The most studied field regarding the environmental protection is the removal of heavy metals from industrial effluents prior to discharge. This extensive research subject still needs a lot of efforts and improvements to achieve desired results. It is particularly true and of great significance when we refer to the treatment of aqueous solutions containing radionuclides contaminants, due to their effect on human health and environment [1–4]. Recently, ionic liquids (ILS) have been widely used, developing good results, in metal ions removal processes from aqueous solutions, especially for radionuclides removal [1–6]. It is well known that the use of ionic liquid represent a good alternative for the replacement of organic solvents because of their good physical properties including higher thermal stability, viscosity, density, lower solubility in water and lower vapor pressure [1–9]. In order to minimize the treatment cost and to enhance the treatment efficiency as an alternative to liquid–liquid extraction processes the use of ionic liquid impregnated support was proposed as a new concept [10–16]. Therefore, was used as a solid support the styrene–6.7%divinylbenzene copolymer that was grafted with aminoethylaminomethyl pendant groups, due to its high surface area, good mechanical stability, ease of regeneration and high adsorption capacity. It was showed that the use of functionalized adsorbent presents a higher loading capacity and a higher interaction with metallic cations compared to low-cost adsorbents [9, 17]. Moreover, if the macromolecular functionalized resin contains an extractant within its lattice will provide the best of advantages of both solvent extraction and ion-exchange separation techniques [9, 12].
The main objective of this paper is to study the influence of the type of ionic liquid impregnated on the styrene–divinylbenzene grafted with aminoethylaminomethyl groups upon the adsorption performance of the obtained adsorbent material in the removal process of thallium and strontium from aqueous solutions. For the adsorption of thallium and strontium ions three ionic liquids were studied: trihexyltetradecylphosphonium chloride ionic liquid – (Cyphos IL-101); 1-octyl-3-methylimidazolium tetrafluoroborate – (OmimBF4) and 1-butyl-3-methylimidazolium hexafluorophosphate – (BmimPF6).
We choose these type of ionic liquids because the imidazolium based ionic liquids were frequently used in the metal separation process through liquid-liquid extraction [18, 19], but they were not studied as impregnated solid support. Contrary various asymmetrical tetraalkylphosphonium ILs were used for the extraction of metal ions, being immobilized on various solid supports [12, 20–23]. Also in our previous researches we studied the adsorption performance of Cyphos IL-101 impregnated onto inorganic solid support in the removal process of strontium and cesium from aqueous solutions [24–27]. The purpose of this study is to establish if the highly studied imidazolium based ionic liquid can be used also as impregnated onto a solid support and we compared their adsorption performance with those of phosphonium based ionic liquid in the removal process of thallium and strontium ions from aqueous solutions.
Also our previous studies showed that ultrasonication method presents the highest efficiency in the removal process of metals ions from aqueous solutions compared with the classical method of impregnation [26, 28, 29]. For the immobilization of the studied ionic liquids on macromolecular supports (styrene–6.7%divinylbenzene) grafted with aminoethylaminomethyl pendant groups this new method (ultrasonication) was used, which needs a shorter time for the impregnation.
Chloromethylated styrene-6.7%divinylbenzene copolymer used as a starting material was supplied by Purolite Victoria (Romania) (S-6.7%DVB,%Cl = 14.22, GF = 4.01 mmoles Cl/g copolymer). The aminoethylaminomethyl grafted on styrene-6.7%divinylbenzene copolymer was obtained by the method previously described .
The studied ionic liquids – trihexyltetradecylphosphonium chloride ionic liquid – (Cyphos IL-101); 1-octyl-3-methylimidazolium tetrafluoroborate – (OmimBF4) and 1-butyl-3-methylimidazolium hexafluorophosphate – (BmimPF6) were provided from Sigma Aldrich. The structures of the used ionic liquids are presented in Scheme 1.
Thallium and strontium solutions were prepared by dilution from stock solution of 1 g/L (Merck standard solutions). All chemical used in the experiments were analytical grade and distilled water was used to prepared the solutions.
The impregnation of the ionic liquids onto the functionalized polymer
The impregnation of the ionic liquids onto the functionalized polymer was realized through ultrasonication for 10 min at amplitude of 100 % . In this purpose various quantity of the studied ionic liquids (range from 0.025 to 0.5 g) were impregnated onto 1 g of solid support. Before impregnation the ionic liquids were dissolved in acetone (0.1 g of IL in 5 mL of acetone) and after impregnation the suspension were dried at 50 °C for 24 h. After establishing of the optimum quantity of ionic liquid impregnated onto the studied polymer a higher quantity of functionalized polymer was impregnated with the studied ionic liquids in these conditions. The obtained impregnated materials were used for the further experiments.
