Metal extraction, concentration dependence

Three causes of extractant solubility in the aqueous phase may be distinguished solubility of un-ionized and ionized extractant and metal-extractant species. For extractants such as acids, amines, and chelating reagents, their polar character will always result in some solubility in the aqueous phase over the pH range in which they are useful for metal extraction. Solubility depends on many factors including temperature, pH, and salt concentration in the aqueous phase, as discussed in Chapter 2. 

Extraction rates of zinc (II) and nickel (II) with ethyldithizone, butyldithizone, or hexyldithizone in an organic phase (chloroform, CCI4, w-heptane, or benzene) showed a first-order dependence on the ligand and metal ion concentration and an inverse-first-order dependence on the proton concentration. The results were explained by chelate formation in the interfacial region [59]. The effects of stirring on the distribution equili-... 

Thus, for a given reagent and solvent, the extraction of the metal chelate is dependent only upon pH and the concentration of reagent in the organic phase and is independent of the initial metal concentration. 

When the solvent is a good solvater, the determination of the solvation number b is difficult, unless the dependence of the extractant concentration on the solvent can be obtained. Solvation numbers can be obtained in mixtures of a solvating extractant and an inert diluent like hexane. Further, in these systems the extraction of the metal commonly requires high concentrations of salt or acid in the aqneons phase, so the activity coefficients of the solutes must be taken into acconnt. 

The amount of metal extracted from the soil by both EDTA and DTPA is dependent on the pH, the metal being extracted, the soiksolution ratio, the concentration of chelating agent, the shaking time, the temperature, and the sample preparation procedure. Clearly, the methodology used should be clearly described and closely followed if repeatable work is to be possible, and comparison of results is to be meaningful. 

Initially the application of this analytical technique to an effluent treatment problem would not appear to be very favorable. In the analytical technique high reagent concentrations relative to the metal salt are employed, and the extraction is dependent upon pH level of the aqueous phase (1,2). Furthermore, organic lead salts in the presence of high concentrations of ions such as Cl will form a series of complexes. As a consequence, any method of extraction must take into account the equilibrium of these species. However, these apparent objections to the use of the analytical technique can be resolved and so the process of combined chemical complexing-solvent extraction does have the potential of successful deployment in the field of the large scale treatment of organic-lead-contaminated waste waters. 

Solvent extraction plays an important role in many commercial processes for the extraction of uranium from ore. In this case, the radioactivity levels are quite low compared with those in spent fuel extraction. The liquors from hy-drometallurgical leaching of ores are typically fairly dilute in uranium (0.5-5 g/L) and contain iron and other metals in solution. Depending on conditions, solvent extraction or ion exchange may be used to separate and concentrate the uranium from the leach liquor. 

The observed extraction rate constants linearly depended on both the metal ion concentration [M +] and the hydrogen ion concentration in the aqueous phase. However, the observed extraction rate constant (k ) did not decrease with an increase in the distribution constant ( Tq) of the ligand as was expected from the mechanism in the aqueous phase. Furthermore, the HSS method revealed that the dissociated form of the n-alkyl-dithizone was adsorbed at the interface generated by vigorous stirring [5]. The following scheme was proposed based on the experimental results, considering both the aqueous... 

According to Eq. (9), the conventional plot of log D against —log[H+] at a constant [(HA)2]D should yield a straight line with a slope of n only when j and aM are equal to unity. For the polymeric extracted species, the distribution ratio, D, depends not only on [H+] and [(HA)2]Q but also on the metal ion concentration. Therefore, the plot according to Eq. (9) yields a curve (Fig. 1). As shown in Fig. 1, the plot falls on the straight... 

Thus, for a given reagent and solvent, the extraction of the metal chelate is dependent only upon pH and the concentration of reagent in the organic phase and is independent of the initial metal concentration. In practice, a constant and large excess of reagent is used to ensure that all the complexed metal exists as MR, and D is then dependent only on pH, i.e. 

Extraction parameters such as solvent type, mixture ratios, metal ion concentration, pH of the aqueous phase, extraction time, and temperature influence the recovery of extracted lipids and must be validated to ensure reliable results. For example, the recovery of the acidic lipids PA and phosphatidylglycerol (PG) can be less than 30% in classic Folch and Bligh Dyer extraction, where these lipids can become bound to proteins tightly (17). Lipids bound to proteins covalently are only released under appropriate conditions, which depend on the type of lipid-protein linkage. For example, ceramides bound to protein of the comi-fled envelop in the human skin (18) can be extracted after mild alkaline hydrolysis of the ester linkage between hpid and protein. Special conditions are required for extraction of more polar lipids such as gangliosides, lysophospholipids and lysosphin-golipids, or phosphatidylinositol-phosphates. 

The HNO3 dependencies of the extraction of U(VI) and Th(IV), shown in Figure 4, should be considered of operational significance only since several parameters vary simultaneously as the equilibrium aqueous HNO3 concentration is varied. For example, NOl activity and nitrato complexing of metals in the aqueous and the free extractant concentration in the organic phase may vary as the aqueous HNO3 concentration is varied. 

The degree of the metal extractions depends on its concentration. Por example, with increasing europium concentration the distribution coefficients in the alkali-DOBTA system decrease, while in the alkali-tartaric acid system a maximum at 7x10 4 M Eu concentration is observed. As we suggested the enhancement in the metal distribution coefficient is evidently due to the metal polymerization in the organic phase, and the decrease is caused by polymerization in the aqueous phase, which eventually results in low extractable polymer form. The latter assumption is supported by the fact that as the alkali concentration increases the maximum on extraction curves undergoes a shift towards lower concentration of the metal. 

The metal extraction curves described above mirror the behavior of these complexing agents when incorporated into coupled transport membranes. Thus, when the product solution pH is maintained at a very low level, the term [MRn] g in Equation 13 is reduced to zero and the flux is determined by the concentration of complexed metal in equilibrium with the feed solution [MRn] q. As a result, the flux vs pH behavior follows the metal extraction curves. This is shown in Figure 9.14, which shows the flux pH (curves) for copper with each of the three complexing agents whose extraction curves are shown in Figure 9.12. The magnitude of the coupled transport flux depends on the liquid viscosity and complexed metal diffusion coefficient, but the pH dependence of the flux is very similar to the metal ion extraction curves. 

Chrome Azurol S was used for the extraction of Fe , Cu, and Sc " with the order Fe " > Cu > Sc" in a PEG-2000/(NH4)2S04 ABS [66. In this study, the (NH/i)2SO/i concentration dependence profile showed that increasing the phase-forming salt concentration decreased the extraction of metal ions. pH studies indicated that pH in the range 0-6 had... 

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