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Solvent extraction of bismuth with 2-ethylhexylphosphonic acid mono-(2-ethylhexyl) ester and 2,2′-bipyridyl
Separation and Purification Technology 104 (2013) 64–67
Contents lists available at SciVerse ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Solvent extraction of bismuth with 2-ethylhexylphosphonic acid mono-(2-ethylhexyl) ester and 2,20 -bipyridyl Naizhong Song, Wanjun Li, Qiong Jia ⇑ College of Chemistry, Jilin University, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 9 August 2012 Received in revised form 29 October 2012 Accepted 3 November 2012 Available online 24 November 2012 Keywords: Solvent extraction Bismuth Acidic phosphorus extractant 2-Ethylhexylphosphonic acid mono-(2ethylhexyl) ester 2,20 -Bipyridyl
a b s t r a c t In the present work, the solvent extraction of bismuth with mixtures of acidic phosphorus extractants, di(2-ethylhexyl) phosphoric acid (D2EHPA) and 2-ethylhexylphosphonic acid mono-(2-ethylhexyl) ester (HEHEHP), with some other extractants was investigated including sec-octylphenoxyacetic acid (CA12), sec-nonylphenoxy acetic acid (CA100), 8-hydroxyquinoline (HQ), and 2,20 -bipyridyl (bipy). The extraction effects from nitric acid medium were studied in detail. Results showed that all other mixing systems except HEHEHP + bipy did not have synergistic effects on bismuth. The methods of slope and constant mole were employed to study the extraction of bismuth in HEHEHP + bipy system. Thermodynamic functions and stripping behaviors were also investigated. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction In the last decades, solvent extraction of metal ions with organophosphorus acids has attracted much attention [1]. >P(O)OH included in such extractants is generally regarded as the active functional group which can react with metal ions. Di(2-ethylhexyl) phosphoric acid (D2EHPA) [2] and 2-ethylhexylphosphonic acid mono-(2-ethylhexyl) ester (HEHEHP) [3] are most frequently reported extractants for metal ions based on this reaction mechanism. Bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex272) and its derivatives, bis(2,4,4-trimethylpentyl) monothiophosphinic acid (Cyanex302,>P(S)OH) and bis(2,4,4-trimethylpentyl) dithiophosphinic acid (Cyanex301,>P(S)SH), are also commonly used in this field [4,5]. Synergistic extraction (extraction with two kinds of extractants) was introduced to improve the extraction efficiency and the extraction selectivity [5–7]. Various organophosphorus acids were used as the extractant in synergistic extraction systems. Neutral organophosphorus reagents, carboxylic acids, amines, b-deketones, crown ethers, and calix[n]arenes were often employed as the other extractant. For instance, Otu investigated the thermodynamics of synergistic extraction of bismuth by 2-ethylhexyl phenylphosphonic acid and micelles of dinonyl naphthalene sulfonic acid from HClO4 [8]. The extraction of bismuth by the mixed ligand system was proved to be entropy controlled. ⇑ Corresponding author. E-mail address: [email protected] (Q. Jia). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.11.002
Bismuth is a ‘‘green’’ heavy metal that is being used to replace environmentally unfavorable metals. However, a literature survey reveals very few synergistic extraction methods for bismuth. In the present work, the extraction of bismuth with mixing extraction systems based on organophosphorus acids was investigated. Mixtures of D2EHPA and HEHEHP with several other reagents, secoctylphenoxyacetic acid (CA12), sec-nonylphenoxy acetic acid (CA100), 8-hydroxyquinoline (HQ), and 2,20 -bipyridyl (bipy) were selected as the extractants. Synergisitic or antagonistic extraction effects of bismuth were studied with the various mixing systems. The synergistic extraction stoichiometry was also investigated. 2. Experimental 2.1. Reagents and apparatus D2EHPA, HEHEHP, CA12, CA100, and bipy were supplied by Shanghai Rare-earth Chemical Co., Ltd. HQ was purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd. All the extractants were used as received and dissolved in n-heptane at the required concentration. Stock solutions of Bi(NO3)3 were prepared with AR chemicals. The pH of the aqueous phase was adjusted by the addition of HNO3 or NaOH solutions. All extraction experiments were performed at a constant ionic strength with NaNO3 (l = 0.6 mol L1). All other reagents were of analytical reagent grade. A pHS-3C digital pH meter was employed for pH measurements (Shanghai Rex Instruments Factory, China). Deionized water was prepared by the Milli-Q SP system (Millipore, Milford, MA, USA).
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N. Song et al. / Separation and Purification Technology 104 (2013) 64–67
For extraction studies, 5 mL aqueous and 5 mL organic solutions were mixed and shaken for 30 min which was sufficient to achieve equilibrium in preliminary experiments. After the separation of the two phases, the concentration of Bi(III) in the aqueous phase was determined by titration with EDTA with xylenol orange as the indicator. The concentration of Bi(III) in the organic phase was then determined by mass balance. 2.3. Stripping test The loaded organic phase was equilibrated with HNO3 of different concentrations. The mixtures were shaken at 293 ± 1 K for 30 min. After separating the aqueous solution from the organic phase, Bi(III) concentrations were quantified to evaluate the stripping ratio, St = ([Bi]a,e/[Bi]o,i) 100%, where [Bi]a,e and [Bi]o,i represented the equilibrium concentrations of Bi(III) in stripping acid and the initial concentration of Bi(III) in the organic phase, respectively [9]. 3. Results and discussion 3.1. Extraction effects of Bi3+ with mixing systems based on D2EHPA and HEHEHP The extraction of Bi3+ with mixtures of D2EHPA and HEHEHP with some reagents, CA12, CA100, HQ, and bipy, was investigated in detail. The distribution ratio, D, was calculated as follows.
