Transport through SLM

Until recently, the role of slow rates of cation release in LM transport was unclear. From U NMR studies, it is known that the complexes can be kinetically stable [65, 66] and, as a consequence, decomplexation rates can be very slow. Influence of slow rates of alkah metal cations release was raised in transport through a BLM [58,64,67-69]. Recently, at cation transport experiments with different calix crown ether derivatives [42,70], was proven that the rate of decomplexation can be rate controlling in the transport through SLMs. 

Metals separation and recovery is always of importance for industry and the environment. The theoretical and fundamental studies on metal transport through SLMs are advanced but still conducted toward implementation of laboratory-scale parameters to industrial applications [191]. This is related to the increased attention to improvement of selectivity and stability... 

The main objectives of this paper are 1) To extend a model for facilitated transport through SLMs, previously presented in the literature for flat-sheet membranes, to allow for transport through hollow fibre membranes, 2) to compare the flat sheet and hollow fibre models and show that the radial geometry must be taken into account when considering hollow fibre units, 3) to demonstrate, by comparison with experimental data, that even with radial variations, the model is still too simple to accurately describe the transport through a hollow fibre unit and 4) to suggest how the model can be improved. 

Transport Through a Bulk Liquid Membrane. All theoretical models concerning carrier-assisted transport through SLMs are based on the theoretical work for carrier-assisted transport through BLM systems reported by Reusch and Cussler (5). They described the transport of different alkali salt mediated by dibenzo-18-crown-6 through a BLM. 

Dense inorganic or metallic membranes for gas separation are usually ion-conducting materials, while membranes with carriers are polymers or supported liquid membranes (SLM). For transport through these materials, different flux equations should be applied. Figure 4.2 sums up and generalizes the various types of transport, which may take place in gas-separation membranes [21]. 

The various steps that characterize the transport of metal species through SLMs can be described with the help of Figure 31.4. Step 1 The metal species, after diffusing to the source-membrane interface, react with the metal carrier, ions are simultaneously released into the feed solution (counter-transport, acidic carrier) or ions accompany the metal ions into the membrane (cotransport, neutral, or basic carriers). Step 2 The metal-carrier complex diffuses across the membrane because its concentration gradient is negative. Step 3 At the membrane-receiver interface the metal-carrier complex releases metal ions into the aqueous phase, ions replace M ions into the membrane (counter-transport) or X ions are simultaneously released together with M ions into the strip solution (cotransport). Step 4 The uncomplexed carrier diffuses back across the membrane. 

Steady-state conditions of the solute transport through the phase interfaces all ffuxes are necessarily the same [36-38]. This assumption is related to SLM and ELM processes, in which the thickness of the... 

Other important toxicological con tarn in ants that can be found in waste-waters are metals. Toxic heavy metal ions are introduced to aquatic streams by means of various industrial activities viz. nfrning, refining ores, fertilizer industries, tanneries, batteries, paper industries, pesticides, etc., and possess a serious threat to the environment. The major toxic metal ions hazardous to humans as well as other forms of hfe are Cr, Fe, Se, V, Cu, Co, Ni, Cd, Hg, As, Pb, Zn, etc. These heavy metals are of specific concern due to their toxicity, bioaccumulation tendency, and persistency in nature [190]. The SLM technique has been widely apphed for the transport and recovery of almost ah important metals from various matrices an exceUent review of ah aspect of metal permeation through SLM (covering both theoretical and practical considerations) is available [191]. Here, only some selected recent examples of the use of SLM for metal separation whl be presented. 

SO.,"] < [MeSO.,"] [40]. These results were quite encouraging and suggested that these SLMs based on ILs could be used for the selective separation of the organic esters from the reaction mixture. SILMs can also be used for the separation of aromatic hydrocarbons from aliphatic hydrocarbons. In this context, the selective separation of benzene, toluene and p-xylene from n-heptane was achieved using SILMs based on [bmim ][PE "], [hmim+][PFg ], [omim+][PFg ] and [Et MeMoEtN [TfjN"] supported on a polyvinyhdene fluoride manbrane [9]. It was found that aromahc hydrocarbons were successfully transported through the membrane based on these ionic liquids, and the maximum selectivity to n-heptane was when benzene used in the aromatic permeation and [bmim+][PFg ] was taken in the hquid membrane phase. 

Besides transport of cations with these modified carriers, also carrier mediated transport of neutral molecules through SLMs has been studied and recently also anionand ditopic (combined anion and cation) receptors as carriers for membrane transport have been studied. 

Diffusion-Limited Transport of Salts. In the case of fast extraction equilibria for salts at the interfaces, the rate of transport is determined by diffusion of the complex through the membrane and the flux for steady state transport through an SLM is given by Equation 9. 

In BLM transport, the most common solvents are chlorinated hydrocarbons. These have low dielectric constants and, as a consequence, the transport is described by ion-pairs. Izatt et al. have extensively studied ion-pair transport through BLMs (18), In SLM transport, cyclohexyl phenyl ether is an important non-polar membrane solvent (19). The extraction equilibrium at the interface in a non-polar membrane... 

Reaction-Rate Limited Transport of Monovalent Ions. From H NMR studies it is known that decomplexation rates can be very slow and, as a consequence, complexes can be kinetically stable (35,36). Until recently the role of slow rates of cation release in SLM transport was unclear. Lehn et al (24) and Fyles (37) theoretically raised the question of the influence of slow rates of alkali metal cation release on transport through a BLM. Experimentally, this phenomenon has only been observed by Yoshida et al (38). They showed that cation transport through a BLM mediated by polynactin was limited by the rate of cation release from the membrane. In 1994, Echegoyen stated that in SLM transport the rate of cation release from the membrane could never... 

The anion effect on BLM transport of metal salts was examined by Lamb et al, 46). From determination of the transport rates for different alkali metal salts by dibenzo-18-crown-6, it became clear that the rate of transport was strongly dependent on the co-transported anion. The relative fluxes of different potassium salts were related to the hydratation energy. Cations which are accompanied by a hydrophilic anion (-AGg. is large) show low fluxes, due to a decrease in the partitioning. A linear relation was obtained by plotting In J vs -AGg. of the anion. For transport through an SLM, a similar relationship was found 47). 

Lamb et al. carried out competitive experiments of monovalent and divalent cations mediated by various crown ethers and cryptands through chloroform BLMs. Relative fluxes from binary mixtures were explained in terms of ring substituents and ring size of the crown ether carriers 50,51). Saito studied the transport of alkali and alkaline earth metal cations as their CIO/ salts through SLMs by tripentyl phosphate and observed a selective transport of Li in the presence of Na, K, and Mg" 52). 

New topics in the field are the assisted transport of anions and transport assisted by ditopic receptors through SLMs. In the future, new studies have to be undertaken to elucidate the corresponding transport mechanisms. 

The transport enhancement which results from incorporation of anionic additives can be striking. Figure 5 shows Cu transport through a supported liquid membrane (SLM) in the presence and absence of an anionic additive (laurate). In this system, transport is enhanced by more than ten times by the addition of laurate. 

In the extraction of the metal ions, the carrier molecule in the membrane picks up metal ion/species from the feed solution forming a complex. This complex diffuses to the other side of the membrane where decomplexation occurs and the metal ion/species are released into the strip solution. Free carrier then diffuses back across the membrane for use in another cycle. This coupled transport through the SLM can take place by ... 

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