Solution-mediated transport mechanism

The solution-mediated transport mechanism has been extensively discussed in the literature. In the middle of the 1960s, according to their studies on the crystallization of zeolite A, Kerr and Ciric proposed the solution-mediated transport mechanism. They believed that the nucleation and growth of zeolite crystals happened in solution. The initial gel was partially or completely dissolved in the solution with the formation of active silicate and aluminate ions. These active silicate and aluminate ions could further form the structural units of zeolite crystal. 

Zhdanov and colleagues for the first time discussed in detail the solution-mediated transport mechanism.[27] They believed that 1) nucleation happened in the solution or at the interface of the solution and solid gel 2) the further growth of zeolite nuclei consumed the silicate and aluminate ions in solution 3) the solution supplied the soluble structural units for the growth of zeolite crystal and 4) the consumption of the liquid component during the crystallization process resulted in the continuous dissolution of solid gel. 

Zhdanov and colleagues found that the growth rate of zeolite crystal depended on the concentration of polysilicate and aluminate ions in the liquid phase and the composition of the liquid phase was not constant during the crystallization process.[27] This study strongly supported the liquid-phase mechanism. 

Powerful evidence for the liquid-phase mechanism comes from the direct crystallization of zeolite from clear solution. In the early 1980s, Koizumi and coworkers carried out an extensive study on this topic. They directly synthesized analcime, hydroxysodalite, zeolite B, mordenite, zeolite P, faujasite,[30] erionite, and potassium chabazite from clear solution. Pang et al. directly crystallized zeolite A[31] and FAPO-5[32] from clear solution as well. The study on the direct synthesis of faujasite [30, 33] from clear solution will be elaborated below. 

In the past 10 years, many characterization techniques have been used in the in situ study of the formation mechanism of zeolite from clear solution. For example, Honssian et al. studied the TPA-silicalite-1 system by using the small-angle X-ray scattering technique [34] Carlsson et al. performed a modeling study on the crystallization of silicalite-I from a liquid-phase system [35] Smaihi and colleagues applied in situ 27A1-, 

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Another name for solid hydrogel transformation mechanism is solid-phase mechanism, while solution-mediated transport mechanism is also called liquid-phase mechanism. The main difference in explaining the formation process of zeolites by these two mechanisms lies in whether the liquid component is involved during the crystallization of zeolites. The views of these two mechanisms are opposite to each other and have their own experimental supporting evidence. To date, the liquid-phase mechanism has more experimental support than does the solid-phase mechanism. 

Figure 5.14 Illustration of solution-mediated transport mechanism...

Important Issues Related to the Solution-mediated Transport Mechanism... 

Depending upon the mechanism that is employed by the organism to accumulate the solute, internalisation fluxes can vary both in direction and order of magnitude. The kinetics of passive transport will be examined in Section 6.1.1. Trace element internalisation via ion channels or carrier-mediated transport, subsequent to the specific binding of a solute to a transport site, will be addressed in Section 6.1.2. Finally, since several substances (e.g. Na+, Ca2+, Zn2+, some sugars and amino acids) can be concentrated in the cell against their electrochemical gradient (active transport systems), the kinetic implications of an active transport mechanism will be examined in Section 6.1.3. Further explanations of the mechanisms themselves can be obtained in Chapters 6 and 7 of this volume [24,245]. 

The sorption of TcOj by roots of hydroponically grown soybean seedlings (Glycine max.) was shown to be linear from 10 M pertechnetate solutions for at least 6 h and to exhibit characteristics of carrier-mediated transport commonly associated with the sorption of nutrient ions in higher plants. Analyses of TcO uptake in the presence of individual nutrient anions revealed the sorption to be competitively inhibited by sulphate, phosphate, selenate. and molybdate indicating the use of common transport mechanisms [54],... 

Boistelle R, Astier JP (1988) Crystallization mechanisms in solution. J Cryst Growth 90 14-30 Boskey AL, Posner AS (1973) Conversion of amorphous calcium phosphate to microcrystalline hydroxylapatite. ApH-dependent, solution-mediated, solid-solid conversion. JPhys Chem77 2313-2317 Boskey AL, Posner AS (1976) Formation of hydroxylapatite at low supersaturation. J Phys Chem 80 40-45 Boudreau AE, Mathez EA, Mccallum IS (1986) Halogen geochemistry of the Stillwater and Bushveld complexes—Evidence for transport of the platinum-group elements by Cl-rich fluids. J Petrol 27 967-986... 

Type 2 (Fig. 19.3-lc). These carry the difliising species across the membrane by incorporating carrier or chelating compounds in the membrane. This kind of canier-mediated transport can be illustrated by the separation of various metal ions, such as cadmium, chromium, copper, and mercuty, from their aqueous solutions by the use of oil-type liquid membranes containing oil-soluble liquid ion-exchange agents. " " These mechanisms have been described in detail elsewhere and are not repeated here. 

Far higher selectivities can be obtained by adding a carrier molecule to the liquid (membrane) which has a high affinity for one of the solutes in phase 1. The carrier accelerates the transport of this specific component. This type of transport is called carrier mediated transport or facilitated transport. The mechanism of facilitated transport can be demonstrated by the simple experiment dqpicted schematically in figure VI > 32. 

Theoretical aspects of mediation and electrocatalysis by polymer-coated electrodes have most recently been reviewed by Lyons.12 In order for electrochemistry of the solution species (substrate) to occur, it must either diffuse through the polymer film to the underlying electrode, or there must be some mechanism for electron transport across the film (Fig. 20). Depending on the relative rates of these processes, the mediated reaction can occur at the polymer/electrode interface (a), at the poly-mer/solution interface (b), or in a zone within the polymer film (c). The equations governing the reaction depend on its location,12 which is therefore an important issue. Studies of mediation also provide information on the rate and mechanism of electron transport in the film, and on its permeability. 

However, despite this lack of a basic understanding of the electrochemistry of these materials, much progress has been made in characterizing polymerization mechanisms, degradation processes, transport properties, and the mediation of the electrochemistry of species in solution. These advances have facilitated the development of numerous applications of conducting polymers, and so it can be anticipated that interest in their electrochemistry will remain high. 

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