Efficient electrocatalysis for denitrification by using TiO2 nanotube arrays cathode and adding chloride ions

Abstract

Electrocatalysis is emerging as a promising alternative to bacterial denitrification for removing nitrate and ammonia from sewage. The technology is highly efficient and robust in actual wastewater treatment scenarios; however, there may be the generation of harmful intermediates (such as nitrite) on the traditional cathode material. In this study, we demonstrated that TiO2 nanotube arrays can be used as an effective cathode to reduce nitrate to ammonia without generation of nitrite. Alongside this, the addition of chloride ions in the solution can further oxidize ammonia to N2. We looked into the key factors influencing the electrocatalytic denitrification, including the current density (2–10 mA/cm2), initial pH values (3–11), and types of anions (HCO3−, Cl−, SO42−). The results showed that 90.8% of nitrate and 59.4% of total nitrogen could be removed in 1.5 h under optimal conditions, with degradation kinetic constants of 1.61 h−1 and 0.79 h−1, respectively. Furthermore, we investigated the formation of intermediate products and explored the electrocatalytic denitrification mechanism: (a) the surface oxygen vacancies and high specific surface area of TiO2 nanotube arrays electrode promote the reduction of nitrate to ammonia and N2; (b) the active chlorine generated at the anode surface can effectively oxidize ammonium to N2.