Thermodynamic Redox Calculations for One and Two Electron Transfer Steps: Implications for Halide Oxidation and Halogen Environmental Cycling

摘要

In oxygenated waters, chloride and bromide are the thermodynamically stable halogen species that exist whereas iodate, the thermodynamically stable form of iodine, and iodide can co-exist. The stability and oxidation of halides in the environment is related to the unfavorable thermodynamics for the first electron transfer with oxygen to form X·atoms. However, reactive oxygen species (ROS) such as ~1O_2, H_2O_2 and O_3 can oxidize the halides to X_2 and perhaps HOX in two electron transfer processes; these reactions become less favorable with increasing pH. Fc(III) and Mn(III,IV) solid phases can oxidize halides with similar patterns as ROS. The ease of oxidation increases from Cl- < Br < I , X_2 can also form HOX in water, and both halogen species can react with natural organic matter with formation of organo-halogcn (R-X) compounds. During the treatment of drinking water. unwanted R-X disinfectant byproducts can form when the oxidant is not capable of quantitatively converting iodide to iodate. Natural and anthropogenic volatile R-X compounds are photochemically active and lead to X·atoms in the atmosphere which undergo reaction with O_3 via an O atom transfer step (two electron transfer step) resulting in O_3 destruction. In the case of iodine, iodine oxide species form aerosol nanoparticlcs leading to cloud condensation nuclei.