Natural nanoparticles of the critical element tellurium

Rahul Ram, Mihoko Maruyama, Xi-Ya Fang, Erik Melchiorre, Christopher G. Ryan, Edgar E. Villalobos-Portillo, Hiram Castillo-Michel, Kiyofumi Nitta, Oki Sekizawa, Jeremiah Shuster, et al.

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Rahul Ram, Mihoko Maruyama, Xi-Ya Fang, Erik Melchiorre, Christopher G. Ryan, et al. Natural nanoparticles of the critical element tellurium. Journal of Hazardous Materials Letters, 2022, 3, pp.100053. 10.1016/j.hazl.2022.100053. hal-05075971

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Natural nanoparticles of the critical element tellurium

Owen P. Missen<sup>a,b,*</sup>, Ella R. Lausberg<sup>a</sup>, Joël Brugger<sup>a</sup>, Barbara Etschmann<sup>a</sup>, Stuart J. Mills<sup>b</sup>, Koichi Momma<sup>c</sup>, Rahul Ram<sup>a</sup>, Mihoko Maruyama<sup>d</sup>, Xi-Ya Fang<sup>e</sup>, Erik Melchiorre<sup>g</sup>, Christopher G. Ryan<sup>f</sup>, Edgar E. Villalobos-Portillo<sup>h</sup>, Hiram Castillo-Michel<sup>h</sup>, Kiyofumi Nitta<sup>i</sup>, Oki Sekizawa<sup>j</sup>, Jeremiah Shuster<sup>j</sup>, Santonu K. Sanyal<sup>k</sup>, Andrew Frieddich<sup>a</sup>, Steve Hunt<sup>l</sup>, Yuka Tsuri<sup>d</sup>, Yuriko Takahashi<sup>d</sup>, Uta Michibata<sup>d</sup>, Sahil Dwivedi<sup>d</sup>, Maria A.D. Rea<sup>m,n,**</sup>

ARTICLE INFO

Keywords:

Tellurium

Biogeochemistry

Nanoparticles

Mineral transformations

Moctezuma, Sonora, Mexico

A B S T R A C T

Tellurium (Te) is a Critical Element that is toxic to microorganisms and humans alike, most notably in its soluble oxyanionic forms. To date, the biogeochemical behaviour of Te in Earth's surface environment is largely unknown. Here, we report the discovery of elemental Te nanoparticles (Te NPs) in regolith samples using Single-Particle Inductively Coupled Plasma Mass Spectroscopy. Tellurium NPs were detected in both proximal and distal locations (bulk concentrations >4 ppm) relative to weathering Te ores. Synchrotron X-ray Fluorescence Mapping and X-ray Absorption Spectroscopy showed that bulk Te in the regolith is generally associated with Fe (oxyhydr) oxides and clay minerals, and mostly found in the oxidation states +IV and +VI. Although Te NPs account for less than 2 mol% of Te in our samples, their detection provides evidence for the active biogeochemical cycling of Te in surface environments. Te NPs are reactive and are likely to have formed in situ in distal samples, most likely via microbially-mediated reduction. Hence, the presence of Te NPs indicates the potential for release of toxic soluble forms of Te even in environments where most Te is "fixed" in forms such as Fe (oxyhydr)oxides that have low solubility and poor bioavailability.

1. Introduction

Tellurium (Te; Z = 52) is classified as a Critical Element due to its use in emerging technologies such as solar panels, and supply risks because of the current coupling of Te production to copper mining (Goldfarb et al., 2017; Hayes and McCullough, 2018; Missen et al., 2020; McNulty

and Jowitt, 2021). Tellurium exists in four common oxidation states (-II, 0, +IV and +VI) in Nature (Brugger et al., 2012; Filella et al., 2019). Of these, the +IV and +VI oxidation states are highly toxic in their soluble oxyanionic forms (tellurite, Te<sup>IV</sup>O<sub>3</sub><sup>2-</sup> and tellurate, Te<sup>VI</sup>O<sub>6</sub><sup>4-</sup>; Tremaroli et al., 2007; Goff et al., 2021). Tellurium is most noted for its toxicity to microbes but is also cytotoxic at low levels to humans in soluble forms

<footnote><sup>a</sup> School of Earth, Atmosphere and Environment, Monash University, 9 Rainforest Walk, Clayton 3800, Victoria, Australia</footnote>

<footnote><sup>b</sup> Geosciences, Museums Victoria, GPO Box 666, Melbourne 3001, Victoria, Australia</footnote>

<footnote><sup>c</sup> National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba, Ibaraki 305-0005, Japan</footnote>

