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Fixed-bed adsorption of ionic liquids onto activated carbon from aqueous phase
Accepted Manuscript Title: Fixed-bed adsorption of ionic liquids onto activated carbon from aqueous phase Authors: Jes´us Lemus, Cristian Moya, Miguel A. Gilarranz, Juan J. Rodriguez, Jose Palomar PII: DOI: Reference:
S2213-3437(17)30514-6 https://doi.org/10.1016/j.jece.2017.10.014 JECE 1924
To appear in: Received date: Revised date: Accepted date:
16-5-2017 2-10-2017 6-10-2017
Please cite this article as: Jes´us Lemus, Cristian Moya, Miguel A.Gilarranz, Juan J.Rodriguez, Jose Palomar, Fixed-bed adsorption of ionic liquids onto activated carbon from aqueous phase, Journal of Environmental Chemical Engineering https://doi.org/10.1016/j.jece.2017.10.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fixed-bed adsorption of ionic liquids onto activated carbon from aqueous phase
Jesús Lemus*, Cristian Moya, Miguel A. Gilarranz, Juan J. Rodriguez, Jose Palomar
S.D. Ingeniería Química, Universidad Autónoma de Madrid, Ctra. de Colmenar Viejo, km 15, 28049 Madrid, Spain. *Corresponding author: [email protected]
Graphical abstract:
Highlights: The separation of ILs from water by fixed-bed adsorption with AC was evaluated. Particle size and superficial liquid velocity are key factor regarding the efficiency. The nature of IL plays a key role in the thermodynamics and kinetics of adsorption. Fixed-bed adsorption with AC was found a feasible for hydrophobic ILs removal.
Abstract Adsorption of ionic liquids (ILs) onto activated carbons (ACs) has been proposed as a thermodynamically and kinetically suitable treatment to remove and/or recover ILs from liquid phase. This work evaluates the potential of AC adsorption in a fixed-bed column to remove ILs from aqueous streams. Different operating conditions were 1
evaluated including AC particle size (0.10 mm to 0.75 mm) and surface liquid velocity (1.4 to 2.0 m·h-1). ILs of different nature were used to assess the influence of cation family (imidazolium- and pyridinium-based) and anion structure (modifying the IL polarity) in the adsorption performance. The adsorption of ILs by ACs in the fixed-bed column was described through the breakthrough curves obtained from experimental tests, which were modelled by the Yoon-Nelson equation. Adsorption capacity (qs), length of the mass transfer zone (HMTZ) and fractional bed utilization (FBU) were estimated from the breakthrough curves. The Yoon-Nelson model describes reasonably well the breakthrough curves, providing a valuable tool design for adsorption. Optimum performance was achieved for hydrophobic ILs at AC particle size between (0.10-0.25) mm and a surface liquid velocity of 1.4 m·h-1. The results of fixed-bed adsorption of ILs were comparable to the obtained with commercial AC for dyes and emergent pollutants.
Keywords: Activated carbon; ionic liquids; breakthrough curve; fixed-bed; adsorption.
1.
Introduction Ionic liquids (ILs) are one of the most rapidly developing areas of modern
chemistry, technology and engineering, focusing on the final objective of industrial applications [1, 2]. ILs present exceptional properties such as negligible vapor pressure and non-flammability under ambient conditions, high thermal and chemical stability, a wide liquid window and high solvent capacity [3-5]. Probably, the most important feature attributed to ILs is the possibility of designing their properties by an adequate selection of the counterions, thus they can be found nowadays in a wide diversity. Because of the 2
enormous number of cation and anion combinations, ILs can possess a wide spectrum of physical and chemical properties (solubility, polarity, viscosity, etc.) and they are already recognized by the chemical industry as new target-oriented reaction and separation media [6, 7]. Nevertheless, most ILs are water soluble or present miscibility with water, which may result in their transfer to aqueous streams with the corresponding environmental risks [8,9]. Therefore, the treatment of aqueous streams containing ILs must be considered for a proper use of ILs in industrial processes, since it has been demonstrated that ILs present a wide range of toxicity and biodegradability [10-12]. The treatment of aqueous streams containing ILs to remove and/or recover them has been extensively studied. Destructive methods, such as advanced oxidation [13-16] or biological treatments [17, 18], have shown high potential for the removal of ILs. However, when visualizing sustainable technologies, the degradation of ILs should be avoided and they should be recovered and recycled instead [19]. Some non-destructive methods have been reported in the literature for the recovery of ILs, such as distillation [20], crystallization [21], nanofiltration [22], pervaporation [23], phase separation [24] and adsorption [25-26]. Among the non-destructive techniques, the adsorption onto activated carbon (AC) has been proved able to recover ILs of different nature from aqueous phase [27-37]. Most of the studies on ILs adsorption onto AC have been conducted in tank operation, including both equilibrium [27,29] and kinetic experiments [30,37]. Moreover, intensification of adsorption by the addition of a salting-out inorganic salt (Na2SO4) to improve the adsorption onto AC has been reported, which is particularly interesting to overcome the difficulty to adsorb hydrophilic ILs [34]. However, to our knowledge, the adsorption of ILs using AC fixed-beds has not been evaluated. Among other advantages, fixed-bed adsorption is one of the most effective and reliable technologies currently available for the removal of different pollutants from water, because of ease to operate, 3
AC and solute loss can be minimized and can be scaled-up to industrial size from laboratory data [38-40]. The current work analyzes the adsorption of representative ILs from water onto a commercial granular AC operating in a fixed bed column under different conditions. First, the adsorption of a reference hydrophobic IL (1-octyl-3-methylimidazolium hexafluorophosphate, OmimPF6) was studied in order to evaluate the potential of this approach and compare its performance with batch-wise operation previously reported [25, 30]. Second, AC particle size (dp) and inlet superficial liquid velocity (u) were checked to
optimize these conditions for adsorption. The experimental breakthrough curves were fitted to the Yoon-Nelson model, which allowed to estimate design parameters such as kinetic constant, mass transfer zone length and percentage of fixed-bed used, and compare them with representative values of industrial operation. Finally, the behavior of different ILs was analyzed under the optimized operation conditions to assess the influence of the nature of the IL in the adsorption process.
