Simultaneous removal of selenite and heavy metals from wastewater and their recovery as nanoparticles using an inverse fluidized bed bioreactor

Occurrence of selenium oxyanions and different heavy metals as co-pollutants in various waste streams is common from industries such as mining, coal burning, thermal power plants, petrochemical refinery, etc. Among the available technologies to treat such wastewater, biological removal using anaerobic microbes is one of the most advantageous. This study examined simultaneous removal and recovery of selenite and heavy metals using an inverse fluidized bed bioreactor (IFBR). The effect of commonly occurring heavy metals namely Zn, Cu and Cd on selenite removal was evaluated at 24 h hydraulic retention time (HRT). The maximum selenite removal of 93.9% was obtained in the absence of any heavy metals. The presence of heavy metals negatively impacted the selenite removal (45.8–85.3%), and the inhibitory effect due to Cu was the most significant among the three metals at 50 mg/L of influent concentration (47% lower). The heavy metal removal efficiency values were more than 70%, except in the case of Cd at 50 mg/L. The recovery values of selenium and heavy metals using the continuous bioreactor were in the range 45.1–73.9% and 59.2–69.8%, respectively, under different experimental conditions. Elemental selenium nanoparticles were formed inside the bioreactor due to selenite bioreduction and the nanoparticle size ranged from 50 to 300 nm. Furthermore, formation of small sized (<10 nm) metal selenide nanoparticles of the respective metals were detected during the bioreactor operation. Overall, this study clearly demonstrated the potential of the IFBR system for the removal of selenite and heavy metals from wastewater as well as their recovery as elemental selenium and metal selenide nanoparticles, respectively.