Effects of CeO₂ nanoparticles on sludge aggregation and the role of extracellular polymeric substances – Explanation based on extended DLVO

The extended DLVO (XDLVO) theory was applied to elucidate the potential effects of CeO₂ nanoparticles (CeO₂ NPs) on sludge aggregation and the role of extracellular polymeric substances (EPS). In this study, seven different concentrations of CeO₂ NPs were added to activated sludge cultured in sequencing batch reactors (SBRs) and compared with a control test that received no CeO₂ NPs. After exposure to 50 mg/L CeO₂ NPs, a negligible change (p>0.1) occurred in the sludge volume index (SVI), whereas the flocculability and aggregation of the sludge decreased by 18.8% and 11.2%, respectively, resulting in a high effluent turbidity. The XDLVO theory demonstrated that the adverse effects of the CeO₂ NPs on sludge aggregation were due to an enhanced barrier energy. Compared to the van der Waals energies (W_A) and the electric double layer (W_R), the acid-base interaction (W_AB) markedly changed for the various concentrations of CeO₂ NPs. The EPS played a decisive role in the sludge surface characteristics, as the removal of EPS equals to the negative effects induced by 5–10 mg/L CeO₂ NPs on the sludge flocculability and aggregation. The presence of CeO₂ NPs induced negative contributions to the tight boundary EPS (TB-EPS) and core bacteria while positive contributions to the total interaction energy of the loose boundary EPS (LB-EPS). - Highlights: • CeO₂ NPs adversely affected the flocculability and aggregation of the sludge. • The presence of CeO₂ NPs increased the energy barrier and led to a stable suspension. • The removal of EPS equals to the negative effects induced by 5–10 mg/L CeO₂ NPs. • The acid-base interaction was dominate and markedly changed for the CeO₂ NPs. • CeO₂ NPs induced negative contributions to the TB-EPS while positive to the LB-EPS.