Molybdenum-based Catalysts For Activation Of Persulfate And Removal Of Organic Pollutants

With the development of society,a large number of refractory organic pollutants are discharged into the water environment,and wastewater treatment has become a necessary prerequisite for human sustainable development.The main sources of refractory organic pollutants are printing and dyeing,pesticides,chemicals,coal,pharmaceuticals,and other industries.These organic pollutants are often highly toxic and highly bio enriched,which seriously threat to human health and the survival of animals and plants.Advanced oxidation technology based on persulfate（PMS）is widely used in the treatment of organic pollutants due to its advantages such as strong oxidation,wide range of p H application,and long average lifetime of free radicals generated.PMS usually needs to be activated under the conditions of additional energy or catalysts to generate active oxygen species,so as to achieve the purpose of oxidative degradation of pollutants in water.The transition metal is an ideal way to activate PMS because of its mild reaction conditions,fast activation speed,and no additional energy required.However,the activation of PMS is mainly limited to common transition metals such as cobalt（Co）,iron（Fe）,copper（Cu）,nickel（Ni）and manganese（Mn）now.Molybdenum（Mo）based compounds are mostly used as cocatalysts in Fenton and Fenton like systems,and there are few reports of directly activated PMS catalytic degradation of organic compounds.Therefore,it is of great significance to develop efficient molybdenum-based catalysts for direct activation of PMS.The catechol group of dopamine can be complexed with the metal to anchor the metal,and it can self-polymerize to produce polydopamine in alkaline environment.By utilizing this characteristic of dopamine,dopamine and molybdenum were complexed,and dopamine self-assembles to form polydopamine during the complexation process.This not only prevent molybdenum migration in the process of calcination,but also improve the dispersion of molybdenum,so as to improve the catalytic performance of molybdenum-based catalyst.In this paper,dopamine and molybdate were selected as the main raw materials,combined with Fe（OH）₃,and Mo O₂@CN catalyst and Mo₂C@Fe catalyst were prepared by grinding and calcination,respectively.This paper investigated the catalytic degradation effect of Mo O₂@CN and Mo₂C@Fe activated PMS on the typical antibiotic carbamazepine（CBZ）.The main research contents are as follows:The MoO₂@CN was designed via one-step calcination from dopamine,ammonium molybdate,and ammonium bicarbonate.Besides,Mo O₂@CN was characterized by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）,Raman spectroscopy,etc.CBZ was selected as the target pollutant and potassium bisulfate（PMS）was selected as the oxidant,the Mo O₂@CN/PMS system could reach 99.2%degradation rate for carbamazepine CBZ in 12 min under p H of 6.5 and temperature at 25℃.Moreover,the apparent rate constant(k_obs)of Mo O₂@CN was calculated to be 0.393 min^-1,about24.0 times higher than that of the commercial Mo O₂(0.016 4 min^-1),which is attributed that Mo O₂@CN possessed better conductivity and larger specific surface area.Mo O₂@CN was capable to degrade CBZ effectively in the p H range of 2.5-10.5,and also exhibited effective degradation performance for most dyes,phenolic compounds,antibiotics,and other pollutants.In addition,the total organic carbon（TOC）degradation rate of CBZ was as high as 74.0%within 60 min in the Mo O₂@CN/PMS system.Electron paramagnetic resonance（EPR）spectroscopy and quenching tests were applied to verify that SO₄·^-and·OH played a major role in the Mo O₂@CN/PMS system.Interestingly,the degradation performance of CBZ was significantly enhanced with high k_obs value of 1.25 min^-1 when Mo O₂@CN was introduced into the Fe²⁺/PMS system,which was about 15.7 times proceeding that of the Fe²⁺/PMS system(0.0797 min^-1).The phenomenon is mainly ascribed that Mo O₂@CN significantly accelerates the transition from Fe³⁺to Fe²⁺,resulting in more·OH production.In order to further improve the catalytic performance of molybdenum-based catalysts while avoiding secondary pollution caused by heterogeneous iron ions.In this paper,Fe（OH）₃was selected as the precursor,the complex of sodium molybdate and dopamine was coated in aqueous solution,and the bimetallic catalyst of Fe-Mo composite was prepared by calcination（Mo₂C@Fe）.Mo₂C@Fe was characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）,Raman spectroscopy（Raman）,etc.The results show that Mo₂C@Fe is composed of coral-like materials,and the catalyst has a high degree of graphitization and conductivity.The Mo₂C@Fe/PMS system could reach 99.5%degradation rate for carbamazepine（CBZ）in 8 min under p H of 6.5 and temperature at 25℃.Moreover,the k_obs of Mo₂C@Fe was calculated to be 0.624 min^-1,about 15.5 times higher than that of the commercial Mo₂C(0.0402min^-1).The total organic carbon（TOC）degradation rate of CBZ was as high as 80.1%within 60 min in the Mo₂C@Fe/PMS system,and it has good removal effect on most common antibiotics,phenols and dyes,it showed that Mo₂C@Fe/PMS system has good application prospects.Combined with EPR technique and free radical quenching experiment,it can be seen that the main active oxygen species in Mo₂C@Fe/PMS system are·OH,·O₂^-and SO₄^·-.The results of XPS analysis showed that molybdenum and iron were the main active sites in the system.Molybdenum and iron can not only react with PMS to generate active oxygen species,but also undergo oxidation and reduction through electron transfer,thus accelerating the degradation of organic pollutants by Mo₂C@Fe/PMS system.