Metal precipitation external

A rather more specific mechanism of microbial immobilization of metal ions is represented by the accumulation of uranium as an extracellular precipitate of hydrogen uranyl phosphate by a Citrobacter species (83). Staggering amounts of uranium can be precipitated more than 900% of the bacterial dry weight Recent work has shown that even elements that do not readily form insoluble phosphates, such as nickel and neptunium, may be incorporated into the uranyl phosphate crystallites (84). The precipitation is driven by the production of phosphate ions at the cell surface by an external phosphatase. 

In the wetlands of Idaho, the formation of an Fe(III) precipitate (plaque) on the surface of aquatic plant roots (Typha latifolia, cat tail and Phalaris arundinacea, reed canary grass) may provide a means of attenuation and external exclusion of metals and trace elements (Hansel et al, 2002). Iron oxides were predominantly ferrihydrite with lesser amounts of goethite and minor levels of siderite and lepidocrocite. Both spatial and temporal correlations between As and Fe on the root surfaces were observed and arsenic existed as arsenate-iron hydroxide complexes (82%). 

We have discussed the oxidation kinetics of metal alloys and of oxide solutions. These reactions lead to dispersed internal products rather than to external product layers. In the present section, let us pose a different question can the reduction of (nonmetallic) solid solutions e.g., (A,B)2Oa to (A,B)304, (A,B)304, to (A,B)0, or (A, B)0 to (A, B)) similarly lead to internally precipitated particles of the reduced product If so, then do these reactions occur in field III, II, or I of the Gibbs triangle plotted in Figure 9-2 We further note that the reaction (A,B)0->(A,B) is the fundamental process of ore reduction. 

AGbo > [ AGa0, almost pure metal A is precipitated in the internal reduction zone. The reaction at the front is induced by a point defect flux which stems from the difference in oxygen potentials (point defect concentration) between the internal reaction front and the external surface. The reaction front and surface act as source and sink for the point defect flux. For example, when we assume that (A,B)0 contains transition-metal ions (e.g., (Ni,Mg)0), the defects are cation vacancies and compensating electron holes. The (reducing) external surface acts as a vacancy sink according to the reaction... 

After thermalization, the defects begin to migrate, recombine, cluster, or precipitate provided the temperature is high enough to activate the motion of point defects. The various possible processes depend on defect concentration and their spatial distribution as well as on defect mobility and their interaction energies. As in non-metallic crystals, internal and external surfaces act as sinks for at least a part of the radiation induced defects in metals. 

In contrast, the thermodynamic template effect in macrocycle synthesis is a process by which the presence of a metal ion template stabilises thermodynamically, or removes (e.g. by precipitation) one particular (usually cyclic) product from an equilibrating mixture, driving the equilibrium towards this thermodynamic minimum. This leads us to the conclusion that any thermodynamically stabilising influence may drive an equilibrium mixture towards a particular product according to Le Chatalier s Principle (in an equilibrating situation, the system will react to diminish the effects of externally applied changes in conditions). 

Introduction of external chemical contamination from the sampling devices into the sample should be avoided because binding of metals to biological molecules is possible in vitro, and this can change the distribution of metal-containing species in the sample. Heavy metal contaminants could cause protein precipitation as well as irreversible deactivation of enzymes. 

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