ARTICLE pubs.acs.org/IECR
Three-Liquid-Phase Extraction and Separation of Ti(IV), Fe(III), and Mg(II) Keng Xie,†,‡,§ Kun Huang,*,† Lin Xu,†,‡,§ Pinhua Yu,†,‡,§ Liangrong Yang,† and Huizhou Liu*,† †
Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People's Republic of China ‡ National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Beijing 100190, People's Republic of China § Graduate University of the Chinese Academy of Sciences, Beijing 100049, People's Republic of China ABSTRACT: Three-liquid-phase extraction has been considered to be a promising method for isolation and separation of multicomponents. Selective extractionseparation of Ti(IV), Fe(III), and Mg(II) in the three-liquid-phase system containing trialkylphosphine oxide (TRPO), poly(ethylene glycol) with an average molecular mass of 2000 (PEG 2000), and (NH4)2SO4 was achieved by adding a water-soluble complexing agent, ethylenediaminetetraacetic acid (EDTA). The simple and environmentally benign complexing method was proved to be an effective strategy for enhancing the selectivity of the PEG-2000-rich middle phase for Fe(III) without reducing the affinity of the TRPO-rich top phase to Ti(IV). The related chemistry was detailed, and effects of some important parameters such as the aqueous phase pH, EDTA amount, mixing time, and temperature were examined. It revealed that Ti(IV) and Fe(III) could be enriched respectively into the TRPO-rich top phase and the PEG-2000-rich middle phase through optimization of the aqueous pH and the EDTA amount. Ti(IV) was extracted into the top phase with a slow kinetics and a positive enthalpy change (ΔH), whereas Fe(III) was rapidly transferred to the middle phase as Fe(III)EDTA complexes formed by an exothermic reaction. Mg(II) tended to remain in the bottom phase.
1. INTRODUCTION Selective purification and concentration of metals in solution has primarily been achieved using wateroil liquidliquid extraction and an aqueous two-phase system (ATPS).13 These techniques have attained a high level of success on the analytical, preparative, and process scales and, consequently, hold an important place in the hierarchy of metal ion separation methods. By integrating a wateroil liquidliquid system with a polymer salt ATPS, a three-liquid-phase system (TLPS) composed of an organic solvent-rich top phase, a polymer-rich middle phase, and a salt-rich bottom phase may be obtained. The novel system provides a new medium with hydrophobicity decreasing gradually from the top phase to the bottom phase and possesses a wide range of polarities and chemical properties, allowing a much higher selectivity than the usual wateroil or aqueous biphasic systems.4 A well-designed TLPS can be used to separate three solutes or three groups of solutes completely in a single step so that the subsequent downstream processing steps are not needed, or at least are reduced in number. Owing to these advantages, the TLPS has been considered to be a promising method of isolation and separation of multicomponents.5 The TLPS was proved suitable for separation and purification of penicillin G,6,7 glycyrrhizic acid, and liquiritin8 as well as phenolic wastewater.9,10 However, no works on the separation of multimetal by a TLPS have hitherto been reported. The main obstacle is that the complicated partition behaviors of various metal ions into different liquid phases are hard to control. Although the polymersalt ATPS exhibits a potential application in the extraction of metal ions,1113 combination of the polymersalt ATPS with a conventional organic phase extraction system makes it difficult to obtain selective enrichment of different metals into different liquid phases. The intention of our research is to provide a three-liquidphase extraction mode in the field of metal ion separation and a r 2011 American Chemical Society
strategy for recovering and analyzing metals. A dilute sulfuric acid solution of three commonly associated metal species, Ti(IV), Fe(III), and Mg(II), was initially taken as the object of study. A TLPS was developed upon mixing the metal solution with an effective titanium extractant, trialkylphosphine oxide (TRPO),1418 and the frequently used poly(ethylene glycol) with an average molecular mass of 2000 (PEG 2000)(NH4)2SO4 ATPS.19,20 Previous experiments showed that Mg(II) was specifically localized at the (NH4)2SO4-rich bottom phase, whereas Ti(IV) and Fe(III) partitioned between the bottom phase and the TRPOrich top phase.21 The extraction of Ti(IV) by TRPO was accompanied by the coextraction of Fe(III), and the PEG2000-rich middle phase exhibited no affinity for metals. However, the partitioning behavior of metals, especially the ferric