Selenite reduction and biogenesis of selenium-nanoparticles by different size groups of aerobic granular sludge under aerobic conditions

Abstract

Microbial transformations play a vital role in Se cycle in the environment and decrease the solubility and toxicity of Se oxyanions by converting to elemental selenium (Se0) nanostructures. Aerobic granular sludge (AGS) has attracted interest due to efficient reduction of selenite to biogenic Se0 (Bio-Se0) and retention in bioreactors. Here, selenite removal, biogenesis of Bio-Se0 and entrapment of Bio-Se0 by different size groups of aerobic granules were investigated to optimize biological treatment process for Se-laden wastewaters. Furthermore, a bacterial strain showing high selenite tolerance and reduction was isolated and characterized. Removal of selenite and conversion to Bio-Se0 were achieved by all the size groups of granules ranging from 0.12 mm to 2 mm and above. However, selenite reduction and Bio-Se0 formation were rapid and more efficient with large aerobic granules (≥0.5 mm). The formed Bio-Se0 was majorly associated with the large granules, due to better entrapment capabilities. In contrast, the Bio-Se0 formed by the small granules (≤0.2 mm) was distributed both in the granules and aqueous phase because of ineffective entrapment. Scanning electron microscope and energy dispersive X-ray (SEM-EDX) analysis confirmed formation of Se0 spheres and association with the granules. Efficient selenite reduction and entrapment of Bio-Se0 was related to prevalent anoxic/anaerobic zones in the large granules. A bacterial strain showing efficient SeO32− reduction of up to 15 mM SeO32− under aerobic conditions was identified as Microbacterium azadirachtae. SEM-EDX analysis confirmed the formation and entrapment of Se0 nanospheres (size: 100 ± 5 nm) in the extracellular matrix. The cells immobilized in alginate beads showed effective SeO32− reduction and Bio-Se0 entrapment. Efficient reduction and immobilization of bio-transformed metalloids by large AGS and AGS-borne bacteria implicates prospective use in bioremediation of metal(loid) oxyanions and bio-recovery.