Preparation Of RuO₂/g-C₃N₄ Photocatalyst And Its Visible Light Photocatalytic Reduction Of Ferric And Hexavalent Chromium In Aqueous Solution

Graphite carbon nitride（g-C₃N₄）is a new visible light photocatalyst,due to its narrow band gap（2.7 e V）,wide source of raw materials and simple preparation method.g-C₃N₄has been widely studied in water splitting to hydrogen,degradation of organic pollutants and reduction of heavy metal ions.However,pure g-C₃N₄suffers from high recombinationof the photogenerated carriers.It is very important to develop efficient carrier separation technology to enhance its photocatalytic activity.Iron in the surface environment is mainly in ferric（Fe（Ⅲ））state.When treating sewage by Fenton method,a large amount of iron sludge will also be produced.If Fe（Ⅲ）in the iron sludge can be reduced to more catalytically active ferrous ion(Fe²⁺),it will not only reduce the treatment cost,but also reduce the generation of secondary pollution.Hexavalent chromium（Cr（Ⅵ））is widely used in leather manufacturing,electroplating,printing and dyeing,polishing and other chemical industries.It is a common pollutant in surface water and groundwater.Reducing the extremely toxic and carcinogenic Cr（Ⅵ）to less harmful Cr（Ⅲ）which can easily be precipitated out of solution is the most effective method to treat Cr（Ⅵ）containing wastewater.In this dissertation,g-C₃N₄was prepared from melamine by thermal condensation polymerization.It was modified by deposition of ruthenium oxide on its surface（RuO₂/g-C₃N₄）.The effects of experimental conditions on the photocatalytic reduction of Fe（Ⅲ）and Cr（Ⅵ）in water by pure g-C₃N₄and RuO₂/g-C₃N₄were studied.The reduction and modification mechanisms were analyzed.The main conclusions are as follows:At pH 1.6-2.8,the reduction rate of Fe（Ⅲ）increases with increasing pH,howhere it decreases when the pH is larger than 2.8;The photocatalytic activity of g-C₃N₄was significantly improved when its precursor is treated by nitric acid;The Fe（Ⅲ）reduction rate in the case of RuO₂/g-C₃N₄increases and then decreases with the increase of RuO₂load.For 200 m L of 5.0 mmol/L Fe（Ⅲ）solution,when the initial pH is 2.8 and the dosage of RuO₂（1.0 wt%）/g-C₃N₄is 0.1 g,the reduction rate can reach 18.4%after 90 min.of 40 W 420 nm LEDirridiation.Keeping pH unchanged at2.8,the reduction rate was further increased to 20.1%.For Cr（Ⅵ）,the reduction rate increases with decreasing pH.When 200 mL of0.5 mmol/L Cr（Ⅵ）solution at pH 2.4 and 0.1 g RuO₂（1.0wt%）/g-C₃N₄was added,the Cr（Ⅵ）removal is 88%and it can be reached to 100%in the presence of 0.5mmol/L Fe（Ⅲ）under 90 min.of irradiation.The stability and catalytic mechanism of g-C₃N₄were studied.When methanol was added,the reduction rates of both Fe（Ⅲ）and Cr（Ⅵ）were greatly enhanced,while when electron capture agentssuch asacetonewere added,the reduction rate of Fe（Ⅲ）decreased,indicating that photogenerated electrons are the main active particles responsible for the reductions.UV-vis diffuse reflectance spectroscopy and theoretical analysis showed that RuO₂forms a heterojunction with g-C₃N₄,which reduces the recombination of photogenerated carriers and enhances its photo-catalytic efficiency.The prepared RuO₂/g-C₃N₄was applied for 5 cycles,and no significant reduction in catalytic efficiency was observed.