Competitive reactions in Cl⁻ solutions treated by plasma-supplied O atoms

Reactions accompanying the hypochlorite formation in Cl⁻ solutions oxidized by plasma-supplied O atoms were studied. The concentration of oxychlorine compounds from reaction of O atoms with solutions of NaCl, NaOCl and NaClO₂ was measured. The concentration dependence of O + Cl⁻ reaction rate was evaluated using the simplified kinetic model leading to estimation of the 2nd order rate constant, k = 1.64 ± 0.01 × 10⁵ M⁻¹s⁻¹. The limiting reaction rate at highly concentrated NaCl corresponds well with the delivery rate of O atoms to the gas-liquid interface calculated from the previously measured O concentration at the plasma jet outlet. Virtually all plasma-supplied O atoms can be captured by Cl⁻ ions in highly concentrated solutions. The measurements confirmed the reactions O + OCl⁻, and O + ClO₂ ⁻ as loss channels decreasing the hypochlorite production in O-treated Cl⁻ solutions. These loss channels are less effective at more acidic pH, where the formation of ClO₂ ⁻ is minimized and as a consequence, also the solution post-discharge reactivity. The validity of described reaction system was confirmed by chemical kinetics modelling. The fitted rate coefficients for the O reactions with ionic Cl forms were 3–4 orders of magnitude lower than those predicted in the literature. (paper)