Mechanically stable bacterial cells encapsulated in Na-alginate hydrogel for effective removal of toxic Se(IV) and its recovery as Se(0) nanostructures

Abstract

Global pollution is causing the release of an excessive amount of toxic compounds into the environment. The use of microorganisms for the removal of toxic compounds, including selenium (Se), lead (Pb), or cadmium (Cd), from the environment has been widely explored in the last decades as one of the most promising methodologies nowadays. Indeed, the immobilization of microorganisms with bioremediation potential in hydrogels is gaining attention, offering several advantages over planktonic or free-living approaches. This study is the first to describe the process of reduction and allotropic transformation of selenite (Se(IV)) to elemental Se(0) nanostructures within eco-friendly Na-alginate hydrogels containing immobilized cells of Stenotrophomonas bentonitica BII-R7. Rheological characterization confirmed the mechanical compliance of hydrogel beads for bacterial cell incorporation, without loss of robustness provoked by the embedded cells. Electron microscopy showed that although the cells are homogeneously distributed across the entire alginate matrix, the surface of the biohydrogels is covered with bacterial cells forming sac-like structures. The biohydrogels removed ~70 % of the total Se(IV) content producing Se(0) nanostructures with different morphologies (nanospheres and hexagonal-shaped), structures and different crystalline phases (t-Se, m-Se). The results obtained herein demonstrate the potential of S. bentonitica hydrogels to remove toxic Se(IV) and recover Se value-added products, enabling the sustainable reuse of wastes according to the circular economy principles.