Electrochemically generated sulfate radicals by boron doped diamond and its environmental applications

Oxidation of recalcitrant organic contaminants via the electrogenerated sulfate radicals (SO4−) with boron doped diamond (BDD) electrode is one of the most efficient techniques. In this review, fundamental mechanisms of sulfate (SO42−), persulfate (PS, S2O82−) and peroxymonosulfate (PMS, HSO5−) activation with BDD anode and its environmental applications are discussed. Significance of BDD electrode properties such as sp3/sp2 ratio, doping agents and substrate material, and the influence of electrolyte medium, current density and the presence of other inorganic ions (chloride, carbonate, nitrate, phosphate) on the electrogeneration of reactive sulfate species and its reactivity is comprehended as followingly, (i) BDD-sulfate process explains the fundamental reaction mechanisms involving in the electrogeneration of SO4−and S2O82− via radical and non-radical routes, (ii) electrochemical activation of electrogenerated/externally added PS and PMS via the direct/indirect pathways and its promotional reactions are detailed, and (iii) mode of reactivity of the electrogenerated reactive sulfate species with organic contaminants in synthetic and real water, wastewater and soil matrices is elaborated. Most of the studies reported the activation of SO42− at current densities ≥10 mA cm−2, and the energy consumption in the range of 41–190 kW h m−3 was reported for the treatment (95–100% removal efficiency) of high organic content real wastewaters. In later section, the importance of coupling other processes such as electrogeneration of H2O2, UV irradiation, and membrane distillation with BDD/sulfate system to enhance the performance efficiency is discussed. Future perspectives of the research and application is presented at the end.