D¼
½MðoÞ ½MðaÞ
ð1Þ
where [M](o) and [M](a) were the concentrations of Bi(III) in the organic and aqueous phases, respectively. Synergistic enhancement coefficient, R, is often employed to evaluate whether a mixing system has synergistic extraction of not [10]. R can be calculated as R ¼ Dmix =ðDA þ DB Þ, where Dmix, DA, and DB denote the distribution ratios when metal ions are extracted with the mixed extractant and the single extractant, A and B, respectively. R > 1 means synergistic extraction whereas R < 1 means antagonistic extraction. The R values in the mixing systems based on D2EHPA and HEHEHP were listed in Table 1, where XD2EHPA or XHEHEHP described the mole fraction of D2EHPA or HEHEHP in the organic phases. It could be concluded from Table 1 that there were not evident synergistic effects with all systems except HEHEHP + bipy mixtures. In this mixing system, the R values were greater than 1 at any XHEHEHP value. The extraction of Bi3+ with HEHEHP, bipy, and HEHEHP + bipy was illustrated in Fig. 1. It should be noted that whether a mixing system has synergistic or antagonistic effect on metal ions is difficult to be predicted. The extraction processes with single extractant and the mixtures exist simultaneously in the mixing system. All these three reactions will affect the final R values and determine the extraction effects. In addition, results showed that the extraction capacity of Bi3+ with single D2EHPA or HEHEHP followed the order: D2EHPA > HEHEHP (Fig. 1). When other experimental conditions were fixed, the distribution ratios of Bi3+ were determined as 0.89 and 0.17 with 0.05 mol L1 D2EHPA and HEHEHP, respectively. This can be explained by the structures of the two acidic organophosphorus extractants. D2EHPA has a structure of (RO)2P(O)OH while that of HEHEHP is R(RO)P(O)OH, where R means CH2CH(C2H5)C4H9. The pKa value of D2EHPA is lower than that of HEHEHP [11], therefore, D2EHPA has higher extraction capacity of Bi3+ than HEHEHP.
Table 1 Synergistic enhancement coefficients of Bi3+ with mixtures based on D2EHPA and HEHEHP.a XD2EHPA or XHEHEHP
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
D2EHPA + CA12 D2EHPA + CA100 D2EHPA + HQ D2EHPA + bipy HEHEHP + CA12 HEHEHP + CA100 HEHEHP + HQ HEHEHP + bipy
0.3 0.6 0.9 0.3 0.4 0.4 0.9 3.3
0.2 0.4 0.7 0.5 0.6 0.5 0.7 2.9
0.2 0.4 0.7 0.5 0.7 0.4 0.8 2.7
0.1 0.3 0.6 0.6 0.8 0.4 0.9 2.5
0.1 0.2 0.6 0.6 0.8 0.3 1.0 2.2
0.1 0.2 0.8 0.7 0.8 0.3 1.1 2.0
0.1 0.1 0.9 0.8 0.9 0.3 1.4 1.8
0.1 0.1 0.9 0.8 0.8 0.3 1.5 1.7
0.1 0.1 0.9 0.9 0.8 0.4 1.2 1.3
a The total concentration of the extractants in every mixing system was 0.05 mol L1.
3.2. Synergistic extraction of Bi3+ with HEHEHP + bipy 3.2.1. Synergistic extraction stoichiometry Since HEHEHP + bipy mixtures had synergistic effects on Bi3+ at any XHEHEHP value, the synergistic extraction stoichiometry was studied in detail. There have been several studies about the extraction of Bi3+ with organophosphorus acids. For example, BiðH3 X6 Þ was determined as the formed complex when 2-ethylhexyl phenylphosphonic acid (H2X2) was employed as the extractant [8]. However, there were few reports about the extraction stoichiometry of Bi3+ with other organophosphorus acids. In the present work, the extraction of Bi3+ with HEHEHP was investigated when the concentration of Bi3+ was fixed at 2.0 103 mol L1. If the following equation was used to express the extraction of Bi3+ with HEHEHP, KA
3þ
BiðaÞ þ mH2 A2ðoÞ BiH2mn A2mðoÞ þ nHþðaÞ
ð2Þ
where ‘‘a’’ and ‘‘o’’ represent the aqueous and organic phases, respectively. H2A2 is the abbreviation of HEHEHP. The equilibrium constant, KA, could be calculated as follows,
log DA ¼ log K A þ m log½H2 A2 ðoÞ n log½Hþ ðaÞ
ð3Þ
m and n could be obtained according to the plots of log DA versus log[H2A2](o) or log DA versus log[H+](a), respectively. KA could thus be obtained with Eq. (3). The following equation was finally determined to express the extraction of Bi3+ with HEHEHP, KA
3þ
BiðaÞ þ 3H2 A2ðoÞ BiH3 A6ðoÞ þ 3HþðaÞ The analytical data of the extraction of Bi shown in Table 2.
