Fundamental Aspects of Metal-Ion Separations by Centrifugal Partition

Chapter 22

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Fundamental Aspects of Metal-Ion Separations by Centrifugal Partition Chromatography Subramaniam Muralidharan and Henry Freiser Strategic Metals Recovery Research Facility, Department of Chemistry, University of Arizona, Tucson, AZ 85721

Centrifugal partition chromatography (CPC), a multistage countercurrent liquid-liquid distribution technique employing discrete stages and two immiscible bulk liquid phases, is ideally suited for the detailed examination, through evaluation of separation factors and efficiencies, of the influence of bulk aqueous and liquid-liquid interfacial equilibria and kinetics on the separations of metal ions. This has been demonstrated by studies of the separation of transition metals, platinum group metals, and trivalent lanthanides. The results indicate that separation efficiencies in CPC are mainly limited by back-extraction kinetics that occur in the bulk aqueous phase and at the organic-aqueous interface as indicated by a direct linear correlation between the half-lives (t 's) of the dissociation reactions and the reduced plate height. In addition, the interfacial areas calculated through this correlation are much larger in many cases than those generated in highly stirred two phase mixtures. Finally, addition of surfactants and interfacial catalysis of the formation and dissociation of the complexes dramatically improve efficiencies. 1/2

Solvent extraction is a particularly appropriate technique for difficult metals separations problems since it incorporates selectivity, versatility, and convenience. It is a powerful separation technique applicable both to trace analytical and macro- or process- scales (1). Because of their great selectivity, solvent extraction techniques can be used to recover metals from multicomponent solutions, which makes them ideal for environmental remediation. Extractants that incorporate chelating functionalities are employed to achieve the highest possible selectivity. Much research has been devoted to the

©1999 American Chemical Society

Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

347


348

elucidation of the many factors that affect metal chelate stability and extractability. Conversely, equilibrium and kinetic studies of metal extraction systems are key to the more complete understanding of separations involving chelation. Similarly, the path to full understanding of multistage separations such as countercurrent distribution and liquid partition chromatography must be, of necessity, based on thoroughly characterized singlestage solvent extraction processes. The separation of metal ions, especially closely related ones such as the trivalent lanthanides, by single stage methods poses daunting challenges even to the most selective of extractants (2). Therefore, the use of multistage methods is necessary for their separation. Multistage methods consist of solid-liquid and liquid-liquid partition, with the former involving solid supports such as silica and cross-linked organic polymers, derivatized, coated, or impregnated with a ligand which serves to separate metal ions by complexation in a conventional liquid chromatographic mode. The latter involves two bulk liquid phases with the extractant dissolved in the organic phase (3). Centrifugal partition chromatography (CPC), which is a liquid-liquid separation technique, is the only multistage technique with discrete stages among the various solid-liquid and liquid-liquid techniques (4). Because of the use of discrete stages and two bulk liquid phases with well-defined volumes, CPC is ideally suited for studying the fundamental factors that influence metal ion separations. CPC, being a multistage technique, is far more sensitive to equilibrium and kinetic phenomena in the bulk aqueous phase and at the organic-aqueous interface than single stage techniques and thus can provide more accurate and detailed information than can batch experiments. In addition, CPC is an excellent model for understanding separations with solid-liquid techniques, since it does not suffer from many of the difficulties associated with solid supports such as diffusion into pores, irreversible adsorption, and so on. We were one of thefirstto adapt CPC, which was originally developed for the separation of organic compounds and biochemicals, to metals separations (5). During the past several years, we have examined the CPC separation of several families of metals (the transition metals, platinum group metals, and the trivalent lanthanides) using a variety of chelating extractants (acylpyrazolones, organophosphorous acids, arylhydroxyoximes, etc.) to discern the influence of bulk and interfacial kinetics, interfacial activities of the ligands and their metal chelates, and interfacial areas generated on the separation efficiencies and resolution of closely related metal ions. A correlation between the half-lives of the slow chemical kinetic steps, and the interfacial areas generated and the inefficiencies of separations have been drawn from these studies. Moreover, the significance and influence of the liquid-liquid interface in multistage separations of metal ions has been illustrated. Centrifugal Partition Chromatography The CPC apparatus (manufactured by Sanki Engineering Company, Japan (4)) consists of a series of cartridges, each of which contains 40 - 400 channels, depending upon the internal volume. These channels serve as stages in the separation experiment. The total number of channels is 400 - 4800, depending upon the number of cartridges employed. These cartridges are arranged in a rotor that is rotated at 700 - 1200 rpm. The centrifugal force generated keeps one of the two phases (usually the organic phase) stationary while the other phase (usually the aqueous phase) is moved through Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


