Counter-coupled transport

Equations (11.1)—(11.10) provide a basis for rationalizing the principal features of coupled transport membranes. It follows from Equation (11.8) that coupled transport membranes can move metal ions from a dilute to a concentrated solution against the metal ion concentration gradient, provided the gradient in the second coupled ion concentration is sufficient. A typical experimental result demonstrating this unique feature of coupled transport is shown in Figure 11.7. The process is counter-transport of copper driven by hydrogen ions, as described in Equation (11.1). In this particular experiment, a pH difference of 1.5 units is... 

As examples of coupled counter-transport (see Figure 13.2d) and coupled cotransport (see Figure 13.2e) the transport of titanium(lV) from low acidity (pH = 1) and high acidity ([H+] = 7 M) feed solutions, respectively using the HUM system [1,2] may be presented. The di-(2-ethyUiexyl) phosphoric acid (DEHPA) carrier reacts with Ti(IV) ion to form complexes on the feed side (see Equations 13.25 and 13.26) and reversible reactions take place on the strip side (see Equations 13.27 and 13.28). Energy for the titanium uphill transport is gained from the coupled transport of protons in the direction opposite to titanium transport from the strip to the feed solutions. In the second case (high-feed acidity), Cl anion cotransported with Ti(IV) cation in the same direction. In both cases fluxes of titanium, protons, and chlorine anion are stoichiometrically coupled. 

In carrier-mediated transport studies, two terms are used, namely, facilitated transport and coupled transport. Facilitated transport is generally referred to as the case where the transport mechanism is independent of any other ion, while in case of coupled transport the transport rate of a particular ion is dependent on the concentration of another ion. The mechanism of facilitated transport is shown in Figure 31.3 a, while those of the two different types of coupled transport (cotransport and counter-transport) are schematically explained in Figure 31.3b and 31.3c. In case of cotransport, the metal ion is transferred along with a counter-anion, while simultaneous transport of another ion from receiver phase to source phase occurs in case of counter-transport. 

Pb Environmental Counter-coupled carrier-mediated transport, D2EHP used as [148]... 

Peptides Pharmaceutical Counter-coupled carrier medieted transport. Aliquot 336 [149]... 

Coupled transport processes can be divided into two categories, depending on the type of reaction occurring between complexing agent and permeant. The first type is called counter transport (shown in Figure 9.6). The key feature of counter transport is that the fluxes of the two permeating ions move counter to each other across the membrane. The reaction in this case is ... 

If more kinds of counter ions are present, it is important to know the relative transference of each species. This depends on the relative concentration of the species and on its mobility. The former factor is determined by the affinity of the ion exchange membrane for the pertinent species. It has been mentioned already that a higher affinity is attended with a lower mobility. In general it is found that the first-mentioned factor predominates, so that a higher affinity is coupled with a relatively higher share in the charge transport. 

Coupled diffusion or transport of counter-ions in the external solution phase. 

The commonly accepted mechanism for the transport of a solute in LM is solution-diffusion. The solute species dissolve in the liquid membrane and diffuse across the membrane due to an imposed concentration gradient. Different solutes have different solubilities and diffusion coefficients in a LM. The efficiency and selectivity of transport across the LM may be markedly enhanced by the presence of a mobile complexation agent (carrier) in the liquid membrane. Carrier in the membrane phase reacts rapidly and reversibly with the desired solute to form a complex. This process is known as facilitated or carrier-mediated liquid membrane separation. In many cases of LM transport, the facilitated transport is combined with coupling counter- or cotransport of different ions through LM. The coupling effect supplies the energy for uphill transport of the solute. 

D Coupled counter-transport E Coupled cotransport F Active transport... 

To maintain electroneutrality and solute uphill pumping many membrane carrier systems require a coupling (from the source or receiving phases) ion to be counter- or cotransported along with the solute ion. Because the coupling ion must also enter and cross the organic phase, it is bound to influence transport efficiency. 

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