<footnote><sup>d</sup> Applied Electro-Physics, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan</footnote>

<footnote><sup>e</sup> Monash Centre for Electron Microscopy, Monash University, Clayton, VIC 3800, Australia</footnote>

<footnote><sup>f</sup> CSIRO Minerals, Normanby Road, Clayton, VIC 3168, Australia</footnote>

<footnote><sup>g</sup> Department of Geological Sciences, California State University of San Bernardino, 5500 University Parkway, San Bernardino, CA 92407, USA</footnote>

<footnote><sup>h</sup> European Synchrotron Radiation Facility, 38043 Grenoble Cedex 9, France</footnote>

<footnote><sup>i</sup> Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan</footnote>

<footnote><sup>j</sup> Tübingen Structural Microscopy Core Facility & Department of Geosciences, Eberhard-Karls Universität Tübingen, Schnarrenbergstraße 94-96, 72076 Tübingen, Germany</footnote>

<footnote><sup>k</sup> Department of Microbiology, Jagannath University, Dhaka 1100, Bangladesh</footnote>

<footnote><sup>l</sup> Gold Eye Claim, Cholla Rd, Quartzsite, AZ 85346, United States</footnote>

<footnote><sup>m</sup> CSIRO Land and Water, Contaminant Chemistry and Ecotoxicology, PMB2, Glen Osmond, SA 5064, Australia</footnote>

<footnote><sup>n</sup> College of Science and Engineering, Flinders University, Bedford Park, South Australia 5042, Australia</footnote>

<footnote>* Corresponding author at: School of Earth, Atmosphere and Environment, Monash University, 9 Rainforest Walk, Clayton 3800, Victoria, Australia.</footnote>

<footnote>** Corresponding author at: CSIRO Land and Water, Contaminant Chemistry and Ecotoxicology, PMB2, Glen Osmond, SA 5064, Australia.
E-mail addresses: omissen@museum.vic.gov.au (O.P. Missen), angel.rea@csiro.au (M.A.D. Rea).</footnote>
(Yarema and Curry, 2005). In fact, the toxicity of the tellurite oxyanion is greater than that of many better-known pollutants (Chasteen et al., 2009). For example, Te<sup>IV</sup>O<sub>2</sub><sup>2-</sup><sub>baq</sub> has been found to be toxic to microorganisms at a concentration of ~1 mg L<sup>-1</sup> (4 µM), a value that is approximately ~100 times lower than the toxicity limits of Cd<sup>II</sup><sub>(aq)</sub>, Hg<sup>II</sup><sub>(aq)</sub> and Se<sup>IV</sup>O<sub>2</sub><sup>2-</sup><sub>baq</sub> (Nies, 1999; Taylor, 1999; Presentato et al., 2019). Humans have historically not come into regular contact with Te, except for gold, copper and lead miners and refinery workers (Shie and Deeds, 1920). Human exposure to Te will likely increase through the 21st Century through two main factors: (1) burgeoning use of cadmium telluride solar panels for cleaner electricity generation (Marwede and Reller, 2012; Cyrs et al., 2014; Zeng et al., 2015; Ramos-Ruiz et al., 2017) and (2) expanding copper mining operations to meet electric vehicle production (Kavlak and Graedel, 2013; Elshkaki et al., 2016; Goldfarb et al., 2017; Calvo and Valero, 2021).

Tellurium is a mobile element in the environment, generally dispersing away from hotspots such as telluride-containing mines (Fil-ella and May, 2019; Gil-Díaz et al., 2020). One prominent mode of dispersal is dissolved transport due to the solubility of its oxyanions (Grundler et al., 2013). Tellurium may also be transported as tellurium nanoparticles (Te NPs), analogous to other natural nanomaterials (e.g., Au nanoparticles; Aiken et al., 2011; Pfeiffer et al., 2014). In general, nanoparticles play a key role in the environment by virtue of their differing properties compared to macro-particles, especially for their mobility and toxic effects towards biota. Advective nanoparticulate transport is important for the dispersion of elements which are insoluble in natural fluids (e.g., plutonium, Cook et al., 2021). In the case of silver (Ag), Ag NPs are a noted anti-bacterial agent (Desireddy et al., 2013; Xu et al., 2020) and their behaviour in the environment is markedly different from macroscopic Ag, which is comparatively unreactive (McGillicuddy et al., 2017; de Souza et al., 2019). Studying metals such as Au (Reith et al., 2013; Kato et al., 2019; Sanyal et al., 2019; Shuster et al., 2021) and Pt (Reith et al., 2014, 2019; Asztemborska et al., 2015; Książyk et al., 2015) has also led to a new understanding of environmental behaviour and toxicity of elements previously thought to be inert in natural contexts.