349 it at a constant flow rate. The injected analyte mixture is carried by the aqueous mobile phase into the cartridges where the mixture components are extracted into the organic stationary phase by simple distribution if they are organic, or by complexation with a suitable ligand if they are metals. When the mobile phase is depleted of the analytes, further flow of the mobile phase of the same (isocratic elution) or different (gradient elution) composition causes the back-extraction of the analytes, which can be detected by a suitable method. If the analytes are completely separated, they appear as discrete peaks such as those observed in conventional chromatographic methods like HPLC (hence the name centrifugal partition chromatography). CPC has a number of unique features, among them a large number of discrete stages (400 - 4800 depending upon the operational volume chosen), high loading capacity for extractants and analytes, negligible loss of stationary phase due to bleeding, flexible organic-aqueous phase volume ratios, a high stationary phase to mobile phase ratio, and ready adaptability to process-scale. Four basic parameters are employed in the analysis of CPC chromatograms: the retention volume, V , which is related to the stationary phase and mobile phase volumes (V and V , respectively) and the distribution ratio of the analyte, D , through equation 1; the chromatographic efficiency, as measured by the number of theoretical plates, N, which is calculated from the retention volume (V ) and the width of the chromatogram (w), as per equation 2; the chromatographic inefficiency, represented by the channel equivalent of a theoretical plate (CETP), which is analogous to reduced plate height and is the ratio of the number of channels, CH (2400 in our experiments), to N, according to equation 3; and the selectivity, a, achieved in the separation of two analytes (designated 1 and 2), which is the ratio of their distribution ratios, D! and D , and is given by equation 4 (6). r

s

m

r

2

V = V m + DV

(1)

V Ν = 16 (—) w

(2)

CETP

2

CH obs

Ν

a=


(3)

(4)

350


Optimization of the CPC Operational Parameters A major difference between CPC and conventional LC is that in the former, even at the smallest attainable ratio of the volume of the stationary phase to the volume of the mobile phase, there are still two bulk phases. Thus, optimizing conditions such as the phase volume ratio, rotational speed, flow rate of the mobile phase, and nature of the stationary and mobile phases, (i.e., organic or aqueous) are important in obtaining the best efficiencies. This is especially true in metals separations, where the CPC bandwidth is determined by the rates of mass transfer and diffusion and by slow chemical kinetics. It is necessary to minimize the contribution to CETPfrommass transfer and diffusion in order to properly elucidate the kinetic factors. The influence of flow rate of the mobile phase, the ratio of the volume of the stationary phase to the volume of the mobile phase (V/V™), and the nature of the stationary phase (organic or aqueous), on the CPC efficiencies have been determined using three organic-aqueous phase pairs (1,2-dichloroethane, toluene, and heptane) with three analytes of different types: 3-picoline (organic), (tetraheptylammonium) IrCl (ion pair), and PdCl (TOPO) (coordination complex; TOPO = trioctylphosphine oxide). The results are summarized below and those for the l,2-dichloroethane-H 0 system are displayed in Figure 1. The efficiencies and hence, CETP values of 3-picoline and Q IrCl (Q = tetraheptylammonium) were very similar at different flow rates and phase volume ratios for the various solvent pairs examined. This strongly suggests that such CETP values represent the contribution of diffusion and mass transfer effects and, therefore, can be termed CETP^f for the metal complexes and ion pairs. The CETP values of PdCl (TOPO) were significantly higher than the values for 3-picoline and Q IrCl . The efficiencies decreased with increasing flow rate for all the analytes, with PdCl (TOPO) exhibiting a much larger decrease compared to the other analytes. This suggests an interfacial contribution to the back-extraction kinetics, as the mobile phase droplet size increases and hence interfacial area decreases with increasing flow rate. Support for this interpretation comesfromthe CPC behavior of the Ni(II)-HPMBP (HPMBP = l-phenyl-3-methyl-4-benzoyl-5-pyrazolone) and the Ni(II)-LIX 860 (LIX 860 = dodecylsalicylaldoxime) systems, which exhibit bulk aqueous and interfacial kinetics, and the trivalent lanthanide-HPMBP and HPMCP (HPMCP = l-phenyl-3methyl-4-capryloyl-5-pyrazolone) systems, which will be discussed in the following sections. The efficiency improved dramatically below V /V of one for all analytes and solvent pairs studied. The best efficiencies were obtained below a V / V of 0.5, with V / V of 0.3 or less being optimum. This is shown in the upper plot of Figure 1 for the three analytes with 1,2-dichloroethane as the stationary phase. The efficiencies when the aqueous phase rather than the organic phase was held stationary were generally much lower (lower plot of Figure 1), by as much as a factor of three. At V / V ratios below 0.5, the efficiencies for the three analytes at a given flow rate of 2