2. Materials and methods 2.1. Materials A commercial granular AC supplied by Merck was used as adsorbent. The textural characteristics of the AC were previously reported: SBET = 927 m2·g-1, SEXT = 155 m2·g-1 and VMicrop. = 0.36 cm3·g-1 [25]. The ILs used in this study were supplied by Iolitec in the highest purity available. The ILs were used without previous purification. Table 1 summarizes the nomenclature used for the ILs together with their main properties associated to adsorption behavior, such as density (kg·L-1), viscosity (cP), molecular 4
volume (Vmolecular, Å3) and octanol-water partition coefficient (LogKow), which informs about the hydrophobicity (high IL LogKow ) or hydrophilicity (low IL LogKow ) of IL. The sample of ILs included in this study present a wide range of water-IL behavior, decreasing the solubility in the order OmimPF6 > BmimPF6 > BpyrPF6 > EmimPF6 > BmimCl and moving from one IL considered clearly hydrophobic (OmimPF6 ) to one considered hydrophilic (BmimCl).
2.2. Fixed-bed adsorption experiments
The adsorption of ILs was evaluated in a fixed bed column (Fig. 1) consisting in a 16 mm i.d. glass tube with a length of 20 mm and a cooling jacket (XK 16 column, GE Healthcare). Glass wool was used as stopper at the bottom of the bed and 2.0 g of AC was placed on it to achieve an adsorbent bed length (Hbed) around 20 mm. The height/diameter ratio (H/D) used in current experiments was 1.3 [41]. The rest of the bed was filled up with glass beads to allow a uniform flow through it. IL solutions with a concentration of ~1300 mg·L-1 were pumped downwards through the bed with a peristaltic pump (Gilson Miniplus3) at the desired flow rate (4-6 mL·min-1). In the case of OmimPF6 the solution concentration was 1000 mg·L-1 due to its lower solubility in water. The inlet superficial liquid velocity (u, m/s) was defined as the ratio of flow to section. The output IL solution was analyzed at regular time intervals (5 minutes) using a double beam UV–visible spectrophotometer (Varian, model Cary 1E) at 212 nm for imidazolium-based ILs and 260 nm for the pyridinium-based ones. The experiments were carried out at (35±1) ºC without any pH adjustment.
5
2.3. Analysis of fixed-bed data
The breakthrough time and the shape of the breakthrough curve are determining issues to analyse the operation and the dynamic response of an adsorption fixed-bed. The breakthrough curves describe the adsorption of IL from the stock solution in the adsorbent fixed-bed and is usually expressed in terms of normalized output concentration (C/C0) versus flow time or elution volume. The area under the breakthrough curve provides the total adsorbate uptake, corresponding to the equilibrium capacity (qs, mg·g-1) at the inlet concentration. It is calculated as: 𝑞𝑠 =
𝐶0 ·𝑄 𝑤
𝑡=𝑠𝑎𝑡
𝐶
∫𝑡=0 (1 − 𝐶 ) · 𝑑𝑡
(1)
0
being w (g) the mass of adsorbent used and Q (L·min-1) the inlet flow rate.
