Anal. Chem. 1999, 71, 4866-4873
Aqua-Impregnated Resins. 2. Separation of Polyvalent Metal Ions on Iminodiacetic and Polyacrylic Resins Using Bis(2-ethylhexyl) Phosphoric and Bis(2-ethylhexyl) Dithiophosphoric Acids as Organic Eluents Maria Oleinikova, Dmitri Muraviev,*,† and Manuel Valiente*
Departament de Quı´mica, Unitat de Quı´mica Analı´tica, Universitat Auto` noma de Barcelona, E-08193 Bellaterra, Barcelona, Spain
This paper dedicated to the further development of the novel aqua-impregnated-resin (AIR) technique reports the results obtained by studying the column separation of equimolar mixtures of Cu2+, Al3+, and Zn2+ on iminodiacetic (IDA) and polyacrylic (PAR) resins by using 0.2 M heptane solutions of either bis(2-ethylhexyl) phosphoric acid (DEHPA) or bis(2-ethyhexyl) dithiophosphoric acid (DEHDTPA) as selective eluents for extractive desorption of metal ions from the resin phase. The quantitative separation of Cu2+-Al3+ mixtures has been achieved by sequential elution of metals from PAR with DEHDTPA and H2SO4 solutions. Purification of Cu2+ from Al3+ and/ or Zn2+ and Al3+ from Cu2+ and Zn2+ is achieved by extractive elution of metal mixtures from IDAR or PAR with DEHPA or DEHDTPA solutions, respectively, followed by stripping of the purified metals with 1.1 M H2SO4. The resulting purity of metals obtained with the yield of >96% exceed 95%. The idea of combined ion-exchange-solvent extraction separation (CIESE) was originated by Korkisch,1-6 who applied mixed water-organic solvents for selective separation of ions on ionexchange resins. The CIESE concept is based on simultaneous ion-exchange (IX) and liquid-liquid extraction by a bifunctional separation mechanism which occurs in applying CIESE-active organic compounds such as, ethers, ketones, and some others in the presence of mineral acids to selectively elute ions from IX resins. Since then, many CIESE systems involving both cation and anion exchangers have been studied.7-15 However, all of them * Corresponding authors: (fax) 34-93-5812379; (e-mail)
[email protected]. † On subbatical leave from Lomonosov Moscow State University, Department of Physical Chemistry, Moscow, Russia. (1) Korkisch, J. Sep. Sci. 1966, 1 (2-3), 159. (2) Korkisch, J. Nature 1966, 210, 626. (3) Korkisch, J. O ¨ sterr. Chem. Ztg. 1966, 67 (9), 309. (4) Korkisch, J. Microchim. Acta 1966, 4-5, 634. (5) Korkisch, J. In Recent Developments in Separation Science; Li, N. N., Navratil, J. D., Eds.; CRC Press: Boca Raton, FL, 1986; Vol. 8, p 105. (6) Korkisch, J. Handbook of Ion Exchange Resins: Their Application to Inorganic Analytical Chemistry; CRC Press: Boca Raton, FL, 1989; Vol. 1, p 88. (7) Kuroda, R.; Hasoi, N. Chromatographia 1981, 14 (6), 359. (8) Sherma, J.; Van Lentan, F. J. Sep. Sci. 1971, 6 (2), 199.