In the first experiments the dependence of the adsorption capacity of the obtained materials, in the removal process of thallium and strontium ions, versus the influence of the ionic liquid quantity impregnated onto the functionalized polymer was studied.
One gram of each sample (impregnated ionic liquid onto the functionalized polymer) was treated with 25 mL of aqueous solutions containing 10 mg/L of the studied metal ions under continuous stirring for 2 h using a Julabo SW23 shaker.
After the time elapsed the samples were filtrated and in the resulted solution the residual concentrations of thallium and strontium ions were analyzed through atomic emission spectrometry using a Varian SpectrAA 280 type atomic absorption spectrometer.
After establishing of the optimum quantity of ionic liquid impregnated onto the studied polymer, the obtained impregnated materials was used in the removal process of thallium and strontium ions from aqueous solutions containing a wide concentration range. In this case was evaluated the influence of the type of ionic liquid impregnated onto styrene–6.7%divinylbenzene grafted with aminoethylaminomethyl groups onto the adsorption process.
The adsorption experiments were done using the batch mode.
The specific surface area (Ssp) of the copolymer used as solid support was determined by the nitrogen adsorption at the boiling temperature of liquid nitrogen, by the Haul–Dümbgen method on a Ströhlein Area Meter apparatus . The apparent density (ρap) was determined using a mercury pycnometer at 1.333 × 10–2 Pa. The specific density (ρr) was measured in n-heptane.
The average pore radius was calculated using the following equation:
The adsorbent materials obtained were subject to energy dispersive X-ray analysis (EDX); scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) in order to evaluate if the impregnation of the solid support occurred. The FTIR spectra (KBr pellets) were recorded on a Shimadzu Prestige- 21 FTIR spectrophotometer in the range 4000–400 cm–1. SEM using a Quanta FEG 250 Microscope, equipped with EDAX ZAF quantifier was used to study the surface morphology of the impregnated materials. Metal ions concentrations were determined using a Varian SpectrAA 280 Fast Sequential Atomic Absorption Spectrometer with an air-acetylene flame.
Results and discussion
Preparation and characterization of the resin
The obtaining of aminoethylaminomethyl groups grafted on styrene–6.7%divinylbenzene copolymer is presented in Scheme 2 :
The morphological characteristics of the aminoethylaminomethyl grafted on styrene–6.7%divinylbenzene copolymer used as substrate for the preparation of ionic liquid impregnated samples showed a microporous material because the average rp is under 50 nm (see Table 1) . The EDX analysis also put in evidence that the nitrogen content from the aminoethylaminomethyl groups grafted on copolymer is 19.65 %.
Morphological characterization of the aminoethylaminomethyl grafted on styrene–6.7%divinylbenzene copolymer.
|Sample||St-6.7%DVB – CH2NH-(CH2)2-NH2|
Influence of the quantity of the studied ionic liquids impregnated onto the solid support
The experimental data regarding the dependence of the adsorption capacities of the studied materials in the removal process of thallium and strontium ions from aqueous solutions function of the quantities of the ionic liquids impregnated onto the styrene–divinylbenzene grafted with aminoethylaminomethyl groups are presented in Fig. 1.
From the experimental data it can be observed that the increasing quantities of the ionic liquid impregnated onto the styrene–6.7%divinylbenzene, grafted with aminoethylaminomethyl groups, lead to the increasing of the adsorption capacities of the obtained material in the removal process of Tl+ ions and Sr2+ ions respectively, from aqueous solutions. This behavior showed that the impregnated ionic liquids are the active species responsible for the adsorption of Tl+ and Sr2+ ions, respectively, from aqueous solutions. For an impregnated quantity of ionic liquids onto 1 g of solid support higher than 0.2 g the samples looked sticker, due to the viscosity of the ionic liquids. Therefore the samples were like some conglomerations which lead to the decreasing of the contact surface with the adsorbates, and we supposed that from this reasons the adsorption capacities decreased for higher quantities of IL impregnated per 1 g of solid support. The samples obtained through impregnation of the styrene-divinylbenzene, grafted with aminoethylaminomethyl groups with the studied ionic liquids in a ratio of solid support:ionic liquid (SS:IL) of 1:0.1, were forward used for the characterization and for the batch experiments.