6

2

2

2

2

2

2

2

2

2

s

m

m

s

m

s

m

6


6


351

Vs/Vm

•

N.lr(IV)

ο

CETP.Ir(IV)

x

N.Picoline

E

CETP.Picoline

*

N,Pd(II)

A

CETP,Pd(I!)

Figure 1. Efficiency and CETP as a function of V / V for the analytes 3-picoline, Q IrCl (Q=tetraheptylammonium), and PdCl (TOPO) in the 1,2-dichloroethaneH 0 phase pair at a flow rate of 2 mL/min, upper: 1,2-dichloroethane as stationary phase, lower: H 0 as the stationary phase. s

2

6

2

m

2

2

2


352 aqueous mobile phase decreased in the order 1,2-dichloroethane > heptane > toluene.


Separation of Platinum Group Metals Separation, extraction, and purification of the platinum group metals (PGM) Pt, Pd, Ir, and Rh in their various oxidation states continue to be challenging and interesting areas of research (1-4). Several techniques have been investigated to recover these precious metals, among them solvent extraction (1-4), ion exchange (5,6), membrane absorption (7- 9), and biomagnetic separation (10,11). The separation of PGM from chloride media by solvent extraction can be achieved either by complexation with a suitable ligand or through ion pair formation with a large cation. Complexation with a ligand is more selective but generally suffers from slow complex formation and dissociation kinetics. By contrast, ion pair formation is diffusion-controlled and not very selective, but is necessary to separate kinetically inert species such as PtCl " and IrCl -. Trioctylphosphine oxide is an organophosphorus compound and is a stable and inexpensive extractant. TOPO, as we have shown {12-14), is unique in that it can function as a monodentate ligand and as a cation for ion pair extraction when protonated. Our initial work focused on the separation by CPC of Pd(II) and Pt(II) from Rh(III) and Ir(III) with TOPO as ligand in the heptane-water solvent pair. We elucidated the extraction equilibria involved and showed PdCl (T0P0) to be the extracted species at [HC1] < 10" M. A very important practical aspect of our work is that these separations were performed under relatively mild conditions in contrast to traditional methods which often involve harsh conditions such as high acidity. The CPC chromatogram for the separation of Pd(II) and Pt(II) with TOPO in a heptane stationary phase, with V /V = 0.2 and at a mobile phase pH of 3 and at 0.08 M chloride concentration, is shown in Figure 2. The separation was performed as a function of the concentrations of TOPO, CI", and pH. Only the neutral complex 2

6

2

6

2

2

3

s

m

K " + 2TOPO ^ MCl (TOPO) ex

MCl

(n_2) n

o

2

2o

+ (n-2) CI"

(5)

