Study On The Mechanism Of Simultaneous Removal Of Organic Micropollutants And Hexavalent Chromium By Visible Light Driven TiO₂

The efficient removal of refractory organic micropollutants and heavy metal ions is the key to advanced sewage treatment and resource utilization.In recent years,the photocatalytic reaction system with titanium dioxide（TiO₂）as the core has attracted much attention in the field of advanced sewage treatment and has very broad application prospects due to its outstanding advantages such as high efficiency,low consumption and environmental protection.Since the visible light has a photon density of more than 50%in the solar spectrum,a lot of research has been devoted to the regulation of the structure and performance of TiO₂ to improve its absorption of visible light and thus enhance the efficiency of catalytic reaction.However,the existing TiO₂ modification strategies and preparation methods are generally complex and costly,which severely limits the practical application of this technical system.Therefore,it is of great theoretical and practical significance to improve the visible light photocatalytic activity of TiO₂ in a simpler way.In this regard,this paper systematically studied the efficiency and catalytic reaction mechanism of TiO₂ without any modification in the simultaneous removal of organic micropollutants and heavy metal hexavalent chromium under visible light,in order to provide new ideas for the development of new visible light catalytic technology.（1）A visible light driven TiO₂ composite catalytic system for simultaneous removal of organic micropollutants and high-priced heavy metal ions was successfully constructed,and the removal characteristics of bisphenol A（BPA）and hexavalent chromium（Cr（Ⅵ））were systematically studied.The results showed that visible light-driven TiO₂ did not have any removal effect when treating BPA or Cr（Ⅵ）solution alone;However,when 50mg/L BPA was mixed with 10 mg/L Cr（Ⅵ）solution,both of them could be effectively removed.After 60 min of visible light driven reaction,the degradation efficiency,reaction rate constant and mineralization degree of BPA were as high as 100%,0.0951 min^-1 and71.91%,respectively.The reduction efficiency and rate constant reached 100%and0.1149 min^-1.At the same time,the removal rate of biological acute toxicity of the BPA and Cr（Ⅵ）mixed solution was as high as 65.27%,and the toxicity to the environment decreased significantly.（2）The influence of key process parameters including initial BPA concentration,TiO₂ Crystal structure,TiO₂ dosage,initial pH value,common inorganic anions,light conditions,organic species and background water quality on the treatment efficiency of the system were systematically investigated.The results showed that increasing the initial concentration of BPA was beneficial to the reduction of Cr（Ⅵ）,and when the concentration ratio of BPA to Cr（Ⅵ）is 5:1,the efficient removal rate of both could be taken into account.Among the three common crystal types of TiO₂,anatase TiO₂ had the most remarkable effect,OPV-P25 TiO₂ was moderate,and rutile TiO₂ was the worst.Acidic conditions were more conducive to the simultaneous removal of BPA and Cr（Ⅵ）,while alkaline conditions significantly inhibited the removal effect of pollutants.Under complex background water quality conditions,the system still had a high effect of simultaneous removal of pollutants.In addition to bisphenol A,many other common organic micropollutants,such as acetaminophen and tetracycline,could be mixed with Cr（Ⅵ）solution to achieve efficient simultaneous removal of organic matter and Cr（Ⅵ）.（3）The catalytic mechanism of visible light driven TiO₂ for simultaneous removal of organic micropollutants and heavy metal Cr（Ⅵ）was analyzed by means of kinetic analysis,correlation evaluation between adsorption and catalytic reaction rate,steady-state fluorescence spectroscopy,in-situ electrochemical characterization and chemical quenching experiments.The specific reaction process is as follows:driven by visible light,charge transfer occurs between the TiO₂ and the organic molecules adsorbed on TiO₂,which generates photo-generated electrons and is transferred from the organic matter to the conduction band of TiO₂.Cr（Ⅵ）as an electron trapping agent can quickly receive the electrons transferred by the conduction band of TiO₂,and then be reduced to Cr（Ⅲ）,and at the same time,it can significantly accelerate the charge transfer efficiency between organic molecules and TiO₂,and finally realize the coordinated and efficient removal of organic matter and Cr（Ⅵ）driven by visible light.