4866 Analytical Chemistry, Vol. 71, No. 21, November 1, 1999
can be eventually characterized by the same extraction mechanism which involves protonation of a CIESE-active component of Lewis base type resulting in the formation of an oxonium salt followed by an interaction of the later with extractable ion from the resin phase due to the formation of ion association complexes in the mobile phase.6 At the same time, application of CIESE-based separation processes in water-imiscible organic solvents is not known,16 although the CIESE systems of this type are of practical importance. For example, the study of IX of transition metal ions from water-insoluble organic solvents can be of great industrial importance for processing of spent motor oil, which must be purified from heavy metals before its reuse.17 On the other hand, further development of novel CIESE techniques may open new routes for increasing the efficiency of single fractionation methods. Recently, it was shown that a CIESE-like effect is observed in biphase systems involving water-imiscible solvents (such as, e.g., carbon tetrachloride) and a water-swollen IX resin.18 A comparison of these IX systems with those involving solvent-impregnated resins (SIR)19-21 has allowed the introduction of the aquaimpregnated resins (AIR) concept, proposed for the first time by Muraviev,22 which is based upon a “symmetry” between AIR and SIR systems. The AIR technique has been effectively applied for carrying out, for example, isotope-exchange reactions.18,22,23 (9) Panse, M.; Khopkar, S. M. J. Sci. Ind. Res. (India) 1975, 34, 5. (10) De, A. K.; Bhattacharyya, C. R. Anal. Chem. 1972, 44, 1686. (11) Henriet, D.; de Gelis, P. Chim. Anal. 1968, 50 (10), 519. (12) Gerwien, U.; Oberhauser, R. Materialpruefung 1978, 20 (6), 233. (13) Orlandini, K. A.; Korkisch, J. Sep. Sci. 1968, 3 (3), 255. (14) Belyavskaya, T. A.; Alimarin, I. P.; Brykina, G. D. Vestn., Mosk. Univ. Ser. Khim. 1967, (No. 1), 53 (Russian). (15) Strelow, F. W. E.; Hanekom, M. D.; Victor, A. H.; Eloff, C. Anal. Chim. Acta 1975, 76, 377. (16) Dorfner, K. In Ion Exchangers; Dorfner, K., Ed.; Walter de Gruyter: Berlin; 1991; Chapter 1.2. (17) Sherman, J. H.; Danielson, N. D.; Taylor, R. T.; Marsh, J. R.; Esterline, D. T. Environ. Technol. 1993, 14, 1097. (18) Muraviev, D.; Rogachev, I.; Bromberg, L.; Warshawsky, A. J. Phys. Chem. 1994, 98, 718. (19) Warshawsky, A. Trans. Min. Metal. 1974, C101. (20) Warshawsky, A. In Ion Exchange and Solvent Extraction; Marinsky, J., Marcus, Y., Eds.; Marcel Dekker: New York; 1981; Vol. 8, Chapter 3. (21) Muraviev, D. Chem. Scr. 1989, 29, 9. (22) Muraviev, D.; Warshawsky, A. React. Polym. 1994, 22, 55. (23) Bromberg, L.; Muraviev, D.; Warshawsky, A. J. Phys. Chem. 1993, 97, 967. 10.1021/ac9902345 CCC: $18.00
© 1999 American Chemical Society Published on Web 09/25/1999
Table 1. System Parameters system (type) S1 (IX) S2 (AIR) S3 (IX) S4 (AIR) S5 (IX) S6 (AIR) S7 (IX) S8 (AIR) a
resin
composition of feed solution (medium)
PAR PAR IDA IDA IDA IDA PAR PAR
Cu:Al ) 0.2 M (SO4 Cu:Al ) 1:1, 0.2 M (SO42-) Cu:Al ) 1:1, 0.2 M (SO42-) Cu:Al ) 1:1,a 0.2 M (SO42-) Cu:Zn ) 1:1,a 0.2 M (SO42-) Cu:Zn ) 1:1,a 0.2 M (SO42-) Cu:Al:Zn ) 1:1:1, 0.3 M (SO42-) Cu:Al:Zn ) 1:1:1, 0.3 M (SO42-) 1:1,a
2-)
composition of stripping solution
purification process
0.1 M and 1.1 M H2SO4 0.2 M DEHDTPA and 1.1 M H2SO4 0.1 and 1.1 M H2SO4 0.2 M DEHPA and 1.1 M H2SO4 0.1 and 1.1 M H2SO4 0.2 M DEHPA and 1.1 M H2SO4 0.1 and 1.1 M H2SO4 0.2 M DEHDTPA, 0.2 M DEHPA, and 1.1 M H2SO4
Al from Cu Al from Cu and Cu from Al Cu from Al Cu from Al Cu from Zn Cu from Zn Al from Cu Cu from Al and Zn; Al from Cu and Zn
The ratio of metal ions is given in moles.
