Recovery And Photocatalytic Performance Of TiO₂ Nano Materials From Titanium-coagulated Algae-rich Sludge

Recently,algae pollution was frequently resulted from water eutrophication which attributed to a large amount of nitrogen,phosphorus and other nutrients from industrial wastewater and domestic sewage discharged into the water environment.Coagulation has gained widespread research in algae-rich surface water purification as a common method.Conventional aluminum-based and iron-based salts as universal coagulants.While,the toxicity of residual aluminum and high effluent turbidity of iron-based coagulants after coagulation were reported to pose a chronic threat to human health.Moreover,there was a non-negligible potential disadvantage of the application for conventional coagulants in algae-rich surface water treatment:a large quantity of algae-rich coagulation sludge will be generated by algae flocs through the sedimentation process after coagulation.At present,more space requirement of the landfill site and over half of the total annual cost of sewage treatment plants are involved in complex sludge disposal process.In addition,a large amount of Microcystins and algal organics which will be accumulated in the algae-rich coagulation sludge on account of release characteristics of algae in the sedimentation process are reported to be harmful to aquatic life and soil environment.Consequently,not only there is an urgent need to develop efficient and innocuous coagulants for water treatment,but also it is increasingly perceived as one of the most severe environmental challenge to rationally dispose algae-rich coagulation sludge.This study proposed polytitanium coagulation for algae-laden surface water treatment,followed by the algae-rich coagulated sludge recovery and reuse.（1）Polytitanium coagulation had advantages over conventional Al-and Fe-coagulation in terms of synergetic removal of algae and organics.（2）Anatase nano-TiO2 spheres were produced from PTC coagulated sludge under high-temperature incineration.Compared with commercially available TiO₂,the nano-TiO₂ was more capable for both Cr（VI）reduction and phenol oxidation in aqueous solutions.Impurities,i.e.,carbon,Mg,Al,Fe,on the surface of nano-TiO2,were speculated to accelerate the separation of hole-electron pairs and the transportation rate of electrons,and therefore improved the photocatalytic activity of nano-TiO2.Moreover,compound contamination caused by chromium and phenol could be degraded by the nano-TiO₂,as indicated by synergetic removal of Cr（VI）and phenol up to 99%when they coexisted.（3）the titanium-coagulated algae-rich sludge was recycled for the production of functional TiO₂nanoflowers through simple hydrothermal and calcination process.Anatase nano-TiO2（particle size of 10–15 nm）with petal-like structures（mesoporous）,relatively high specific surface areas,i.e.299.4 m²g^-1,and low band gaps,i.e.2.67 eV（compared to P-25）,were obtained.Additionally,oxygen vacancy was generated on the surface of the recycled TiO2nanoflowers based on electron paramagnetic resonance results,which were verified by the first-principles calculations within density-functional theory.The recycled TiO2 nanoflowers display high photocatalytic performance for the degradation of diverse phenolic organic contaminants,such as bisphenol A,diphenyl phenol,p-tert-butyl phenol,and resorcinol,i.e.>95%,under mild ultraviolet light irradiation and without any sacrificial reagents.Formation of oxygen vacancy on the recycled TiO2 nanoflowers not only efficiently inhibited the recombination of photo-generated electrons and holes but also facilitated contaminant adsorption and photo-generated electron transfer on the surface of the recycled TiO₂nanoflowers,thereby promoting the generation of holes and hydroxyl and superoxide radicals which were regarded as the reactive oxygen species for attacking contaminants in the reactions.This study utilises polytitanium coagulation as an alternative to conventional Al and Fe coagulation,presenting expanded feasibility and a new perspective on recycling chemical-coagulated sludge for producing functional nanomaterials as photocatalysts with remarkable photocatalytic performance and stability.This study has great impetus to suit for practical application in the resource recycling and water purification.