Highly efficient removal of total nitrogen and dissolved organic compound in waste reverse osmosis concentrate mediated by chlorine radical on 3D Co3O4 nanowires anode

Reverse osmosis concentrate (ROC) from wastewater reclamation has posed significant disposal challenges due to its highly concentrated NH3-N, chloride ion and bio-refractory organics, and developing technologies for their removal are essential. Herein, we developed an efficient electrochemical system to remove total nitrogen and dissolved organic compound (DOC) simultaneously mediated by chlorine radical (Cl•), which is generated by activation of chloride ion existing in ROC on an inexpensive, three-dimensional Co3O4 nanowires. Results showed that the total nitrogen and total organic carbon removal were 98.2% and 56.9% in 60 min for synthetic ROC with 56 mg/L of NH3-N and 20 mg/L of DOC. The utilization of Co3O4 nanowires enhanced NH3-N degradation by 2.58 times compared with Co3O4 nanoplates, which were 1.69 and 17.5 times these of RuO2 and Pt. We found that structural Co3+/Co2+ acts as cyclic catalysis to produce Cl• via single-electron transfer, which convert NH3-N to N2 and lead to faster DOC degradation. This architecture provides abundant catalytic sites and sufficient accessibility of reactants. Small amount of nitrate generated by oxidation of NH3-N was further reduced to N2 on Pd-Cu/NF cathode. These findings provide new insights for utilization of waste Cl- and development of novel electrochemical system for ROC disposal.