Accepted Manuscript Efficient separation of vanadium from chromium by a novel ionic liquid-based synergistic extraction strategy Junmei Zhao, Qiaoyu Hu, Yingbo Li, Huizhou Liu PII: DOI: Reference:
S1385-8947(14)01530-7 http://dx.doi.org/10.1016/j.cej.2014.11.071 CEJ 12926
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
26 September 2014 10 November 2014 13 November 2014
Please cite this article as: J. Zhao, Q. Hu, Y. Li, H. Liu, Efficient separation of vanadium from chromium by a novel ionic liquid-based synergistic extraction strategy, Chemical Engineering Journal (2014), doi: http://dx.doi.org/ 10.1016/j.cej.2014.11.071
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Efficient separation of vanadium from chromium by a novel ionic liquid-based synergistic extraction strategy Junmei Zhaoa,*, Qiaoyu Hua, Yingbo Lia, Huizhou Liua,*
a
Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese
Academy of Sciences, No. 1 Bei Er Tiao, Zhong Guan Cun, Beijing, P. R. China 100190 * Correspondence to: Junmei Zhao and Huizhou Liu, E-mail:
[email protected] (Zhao JM), telephone: +86-10-82544911, fax: +86-10-62554264
ABSTRACT Ionic liquid (IL)-based extraction is a promising and environmentally benign separation technology. To develop sustainable extraction technologies, quaternary ammonium-based IL extraction strategy is attractable. In this work, the separation of vanadium(V) from chromium(VI) by pure tricaprylmethylammonium nitrate ([A336][NO3]) and organic acidified primary amine N1923 ([RNH3][NO3]) was systematically investigated. The optimal proportion of [A336][NO3] and [RNH3][NO3] was studied and results showed that the mixed [A336][NO3] and [RNH3][NO3 ] exhibited an obvious synergistic-effect for V(V). The extraction of V(V) was strongly dependent on the acidity of the aqueous phase and reaches maximum at pH 2.5 - 3, while the maximum separation coefficient (βV/Cr) was located at about pH 9.0. Moreover, βV/Cr could be improved through adjusting the molar concentration ratio of V/Cr. The interference of coexisting anions (nitrates, chlorides, sulphate and phosphate) on the extraction of V(V) was examined and the results showed that PO43-, NO3- and Cl- had negative effects at various degrees except for SO42-. The V(V) extraction
behaviors
could
be
properly
described
by
Langmuir
and
pseudo-second-order rate equations . The maximum extraction capacity for V(V) was estimated as 1.877 mmol/g at 303K. Increased temperature had little effect on the extraction capacity, but greatly improved the extraction rate. The typical anion 1
exchange mechanism between NO3- and V4O124- (or V3O93-) was proposed for the current extraction system. The IL phase could be renewed through stripping the loaded vanadium by a 0.5M NaNO3 solution. This work demonstrated that quaternary ammonium IL containing a commercial organic extractant is an efficient and sustainable IL-based extraction strategy for the separation of vanadium from chromium, and as a result, the development of an IL-based extraction process is straightforwardly envisaged.
Keywords: [A336][NO3], acidified N1923, vanadium(V), chromium(IV), IL-based extraction
1. Introduction Liquid-liquid extraction is the most widely applied technology for metal ion separations due to its advantages including operation in a continuous mode, employment of simple equipment, achievement of high sample throughput and easy scale-up. However, the conventional liquid-liquid extraction process utilizes water-immiscible molecular solvents which are flammable, volatile or toxic. Without doubt, innovative and green extraction processes are highly desired with growing concern about safety issues and environmental impact related with the use of these volatile organic compounds (VOCs). Ionic liquids (ILs) are a type of organic salts with a melting point below 100 oC. Their negligible vapor pressure and non-flammability make this class of solvents safer and more environmentally friendly than molecular solvents. Since Rogers and his co-workers firstly reported the use of room temperature ionic liquids to separate substituted-benzene derivatives [1], ionic liquid as novel media for ‘clean’ liquid-liquid extraction has gained increasing attention in the field of the development of sustainable separation processes [2-6]. In particular, studies on the use ILs either as ion-type extractant or as suitable candidate for replacement of volatile organic solvent for separation of metal ions have been well documented [6]. As a kind of ion-type 2
extractant, the extraction of metal ions often takes place via an ion-exchange mechanism [7-13]. This means that loss of ionic liquid components occurs inevitably upon extraction, which would hamper the regeneration of ionic liquids. What’s more, it will contaminate the water, especially in the case of the ionic liquids containing fluoride. For example, PF6- of [Cnmim][PF6] can hydrolyze into toxic and corrosive PO43-, HF, POF3, H2PO3F, HPO2F2 at acidic media [14, 15], and these compounds can pollute water body persistently. Recently, task-specific ionic liquids (TSIL) with non-fluorinated anions and functional cation for task-specific purpose, such as tricaprylmethylammonium chloride (Aliquat 336, [A336][Cl]) [16] and tri(hexyl)tetradecylphosphonium chloride (Cyphos IL 101) [17], draw wide attention owing to the above reasons. As far as [A336][Cl] or [A336][NO3] is concerned, their application in the extraction of metal ions can be categorized in three aspects. Firstly, these ILs are used as single extractant or synergistic extractant in the traditional VOC-based extraction of metal ions [18-20]. In this case, the prominent advantage of using ionic liquids for extraction is lost. Secondly, these ILs have been employed to tailor new TSILs as efficient extractant [21-24],
such
as
[A336][P507],
[A336][P204],
[A336][Cyanex272]
and
