Hydrous oxides aluminium

Many of the important chemical reactions controlling arsenic partitioning between solid and liquid phases in aquifers occur at particle-water interfaces. Several spectroscopic methods exist to monitor the electronic, vibrational, and other properties of atoms or molecules localized in the interfacial region. These methods provide information on valence, local coordination, protonation, and other properties that is difficult to obtain by other means. This chapter synthesizes recent infrared, x-ray photoelectron, and x-ray absorption spectroscopic studies of arsenic speciation in natural and synthetic solid phases. The local coordination of arsenic in sulfide minerals, in arsenate and arsenite precipitates, in secondary sulfates and carbonates, adsorbed on iron, manganese, and aluminium hydrous oxides, and adsorbed on aluminosilicate clay minerals is summarized. The chapter concludes with a discussion of the implications of these studies (conducted primarily in model systems) for arsenic speciation in aquifer sediments. 

Aluminium hydrous oxide sols consisting of spherical particles of narrow size distribution are prepared by ageing aluminium salt solutions containing complexing anions (e.g. sulphate) at 98 °C for many hours.419 Once the particles are formed, the sulphate ions are removed by exchange with OH" from an added base. 

Catone, D.L. and Matijevich, E. (1974) Aluminium hydrous oxide sols. Journal of Colloid and Interface Science, 48, 291-301. 

Scandium is not an uncommon element, but is difficult to extract. The only oxidation state in its compounds is -I- 3, where it has formally lost the 3d 4s electrons, and it shows virtually no transition characteristics. In fact, its chemistry is very similar to that of aluminium (for example hydrous oxide SC2O3, amphoteric forms a complex [ScFg] chloride SCCI3 hydrolysed by water). 

In addition to the soluble chemical species and possible solid phase species described in the previous sections no discussion on speciation can be complete without the consideration of surface species. These include the inorganic and organic ions adsorbed on the surface of particles. Natural systems such as soils, sediments and waters abound with colloids such as the hydrous oxides of iron, aluminium, manganese and silicon which have the potential to form surface complexes with the various cationic and anionic dissolved species (Evans, 1989). 

Kinniburgh, D. G., Jackson, M. L., and Syers, J. K. (1976). Adsorption of alkaline earth, transition, and heavy metal cations by hydrous oxide gels of iron and aluminium. Soil Sci. Soc. Am.J. 40, 796—799. 

Surface Complex Formation. Metal ions form both outer and inner sphere complexes with solid surfaces, e.g. hydrous oxides of iron, manganese, and aluminium. In addition, metal ions, attracted to charged surfaces, may be held in a diffuse layer, which, depending upon ionic strength, extends several nanometres from the surface into solution. 

Hydrous Oxides. This term is generally taken to include the oxides, hydroxides, and oxyhydroxides of aluminium, iron and manganese, which form in soil when these elements are released from primary minerals by weathering. They exist mainly as small particles in the claysized fraction of a soil (<2 pm), and also as coatings on other soil minerals or as components of larger aggregates. 

Reisenauer, H. M., Tabikh, A. A., and Stout, P. R. (1962). Molybdenum reactions with soils and hydrous oxides of iron, aluminium and titanium. Proc. Soil Set Soc. Am. 26 23-7. 

R. M. McKenzie, The adsorption of lead and other heavy metals on oxides of manganese and iron, Aust. J. Soil Res. 18 61 (1980). H. Kerndorf and M. Schnitzer, Sorption of metals on humic acid, Geochim. Cosmochim. Acta 44 1701 (1980). D. G. Kinniburgh, M. L. Jackson, and J. K. Syers, Adsorption of alkaline earth, transition, and heavy metal cations by hydrous oxide gels of iron and aluminium, Soil Sci. Soc. Am. J. 40 796 (1976). H. Farrah and W. F. Pickering, Influence of clay-solute interactions on aqueous heavy metal ion levels, Water, Air and Soil Pollution 8 189 (1977). 

Until comparatively recently, the only reported stable inorganic hydrosols were primarily sols of elements such as gold, sulphur, selenium, etc. and compoimds such as silica, lead iodate, silver halides, etc. A considerable amount of attention is now being paid, however, to the preparation of mono-dispersed hydrous metal oxides, which are chemically considerably more complex than other crystalline or stoichiometrically well-defined materials and are of interest as potential catalysts. Examples include the hydrous oxides of chromium and aluminium (spheres) and copper and iron (polyhedra) with particle sizes < 1 pm. One manufacturing procedure consists of ageing aqueous... 

The present review is concerned mainly with the electrochemical formation and redox behavior of the hydrous oxides of those transition metals centered within and around Group VIII of the periodic table. There have been a number of recent reviews of monolayer oxide growth on these metals so that this area will not be treated here in an exhaustive manner. Structural data for many of the systems (especially direct evidence obtained by investigation of hydrous films themselves) are very sparse at the present time. However, some idea of the type of material involved can be obtained from structural studies of oxide battery materials a useful introduction to the structural complexities in this area in general is Alwitt s account of the aluminium oxide system. An important feature of hydrous oxides, not normally as evident with their anhydrous analogs, is their acid-base behavior and in particular the influence of the latter on the redox properties of the hydrous material. Because of its central role in many oxide (especially hydrous oxide) processes, and its relative neglect in the electrochemistry of these systems until quite recently, this add-base character of oxide systems will be reviewed here in some detail. 

Cabrera et al. [95] determined total dissolved and suspended phosphorus in natural waters by a method involving digestion with hydrogen peroxide and sulphuric acid, errors may be caused by adsorption of phosphorus on hydrous iron and aluminium oxides formed during neutralization prior to filtration. It is proposed that this can be prevented by adding extra sulphuric acid after neutralization, to dissolve such oxides and release the adsorbed phosphorus into solution. 

Instead of hydrous aluminum oxide, aluminum foils have very recently been proposed for the extraction of lithium from sea water. However, not the aluminium metal itself, but its corrosion product hydrous aluminum oxide formed in sea water on the metal surface was found to be the effective lithium binding agent15) thus, there seems to be no fundamental difference compared to the direct application of pure hydrous aluminum oxide. 

Hgure 2 (A) Calculated adsorption by a hydrous ferric oxide of several metal cations at a tolal added melal concentration of 10 mol I using the diffuse double layer surface complexation model. (From Dzombak DA and Morel FMM (1990) Surface Complexation Modeling Hydrous Ferric Oxide. New York Wiley.) (B) Experimentally measured cadmium ([Cd]total = 0.3 mmol I h adsorption by O (open circles) and B (filled circles) horizons (18.5gdm ) in 0.01 moll NaNOs. (Data from Lumsdon DG (2004) Partitioning of organic carbon, aluminium and cadmium between solid and solution in soils application of a mineral-humic particle additivity model. European Journal of Soil Science, in press.)... 

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