Abstract

In our modern economy, tantalum has become a 'technology-critical element' which is increasingly used in new technologies. This has led to a need to evaluate potential environmental impacts, which, in turn, requires knowledge of its concentration in the environment along with better understanding of the chemical processes underlying its environmental behaviour. This review is an attempt to summarize our current knowledge about tantalum in the environment. The attention paid to tantalum in the environmental field lags far behind what it attracts in some geosciences: while a wealth of tantalum concentration values in meteorites and rocks is available - allowing for good estimates of its content in the continental crust and of the Nb/Ta ratio (a precious geochemical tool) - data on tantalum concentrations in the different environmental compartments is scarce, particularly in natural waters where reliable estimates of 'dissolved' tantalum concentrations in seawater and freshwaters cannot even be produced. What is more, tantalum appears to be mostly present in 'particulate' form in natural waters, not 'dissolved'. Values exist for concentrations in soils, bed sediments and atmospheric aerosols; these are close to tantalum UCC values, thus pointing to a detrital origin. No signs of pollution effects linked to human use of the element are visible. At first sight, tantalum appears to be a very conservative element in biogeochemical terms but real understanding of its cycling and reactivity would require extensive and meaningful data collection. This is hindered by analytical difficulties, mainly: (i) 'dissolved' concentrations are well below current analytical capabilities in natural waters and require pre-concentration procedures that, for the moment, do not give consistent results; (ii) solid samples require complex mineralisation procedures that exclude tantalum from routine multielement studies. Lastly, there is scant reliable data on stability constants (i.e., absence or highly uncertain values) for relevant tantalum complexes. No sorption studies exist on tantalum binding by natural organic matter or inorganic particles.