[A336][DGA]. Unfortunately, the synthetic TSILs usually exhibit high viscosity and limited solubility. Sometimes, these ILs are solely used as solvent [23, 25]. Consequently, there are few reports on the use of [A336][Cl] or [A336][NO3] in a pure state as both extractant and solvent. These ILs can adopt ion association or anion-exchange mechanism during the extraction of metal ions, so that no organic ligand (cation component of IL) is lost. In this viewpoint, their toxicities are lower than those of imidazolium-type ILs. It is worth mentioning that their costs are much cheaper than those of [Cnmim]+-type ILs. Undoubtedly, such advantages are much preferred for IL-based extraction. Vanadium is an important metal for high-tech fields such as aerospace, catalyst, steel and iron industries. [26] Furthermore, vanadium and chromium are a pair of adjacent transition metals and co-exist in nature. It is not an easy task for the separation of vanadium from chromium due to their similarly physicochemical 3
properties. In the past decades, the separation of V(V) from Cr(VI) has drawn considerable attention. Although successful separation of V(V) from Cr(VI) could be achieved by traditional liquid-liquid extraction by amines [27-30], environmental problems as well as generation of interfacial emulsion and crud caused by the use of huge amount of VOCs as diluents have been long realized [29, 30]. Moreover, Cr(VI) is a well-known strong oxidant. It can cause the oxidation of alcohols [31], alkynes [32], aldehydes [33] and amines [34], which takes place more readily under acidic conditions. Currently, most of the V-Cr separation are performed in the acidic or neutral aqueous solution. As a result, the extractants (amines) could be destroyed by Cr(VI). Indeed, in our preliminary experiments, it has been observed that the organic extractant phase became darker and darker from the light yellow if contacting the Cr(VI) aqueous phase in acid media for a longer time. Recently, we reported the separation of V(V) from Cr(VI) using [C8mim][PF6] [35]. Although this separation is efficient, [C8mim][PF6] suffers from the loss of anion PF6due to an anion exchange mechanism between PF6- and vanadate. In this work, a novel V(V)-Cr(VI) extraction and separation strategy from near-alkaline solution was designed by using pure [A336][NO3] containing a commercial extractant N1923 as mixed extractants. Where, it is expected that [A336][NO3] plays not only a kind of solvent for the ionic extracted species but also an efficient extractant for vanadate. Moreover, since ion liquids are ready to be stable for large cation or large anion [36], it is expected that the big cation [A336]+ has a stronger affinity to the larger vanadate than that to the smaller chromate. Due to the solubilization of IL for the ionic species, the extraction and separation performance of V(V) and Cr(VI) should be enhanced. Hence, it can be expected that the novel IL-based extraction system consisting of quaternary ammonium nitrate IL containing a commercial extractant is an efficient and sustainable strategy for the separation of vanadate and chromate.
2. Material and methods
4
2.1. Materials
Aliquat 336 ([A336][Cl]), tricaprylmethylammonium chloride is a mixed quaternary ammonium salt containing mainly trioctylmethylammonium chloride (Aldrich, 97%). Primary amine N1923 (93%) was kindly supplied by Shanghai Rare-earth Chemical Co., Ltd. Their structures of the main components were shown in Scheme 1. The stock solutions of V(V) and Cr(VI) were prepared by dissolving NaVO3 (analytical grade, Sinopharm Chemical Reagent Co. Ltd. China) and Na2CrO4 (analytical grade, TianJin GuangFu Fine Chemical Research Institute, China) in deionized water, respectively. The other chemicals, such as KNO3, NaCl, Na2SO4, NaNO3 and Na3PO4, are analytical grade reagents. All chemicals were used as received, without further purification. Many developed ionic liquids including Aliquat 336 had relatively high viscosity, which is disadvantageous to industrial application. To decrease the viscosity of Aliquat 336 and to improve mass transfer and kinetics, [A336][Cl] was replaced by [A336][NO3] in this work. In order to prepare [A336][NO3], 100 g of [A336][Cl] was pre-equilibrated 4 times with each 200 ml 2.5 M KNO3 solution to exchange the chloride ions to be nitrate ions. The decline in chloride concentration after each equilibration is shown in Table S1. After the fourth equilibration, chloride levels remained a very low extent and almost unchanged, which shows that most of [A336][Cl] has been changed to be [A336][NO3]. In order to control extraction equilibrium acidity, N1923 was pre-equilibrated with 0.5 M HNO3 according to a stoichiometric ratio of 1.05 between HNO3 and -NH2, then the obtained acidified N1923 (abbreviated as [RNH3][NO3]) was washed till neutral by deionized water. Physical properties of Aliquat 336, [A336][NO3], N1923 and [RNH3][NO3], including density (g/cm3), viscosity (mpa·s) and average molecular weight, were listed in Table S2. As shown in Table S2, the viscosity of [A336][ NO3] was much less than that of [A336][Cl], which would thus improve the extraction and save energy.
5
(a) H3C
(b)
C8H17 N
+
C8H17 Cl-
H2N
C 9-11H19-23 H C C9-11H19-23
C8H17
Scheme 1. Structures of main components for ionic liquid Aliquat 336 and extractant N1923 used in this study.
2.2. Instruments and measurements
The pH measurements were performed using HANNA pH211 digital pH meter (Italy). The concentrations of metal ions in aqueous phase were determined by OPTIMA 7000DV inductively coupled plasma-optical emission spectrometer (ICP-OES) (PekinElmer, USA). The concentration of Cl- ions in aqueous phase was determined by ion chromatograph (ICS-1000, Dionex). Fourier transform infrared spectroscopy (FT-IR) measurements were performed on a Bruker Tensor 27 Spectrometer using KBr pellets (32 scans at a resolution of 2 cm-1) in transmission mode. Densities and viscosities of ILs or extractant were measured by a density meter and a viscosity meter (Anton Paar DMA 5000 / AMVn, Anton Paar Co., Austria) with a reproducibility < 0.5% and repeatability < 0.1%, respectively. The calibration was carried out using ultrapure water or viscosity standard oils (No. H117, Anton Paar Co., Austria).
