Methodologic insights aimed to set-up an innovative Laser Direct InfraRed (LDIR)-based method for the detection and characterization of microplastics in wastewaters

Wastewater treatment plants (WWTPs) are generally reported to be effective in removing microplastics (MPs). Nevertheless, the lack of standardized methodologies for their counting and characterization hinders direct comparison across literature reports, limiting the establishment of reliable benchmarks. In this perspective, this work aimed to provide methodological insights on a feasible approach for detecting and characterizing MPs in both raw and treated wastewater by exploiting the innovative Laser Direct InfraRed (LDIR) technique. MPs of various polymeric nature, size and shape were specially produced and used to fine-tune and validate a LDIR-based method for both their chemical identification and size/morphology description, while well-established techniques were employed to evaluate the reliability of collected data. The robustness of the tailored protocol was then assessed through a monitoring campaign conducted at a large municipal WWTP in Tuscany (Italy), for which an average MPs removal efficiency of 82 % was estimated. Various polymers were detected in the processed samples, with a high relative content of cellulose-based materials in both influent and effluent (32 % and 54 % of particles, respectively). Most MPs had a characteristic size lower than 100 μm, with particles <30 μm representing about 45 % and 29 % of MPs in the influent and effluent, respectively. MPs were in the form of fibers (25-39 %), fragments (32-43 %) and pellets (29-32 %). The consistency of the obtained results suggested the robustness and reliability of the proposed LDIR-based method, highlighting its potential for more in-depth monitoring of MPs in WWTPs.