Applications of Osmium and Iridium as Biogeochemical Tracers in the Environment

Abstract

Osmium (Os) and Iridium (Ir) and are among the rarest elements on the surface of the earth and ones whose applications in modern industry are quite limited. However, their environmental burden has been increasing as they occur in nature with other platinum group elements, which have a wide variety of industrial, chemical, electrical and pharmaceutical applications. This review traces the development of the analytical techniques used to precisely measure Os and Ir concentrations and Os isotope composition in the environmental samples from their roots in geochemical and cosmochemical investigations. We then examine the distribution of Os and Ir in natural samples and review recent literature applying these elements as biogeochemical tracers. The primary environmental applications of Os and Ir arise from the fact that these elements are extremely rare on the surface of the earth and their introduction into the environment leads to an increase in their concentration in surface materials. In addition, a unique isotope fingerprint is present for Os introduced into the environment in that it comes from ores mined primarily in South Africa and Russia. In particular, we examine studies where (a) Os isotopes have been utilized to track the dispersal of platinum group elements from automobile catalysts and also to assess dispersal of Os itself and (b) Ir has been introduced as an intentional tracer to evaluate soot contribution from burning of fossil fuels.