Oxidation of bromide by heat-activated persulfate – Effects of temperature and the organic matter surrogate phenol on kinetics and stoichiometry

Abstract

• Oxidation of persulfate by sulfate radical affects persulfate decomposition kinetics. • Bromide oxidation to bromate was hindered by phenol. • Sulfate radicals reacting with Br-species decreased at higher temperature. • Insoluble phenol oxidation byproducts formation is favored at high temperature. Reaction mechanisms of sulfate radical with bromide and organic substances can be largely altered at elevated temperature in heat-activated persulfate process due to different activation energies. This study investigated kinetics and stoichiometry of bromide oxidation in the heat-activated persulfate process. We postulated that reaction of sulfate radical with persulfate, which has relatively low reaction rate (6.3 × 10 5 M −1 s −1 ), may become an important reaction at elevated temperatures. Persulfate decomposition was inhibited in the presence of sulfate radical scavenger phenol at low temperature (40 °C) whereas phenol affected persulfate decomposition only at < 50 °C but not at higher temperatures. This was because the second order reaction rate constant of sulfate radical with persulfate became higher (assumed as 10 10 M −1 s −1 ) than with phenol at a temperature > 50 °C, which can be explained by a high activation energy (estimated activation energy: 311 kJ/mol). Bromate formation was inhibited in the presence of phenol. However, phenol oxidation was not affected by the presence of Br − , even though Br − reacts faster with sulfate radicals than phenol and was present in excess. This indicated that formed reactive bromine species such as Br and Br 2 − oxidize phenol under reformation of bromide. Formation of insoluble products was also observed indicating polymerization of phenol. This can be explained by lack of dissolved oxygen under conditions of heat-activation.