Study On The Photocatalytic Reduction Of Tc（Ⅶ）/Re（Ⅶ） In Amorphous TiO₂/g-C₃N₄ Composites

Technetium-99(⁹⁹Tc),as a typically long-livedβemitting radionuclide,mainly exists in the form of highly soluble anion（Tc O₄^-）,which is hard to be absorbed by natural soils and minerals and therefore easily to migrate in the environment,posing a certain long term threat to the human ecosystem.As an efficient and sustainable envi-ronmental remediation method based on solar energy conversion,photocatalysis has been widely used in the removal of heavy metal ions,and it is expected to play im-portant roles in the practical treatment of radionuclides.In this dissertation,taking into account of drawbacks of low photogenerated carrier utilization rate and non-visible light absorption of amorphous TiO₂,heterojunction composite photocatalytic materials were prepared by combining amorphous TiO₂ with g-C₃N₄.In addition,the related structures and performancesof as-synthesized materials were characterizied.The pho-tocatalytic reduction behaviors and related mechanism of Re（Ⅶ）（Re as a chemical analogue of Tc）with composite materials were systematically explored and the major research results are as follows:First of all,a series of amorphous TiO₂/g-C₃N₄（TCN）composites were fabricated by a facile hydrolysis route.The obtained amorphous TiO₂/30 wt%g-C₃N₄（TCN-3）photocatalysts has a tremendous specific surface area(～456 m² g^-1),narrow band gap（～2.67 e V）and a high utilization rate of photogenerated carriers,which can provide more active sites,better light capturing performance and excellent photocatalytic activ-ity.EPR of TCN-3 showed enhanced DMPO-·OH and DMPO-·O₂^-signals,proving that the heterojunction type is Z-scheme heterojunction,thus exhibiting strong transient photocurrent density and low photoluminescence spectroscopy（PL）intensity and elec-trochemical impedance（EIS）.Next,the photocatalytic performance of the as-prepared amorphous TiO₂/g-C₃N₄（TCN）composites were evaluated by the photocatalytic reduction of Re（Ⅶ）.The ef-fects of different amounts of carbon nitride and p H on the removal efficiency of photo-catalytic reduction were studied.It was found that,at p H 3,TCN-3 displayed excellent photocatalytic reduction activity toward Re（Ⅶ）in air（90%）and the material still maintains good stability after 8 cycles.XPS and XAFS results showed that Re（Ⅶ）was reduced to Re（Ⅳ）,and the re-dissolution process slowed down after the reduction.Be-cause of both shortening of Re O₂·n H₂O chains（decrease in coordination number of Re-Re）and association of Re（Ⅳ）with amorphous TiO₂ in TCN-3（formation of Re-Ti bond）,the reoxidation and dissolution of Re（Ⅳ）is effectively hindered,which is fa-vorable for the removal of Re（Ⅶ）/Tc（Ⅶ）.Finally,to clarify the effect of different N sources on the photocatalytic reduction of Re（Ⅶ）in amorphous TiO₂/g-C₃N₄（TCN）composites,g-C₃N₄ were prepared via a thermal decomposition of three precursors（Urea,Thiourea and Melamine）.The exper-imental results show that U-TCN（Urea as the N source）has a more uniform appearance,the largest specific surface area（474 m²/g）,and the highest light absorption perfor-mance.The removal percentage（90%）of U-TCN photocatalytic reduction Re（Ⅶ）is superior to that of T-TCN（20%）and M-TCN（15%）.