Fate of biological selenium(0) nanoparticles at elevated temperature

Selenium is toxic and thus its fate in the environment has to be ascertained. The bio-transformation of Se oxyanions by microorganisms will affect its (bio)availability in the environment. The microorganisms are known to reduce Se oxyanions to biological Se(0) nanoparticles (BioSe-Nanospheres) or biological Se(0) nanorods (BioSe-Nanorods) under mesophilic (30 °C) and thermophilic (55 °C) conditions, respectively. Though both types exhibit a coating of extracellular polymeric substances, their colloidal properties differs, leading to a different mobility in environmental waters. [1] Consequently, understanding the underlying mechanisms of the formation of amorphous BioSe-Nanospheres and trigonal BioSe-Nanorods is essential to understand the fate of selenium in the environment.
This study identified BioSe-Nanospheres produced by various microorganisms behaving differently when exposed to thermophilic conditions (55 °C). The bacteria strain E.coli K12 and the anaerobic granular sludge from a full scale Upflow Anaerobic Sludge Bioreactor were used to produce BioSe-Nanospheres at 30 °C. After purification (to separate the particles from the cells) the BioSe-Nanospheres were heated for 7 days at 55 °C with shaking. The Raman and Scanning Electron Microsocpy data evidenced a transformation of the anaerobic granular sludge Se(0) particles to trigonal BioSe-Nanorods upon heating, while the BioSe-Nanospheres of the bacteria strain E.coli K12 remained spherical and amorphous.
The CD-Spectrosopy data revealed that the proteins coating the anaerobic granular sludge BioSe-Nanospheres gets unstable during a heating time of 23 hours, suggesting a denaturating process. Such a behavior was not observed for particles produced via E. coli. Unravelling the differences in particles coating and selenium allotropy which results in changes of mobility and toxicity will further increase the knowledge on the environmental fate of selenium.
[1] Jain, R. et al., Environ. Sci.: Nano, 2017.