Missen et al. (2022) observed Te NPs on the surface of native

tellurium grains (i.e., coarse-grained Te⁰ formed from hydrothermal fluids) undergoing weathering at the Bambolla mine, Moctezuma, Mexico (Fig. 1). To date, no Te NPs were observed away from this specific micro-environment, even in regolith located centimetres away from native tellurium (Missen et al. 2022), and the role of Te NPs in Te transport and toxicity once separated from Te-containing ore remains unknown. In the present study, we develop a Single-Particle Inductively Coupled Plasma Mass Spectroscopy (SP-ICP-MS) protocol (Mozhayeva and Engelhard, 2020) for detecting low concentrations of environmental Te NPs in regolith samples. We analyse samples from both proximal and distal environments around relatively undisturbed natural anomalies at several North American Te-enriched mine sites (Table 1). We establish the existence of Te NPs in many of the samples and determine their size distributions and concentrations. We combine this information with a characterisation of the dispersion and speciation (oxidation state and association with main regolith phases) of Te in selected regolith samples to discuss the function of Te NPs as a short-term environmental sink, and their significance for Te mobility away from Te-rich mineralisation as an analogue for dispersion away from anthropogenic sources.

## 2. Experimental

We collected fresh regolith samples from two sites in northern Mexico and four sites in the south-western United States of America (see Table 1; Supplementary Table S1). All sites feature Te dispersion away from varying grades of Te-Au±Cu±Ag mineralisation. Even though all sites have seen small-scale mining in the past, parts of the surrounding regolith remain mostly undisturbed. Analytical techniques used include Single-Particle Inductively Coupled Plasma Mass Spectroscopy (SP-ICP-MS) to quantitatively detect individual Te NPs in the regolith samples; Scanning Electron Microscopy (SEM) for deciphering regolith mineralogy and texture; and Synchrotron-based X-ray analyses for elemental mapping and for deciphering the bulk chemical state of Te. The synchrotron analyses were undertaken around the Te K-edge (31.814 keV) at the BL37XU beamline of the SPring-8 Synchrotron, Japan (beam size 2 × 2 µm in cross-section); and the Te L<sub>3</sub>-edge (4.341 keV) at the ID21 beamline of the European Synchrotron Radiation Facility (ESRF), France

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Schematic of the study showing Te NPs in situ and their detection in regolith environments using SP-ICP-MS. Abbreviations in sampling locations subfigure are as described in Table 1.</figcaption></figure>
<table><caption>**Table 1**
Summary table of number of particles and mean nanoparticle size.</caption><thead><tr><th>Site code</th><th>Site name</th><th>Climate (Köppen code)</th><th>Sampling locality description and distance from Te-rich veins</th><th>n*</th><th>Number of particles per g of regolith</th><th>Mean NPs size (nm)</th><th>Weight of Te NPs (g) per g of regolith</th><th>Average regolith Te (ppm)</th><th>Te NPs mole% of total Te</th></tr></thead><tbody><tr><td>Proximal M1</td><td>Bambolla, Sonora, Mexico</td><td>BSh</td><td>Former exploration pit; 0–5 m</td><td>2</td><td>3.10(24) × 10<sup>7</sup></td><td>127.5(9)</td><td>2.10 × 10<sup>-7</sup></td><td>854</td><td>0.25</td></tr><tr><td>Proximal M2</td><td>Bambolla, Sonora, Mexico</td><td>BSh</td><td>Washout contacting crumbly ore; 0–5 m</td><td>2</td><td>4.76(52) × 10<sup>8</sup></td><td>94.8(1)</td><td>1.54 × 10<sup>-6</sup></td><td>1000</td><td>1.54</td></tr><tr><td>Proximal M3</td><td>Bambolla, Sonora, Mexico</td><td>BSh</td><td>Regolith near exploration pit; 20 m</td><td>3</td><td>3.25(48) × 10<sup>4</sup></td><td>107(5)</td><td>1.72 × 10<sup>-10</sup></td><td>234</td><td>0.00074</td></tr><tr><td>Mid M4</td><td>Bambolla, Sonora, Mexico</td><td>BSh</td><td>Regolith contacting crumbly ore; 5 m</td><td>2</td><td>2.82(13) × 10<sup>6</sup></td><td>115(6)</td><td>2.10 × 10<sup>-8</sup></td><td>1620</td><td>0.013</td></tr><tr><td>Distal M5a</td><td>Bambolla, Sonora, Mexico</td><td>BSh</td><td>Averaged profiles trending away from mine; 30–200 m</td><td>1</td><td>9.47 × 10<sup>4</sup></td><td>85.2</td><td>3.14 × 10<sup>-10</sup></td><td>11.5</td><td>0.027</td></tr><tr><td>Proximal S1a</td><td>San Miguel, Sonora, Mexico</td><td>BSh</td><td>Regolith near mine entrance; 10 m</td><td>3</td><td>2.01(13) × 10<sup>7</sup></td><td>136.9(4)</td><td>3.04 × 10<sup>-7</sup></td><td>490</td><td>0.62</td></tr><tr><td>Distal S2</td><td>San Miguel, Sonora, Mexico</td><td>BSh</td><td>Ridge trending up-slope; 25–125 m</td><td>1</td><td>5.51 × 10<sup>5</sup></td><td>85.2</td><td>2.18 × 10<sup>-9</sup></td><td>4.23</td><td>0.52</td></tr><tr><td>Varying BB</td><td>Blue Bell, California, USA</td><td>BWh</td><td>Regolith downslope of adits; 10–100 m</td><td>1</td><td>1.66 × 10<sup>5</sup></td><td>68.3</td><td>3.94 × 10<sup>-10</sup></td><td>180</td><td>0.0022</td></tr><tr><td>Distal OM</td><td>Otto Mountain, California, USA</td><td>BWh</td><td>Regolith downslope of Aga mine; 50–200 m</td><td>1</td><td>≤ 1.93 × 10<sup>6</sup></td><td>≤ 43.5 **</td><td>≤ 1.10 × 10<sup>-9</sup></td><td>0.85</td><td>1.30</td></tr><tr><td>Distal MC</td><td>Middle Camp, Arizona, USA</td><td>BWh</td><td>Surface placer deposits; > 50 m</td><td>1</td><td>≤ 3.83 × 10<sup>5</sup></td><td>≤ 43.6 **</td><td>≤ 2.53 × 10<sup>-10</sup></td><td>0.88</td><td>0.29</td></tr><tr><td>Distal GE</td><td>Gold Eye Claim, Arizona, USA</td><td>BWh</td><td>Surface placer deposits; > 50 m</td><td>1</td><td>≤ 6.36 × 10<sup>6</sup></td><td>≤ 43.7 **</td><td>≤ 4.51 × 10<sup>-9</sup></td><td>0.028</td><td>161</td></tr></tbody></table>

