J Mol Model (2014) 20:2186 DOI 10.1007/s00894-014-2186-8

ORIGINAL PAPER

A theoretical and experimental evaluation of imidazolium-based ionic liquids for atmospheric mercury capture Cristina Iuga & Corina Solís & J. Raúl Alvarez-Idaboy & Miguel Ángel Martínez & Ma. Antonieta Mondragón & Annik Vivier-Bunge

Received: 6 December 2013 / Accepted: 24 February 2014 / Published online: 15 April 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract In this work, the capacity of three different imidazolium-based ionic liquids (ILs) for atmospheric mercury capture has been evaluated. Theoretical calculations using monomer and dimer models of ILs showed that [BMIM]+[SCN]− and [BMIM]+[Cl]− ionic liquids capture gaseous Hg0, while [BMIM]+[PF6]− shows no ability for this purpose. These findings are supported by experimental data obtained using particle induced X-ray emission (PIXE) trace element analysis. Experimental and theoretical infrared data of the ILs were obtained before and after exposure to Hg. In all cases, no displacement of the bands was observed, indicating that the interaction does not significantly affect the force constants of substrate bonds. This suggests that van der Waals forces are the main forces responsible for mercury capture. Since the anion-absorbate is the driving force of the interaction, the largest charge-volume ratio of [Cl]− could explain the

This paper belongs to Topical Collection QUITEL 2013 C. Iuga (*) Universidad Autónoma Metropolitana-Xochimilco, Calzada del Hueso 1100, 04960 México, D.F., Mexico e-mail: [email protected] C. Solís Instituto de Física, Universidad Nacional Autónoma de México, Av. Insurgentes Sur s/n. Ciudad Universitaria, México, D.F., Mexico J. R. Alvarez-Idaboy : M. Á. Martínez Facultad de Química, Universidad Nacional Autónoma de México, Av. Insurgentes Sur s/n. Ciudad Universitaria, México, D.F., México M. A. Mondragón Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México, Boulevard Juriquilla 3001, 76230 Querétaro, Mexico A. Vivier-Bunge Universidad Autónoma Metropolitana-Iztapalapa, Av San Rafael Atlixco No.186, 09340 México, D.F., Mexico

higher affinity for mercury sequestration of the [BMIM]+[Cl]− salt. Keywords Ionic liquids . Atmospheric mercury . Quantum chemistry

Introduction Mercury is a neurotoxic metal that causes damage to the environment and to human health, and represents a high priority in environmental legislation. Mercury is emitted to the atmosphere from natural sources, such as volcanoes and forest fires, and by human activities, primarily coal-fired utilities and waste incineration. Human activities account for about 30–55 % of global atmospheric mercury emissions and are estimated to amount to 1,000–6,000 tons mercury per year [1]. Mercury is present in the atmosphere mainly in its elemental form (Hg0) [2, 3]. Its lifetime is of the order of 1–2 years, which provides sufficient time for long-range transport, and explains the observation of nearly uniform mixing ratios of Hg within the Earth’s atmosphere. Its assessment in the environment is highly desirable, but this element is difficult to detect by conventional sampling methods. Ionic liquids (ILs) exhibit extremely low vapor pressure, thermal stability up to 400 °C, and relatively low flammability. Because of their wide variety of properties, ILs have been applied in most fields of chemistry and chemical engineering, including organic synthesis and catalysis, biochemistry, electrochemistry, extraction, and other industrial processes. Large changes in their physicochemical properties, such as melting temperature, density, viscosity, conductivity, and solvent properties, are observed when the cation or anion are modified, and therefore the properties of ILs can be readily designed to fit the requirements needed.

