Transformations of Microplastics in Biosolids Through Hydrothermal Carbonization: A Morphological SEM Study

Hydrothermal Carbonization (HTC) has emerged as a promising technology for treating biosolids. Recently, HTC has gained significant attention in mitigating Microplastic contamination [1]. This study investigates the impact of HTC on the morphology and distribution of Microplastics in biosolids by using scanning electron microscopy (SEM) as a key analytical tool. Biosolid samples were subjected to HTC at three different temperatures:  200, 210, and 220 °C and autogenous pressure to assess the structural transformations of Microplastic. Polymer particles were extracted by 15% H₂O₂ chemical digestion, separated by density using saturated CaCl₂ solution and filtered by anodic alumina membrane microfilters. It has been proven that the HTC process causes significant morphological alterations in Microplastics, which are dependent on the severity of the HTC process parameters [1]. Based on previous research, higher temperatures (>220 °C) promote the decomposition and embrittlement of Microplastics the most, reducing particle size and affecting their chemical composition [2]. In this study, the SEM analysis was applied to assess morphological changes, as it can be used to evaluate Microplastic transformations under hydrothermal conditions [3].  After that, the interaction between Microplastics and biosolid matrices during HTC was explored, highlighting the encapsulation and immobilisation of residual particles in hydrochars. This study contributes to the understanding of Microplastic behaviour under hydrothermal conditions and supports the adoption of HTC as an innovative solution for the management of sewage sludge.

Acknowledgements: This research project was supported by the programme "Excellence Initiative – Research University" for the AGH University of Krakow, Poland. The research was partially supported by Research Subsidy AGH 16.16.210.476.

References:

[1] Prus, Z., Wilk, M. Microplastics in Sewage Sludge: Worldwide Presence in Biosolids, Environmental Impact, Identification Methods and Possible Routes of Degradation, Including the Hydrothermal Carbonization Process. Energies 2024, 17, 4219. https://doi.org/10.3390/en17174219

[2] Xu, Z., Bai, X. Microplastic Degradation in Sewage Sludge by Hydrothermal Carbonization: Efficiency and Mechanisms. Chemosphere 2022, 297, 134203. https://doi.org/10.1016/j.chemosphere.2022.134203

[3] Akaniro, I. R., Zhang, R., Tsang, C. H. M., Wang, P., Yang, Z., & Zhao, J. Exploring the potential of hydrothermal treatment for microplastics removal in digestate. ACS Sustainable Chemistry & Engineering 2024, 12, 38, 14187–14199. https://doi.org/10.1021/acssuschemeng.4c04124