Weathered Microplastics Induce Silver Nanoparticle Formation

Microplastics (MPs) are ubiquitous in marine, freshwater, and terrestrial systems. Despite MPs acting as vectors of contaminants in the environment, it remains unknown whether MPs alter the chemical speciation of coexisting contaminants. Here, we show the photoreduction of dissolved silver (Ag(I)) to metallic silver nanoparticles (Ag(0)-NPs) in surface freshwater and in a sand matrix by polystyrene (PS) upon solar irradiation for 3 and 7 days, respectively. The reduction rate constants range from 2.3 to 6.1 × 10–3 h–1 in borate buffer under simulated solar irradiation. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy identify carbonyl groups including aldehydes on weathered PS as a key determinant for Ag(I) reduction. In particular, the contents of carbonyl groups correlate linearly with the amount of newly formed Ag(0)-NPs (k = 8.2, R2 = 0.97). We propose that the photoreduction of Ag(I) involves its binding to carbonyl groups, as a result of PS oxidation by environmental oxygen, on weathered PS and then electron shuttling. These findings clearly reveal the chemical reactivity of weathered PS. Tentative estimates show an unexpectedly large pool of reductive PS in the natural environment (18.1─39.4 MT per day), and it plays a significant uncharacterized role that will impact the biogeochemistry of silver and other environmental contaminants reactive with surface carbonyl groups.