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Species dependent aqueous biphasic extraction of some heavy metals
Journal of Industrial and Engineering Chemistry 18 (2012) 855–859
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Species dependent aqueous biphasic extraction of some heavy metals Debashree Das a, Kamalika Sen b,* a b
Structural Genomics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700 064, India Department of Chemistry, University of Calcutta, 92 APC Road, Kolkata 700009, India
A R T I C L E I N F O
Article history: Received 3 September 2011 Accepted 12 November 2011 Available online 21 December 2011 Keywords: Liquid–liquid extraction Aqueous biphasic Separation Nonionic surfactant Metal speciation
A B S T R A C T
A clean liquid–liquid extraction system was designed for extraction of three different heavy metals (Hg2+, Pb2+ and Bi3+) using an Aqueous Biphasic System consisting of a nonionic surfactant Triton X-100 in combination with two different salts viz., magnesium sulphate and sodium citrate. The extraction was monitored in two ways, by direct partitioning as well as by forming a complex with diphenylthiocarbazone (dithizone) for each metal ion in the Triton phase and measuring the colour intensity using a spectrophotometer. The change in extraction pattern at different pH of the salt rich phase was monitored. The possible change in speciation of these metal ions with pH was checked with the software programme ‘CHEAQS’ (Chemical Equilibria in Aquatic Systems). These heavy metals were also found to have higher extraction in the Triton phase when a prior complexation with dithizone was performed. The method may find extensive applications for the removal and decontaminations of the above mentioned toxic heavy metals from a sample. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Aqueous Biphasic System (ABS) is a greener alternative for the traditional organic-aqueous solvent extraction system. ABS has drawn the attention of many researches because of its environmental and economic viability. The formation of two distinct phases is affected the size, charge, hydrophobicity, temperature, pH and ionic strength of the medium. The properties of the biphasic systems can be greatly attributed to the incompatibility between the aqueous pools of higher and lower densities of water. ABS offers a mild condition due to low interfacial tension between the phases than that between water and any immiscible organic solvent, allowing small droplet size and larger interfacial area. Recently a thermodynamic investigation of a new surfactant based ABS consisting of Triton X-100, a non-ionic surfactant, and two different salts, MgSO4 and Na-citrate has been reported [1]. Triton X-100, i.e. octyl phenol polyethoxylene forms neutral adducts. Again this type of aqueous two phase has multifold advantages over the traditional liquid–liquid extraction systems as Triton X100 is almost non-toxic, non-volatile and biodegradable in nature. To the best of our knowledge the applications of this ABS consisting of Triton X-100 have not been studied till now. Growing industrialization generates a number of toxic metal ions in effluent wastewater as by-products and high levels of
* Corresponding author. E-mail addresses: [email protected], [email protected] (K. Sen).
different toxic metals enter in the environment in different chemical forms. Their toxicity levels vary according to the species concerned. Mercury is a distinctly ‘‘soft’’ cation, showing a strong preference for Cl, Br, I, P, S, Se and certain N-terminal ligands. Moore was the first to suggest that solvent extraction methods could be used to remove mercury from industrially generated wastewater [2]. Since then many methods were developed so far. But all these methods involve various organic reagents or solvents which are contrary to greener approach now almost mandatory for chemist community. Recently extraction of mercury has been studied using Aqueous Biphasic Systems. Extraction and speciation study of mercury was done using a PEG-salt type ABS [3,4]. Lead is another major pollutant of the atmosphere, entering mainly from smelters and car exhausts. Recently, extraction of lead was carried out using cyclohexane/water/ionic-liquid (1-butyl 3 methylimidazolium hexafluorophosphate, (bmim) (PF6) [5]. But extraction of lead in a considerable percentage using a greener system like ABS has not been observed yet. On the contrary certain Bi compounds like bismuth subnitrate and bismuth subcarbonate are used in medicine [6]. Bismuth subsalicylate is used as an antidiarrhoeal and to treat some other gastro-intestinal diseases. A solvent extraction method was developed for determination of bismuth in the form of tetra-nbutylammonium tetraiodobismuthate (III) [7]. Here we report experimental results of ABS with three different heavy metals (Hg2+, Pb2+ and Bi3+) using an ABS consisting of a nonionic surfactant Triton X-100 in combination with two different salts viz., magnesium sulphate and sodium citrate. The
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.11.142
856
D. Das, K. Sen / Journal of Industrial and Engineering Chemistry 18 (2012) 855–859
