1 REVISION 1 2 Love is in the Earth: a review of tellurium (bio)geochemistry in Earth 3 surface environments 4 5 O.P. Missen a,b,*, R. Ram a, S.J. Mills b, B. Etschmann a, F. Reith c,d , J. Shuster c,d, D.J. Smith e, 6 J. Brugger a,* 7 8 a School of Earth, Atmosphere and Environment, 9 Rainforest Walk, Monash University, 9 Clayton 3800, Victoria, Australia 10 b Geosciences, Museums Victoria, GPO Box 666, Melbourne 3001, Victoria, Australia 11 c School of Biological Sciences, The University of Adelaide, Adelaide, SA 5005, Australia 12 d CSIRO Land and Water, Contaminant Chemistry and Ecotoxicology, PMB2, Glen Osmond, 13 SA 5064, Australia 14 e School of Geography, Geology & the Environment, University of Leicester, Leicester, UK 15 16 *Corresponding authors’ e-mails: [email protected], [email protected] 17 18 19 Keywords: Tellurium cycling, environments, (bio)geochemistry, mobility, mineralogy 20 21 22 ABSTRACT 23 Tellurium (Te) is a rare metalloid in the chalcogen group of the Periodic Table. Tellurium is 24 regularly listed as a critical raw material both due to its increased use in the solar industry, and 25 to the dependence on other commodities in its supply chain. A thorough understanding of the 26 geo(bio)chemistry of Te in surface environments is fundamental for supporting the search for 27 future sources of Te (geochemical exploration); developing innovative processing techniques 28 for extracting Te; and quantifying the environmental risks associated with rapidly increasing 29 anthropogenic uses. The present work links existing research in inorganic Te geochemistry and 30 mineralogy with the bio(geo)chemical and biological literature towards developing an 31 integrated Te cycling model. 32 Although average crustal rocks contain only a few µg/kg of Te, hydrothermal fluids and 33 vapours are able to enrich Te to levels in excess of mg/kg. Tellurium is currently recovered as 34 a by-product of base-metal mining; in these deposits, it occurs mainly in common sulphides 35 substituting for sulphur. Extreme Te enrichment (up to wt.%) is found in association with the 36 precious metals Au and Ag in the form of telluride and sulphosalt minerals. Tellurium also 37 forms a large variety of oxygen-containing secondary minerals as a result of weathering of Te- 38 containing ores in (near-)surface environments. Anthropogenic activities introduce significant 39 amounts of Te into surficial environments, both through processing materials that contain 40 minor Te, and through breakdown of used Te-containing materials. Additionally, radioactive 41 132Te is produced in nuclear reactors, and can contaminate surrounding and distal environments. 42 Environmental contamination of Te poses concern to organisms due to the acute toxicity of 43 some Te compounds, especially the soluble tellurite and tellurate anions. A small percentage 44 of microorganisms, however, are able to tolerate elevated levels of Te by detoxifying it through 45 precipitation or volatilisation. Bioaccumulation of Te compounds can occur in some plants of 46 the garlic family. A variety of interlinked processes govern Te environmental chemistry, 47 linking them into a cycle which encompasses both inorganic and organic processes. The Te 48 cycle in surface environments incorporates (oxidative) dissolution of Te from primary ore 49 minerals, inorganic precipitation and redissolution processes in which secondary minerals are 50 formed, and bioreductive reprecipitation and volatilisation processes governed mainly by 51 microbes. Our integrated Te cycling model highlights the interplay between anthropogenic, 52 geochemical and biogeochemical processes on the distribution and mobility of Te in surface 53 environments. 54 HIGHLIGHTS 55 • Tellurium has a complex environmental geochemistry 56 • The many modes of Te bonding govern its surface behaviour 57 • Outcropping Te deposits are an analogue for anthropogenic contamination 58 • We propose a cycling model for tellurium in surface environments 59 • We highlight future areas for tellurium biogeochemical research 60 TABLE OF CONTENTS 61 1. Introduction 62 2. Physical and chemical properties of tellurium 63 3. Tellurium mineralogy 64 4. Tellurium distribution and ore deposits 65 5. Tellurium in the environment 66 6. Biogeochemistry of tellurium 67 7. A tellurium biogeochemical cycling model 68 Acknowledgements 69 Footnotes 70 References 71 1. INTRODUCTION 72 Tellurium (Te) was discovered in 1783 by Franz Joseph Müller von Reichenstein, but fully 73 publicised only over a decade later by Martin Heinrich Klaproth, who named the new element 74 after the Latin word for "earth", tellus (Emsley, 2011; Klaproth, 1798). With an estimated 75 crustal abundance of ~5 µg/kg (reported range 1 to 27 µg/kg; Emsley, 2011; Vaigankar et al., 76 2018; Wedepohl, 1995), it is one of the least abundant elements in the lithosphere and 77 comparable to crustal abundances of precious metals gold (Au) and platinum (Pt) (Emsley, 78 2011). Recently, Te has come into prominence due to new industrial applications, including in 79 cadmium telluride (CdTe) solar panels (Diso et al., 2016; Goldfarb, 2014; Reese et al., 2018), 80 thermoelectric devices (Bae et al., 2016; Knockaert, 2011; Lin et al., 2016), batteries (Ding et 81 al., 2015; He et al., 2016; He et al., 2017), and nanomaterials like CdTe quantum dots (Mahdavi 82 et al., 2018). Furthermore, the recent nuclear incident at Fukushima led to severe contamination 83 by the radioisotope 132Te, renewing interest in the biogeochemical mechanisms involved in Te 84 transport in the environment (e.g. Gil-Díaz, 2019; Tagami et al., 2013). 