Biological Removal of Selenate and Selenite from Wastewater: Options for Selenium Recovery as Nanoparticles

PURPOSE OF REVIEW: Different selenium species released into the environment by anthropogenic activities pollute surface and ground water resources and can cause severe damage to the environment and ecosystems due to bio-accumulation. Though several physico-chemical methods are available to treat selenium oxyanion-containing wastewater, biological methods have gained significant importance due to their capability to convert selenium oxyanions to elemental selenium nanoparticles. The purpose of this review is to summarize the literature available on biological removal of selenium oxyanions and their recovery as elemental selenium nanoparticles. RECENT FINDINGS: In recent times, the capability of several bacterial and fungal strains to reduce selenate and selenite to form elemental selenium nanoparticles has been reported. The shape, size and location of these selenium nanoparticles along with the selenium oxyanion removal efficiency depend on the operating parameters. Moreover, bioreactor configurations and operation strategies greatly influence the selenium removal and recovery efficiency. Several conventional bioreactor systems can be used to remove selenate and selenite from wastewater and form selenium nanoparticles. However, the selenium nanoparticles are mostly entrapped in the biomass and require a secondary treatment to recover them. On the other hand, some novel bioreactors, viz. inverse fluidized bed bioreactor, rotating biological contractor, horizontal rotating packed bed bioreactor, moving bed biofilm reactor, and hybrid bioreactor, can possibly recover selenium nanoparticles following bioreduction of selenium oxyanions in a single stage system. Thus, this review will help in finding research gaps in this area and providing solutions for resource recovery from selenium oxyanion-containing wastewater.