The FTIR analysis of the styrene-divinylbenzene grafted with aminoethylaminomethyl groups and impregnated with the various studied ionic liquids was studied and evaluated. In Fig. 2 in the IR spectrum of the compound a) are found the absorption bands from Chyphos IL 101 vibrational spectrum (the intense bands between 2950 to 2800 cm–1 are stretching vibrations of CH3 and CH2, the bands around 1400, 1100, 1050, and 800 cm–1 are attributed to the P–C vibrations) , in the IR spectrum of the compound b) are found the absorption bands from OmimBF4 vibrational spectrum (the intense bands between 2960 to 2860 cm–1 are stretching vibrations of CH3 and CH2, the imidazolium CH stretching: 3160 cm–1, 3100 cm–1; the bands between 850 to 740 cm–1 are attributed to stretching vibrations of BF4 anion)  and into IR spectrum of the compound c) are found of the absorption bands from Bmim PF6 vibrational spectrum (the intense bands between 2970 to 2850 cm–1 are stretching vibrations of CH3 and CH2, the imidazolium CH stretching: 3100 cm–1, 3160 cm–1; the bands around 840 and 760 cm–1 are attributed to the antisymmetric respectively symmetric stretching vibrations of PF6 anion) .
SEM analysis was used to study the morphology changes on the surface of the styrene-divinylbenzene, grafted with aminoethylaminomethyl groups, produced by the impregnation with the ionic liquids. Figure 3 shows the SEM images magnified to 5000×.
It is evident from Fig. 3a that the impregnation of the solid support with Cyphos IL-101 ionic liquid has led to a pronounced and characterized change of the surface morphology. The particles of the ionic liquids (white particles) are distributed onto the surface of the solid support, but present some conglomeration zones. The image of the functionalized polymer impregnated with OmimBF4 [Fig. 3b] is clearly showing the particles of the ionic liquid as individually distribute in uniform and homogeneous shapes. In this case it seems that the bounding of the ionic liquids onto the surface of the solid support is stronger in comparison with the other two studied ionic liquids. OmimBF4 reached the cavities of the solid support and the impregnation occurred deeper. In case of the impregnation of the functionalized polymer with BmimPF6 [Fig. 3c] through the ultrasonication method of impregnation onto the surface of the solid support is bounded some patchy shapes of the ionic liquid having various sizes. Moreover, the SEM images of the new obtained adsorbent materials provide a direct evidence for the existence of the styrene-divinylbenzene, grafted with aminoethylaminomethyl groups in its solid form and not a dissolved species or ions in the ionic liquids compounds. The results are clearly denoting the ability of the studied ionic liquid to absorb of the surface of styrene–divinylbenzene, grafted with aminoethylaminomethyl groups.
The EDX analysis also put in evidence that the impregnation of the styrene–divinylbenzene, grafted with aminoethylaminomethyl groups with the studied ionic liquids occurred, because in the EDX spectrum were found the specific peaks of the elements which originate from the impregnated ionic liquids. In Table 2 are presented the quantification of these compounds for each obtained impregnated materials.
The quantification of the elements from the IL onto the obtained impregnated materials.
|Functionalized polymer impregnated with Cyphos IL-101||P, wt% = 0.33||Cl, wt% = 0.23||N, wt% = 5.26|
|Functionalized polymer impregnated with OmimBF4||B, wt% = 4.76||F, wt% = 3.87||N, wt% = 7.31|
|Functionalized polymer impregnated with BmimPF6||P, wt% = 0.16||F, wt% = 7.42||N, wt% = 6.56|
Adsorption experiments of thallium and strontium
The obtained adsorbent materials were used in the removal process of thallium and strontium ions from solutions containing wide range of concentration. The effect of the initial metals ions concentrations upon the adsorption capacities of the studied materials are presented in Fig. 4.
As shown in Fig. 4, the adsorbed amount of Tl+ and Sr2+, respectively increased with increase in their initial concentration. This is expected because at higher initial concentration more efficient utilization of active sites is envisaged due to a greater driving force by a higher concentration gradient. The collision between the metal ions and the active sites of the adsorbent is the major factor in kinetics which can lead to the increase in the rate of chemical reactions. This effect is positively influenced by the increasing of the initial concentration, which lead to a higher availability of metals ions for sorption. These sorption characteristics indicate that surface saturation is dependent on the initial metal ion concentration, as this determines the amount of metal ions adsorbed by the impregnated solid support in the presence of available active sites. It can be observed from Fig. 4 that the functionalized polymer, without impregnation with ionic liquids, develops a very low efficiency in the removal process of Tl+ and Sr2+ ions from aqueous solutions. This behavior is in accordance with the conclusions raised from the influence of the quantity of the ionic liquids impregnated onto the solid support, where it was observed that the impregnated ionic liquids are responsible for the removal of studied metal ions from aqueous solutions.
