Global Selenium Cycle

The overview of the global cycle of selenium was presented by Nriagu (1989) for principal biogeochemical concentrations and pools (Table 11). 

An understanding of compartmental concentrations has only preliminary use in considering the global Se cycle, because it is the fluxes, driven by both natural (biogenic) and human (anthropogenic) forces, which still have many uncertainties in estimates. 

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Hypothetical problem for chemists consider the global cycle of selenium which has many chemical similarities to sulfur. Construct a box diagram for the global selenium cycle based on known similarities and differences of Se and S. 

Merian E. 1984. Introduction on environmental chemistry and global cycles of chromium, nickel, cobalt, beryllium, arsenic, cadmium and selenium, and their derivatives. Toxicol Environ Chem 8 9-38. 

The global flux of selenium from land to the oceans via rivers has been estimated as 1.538 X 10" tyr (Haygarth, 1994). The cycling of selenium from land to water is poorly understood, but —85% of the selenium in rivers is thought to be in particulate rather than dissolved form. 

Haygarth P. M. (1994) Global importance and global cycling of selenium. In Selenium in the Environment (eds. W. T. Frankenberger and S. Benson). Dekker, New York, chap. 1, pp. 1-27. 

Meeian E (1985) Introduction on Environmental Chemistry and Global Cycles of Chromium, Nickel, Cobalt, Beryllium, Arsenic, Cadmium, and Selenium. In Merian E, Erei RW, Haerdi W and Schlatter C, eds. Carcinogenic and Mutagenic Compounds, pp. 25-32. Gordon Breach, London. 

Both oxidation and methylation are microbial transformations involved in the redistribution and global cycling of arsenic. Oxidation involves the conversion of toxic arsenite to less toxic arsenate. Bacterial methylation of inorganic arsenic under anaerobic conditions may be a mechanism of arsenic detoxification. Fungi also transform inorganic and organic arsenic compounds into volatile methylar-sines. However, unlike methylated selenium which is nontoxic, the volatile arsine... 

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