Insights into spatial effects of ceria nanoparticles on oxygen mass transfer in wastewater biofilms: Interfacial microstructure, in-situ microbial activity and metabolism regulation mechanism

Growing international exploitation of ceria nanoparticles (CeO2 NPs) for varied applications has increased their release into wastewater treatment plants. Mass transfer of oxygen (MTO) in wastewater biofilm is of considerable importance to influence the activity and purification ability of biofilm. Herein, we investigated the spatial distribution of oxygen in gas-liquid-biofilm phases, the microstructure of interfaces and the in-situ microbial activity to reveal the impacts of CeO2 NPs on MTO in wastewater biofilm and the related mechanisms. After exposure to 1 and 10 mg/L CeO2 NPs, the oxygen transfer coefficient (K-La) from gas to wastewater increased by 28.1% and 75.3% with a reduction of thickness in gas-liquid boundary layer, indicating the enhanced MTO in gas-liquid interface. In contrast, the MTO in liquid-biofilm interface was negatively affected and the thickness of liquid-biofilms boundary layer significantly increased, which was mainly attributed to the smoother surface and the decreased surface area difference of biofilm. Within biofilm, the microbial activity was inhibited by 10 mg/L CeO2 NPs, whereas the production of extracellular polymeric substance (EPS) was significantly improved, leading to a decline of 35.0% in the internal effective diffusivity (D-B) and a 300-mu m reduction of oxygen penetration depth. Moreover, the relative activities of key enzymes involved in glycometabolism indicated the transition of Embden-Meyerhof pathway to pentose phosphate pathway, which probably contributed to the enhanced EPS production and consequently increased mass transfer resistance in liquid-biofilm interface and inner biofilm. These results could potentially expand the knowledge on mass transfer of nutrients or pollutants in wastewater biofilm in response to NPs exposure. (C) 2020 Published by Elsevier Ltd.