(beam size 0.4 µm vertical × 0.9 µm horizontal) with data preserved online (Etschmann et al., 2021; Mills et al., 2022). These analyses combined X-ray Fluorescence (XRF) to determine micro-scale elemental associations and Te distribution; X-ray Absorption Near-Edge Spectroscopy (XANES) to measure the oxidation state of Te; Extended X-ray Absorption Fine Structure (EXAFS, SPring-8 only) to investigate Te bonding environments; and micro- X-Ray Diffraction (µ-XRD, SPring-8 only) to analyse mineralogy. Further details of these analyses and data reduction are provided in the Supplementary Material section S5.

# 3. Results

## 3.1. Tellurium nanoparticles

Using the first application of SP-ICP-MS to study Te NPs from environmental samples, we discovered that small quantities of Te NPs occur in regolith samples away from Te-rich ore veins, not in association with relict ore minerals. Tellurium NPs were found in varying concentrations and size distributions at the different sites (Table 1). The concentration of Te NPs in the regolith (4.8(5) × 10⁸ particles·g⁻¹) is greatest by an order of magnitude at Bambolla mine location M2: a washout contacting Te-rich veins. Bambolla mine location M1 (former exploration pit) contains the next highest concentration of NPs (3.1(2) × 10⁷ particles·g⁻¹), an amount similar to the S1a location (regolith near mine entrance) at San Miguel (2.0(1) × 10⁷ particles·g⁻¹). Both locations are close to the mineral veins, though the M1 location directly contacts a vein, while the S1a location is found 10 m from the nearest vein. The NPs in these proximal samples from Bambolla and San Miguel are similar in size (95–137 nm), with the nanoparticles from S1a being the largest NPs of all analysed locations (Fig. 2). In general, the Te NPs are larger in regolith proximal to the mineralised veins, while their size decreases

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** NP size distribution profile(s) from SP-ICP-MS, showing varying Te NP size distribution profiles.</figcaption></figure>

distal to the mineralised veins. The M5 and S2 locations, which are in regolith profiles between 25 and 200 m from the veins, have an average Te NP size of ~85 nm (Fig. 2).

Tellurium nanoparticles were also detected in an averaged (10–100 m from source) regolith sample from Blue Bell in South-western USA, the most Te-rich of the USA samples. Otto Mountain, Middle Camp and Gold Eye claim had lower regolith Te concentrations (≤0.88 ppm) and the Te NPs were below detection limit (<44 nm) for our SP-ICP-MS technique (Lee et al., 2014).