MCl (TOPO) (M = Pd(II) Pt(TI)), was extracted (equation 5 where η = 2 - 4). The values for PdCl , PdCl ", and PdCl " are 794.3 M ' , 2.75 M , and 0.14 respectively. A single peak was observed in the CPC chromatogram of Pd(II) at any concentration of CI" as its hydrolytic equilibria are rapid. The corresponding values for the three Pt(II) chloro species are 48 M" , 0.047 M and 0.018, clearly indicating the better extractability of Pd(II) over Pt(II), also evidentfromFigure 2. The difference in the Kç values for the Mcl " species can be exploited to obtain an efficient separation of Pt(II) and Pd(II) from Rh(III) and Ir(III). This was achieved using stepwise CI" 2

2

5

2

2

3

2

1

4

2

1

2

x4

4


353 gradients of 0.001, 0.08, and 0.5 M in the mobile phase, Figure 3. Formation of HTOPO , at HC1 concentrations ^ 0.1 M , resulted in the extraction of (HTOPO) MCl (M = Pt or Pd): +

2

4

Κ ex 2

MCI "

+

+

2 H + 2TOPO

Q

^

(HTOPO) MCl 2

(6)

4 o


3

2

2

+

The chromatogram of the separation of RhCl \ PdCl ", and PtCl " by HTOPO is shown in Figure 4. The K„ values of Pd(n) and Pt(H) are 93.3 M" and 1961 M" , respectively, indicating that Pd(II) elutes ahead of Pt(El) in the ion pair separation while the opposite is true in the separation by complexation. While the chromatogram of Pt(II) involves only the formation of (HTOPO) PtCl , the chromatogram of Pd(II) also involves the formation of (HTOPO)PdCl . In fact, under the experimental conditions employed in these separations, this is the major Pd ion pair that is extracted. The extraction equilibrium constant for (HTOPO)PdCl is 18.25 M . Similarly, Pt(IV) and Ir(IV) could be separated by HTOPO by ion pair formation with their MC1 " species. The values for the Pt(IV) and Ir(IV) species are 1576 Mr and 8035 M' , respectively. The concentrations of CI" and H can be simultaneously varied to separate PtCl ", PdCl ", and PtCl " by gradient elution, and such a separation is shown in Figure 5. Here the extraction of Pt(II) occurs by formation of PtCl (TOPO) , that of Pt(IV) by formation of (HTOPO) PtCl , and that of Pd(Q) by the formation of PdCl (TOPO) , (HTOPO)PdCl , and (HTOPO) PdCl . The ion pair extraction of the chloro anions of PGM by QpTS (pTS = ptoluenesulfonate) in a 1,2-dichloroethane stationary phase also afforded the separation of these species under relatively mild conditions. The overall extraction equilibrium for the MC1 ' species is given in equation 7. 6

4

4

4

2

4

4

3

1

3

+

2

6

4

4

+

2

2

4

2

4

6

2

2

2

2

2

6

3

2

4

2

6

Κ 2

MCI " +

2QpTS

o

^

Q MCl 2

g o

+

2pTS "

(7)

The values for Pt(IV) and Ir(IV) are 3890 and 7760, respectively, and despite the similarity of these values, baseline separation of these ions can be obtained. The CPC behavior of both Pt(II) and Pd(II) are dependent on [CI] in the mobile phase. In the absence of any added NaCl (C = 0.0032 M) in the mobile phase, Pt(II) exhibited two peaks and Pd(II) exhibited a broad peak. Addition of 0.1 M NaCl to the mobile phase resulted in three peaks for Pt(II) (Figure 6), and a narrower single peak for Pd(II). The difference in the behavior of Pt(II) and Pd(II) stemfromthe more sluggish interconversion of the various MCl / " species when M = Pt. The interconversion of the chloro species is rapid (relative to the time scale of HC1

i2)