2.3. Extraction experiments
In a typical extraction experiment, 60 ml aqueous phase and 0.5 ml or 1.0 ml of ionic liquid were shaken in a conical flask using a mechanical shaker at 250 rpm for 60 min at 303 K except for the temperature experiments. After the extraction equilibrium, the metal ion concentration in the bottom phase (aqueous phase) was measured by ICP-OES and the amount of metal ion extracted into IL phase was determined by mass balance. The pH was adjusted by diluted HNO3 or NaOH for the 6
studies of pH effect. In addition, IL-based extraction can also be regarded as a kind of liquid-liquid adsorption. Adsorption capacity (qe, mmol/g) has also been used for the discussion of extraction isotherms and kinetics. The distribution ratio (D), extraction efficiency (E%), adsorption capacity (qe) and separation coefficents ( β ) were calculated according to the following equations: D=
(C0 − Ce ) Vaq × Ce VIL
E% =
(1)
(C 0 − C e ) × 100 C0
(2)
qe =
(C 0 − C e ) × Vaq × 10 −3 W
(3)
β=
DV DCr
(4)
Where, D is the distribution ratio. C0 and Ce (mmol/l) are the initial and equilibrated concentrations of metal ion in the aqueous phase, respectively. Vaq (ml) represents the volume of the aqueous phase. W (g) is the mass of ionic liquid phase. βis the separation coefficient. DV and DCr signify the distribution ratio of V(V) and Cr(VI), respectively.
3. Results and discussion
3.1. Effect of ionic liquid phase composition on the extraction of V(V) and Cr(VI)
Vanadium and chromium belong to amphoteric metals, and alkaline leaching is a main means in the hydrometallurgical process. In this point, the separation of V(V) and Cr(VI) from alkaline solution is very significant. Here, the mixture of V(V) and Cr(VI) at pH 9.0 was used for the study of the effect of ionic liquid phase composition on the extraction of these two metal ions. The total volume of [A336][NO3] and [RNH3 ][NO3] was fixed at 1.0 ml, in which the volume proportion of [A336][NO3] 7
and [RNH3][NO3] was varied.
600
DV DCr
D
450 300 150 0 0.0
0.2 0.4 0.6 0.8 XN1923 (volume percent)
1.0
Fig. 1. The effect of ionic liquid phase composition on the extraction of V(V) and Cr(VI). V[A336][NO3] + V[RNH3][NO3] = 1.0 ml, pH = 9.0, CV(V) = CCr(VI) = 11.5 mmol/l, Vaq = 60 ml.
As shown in Fig. 1, the distribution ratio of V(V) varied with the increasing volume percentages of [RNH3][NO3] while that of Cr(VI) remained almost unchanged and a very low value in the whole range. The extraction of V(V) reached maximum at a volume percent of 0.4 for [RNH3][NO3]. In addition, it was obvious that DV (630.4, X[A336][NO3] = 0.4) > DV (341.9, XN1923 = 0) + DV (101.4, XN1923 = 1), which shows a prominent synergistic effect for the extraction of V(V). This synergistic effect is probably due to the enhanced solubilization of ionic liquid for the extracted ion-type species. Here, [A336][NO3] is not only an efficient synergistic extractant for V(V) but also an effective solvent for the extracted species. Consequently, the optimal composition, i.e. X N1923 = 0.4, was adopted in the following experiments.
3.2. Effect of acidity on the extraction of V(V) and Cr(VI)
Adjusting acidity of aqueous phase is a widely used method to achieve efficient separation in liquid-liquid extraction. The extraction behaviors of V(V) and Cr(VI) by 8
[A336][NO3]-[RNH3][NO3] and [A336][NO3] at the various acidity (pH 1 - 13) was investigated (Fig. 2).
600
A336[NO3]+[RNH3][NO3], V(V)
500
A336[NO3], V(V)
400
A336[NO3], Cr(VI)
A336[NO3]+[RNH3][NO3], Cr(VI)
D
300 200 100 0 0
2
4
6
pHe
8
10
12
14
Fig. 2. The effect of acidity on the extraction of V(V) and Cr(VI). CV(V) = CCr(VI) = 27.5 mmol/l, Vaq = 60 ml, the mixed IL-extraction system: 0.3 ml [A336][NO3] and 0.2 ml [RNH3][NO3], the single IL-extraction system: 0.3 ml [A336][NO3].
