Solid metal phase fractionation

Shan and Chen [32] reported that various proportions of metals released from exchangeable, carbonate-bound iron, manganese oxide-bound and organic-bound fractions were readsorbed onto the other solid geochemical phases during sequential extractions. 

Terrestrial materials (river sediments, lake sediments, and urban particulate matter) appear to have between 50% and 70% exchangeable Pb and Zn while marine sediments contain very little exchangeable metal but appreciably more reducible and much more residual Pb and Zn (Kersten and Forstner, 1995). This may not be too surprising as exchangeable metals are released once freshwater mixes with salt water and redistribution in the marine environment results in some precipitated phases (carbonates, Fe/Mn oxyhydroxides) and the relative increase in the lithogenic fraction. In future, the solid-phase identification techniques should be used to classify the sediments that are to be subjected to selective extraction techniques for the purpose of understanding the heavy metal phase associations. 

What are the absolute and relative abundances of important sorbent solids and what fraction of their surface areas are exposed to flowing water Any adsorption model we select that assumes a finite number of sorption sites, requires, as input, the area of a sorbing phase exposed to a given volume of water I.e.g., Cs(g/L) x 5 (m /g)] and a surface site density [ (sites/m-)] for that phase. Can we measure or estimate these values Such measurements and estimates are extremely difficult for metal adsorption by modern stream sediments, which may be mix-... 

Courchesne, F., Seguin, V., and Dufresne, A. (2001). Solid phase fractionation of metals in the rhizosphere of forest soils. In Trace Elements in the Rhizosphere, ed. Gobran, G. R., Wenzel, W. W., and Lombi, E., CRC Press, Boca Raton, FL, 189-206. 

The principal commercial source of rubidium is accumulated stocks of a mixed carbonate produced as a byproduct in the extraction of lithium salts from lepidohte. Primarily a potassium carbonate, the byproduct also contains ca. 23 wt.% rubidium and 3 wt.% cesium carbonates. The primary difficulty associated with the production of either pure rubidium or pure cesium is that these two elements are always found together in nature and also are mixed with other alkali metals because these elements have very close ionic radii, their chemical separation encounters numerous issues. Before the development of procedures based on thermochemical reduction and fractional distillation, the elements were purified in the salt form through laborious fractional crystallization techniques. Once pure salts have been prepared by precipitation methods, it is a relatively simple task to convert them to the free metal. This is ordinarily accomplished by metallothermic reduction with calcium metal in a high-temperature vacuum system in which the highly volatile alkali metal is distilled from the solid reaction mixture. Today, direct reduction of the mixed carbonates from lepidolite purification, followed by fractional distillation, is perhaps the most important of the commercial methods for producing rubidium. The mixed carbonate is treated with excess sodium at ca. 650 C, and much of the rubidium and cesium passes into the metal phase. The resulting crude alloy is vacuum distilled to form a second alloy considerably richer in rubidium and cesium. This product is then refined by fractional distillation in a tower to produce elemental rubidium more than 99.5 wt.% pure. 

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