<footnote>*n = measurement replicate for SP-ICP-MS; **NP size (assumed spherical) ≤ blank controls.</footnote>

<footnote>Notation: 3.10(24) × 10⁷ is a short-hand for 3.10 × 10⁷ ± a standard deviation of 0.24 × 10⁷ (i.e. 2.4 × 10⁶) particles g⁻¹; 127.5(9) means 127.5 ± 0.9 nm; and so on.
Location coordinates: Bambolla: 29.69169°N, 109.71220°W; San Miguel: 29.76366°N, 109.75112°W; Blue Bell: 35.24079°N, 116.20281°W; Otto Mountain: 35.272783°N, 116.09515°W; Middle Camp: 33.66505°N, 114.32777°W and Gold Eye Claim: 33.60074°N, 114.29948°W.</footnote>
## 3.2. Te mineralogy and speciation

Two previous studies investigated Te speciation in Te-rich mine tailings and associated contaminated soils. At the Kawazu mine, Japan, Qin et al. (2017) showed that Te was present as a mixture of TeIV and TeVI species; FeIII hydroxides were the host phases for TeIV and TeVI, but some TeVI was also associated with illite. In contrast, Hayes and Ramos (2019) found that Te was present as TeVI and associated with FeIII hydroxides within the Delamar mine tailings, Nevada, USA. Both studies used synchrotron-based X-ray Absorption Spectroscopy (XAS) to decipher the nature of Te (μ-XAS in Qin et al., 2017; bulk XAS in Hayes and Ramos, 2019). This method is well suited for studying the chemical state of minor to trace elements in complex materials, due to its low detection limits (ppm), element specificity, and suitability for characterisation of disordered materials (e.g., sorbed species; Brugger et al., 2010; Etschmann et al., 2010, 2014; Qin et al., 2017).

XRF elemental distribution maps show that the association of Te with other elements and the size of Te hotspots varies in proximal and distal samples (Fig. 3). Te is associated with Fe (oxyhydr)oxides and with clay (using Rb as a proxy, as Rb is associated primarily with interlayer sites in clay minerals), especially in distal regolith. Although Te may also associate with these elements in proximal and intermediate samples, sometimes bulk Te is associated with no other elements (Fig. 3b,c,e at Bambolla M1). Regardless, Te is correlated with Fe in both proximal (Fig. 3d at Blue Bell), intermediate (Fig. 3c at Bambolla M3) and distal samples (Fig. 3f at Bambolla M5a, as a <20 µm grain of Te-rich Fe (oxyhydr)oxide). Quartz and illite dominate the μ-XRD scans collected at Bambolla, which is expected as bulk Fe concentrations are below 2 wt% in all samples (Missen et al., 2022). Minor goethite was sometimes

detected via μ-XRD in proximal Bambolla samples in association with Te-rich zones; goethite is more prominent in some Blue Bell μ-XRD scans, where Te has an overall better correlation with Fe than at Bambolla (Fig. 3d) based on XRF map statistics. Te tends to be more disseminated in clays (whether in proximal or distal samples) as < 10 µm Te ‘hotspots’ (green hotspots of Te amongst blue areas of clay minerals in Fig. 3).

The average oxidation states of Te at selected points of interest was determined using the shift in edge position of the Te K-edge XANES spectra. Specifically, this was measured by fitting the position of the maximum in the first derivative of the XANES spectrum (see Supplementary Material section S5.4 for details). Quantitative determination of Te oxidation state was not possible based on Te L3-edge data due to insufficient separation between the edge positions of the TeIV and TeVI oxidation states and complex background; but by matching the shape and position of key features of the standards with features of our regolith samples, we were able to corroborate the oxidation states determined at the K-edge with those determined at the L3-edge. A compilation of the oxidation state results from K-edge Te XANES is shown in Fig. 3a, with representative spectra shown in Supplementary Fig. S1. The only spectrum (with sufficient statistics) corresponding to metallic tellurium was found in proximal sample MOC1001 (Fig. 3b; marked as Te⁰). This corresponds to a hot spot ~15 µm in diameter, also enriched in As and Se, and hence is most probably a fragment of primary hydrothermal native tellurium from the ore vein. The other three XANES spectra from this sample cover a TeIV/Teₜot range of 0.21–0.93, with Te associated either with no other elements (weathering native tellurium) or with clay minerals. In contrast, the Blue Bell BB917 location is the most oxidised among the studied sites, with an average TeIV/Teₜot of 0.05(4) (n = 7);

