Understanding Chlorite and Chlorate Formation Associated with Hypochlorite Generation at Boron Doped Diamond Film Anodes

This research investigated reaction pathways for formation of chlorite and chlorate when using boron doped diamond (BDD) film anodes for generating hypochlorite. Batch electrolysis and voltammetry experiments were performed to investigate the rates and potential dependency of hypochlorite and chlorite oxidation. Density functional theory (DFT) modeling was used to investigate possible reaction pathways. The DFT simulations included reactions with hydrogen terminated surfaces, and with surface sites produced by anodic polarization, namely: ≡C•, =C•H, ≡C–O• and =C•HO. Oxychlorine radicals (ClO•, ClO2 •) were found to chemically adsorb to both secondary and tertiary carbon atoms on the BDD surface. These chemisorbed intermediates could react with hydroxyl radicals to regenerate the original chlorine oxyanion (ClO− or ClO2 −), and produce ≡C–O• and =C•HO sites on the BDD surface. The ≡C–O• and =C•HO sites also reacted with oxychlorine radicals to form chemisorbed intermediates, which could then be converted to higher oxidation states (ClO2 −, ClO3 −) via reaction with hydroxyl radicals. The predominant pathway for chlorite and chlorate production appears to involve oxidation of HOCl or HClO2 via direct electron transfer, followed by reaction of ClO• or ClO2 • with a hydroxyl radical.