Photochemical oxidation of chloride ion by ozone in acid aqueous solution

Abstract

The experimental investigation of chloride ion oxidation under the action of ozone and ultraviolet radiation with wavelength 254 nm in the bulk of acid aqueous solution at pH 0-2 has been performed. Processes of chloride oxidation in these conditions are the same as the chemical reactions in the system O3 - OH - Cl(-)(aq). Despite its importance in the environment and for ozone-based water treatment, this reaction system has not been previously investigated in the bulk solution. The end products are chlorate ion ClO3(-) and molecular chlorine Cl2. The ions of trivalent iron have been shown to be catalysts of Cl(-) oxidation. The dependencies of the products formation rates on the concentrations of O3 and H(+) have been studied. The chemical mechanism of Cl(-) oxidation and Cl2 emission and ClO3(-) formation has been proposed. According to the mechanism, the dominant primary process of chloride oxidation represents the complex interaction with hydroxyl radical OH with the formation of Cl2(-) anion-radical intermediate. OH radical is generated on ozone photolysis in aqueous solution. The key subsequent processes are the reactions Cl2(-) + O3 → ClO + O2 + Cl(-) and ClO + H2O2 → HOCl + HO2. Until the present time, they have not been taken into consideration on mechanistic description and modelling of Cl(-) oxidation. The final products are formed via the reactions 2ClO → Cl2O2, Cl2O2 + H2O → 2H(+) + Cl(-) + ClO3(-) and HOCl + H(+) + Cl(-) ⇄ H2O + Cl2. Some portion of chloride is oxidized directly by O3 molecule with the formation of molecular chlorine in the end.

Mechanism and Kinetic Model of Chlorate and Perchlorate Formation during Ozonation of Aqueous Chloride Solutions

“…Ozone enters the reaction solution from the gas phase, and thus we have a typical gas−liquid reaction. It was demonstrated in a publication 27 by estimating the Hatta number that in such reaction systems, the rates of chemical reactions of ozone at the gas−liquid interface are negligible compared to the rate of its mass transfer through the interface by diffusion. This means that the rates of ozone dissolution and chemical reactions are independent of each other and can be represented as separate terms in kinetic equations.…”

“…15,17,18 The kinetics of the oxidation of chloride ion by ozone with the formation of molecular chlorine in acidic medium was studied in detail in our works. 19−26 The generation of significant amounts of chlorate during the photochemical interaction between ozone and chloride ion in acidic aqueous solutions was first discovered in a previous study, 27 and the kinetics and mechanism of this process were studied. An extensive series of investigations [5][6][7][8]28,29 have been devoted to the study of the mechanism of formation of molecular chlorine during the interaction of chloride ions contained in the liquid phase of aqueous aerosol with gaseous ozone and hydroxyl radicals formed during O 3 photolysis.…”

“…A mechanism of chloride ion oxidation during ozonation and UV irradiation in a strongly acidic medium was proposed in our work. 27 The important primary processes are the reactions of chloride not only with the O 3 molecule but also with OH • free radical. However, this mechanism does not contain many reactions that are significant in neutral and alkaline solutions.…”

Halide Photoredox Chemistry

“…A large body of literature has described the formation of halogen atoms, such as Cl • , ,,− Br • , ,,,,,,,− ,,,,,,− and I • , ,,,,,,,,− in water or organic solvents. In pulse radiolysis, Cl • is formed by reaction with SO 4 • or HO • that are generated by electron beam ...…”

ZVI (Fe0) desalination: catalytic partial desalination of saline aquifers

“…The H 2 O 2 will decompose to form 2OH (Pourbaix 1974). Some of the H 2 O 2 will react with the ClO (i.e., ClO + H 2 O 2 = HOCl + HO 2 Levanov et al 2015).…”