Diffusion-limited uptake metal

Figure 9. Schematic representation of concentration profiles at the biological surface in the case of a diffusion-limited uptake. Note that the ratio of bound metal to free metal is not drawn to scale in reality, the ratio at the biological surface is always larger than that in solution. The figure assumes that the total concentration of ligand is much greater than the total concentration of metal. For further details, refer to [142,331,333]...

For the biological limitation of trace metal internalisation, complex formation will invariably decrease the concentration of free metal ion and thus decrease the biouptake fluxes and carrier-bound metal (FIAM, BLM). In the case of a diffusion-limited internalisation, complex labilities and mobilities become much more pertinent when determining uptake fluxes. As shown earlier, few experiments have been designed to identify diffusion limitation of metal uptake fluxes, despite the fact that such a limitation is possible (Figure 10). Competition experiments that can distinguish a kinetic from a thermodynamic control are rare. In these areas, an important research focus is... 

In the case of a diffusion limitation, the free metal ion is largely consumed at the surface of the organism such that the concentration gradient of M in the external medium is strongly perturbed by biological uptake. The flux will depend on the concentration gradient of M that occurs between the bulk... 

The general principles of metal uptake by biota from soil systems are treated in chapters 7 to 10 of the book. The same rules govern radionuclide uptake by biological systems. Uptake and assimilation depend on the chemical and biological properties of the element. The rate-limiting step for the uptake of many radionuclides, those strongly immobilized by soil constituents, is the movement to the interface between the soil solution and the biological membrane. If absorption is more rapid than the movement of the solute to the interface, uptake becomes diffusion limited. The adsorption properties of the soil are therefore determinant. 

To describe the dynamics of metals at biological interphases in the presence of various ligands, the kinetics of dissociation of the complexes have to be taken into account in relation to the diffusion and to the uptake kinetics ([14] and Chapters 3 and 10 in this volume). Based on kinetic criteria, labile and inert complexes can be distinguished as limiting cases with regard to biological uptake ([14] and Chapter 3, this volume). 

It is perhaps wise to begin by questioning the conceptual simplicity of the uptake process as described by equation (35) and the assumptions given in Section 6.1.2. As discussed above, the Michaelis constant, Km, is determined by steady-state methods and represents a complex function of many rate constants [114,186,281]. For example, in the presence of a diffusion boundary layer, the apparent Michaelis-Menten constant will be too large, due to the depletion of metal near the reactive surface [9,282,283], In this case, a modified flux equation, taking into account a diffusion boundary layer and a first-order carrier-mediated uptake can be taken into account by the Best equation [9] (see Chapter 4 for a discussion of the limitations) or by other similar derivations [282] ... 

It is suggested that whereas metal ions usually penetrate poorly into a microbial cell the formation of a metal chelate may facilitate metal ion uptake from the medium in which the cell is growing, leading to toxicity. Alternatively, diffusion of the chelating agent into the cell may lead to a critical depletion of essential metal ions in the cell. Metal [Fe(II), Cu(II), Cd, Ni, Ru(II)] chelates of 1,10-phenanthroline bases are lethal to cultured mammalian cells at concentrations down to 0.1 /uM98). This cytotoxicity limits their possible medical uses to topical applications. 

The metal ion uptake profiles are shown in Fig. 11.1 for variations of NaCNS concentration (Fig. 11.1a), temperature (Fig. 11.1b) and plasticisation drawing (Fig. 11.1c) of the precipitation bath for Co uptake. Similar curves were obtained with Ni. Table 11.2 shows the data for different parameters related to a fully metallised fibre obtained after metallisation of PAN fibres, produced under different experimental conditions of the precipitation bath. Despite the fact that the uptake profiles are considerably different and the data obtained (diffusion coefficient) confirms this, no remarkable changes are observed in the total amount of metal absorbed by the fibre. This means that saturation for metal uptake is obtained independently of the precipitation bath parameters. The role of these parameters is limited to the rate of metal uptake, and a choice for the optimal value of these parameters should be based on economic reasons first the consumption of chemicals and energy and, secondly, the processing time. Taking these two criteria into account, a NaCNS concentration of about 12%, a temperature of 283 K and a plasticisation drawing of 500% are further used. 

This is a case of diffusion from a stirred solution with limited volume. The CIM is considered as a sheet of uniform material of thickness 2w placed in the solution containing the solute, which is allowed to diffuse into the sheet. The sheet occupies the space -w < X < while the solution is of limited extent and occupies the space —w — a < x < —w, w < x < + w + a. The concentration of the solute in the solution is always uniform and is initially Cq while initially the sheet is free from solute. Considering an apparent metal-ion diffusivity within the CIM phase, the PDF that must be solved in this case of metal uptake through a plane sheet (the thickness of the CIM) from a solution of limited volume is given as ... 

The chemical properties of the other essential transition elements simplify their transport properties. For zinc there is only the -f 2 oxidation state, and the hydrolysis of this ion is not a limiting feature of its solubility or transport. Zinc is an essential element for both animals and plants.In general, metal ion uptake into the roots of plants is an extremely complex phenomenon. A cross-sectional diagram of a root is shown in Figure 1.6. It is said that both diffusion... 

Bruemmer et al. (55) studied Ni, Zn, and Cd sorption on goethite, a porous iron oxide known to have defects within the structure in which metals can be incorporated to satisfy charge imbalances. At pH 6, as reaction time increased from 2 hours to 42 days (at 293K), sorbed Ni increased from 12 to 70% of Ni removed from solution, and total increases in Zn and Cd sorption over this period increased 33 and 21%, respectively. The kinetics of Cd, Zn, and Ni were described well with a solution to Pick s second law (a linear relation with the square root of time). Bruemmer et al. (55) proposed that the uptake of the metal follows three-steps (i) adsorption of metals on external surfaces (ii) solid-state diffusion of metals from external to internal sites and (iii) metal binding and fixation at positions inside the goethite particle. They suggest that the second step is the rate-limiting step. However, they did not conduct microscopic level experiments to confirm the proposed mechanism. In view of more recent studies, it is likely that the formation of metal-nucleation products could have caused the slow metal sorption reactions observed by Bruemmer et al. (55). 

It is interesting to observe that also in these cases, cyanide ion depresses the oxidation rates and that the rates of oxygen uptake with these metal complexes exceed in several cases the limiting rate of oxygen diffusion. 

Ingestion. The absorption of swallowed materials is a complex phenomenon and during the course of evolution elaborate systems have developed to control the uptake of substances from the gut, principally the so-called essential trace elements which are themselves toxic if their concentration in the blood exceeds certain limits. There are thus a number of specific transport mechanisms for the transfer of essential metals across the gut wall, and some toxic metals, such as lead and cadmium, can take advantage of these to gain access to the body. Even so, it is necessary for the metals to be in the correct chemical state for absorption to take place. Highly soluble metal compounds may cross the gut wall by simple diffusion, and organic compounds which are fat soluble can also... 

---