Deciphering the effects of CeO2 nanoparticles on Escherichia coli in the presence of ferrous and sulfide ions: Physicochemical transformation-induced toxicity and detoxification mechanisms.

The physicochemical transformations as well as the redox reaction-induced toxicity changes of ceria nanoparticles (CeO2 NPs) in reducing conditions is extremely lacking. Herein, the behaviors, chemical modifications and toxicity of CeO2 NPs in the presence of reduction-active ions (namely Fe2+ and S2-) were investigated, with a particular emphasis on the cytotoxicity mechanism associated with their physicochemical transformations. The presence of Fe2+ and S2- differently altered the surface properties and toxicity of CeO2 NPs. Redox reactions with Fe2+ led to form small aggregates, boosted the reduction of CeIVO2 and enhanced dissolved Ce3+ concentration. Moreover, CeO2 NPs possessed a high affinity for Escherichia coli (E. coli) and induced the generation of •OH abiotically after reaction with Fe2+, provoking serious disruption of cell membranes and causing high toxicity to E. coli. In contrast, the amending of S2- protected E. coli from direct contact with CeO2 NPs by creating new Ce2S3 precipitated on the surface, accelerating the aggregation of NPs and reducing the concentration of dissolved Ce3+. This study suggested that the chemical interactions between the reactive surfaces of CeO2 and reduction-active ions highly determined the stability and cytotoxicity of CeO2 NPs, which provides fundamental insights into the environmental risks of CeO2 NPs.