Catalytical dehalogenation of HAAs through electrolysis on VB12-modified electrode: kinetics, intermediates, and mechanisms

Haloacetic acids (HAAs) are prevalently occurred in drinking water after disinfection by chlorine or chloramine, classified as the non-volatile and thermally stable disinfection byproducts (DBPs). To minimize the existed HAAs, catalytical electrolysis based on effective cathode was employed. Dehalogenation of HAAs were improved by vitamin B12 (VB12) coating on the stainless steel (SS), iron, copper, and aluminum electrodes. The VB12/SS composite electrode proved to be the optimal for the synergistic effect of VB12 and metal substrate on the DCAA degradation. Target chloro-, bromo-, and iodo-HAAs were catalytically decomposed, and their dehalogenation efficiency and mechanistic pathway were compared. The results show that HAAs electrolytic dehalogenation rates were significantly related with both the degree of halogenation (tri > di > mono) and the halogen atomic weight (I > Br > Cl). Dehalogenation of HAAs follows a first-order kinetic model, and kinetic variations were well described by a linear model. The logarithm base 10 of the dehalogenation rate constant k was proportional to its HAA molecular weight: log 10 (k) =0.00237 MW -0.85. The intermediate dehalogenation products were identified as less-halogenated acetic acids, and the final were acetic acid and halide ions. No mineralization occurred according to carbon conservation analysis. ESR detection and quenching experiments confirmed that HAAs dehalogenation was mainly an indirect reduction mediated by atomic H*. The synergistic effect between atomic H* and VB12 occurred through the electron transfer. This comparative study of various HAAs dehalogenation provides a mechanism of dehalogenation and a predictive linear model for electrolysis of HAAs.