ions, changed after addition of a water-soluble complexing agent, ethylenediaminetetraacetic acid (EDTA). A portion of Fe(III) was distributed into the PEG-2000-rich middle phase and inhibited from distributing into the top phase. The transfer of Fe(III) greatly improved the selectivity of TRPO for Ti(IV) and made a positive impact on the separation of Ti(IV), Fe(III), and Mg(II). Several reports22,23 showed that the extractionseparation of metals in the presence of EDTA depended on the pH value of the aqueous phases, the complex formation constants, and the EDTA amount. The separation of Ti(IV), Fe(III), and Mg(II) should therefore be optimized when the above conditions can be properly combined in the TLPS. Moreover, in the TLPS, there is competition between solvent molecules (H2O, TRPO, PEG 2000) and the ligands (EDTA4, SO42) for a metal ion and Received: November 5, 2010 Accepted: April 15, 2011 Revised: March 28, 2011 Published: April 15, 2011 6362
dx.doi.org/10.1021/ie1022354 | Ind. Eng. Chem. Res. 2011, 50, 6362–6368
Industrial & Engineering Chemistry Research
ARTICLE
simultaneous competition between different metal ions for a solvent molecule or a ligand. Knowledge of chemistry underlying the overall extraction is necessary and useful for better understanding of the partitioning of Ti(IV), Fe(III), and Mg(II). In the present paper, details such as the metal speciation, binary interactions, and extraction equilibrium in the TLPS were first explored. Partitioning behavior of Ti(IV), Fe(III), and Mg(II) in the TRPOPEG 2000(NH4)2SO4 TLPS in the presence of EDTA was studied as a function of the aqueous phase pH, EDTA amount, mixing time, and temperature. By resorting to these parameters, it is hoped to find a condition under which the TLPS has both enhanced selectivity and high extraction efficiency for Ti(IV), Fe(III), and Mg(II).
2. EXPERIMENTAL SECTION 2.1. Reagents and Chemicals. TRPO was purchased from Shanghai Laiyashi Chemical Co., Ltd., and was used without further purification. This extractant is a mixture containing four major components, which are trihexylphosphine oxide, dihexylmonooctylphosphine oxide, dioctylmonohexylphosphine oxide, and trioctylphosphine oxide, respectivey. PEG 2000 and (NH4)2SO4 were purchased from Sinopharm Chemical Reagent Co., Ltd. PEG 2000 was used as received and (NH4)2SO4 stock of 50% (w/w) was prepared. A stock solution of 10 mmol/L EDTA was prepared by dissolving a weighed amount of dried disodium ethylenediamintetraacetate (Na2H2EDTA, Guangdong Xilong Chemical Reagent Factory) in deionized water. A stock solution of Ti(IV), Fe(III), and Mg(II) was prepared with dilute sulfuric acid from their analytical grade sulfates. The concentrations of Ti(IV), Fe(III), and Mg(II) in the solution were maintained at 30, 5, and 15 mmol/L, respectively. This initial molar ratio of metals equals that of actual leaching solution derived from the titanium-bearing blast furnace slag at Pangang Corporation, China. All other used chemicals were of analytical grade. 2.2. Partitioning and Analytical Procedures. Four grams of PEG 2000 and required volumes of (NH4)2SO4 stock solution and EDTA stock solution were placed in a 50 mL graduated tube. The mixture was made up to volume with deionized water and shaken to make PEG 2000 dissolve. The desired pH value of the mixture was achieved by addition of dilute NH4OH solution or sulfuric acid under the control of a pH211 acidometer (Hanna, Italy). Then, 5 mL of TRPO and 1 mL of metal stock solution were added. The system was prepared with a global volume of 25 mL. A thermostatic shaker was used to study the effect of mixing time and temperature on the partitioning. The system was mechanically shaken for some period of time at a specific temperature followed by 10 min of centrifugation at a speed of 4000 rpm. After phase separation, volumes of immiscible phases could be directly read from the scale of the tube. A sample of each separated phase was carefully withdrawn by a syringe and used for analysis. Concentrations of Ti(IV), Fe(III), and Mg(II) in middle and bottom phases were analyzed by an OPTIMA 7000DV inductively coupled plasma optical emission spectrometer (ICP-OES, PerkinElmer, USA) at wavelengths of 335, 238, and 285 nm, respectively. The concentrations of metal ions in the top phase were determined by difference. The metals present in the top phase were also analyzed to check the mass balance. The metal extraction experiments followed by analyses of the samples were conducted several times to check the repeatability and the accuracy of measurements. The error in
Figure 1. Schematic ternary graph of mass fractions of a metal in the three-liquid-phase system.