Different approaches have been used to model breakthrough curves [42-44]. The one developed by Yoon and Nelson [45] gives the expression: 𝑡 = 𝑡0.5 + 𝑘
1
𝑌−𝑁
ln 𝐶
𝐶
0 −𝐶
(2)
where t is the operation time (min) and t0.5 is the time at which the output concentration is one half of the input one and kY-N is a constant (h-1), that is named effective kinetic constant (higher kY-N values imply higher adsorption rates). These parameters are obtained from the intercept and slope, respectively, in the ln[C/(C0-C)] vs t plot, taking into account the 0.05 to 0.5 C/C0 range (Figs. S1-3 of Supporting Information). This model has been successfully applied to describe the breakthrough curves obtained in the aqueous phase adsorption of different adsorbates [43-44]. In addition to this, the length of the mass
6
transfer zone (HMTZ), which characterizes the wave front of the fixed-bed, can be estimated from the breakthrough curves using the expression [39-40]: 𝐻𝑀𝑇𝑍 = 𝐻 ·
(𝑡0.95 −𝑡0.05 )
(3)
𝑡0.95
where H is the total length of adsorbent in the fixed-bed, t0.95 and t0.05 are the times at which the output IL concentration is 95 % and 5 % of the input one, respectively. Finally, the fractional bed utilization (FBU), defined as the ratio between the capacity at breakthrough time and at saturation time (qs, fixed-bed), is described by Equation (4) [3940]. For comparison purposes, in this work the breakthrough and the saturation concentrations were set at 5 % and 95 % of the input concentration, respectively. % 𝐹𝐵𝑈 =
𝑞𝑏𝑟𝑒𝑎𝑘𝑡ℎ𝑟𝑜𝑢𝑔ℎ 𝑞𝑠,𝑓𝑖𝑥𝑒𝑑 𝑏𝑒𝑑
· 100
(4)
3. Results and discussion The breakthrough curve of a reference IL, OmimPF6, obtained operating at standard conditions: 1.7 m·h-1 superficial liquid velocity, 1000 mg·L-1 of input OmimPF6 concentration, 0.175 mm of mean AC particle size and 35 ºC can be seen in Fig. 2 (black circle series). The resulting breakthrough curve shows a typical S-shaped profile, but the breaking zone has a higher slope than the saturation one. The fixed-bed column experiment revealed an adsorption equilibrium capacity similar to that obtained previously in stirred tank experiments (447 and 450 mg·g−1, respectively) for OmimPF6 on AC at equivalent aqueous phase concentration [25]. Regarding to adsorption kinetics, the effective kinetic constant of the Yoon-Nelson model (kY-N) was 1.6 h-1, close to the one (1.75 h-1) reported for methylene blue (an organic solute commonly separated by adsorption) under similar operating conditions [44]. Moreover, the value of HMTZ was 16 7
mm and FBU was 71%, for the conditions tested, similar to those found for other solutes, as reactive dyes or organic compounds (caffeine, atenolol), separated from water using commercial AC in fixed-bed column [39-40, 46].
Fig. 2 depicts the influence of the AC particle size in the behaviour of the fixedbed column for the adsorption of OmimPF6. Table 2 collects the parameters obtained from the breakthrough curves. When decreasing the AC particle size, the apparent adsorption capacity (as defined in the Yoon and Nelson model) becomes closer to the obtained in stirred tank experiments at equilibrium with a liquid-phase concentration equal to the input concentration to the bed (qe, 450 mg·g-1). Smaller AC particles also imply a higher adsorption rate (higher kY-N values), due to the shorter diffusion path. These results are in agreement with previous works indicating that intraparticle diffusion is the prevailing rate-controlling step in the adsorption of OmimPF6 onto AC and that adsorption rate can be increased by using smaller adsorbent particle size [30]. In addition, it was demonstrated that the IL adsorption capacity of ACs is closely related to available narrow mesopores within the size range between 2 and 8 nm [25,37]. In consequence, the observed decreasing of apparent IL adsorption capacity of AC with larger particle size in fixed-bed operation seems to indicate that the adsorption onto the narrowest mesopores is affected by the longer diffusion path and maybe by the loss of pore connectivity because of pore filling. This behaviour is evidenced by the change in the slope of the breakthrough curve at high C/C0 values. Accordingly, decreasing the diameter particle allows decreasing the mass-transfer zone length (HMTZ) from 27 to 16 mm and increasing the fractional bed utilization (FBU) from 31 to 71%. The mean particle size of AC selected further in this study was 0.175 mm, which is still within the particle size range of industrial adsorbents (0.06-6.00 mm) [46]. 8
Fig. 3 shows the breakthrough curves for the adsorption of OmimPF6 at different superfical liquid velocities. As can be seen from the corresponding parameters in Table 2, the saturation capacity (qs) is not highly affected by the superficial liquid velocity, with values ranging between 413 and 447 mg·g-1. In contrast, the parameters more directly related to adsorption kinetics are affected by superficial liquid velocity. Thus, HMTZ and FBU significantly decreases and kY-N values clearly increases when using lower superficial liquid velocities, indicating that velocities lower 2 m·h-1 are more adequate for this internal diffusion-controlled adsorption phenomena of IL onto AC [47-48]. A superfical liquid velocities of 1.7 m·h-1 was selected for the further experiments and proposed as more favourable operating conditions (compromise between adsorption efficiency and operating time) for scaling column.
Fig. 4 compares the results of adsorption tests for 5 ILs with remarkably different chemical nature. The breakthrough curves show substantially different breakthrough and saturation times, and different slopes in the breakthrough region, thus evidencing the influence of the nature of the ILs adsorbed. As can be seen in Table 2, the adsorption capacities obtained now in fixed-bed experiments increase with the hydrophobicity of the ILs tested, in good agreement with previous results obtained in batch experiments [25, 30]. Interestingly, the HMTZ decreased with the hydrophobicity of the IL (see LogKow values presented in Table 2). Thus, adsorption of OmimPF6 onto AC is both thermodynamically and kinetically favored. In the case of hydrophilic ILs a substantially higher amount of adsorbent would be required or alternative approaches would be needed. The modification of the AC surface, which get double the adsorption capacity of BmimCl [25, 27] or the addition of salting-out inorganic salt (e.g. Na2SO4) to increase the adsorption efficiency, in the case the adsorption capacity is up to five times [34], are propose as promising alternatives. 9
The next step in the development of this technology would be the regeneration of the exhausted bed and the recovery of the IL. In previous work [25], it was demonstrated by batch experiments that adequate organic solvents, as acetone, can be successfully used to regenerate the exhausted AC beds. Removing IL adsorbate from solid phase by washing with acetone allowed the recovery of the IL (checked by 1H-NMR), keeping virtually intact the porous structure of the AC (checked by N2 adsorption isotherms) [27]. Future work will be devoted to evaluate different regeneration alternatives in fixed bed experiments.
