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Langmuir 1992,8, 366-368
An Attenuated Total Internal Reflection Spectroscopy Study of the Kinetics of Metal Ion Extraction at the Decane-Aqueous Solution Interface Jilska M. Perera, Jennifer K. McCulloch, Brent S. Murray, Franz Grieser, and Geoffrey W. Stevens* Department of Chemical Engineering and School of Chemistry, The University of Melbourne, Parkville, Victoria 3052,Australia Received September 23, 1991. I n Final Form: November 18, 1991 UV-visible attenuated total internal reflection spectroscopy at a free liquid interface has been used to investigate the kinetics of metal ion extraction from an aqueous phase into a n-decane phase. Kinetic data on the Co(II)/Ionquest 801 (an organophosphate) system as a function of pH, ligand concentration, and metal ion concentration is presented.
Introduction Solvent extraction is a process for separating components in solution.lp2 It relies on unequal partitioning of various solutes between two immiscible phases. For the case where the solute to be extracted is a metal ion in an aqueous solution of other metal ions, such as occurs in the hydrometallurgical industry, the extraction is done into an organic phase, typically kerosene, containing a ligand that selectively complexes with the particular metal ion of interest. In this process the metal ion reacts with ligands which are oil soluble and forms a complex which is also oil soluble. This reaction occurs at the interface between the oil and aqueous solution and is the limiting step in many industrial processes, yet there is very little information available at the molecular level relating to the dynamic events occurring at the liquid-liquid i n t e r f a ~ e . ~ This paper describes a new technique for obtaining information about the metal-ligand reaction. The technique is based on using UV-visible attenuated total internal reflection (ATR) spectroscopy at free liquid interfaces. The kinetics of extracting Co2+ from water using an organophosphate ligand is described and the virtues of this novel ATR technique are discussed. Experimental Details Chemicals. ACS reagent grade cobalt chloride hexahydrate and AR grade sodium chloride, hydrochloric acid, and sodium hydroxide were used as received. Sigma-Aldrich solvent n-decane (>99% pure) and 2-ethylhexylphosphonic acid mono-2ethylhexyl ester (HEHEHP), marketed as Ionquest 801 (97 w t %), from Albright and Wilson, Australia, were used as supplied. Apparatus. A schematic diagram of the ATR cell and light passage through the cell to the detector is shown in Figure 1.The sourcelight from an Oriel 150-Wxenon arc lamp is passed through a 10 cm path length water filter (to remove IR radiation) down a in. diameter optical fiber bundle (Dolan-Jenner),fitted with a beam collimator and into the ATR cell. The light is then reflected off the water-oil interface, a number of times, and fed into a second optical fiber bundle to an Oriel 1/4-m monochromator. The output from the monochromator is spread onto an Oriel Instaspec I1 diode array detector. The absorbance information is processed using an IBM compatible 386 PC with a DT-2801A interface board in conjunction with Instaspec V1.33 software.
* Author to whom correspondence should be addressed.
(1) Lo, T. E.; Baird, M. H. I.; Hanson, C. Handbook of Soluent Extraction; Wiley: New York, 1983. (2)Ritcey, G.M.;Ashbrook, A. W. Soluent Extraction-Principles and ADDlications to Process Metallurgy: - _ Elsevier: Amsterdam, Part 1, 1984, Part 2, 1979. (3) Grubb, S.G.;Kim, M. W.; Rasing, Th.; Shen, Y. R. Langmuir 1988, 4, 452.
P e p e x lid
Ilg-ht In
/
aqueous phase
'
llpht oul
I
Teflon coaled stalnlesssteel cell
Figure 1. Schematic diagram of the ATR cell.
The cell is glass fronted, stainless steel (Teflon coated) and enveloped in a water jacket for temperature control. A mirror is suspended from a Perspex lid attachedto screwswhich enable the height and level of the mirror to be adjusted as required. The light through the optical fiber bundles enters and exits the cell through glass side arms fitted with quartz windows. The glass front and side arms are sealed to the cell with Viton gaskets. This type of fitting provides a certain amount of flexibility for the side arms and allows the adjustment of the angle at which the light enters the cell. These side arms are fixed in position once the correct angle is selected. Procedure. HEHEHP in n-decane of the required concentration and 0.1 M aqueous NaCl solution were introduced into the cell to form a two-phasesystem as seen in Figure 1. The pH of the aqueous phase was then adjusted by addition of NaOH or HCl as required. A reference scan was then taken and stored. A known volume of concentrated CoClz.6Hz0 was then injected into the aqueousphase and, after initially stirring for 10s, spectra were measured at known time intervals.
