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Kinetics and Mechanism of Selenium(IV) Oxidation by Aqueous Bromine Solution
György Csekő1, Boglárka Nyitrai1, Attila K Horváth1
1Department of General and Inorganic Chemistry, Faculty of Sciences, University of Pécs, Ifjúság útja 6, Pécs H-7624, Hungary.
ACS Omega
|May 8, 2023
Summary
The complex bromine-selenite reaction kinetics in acidic solutions were elucidated. A novel mechanism involving bromonium ion transfer and intermediate hydrolysis explains the observed autoinhibition by bromide ions.
Area of Science:
- Analytical Chemistry
- Chemical Kinetics
- Environmental Chemistry
Background:
- The reaction between bromine and selenite in acidic media is crucial for understanding selenium speciation and removal in water treatment.
- Previous kinetic models failed to fully explain the complex reaction behavior, particularly the autoinhibition by bromide ions.
Purpose of the Study:
- To investigate the complex kinetics of the bromine-selenite reaction under strongly acidic conditions.
- To elucidate the reaction mechanism, including the role of hydrogen and bromide ions and selenium(IV) species.
- To develop a comprehensive kinetic model applicable across a wide pH range for water treatment applications.
Main Methods:
- Spectrophotometric monitoring of absorbance-time traces at the isosbestic point of the bromine-tribromide system.
- Kinetic analysis considering formal reaction orders, ionic strength, and temperature.
- Development and validation of a detailed reaction mechanism involving multiple equilibria and intermediate hydrolysis.
Main Results:
- The reaction exhibits complex kinetics with formal orders of 1 for bromine and selenite, but variable negative orders for hydrogen ions.
- Bromide ions significantly inhibit the reaction, a phenomenon explained by a proposed bromonium ion transfer mechanism.
- The active selenium(IV) species was identified as HSeO3-, and a mechanism involving its reaction with bromine and subsequent hydrolysis of SeO3Br- was established.
- The developed kinetic model accurately fits experimental data across a wide pH range (1-13).
Conclusions:
- A novel mechanism involving bromonium ion transfer and hydrolysis of a short-lived intermediate (SeO3Br-) explains the complex kinetics and autoinhibition.
- The kinetic model provides an accurate prediction of selenium(IV) behavior in acidic aqueous solutions.
- This research offers a valuable tool for optimizing selenium removal in water treatment processes.
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