Dft Studies Of Adsorption Behivior Of Persistent Organic Pollutants （POPs） PFOS/PFOA On Anatase TiO₂Surfaces

It is an important part of environment protection to get rid of persistent organicpollutants, while PFOS and PFOA belong to a sorts of organic pollutants with higherpersistence and bioaccumulation than organochlorine pesticides and dioxin, and it isof great significance to study the function mechanism of PFOS and water-solidinterface from molecular level to investigate the environmental trend of PFOS anddevelop high-efficiency adsorption technology.Polyfluorinated compounds （PFCs） including perfluorooctane sulfonate （PFOS）and perfluorooctanoic acid （PFOA） were studied with DFT-B3LYP method. First ofall, adsorption behavior of PFOS （perfluorooctane sulfonate） on anatase TiO₂surfacesin water environment was primarily studied with polar solvent model, microscopicfeatures of the PFOS/anatase system were investigated by discussing structural,electronic, and thermodynamic properties of the products. Furthermore, influences ofpH on PFOS adsorption behavior were also analysed. Moreover, in an effort to betterunderstand linear PFOA and its trifluoromethyl-branched isomers （i.e.1-CF3-to5-CF3-PFOA）, discussions were focused on finding relations between the arrangementof the atoms in the various isomers and the calculated electronic characters（i.e.potential maps, highest occupied and lowest unoccupied molecular orbitals and theirHOMO-LUMO gaps）. In addition, the reasons why distinctions existed among thedifferent PFOA isomers were analysed by calculations of their thermodynamicproperties. The main results are as follows:（1） Surface complexes of PFOS and TiO₂surface could be formed by two path, onewas sharing oxygen atoms with the surface, the other was absorbed by hydrogen-bondinteraction, which were belonged to chemisorption and physisorption respectively.Calculations of adsorption energy（Eads） and Gibbs free energy（ΔGads） was H-bonded（hydrogen bonded adsorption）> MM1（reacted with one-H2O of surface）>BB1（reacted with two-H2O of surface）>MM2（reacted with one-OH of surface）>BB2（reacted with one-H2O and one-OH of surface）, that is, physisorptionwas easier to occur. MM1surface complex was the most thermodynamically favorablechemi-adsorption mode. Results of bond length analysis indicated that interactionbetween-H2O/-OH function group on TiO₂surface and sulfonate group in PFOSresulted in the formation of moderate intensity hydrogen bond.（2） Influences of pH on PFOS adsorption behavior were definitely significant. Weconcluded that it was hard to form bidentate binuclear （BB） complexes for PFOS andTiO₂surface in pH range considered, the most thermodynamically favorablechemisorption surface complex were monodentate mononuclear （MM） complexes atlow and intermediate pH, and there no chemisorption complexes formed under highpH condition. H-bonded （Hb） PFOS adsorption was predicted to be energeticallyfavorable, at low pH, and the thermodynamic favorability of hydrogen bondedadsorption was weaker and weaker with the increasing of pH value.（3）PFOA in linear form was in an helical conformation. Calculated results of thebranched PFOA suggested that the position of the trifluoromethyl-brancheddetermined the shape of the molecules. The Gibbs free energy（G） indicated thatmolecules with the most regular atomic arrangement were the most stable structures.In the protonated and sodium salt of linear and branched PFOA, insignificant changeswere discovered with respect to their anionic counterparts. All of the protonated andsodium salt compounds were not capable of reacting with charged species because oftheir electronic densities were evenly distributed along their molecular structures. Inaddition, from the study, we find LUMO orbitals of some branch isomers anion werewell localized and accessible, indicating that they are able to interact with radicals.