Removing ammonium from underground water by strengthening Mn(III) generation through electrochemical reduction

Manganese oxide has been found to be an active catalyst for ammonium oxidation. This work presented an innovative electrochemical system to improve the removal efficiency of ammonium by strengthening the generation of Mn(III). In this system, the manganese oxide coating activated carbon (MnO_x /AC) particles were used as the cathode of a fuel cell (MnO_x /AC-Ca). Compared to the conventional method using MnO_x in a nonelectrochemical system (MnO_x /AC-Control), the ammonium removal efficiency was doubled in the MnO_x /AC-Ca system. Conversely, when using MnO_x as the anode of a fuel cell (MnO_x /AC-An), the ammonium removal efficiency was lower than that in the MnO_x /AC-Control system. XPS results showed that Mn(III) in the MnO_x /AC-Ca system was obviously higher than that in the MnO_x /AC-Control system. Conversely, more Mn(IV), which was less active than Mn(III) for ${\text{NH}}_4^{+}-\text{N}$ removal, was detected in the MnO_x /AC-An system. The possible pathway for the ammonium removal by MnO_x was that Mn(III) reacted with ${\text{NH}}_4^{+}-\text{N}$ and produced Mn²⁺ . These Mn²⁺ were then reoxidized into new active MnO_x under the self-catalysis of MnO_x , resulting in a continuous ammonium removal. Moreover, nitrite, the intermediate product of ammonium oxidation, can be oxidized by MnO_x , thus helping the ammonium oxidation process. PRACTITIONER POINTS: Mn(III) was generated by using MnO_x as the cathode of fuel cells. Ammonium removal efficiency went up with the content of Mn(III) in MnO_x . Mechanism of the ${\text{NH}}_4^{+}-\text{N}$ removal by MnO_x was discussed.