ð4Þ 3+
with HEHEHP were
1.0
0.8
HEHEHP bipy HEHEHP + bipy D2EHPA
0.6 D
2.2. Procedures
0.4
0.2
0.0 0.0
0.2
0.4 0.6 XHEHEHP or XD2EHPA
0.8
1.0
Fig. 1. Extraction of Bi3+ with HEHEHP, bipy, HEHEHP + bipy, and D2EHPA. [Bi3+] = 2.0 103 mol L1, pH = 0.5, l = 0.6 mol L1, CHEHEHP + Cbipy = 0.05 mol L1, CD2EHPA = 0.05 mol L1.
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Table 2 Analytical data of the extraction of Bi3+ with HEHEHP (pH = 0.5, l = 0.6 mol L1). No.
VHEHEHP (mL)
log[HEHEHP](o) (mol L1)
log DA 3pHeq
log KA
1 2 3 4 5
1 2 3 4 5
2.00 1.70 1.53 1.40 1.31
4.94 4.04 3.51 3.17 2.89
1.06 1.07 1.07 1.04 1.03
Average log KA
The following equation could thus be used to describe the relationship between the distribution ratio DAB and the equilibrium constant KAB.
log DAB ¼ x log½H2 A2 ðoÞ þ y log½BðoÞ þ ipH þ log K AB þ ð3 iÞ log½NO3 ðaÞ According to Eqs. (4) and (5),
DA ¼ -1.0
logDAB - 3pH
½BiH3 A6 ðoÞ C Bi3þ ½BiH3 A6 ðoÞ ½BiðNO3 Þ3i H2xi A2x By ðoÞ ½BiðNO3 Þ3i H2xi A2x By ðoÞ
-1
[B] = 0.02 mol L -1 [B] = 0.01 mol L
DAB ¼
-1.5
CBi3þ ½BiH3 A6 ðoÞ ½BiðNO3 Þ3i H2xi A2x By ðoÞ
½BiH3 A6 ðoÞ ¼
DA CBi3þ 1 þ DA þ DAB
½H2 A2 ðoÞ ¼ C H2 A2 C Bi3þ -2.0
-1.5
-1.0
log[H2A2](o) Fig. 2. Relationship between HEHEHP concentration and distribution ratio. [Bi3+] = 2.0 103 mol L1, pH = 0.5, l = 0.6 mol L1.
-1
[H2A2] = 0.02 mol L
-1
[H2A2] = 0.01 mol L -1.5
logDAB - 3pH
ð10Þ
3DA þ xDAB 1 þ DA þ DAB
ð11Þ
Therefore,
-2.5
slope: 0.98 -2.0
-2.5
-2.0
-1.5
-1.0
log[B](o) Fig. 3. Relationship between bipy concentration and distribution ratio. [Bi3+] = 2.0 103 mol L1, pH = 0.5, l = 0.6 mol L1.
Bipy, with a similar structure with phen, has found its applications in the synergistic extraction of metal ions [12,13]. In the present work, the extraction of Bi3+ with HEHEHP, bipy, and their mixtures was carried out (Fig. 1). It was observed that the extraction of Bi3+ with bipy could be neglected compared with that with HEHEHP or HEHEHP + bipy. The extraction of Bi3+ with HEHEHP + bipy could be expressed as follows, 3þ
BiðaÞ þ ð3 iÞNO3ðaÞ þ xH2 A2ðoÞ þ yBðoÞ BiðNO3 Þ3i H2xi A2x ByðoÞ þ
iHþðaÞ
B was the abbreviation of bipy.
½BðoÞ ¼ C B C Bi3þ
yDAB 1 þ DA þ DAB
ð5Þ
ð12Þ
In order to obtain x, y, and i values in Eq. (5), a series of experiments were designed and carried out. Firstly, the effects of aqueous acidity on the extraction were studied when other experimental conditions were fixed including ionic strength and H2A2 and B concentrations. A straight line with the slope of about 3.0 was obtained, indicating that i equals to 3.0. Secondly, x and y were determined similarly, i.e., the aqueous acidity, ionic strength, and the concentration of a certain extractant were fixed. x was determined when B concentration was fixed while y was determined when H2A2 concentration was fixed. Results were shown in Figs. 2 and 3. It could be seen that the values of x and y were 1.5 and 1.0, respectively. Therefore, Eq. (5) could be changed as, K AB 3 3þ BiðaÞ þ H2 A2ðoÞ þ BðoÞ BiA3 BðoÞ þ 3HþðaÞ 2
slope: 1.03
K AB
DAB C Bi3þ 1 þ DA þ DAB
½BiðNO3 Þ3i H2xi A2x By ðoÞ ¼
slope: 1.57
-3.0 -2.5
ð8Þ
ð9Þ
-2.0
-1.0
ð7Þ
½BiH3 A6 ðoÞ and ½BiðNO3 Þ3i H2xi A2x By ðoÞ could be expressed as, slope: 1.52
-3.0 -2.5
ð6Þ
1.06 ± 0.03
ð13Þ
The equilibrium constant, KAB, could be calculated as 2.53 ± 0.03. It should be noted that the ratio of x and y was determined as 1.5:1, which was not absolutely in accordance with the results shown in Fig. 1 [14]. In Fig. 1, the maximum distribution ratio appeared at XHEHEHP value of about 0.8, where the fractional ratio of HEHEHP and bipy was 4:1. However, as mentioned above, the synergistic extraction was determined by three extraction processes, i.e., the extraction of Bi3+ with single HEHEHP, single bipy, and HEHEHP + bipy. The three reactions existed simultaneously and decided the final extraction effects. A possible explanation about the synergistic extraction is as follows. As is known, HEHEHP takes part in the solvent extraction reaction as a cationic extractant. In the solvent extraction of metal by a mixture of cationic extractant and solvating extractant, e.g., bipy, the solvating extractant may replace the self-adducted cationic extractant, which is in the form of BiH3A6 in the present system. This replacement results in synergistic effect. According to Eqs. (4) and (13), the following formation reaction could be derived. b 3 BiH3 A6ðoÞ þ BðoÞ BiH3 A6 BðoÞ þ H2 A2ðoÞ 2
ð14Þ
The equilibrium constant, log b, could be calculated as 2.47 (log b = log KABlog KA).
