Kinetics and Mechanism of Selenium(IV) Oxidation by Aqueous Bromine Solution

Abstract

The bromine−selenite reaction at strongly acidic conditions was investigated by monitoring the absorbance−time traces at the isosbestic point of the bromine−tribromide system at a constant ionic strength (0.5 M adjusted by sodium perchlorate) and temperature. Despite the simplicity of the stoichiometry, the kinetics was found to be very complex. Although the formal kinetic orders of the reactants bromine and selenite are strictly 1, that of the hydrogen ion varies from −2 to less than −3 and notably depends on the initial bromide concentration as well. The bromide ion also inhibits the reaction, making the whole system as a sound example of efficient autoinhibition. We have clearly shown that the inhibitory effect of the bromide ion cannot be explained quantitatively by either exclusively considering the unreactivity of the tribromide ion over elemental bromine or driving the reaction via hypobromous acid formed from the well-known hydrolysis of bromine in aqueous solutions. Instead of that, bromonium ion transfer initiating equilibrium is suggested between the selenium(IV) and bromine species to produce bromide ion and SeO 3 Br − followed by the hydrolysis of this short-lived intermediate. This hydrolytic transformation was found to be catalytic with respect to hydroxide and bromide ions as well. We have also demonstrated that, among the wide variety of selenium species present in the acidic aqueous solution, the best result can be obtained by considering HSeO 3 − as the kinetically active species toward bromine. The proposed mechanism containing 10 acid−base equilibria with known equilibrium constants, the above-mentioned initiating equilibrium, and the hydrolysis of SeO 3 Br − is able to fit all 49 kinetic absorbance−traces simultaneously, taking into account properly the most important characteristics of the measured data at strongly acidic conditions. Furthermore, this kinetic model was further extended by the direct reactions of hypobromous acid with selenium(IV) species suggested previously with reasonably modified rate coefficients to describe the pH dependence of the apparent second-order rate coefficients over the pH = 1−13 range, providing a useful tool to predict more accurately the kinetic behavior of selenium(IV) species in water treatment process conditions.

Bromide Ion Impurity-Induced Reaction between Selenium(IV) and Acidic Bromate: Prototype of a Cycle with Autocatalytic Behavior

“…The kinetics of eq 4 was found to be very complex, recently an overall model was proposed to describe all the characteristic features of this reaction including hydrogen and bromide inhibited pathways. 15 It was also demonstrated that this mechanism is capable of explaining the pH-dependence of the reaction at the pH range of 1−13. Therefore, here, we directly implemented it in the proposed model without any simplification to preserve all the characteristics of the selenite−bromine reaction.…”

“…The kinetics of eq was found to be very complex, recently an overall model was proposed to describe all the characteristic features of this reaction including hydrogen and bromide inhibited pathways . It was also demonstrated that this mechanism is capable of explaining the pH-dependence of the reaction at the pH range of 1–13.…”

“…14 Their major conclusion about the selenite−bromine system seemed to be quite dubious, especially at lower pHs; therefore, Csekőet al very recently reported the modified kinetic model in the given system that explains quantitatively the pH dependence over a wide pH range of 1−13. 15 Importance of the latter study is not limited to elucidating the kinetics and mechanism of the selenite−bromine reaction in itself because our preliminary experiments on the selenite− ■ EXPERIMENTAL SECTION Chemicals. All the reagents, including sodium selenite, sodium perchlorate, sodium dihydrogen phosphate, phosphoric acid, sodium bromide, and sodium bromate were analytical grade reagents and used without further purification.…”

“…In addition to that, Liu et al investigated the kinetic and mechanistic aspects of selenite oxidations by various oxidizing agents like chlorine, bromine, monochloramine, ozone, permanganate, and hydrogen peroxide . Their major conclusion about the selenite–bromine system seemed to be quite dubious, especially at lower pHs; therefore, Csekő et al very recently reported the modified kinetic model in the given system that explains quantitatively the pH dependence over a wide pH range of 1–13 . Importance of the latter study is not limited to elucidating the kinetics and mechanism of the selenite–bromine reaction in itself because our preliminary experiments on the selenite–bromate acidic system have revealed the transient formation of bromine in the excess of selenite.…”

“… 14 Their major conclusion about the selenite–bromine system seemed to be quite dubious, especially at lower pHs; therefore, Csekő et al very recently reported the modified kinetic model in the given system that explains quantitatively the pH dependence over a wide pH range of 1–13. 15 Importance of the latter study is not limited to elucidating the kinetics and mechanism of the selenite–bromine reaction in itself because our preliminary experiments on the selenite–bromate acidic system have revealed the transient formation of bromine in the excess of selenite. As a result, the modified kinetic model must be a crucial part of the mechanism of the title reaction itself.…”