Immobilization isomorphic substitution

Other metal sulfides, such as galena (PbS) and sphalerite (ZnS), may affect the mobility of arsenic in anoxic environments. However, immobilization depends on surface complexation rather than precipitation. In contrast to iron (oxy)(hydr)oxides (discussed later), As(III) adsorption on galena and sphalerite increases with pH (Bostick, Fendorf and Manning, 2003). Surface complexation does not occur by isomorphic substitution of lead or zinc, or by a ligand exchange mechanism. Instead, multinuclear, inner-surface arsenic-thiosulfide complexes probably form on galena or sphalerite surfaces (Bostick, Fendorf and Manning, 2003). 

The most straightforward immobilization method for catalytically active redox elements for liquid phase oxidation reactions consists of isomorphic substitution. Well-known systems with very peculiar properties that will not be treated in further detail are ... 

Zeolites are crystalline but versatile materials. They may be modified in many ways they can be tuned over a wide range of acidity and basicity, and of hydrophylicity and hydrophobicity, many cations can be introduced by ion exchange and isomorphous substitution is possible also allowing build-in of isolated redox centers (e.g. Ti) in the lattice. Moreover metal crystallites and metal complexes can be entrapped within the microporous environment. There is for instance much progress in enantioselective synthesis on chiral catalysts immobilized in microporous or mesoporous materials [16]. 

Rivers transport large amounts of dissolved substances to the sea. The residence times of every element are short with respect to the age of the oceans, yet the oceans are not a saturated solution for most of these elements. This means that they are eliminated from the ocean systems by other mechanisms than supersaturation and precipitation of one of their compounds. Most trace elements are incorporated by isomorphic substitution in the lattice of compounds of major elements, and thereby immobilized. Immobilization mechanisms like these keep the concentrations of many elements in the oceans at very low levels. 

The most efficient catalysts in liquid-phase oxidation of organic compoimds were crystalline mked oxides [1]. They are ionic mixed oxides or mixed oxides containing oxides supported on oxides. In the latter case, the catalytic activity of the oxide support is increased by adding one or more metal components or is obtained by immobilization of metal oxides on inactive oxide support. Metal ions were isomorphously substituted in framework positions of molecular sieves, for example, zeolites, silicalites, silica, aluminosilicate, aluminophosphates, silico-aluminophosphates, and so on, via hydrothermal synthesis or postsynthesis modification. Among these many mixed oxides with crystalline microporous or mesoporous structure, perovskites were also used as catalysts in liquid-phase oxidation. 

Much effort has been devoted to aluminophosphates with isomorphous Co2+ substitution (Co-AlPOs) (162). These structures are of interest for two reasons. First, Co is immobilized at lattice sites, and formation of clusters of Co is therefore impossible. Such clusters, which are catalytically less active, can in contrast be formed in materials with mobile Co, for example, Co-exchanged zeolite Y. Second, calcination of Co-AlPOs brings at least... 

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