Understanding mercury extraction mechanism in ionic liquids

Understanding mercury extraction mechanism in ionic liquids

Separation and Purification Technology 116 (2013) 294–299 Contents lists available at SciVerse ScienceDirect Separation and Purification Technology jo...

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Separation and Purification Technology 116 (2013) 294–299

Contents lists available at SciVerse ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Understanding mercury extraction mechanism in ionic liquids Maria Vincenza Mancini a, Nicoletta Spreti a,c, Pietro Di Profio b,c, Raimondo Germani c,⇑ a

Department of Physical and Chemical Sciences, University of L’Aquila, Via Vetoio, I-67100 Coppito, L’Aquila, Italy Department of Pharmacy, University of Chieti-Pescara ‘‘G. D’Annunzio’’, Via dei Vestini 31, I-66013 Chieti, Italy c CEMIN, Center of Excellence on Innovative Nanostructured Materials, Department of Chemistry, Biology and Biotechnology, University of Perugia, Via Elce di Sotto 8, I-06123 Perugia, Italy b

a r t i c l e

i n f o

Article history: Received 19 November 2012 Received in revised form 3 June 2013 Accepted 6 June 2013 Available online 13 June 2013 Keywords: Ionic liquids Liquid–liquid extraction Mercury ion Salt effect

a b s t r a c t In this paper the complete removal of mercury ions from aqueous solutions using hydrophobic ionic liquids in the absence of chelating agents is reported. Several parameters were studied; in particular, the anionic component of ionic liquid, the anion of the metal salt and the nature of the aqueous phase used to dissolve the mercuric salt were varied to understand the transfer mechanism of Hg(II) ions into ‘‘classical’’ imidazolium-based ionic liquid. Results seem to suggest that metal ion partition into 1-octyl-3methylimidazolium salts involves neutral and/or anionic Hg(II) species and that the rate of the process is strongly dependent on the experimental conditions, such as the working temperature and the nature of the anionic component of the ionic liquid, of the buffer solution and of the counterion of Hg(II). Moreover, addition of inorganic salts, such as NaCl or NaBr, to the aqueous phase increases metal ion extraction rate, with more than 90% of metal ion being transferred into the organic phase within 30 min and total ion removal accomplished in just four hours. Salt effects could even overcome the high viscosity of the ionic liquid and then a simple method for large-scale Hg(II) extraction was developed. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Considerable attention has been recently focused onto ionic liquids (ILs) since they are considered as a promising alternative to classical organic solvents. Therefore they have been applied in many fields of chemical and industrial applications, as reported in some recent reviews [1–4]. Ionic liquids are a large class of low-melting salts that are liquids below 100 °C, usually comprised of large asymmetric organic cations and different inorganic or organic anions, with shielded or delocalised charges. Their most important feature is their remarkable structural tunability, and even slight changes in both the cationic and/or anionic components have been shown to produce different physicochemical properties of ILs [5]. Application of ionic liquids to separation technology has attracted great attention [2–4]; recently, liquid–liquid extraction of actinides and lanthanides has been reviewed and considerable attention has been focused to the extraction mechanism [6]. Understanding underlying mechanisms and finding the variables involved in the control of the processes is therefore a key-step to identify the ionic liquids best suited to a specific application and for a better design of novel extraction systems [7]. Efficient liquid/liquid extraction systems can be obtained by replacing traditional molecular solvents with water-immiscible ⇑ Corresponding author. Tel.: +39 075 5855632; fax: +39 075 5855560. E-mail address: [email protected] (R. Germani). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.06.006

