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toluene mixtures as novel extractants
Separation and Purification Technology 66 (2009) 19–24
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Extractive removal of zinc(II) from chloride liquors with phosphonium ionic liquids/toluene mixtures as novel extractants Magdalena Regel-Rosocka ∗ Institute of Chemical Engineering and Technology, Poznan University of Technology, pl. M. Sklodowskiej – Curie 2, 60-965 Poznan, Poland
a r t i c l e
i n f o
Article history: Received 28 August 2008 Received in revised form 1 December 2008 Accepted 2 December 2008 Keywords: Zinc(II) removal Ionic liquids Quaternary phosphonium salt Chloride media Extraction
a b s t r a c t Trihexyl(tetradecyl)phosphonium chloride (Cyphos® IL 101) and bis(trifluoromethylsulphonyl)imide (Cyphos® IL 109) – phosphonium ionic liquids – were used as novel reagents mixed with toluene to extract zinc(II) from chloride media. Extraction of zinc(II) was very fast and efficient (EZn over 95%) for molar ratio of Cyphos® IL 101/Zn(II) more than 2. It was found that the presence of HCl in the feed enhanced Zn(II) extraction. The reactions of Zn(II) extraction mechanism were proposed. The values of H◦ were estimated to be 32.93 (standard deviation (s.d.) = 2.81) and 52.85 (s.d. = 1.90) kJ mol−1 and S◦ amounted to 120.65 (s.d. = 8.84) and 182.13 (s.d. = 6.02) J mol−1 K−1 for reaction without HCl and with 0.58 M acid, respectively. The extraction of zinc(II) with Cyphos® IL 101/toluene mixture is an endothermic reaction. Successful stripping of zinc(II) from the loaded organic phase was achieved with 1 M sulphuric acid. Cyphos® IL 101 can be reused at least in 3 cycles of extraction-stripping process. Due to low extraction of Zn(II) Cyphos® IL 109 cannot be considered as effective extractant in the studied system. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The removal of toxic zinc(II) is a very important issue in the case of spent pickling solutions (SPS) from hot-dip galvanizing plants. As a result of pickling, zinc(II) concentration in spent solutions increases even up to 130 g dm−3 , iron content to 100 g dm−3 , HCl to 10% [1]. Composition of the waste depends upon a plant and the way of pickling applied, while according to the European and national standards permissible content of waste after neutralization is as follows: 2 mg dm−3 Zn, 10 mg dm−3 Fe, 1 g dm−3 Cl− , pH 6–9. Thus, regeneration of SPS is a crucial issue regarding both environmental protection and economy of the process [2]. Solvent extraction belongs to these separation methods that are applied for removal of metal ions from wastewaters. Several extractants (basic, acidic and neutral) have been proposed for zinc(II) extraction from hydrochloric acid solutions in previous works [3–5]. Recently ionic liquids (ILs) as a new generation of solvents, considered as “green solvents”, have been proposed for separation processes. Due to some specific features such as: non-flammability, high thermal stability and non-volatility, ionic liquids account for their application in many fields, e.g., as solvents and catalysts for organic reactions, electrolytes in chemical sources of energy, plasticizers, bactericides, fungicides or
∗ Tel.: +48 61 665 37 71; fax: +48 61 665 36 49. E-mail address: [email protected]. 1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2008.12.002
antistatic agents [6–10]. Numerous studies on the application of imidazolium ILs as solvents in separation processes have been carried out for the last ten years. The following ILs are most frequently applied: 1-alkyl-3-methylimidazolium hexafluorophosphate [Cn mim][PF6 ], tetrafluoroborate [Cn mim][BF4 ] and bis[(trifluoromethyl)sulfonyl]imide [Cn mim][Tf2 N]. They have replaced molecular solvents, such as chloroform, n-dodecane or 1octanol, in pyridinecalix-4-arene (extraction of Ag(I)) [11], in CMPO (octyl(phenyl)-N,N-diisobutylcarbamylmethylphosphine oxide) – extraction of Ce(III), Eu(III), Y(III) [12] – in PAN (1-(2-pyridylazo)2-naphthol) and TAN (1-(2-tiazolylazo)-2-naphthol) (extraction of Hg(II)) [13], and in crown ethers 18C6 and DCH18C6 (extraction of alkali metals and Sr(II)) [14–16] resulting in a significant improvement in the extraction efficiency of the metal cations referred to above. Moreover, Dietz and Stepinski [17] have studied distribution of uranyl ion between nitrate-containing aqueous phases and various N,N-dialkylimidazolium-based room-temperature ILs in the presence of tri-n-butyl phosphate. Phosphonium ionic liquids are considered to be prospective for separation of substances [18]. Up to now extraction of lactic acid with trihexyl-(tetradecyl)phosphonium-bis-2,4,4-trimethylpentylphosphinate (Cyphos® IL 104) has been reported by Martak and Schlosser [19]. Turanov et al. [20] has studied partition of lanthanide chlorides between HCl and organic solutions of neutral organophosphorous compounds and IL – e.g. butyldiphenylphosphonium hexafluorophosphate and bis(trifluoromethylsulfonyl) imide. Trihexyl-(tetradecyl)phosphonium chloride (Cyphos® IL 101) has been used recently to impregnate Amberlite XAD-7 and
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M. Regel-Rosocka / Separation and Purification Technology 66 (2009) 19–24
Table 1 The structures of the phosphonium ILs used.
, ®
Cyphos IL 101, Trihexyl(tetradecyl)phosphonium chloride
®
Cyphos IL 109, Trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulphonyl)imide
biopolymer capsules and to remove efficiently Zn(II), Hg(II) and Au(III) from HCl solutions [21–23]. Looking for new and more efficient solvents the aim of the work is to present fundamental studies on extractive zinc(II) removal from chloride liquors with mixtures of selected phosphonium ILs/toluene as novel extractants. 2. Experimental Two phosphonium ionic liquids supplied by Cytec Industries Inc. (Canada) were mixed with toluene and used as extractants (0.04–0.86 M). Toluene was applied to overcome some drawbacks caused by the high viscosity of ILs. Structures of the applied ILs are presented in Table 1. Extraction was carried out in a typical way: equimolar aqueous feeds, containing 0.5–90 g dm−3 of zinc(II) (added as a chloride) in 0 or 0.58 M HCl were mechanically shaken with IL phase (volume ratio w/o = 1) for a period of time between 10 s and 60 min at a room temperature in glass separatory funnels and then allowed to stand for phase separation. The chloride concentration adjusted with NaCl was equal to 5 M in the initial aqueous feed. Loaded organic phase was stripped with various stripping solutions such as: 0.5 M NH4 OH, 0.5 and 1 M H2 SO4 , 4 M HCl, water (w/o = 1). Stripping was repeated three times consecutively using fresh stripping solution. Potentiometric titration was used for determination of metal, hydrochloric acid and chloride concentrations. The content of water in the organic phase was determined by Karl–Fischer titration. Each experiment was carried out twice and the error did not exceed 5%. Distribution coefficient (D) was defined as the ratio of the zinc ion concentrations in the organic [Zn]*(o) and aqueous phase [Zn]*(aq) after extraction. Percentage extraction (EZn ) was calculated from the contents of metal ions in the aqueous phases before [Zn]i and after [Zn]*(aq) extraction: EZn = (([Zn]i − [Zn]∗(aq) )/[Zn]i ) × 100%, assuming that the volumes of phases did not change. This assumption was valid.