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** (a) Histogram showing distribution of TeIV/(TeIV+TeVI) ratios in analyses of regolith, determined at the K-edge. (b-f) comparison of X-ray fluorescence (XRF) maps collected at 32 keV, Spring-8. TeIV/(TeIV+TeVI) ratios at specific micro-XANES points are shown next to stars indicating analysis points. All scale bars are 100 µm.</figcaption></figure>
Te is closely associated with Fe (specifically goethite in this near-proximal sample). EXAFS analysis of selected K-edge spectra showed that the Te bonding environment in the regolith samples is usually most similar to that of synthetic Te<sup>VI</sup> in goethite (Supplementary Material Section S5.6 & Fig. S1).

## 4. Discussion

Our study reveals that Te NPs occur not only on the surface of native tellurium grains subjected to biogeochemical weathering (Missen et al., 2022), but also in regolith environments including distal locations away from the Te source. The detection of Te NPs is particularly notable because of the scarcity and toxicity of Te in the environment. Although Te NPs make up less than 2 mol‰ of total Te in all studied samples, with the remainder of regolith Te found as oxidised Te<sup>IV</sup> or Te<sup>VI</sup>, they are likely to be an important sink for Te in surface environments. Nanoparticles have markedly different properties to macroscopic versions of the same phase (Hochella et al., 2008). The detection of elemental Te NPs in both proximal and distal environments could affect various aspects of Te surface (bio)geochemistry including transport and environmental toxicity, based on comparison with other metals like Cu (Ameh and Sayes, 2019), Pt (Reith and Cornelis, 2017), and Au (Bohu et al., 2019; Reith et al., 2010); and metalloids such as Se (Nancharaiah and Lens, 2015).

**Tellurium NPs in the regolith.** In general, the analysis of any NPs presents challenges related to detection accuracy and limits (Leppard, 2008; Kuhlbusch et al., 2010), coupled to the difficulty of analysing scarce Te NPs from environmental samples (Wang and Becker, 2014; Llaver et al., 2021). In this study, Te NPs were detected in all regolith samples that were highly enriched in Te (bulk concentrations >4 ppm, at least 3 orders of magnitude greater than the 1 ppb crustal abundance of Te; Wedepohl, 1995). NPs were detected in highest concentrations at sampling locations with higher levels of Te (>100 ppm), suggesting that they are a significant form of Te in the regolith environment around Te hotspots. The average size of Te NPs proximal to richly mineralised veins are on average > 100 nm, while further away from mineralisation (25–200 nm) the average NP size drops to ~85 nm (Table 1; Fig. 2). Sampling locations with lower concentrations of Te (<4 ppm) do not contain detectable NPs. Thus, while detecting Te NPs in environments in which Te concentrations are < 4 ppm is for now a challenge, we should expect to find nanoparticulate Te in areas such as the surrounds of Te-bearing mines, especially some base- and precious-metals mines (Goldfarb et al. 2017); refineries and smelters (e.g., Ojebuoboh (2008)); and anthropogenic stockpiles of Te-bearing devices, including CdTe solar panels.

**Tellurium NPs: A minor but significant component of total Te.** XAS spectroscopy indicates that Te is present mostly in its oxidised forms (Te<sup>IV</sup> and Te<sup>VI</sup>) at Moctezuma, San Miguel, and Blue Bell, while other localities had Te concentrations too low to collect reliable XANES spectra (Fig. 3a). The apparent disparity between the XAS data and the presence of Te NPs is explained by the ratio of Te NPs to total Te, which is < 2 mol‰ in all analysed samples. Detecting reduced Te with < 2 mol ‰ fraction compared to the oxidised Te is below the XANES methods detection limit (~5 mol%), and point µ-XANES data at both the K- or L<sub>3</sub>-edges indicate that Te NPs do not aggregate to form 'hot spots' with concentrations high enough to be detected alongside the dominant oxidised Te component. Metallic tellurium was identified only in one µ-XAS spectrum from a sample taken proximal to the exploration pit at Moctezuma, most likely a relict grain of hydrothermal native tellurium (Fig. 3b). The Te NPs proportion was highest at the Bambolla proximal M2 location (1.54 mol‰) and decreased in distal locations to 0.027 mol ‰ at the Bambolla M5a location and 0.52 mol‰ at the San Miguel S2 location. Our results also confirm the climatic control on the oxidation state of Te in tailings and contaminated soils reported previously by Hayes and Ramos (2019) and Qin et al. (2017). Neither study detected elemental Te, and Qin et al. (2017) found both Te<sup>IV</sup> and Te<sup>VI</sup> in the