85 Tellurium is distributed unevenly through the Earth’s crust. Hydrothermal and magmatic 86 processes are the key mechanisms leading to high Te concentrations and the formation of 87 primary Te minerals (Brugger et al., 2016). Tellurium is an essential element in over 170 88 minerals, making it the most anomalously diverse element in mineralogy, i.e. it forms the 89 greatest number of minerals relative to its crustal abundance (Christy, 2015; Pasero, 2019). 90 Tellurides are primary minerals containing reduced Te (formal oxidation state -II to 0; e.g., 91 calaverite, krennerite, sylvanite; Figure 1a-c) and elemental tellurium (Figure 1d); secondary 92 minerals comprise tellurites (oxidation state +IV) and tellurates (+VI) (e.g., teineite, zemannite, 93 and jensenite, Figure 1d-f). The designations ‘primary’ and ‘secondary’ minerals relate to 94 formation conditions. Primary minerals form deeper in the crust under anoxic conditions from 95 hydrothermal fluids or silicate melts (Ciobanu et al., 2006; Zhang and Spry, 1994); whereas 96 secondary minerals form via weathering of primary minerals under the oxidising conditions of 97 near-surface environments (Christy et al., 2016a). Some tellurites and tellurates possess non- 98 linear optical properties (Norman, 2017; Weil, 2018; Yu et al., 2016) with potential 99 applications in the electronics industry. Nonetheless, most industrial applications for Te utilise 100 tellurides (Amatya and Ram, 2012; Woodhouse et al., 2013; Yeh et al., 2008). 101 In 2010, the US Department of Energy classified Te as a critical metal with an anticipated 102 global supply shortfall by 2025 (Bauer et al., 2010), and Te remains on the list of critical metals 103 published by the US Department of the Interior in 2018 (USDOI, 2018). The global Te industry 104 is still in its infancy with a global production of 440 metric tonnes and estimated reserves of 105 31,000 metric tonnes from Te contained in copper ores (Anderson, 2019). Currently, >90% of 106 Te (along with Se) is recovered from copper anode slimes as a by-product of the electrolytic 107 refining of copper (Kyle et al., 2011; Makuei and Senanayake, 2018), and Te supply is thus 108 intrinsically linked to the Cu mining industry. Recent advances in industrial uses of Te focus 109 on CdTe solar panels, which currently supply five percent of the global solar panel market 110 (USDOE, 2019). Due to a growing world population and concurrent attempts to limit man- 111 made climate change, renewable energy industries including CdTe solar panels are growing in 112 prominence (see Figure 2; Frishberg, 2017; Nuss, 2019; Wang, 2011). 113 The increased demand for Te will inevitably result in increasing Te contamination around 114 mining and industrial sites (e.g. Kagami et al., 2012), and the decommissioning of CdTe solar 115 panels also has the potential to be a source of contamination, in particular to 116 groundwater (Fthenakis and Wang, 2006; Marwede and Reller, 2012; Ramos-Ruiz et al., 2017b; 117 Xu et al., 2018). Another short-term anthropogenic source of Te contamination are the 118 radiogenic isotopes 132Te and 129mTe released to the environment in nuclear spills or explosions. 119 This comprises both nuclear weapons testing (particularly from the 1940s to the 1970s) and 120 accidental spillage from power plant failure such as the Chernobyl and Fukushima Daiichi 121 nuclear disasters (Dickson and Glowa, 2019; Yoschenko et al., 2018). The radioactive and 122 biologically active decay product of 132Te, 132I, is of most concern, and a greater understanding 123 of Te biogeochemical cycling could have provided better and longer-lasting solutions for 124 cleaning up radioactive materials following the Fukushima spill (Gil-Díaz, 2019). 125 Research in Te biogeochemistry continues to be of scientific interest because the cycling of 126 this element in near-surface environments is dynamic, as the transformation of Te oxidative 127 states can form both inorganic and organic compounds (Belzile and Chen, 2015; Bonificio and 128 Clarke, 2014; Chasteen et al., 2009). In terms of biogeochemical processes at the cellular level, 129 mechanisms for detoxifying often involve reduction of soluble Te oxyanions (i.e., tellurite and 130 tellurate anions) (Piacenza et al., 2017; Taylor, 1996; Taylor, 1999). These soluble Te 131 oxyanions are toxic to some microorganisms at low concentrations, i.e., 1 mg/L or an 132 equivalent 4 µM (Presentato et al., 2019), which are orders of magnitude less than known 133 cytotoxic concentrations of mercury or cadmium (Chasteen et al., 2009; Presentato et al., 2016).