Selenium (Se) in the aquatic environment is predominantly found as the soluble selenium oxyanions; selenate (SeO42-), and selenite (SeO32-). These oxyanions are toxic, and they readily bio-accumulate in the food chain. However, numerous studies have proven the viability of microbial remediation in reducing them to elemental selenium (Se0) which is considered to be biologically inert and relatively less toxic. Of the various microorganisms that have been employed in Se bioremediation, Pseudomonas stutzeri NT-I has shown great potential in removing high concentrations of SeO42- and SeO32-. Of these two selenium oxyanions, SeO32- is more toxic, is the most reactive and is usually found in mildly oxidising acidic environments. Therefore, the focus of this study is on selenite. In this study Pseudomonas stutzeri NT-I was used in aerobic batch reduction of various SeO32- concentrations (0.5 to 10 mM) to Se0 under already known optimum conditions of temperature 35±2 °C, pH 7 – 8 and salinity 5 g.L-1 NaCl. The selenium (Se) reduction efficiency of strain NT-I was assessed under varying conditions, such as the presence and absence added nitrogen and glucose substrates, inhibited metabolic activity and bacterial cells. Moreover, the variation in the different parameters such the oxidation reduction potential (ORP), metabolic activity (MA) and the concentrations of SeO32-, glucose and total organic carbon (TOC) were also monitored. Key results indicated that the rate and amount of SeO32 reduction was influenced by the concentration to be reduced. For an initial SeO32- concentration of 0.5 mM, reduction was gradual and after 36 h, approximately 75 % had been reduced to Se0 which was equivalent to 0.375 mM. In the reduction of 10 mM SeO32- however, the reduction rate was rapid and even though the overall percentage reduction averaged around 18 %, this was equivalent to 1.8 mM This indicated that the increased initial reduction rate was a result of increased biomass activity in response to increased selenite concentration. This response is likely a defence mechanism employed by strain NT-I to detoxify its surrounding in elevated SeO32- concentrations. The results of these biological experiments were modelled, and the non-growth kinetics were found to fit the adapted Monod equation with k_s and k_Se values of 4.723 mM and 2.869 mmol.(h.g)-1 respectively. Pseudomonas stutzeri NT-I’s capability of being able to survive in very high selenium concentrations make it an attractive and versatile microbial species suitable for the bioremediation of selenium laden industrial wastewater. / Dissertation (MSc (Applied Science: Environmental Technology))--University of Pretoria, 2020. / Chemical Engineering / MSc (Applied Science: Environmental Technology) / Restricted

Bioremediation of seleniferous water is gaining more momentum, especially when it comes to bacterial reduction of the selenium oxyanions. More and more bacterial strains that are able to reduce selenium are being isolated. These bacteria need to be studied further to determine whether they are suited for industrial application. In this study, the reduction of Se(VI) to Se(0) by Pseudomonas stutzeri NT-I was examined using batch experiments with the bacteria suspended in MSM. For the determination of the optimum conditions for the growth of the bacteria, the linearized rate during the exponential phase for different conditions were compared. A pH of 7, temperature of 37 ⁰C, salinity of 20 g.L-1 NaCl and initial concentration of 5 mM selenate were found to be the best at promoting growth. To determine the optimum conditions for the reduction of selenium, the amount of Se(0) recovered from the plug after 16 hours of incubation was measured. A pH of 8, temperature of 37 ⁰C and salinity of 5 g.L-1 resulted in the most Se(0) recovered. The kinetics of the reduction of Se(VI) to Se(0) was found to follow the adapted Monod equation. An increase in the initial Se(VI) concentration positively affected the reduction rate indicating that substrate saturation had not yet been reached. One kmax could be fitted to each of the two reactions but not one Ks. It was found that Ks decreased with increasing initial selenate concentration. Visually it can be deduced that inhibition starts playing a role in the reduction of selenate at a concentration of 4 mM. Pseudomonas stutzeri NT-I is an exemplary selenium reducing agent and deserves more attention, not only for industrial application but also in the research world, for further understanding of the complex mechanism behind metal reduction in bacteria. / Dissertation (MEng)--University of Pretoria, 2017. / Chemical Engineering / MEng / Unrestricted