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Ionic liquids have been investigated as alternative media for sequestering mercury from both liquid and gas streams [4–9]. In general, for the specific application of metal extraction from wastewater, ILs with low melting temperatures, moderate to low viscosities and very low water solubilities, are required. Extraction of heavy metals using hydrophobic ILs has been reported in numerous publications [8–13]. In particular, ILs containing imidazolium cations [14] have been employed for this purpose. 1-Butyl-3-methylimidazolium chloride IL has been investigated as a non-aqueous electrolyte medium for the recovery of metals from spent nuclear fuel and other sources [15, 16]. Imidazolium-based ILs are the most typical and widely studied ILs. 1-Butyl-3-metylimidazolium [BMIM]+ is one of the cations most commonly used in ILs (Scheme 1) [17–19]. The relative positioning of the anion and cation in the IL is an important structural factor. As an example, the observed melting point changes for ILs with different anions have been explained in terms of changes in the interaction energy and in the relative position of the ions [20, 21]. Calculations have shown that the relative position of the anion with respect to the cation changes with anion type [22, 24] and, in the case of imidazolium-based ILs, the interaction between the anion and H–C(2) is crucial in determining this structure [21, 25–27]. The recent simulated IR spectrum in the 2,800–3,200 cm−1 range was shown to change with anion position [21]. Turner et al. [20] reported ab initio calculations for the [BMIM]+X− complexes (X=F, Cl, Br, and I). Wang et al. [28] also reported density functional theory (DFT) calculations for the [BMIM]+X− complexes. Both conclude that the anion has close contact with a C–H bond of imidazolium in the optimized structures for these complexes. These studies focused mainly on the positions of the anion in the stable structures. The interaction between the imidazolium and an anion is very strong; the effects of the anion on the conformation of the [BMIM]+ cation have been discussed by Skarmoutsos et al. [29]. It is well known that hydrogen bonding significantly intensifies the formation of ion pairs in electrolyte solutions when compared to systems without specific interactions. Thus, it is assumed that hydrogen bonding leads to the formation of ion pairs or even higher aggregates within an IL. In the latter, aggregates with a kind of layer structure are formed, in which the anions are located mainly above and below the aromatic ring near C2. The occurrence of hydrogen bonding in addition

H3C

N

+

N

C4 H9

-

X

Scheme 1 Chemical structure of 1-butyl-3-methylimidazol [BMIM]+X ionic pair

to Coulombic interactions between the ions might explain the high viscosity and some of the other specific macroscopic properties of ionic liquids. In ILs in general, electrostatic interactions are a major source of cohesion. However, the bulky characteristic of the cations (and in some cases also of the anions) marks a significant difference with respect to traditional inorganic salts such as NaCl. In imidazolium-based salts, the cation’s charge is concentrated on the aromatic ring, while the side alkyl chains exhibit mostly a non-polar, closed-shell character. The latter are, in general, quite large, and they contribute significantly to cohesion by means of dispersion and van der Waals-type interactions. Recently, the particle induced X-ray emission (PIXE) technique has been used to measure Hg incorporated by ionic liquids [30, 31]. Trace element analysis by PIXE is based on a rapid, non-destructive, and multi-element analytical capability, and is applied frequently to analyze inorganic pollutants associated to particulate matter in aerosols [32]. In these studies, PIXE is particularly useful since it is sensitive enough to analyze small samples such as aerosol filters, where the adsorbed mass can be smaller than 1 mg. In the present study, we investigated the capacity of three imidazolium-based ILs to bind gas-phase mercury, using both theoretical and experimental techniques. Theoretical calculations at the molecular level were performed within the DFT framework. Experimentally, total trapped Hg determination was performed using PIXE. The following salts of the [BMIM] + cation were selected: 1-butyl-3-methylimidazolium chloride [BMIM] + [Cl] − ; 1-butyl-3methylimidazolium thiocyanate [BMIM]+[SCN]−; and 1butyl-3-methyl-imidazolium hexafluorophosphate [BMIM]+[PF6]− ionic liquids. As [BMIM]+[Cl]− is solid at room temperature [33], experimentally we used a combination of [BMIM]+[SCN]− with [BMIM]+[Cl]− 1:1(w/w), which is liquid. The nature of the interaction between uncharged mercury (Hg0) and the studied ILs is discussed.

Theoretical methodology All electronic calculations were performed with the Gaussian 09 package [34]. Geometry optimizations were carried out using the M06-2X functional in conjunction with the Stuttgart-Dresden (SDD) [35] basis set that includes relativistic effects, which are necessary to represent heavy metal atoms [36]. We chose the M06-2X functional because it has been calibrated [37] to take into account dispersion forces (including dipole induced-dipole) at an intermediate range, thus improving one of the biggest deficiencies in DFT methods. Frequency calculations at 298.15 K at all stationary points were carried out at the same level of theory as the geometry optimizations, in order to ascertain the nature of the stationary

