Sulfide induces physical damages and chemical transformation of microplastics via radical oxidation and sulfide addition

Transformation of microplastics in aquatic environments and engineered systems (e.g., wastewater treatment plants) significantly affects their transport, fate and effects. Here, we present the counterintuitive finding that sulfide, a prevalent nucleophile and reductant, can result in oxidation of microplastics, in addition to sulfide addition. Treating four model microplastics (thermoplastic polyurethane, polystyrene, polyethylene terephthalate and polyethylene) with 0.1 mM sulfide in a Tris-buffer solution (pH 7.2, 25 degrees C) resulted in physical damages (embrittlement and cracking) and chemical transformation (increased O/C ratio and formation of C-S bonds) of the materials. Pre-aging of the microplastics with O-3 or UV treatment had varied effects on their reactivities toward sulfide, depending on the specific structural and surface chemistry properties of the polymers. Electron paramagnetic resonance and radical trapping/quenching experiments showed that sulfide underwent spontaneous oxidation to form center dot OH radicals, which acted as the primary oxidant to attack the carbon atoms in the polymer chains, leading to surface oxidation and chain scission. Notably, sulfide addition, verified with X-ray photoelectron spectroscopy and 13C-nuclear magnetic resonance spectroscopy analyses, likely contributed to the physicochemical transformation of microplastics together with radical oxidation in a synergistic manner. The findings unravel an important transformation route (and a potential source) of microplastics in the environment. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The utilization of sulfide in aquatic environments and engineered systems may lead to the oxidation and transformation of microplastics, causing physical and chemical alterations to the materials and affecting the properties of microplastics. Sulfide oxidation may also synergistically interact with radical oxidation, contributing to the transformation of microplastics.