Photocatalytic Inactivation Mechanism Of Non-Noble Metal Modified TiO₂ Supported On A Metal Organic Skeleton Against Karenia Mikimotoi Under Visible Light

Frequent water eutrophication has been threatening the healthy development of biodiversity.In recent years,photocatalytic technology,which has attracted much attention from researchers,has shown great research prospects because it can not only inactivate algal cells in large quantities,but also efficiently degrade algal toxins.In this study,traditional photocatalyst TiO₂ was selected.It was modified by non-precious metal doping（S,N,P）.On this basis,it was loaded on the surface of Cu foundation metal organic skeleton（Cu-MOF）,and SNP-TiO₂@Cu-MOF adsorption-photocatalytic composite material was prepared successfully.In order to solve the problems of powder photocatalyst application,four kinds of floating carriers were selected to prepare floating photocatalysts,which were applied to photocatalytic inactivation of K.mikimotoi under visible light.Through measuring the changes of various growth and metabolic indexes of K.mikimotoi during the photocatalytic process,the algae removal efficiency and inactivation principle of the obtained photocatalysts were explored.The results were as follows:1.Photocatalytic inactivation of K.mikimotoi by SNP-TiO₂ under visible lightThe incorporation of S,N and P reduced the band gap width of TiO₂ from 3.2 eV to 3.08 eV,and the optical absorption range extended to the visible range.It was found that the incorporation of N could enhance the optical absorption capacity of the photocatalysts in the UV region,and the incorporation of P could adjust the crystal size of the photocatalysts,demonstrated the integrity of the doping process.The optimal molar ratio of each element was S_0.7N_1.4P_0.05.Under certain conditions,the inhibition rate of SNP-TiO₂ photocatalyst on the growth of K.mikimotoi was 81.80%.With the high concentration photocatalyst,the photosynthetic pigments in algae cells were degraded,affected the normal operation of the photosynthetic system.At the same time,the catalyst had a certain impact on the oxidation system,thereby accelerated the inactivation of algae cells.2.Photocatalytic inactivation of K.mikimotoi by SNP-TiO₂@Cu-MOF under visible lightThe SNP-TiO₂@Cu-MOF photocatalyst prepared by hydrothermal synthesis was an intact regular octahedron with a particle size of 12-15μm,and the surface of the photocatalyst became rough due to the adhesion of SNP-TiO₂.It contained Cu-O,-COOH,N-O and P-O functional groups,it indicated that the doping process did not change the original functional group type,showed structural stability.The incorporation of S,N,P elements with Cu-MOF materials reduced the intrinsic band gap of TiO₂ to2.82 eV,the visible light absorption capacity of the photocatalyst was improved significantly,and the recombination efficiency of photogenerated electron hole pairs was reduced.The obtained composites were confirmed to be mesoporous adsorption materials,with high specific surface area of 245 m²/g and mesoporous pore size of about 4 nm,wnich was better than Cu-MOF composites prepared by predecessors.Under certain conditions,the visible light photocatalytic inactivation efficiency of SNP-TiO₂@Cu-MOF composite material on K.mikimotoi was 93.75%,the photosynthetic pigment content and protein content of the algae cells were significantly lower than other experimental groups,and the unit enzyme activity was significantly higher than other experimental groups,showed the strongest photocatalytic algaecidal activity.3.Selection of floating photocatalyst carriers and exploration of algal inhibition effectIt was found that the loading between the photocatalyst and the supports was firm,the hydrothermal synthesis method can well retained the octahedral structure of SNP-TiO₂@Cu-MOF photocatalyst,sodium alginate fixation method had the advantages of simple preparation process and low cost.BET results showed that the floating photocatalyst（MF-P）with melamine sponge as the support had the highest specific surface area（28.47 m²/g）,and MF-P also had the best algae removal effect,with the inactivation rate of 96.70%in 1 h and 98.68%in 6 h.All the four photocatalysts showed good recyclability,and the removal efficiency of MF-P could still reach 94.12%after four cycles of experiments.In summary,SNP-TiO₂@Cu-MOF photocatalyst showed excellent activity of visible light photocatalytic algal removal.In the photocatalytic process,the normal metabolic activity of algal cells was destroyed,algal growth was inhibited and eventually led to death.Furthermore,the practical application of the photocatalyst became possible through the loading of photocatalyst on the floating carrier.This study was expected to provide a new idea and reference for the in-situ treatment of harmful algae polluted water.