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points. Interaction Gibbs free energies are reported and are calculated with respect to the sum of the Gibbs free energies of the separated reactants, according to:
ΔGinteraction ¼ Gcomplex − GIL þ GHg0 Thermal corrections to the energy (TCE) are included. Liquid phase effects have been included according to the corrections proposed by Okuno [38], by taking into account the free volume theory [39]. These are in good agreement with those independently obtained by Ardura et al. [40] and have been used successfully by other authors [41–47]. The expression used to correct the Gibbs free energy is: n h i o 0 ð2n−2Þ ΔGFV −ðn−1Þ ð1Þ sol ≅ΔGsol −RT In n 10 where n represents the molecularity of the reaction. According to Eq. (1), the entropy loss effects in solution cause ΔG to decrease by 2.54 kcal mol−1 for bimolecular reactions, at 298.15 K. This correction is important because the packing effects of the solvent reduce the entropy loss associated with any chemical reaction whose molecularity is equal or larger than two. Partial charges are obtained using Hirshfeld population analysis (HPA) [48, 49].

Theoretical results The problem of adequate modelling of the interaction between a metal atom and an IL is very complex, as a single IL pair is probably not sufficient and, at the same time, consideration of several IL pairs may be computationally unaffordable. In previous work, Gao et al. [50] used the M05-2X and the 631+g(d,p) basis set to model [BMIM]+[Cl]− and its interaction with volatile organic compounds. They found that the smallest model that adequately yields Gibbs free energies consists of two IL molecules in a stacked configuration. Of course, this methodology is based on gas-phase calculations, and the peculiarities of condensed phases are not included. In this work, one and two ionic pairs were used as models to mimic the proposed ionic liquids and their interaction with Hg0 atoms. In a first step, we calculated the conformational energies of the three different ion pairs and obtained the optimized structures of their most stable conformers. Next, we studied the interaction between atomic Hg and the monomer and dimer ion pairs. IL monomer models In a first step, we calculated the conformational energies of an isolated [BMIM]+ cation, and obtained the optimized structure of its most stable conformer (Fig. 1). In order to facilitate the

Fig. 1 Optimized structure of the [BMIM]+ cation and numbering scheme

discussion, the atomic numbering scheme is indicated on the figure. It is well-known that the proton at position 2 of an imidazolium ring interacts strongly with anions. Therefore, this proton plays an important role in the physicochemical properties, structure, and dynamics of the IL. Computational results for the [BMIM]+ cation charge distribution were obtained using Hirshfeld charges. They show that the positive charge is indeed located mainly on the ring C2 carbon and on the methyl carbon, C6. On both of these atoms, the computed Hirshfeld charge is approximately +0.24. Hence, since the electrostatic interaction is the major source of attraction between ions, one can expect that C4 and C6 will be the main points of interaction with anions in the ILs [23, 51]. However, the charges on the other ring carbon atoms are about 0.22 and, in fact, it is clear that the positive charge is distributed almost uniformly among all of them. Similar tendencies are obtained using other population analysis methods, as the calculated atomic charges are not quantitative values. Next, we modeled the three ion pairs: [BMIM]+[Cl]−, [BMIM]+[SCN]− and [BMIM]+[PF6]−. As mentioned before, the [BMIM]+[Cl]− is solid at room temperature. However, in the theoretical calculations, we used both [BMIM]+[Cl]− and [BMIM]+[SCN]- molecular models, in order to compare their affinity with mercury. The interactions within the IL are not very specific and the anion could, in principle, interact with several atoms on the cation. Indeed, when the [BMIM]+[Cl]−, [BMIM]+[SCN]− and [BMIM]+[PF6]− ionic pairs were modeled, different configurations were found to have similar energies. As a consequence, potential energy surfaces were extremely difficult to obtain as they involve many local minimum energy structures. The optimized minimum energy structure of the most stable IL conformers are shown in Fig. 2. The interaction free energies are obtained as the difference between the free energy of the ion pairs and the one of the corresponding isolated ions, taking into account the free volume theory correction described in the methodology section.