The data deduced from the effect of Tl+ and Sr2+ ions initial concentrations were used to obtain various adsorption isotherms, which gives important data like: adsorption capacity of the studied materials and equilibrium coefficient for adsorption that could be used for adsorption design purpose. In the present study four adsorption isotherm models were utilized: Langmuir, Freundlich, Temkin and Dubinin-Radushkevich. These isotherms are useful for comparing results from different sources on a quantitative basis, providing information on the adsorption potential of a material with easily interpretable constants [4, 7, 8, 11, 23–25].
Langmuir isotherm model is probably the best known and most widely applied sorption isotherm, is used for monolayer adsorption onto a surface containing a finite number of identical binding sites. Applying this model is assumed that once an adsorbate molecule occupies a site no further adsorption takes place and the forces of interaction between adsorbed molecules are negligible. The linearized form of the Langmuir equation is given in Eq. (3) [4, 7, 8, 11, 23–25]:
Where qm is the maximum adsorption capacity for a monolayer coverage, KL is a coefficient related to the affinity between the adsorbent and the adsorbate. The constants qm and KL were obtained from the slope and intercept of the plot of Ce/qe against Ce shown in Fig. 5 and the Langmuir isotherm parameters are given in Table 3.
Equilibrium isotherm parameters.
|Isotherm||Parameter||Functionalized polymer impregnated with Cyphos IL-101||Functionalized polymer impregnated with OmimBF4||Functionalized polymer impregnated with BmimPF6|
The Freundlich isotherm model is the earliest known relationship describing the adsorption equation and has widely been used for many years in case of non-ideal adsorption. In this case is assumed that the adsorption of the studied adsorbate onto the surface of the adsorbent is heterogeneous [4, 7, 8, 25]. This isotherm was applied in order to determine the adsorption intensity of the adsorbent for the adsorbate. The linear form of the Freundlich isotherm equation is expressed in Eq. (4).
Where KF is a constant describing the adsorption capacity (L/mg) and 1/n is an empirical parameter related to the adsorption intensity. The plot of lnqe against lnCe is shown in Fig. 6 and the Freundlich isotherm parameters obtained are given in Table 3.
The Temkins isotherm model was also applied to the experimental data, unlike the Langmuir and Freundlich isotherm models, this isotherm takes into account the interactions between adsorbents and metal ions to be adsorbed and is based on the adsorption that the free energy of adsorption is simply a function of surface coverage [11, 26]. The derivation of the Temkin isotherm model assumes that the fall in the heat of adsorption is linear rather than logarithmic. The linear form of the Temkins isotherm model equation is given in Eq. (5).
Where RT/bT in (J/mol) corresponding to the heat of adsorption, R is the ideal gas constant, T(K) is the absolute temperature, bT is the Temkins isotherm constant. This isotherm was applied by a linear plot of qe against lnCe shown in Fig. 7 and the KT and bT were calculated from the slope and intercept respectively. The Temkins isotherm parameters and correlations coefficients are presented in Table 3.
In order to deduce the heterogeneity of the surface energies of adsorption and the characteristic porosity of the adsorbent the Dubinin–Radushkevich (D–R) isotherm model was applied to the data . The linear form of the D–R isotherm is given in Eq. (6).
The apparent energy of adsorption, E was calculated using Eq. (7):
The constants qD(mol/g) is the D–R constant representing the theoretical saturation capacity and BD(mol2/J2) is a constant related to the mean free energy of adsorption per mol of the adsorbate, R is the ideal gas constant, (8.314 J/molK), T(K) is the temperature of adsorption and E(kJ/mol) is the mean free energy of adsorption per molecule of the adsorbate when transferred to the surface of the solid from infinity in solution. The plot of lnqe against [RTln(1 + 1/Ce)]2 is shown in Fig. 8 and the constants qD and BD were calculated from the intercept and slope respectively. The D–R isotherm parameters are given in Table 3.
Comparing the isotherms applied, the Langmuir gave the best fit, followed by the Temkin and Freundlich isotherms and then the least fit was obtained with the Dubinin–Radushkevich isotherm. It can be notice that the Langmuir isotherm effectively describes the Tl+ and Sr2+ adsorption onto the studied adsorbent because is obtained a correlation coefficient closed to unity. The low values obtained for the Langmuir constant indicate that the functionalized polymer impregnated with the studied ionic liquids has a high affinity for Tl+ and Sr2+ ions. Also in this case the obtained maximum adsorption capacities of the studied adsorbent in the removal process of Tl+ and Sr2+ ions from aqueous solutions are closed with those experimentally obtained. The highest adsorption capacity of the studied radionuclides is obtained in case of the styrene-divinylbenzene, grafted with aminoethylaminomethyl groups and impregnated with OmimBF4. Also for the all three studied adsorbents are obtained better results in the process of Sr2+ ions removal process from aqueous solutions compared with the results obtained in the adsorption of Tl+ ions. An important characteristic of the Langmuir isotherm is expressed in a dimensionless constant equilibrium parameter RL. The RL value indicates the shape of the isotherm and is given in Eq. (8).