N. Song et al. / Separation and Purification Technology 104 (2013) 64–67
-0.3
3.2.3. Stripping studies The stripping behaviors of the synergistic extraction of Bi3+ with HEHEHP + bipy were studied. The stripping efficiency versus HNO3 concentrations was shown in Fig. 5, indicating that the majority of Bi3+ could be stripped by one stage when the concentration of HNO3 was greater than 0.03 mol L1. This implied that HEHEHP + bipy mixing system has potential to be applied in the hydrometallurgical field.
-0.6
logD
67
-0.9 slope: 1.39
4. Conclusions
-1.2
-1.5 2.8
3.0
3.2
3.4
3.6
-1
1000 / T, K
Fig. 4. Relationship between temperature and distribution ratio. [Bi3+] = 2.0 103 mol L1, pH = 0.5, l = 0.6 mol L1, CHEHEHP = 0.04 mol L1, Cbipy = 0.01 mol L1.
100
Acknowledgements
80
The authors wish to thank Mr. Yong-Hui Dai and Prof. ZhenFeng Cui of Changchun Institute of Technology.
St%
60
References
40
20
0 0.00
0.02
0.04 CHNO3, mol L
0.06
0.08
-1
Fig. 5. Stripping behavior of Bi3+ with HNO3.
3.2.2. Influence of temperature on the extraction of Bi3+ with HEHEHP + bipy When other experimental conditions were fixed, the influence of temperature on the extraction of Bi3+ with HEHEHP + bipy was investigated. Results were shown in Fig. 4. Thermodynamic functions, the change of enthalpy (DH°), Gibbs free energy (DG°), and entropy (DS°), can thus be obtained as the followings,
D log D DH ¼ 2:303R D 1T
ð15Þ
DG ¼ RT ln K
ð16Þ
Among the mixtures of D2EHPA / HEHEHP with CA12, CA100, HQ, and bipy, only the HEHEHP + bipy mixing system had synergistic effects for bismuth extraction from nitric solutions. The synergistic coefficients were greater than 1.0 at any mole fractional ratio of HEHEHP and bipy. Although the mechanism of the synergistic extraction was difficult to be obtained, the extraction stoichiometry could be investigated with the methods of slope and constant mole. The formed complex could also be determined together with equilibrium constants. The synergistic extraction was proved to be an exothermic and spontaneous process. Furthermore, stripping tests illustrated that bismuth could be stripped by one stage with 0.03 mol L1 HNO3.
DG ¼ DH T DS ) DS ¼
DH DG T
ð17Þ
DH° was calculated as 26.7 kJ mol1. The sign of DH°, , illustrated that the synergistic extraction was an exothermic process. DG° and DS° when T is 293 K could be determined as 14.2 kJ mol1 and 42.6 J mol1 K1, respectively, implying that the synergistic extraction was a spontaneous process.
[1] M. Ahmadipour, F. Rashchi, B. Ghafarizadeh, N. Mostoufi, Synergistic effect of D2EHPA and Cyanex 272 on separation of zinc and manganese by solvent extraction, Sep. Sci. Technol. 46 (2011) 2305–2312. [2] T.S. Hannel, E.O. Otu, M.P. Jensen, Thermochemistry of the extraction of bismuth(III) with bis(2-ethylhexyl) phosphoric and 2-ethyhexylphenylphosphonic acids, Solvent Extr. Ion Exch. 25 (2007) 241–256. [3] D.B. Wu, Q. Zhang, B.R. Bao, Synergistic effects in extraction and separation of praseodymium(III) and neodymium(III) with 8-hydroxyquinoline in the presence of 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester, Ind. Eng. Chem. Res. 46 (2007) 6320–6325. [4] S.G. Sarkar, P.M. Dhadke, Solvent extraction separation of antimony(III) and bismuth(III) with bis (2,4,4-trimethylpentyl) monothiophosphinic acid (Cyanex 302), Sep. Purif. Technol. 15 (1999) 131–138. [5] X. Wang, M. Du, H. Liu, Synergistic extraction study of samarium(III) from chloride medium by mixtures of bis(2,4,4-trimethylpentyl)phosphinic acid and 8-hydroxyquinoline, Sep. Purif. Technol. 93 (2012) 48–51. [6] Y. Xiong, W.J. Shan, Z.N. Lou, Z.Q. Xing, D.W. Fang, G.X. Han, Synergistic extraction of rhenium(VII) and molybdenum(VI) with mixtures of bis-(3,5dimethylhexyl-4-methylhexyl) amine and tributylphosphate, J. Phase Equilib. Diffus. 32 (2011) 193–197. [7] Q. Zhou, J. Wang, W. Ruan, Synergistic extraction of bismuth(lII) with oxine and b-diketone, Yantai Normal Univ. J. 9 (1993) 64–67. [8] E.O. Otu, The thermodynamics of synergistic extraction of bismuth by 2ethylhexyl phenylphosphonic acid and micelles of dinonyl naphthalene sulfonic acid, Thermochim. Acta 329 (1999) 117–121. [9] W.W. He, W.P. Liao, C.J. Niu, D.Q. Li, Synergistic extraction of rare earths using acid-base coupling extractants of calix[4]arene carboxyl derivative and primary amine N1923, Sep. Purif. Technol. 62 (2008) 674–680. [10] G.X. Xu, W.Q. Wang, J.G. Wu, B.L. Li, G.B. Wu, N. Shi, Chemistry of nuclear fuels extraction, At. Energy, Sci. Technol. (1963) 487–508. [11] Z.L. Gao, S.X. Sun, J.L. Shen, Chemistry of Solvent Extraction, Science Press, Beijing, 1991. [12] S.J. Fan, M.M. Tian, N.Z. Song, Q. Jia, W.P. Liao, Synergistic extraction and separation study of rare earth elements from nitrate medium by mixtures of sec-nonylphenoxy acetic acid and 2,20 -bipyridyl, J. Chem. Technol. Biotechnol. 86 (2011) 719–723. [13] M.M. Tian, Q. Jia, N.Z. Song, X.J. Quan, C. Liu, Extraction and separation of rare earth elements from nitrate medium with mixtures of sec-octylphenoxyacetic acid and 2,20 -bipyridyl, J. Chem. Eng. Data 55 (2010) 4281–4284. [14] X.B. Sun, J.M. Zhao, S.L. Meng, D.Q. Li, Synergistic extraction and separation of yttrium from heavy rare earths using mixture of sec-octylphenoxy acetic acid and bis(2,4,4-trimethylpentyl)phosphinic acid, Anal. Chim. Acta 533 (2005) 83–88.