ionic liquids, since they have no detectable vapour pressure and are relatively thermally stable, allowing to avoid many environmental and safety problems related to the use of volatile organic compounds (VOCs). Therefore, IL-based extraction systems of a wide range of metal ions have been extensively investigated [8– 10], generally with encouraging results. Two main approaches have been adopted in liquid/liquid extraction of metal ions: (i) proper chelating agents can be solubilised in ionic liquid phase [11–14], or (ii) task-specific ionic liquids (TSILs) can be designed and synthesised not only to increase the affinity of a target species for the IL over a second phase but also to greatly diminish the chance for ligand loss to the aqueous phase. In fact, TSILs contain specific chelating groups covalently incorporated within one of the ionic liquid components, typically within the cation [15–18]. Recently, de los Ríos et al. [19,20] reported the extraction of Zn(II), Cd(II) and Fe(III) in ionic liquids in the absence of chelating agents. Among the selected ionic liquids, methyltrioctylammonium chloride ([MTOA][Cl]) allowed almost complete removal of the three metal ions from acidic aqueous solution, while the selective separation of Zn(II) and Cd(II) over Fe(III) and Cu(II) was accomplished with 1-methyl-3-octylimidazolium tetrafluoroborate [C8MIM][BF4]. Mercury is considered the most toxic nonradioactive metal and, taking into account its water-solubility and then its bioavailability, its removal from aqueous solutions is a very important issue. To date, the partition of Hg(II) into an IL extracting phase in

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liquid/liquid systems is accomplished by the use of TSILs containing a bis-imidazolium cation incorporating a short ethylene-glycol spacer [21] or thioether, thiourea and urea functional groups incorporated in the alkyl chains of the imidazolium cation [15,22]. In a previous work [23], we reported the first example of the complete spontaneous partition of Hg(II) ions into a ‘‘classical’’ imidazolium-based ionic liquid, i.e. 1-alkyl-3-methylimidazolium hexafluorophosphate, where the only ‘‘functionalisation’’ is related to the hydrophobic moiety. Interestingly, binding and transport properties of metal ions in such peculiar media could be controlled, by the working temperature and by the alkyl chain length on the imidazole ring. In this work, a thorough study of the partitioning properties of ionic liquids towards mercuric ions is reported. The anionic component of ionic liquid, the anion of the metal salt and the nature of the aqueous phase used to dissolve the mercuric salt were varied to understand the transfer mechanism of Hg(II) ions in imidazolium-based ionic liquid. A better knowledge on these issues is indeed of primary importance not only in view of developing efficient back-extraction systems but also to understand if other metallic species could be spontaneously partitioned in ionic liquids by modifying specific experimental variables. 2. Experimental 2.1. Metal ion analysis The concentration of Hg(II) ions in the aqueous phase was measured by following the sensitive spectrophotometric method described in an earlier paper [24]. A diode array Hewlett Packard 8452 A spectrophotometer was used for the quantitative determinations.

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J = 7.4 Hz, 2H, N-CH2); 7.51 (d, J = 1.8 Hz, 1H, CH); 7.59 (d, J = 1.8 Hz, 1H, CH); 8.95 (s, 1H, N-CH-N). 2.4. Synthesis of [C8MIM][PF6] To a stirring solution of [C8MIM][Br] (186.41 g, 0.6773 mol) and water (150 mL) at 0 °C, 50% excess hexafluorophosphoric acid (200 mL of 60% aqueous solution) was slowly added. The resulting biphasic system was stirred for 4 h followed by dichloromethane extraction (5  80 mL) to remove the ionic liquid. Water wash (10  100 mL) using distilled water was used to eliminate unreacted acid. The dichloromethane and water were removed under vacuum overnight at 120 °C. The absence of bromide ions in the colourless liquid was verified by testing with silver nitrate. 2.5. Synthesis of [C8MIM][BF4] Fluoroboric acid (89.36 g of 48% aqueous solution, 0.44 mol) was added to an aqueous solution of [C8MIM][Br] (121.10 g, 0.44 mol in 140 mL) at 0 °C. Addition of the acid was gradually done with continuous stirring. When addition of the fluoroboric acid was completed, the temperature of the reaction medium was increased to 20 °C and the mixture left overnight. Traces of unreacted fluoroboric acid were removed with distilled water (10  100 mL); then, portions of dichloromethane (5  80 mL) were used to extract the ionic liquid. Removal of the remaining dichloromethane and water occurred in vacuum at 110 °C overnight. The absence of bromide ions in the liquid was verified by testing with silver nitrate. 2.6. Synthesis of [C8MIM][Tf2N]

In a screw cap, flat-bottomed vial, 1 mL of ionic liquid and 1 mL of a 5  103 M HgX2 solution in 0.15 M sodium acetate buffer, pH 4.68 were added. They were kept under stirring in a glycerine/ water bath at constant temperature and the metal ion concentration in the upper aqueous phase was monitored. Then, Hg(II) ion concentration in ionic liquid was calculated as a difference. Each experiment was done at least in triplicate and results agreed to within 5%.

