Detailed Kinetics and Mechanism of the Oxidation of Thiocyanate Ion (SCN^–) by Peroxomonosulfate Ion (HSO₅^–). Formation and Subsequent Oxidation of Hypothiocyanite Ion (OSCN^–)

Abstract

The haloperoxidase-catalyzed in vivo oxidation of thiocyanate ion (SCN(-)) by H(2)O(2) is important for generation of the antimicrobial hypothiocyanite ion (OSCN(-)), which is also susceptible to oxidation by strong in vivo oxidizing agents (i.e., H(2)O(2), OCl(-), OBr(-)). We report a detailed mechanistic investigation on the multistep oxidation of excess SCN(-) with peroxomonosulfate ion (HSO(5)(-) in the form of Oxone) in the range from pH 6.5 to 13.5. OSCN(-) was detected to be the intermediate of this reaction under the above conditions, and a kinetic model is proposed. Furthermore, by kinetic separation of the consecutive reaction steps, the rate constant of the direct oxidation of OSCN(-) by HSO(5)(-) was determined: k(2) = (1.6 ± 0.1) × 10(2) M(-1) s(-1) at pH 13.5 and k(2)(H) = (3.3 ± 0.1) × 10(3) M(-1) s(-1) at pH 6.89. A critical evaluation of the estimated activation parameters of the elementary steps revealed that the oxidations of SCN(-) as well as the consecutive OSCN(-) by HSO(5)(-) are more likely to proceed via 2e(-)-transfer steps rather than 1e(-) transfer.

Oxidation of Ru(III)-Bound Thiocyanate with Peroxomonosulfate: Kinetic and Mechanistic Studies

“…Moreover, the most reactive form of the ruthenium(III) complex, [Ru III (edta)(H 2 O)] − , exists predominantly in the pH range 4–6 . At pH 6.9 , HSO 5 − solely oxidizes SCN − (at [SCN − ] ≫ [HSO 5 − ]) to produce the hypothiocyanite ion (OSCN − ) . It may be noted here that OSCN − is further oxidized by HSO 5 − to O 2 SCN − (which finally decomposes rapidly to yield SO 4 2− , SO 3 2− , CN − , and OCN − ) at a much higher rate (3300 M −1 s −1 ) than the rate of oxidation of SCN − by HSO 5 − (200 M −1 s −1 ) .…”

“…KHSO 5 commercially known as “oxone” is readily available and stable in the solid state as a “triple salt” (2KHSO 5 , K 2 SO 4 , KHSO 4 ). Noteworthy here that ability of HSO 5 − alone to oxidize SCN − (when present in excess over the concentration of HSO 5 − ) to OSCN − has been reported recently . The goal of the present work is to contribute to the mechanistic understanding of the reactivity of the [Ru III (edta)(SCN)] 2− toward the oxidation in general by ROOH (ROOH = H 2 O 2 , HSO 5 − ).…”

Reaction Schemes That Are Easily Confused with a Reversible First-Order Reaction

“…For the concentration of A and P, the deviation from a pseudo–first–order curve is much larger and would probably be detectable in experiments as well. Indeed, in the cited experimental example, the spectrophotometric detection was selective to species B (HOSCN) .…”

“…In the recent literature, two cases have been identified when the procedure described in the preceding paragraph was successful, yet the possibility of a reversible second–order reaction was ruled out based on independent experimental observations or plausible chemical considerations . In both cases, numerical integration of alternative kinetic schemes was used to show that other interpretations of the experimental data are also in agreement with the kinetic observations.…”

Degradation of thiocyanate in aqueous solution by persulfate activated ferric ion

“…Therefore, in recent years, interest in using the alkaline chlorination process has been decreasing. Peroxygen compounds such as hydrogen peroxide and peroxomonosulfuric acid (commonly known as Caro's acid) can be proposed as effective alternatives to alkaline chlorination for destroying cyanide and thiocyanate (Young and Jordan, 1995;Adams, 2013;Kalmar et al, 2013;Teixeira et al, 2013). This peroxygen compounds do not preferentially react with thiocyanate alone, although about 10% could be oxidized during treatment.…”