method may find extensive applications for the removal and decontaminations of the above mentioned toxic heavy metals from a sample. 2. Experimental The non-ionic surfactant Triton X-100 (octylphenol poly ethoxylene, C34H17O7) was purchased from Spectrochem Co. and used without further purification. The average number of ethylene (EO) units per molecule, the average molecular weight and the critical micellar concentration (CMC) of Triton X-100 are about 10, 646.85 g mol 1, and 130 g L 1, respectively. Magnesium sulphate (G.R., min 99.5%) and sodium citrate, Na3C6H5O7 (G.R., min 99.5%) were obtained from Merck. The complexing agent dithizone was also obtained from Merck. The experiments were performed in 15 ml graduated test tubes. The ABSs were prepared from the nonionic surfactant and salts of magnesium sulphate or sodium citrate in doubly distilled water. A double beam Perkin-Elmer Lambda 25 UV–visible spectrophotometer was used for determination of the percentage of extracted heavy metals complexed with dithizone in the Triton phase from the salt-rich phase. A digital pH metre Systronics (type no. 335) was used to measure and adjust pH of different solutions. The ABS was prepared by mixing 3 mL of Triton X-100 and 3 mL of either of the two different salt rich phases, MgSO4 (2 M) or Na3C6H5O7 (2 M). These two salts were found to form an equal volume phase separation with Triton X-100 at an optimum concentration of 2 M. HgCl2 (0.0209 M) and Bi2(SO4)3 (0.0200 M) were dissolved in measured volumes of 4(M) HCl to make the stock solutions. Pb(NO3)2 (0.0204 M) was dissolved in measured volumes of double distilled water. 2 mL aliquot of each species was introduced to the two different types of ABS, one consisting of Triton X-100 and sodium citrate as salt rich phase (ABS-1) and the other made of Triton X-100 and magnesium sulphate (ABS-2). All the sets were shaken mechanically for 10 min and then allowed to settle for some time to attain the chemical equilibrium to have the phase separation and partitioning. Phase separation was judged visually. Phase equilibrium is attained when a clear interfacial boundary vertical to the wall is formed; the volumes and the appearance of the top phase and the bottom phase are not changed with time. 2 mL from the Triton phase of all the prepared sets was carefully taken out for UV–visible spectrophotometric studies. The pHs of the salt rich phases were adjusted using dilute HCl and the effect of pH on the extraction pattern of different heavy metals were studied. Extraction of three different heavy metals in the Triton phase from sodium citrate and magnesium sulphate as salt rich phases in individual experiment were studied. Firstly, direct extraction of the metals in the surfactant phase was carried out and the extracted portions of metals in Triton phase were then allowed to form complex with dithizone for spectrophotometric analysis. In the second step, the extraction of metals was done with prior complexation with dithizone and the percentage of metals extracted by the surfactant was determined by UV–visible spectrophotometry. This was done to find out the effect of complexation on extraction of these metals onto the surfactant phase. Finally the direct extraction of all the three metals from a mixture was studied at varying pHs using both sodium citrate and magnesium sulphate as the salt rich phase. The dithizone complex of the three metal mixture was also studied for a similar extractions.
concentration in the upper (low-density) phase in spite of its inherent density being slightly greater than water. This, together with the 2 M solutions of either sodium citrate or magnesium sulphate creates a predominantly low-density water environment of this surfactant phase, due to its partially hydrophobic character. The results of ABS with different heavy metal salts from two different salt rich phases reveal differential extraction behaviour. The salts used to generate the ABS have interesting influence on the system and the resulting separations. The spectrophotometric estimation of these three metal salts with dithizone in Triton phase was done by a usual calibration procedure followed by the determination of unknown metal in our extracted solutions. The wavelength of maximum absorption (lmax) of the dithizone in Triton was referred to as the blank and that of the metal–dithizone complex in Triton was considered for calibration and estimation. The spectra showed lmax position of blank at 420 nm and that of the Hg–dithizonate, Bi–dithizonate and Pb–dithizonate complexes at 500 nm, 350 nm and 480 nm respectively. 3.1. Results of ABS with mercury From the experiment it is proved that the extraction of mercury salts from different salt rich phases depend on the pH of the salt rich phases and mercury can be extracted appreciably from both of the salt rich phases but at different pH in each case. The pH dependence of metal ion extraction in different systems has long been evidenced in separation sciences. With change of pH, the species of metal ions in the salt rich phase and the hydration sphere change, as a result percentage amount of the extracted metal ions in the surfactant rich phase changes. The pH of the sodium citrate rich phase was varied, complete extractions of mercury was observed at pH 8.6 in both cases when the metal ion was extracted directly in the Triton rich phase and also when the complexing agent, diphenyl dithiocarbazone was used (Fig. 1). But a different extraction pattern was observed at lower pH (5.3). When the complexing agent was used, percentage extraction of mercury in upper phase decreased as compared to the direct extraction of the metal by Triton X-100. The stability of dithizone complexes of metal ions decreases in the order Ag(I) > Hg(I, II) > Pd(II) > Pt(II) > Au(I, III) > Cu(I, II) > Bi(III) > In(III) >Sn(II) > Zn > Cd > Co(II) > Pb(II) > Ni(II) > Fe(II) >Mn(II) > Tl(I) [8]. Though it can be seen that mercury dithizonate is second only to silver dithizonate in stability, Triton X was taken in the most concentrated form. As a matter of fact, Triton, a liquid polymer of high molecular weight, contains phenyl ring in the long chain of the surfactant. Again mercury dithizonate complex having a large size may face steric repulsion and less amount of mercury as dithizonate complex is extracted from the salt rich phase.