To an aqueous solution of lithium bis(trifluoromethylsulfonyl) imide (Li(Tf2N)) (25.00 g, 0.087 mol in 50 mL) an aqueous solution of [C8MIM][Br] (23.67 g, 0.086 mol in 30 mL) was added and the mixture was stirred for 2 h at RT. The [C8MIM][Tf2N] produced was separated from the aqueous layer and washed with distilled water (5  100 mL) to remove residual LiBr salt and traces of unreacted Li(Tf2N). The colourless ionic liquid was subsequently heated at 110 °C under vacuum overnight to remove traces of water. The absence of bromide ions in the liquid was verified by testing with silver nitrate.

2.3. Materials

3. Results and discussion

All chemicals were analytical grade, purchased from Sigma– Aldrich and used as received. Ionic liquids were synthesised by following the reported procedures [8,25]; 1-octyl-3-methylimidazole bromide [C8MIM][Br] was synthesised and used as a precursor to produce 1-octyl-3-methylimidazole hexafluorophosphate [C8MIM[[PF6], 1-octyl-3-methylimidazole tetrafluoroborate [C8MIM][BF4] and 1-octyl-3-methylimidazole bis[trifluoromethyl)sulfonyl]imide [C8MIM][Tf2N]. [C8MIM][Br] was prepared by adding equimolar amounts of freshly distilled 1-methylimidazole and 1-octylbromide into a 500 mL round-bottomed flask fitted with a reflux condenser for 48 h at 60–70 °C with stirring until two phase formed. Unreacted starting materials were extracted with ethyl acetate and traces of solvent were removed under vacuum (50 mTorr) at 80 °C. Finally a pale yellow and slightly viscous liquid was obtained. The 1H NMR spectrum (200 MHz, CD3OD), consists of the following peaks: d = 0.87 (t, J = 7.3 Hz, 3H, CH3); 1.09–1.29 (m, 10H, 5CH2); 1.81–1.87 (m, 2H, CH2); 3.85 (s, 3H, N-CH3); 4.15 (t,

As previously reported [23], water/ionic liquid biphasic systems can be successfully employed to obtain a quantitative transfer of Hg(II) ions into the ionic liquid phase even in the absence of a chelating agent. In the previous paper we have shown that Hg(II) extraction from the buffered aqueous phase depends on both the working temperature and hydrophobicity of ionic liquid; when [C8MIM][PF6] was used, complete partitioning occurred in 12 h at 60 °C, while at lower temperatures the process was much slower (for example, just at 40 °C, the quantitative transfer of metal ions required 3 days to be completed). The extraction efficiency was evaluated solely by analysing the effects of working temperature variation (range 25–60 °C), and those related to the alkyl chain length linked to the imidazolium ring. In this paper, 1-octyl-3-methylimidazolium, as cation of ionic liquid, and the operating temperature of 60 °C were selected; the effects of others factors and of experimental conditions on Hg(II) extraction were here evaluated, to better understand the extraction mechanism. In particular, extraction efficiency was investigated as a function of the anion of both ionic liquid and metal

