Weathering alters surface characteristic of TiO2-pigmented microplastics and particle size distribution of TiO2 released into water

In natural environment, microplastics (MPs) undergo varying degrees of aging. It is believed that aged MPs show different surface properties and leaching behaviors from unaged ones. Here, the effects of aging on surface characteristics of TiO-pigmented MPs and particle size distribution of TiO leached from MPs were systematically investigated. Experimental results show that the carbonyl index and color difference of MPs increased after aging. The increased carbonyl content in MPs was due to photo-oxidation, which further caused the fragility of MPs, the loss of TiO pigments and the formation of surface cracks. The decreased weight loss for aged MPs was due to the photo-transformation of LDPE into other compounds. Aged MPs could release more and faster TiO particles than unaged MPs, and aged MPs tended to release more large particles (>5 μm) and fewer small particles (<5 μm) than unaged MPs. Aged MPs released TiO with particle size uniformly distributed in each size fraction, but unaged MPs mainly released small particles (57.6%-86.2%). With increasing leaching time, the proportion of small particles leached from MPs especially aged MPs decreased and the proportion of large particles increased. These findings will help us better understand how aging process impacts the properties and leaching of MPs and the size distribution of particulate additives leached from MPs.