Combined Method of Froth Flotation Separation–Raman Fingerprint and Its Potential on Sources Identification of Microplastics

Source identification of microplastics (MPs) is important for controlling pollution, however, an effective method is still lacking. In the present paper, a low-dosage froth flotation separation protocol was developed to separate MPs efficiently, and coupled with Raman fingerprints to identify the source. By analyzing the alterations in surface properties of MPs, the inhibitory mechanism of activated sludge on froth flotation efficiency of MPs was elucidated, attributed to the reduced hydrophobicity, adsorption of humic acid (HA), and the concentration of HA in flotation solution. Sieving pretreatment was applied to mitigate the inhibition caused by HA, while the flotation efficiency (exceeding 70%) was enhanced by the addition of 16 mg/L cetyltrimethylammonium bromide (CTAB). The concentration of MPs in actual samples was quantified by FT-IR spectroscopy at 38-46 particles/L, with identified polymers, including polyamide (PA), polyethylene terephthalate (PET), polystyrene (PS), and polyurethane (PU). MPs in different treatment units were identified using 20 mM KBr-modified SERS via detection of surface-adsorbed substances on MPs (detection limit: 10 μg/L). Raman fingerprints were obtained by froth flotation coupled with SERS, enabling the successful detection of surface substances and source identification in actual samples. This integrated approach offers high anti-interference capability for analyzing surface-adsorbed substances on MPs, thereby improving the accuracy of environmental fate tracing in treatment systems, and promising to be an efficient and accurate traceability method for MPs.