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 multiwalled 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, ∼11 s) resulted in 73, 94, and 96% DBAA reduction for Ti₄O₇, PAC-Ti₄O₇, and MWCNT-Ti₄O₇ REMs, respectively. The reactive-transport model yielded <i>k</i>_obs values between 9.16 and 33.3 min^-1, which were 2 to 4 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 was 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.