**5. Summary**

A following study by the same research group evaluated the feasibility to use UASB reactors

tium in granular sludge, and the subsequent separation of the nanomaterials from the water effluent [164]. In this study, ethanol was added to the system as exogenous source of electrondonating substrate, while riboflavin was supplied as redox mediator during the biological process [164]. UASB reactors were operated with hydraulic retention time of 14 h at 28°C and

version, lowering the toxicity of this oxyanion toward the microbial consortium. Moreover,

UASB reactor, showing a bioreduction efficiency ranging from 83%, when riboflavin was

increased after 42 days to 20 mg L−1. Te-oxyanion removal started immediately after the initial

tion of the microbial consortium to the presence of this oxyanion [165]. Furthermore, TeO3

opens new possibility to combine oxyanion removal with the recovery of Te0

**4. Microbial generation of Se- and Te-nanostructures**

be emphasized in the nanosized material containing Se0

less toxic and bioavailable elemental form of these chalcogens (i.e., Se0

was almost completely bioconverted to its elemental state in the form of Te-nanostructures associated with the loosely bound EPS fraction surrounding the sludge, suggesting a pivotal role played by EPS and its functional groups in the biogenesis of Te-nanoprecipitates. In this regard, the possibility to easily recover Te-nanostructures associated with the EPS fraction

It is nowadays recognized the key role played by bacteria not only as tool for bioremediation purposes of highly contaminated Se- and Te-matrices, but also as a mean by which the

ated and recovered. Indeed, yet Se and Te are elements featured by unique chemical-physical (i.e., semiconductive, photoconductive and catalytic) properties [166–169], which result to

nanoparticles (NPs) and/or nanorods (NRs). Se and Te as nanoscale structures are characterized by a large surface-to-volume ratio and a large surface energy as compared to their bulk counterparts [8], which make them suitable for biotechnological applications, such as: biomedicine, electronics, environmental engineering and agricultural industries [168, 170], to name a few. Since bacteria are considered inexpensive catalysts, their use for the production of Se- and Te-based nanostructures is an attractive choice over the chemical synthesis processes [79]. Thus, microorganisms capable of generating biogenic nanomaterials are seen as *green* and cost-effective exploitable methods to synthesize high-quality nanostructures [10],

2− addition [165]. Particularly, after the first 3–4 weeks of sludge incubation in the reactor,

2− removal from wastewater using a UASB bioreactor was also recently investigated by Mal and coworkers, which inoculated a UASB reactor with anaerobic granular sludge fed with lactate as carbon source, with a hydraulic retention time of 12 h at 30°C [165]. In the UASB

[164], the presence of riboflavin as redox mediator enhanced the efficiency of TeO<sup>3</sup>

absent, to 99.5%, when riboflavin was added to the system [164].

reactor, firstly a concentration of 10 mg L−1 of TeO3

2− to Te-nanoprecipitates using a methanogenic microbial consor-

2− by the anaerobic microbial consortium was observed in the

2− removal efficiency was observed, suggesting an adapta-

and Te0

2− [164]. Similarly to the above-mentioned study

2− was added, which was subsequently

[165].

and Te0

as building blocks, forming

) are gener-

2− biocon-

2−

for the bioconversion of TeO3

a continuous removal of TeO3

a significant improvement of TeO<sup>3</sup>

TeO3

130 Biosorption

TeO3

supplemented with up to 20 mg L−1 of TeO3

Bioremediation strategies of Se- and Te-polluted environments based on the ability of microorganisms to bioprocess these toxic oxyanion species is an environmental-sustainable choice to reclaim contaminated soils, groundwater, surface water bodies and sediments. The primary microbial process after biosorption is the bioreduction of chalcogen-oxyanions into their less toxic and bioavailable elemental forms (i.e., Se0 and Te0 ) generating, as end-products nanoscale materials, which can be recovered from the biomasses and used for technological purposes.