Fig. 1. Effect of contact time on zinc(II) extraction with Cyphos® IL 101 and Cyphos® IL 109 (feed: 5 g dm−3 Zn(II), 0 or 0.58 M HCl, 5 M Cl− ).
slowly (0.01 mol dm−3 s−1 ) and reaches only 20% of Zn(II) extracted after 60 min. In the case of Cyphos® IL 101 in toluene constant value of the Zn(II) percentage extraction (EZn = 100%) is achieved after 15 min. Phase separation after extraction is very fast and good, no formation of emulsions is observed. 3.2. Extraction equilibrium The isotherms of zinc(II) extraction from feed with 0.58 M HCl and without acid indicate that zinc(II) is very well extracted in both cases with Cyphos® IL 101/toluene (Fig. 2). The presence of HCl in the feed slightly enhances zinc(II) extraction. Maximum loading capacity of 0.86 M Cyphos® IL 101 amounts to 30 and 40 g dm−3 of Zn(II) from feed without HCl and with 0.58 M acid, respectively. The isotherm for Cyphos® IL 109 is very flat. Zn(II) remains in the aqueous phase, only 2.5 g dm−3 Zn(II) is extracted. Zinc(II) distribution and percentage extraction decrease with increasing Zn(II) content in feed due to decrease of molar ratio between 0.86 M Cyphos® IL 101 and Zn(II) content (Table 2 and Fig. 3). At 90 g dm−3 (1.38 M) Zn(II) percentage extraction reaches less than 40% whereas at 5 g dm−3 (0.076 M) EZn is higher than 95%. At molar ratio Cyphos® IL 101/Zn(II) less than 2 a distinct decrease in Zn(II) extraction is observed. Above the ratio of 2 zinc(II) extraction amounts to 100%. The issue of extraction mechanism will be
3. Results and discussion 3.1. Effect of contact time The studies on kinetics of Zn(II) extraction have been carried out by equilibrating the aqueous feed containing 5 g dm−3 Zn(II), 0 or 0.58 M HCl, 5 M Cl− with ILs in toluene. An initial extraction rate has been calculated from the slopes at the beginning of the extraction [24]. As shown in Fig. 1, zinc(II) extraction with Cyphos® IL 101 is very fast, the initial rate of metal transfer to the IL phase equals 0.3 mol dm−3 s−1 . Cyphos® IL 109 in toluene extracts Zn(II) rather
Fig. 2. Extraction isotherms of zinc(II) with Cyphos® IL 101 and Cyphos® IL 109 (feed, 0.5–58 g dm−3 Zn(II), 0.58 M HCl, 5 M Cl− ).
M. Regel-Rosocka / Separation and Purification Technology 66 (2009) 19–24
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Table 2 Extraction parameters and water content (H2 O) in the IL phase after extraction of zinc(II) from feed in the presence and absence of HCl. Extractant ®
Cyphos IL 101
[Zn]i (g dm−3 ) 5 90
Cyphos® IL 109
5
HCl (M)
log DZn
EZn (%)
H2 O (%)
0 0.58 0 0.58 0 0.58
1.5 1.8 −0.28 −0.20 −0.99 −1.1
96.6 98.1 35.4 38.5 9.38 7.79
3.9 3.0 1.0 1.3 0.04 0.12
discussed in the next section. Both DZn and EZn are slightly affected by the presence of HCl. Zn(II) extraction increases with increasing HCl content. The observed decrease in the distribution ratio for Cyphos® IL 109/toluene mixture compared with Cyphos® IL 101, results probably from the high hydrophobicity of the imide anion. The affinity of the IL phase to the aqueous phase is very low and the transfer of zinc(II) species is difficult. Conversely, the hydrophilic chloride anion from Cyphos® IL 101 is easy to be exchanged into zinc(II) anionic chlorocomplexes. Water content in the organic phase after extraction confirms differences in the hydrophilicity of the IL phase. Water content in Cyphos® IL 109 is very small and does not reach 0.15 wt.%, whereas Cyphos® IL 101 after extraction contains, depending on Zn(II) concentration, from 1 to almost 4 wt.% of water (Table 2). The presented equilibrium data indicate that the Cyphos® IL 101/toluene mixture is a very effective phase that enables the 0.86 M IL phase to be loaded with Zn(II) up to as much as 40 g dm−3 . Most importantly, Cyphos® IL 101 can be considered as an active extractant if mixed with a neutral solvent. Due to low extraction of Zn(II) Cyphos® IL 109 has not been considered for further investigations.
3.3. Mechanism of Zn(II) extraction On the basis of literature data it has been assumed that extraction can be realized according to two different mechanisms [25], depending on the concentration of chloride ions and therefore on the distribution of zinc chlorocomplexes:
− ZnCl2− 4(aq) + nR3 R PCl ⇔ (R3 RP)n ZnCl4 + nCl(aq)
(1)
(2)
where subscript (aq) stands for the aqueous phase and a horizontal bar denotes the organic phase. Previously studied systems support the extraction of ZnCl2 and ZnCl4 2− [26–28] depending upon chloride concentration and acidity of the aqueous feed. However, El Dessouky et al. [29] report extraction of ZnCl3 − species with neutral extractants. In this paper it is assumed that for feed containing Zn(II), 0.58 M HCl and 5 M Cl− the predominant species existing in the aqueous feed is ZnCl4 2− (extraction according to Eq. (2)) and for the feed without HCl–ZnCl2 (Eq. (1)). During extraction chlorocomplexes of zinc(II) form ionic pairs with the phosphonium cations. Molar ratios of chloride to zinc(II) ions transported to the loaded Cyphos® IL 101, calculated from the mass balance of the aqueous phase before and after extraction, are presented in Table 3. Average values of the ratios are equal to 1.65 and 2.11 for feed without HCl and with 0.58 M acid, respectively. They are close to 2 and fit to both mechanisms according to Eqs. (1) and (2). For Eq. (1) the extraction constant, Kex(1) , can be written as follows (indices (1) and (2) assign the appropriate reactions): Kex(1) =
[(R3 R P)m ZnCl2+m ]
(3)
m
[R3 R PCl] [ZnCl2 ](aq)
or for Eq. (2): n
Kex(2) =
(a) addition of neutral chlorocomplex to IL molecule (R3 R PCl) ZnCl2(aq) + mR3 R PCl ⇔ (R3 R P)m ZnCl2+m
(b) anion exchange of chlorides into a negatively charged zinc tetrachlorocomplex
[(R3 R P)n ZnCl4 ][Cl− ](aq)
(4)
n
[R3 R PCl] [ZnCl2− 4 ](aq)
Taking into account formation constants of zinc(II) chlorocomplexes the distribution coefficient, DZn , is given as follows: DZn =
[ionic pair] ([Zn2+ ](1 +
4
(5)
i ˇ [Cl− ] ))(aq) i=1 i
where ˇi is stability constant of zinc(II) chlorocomplex; [ionic pair] = [(R3 R P)m ZnCl2+m ] or [(R3 R P)n ZnCl4 ]. Table 3 Loading of Cyphos® IL 101 with zinc(II) and chloride ions.
