Interfacial Interactions of Uranium and Arsenic with Microplastics: From Field Detection to Controlled Laboratory Tests

We studied the co-occurrence of microplastics (MPs) and metals in field sites and further investigated their interfacial interaction in controlled laboratory conditions. First, we detected MPs in freshwater co-occurring with metals in rural and urban areas in New Mexico. Automated particle counting and fluorescence microscopy indicated that particles in field samples ranged from 7 to 149 particles/L. The urban location contained the highest count of confirmed MPs, including polyester, cellophane, and rayon, as indicated by Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy analyses. Metal analyses using inductively coupled plasma (ICP) revealed that bodies of water in a rural site affected by mining legacy contained up to 332.8 mu g/L of U, while all bodies of water contained As concentrations below 11.4 mu g/L. These field findings motivated experiments in laboratory conditions, reacting MPs with 0.02-0.2 mM of As or U solutions at acidic and neutral pH with poly(methyl-methacrylate), polyethylene, and polystyrene MPs. In these experiments, As did not interact with any of the MPs tested at pH 3 and pH 7, nor U with any MPs at pH 3. Experiments supplied with U and MPs at pH 7 indicated that MPs served as substrate surface for the adsorption and nucleation of U precipitates. Chemical speciation modeling and microscopy analyses (i.e., Transmission Electron Microscopy [TEM]) suggest that U precipitates resemble sodium-compreignacite and schoepite. These findings have relevant implications to further understanding the occurrence and interfacial interaction of MPs and metals in freshwater.