2.2. Extraction experiments

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3.1. Effect of the ionic liquid anion Firstly, the anionic component of the ionic liquid was varied and 1-methyl-3-octylimidazolium tetrafluoroborate ([C8MIM][BF4]) and 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide ([C8MIM][Tf2N]) were selected as the organic phase for the extraction of Hg(II) from acetate buffer aqueous solution. As known from literature [27], mutual miscibility of [C8MIM][BF4] with water increased with temperature and at the selected working temperature, i.e. 60 °C, a monophasic system was obtained. Therefore, the experimental procedure was properly changed: equal volumes of ionic liquid and HgCl2 buffered solution were kept a few minutes under stirring at 60 °C until only one phase was observed. Then, the system was cooled to 4 °C in order to achieve phase separation and Hg(II) concentration in aqueous solution was determined: in a few minutes a complete extraction of metal ion was attained. Transfer rate of Hg(II) in [C8MIM][Tf2N] at 60 °C is reported in Fig. 1, along with the results previously obtained in [C8MIM][PF6] [23]. The quantitative Hg(II) removal from aqueous phase was obtained also in [C8MIM][Tf2N], even if the required time was significantly higher. In detail, after 4 h, only 50% metal ion was transferred in the organic phase, as compared to 90% extraction obtained in [C8MIM][PF6], and the process was completed in 24 h. The just described results highlight that kinetics of mercury  transfer in ionic liquid followed the order Tf 2 N < PF 6 < BF4 and then it was strongly dependent on the anionic component of ionic liquid itself. The observed trend could be related to the progressive increase of the anion size which leads to a more delocalised charge and then to a decrease in the ability to hydrogen bonding. In fact, water solubility of both the anion [28] and the corresponding imidazolium salt [27] progressively decreases following the same or der Tf 2 N < PF 6 < BF4 . 3.2. Effect of the metal salt anion The role of the anion of the metal salt on Hg(II) extraction was then investigated; HgBr2, Hg(CH3COO)2 and HgSO4 were selected and metal ion partitioning rates in [C8MIM][PF6] at 60 °C are reported in Fig. 2. Data related to HgCl2 are shown for comparison.

5

4

[Hg(II)], mM

salt, the aqueous phase composition (buffer nature) and NaCl (NaBr) addition to the aqueous phase. Some hypotheses about the transfer mechanism of mercury ions can be advanced: (1) a cation exchange between mercury and imidazolium ions could occur; (2) an ion pair Hg(II)-counter ion could partition in ionic liquid as such; and (3) transfer could involve molecular forms of mercury; in any case, electroneutrality must be maintained in both phases. If the mechanism of cation exchange occurred, this process should become increasingly difficult as the hydrophobicity of the ionic liquid cation is increased, since the cation partitioning in the aqueous phase decreased from n-butyl to n-octyl [26]. Our previous results [23] highlighted that transfer rate of Hg(II) in ionic liquid phase follows the order C4 < C6 < C8, i.e. the longer the alkyl chain on the imidazolium ring, the faster the process; therefore, a cation exchange mechanism should be considered the less probable. Mercuric ions could transfer to the organic phase together with an anion, as an ion pair, and the counterion should come from the metal salt, the buffer or the anion of the ionic liquid. To evaluate how the nature of each species involved in the extraction process affects Hg(II) transfer rate, tests were performed following the experimental procedure formerly optimised [23].

3

2

1

0 0

4

8

12

16

20

24

time, h Fig. 1. [Hg(II)] in ionic liquid versus time at 60 °C. Aqueous phase: 5  103 M HgCl2 in 0.15 M sodium acetate buffer, pH 4.68. Organic phase: (j) [C8MIM][PF6] and (d) [C8MIM][Tf2N].

Complete metal ion extraction was obtained again within 24 h, but the rate of the process is strongly influenced by the mercury accompanying counter-anion. The different curve slopes clearly highlight that, in the presence of acetate or sulphate anions, Hg(II) removal was noticeably slower with respect to chloride, while only 4 h were needed in the case of bromide salts. This result could be firstly explained by considering the ‘‘soft’’ nature of mercury ion, able to ‘‘ignore’’ the higher concentration of acetate, used as anion buffer, than the metal counter-anion, (about two orders of magnitude). On the other hand, the different water solubility of metal salts could be also considered; in fact, when mercuric salt is dissolved in the aqueous solution, it undergoes the following dissociations:

HgX2 ¢ HgXþ þ X ¢ Hg2þ þ 2X

ð1Þ

The extent of both primary and secondary dissociations is strongly affected by the nature of the anion; in the case of chloride, and even more of bromide, secondary dissociation is negligible and the equilibrium is almost completely shifted to the left [29,30]. On the other hand, acetate and sulphate salts are more soluble than halides in acidic solutions and then their dissociation in water is more marked. Further experiments will be conducted to confirm

5

4

[Hg(II)], mM

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3

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1

0 0

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16

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time, h Fig. 2. [Hg(II)] in ionic liquid versus time at 60 °C. Aqueous phase: 5  103 M (j) HgCl2, (d) HgBr2, (N) Hg(CH3COO)2 and (.) HgSO4 in 0.15 M sodium acetate buffer, pH 4.68. Organic phase: [C8MIM][PF6].

