Geochemical enrichment

Szalay, A., 1958. The significance of humus in the geochemical enrichment of uranium. Geneva, Proc. Internat. Conf. Peaceful Uses Atom. Energy, 2nd, 2 182—186. 

Szalay, A., 1964. Cation-exchange properties of humic acids and their importance in the geochemical enrichment of UOl and other cations. Geochim. Cosmochim. Acta, 28 1605-1614. 

Acid magmas from the interior of the earth react with basic rocks and start to sohdi-fy. Uranium is enriched in the remaining melts. When these finally solidify, the uranium content may be 100 g/tonne (100 ppm, 0.01%). Compared to the mean content in the earth s crust, 2.7 ppm, this is a considerable geochemical enrichment. In addition, uranium is concentrated into phosphate minerals and coal, especially brown coal. This is the background to the fact that coal-fired power stations emit more uranium than nuclear power stations ... 

Today it has become clear that the effect of trace elements in living systems, in food, and in the environment depends on the chemical form in which the element enters the system and the final form in which it is present. The form, or species, clearly governs its biochemical and geochemical behaviour. lUPAC (the International Union for Pure and Applied Chemistry) has recently set guidelines for terms related to chemical speciation of trace elements (Templeton et al. 2000). Speciation, or the analytical activity of measuring the chemical species, is a relatively new scientific field. The procedures usually consist of two consecutive steps (i) the separation of the species, and (2) their measurement An evident handicap in speciation analysis is that the concentration of the individual species is far lower than the total elemental concentration so that an enrichment step is indispensable in many cases. Such a proliferation of steps in analytical procedure not only increases the danger of losses due to incomplete recovery, chemical instability of the species and adsorption to laboratory ware, but may also enhance the risk of contamination from reagents and equipment. 

There are also natural geochemical anomalies where soils are enriched by cadmium, for example, in the central parts of Sweden. Here the cultivation of crops accumulating cadmium (grains, potato, some grasses) is not recommended. In the coastal marine areas the cadmium mobility in soils is stimulated by its complexation with chlorine. 

Figure 1. Bio geochemical sub-region and provinces enriched by selenium. 1—sub-region with Se content in soil from 0.2 to 0.5 ppm and in plant species from 0.08 to 0.5 ppm 2—Ulug-Hemsk and Turan-Uluks biogeochemical provinces with Se content in soil as much as 03-6.0ppm and in plant species from 0.1 to 13.1 ppm by dry weight (Ermakov, 1993).

Evidence also exists for a terrestrial source of the iridium enrichment as volcanic ejecta is enriched in this rare element. Thus, the enriched sediment layer could also have been caused by an abrupt and large increase in volcanic activity. Evidence for this is suggested by high levels of volcanic ash, soot, and shocked minerals in the iridium-enriched layer. Other geochemical characteristics of this sediment layer appear to have been caused by acid rain and tsunamis, both of which are by-products of volcanic activity. 

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