EFFECT OF ELECTRONEGATIVITY ON ENVIRONMENTALLY PERSISTENT FREE RADICAL (EPFR) FORMATION, PERSISTENCE, AND TOXICITY ON TRANSITION METAL OXIDES

Environmentally persistent free radicals (EPFRs) are a group of emerging environmental pollutants. Owing to their resonance structures, EPFRs have significantly higher lifetimes than typical free radicals, and can induce the formation of reactive oxygen species (ROS) known to cause pulmonary and cardiovascular dysfunction. EPFRs form by the adsorption of organic precursors on a transition metal oxide (TMO) surface and exhibit enough stability to cause negative health and environmental effects. While aromatic and substituted aromatic precursors form EPFRs on TMO surface at elevated temperatures and numerous substituted organics are ubiquitous in various industrial processes and household products, studies have largely overlooked the significance of the substituents. Due to the occurrence of an electron transfer between the organic precursor and the transition metal oxide during EPFR formation, the influence of electronegativity on EPFR formation has been studied utilizing 1,2-dibromobenzene (DBB), 1,2-dichlorobenzene (DCB), and 1,2-difluorobenzene (DFB). TiO2 and ZnO nanoparticles (NPs) were chosen as the transition metal oxides based on reports that TiO2 is reduced and ZnO is oxidized during EPFR formation. Electron paramagnetic resonance (EPR) studies confirmed the formation of organic free radicals for both TiO2 and ZnO nanoparticles, with the radical concentrations ranged from 1016-1017 spins/g being the highest for DBB and lowest for DFB. X-ray absorption spectroscopy (XAS) studies for both ZnO and TiO2, dosed with the aforementioned organic precursors, confirmed the reduction of Zn2+ and Ti4+ metal centers. Computational results were in line with experimental findings providing atomistic details about the radical formation mechanism. The formed EPFRs demonstrated noteworthy persistency of months, and ability to form ROS making these radicals potentially toxic and harmful for human health. In concert, these findings illustrate the significance of the polarity of the organic precursors in surface mediated EPFR formation, as well as EPFRs’ potential to impose serious health risks.