4. Conclusions Previous studies performed in batch proved that adsorption onto activated carbon (AC) is a promising non-destructive treatment to remove ionic liquids (ILs) at low concentrations from aqueous solutions. In the current work, the evaluation was extended to fixed-bed tests using granular AC, for the sake of obtaining more realistic information to analyze the feasibility of adsorption as a potential solution for IL-bearing aqueous effluents to remove ILs from water. The breakthrough curves were obtained at different operating conditions (such as AC particle size and superfical liquid velocity) and with ILs of different chemical nature. Fixed-bed adsorption with AC proved a reasonably good performance yielding values of the representative parameters close to the reported for other organic compounds under similar operating conditions. Relatively low AC particle size (0.175 mm) and superfical liquid velocity (1.7 m·h-1) are recommended, due to the mass-transfer restrictions of IL adsorption. The adsorption capacity and FBU increases for ILs of lower polarity, so that hydrophobic ILs (such as OmimPF6) are better candidates for the application of this technology.
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Acknowledgements The authors are grateful to the Spanish “Ministerio de Ciencia e Innovación (MICINN)” for financial support (project CTQ2014-52288-R) and Comunidad de Madrid (project S2013-MAE-2800). We are very grateful to “Centro de Computación Científica de la Universidad Autónoma de Madrid” for computational facilities.
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Peristaltic Pump 4–6 mL·min-1
AC fixed-bed H = 20 mm
2g
UV spectrophotometer. Outlet IL quantification
Stock Solution ~ 1300 ppm Sampling collector
Fig. 1. Schematic diagram of the experimental set-up.
16
1.0
0.8
C/C0
0.6
0.4 0.625 mm
0.2
0.375 mm 0.175 mm
0.0 0
100
200
300
400
500
Time (min)
Fig. 2. Experimental (dots) and predicted (lines) breakthrough curves for OmimPF6 adsorption in fixed-bed columns using AC with different particle size (u: 1.7 m·h-1 and C0: 1000 mg·L-1).
17
Fig. 3. Experimental (dots) and predicted (lines) breakthrough curves for OmimPF 6 adsorption at different superfical liquid velocities (dp: 0.175 mm and C0: 1000 mg·L-1 ).
18
Fig. 4. Experimental (dots) and predicted (lines) breakthrough curves for the adsorption of different ILs (C0: 1300 mg·L-1, dp: 0.175 mm and u: 1.7 m·h-1).
19
Table 1. Properties of the ionic liquids used.
Name
BpyrPF6
BmimPF6 1-Butyl-3-methyl imidazolium hexafluoroborate
340
284
281
256
175
1.22
1.37
1.32 (50ºC)
1.47
1.08
720
235
Solid at 25ºC
Solid at 25ºC
4147
389
303
304
256
236
2.4
0.8
0.1
-0.4
-3.3
1-Butylpyridinium hexafluoroborate
EmimPF6 1-Ethyl-3-methyl imidazolium hexafluoroborate
BmimCl
OmimPF6 1-Methyl-3-octyl imidazolium hexafluoroborate
1-Butyl-3-methyl imidazolium chloride
Chemical structure
M weight (g·mol-1) Densitya (g·mL-1) Viscositya (cP) V molecular a (Å3) LogK ow b
a
Obtained at 25ºC from ILthermo web site. bCalculated by COSMO-RS (computational details available in the Supporting Information)
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Table 2. Operating conditions for the adsorption experiments and fitting parameters obtained from breakthrough curves. qs,
qs,
fixed-bed
tank
(mg·g-1)
1.7
0.375
FBU
t0.5
kY-N
(mg·g-1)a (mm)
(mm)
(%)
(h)
(h-1)
447
450 [25]
20
16
71
3.0
2.2
0.99
1.7
421
--
21
25
47
2.0
2.0
0.97
0.625
1.7
309
--
22
27
31
1.4
1.9
0.97
0.175
2.0
413
--
20
24
58
2.2
1.6
0.95
0.175
1.4
436
--
20
14
75
3.7
2.6
0.98
BmimPF6
0.175
1.7
306
320
20
18
70
1.9
3.4
0.98
BPyrPF6
0.175
1.7
222
260
20
20
62
1.9
2.3
0.99
0.175 1.7 127 140 20 21 60 1.1 0.175 1.7 27 30 20 22 58 0.5 BmimCl a obtained from the isotherms depicted in Fig. S4 of Supporting Information. b fitting to Yoon-Nelson model depicted in Figs. S1-3 of Supporting Information.
4.0 6.2
0.98 0.99
EmimPF6
u
(mm)
(m·h-1)
0.175
Hbed
R2, b
HMTZ
OmimPF6
dp
21
S2213-3437(17)30514-6 https://doi.org/10.1016/j.jece.2017.10.014 JECE 1924
To appear in: Received date: Revised date: Accepted date:
16-5-2017 2-10-2017 6-10-2017
Please cite this article as: Jes´us Lemus, Cristian Moya, Miguel A.Gilarranz, Juan J.Rodriguez, Jose Palomar, Fixed-bed adsorption of ionic liquids onto activated carbon from aqueous phase, Journal of Environmental Chemical Engineering https://doi.org/10.1016/j.jece.2017.10.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fixed-bed adsorption of ionic liquids onto activated carbon from aqueous phase
Jesús Lemus*, Cristian Moya, Miguel A. Gilarranz, Juan J. Rodriguez, Jose Palomar
S.D. Ingeniería Química, Universidad Autónoma de Madrid, Ctra. de Colmenar Viejo, km 15, 28049 Madrid, Spain. *Corresponding author: [email protected]
Graphical abstract:
Highlights: The separation of ILs from water by fixed-bed adsorption with AC was evaluated. Particle size and superficial liquid velocity are key factor regarding the efficiency. The nature of IL plays a key role in the thermodynamics and kinetics of adsorption. Fixed-bed adsorption with AC was found a feasible for hydrophobic ILs removal.