In a previous paper, dedicated to elucidation of the mechanism of mass transfer in AIR systems, we demonstrated the applicability of the AIR concept to separation of metal ions under batch conditions.24 The present paper reports the results obtained by applying the AIR technique for column separation of polyvalent metal ions such as, Cu2+, Zn2+, and Al3+ on iminodiacetic and carboxylic resins using heptane solutions of bis(2-ethylhexyl) phosphoric and bis(2-ethylhexyl) dithiophosphoric acids25,26 as organic eluents. The results obtained in this work represent the first successful application of the CIESE concept for the quantitative separation of metal ions on columns in water-imiscible organic media and fill the gap between the CIESE and AIR concepts. EXPERIMENTAL SECTION Reagents, Ion Exchangers, and Extractants. Copper, zinc, aluminum, and nickel sulfates, and sulfuric and hydrochloric acids (all of p.a. quality) were purchased from Panreac P.A. (Barcelona, Spain) and used as received. Iminodiacetic ion exchanger, Lewatit TP-207, and carboxylic resin Lewatit R-250K, were kindly supplied by Bayer Hispania Industrial, S.A. Prior to use, the resins were washed, purified, and conditioned by using conventional procedures.21 Bis(2-ethylhexyl) phosphoric acid (DEHPA, 99%) was purchased from Carlo Erba R.S. It was purified prior to use as described elsewhere.27 Bis(ethylhexyl) dithiophosphoric acid (DEHDTPA) was synthesized as described elsewhere.28 Further purification of DEHDTPA was carried out according to a previously reported procedure.29 The purity of the final product, determined by potentiometric titration of DEHDTPA samples dissolved in a water-ethanol mixture with 0.05 M NaOH, appeared to be higher than 95%. Doubly distilled water was used in all experiments. Prior to the experiments, aqueous solutions were degassed using an ultrasonic bath (Branson 1200) and a vacuum pump. The pH of the stock solutions of metal ion mixtures was adjusted to 3.0 ( 0.05 with 0.1 M H2SO4 by using a Crison pH meter 507 (Barcelona, Spain), supplied with a combined glass (24) Muraviev, D.; Oleinikova, M.; Valiente, M. Langmuir 1997, 13, 4915. (25) Separation of copper, zinc, and aluminum, known to be the main components of copper alloys such as, brasses and bronzes, is an essential problem in the processing of nonferrous scraps aiming to the recovery of secondary metals.26 (26) Muraviev, D.; Noguerol, J.; Valiente, M. Anal. Chem. 1997, 69, 4234. (27) Kolarik, Z.; Pankova, H. J. Inorg. Nucl. Chem. 1966, 28, 2325. (28) Handley, T. H.; Dean, J. A. Anal. Chem. 1962, 34, 1312. (29) Levin, I. S.; Sergeeva, V. V.; Tarasova, V. A.; Varentsova, V. I.; Rodina, T. F.; Vorsina, I. A.; Kozlova, N. E.; Kogan, B. I. Zh. Neorg. Khim. 1973, 18, 1643 (in Russian).
electrode. Standard precautions recommended for handling sulfuric acid solutions30 were followed when adjusting pH and preparing 0.1 M H2SO4 solution from concentrated acid. The concentration of metal ions was determined by the ICP technique using an ARL model 3410 spectrometer with minitorch (Fisons Instruments, Valencia, CA). The emission lines used for spectrochemical analysis were 224.7 nm for Cu2+, 206.91 nm for Zn2+, and 394.40 nm for Al3+. The relative uncertainty of metal ions determination was 1. The relative uncertainties on P and R determination did not exceed 3 and 7%, respectively. RESULTS The main parameters characterizing the systems studied are collected in Table 1. As seen, the IX and AIR systems (i.e., S1 and S2) differ from each other only by the composition of the stripping solution (eluent) used. This allows one to compare the separation behavior of target ions in IX and AIR systems and to elucidate the main features of the fractionation technique under study. (31) Densities of H2SO4 solutions were taken from CRC Handbook of Chemistry and Physics, 63rd ed.; Weast, R. C., Astle, M. J., Eds.; CRC Press: Boca Raton, FL, 1982-1983. Densities of DEHPA and DEHDTPA heptane solution were considered to be close to that of heptane (0.684 g/cm3).