3. Results and discussion The most interesting fact about ABS is that both the phases are water soluble, so they partly mix with each other though they form two distinct phases. So the upper phase is surfactant rich phase and lower phase is salt rich phase. Triton X-100 usually has a far higher
Fig. 1. Extraction profile of mercury in Triton from varying pH of 2 M sodium citrate phase.
D. Das, K. Sen / Journal of Industrial and Engineering Chemistry 18 (2012) 855–859
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different pH in 2 M MgSO4 reveals the percentage of different forms of mercury present in the medium as dissolved species and they are presented below (Fig. 3). So it can be seen that with the increase in pH mercury tends to form soluble hydroxide species and thus it prefers to remain in salt rich phase possibly due to the fact that it is a harder species than the chloride form and forms hydrogen bonding in the high density water medium. 3.2. Results of ABS with bismuth
Fig. 2. Extraction profile of mercury in Triton X-100 from 2 M magnesium sulphate phase.
Literature studies indicate that citric acid at a concentration of 10 2 M has a significant retarding effect on the adsorption of Hg by kaolinite at pHs of 6.0 and 8.0 [9]. This may be due complexation/ stabilization of Hg in presence of citrate ions. The lower extraction of Hg in Triton at a pH lower than 8 may be a possible reflection of this fact. Again a different scenario was observed when magnesium sulphate was used as the salt rich phase (Fig. 2). The mercury dithizone complex being very bulky, experiences steric crowding in the Triton rich phase and preferably remains in the salt rich phase. Complete extraction of mercury was observed at lower pH conditions when extracted directly. Using the software programme, CHEAQS for calculating the speciation of an element in solution some possible explanations for observation of decreasing extraction of mercury with the increase in pH of the salt solution were found [10]. When a metal solution is introduced in the two phase system, it first goes to that salt rich phase, and then depending on the salting out ability of the salt rich phase and various interactions mentioned earlier, it partitions to the top phase which is hydrophobic in nature. So the partitioning behaviour may depend on the speciation of the metal in salt rich phase as different species of same metal may have different solubility in a medium. The solubility of the species depends on several factors like polarity, hydrogen bonding, size, their water structuring property, etc. The results of CHEAQS for mercury at
The total concentration of Bi at all the pH of the salt rich phases studied was found to remain in the bottom phase. It was not extracted in the Triton phase though different salt rich phases were used in different concentrations and different pHs. In case of direct extraction of Bi from sodium citrate salt rich phase, it is expected from a lot of literature studies that Bi-citrate is a very stable complex at a wide range of pH having lots of utility in pharmaceuticals. In addition Bi(III) being a hard cation does not get extracted in the non ionic surfactant medium. Hence as expected Bi prefers the citrate phase [11]. The complexing agent dithizone was also used to examine its extraction behaviour. But Bi was not at all extracted even after complexation. This is probably due to the fact that Bi is in trivalent state, so it has very small cationic radius and has large Gibbs free energy of hydration. So it prefers the polar salt rich phase and hence it was not extracted in top phase directly. Again if we look at the stability order of the metal dithizonate complexes it is found that Bi dithizonate complex has much lower stability and it forms a tris complex with dithizone, so steric factor also plays a vital role in the extraction behaviour and hence the Bi dithizone complex encounters steric hindrance to get partitioned to the polymeric medium. The expected forms of Bi present in the MgSO4 medium at different pH as calculated by CHEAQS also reveals that Bi remains mostly as the hard species forms both with respect to cation and anions as Bi(III) hydroxide and sulphates. Hence the phase preference of Bi is once again justified (Fig. 4). 3.3. Results of ABS with lead Lead was extracted in Triton X-100 phase directly as well as in the form of lead dithizonate complex from the salt rich phases,
Fig. 3. Species distribution of Hg at different pH of MgSO4 as obtained from CHEAQS.
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Fig. 4. Species distribution of Bi at different pH of MgSO4 as obtained from CHEAQS.
