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

Highlights

• Tellurium has a complex environmental geochemistry.

• The many modes of Te bonding govern its surface behaviour.

• Outcropping Te deposits are an analogue for anthropogenic contamination.

• We propose a cycling model for tellurium in surface environments.

• We highlight future areas for tellurium biogeochemical research.

Abstract

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 (bio)geochemistry 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 sulfides substituting for sulfur. Extreme Te enrichment (up to wt.%) is found in association with the precious metals Au and Ag in the form of telluride and sulfosalt 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 govern 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.

Introduction

Tellurium (Te) was discovered in 1783 by Franz Joseph Müller von Reichenstein, but fully publicised only over a decade later by Martin Heinrich Klaproth, who named the new element after the Latin word for "earth", tellus (Emsley, 2011; Klaproth, 1798). With an estimated crustal abundance of ~5 μg/kg (reported range 1 to 27 μg/kg; Emsley, 2011; Vaigankar et al., 2018; Wedepohl, 1995), it is one of the least abundant elements in the lithosphere and comparable to crustal abundances of precious metals gold (Au) and platinum (Pt) (Christy, 2015, Emsley, 2011). Recently, Te has come into prominence due to new industrial applications, including in cadmium telluride (CdTe) solar panels (Diso et al., 2016; Goldfarb, 2014; Reese et al., 2018), thermoelectric devices (Bae et al., 2016; Knockaert, 2011; Lin et al., 2016), batteries (Ding et al., 2015; He et al., 2016; He et al., 2017) and nanomaterials like CdTe quantum dots (Mahdavi et al., 2018). Furthermore, the recent nuclear incident at Fukushima led to severe contamination by the radioisotope ¹³²Te, renewing interest in the biogeochemical mechanisms involved in Te transport in the environment (e.g. Gil-Díaz, 2019; Tagami et al., 2013).

Tellurium is distributed unevenly through the Earth’s crust. Hydrothermal and magmatic processes are the key mechanisms leading to high Te concentrations and the formation of primary Te minerals (Brugger et al., 2016). Tellurium is an essential element in over 180 minerals, making it the most anomalously diverse element in mineralogy, i.e. it forms the greatest number of minerals relative to its crustal abundance (Christy, 2015; Pasero, 2020). Tellurides are primary minerals containing reduced Te (formal oxidation state -II to 0; e.g. calaverite, krennerite and sylvanite; Fig. 1a-c) and elemental tellurium (Fig. 1d); secondary minerals comprise tellurites (oxidation state +IV) and tellurates (+VI) (e.g. teineite, zemanniteand jensenite; Fig. 1d-f). The designations ‘primary’ and ‘secondary’ minerals relate to formation conditions. Primary minerals form deeper in the crust under anoxic conditions from hydrothermal fluids or silicate melts (Ciobanu et al., 2006; Zhang and Spry, 1994); whereas secondary minerals form via weathering of primary minerals under the oxidising conditions of near-surface environments (Christy et al., 2016a). Some tellurites and tellurates possess non-linear optical properties (Norman, 2017; Weil, 2018; Yu et al., 2016) with potential applications in the electronics industry. Nonetheless, most industrial applications for Te utilise tellurides (Amatya and Ram, 2012; Woodhouse et al., 2013; Yeh et al., 2008).

In 2010, the US Department of Energy classified Te as a critical metal with an anticipated global supply shortfall by 2025 (Bauer et al., 2010) and Te remains on the list of critical metals published by the US Department of the Interior in 2018 (U.S. Department of the Interior, 2018). The global Te industry is still in its infancy with a global production of 440 metric tonnes and estimated reserves of 31,000 metric tonnes from Te contained in copper ores (Anderson, 2019). Currently, >90% of Te (along with Se) is recovered from copper anode slimes as a by-product of the electrolytic refining of copper (Green, 2009; Kyle et al., 2011; Makuei and Senanayake, 2018) and Te supply is thus intrinsically linked to the Cu mining industry. Recent advances in industrial uses of Te focus on CdTe solar panels, which currently supply five percent of the global solar panel market (U.S. Department of Energy, 2019). Due to a growing world population and concurrent attempts to limit man-made climate change, renewable energy industries including CdTe solar panels are growing in prominence (see Fig. 2; Frishberg, 2017; Nuss, 2019; Wang, 2011).

