Abstract:
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.