Fig. 5. Extraction profile of lead in Triton X-100 from varying pH of 2 M sodium citrate phase.
sodium citrate and magnesium sulphate at a particular pH of the salt rich phase. Only at pH greater than 8 lead was extracted. Again another information was found that lead was extracted completely in Triton X-100 phase from sodium citrate rich phase and the percent extracted was much less in case of MgSO4 salt rich phase (Figs. 5 and 6). When complexing agent was used no extraction was found, this may be due to a similar reason, that stability of lead dithizonate complex is very low and hence it cannot overcome the steric repulsion caused by the polymeric surfactant phase.
Fig. 6. Extraction profile of lead in Triton X-100 from varying pH of 2 M magnesium sulphate phase.
Literature search revealed that a comparative study of complexation and dissolution of insoluble lead salt was done using different organic acids. Much higher complexing ability and very low dissociation constant of the citrate complex in the pH range 6.95–7.45 was found. In the case of citrate a precipitate (presumably Pb3Cit2) forms upon the addition of an equivalent
Fig. 7. Species distribution of lead at different pH of MgSO4 as obtained from CHEAQS.
D. Das, K. Sen / Journal of Industrial and Engineering Chemistry 18 (2012) 855–859
Fig. 8. Extraction profile of mercury, bismuth and lead from a mixture in Triton X100 from varying pH of 2 M sodium citrate phase.
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Fig. 10. Extraction profile of mercury, bismuth and lead from a mixture in Triton X100 from varying pH of 2 M magnesium sulphate phase.
Hg with increasing pH. These results are in accordance with those when extraction was carried out with single elements. This is also supported by the fact that the speciation of the elements remains the same in similar solutions though other metals are present in it. Finally when a dithizone complex of the metal mixture was extracted it was observed that none of the metal complex could get extracted in the Triton phase (figure not shown). This is due to high steric crowding created due to all the bulky complexes formed and they prefer to remain in the magnesium sulphate phase. 4. Conclusion
Fig. 9. Extraction profile of dithizone complexes of mercury, bismuth and lead from a mixture in Triton X-100 from varying pH of 2 M sodium citrate phase.
amount of citrate which rapidly and completely dissolves in an excess of citrate [12]. The results of calculation by CHEAQS programme at different pH in MgSO4 for Pb shows that at lower pH Pb remains as hard ionic species viz., lead sulphate in the anionic form and free lead(II) which tend to remain in the hard magnesium sulphate solution owing to hard-hard interactions (Fig. 7). At pH 9.5, however, there is a considerable amount of a diffused and soft cationic species Pb6(OH)84+ which has a large hydration sphere that is responsible for the extraction of lead in the Triton phase at this pH. 3.4. Results of ABS with all the three metals together When a direct extraction was carried out using a mixture containing all the three metals from sodium citrate phase (Fig. 8) a reflection of the extraction with individual metals could be seen due to similar type of speciation in the solution. Bi was not extracted at all. Pb and Hg were found to get increasingly extracted in the Triton phase with increasing pH of the sodium citrate salt rich phase. When the metal mixture was complexed with dithizone (Fig. 9), Bi and Pb complexes were not extracted in the Triton phase from any pH of the sodium citrate phase. However at neutral to basic pH, Hg dithizonate was extracted considerably. This is also in accordance with the experiment with single Hg dithizonate complex. From magnesium sulphate salt rich phase (Fig. 10), the direct extraction of a combination of metals shows no extraction for bismuth, increasing extraction of Pb and a decreasing extraction of
In this report we have tried to find a greener alternative for extraction of heavy metals like Hg2+, Pb2+, Bi3+ using a nonconventional liquid liquid extraction system (Aqueous Biphasic System) consisting of Triton X-100, a non-ionic surfactant and two different salts, MgSO4 (an inorganic salt) and Na3C6H5O7 (an organic salt). We found that mercury can be extracted directly in Triton phase using both of the salts at a certain pH of the salt rich phase. Again we found an interesting characteristic about bismuth that it was not extracted at all in the Triton phase. Sodium citrate was more effective in the partitioning behaviour of lead at a definite pH of the medium. It was also observed that from a mixture containing all the three metals, Hg can be extracted from acidic medium and Pb can be extracted from a basic medium of magnesium sulphate salt rich phase. So by summing up all the observations found in this report we may conclude this new type of ABS can be used for removing and purifying the heavy metals in a mixture as well as to separate these metals from one another. This ABS, as it offers a greener approach, may be a useful alternative for separation media in future. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
S. Alireza, T.M. Somayeh, R.F. Mina, Calphad 34 (2010) 81. F.L. Moore, Environ. Sci. Technol. 6 (1972) 525. R.D. Rogers, S.T. Griffin, J. Chromatogr. B 711 (1998) 277. K. Roy, S. Lahiri, Appl. Radiat. Isot. 67 (2009) 1781. F. Macfigek, P. Bartog, P. Gerhart, J. Radioanal. Nucl. Chem. 229 (1998) 87. R. Hammond, Handbook of Chemistry and Physics, 81st edition, CRC Press, 2004. K. Hasebe, M. Taga, Talanta 29 (1982) 1135. N.A. Gavrilenko, N.V. Saranchina, G.M. Mokrousov, J. Anal. Chem. 62 (2007) 832. J. Singh, P.M. Huang, U.T. Hammer, W.K. Liaw, Clays Clay Miner. 44 (1996) 41. W. Verweij, CHEAQS PRO: a program for calculating chemical equilibria in aquatic systems, 2005.