It could be seen from Fig. 2, DV by [A336][NO3]-[RNH3][NO3] was much higher than that by [A336][NO3] in the pH range of 1 - 10. While for DCr, there is only slight increase at pH 1 - 6. It was also shown that there are two peaks for DV. One is at the acidic region (pH about 3) and the other one is at the alkaline region (pH about 9.0). DV in the acidic region is much higher than that in the alkaline region, which indicated different extraction mechanism at different acidity. This difference is probably derived from [RNH3][NO3]. As shown in Fig. S1, vanadium and chromium could exist as different forms at the varying pH [37, 38]. Accordingly, when the concentration of metal ions is higher than 1 g/L, V mainly exists as V4O124- (or V3O93-) at pH 9.0 and HV10O273- (or HV10O273-) at pH 3.0, while Cr mainly exists as CrO42- at pH 9.0 and Cr2O72- at pH 3.0. Hydroxo-complexes are much easier to form hydrogen bonds with N1923 than oxo-complexes due to two active hydrogen atoms attached to nitrogen atom for primary amines. Therefore, for the extraction of V(V) by [RNH3 ][NO3], there exists hydrogen-bond interaction besides anion exchange 9
mechanism at pH 3.0. While at pH 9.0, the extraction mainly belongs to anion exchange mechanism. However, for the extraction of Cr, there exists only anion exchange mechanism no matter what the acidity is. More detailed vanadium extraction mechanism will be discussed later. The calculated βV/Cr varies with the increasing pH was shown in Fig. 3. Correspondingly, there are also two maximum values for βV/Cr at the acidic region (pH about 3) and the alkaline region (pH about 9.0), respectively. Compared with [A336][NO3] alone, βV/Cr gets much improved during pH 8 - 10 by [A336][NO3] -[RNH3][NO3]. From Fig. 3, it can be concluded that the species V4O124- (or V3O93-) are more superior to exchange with the anion NO3- at pH 9.0 than CrO42-. The thermodynamic radius of vanadate (> 3.40 Å) is much larger than that of chromate (~ 2.40 Å) [28]. The excellent extraction selectivity can be attributed to the stronger affinity between cation [A336]+ or [RNH3]+ and the larger metal oxo-anion V4O124(or V3O93-) instead of the smaller anion CrO42-. Although high extractability of V(V) is preferred at acidic condition (Fig.2), the oxidizability of Cr(VI) in acidic solution is very strong, which will destroy extractant and decrease the extractant’s working life. Moreover, the value of separation factor at alkine condition is much higher than that at acidic condition. As a consequence, the current extraction studies focus on an aqueous solution at pH 9.0. 40 βV/Cr(A336[NO3]+[RNH3][NO3] βV/Cr(A336[NO3])
β V/Cr
30
20
10
0 0
2
4
6
8 pHe
10
12
Fig. 3. The effect of acidity on the separation coefficients βV/Cr. 10
14
In addition, the concentration ratio of V/Cr in the feeding solution is another important impact factor for βV/Cr. Therefore, the influence of Cr(VI) oxo-anions on the extraction of V(V) by ILs phase [A336][NO3]-[RNH3 ][NO3] has been studied through varying V/Cr at a fixed total concentration 19.6 mmol/l of V(V) and Cr(VI) (Fig. 4). Fig. 4A and 4B showed the variation of distribution ratio, separation coefficients and extraction capacities with the increasing V/Cr. As shown in Fig. 4A, the extraction distribution ratios of V(V) firstly increased, and then declined, finally reached a maximum when V/Cr is close to 0.5. Whereas the extraction distribution ratios of Cr(VI) remained a low level. Accordingly, the separation coefficients (βV/Cr ) reached maximum when V/Cr is between 0.1 and 0.5. As far as extraction capacities are concerned, V(V) increased while Cr(VI) droped slowly with the increasing V/Cr (Fig. 4B). It is obvious that there is a competitive extraction between V(V) and Cr(VI). Therefore, the molar concentration ratio of V/Cr at 0.5 - 1.0 in the feeding solution could guarantee a good separation effect.
1500
1.8
A
V (V) Cr(VI)
1.5 1.2
45
q(mmol/g)
1200
β V/Cr
D
900
30
600 300
V(V)
B
60
0.9 0.6
15
0.3
0
0.0
Cr(VI) 0 0
2
4 V/Cr
6
8
0
2
4
V/Cr
6
8
Fig. 4. Influence of the molar concentration ratio V/Cr on the extraction of V(V) and Cr(VI) by [A336][NO3]-[RNH3][NO3]. The mixed IL phase: 0.3 ml [A336][NO3] and 0.2 ml [RNH3][NO3], CV(V) + CCr(VI) = 19.6 mmol/l, Vaq = 60 ml, pH = 9.0.
3.3. Effect of co-existing anions on the extraction of V(V)
Other anions, such as Cl-, SO42-, NO3- and PO43-, would also influence the 11
extraction of V(V). Hence, the effect of co-existing anions (Cl-, SO42-, NO3- and PO43-) on the extraction of V(V) by ionic liquid phase [A336][NO3]-[RNH3][NO3] was examined at the varied concentrations of other anion species (Fig. 5). The results showed that the affinity of the tested anion species to amine cations ranks as PO43- > NO3- > Cl- > SO42-. The effect of SO42- on the extraction of V(V) can almost be negligible, while Cl- has a slightly negative effect on the extraction of V(V). However, the presence of PO43- or NO3- rapidly decreased the extraction. It can be probably attributed to the strong affinity between [A336]+ / [RNH3]+ and PO43-. Due to the release of NO3- during the extraction of V(V), it is reasonable that the extraction declines rapidly with the increasing NO3- concentrations. This suggests that NaNO3 solution could be used for the stripping of V(V) from the loaded ILs phase. 100 80
E%
60 40
NaCl
Na2SO4
NaNO3
Na3PO4
20 0 0.0
0.2
0.4 0.6 C(mol/L)
0.8
1.0
Fig. 5. Influence of co-existing interfering anions on the extraction of V(V) by ILs phase [A336][NO3]-[RNH3][NO3]. The mixed ILs phase: 0.3 ml [A336][NO3] and 0.2 ml [RNH3 ][NO3], CV(V) = 8.4 mmol/l, Vaq = 60 ml, pH = 9.0.