Fig. 3. Effect of IL101/Zn in feed ratio on zinc(II) extraction to the IL phase (feed, 0.5–58 g dm−3 Zn(II), 0.58 M HCl, 5 M Cl− ; organic: 0.86 M Cyphos® IL 101 in toluene).
HCl in feed (M)
[Zn]*o (g dm−3 )
Cl− o (M)
Molar ratio Cl− o /[Zn]*o
0
34.4 33.3 33.4 31.2 30.2
0.843 0.815 0.849 0.803 0.775
1.60 1.60 1.66 1.69 1.68
0.58
28.1 32.6 32.2 39.7 36.0
0.780 1.25 1.11 1.34 0.961
1.81 2.51 2.25 2.21 1.75
“o” denotes the organic phase.
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M. Regel-Rosocka / Separation and Purification Technology 66 (2009) 19–24
3.4. Influence of temperature on Zn(II) extraction The extraction of Zn(II) from feed containing 0.58 M HCl and without acid has been carried out at temperature range 20–70 ◦ C (Fig. 5). With increasing temperature increases zinc(II) distribution between the organic and aqueous phase. The effect of temperature is studied to estimate values of extraction enthalpies H◦ and entropies S◦ . The dependence of Kex on the temperature is expressed by the Gibbs–Helmholtz equation [30]: log Kex =
−H ◦ S◦ +
2.303 RT
(9)
2.303 R
where R is the gas constant (8.314 J mol−1 K−1 ). Introducing Gibbs–Helmholtz equation and Eq. (7) or (8) to Eq. (6) a dependence of DZn upon temperature is obtained for reaction (1): Fig. 4. Dependence of zinc(II) distribution ratio on Cyphos® IL 101 concentration for the feed () with 0.58 M HCl and () without acid (feed: 5 g dm−3 Zn(II), 5 M Cl− ; organic: 0.04–0.7 M Cyphos® IL 101 in toluene).
The following equations for the logarithm of distribution coefficient can be derived after introducing the extraction constant to Eq. (5): log DZn = log K + m log[R3 R PCl]
(6)
As the total chloride ion concentration has been established for 5 M in the feed, it can be considered as constant, because the amount of the chloride ion is much higher in the aqueous phase than the extracted amount of zinc(II) and chloride. Thus, for Eq. (6) a constant K has been introduced: 2
K(1) = Kex(1) [Cl− ] ×
− 4−n
K(2) = Kex(2) [Cl ]
1+ ×
4
1+
ˇ2
ˇ i=1 i
4
× [Cl− ]
i
− i
Based on Eq. (6), the number of molecules of extractant engaged in Zn(II) extraction has been read from the slope of the plot (Fig. 4). Values for both reactions (1) and (2) equal m = n = 2. It means that Zn(II) is extracted to the IL phase as the following ionic pair: [(R3 R P)2 ZnCl4 ].
2.303 RT
1+
+
2.303 R
4
+ 2 log[Cl− ]
ˇ2
ˇ × [Cl− ] i=1 i
i
+ 2 log[R3 R PCl]
(10)
and for reaction (2): log DZn(2) =
−H ◦ S◦ 2.303 RT + log
1+
+
2.303 R
4
+ 2 log[Cl− ]
ˇ4 − i
ˇ × [Cl ] i=1 i
+ 2 log[R3 R PCl]
(11)
The standard molar enthalpy H◦ is the slope of the log DZn versus 1/T (Fig. 5) and the standard entropy S◦ of extraction is the intercept of the plot calculated from the following ◦ S /2.303 R + 2 log[Cl− ] + expression for reaction (1):
(8)
× [Cl ]
−H ◦ S◦ + log
log ˇ2 / 1 +
ˇ4
ˇ i=1 i
(7)
log DZn(1) =
(2):
4
ˇ × [Cl− ] i=1 i
S ◦ /2.303 R
i
+ 2 log[R3 R PCl]; and for reaction
−
+ 2 log[Cl ] + log ˇ4 /
1+
4
ˇi
i · [Cl− ]
+
i=1
2 log[R3 R PCl]. Values of formation constants ˇi reported by various authors differ significantly [31]. Despite ˇi constants depend on the temperature, the values obtained from the logarithm expression are small and do not significantly affect S◦ values. Thus, exemplary values ˇ1 = 0.0575; ˇ2 = 0.0355; ˇ3 = 0.0324; ˇ4 = 0.0214 are assumed according to [32] at 3 M ionic strength and 20 ◦ C. The values of H◦ are found to be 32.93 (s.d. = 2.81) and 52.85 (s.d. = 1.90) kJ mol−1 , whereas S◦ equals 120.65 (s.d. = 8.84) and 182.13 (s.d. = 6.02) J mol−1 K−1 for reactions (1) and (2), respectively. The extraction of zinc(II) with Cyphos® IL 101/toluene mixture is an endothermic reaction both from the feed with HCl and without acid. Standard Gibbs free energy (G◦ in kJ/mol) of zinc(II) extraction with Cyphos® IL 101/toluene according to reactions (1) and (2) can be expressed as follows: ◦ = 32.93 − 120.65 × 10−3 T G(1)
(12)
◦ = 52.85 − 182.13 × 10−3 T G(2)
(13)
3.5. Stripping of Zn(II) from loaded Cyphos® IL 101/toluene
Fig. 5. The effect of temperature on zinc(II) distribution coefficient (feed: 5 g dm−3 Zn(II), 0 or 0.58 M HCl, 5 M Cl− ; organic: 0.86 M Cyphos® IL 101 in toluene).
The ionic bonds in the IL phase are very strong, therefore simple stripping of Zn(II) with water is not possible. Another stripping solution, forming complexes with Zn(II) strong enough to attract them from IL, should be used. Recovery results for various investigated stripping solutions are presented in Table 4. Percentage
M. Regel-Rosocka / Separation and Purification Technology 66 (2009) 19–24
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Table 4 Zn(II) stripping from loaded Cyphos® IL 101 with various stripping solutions. Stripping solution
S1 (%)
S1+2 (%)
S1+2+3 (%)
1 M H2 SO4 0.5 M H2 SO4 4 M HCl 0.5 M NH4 OH
65.6 56.4 4.66 100
100 90.3 5.42 –
– 97.7 5.42 –
stripping of Zn(II) from loaded Cyphos® IL 101 after first (S1 ), second (S1+2 ) and third stage (S1+2+3 ) is defined as the sum of Zn(II) stripped in one, two or three stages to Zn(II) content in the organic phase prior to first stripping. Based on the preliminary results, 0.5 M NH4 OH has been proposed as an efficient stripping solution as it enables up to 100% of zinc(II) to be removed from the loaded Cyphos® IL 101. However, formation of white precipitation at the interface, when the organic phase is loaded with big amount of Zn(II), is the main drawback of NH4 OH. Hydrochloric acid has been applied due to its efficient stripping of metals from quaternary ammonium salts [25]. However, in case of phosphonium salts Zn(II) stripping with 4 M HCl is ineffective (only 5.42% of Zn(II) recovered). Sulfuric acid seems to be the best stripping solution and its effectiveness increases with increasing acid concentration. After two stages of stripping with 1 M H2 SO4 zinc is completely removed from loaded Cyphos® IL 101/toluene mixture. Moreover, there are no problems with emulsions and/or precipitation, both phases are transparent. 3.6. Regeneration of Cyphos® IL 101/toluene The ability to reuse the IL phase is of critical importance in determining the feasibility of IL-based separation process. The issue of IL/toluene recycling concerns Zn(II) recovery from loaded Cyphos® IL 101 and further changes in extraction ability of regenerated IL phase. The successful recovery of Zn(II) with 1 M H2 SO4 has been presented in Section 3.5. In order to investigate extraction with regenerated IL, 0.1 Cyphos® IL 101 has been loaded with zinc(II). After 5 stages of extraction each time with fresh aqueous feed (5 g dm−3 Zn(II), 0.58 M HCl, 5 M Cl− ) the organic phase contains constant amount of Zn(II) 3.73 g dm−3 (Fig. 6). Further the loaded IL has been stripped three times with 1 M H2 SO4 to remove completely Zn(II) from IL phase. Regenerated Cyphos® IL 101 has been used again for Zn(II) extraction (2nd cycle) and than stripped three times with 1 M H2 SO4 . The procedure of extraction-stripping has been repeated 4 times (Fig. 7). Extraction
Fig. 6. Loading capacity of 0.1 M Cyphos® IL 101 (feed: 5 g dm−3 Zn(II), 0.58 M HCl, 5 M Cl− ).
