Abstract
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 7, temperature of 37°C, 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°C 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.
Highlights
BackgroundFirst discovered in 1818, is a metalloid and chalcogen. It has multiple oxidation states, beginning with the most reduced state, namely selenate (SeO 42-, Se(VI)), followed by selenite (SeO32-, Se(IV)), elemental selenium (Se0, Se(0)) and selenide (Se2-)
Selenium was discovered in 1818 by Jons Jakob Berzelius (Lenz & Lens, 2009)
Even though elemental selenium is less of a threat to water users, it is still important that the elemental selenium be removed from waste water before it is discharged into water bodies This is due to elemental selenium being reoxidised to selenate and selenite in water bodies with a high redox potential, such as surface waters (Zhang et al, 2004)
Summary
First discovered in 1818, is a metalloid and chalcogen. It has multiple oxidation states, beginning with the most reduced state, namely selenate (SeO 42-, Se(VI)), followed by selenite (SeO32-, Se(IV)), elemental selenium (Se0, Se(0)) and selenide (Se2-). There are many natural pathways through which selenium is released into the environment and redistributed These processes range from volcanic activity to wildfires, volatilisation from soils, plants and water bodies, and weathering of rocks (Sandy & DiSante, 2010). Selenium is mostly present in water as selenate or selenite This poses a problem, since these two oxyanions are toxic and bio-accumulate readily. Selenium poisoning – referred to as selenosis – can have many adverse effects, including respiratory difficulty, gastrointestinal distress and liver damage. It has a negative impact on the central nervous system (Kenward et al, 2006)
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