Kawazu mine tailings, Japan (Cfa zone, Humid subtropical climate), whereas only Te<sup>VI</sup> was identified in the Delamar mine tailings, Nevada, USA (BSk, cold semi-arid climate) by Hayes and Ramos (2019). Similarly, we identified a mixture of Te<sup>IV</sup> and Te<sup>VI</sup> at Bambolla and San Miguel associated with both Fe<sup>III</sup> (oxyhydr)oxides and illite similar to Qin et al. (2017). These localities are within the hot semi-arid Köppen climate zone (BSh), characterised by hot, wetter summers and cool, dry winters (Weatherspark, 2021; World Weather Online, 2021) with soil samples collected in late summer (September 2019). In contrast, the Blue Bell mine is located in a hot desert climate (BWh) similar to Delamar, and Te is similarly present almost entirely as Te<sup>VI</sup>, mostly associated with Fe<sup>III</sup> (oxyhydr)oxide minerals. These observations suggest a strong climatic control for Te speciation in the regolith surrounding Te hotspots.

**Formation mechanism of Te NPs.** Environmental nanoparticles form via a variety of different methods, including biological, inorganic, or physical pathways (Sharma et al., 2015). Physical production of Te NPs (e.g., by wind-driven erosion of larger native tellurium crystals) has to our knowledge not been reported, but Te NPs are readily obtained using both inorganic and microbially mediated pathways. Inorganic production uses simple reduction of soluble Te oxyanions with reductants such as hydrazine (Lin et al., 2012), potassium borohydride (Panahi-Kalamuei et al., 2014), or citric acid (Cruz et al., 2019), while biological formation is achieved by microbially-mediated reduction of Te oxyanions (Zare et al., 2012; Zonaro et al., 2017; Vaigankar et al., 2018). Based on in vitro experiments performed using pure bacterial cultures and soluble Te<sup>IV</sup> or Te<sup>VI</sup> as the Te sources, several authors argued that the microbes form Te NPs through a detoxification mechanism aimed at transforming toxic soluble Te oxyanions to Te NPs (e.g., Borghese et al., 2016; Espinosa-Ortiz et al., 2017), providing a direct link between microbes and biomineralisation (Tremaroli et al., 2007; Zonaro et al., 2017; Presentato et al., 2019; Goff et al., 2021). Thus, Te NPs may indirectly indicate the bioreduction of soluble Te, acting as a proxy for biological cycling of Te. Analogous examples include the presence of uraninite (UO<sub>2</sub>) NPs in surface environments showing evidence of biological cycling of uranium (Bargar et al., 2008). As one of the most toxic oxyanions, even low concentrations of soluble tellurite in regolith environments will cause a high degree of oxidative stress. Since over 90% of microorganisms have no resistance mechanism to Te oxyanions (Ollivier et al., 2008), soluble Te in the regolith pore water is likely to select for microorganisms that have a generalised resistance to oxidative stress (Arenas et al., 2014). Therefore, resistance to Te is not confined to one biological niche, with microorganisms in many different phyla able to reduce Te (Presentato et al., 2019), depending on the solubility and concentration of Te oxyanions in each microenvironment. Tellurium NPs are abundant on the surface of hydrothermal native tellurium undergoing weathering, where they are associated with carbon-rich organic material (Missen et al., 2022), as expected for bio-mediated reduction. In this microenvironment, high concentrations of Te oxyanions are the result of the oxidation of native tellurium under the influence of oxidised ground- and surface waters (Missen et al., 2022). An analogous process is well described on the surface of native gold grains in the regolith, resulting in the (bio)transformation of coarse hydrothermal gold into nano- to micro-particulate forms, a process driven by the high toxicity of dissolved ionic forms of gold (Fairbrother et al., 2013; Etschmann et al., 2016; Sanyal et al., 2019). Inorganic formation of Te NPs is unlikely to be dominant in the regolith due to a lack of suitable reductants (for instance, most Fe is found as oxidised Fe<sup>III</sup> rather than as Fe<sup>II</sup>).