3.4. Extraction isotherms
IL-based extraction can also be regarded as a kind of liquid-liquid adsorption. The extraction
isotherms
of
V(V)
and
Cr(VI)
by
ionic
liquid
phase
[A336][NO3]-[RNH3][NO3] at different temperatures were investigated to evaluate the extraction capacities (Fig. 6). As seen in Fig. 6(A), the adsorption amounts of V(V) increase with increasing equilibrium concentration and tend to approach constant 12
when its concentration is higher than 3 mmol/l. On the contrary, there is an declining trend for Cr(VI). Moreover, the effect of temperature on the adsorption of Cr(VI) could be negligible. However, increasing temperature has a slightly negative effect on the adsorption of V(V), which indicates that the adsorption process of V(V) by the ionic liquid phase is exothermic. The maximum adsorption capacity for V(V) was estimated as 1.877 mmol/g at 303 K.
12
A
B
q (mmol/g)
1.5
9
o
V, 30 C o V, 40 C o V, 50 C o Cr, 30 C o Cr, 40 C o Cr, 50 C
1.0
0.5
C e/q (g/l)
2.0
6 y=0.5329x+0.2361, 2 o R =0.999 (30 C) y=0.5399x+0.2627, 2 o R =0.999 (40 C) y=0.5640x+0.3134, 2 o R =0.999 (50 C)
3
0 0.0 0
3
6 9 Ce(mmol/l)
12
15
18
0
6 12 Ce (mmol/l)
18
Fig. 6. (A) Extraction isotherms of V(V) and Cr(VI) by ionic liquid phase 0.3 ml [A336][NO3] and 0.2 ml [RNH3][NO3]. CV(V) = CCr(VI) = 3.90 - 29.5 mmol/l, Vaq = 60 ml, pH = 9.0. (B) The fitting plots of Ce/ qe vs Ce for the extraction of V(V) according to Langmuir model.
The extraction isotherms data for the extraction of V(V) at three different temperatures were fitted to Langmuir [39] and Freundlich isotherm [40] equations expressed by eqn. (5) and (6), respectively.
Ce 1 c = + e qe qmax b qmax
(5)
1 log qe = log K f + log Ce n
(6)
where Ce (mmol/l) and qe (mmol/g) represent the equilibrium aqueous concentration and the equilibrium adsorption capacity, respectively. qmax (mmol/g) and b (L/mmol) are the maximum adsorption capacity and adsorption equilibrium constant related to the adsorption energy, respectively. Kf and n are Freundlich isotherm constants. The 13
values of qmax and b were evaluated from the slope and intercept of the fitting plots of Ce/ qe vs Ce while those of n and Kf were evaluated from those of logqe vs logCe. It can be concluded that Langmuir model was the best fit in terms of coefficients (R2), as shown in Fig. 6B. Langmuir adsorption suggests that monolayer adsorption takes place during the adsorption process. When small volumetric ionic liquid phase shakes with large quantity of aqueous phase during the extraction operation, hydrophobic ionic liquid phase will be dispersed into numerous small oil beads. Each oil bead could be regarded as an adsorbent gel. On the surface of the adsorbent gel, it is possible for monolayer adsorption to occur. The comparison of V(V) adsorption capacities on various adsorbents [41-46] was listed in Table 1. Obviously, the current ionic liquid has a superior extraction capacity for V(V), which is only lower than that of the commercial D201 resin (4.412 mmol/g). However, the separation coefficient (βV/Cr) is much superior to D201 resin [43].
Table 1 Comparison of V(V) adsorption capacities on various adsorbents.
Adsorbent
qmax (mmol/g)
Refs.
PGTFS-NH3+Cl−
0.899
Ref [41]
D201 resin
4.412
Ref [42]
Zr(IV)-SOW
1.003
Ref [43]
Black wattle tannin
1.598
Ref [44]
Amberlite IRA-904 resin
0.931
Ref [45]
quinine modified resin
0.149
Ref [46]
1.85
Present work
2.76
Present work
[A336][NO3]-[RNH3][NO3] (pH = 9.0) [A336][NO3]-[RNH3][NO3] (pH = 3.0), calculated
14
From the temperature dependency of Langmuir adsorption equilibrium constant (b), the thermodynamics parameters such as the free energy (∆Go), enthalpy (∆Ho) and entropy (∆So) changes associated to the adsorption of V(V) by ionic liquid phase were evaluated according to eqn. (7) - (8).
∆G o = − RT ln b
(7)
ln b = −∆G o / RT = −∆H o / RT + ∆S o / R
(8)
The plot lnb as a function of 1/T yields a straight line as depicted in Fig. S2, from which ∆Ho and ∆So were calculated from the slope and intercept, respectively. The calculated thermodynamic parameters were listed in Table 2. The negative values of enthalpy changes (∆Ho) and entropy changes (∆So) demonstrate the exothermic nature of the extraction process and the decreased randomness because metal ions are bound into the ionic liquid phase. The negative values of Gibbs free energy (∆Go) at all temperatures confirm the feasibility and spontaneous nature of the extraction process. Since the adsorption equilibrium constant b declines with the increasing temperature, ∆G is more and more positive, which illustrates that the extraction reaction is
unfavorable with the increasing temperature.