2+


354

0LO9 αοβ

ω υ

αο6

RBΑΝ


αο7

0.05

0

0.04 0.03 0.020.01 0

50

100

150

200

250

350

300

RETENTION VOLUME (mL) Figure 2. Separation of Rh(III), Ir(III), Pt(II), and Pd(II) using 0.5 M TOPO in heptane, 0.08 M Cl", V / V = 0.2, and flow rate 0.87 mL/min. (a) 5 χ ΙΟ" M Rh(III) and Ir(III); (b) ΙΟ" M Pt(II); (c) 10" M Pd(II). 4

s

m

4

3

50

100

150

200

250

300

RETENTION VOLUME (mL) 2

2

2

2

Figure 3. Separation of PdCl "and PtCl from IrCl ' and RhCl - by stepwise CI" gradient in the aqueous mobile phase with 0.5 M TOPO, 10" M HC1, and 2.0 mL/min flow rate. Eluting species are (a) 10" M IrCl " and RhCl " (no added CI"), (b) 10" M PtCl " (0.08 M NaCl), and (c) 10" M PdCl " (0.5 M NaCl). 4

4

6

6

3

3

2

2

6

4

2

4

6

4

2

4


355 a


b

Ο

100

200

300

400

RETENTION V O L U M E (mL) -4

2

2

3

2

Figure 4. Separation of 10 M IrCl ", PtCl ", and 10" M PdCl " as their ion pairs with HTOPO as a fonction of [Cl] with 0.5 M TOPO at 0.1 M HC1 and 4.0 mL/min flow rate. Eluting species are (a) IrCl ", (b) PdCl ", and (c) PtCl ". 6

4

4

+

2

2

6

2

4

4

0.12-

R E T E N T I O N V O L U M E (mL) 3

2

2

2

+

Figure 5. Separation of 10" M PtCl ", PdCl ", and PtCl " using H and Cl" gradients in the mobile phase with 0.5 M TOPO and mobile phase flow rate of 4.0 mL/min. The eluting peaks and gradient conditions are: (a) PtCl ", 0.01 M HC1, 0.5 M NaCl; (b) PdCl \ 0.1 M HC1, 0.5 M NaCl; (c) PtCl ", 0.1 M HC1, 0.8 M NaCl. 4

4

6

2

4

2

4

2

6


356 the experiment) for Pd(II), resulting in a broad peak at low [CI] where all the species tend to be present, and a narrower peak at high [CI] where PdCl " is the predominant species. These processes are slow in the case of Pt(H), resulting in the multiple peaks. 2


4

Separation Efficiencies of Platinum Group Metals. We observed early on that CETP values are significantly larger for metal ion separations than those for simple organic analytes under the same conditions (Figure 1). They are far larger than could be explained in terms of mass transfer and diffusion factors. Moreover, they increase more rapidly with increasing flow rate than those of organic analytes, indicating a chemical kinetic component affecting the CETP. The CETP values observed with metal ions, after correction for mass transport and diffusion (achieved using an organic analyte with similar distribution characteristics), reflect the half lives of chemical reactions causing the added inefficiencies. Metal complex formation and dissociation reactions with half -lives of milliseconds, that is, rapid enough that in batch experiments they reach equilibrium "instantaneously", will lower the efficiencies of CPC chromatograms. Conversely, CETP values can be used to study rapid reaction kinetics if this relationship is found to be generally valid. Thus CPC is a useful tool not only for uncovering kinetics of metals separations but also for obtaining detailed mechanisms of those reactions responsible for inefficiencies in multistage metals separations. This demonstrates the utility of CPC for examining the kinetics of metal complex formation and dissociation reactions in two-phase systems that are too rapid for the automated membrane extraction system (AMES). It was evident from the separations of PGM that their experimental CETP values were much larger compared to that for an organic analyte at identical distribution ratios. This is illustrated in Figure 7 where the CPC chromatograms of 3picoline and PdCl (TOPO) at the same D values are shown. These results indicated that factors other than mass transfer and diffusion were responsible for the additional bandwidths in the case of the metal ions. The most likely factor is the slow kinetics of back-extraction of the metal ions, as the forward extraction reactions are usually rapid. To test this hypothesis, 3-picoline was used as the model compound for the determination of the CPC bandwidth due to mass transfer and diffusion (CETP^), and the CETP value due to slow chemical kinetics (CETP ) was derived by expressing the experimental CETP (CETP ) as a sum of CETP^ and CETP , equation 8. 2

