Electrochemical Destruction of N-Nitrosodimethylamine in Reverse Osmosis Concentrates using Boron-doped Diamond Film Electrodes

Boron-doped diamond (BDD) film electrodes were used to electrochemically destroy N-nitrosodimethylamine (NOMA) in reverse osmosis (RO) concentrates. Batch experiments were conducted to investigate the effects of dissolved organic carbon (DOC), chloride (Cl-), bicarbonate (HCO3-), and hardness on rates of NOMA destruction via both oxidation and reduction. Experimental results showed that NDMA oxidation rates were not affected by DOC, Cl-, or HCO3- at concentrations present in RO concentrates. However, hydroxyl radical scavenging at 100 mM concentrations of HCO3- and Cl- shifted the reaction mechanism of NDMA oxidation from hydroxyl radical mediated to direct electron transfer oxidation. In the 100 mM Cl- electrolyte experimental evidence suggests that the in situ production of ClO3 center dot also contributes to NDMA oxidation. Density functional theory calculations support a reaction mechanism between ClO3 center dot and NDMA, with an activation barrier of 7.2 kJ/mol. Flow-through experiments with RO concentrate yielded surface area normalized first-order rate constants for NDMA (40.6 +/- 3.7 L/m(2) h) and DOC (as C) (38.3 +/- 2.2 L/m(2) h) removal that were mass transfer limited at a 2 mA/cm(2) current density. This research shows that electrochemical oxidation using BDD electrodes has an advantage over other advanced oxidation processes, as organics were readily oxidized in the presence of high HCO3- concentrations.