Enhanced Catalytic Performance Of Co₃O₄ Towards Peroxymonosulfate Activation For The Degradation Of Methylene Blue

Advanced oxidation processes（AOPs）is one of the typically chemical means for efficient treatment of polluted water,which is benefit by its super non-selective oxidation capacity to oxidize or even completely mineralize organic pollutants into inorganic substances such as water and carbon dioxide.In recent years,advanced oxidation technology based on Peroxymonosulfate（PMS）has attracted wide attention because of its advantages of low reaction conditions,simple operation and high safety.The key to the practical application of this technology lies in the prioritization of PMS activation method.The activation methods for PMS mainly include energy activation and catalytic activation.In the former,external energy such as light,heat,ultrasonic and microwave activation are adopted as PMS activator.While in the latter,homogeneous or heterogeneous catalysts are used to activate PMS.In practices,compare with homogeneous PMS activation systems that contain transition mental ions,heterogeneous PMS activation systems use transition mental oxides as catalyst have more advantages such as convenient catalyst recovery and less secondary pollution.However,the immanent defects of heterogeneous reactions result in a weaker catalytic performance of transition mental oxides,which is the key problem to solve.This study focused on the heterogeneous catalytic activation technology of PMS.Co₃O₄was selected as the research object,and Methylene Blue（MB）solution of simulated dye wastewater was used as degradation model.Series of hierarchic micro/nano-structure catalysts which have different micro topography or chemical composition were designed and synthesized,and compared them in their catalytic performance.On this basis,the influence of different parameters on MB degradation processes were explored,and the primary reaction mechanism was also proposed according to the reactive oxidation species detecting results.The main contents include:1)Three kinds of hierarchic micro/nano-structure Co₃O₄powers were synthesized through the urea hydrothermal-calcination method（named as Co₃O₄-A）,chemical bath deposition-calcination method（Co₃O₄-B）and oxalate pyrolysis method（Co₃O₄-C）,respectively.And then compared them in catalytic performance for activating PMS as well as further degrading MB.As indicated from the experiment results,Co₃O₄-B that prepared via chemical bath deposition-calcination method was endowed with the biggest specific surface area,surface oxygen vacancy concentration,and surface hydroxyl density,thus showing the highest PMS activation performance.The decomposition process of PMS under its activation fits well with the first-order reaction kinetic model with a reaction rate constant of 0.217 4 min^-1.The optimized parameters for treating MB with Co₃O₄-B/PMS advanced oxidation system are also discussed out as follow:a catalyst dosage of 0.02 g·L^-1and a PMS dosage of 0.6 mmol·L^-1.In which condition the MB degradation ratio can be reached as high as 98.33%within 25 min.Moreover,four reactive oxygen species,·SO₄^-,·OH,·O₂^-and ¹O₂were detected in Co₃O₄-B/PMS advanced oxidation system.2)Series of cobalt/zinc bimetallic oxide materials were prepared by urea hydrothermal-calcination method or oxalate pyrolysis method,and their properties in activating PMS to degrade MB were all discussed.As indicated by the experiment results,the uniform composite Co/Zn bimetallic oxide materials with hierarchic micro/nano structures can be synthesized though both of the two methods.The ingredient ratio between Co and Zn is all optimized to be 1:5.Catalyst U-1 that prepared though urea hydrothermal-calcination method is microspheric,and its parameters for treating MB by activating PMS are optimized as follows:catalyst dosage of 0.04 g·L^-1and PMS dosage of 0.6 mmol·L^-1,where the corresponding MB degradation ratio is 94.65%.While catalyst O-1 which prepared though oxalate pyrolysis method is prismatic,with the parameters optimized as:catalyst dosage of0.02 g·L^-1and PMS dosage of 0.6 mmol·L^-1,where the corresponding MB degradation ratio is 98.49%.Meanwhile,the presence of·SO₄^-is the primary factor for the efficient degradation of MB in both U-1/PMS and O-1/PMS AOP system,and the introduction of either Cl^-or C₂O₄^2-can inhibit MB degradation process.Zn O that serves as a catalyst carrier in the composite facilitates a high dispersion of the Co₃O₄.Meanwhile,the partial dissolution of Zn O in the PMS solution that causes the increase of p H and more exposure of Co₃O₄,which improved the activation performance of Co₃O₄towards PMS.3)Preparing the S-modified Co₃O₄catalyic material via a modified oxalate pyrolysis method by using Na₂S₂O₃as the sulfur source,and further investigated its performance in activating PMS for treating of MB solution.As indicated from the experiment results,the S element used to modify Co₃O₄is mainly bonded to the surface of the catalyst in the form of SO₄^2-.And the catalytic performance of S-modified Co₃O₄increases with the addition amount of Na₂S₂O₃within the scope of this study,where catalyst that named Co₃O₄@S₁showed the highest catalytic activity.MB degradation ratio in Co₃O₄@S₁/PMS system under the optimal process parameters(catalyst dosage of 0.04 g·L^-1and PMS dosage of 0.6 mmol·L^-1)is up to 98.35%,while it is only 17.45%when using pure Co₃O₄as PMS activator.Besides,although the degradation effect for MB in Co₃O₄@S₁/PMS system exhibits decreasing tendency in continuous recycling tests,this AOP system shows a better tolerance to common inorganic anions,the MB degradation process is less affected by anions.The improvement of catalytic performance of Co₃O₄is not only due to the increases of specific surface area and oxygen vacancy content,but also related to the changes of physicochemical properties in its surface that caused by SO₄^2-bonding.4)Introducing strong Lewis acid Eu³⁺into the Co₃O₄catalysts,for enhancing its performance in activating PMS for treating with MB solution.As indicated from the experiment results,the oxalate pyrolysis method is a promising method for gaining homogeneous composite Co/Eu bimetallic oxide material.In those composite catalysts,the electron-deficient properties of Eu³⁺can polarize the PMS that adsorbed on their surface,which makes it more likely to be activated by the Co²⁺sites in Co₃O₄.Furthermore,materials named as Co₉Eu₁which is synthesized with a Co/Eu ingredient ratio of 9:1 is considered to be the most economical and practical catalyst.The degradation ratio of MB is up to 86.66%in Co₉Eu₁/PMS under the reaction conditions:catalyst addition of 0.10 g·L^-1and PMS concentration of 0.6 mmol·L^-1.And the degradation efficiency increases with the increasing addition of catalyst dosage or PMS dosage.The catalyst Co₉Eu₁perform well in stability as no significant changes in the catalytic performance during four consecutive cycles.And it is found that both C₂O₄^2-and HCO₃^-have obvious inhibitory effects on the degradation efficiency of MB in Co₉Eu₁/PMS system.Both radical reactive oxidation species（include·SO₄^-,·OH and·O₂^-）and non-radical reactive oxidation specie（¹O₂）are exist in Co₉Eu₁/PMS system,while·SO₄^-plays a key role in MB oxidative degradation process.