Depletion zones

Dissolved oxygen is the enemy of anaerobic bacteria. Naturally occurring corrosion consumes oxygen. Without convective replenishment, oxygen-depleted zones form and corrosion by anaerobes is usually worse. 

Fig. 1.8(a) Intergranular precipitation of chromium carbide particles in a sensitised austenitic stainless steel and the consequent chromium-depleted zones adjacent to the grain boundaries, (b) variation of the chromium content across a grain boundary in a sensitised austenitic stainless steel (l8Cr) and (c) intergranular corrosion of a sensitised austenitic stainless steel... 

A non-adsorbing polymer in solution can also destabilise a dispersion through a mechanism called depletion flocculation. When polymer molecules do not interact favourably with the particle surfaces from an enthal-pic perspective, they are repelled from the surface regions due to entropic reasons. A depletion zone around the particles is created which has a lower average polymer concentration than the bulk solution. The osmotic... 

X. L. Li. E. George, and H. Marschner, Extension of the phosphorus depletion zone in VA-mycorrhizal white clover in calcareous. soil. Plant and Soil I3I 4 (1991). X. L. Li, E. George, and H. Marschner, Acquisition of phosphorus and copper by VA-mycorrhizal hyphac and root-to-shoot transport in white clover. Plant and Soil 135 49 (1991). 

Many more papers deal with rhizosphere phosphatase activity (63-83) in the presence of a number of different plant species this will partly be due to the simplicity of the enzyme activity assay (85,86) and the generally reported, well-correlated variation trends among organic and inorganic phosphorus content and phosphatase activity. More precisely, closer to the roots, the inorganic P depletion zone in comparison with bulk soil is more pronounced in addition, organic and inorganic P contents are inversely correlated, and the mineralization rate of or-... 

Figure 11 Root systems as simulated by the computer program Simroot. The systems display herriitgbone architecture (a) dichotomous architecture (b) and bean root architecture (c). The bean root system (c) is shown in

It was shown in [98] that during acceptor adsorption in a broad band semiconductor of -type characterized by availability of an a priori surface-adjacent depletion zone developing depletion of BSS levels slows... 

The rate of electron accumulation at ionized traps in the depletion zone of the Schottky barrier in the Au/ZnO contact is in proportion to the concentration of unoccupied traps, frequency of metal parti-cle/metastable atom interaction events, and to the probability of electron capture per a trap in a single event of interaction between metastable atoms and metal particle. 

As stated earlier, mycorrhizae enhance nutrient absorption. Greater soil exploitation by mycorrhizal roots as a means of increasing phosphate uptake is well established. The normal phosphate depletion zone around non-mycorrhizal roots is 1-2 mm, but an endomycorrhizal root symbiont increased this zone to 7 cm (140). This ability to increase the nutritional level (particularly with regard to phosphorus), and subsequently the overall better growth dynamics of the mycorrhizal plant has been suggested as the reason for the salt (43) and drought (44-46) tolerance and increased nodulation (47) observed in mycorrhizal associations. Another interesting aspect of this enhanced nutrient uptake is the possible effect of mycorrhizae on competitive ability between two plant species. Under some conditions, mycorrhizal... 

Finally, in this section we consider the case where Xs = 0 i.e. where no adsorption takes place. There is then a depletion of segments in the region near the interface (figure 5). It is useful to define an effective depletion zone of thickness, <5, defined such that the two shaded areas are equal, and given by,... 

The principle of depletion is illustrated in Figure 1. If a surface is in contact with a polymer solution of volume fraction , there is a depletion zone near the surface where the segment concentration is lower than in the bulk of the solution due to conformational entropy restrictions that are, for nonadsorbing polymers, not compensated by an adsorption energy. The effective thickness of the depletion layer is A. Below we will give a more precise definition for A. 

Figure A. Overlap of depletion zones. The hatched region corresponds to the volume of solvent that is transferred when two particles of radius a and depletion thickness A come into close contact.

Fig. 5.18 Anode life calculations using Equations 5.10-5.13, with a depletion zone of 1 jim, for a coating composition of 30% Ru02 + 70% Ti02 (total Ru loading = 5g nrT2 coating thickness = 10 jam operating current density = 3 kA nrT2). (A) Effect of shutdowns (S/D) (B) effect of increased shutdowns.

Figure 6.22 Profiles of Zn fractions in anaerobic soil near a planar layer of rice roots after indicated times of root-soil contact. The dashed lines are the hypothetical profiles if no Zn had been removed by the plants, estimated by interpolation of the lines beyond the depletion zones. Corresponding profiles of Fe(ll), Fe(lll) and pH are in Figure 6.16 (Kirk and Bajita, 1995). Reproduced by permission of Blackwell Publishing...

Figure 2. Two morphological models used to describe the origin of the ionic SAXS maximum observed for Nafion (a) the modified hard-sphere model depicting interparticle scattering and (b) the depleted-zone core—shell model depicting intraparticle scattering. (Adapted with permission from ref 36. Copyright 1981 American Chemical Society.)...

Figure 5.4. Scheme of (a) a spatial fluctuation and (b) a local depletion zone due to monolayer density fluctuations in thin liq-nid tihns. 

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