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Fig. 2 Optimized structures of the [BMIM]+[Cl]−, [BMIM]+[SCN]− and [BMIM]+[PF6]− ion pair complexes

Some structural changes are observed in the geometry of the anions and the [BMIM]+ cation when the ion pair is formed. In all these structures, the aliphatic chain changes its orientation to make room for the interacting anion. In the case of the thiocyanate anion, two structures have to be considered, as this anion is known to bind either through the sulfur atom, or the nitrogen atom. In fact, non-metallic thiocyanates are usually S-bonded. In the most stable complex, the N atom binds to a methyl H, while the S atom binds mainly to the H atom on C2 at a distance 2.6 Å, and also to several aliphatic chain H atoms (2.9 Å). The S–C and C–N distances remain practically unchanged in the free anion and in the complex. For [PF6]−, while the average P–F distance for any of the F atom in [PF6]− is 1.713 Å, in the complex, three of the fluorine atoms interact with the cation and separate significantly from it, their average P–F distance being 1.74 Å. Instead, the other three fluorine atoms are closer to P, at a distance of only 1.68 Å. Also, the charge on the anion is redistributed over the whole complex, when the IL is formed. The structural data of the studied ion pairs clearly show the occurrence of unconventional (i.e., involving a carbon atom)

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hydrogen bonding between the anion and the [BMIM]+ cation. It can be observed that, in all cases, the anion associates preferentially with the hydrogen atom at the imidazolium ring C2 position and with that of the methyl group. The distance between the [Cl]− anion and the imidazol C(2) H atom is 2.59 Å; the shortest distance between the [SCN]− anion and C– H(2) is about 2.53 Å. In the case of [PF6]–, the phosphorus atom is located above the C2 atom, at a distance of 3.44 Å, but two of the fluorine atoms are found at only 2.31 Å and 2.46 Å from the C2 hydrogen. As noted by Tsuzuki et al. [52], although the atomic distances in the interactions above are similar to those of conventional hydrogen bonds, the nature of the interaction between the imidazolium cation and any of the anions is completely different, both in terms of directionality and energy. Electrostatic interactions between ions are not, in general, highly orientation-dependent, because the dominant term of the electrostatic interaction is the isotropic charge–charge interaction. Also, while conventional hydrogen bond interaction energies are about −5 kcal mol−1 [53], ab initio calculations interaction energies between imidazolium and anions ([Cl]−, [SCN]−, or [PF6]−) in ionic liquids are reported to be more than 20 times larger, about −70 to −90 kcal mol−1 [23]. These energies should be dependent mainly on the charge and distance between the imidazolium ring and the anions. On the [Cl]−, the charge is a point-charge, while in [SCN]− and [PF6]− it is distributed over the molecules. In Table 1 the relationship between the interaction free energies and the molar volumes is presented. A clear correlation is observed between these quantities: for the same charge, the value of the molar volume is inversely proportional to the interaction free energy. Thus, the charge-volume ratio does explain the observed experimental results. As the charge in large anions is stabilized by delocalization, their interaction with the cation is consequently weaker. In other words, Cl− is more nucleophilic, i.e., it is a better electron donor than the other two anions, and the interactions of Cl− with an electrophilic absorbate is expected to be strongest. Interaction free energies (ΔG) between the anion and cation in the ILs, in kcal mol−1, and molar volumes of the separated anions, in cm3 mol−1, are presented in Table 1.

Table 1 Interaction free energies (ΔG) between the anion and cation in the ionic liquids (ILs), in kcal mol−1, and molar volumes of the separated anions, in cm3 mol−1 IL

ΔG

Molar volume of the anion

[BMIM]+[Cl]− [BMIM]+[SCN]− [BMIM]+[PF6]−

−101.26 −82.95 −73.75

18.96 41.52 65.56

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IL dimer models The existence of cooperative effects between ions reported in previous work [50] is necessary to explain, for example, the high viscosity and other specific macroscopic properties of the ILs. This suggests that ion pair dimers could be more realistic models than single pairs. Thus, in a third step, we modeled the double ion pairs complexes of [BMIM]+[Cl]−,

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[BMIM]+[SCN]− and [BMIM]+[PF6]−. In these structures, the occurrence of hydrogen bonding and van der Waals-type interactions, in addition to Coulombic interactions between the ions, were observed. For the double ion pairs, it was possible to identify various different manners of stacking the individual anions. In this work, the minimum energy structure was identified as one in which the imidazolium cations face each other, the side chains extend away from the rings, and the anions lie on each side of the structure. The anions are located on the edge of the space between the stacked cations, one on each side. These structures are shown in Fig. 3. Other possible secondary structures are less stable and are not presented here. The interaction energies were calculated following a procedure similar to that described above for monomers. Hg0 capture The capture of Hg0 by ILs gives rise to a Hg0-IL complex intermediate, which could involve a donor–acceptor interaction. The most stable geometries of the [BMIM]+[Cl]−⋯Hg0,