Abstract Adsorption of ionic liquids (ILs) onto activated carbons (ACs) has been proposed as a thermodynamically and kinetically suitable treatment to remove and/or recover ILs from liquid phase. This work evaluates the potential of AC adsorption in a fixed-bed column to remove ILs from aqueous streams. Different operating conditions were 1
evaluated including AC particle size (0.10 mm to 0.75 mm) and surface liquid velocity (1.4 to 2.0 m·h-1). ILs of different nature were used to assess the influence of cation family (imidazolium- and pyridinium-based) and anion structure (modifying the IL polarity) in the adsorption performance. The adsorption of ILs by ACs in the fixed-bed column was described through the breakthrough curves obtained from experimental tests, which were modelled by the Yoon-Nelson equation. Adsorption capacity (qs), length of the mass transfer zone (HMTZ) and fractional bed utilization (FBU) were estimated from the breakthrough curves. The Yoon-Nelson model describes reasonably well the breakthrough curves, providing a valuable tool design for adsorption. Optimum performance was achieved for hydrophobic ILs at AC particle size between (0.10-0.25) mm and a surface liquid velocity of 1.4 m·h-1. The results of fixed-bed adsorption of ILs were comparable to the obtained with commercial AC for dyes and emergent pollutants.
Keywords: Activated carbon; ionic liquids; breakthrough curve; fixed-bed; adsorption.
1.
Introduction Ionic liquids (ILs) are one of the most rapidly developing areas of modern
chemistry, technology and engineering, focusing on the final objective of industrial applications [1, 2]. ILs present exceptional properties such as negligible vapor pressure and non-flammability under ambient conditions, high thermal and chemical stability, a wide liquid window and high solvent capacity [3-5]. Probably, the most important feature attributed to ILs is the possibility of designing their properties by an adequate selection of the counterions, thus they can be found nowadays in a wide diversity. Because of the 2
enormous number of cation and anion combinations, ILs can possess a wide spectrum of physical and chemical properties (solubility, polarity, viscosity, etc.) and they are already recognized by the chemical industry as new target-oriented reaction and separation media [6, 7]. Nevertheless, most ILs are water soluble or present miscibility with water, which may result in their transfer to aqueous streams with the corresponding environmental risks [8,9]. Therefore, the treatment of aqueous streams containing ILs must be considered for a proper use of ILs in industrial processes, since it has been demonstrated that ILs present a wide range of toxicity and biodegradability [10-12]. The treatment of aqueous streams containing ILs to remove and/or recover them has been extensively studied. Destructive methods, such as advanced oxidation [13-16] or biological treatments [17, 18], have shown high potential for the removal of ILs. However, when visualizing sustainable technologies, the degradation of ILs should be avoided and they should be recovered and recycled instead [19]. Some non-destructive methods have been reported in the literature for the recovery of ILs, such as distillation [20], crystallization [21], nanofiltration [22], pervaporation [23], phase separation [24] and adsorption [25-26]. Among the non-destructive techniques, the adsorption onto activated carbon (AC) has been proved able to recover ILs of different nature from aqueous phase [27-37]. Most of the studies on ILs adsorption onto AC have been conducted in tank operation, including both equilibrium [27,29] and kinetic experiments [30,37]. Moreover, intensification of adsorption by the addition of a salting-out inorganic salt (Na2SO4) to improve the adsorption onto AC has been reported, which is particularly interesting to overcome the difficulty to adsorb hydrophilic ILs [34]. However, to our knowledge, the adsorption of ILs using AC fixed-beds has not been evaluated. Among other advantages, fixed-bed adsorption is one of the most effective and reliable technologies currently available for the removal of different pollutants from water, because of ease to operate, 3
AC and solute loss can be minimized and can be scaled-up to industrial size from laboratory data [38-40]. The current work analyzes the adsorption of representative ILs from water onto a commercial granular AC operating in a fixed bed column under different conditions. First, the adsorption of a reference hydrophobic IL (1-octyl-3-methylimidazolium hexafluorophosphate, OmimPF6) was studied in order to evaluate the potential of this approach and compare its performance with batch-wise operation previously reported [25, 30]. Second, AC particle size (dp) and inlet superficial liquid velocity (u) were checked to
optimize these conditions for adsorption. The experimental breakthrough curves were fitted to the Yoon-Nelson model, which allowed to estimate design parameters such as kinetic constant, mass transfer zone length and percentage of fixed-bed used, and compare them with representative values of industrial operation. Finally, the behavior of different ILs was analyzed under the optimized operation conditions to assess the influence of the nature of the IL in the adsorption process.
