Formation of environmentally persistent free radicals on molecular sieves: The role of Lewis acid sites

Abstract

Environmental persistent free radicals (EPFRs) generated on metal oxide surfaces have garnered significant research interest due to their environmental persistence and biological toxicity. While the catalytic role of Lewis acid sites (LAS) in EPFR formation has been hypothesized, the specific mechanisms underlying LAS-mediated generation at ambient temperature remain poorly understood. This investigation systematically elucidates the critical role of LAS concentration in molecular sieves for EPFR formation through polycyclic aromatic hydrocarbons (PAHs) adsorption under ambient conditions. Our results demonstrate that EPFR generation efficiency follows a distinct hierarchy among ultra-stable Y-type (USY) molecular sieves: USY(5.4) > USY(11) > USY(8), showing a strong positive correlation (p < 0.05) with LAS concentration. The absence of detectable EPFR signals on silica controls confirms LAS as essential active sites for radical stabilization. The concentration of EPFRs generated by different PAHs on the molecular sieve follows the order of pyrene (PYR) > anthracene (ANT) > phenanthrene (PHE) > naphthalene (NAP), which was negatively correlated (p < 0.05) with the ionization potential (IP) of PAHs. This indicates that the electron transfer ability of PAH also affects the generation of EPFRs. In addition, the acute toxicity of EPFRs was evaluated using Photobacterium phosphoreum T3 (PPT₃), and the inhibition rate of the luminescent bacteria was positively correlated with concentration of EPFRs and the mediated generation of ROS. The results of this study can contribute to the understanding of the generation mechanism and environmental risks of EPFRs.

Keywords: Generation mechanisms; Ionization potential; Lewis acid sites; Persistent free radicals.