Atomic Force Microscopy Nanomechanicsfor the Detection of Microplastics in Groundwater: Case Study from Shenzhen (Guangdong, China)

Microplastic pollution is a severe environmental issue with potential impacts on human health. Despite the worldwide diffusion of plastic debris in the environment, the effective characterization of microplastics (MPs) in groundwater is still under debate. During the seepage processes throughout the aquifers, subsoils have the capacity to block the larger MPs particles, while the finer fragments are difficult to detect with the standard techniques. Here, we propose a new protocol based on multifrequency Atomic Force Microscopy (AFM) and fluorescence methods, to detect and quantify plastic particles from microscale down to nanoscale according to their nanomechanical properties. The proposed methodology was tested on reconstituted MPs to distinguish potential false-positive detection of MPs in natural samples. Subsequently, our protocol was applied on groundwater samples collected in water wells and springs in Asian megacity (Shenzhen, China). Shenzhen The local aquifer consists of marine to alluvial sequences interdigitated with laterite soils, lying on crystalline bedrock with low hydraulic conductivity. Collected water samples were pretreated to remove mineral fragments and organic matter before the analysis. MPs were detected and characterized in three groundwater samples both using AFM and spectroscopic techniques. This methodology represents an advancement in the mechanical characterization of MPs that can circulate in the aquifers but are difficult to detect via traditional analytical methods, providing also information on the degradation of the detected microplastics.