Effective and selective nitrate electroreduction into nitrogen through synergistic parameter interactions

Effective and selective nitrate electroreduction with low energy consumption was achieved at room temperature through synergistic interactions of the distance between the electrodes (D), voltage applied (U), initial pH and reaction time. Almost 90–95% of nitrates were converted with energy consumption exceeding 8 × 10−3 kWh/g-N for D = 3–5 mm and U = 4.0–4.5 V, when using 70 cm2 stainless steel electrodes in a batch reactor containing 150–400 mL of aqueous KNO3 (100–1000 mg-N/L). Further investigations through 34 factorial experiment designs demonstrated that only D and U displayed strong individual effects, but all parameters exhibited appreciable interactions. The effectiveness and selectivity of the nitrate conversion into nitrogen were discussed in terms of changes in the reaction scheme. Energy consumption of up to 0.87 × 10−3 kWh/g-N with conversion yields of 55–60% were noticed at pHs of 6–7 after 120 min. Lower energy consumption of 0.1 × 10−3 kWh/gN resulted in nitrate conversion yields of almost 43% after only 40–80 min. No nitrite or ammonia appeared to form at 5.3 V for D = 7.2 mm and neutral pH. Parameters simulation allowed to predict the nitrate conversion yield in correlation with the process selectivity according to the energy constraint, and conversely. This opens promising prospects for low-cost nitrate removal without generating nitrite nor ammonia.

► Nitrate electroreduction was optimized through factorial designs of experiments. ► Stainless steel electrodes exhibited stability below 6.0 V electric potential. ► The electrode distance and voltage showed strong individual effects and interactions. ► Nitrates were removed at 60–95% with an energy consumption of 0.1–10 × 10−3 kWh/g-N. ► High conversion with low energy required an optimization of parameter interactions.