Biogenic nanoparticles of elemental selenium : synthesis, characterization and relevance in wastewater treatment
Nanoparticles exhibit many unique properties as compared to bulk materials owning to their high surface to volume ratio. Elemental selenium nanoparticles also exhibit novel properties that are exploited in the fabrication of solar cells, semiconductor rectifiers and removal of mercury and copper. The chemical synthesis of elemental selenium nanoparticles is costly, requires specialized equipment and uses toxic chemicals. On the other hand, biological production of elemental selenium nanoparticles (BioSeNPs) can be a green replacement for their chemical synthesis.
BioSeNPs are produced by microbial reduction of selenite and selenate. The source of the selenium oxyanions can be wastewater, where microbial reduction is employed as a remediation technology for the removal of selenium. The formed BioSeNPs are colloidal poly-disperse particles with negative surface charge and are present in the effluent of the bioreactor. Thus far, the properties of these BioSeNPs are not very well understood. This knowledge will help to produce better quality selenium nanomaterials, exploit BioSeNPs applications in wastewater treatment and control the fate of these BioSeNPs in the microbial reactors and the environment.
The characterization of BioSeNPs revealed the presence of extracellular polymeric substances (EPS) on the surface of BioSeNPs. The EPS was identified to control the surface charge and to some extent the shape of the BioSeNPs. Microbial reduction at 55 and 65 °C can lead to the formation of selenium nanowires as compared to nanospheres when the reduction takes place at 30 °C. These selenium nanowires are present in a trigonal crystalline structure and form a colloidal suspension, unlike the chemically formed trigonal selenium nanorods. The colloidal nature is due to negative ζ-potential values owning to the presence of EPS on the surface of biogenic selenium nanowires. Since proteins are a major component present in the EPS, the presence of various proteins on the surface of BioSeNPs was determined. The interaction of various amino acids with the BioSeNPs was also evaluated.
The interaction of heavy metals and BioSeNPs was studied with a view of developing a technology where BioSeNPs present in the effluent of an upflow anaerobic sludge blanket (UASB) reactor are mixed with heavy metals containing wastewater leading to the removal of both BioSeNPs and heavy metals. It was found that Cu, Cd and Zn can be effectively adsorbed onto BioSeNPs. Cu was 4.7 times preferentially adsorbed onto BioSeNPs. The interaction of BioSeNPs with the heavy metals led to a less negative ζ-potential of BioSeNPs loaded with heavy metals and thus better settling of BioSeNPs.
The presence of BioSeNPs in the effluent of a bioreactor treating selenium oxyanions containing wastewaters is undesirable due to higher total selenium concentrations. When a UASB reactor was operated under mesophilic and thermophilic conditions, the total selenium concentration in the effluent under thermophilic conditions were lower than in the mesophilic bioreactor effluent, suggesting better trapping of BioSeNPs at elevated temperatures. When an activated sludge reactor system was investigated to aerobically reduce selenite to BioSeNPs and trap them in the activated sludge flocs, around 80% of the fed selenium was trapped in the biomass. Sequential extraction procedure revealed that the trapped selenium is in form of BioSeNPs. The trapping of BioSeNPs in the activated sludge improved its settleability and hydrophilicity.