Abstract
BackgroundSelenium and Tellurium have many common chemical properties as both belong to group 16 of the periodic table. High toxicities of Se and Te oxyanions cause environmental problems in contaminated soils and waters. Three strains (C4, C6 and C7) of selenite reducing and nanoparticle forming aerobic bacteria which were isolated from agricultural soils of India containing high concentrations of Se were investigated after 3.5 months of freeze-storage for their resistance against the toxic oxyanion tellurite and its reduction to non toxic elemental form Te0 as well as nanoparticles formation.ResultsStrains C4, C6 and C7 reduced tellurite at maximum reduction rates of 2.3, 1.5 and 2.1 mg Te (IV)/L/d, respectively and produced extracellular Te0 nanospheres as revealed from SEM-EDX analysis. Production of extracellular Te nanospheres has been described seldom. Further, concurrent reduction of both selenite and tellurite by bacteria was examined as these toxic oxyanions are often present together in natural environments, mine tailings or wastewater from copper refining. Interestingly, bioreduction of 100 mg/L selenite in shake flasks was not much affected by the presence of 10 mg/L tellurite but tellurite reduction rate increased 13 fold with selenite in the medium. The concurrent reduction of these oxyanions resulted in rarely described bioformation of extracellular nanoparticles composed of both Se and Te, reported first time for aerobically growing heterotrophic non-halophilic bacterial cultures. Duganella violacienigra, the closely related strain to C4 was also found to be resistant to oxyanions of Se and Te.ConclusionsSelenite reducing heterotrophic non-halophilic aerobic bacteria revived from 3.5 months freeze storage could successfully reduce toxic tellurite to non toxic elemental form and produced extracellular nanospheres during detoxification. Presence of relatively less toxic selenite in the medium triggers bioreduction of more toxic tellurite leading to formation of extracellular SeTe nanospheres which are sought by solar and optical recording media industry because of their excellent photovoltaic and optical properties. The bacterial cultures investigated in this study could be exploited commercially to remediate not only selenite and tellurite-contaminated soil and water but also for green synthesis of extracellular Se, Te and Se + Te nanospheres.
Highlights
Selenium and Tellurium have many common chemical properties as both belong to group 16 of the periodic table
Selenite reduction after revival of freezed cultures Pure cultures of selenite-reducing aerobic bacteria that have been stored for 3.5 months in a deep freezer were revived by thawing at room temperature (25 ± 2°C) and incubation in media with glucose but without Se (IV)
Bacterial cultures used in the present study were isolated from soil [1,18] and after freeze-storage, successive long incubations were necessary to revive the capability of the bacteria to form elemental nanoparticles by adapting them again to increasing concentration of Se (IV)
Summary
Selenium and Tellurium have many common chemical properties as both belong to group 16 of the periodic table. Three strains (C4, C6 and C7) of selenite reducing and nanoparticle forming aerobic bacteria which were isolated from agricultural soils of India containing high concentrations of Se were investigated after 3.5 months of freeze-storage for their resistance against the toxic oxyanion tellurite and its reduction to non toxic elemental form Te0 as well as nanoparticles formation. Elemental (zero valent) Te and Se are non soluble Both Te and Se are found in relatively low abundances in earth’s crust, yet high toxicities of these elements cause environmental problems in contaminated soils and waters e.g. in agricultural lands or wastewater discharges from industrial activities or mine tailings [1,2]. Concurrent Te and Se reduction has not been investigated for chalcogen reducing heterotrophic aerobic bacterial cultures except for a few studies describing synergistic reduction of Te + Se by halophilic bacteria [6,16,17]
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