Fig. 7. Extraction of Zn(II) with regenerated Cyphos® IL 101.
of Zn(II) with fresh and once regenerated IL (2nd cycle) is comparable and amounts to 60%. As 0.1 M Cyphos® IL 101 is used for recycling studies, instead of 0.8 M, thus about 65% of zinc(II) can be extracted as maximum from feed containing 5 g dm−3 Zn(II) (0.077 M). However, in the next cycles EZn increases up to 69 and 75% in 3rd and 4th cycles, respectively. It is positive that the IL can be reused several times, however increase in percentage extraction indicates possible changes in the extraction mechanism or structure of complexes. This issue needs further investigations. Compared with commercial extractants [3,5,27–29] Cyphos® IL 101 indicates very good zinc(II) extraction and stability even after several cycles of regeneration. Stripping with acid instead of water should be pointed out as a drawback of this quaternary phosphonium salt. 4. Conclusions • Due to low extraction of Zn(II) Cyphos® IL 109 cannot be considered as effective extractant in the studied system. The affinity of Cyphos® IL 109 phase to the aqueous phase is very low and the transfer of zinc(II) species is difficult. • Results of the presented investigations prove that Cyphos® IL 101/toluene mixture can be successfully used as the extractant for zinc(II) removal from chloride media. • Extraction of zinc(II) is very fast and efficient (EZn over 95%) for molar ratio Cyphos® IL 101/Zn(II) more than 2. The presence of HCl in the feed enhances Zn(II) extraction. • The following reactions of Zn(II) extraction mechanism are proposed: ZnCl2(aq) + 2R3 R PCl ⇔ (R3 R P)2 ZnCl4
(1)
− ZnCl2− 4(aq) + 2R3 R PCl ⇔ (R3 R P)2 ZnCl4 + 2Cl(aq)
(2)
• The extraction of zinc(II) with Cyphos® IL 101/toluene mixture is an endothermic reaction from the feed both with HCl and without acid. • The estimated values H◦ amount to 32.93 and 52.85 kJ mol−1 , whereas S◦ equals 120.65 and 182.13 J mol−1 K−1 for reactions (1) and (2), respectively. • Sulfuric acid is the best stripping solution among the studied and its effectiveness increases with increasing acid concentration. • The feasibility of Cyphos® IL 101-based separation process is proven due to the ability to reuse the Cyphos® IL 101/toluene mixture in several cycles of Zn(II) extraction-stripping.
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Acknowledgments We thank Cytec Industries Inc. for providing us with free samples of Cyphos® IL 101 and Cyphos® IL 109. This work was supported by the Polish Ministry of Science and Higher Education with its grant no. 1T 09B 081 30. References [1] P. Maass, P. Peissker, Hot-dip galvanizing, Agencja Wydawnicza Placet, Warsaw, 1998 (in Polish). [2] M. Regel-Rosocka, M. Wisniewski, in: J. Kudelko, J. Kulczycka, H. Wirth (Eds.), Sustainable Mineral Supply for Europe—From Waste to Resources, IGSMiE PAN, Krakow, 2007, pp. 46–53. [3] A. Grzeszczyk, M. Regel-Rosocka, Hydrometallurgy 86 (2007) 72–79. [4] M. Regel-Rosocka, J. Szymanowski, Solvent Extr. Ion Exch. 23 (2005) 411–424. [5] R. Cierpiszewski, I. Miesiac, M. Regel-Rosocka, A.M. Sastre, J. Szymanowski, Ind. Chem. Eng. Res. 41 (2002) 598–603. [6] K.R. Seddon, A. Stark, M.J. Torres, Pure Appl. Chem. 72 (2000) 2275–2287. [7] C.J. Adams, in: R.D. Rogers, K.R. Seddon (Eds.), Ionic Liquids. Industrial Applications for Green Chemistry, ACS Symposium Series, vol. 818, American Chemical Society, Washington, DC, 2002, pp. 13–29 (Chapter 2). [8] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH, Weinheim, 2003. [9] T. Welton, Chem. Rev. 99 (1999) 2071–2083. [10] A. Cieniecka-Roslonkiewicz, J. Pernak, J. Kubis-Feder, A. Ramani, A.J. Robertson, K.R. Seddon, Green Chem. 7 (2005) 855–862. [11] K. Shimojo, M. Goto, Anal. Chem. 76 (2004) 5039–5044. [12] K. Nakashima, F. Kubota, T. Maruyama, M. Goto, Ind. Eng. Chem. Res. 44 (2005) 4368–4372.
