| Summary: | Spent caustic wastewater produced in a soda plant has a high concentration of ammonia nitrogen (NH<sub>4</sub><sup>+</sup>-N). As excessive NH<sub>4</sub><sup>+</sup>-N discharging into water bodies would cause eutrophication as well as destruction to the ecology balance, developing an efficient technology for NH<sub>4</sub><sup>+</sup>-N removal from the spent caustic wastewater is imperative in the current society. In this study, an electrochemical process with graphene electrodes was designed for the NH<sub>4</sub><sup>+</sup>-N removal in the spent caustic wastewater. The removal efficiency of the NH<sub>4</sub><sup>+</sup>-N during the electrochemical process could reach 98.7% at 4 A in a short treatment time (within 120 s) with an acceptable energy consumption (6.1 kWh/m<sup>3</sup>-order). NO<sub>3</sub><sup>−</sup> and NO<sub>2</sub><sup>−</sup> were not detected during the electrochemical process. An insignificant amount of NH<sub>2</sub>Cl, NHCl<sub>2</sub>, and NCl<sub>3</sub> produced in the treatment suggested that little of the NH<sub>4</sub><sup>+</sup>-N reacted with chlorine, that is, chlorination played a negligible role in the NH<sub>4</sub><sup>+</sup>-N removal. By electron equilibrium and nitrogen conversion analysis, we think that NH<sub>4</sub><sup>+</sup>-N was primarily converted to NH<sub>2</sub>(ads) on the surface of a graphene electrode by one-electron transfer during the direct oxidation of the electrochemical process. Due to the high calcium ion (Ca<sup>2+</sup>) in the spent caustic wastewater, the electrode scale significantly increased to 1.4 g after treatment of 240 s at 4 A. By X-ray diffraction (XRD) analysis, the composition of the electrode scale is portlandite Ca(OH)<sub>2</sub>. Although the electrode scale was obvious during the electrochemical treatment, it could be alleviated by alternating the electrode polarity. As a result, the life and efficiency of the graphene electrode for NH<sub>4</sub><sup>+</sup>-N removal could remain stable for a long time. These results suggest that the electrochemical process with a graphene electrode may provide a competitive technology for NH<sub>4</sub><sup>+</sup>-N removal in spent caustic wastewater treatment.
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