2. Materials and methods 2.1. Materials A commercial granular AC supplied by Merck was used as adsorbent. The textural characteristics of the AC were previously reported: SBET = 927 m2·g-1, SEXT = 155 m2·g-1 and VMicrop. = 0.36 cm3·g-1 [25]. The ILs used in this study were supplied by Iolitec in the highest purity available. The ILs were used without previous purification. Table 1 summarizes the nomenclature used for the ILs together with their main properties associated to adsorption behavior, such as density (kg·L-1), viscosity (cP), molecular 4
volume (Vmolecular, Å3) and octanol-water partition coefficient (LogKow), which informs about the hydrophobicity (high IL LogKow ) or hydrophilicity (low IL LogKow ) of IL. The sample of ILs included in this study present a wide range of water-IL behavior, decreasing the solubility in the order OmimPF6 > BmimPF6 > BpyrPF6 > EmimPF6 > BmimCl and moving from one IL considered clearly hydrophobic (OmimPF6 ) to one considered hydrophilic (BmimCl).
2.2. Fixed-bed adsorption experiments
The adsorption of ILs was evaluated in a fixed bed column (Fig. 1) consisting in a 16 mm i.d. glass tube with a length of 20 mm and a cooling jacket (XK 16 column, GE Healthcare). Glass wool was used as stopper at the bottom of the bed and 2.0 g of AC was placed on it to achieve an adsorbent bed length (Hbed) around 20 mm. The height/diameter ratio (H/D) used in current experiments was 1.3 [41]. The rest of the bed was filled up with glass beads to allow a uniform flow through it. IL solutions with a concentration of ~1300 mg·L-1 were pumped downwards through the bed with a peristaltic pump (Gilson Miniplus3) at the desired flow rate (4-6 mL·min-1). In the case of OmimPF6 the solution concentration was 1000 mg·L-1 due to its lower solubility in water. The inlet superficial liquid velocity (u, m/s) was defined as the ratio of flow to section. The output IL solution was analyzed at regular time intervals (5 minutes) using a double beam UV–visible spectrophotometer (Varian, model Cary 1E) at 212 nm for imidazolium-based ILs and 260 nm for the pyridinium-based ones. The experiments were carried out at (35±1) ºC without any pH adjustment.
5
2.3. Analysis of fixed-bed data
The breakthrough time and the shape of the breakthrough curve are determining issues to analyse the operation and the dynamic response of an adsorption fixed-bed. The breakthrough curves describe the adsorption of IL from the stock solution in the adsorbent fixed-bed and is usually expressed in terms of normalized output concentration (C/C0) versus flow time or elution volume. The area under the breakthrough curve provides the total adsorbate uptake, corresponding to the equilibrium capacity (qs, mg·g-1) at the inlet concentration. It is calculated as: 𝑞𝑠 =
𝐶0 ·𝑄 𝑤
𝑡=𝑠𝑎𝑡
𝐶
∫𝑡=0 (1 − 𝐶 ) · 𝑑𝑡
(1)
0
being w (g) the mass of adsorbent used and Q (L·min-1) the inlet flow rate.
Different approaches have been used to model breakthrough curves [42-44]. The one developed by Yoon and Nelson [45] gives the expression: 𝑡 = 𝑡0.5 + 𝑘
1
𝑌−𝑁
ln 𝐶
𝐶
0 −𝐶
(2)
where t is the operation time (min) and t0.5 is the time at which the output concentration is one half of the input one and kY-N is a constant (h-1), that is named effective kinetic constant (higher kY-N values imply higher adsorption rates). These parameters are obtained from the intercept and slope, respectively, in the ln[C/(C0-C)] vs t plot, taking into account the 0.05 to 0.5 C/C0 range (Figs. S1-3 of Supporting Information). This model has been successfully applied to describe the breakthrough curves obtained in the aqueous phase adsorption of different adsorbates [43-44]. In addition to this, the length of the mass
6
transfer zone (HMTZ), which characterizes the wave front of the fixed-bed, can be estimated from the breakthrough curves using the expression [39-40]: 𝐻𝑀𝑇𝑍 = 𝐻 ·
(𝑡0.95 −𝑡0.05 )
(3)
𝑡0.95
where H is the total length of adsorbent in the fixed-bed, t0.95 and t0.05 are the times at which the output IL concentration is 95 % and 5 % of the input one, respectively. Finally, the fractional bed utilization (FBU), defined as the ratio between the capacity at breakthrough time and at saturation time (qs, fixed-bed), is described by Equation (4) [3940]. For comparison purposes, in this work the breakthrough and the saturation concentrations were set at 5 % and 95 % of the input concentration, respectively. % 𝐹𝐵𝑈 =
𝑞𝑏𝑟𝑒𝑎𝑘𝑡ℎ𝑟𝑜𝑢𝑔ℎ 𝑞𝑠,𝑓𝑖𝑥𝑒𝑑 𝑏𝑒𝑑
· 100
(4)
3. Results and discussion The breakthrough curve of a reference IL, OmimPF6, obtained operating at standard conditions: 1.7 m·h-1 superficial liquid velocity, 1000 mg·L-1 of input OmimPF6 concentration, 0.175 mm of mean AC particle size and 35 ºC can be seen in Fig. 2 (black circle series). The resulting breakthrough curve shows a typical S-shaped profile, but the breaking zone has a higher slope than the saturation one. The fixed-bed column experiment revealed an adsorption equilibrium capacity similar to that obtained previously in stirred tank experiments (447 and 450 mg·g−1, respectively) for OmimPF6 on AC at equivalent aqueous phase concentration [25]. Regarding to adsorption kinetics, the effective kinetic constant of the Yoon-Nelson model (kY-N) was 1.6 h-1, close to the one (1.75 h-1) reported for methylene blue (an organic solute commonly separated by adsorption) under similar operating conditions [44]. Moreover, the value of HMTZ was 16 7
mm and FBU was 71%, for the conditions tested, similar to those found for other solutes, as reactive dyes or organic compounds (caffeine, atenolol), separated from water using commercial AC in fixed-bed column [39-40, 46].
