extractant constitutes an efficient, green and lowcost methodology for the extraction of Cd and Ag from saline water samples and hyper-saline waters. Moreover, due to its high efficacy in short operation times, it can be considered as a potential tool to overcome the problem of inefficacy of metals extraction from saline matrices. Acknowledgments The present work was financed by the Spanish Ministry of Economy and Competitiveness. Project CTM-2013-47549-P. References [1] Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy, (n.d.). [2] Directive 2006/11/EC of the European Parliament and of the Council of 15 February 2006 on pollution caused by certain dangerous substances discharged into the aquatic environment of the Community, (n.d.).
1
NaCl (g L 0.08 0.03 37.82 70.39
)
Ag
Cd
9.14 ± 1.17 29.24 ± 4.63 61.46 ± 3.00 64.23 ± 2.85
1.79 ± 1.08 5.09 ± 2.58 88.10 ± 4.14 92.73 ± 5.37
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