Contents lists available at SciVerse ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Solvent extraction of bismuth with 2-ethylhexylphosphonic acid mono-(2-ethylhexyl) ester and 2,20 -bipyridyl Naizhong Song, Wanjun Li, Qiong Jia ⇑ College of Chemistry, Jilin University, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 9 August 2012 Received in revised form 29 October 2012 Accepted 3 November 2012 Available online 24 November 2012 Keywords: Solvent extraction Bismuth Acidic phosphorus extractant 2-Ethylhexylphosphonic acid mono-(2ethylhexyl) ester 2,20 -Bipyridyl
a b s t r a c t In the present work, the solvent extraction of bismuth with mixtures of acidic phosphorus extractants, di(2-ethylhexyl) phosphoric acid (D2EHPA) and 2-ethylhexylphosphonic acid mono-(2-ethylhexyl) ester (HEHEHP), with some other extractants was investigated including sec-octylphenoxyacetic acid (CA12), sec-nonylphenoxy acetic acid (CA100), 8-hydroxyquinoline (HQ), and 2,20 -bipyridyl (bipy). The extraction effects from nitric acid medium were studied in detail. Results showed that all other mixing systems except HEHEHP + bipy did not have synergistic effects on bismuth. The methods of slope and constant mole were employed to study the extraction of bismuth in HEHEHP + bipy system. Thermodynamic functions and stripping behaviors were also investigated. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction In the last decades, solvent extraction of metal ions with organophosphorus acids has attracted much attention [1]. >P(O)OH included in such extractants is generally regarded as the active functional group which can react with metal ions. Di(2-ethylhexyl) phosphoric acid (D2EHPA) [2] and 2-ethylhexylphosphonic acid mono-(2-ethylhexyl) ester (HEHEHP) [3] are most frequently reported extractants for metal ions based on this reaction mechanism. Bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex272) and its derivatives, bis(2,4,4-trimethylpentyl) monothiophosphinic acid (Cyanex302,>P(S)OH) and bis(2,4,4-trimethylpentyl) dithiophosphinic acid (Cyanex301,>P(S)SH), are also commonly used in this field [4,5]. Synergistic extraction (extraction with two kinds of extractants) was introduced to improve the extraction efficiency and the extraction selectivity [5–7]. Various organophosphorus acids were used as the extractant in synergistic extraction systems. Neutral organophosphorus reagents, carboxylic acids, amines, b-deketones, crown ethers, and calix[n]arenes were often employed as the other extractant. For instance, Otu investigated the thermodynamics of synergistic extraction of bismuth by 2-ethylhexyl phenylphosphonic acid and micelles of dinonyl naphthalene sulfonic acid from HClO4 [8]. The extraction of bismuth by the mixed ligand system was proved to be entropy controlled. ⇑ Corresponding author. E-mail address: [email protected] (Q. Jia). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.11.002
Bismuth is a ‘‘green’’ heavy metal that is being used to replace environmentally unfavorable metals. However, a literature survey reveals very few synergistic extraction methods for bismuth. In the present work, the extraction of bismuth with mixing extraction systems based on organophosphorus acids was investigated. Mixtures of D2EHPA and HEHEHP with several other reagents, secoctylphenoxyacetic acid (CA12), sec-nonylphenoxy acetic acid (CA100), 8-hydroxyquinoline (HQ), and 2,20 -bipyridyl (bipy) were selected as the extractants. Synergisitic or antagonistic extraction effects of bismuth were studied with the various mixing systems. The synergistic extraction stoichiometry was also investigated. 2. Experimental 2.1. Reagents and apparatus D2EHPA, HEHEHP, CA12, CA100, and bipy were supplied by Shanghai Rare-earth Chemical Co., Ltd. HQ was purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd. All the extractants were used as received and dissolved in n-heptane at the required concentration. Stock solutions of Bi(NO3)3 were prepared with AR chemicals. The pH of the aqueous phase was adjusted by the addition of HNO3 or NaOH solutions. All extraction experiments were performed at a constant ionic strength with NaNO3 (l = 0.6 mol L1). All other reagents were of analytical reagent grade. A pHS-3C digital pH meter was employed for pH measurements (Shanghai Rex Instruments Factory, China). Deionized water was prepared by the Milli-Q SP system (Millipore, Milford, MA, USA).