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these hypotheses; anyway, these results suggest that the higher the concentration of undissociated or partially dissociated forms, the faster the transfer process. 3.3. Effect of aqueous phase composition To investigate the role of the buffer, tests in phosphate and phthalate buffers were performed at the same pH, i.e. 4.68; the effect of the nature of buffer species used to dissolve HgCl2 on metal ion partition in [C8MIM][PF6] at 60 °C is shown in Fig. 3. As already observed for the anion of the salt, the nature of buffer plays an important role in the transfer of Hg(II) ion to the organic phase. In particular, the replacement of acetate buffer with phthalate or phosphate caused a reduction in the extraction rate, even if such effect was much more evident in the presence of phosphate. In fact, after 12 h the amount of Hg(II) transferred to the organic phase is slightly higher than 50%, while in phthalate buffer the process was almost complete. Unlike acetate and phthalate buffers, in the case of phosphate, two anionic species are present in solution and they can alter both ionic exchange and partition equilibria between aqueous and organic phases and this probably adversely affects the extraction process. Tests were then carried out by dissolving HgCl2 and HgBr2 in water and data reported in Table 1 clearly shows that the presence of the buffer salt is helpful for the performance of the extraction system, probably because of the mass effect exerted by the buffer species in the equilibrium of HgX2 dissociation. In particular, Hg(II) removal from the aqueous phase was not only very slow, but also incomplete. In fact, after 96 h in the aqueous phase about 30% and 10% metal ion was still detected for chloride and bromide salts, respectively. Data in pure aqueous phase seem to confirm the hypothesis of a transfer that would involve different species of mercury; in fact, also in these experiments, taking into account the solubility product constants of HgCl2 (Ksp = 2.6  1015) and HgBr2 (Ksp = 6.2  1020), the higher the concentration of undissociated species, the higher the removal rate.

Table 1 Mol percentage of Hg(II) extracted from the aqueous phase to the organic phase.a. Time (h)

HgCl2

HgBr2

1 2 8 24 48 96

24.8 26.2 37.6 49.6 59.8 65.4

63.6 69.4 76.0 85.1 86.4 88.2

a Conditions: 60 °C; source phase: 1 mL of 5  103 M HgX2 in water; organic phase: 1 mL of [C8MIM][PF6].

Cl

  Cl

2

HgCl2 ¢ HgCl3 ¢ HgCl4

ð2Þ 

In particular, at 0.1 M Cl approximately equal amounts of  2 HgCl2 , HgCl3 and HgCl4 are present, but, at 1 M Cl, the equilibrium shown in Eq. (2) is almost completely shifted to the right [29]. Fig. 4 shows the effect of NaCl concentration on Hg(II) transfer rate in [C8MIM][PF6] at 60 °C. The complete Hg(II) partition in the organic phase was observed again, but the rate of the process significantly increased in the presence of salt and the higher the Cl concentration in the aqueous phase, the faster the ion transfer. Indeed, after only 30 min the Hg(II) percentage in the organic phase raised from 30, in the absence of salt, to 60%, 80% and 95% at 0.01, 0.05 and 0.1 M NaCl, respectively; moreover, in the latter case, ion removal was accomplished after just 4 h. The dependence of Hg(II) transfer rate on Cl concentration could be due to a higher partition coefficient of anionic mercury  2 complexes, i.e. HgCl3 and HgCl4 , in [C8MIM][PF6] with respect to the neutral species. Moreover, at high salt concentration, an ion exchange process can occur between [C8MIM][PF6] and chloride, as previously reported for [C6MIM][PF6] [31], which made [C8MIM][Cl] more soluble in water. Imidazolium ion, partitioned into the aqueous phase, could form a complex with neutral and/or anionic Hg(II) species and then, as a complex, moves back to the organic phase [32].