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Species dependent aqueous biphasic extraction of some heavy metals Debashree Das a, Kamalika Sen b,* a b
Structural Genomics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700 064, India Department of Chemistry, University of Calcutta, 92 APC Road, Kolkata 700009, India
A R T I C L E I N F O
Article history: Received 3 September 2011 Accepted 12 November 2011 Available online 21 December 2011 Keywords: Liquid–liquid extraction Aqueous biphasic Separation Nonionic surfactant Metal speciation
A B S T R A C T
A clean liquid–liquid extraction system was designed for extraction of three different heavy metals (Hg2+, Pb2+ and Bi3+) using an Aqueous Biphasic System consisting of a nonionic surfactant Triton X-100 in combination with two different salts viz., magnesium sulphate and sodium citrate. The extraction was monitored in two ways, by direct partitioning as well as by forming a complex with diphenylthiocarbazone (dithizone) for each metal ion in the Triton phase and measuring the colour intensity using a spectrophotometer. The change in extraction pattern at different pH of the salt rich phase was monitored. The possible change in speciation of these metal ions with pH was checked with the software programme ‘CHEAQS’ (Chemical Equilibria in Aquatic Systems). These heavy metals were also found to have higher extraction in the Triton phase when a prior complexation with dithizone was performed. The method may find extensive applications for the removal and decontaminations of the above mentioned toxic heavy metals from a sample. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Aqueous Biphasic System (ABS) is a greener alternative for the traditional organic-aqueous solvent extraction system. ABS has drawn the attention of many researches because of its environmental and economic viability. The formation of two distinct phases is affected the size, charge, hydrophobicity, temperature, pH and ionic strength of the medium. The properties of the biphasic systems can be greatly attributed to the incompatibility between the aqueous pools of higher and lower densities of water. ABS offers a mild condition due to low interfacial tension between the phases than that between water and any immiscible organic solvent, allowing small droplet size and larger interfacial area. Recently a thermodynamic investigation of a new surfactant based ABS consisting of Triton X-100, a non-ionic surfactant, and two different salts, MgSO4 and Na-citrate has been reported [1]. Triton X-100, i.e. octyl phenol polyethoxylene forms neutral adducts. Again this type of aqueous two phase has multifold advantages over the traditional liquid–liquid extraction systems as Triton X100 is almost non-toxic, non-volatile and biodegradable in nature. To the best of our knowledge the applications of this ABS consisting of Triton X-100 have not been studied till now. Growing industrialization generates a number of toxic metal ions in effluent wastewater as by-products and high levels of
* Corresponding author. E-mail addresses: [email protected], [email protected] (K. Sen).
different toxic metals enter in the environment in different chemical forms. Their toxicity levels vary according to the species concerned. Mercury is a distinctly ‘‘soft’’ cation, showing a strong preference for Cl, Br, I, P, S, Se and certain N-terminal ligands. Moore was the first to suggest that solvent extraction methods could be used to remove mercury from industrially generated wastewater [2]. Since then many methods were developed so far. But all these methods involve various organic reagents or solvents which are contrary to greener approach now almost mandatory for chemist community. Recently extraction of mercury has been studied using Aqueous Biphasic Systems. Extraction and speciation study of mercury was done using a PEG-salt type ABS [3,4]. Lead is another major pollutant of the atmosphere, entering mainly from smelters and car exhausts. Recently, extraction of lead was carried out using cyclohexane/water/ionic-liquid (1-butyl 3 methylimidazolium hexafluorophosphate, (bmim) (PF6) [5]. But extraction of lead in a considerable percentage using a greener system like ABS has not been observed yet. On the contrary certain Bi compounds like bismuth subnitrate and bismuth subcarbonate are used in medicine [6]. Bismuth subsalicylate is used as an antidiarrhoeal and to treat some other gastro-intestinal diseases. A solvent extraction method was developed for determination of bismuth in the form of tetra-nbutylammonium tetraiodobismuthate (III) [7]. Here we report experimental results of ABS with three different heavy metals (Hg2+, Pb2+ and Bi3+) using an ABS consisting of a nonionic surfactant Triton X-100 in combination with two different salts viz., magnesium sulphate and sodium citrate. The
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.11.142
856
D. Das, K. Sen / Journal of Industrial and Engineering Chemistry 18 (2012) 855–859
