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Electron bridges and electron sinks: Molecular regulation of PVC aging by aromatic plastic additives in an nZVI-driven electron-donor system
Summary
Chemicals added to plastics (like BPA and flame retardants) aren't just harmless extras — they actually change how fast microplastics break down in the environment, with some speeding up degradation and others slowing it down. This matters because it means the mix of additives in a plastic product could affect how long microplastics persist and how they release chlorine-based byproducts, which could influence human exposure to these particles and their chemical fragments over time.
Additives are usually required to be chemically inert and non-reactive with plastics, leading to their role in plastic aging being largely overlooked. However, their electronic structure may play a far more than expected role in regulating reactions. This study demonstrates that aromatic plastic additives can modulate the degradation pathways of polyvinyl chloride (PVC) at the molecular scale by acting as electron bridges or forming electron sinks. In anaerobic systems mediated by nanoscale zero-valent iron (nZVI), bisphenol A (BPA) and bisphenol B (BPB) promote electron transfer from nZVI to PVC via orbital delocalized, accelerating dechlorination and chain scission. In contrast, decabromodiphenyl ether (BDE209), with its high electron affinity, preferentially scavenges electrons and blocks the reaction chain, significantly suppressing aging. Similar phenomena have also been observed in other types of microplastics (MPs), not just limited to PVC. Under aerobic conditions, dissolved oxygen competes for electrons, accelerates nZVI passivation, and induces the formation of radicals, thereby shifting the reaction pathway from reductive dechlorination to additives degradation. In addition, the formation of phenolic intermediates, which further weaken the electron tranfer of additives and consequently suppress PVC dechlorination. Based on electrochemical, spectroscopic, and theoretical analyses, we propose a mechanistic centered on electron bridging-electron sinking, showing that additives are not passive background components but active regulators in MPs aging. This work helps to clarify the role of electron-driven processes and plastic additives aging of MPs.