What do we know about the natural sources, transport and sinks of antimony in the environment?

Abstract

Limited attention has been given to antimony present in detrital form in the different environmental compartments except for highly polluted systems related in some way to ore deposits. In highly polluted systems, the ultimate sinks of Sb may be the minerals tripuhyite (FeSbO₄) or perhaps schafarzikite (FeSb₂O₄) but how about Sb dynamics in the much more abundant, weakly polluted or ‘non-polluted’ systems? This deficiency in our knowledge is probably related to the perception that the element is mostly present ‘dissolved’ in waters and to a focus on the role of its binding to iron oxyhydroxides in solid phases. Here we evaluate the state of our knowledge in the Sb journey from geological matrices to detrital forms in soils and waters and identify key aspects that require further investigation. In high-temperature environments, Sb demonstrated its striking incompatibility by fractionation into aqueous fluids or sulfide/metallic melts, or by uptake in a few common minerals that accept this element (e.g., rutile or pyrite). In low-temperature environments, Sb enters the structures of minerals with different formation rates and solubilities, creating a confusing impression of being mobile and immobile at the same time. The estimates of Sb concentration in the upper continental crust are scattered and the Sb-bearing mineral(s) there have not yet been identified. Given that sedimentary rocks are consistently enriched in Sb, the carriers could be the clay minerals. In surface water bodies, Sb could be carried predominantly in the particulate fraction, despite the popular belief of the opposite. An important point to consider is the transport of Sb within the suspended particulate matter, not on its surface. In soils, many studies employed sequential extractions to show that Sb accumulates in the ‘residual’ fraction, without ever asking what the nature of this fraction is. Based on these facts (i.e., knowns), we have identified the unknowns regarding detrital Sb on our planet that should preferentially be addressed by future projects if our understanding is to improve.