Removal and recovery of selenium species from wastewater: Adsorption kinetics and co-precipitation mechanisms

Abstract

This study investigates the effectiveness of four adsorbent candidates, including nano-zerovalent iron (nZVI), magnetite (Fe3O4), ferric chloride (FeCl3) and granular activated carbon (GAC) to remove inorganic and organic selenium species from contaminated water. The efficacy of the modified co-precipitation method using a combination of ferrous and ferric chloride was also investigated. Among the adsorbents tested, ferric oxyhydroxide flocs formed by the precipitation of ferric chloride was found to be most efficient as more than 95 % of Se (VI) was removed using ferric chloride dose of 1 g/L. The optimum pH for removal of Se (VI) irrespective of the adsorbent was found to be in the range of 4.0–5.5. Comparatively, Se (IV) was removed faster and required lesser adsorbent doses for all adsorbents tested. The experimental data fitted well with Langmuir model compared to Freundlich model indicating the monolayer adsorption. While metallic adsorbents were found to show better performance for removal of inorganic selenium species, GAC showed a higher affinity towards the removal of selenomethionine, an organo-selenium compound. About 50 % of selenomethionine was removed within a contact time of one hour for GAC dose of 0.5 g/L, while about 9% and 7% removal was observed for the same doses of Fe3O4 and nZVI powders respectively. In contrast to electrostatic attraction, which is considered to be the main drive for adsorption of inorganic selenium, van der Waals attraction is supposed to drive adsorption of selenomethionine.