In all the cases the obtained RL values are between 0 and 1 indicating a favorable adsorption process [21–24]. This suggests the applicability of these adsorbents in the removal process of Tl+ and Sr2+ from aqueous solutions.
One important characteristic of the Freundlich isotherm is the 1/n parameter which is below one indicating also a high affinity of the studied adsorbents for the Tl+ and Sr2+ ions.
KT was used to determine the value of the Gibbs free energy of adsorption as follows:
The negative values of ΔG° confirmed the feasibility of the process and the spontaneous nature of sorption.
The D–R plot as indicate from the regression parameter showed that this isotherm did not provide a very good fit to the experimental data. If the value of the E is higher than 8 KJ/mol the sorption process is a chemisorptions one, while values of below 8 KJ/mol indicates a physical adsorption process . The value of the apparent energy of adsorption obtained in all the cases indicate chemisorptions between the functionalized polymer impregnated with the studied ionic liquids and Tl+ and Sr2+ ions.
Because all the studied adsorbent presented a higher efficiency in the removal process of Sr2+ ions compared with the efficiency developed in the removal process of Tl+ ions we used these results in order to compare the Langmuir adsorption capacity of different adsorbents for Sr2+ adsorption (Table 4).
Comparison of the Langmuir sorption capacity (qm in mg/g) of different adsorbents for Sr2+ adsorption.
|Styrene–divinylbenzene, grafted with aminoethylaminomethyl groups and impregnated with OmimBF4||15.6||Present work|
|PAN/zeolite||0.011||S. Yusan et al., 2011 |
|Dolomite||1.172||A. Ghaemi et al., 2011 |
|Hydrous ceric oxide||0.106||S.P. Mishra et al., 1995 |
|Chromium (IV) oxide||0.055||S.P. Mishra et al., 1995 |
|Florisil impregnated with Cyphos IL-101||2.94||A. Negrea et al., 2013 |
|Silica impregnated with Cyphos IL-101||3.97||A. Negrea et al., 2013 |
The impregnation of the styrene–divinylbenzene grafted with aminoethylaminomethyl groups with ionic liquid lead to the obtaining of an efficient adsorbent used in the removal process of radionuclides from aqueous solutions.
The present paper showed that the type of ionic liquid impregnated onto the styrene–divinylbenzene grafted with aminoethylaminomethyl groups influence the adsorption properties of the obtained adsorbent in the removal process of Tl+ and Sr2+ ions from aqueous solutions. It was observed that the ionic liquid impregnated onto the functionalized polymer increase the adsorption properties of the studied materials in the removal process of Tl+ and Sr2+ ions from aqueous solutions. The most efficient adsorbents proved to be those obtained through impregnation of 0.1 g of ionic liquids onto 1 g of solid support. The FTIR, SEM, and energy dispersive EDX applied to the obtained adsorbents proved the fact that the studied solid support was impregnated with the studied ionic liquids and also put in evidence the morphology changes of the functionalized polymers produced by its impregnation with these ionic liquids. In order to determine the effect of the type of ionic liquid impregnated onto the functionalized polymer, the obtained adsorbent materials were used in the removal process of thallium and strontium ions from solutions containing wide range of concentration. The experimental data showed good fit to the Langmuir isotherm, followed by the Temkin and Freundlich isotherms and then the least fit was obtained with the Dubinin–Radushkevich isotherm.
The imidazolium based ionic liquids impregnated onto the functionalized polymer have a higher efficiency compared with the phosphonium ionic liquid in the removal process of Tl+ and Sr2+ ions. This effect can be explained by the synergic effect realized by the fact that the impregnated ionic liquid has the same cation (nitrogen) as the functionalized groups grafted onto the styrene–divinylbenzene. From the imidazolium based ionic liquids 1-octyl-3-methylimidazolium tetrafluoroborate – (OmimBF4) presented a higher efficiency compared with 1-butyl-3-methylimidazolium hexafluorophosphate – (BmimPF6). All the studied adsorbents developed higher efficiency in the removal process of Sr2+ ions from aqueous solutions compared with the efficiency developed in the removal process of Tl+ ions from aqueous solutions.
This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS – UEFISCDI, project number PN-II-RU-TE-2012-3-0198.
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