Love is in the Earth: A review of tellurium (bio)geochemistry in Earth surface environments

Tellurium (Te) is a rare metalloid in the chalcogen group of the Periodic Table. Tellurium is regularly listed as a critical raw material both due to its increased use in the solar industry, and to the dependence on other commodities in its supply chain. A thorough understanding of the geo(bio)chemistry of Te in surface environments is fundamental for supporting the search for future sources of Te (geochemical exploration); developing innovative processing techniques for extracting Te; and quantifying the environmental risks associated with rapidly increasing anthropogenic uses. The present work links existing research in inorganic Te geochemistry and mineralogy with the bio(geo)chemical and biological literature towards developing an integrated Te cycling model.

Although average crustal rocks contain only a few μg/kg of Te, hydrothermal fluids and vapours are able to enrich Te to levels in excess of mg/kg. Tellurium is currently recovered as a by-product of base-metal mining; in these deposits, it occurs mainly in common sulphides substituting for sulphur. Extreme Te enrichment (up to wt.%) is found in association with the precious metals Au and Ag in the form of telluride and sulphosalt minerals. Tellurium also forms a large variety of oxygen-containing secondary minerals as a result of weathering of Te-containing ores in (near-)surface environments. Anthropogenic activities introduce significant amounts of Te into surficial environments, both through processing materials that contain minor Te, and through breakdown of used Te-containing materials. Additionally, radioactive 132Te is produced in nuclear reactors, and can contaminate surrounding and distal environments.

Environmental contamination of Te poses concern to organisms due to the acute toxicity of some Te compounds, especially the soluble tellurite and tellurate anions. A small percentage of microorganisms, however, are able to tolerate elevated levels of Te by detoxifying it through precipitation or volatilisation. Bioaccumulation of Te compounds can occur in some plants of the garlic family. A variety of interlinked organic and inorganic processes governs Te environmental chemistry. The Te cycle in surface environments incorporates (oxidative) dissolution of Te from primary ore minerals, inorganic precipitation and redissolution processes in which secondary minerals are formed, and bioreductive reprecipitation and volatilisation processes governed mainly by microbes. Our integrated Te cycling model highlights the interplay between anthropogenic, geochemical and biogeochemical processes on the distribution and mobility of Te in surface environments.