[13] A.E. Visser, R.P. Swatloski, S.T. Griffin, D.H. Hartman, R.D. Rogers, Sep. Sci. Technol. 36 (2001) 785–804. [14] D.C. Stepinski, M.P. Jensen, J.A. Dzielawa, M.L. Dietz, Green Chem. 7 (2005) 151–158. [15] H. Luo, S. Dai, P.V. Bonnesen, Anal. Chem. 76 (2004) 2773–2779. [16] M.L. Dietz, D.C. Stepinski, Green Chem. 7 (2005) 747–750. [17] M.L. Dietz, D.C. Stepinski, Talanta 75 (2008) 598–603. [18] C.J. Bradaric, A. Downard, C. Kennedy, A.J. Robertson, Y. Zhou, Green Chem. 5 (2003) 143–152. [19] J. Martak, S. Schlosser, Sep. Purif. Technol. 57 (2007) 483–494. [20] A.N. Turanov, V.K. Karndashev, V.E. Baulin, Russ. J. Inorg. Chem. 53 (2008) 970–975. [21] V. Gallardo, R. Navarro, I. Saucedo, M. Ávila, E. Guibal, Sep. Sci. Technol. 43 (2008) 2434–2459. [22] E. Guibal, K. Gavilan, P. Campos, T. Bunio, A. Vincent, Trochimczuk, Sep. Sci. Technol. 43 (2008) 2406–2433. [23] K. Campos, T. Vincent, P. Bunio, A. Trochimczuk, E. Guibal, Solvent Extr. Ion Exch. 26 (2008) 570–601. [24] I. Szczepanska, A. Borowiak-Resterna, M. Wisniewski, Hydrometallurgy 68 (2003) 159–170. [25] T. Sato, T. Nakamura, Hydrometallurgy 6 (1980) 3–12. [26] D.F.C. Morris, E.L. Short, J. Chem. Soc. (1962) 2662–2671. [27] J. Niemczewska, R. Cierpiszewski, J. Szymanowski, Desalination 162 (2004) 169–177. [28] M.B. Mansur, S.D.F. Rocha, F.S.J. Magalhaes, d.S. Benedetto, J. Hazard. Mater. 150 (2008) 669–678. [29] S.I. El Dessouky, Y.A. El-Nadi, I.M. Ahmeda, E.A. Saad, J.A. Daoud, Chem. Eng. Process 47 (2008) 177–183. [30] A. Mellah, D. Benachour, Sep. Purif. Technol. 56 (2007) 220–224. [31] B. Wassink, D. Dreisinger, J. Howard, Hydrometallurgy 57 (2000) 235–252. [32] A. Masana, M. Valiente, Anal. Sci. 4 (1988) 63–68.
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Extractive removal of zinc(II) from chloride liquors with phosphonium ionic liquids/toluene mixtures as novel extractants Magdalena Regel-Rosocka ∗ Institute of Chemical Engineering and Technology, Poznan University of Technology, pl. M. Sklodowskiej – Curie 2, 60-965 Poznan, Poland
a r t i c l e
i n f o
Article history: Received 28 August 2008 Received in revised form 1 December 2008 Accepted 2 December 2008 Keywords: Zinc(II) removal Ionic liquids Quaternary phosphonium salt Chloride media Extraction
a b s t r a c t Trihexyl(tetradecyl)phosphonium chloride (Cyphos® IL 101) and bis(trifluoromethylsulphonyl)imide (Cyphos® IL 109) – phosphonium ionic liquids – were used as novel reagents mixed with toluene to extract zinc(II) from chloride media. Extraction of zinc(II) was very fast and efficient (EZn over 95%) for molar ratio of Cyphos® IL 101/Zn(II) more than 2. It was found that the presence of HCl in the feed enhanced Zn(II) extraction. The reactions of Zn(II) extraction mechanism were proposed. The values of H◦ were estimated to be 32.93 (standard deviation (s.d.) = 2.81) and 52.85 (s.d. = 1.90) kJ mol−1 and S◦ amounted to 120.65 (s.d. = 8.84) and 182.13 (s.d. = 6.02) J mol−1 K−1 for reaction without HCl and with 0.58 M acid, respectively. The extraction of zinc(II) with Cyphos® IL 101/toluene mixture is an endothermic reaction. Successful stripping of zinc(II) from the loaded organic phase was achieved with 1 M sulphuric acid. Cyphos® IL 101 can be reused at least in 3 cycles of extraction-stripping process. Due to low extraction of Zn(II) Cyphos® IL 109 cannot be considered as effective extractant in the studied system. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The removal of toxic zinc(II) is a very important issue in the case of spent pickling solutions (SPS) from hot-dip galvanizing plants. As a result of pickling, zinc(II) concentration in spent solutions increases even up to 130 g dm−3 , iron content to 100 g dm−3 , HCl to 10% [1]. Composition of the waste depends upon a plant and the way of pickling applied, while according to the European and national standards permissible content of waste after neutralization is as follows: 2 mg dm−3 Zn, 10 mg dm−3 Fe, 1 g dm−3 Cl− , pH 6–9. Thus, regeneration of SPS is a crucial issue regarding both environmental protection and economy of the process [2]. Solvent extraction belongs to these separation methods that are applied for removal of metal ions from wastewaters. Several extractants (basic, acidic and neutral) have been proposed for zinc(II) extraction from hydrochloric acid solutions in previous works [3–5]. Recently ionic liquids (ILs) as a new generation of solvents, considered as “green solvents”, have been proposed for separation processes. Due to some specific features such as: non-flammability, high thermal stability and non-volatility, ionic liquids account for their application in many fields, e.g., as solvents and catalysts for organic reactions, electrolytes in chemical sources of energy, plasticizers, bactericides, fungicides or
∗ Tel.: +48 61 665 37 71; fax: +48 61 665 36 49. E-mail address: [email protected]. 1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2008.12.002
antistatic agents [6–10]. Numerous studies on the application of imidazolium ILs as solvents in separation processes have been carried out for the last ten years. The following ILs are most frequently applied: 1-alkyl-3-methylimidazolium hexafluorophosphate [Cn mim][PF6 ], tetrafluoroborate [Cn mim][BF4 ] and bis[(trifluoromethyl)sulfonyl]imide [Cn mim][Tf2 N]. They have replaced molecular solvents, such as chloroform, n-dodecane or 1octanol, in pyridinecalix-4-arene (extraction of Ag(I)) [11], in CMPO (octyl(phenyl)-N,N-diisobutylcarbamylmethylphosphine oxide) – extraction of Ce(III), Eu(III), Y(III) [12] – in PAN (1-(2-pyridylazo)2-naphthol) and TAN (1-(2-tiazolylazo)-2-naphthol) (extraction of Hg(II)) [13], and in crown ethers 18C6 and DCH18C6 (extraction of alkali metals and Sr(II)) [14–16] resulting in a significant improvement in the extraction efficiency of the metal cations referred to above. Moreover, Dietz and Stepinski [17] have studied distribution of uranyl ion between nitrate-containing aqueous phases and various N,N-dialkylimidazolium-based room-temperature ILs in the presence of tri-n-butyl phosphate. Phosphonium ionic liquids are considered to be prospective for separation of substances [18]. Up to now extraction of lactic acid with trihexyl-(tetradecyl)phosphonium-bis-2,4,4-trimethylpentylphosphinate (Cyphos® IL 104) has been reported by Martak and Schlosser [19]. Turanov et al. [20] has studied partition of lanthanide chlorides between HCl and organic solutions of neutral organophosphorous compounds and IL – e.g. butyldiphenylphosphonium hexafluorophosphate and bis(trifluoromethylsulfonyl) imide. Trihexyl-(tetradecyl)phosphonium chloride (Cyphos® IL 101) has been used recently to impregnate Amberlite XAD-7 and
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M. Regel-Rosocka / Separation and Purification Technology 66 (2009) 19–24
Table 1 The structures of the phosphonium ILs used.