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N. Song et al. / Separation and Purification Technology 104 (2013) 64–67
For extraction studies, 5 mL aqueous and 5 mL organic solutions were mixed and shaken for 30 min which was sufficient to achieve equilibrium in preliminary experiments. After the separation of the two phases, the concentration of Bi(III) in the aqueous phase was determined by titration with EDTA with xylenol orange as the indicator. The concentration of Bi(III) in the organic phase was then determined by mass balance. 2.3. Stripping test The loaded organic phase was equilibrated with HNO3 of different concentrations. The mixtures were shaken at 293 ± 1 K for 30 min. After separating the aqueous solution from the organic phase, Bi(III) concentrations were quantified to evaluate the stripping ratio, St = ([Bi]a,e/[Bi]o,i) 100%, where [Bi]a,e and [Bi]o,i represented the equilibrium concentrations of Bi(III) in stripping acid and the initial concentration of Bi(III) in the organic phase, respectively [9]. 3. Results and discussion 3.1. Extraction effects of Bi3+ with mixing systems based on D2EHPA and HEHEHP The extraction of Bi3+ with mixtures of D2EHPA and HEHEHP with some reagents, CA12, CA100, HQ, and bipy, was investigated in detail. The distribution ratio, D, was calculated as follows.
D¼
½MðoÞ ½MðaÞ
ð1Þ
where [M](o) and [M](a) were the concentrations of Bi(III) in the organic and aqueous phases, respectively. Synergistic enhancement coefficient, R, is often employed to evaluate whether a mixing system has synergistic extraction of not [10]. R can be calculated as R ¼ Dmix =ðDA þ DB Þ, where Dmix, DA, and DB denote the distribution ratios when metal ions are extracted with the mixed extractant and the single extractant, A and B, respectively. R > 1 means synergistic extraction whereas R < 1 means antagonistic extraction. The R values in the mixing systems based on D2EHPA and HEHEHP were listed in Table 1, where XD2EHPA or XHEHEHP described the mole fraction of D2EHPA or HEHEHP in the organic phases. It could be concluded from Table 1 that there were not evident synergistic effects with all systems except HEHEHP + bipy mixtures. In this mixing system, the R values were greater than 1 at any XHEHEHP value. The extraction of Bi3+ with HEHEHP, bipy, and HEHEHP + bipy was illustrated in Fig. 1. It should be noted that whether a mixing system has synergistic or antagonistic effect on metal ions is difficult to be predicted. The extraction processes with single extractant and the mixtures exist simultaneously in the mixing system. All these three reactions will affect the final R values and determine the extraction effects. In addition, results showed that the extraction capacity of Bi3+ with single D2EHPA or HEHEHP followed the order: D2EHPA > HEHEHP (Fig. 1). When other experimental conditions were fixed, the distribution ratios of Bi3+ were determined as 0.89 and 0.17 with 0.05 mol L1 D2EHPA and HEHEHP, respectively. This can be explained by the structures of the two acidic organophosphorus extractants. D2EHPA has a structure of (RO)2P(O)OH while that of HEHEHP is R(RO)P(O)OH, where R means CH2CH(C2H5)C4H9. The pKa value of D2EHPA is lower than that of HEHEHP [11], therefore, D2EHPA has higher extraction capacity of Bi3+ than HEHEHP.
Table 1 Synergistic enhancement coefficients of Bi3+ with mixtures based on D2EHPA and HEHEHP.a XD2EHPA or XHEHEHP
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
D2EHPA + CA12 D2EHPA + CA100 D2EHPA + HQ D2EHPA + bipy HEHEHP + CA12 HEHEHP + CA100 HEHEHP + HQ HEHEHP + bipy
0.3 0.6 0.9 0.3 0.4 0.4 0.9 3.3
0.2 0.4 0.7 0.5 0.6 0.5 0.7 2.9
0.2 0.4 0.7 0.5 0.7 0.4 0.8 2.7
0.1 0.3 0.6 0.6 0.8 0.4 0.9 2.5
0.1 0.2 0.6 0.6 0.8 0.3 1.0 2.2
0.1 0.2 0.8 0.7 0.8 0.3 1.1 2.0
0.1 0.1 0.9 0.8 0.9 0.3 1.4 1.8
0.1 0.1 0.9 0.8 0.8 0.3 1.5 1.7
0.1 0.1 0.9 0.9 0.8 0.4 1.2 1.3
a The total concentration of the extractants in every mixing system was 0.05 mol L1.
