Development of a new methodology for the determination of PET microplastics in sediment, based on microwave-assisted acid digestion

Analytical methods for the determination of microplastics in sediments typically involve matrix drying, sieving, grinding, and flotation as part of the sample treatment. However, the real need for these steps and analytical validation studies are scarce. This work proposes a method that avoids the drying, sieving, and flotation procedures by using a direct acid attack of HNO₃/HCl (3:1) on wet sediment samples, assisted by microwave digestion. For detection, induced fluorescence using a UV camera, with Nile Red (NR) as the fluorophore and a cell phone camera for image capture were used. The results showed that when the digestion temperature was raised to 120°C, PET recovery decreased due to plastic particle fusion. However, at 60°C, microwave digestion resulted in a 97% recovery of PET particles, eliminating chitin interference and canceling cellulose fluorescence without the need for flotation. This method proved effective for monitoring plastic microparticles in sediments from the Loa River, Chile, revealing that the river is predominantly contaminated with PET microparticles, particularly upstream in the Taira area.