4868 Analytical Chemistry, Vol. 71, No. 21, November 1, 1999
Figure 1. Concentration-volume histories obtained by elution of Cu2+ and Al3+ in S1 (a) and S2 (b).
Typical concentration-volume histories obtained by the stripping of Cu2+ and Al3+ in S1 and S2 systems are shown in panels a and b of Figure 1, respectively. A comparison of these results reveals a partial separation of metal ions when sulfuric acid solution is used. At the same time, elution with DEHDTPA leads to a selective and quantitative desorption of Cu2+, resulting in the complete removal of this ionic species from the resin phase. The overall process allows one to carry out a selective stripping of copper-free Al3+ with 1.1 M H2SO4 and to obtain both metal components in a highly pure state. Figure 2 shows concentration-volume histories obtained by the stripping of Cu2+ and Al3+ from IDAR in S3 (Figure 2a) and S4 (Figure 2b and c). As seen in Figure 2a, the stripping of metal ions with 0.1 M H2SO4 leads to a significant concentration of copper in the first portions of eluate. Aluminum is also concentrated in the same eluate samples, what causes Al3+ contamination of the most concentrated Cu2+ solution. After complete removal of Al3+, copper is stripped as a pure product; nevertheless the rate of Cu2+ desorption by elution with 0.1 M H2SO4 gradually decreases, leading to a remarkable drop of copper concentration in the solution samples collected. The increase of H2SO4 concentration to 1.1 M results in the stripping of the residual Cu2+ from the resin phase.
Figure 2. Concentration-volume histories obtained by elution of Al3+ and Cu2+ in S3 (a) and S4 (b, c).
As clearly seen in Figure 2b, in AIR system S4, the separation of Cu2+ from Al3+ proceeds far more effectively. Indeed, the complete elution of Al3+ with 0.2 M DEHPA is accompanied by extraction of the trace amounts of Cu2+, while the main part of the purified copper is then stripped with 1.1 M H2SO4. Separation of Cu2+ from Zn2+ in S5 and S6 is shown in Figure 3a and Figure 3b and c, respectively. A comparison of the metal ion behavior in S5 with that in S3 (cf. Figure 2a and Figure 3a) testifies to a remarkable similarity between these two systems. In S5, Zn2+ is accumulated in the head part of the eluate together
Figure 3. Concentration-volume histories obtained by elution of Zn2+ and Cu2+ in S5 (a) and S6 (b, c). Head part b and complete breakthrough curve c obtained in S6.
with Cu2+ in solution samples with the maximum copper content. As seen in Figure 3b and c, separation of Zn2+ from Cu2+ in S6 follows, in fact, the same “scenario” as that of Al3+ from Cu2+ in S4 (see Figure 2b). As the result, a practically pure copper is stripped from the resin with 0.1 M H2SO4 after selective removal of Zn2+ with DEHPA. Figure 4 presents the results obtained by separation of Al3+, 2+ Cu , and Zn2+ in the S7 and S8 systems. As follows from the comparison of Figures 4a and 1a, the efficiency of separation in Analytical Chemistry, Vol. 71, No. 21, November 1, 1999
4869
Table 2. Separation Factors r for Different Metal Ion Couples Determined in S1-S8 Systems
Figure 4. Concentration-volume histories obtained by elution of Zn2+, Al3+, and Cu2+ in S7 (a) and S8 (b).
