Insights into the Effects of CeO2 Nanoparticles on Medium-Chain Carboxylates Production from Waste Activated Sludge

The synthesis of medium-chain carboxylates (MCCs) from waste-activated sludge (WAS) upgrading has received considerable attention. However, limited research has been conducted on the effects of CeO2 nanoparticles (NPs) on this process. This study showed that 1 mg/g-TS of CeO2 NPs improved the solubilization of WAS, resulting in higher production of MCCs. At 5 mg/g-TS, CeO2 NPs weakly inhibited 3 biological steps. Despite this, there was an enhancement in WAS solubilization, thus the overall production of MCCs was similar to the control. However, doses of CeO2 NPs ranging from 25-100 mg/g-TS were unable to offset biological inhibition, leading to a decrease in MCC production. The toxic mechanisms involved were not the generation of reactive oxygen species or Ce ions from CeO2 NPs to anaerobic sludge, but instead the decline of extracellular polymeric substance (EPS) and destruction of the cell membrane through physical penetration. Microbial community analysis confirmed that 1 mg/g-TS of CeO2 NPs increased the relative abundance of key bacteria involved in the anaerobic fermentation of WAS. The MCC microbe Clostridium sensu stricto was enriched in the control group, while the relative abundance of this genus was significantly reduced with 100 mg/g-TS CeO2 NPs.