Adsorption of Extractants

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. 

From these results, one can understand that the liquid-liquid interface can assist effectively in the interfacial reaction through the adsorption of extractants like a solid catalyst. The whole extraction scheme of the chelate extraction system is represented in Scheme 1. 

Interfacial adsorption of extractant increases the interfacial concentration, thus accelerating the interfacial complexation and extraction rate. 

Unfortunately, little direct information is available on the physicochemical properties of the interface, since real interfacial properties (dielectric constant, viscosity, density, charge distribution) are difficult to measure, and the interpretation of the limited results so far available on systems relevant to solvent extraction are open to discussion. Interfacial tension measurements are, in this respect, an exception and can be easily performed by several standard physicochemical techniques. Specialized treatises on surface chemistry provide an exhaustive description of the interfacial phenomena [10,11]. The interfacial tension, y, is defined as that force per unit length that is required to increase the contact surface of two immiscible liquids by 1 cm. Its units, in the CGS system, are dyne per centimeter (dyne cm" ). Adsorption of extractant molecules at the interface lowers the interfacial tension and makes it easier to disperse one phase into the other. 

Analogous to above mentioned method, the adsorption of extract on a suitable solid material, is possible (see Fig. 6.6-4). The disadvantage of adsorption is that regeneration of the adsorbent is difficult or impossible. Therefore, no extract is produced and adsorption can only be used if the residual material is the product and not the extract. 

The most important step in the interfacial catalysis in complex formation is the adsorption of extractant, which increases the interfacial concentration thus, the interfacial complexation and extraction rate are accelerated. The kinetic solvent effect of the liquid-liquid interface is very sensitive to the location where the ligand molecule is adsorbing. The interfacial solvent effect in the nanometer region has to be studied more extensively. 

The density of population and orientation of surfactants at the interface depends upon their structure and concentration and the type of diluent. The effect of the latter is decisive. Molecules of aromatic hydrocarbons that have n electrons compete with extractant molecules for access to the hypothetical interface, and solvating diluents disturb the adsorption of extractant molecules and the penetration of... 

Prochaska, K Adsorption of extractants and modifiers in mixed binary- model systems, Solvent Extr. Ion Exch., 14(6), 1057-1076(1996). 

Aromatic and Nonaromatic Hydrocarbon Separation. Aromatics are partially removed from kerosines and jet fuels to improve smoke point and burning characteristics. This removal is commonly accompHshed by hydroprocessing, but can also be achieved by Hquid-Hquid extraction with solvents, such as furfural, or by adsorptive separation. Table 7 shows the results of a simulated moving-bed pilot-plant test using siHca gel adsorbent and feedstock components mainly in the C q—range. The extent of extraction does not vary gready for each of the various species of aromatics present. SiHca gel tends to extract all aromatics from nonaromatics (89). 

The soliihility of the inert, adsorption of solute on the inert, and complexity of solvent and extracted material can he taken into account if necessary. Their consideration is heyond the scope of this treatment. 

Despite the recent efforts for settling operational conditions for metal and metalloid fractionation, conventional batch sequential extraction schemes lack automation and are rather time consuming and laborious. Two additional main problems are the phase overlapping and possible re-adsorption of released elements. 

SG sols were synthesized by hydrolysis of tetraethyloxysilane in the presence of polyelectrolyte and surfactant. Poly (vinylsulfonic acid) (PVSA) or poly (styrenesulfonic acid) (PSSA) were used as cation exchangers, Tween-20 or Triton X-100 were used as non- ionic surfactants. Obtained sol was dropped onto the surface of glass slide and dried over night. Template extraction from the composite film was performed in water- ethanol medium. The ion-exchange properties of the films were studied spectrophotometrically using adsorption of cationic dye Rhodamine 6G or Fe(Phen) and potentiometrically by sorption of protons. 

Extraction (discussed in Chapter 5) uses the selective adsorption of a component in a liquid to separate specific molecules from a stream. In application extraction may be coupled with its cousins, extractive distillation and azeotropic distillation, to improve extraction efficiency. Typical refinery extraction applications involve aromatics recovery (UDEX) and lubricants processing (furfural, NMP). Extractive distillation and azeotropic distillation are rarely employed in a refinery. The only... 

This chapter provides details on a number of commonly used process units reactors, heat exchangers, columns of various types (distillation, absorption, adsorption, evaporation, extraction), dryers, and grinders. The purpose of each unit or operation and the many configurations in which the units can be found are also discussed. 

These are used to provide adsorption of gases and vapors, most commonly to remove odors from extracts from kitchens or industrial processes. An efficiency of 95 per cent is obtainable and the carbon has a long life since heating can reactivate it. 

Adsorption, like extraction, depends on equilibrium relationships. Isothermal adsorption is projected by Langmuir isotherms. The model is shown in Figure 7.14, which is based on the linear model of the following equation ... 

The whole sample can be extracted where this is possible or, again, the two phases separated and both extracted by different techniques. As stated before, adsorption of the substances to be determined on the solid phase can become difficult when they are present at very low concentrations and the adsorbed material is a significant proportion of the total sample. 

Distillation usually is the most economical method of separating liquids, superior to extraction, adsorption, crystallization, or others. Exceptions to this rule include Flash separation when flash separation is sufficient and Settling (decanting or coalescing) when the mixture has LL immiscibility without addition of extraction solvent. 

A sequential analysis protocol includes three steps (1) extraction in water or other appropriate solvent for the colorant, (2) purification or concentration of the colorant, and (3) separation coupled with detection of the target molecule. Different methods of extracting synthetic colorants from foods have been developed using organic solvents followed by SPE protocols using as adsorption support RP-C18, amino materials, or Amberlite XAD-2. Eor qualitative evaluations, the easiest option for separating colorant molecules from unwanted ingredients found in an extract is SPE on polyamide or wool. 

Two examples will be presented as an example of groundwater analyses at Bayer CropScience. Both analyses were carried out using DAI with the aid of a stable isotope IS. Attempts at conventional SEP extractions presented special problems because of adsorption of metabolites on the SEP media or oxidative instability during extraction. A brief description of the procedures is outlined below. 

The heptane water and toluene water interfaces were simulated by the use of the DREIDING force field on the software of Cerius2 Dynamics and Minimizer modules (MSI, San Diego) [6]. The two-phase systems were constructed from 62 heptane molecules and 500 water molecules or 100 toluene molecules and 500 water molecules in a quadratic prism cell. Each bulk phase was optimized for 500 ps at 300 K under NET ensemble in advance. The periodic boundary conditions were applied along all three directions. The calculations of the two-phase system were run under NVT ensemble. The dimensions of the cells in the final calculations were 23.5 A x 22.6 Ax 52.4 A for the heptane-water system and 24.5 A x 24.3 A x 55.2 A for the toluene-water system. The timestep was 1 fs in all cases and the simulation almost reached equilibrium after 50 ps. The density vs. distance profile showed a clear interface with a thickness of ca. 10 A in both systems. The result in the heptane-water system is shown in Fig. 3. Interfacial adsorption of an extractant can be simulated by a similar procedure after the introduction of the extractant molecule at the position from where the dynamics will be started. 

TABLE 1 Parameters in the Adsorption and Extraction of Ni(II) with 2-Hydroxy Oxime in Heptane-0.1 M(H,Na)C104 at 25°C... 

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