analysis was within (5%. Fourier transform infrared spectroscopy (FTIR) analysis was taken on a Bruker Vector 22 FTIR spectrometer in the range 8504000 cm1 using BaF pellets. 2.3. Determination of Partitioning of Ti(IV), Fe(III), and Mg(II). To evaluate the partitioning of Ti(IV), Fe(III), and Mg(II) in the TLPS, the mass fraction (w) of metal ions in each phase was determined according to the following equation: wM, i ¼
CM , i V i 100% CM , 0 V 0
ð1Þ
where M denotes metal ions such as Ti(IV), Fe(III), and Mg(II) and i represents the top (t) middle (m), or bottom (b) phases. CM,i and Vi are the concentration of metal ion in the i phase and the volume of the i phase, respectively. CM,0 is the original concentration of the metal ion in the stock solution, and V0 is the volume of stock solution added to the system. Points in a ternary graph were unprecedentedly introduced to depict the mass fractions of solutes in the three phases of the TLPS under varying conditions. The ternary graph is drawn as an equilateral triangle. Each base of the triangle represents a mass fraction of 0% (w = 0%), with the point of the triangle opposite that base representing a mass fraction of 100% (w = 100%), as shown in Figure 1. Symbols of different colors in the triangle represent different metals. Also, different geometries of the points are employed to represent levels of the parameter investigated. For example, in Figure 1, the black1 symbolizes the mass fractions (wM,t = 24.8%, wM,m = 23.2%, and wM,b = 52.0%) of a metal in the TLPS at a specific condition, and the black 2 symbolizes the mass fractions of the same metal at another specific condition. From the concentration values in different phases, the distribution ratio was calculated. Specifically, distribution ratios of Ti(IV), Fe(III), and Mg(II) between two phases in the TLPS are
6363
DM, t=m ¼
CM , t CM , m
ð2Þ
DM, t=b ¼
C M, t CM , b
ð3Þ
DM, m=b ¼
C M, m CM , b
ð4Þ
dx.doi.org/10.1021/ie1022354 |Ind. Eng. Chem. Res. 2011, 50, 6362–6368
Industrial & Engineering Chemistry Research
ARTICLE
Figure 2. Representative FTIR spectra of TRPO, Ti(IV) loaded TRPO, and Fe(III) loaded TRPO.
where M represents metal ions; CM,t, CM,m, and CM,b are concentrations of metal ion M in the top, middle, and bottom phases, respectively. Furthermore, the separation factor (β) between two metal ions was calculated by the ratio of their distribution ratios. The Ti(IV)/Fe(III) separation factor between the upper two phases is defined as βTiðIVÞ=FeðIIIÞ, t=m ¼
DTiðIVÞ, t=m DFeðIIIÞ, t=m
ð5Þ
3. RESULTS AND DISCUSSION 3.1. Chemistry of Partitioning. The aqueous chemistry of Ti(IV), Fe(III), and Mg(II) has been studied by electromigration, ion-exchange chromatography, and dialysis techniques24 and has shown that Ti(IV), Fe(III), and Mg(II) exist as TiO2þ, [Fe(H2O)6]3þ, and [Mg(H2O)6]2þ in aqueous solution in the absence of complexing agents.25,26 In sulfate solution, Ti(IV), Fe(III), and Mg(II) form electrically neutral and anionic complexes, since the SO42 and HSO4 ions take part in complex formation. The important species of Ti(IV) in sulfuric acid solutions are TiOSO4 and TiO(SO4)22.27 Fe(III) distributes as FeSO4þ and Fe(SO4)2 complex compounds.28 These titanium species and ferric species can transport into the TRPO phase, because the tetravalent titanium and trivalent iron ions can coordinate to the oxygen lone electron couple in the PdO group of TRPO to form extracted adducts.15,17 This was verified by the FTIR spectra of the compounds recovered from the organic layer after phase transfer. As shown in Figure 2, the stretching frequency of PdO was shifted from 1151 cm1 in pure TRPO to 1139 cm1 in Ti(IV) loaded TRPO and 1143 cm1 in Fe(III) loaded TRPO, indicating the interaction between the metal ions and the phosphoryl group of TRPO. Divalent magnesium showed an overwhelming hydration tendency and could not coordinate to the phosphoryl oxygen atom, leading to the stay of magnesium in the salt solution. However, the extraction equilibrium of metal by TRPO will change when EDTA is added into the system. It is well-known that EDTA contains two iminodiacetic acid (IDA) units with the
Figure 3. Effect of pH on mass fractions of Ti(IV), Fe(III), and Mg(II) in the three-liquid-phase system at 30 °C. The system had a global volume of 25 mL and contained 5 mL of TRPO, 4 g of PEG 2000, 5 g of (NH4)2SO4, and 30 μmol of EDTA. Mixing time was 40 min.
