Nitrate Removal by an Optimized Paired Electrolysis Process

Ni, Cu, Cu90Ni10 and Cu70Ni30 were evaluated as cathode materials for the conversion of nitrate to nitrogen by a paired electrolysis process using an undivided flow-through electrolyzer. In a first step, polarization curves measurements revealed that Ni and Cu-Ni electrodes have corrosion rates about four times and ten times slower than pure copper in presence of nitrate and ammonia, respectively. In a second step, nitrate electroreduction experiments showed that the Cu70Ni30 electrode is the most efficient for reducing nitrate to ammonia with a selectivity of 100%. Then, paired electrolysis using Cu70Ni30 cathodes and Ti/IrO2 anodes revealed that the nitrate removal efficiency decreases when a large excess of chloride (e.g., 100 mM NaCl vs. 10 mM NaNO3) is present due to the competitive reduction of hypochlorite ions at the cathode side. However, with an appropriate amount of chlorine (e.g., 50 mM NaCl), the paired electrolysis process allowed the conversion of nitrate to nitrogen from 620 ppm NO3− to less than 50 ppm NO3− with a power consumption as low as 20 kWh/kg NO3−.