2

ck

obs

CETP . obs

ck

= CETP„., + C E T P dif

u

(8)

ck

v°/

The CETP values determined by varying the concentrations of the species in the aqueous and organic phases clearly showed that the slow back-extraction kinetics of the metal complexes were indeed responsible for the broad bands in the CPC chromatograms. On the basis of these results, a mechanism of the dissociation step could be deduced. For example, the mechanism of dissociation of PdCl (TOPO) and ck

2


2


357

1

1

0

1

1

100

1

1

200

1

Γ

300

RETENTION VOLUME (mL) Figure 6. The separation of different chloro species of Pt(II) at 0.1 M NaCl with 0.002 M QpTS (QpTS = tetraheptylammonium p-toluenesulfonate) in 1,2 dichloroethane-H 0 phase pair, V / V = 1, and aqueous mobile phase flow rate = 2 mL/min. 2

s

m

0.5-

o-i

0

,

,

50

,

1

,

1

100 150 200 250 300 R E T E N T I O N V O L U M E (mL)

j

i

350

400

Figure 7. Chromatograms of 3-picoline and PdCl (TOPO) at identical distribution ratios in the heptane-H 0 phase pair; V / V = 0.2, flow rate = 2 mL/min. (a) 10" M Pd(II), 0.3 M TOPO, 0.1 M Cl", pH = 3, (b) 3-picoline, pH =6.1, (c) dead volume peak. 2

2

3

2

s

m


358 2+

2+

PtCl (TOPO) can be shown by equations 9 -11 (M = Pd or Pt ), where equation 10 is the rate-limiting step (5). 2

2

K MCl (TOPO) 2

;

MCl (TOPO) + TOPO

2

(9)

2


fast

MCl (TOPO) + CI"

-

2

MC1 " + T O P O

(10)

3

slow

M C I " + CI "

MCI fast

(11)

2

k .K^Cl"] b

k

obs

Κ

=

1

k

b

(12)

+ [TOPO]

K

2

'

_

E

(13)

1

[TOPO]

The pseudo-first-order dissociation rate constant is given by equation 12, which has two limiting cases: K ' » [TOPO] and K ' « [TOPO], and the latter condition leads to equation 13. This was independently verified by studying the dissociation of MCl (TOPO) in Triton X-100 micelles (as a model for the two phase system) using the stopped flow technique, as this reaction is too fast for conventional spectrophotometric kinetic measurements. As evidentfromequation 12, the k_ K', k_ and K ' values can be obtainedfromthe k values and for Pt and Pd these are: k. K' = 50.24 ± 2.2 s" (Pt ), 0.67 ± 0.02 s (Pd ); k. = 91.52 ± 4.8 M ' V (Pt ), 168 ± 8 M ' V (Pd ); and K ' = 0.55 ± 0.004 M (Pt ), 0.004 ± 0.0002 M (Pd ). According to equation 13, the t (=0.693/k ) for the dissociation reaction, equations 9-11, should be proportional to [TOPO] and 1/[C1'] which was determined to be the case for CETP from CPC experiments and t from the stopped flow experiments. The major difference between the PdCl (TOPO) and PtCl (TOPO) dissociation kinetics lies in the preequilibrium step, which involves the dissociation of a TOPO molecule. This equilibrium constant is about two orders of magnitude larger for PtCl (TOPO) compared to PdCl (TOPO) , resulting in higher dissociation 2

2

2

obs

2+

2

2+

b

1

2+

A

2+

1

2

2+

2

1

2+

2+

2+

obs

1/2

b

ck

1/2

2

2

2

2

2

2


2

2

359 rate constants and CPC efficiencies for the former. A plot of the CETP values for PtCl (TOPO) against the t values yields a straight line. Further, these points and those for the Pd(IT) system fall on a single line indicating a general correlation for the separation of these two metals using TOPO in the heptane-H 0 phase pair. While the experimental conditions employed in CPC provide the mechanistic and kinetic information on the dissociation of MCl (TOPO) (M = Pt(II), Pd(II)) complexes, we can also derive information on the formation reaction using the principle of microscopic reversibility (

Fundamental Aspects of Metal-Ion Separations by Centrifugal Partition

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