Investigation of persulfate production on BDD anode by understanding the impact of water concentration

The formation of persulfate (S2O82−) in aqueous solution has been frequently considered as a direct oxidation of SO42− ions on boron-doped diamond (BDD) surface. Conversely, some authors have also suggested that slower reactions between OH radicals and SO42− ions are feasible. However, no attempts have confirmed this electrochemical pathway because the studies have been frequently performed at diluted sulfuric acid solutions. Thus, the purpose of this work was to study the effect of the ratio H2O/H2SO4 on the electrochemical behavior at BDD surface in order to verify the possible participation of hydroxyl radical on the production of persulfate. Different electrochemical techniques (cyclic voltammetry, polarizations curves and impedance) were employed to understand the electrochemical behavior when different concentrations of H2O were added to concentrated H2SO4. Results clearly suggest that, intermediate species are necessary to promote the formation of S2O82−. The predominance of HSO4− ions at concentrated H2SO4 system with lowest water content (2%) suggests that sulfate radical specie (SO4−) may be produced via the reaction of HSO4− with OH radicals. Then, S2O82− production depends principally on the particular formation of OH on BDD surface or BDD(SO4−) sites rather than on the direct anodic oxidation of SO42− at BDD surface. These results are described and discussed in the light of current literature. •No direct formation of S2O82− is favored at BDD surface.•S2O82− production depends on the formation of BDD(OH) or BDD(SO4−) sites.•SO4− may be produced via the reaction of HSO4− with OH radicals.