system S7 does not differ remarkably from that observed in S1. Such an analogy can be attributed to the low sorbability of Zn2+ on PAR in the presence of Al3+ and Cu2+. Nevertheless, as seen in Figure 4a, trace amounts of zinc are eluted with 0.1 M H2SO4 from the resin phase together with copper. Practically no separation of the mixture components is observed in S7. A similar analogy can be made between S8 and S2. Indeed, as seen in Figure 4b, elution of the resin with 0.2 M DEHDTPA solution results in the selective stripping of Cu2+ from the resin phase. It is interesting to note that Zn2+ appears to be also almost completely extracted from the resin with organic eluent. Additional stripping with 0.2 M DEHPA solution gives essentially zinc- and copper-free eluent, which contains only trace amounts of Al3+. The final stripping with 1.1 M H2SO4 solution allows for obtaining a sufficiently pure aluminum solution. DISCUSSION The results shown in Figures 1-4 indicate that the separation of metal ions proceeds far more efficiently in the AIR systems in comparison with the corresponding IX analogues (see Table 1). This difference in the separation efficiency is associated with the different mechanisms of ion separation in AIR and IX systems. 4870 Analytical Chemistry, Vol. 71, No. 21, November 1, 1999
system (type)
resin
ion couple (M1- M2)
M1 RM 2
S1 (IX) S2 (AIR) S3 (IX) S4 (AIR) S5 (IX) S6 (AIR) S7 (IX) S8 (AIR)
PAR PAR IDA IDA IDA IDA PAR PAR
Al-Cu Al-Cu Cu-Al Cu-Al Cu-Zn Cu-Zn Al-Cu Al-Cu
1.72 1.78 51.5 57.8 18.9 19.5 2.33 2.35
Indeed, the stripping of metal ions from the resin with H2SO4 solution in IX systems (see panels a in Figures 1-4) can be qualified as “nonselective” as the stability constants, K, of sulfate complexes of Cu2+, Al3+, and Zn2+ are very close to each other; i.e., respective log K values reported by Izatt et al.32 are 2.26 (CuSO4), 2.49 (ZnSO4), and 3.01 (Al2(SO4)3). Hence, the separation in this case is provided only by the selectivity of the resin. In other words, the separation in conventional IX system proceeds due to the action of “one separation force”. On the other hand, in the AIR systems, the separation of metal ions can be defined as proceeding due to a “couple of separation forces” acting in opposite directions (similar to the action of a couple of forces in physics). This conclusion becomes clearer after comparison of the resin selectivities with those of extractants toward metal ions in the systems under study. The selectivity factors (R) for different metal ion couples determined in both IX and AIR systems are collected in Table 2. For example, as seen in Table 2, in both IX (S1) and AIR (S2) systems, the selectivity of the resin (PAR) is higher toward Al3+ over Cu2+. This means that the former is retained by the resin phase stronger than the latter. This results in the partial separation of Al3+ and Cu2+ by their elution from the resin with H2SO4 in S1 (see Figure 1a), which proceeds by the reverse frontal separation mechanism.26 At the same time, the selectivity of DEHDTPA toward Cu2+ is much higher than that for Al3+ ion.33,34 Indeed, the extraction constant, Kex, for Cu2+ was reported by Levin et al.33 to equal log Kex ) 12.3 while, according to Alimarin et al.,34 Al3+ is nonextractable by DEHDTPA ionic species. Hence, a combined action of two “separation forces”, one of which is determined by a higher selectivity of the resin toward Al3+ (separation force directed to the resin phase) and the other one due to far higher selectivity of extractant for Cu2+ (separation force directed out of the resin), results in the complete separation of Cu2+ and Al3+ in the AIR system S2 (see Figure 1b). The above reasoning can be applied to explain the results obtained in other AIR and IX systems studied. For example, as seen in Table 2, Zn2+-Cu2+ exchange on IDA resin in S5 and S6 is characterized by R ≈ 19, which indicates sufficiently high selectivity of the resin toward Cu2+. On the other hand, the log Kex values of DEHPA for Zn2+ and Cu2+ (extraction from the sulfate media) have been reported by Sastre et al.35,36 to be 9.76 (32) Izatt, R. M.; Eatough, D. J.; Christensen, J. J.; Bartolomew, C. H. J. Chem. Soc. A 1969, 47. (33) Levin, I. S.; Sergeeva, V. V. Izv. Sib. Otd. Acad. Nauk SSSR, Ser. Khim. Nauk 1974, (No. 3), 53 (in Russian). (34) Alimarin, I. P.; Rodionova, T. V.; Ivanov, V. M. Russ. Chem. Rev. 1989, 58, 863.
