Behavior And Mechanism Of Adsorption And Photocatalytic Degradation Of Emerging Contaminants In Water By Titanium-based Materials

With the continuous development of society,human activities not only bring technological progress,but also cause serious pollution to the environment.Traditional wastewater treatment technology is difficult to effectively remove emerging contaminants including pharmaceuticals and personal care products（PPCPs）,pesticides and endocrine disrupting chemicals（EDCs）.As a green water treatment technology,adsorption and photocatalysis technology have attracted wide attention.Due to the controllable of structure and composition,good photoelectric response,and excellent sedimentation and separation performance,titanium-based materials show unique advantages in water pollution control.In this paper,the application and mechanism of titanium-based materials for the removal of emerging pollutants in water had been explored,with 17β-estradiol（E2）and ciprofloxacin（CIP）as the target pollutants.Moreover,characterization method and density functional theory（DFT）calculation were combined to discussed that the detail removal mechanism and approach of the removal of emerging contaminants.The specific objects and conclusions were as follows:1)A low-cost composite of activated charcoal supported titanate nanotubes（TNTs@AC）was developed via a facile hydrothermal method,which was applied to remove the17β-estradiol in water matrix,through initial adsorption and subsequent photo-degradation.Characterizations indicated that the modification occurred,i.e.,the titanate nanotubes（TNTs）will be grafted onto the activated charcoal surface and the micro-carbon can modify the tubular structure of TNTs.E2 was rapidly adsorbed onto TNTs@AC and the uptake reached 1.87 mg/g from the dual-mode model fitting.Subsequently,the adsorbed E2 can be degraded 99.8%within2 h under ultraviolet light irradiation.TNTs@AC was attributed unique hybrid structure,providing the hydrophobic effect,π-πinteraction,and capillary condensation for E2 adsorption,and facilitating the electron transfer and then enhancing photocatalytic capacity for E2-degradation.In addition,the removal mechanism of E2 was elucidated through the DFT calculation.2)The adsorption behaviors and mechanisms of CIP with different dissociation species by TNTs were studied through both experimental and theoretical calculations.The multilayered TNTs with high BET surface area（272.3 m²/g）and large pore volume（1.26 cm³/g）exhibited good adsorption property for CIP.The CIP species（i.e.,CIP⁺,CIP^±,CIP^-）at various p H exhibited significantly different adsorption favorability.Adsorption isotherm results revealed that TNTs offered high uptake capacity for CIP⁺(Q_max=464.47μmol/g or 153.90 mg/g at p H 5)than CIP^±and CIP^-.Characterizations indicated the formation of Ti-O-N linkage between CIP molecules and TNTs after adsorption,suggesting the chemical interaction between CIP and TNTs.DFT calculations revealed p H affected the protonation/deprotonation state of CIP,and then changed the distribution of molecular orbitals and the electrostatic potential（ESP）energy of CIP.The ESP value of CIP species follows the trend of CIP⁺（180.57 kcal/mol）>CIP^±（146.78 kcal/mol）>CIP^-（12.30 kcal/mol）,indicating the side of piperazine ring in CIP oriented to TNTs dominates the CIP adsorption.Combining experimental and theoretical results,for the first time,we suggested that ESP energy can serve as the indicator for adsorption ability of the PPCPs molecules with various species.3)In this study,the photodegradation kinetics,transformation products and degradation pathways of CIP with different dissociation species（cations,zwitterions and anions）were investigated in photocatalytic system.Based on DFT calculation,Fukui index interpreted the active sites of different CIP species,and potential energy surface further elucidated the reaction transition state evolution for the radical attack（·OH）.Zwitterionic form of CIP exhibited the highest pseudo-first order rate constant(0.2217±0.0179 min^-1)during photocatalytic degradation.Combining the analysis of high performance liquid chromatography-mass spectrometry and DFT calculation,cationic and anionic CIP mainly experienced the cyclodealkylation of piperazine,while zwitterionic CIP mainly experienced the defluorination and epoxidation of piperazine.Toxicity evaluation showed that most of the transformation intermediates/products decomposed by defluorinated and hydroxylated CIP had low risk to aquatic organisms.