Fig. 2 depicts the influence of the AC particle size in the behaviour of the fixedbed column for the adsorption of OmimPF6. Table 2 collects the parameters obtained from the breakthrough curves. When decreasing the AC particle size, the apparent adsorption capacity (as defined in the Yoon and Nelson model) becomes closer to the obtained in stirred tank experiments at equilibrium with a liquid-phase concentration equal to the input concentration to the bed (qe, 450 mg·g-1). Smaller AC particles also imply a higher adsorption rate (higher kY-N values), due to the shorter diffusion path. These results are in agreement with previous works indicating that intraparticle diffusion is the prevailing rate-controlling step in the adsorption of OmimPF6 onto AC and that adsorption rate can be increased by using smaller adsorbent particle size [30]. In addition, it was demonstrated that the IL adsorption capacity of ACs is closely related to available narrow mesopores within the size range between 2 and 8 nm [25,37]. In consequence, the observed decreasing of apparent IL adsorption capacity of AC with larger particle size in fixed-bed operation seems to indicate that the adsorption onto the narrowest mesopores is affected by the longer diffusion path and maybe by the loss of pore connectivity because of pore filling. This behaviour is evidenced by the change in the slope of the breakthrough curve at high C/C0 values. Accordingly, decreasing the diameter particle allows decreasing the mass-transfer zone length (HMTZ) from 27 to 16 mm and increasing the fractional bed utilization (FBU) from 31 to 71%. The mean particle size of AC selected further in this study was 0.175 mm, which is still within the particle size range of industrial adsorbents (0.06-6.00 mm) [46]. 8
Fig. 3 shows the breakthrough curves for the adsorption of OmimPF6 at different superfical liquid velocities. As can be seen from the corresponding parameters in Table 2, the saturation capacity (qs) is not highly affected by the superficial liquid velocity, with values ranging between 413 and 447 mg·g-1. In contrast, the parameters more directly related to adsorption kinetics are affected by superficial liquid velocity. Thus, HMTZ and FBU significantly decreases and kY-N values clearly increases when using lower superficial liquid velocities, indicating that velocities lower 2 m·h-1 are more adequate for this internal diffusion-controlled adsorption phenomena of IL onto AC [47-48]. A superfical liquid velocities of 1.7 m·h-1 was selected for the further experiments and proposed as more favourable operating conditions (compromise between adsorption efficiency and operating time) for scaling column.
Fig. 4 compares the results of adsorption tests for 5 ILs with remarkably different chemical nature. The breakthrough curves show substantially different breakthrough and saturation times, and different slopes in the breakthrough region, thus evidencing the influence of the nature of the ILs adsorbed. As can be seen in Table 2, the adsorption capacities obtained now in fixed-bed experiments increase with the hydrophobicity of the ILs tested, in good agreement with previous results obtained in batch experiments [25, 30]. Interestingly, the HMTZ decreased with the hydrophobicity of the IL (see LogKow values presented in Table 2). Thus, adsorption of OmimPF6 onto AC is both thermodynamically and kinetically favored. In the case of hydrophilic ILs a substantially higher amount of adsorbent would be required or alternative approaches would be needed. The modification of the AC surface, which get double the adsorption capacity of BmimCl [25, 27] or the addition of salting-out inorganic salt (e.g. Na2SO4) to increase the adsorption efficiency, in the case the adsorption capacity is up to five times [34], are propose as promising alternatives. 9
The next step in the development of this technology would be the regeneration of the exhausted bed and the recovery of the IL. In previous work [25], it was demonstrated by batch experiments that adequate organic solvents, as acetone, can be successfully used to regenerate the exhausted AC beds. Removing IL adsorbate from solid phase by washing with acetone allowed the recovery of the IL (checked by 1H-NMR), keeping virtually intact the porous structure of the AC (checked by N2 adsorption isotherms) [27]. Future work will be devoted to evaluate different regeneration alternatives in fixed bed experiments.
4. Conclusions Previous studies performed in batch proved that adsorption onto activated carbon (AC) is a promising non-destructive treatment to remove ionic liquids (ILs) at low concentrations from aqueous solutions. In the current work, the evaluation was extended to fixed-bed tests using granular AC, for the sake of obtaining more realistic information to analyze the feasibility of adsorption as a potential solution for IL-bearing aqueous effluents to remove ILs from water. The breakthrough curves were obtained at different operating conditions (such as AC particle size and superfical liquid velocity) and with ILs of different chemical nature. Fixed-bed adsorption with AC proved a reasonably good performance yielding values of the representative parameters close to the reported for other organic compounds under similar operating conditions. Relatively low AC particle size (0.175 mm) and superfical liquid velocity (1.7 m·h-1) are recommended, due to the mass-transfer restrictions of IL adsorption. The adsorption capacity and FBU increases for ILs of lower polarity, so that hydrophobic ILs (such as OmimPF6) are better candidates for the application of this technology.
