Effect of Select Organic Compounds on Perchlorate Formation at Boron-doped Diamond Film Anodes

Abstract

Rates of ClO₄^– formation from ClO₃^– oxidation were investigated in batch experiments as a function of organic compounds (p-nitrophenol, p-benzoquinone, p-methoxyphenol, and oxalic acid) and current density using boron-doped diamond film anodes. Excluding organics, ClO₄^– formation rates ranged from 359 to 687 μmoles m^–2 min^–1 for current densities of 1—10 mA cm^–2. The presence of p-substituted phenols inhibited ClO₄^– formation rates between 13.0 and 99.6%. Results from a reactive-transport model of the diffuse layer adjacent to the anode surface indicate that competition between organics and ClO₃^• for OH^• within a reaction zone (0.02—0.96 μm) adjacent to the anode controls ClO₄^– formation. Under kinetic-limited conditions (1.0 mA cm^–2), organics reach the anode surface and substrates with higher OH^• reaction rates demonstrate greater inhibition of perchlorate formation (IPF). At higher current densities (10 mA cm^–2), organic compound oxidation becomes mass transfer-limited and compounds degrade a small distance from the anode surface (∼ 0.26 μm for p-methoxyphenol). Therefore, OH^• scavenging does not occur at the anode surface and IPF values decrease. Results provide evidence for the existence of desorbed OH^• near the anode surface and highlight the importance of controlling reactor operating conditions to limit ClO₄^– production during anodic treatment of organic compounds.