method may find extensive applications for the removal and decontaminations of the above mentioned toxic heavy metals from a sample. 2. Experimental The non-ionic surfactant Triton X-100 (octylphenol poly ethoxylene, C34H17O7) was purchased from Spectrochem Co. and used without further purification. The average number of ethylene (EO) units per molecule, the average molecular weight and the critical micellar concentration (CMC) of Triton X-100 are about 10, 646.85 g mol 1, and 130 g L 1, respectively. Magnesium sulphate (G.R., min 99.5%) and sodium citrate, Na3C6H5O7 (G.R., min 99.5%) were obtained from Merck. The complexing agent dithizone was also obtained from Merck. The experiments were performed in 15 ml graduated test tubes. The ABSs were prepared from the nonionic surfactant and salts of magnesium sulphate or sodium citrate in doubly distilled water. A double beam Perkin-Elmer Lambda 25 UV–visible spectrophotometer was used for determination of the percentage of extracted heavy metals complexed with dithizone in the Triton phase from the salt-rich phase. A digital pH metre Systronics (type no. 335) was used to measure and adjust pH of different solutions. The ABS was prepared by mixing 3 mL of Triton X-100 and 3 mL of either of the two different salt rich phases, MgSO4 (2 M) or Na3C6H5O7 (2 M). These two salts were found to form an equal volume phase separation with Triton X-100 at an optimum concentration of 2 M. HgCl2 (0.0209 M) and Bi2(SO4)3 (0.0200 M) were dissolved in measured volumes of 4(M) HCl to make the stock solutions. Pb(NO3)2 (0.0204 M) was dissolved in measured volumes of double distilled water. 2 mL aliquot of each species was introduced to the two different types of ABS, one consisting of Triton X-100 and sodium citrate as salt rich phase (ABS-1) and the other made of Triton X-100 and magnesium sulphate (ABS-2). All the sets were shaken mechanically for 10 min and then allowed to settle for some time to attain the chemical equilibrium to have the phase separation and partitioning. Phase separation was judged visually. Phase equilibrium is attained when a clear interfacial boundary vertical to the wall is formed; the volumes and the appearance of the top phase and the bottom phase are not changed with time. 2 mL from the Triton phase of all the prepared sets was carefully taken out for UV–visible spectrophotometric studies. The pHs of the salt rich phases were adjusted using dilute HCl and the effect of pH on the extraction pattern of different heavy metals were studied. Extraction of three different heavy metals in the Triton phase from sodium citrate and magnesium sulphate as salt rich phases in individual experiment were studied. Firstly, direct extraction of the metals in the surfactant phase was carried out and the extracted portions of metals in Triton phase were then allowed to form complex with dithizone for spectrophotometric analysis. In the second step, the extraction of metals was done with prior complexation with dithizone and the percentage of metals extracted by the surfactant was determined by UV–visible spectrophotometry. This was done to find out the effect of complexation on extraction of these metals onto the surfactant phase. Finally the direct extraction of all the three metals from a mixture was studied at varying pHs using both sodium citrate and magnesium sulphate as the salt rich phase. The dithizone complex of the three metal mixture was also studied for a similar extractions.
concentration in the upper (low-density) phase in spite of its inherent density being slightly greater than water. This, together with the 2 M solutions of either sodium citrate or magnesium sulphate creates a predominantly low-density water environment of this surfactant phase, due to its partially hydrophobic character. The results of ABS with different heavy metal salts from two different salt rich phases reveal differential extraction behaviour. The salts used to generate the ABS have interesting influence on the system and the resulting separations. The spectrophotometric estimation of these three metal salts with dithizone in Triton phase was done by a usual calibration procedure followed by the determination of unknown metal in our extracted solutions. The wavelength of maximum absorption (lmax) of the dithizone in Triton was referred to as the blank and that of the metal–dithizone complex in Triton was considered for calibration and estimation. The spectra showed lmax position of blank at 420 nm and that of the Hg–dithizonate, Bi–dithizonate and Pb–dithizonate complexes at 500 nm, 350 nm and 480 nm respectively. 3.1. Results of ABS with mercury From the experiment it is proved that the extraction of mercury salts from different salt rich phases depend on the pH of the salt rich phases and mercury can be extracted appreciably from both of the salt rich phases but at different pH in each case. The pH dependence of metal ion extraction in different systems has long been evidenced in separation sciences. With change of pH, the species of metal ions in the salt rich phase and the hydration sphere change, as a result percentage amount of the extracted metal ions in the surfactant rich phase changes. The pH of the sodium citrate rich phase was varied, complete extractions of mercury was observed at pH 8.6 in both cases when the metal ion was extracted directly in the Triton rich phase and also when the complexing agent, diphenyl dithiocarbazone was used (Fig. 1). But a different extraction pattern was observed at lower pH (5.3). When the complexing agent was used, percentage extraction of mercury in upper phase decreased as compared to the direct extraction of the metal by Triton X-100. The stability of dithizone complexes of metal ions decreases in the order Ag(I) > Hg(I, II) > Pd(II) > Pt(II) > Au(I, III) > Cu(I, II) > Bi(III) > In(III) >Sn(II) > Zn > Cd > Co(II) > Pb(II) > Ni(II) > Fe(II) >Mn(II) > Tl(I) [8]. Though it can be seen that mercury dithizonate is second only to silver dithizonate in stability, Triton X was taken in the most concentrated form. As a matter of fact, Triton, a liquid polymer of high molecular weight, contains phenyl ring in the long chain of the surfactant. Again mercury dithizonate complex having a large size may face steric repulsion and less amount of mercury as dithizonate complex is extracted from the salt rich phase.