Results and Discussion The Co'lHEHEHP complex has a reasonably pronounced absorption spectrum in the visible region from 450to 675nm. The extinction coefficient at 628nm is 314 M-' cm-'. The spectrum shown in Figure 2 is virtually the same as that obtained in pure n-decane and in nonionic micellar solutions of octaethylene glycol n-dodecyl ether (C12Es). As outlined in the Experimental Details, the ATR system has been designed to measure the formation of the complex at the oil-aqueoussolution interface and the spectra shown in Figure 2 display the temporal evolution of the complex in the interfacial region as well as its diffusion into the bulk organic phase. If the interface is taken to be a single monolayer of the complex, the absorbance due to this monolayer will constitute only a minor portion (maximum of about 0.001absorbance unit) of the absorbance changes seen in Figure 2. Nonetheless, in principle, if the spectrum of the complex at the interface were different to that of complex in the bulk oil phase, then it would be possible to quantitatively determine the amount of the complex in 0 1992 American Chemical Society
Langmuir, Vol. 8, No. 2, 1992 367
Letters
,
1.0
0.0
,
-2.5
1
-0.5
i -3.0
'
-2.0 1 0
0.0
10000
Time
20000
1
5000
Time
(9)
10000 (9)
a 2
-
-3.5
M
-4.0
-4.5
I 2
--
I
3
4
5
6
PH
Figure 3. Plot of loglo hob as a function of pH at different cobalt and HEHEHP concentrations.
Scheme I O'O
500
550
600
650
Wavelength (nm)
Figure 2. Absorbance spectra of the cobalt-HEHEHP complex as a function of time. Conditions were as follows: 0.0084 M CoCl~6Hz0,0.5 M HEHEHP in n-decane,0.1 M NaC1, pH = 2.5, temp = 298 K. Curves a-h correspond to times, 300,1200,4200, 5700,8100,9600,and 13500a, after mixing. Inserts a and b show the change in absorbance (at 628 nm), and the first-order treatment of the absorbance,respectively, as a function of time. the two regions. Unfortunately the spectrum in the C12E8 micelles, which is essentially the spectrum of the complex at a liquid-liquid interface, is identical to the bulk oil phase spectrum. The change in the absorbance signal can be processed, as shown in the inserts to Figure 2, to yield an apparent first-order rate constant for the establishment of equilibrium between the oil and aqueous phases. It should be remembered that the absorbance measured by the ATR beam is not sensitive to the distribution profile of the complex in the organic phase. The ATR absorbance is simply a measure of the total amount of complex in the oil phase. The apparent kobs is then a mixed rate constant that is dependent on both the reactions involved in the formation of the metal ligand complex at the interface, as well as ita subsequent diffusion into the bulk oil phase. The dependence of kob on the ligand concentration, pH, and the metal ion concentration is shown in Figure 3. To help understand the significance of the data shown in this figure, it is instructive to consider the reaction scheme below. These reactions, to give a balanced view, are not the only processes that have been suggested in the literature; however currently there is little mechanistic evidenceavailable to support any detailed reaction scheme. To justify the reaction scheme presented, some additional information is required. The ligand has a very low solubility in water and is believed to predominantly exist as a dimer in the oil ~ h a s e . ~Its . ~chemical structure is that of a surfactant and therefore there will be somesurface activity a t the oil-aqueous solution interfa~e~>~-at most, a closely packed monolayer. A t the interface the dimer ligand may partly ionize, depending of course on the surface pH, or dissociate and then ionize (SchemeI). The existence of a protonated ligand dimer has not been established, (4) Vandergrift, G. F.; Horwitz, E. P. J. Znorg. Nucl. Chem. 1980,42, 119. (5) Miyake, Y.; Matauyama, H.; Nishida, M.; Nakai, M.; Nagase, N.; Teramato, M. Hydrometallurgy 1990,23, 19.
11 11 H+ Coz+
Aqueous phase
although the analogous alkyl carboxylate dimer has been claimed to exist6 at the air-water monolayer interface. Support for the dimer dissociation and then ionization step in Scheme I comes from the modeling of the titration of octadecyl-1-naphthoicacid (ONA) at both the air-water' and solid-water interfacese8 In this latter study it wae shown that the ionized dimer of ONA could not be used to mimic the titration of the surface-adsorbed carboxylic acid. On the basis of this information we are inclined to prefer the chemical species shown in Scheme I; although without direct evidence for the nonexistence of the ionized organophosphate dimer we cannot exclude this latter species. Reaction of the metal with the ligand to form the complex will occur a t the interface through a number of intermediate species (not shown). It has also been postulated that the ligand enters the aqueous phase, reacts with metal, and then diffuses into the oil phase.g Considering the low water solubility of HEHEHP,4y5 we do not consider this to be an important process. Over the pH range studied the amount of ionized form of the ligand (PKa 3) in the aqueous phase does not exceed lo4 M.6 In comparison, and assuming that the degree of ionization of the ligand at the interface is the same as in bulk aqueous phase, the analytical concentration of ionized ligand at the oil-water interface is in the range of 0.1-1 M. This is based on the measured surface excess of about (1-2) X 10-lomol cm-2 (1nm2/molecule)and monolayer thickness of 25 A. Hence, the rate of the metal-ionized ligand reaction at the interface can be expected to dominate over the bulk solution reaction. Further, the possibility that there is a significant absorption signal from the complex formed in the bulk solution phase compared with a mono-
-
(6)Goddard, E. D. Adv. Colloid Interface Sci. 1974,4, 45. (7) Hall, R.; Hayes, D.; Thistlethwaite, P. J.; Grieser, F. Colloids Surf. 1991,56,339.
