Interpretation of high perchlorate generated during electrochemical disinfection in presence of chloride at BDD anodes.

Perchlorate is a disinfection by-product (DBP) of serious health concern. Herein, the long sought mechanism of high perchlorate production during electrochemical disinfection at boron-doped diamond (BDD) anode in the presence of chloride was elucidated. The generated perchlorate at BDD during electrochemical disinfection (in 10 mM NaCl) in 60 min reached 0.125 mM, which was 830 times higher than the EPA standard. In contrast, perchlorate at PbO2 and SnO2 anodes was below the detection limit. Further experiments employing NaClO3 revealed that the conversion ratio from ClO3- to ClO4- in 10 h at BDD (98%) was considerably higher than PbO2 (13%) and SnO2 (12%). Such significant difference among anodes was fully interpreted with a two-step mechanism. The first step is essential to produce ·ClO3 by oxidizing ClO3- at electrodes. Otherwise, the conversion to perchlorate would be impossible even with excessive ·OH, which was verified with the photocatalysis process. The second step is the perchlorate generation with radical reaction between ·ClO3 and ·OH, where the primary role of ·OH was substantiated by scavenging test. Interestingly, the capability of perchlorate production was correlated with free ·OH instead of the total amount of ·OH. Despite the similar abilities of electron transfer between anodes and ClO3-, much higher free ·OH exists at BDD anode than at PbO2 and SnO2 anodes through chronoamperometry experiments and work function characterization, which reasonably provides interpretation of high perchlorate production at BDD anode.