Extraction kinetics

Because the metal-extraction separation process irtvolves chemical reactions, rates may be slow compared with ordinary liquid extraction. Although slow kinetics have important process design inqrlications, more attention has bera focused on mechanistic interpretations than on quantitative phenomenonological characterization. Cox and Flett have reviewed sortK chemical aspects of extraction kinetics. 

Furthermore, for a large ratio of bulk metal ion concentration to the interfacial aqueous extractant concentration, the reaction zone is located very close to the liquid-liquid interfoce, and the extraction rate per unit area of interface becomes independent of the aqueous-film mass transfer coefficient as well as the system volume. Under these conditions a duly heterogeneous reaction and a homogeneous process are indistinguiriiable in the sense fiut the rate of each will be a definite function of reactant concentrations in the vicinity of the interfiice. In either case, the functional form of die kinetics and its parameters must be determined experimentally. 

Fbr a single cation-exchange extraction, such as reaction (8.2-3), concentratkm profiles near the in-terfece are rqitesemed by Eifi- 8.4-1. The fluxes at the interface of all four species are related by the reaction stokhiometry. The interfacial concentrations depend on the bulk-solution concentrations, die in-terfacial metal flux, and the respective mass transfer resistances. Requiring the interfaciai concentrations to be in equilibrium accordii to Eq. (8.3-1) yields the follovring equation for the metal-extraction rate  

FIGURE 8.4-1 Concentration profiles near the interfile during metal extraction by an acidic extractant. 

FIGURE 8.4-2 Dimensionless extraction flux as a function of distance of bulk-phase concentrations from equilibrium as computed from Eq. (8.4-1). Curves 1 and 2 represent different bulk concentration ratios. Curve P-B represents a pseudobinary calcuiation where gradients in H and HR are neglected. From Ref. 6, with permission. 

FIGURE 8.4-3 The me of extinction of copper from sulfate solution by Kelex 100 in xylene. The experimental flux is presented relative to the mass-lransfer-louited metal flux for kM2. = 0 2 cm/s. From Ref. 6. with permission. 

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J.L. Leblanc and B. Stragliati, An extraction kinetics method to study the morphology of carbon black filled mbber compounds, J. Appl. Polym. Sci., 63, 959-970, 1997. 

Although the improved extraction kinetics also increase the concentration of coextractives in the final extract, some degree of selectivity can be achieved by careful selection of the solvent or solvents used. Matrix co-extractives may be removed, or at least partially removed, by placing a suitable sorbent, such as alumina, at the exit of the extraction cell to remove lipid co-extractives. Excellent recoveries of both polar and nonpolar pesticides from a wide range of foodstuffs have been reported. Specific applications include organophosphorus and A-methylcarbamate pesticides. 

Catalytic Effect of the Liquid-Liquid Interface in Solvent Extraction Kinetics Hitoshi Watarai... 

Although the Lewis cell was introduced over 50 years ago, and has several drawbacks, it is still used widely to study liquid-liquid interfacial kinetics, due to its simplicity and the adaptable nature of the experimental setup. For example, it was used recently to study the hydrolysis kinetics of -butyl acetate in the presence of a phase transfer catalyst [21]. Modeling of the system involved solving mass balance equations for coupled mass transfer and reactions for all of the species involved. Further recent applications of modified Lewis cells have focused on stripping-extraction kinetics [22-24], uncatalyzed hydrolysis [25,26], and partitioning kinetics [27]. 

This technique has been used to study the extraction kinetics of rare-earth elements from an aqueous phase into heptane by 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (HEH/EHP) [31]. The linear dependence of f c ) on t in Fig. 5 was considered to indicate that the extraction of ErCl3 by HEH/EHP is a diffusion-controlled process. 

The hydrodynamics of the moving drop are difficult to calculate, particularly the flow characteristics within the droplet itself. However, this technique is still used widely, because it is a simple and straightforward method. It was recently applied to study the stripping-extraction kinetics of Mn(II) in an aqueous-kerosene system [50,51]. The effect of anionic surfactants on the kinetics of extraction of lactic acid from an aqueous phase by Alamine 336 in a toluene phase was also studied by this technique [52]. 

