Emulsion liquid membranes extractant agents

In contrast to osmotic, dialysis, filtration or size-exclusion type membranes, PIMs as other liquid membranes (i.e., bulk liquid membranes, emulsion liquid membranes and supported liquid membranes) rely on the action of a chemical agent to extract the solute of interest from an aqueous phase (Kolev, 2005). The action of this chemical agent is the most important factor in the performance of any PIM and its behavior shares considerable similarities with SX apphcations (e.g., hydrometaUurgy). 

Figure 1 A schematic representation of copper ion extraction with an emulsion liquid membrane. Copper(II) is transported to the emulsion/feed phase interface and reacts with the complexing agent (RH) to form a soluble copper complex (CuRj). This complex diffuses to the interior of the emulsion droplet until it encounters a droplet of the internal phase where the metal ion is exchanged for a hydrogen ion. The net effect is a unidirectional mass transport of the cation from the original fe to the receiving phase with counter-transport of hydrogen ions. Mercury exhibits a comparable mechanism for transport in these systems.

The effect of viscosity is important in the production of liquid membranes. These are, to a limited extent, employed in the extraction of non-ferrous metal salts (particularly Zn, Ni, Cu) from process efluents. In their manufacture a prepared water/oil emulsion (e.g. 1/3 kerosene with 2% of a surfactant and 2/3 aqueous NiSO4 with a homogenizing agent) is stirred into the non-ferrous metal salt containing effluent in a ratio of ca. 1 5. It emerged [404], that it is by no means unimportant, how the prepared water/oil emulsion is stirred into this solution. It can be carefully added layer-wise over the aqueous solution and then the stirrer switched on (A), or immediately added to the rotating stirrer (B). 

Figure 9.41 Variation of copper concentration in the raffinate during extraction with a liquid emulsion-membrane complexing agent system.43...

The information provided by the equilibrium isotherms depicted in Figure 29.5 confirms the commercial selective carrier bis(2,4,4-trimethylpenthyl) phosphinic acid (Cyanex 272) as a suitable reagent to formulate the liquid membrane due to its capacity to selectively separate iron and zinc from chromium under the typical pH conditions of the passivation baths. Therefore, the typical composition of the pseudo emulsion employed in the EPT regeneration process contains Cyanex 272 and sulfuric acid as stripping agent, and the extraction and BEX of zinc and iron are described, respectively, by the following reversible chemical reactions ... 

Various substances such as amino acids, organic acids, NaOH, NaCl, carbon dioxide, oxygen, metals, and various ions, such as Cd(II), Cu(II), Co(II), and Fe(III), can be separated by using suitable carrier agents in liquid or solid composite membranes. Liquid membranes behave like double liquid-liquid extraction systems where the usage of organic solvent is minimized. Such devices are generally prepared as bulk liquid, emulsion liquid, and supported liquid membranes. 

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