Electrocatalytic Conversion of Wastewater Nitrate to Valuable Ammonium

Nitrate contaminated wastewater and drinking water negatively impact the environment and human health. For example, nitrate contributes to eutrophication and disruption of the nitrogen cycle as well as certain diseases and disorders such as methemoglobinemia and endocrine disorders. Conventional nitrate removal methods include ion exchange, reverse osmosis, and biological denitrification. These processes often produce concentrated A promising alternative method for nitrate removal is electrocatalytic nitrate reduction. Through this process, nitrate may be transformed either into N₂, a benign gas, or into ammonium, which enables recovery of reactive nitrogen in wastewaters for reuse as fertilizer or energy storage in nitrogen-based fuels. However, this method often requires expensive precious metals such as Pt or Pd, or metals with low electrochemical stability such as Cu.

Therefore, we investigated the use of transition metal oxynitrides (MO_xN_y) and MO_xN_y on reduced graphene oxide supports (MO_xN_y /rGO), as novel catalysts for the nitrate reduction reaction to address the lack of cost-efficient, active, and stable electrocatalysts with high selectivity to ammonium. Following a urea glass synthesis, the MO_xN_y and MO_xN_y/rGO catalysts were used as cathodes in a batch electrolysis reactor containing different initial nitrate concentrations (1 mM, 10 mM, and 50 mM) representative of those found in nitrate-containing wastewaters (polluted groundwater, textile wastewater, and industrial wastewater, respectively). Results showed increasing nitrate conversion with increasing nitrate concentration and higher conversions with MO_xN_y/rGO electrodes compared to those coated with MO_xN_y. Further electrochemical analyses with linear sweep voltammetry and electrochemical impedance spectroscopy confirmed the improved activity of MO_xN_y/rGO compared to MO_xN_y, characterized by a lower onset potential, a lower charge transfer resistance, and higher electrochemically active surface area. Furthermore, electrochemical membranes (EMs) functionalized with MO_xN_y and synthesized with reduced graphene oxide were prepared and tested for nitrate reduction. Results suggest that modular EMs for nitrate electrochemical conversion may enable recovery of reactive nitrogen as valuable ammonia.