and 7.55, respectively, which indicates a higher selectivity of the extractant for Zn2+. In other words, the action of the “couple of separation forces” in the AIR system S6 results in effective separation of zinc and copper so that Zn2+ is quantitatively concentrated in the eluate (DEHPA) while Cu2+ is almost quantitatively retained by the resin (IDA). At the same time, a relatively high affinity of DEHPA toward Cu2+ leads to a slight coextraction of this ionic species in the course of zinc elution from the resin phase (see Figure 3b). Nevertheless, the main part of zinc-free copper (∼97%) remains in the resin phase (see below). Note that a similar behavior is observed in the AIR system S4 (see Figure 2b). One paramount feature of separation of metal ions by the AIR systems under study must be emphasized. This feature clearly follows from the comparison of the shape of Cu2+ peaks obtained in elution of this metal ion from PAR with H2SO4 (in S1, see Figure 1a) and DEHPA (in S2, see Figure 1b). Indeed, despite the concentration of both eluents being identical (0.2 equiv/dm3; see Table 1), the width of elution peak for Cu2+ appears to be approximately half in the AIR system S2 in comparison with that in S1, while the height of the peak obtained in S2 differs from that in S1 by a factor of 1.5. This difference in the elution efficiency can be attributed to the specific interaction mechanism of an organic solution containing an extractant with the water-swollen (aqua-impregnated) ion exchanger. This type of ion-exchange mechanism was studied first by Muraviev and Omarova37 and is known as a “contact granulo-micellar exchange”,37,38 which can be considered as a version of a “contact ion exchange” (proceeding between contacting resin beads in different ionic forms) investigated by Nikolaev and Bogatyrev39-41 and later by Bunzl and Schultz.42 The extractants under study (both DEHPA and DEHDTPA) exist in the organic (heptane) solution in the form of reverse micelles (RM)38,43 (hydrophilic >PSSH functional groups facing toward the hydration nuclei) as their concentration (0.2 mol/dm3) substantially exceeds the critical micellization concentration (cmc). Hence, the interaction between metal bearing resin and the extractant can only occur through collisions between the resin beads and the extractant RM. The collisions must be accompanied by the “partial opening” of RM when approaching the hydrophilic surface of the resin bead as this is, in fact, the necessary condition for such interaction to proceed. A schematic diagram of the contact granulo-micellar exchange in an AIR system is shown in Figure 5. As seen, the interaction of the extractant RM with resin beads can be considered to proceed through the following steps: (1) diffusion of RM to the surface of the resin bead, (2) opening the RM at the bead surface followed by interaction of extractant with metal ions (denoted by M2+ in Figure 5), and (3) closing the RM (35) Sastre, A.; Muhammed, M. Hydrometallurgy 1984, 12, 177. (36) Sastre, A.; Muralles, N.; Aguilar, M. Chem. Scr. 1984, 24, 44. (37) Muraviev, D.; Omarova, F. In Applications of Ion-Exchange Materials in Industry and Analytical Chemistry; Proceedings of the National Conference; VGU, Voronezh, Russia, 1986; Vol. 1, p 21 (in Russian). (38) Muraviev, D. Solv. Extr. Ion Exch. 1998, 16, 381. (39) Nikolaev, A. V.; Bogatyrev, V. L.; Zhurko, F. V. Dokl. AN SSSR 1971, 200, 886 (in Russian). (40) Tobolov, A. A.; Bogatyrev, V. L.; Zhurko, F. V. Izv. Sib. Otd. AN SSSR, Ser. Khim. Nauk 1976, 12, 20 (in Russian). (41) Tobolov, A. A.; Bogatyrev, V. L. Izv. Sib. Otd. AN SSSR, Ser. Khim. Nauk 1976, 12, 14 (in Russian). (42) Bunzl, K.; Schultz, W.J. Inorg. Nucl. Chem. 1981, 43, 791. (43) Friedrichsberg, D. A. Colloid Chemistry, 2nd ed.; Khimia: Leningrad, 1984; p 325 (in Russian).