Table 2 Thermodynamic parameters of V(V) extraction by ionic liquid phase 0.3 ml
[A336][NO3] and 0.2 ml [RNH3][NO3]. Thermodynamic parameters T(K)
b
qmax
∆Go
∆Ho
∆So
l/mmol
mmol/g
kJ/mol
kJ/mol
J/(K·mol)
303
2.257
1.877
-2.051
313
2.054
1.852
-1.874
-9.195
-23.52
323
1.800
1.773
-1.578
3.5. Extraction kinetics
15
The extraction kinetics is of great importance for the design of a suitable extraction process.The extraction kinetics of V(V) and Cr(VI) by ionic liquid phase [A336][NO3]-[RNH3][NO3] were thus investigated at various temperatures. The extraction capacities of V(V) initially increased with increasing contacting time and then reached equilibrium after 50 min, indicating that 1 h is sufficient to attain the extraction equilibrium (Fig. 7A). Compared with traditional VOC-based extraction (~5 min), a longer time to get equilibrium for vanadate and chromate might be correlated with their stronger steric hindrance and the larger viscosity of ionic liquid phase. Vanadium extraction kinetic data were analyzed in terms of various kinetics models including pseudo-first-order (eq.9) [47], pseudo-second-order (eq.10) [48] and intraparticle diffusion models (eq.11) [49], and among them the pseudo-second-order rate model was the best fit in terms of coefficients (R2), as shown in Fig. 7B. log( qe − qt ) = log qe −
k1 t 2.303
(9)
t 1 1 = + t 2 qt k2 qe qe
(10)
qt = k p t1 / 2 + I
(11)
In the pseudo-second-order rate equation, qe (mmol/g) and qt (mmol/g) represent the adsorption amount of metal ions at equilibrium and time t (h), respectively. k2 (g/(mmol·h)) is the rate constant of the pseudo-second-order model. The values of qe and k2 can be evaluated from the slope and intercept of the fitting plots of t/qt vs. t, and these values were tabulated in Table 3.
A
3.00
0.6
t/qt (h/(mmol.g))
303K,V(V) 313K,V(V) 323K,V(V) 303K,Cr(VI) 313K,Cr(VI) 323K,Cr(VI)
0.8 qt(mmol/g)
B 2
1.0
0.4
2.25
303 K, y=0.9093x+0.02673, R =0.9999 2 313 K, y=0.8969x+0.01672, R =0.9999 2 323 K, y=0.9379x+0.00239, R =0.9999
1.50 V(V)
0.75 0.00
0.2 0.0
0.5
1.0
1.5 t(h)
2.0
2.5
3.0
0.00
16
0.75
1.50 t(h)
2.25
3.00
Fig. 7. (A) Extraction kinetics of V(V) and Cr(VI) by ionic liquid phase 0.6 ml
[A336][NO3] and 0.4 ml [RNH3][NO3]. CV(V) = 9.6 mmol/l, CCr(VI) = 11.5 mmol/l, Vaq = 120 ml, pH = 9.0. (B) The fitting plots of t/qt vs t for the extraction of V(V) according to pseudo-second-order rate equation.
Table 3
Kinetic parameters of V(V)
extraction by ionic liquid phase
[A336][NO3]-[RNH3][NO3] Pseudo-second-order qe.exp T(K)
V(V)
(mmol/g)
qe
k2
mmol/g
g/(mmol·h)
R2
303
1.089
1.100
30.97
0.9999
313
1.108
1.115
54.82
0.9999
323
1.083
1.066
366.5
0.9999
From Table 3, the calculated reaction rate constants (k2) rapidly increase with the increasing temperature. Probably it is related with the rapidly decreasing viscosity with the increasing temperature. Furthermore, from the relationship between rate constant and temperature (as shown in Fig. S3), the activation energy, Ea, was calculated to be 99.9 kJ·mol-1 by the Arrhenius plot: ln k 2 = −
Ea +B RT
(12)
Where k2, Ea, R, T and B are the pseudo-second-order rate constant (g/(mmol·h)), activation energy (kJ/mol), universal gas constant (J/(mol·K)), temperature (K) and preexponential factor, respectively. The value of activation energy (99.9 kJ·mol-1) indicates that the extraction of V(V) by ionic liquid phase [A336][NO3]-[RNH3][NO3] belongs to a chemical reaction control process.
17
3.6. Extraction mechanism analysis
It had been reported that extraction of metal ions by amines follows two typical mechanisms [28]. One is solvation mechanism with molecular association through hydrogen bonds, such as: + xRNH 2 (o ) + 4 yH aq + yV4O124− ⇔ ( RNH 2 ) x ⋅ ( H 4V4O12 ) y ( o )
Another is anion exchange mechanism to form ion association compounds. The former is related with the number of active hydrogen atoms attached to nitrogen atom of amines. It is necessary for the addition of acid into the aqueous phase to make hydroxo-complexes generated for the solvation mechanism. Hence, primary amines are inclined to follow solvation mechanism. However, when primary amines are pre-acidified before extraction, they should follow an anion exchange mechanism, such as: +
RNH 2( o ) + HNO3 ⇔ [ RNH 3 ] x ⋅ NO3 +
x[ RNH 3 ] ⋅ NO 3
−
( o)
+ yV4O12
4−
−
4−
⇔ [ RNH 3+ ] x ⋅ [V4O12 ] y ( o ) + xNO3
−
But for quaternary amines, an anion exchange mechanism between nitrate and oxo-anions is reasonable. In this work, quaternary amine [A336][NO3] containing an acidified primary amine N1923 ([RNH3][NO3]) have been used for the extraction of vanadium. Therefore, the extraction mechanism should follow the anion exchange mechanism in theory. For the traditional VOC-based extraction with organic diluent, the solubility of the formed ion association compounds in inert organic diluent is limited. Thus, it will decrease its extractability. However, the ionic liquid phase can solubilize the formed ion association compounds. Therefore, IL-based extraction could be intensified when the extraction reaction belongs to the ion exchange mechanism. In order to further confirm the above speculation, [A336][NO3], [RNH3 ][NO3] and mixed [A336][NO3]-[RNH3][NO3] have been used to extract V(V) with varying extractant concentrations under the same experimental conditions. After the extraction gets equilibrium, the concentrations of V and Na in organic phase, the concentration of NO3- released in aqueous phase and equilibrium pH were determined. 18
The results were listed in Table 4.