nitrogen atoms linked by two methylene groups.29,30 It may exist in various forms including H6EDTA2þ, H5EDTAþ, H4EDTA, H3EDTA, H2EDTA2, HEDTA3, and EDTA4 in aqueous solution.31 At pH 11 condition the fully deprotonated EDTA4 form is prevalent. In the paper, the term EDTA is used to mean H4xEDTAx, whereas in its complexes EDTA4 stands for the tetradeprotonated ligand. Because of its high denticity, EDTA has a high affinity for metal cations and usually binds to a metal cation through its two amines and four carboxylates. The formation constant of Fe(III)EDTA chelate (24.23) is much larger than that of Ti(IV)EDTA (17.3) or Mg(II)EDTA (8.64) complex.32 Thus, the EDTA which is being added is first chelating Fe(III) ions. Since the Fe(III)EDTA chelate is more stable than the Fe(III)TRPO complex, EDTA can react with Fe(III) to form hydrophilic Fe(III)EDTA chelate which will not be extracted by TRPO. In that case, EDTA plays the role of a masking agent. By the masking effect, Fe(III), which can interfere in the extraction of Ti(IV), may be removed from the TRPO-rich top phase. Besides, the two hydrophobic methylene groups of EDTA enable the chelate to dissolve easily in the similar polar PEG-2000-rich phase. For this reason, EDTA acts as an extractant which is capable of transferring Fe(III) to the middle phase. 3.2. Effect of pH Value. Preliminary experiments of the effect of pH value on the formation of the TLPS indicated that with increasing acid concentration the TLPS became unstable and a large amount of (NH4)2SO4 was required for the TLPS formation. In addition, Ti(IV) was subjected to hydrolysis at pH value greater than 2. Thus, the pH range 02 was used under the current experimental conditions. Partitioning of Ti(IV), Fe(III), and Mg(II) in the TLPS was studied at four different pH values: pH 0.5, 1.0, 1.5, and 2.0. It can be seen from Figure 3 that high pH raised extraction of Fe(III) into the middle phase and decreased its distribution into the top phase. wFe(III),m increased from 2.3% at pH 0.5 to 52.6% at pH 2.0, while wFe(III),t decreased from 61.0% at pH 0.5 to 5.4% at pH 2.0. It is demonstrated that the pH value of the aqueous phases is a very essential factor controlling the partitioning of Fe(III) and Hþ ion plays an important role in the Fe(III) EDTA formation. As stated above, EDTA4 is basic and binds to Hþ ions over a wide range of pH values. Some of these Hþ ions 6364
dx.doi.org/10.1021/ie1022354 |Ind. Eng. Chem. Res. 2011, 50, 6362–6368
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Effect of EDTA Amount on Partitioning of Ti(IV), Fe(III), and Mg(II) in the Three-Liquid-Phase System at 30 °C and pH 2.0a Ti(IV)
a
Fe(III)
Mg(II) βTi(IV)/Fe(III),t/m
EDTA amt (μmol)
wt
wm
Dt/m
Dt/b
wt
wm
Dt/m
Dm/b
5
74.9
0.9
118.3
7.1
26.9
35.7
1.13
1.5
99.6
104.3
6
75.2
1.0
105.3
7.3
16.3
42.5
0.68
1.8
99.6
155.9
7 7.5
75.8 76.8
1.1 1.2
100.1 98.9
7.6 8.0
11.5 8.2
48.7 51.0
0.36 0.24
1.9 2.0
99.5 99.5
275.0 409.9
8
76.6
4.2
27.3
9.2
6.0
51.3
0.17
1.9
99.5
157.9
9
76.2
6.7
17.1
10.2
5.6
51.5
0.16
1.8
99.4
105.7
10
76.3
9.1
12.6
12.1
5.2
51.8
0.15
1.8
99.2
83.1
wb
The system had a global volume of 25 mL and contained 5 mL of TRPO, 4 g of PEG 2000, and 5 g of (NH4)2SO4. Mixing time was 40 min.