3. Results and discussion The most interesting fact about ABS is that both the phases are water soluble, so they partly mix with each other though they form two distinct phases. So the upper phase is surfactant rich phase and lower phase is salt rich phase. Triton X-100 usually has a far higher
Fig. 1. Extraction profile of mercury in Triton from varying pH of 2 M sodium citrate phase.
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different pH in 2 M MgSO4 reveals the percentage of different forms of mercury present in the medium as dissolved species and they are presented below (Fig. 3). So it can be seen that with the increase in pH mercury tends to form soluble hydroxide species and thus it prefers to remain in salt rich phase possibly due to the fact that it is a harder species than the chloride form and forms hydrogen bonding in the high density water medium. 3.2. Results of ABS with bismuth
Fig. 2. Extraction profile of mercury in Triton X-100 from 2 M magnesium sulphate phase.
Literature studies indicate that citric acid at a concentration of 10 2 M has a significant retarding effect on the adsorption of Hg by kaolinite at pHs of 6.0 and 8.0 [9]. This may be due complexation/ stabilization of Hg in presence of citrate ions. The lower extraction of Hg in Triton at a pH lower than 8 may be a possible reflection of this fact. Again a different scenario was observed when magnesium sulphate was used as the salt rich phase (Fig. 2). The mercury dithizone complex being very bulky, experiences steric crowding in the Triton rich phase and preferably remains in the salt rich phase. Complete extraction of mercury was observed at lower pH conditions when extracted directly. Using the software programme, CHEAQS for calculating the speciation of an element in solution some possible explanations for observation of decreasing extraction of mercury with the increase in pH of the salt solution were found [10]. When a metal solution is introduced in the two phase system, it first goes to that salt rich phase, and then depending on the salting out ability of the salt rich phase and various interactions mentioned earlier, it partitions to the top phase which is hydrophobic in nature. So the partitioning behaviour may depend on the speciation of the metal in salt rich phase as different species of same metal may have different solubility in a medium. The solubility of the species depends on several factors like polarity, hydrogen bonding, size, their water structuring property, etc. The results of CHEAQS for mercury at
The total concentration of Bi at all the pH of the salt rich phases studied was found to remain in the bottom phase. It was not extracted in the Triton phase though different salt rich phases were used in different concentrations and different pHs. In case of direct extraction of Bi from sodium citrate salt rich phase, it is expected from a lot of literature studies that Bi-citrate is a very stable complex at a wide range of pH having lots of utility in pharmaceuticals. In addition Bi(III) being a hard cation does not get extracted in the non ionic surfactant medium. Hence as expected Bi prefers the citrate phase [11]. The complexing agent dithizone was also used to examine its extraction behaviour. But Bi was not at all extracted even after complexation. This is probably due to the fact that Bi is in trivalent state, so it has very small cationic radius and has large Gibbs free energy of hydration. So it prefers the polar salt rich phase and hence it was not extracted in top phase directly. Again if we look at the stability order of the metal dithizonate complexes it is found that Bi dithizonate complex has much lower stability and it forms a tris complex with dithizone, so steric factor also plays a vital role in the extraction behaviour and hence the Bi dithizone complex encounters steric hindrance to get partitioned to the polymeric medium. The expected forms of Bi present in the MgSO4 medium at different pH as calculated by CHEAQS also reveals that Bi remains mostly as the hard species forms both with respect to cation and anions as Bi(III) hydroxide and sulphates. Hence the phase preference of Bi is once again justified (Fig. 4). 3.3. Results of ABS with lead Lead was extracted in Triton X-100 phase directly as well as in the form of lead dithizonate complex from the salt rich phases,
Fig. 3. Species distribution of Hg at different pH of MgSO4 as obtained from CHEAQS.