(8) Lovelock, B.; Grieser, F.; Healy, T. W. J. Phys. Chem. 1985, 89, 501. (9) Al-Bazi, S. J.; Freiser, H. Inorg. Chem. 1989, 28, 417.
368 Langmuir, Vol. 8, No. 2, 1992
layer of complex at the oil-water interface can also be dismissed. The reflected wave, at say 628 nm, "samples" the aqueous phase to a depth of -370 nm under our experimental conditions.1° Using this information, and the equations given by Sperline et al." at a concentration M of complex in the aqueous phase, shows that the of absorption signal from a monolayer of complex is far in excess of the signal from complex in the bulk aqueous phase. A largely neglected aspect of reaction at an interface is the influence of the surface electrostatic potential created by the presence of ionized species at the oil-water surface. This potential will influence directly the surface concentration of H+ and Co2+and indirectly the concentration of the ionized ligand. Hence, variation in the surface potential, e.g., by a change in the solution electrolyte, will affect the rate of the reaction forming the complex. This aspect of the interfacial kinetics can only be quantitatively treated when more information is available on both the interfacial dielectric constant (or equivalent solvent environment parameter) and electrostatic surface potential. To minimize the influence of an electrostatic surface potential effect, our experiments have been conducted in the presence of 0.1 M NaCl. As can be seen from the scheme presented there are a number of possible rate controlling steps which could be involved in the temporal growth of the metal complex in the oil phase: (1)chemical reaction; (2) rate of supply of metal ion or/and ligand, from bulk phase to interface; (3) rate of dissociation/ionization of neutral ligand; (4) rate of diffusion of metal-ligand complex away from interface in bulk oil. At the moment we are not able to decide on which of the pathways is rate limiting. It is however worth making the observation that for the conditions studied preliminary modeling calculations appear to indicate that the ratelimiting step in the extraction process is the diffusion of the complex into the bulk oil phase, not chemical reaction or component supply, to form the complex. This conclusion is the same as that reached by Miyake et alS5and Chen et al,12 who investigated the same system we have studied using a Lewis cell and obtained results comparable to those presented here. (IO) Harrick, N. J. Internal Reflection Spectroscopy,2nd ed.; Harrick Scientific Corp.: Ossining, NY, 1979.
(11) Sperline, R. P.; Muralidharan, S.; Freiser, H. Langmuir 1987,3, 198. Proceedings ZSEC '86, ACS, (12) Chen, C.-q.; Zhu, T.; Chen, J.-Y. 1986; vol. 11, p 339.
Letters
If our assumptions in the modeling exercises are correct, then the results of Figure 3 allow an interesting observation to be made. The modeling showsthat the rate of diffusion into the bulk oil phase increases with an increase in the surface Concentration of complex. Adsorption of the ligand at the interface is expected to follow a Langmuir isotherm, i.e., the fraction (e) of the surface covered by the ligand is given by the expression
e = K[LI/(I + K ~ L I ) (1) where K is the equilibrium constant between the ligand in the oil phase and the oil-aqueous solution surface and [Ll is the concentration of the ligand in the oil. If the interface were saturated with ligand, i.e., e = 1, then a change in [Ll over the range of the experiments shown in Figure 3 would not affect k&, which is clearly not the case. The observation therefore implies that the interface is only partially covered with ligand. This is not quite in accord with measured oil-water interfacial tensions of HEHEHP which suggest that the surface is saturated with ligand, albeit at quite large molecular areas of the order of 1nm2 per molecule, in the concentration and pH range we have ~ s e d . ~ J ~ We are currently extending the method presented here to other metal ion-ligand systems and to shorter times in order to monitor the formation of the complex directly at the free liquid-liquid interface, at the monolayer level, before diffusion into the bulk oil has commenced. With this type of interfacial resolution we are potentially capable of differentiating between the various models proposed to explain the mechanism by which metal ion extraction occurs. In addition, the experimental arrangement provides the scope for determining both interfacial electrostatic potentials and interfacial solvent environment using solvatochromic probes as we have done for micelles,14mic r o e m ~ l s i o n sand , ~ ~ air-water monolayers.16 Acknowledgment. This work has been supported by the Australian Research Council, Advanced Mineral Products Centre and the Generic Industrial Research and Development Council. We thank Professor T. W. Healy for helpful discussions. Registry No. HEHEHP,14802-03-0; Co, 7440-48-4. (13) Cox, M.; Elizalde, M.; Castresana, J.; Miralles, N. Proceedings of ISEC '83, ACS, 1983; p 268. (14) Grieser, F.; Drummond, C. J. J. Phys. Chem. 1988, 92, 5580. (15) Murray, B. S.;Drummond, C. J.;Grieser,F.; White, L. R. J.Phys. Chem. 1990,94,6804. (16) Kibblewhite, J.; Grieser,F.; Drummond, C. J.; Thistlethwaite, P. J. J.Phys. Chem. 1989,93, 7464.