3.2. Synergistic extraction of Bi3+ with HEHEHP + bipy 3.2.1. Synergistic extraction stoichiometry Since HEHEHP + bipy mixtures had synergistic effects on Bi3+ at any XHEHEHP value, the synergistic extraction stoichiometry was studied in detail. There have been several studies about the extraction of Bi3+ with organophosphorus acids. For example, BiðH3 X6 Þ was determined as the formed complex when 2-ethylhexyl phenylphosphonic acid (H2X2) was employed as the extractant [8]. However, there were few reports about the extraction stoichiometry of Bi3+ with other organophosphorus acids. In the present work, the extraction of Bi3+ with HEHEHP was investigated when the concentration of Bi3+ was fixed at 2.0 103 mol L1. If the following equation was used to express the extraction of Bi3+ with HEHEHP, KA
3þ
BiðaÞ þ mH2 A2ðoÞ BiH2mn A2mðoÞ þ nHþðaÞ
ð2Þ
where ‘‘a’’ and ‘‘o’’ represent the aqueous and organic phases, respectively. H2A2 is the abbreviation of HEHEHP. The equilibrium constant, KA, could be calculated as follows,
log DA ¼ log K A þ m log½H2 A2 ðoÞ n log½Hþ ðaÞ
ð3Þ
m and n could be obtained according to the plots of log DA versus log[H2A2](o) or log DA versus log[H+](a), respectively. KA could thus be obtained with Eq. (3). The following equation was finally determined to express the extraction of Bi3+ with HEHEHP, KA
3þ
BiðaÞ þ 3H2 A2ðoÞ BiH3 A6ðoÞ þ 3HþðaÞ The analytical data of the extraction of Bi shown in Table 2.
ð4Þ 3+
with HEHEHP were
1.0
0.8
HEHEHP bipy HEHEHP + bipy D2EHPA
0.6 D
2.2. Procedures
0.4
0.2
0.0 0.0
0.2
0.4 0.6 XHEHEHP or XD2EHPA
0.8
1.0
Fig. 1. Extraction of Bi3+ with HEHEHP, bipy, HEHEHP + bipy, and D2EHPA. [Bi3+] = 2.0 103 mol L1, pH = 0.5, l = 0.6 mol L1, CHEHEHP + Cbipy = 0.05 mol L1, CD2EHPA = 0.05 mol L1.
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N. Song et al. / Separation and Purification Technology 104 (2013) 64–67
Table 2 Analytical data of the extraction of Bi3+ with HEHEHP (pH = 0.5, l = 0.6 mol L1). No.
VHEHEHP (mL)
log[HEHEHP](o) (mol L1)
log DA 3pHeq
log KA
1 2 3 4 5
1 2 3 4 5
2.00 1.70 1.53 1.40 1.31
4.94 4.04 3.51 3.17 2.89
1.06 1.07 1.07 1.04 1.03
Average log KA
The following equation could thus be used to describe the relationship between the distribution ratio DAB and the equilibrium constant KAB.
log DAB ¼ x log½H2 A2 ðoÞ þ y log½BðoÞ þ ipH þ log K AB þ ð3 iÞ log½NO3 ðaÞ According to Eqs. (4) and (5),
DA ¼ -1.0
logDAB - 3pH
½BiH3 A6 ðoÞ C Bi3þ ½BiH3 A6 ðoÞ ½BiðNO3 Þ3i H2xi A2x By ðoÞ ½BiðNO3 Þ3i H2xi A2x By ðoÞ
-1
[B] = 0.02 mol L -1 [B] = 0.01 mol L
DAB ¼
-1.5
CBi3þ ½BiH3 A6 ðoÞ ½BiðNO3 Þ3i H2xi A2x By ðoÞ
½BiH3 A6 ðoÞ ¼
DA CBi3þ 1 þ DA þ DAB
½H2 A2 ðoÞ ¼ C H2 A2 C Bi3þ -2.0
-1.5
-1.0
log[H2A2](o) Fig. 2. Relationship between HEHEHP concentration and distribution ratio. [Bi3+] = 2.0 103 mol L1, pH = 0.5, l = 0.6 mol L1.
-1
[H2A2] = 0.02 mol L
-1
[H2A2] = 0.01 mol L -1.5
logDAB - 3pH
ð10Þ
3DA þ xDAB 1 þ DA þ DAB
ð11Þ
Therefore,
-2.5
slope: 0.98 -2.0
-2.5
-2.0
-1.5
-1.0
log[B](o) Fig. 3. Relationship between bipy concentration and distribution ratio. [Bi3+] = 2.0 103 mol L1, pH = 0.5, l = 0.6 mol L1.
Bipy, with a similar structure with phen, has found its applications in the synergistic extraction of metal ions [12,13]. In the present work, the extraction of Bi3+ with HEHEHP, bipy, and their mixtures was carried out (Fig. 1). It was observed that the extraction of Bi3+ with bipy could be neglected compared with that with HEHEHP or HEHEHP + bipy. The extraction of Bi3+ with HEHEHP + bipy could be expressed as follows, 3þ
BiðaÞ þ ð3 iÞNO3ðaÞ þ xH2 A2ðoÞ þ yBðoÞ BiðNO3 Þ3i H2xi A2x ByðoÞ þ
iHþðaÞ
B was the abbreviation of bipy.
