Biotoxicity evaluation of zinc oxide nanoparticles on bacterial performance of activated sludge at COD, nitrogen, and phosphorus reduction

Highlights

• ZnO-NP disrupted metabolic/catabolic balance of bacteria by affecting DHA activity.

• ZnO-NPs toxicity was related to Zn²⁺ ion, interaction with cell and ROS generation.

• Exposure to ZnO-NPs resulted in changed bacterial community structure at sludge.

• The change in the EPS content was observed during exposure to ZnO-NPs.

Abstract

The unique properties and growing usage of zinc oxide nanoparticles increase their release in municipal wastewater treatment plants. Therefore, these nanoparticles, by interacting with microorganisms, can fail the suitable functioning of biological systems in treatment plants. For this reason, research into the toxicity of ZnO is urgent. In the present study, the toxicity mechanism of ZnO-NPs towards microbial communities central to granular activated sludge (GAS) performance was assessed over 120-day exposure. The results demonstrate that the biotoxicity of ZnO-NPs is dependent upon its dosage, exposure time, and the extent of reactive oxygen species (ROS) production. Furthermore, GAS performance and the extracellular polymeric substances (EPS) content were significantly reduced at 50 mg/L ZnO-NPs. This exposure led to decreases in the activity of ammonia monooxygenase (25.2%) and nitrate reductase (11.9%) activity. The Field emission scanning electron microscopy images confirmed that ZnO-NPs were able to disrupt the cell membrane integrity and lead to cell/bacterial death via intracellular ROS generation which was confirmed by the Confocal Laser Scanning Microscopy analysis. After exposure to the NPs, the bacterial community composition shifted to one dominated by Gram-positive bacteria. The results of this study could help to develop environmental standards and regulations for NPs applications and emissions.

Graphical abstract

Keywords

Granular sludge / Biotoxicity / Reactive oxygen species / Extracellular polymeric substances

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Acknowledgements

This work was supported by the vice-chancellor for Research and Technology of Kerman University of Medical Sciences ( Grant No. 98001185) and the code of research ethics certificate IR.KMU.REC.1399.415. The authors would like to thank the Environmental Health Engineering Research Center, the Kerman University of Medical Sciences, for their scientific supports.

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