are frequently displaced from EDTA by the metal during the chelate formation. Fe(III)EDTA speciation distribution diagrams provided by Shimizu et al.33 showed that FeHEDTA and FeEDTA are the predominant species in the pH range 02. Complexation between Fe(III) and Hþ for EDTA can be defined as Fe3þ þ HEDTA 3 S FeEDTA þ Hþ
ð6Þ
From eq 6 it is clear that the stability of the Fe(III)EDTA complex is pH dependent. The lower the pH of the aqueous phases is, the lesser the stability of the complex will be, because more Hþ ions are available to compete with Fe(III) for the ligand. It can also be seen from Figure 3 that wTi(IV),t increased from 63.4% at pH 0.5 to 75.9% at pH 2.0. This is attributed to the fact that Fe(III) is masked from TRPO by EDTA, leading to an increase in number of free molecules of TRPO available to extract Ti(IV). At the same time, Mg(II) showed an absolute preference for the salt-rich bottom phase, because the chelate of Mg(II) is completely dissociated in the acidic solution. For the sake of obtaining high extractions of Ti(IV) and Fe(III) in their respective receiving top phase and middle phase, the optimum pH should be set at 2.0. However, transfer of Ti(IV) to the middle phase occurred more readily at pH 2.0, with a mass fraction of 14.9%. The EDTA amount accounted for the interference of Ti(IV) in the middle phase, and its effect will be discussed in section 3.3. 3.3. Effect of EDTA Amount. Theoretically, EDTA forms complexes with most metal cations in a 1:1 molar ratio, irrespective of the valence of the ion. In practice, however, the molar amount of EDTA added is not equivalent to that of metal present, and an excess amount of EDTA relative to metal ions was usually used.31 In the study, EDTA was used as a complexing agent special for Fe(III); thus the EDTA amount should be determined according to the Fe(III) amount in the TLPS. The treatment of a solution containing a single metal Fe(III) offers little difficulty. Problems arise when the interfering species Ti(IV) is present. Some Ti(IV) ions can be coextracted with EDTA in the form of chelated anions from TRPO by PEG 2000, as long as all the Fe(III) is chelated and a surplus of EDTA exists. Hence, when selecting an EDTA amount, attention must be paid to avoid Ti(IV)EDTA complexation, which can affect the retention of Ti(IV) in the top phase and negatively affect the separation. At the optimum pH 2.0 as determined before, the
Figure 4. Effect of EDTA amount on (a) mass fractions of Ti(IV), Fe(III), and Mg(II) and (b) the separation factor of Ti(IV) and Fe(III) in the upper two phases in the three-liquid-phase system at 30 °C and pH 2.0. The system had a global volume of 25 mL and contained 5 mL of TRPO, 4 g of PEG 2000, and 5 g of (NH4)2SO4. Mixing time was 40 min.
criteria for the selection of a desired EDTA amount are the purity of Fe(III) in the middle phase and the largest separation factor of Ti(IV) and Fe(III) in the upper two phases. Varying amounts of EDTA were added into the TLPS at pH 2.0, and their effect on the partitioning of Ti(IV), Fe(III), and Mg(II) is shown in Table 1 and Figure 4. From Figure 4a, wFe(III),m increased with increasing EDTA amount from 0 to 7.5 μmol and held almost constant thereafter. Meanwhile, a corresponding decrease of wFe(III),t was detected. wTi(IV),m was negligible and seemed nearly unchanged when the added amount of EDTA was 6365
dx.doi.org/10.1021/ie1022354 |Ind. Eng. Chem. Res. 2011, 50, 6362–6368
Industrial & Engineering Chemistry Research
ARTICLE
Figure 5. Effect of mixing time on mass fractions of Ti(IV), Fe(III), and Mg(II) in the three-liquid-phase system at 30 °C and pH 2.0. The system had a global volume of 25 mL and contained 5 mL of TRPO, 4 g of PEG 2000, 4 g of (NH4)2SO4, and 7.5 μmol of EDTA.
less than 7.5 μmol but gained an increase within EDTA amount range 7.510 μmol, reaching a value of 9.1% when the added EDTA amount was 10 μmol. This suggested that the EDTA amount of 7.5 μmol was sound to achieve a high separation factor of Ti(IV) and Fe(III) in the upper two phases. As shown in Figure 4b, βTi(IV)/Fe(III),t/m reached the maximum value of 410 or so when 7.5 μmol EDTA was added. Considering the Fe(III) amount in the feed solution was 5 μmol, the EDTA/Fe(III) molar ratio was calculated to be 3:2 in this situation. 3.4. Effect of Mixing Time. The partitioning of Ti(IV), Fe(III), and Mg(II) in the TLPS was carried out with various periods of time to confirm its influence. The results of wM,i within the mixing time range 340 min are shown in Figure 5. From Figure 5, at the shortest tested time of 3 min, more than half Fe(III) and 12% Ti(IV) were transported into the middle phase. The relatively high values indicated that the complexing reaction between EDTA and metal is extremely fast and their complexes could diffuse rapidly into the PEG-2000-rich environment. wFe(III),m was not significantly affected by the mixing periods of time, and it held constant around 52% throughout the mixing time range 340 min, demonstrating that the equilibrium time for Fe(III) extraction by PEG 2000 in the presence of EDTA is less than 3 min. Fe(III) partition has a very fast kinetics. In contrast, the extraction of Ti(IV) by TRPO was slow. wTi(IV),t increased with prolonging the mixing time until about 30 min. At the same time, wTi(IV),m decreased as the mixing time increased. When the mixing time was 30 min, the last trace of Ti(IV) in the middle phase was replaced by Fe(III) and distributed into the top phase and wTi(IV),t was calculated to be 74.2%. It is therefore concluded that Ti(IV) partitioning required 30 min to achieve equilibrium. Evidence in the literature34 indicated that extraction of Ti(IV) from sulfate media required an equilibrium time of 1015 min for TRPO in the liquidliquid system. In comparison, the extraction of Ti(IV) by TRPO in the TLPS is much slower. This is possibly due to the existence of PEG 2000, which could hinder the contact between TRPO molecules and metal ions. Based on these results, a time of 30 min should be chosen to ensure sufficient mixing. 3.5. Effect of Temperature. Mass fractions of Ti(IV), Fe(III), and Mg(II) in the TLPS at different mixing temperatures are shown in Figure 6a. On one hand, extraction of Ti(IV) and Fe(III) into the top phase increased with the increase of temperature.