½BðoÞ ¼ C B C Bi3þ
yDAB 1 þ DA þ DAB
ð5Þ
ð12Þ
In order to obtain x, y, and i values in Eq. (5), a series of experiments were designed and carried out. Firstly, the effects of aqueous acidity on the extraction were studied when other experimental conditions were fixed including ionic strength and H2A2 and B concentrations. A straight line with the slope of about 3.0 was obtained, indicating that i equals to 3.0. Secondly, x and y were determined similarly, i.e., the aqueous acidity, ionic strength, and the concentration of a certain extractant were fixed. x was determined when B concentration was fixed while y was determined when H2A2 concentration was fixed. Results were shown in Figs. 2 and 3. It could be seen that the values of x and y were 1.5 and 1.0, respectively. Therefore, Eq. (5) could be changed as, K AB 3 3þ BiðaÞ þ H2 A2ðoÞ þ BðoÞ BiA3 BðoÞ þ 3HþðaÞ 2
slope: 1.03
K AB
DAB C Bi3þ 1 þ DA þ DAB
½BiðNO3 Þ3i H2xi A2x By ðoÞ ¼
slope: 1.57
-3.0 -2.5
ð8Þ
ð9Þ
-2.0
-1.0
ð7Þ
½BiH3 A6 ðoÞ and ½BiðNO3 Þ3i H2xi A2x By ðoÞ could be expressed as, slope: 1.52
-3.0 -2.5
ð6Þ
1.06 ± 0.03
ð13Þ
The equilibrium constant, KAB, could be calculated as 2.53 ± 0.03. It should be noted that the ratio of x and y was determined as 1.5:1, which was not absolutely in accordance with the results shown in Fig. 1 [14]. In Fig. 1, the maximum distribution ratio appeared at XHEHEHP value of about 0.8, where the fractional ratio of HEHEHP and bipy was 4:1. However, as mentioned above, the synergistic extraction was determined by three extraction processes, i.e., the extraction of Bi3+ with single HEHEHP, single bipy, and HEHEHP + bipy. The three reactions existed simultaneously and decided the final extraction effects. A possible explanation about the synergistic extraction is as follows. As is known, HEHEHP takes part in the solvent extraction reaction as a cationic extractant. In the solvent extraction of metal by a mixture of cationic extractant and solvating extractant, e.g., bipy, the solvating extractant may replace the self-adducted cationic extractant, which is in the form of BiH3A6 in the present system. This replacement results in synergistic effect. According to Eqs. (4) and (13), the following formation reaction could be derived. b 3 BiH3 A6ðoÞ þ BðoÞ BiH3 A6 BðoÞ þ H2 A2ðoÞ 2
ð14Þ
The equilibrium constant, log b, could be calculated as 2.47 (log b = log KABlog KA).
N. Song et al. / Separation and Purification Technology 104 (2013) 64–67
-0.3
3.2.3. Stripping studies The stripping behaviors of the synergistic extraction of Bi3+ with HEHEHP + bipy were studied. The stripping efficiency versus HNO3 concentrations was shown in Fig. 5, indicating that the majority of Bi3+ could be stripped by one stage when the concentration of HNO3 was greater than 0.03 mol L1. This implied that HEHEHP + bipy mixing system has potential to be applied in the hydrometallurgical field.
-0.6
logD
67
-0.9 slope: 1.39
4. Conclusions
-1.2
-1.5 2.8
3.0
3.2
3.4
3.6
-1
1000 / T, K
Fig. 4. Relationship between temperature and distribution ratio. [Bi3+] = 2.0 103 mol L1, pH = 0.5, l = 0.6 mol L1, CHEHEHP = 0.04 mol L1, Cbipy = 0.01 mol L1.
100
Acknowledgements
80
The authors wish to thank Mr. Yong-Hui Dai and Prof. ZhenFeng Cui of Changchun Institute of Technology.
St%
60
References
40
20
0 0.00
0.02
0.04 CHNO3, mol L
0.06
0.08
-1
Fig. 5. Stripping behavior of Bi3+ with HNO3.
3.2.2. Influence of temperature on the extraction of Bi3+ with HEHEHP + bipy When other experimental conditions were fixed, the influence of temperature on the extraction of Bi3+ with HEHEHP + bipy was investigated. Results were shown in Fig. 4. Thermodynamic functions, the change of enthalpy (DH°), Gibbs free energy (DG°), and entropy (DS°), can thus be obtained as the followings,
D log D DH ¼ 2:303R D 1T
ð15Þ
DG ¼ RT ln K
ð16Þ
Among the mixtures of D2EHPA / HEHEHP with CA12, CA100, HQ, and bipy, only the HEHEHP + bipy mixing system had synergistic effects for bismuth extraction from nitric solutions. The synergistic coefficients were greater than 1.0 at any mole fractional ratio of HEHEHP and bipy. Although the mechanism of the synergistic extraction was difficult to be obtained, the extraction stoichiometry could be investigated with the methods of slope and constant mole. The formed complex could also be determined together with equilibrium constants. The synergistic extraction was proved to be an exothermic and spontaneous process. Furthermore, stripping tests illustrated that bismuth could be stripped by one stage with 0.03 mol L1 HNO3.
DG ¼ DH T DS ) DS ¼
DH DG T
ð17Þ
DH° was calculated as 26.7 kJ mol1. The sign of DH°, , illustrated that the synergistic extraction was an exothermic process. DG° and DS° when T is 293 K could be determined as 14.2 kJ mol1 and 42.6 J mol1 K1, respectively, implying that the synergistic extraction was a spontaneous process.
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