Three-dimensional Co/Ni bimetallic organic frameworks for high-efficient catalytic ozonation of atrazine: Mechanism, effect parameters, and degradation pathways analysis

Herein, the potential of bimetallic MOFs in catalytic ozonation was investigated for the first time. Three novel ozonation catalysts, i.e. cobalt-based, nickel-based and cobalt/nickel-based metal-organic frameworks (Co-MOF, Ni-MOF and Co/Ni-MOF), were synthesized, characterized by XRD, SEM, N2 sorption-desorption isotherms, FTIR and XPS, and applied in catalytic ozonation for atrazine removal. It was found that the catalysts showed outstanding performance in the catalytic ozonation, especially Co/Ni-MOF which was attributed to multiple metal sites, higher coordination unsaturation, metal centers with larger electron density, and better efficiency in electron transfer than its single-metal counterparts. Under specific experimental conditions, 47.8%, 67.0%, 75.5%, and 93.9% of atrazine were removed after adsorption and degradation in the ozonation system without catalyst, and the catalytic ozonation systems with Co-MOF, Ni-MOF and Co/Ni-MOF, respectively. Higher removal rates could be achieved by growing initial pH, increasing oxidant dosage and reducing pollutant concentration, while an excess of Co/Ni-MOF was not favorable for the catalytic ozonation. Surface hydroxyl groups and acid sites were considered as the critical catalytic sites on Co/Ni-MOF. From the results of EPR tests, O2·-, 1O2 and ·OH were ascertained as the main reactive species in the degradation. It was suspected that O2·- and H2O2 played important roles in the formation of ·OH and the cycle of Co(II)/Co(III) and Ni(II)/Ni(III). Additionally, Co/Ni-MOF displayed good stability and reusability in cycling experiments, ascribed to the enhancement of the porosity and pore hydrophobicity. Finally, based on MS/MS analysis at different reaction times, major degradation pathways for atrazine were proposed.