Atmospheric oxidation of fluoroalcohols initiated by ˙OH radicals in the presence of water and mineral dusts: mechanism, kinetics, and risk assessment

Abstract

The transport and formation of fluorinated compounds are greatly significant due to their possible environmental risks. In this work, the ˙OH-mediated degradation of CF₃CF₂CF₂CH₂OH and CF₃CHFCF₂CH₂OH in the presence of O₂/NO/NO₂ was studied by using density functional theory and the direct kinetic method. The formation mechanisms of perfluorocarboxylic/hydroperfluorocarboxylic acids (PFCAs/H-PFCAs), which were produced from the reactions of α-hydroxyperoxy radicals with NO/NO₂ and the ensuing oxidation of α-hydroxyalkoxy radicals, were clarified and discussed. The roles of water and silica particles in the rate constants and ˙OH reaction mechanism with fluoroalcohols were investigated theoretically. The results showed that water and silica particles do not alter the reaction mechanism but obviously change the kinetic properties. Water could retard fluoroalcohol degradation by decreasing the rate constants by 3–5 orders of magnitude. However, the heterogeneous ˙OH-rate coefficients on the silica particle surfaces, including H₄SiO₄, H₆Si₂O₇, and H₁₂Si₆O₁₈, are larger than that of the naked reaction by 1.20–24.50 times. This finding suggested that these heterogeneous reactions may be responsible for the atmospheric loss of fluoroalcohols and the burden of PFCAs. In addition, fluoroalcohols could be exothermically trapped by H₁₂Si₆O₁₈, H₆Si₂O₇, and H₄SiO₄, in which the chemisorption on H₁₂Si₆O₁₈ is stronger than that on H₆Si₂O₇ or H₄SiO₄. The global warming potentials and radiative forcing of CF₃CF₂CF₂CH₂OH/CF₃CHFCF₂CH₂OH were calculated to assess their contributions to the greenhouse effect. The toxicities of individual species were also estimated via the ECOSAR program and experimental measurements. This work enhances the understanding of the environmental formation of PFCAs and the transformation of fluoroalcohols.