Table 4 The relative species contents at the extraction equilibrium
Extractant
V(V)org
NO3-,aq
Naorg
(mmol)
(mmol)
(mmol)
(mmol)
0.2113
0.1440
0.1448
0.02125
8.46
0.4225
0.2006
0.2200
0.02109
8.28
[A336][NO3] 0.5281
0.2059
0.2294
0.01937
8.18
0.6338
0.2086
0.2323
0.01859
7.87
0.8450
0.2119
0.2407
0.01526
7.33
0.2694
0.2040
0.2260
0.01867
8.02
0.5388
0.2158
0.2620
0.01953
6.61
[RNH3 ][NO3] 0.8083
0.2158
0.2643
0.01835
6.24
1.0777
0.2159
0.2658
0.01804
6.03
1.3471
0.2159
0.2664
0.01311
5.95
0.235
0.1944
0.1890
0.0005
8.96
0.469
0.3185
0.3485
0.0157
7.53
0.704
0.3361
0.3791
0.0161
7.07
0.938
0.3369
0.3885
0.0122
6.74
1.173
0.3378
0.3877
0.0091
6.69
[A336][NO3] and [RNH3 ][NO3]
pHe
Note: for [A336][NO3] or [RNH3][NO3], CV(V) = 21.6 mmol/l, Vaq = 10 ml, pH = 9.0; for the mixed [A336][NO3] and [RNH3][NO3], CV(V) = 34.0 mmol/l, Vaq = 10 ml, pH = 9.0.
19
According to Fig. S1, V(V) mainly exists as the species V4O124- or V3O93- in the initial aqueous solution. From Table 4, the amount of V extracted into the organic phase is close to the amount of NO3- released into the aqueous phase whether for single or mixed system. Therefore, the main extraction reaction is the anion exchange between NO3- and V4O124- (or V3O93-). However, the released NO3- is increasingly higher than the equivalent amounts of the extracted V with the increasing dosage of extractant, which illustrates that the main ion exchange reaction should accompany with some possible side-reactions. According to the decreasing pHe, the side-reactions are probably caused by a small amount of hydrolysis from [A336][NO3] or [RNH3 ][NO3]. Slight hydrolysis of [A336][NO3] with the increasing pH has been reported [25]. Furthermore, the content of Na in organic phase is negligible probably due to entrainment. Therefore, Na is not involved into the current extraction reaction. Based on the above discussion, the main and side-reactions can be written as follows: For [A336][NO3], main reaction is: −
4 x[ A336+ ][ NO3 ]( o) + xV4O12
4−
4−
⇔ [ A336+ ]4 x ⋅ [V4O12 ]x ( o) + 4 xNO3
−
( aq )
Side-reaction is: −
[ A336 + ][ NO3 ]( o) + H 2O ⇔ [ A336 + ] ⋅ [OH − ]( o ) + HNO3( aq )
For [RNH3][NO3], main reaction is: 4 y[ RNH 3+ ][ NO3− ]( o ) + yV4O124−( aq ) ⇔ [ RNH 3+ ]4 y ⋅ [V4O124− ] y (o ) + 4 yNO3−( aq )
Side-reaction is: [ RNH 3+ ][ NO3− ]( o ) ⇔ RNH 2 (o ) + HNO3( aq ) However, there is no proof for the interaction between [A336][NO3] and [RNH3 ][NO3] in the mixed system. The improved extractability for the mixed system might be derived from the solubilization of ILs for the formed ion association compounds. Based on the above discussion, the solution of NaNO3 could be used for the stripping ofV from the loaded IL phase and thus renew IL phase.
20
3.7. Stripping and Recycling
Stripping and recycling are very significant for an extraction system. The stripping properties and recyclability of the current ionic liquid phase had been investigated. An ideal strippant should not only have nearly 100% stripping for the loaded metal ions but also be helpful for the recycling. Here, several solutions have been used as strippant and the results were shown in Fig. 8. Results showed that water had no stripping ability. 1M HCl was not good enough for the stripping because the extractants are prone to be protonated at higher acidity, which is good for extraction but not for stripping. Higher concentration of NaOH favored the stripping and 2.0 M NaOH could result in complete stripping. These results were consistent with those from the effect of acidity on the extraction of V(V). By contrast, NaNO3 solution exhibited a satisfied stripping ability. A stripping percentage of 50.2% can be obtained by 0.2 M NaNO3. 0.5 M NaNO3 or a higher concentration could result in 100% stripping. Hence, 0.5 M NaNO3 was used to study the recycle test. As shown in Fig. 9, the [A336][NO3]-[RNH3][NO3] extraction system maintained relatively high extraction efficiency (> 80%) even through seven cycles, which indicated the developed system is efficient and sustainable for the extraction and separation of vanadium from chromium.
21
100
S%
80 60 40 20 0 1M HCl
H2O 0.05 0.1
0.2 0.5 1.0 NaOH (M)
2.0
0.2
8.
Stripping
percentages
(S%)
of
1.0
NaNO3 (M)
strippant
Fig.
0.5
V(V)
from
the
V(V)-loaded
[A336][NO3]-[RNH3][NO3] system by different strippant. The concentration of V(V) in ILs phase is 1.5 mmol/g. The stripping phase ratio: 0.5 g of V(V)-loaded ILs phase with 5 ml of strippant solution.
100 80
E%
60 40 20 0 1
2
3 4 5 recycling number
6
7
Fig. 9. Extraction efficiencies of [A336][NO3]-[RNH3 ][NO3] system for V(V) at each
cycle, V[A336][NO3] + V[RNH3][NO3] = 1.0 ml. Each cycle with a fresh aqueous phase: pH = 9.0, CV(V) = 12.5 mmol/l, Vaq = 60 ml. Stripping of V(V)-loaded ionic liquid phase (1.0 ml) using 10 ml 0.5 M NaNO3 at each cycle.
