Photocatalytic Activation Of Sulfite And Thiosulfate By Graphite Carbon Nitride For Organic Pollutants Degradation In Aqueous Solution

Advanced oxidation techniques such as Fenton,Fenton and photocatalytic techniques are an effective way to degrade organic pollutants.The photocatalytic oxidation technology bases on graphite carbon nitride（g-C₃N₄）can effectively degrade a variety of organic pollutants under visible light.However,this technology has the disadvantages of low quantum yield and poor activity.Adding oxidants such as hydrogen peroxide,persulfate and ozone is an effective way to increase the activity of g-C₃N₄.The oxidation process of sulfite and thiosulfate can also produce highly active free radicals,and the sources are more extensive and the price is lower.In this thesis,different g-C₃N₄ catalysts are prepared by calcining melamine and urea,which are used to activate sodium bisulfite（Na HSO₃）and sodium thiosulfate（Na₂S₂O₃,STS）under visible light to degrade organic wastewater pollutants.It is found that when these two substances exist,the performance of the g-C₃N₄ photocatalytic system is significantly improved,and the reaction mechanism and structure-activity relationship are further explored.The specific work carried out is summarized as follows:（1）The g-C₃N₄+HSO₃+Vis is system could achieve remarkably enhanced degradation of dyes such as methyl orange（MO）,rhodamine B（Rh B）and acid orange7,and organic pollutants in wastewater such as phenol.The excellent reusability of the metal free catalyst was also observed during ten successive cycles.The effifi-ciency of the system was dependent on the reaction conditions,which fifirst increased and then decreased with the increase of HSO₃^-concentration and initial solution p H.The addition of HCO₃^-stimulated the pollutant degradation,but other water matrix components such as Cl^-and humic acid showed nearly no inflfluence on the reaction.The mechanism researched by electron spin-resonance spectroscopy and free radical suppression experiments shows that HSO₃^-is oxidized by photogenerated holes into SO₃^●-radicals in the system,and photogenerated electrons can promote the reduction of oxygen to produce O₂^●-radicals.SO₃^●-radicals can react with oxygen and superoxide radicals to form SO₅^●-radicals and HSO₅^-;Sulfites or photogenerated electrons can also reduce SO₅^●-radicals to HSO₅^-.HSO₅^-can further generate SO₄^●- radicals through photoelectron reduction,SO₄^●-radicals and O₂^●-radicals together and serve as the main active species in the system for dye degradation.（2）The Mn-doped g-C₃N₄（Mn CN）material was synthesized by a simple roasting method and make it as a catalyst to activate sulfite for MO degradation.Fabricated Mn CN were characterized by various characterization methods.Results showed that Mn exists in divalent and trivalent states and Mn is uniformly dispersed in the structure of g-C₃N₄ mostly in the form of Mn-O-C/N coordination.Compared with g-C₃N₄,the activity of Mn CN is greatly improved,and the first-order reaction rate constant increases from 0.026 min^-1 to 0.16 min^-1.The catalyst also has good repeated use performance.In addition,factors such as the p H of the solution,the amount of Mn doping,the amount of sulfite and the amount of catalyst have an impact on the photocatalytic performance of the Mn CN system.Mechanism studies have shown that the existence of Mn species reduces the recombination probability of photogenerated electrons and holes,and increases the amount of valence electrons,thereby promoting the generation of O₂^●-radicals.（3）STS is activated by g-C₃N₄ under visible light irradiation,and increase the degradation rate of rhodamine B and other pollutants.The performance of g-C₃N₄ prepared from urea was much higher than that from melamine,due to the higher surface area and more negative conduction band potential of the former catalyst.During ten successive cycles,the excellent reusability of the catalyst was also obtained.The effect of different concentrations of STS and g-C₃N₄,and initial solution p H on the performance of the system was also studied.The mechanism study suggests that STS is first oxidized to S₂O₃^●-radicals by photogenerated hole,which will be transformed to other oxysulfur radicals such as SO₃^●-and finally to SO₄^2-ions during the reaction.At the same time,the rate of O₂ reduction by photoelectrons to O₂^●-radicals as well as Rh B degradation increases.The finding of this study provides a promising advanced oxidation process for organic pollutants degradation via STS activation.