3.4. Effect of NaCl addition 3.5. Optimisation of transfer process Data collected so far seem to suggest that different species containing mercury, i.e. polyanions, undissociated and/or partially

5

5

4

4

[Hg(II)], mM

[Hg(II)], mM

In aqueous solutions mercury forms complexes HgXnþ2 for n n = 1/4; for example, at high Cl concentrations, polyanion species can be formed, as indicated by the following equilibrium:

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0 0

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36

48

60

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time, h Fig. 3. [Hg(II)] in ionic liquid versus time at 60 °C. Aqueous phase: 5  103 M HgCl2 in 0.15 M (j) sodium acetate buffer, (d) sodium phthalate buffer and (N) sodium phosphate buffer, pH 4.68. Organic phase: [C8MIM][PF6].

0

2

4

6

8

10

12

time, h Fig. 4. [Hg(II)] in [C8MIM][PF6] versus time at 60 °C in the presence of different NaCl concentrations: (j) 0, (d) 0.01 M, (N) 0.05 M and (.) 0.1 M. [HgCl2] = 5  103 M in 0.15 M sodium acetate buffer, pH 4.68.

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dissociated forms, are involved in the spontaneous mercury partition in imidazolium-based ionic liquids and that the rate of the process is deeply controlled by the experimental conditions. Moreover, as previously seen, a complete removal of Hg(II) was almost always observed, although with different times, while if mercuric salts were dissolved in pure water, not only the transfer process was very slow, but it was also incomplete. On the other hand, the addition of an inorganic salt to the aqueous phase, such as NaCl, greatly increases Hg(II) removal rate. Experiments were then performed to make the process of Hg(II) extraction more suitable for application purposes and the possibility of quantitatively transferring the metal ion in the absence of buffer was evaluated. When extraction processes were performed at 60 °C using HgCl2 solubilised in 0.1 M NaCl or NaBr aqueous solutions, metal ion transfer rate was very similar to that reported in Fig. 4: after 30 min, more than 90% Hg(II) was in the organic phase and a quantitative removal was observed in 24 and only 6 h in the presence of Cl and Br, respectively. Moreover, as previously reported [23], temperature strongly affects Hg(II) partition in [C8MIM][PF6] and the mass transfer between the aqueous and organic phases is closely controlled by the ionic liquid viscosity. In fact, as the temperature increases from 25 to 60 °C, not only the viscosity decreases from 510 to 95 cP [33], but also the requested time to completely extract Hg(II) ions from the aqueous phase drops from 7 days to 12 h. Unexpectedly, despite the high viscosity of the organic phase, in the presence of aqueous 0.1 M NaBr or NaCl, metal ion removal rate at 30 °C was quite comparable to that at 60 °C (data not shown), suggesting that a salt effect could overcome the high viscosity of the medium. We believe that the findings reported in this paper can be developed into a simple and cheap method for mercury extraction, which allows the complete removal of ions from aqueous compartments in short times and at room temperature, with interesting perspectives for large-scale applications.

4. Conclusions The transfer mechanism of mercuric ions from aqueous phase to hydrophobic ionic liquids in the absence of chelating agents is herein reported. Several experimental variables were changed, i.e. the anionic component of 1-octyl-3-methylimidazolium salt, the anion of the metal salt and the nature of the aqueous phase used to dissolve the mercuric salt. The complete removal of Hg(II) ions is always attained, but different times were requested to thoroughly extract metal ions from aqueous phase. In particular, different species of mercuric ions may be present in aqueous solution and both the nature and the concentration of the anions in the system affected the chemical equilibria to which Hg(II) is subjected. Such dissociated and undissociated species could partition differently between aqueous and organic phases, resulting in different extraction rate. Our results seem to suggest that the higher the concentration of neutral and/or anionic species, the faster the transfer process. Moreover, the addition of inorganic salts to the aqueous solution allowed us to obtain a complete removal of mercuric ions within a few hours, in the absence of buffer and at low temperature, thus making Hg(II) extraction from wastewaters perspectively suitable for large-scale applications.

Acknowledgement Support of the Ministero dell’Università e Ricerca (MIUR), (PRIN 2008, Grant Number 2008AZT7RK_002) is gratefully acknowledged.

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