Construction Of Polydopamine-modified Bismuth Tungstate Composites And Study Of Visible Light Reduction Of Cr(VI

Hexavalent chromium[Cr（VI）]is a common heavy metal ion in wastewater from textile industry,which poses a great threat to water safety and human health.Photocatalytic technology plays an important role in the field of wastewater treatment.Bismuth tungstate has attracted extensive attention in recent years due to its high stability and visible light response.However,the photocatalytic activity of bismuth tungstate is severely restricted by the high electron-hole recombination rate.In addition,it is difficult to recycle the powder photocatalyst and easy to cause secondary pollution.In order to solve the above-mentioned problems,a series of new photocatalytic materials were developed in this study,and the properties and mechanism of the photocatalytic reduction of Cr（VI）were explored.The details are as follows:Firstly,bismuth self-doped bismuth tungstate(Bi_2.15WO₆,denoted as BWO)was successfully synthesized by the hydrothermal method.Based on ligand-to-metal charge transfer（LMCT）mechanism,polydopamine-modified bismuth tungstate composite（PDA/BWO）with core-shell structure was prepared by using dopamine（DA）with abundant functional groups.The effects of DA concentration,p H,initial concentration of Cr（VI）,catalyst concentration and light intensity on the photocatalytic reduction of Cr（VI）by PDA/BWO were systematically studied.The results show that under the optimal experimental parameters,the reduction ratio of Cr（VI）reaches 99.7%after 90 min reaction under visible light,and the rate constant is0.0701 min^-1,exhibiting good photocatalytic activity.Based on the capture experiments,spectroscopic analysis and electrochemical characterization,it was proposed that the main active substances involved in the reduction of Cr（VI）by PDA/BWO were photogenerated electrons,and the generation pathways of electrons included dye sensitization,LMCT and BWO bandgap excitation.Secondly,in order to solve the poor recycling and easy agglomeration of PDA/BWO powder catalyst,cotton cellulose was dissolved in Na OH/urea system to prepare regenerated cellulose film（RC）.PDA/BWO/RC composite membrane（RCPB）with visible light catalytic activity was synthesized by blending PDA/BWO onto regenerated cellulose film.The effects of p H,film thickness and dosage on the photocatalytic reduction of Cr（VI）by RCPB were investigated by kinetic experiments.The main active substances for Cr（VI）reduction by RCPB were also electrons.In addition,the reduction percent of Cr（VI）can reach 80.4%after repeated use of RCPB for four times,indicating that the film has good stability.Finally,polyacrylonitrile（PAN）nanofiber membrane was introduced to further improve the mechanical properties and recycling of RCPB.The PAN nanofiber membrane was used as the core layer,and the upper and lower layers were coated with RCPB to form the RCPB/PAN/RCPB sandwich structure membrane.Compared with RCPB,the respective tensile strength and elongation at break of the sandwich membrane are increased by 1.1 times and 2.5 times,indicating that RCPB/PAN/RCPB has good mechanical properties.In addition,the reduction percent of Cr（VI）by RCPB/PAN/RCPB maintained at 84.6%after four cycles.The photocatalytic materials developed in this study provide a new method for the treatment of wastewater containing Cr（VI）.