Electrolytic denitrification with an ion-exchange membrane in groundwater

Nitrate contamination of groundwater is an important issue in rural areas. In this study, an electrolytic method for the denitrification of groundwater was investigated in a laboratory reactor. We used an ion-exchange membrane, employing a titanium oxide anode and five kinds of cathode for the investigation. The nitrate removal efficiencies with Cu, Pb, Ti/Ir/Ru, Ti/Ir/Ta, and Zn cathodes were 30.0%, 59.9%, 73.8%, 23.3%, and 80.6%, respectively. A cation-selective membrane was employed to separate the electrode compartments, and only the proton produced in the anode side was permitted to the cathode side in order to avoid reversible reactions. In terms of the nitrate removal, good experimental results were obtained by using the membrane reactor equipped with the Zn cathode. The nitrate concentration level dropped from 30 to 6 mg/L, which is below the limit for drinking water. The pH was not affected significantly by the current density and it increased up to 9.0 at 100 mA/cm². The nitrate removal efficiency showed the highest value with 1 mM of NaCl as an electrolyte. The impression was that membrane electrolysis made it possible to avoid the re-oxidation problem. It appears clear that electrolytic denitrification with an ion-exchange membrane is a very effective process with a mean energy consumption of only between 26.9 and 112.9 kWh/kg-NO₃.