Kinetics of peroxynitric acid reactions with halides at low pH

The oxidation of iodide, bromide, and chloride by peroxynitric acid (HOONO2) was studied by spectroiodometry. The second-order rate constants were found to be 890 (I-, 295 K), 0.54 (Br-, 295 K), and 0.0014 M-1 s(-1) (Cl-, 298.2 K). No pH dependence was observed in any of the systems (pH 1.2-4.9). The temperature and ionic-strength dependencies of the chloride oxidation rate constant are 4.8 x 10(7) M-1 s(-1) exp(-60 kJ mol(-1)/(RT)) and 6.0 x 10(-4) M-1 s(-1) + 1.7 x 10(-4) M-2 s(-1) mu, respectively, where mu is the ionic strength. HOONO2 also reduces halogens (X-2), the active species being HO2, a radical in constant equilibrium with HOONO2. Under most conditions, the reduction can be explained quantitatively with a free-radical mechanism using known rate constants. Reduction by H2O2 was not significant. These systems also seem to be affected by reactions in addition to the direct oxidation and free-radical reduction. First, some iodine atoms are stored in a reservoir when initial concentrations of iodide and peroxynitric acid are near stoichiometric values, but we could not identify this reservoir. Second, reaction of HOONO2 with large excesses of KBr (in the 10(-2) M range) gave inordinately fast and variable bromide oxidation. The addition of Cu2+ suppressed this at pH 1.7, suggesting an additional oxidation mechanism that involves HO2. On the basis of the above results, the potential role of HOONO2 in sea-salt chemistry has been evaluated. Given typical marine boundary layer conditions, it should be negligible in warm, clean, remote oceanic areas. In polluted coastal regions and/or at low temperatures, it might become marginally significant compared to other known reactions leading to halogen release from sea-salt particles.