Enhancing removal of nitrates from highly concentrated synthetic wastewaters using bipolar Si/BDD cell: Optimization and mechanism study

Electrochemical denitrification of highly concentrated synthetic wastewaters was investigated using bipolar Boron-Doped Diamond on silicone substrate (Si/BDD) electrodes under various experimental conditions. The effects of the key operational parameters on the removal efficiency of nitrates were assessed using the Response Surface Methodology. Results showed that the efficiency of removal of nitrates was mostly sensitive to the applied current density and the duration of treatment, which are optimized, respectively, at 35.7 mA cm^− 2 and 120 min. Under these experimental conditions, the removal yield of nitrates was evaluated at 91% with formation of by-products of 0.4% and 3.6% of nitrites and ammonia, respectively. Furthermore, average current efficiency is optimized at 59% with an energy consumption estimated at 0.35 × 10^− 3 kWh(g-N)^− 1. To better understand the role of BDD anode/cathode, by-products electrogeneration and disappearance, as well as the effect of chloride ions on pathway electrochemical denitrification, series of experiments were carried out at three concentrations of NaCl. The kinetic constants were calculated using a pseudo first-order kinetic model based on the fitting of the experimental data to established numerical approximations using Matlab software. At first, we noted that the direct reduction to N₂ in the absence of NaCl was very low (k = 1.7 × 10^− 9 s^− 1) compared to NH₃ and NO₂ ⁻ electrogeneration (k = 1.7 × 10^− 5 and 5.4 × 10^− 4 s^− 1, respectively), where N₂ seemed to be the result of a reduction in by-products. The presence of hypochlorous species had a negligible effect on the kinetic rates of NO₃ ⁻ reduction.