Figure 6. Effect of temperature on (a) mass fractions of Ti(IV), Fe(III), and Mg(II) and (b) distribution ratios of Ti(IV) and Fe(III) in the threeliquid-phase system at pH 2.0. The system had a global volume of 25 mL and contained 5 mL of TRPO, 4 g of PEG 2000, 4 g of (NH4)2SO4, and 7.5 μmol of EDTA. Mixing time was 30 min.
wTi(IV),t was as high as 85% at 60 °C, compared to 56% at 20 °C. Meanwhile, wFe(III),t increased gradually from 5.7% at 20 °C to 10.4% at 60 °C. On the other hand, extraction of Fe(III) into the middle phase decreased as temperature increased. wFe(III),m decreased from 59.8% at 20 °C to 42.6% at 60 °C. This is because an increase in temperature causes a slight increase in the protonation of EDTA and a slight lowering of the stability constant of the Fe(III)EDTA complex. It is worth mentioning that all Mg(II) was concentrated in the bottom phase regardless of the temperature change. The plots of log DTi(IV),t/b and log DFe(III),m/b versus (1/T K) 103 are given in Figure 6b. The experimental points for Ti(IV) do not fall on a straight line, but a curve is obtained with greater limiting slope in the lower temperature region and lower limiting slope in the higher temperature region. Virtually, the extraction of Ti(IV) is not enhanced appreciably above 30 °C (1/T K = 3.3 103). At 30 °C the tangential slope is about 2.09 103, which gives a ΔH value of 39.5 kJ/mol. The slope (in negative sense) and consequently the enthalpy change are increased with decrease in temperature. The tangential slope of the curve at 20 °C is about 2.95 103, giving a ΔH value of 55.8 kJ/mol. The positive ΔH value indicates that the extraction of Ti(IV) with TRPO is endothermic in nature. The variation of ΔH value with temperature indicates the change in the extraction mechanism from a chemically controlled process to a diffusion controlled process. The experimental points for Fe(III) fall on a straight line 6366
dx.doi.org/10.1021/ie1022354 |Ind. Eng. Chem. Res. 2011, 50, 6362–6368
Industrial & Engineering Chemistry Research which has a positive slope 0.68 103, giving a ΔH value of 13 kJ/mol. This indicates that the complexation between EDTA and metal is exothermic.
4. CONCLUSION In the TRPOPEG 2000(NH4)2SO4 three-liquid-phase system with EDTA, each phase has its own specific selectivity for dissolving component Ti(IV), Fe(III), and Mg(II) from their mixture. Ti(IV) in aqueous sulfuric solution existing as titanyl sulfate complex species was susceptible to extraction into the TRPO-rich top phase, while Fe(III) was bound to EDTA and preferably extracted by the PEG-2000-rich middle phase against the other two metals, leaving behind Mg(II) in the salt-rich bottom phase. The facilitated partitioning of Ti(IV), Fe(III), and Mg(II) in the TLPS could be achieved by adjusting the aqueous pH value, EDTA amount, mixing time, and mixing temperature. The aqueous pH and EDTA amount play a key role in the selectivity: an appropriate combination of pH value and EDTA amount can improve Fe(III) transfer into the middle phase and in turn realize the one-step extractionseparation of Ti(IV) and Fe(III) in the upper two phases. The experimental results obtained highlight the validity of the TLPS for the treatment of multimetal solutions. However, the practical utility of the current process requires further improvement of extraction kinetics and assessment of new phase-forming substances. Research on the partition behavior of metal ions in the TLPS remains in its early stages, and more analytical efforts are still needed as well. For satisfactory separation of Ti(IV), Fe(III), and Mg(II), further studies are now under way to find a class of extractants that has a clear affinity for each metal. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (K.H.);
[email protected] (H.L.). Tel./fax: þ86-10-62554264.