Figure 5. Schematic diagram of contact granulo-micellar exchange in AIR systems (see text).
bearing captured metal ions followed by their diffusion from the bead surface. According to the mechanism presented in Figure 5, metal ions desorbed from the resin by the extractant appear to be “entrapped” by the RM. The interaction between extractant RM in nonpolar solvent is negligibly weaker in comparison with the interionic interactions in aqueous solutions.43 Hence, the dispersion of peaks in the course of elution of stripped metals along the column in AIR systems appears to be far less than in their IX analogues, what results in the formation of sharper peaks in the first case in comparison with the second. It seems quite obvious that this feature of AIR systems makes this separation mode of great interest for application in HPLC and we intend to continue our investigations in this direction. The higher efficiency of separation in AIR systems can be understood through the consideration of the ion-exchange equilibria characterizing the redistribution of metal ions between the resin and extractant phases. Let us consider for this purpose a hypothetical triphase system composed with an aqueous solution (AS) containing a couple of divalent metal ions M12+ and M22+, an ion-exchange resin, IXR (e.g., IDA), and an organic solution of extractant (OSE). The equilibrium in this system can be described by the following ion-exchange reactions associated with the corresponding R values (see eq 1):24
(1) IXR-AS equilibrium IXR-M2 + M12+ S IXR-M1 + M22+ RIXR )
[M12+]IXR [M22+]AS [M22+]IXR [M12+]AS
Analytical Chemistry, Vol. 71, No. 21, November 1, 1999
(3) (4) 4871
(2) OSE-AS equilibrium OSE-M2 + M12+ S OSE-M1 + M22+ ROSE )
[M12+]OSE [M22+]AS [M22+]OSE [M12+]AS
(5) (6)
(3) IXR-OSE equilibrium OSE-M2 + IXR-M1 S OSE-M1 + IXR-M2 ROSE IXR )
[M12+]OSE [M22+]IXR [M22+]OSE [M12+]IXR
(7) (8)
Here and above IXR, OSE, and AS subscripts denote the IX resin, organic solution of extractant, and aqueous solution, respectively. The aqueous solution in contact with IXR and OSE is the same; therefore eq 7 can be obtained by subtracting eq 3 from eq 5. Hence, the aqueous phase can be withdrawn from the triphase system under consideration and it appears to be reduced to the respective AIR (IXR-OSE) one. Then eq 8 can be rewritten in the following form:24
RAIR ) ROSE/RIXR
Figure 6. Purity vs amount of copper stripped in S4 (1) and S3 (2) systems.
(9)
Relationship 9 is the fundamental equation describing the shift of IX equilibrium in AIR systems of different types to the desired direction in comparison with respective IX equilibria in a single IXR-AS or OSE-AS system. Indeed, as seen from eq 9, the necessary condition for RAIR > ROSE is RIXR < 1. For example, ROSE ) RDEHPA for Cu2+-Zn2+ exchange (RZn Cu) can be estimated from the above Kex values as follows: RZn (OSE) ) Kex(Zn)/Kex(Cu) ) Cu 162. On the other hand, as seen in Table 2, for IDA resin Cu 2+ 2+ exchange RZn Cu(IXR) ) 1/RZn ) 0.053. Hence, RAIR of Cu -Zn in S6 (see Table 1) can be estimated to be 3057. The same eq 9 can also be applied to preselect the conditions for the most efficient separation of a given metal ion mixture by using the AIR technique. This preselection is based on the literature data on IX and extraction equilibrium, from which one can estimate the efficiency of separation (RAIR) through the use of respective R and Kex values. The completeness of desorption of metal ions in both AIR and IX systems under study can be estimated from the data tabulated in Table 2, where R values for different ion couples calculated from the results of the stripping solution analyses in the AIR and IX systems studied are shown. As seen in Table 2, R values for the same ion couples determined in systems of both types correlate with each other within the corridor of uncertainties on their determination. This correlation testifies to the complete stripping of all ionic species in all experiments carried out. The efficiency of purification of metals in all AIR systems appears to be far higher than in that respective IX ones (see Figures 1b, 2b, 3b, and 3c). To quantify this conclusion, we reproduce the concentration-volume histories presented in Figures 2-4 in terms of the purity vs amount of the stripped metal dependencies, which are shown in Figures 6-8. The purities of copper (see Figures 6 and 7) and aluminum (see Figure 8), obtained in S3-S8 systems, are plotted against the amount of metals desorbed. As seen in Figures 6 and 7, the stripping of Cu 4872 Analytical Chemistry, Vol. 71, No. 21, November 1, 1999
Figure 7. Purity vs amount of copper stripped in S6 (1) and S5 (2) systems.
