Cobalt And Nickel Bimetallic Organic Frameworks For High-efficiency Catalytic Ozonation To Remove Bio-refractory Pollutants

In practical engineering applications,after biological treatment,industrial wastewater remains organic pollutants with low concentration,carrying potential ecological risks.Besides,the water treatment industry is generally facing more stringent emission standards and more urgent requirements for emission reduction and reuse,so it is imperative to carry out advanced treatment of high-risk wastewater.Characteristic with high efficiency,economical use,well development and eco-friendliness,catalytic ozonation can overcome the drawback-selective oxidation of unsaturated organic compounds by O₃,and effectively remove the refractory organic substances in industrial wastewater.Therefore,it is widely used in the advanced treatment of wastewater.Combining the advantages of traditional metal-based and metal-free based ozonation catalysts,metal-organic frameworks?MOFs?-Co-MOF,Ni-MOF,and Co/Ni-MOF,were synthesized via a hydrothermal method,and their feasibility as ozonation catalysts for catalytic ozonation of typical refractory pollutants or wastewater was investigated in this work.With the typical refractory pesticide atrazine as the target pollutant,the catalytic ozonation performance of these MOFs was investigated;and their physicochemical properties were analyzed to establish the relationship between catalytic activity and catalyst properties,through a serious of characterization methods.It was found that the MOFs showed outstanding performance in the catalytic ozonation.Notably,Co/Ni-MOF displayed better catalytic performance than its single-metal counterparts,which was attributed to multiple metal sites,metal centers with larger electron density,higher coordination unsaturation,and better efficiency in electron transfer.The testing results showed that surface hydroxyl groups and Lewis acid sites were the critical adsorption sites and catalytic sites on Co/Ni-MOF.From the results of EPR tests and quenching experiments,atrazine removal in the Co/Ni-MOF/O₃system mainly depended on O₂·^-,¹O₂ and·OH,and Co/Ni-MOF could highly promote the formation of these reactive species.Particularly,O₂·^-and H₂O₂ generated not only promote the development of free radical chain reaction but act as electron acceptors/donors to accelerate the cycle of Co?II?/Co?III?and Ni?II?/Ni?III?.Besides,higher removal rates could be achieved by growing initial p H,increasing O₃ dosage,and reducing atrazine concentration,while an excess of catalyst might become a quencher for reactive species.In cycling experiments,Co/Ni-MOF displayed good stability in catalytic activity and structure,but still had the potential for improvement.According to the results of HPLC-MS/MS,three major degradation pathways for atrazine in the catalytic system were proposed.With the help of T.E.S.T,it was found that the oral rat lethal dose LD₅₀ of intermediate products generally showed an upward trend,and the developmental toxicity of them showed a downward trend,indicating that the toxicity of pollutants has been reduced after catalytic ozonation,and the environmental risks have been controlled.With the bio-treated coking wastewater as the treatment object,the effectiveness of the Co/Ni-MOF/O₃ system for advanced treatment of the wastewater was investigated.Compared with the O₃ system,the catalytic system could treat the wastewater more efficiently with reactive oxygen species through hydrogen abstraction,substitution,electron transfer,etc.,and the mean oxidation number of carbon of the tailwater increased from 1.73 to 2.45.Besides,increasing the dosage of O₃ and the catalyst could improve the treatment of the bio-treated coking wastewater.Spectral analysis showed that the catalytic system could further reduce the monocyclic,heterocyclic and polycyclic aromatic hydrocarbons with characteristic ultraviolet absorption,and remove the tryptophan-like substances and soluble bio-metabolic products with fluorescent properties in the wastewater.At the same time,SUVA₂₅₄ dropped from 4.71L/?mg·m?to 2.93 L/?mg·m?,and the formation potential of disinfection by-products would also decrease,indicating that the ecological risks of the bio-treated coking wastewater were reduced.