Fig. 3 Optimized structure of the double ion pair complexes

Fig. 4 Mercury interaction with [BMIM]+[Cl]−, [BMIM]+[SCN]− and [BMIM]+[PF6]− single pair ILs

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[BMIM]+[SCN]−⋯Hg0, and [BMIM]+[PF6]−⋯Hg0 single ion pair complexes were obtained and are shown in Fig. 4. In the monomer models, the Hg0 atom is always located close to the anion and over the imidazolium ring. It interacts both with the negatively charged anions and the positively charged cation, thus minimizing a pure Coulomb type interaction. Moreover, interaction distances are long, larger than 3.3 Å. The interaction energies between Hg0 and the ILs are negative in all cases, although the one for the [PF6]− complex is close to zero. The interaction distances are smallest for Hg0⋯Cl in spite of the fact that the sulfur atom radius is similar to that of chlorine and much larger than that of fluorine. This is evidence that this kind of interaction Hg0⋯Cl is the one making a larger contribution to the overall interaction energy.

The interaction strength between Hg0 and the ILs should depend on the overlap between the highest occupied molecular orbital (HOMO) on the IL and the lowest unoccupied molecular orbital (LUMO) of Hg0. Consequently, in a complex with a certain MO interaction, its frontier molecular orbitals should be common to both moieties, i.e., the HOMO electrons, for example, should be delocalized on both the anion and the cation. HOMO frontier orbitals of the Hg0-IL monomer complexes are shown in Fig. 5. It can be observed that the HOMO of the [PF6] − complex is located exclusively on the Hg atom, as no MO interaction has occurred. We therefore can presume that the interaction of Hg0 with ILs with smaller anions includes a small amount of MO (or donor–acceptor) character. The structures for the double ion pair complexes with Hg are shown in Fig. 6. In the dimer models, the Hg atom is also always located close to one anion and on the edge of the space between the imidazolium rings. The interaction distances are

Hg

Hg

[BMIM]+[Cl]−...Hg0 [BMIM]+[Cl]−...Hg0

Hg

[BMIM]+[SCN]−...Hg0

[BMIM]+[SCN]−...Hg0

Hg

[BMIM]+[PF6]−...Hg0 Fig. 5 Highest occupied molecular orbital (HOMO) frontier orbitals of the Hg0-IL monomer complexes

Hg

Hg

[BMIM]+[PF6]−...Hg0

Fig. 6 Mercury interaction with [BMIM]+[Cl]−, [BMIM]+[SCN]− and [BMIM]+[PF6]− double pair ILs

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similar to those observed for the monomers, although slightly smaller. The HOMO frontier orbitals of the Hg0-IL complexes in the dimer models are shown in Fig. 7. The interaction free energies between the IL and Hg0 for the monomer and dimer models used in this study are presented in Table 2, in kcal mol−1.

Page 7 of 9, 2186 Table 2 Interaction free energies (ΔG) between the ILs and Hg0, in kcal mol−1 IL

ΔG monomer⋯Hg0

ΔG dimer⋯Hg0

[BMIM]+[Cl]− [BMIM]+[SCN]− [BMIM]+[PF6]−

−5.57 −3.23 −0.72

−2.46 −0.38 0.54

We note that, using the monomer model, we obtain Hg0-IL interaction free energies that are negative for the three ILs considered. However, when the dimer model is used, the chlorine and thiocyanate ILs present negative interaction free energies, while a positive value is obtained for the [PF6]− IL. These results are in perfect agreement with the experimental results, which indicate that the [PF6]− IL does not capture Hg0 even after several weeks exposure. As proposed before [50], a model with only one IL pair overestimates the interactions with any absorbate.