10
Acknowledgements The authors are grateful to the Spanish “Ministerio de Ciencia e Innovación (MICINN)” for financial support (project CTQ2014-52288-R) and Comunidad de Madrid (project S2013-MAE-2800). We are very grateful to “Centro de Computación Científica de la Universidad Autónoma de Madrid” for computational facilities.
11
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[42] Y.H. Yoon, J.H. Nelson, Application of Gas-Adsorption Kinetics .1. a TheoreticalModel for Respirator Cartridge Service Life, Am. Ind. Hyg. Assoc. J. 45 (1984) 509-516. [43] M. Auta, B.H. Hameed, Chitosan-clay composite as highly effective and low-cost adsorbent for batch and fixed-bed adsorption of methylene blue. Chem. Eng. J. 237 (2014) 352-361. [44] M.T. Yagub, T.K. Sen, S. Afroze. H.M. Ang. Fixed-bed dynamic column adsorption study of methylene blue (MB) onto pine cone. Desal. Wat. Treat. (2014) 1-14. [45] J.L. Sotelo, G. Ovejero, A. Rodriguez, S. Alvarez, J. Galan, J. Garcia. Competitive adsorption studies of caffeine and diclofenac aqueous solutions by activated carbon. Chem. Eng. J. 240 (2014) 443-453. [46] Y.S. Al-Degs, M.A.M. Khraisheh, S.J. Allen, M.N. Ahmad. Adsorption characteristics of reactive dyes in columns of activated carbon. J. Hazard. Mat. 165 (2009) 944–949. [47] J. Chern, Y. Chien. Adsorption of nitrophenol onto activated carbon: Isotherms and breakthrough curves. Water Res. 36 (2002) 647–655. [48] D.W. Hendricks. Water Treatment Unit Processes: Physical and Chemical. Taylor & Francis, 2006.
15
Peristaltic Pump 4–6 mL·min-1
AC fixed-bed H = 20 mm
2g
UV spectrophotometer. Outlet IL quantification
Stock Solution ~ 1300 ppm Sampling collector
Fig. 1. Schematic diagram of the experimental set-up.
16
1.0
0.8
C/C0
0.6
0.4 0.625 mm
0.2
0.375 mm 0.175 mm
0.0 0
100
200
300
400
500
Time (min)
Fig. 2. Experimental (dots) and predicted (lines) breakthrough curves for OmimPF6 adsorption in fixed-bed columns using AC with different particle size (u: 1.7 m·h-1 and C0: 1000 mg·L-1).
17
Fig. 3. Experimental (dots) and predicted (lines) breakthrough curves for OmimPF 6 adsorption at different superfical liquid velocities (dp: 0.175 mm and C0: 1000 mg·L-1 ).
18
Fig. 4. Experimental (dots) and predicted (lines) breakthrough curves for the adsorption of different ILs (C0: 1300 mg·L-1, dp: 0.175 mm and u: 1.7 m·h-1).
19
Table 1. Properties of the ionic liquids used.
Name
BpyrPF6
BmimPF6 1-Butyl-3-methyl imidazolium hexafluoroborate
340
284
281
256
175
1.22
1.37
1.32 (50ºC)
1.47
1.08
720
235
Solid at 25ºC
Solid at 25ºC
4147
389
303
304
256
236
2.4
0.8
0.1
-0.4
-3.3
1-Butylpyridinium hexafluoroborate
EmimPF6 1-Ethyl-3-methyl imidazolium hexafluoroborate
BmimCl
OmimPF6 1-Methyl-3-octyl imidazolium hexafluoroborate
1-Butyl-3-methyl imidazolium chloride
Chemical structure
M weight (g·mol-1) Densitya (g·mL-1) Viscositya (cP) V molecular a (Å3) LogK ow b
a
Obtained at 25ºC from ILthermo web site. bCalculated by COSMO-RS (computational details available in the Supporting Information)
20
Table 2. Operating conditions for the adsorption experiments and fitting parameters obtained from breakthrough curves. qs,
qs,
fixed-bed
tank
(mg·g-1)
1.7
0.375
FBU
t0.5
kY-N
(mg·g-1)a (mm)
(mm)
(%)
(h)
(h-1)
447
450 [25]
20
16
71
3.0
2.2
0.99
1.7
421
--
21
25
47
2.0
2.0
0.97
0.625
1.7
309
--
22
27
31
1.4
1.9
0.97
0.175
2.0
413
--
20
24
58
2.2
1.6
0.95
0.175
1.4
436
--
20
14
75
3.7
2.6
0.98
BmimPF6
0.175
1.7
306
320
20
18
70
1.9
3.4
0.98
BPyrPF6
0.175
1.7
222
260
20
20
62
1.9
2.3
0.99
0.175 1.7 127 140 20 21 60 1.1 0.175 1.7 27 30 20 22 58 0.5 BmimCl a obtained from the isotherms depicted in Fig. S4 of Supporting Information. b fitting to Yoon-Nelson model depicted in Figs. S1-3 of Supporting Information.
4.0 6.2
0.98 0.99
EmimPF6
u
(mm)
(m·h-1)
0.175
Hbed
R2, b
HMTZ
OmimPF6
dp
21