Immobilization, of metals

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. 

Successful applications of the oxygen-modified CNFs are reported on immobilization of metal complexes ]95], incorporation of small Rh particles [96], supported Pt and Ru CNFs by adsorption and homogeneous deposition precipitation ]97, 98], Co CNFs for Fischer-Tropsch synthesis ]99], and Pt CNFs for PEM fuel cells [100]. 

The immobilization of metal catalysts onto sohd supports has become an important research area, as catalyst recovery, recycling as well as product separation is easier under heterogeneous conditions. In this respect, the iron complex of the Schiff base HPPn 15 (HPPn = iVA -bis(o-hydroxyacetophenone) propylene diamine) was supported onto cross-linked chloromethylated polystyrene beads. Interestingly, the supported catalyst showed higher catalytic activity than the free metal complex (Scheme 8) [50, 51]. In terms of chemical stability, particularly with... 

THE DIFFERENT PROCEDURES OF IMMOBILIZATION OF METAL COMPLEXES ON SOLID SUPPORTS... 

The utility of such reagents in the oxidation processes is compromised due to their inherent toxicity, cumbersome preparation, potential danger in handling of metal complexes, difficulties encountered in product isolation and waste disposal problems. Immobilization of metallic reagents on solid supports has circumvented some of these drawbacks and provided an attractive alternative in organic synthesis because of the selectivity and associated ease of manipulation. Further, the localization of metals on the mineral oxide surfaces reduces the possibility of their leaching into the environment. 

The immobilization of metal nanoparticles with a water-soluble polymeric material such as PVP has also been described. The groups of Choukroun and Chaudret have described the hydrogenation of benzene in a biphasic mixture with PVP-protected native Rh nanoparticles synthesized from the organometal-... 

E. F. Murphy, L. Schmid, T. Burgi, M. Maciejewski, A. Baiker, D. Gunther and M. Schneider, Nondestructive Sol-Gel Immobilization of Metal (salen) Catalysts in Silica Aerogels and Xerogels, Chem. Mater., 2001, 13, 1296. 

Figure 4.5 Ligands used to improve immobilization of metal catalysts in ionic liquids...

In summary, the examples given above demonstrate that immobilization of metal salts in a block copolymer micellar system followed by a reduction step is a suitable method to synthesize stable colloids with small particle sizes and narrow size distributions. Moreover, such systems are very interesting for catalytic applications because they offer the possibility of designing tailored catalysts for special demands and can be easily tuned by the choice and combination of different polymer block types and lengths, different types of the metal precursor and of the reduction method used. Additional introduction of further functionalities such as charges or chiral groups could make these catalyst systems even more versatile and effective. 

The HAZCON solidification process is an ex situ technology for the immobilization of metals and inorganic hazardous wastes in wet or dry soil and sludges. The technology is a cement-based process in which the contaminated material is mixed with pozzolanic materials such as Portland cement, a patented additive called Chloranan, and water. The process is capable of treating solids, sludges, semisolids, or liquids. The mixture hardens into a cohesive mass that immobilizes heavy metals. 

Natural pH control and immobilization of metals to protect the enriched population of methanogens... 

To conclude, it must be pointed out that the releases of metals in the glassfill / glassroad scenarios, although realistic, do not take into account the complex mechanisms of immobilization of metals by precipitation of stable secondary solid phases, either at the surface of the corroded HT material or in solution. These mechanisms of immobilization are difficult to... 

The third investigation track demonstrated the immobilization of metal-salen complexes in mesoporous materials and their use in the hydrolytic kinetic resolution of meso and terminal epoxides. The best results were obtained over cobalt-Ja-cobsen catalysts. The catalytic activity of the (S,S)-Co(II)-Jacobsen complex immobilized on Al-MCM-41 was comparable with that of the homogeneous counterpart. Several other immobilization methods are still under investigation. 

Francis (1990) has summarized the numerous possible microbially mediated reactions resulting in the mobilization or immobilization of metals and found that major interactions include oxidation-reduction processes, biosorption and immobilization by cell biomass and exudates, and mobilization by microbial metabolites. A profound issue in metal remediation is that through microbial action, metals can readily be re-mobilized, creating toxicity issues in sites where metals are not completely removed. 

Current approaches to metal bioremediation are based upon the complexation, oxidation-reduction, and methylation reactions just discussed. Until recently, interest was focused on technologies that could be applied to achieve in situ immobilization of metals. However, within the last few years, the focus has begun to shift toward actual metal removal, because it is difficult to guarantee that metals will remain immobilized indefinitely. 

Immobilization of a catalyst in (or on) a membrane Immobilization of enzymes or cells on polymeric membranes Immobilization of metals (Pd, Pt) on ceramic membranes... 

The chemical modification of the surface of solids has led to increased possibilities in a number of fields on laboratory as well as on industrial scale. Applications of modified silicas may be classified according to the field in which they are of interest. In each field the interaction with a specific type of molecules is effectuated. In the analytical field organic compounds and metal ions are selectively adsorbed. The chemical field aims at the immobilization of metal complexes for use as catalyst... 

Recently, we have successfully used these thermosensitive core-shell microgel particles as templates for the deposition of metal nanoparticles (Ag, Au, Pd, Pt, and Rh) [29, 59, 60], The reduction to metallic nanoparticles in the presence of microgel particles was done at room temperature via the addition of NaBPL and could be followed optically by the color change of the suspensions, as shown in Fig. 3. The immobilization of metal nanoparticles might be due to the strong localization of... 

Fig. 10.8. Simple biogeochemical model for metal mineral transformations in the mycorhizosphere (the roles of the plant and other microorganisms contributing to the overall process are not shown). (1) Proton-promoted (proton pump, cation-anion antiport, organic anion efflux, dissociation of organic acids) and ligand-promoted (e.g. organic adds) dissolution of metal minerals. (2) Release of anionic (e.g. phosphate) nutrients and metal cations. (3) Nutrient uptake. (4) Intra- and extracellular sequestration of toxic metals biosorption, transport, compartmentation, predpitation etc. (5) Immobilization of metals as oxalates. (6) Binding of soluble metal species to soil constituents, e.g. clay minerals, metal oxides, humic substances.

The imprinting of metal oxides is a relatively new method for the immobilization of metal complexes. The case of Rh2(CO)4Cl2 on silica is illustrative. The dimer is first adsorbed onto the surface of silica, most likely through a surface hydroxyl group, and then reacted with trimethylphosphite, P(OMe)3. Amorphous silica is then... 

Polymers can also be used for the physical immobilization of metal complexes. For example, the degree of swelling observed for 1% crosslinked polystyrene varies with solvents more swelling is observed in THF than in methanol. Thus, the cationic complex [(diphos)Rh(NBD)]PF6 can be adsorbed onto polystyrene in THF. When dried and extracted with methanol, the complex is bound within the polystyrene matrix. The immobilized complex is a catalyst for the hydrogenation of 1-hexene in methanol. ... 

---