Performance And Mechanisms Of Metal Sulfide Modified TiO₂ Nanotubes Photocatalytic Reduction Of Hexavalent Uranium

As a sustainable clean energy,nuclear energy plays an indispensable role in human industrial production and daily life.In the nuclear fuel cycle,such as uranium mining and smelting,nuclear fuel processing,and spent fuel post-processing,a large amount of uranium-containing radioactive waste water will inevitably be produced,which may break human health and aquatic ecosystems.The commonly existing valence state of uranium in radioactive wastewater are U（Ⅵ）and U（IV）.U（Ⅵ）is the predominant form of uranium.Due to the difference in solubility of U（Ⅵ）and U（IV）,reducing the soluble and easy migration of U（Ⅵ）to insoluble U（IV）precipitation has been considered as a promising strategy for the elimination of radioactive contamination.Photocatalytic reduction technology has been regarded as one of the effective patterns to deal with the energy crisis and water pollution due to the advantages of mild reaction conditions and no secondary pollution.Ti O₂,especially one-dimensional Ti O₂nanotubes arrays（TNTAs）semiconductor with electrical and optical properties,has been wildly utilized in water treatment.Unfortunately,the rapid recombination of photogenerated charge carriers and intrinsically wide band gap（3.2 e V）significantly constrain its photocatalytic activity.To address these issues,coupling TNTAs with different narrow band gap metal sulfides to prepare new high-efficiency photocatalysts that respond to visible light and apply them to the photoreduction and removal of U（Ⅵ）is of great significance to the treatment of uranium-containing radioactive wastewater.The specific research contents of paper were as follows:（1）Herein,CdS/TNTAs heterojunction photocatalyst were synthesized readily via an anodized method and successive ionic layer adsorption reaction method（SILAR）.The investigations of scanning electron microscopy（SEM）and transmission electron microscopy（TEM）indicated that CdS nanoparticles successfully loaded onto TNTAs.UV–vis diffuse reflectance spectra showed the band gap of CdS/TNTAs was narrowed and improved the visible light absorption.Photo-luminescence,transient photocurrent responses and electrochemical impedance spectroscopy proved that the CdS-sensitized heterojunction structure formed by TNTAs can effectively improve electron transmission and inhibit the recombination of electrons and holes,whereby the photocatalytic removal of U（Ⅵ）is greatly improved.The experimental results suggest that the high photocatalytic activity can be attributed to the well-matched band energy edge of the heterojunction interfaces literally establish efficient electron transportation channels.Furthermore,the possible photocatalytic electron transfer path and the photocatalytic mechanism of CdS/TNTAs heterojunction was proposed.（2）CuS/TNTAs nanocomposite with p-n type heterostructure was developed to further improve the density and separation performance of photo-induced carriers.The CuS/TNTAs-10 with 20.59 wt%CuS exhibited the best photocatalytic activity for reduction of U（Ⅵ）（94%at 180 min）and the photocatalytic activity was still 87%after 5cycles.These improvements probably stem from the porous tubular structure of TNTAs acts as the protective shell which can effectively inhibit the photocorrosion of CuS and promote the photoinduced interfacial electron-hole pairs separation.Furthermore,the possible the photocatalytic electron transfer path was proposed,and the photogenerated electrons（e^-）and the superoxide radicals（O₂^-）are the predominant active free radicals.（3）In order to realize the utilization of near infrared light（NIR）,Ag₂S/TNTAs heterojunction photocatalyst were designed and synthesized.Current results suggested that Ag₂S/TNTAs displayed excellent photoelectrochemical performance,due to its combination with Ag₂S effectively extended the optical response range and promoted separation and transfer of photoinduced carriers.Ag₂S/TNTAs-7 exhibited excellent photoreduction activity,and the removal efficiency was up to 94%at the optimal p H 6.0and the solid-to-liquid ratio 0.4 g/L.The existence form of photocatalytic reduction of U（Ⅵ）is（UO₂）O₂·₂H₂O by using X-ray photoelectron spectroscopy（XPS）and X-ray diffraction（XRD）.