in AIR systems S3 and S5 allows for yielding copper of higher purity than that obtained in the IX systems S4 and S6. The same conclusion follows from the results presented in Figure 8, where the purity of aluminum obtained in systems S7 and S8 is shown. The highly selective quantitative extraction of metal ions in the AIR systems also results in much higher yields of the pure products obtained. This is clearly seen from the results collected in Table 3, where the initial, maximum, and average purities of the metals purified and the yields of >95% purity products are given. As follows from the data presented in Table 3, the yields and the purity of metals purified in the AIR systems are essentially higher than those obtained under identical conditions in the corresponding IX systems. The meaning of the results obtained must be also discussed from the practical viewpoint. Combination of ion exchange and solvent extraction in one separation process may be of particular importance, for example, in processing of industrial wastewaters and effluents containing nonferrous and heavy metal ions. Ion exchange is known to be highly effective for removal of all ionic contaminants from diluted solutions (in the countercurrent mode of operation, in particular) and can be applied at the first stage of
Figure 8. Purity vs amount of aluminum stripped in S8 (1) and S7 (2) systems. Table 3. Results Obtained by Separation of Metal Ions in S1-S8 Systems system (type)
product (initial purity, %)
yield of product (%) with final purity >95%
av final purity obtained (%)
S1 (IX) S2 (AIR)
Al (29.8) Al (29.8) Cu (70.2) Cu (70.2) Cu (70.2) Cu (49.3) Cu (49.3) Al (17.3) Al (17.3) Cu (40.8)
0 98.5 96 87.4 98 72 97 55.4 98.7 96
68 99.2 98.6 98 98.8 95.2 97.2 77 98.1 97.3
S3 (IX) S4 (AIR) S5 (IX) S6 (AIR) S7 (IX) S8 (AIR)
the process for the concentration of all metal ions in the resin phase. The main limitation of ion exchange as a separation tool deals (at least in nowadays) with the limited number of commercially available ion exchangers characterized by sufficiently high selectivity toward certain ionic species such as, for example, single heavy metal ions. On the other hand, the spectrum of (44) Ho ¨gfeldt, E. In Ion Exchangers; Dorfner, K., Ed.; Walter de Gruyter: Berlin, 1991; Chapter 1.9. (45) Nakagawa, T. In Membrane Science and Technology; Osada, Y., Nakagawa, T., Eds.; Marcel Dekker: New York, 1992; p 239.
commercially available highly selective extractants is far wider; therefore, the separation power of the CIESE fractionation method must be much higher than that of a single ion exchange or solvent extraction. Extraction techniques are known to be preferentially used for the treatment of relatively concentrated solutions.44 Hence, selective extraction of metal ions (after their concentration in the ion exchanger) directly from the resin phase (AIR concept) may be considered as one of the routes for combining ion exchange and solvent extraction in one process. Such a combination allows for recovering the most valuable and/or toxic constituents from the resin phase with highly selective organic extractants followed by the stripping of the residual (and also purified) metals with acid solutions. Finally, in the large-scale applications of the AIR technique, the problems dealing with the use of organic eluents and solvents can be minimized (1) by using environmentally safe solvents (e.g., ethanol and heptane) and (2) by recycling of solvents and extractants. The last problem can be easily solved by reuse of the metal-free extractants obtained after stripping of metal ions from the eluate collected. The water-ethanol and ethanolheptane mixtures obtained after rinsing of the resin phase from water and ethanol, respectively, can be collected separately followed by quantitative recuperation of organic components (ethanol and heptane) by using a membrane pervaporation technique.45 Acidic stripping solutions can also be reused following quantitative recovery of pure metals by electriwinning methods. Hence, an essentially wasteless AIR-based separation process can be easily designed. ACKNOWLEDGMENT This work was supported by Research Grant QUI-96-1025-C0301 from the Spanish Commission for Research and Development. Bayer Hispania Industrial, S.A. is gratefully acknowledged for kindly supplying samples of Lewatit resins. M.O. is the recipient of a fellowship from CIRIT (Commission for Science and Technology of Catalonia). The Catalonian Government is acknowledged with thanks for the financial support of D.M. during his Visiting Professorship in the Autonomous University of Barcelona. Received for review March 1, 1999. Accepted July 21, 1999. AC9902345
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