, ®
Cyphos IL 101, Trihexyl(tetradecyl)phosphonium chloride
®
Cyphos IL 109, Trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulphonyl)imide
biopolymer capsules and to remove efficiently Zn(II), Hg(II) and Au(III) from HCl solutions [21–23]. Looking for new and more efficient solvents the aim of the work is to present fundamental studies on extractive zinc(II) removal from chloride liquors with mixtures of selected phosphonium ILs/toluene as novel extractants. 2. Experimental Two phosphonium ionic liquids supplied by Cytec Industries Inc. (Canada) were mixed with toluene and used as extractants (0.04–0.86 M). Toluene was applied to overcome some drawbacks caused by the high viscosity of ILs. Structures of the applied ILs are presented in Table 1. Extraction was carried out in a typical way: equimolar aqueous feeds, containing 0.5–90 g dm−3 of zinc(II) (added as a chloride) in 0 or 0.58 M HCl were mechanically shaken with IL phase (volume ratio w/o = 1) for a period of time between 10 s and 60 min at a room temperature in glass separatory funnels and then allowed to stand for phase separation. The chloride concentration adjusted with NaCl was equal to 5 M in the initial aqueous feed. Loaded organic phase was stripped with various stripping solutions such as: 0.5 M NH4 OH, 0.5 and 1 M H2 SO4 , 4 M HCl, water (w/o = 1). Stripping was repeated three times consecutively using fresh stripping solution. Potentiometric titration was used for determination of metal, hydrochloric acid and chloride concentrations. The content of water in the organic phase was determined by Karl–Fischer titration. Each experiment was carried out twice and the error did not exceed 5%. Distribution coefficient (D) was defined as the ratio of the zinc ion concentrations in the organic [Zn]*(o) and aqueous phase [Zn]*(aq) after extraction. Percentage extraction (EZn ) was calculated from the contents of metal ions in the aqueous phases before [Zn]i and after [Zn]*(aq) extraction: EZn = (([Zn]i − [Zn]∗(aq) )/[Zn]i ) × 100%, assuming that the volumes of phases did not change. This assumption was valid.
Fig. 1. Effect of contact time on zinc(II) extraction with Cyphos® IL 101 and Cyphos® IL 109 (feed: 5 g dm−3 Zn(II), 0 or 0.58 M HCl, 5 M Cl− ).
slowly (0.01 mol dm−3 s−1 ) and reaches only 20% of Zn(II) extracted after 60 min. In the case of Cyphos® IL 101 in toluene constant value of the Zn(II) percentage extraction (EZn = 100%) is achieved after 15 min. Phase separation after extraction is very fast and good, no formation of emulsions is observed. 3.2. Extraction equilibrium The isotherms of zinc(II) extraction from feed with 0.58 M HCl and without acid indicate that zinc(II) is very well extracted in both cases with Cyphos® IL 101/toluene (Fig. 2). The presence of HCl in the feed slightly enhances zinc(II) extraction. Maximum loading capacity of 0.86 M Cyphos® IL 101 amounts to 30 and 40 g dm−3 of Zn(II) from feed without HCl and with 0.58 M acid, respectively. The isotherm for Cyphos® IL 109 is very flat. Zn(II) remains in the aqueous phase, only 2.5 g dm−3 Zn(II) is extracted. Zinc(II) distribution and percentage extraction decrease with increasing Zn(II) content in feed due to decrease of molar ratio between 0.86 M Cyphos® IL 101 and Zn(II) content (Table 2 and Fig. 3). At 90 g dm−3 (1.38 M) Zn(II) percentage extraction reaches less than 40% whereas at 5 g dm−3 (0.076 M) EZn is higher than 95%. At molar ratio Cyphos® IL 101/Zn(II) less than 2 a distinct decrease in Zn(II) extraction is observed. Above the ratio of 2 zinc(II) extraction amounts to 100%. The issue of extraction mechanism will be
3. Results and discussion 3.1. Effect of contact time The studies on kinetics of Zn(II) extraction have been carried out by equilibrating the aqueous feed containing 5 g dm−3 Zn(II), 0 or 0.58 M HCl, 5 M Cl− with ILs in toluene. An initial extraction rate has been calculated from the slopes at the beginning of the extraction [24]. As shown in Fig. 1, zinc(II) extraction with Cyphos® IL 101 is very fast, the initial rate of metal transfer to the IL phase equals 0.3 mol dm−3 s−1 . Cyphos® IL 109 in toluene extracts Zn(II) rather
Fig. 2. Extraction isotherms of zinc(II) with Cyphos® IL 101 and Cyphos® IL 109 (feed, 0.5–58 g dm−3 Zn(II), 0.58 M HCl, 5 M Cl− ).
M. Regel-Rosocka / Separation and Purification Technology 66 (2009) 19–24
21
Table 2 Extraction parameters and water content (H2 O) in the IL phase after extraction of zinc(II) from feed in the presence and absence of HCl. Extractant ®
Cyphos IL 101
[Zn]i (g dm−3 ) 5 90
Cyphos® IL 109
5
HCl (M)
log DZn
EZn (%)
H2 O (%)
0 0.58 0 0.58 0 0.58
1.5 1.8 −0.28 −0.20 −0.99 −1.1
96.6 98.1 35.4 38.5 9.38 7.79
3.9 3.0 1.0 1.3 0.04 0.12
discussed in the next section. Both DZn and EZn are slightly affected by the presence of HCl. Zn(II) extraction increases with increasing HCl content. The observed decrease in the distribution ratio for Cyphos® IL 109/toluene mixture compared with Cyphos® IL 101, results probably from the high hydrophobicity of the imide anion. The affinity of the IL phase to the aqueous phase is very low and the transfer of zinc(II) species is difficult. Conversely, the hydrophilic chloride anion from Cyphos® IL 101 is easy to be exchanged into zinc(II) anionic chlorocomplexes. Water content in the organic phase after extraction confirms differences in the hydrophilicity of the IL phase. Water content in Cyphos® IL 109 is very small and does not reach 0.15 wt.%, whereas Cyphos® IL 101 after extraction contains, depending on Zn(II) concentration, from 1 to almost 4 wt.% of water (Table 2). The presented equilibrium data indicate that the Cyphos® IL 101/toluene mixture is a very effective phase that enables the 0.86 M IL phase to be loaded with Zn(II) up to as much as 40 g dm−3 . Most importantly, Cyphos® IL 101 can be considered as an active extractant if mixed with a neutral solvent. Due to low extraction of Zn(II) Cyphos® IL 109 has not been considered for further investigations.