’ ACKNOWLEDGMENT The study was financially supported by the National Basic Research Program of China (973 project 2007CB613507) and the National Natural Science Foundation of China (Grants 51074150 and 21027004). ’ REFERENCES (1) Rydberg, J.; Cox, M. Solvent Extraction Principles and Practice; CRC: Boca Raton, FL, USA, 2004. (2) Bulgariu, L.; Bulgariu, D. Extraction of metal ions in aqueous polyethylene glycol-inorganic salt two-phase systems in the presence of inorganic extractants: correlation between extraction behaviour and stability constants of extracted species. J. Chromatogr., A 2008, 11961197, 117–124. (3) Rodrigues, G. D.; da Silva, M. d. C. H.; da Silva, L. H. M.; Paggioli, F. J.; Minim, L. A.; Reis Coimbra, J. S. d. Liquid-liquid extraction of metal ions without use of organic solvent. Sep. Purif. Technol. 2008, 62 (3), 687–693. (4) Silva, L.; Loh, W. Polymer induced multiphase generation in water/organic solvent mixtures. Strategies towards the design of triphasic and tetraphasic liquid systems. Chem. Commun. 1998, 7, 787–788. (5) Mojski, M.; Gluch, I. Characteristics and applications of threephase extraction systems. J. Anal. Chem. 1996, 51 (4), 329–342. (6) Chen, J.; Liu, H.; Wang, B.; An, Z.; Liu, Q. Study on the ThreePhase Extraction of Penicillin G with a Single-Step Method. In
ARTICLE
Proceedings of the International Solvent Extraction Conference, Johannesburg, 2002; Sole, K. C., Cole, P. M., Preston, J. S., Eds.; Chris Van Rensburg Publications (Pty) Ltd.: Johannesburg, South Africa, 2002; pp 602606. (7) Tan, X.; Ji, Q.; Chang, Z. Partition Behavior of Penicillin in Three-liquid-phase Extraction System. Chin. J. Process Eng. 2006, 6 (3), 363–368. (8) Shen, S.; Chang, Z.; Liu, J.; Sun, X.; Hu, X.; Liu, H. Separation of glycyrrhizic acid and liquiritin from Glycyrrhiza uralensis Fisch extract by three-liquid-phase extraction systems. Sep. Purif. Technol. 2007, 53 (3), 216–223. (9) Shen, S.; Chang, Z.; Liu, H. Three-liquid-phase extraction systems for separation of phenol and p-nitrophenol from wastewater. Sep. Purif. Technol. 2006, 49 (3), 217–222. (10) Yu, P.; Chang, Z.; Ma, Y.; Wang, S.; Cao, H.; Hua, C.; Liu, H. Separation of p-Nitrophenol and o-Nitrophenol with three-liquid-phase extraction system. Sep. Purif. Technol. 2009, 70 (2), 199–206. (11) Rogers, R.; Bauer, C. Partitioning behavior of group 1 and 2 cations in poly(ethylene glycol)-based aqueous biphasic systems. J. Chromatogr., B 1996, 680 (12), 237–241. (12) Rogers, R.; Bond, A.; Bauer, C.; Zhang, J.; Griffin, S. Metal ion separations in polyethylene glycol-based aqueous biphasic systems: correlation of partitioning behavior with available thermodynamic hydration data. J. Chromatogr., B 1996, 680 (12), 221–229. (13) Rogers, R.; Bond, A.; Bauer, C. The crown ether extraction of group 1 and 2 cations in polyethylene glycol-based aqueous biphasic systems at high alkalinity. Pure Appl. Chem. 1993, 65, 567–567. (14) Saji John, K.; Saji, J.; Reddy, M. L. P.; Ramamohan, T. R.; Rao, T. P. Solvent extraction of titanium(IV) from acidic chloride solutions by Cyanex 923. Hydrometallurgy 1999, 51 (1), 9–18. (15) Remya, P. N.; Reddy, M. L. Solvent extraction separation of titanium(IV) vanadium(V) and iron(III) from simulated waste chloride liquors of titanium minerals processing industry by the trialkyphosphine oxide Cyanex 923. J. Chem. Technol. Biotechnol. 