Metal oxides, short-range-ordered

Metal oxides have a significant role in influencing physical, chemical, and biological properties of soils. They may exist as ciystalline minerals, as short-range ordered (SRO) mineral colloids, or as surface coatings on clay minerals and organic matter. Organic compounds influence the formation, transformation, and surface properties of these metal oxides. The SRO A1... 

Xue, J. Huang, P.M. (1995) Zinc adsorption-desorption on short-range ordered iron oxide as influenced by citric add during its formation. Geoderma 64 343—356 Yamada, K. Harato,T. Shiozaki,Y. (1980) Flocculation and sedimentation of red mud. Light Metals 39-50... 

Traditionally, X-ray absorption edge measurements have been used to determine oxidation states of metals in complex materials. The extended X-ray absorption fine structure (EXAFS), on the other hand, provides structural information such as bond distances and coordination numbers even with powdered samples, crystalline or amorphous, the fine structure essentially resulting from short-range order around the absorbing atom. The technique is also useful for studying solid surfaces (SEXAFS). The observation of fine structure beyond the K-absorption edges of materials dates back to... 

Fused catalysts allow the combination of compounds and elements in atomic dispersions which do not mix either in solution (e.g. oxides) or in the solid state. Melting provides the necessary means to generate an intimate, eventually atomically disperse distribution a carefully controlled solidification can preserve the mctastablc situation in the melt down to operation temperature. In the melt the preformation of molecules such as oxo complexes or alloy clusters can occur. The final short-range order of the catalyst is predetermined. Examples are alloys of noble metals with elements located in the main group sections or in... 

While it is to some extent arbitrary, a classification of this kind provides a means of discussing some of the general features now emerging from studies of metallic oxides. We have stressed the evidence that a nonstoichiometric phase is disordered, but may be related to chemically similar phases of fixed composition where an anomaly of structure is ordered and identifiable by x-ray diffraction methods. Where such ordered phases are found, it is possible that features of them are retained as blocks or domains with short range order in the related berthollide. Efforts should be directed towards order-disorder effects, with a view to reconsidering the status of the nonstoichiometric compound with a very wide composition range. 

Formation of short-range-ordered metal oxides 7... 

Interactions of soil minerals with organic matter and microorganisms exert a vital role in the formation of short-range-ordered metal oxides, humic substances, mineral colloid-humic complexes, aggregates, and the structure and porosity of soils. These nonliving and living reactive components and the structural stability... 

Formation of Short-Range-Ordered Metal Oxides... 

Adsorption of trace elements in cationic form is pH-dependent and is characterized by a pH range, where the amount of a heavy metal that is bound to a sorbent increases abruptly to nearly 100%, known as adsorption edge. Fig. 1 shows the adsorption of Pb, Cu, Zn, and Co at different pH values onto selected short-range-ordered oxides (a noncrystalline A1 precipitation product [RO], ferrihydrite [Roo], and mixed Fe-Al oxides formed at different initial Fe/Al molar ratios [Rl-RlO]) (Table 1), whereas Fig. 2 shows the adsorption of Cu onto well-crystallized metal oxides, goethite, and bayerite. In the region in which adsorption increases rapidly, the species MeOH and Me(OH)2 of each metal were negligible. 

Value of pHjo" for Pb, Cu, Zn short-range-ordered metal oxides and Co adsorbed onto selected crystalline and... 

Heavy metals and metalloids are selectively adsorbed on variable-charge minerals (e.g., Al, Fe and Mn oxides), which occur in soils in advanced stages of weathering and in rhizosphere soils. Weathering induced hy LMMOAs and other biomolecules facilitates the release of Fe and Al from primary and secondary minerals, promoting the formation of Fe and Al oxides, usually of short-range order. 

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