A comparison of fate and toxicity of selenite, biogenically, and chemically synthesized selenium nanoparticles to zebrafish (Danio rerio) embryogenesis

Microbial reduction of selenium (Se) oxyanions to elemental Se is a promising technology for bioremediation and treatment of Se wastewaters. But a fraction of biogenic nano-Selenium (nano-Se-b) formed in bioreactors remains suspended in the treated waters, thus entering the aquatic environment. The present study investigated the toxicity of nano-Se-b formed by anaerobic granular sludge biofilms on zebrafish embryos in comparison with selenite and chemogenic nano-Se (nano-Se-c). The nano-Se-b formed by granular sludge biofilms showed a LC50 value of 1.77mg/L, which was 3.2-fold less toxic to zebrafish embryos than selenite (LC50=0.55mg/L) and 10-fold less toxic than bovine serum albumin stabilized nano-Se-c (LC50=0.16mg/L). Smaller (nano-Se-cs; particle diameter range: 25-80nm) and larger (nano-Se-cl; particle diameter range: 50-250nm) sized chemically synthesized nano-Se-c particles showed comparable toxicity on zebrafish embryos. The lower toxicity of nano-Se-b in comparison with nano-Se-c was analyzed in terms of the stabilizing organic layer. The results confirmed that the organic layer extracted from the nano-Se-b consisted of components of the extracellular polymeric substances (EPS) matrix, which govern the physiochemical stability and surface properties like zeta-potential of nano-Se-b. Based on the data, it is contented that the presence of humic acid like substances of EPS on the surface of nano-Se-b plays a major role in lowering the bioavailability (uptake) and toxicity of nano-Se-b by decreasing the interactions between nanoparticles and embryos.