2004, 79 (7), 734–741. (16) Shi, J.; Gu, G.; Fu, X.; Wang, M.; Hu, Z. Extraction behavior of TOPO (or TRPO)-kerosene/Ti(IV)-H2SO4 systems and the preparation of TiO2 by predispersed-hydrolytic method. Colloids Surf., A 2001, 194 (13), 207–212. (17) Reddy, M. L. P.; Saji, J. Solvent extraction of tetravalent titanium with organophosphorus extractants. Miner. Process. Extr. Metall. Rev. 2003, 23 (3), 199–227. (18) Sole, K. C. Recovery of titanium from the leach liquors of titaniferous magnetites by solvent extraction: Part 1. Review of the literature and aqueous thermodynamics. Hydrometallurgy 1999, 51 (2), 239–253. (19) Wu, Y.; Lin, D.; Zhu, Z. Thermodynamics of aqueous twophase systems: the effect of polymer molecular weight on liquid-liquid equilibrium phase diagrams by the modified NRTL model. Fluid Phase Equilib. 1998, 147 (12), 25–43. (20) Rogers, R. D.; Zhang, J. Effects of increasing polymer hydrophobicity on distribution ratios of TcO4 in polyethylene/poly(propylene glycol)-based aqueous biphasic systems. J. Chromatogr., B 1996, 680 (12), 231–236. (21) Xie, K.; Zhao, J.; Yang, L.; Yu, P.; Liu, H. Investigation of three-liquid-phase extraction systems for the separation of Ti(IV), Fe(III) and Mg(II). Sep. Purif. Technol. 2010, 76 (2), 191–197. (22) Nishihama, S.; Hirai, T.; Komasawa, I. Selective extraction of Y from a Ho/Y/Er mixture by liquid-liquid extraction in the presence of a water soluble complexing agent. Ind. Eng. Chem. Res. 2000, 39 (10), 3907–3911. (23) Nishihama, S.; Hirai, T.; Komasawa, I. Review of Advanced LiquidLiquid Extraction Systems for the Separation of Metal Ions by a Combination of Conversion of the Metal Species with Chemical Reaction. Ind. Eng. Chem. Res. 2001, 40 (14), 3085–3091. (24) Lincoln, S. F.; Richens, D. T.; Sykes, A. G. Metal aqua ions. In Comprehensive Coordination Chemistry II; McCleverty, J. A., Meyer, T. J., Eds.; Pergamon: Oxford, U.K., 2003; pp 515555. 6367
dx.doi.org/10.1021/ie1022354 |Ind. Eng. Chem. Res. 2011, 50, 6362–6368
Industrial & Engineering Chemistry Research
ARTICLE
(25) Nabivanets, B. I. Electromigration of titanium(IV) ions in nitric, hydrochloric, and sulphuric acids. Russ. J. Inorg. Chem. 1962, 7 (2), 210–212. (26) Pye, C. C.; Rudolph, W. W. An ab initio and Raman investigation of magnesium(II) hydration. J. Phys. Chem. A 1998, 102 (48), 9933–9943. (27) Szilagyi, I.; K€onigsberger, E.; May, P. M. Characterization of Chemical Speciation of Titanyl Sulfate Solutions for Production of Titanium Dioxide Precipitates. Inorg. Chem. 2009, 48 (5), 2200–2204. (28) Casas, J. M.; Crisostomo, G.; Cifuentes, L. Speciation of the Fe(II)-Fe(III)-H2SO4-H2O system at 25 and 50°C. Hydrometallurgy 2005, 80 (4), 254–264. (29) Harris, D. Quantitative Chemical Analysis; W. H. Freeman: New York, 2003. (30) Jeffery, G. H.; Bassett, J.; Mendham, J.; Denney, R. C. Vogel’s Text Book of Quantitative Inorganic Analysis; ELBS Longman: London, 1997. (31) Wang, M.; Woo, K. D.; Kim, I. Y.; Woong, K.; Sui, Z. Separation of Fe3þ during hydrolysis of TiO2þ by addition of EDTA. Hydrometallurgy 2007, 89 (34), 319–322. (32) Patnaik, P. Preliminary separation methods. In Dean’s Analytical Chemistry Handbook, 2nd ed.; McGrawHill: New York, 2004. (33) Shimizu, K.; Hutcheson, R.; Engelmann, M.; Francis Cheng, I. Cyclic voltammetric and aqueous equilibria model study of the pH dependent iron(II/III) ethylenediamminetetraacetate complex reduction potential. J. Electroanal. Chem. 2007, 603 (1), 44–50. (34) Sole, K. C. Recovery of titanium from the leach liquors of titaniferous magnetites by solvent extraction: Part 2. Laboratory-scale studies. Hydrometallurgy 1999, 51 (3), 263–274.
6368
dx.doi.org/10.1021/ie1022354 |Ind. Eng. Chem. Res. 2011, 50, 6362–6368
