Mechanistic Investigation of Haloacetic Acid Reduction using Carbon-Ti4O7 Composite Reactive Electrochemical Membranes.

Abstract

Carbon-Ti4O7 composite reactive electrochemical membranes (REMs) were studied for adsorption and electrochemical reduction of haloacetic acids (HAAs). Powder activated carbon (PAC) or multi-walled carbon nanotubes (MWCNTs) were used in these composites. Results from flow-through adsorption experiments with dibromoacetic acid (DBAA) as a model HAA were interpreted with a transport model. It was estimated that ~46% of C in the MWCNT-REM and ~10% of C in the PAC-REM participated in adsorption reactions. Electrochemical reduction of 1 mg L-1 DBAA in 10 mM KH2PO4/K2HPO4 at -1.5 V/SHE (hydraulic residence time ~ 11s) resulted in 73, 94, and 96% DBAA reduction for the Ti4O7, PAC-Ti4O7, and MWCNT-Ti4O7 REMs, respectively. The reactive-transport model yielded kobs values between 9.16 to 33.3 min-1, which were two to four orders of magnitude higher than previously reported. PAC-Ti4O7 REM was tested with tap water spiked with 0.11 mg L-1 of nine different HAAs in a similar reduction experiment. The results indicated that all HAAs were reduced to < 20 µg L-1. Moreover, the total combined concentration of five regulated HAAs were lower than the regulatory limit (60 µg L-1). Density functional theory simulations suggest that a direct electron transfer reaction was the probable rate-determining step for HAA reduction.