Electrochemical activation of persulfates at BDD anode: Radical or nonradical oxidation?

The combination of persulfates (peroxydisulfate (PDS) and peroxymonosulfate (PMS)) and electrolysis using boron-doped diamond (BDD) anode is a promising green advanced oxidation process. In comparison with electrolysis alone, electrochemical activation of persulfates at BDD anode considerably enhanced the degradation of carbamazepine (CBZ). The experimental results indicate that the surface-adsorbed hydroxyl radical (HO) played the dominant role. The generally proposed nonradical oxidation mechanism ignored hydroxyl radical (HO) oxidation because low concentration of radical scavenger (<10 M methanol or 5 M tertbutanol) could not effectively scavenge the surface-adsorbed HO. The quasi steady-state concentration of HO was estimated to be about 5.0-9.1 × 10^-12 M for electrolysis with BDD anode, and it was increased to 1.1-1.6 × 10^-11 M and 3.2-5.0 × 10^-11 M for addition of 5 mM PDS and PMS, respectively. The results of cyclic voltammetry (CV) and chronoamperometry as well as evolution of dissolved oxygen (DO) reveal that the electrochemically activated persulfates molecule (PDS^∗/PMS^∗) promoted the production of HO via water dissociation at BDD anode and enhanced the direct electron transfer (DET) reaction, which otherwise inhibited the oxygen evolution side reaction. Therefore, higher current efficiency was achieved in electrochemical activation of persulfates process compared with electrolysis process. Additionally, the transformation products of CBZ were also investigated and their formation pathways were proposed.