Indirect electrochemical reduction of perchlorate and nitrate in dilute aqueous solutions at the Ti–water interface

Abstract

Perchlorate and nitrate are contemporary contaminants in drinking water. In most perchlorate containing surface or ground waters, nitrate was also present at significant concentrations. The present study investigated the removal of perchlorate and nitrate at the Ti–water interface by indirect electrochemical reduction process. Results indicated that perchlorate and nitrate could be reduced readily at the surface of a Ti anode simultaneously. Upon the application of an anodic current at the Ti electrode, Ti species such as Ti(III) or Ti(II) were generated. These multivalent Ti species are strong reducing agent that can reduce perchlorate and nitrate ion. The dominant end product of the indirect electrochemical reduction was chloride or nitrite. However, when these two anions were co-existing, the nitrite concentration was negligible. The reduction for both anions followed typical Langmuir–Hinshelwood kinetics; zero-order and first-order reaction at high and low concentrations, respectively. The first-order reaction rate constant was in the range of 10 −5 s −1. Perchlorate and nitrate at initial concentration of 200 and 1000 ppm individually were reduced to final concentration of <20 and <200 ppb, respectively, over a short reaction time of 6–8 h. The reaction mechanism was studied with the aid of surface analysis of the solid end products (e.g., TiO 2 particles) remaining on the Ti electrode using XPS and SEM/EDX. It was observed that doping of chlorine and nitrogen atoms onto the solid TiO 2 particles contributed partially to imbalance of the total Cl and N mass in the system. Additionally, formation of volatile chlorine and hypochlorite species brought mass imbalance to total chlorine.