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Fig. 4. Species distribution of Bi at different pH of MgSO4 as obtained from CHEAQS.
Fig. 5. Extraction profile of lead in Triton X-100 from varying pH of 2 M sodium citrate phase.
sodium citrate and magnesium sulphate at a particular pH of the salt rich phase. Only at pH greater than 8 lead was extracted. Again another information was found that lead was extracted completely in Triton X-100 phase from sodium citrate rich phase and the percent extracted was much less in case of MgSO4 salt rich phase (Figs. 5 and 6). When complexing agent was used no extraction was found, this may be due to a similar reason, that stability of lead dithizonate complex is very low and hence it cannot overcome the steric repulsion caused by the polymeric surfactant phase.
Fig. 6. Extraction profile of lead in Triton X-100 from varying pH of 2 M magnesium sulphate phase.
Literature search revealed that a comparative study of complexation and dissolution of insoluble lead salt was done using different organic acids. Much higher complexing ability and very low dissociation constant of the citrate complex in the pH range 6.95–7.45 was found. In the case of citrate a precipitate (presumably Pb3Cit2) forms upon the addition of an equivalent
Fig. 7. Species distribution of lead at different pH of MgSO4 as obtained from CHEAQS.
D. Das, K. Sen / Journal of Industrial and Engineering Chemistry 18 (2012) 855–859
Fig. 8. Extraction profile of mercury, bismuth and lead from a mixture in Triton X100 from varying pH of 2 M sodium citrate phase.
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Fig. 10. Extraction profile of mercury, bismuth and lead from a mixture in Triton X100 from varying pH of 2 M magnesium sulphate phase.
Hg with increasing pH. These results are in accordance with those when extraction was carried out with single elements. This is also supported by the fact that the speciation of the elements remains the same in similar solutions though other metals are present in it. Finally when a dithizone complex of the metal mixture was extracted it was observed that none of the metal complex could get extracted in the Triton phase (figure not shown). This is due to high steric crowding created due to all the bulky complexes formed and they prefer to remain in the magnesium sulphate phase. 4. Conclusion
Fig. 9. Extraction profile of dithizone complexes of mercury, bismuth and lead from a mixture in Triton X-100 from varying pH of 2 M sodium citrate phase.
amount of citrate which rapidly and completely dissolves in an excess of citrate [12]. The results of calculation by CHEAQS programme at different pH in MgSO4 for Pb shows that at lower pH Pb remains as hard ionic species viz., lead sulphate in the anionic form and free lead(II) which tend to remain in the hard magnesium sulphate solution owing to hard-hard interactions (Fig. 7). At pH 9.5, however, there is a considerable amount of a diffused and soft cationic species Pb6(OH)84+ which has a large hydration sphere that is responsible for the extraction of lead in the Triton phase at this pH. 3.4. Results of ABS with all the three metals together When a direct extraction was carried out using a mixture containing all the three metals from sodium citrate phase (Fig. 8) a reflection of the extraction with individual metals could be seen due to similar type of speciation in the solution. Bi was not extracted at all. Pb and Hg were found to get increasingly extracted in the Triton phase with increasing pH of the sodium citrate salt rich phase. When the metal mixture was complexed with dithizone (Fig. 9), Bi and Pb complexes were not extracted in the Triton phase from any pH of the sodium citrate phase. However at neutral to basic pH, Hg dithizonate was extracted considerably. This is also in accordance with the experiment with single Hg dithizonate complex. From magnesium sulphate salt rich phase (Fig. 10), the direct extraction of a combination of metals shows no extraction for bismuth, increasing extraction of Pb and a decreasing extraction of
In this report we have tried to find a greener alternative for extraction of heavy metals like Hg2+, Pb2+, Bi3+ using a nonconventional liquid liquid extraction system (Aqueous Biphasic System) consisting of Triton X-100, a non-ionic surfactant and two different salts, MgSO4 (an inorganic salt) and Na3C6H5O7 (an organic salt). We found that mercury can be extracted directly in Triton phase using both of the salts at a certain pH of the salt rich phase. Again we found an interesting characteristic about bismuth that it was not extracted at all in the Triton phase. Sodium citrate was more effective in the partitioning behaviour of lead at a definite pH of the medium. It was also observed that from a mixture containing all the three metals, Hg can be extracted from acidic medium and Pb can be extracted from a basic medium of magnesium sulphate salt rich phase. So by summing up all the observations found in this report we may conclude this new type of ABS can be used for removing and purifying the heavy metals in a mixture as well as to separate these metals from one another. This ABS, as it offers a greener approach, may be a useful alternative for separation media in future. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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