3.3. Mechanism of Zn(II) extraction On the basis of literature data it has been assumed that extraction can be realized according to two different mechanisms [25], depending on the concentration of chloride ions and therefore on the distribution of zinc chlorocomplexes:
− ZnCl2− 4(aq) + nR3 R PCl ⇔ (R3 RP)n ZnCl4 + nCl(aq)
(1)
(2)
where subscript (aq) stands for the aqueous phase and a horizontal bar denotes the organic phase. Previously studied systems support the extraction of ZnCl2 and ZnCl4 2− [26–28] depending upon chloride concentration and acidity of the aqueous feed. However, El Dessouky et al. [29] report extraction of ZnCl3 − species with neutral extractants. In this paper it is assumed that for feed containing Zn(II), 0.58 M HCl and 5 M Cl− the predominant species existing in the aqueous feed is ZnCl4 2− (extraction according to Eq. (2)) and for the feed without HCl–ZnCl2 (Eq. (1)). During extraction chlorocomplexes of zinc(II) form ionic pairs with the phosphonium cations. Molar ratios of chloride to zinc(II) ions transported to the loaded Cyphos® IL 101, calculated from the mass balance of the aqueous phase before and after extraction, are presented in Table 3. Average values of the ratios are equal to 1.65 and 2.11 for feed without HCl and with 0.58 M acid, respectively. They are close to 2 and fit to both mechanisms according to Eqs. (1) and (2). For Eq. (1) the extraction constant, Kex(1) , can be written as follows (indices (1) and (2) assign the appropriate reactions): Kex(1) =
[(R3 R P)m ZnCl2+m ]
(3)
m
[R3 R PCl] [ZnCl2 ](aq)
or for Eq. (2): n
Kex(2) =
(a) addition of neutral chlorocomplex to IL molecule (R3 R PCl) ZnCl2(aq) + mR3 R PCl ⇔ (R3 R P)m ZnCl2+m
(b) anion exchange of chlorides into a negatively charged zinc tetrachlorocomplex
[(R3 R P)n ZnCl4 ][Cl− ](aq)
(4)
n
[R3 R PCl] [ZnCl2− 4 ](aq)
Taking into account formation constants of zinc(II) chlorocomplexes the distribution coefficient, DZn , is given as follows: DZn =
[ionic pair] ([Zn2+ ](1 +
4
(5)
i ˇ [Cl− ] ))(aq) i=1 i
where ˇi is stability constant of zinc(II) chlorocomplex; [ionic pair] = [(R3 R P)m ZnCl2+m ] or [(R3 R P)n ZnCl4 ]. Table 3 Loading of Cyphos® IL 101 with zinc(II) and chloride ions.
Fig. 3. Effect of IL101/Zn in feed ratio on zinc(II) extraction to the IL phase (feed, 0.5–58 g dm−3 Zn(II), 0.58 M HCl, 5 M Cl− ; organic: 0.86 M Cyphos® IL 101 in toluene).
HCl in feed (M)
[Zn]*o (g dm−3 )
Cl− o (M)
Molar ratio Cl− o /[Zn]*o
0
34.4 33.3 33.4 31.2 30.2
0.843 0.815 0.849 0.803 0.775
1.60 1.60 1.66 1.69 1.68
0.58
28.1 32.6 32.2 39.7 36.0
0.780 1.25 1.11 1.34 0.961
1.81 2.51 2.25 2.21 1.75
“o” denotes the organic phase.
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M. Regel-Rosocka / Separation and Purification Technology 66 (2009) 19–24
3.4. Influence of temperature on Zn(II) extraction The extraction of Zn(II) from feed containing 0.58 M HCl and without acid has been carried out at temperature range 20–70 ◦ C (Fig. 5). With increasing temperature increases zinc(II) distribution between the organic and aqueous phase. The effect of temperature is studied to estimate values of extraction enthalpies H◦ and entropies S◦ . The dependence of Kex on the temperature is expressed by the Gibbs–Helmholtz equation [30]: log Kex =
−H ◦ S◦ +
2.303 RT
(9)
2.303 R
where R is the gas constant (8.314 J mol−1 K−1 ). Introducing Gibbs–Helmholtz equation and Eq. (7) or (8) to Eq. (6) a dependence of DZn upon temperature is obtained for reaction (1): Fig. 4. Dependence of zinc(II) distribution ratio on Cyphos® IL 101 concentration for the feed () with 0.58 M HCl and () without acid (feed: 5 g dm−3 Zn(II), 5 M Cl− ; organic: 0.04–0.7 M Cyphos® IL 101 in toluene).
The following equations for the logarithm of distribution coefficient can be derived after introducing the extraction constant to Eq. (5): log DZn = log K + m log[R3 R PCl]
(6)
As the total chloride ion concentration has been established for 5 M in the feed, it can be considered as constant, because the amount of the chloride ion is much higher in the aqueous phase than the extracted amount of zinc(II) and chloride. Thus, for Eq. (6) a constant K has been introduced: 2
K(1) = Kex(1) [Cl− ] ×
− 4−n
K(2) = Kex(2) [Cl ]
1+ ×
4
1+
ˇ2
ˇ i=1 i
4
× [Cl− ]
i
− i
Based on Eq. (6), the number of molecules of extractant engaged in Zn(II) extraction has been read from the slope of the plot (Fig. 4). Values for both reactions (1) and (2) equal m = n = 2. It means that Zn(II) is extracted to the IL phase as the following ionic pair: [(R3 R P)2 ZnCl4 ].
2.303 RT
1+
+
2.303 R
4
+ 2 log[Cl− ]
ˇ2
ˇ × [Cl− ] i=1 i
i
+ 2 log[R3 R PCl]
(10)
and for reaction (2): log DZn(2) =
−H ◦ S◦ 2.303 RT + log
1+
+
2.303 R
4
+ 2 log[Cl− ]
ˇ4 − i
ˇ × [Cl ] i=1 i
+ 2 log[R3 R PCl]
(11)
The standard molar enthalpy H◦ is the slope of the log DZn versus 1/T (Fig. 5) and the standard entropy S◦ of extraction is the intercept of the plot calculated from the following ◦ S /2.303 R + 2 log[Cl− ] + expression for reaction (1):
(8)
× [Cl ]
−H ◦ S◦ + log
log ˇ2 / 1 +
ˇ4
ˇ i=1 i
(7)
log DZn(1) =
(2):
4
ˇ × [Cl− ] i=1 i
S ◦ /2.303 R
i
+ 2 log[R3 R PCl]; and for reaction
−
+ 2 log[Cl ] + log ˇ4 /
1+
4
ˇi
i · [Cl− ]
+
i=1
2 log[R3 R PCl]. Values of formation constants ˇi reported by various authors differ significantly [31]. Despite ˇi constants depend on the temperature, the values obtained from the logarithm expression are small and do not significantly affect S◦ values. Thus, exemplary values ˇ1 = 0.0575; ˇ2 = 0.0355; ˇ3 = 0.0324; ˇ4 = 0.0214 are assumed according to [32] at 3 M ionic strength and 20 ◦ C. The values of H◦ are found to be 32.93 (s.d. = 2.81) and 52.85 (s.d. = 1.90) kJ mol−1 , whereas S◦ equals 120.65 (s.d. = 8.84) and 182.13 (s.d. = 6.02) J mol−1 K−1 for reactions (1) and (2), respectively. The extraction of zinc(II) with Cyphos® IL 101/toluene mixture is an endothermic reaction both from the feed with HCl and without acid. Standard Gibbs free energy (G◦ in kJ/mol) of zinc(II) extraction with Cyphos® IL 101/toluene according to reactions (1) and (2) can be expressed as follows: ◦ = 32.93 − 120.65 × 10−3 T G(1)
(12)
◦ = 52.85 − 182.13 × 10−3 T G(2)
(13)
3.5. Stripping of Zn(II) from loaded Cyphos® IL 101/toluene
Fig. 5. The effect of temperature on zinc(II) distribution coefficient (feed: 5 g dm−3 Zn(II), 0 or 0.58 M HCl, 5 M Cl− ; organic: 0.86 M Cyphos® IL 101 in toluene).
