Sulfate radical-based water treatment in presence of chloride: Formation of chlorate, inter-conversion of sulfate radicals into hydroxyl radicals and influence of bicarbonate

Sulfate radical (SO4−) based oxidation is discussed as a potential water treatment option and is already used in ground water remediation. However, the complex SO4− chemistry in various matrices is poorly understood. In that regard, the fast reaction of SO4− with Cl− is of high importance since Cl− belongs to the main constituents in aqueous environments. This reaction yields chlorine atoms (Cl) as primary products. Cl initiate a cascade of subsequent reactions with a pH dependent product pattern. At low pH (<5) formation of chlorine derived oxidation products such as chlorate (ClO3−) is favoured. This is undesired because ClO3− may reveal adverse effects on the environment and human health. At pH > 5 Cl mainly react with water yielding hydroxyl radicals. Thus, at moderate Cl− concentrations (mM range) the SO4−-based process may be converted into a conventional (hydroxyl radical -based) advanced oxidation process. The conversion of SO4− into OH, however, is interrupted in presence of bicarbonate by scavenging of Cl.