[BMIM]+[Cl]−... Hg0

[BMIM]+[SCN]−... Hg0

[BMIM]+[PF6]−... Hg0 Fig. 7 HOMO frontier orbitals of the Hg0-IL dimer complexes

Experimental section Known amounts of ILs were introduced, in the form of bulk, in a 70 mL glass tube with a known amount of metallic mercury, which was then sealed with a PTFE cap (Fig. 8). The concentration of mercury in the IL was measured at different times for a total of 12 weeks of exposure. Total trapped Hg determination was performed by PIXE, by encapsulating the IL between two layers of 4 μm thick Prolene film immobilized in a Mylar holder (Fig. 8). The following ionic liquids were used: [BMIM]+[PF6]−, [BMIM]+[SCN]− and a combination of [BMIM] + [SCN] − with [BMIM] + [Cl] − 1:1(w/w). The reason for combining [BMIM]+[SCN]− with [BMIM]+[Cl]− 1:1(w/w) is that the latter is solid at room temperature, while its mixture with [BMIM]+[SCN]− is liquid. The amount of mercury captured by [BMIM]+[PF6]−, by [BMIM]+[SCN]− and by a combination of [BMIM]+[SCN]− with [BMIM]+[Cl]− 1:1(w/w), as a function of time of exposure is shown in Fig. 9. The results show clearly that the [BMIM]+[PF6]− sorption efficiency is very low, as previously reported [54]. The thiocyanate IL is much more efficient, and sorption improves slightly with the addition of the chlorine anion. Possible changes in the molecular structure of the ILs due to their interaction with the Hg atom were investigated using infrared (IR) spectroscopy. No changes in the peak positions in the IR were observed; neither did the calculated IR spectra produce changes in the position of the bands. The absence of new band structures after exposure to mercury indicates that Hg0 does not form a complex with the ILs. Indeed, the [SCN]− anion is the strongest ligand, and yet, when combined with [Cl] anion, which is a weak ligand, it shows the best capture capacity. The implication is that Hg0 capture must be due to a

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Fig. 8 Left An artificial Hg0 vapor atmosphere was created by introducing an aliquot of liquid Hg0 (1 g approximately), and a drop of ionic liquid (IL) was supported on a 4 μm thick Prolene film. Right Drop of IL covered by a Prolene layer, for particle induced X-ray emission (PIXE) analysis

weak interaction probably involving Van der Waals-type dispersion forces. Hg0 oxidation by ILs has been ruled out by experimental results.

Conclusions In order to investigate potential new IL candidates for extraction applications, the focus of this work was to evaluate the capacity of three different ILs for atmospheric mercury capture and to explain, at the molecular level, the interactions involved. The following salts of the [BMIM]+ cation were selected: 1-butyl-3-methyl-imidazolium chloride [BMIM]+[Cl]−; 1-butyl-3-methylimidazolium thiocyanate [BMIM] + [SCN] − and 1-butyl-3-methyl-imidazolium hexafluorophosphate [BMIM]+[PF6]− ILs. Experimental sorption results based on PIXE determinations, and computational work based on DFT calculations, agree on the evaluation of the three ILs considered. Theoretical and experimental results show that [BMIM]+[SCN]− and [BMIM]+[Cl]− ILs are effective to capture gaseous Hg0, while the [BMIM]+[PF6]− shows no ability for this purpose, and the tendency is as follows:

½BMMŠþ ½C1Š− > ½BMMŠþ ½SCNŠ− > ½BMMŠþ ½PF6 Š−

As [BMIM]+[Cl]− is solid at room temperature, experimentally we have used a combination of [BMIM]+[SCN]− with [BMIM]+[Cl]− 1:1(w/w), which is liquid. However, in the theoretical calculations we have used both [BMIM]+[Cl]− and [BMIM]+[SCN]− molecular models, in order to compare their affinity with mercury. Our results show that [BMIM]+[Cl]− is more active than the other complex, which means that, in the experiment, when using the mixture of the two ILs, mercury will probably bind to the [BMIM]+[Cl]− ionic pair. The theoretical calculations suggest that Hg0 capture results from the formation of a complex involving Hg0 and ions in the IL complex. In the case of the [BMIM]+[Cl]− IL, the [Cl]− large charge-volume ratio explains its higher affinity for mercury sequestration. Acknowledgments This work is a result of the FONCICYT (Fondo de Cooperación Internacional en Ciencia y Tecnología) Mexico-EU ‘RMAYS’ network, Project Nº 94666. Partial support was also received by DGAPA UNAM (Dirección General Asuntos del Personal Académico Universidad Nacional Autónoma de México) under grants IN112609 and IN219609. We gratefully acknowledge the Laboratorio de Supercómputo y Visualización en Paralelo at Universidad Autónoma MetropolitanaIztapalapa and the Dirección General de Cómputo y de Tecnologías de Información y Comunicación (DGTIC) at Universidad Nacional Autónoma de México for computer time.

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Fig. 9 Concentration of Hg (in micrograms) in different ILs after 35 days of exposure

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