Germanium: environmental occurrence, importance and speciation

This review discusses the various aspects of the bio-geochemistry of germanium, and of its technological, economical and environmental importance. Despite the relatively low annual production and consumption of this semi-metal (ca. 80 metric tons/a) there are important technological applications of this element in the semiconductor, infrared optics and fibre optics/telecommunication industries. A small, but not insignificant fraction of this element is used for the production of pharmaceuticals and nutritional supplements, although its actual merits have not been fully demonstrated yet, while they are opposed to chronic toxicity of the element when being administrated at relatively high doses for an extended period of time. Neither the exact mechanism of action in the case of cancer treatment or the treatment of infectious diseases is known, nor the reason for the toxicity of inorganic species of this element. In plants, Ge can partially substitute for B in the case of boron deficiency, although deficiency symptoms are still seen with a lag period of ca. one to three weeks. In biogeochemical respect, germanium and silicon react very similar, as if Ge were a very heavy isotope of Si. Their molar ratio is typically in the order of 0.6 × 10⁻⁶, with significant deviations only where germanium is complexed and transported, e.g., by humic-rich waters. Germanium is a very conservative element in biogeochemical terms: It hardly shows involvement in any biogeochemical reaction cycles and is mainly present in the form of complexes or hydroxo-compounds of the tetravalent germanium. The only naturally occurring organogermanium compounds are mono- and dimethylgermanium which are believed to be formed by microbiological activity in continental zones containing Ge-rich minerals, and then are leached into rivers, and finally into the open sea. It becomes evident that, although very sophisticated technological uses of germanium exist, a better understanding of its biogeochemical importance, cycling and reactivity must still be developed.