Understanding the role of cations and hydrogen bonds on the stability of aerobic granules from the perspective of the aggregation and adhesion behavior of extracellular polymeric substances

Extracellular polymeric substances (EPS) were essential for the granulation and stability of aerobic granular sludge (AGS). In this study, the effects of electrostatic interactions, bridging effect of divalent cations, and hydrogen bonds on the EPS-EPS and EPS-surface interaction were verified by enhancing or reducing the specific interaction with the addition of cations or urea. The size and the surface properties of EPS aggregates were investigated, the adhesion behavior and viscoelasticity of EPS were analyzed by quartz crystal microbalance with dissipation monitoring. The changes of EPS in response to the various condition were analyzed by infrared spectroscopy and fluorescence spectrum. The electrostatic repulsion between EPS could be significantly reduced by Ca2+ addition. With the bridging effect, 10 mu M of Ca2+ could reduce the negative charge of EPS more effectively than 200 mu M of Na+. As Ca2+ could form the complex with the protein and Ca2+ was more inclined to bind with COO-, the Ca2+ took advantage of boosting the EPS-EPS and EPS-surface interaction than Mg2+ at the same ionic strength, which resulted in the denser structure of calcium-treated EPS. The destruction of hydrogen bonds by urea addition reduced the EPS-EPS and EPS-surface interaction, which confirmed the potential existence of hydrogen bonds in the interaction of EPS-EPS and EPS-surface. The removal of hydrogen bonds of EPS destroyed the protein's secondary structure and caused the unfolded state of the protein, which led to the looser structure of the EPS layer. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study demonstrated the effects of electrostatic interactions, bridging effect of divalent cations, and hydrogen bonds on EPS-EPS and EPS-surface interactions. Ca2+ was more effective in enhancing the interactions than Mg2+, leading to a denser structure, while urea disrupted hydrogen bonds resulting in a looser structure of EPS.