3.8. IR spectra 22
A
2920
Absorbance
2853
1330 nVxOy
1463 1380 1532
1630
~3429
[RNH3][NO3]-V(V) [RNH3][NO3] raw N1923
1000
1500 2000 2500 3000 -1 Wavenumber (cm )
3500
4000
B 2925
Absorbance
2855
1330 1463 1382 n1646 VxOy
~3429
[A336][NO3]-V
[A336][NO3]
1000
1500
2000 2500 3000 -1 Wavenumber (cm )
23
~3429
3500
4000
C
Absorbance
~3429 [A336][NO3]+[RNH3][NO3]-V(V)-stripping
~3429
VxOyn-
[A336][NO3]+[RNH3][NO3]-V(V) ~3429 raw [A336][NO3]+[RNH3][NO3] ~3429 raw [A336][NO3]+N1923
1000
D
1500 2000 2500 3000 -1 Wavenumber (cm )
3500
4000
1334 1467
Absorbance
1328
1200
1377
1635
1328
1540 1540
1635
1540
1635
1337
1300
1400 1500 1600 -1 Wavenumber (cm )
1700
Fig. 10. IR spectra. (A) N1923, acidified N1923 ([RNH3][NO3]) and V(V) - loaded
[RNH3 ][NO3]. (B) [A336][NO3] and V(V) - loaded [A336][NO3]. (C) [A336][NO3] + N1923, [A336][NO3] + [RNH3][NO3], V(V) - loaded [A336][NO3] + [RNH3][NO3 ] and recovered [A336][NO3] + [RNH3][NO3] after V(V) stripping by NaNO3 solution. (D) Enlarged view of the red dashed box area in (C).
Fig. 10 showed IR spectra of the referred extractant or ionic liquid before and after the extraction of V(V). As seen in Fig. 10A, when pure N1923 is acidified by HNO3 24
to be [RNH3][NO3], some new peaks appear, such as 1532 cm-1 (δs -NH3+), 1630 cm-1 (δas -NH3 +) and ~ 1380 cm-1 (νas NO3-). 1463 cm-1 is attributed to δs -CH3 and δCH2. ~ 3076 cm-1 belongs to νas -NH3+ and νs -NH3 +. A wide strong peak at ~3429 cm-1 is derived from H2O. The enhanced stretching bands between 2850 and 3000 cm-1 correspond to CH-, CH2- and –CH3 groups in addition to bending bands at 1463 cm-1. Therefore, it is confirmed that [RNH3][NO3] is formed by acidifying N1923 using HNO3, and H2O is involved into the organic phase simultaneously. To be noticed, νC-N (1337 cm-1) shifts to a lower wave number 1328 cm-1 after acidification due to NH2 transferring to be NH3+ (Fig. 10D). In the spectra of vanadate-loaded [RNH3][NO3 ] (Fig. 10A), [A336][NO3] (Fig. 10B) or [A336][NO3]-[RNH3 ][NO3] (Fig. 10C), new peaks at 800 and 921 cm-1 are attributed to the infrared characteristic peak of vanadate anions [50]. When the loaded vanadate was stripped by 0.5 M NaNO3 solution from the mixed ILs phase, the spectra of the recovered ILs have also been compared with the original [A336][NO3] + [RNH3][NO3] and [A336][NO3] + N1923 in Fig. 10C and Fig. 10D, respectively. Since νC-N (1334 cm-1) in the recovered ILs is between [A336][NO3] + N1923 (1337 cm-1) and [A336][NO3] + [RNH3][NO3] (1328 cm-1), it can be concluded that part of [RNH3][NO3] releases HNO3 to be N1923 molecule during the stripping process. However, the effective stripping exhibits that the current IL phase can be recycled after the addition of a small amount of HNO3 before carrying out the next extraction.
4. Conclusions
The newly developed IL-based separation strategy using tricaprylmethylammonium nitrate ([A336][NO3]) and organic acidified primary amine N1923 ([RNH3][NO3]) as ionic liquid phase exhibits a notable synergistic effect for V(V) and a prominent separation ability for V(V) from Cr(VI). Compared to [A336][NO3] alone, the extraction distribution ratio of V(V) was enhanced significantly from 341.9 to 630.4 and the separation coefficients βV/Cr also increased from 10 to 35 at pH 9.0 by 25
[A336][NO3] and [RNH3][NO3]. In the current extraction system, IL [A336][NO3] is not only an efficient synergistic extractant for V(V) but also an effective solvent for the ion-type extracted species. Most importantly, the typical anion exchange mechanism between NO3- and V4O124- (or V3O93-) can effectively hamper the release of organic cation-ligands into aqueous phase, thus make this extraction process sustainable. The present study is expected to be helpful to designing IL-based synergistic extraction system for green and sustainable extraction process, which would extent the application of IL in the field of hydrometallurgical field. Finally, application of the current IL-based extraction system for the recovery of V(V) from an industry effluent is under the way.
Acknowledgements
This work was supported by the Major Project of NSFC (No. 51090382), NSFC (No. 51104138), “973” Project (2012CBA01202) and Beijing Natural Science Foundation (2142030). This work is also supported by the State Scholarship Fund from China Scholarship Council.
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29
Highlights: 1. Pure IL [A336][NO3] plus [RNH3][NO3] was used to extract and separate V from Cr. 2. It shows a notable synergistic effect for V with improved separation factor βV/Cr. 3. IL is not only extractant but also good solvent for the ion-type extracted species. 4. Anion exchange mechanism between NO3- and V4O124- (or V3O93-) was confirmed. 5. IL phase can be recycled through stripping the loaded vanadium by 0.5M NaNO3.
30