The increased demand for Te will inevitably result in increasing Te contamination around mining and industrial sites (e.g. Kagami et al., 2012) and the decommissioning of CdTe solar panels also has the potential to be a source of contamination, in particular to groundwater (Fthenakis and Wang, 2006; Marwede and Reller, 2012; Ramos-Ruiz et al., 2017b; Xu et al., 2018). Recent improvements to the detection of low levels of Te via cathodic stripping voltammetry (Biver et al., 2015) and inductively-coupled plasma mass spectrometry (Filella and Rodushkin, 2018) have been made, although further improvements in the detection limits of environmental Te are ideally required (Filella, 2019). Another short-term anthropogenic source of Te contamination are the radiogenic isotopes ¹³²Te and ^129mTe released to the environment in nuclear spills or explosions. This comprises both nuclear weapons testing (particularly from the 1940s to the 1970s) and accidental spillage from power plant failure such as the Chernobyl and Fukushima Daiichi nuclear disasters (Dickson and Glowa, 2019; Yoschenko et al., 2018). The radioactive and biologically active decay product of ¹³²Te and ¹³²I, is of most concern, and a greater understanding of Te biogeochemical cycling could have provided better and longer-lasting solutions for cleaning up radioactive materials following the Fukushima spill (Gil-Díaz, 2019).

Research in Te biogeochemistry remains overall in its infancy. The cycling of this element in near-surface environments is dynamic, as the transformation of Te oxidative states can form both inorganic and organic compounds (Belzile and Chen, 2015; Bonificio and Clarke, 2014; Chasteen et al., 2009). In terms of biogeochemical processes at the cellular level, mechanisms for detoxifying often involve reduction of soluble Te oxyanions (i.e. tellurite and tellurate anions; Piacenza et al., 2017; Taylor, 1996; Taylor, 1999). These soluble Te oxyanions are toxic to most microorganisms at low concentrations, i.e. 1 mg/L or an equivalent 4 μM (Presentato et al., 2019), which are orders of magnitude less than cytotoxic concentrations of mercury or cadmium (Chasteen et al., 2009; Presentato et al., 2016). As such, Te solubility is a key factor contributing to its toxicity; reduced Te compounds (e.g. metallic Te) have low solubility and are therefore considered less toxic as they are not bioavailable. Reduction of Te oxyanions by microorganisms follows two major pathways, which may be either active or passive: bioprecipitation and, to a lesser extent, biovolatilisation (Chasteen and Bentley, 2003). Active bioprecipitation (also known as biomineralisation) results in the formation of nanoparticles of elemental Te, and biovolatilisation in the formation of volatile, organic forms of Te such as dimethyl telluride. In terms of passive bioprecipitation/biomineralisation, microorganisms as well as other organic material, e.g. extracellular polymeric substances (EPS), can act as a sorbent material as well as a reductant for Te oxyanions. As such, the accumulation of reduced Te can occur over time as long as Te oxyanions are supplied to the system and a consistent presence of biomass is available to serve as a sorbent material and reductant (Tanaka et al., 2010). Biooxidation of Te leading to its solubility (and intuitively, its mobility in the environment) is the less-studied process of biogeochemical cycling relative to Te oxyanion reduction. Metal-tolerant microorganisms are capable of indirectly oxidising tellurides by producing an oxidant (e.g. Fe-oxidisers producing Fe³⁺) as a by-product of their metabolism (Climo et al., 2000b). Collectively, the summation of reduction and oxidative processes contribute to the biogeochemical cycle of Te under near-surface environmental conditions and provide both organic and inorganic pathways for precipitation, sorption and oxidation (Filella et al., 2019).

Here we focus on the environmental (bio)geochemistry of Te, by first tying together existing research in mineralogy, geochemistry and microbiology – each discipline extensively studied in their own right, but not often explicitly linked. Over the past decade, various aspects of the biosphere and lithosphere have been shown to influence the dissolution, re-precipitation and mobility of Se, the chalcogen element located above Te in the Periodic Table (Bailey, 2017; Nancharaiah and Lens, 2015; Sharma et al., 2015; Tan et al., 2016; Ullah et al., 2018). In addition, precious metals such as Au (Reith et al., 2013; Sanyal et al., 2019; Southam et al., 2009) and Pt (Reith et al., 2014; Reith et al., 2016; Reith et al., 2019), which have had been generally considered to be inert in the biosphere, have been shown to display complex biogeochemical cycling. To our knowledge, an interdisciplinary research perspective of Te cycling is still lacking in the literature; therefore, here we develop a model for a global biogeochemical cycling of Te in near-surface environments. This Te biogeochemical cycling model helps to identify gaps in our current understanding of Te biogeochemistry, and allows for critical evaluation of various environmental factors contributing to Te mobility, which is becoming increasingly important as anthropogenic activity associated with applications of Te becomes a greater factor in environmental contamination.