A significant advance was made in this field by Watarai and Freiser [58], who developed a high-speed automatic system for solvent extraction kinetic studies. The extraction vessel was a 200 mL Morton flask fitted with a high speed stirrer (0-20,000 rpm) and a teflon phase separator. The mass transport rates generated with this approach were considered to be sufficiently high to effectively outrun the kinetics of the chemical processes of interest. With the aid of the separator, the bulk organic phase was cleanly separated from a fine dispersion of the two phases in the flask, circulated through a spectrophotometric flow cell, and returned to the reaction vessel. 

One of the most attractive roles of liquid liquid interfaces that we found in solvent extraction kinetics of metal ions is a catalytic effect. Shaking or stirring of the solvent extraction system generates a wide interfacial area or a large specific interfacial area defined as the interfacial area divided by a bulk phase volume. Metal extractants have a molecular structure which has both hydrophilic and hydrophobic groups. Therefore, they have a property of interfacial adsorptivity much like surfactant molecules. Adsorption of extractant at the liquid liquid interface can dramatically facilitate the interfacial com-plexation which has been exploited from our research. 

II. HOW TO STUDY THE INTERFACIAL REACTION IN SOLVENT EXTRACTION KINETICS... 

In this section we will focus on the methods applicable to the study of interfacial extraction kinetics which were developed in our group. 

III. SOLVENT EXTRACTION KINETICS AND CATALYTIC INTERFACIAL COMPLEXATION... 

The extraction system which was measured by the HSS method for the first time was the extraction kinetics of Ni(II) and Zn(II) with -alkyl substituted dithizone (HL) [14]. The observed extraction rate constants linearly depended on both concentrations of the metal ion [M j and the dissociated form of the ligand [L j. This seemed to suggest that the rate determining reaction was the aqueous phase complexation which formed a 1 1 complex. However, the observed extraction rate constant k was not decreased with the distribution constant Kj of the ligands as expected from the aqueous phase mechanism. 

When the extraction kinetics is governed by the aqueous phase reaction,... 

In the ion-association extraction systems, hydrophobic and interfacially adsorbable ions are encountered very often. Complexes of Fe(II), Cu(II), and Zn(II) with 1,10-phenanthro-line (phen) and its hydrophobic derivatives exhibited remarkable interfacial adsorptivity, although the ligands themselves can hardly adsorb at the interface, except for protonated species [19-21]. Solvent extraction photometry of Fe(II) with phen is widely used for the determination of trace amounts of Fe(II). The extraction rate profiles of Fe(II) with phen and its dimethyl (DMP) and diphenyl (DPP) derivatives into chloroform are shown in Fig.9. In the presence of 0.1 M NaC104, the interfacial adsorption of phen complex is most remarkable. The adsorption of the extractable complex must be considered in the analysis of the extraction kinetic mechanism of these systems. The observed initial rate r° shows the relation... 

The actual SFE extraction rate is determined by the slowest of these three steps. Identification of the ratedetermining step is an important aspect in method development for SFE. The extraction kinetics in SFE may be understood by changing the extraction flow-rate. Such experiments provide valuable information about the nature of the limiting step in extraction, namely thermodynamics (i.e. the distribution of the analytes between the SCF and the sample matrix at equilibrium), or kinetics (i.e. the time required to approach that equilibrium). A general strategy for optimising experimental parameters in SFE of polymeric materials is shown in Figure 3.10. 

Al-Bazi, S. J. Freiser, H. Phase-transfer catalysts in extraction kinetics palladium extraction by dioctyl sulfide and KELEX 100. Inorg. Chem. 1989, 28, 417 120. 

There is only limited information in the literature on determinations of carrier numbers for the uptake of trace metals. Hudson and Morel [7] and later Sunda and Huntsman [200] argued that to enable Fe uptake, marine diatoms required extremely large numbers of carriers (enough to cover 50% of the cell surface area in some cases). For Pb uptake by Chlorella kesslerii, Slaveykova and Wilkinson [201] have also estimated large carrier numbers of 1.5 x 10 11 mol cm-2 (> 106 carriers per cell) using kinetic EDTA extraction techniques. Finally, using similar extraction kinetics, saturation of Zn... 