**Active biogeochemical cycling of Te.** The presence of reduced Te (in the form of Te NPs) alongside oxidised Te in the regolith provides strong evidence for active cycling of Te, from soluble, oxidised Te to reduced, solid forms of Te. Tellurium NPs oxidise rapidly (in a matter of hours) in the presence of oxidants such as atmospheric O<sub>2</sub>, especially if not stabilised by capping agents (Piacenza et al., 2017; Presentato et al., 2019), and hence are unlikely to survive for extensive periods of time in the
regolith, where conditions may change rapidly. Moreover, if Te oxidation states were environmentally irreversible and related entirely to inorganic oxidation, we would expect to see entirely Te^IV^ and Te^VI^ without any elemental Te, a hypothesis invalidated by the presence of elemental Te NPs detected by SP-ICP-MS. While oxidised Te predominates in regolith environments, the consistent presence of elemental Te NPs in both proximal and distal samples suggests that Te NPs are continuously formed in surface environments as part of an active Te cycling. Smaller average Te NP sizes in distal environments may also reflect the lower concentrations of Te, which limits biogenic formation of NPs due to a lack of suitable Te-enriched microenvironments. Biological activity is also limited under arid conditions (e.g., Fairbrother et al., 2012); hence, the higher level of Te cycling under more humid conditions can reflect the high level of biogenic cycling in general under these conditions.

*Role of Te NPs in Te mobility.* While coarsely crystalline native tellurium has been known as a hydrothermal mineral for over two centuries (Nakata and Komuro, 2011), this study shows that ‘secondary’ elemental Te is part of the surface cycling of Te. Consequently, some Te mobility in surface environments may proceed by colloidal transport of Te NPs (Hartland et al., 2013), analogous to Se (Nancharaiah et al., 2016); however, the reactivity of Te NPs is likely to limit their ability to be transported over significant distances.

*Environmental Implications.* More importantly, the detection of Te NPs has significant implications for Te toxicity and bioavailability in soils. Considering the high reactivity of Te NPs, their presence in distal regolith provides strong evidence for the cycling of Te from soluble oxyanionic forms back to elemental Te NPs in situ. This active (bio) geochemical cycling is evidence that toxic soluble Te is present in contaminated soils. It also indicates that nominally poorly soluble forms of Te sorbed on, or co-precipitated with, Fe (oxyhydr)oxides and illite may become bioavailable to microbes at least locally in specific micro-environments, e.g., via dynamic recrystallisation of these minerals (Gorski and Fantle, 2017; Friedrich et al., 2019). Therefore, while Te NPs make up only 0.1–1 mol% in distal regolith, they are an important contributor to Te behaviour.

## 5. Conclusions

Whilst Te NPs are not the major form of Te in the regolith (generally <2 mol% of total Te), they are important to consider when assessing the environmental chemistry of this semi-metal. Unlike Te that is locked up in secondary minerals or sorbed to clay minerals, Te NPs are reactive (susceptible to rapid re-oxidation and/or aggregation) and a short-lived sink of Te. Their detection in regolith distal from Te-containing ores provides further strong evidence for active Te biogeochemical cycling. Te NPs over 100 m from the Te source can only be produced in situ by the biological action of microbes on soluble Te oxyanions, otherwise all Te would be expected to be Te^IV^ and Te^VI^ under highly oxidising surface conditions (Qin et al., 2017; Missen et al., 2020). Yet despite the harsh desert conditions and low moisture, Te NPs have been detected in environmental samples for the first time, opening a new nano-frontier for the cycling of this Critical Element.

Furthermore, our study is a reminder that the behaviour of most Critical Elements in the environment require analysis at the nanoscale, otherwise crucial detail may be omitted from a biogeochemical study regarding important factors such as environmental mobility and reactivity.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

The authors acknowledge support funding provided to OPM by an Australian Government Research Training Program (RTP) Scholarship, a Monash Graduate Excellence Scholarship (MGES), a Monash-Museums Victoria Scholarship (Robert Blackwood) and internal support through Monash University's Faculty of Science Research Reactivation Small Grant Scheme. The authors further acknowledge The Ian Potter Foundation grant 'tracking tellurium' to SJM. We also thank A/Prof Frank Reith (University of Adelaide, dec.) for his pioneering work in gold biogeochemistry, Marek Chorazewicz, Gregor Losson, Dian Hare and the Melchiorre lab for assistance with fieldwork, Dr Manuel Montaño (Western Washington University) for his assistance with SP-ICP-MS data processing and Dr Marine Cotte (ESRF, France) for assistance with synchrotron experiments.

The authors acknowledge use of the facilities at the Monash Centre for Electron Microscopy, the Monash X-ray Platform and Flinders Analytical Services (Flinders University). This research used equipment funded by Australian Research Council grant LE0882821. The authors would like to acknowledge the ARC Research Hub on Australian Copper-Uranium (project number: IH130200033), funded by the Australian Research Council, BHP Olympic Dam and the South Australian Department of State Development; for their support and assistance. XRF and XAS measurements were performed at SPing-8, Japan (Proposal No. 2021A1077) and the ESRF, France (Proposal Nos. EV-447 & EV-449).

## Appendix A. Supplementary material

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.hazl.2022.100053.

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