Basnayake, Rukma ST, Inorganic selenium and tellurium speciation in aqueous medium of biological samples, Master of Science (Chemistry), December 2001, Sam Houston State University, Huntsville, Texas, 60 pp. (pdf version of this thesis)

The purpose of this research was to develop methods to study the ability of bacteria, Pseudomonas fluorescens K27 to detoxify tellurium and selenium salts by biotransformation processes under anaerobic conditions. Another purpose was to make an effort to separate biologically produced Se0 from cells.

Pseudomonas fluorescens K27 was grown in TSN3 medium (tryptic soy broth with 0.3% nitrate) under anaerobic conditions and the production of elemental tellurium and elemental selenium was observed when amended with inorganic tellurium salts and selenium salts, respectively. The amount of soluble tellurium species in the culture medium also was determined. Samples from a 2.75 L bioreactor were taken after cultures had reached the stationary growth phase and were centrifuged in order to separate insoluble species (elemental tellurium, elemental selenium) from soluble species (oxyanions of tellurium, oxyanions of selenium). In tellurium samples, supernatant consisting of soluble tellurium and the sediment consisting of insoluble tellurium were analyzed separately by oxidation and reduction procedures followed by using hydride generation atomic absorption spectroscopy (HGAAS). Sediment from selenium bioreactor samples was used to find free elemental selenium and elemental selenium inside and/or on K27 cells. A sucrose density gradient with overlaid precipitate was prepared, centrifuged, fractionated and analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) to find the concentration of Se0 and also analyzed by UV/Vis spectrometry to find the distribution of cells in the sucrose gradient.

K27 grew well in TSN3 medium under anaerobic conditions. Bacterial cells survived and continued their growth when poisoned with 1 mM tellurate or 10 mM selenite and produced elemental tellurium or elemental selenium. The calibration range for Te analysis using the HGAAS instrumental method was narrow (~3 to 20 ppb) with a detection limit of 3 ppb. The calibration range for Se analysis using ICP-AES instrumental method was wide (~500 to 1000 ppb) with a detection limit of 500 ppb. Approximately 66% of added tellurium was recovered in the liquid medium and 34% was recovered in the solid phase. According to the sucrose density gradient experiment, it was not clear whether selenium was distributed outside the K27 cells as well as inside the K27 cells; either Se0 has the same density as cells or Se0 and cells are bound together.

Thomas G. Chasteen
Thesis Director

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