The largest research effort in extraction kinetics is likely to be in the development of solvent extraction related techniques, such as various versions of liquid chromatography, liquid membranes, etc. These techniques require a detailed knowledge of the kinetics of the system to predict the degree of separation. 

These two thin liquid films, which are also called diffusion films, diffusion layers, or Nernst films, have thicknesses that range between 10 and 10 cm (in this chapter centimeter-gram-second (CGS) units are used, since most published data on diffusion and extraction kinetics are reported in these units comparison with literature values is, therefore, straightforward). 

Diffusion is a complex phenomenon. A complete physical description involves conceptual and mathematical difficulties associated with the need to involve theories of molecular interactions and to solve complicated differential equations [3-6]. Here and in sections 5.8 and 5.9, we present only a simplified picture of the diffusional processes, which is valid for hmiting conditions. The objective is to make the reader aware of the importance of this phenomenon in connection with solvent extraction kinetics. 

Equations (5.16) of Table 5.1 refer to series first-order reactions. Of interest for the solvent extraction kinetics is a special case arising when the concentration of the intermediate, [Y], may be considered essentially constant (i.e., d[Y]/dt = 0). This approximation, called the stationary state or steady-state approximation, is particularly good when the intermediate is very reactive and present at very small concentrations. This situation is often met when the intermediate [Y] is an interfacially adsorbed species. One then obtains... 

Many extractants reach a constant interfacial concentration at bulk organic concentrations far below the practical concentrations that are generally used to perform extraction kinetic studies. This means that when writing a rate law for an extraction mechanism that is based on interfacial chemical reactions, the interfacial concentrations can often be incorporated into the apparent rate constants. This leads to simplifications in the rate laws and to ambiguities in their interpretation, which are discussed in later sections. 

Section 5.1 describes how, in a stirred system, solvent extraction kinetics can be controlled only by slow chemical reactions or only by diffusion through the interfacial films. An intermediate situation can also occur whereby both the rates... 

When one or more of the chemical reactions is sufficiently slow in comparison with the rate of diffusion to and away from the interface of the various species taking part in an extraction reaction, such that diffusion can be considered instantaneous, the solvent extraction kinetics occur in a kinetic regime. In this case, the extraction rate can be entirely described in terms of chemical reactions. This situation may occur either when the system is very efficiently stirred and when one or more of the chemical reactions proceeds slowly, or when the chemical reactions are moderately fast, but the diffusion coefficients of the transported species are very high and the thickness of the two diffusion films is close to zero. In practice the latter situation never occurs, as diffusion coefficients in liquids generally do not exceed 10 cm s, and the depth of the diffusion films apparently is never less than 10 cm. 

The experimental identification of the regime that controls the extraction kinetics is, in general, a problem that cannot be solved by reference to only one set... 

In this section, we describe three simple cases of rates and mechanisms that have been found suitable for the interpretation of extraction kinetic processes in kinetic regimes. These simple cases deal with the exuaction reaction of a monovalent metal cation (solvation water molecules are omitted in the notation) with a weakly acidic solvent extraction reagent, BH. The overall extraction reaction is... 

The diffusion-controlled, extraction kinetics of A, therefore, can be described as a pseudo-first-order rate process with apparent rate constants... 

Film diffusion usually is slower than the rate of many ligand-substitution reactions. Therefore, when rate laws, snch as Eqs. (5.63) or (5.89), are found for the extraction kinetics of metal species, preference shonld, in general, be... 

A mathematically simple case, that occurs frequently in solvent extraction systems, in which the extracting reagent exhibits very low water solubility and is strongly adsorbed at the liquid interface, is illustrated. Even here, the interpretation of experimental extraction kinetic data occurring in a mixed extraction regime usually requires detailed information on the boundary conditions of the diffusion equations (i.e., on the rate at which the chemical species appear and disappear at the interface). 

Fig. 5.10 Computer-assisted extraction kinetics-measuring apparatus for highly stirred phases (A) high-speed stirrer (B) stirrer shaft (C) sample inlet (D) Teflon stirring har (E) Teflon phase separator (F) water hath (G) flow-cell (H) spectrophotometer (I) peristaltic pump (J) chart recorder (K) A/D converter (L) clock (M) minicomputer (N) dual-floppy disk drive (O) printer, (P) plotter. (From Ref. 16.)...

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