Effects of advanced oxidation processes on leachates and properties of microplastics

Microplastics (MPs) in natural environments undergo various aging processes. So far, little is known about the effects of chemical oxidation on leachates and properties of MPs. Here, we investigated the removal of pigment red from MPs by ozonation, Fenton, and heat-activated persulfate treatments, and further explored the nanoscale surface properties of treated MPs. Experimental results indicated that advanced oxidation processes effectively degraded pigment red released from MPs and the degradation rate was much faster than the leaching rate of pigments. Dominant reactive oxygen radicals in the ozone, Fenton, and heat-activated persulfate systems were identified as O₂^•-, HO•, and SO₄^•-, respectively. Height ranges of untreated, ozone-treated, Fenton-treated, and persulfate-treated MPs were 73 nm, 163 nm, 195 nm, and 206 nm, respectively. Oxidation of the -CH₃ and -CH₂ bonds occurred on the surface of treated MPs and the persulfate system achieved more serious oxidation degree than the ozone and Fenton systems. Addition of pigment red to the plastic polymer increased the glass transition temperature of MPs, which then showed a decline after advanced oxidation treatments except Fenton. The surface of persulfate-treated MPs was the stiffest, but the stiffness distribution of the ozone-treated and Fenton-treated MPs was more uneven. These research findings provide promising strategies to accelerate the aging process of MPs and contribute to a better understanding of the effects of aging on the environmental behavior of MPs.