**1. Introduction**

The chalcogens tellurium (Te) and selenium (Se) are naturally occurring rare elements of the Earth crust belonging to the group 16 of the periodic table that are defined as metalloids,

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due to their intermediate chemical–physical properties between metal and non-metals [1]. Te estimated average amount in the environment is around 0.027 ppm [2], while Se is unevenly distributed on the Earth's surface with a concentration ranging from 0.01 to 1200 ppm [3, 4]. These elements can be found in natural rocks and ores, soils, sediments or in association with rare minerals (e.g., calaverite AuTe2 , sylvanite AgAuTe4 , crooksite CuTlSe, calusthalite PbSe) [4–6]. Moreover, Se is an essential micronutrient for living systems, being part of the structure of several important enzymes, (i.e., glutathione peroxidases and thioredoxin reductases), as the 21st amino acid seleno-cysteine, in at least 25 human selenoproteins [7], while, to date, any biological function has been ascribed to Te [8]. Both these chalcogens exist in four different valence states in the environment (i.e., +VI, +IV, 0 and –II), and among them the oxyanion forms of Selenate (SeO4 2−), Tellurate (TeO4 2−), Selenite (SeO3 2−) and Tellurite (TeO3 2−) are the most abundant in soils and waters [9, 10].

there is a strong movement toward eco-friendly approaches for bioremediation of chalcogencontaminated areas of interest. Moreover, among bacterial strains able to bioconvert Se- and

them were characterized for the generation of either intra- or extracellular precipitates and/or

Here, we will overview the microbial-based strategies that, to date, are applied as tools for bioremediation purposes of chalcogens polluted environments, and briefly will be described the valuable role of bacteria for the recovery of metalloids in their zero-valence state in the

Annually, the total average amount of either Se or Te produced worldwide is 2500–2800 or 220 tons, respectively, with USA, Japan, Russia, Canada, Germany, Belgium and Sweden as main manufacturers [7, 30]. The accumulation of Se- or Te-compounds in the environment mainly relies on their anthropogenic use in several application fields, causing therefore their emission in the atmosphere [31, 32]. Se-accumulation derives from metallurgic industries, glass manufactures, pigments production, electronics and agriculture applications [33], while Te-containing compounds are used in copper refining [19], tarnishing metals [34], vulcanization of rubber [8], production of color glass or ceramics [19], photovoltaic cells and solar panels [8], as well as catalysis of several reactions [19]. Recently, the possibility to develop new Te-based nanomaterials such as fluorescent quantum dots (QDs) has been extensively investigated to create new high-tech probes in biological detection [8, 35], exasperating the already

Among the different Se-species present in the environment, the inorganic forms of Se2−, SeO4

and volatile ones (i.e., methylselenides, trimethylselenonium ions and selenoamino acids) occur in air and soils [37]. Similarly, Te-compounds result to be highly concentrated either

The presence of Se- and Te-compounds in water reservoirs has become a problem for both human health and ecological wildlife [42–45], which led to the development of several strategies aimed to protect aquatic and human life [46], as Se-poisoning events have occurred in the last few years worldwide, such as in the Kesterson Wildlife reservoir (California) [47], the uranium mine in Saskatchewan (Canada), and the Lake Sutton (USA) [48], causing physical deformities and mutations [46]. The major areas of the world affected by water contamination

while higher level of Te-oxyanions has been detected in the surface waters of Te-contaminated basins in Angola and Panama as compared to the deep ones, indicating a difference in behavior between Te and Se, which, as nutrient, is usually highly concentrated in the deep ocean [49]. Finally, Te-compounds emission in the atmosphere is now investigating, even if the implication related to the presence of Te-species in the air has not been established yet [19].

2− are generally found in surface and ground waters as pollutants [36], while the organic

2− and TeO3

2− are North America, Australia and New Zealand [23],

and Te0

http://dx.doi.org/10.5772/intechopen.72096

Microbial-Based Bioremediation of Selenium and Tellurium Compounds

), some of

119

2−,

2−, being the latter highly

Te-oxyanions in their less toxic and less bioavailable elemental form (i.e., Se0

nanomaterials, for example, nanoparticles (NPs) and nanorods (NRs) [8, 19].

**1.1. Environmental toxicity of selenium and tellurium compounds**

form of nanomaterials.

dramatic waste disposal circumstances.

soluble and toxic [35, 40, 41].

due to the presence of SeO4

in soils [38, 39] or waters [34] mainly in the form of TeO4

2− and SeO3

or SeO3

The wide spread use of Se- and Te-compounds by anthropogenic activities related to oil refining, phosphate and metal ore mining, electronics and industrial glasses, have led to an increase in the presence of these chemicals in the environment [6, 11]. In this regard, although Se is an essential micronutrient, it is toxic at concentrations higher than the human dietary requirement (25–30 μg day−1) [10], while the toxicity exerted by Te is even more dramatic, negatively affecting both prokaryotes and eukaryotes at concentration as low as 1 μg mL−1 [6]. Particularly, Se- and Te- oxyanions are recognized as harsh toxicants of public health and environmental concern due to their association with oxygen, which makes them highly bioavailable, enabling the mobilization of Se- and Te-compounds through water and soil [12, 13]. On the contrary, Se and Te organic forms (e.g., dimethyl selenide, trimethyl selenonium, selenomethionine, selenocysteine, Se-methilselenocysteine, dimethyl telluride), as well as their zero-valence states (Se0 and Te0 ) showed lower toxicity levels [2, 12, 14]. Considering the shared physical–chemical features of Se and Te, the suggested mechanism of toxicity exerted by the chalcogen-oxyanions is based on their interaction with glutathione molecules (GSHs) and related molecules, which are likely responsible for their reduction [8, 13, 15]. This bioconversion mechanism leads to the generation of reactive oxygen species (ROS), such as hydrogen peroxide (H2 O2 ) [16] or superoxide ions (O2−) [17], therefore causing cell death [18–20]. An additional target of TeO3 2− is the impairment of the heme metabolism in *E. coli* K-12 cells, by which this oxyanion is responsible for the accumulation of the heme precursor protoporphyrin IX, causing iron depletion and, subsequently, cell death [21].

Despite the toxic effects of Se- and Te-oxyanions, in the last 20 years several microorganisms able to sequester, bioconvert or biomethylate these chalcogen-ions have been isolated from extreme environments, such as ocean hydrothermal vents and the highly alkaline water Monolake (California), to name a few [22]. Mainly anaerobic or facultative-anaerobic bacteria capable of growing phototrophycally or chemotrophycally under oxic and anoxic conditions have been described for their metabolic potential in bioconverting these species, while much less is known about strictly aerobic microorganisms [23]. In this regard, anaerobic microorganisms have been described for their use of chalcogen-oxyanions as terminal electron acceptors to sustain their growth [19, 22, 24–29]. Although the exact biochemical mechanisms behind Se and Te metabolism and bioconversion in these microorganisms have not been fully elucidated, there is a strong movement toward eco-friendly approaches for bioremediation of chalcogencontaminated areas of interest. Moreover, among bacterial strains able to bioconvert Se- and Te-oxyanions in their less toxic and less bioavailable elemental form (i.e., Se0 and Te0 ), some of them were characterized for the generation of either intra- or extracellular precipitates and/or nanomaterials, for example, nanoparticles (NPs) and nanorods (NRs) [8, 19].

Here, we will overview the microbial-based strategies that, to date, are applied as tools for bioremediation purposes of chalcogens polluted environments, and briefly will be described the valuable role of bacteria for the recovery of metalloids in their zero-valence state in the form of nanomaterials.