The ionic bonds in the IL phase are very strong, therefore simple stripping of Zn(II) with water is not possible. Another stripping solution, forming complexes with Zn(II) strong enough to attract them from IL, should be used. Recovery results for various investigated stripping solutions are presented in Table 4. Percentage
M. Regel-Rosocka / Separation and Purification Technology 66 (2009) 19–24
23
Table 4 Zn(II) stripping from loaded Cyphos® IL 101 with various stripping solutions. Stripping solution
S1 (%)
S1+2 (%)
S1+2+3 (%)
1 M H2 SO4 0.5 M H2 SO4 4 M HCl 0.5 M NH4 OH
65.6 56.4 4.66 100
100 90.3 5.42 –
– 97.7 5.42 –
stripping of Zn(II) from loaded Cyphos® IL 101 after first (S1 ), second (S1+2 ) and third stage (S1+2+3 ) is defined as the sum of Zn(II) stripped in one, two or three stages to Zn(II) content in the organic phase prior to first stripping. Based on the preliminary results, 0.5 M NH4 OH has been proposed as an efficient stripping solution as it enables up to 100% of zinc(II) to be removed from the loaded Cyphos® IL 101. However, formation of white precipitation at the interface, when the organic phase is loaded with big amount of Zn(II), is the main drawback of NH4 OH. Hydrochloric acid has been applied due to its efficient stripping of metals from quaternary ammonium salts [25]. However, in case of phosphonium salts Zn(II) stripping with 4 M HCl is ineffective (only 5.42% of Zn(II) recovered). Sulfuric acid seems to be the best stripping solution and its effectiveness increases with increasing acid concentration. After two stages of stripping with 1 M H2 SO4 zinc is completely removed from loaded Cyphos® IL 101/toluene mixture. Moreover, there are no problems with emulsions and/or precipitation, both phases are transparent. 3.6. Regeneration of Cyphos® IL 101/toluene The ability to reuse the IL phase is of critical importance in determining the feasibility of IL-based separation process. The issue of IL/toluene recycling concerns Zn(II) recovery from loaded Cyphos® IL 101 and further changes in extraction ability of regenerated IL phase. The successful recovery of Zn(II) with 1 M H2 SO4 has been presented in Section 3.5. In order to investigate extraction with regenerated IL, 0.1 Cyphos® IL 101 has been loaded with zinc(II). After 5 stages of extraction each time with fresh aqueous feed (5 g dm−3 Zn(II), 0.58 M HCl, 5 M Cl− ) the organic phase contains constant amount of Zn(II) 3.73 g dm−3 (Fig. 6). Further the loaded IL has been stripped three times with 1 M H2 SO4 to remove completely Zn(II) from IL phase. Regenerated Cyphos® IL 101 has been used again for Zn(II) extraction (2nd cycle) and than stripped three times with 1 M H2 SO4 . The procedure of extraction-stripping has been repeated 4 times (Fig. 7). Extraction
Fig. 6. Loading capacity of 0.1 M Cyphos® IL 101 (feed: 5 g dm−3 Zn(II), 0.58 M HCl, 5 M Cl− ).
Fig. 7. Extraction of Zn(II) with regenerated Cyphos® IL 101.
of Zn(II) with fresh and once regenerated IL (2nd cycle) is comparable and amounts to 60%. As 0.1 M Cyphos® IL 101 is used for recycling studies, instead of 0.8 M, thus about 65% of zinc(II) can be extracted as maximum from feed containing 5 g dm−3 Zn(II) (0.077 M). However, in the next cycles EZn increases up to 69 and 75% in 3rd and 4th cycles, respectively. It is positive that the IL can be reused several times, however increase in percentage extraction indicates possible changes in the extraction mechanism or structure of complexes. This issue needs further investigations. Compared with commercial extractants [3,5,27–29] Cyphos® IL 101 indicates very good zinc(II) extraction and stability even after several cycles of regeneration. Stripping with acid instead of water should be pointed out as a drawback of this quaternary phosphonium salt. 4. Conclusions • Due to low extraction of Zn(II) Cyphos® IL 109 cannot be considered as effective extractant in the studied system. The affinity of Cyphos® IL 109 phase to the aqueous phase is very low and the transfer of zinc(II) species is difficult. • Results of the presented investigations prove that Cyphos® IL 101/toluene mixture can be successfully used as the extractant for zinc(II) removal from chloride media. • Extraction of zinc(II) is very fast and efficient (EZn over 95%) for molar ratio Cyphos® IL 101/Zn(II) more than 2. The presence of HCl in the feed enhances Zn(II) extraction. • The following reactions of Zn(II) extraction mechanism are proposed: ZnCl2(aq) + 2R3 R PCl ⇔ (R3 R P)2 ZnCl4
(1)
− ZnCl2− 4(aq) + 2R3 R PCl ⇔ (R3 R P)2 ZnCl4 + 2Cl(aq)
(2)
• The extraction of zinc(II) with Cyphos® IL 101/toluene mixture is an endothermic reaction from the feed both with HCl and without acid. • The estimated values H◦ amount to 32.93 and 52.85 kJ mol−1 , whereas S◦ equals 120.65 and 182.13 J mol−1 K−1 for reactions (1) and (2), respectively. • Sulfuric acid is the best stripping solution among the studied and its effectiveness increases with increasing acid concentration. • The feasibility of Cyphos® IL 101-based separation process is proven due to the ability to reuse the Cyphos® IL 101/toluene mixture in several cycles of Zn(II) extraction-stripping.
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Acknowledgments We thank Cytec Industries Inc. for providing us with free samples of Cyphos® IL 101 and Cyphos® IL 109. This work was supported by the Polish Ministry of Science and Higher Education with its grant no. 1T 09B 081 30. References [1] P. Maass, P. Peissker, Hot-dip galvanizing, Agencja Wydawnicza Placet, Warsaw, 1998 (in Polish). [2] M. Regel-Rosocka, M. Wisniewski, in: J. Kudelko, J. Kulczycka, H. Wirth (Eds.), Sustainable Mineral Supply for Europe—From Waste to Resources, IGSMiE PAN, Krakow, 2007, pp. 46–53. [3] A. Grzeszczyk, M. Regel-Rosocka, Hydrometallurgy 86 (2007) 72–79. [4] M. Regel-Rosocka, J. Szymanowski, Solvent Extr. Ion Exch. 23 (2005) 411–424. [5] R. Cierpiszewski, I. Miesiac, M. Regel-Rosocka, A.M. Sastre, J. Szymanowski, Ind. Chem. Eng. Res. 41 (2002) 598–603. [6] K.R. Seddon, A. Stark, M.J. Torres, Pure Appl. Chem. 72 (2000) 2275–2287. [7] C.J. Adams, in: R.D. Rogers, K.R. Seddon (Eds.), Ionic Liquids. Industrial Applications for Green Chemistry, ACS Symposium Series, vol. 818, American Chemical Society, Washington, DC, 2002, pp. 13–29 (Chapter 2). [8] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH, Weinheim, 2003. [9] T. Welton, Chem. Rev. 99 (1999) 2071–2083. [10] A. Cieniecka-Roslonkiewicz, J. Pernak, J. Kubis-Feder, A. Ramani, A.J. Robertson, K.R. Seddon, Green Chem. 7 (2005) 855–862. [11] K. Shimojo, M. Goto, Anal. Chem. 76 (2004) 5039–5044. [12] K. Nakashima, F. Kubota, T. Maruyama, M. Goto, Ind. Eng. Chem. Res. 44 (2005) 4368–4372.
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