| Environmentally persistent free radicals (EPFRs) have recently been paid extensive attention due to their unique properties and toxic effects. Studies have systematically provided evidence that EPFRs are more toxic than the parent chemicals. The presence of EPFRs will challenge the regular method based on concentration to assess the environmental risks of contaminants, which neglected the contribution of free radicals. Most studies were generally discussed the formation mechanisms of EPFRs in combustion and high temperature systems, but the EPFRs may be well present in the ambient. However, rarely study was specifically designed to interpret the EPFRs generation mechanisms under environmental conditions, nor the potential impacts on organic contaminant behavior, which may be a missing link in understanding their environmental behavior.We propose a hypothesis that with the wide occurrence of organic contaminants and the ubiquitous of transition metals in soils (such as iron); EPFRs formation may be common in nature conditions (such as UV irradiation) and will involve in degradation of organic chemicals. Catechol was selected as the model chemical because it is a common byproduct of organic compound degradation and ubiquitously present in the environment. Hematite loaded on silica instead of natural soil was selected as the model particle to avoid interferences from complex soil components such as humic substances and Mn oxides. We observed that catechol degradation after 10 h UV irradiation was decreased over 20% on silica particles coated with 1% hematite in comparison to uncoated silica particles. Electron paramagnetic resonance spectroscopy (EPR) was applied to monitor the generation of radicals with UV irradiation in situ. Typical and strong organic free radical signals were observed in both silica and HMT-silica systems. The g-values were 2.0049-2.0050 for silica system and 2.0043-2.0046 for HMT-silica system. Semiquinone radicals were generated in both systems, in addition, phenoxyl radicals also formed in HMT-silica system. The apparent EPR signal decreased with the extended UV irradiation time in silica system, especially dramatically decreased after the UV light shut down, but rather stable in HMT-silica system. EPFRs formation probably blocked catechol degradation then reduced catechol degradation on HMT-silica surface under UV irradiation at ambient temperature. EPFRs should be incorporated in the studies of organic contaminant geochemical behavior, and will be a new input in their environmental fate modeling.The previously observed the stabilization of catechol related radicals on a HMT-silica surface, but only in air without a clear description of the role of the atmosphere. We hypothesized that EPFRs formation could block chemical degradation in O2-limited dry environments, but the Fe(Ⅲ)/Fe(Ⅱ) cycles in O2-enriched conditions may increase the chemical degradation. Our results showed that 1% hematite coating on a silica surface inhibited catechol degradation in N2, notably at low catechol loadings on the solid particles (SCT)-However, under an O2 environment, catechol degradation decreased when SCT< 1 μg/mg but increased when SCT> 1 μg/mg. Stable organic free radicals were observed in the N2 system, with g factors in the range of 2.0035-2.0050, suggesting the dominance of oxygen-centered free radicals. Introducing O2 into the catechol degradation system dramatically decreased the free radical signals and decreased the Fe(Ⅱ) content to a lesser extent. The surface Fe(Ⅱ)/Fe(Ⅲ) stoichiometric ratio of 0.5 suggested the formation of crystalline magnetite. These results indicate the presence of EPFRs and Fe(Ⅲ)/Fe(Ⅱ) cycles will definitely regulate organic chemical fates.In order to better mimic the conditions in natural environment, the effect of light and humidity on degradation of catechol and related EPFRs formation were investigated. Our results showed that utilized light wavelength above 340 nm exhibited lower degradation than the one above 280 nm, but the degradation trend was not changed when the wavelength was changed, HMT protected the catechol from degrading at low SCTs and accelerated catechol degradation at higher SCTs. In addition, the formation of EPFRs and the effect on catechol degradation in the dark system were similar to UV irradiation system, indicating that the EPFRs could generate without light irradiation. Introducing of water enhanced the degradation of catechol on HMT-silica, this may due to the generation of ROS via EPFRs in water molecular layer on the surface of HMT-silica particle. These results indicate that the EPFRs could ubiquitous formed in ambient environment and regulate organic chemical fates. Ignoring this process may lead to potential deviation on understanding the environmental behaviors and risks of organic pollutants. Further extended studies are urgently required to investigate the formation mechanism of EPFRs, the effect of environmental conditions on formation of EPFRs, and also the behavior and risk of EPFRs. |
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