Theoretical Study On The Degradation Reaction Mechanism And The Optoelectronic Properties Of Metal Complexes

Perfluorooctanoic acid?PFOA?,as a typical perfluorocarboxylic acid?PFCAs?compounds,is one kind of emerging persistent pollutants appearing in natural water.It is of theoretical guiding significance to research the degradation mechanism of PFOA for experimentally exploring effective degradation methods.In this paper,the thermal degradation reaction mechanism of PFOA,the degradation mechanism of elimination fluorine from PFPA with metal-catalyzed·OH attack and the structure,electronic property,ultraviolet spectrum of PFOA metal complexes were studied systematically with the quantum methods.It is expected providing some theoretical datas for the researching on degradation methods of PFCAs.?1?The mechanism of elimination hydrogen fluoride from PFOA was studied by Gaussian09 ONIOM layered calculation method.Seven possible reaction channels are obtained at the B3LYP/aug-cc-PVTZ//B3LYP/6-31G?d,p?+ZPVE levels.The results show that the thermal degradation reaction of PFOA can proceed through the six-membered ring mechanism,five-membered ring mechanism,direct CO/CO₂/CF₂ elimination mechanism.The process of generating CO and CF₃?CF₂?₅CFO via a five-membered ring transition state has relatively low activation energy,thus becoming the main channel for reaction,but the production rate of CO₂ and CF₃?CF₂?₄CF=CF₂ is low.The rate-determining step is the process of eliminating HF and forming a epoxy intermediate for the H-isomerized product of PFOA through five-membered ring transition state with the energy barrier of 183.4 kJ/mol.Theoretical prediction of the main product is consistent with the experimental results.The results also show that the strong acidic conditions are beneficial to the degradation of PFOA.?2?The defluorinated degradation mechanism of perfluoropropionic acid?PFPA?with metal-catalyzed·OH attack was studied by using Gaussian09calculation method.The gradual degradation reaction mechanism of PFPA was obtained at the B3LYP/aug-cc-PVTZ//B3LYP/6-31G?d,p?+ZPVE levels.The results show that the thermal degradation reaction of PFPA can proceed through isomerization of H,elimination of HF,elimination of CO,combination of·OH and then elimination of HF.The rate-determining step is the process of eliminating HF and forming a epoxy intermediate for the H-isomerized product of PFOA through five-membered ring transition state with the energy barrier of 194.2 kJ/mol.Degradation intermediates are mainly short-chain perfluorinated carboxylic acids?PFCAs?and F^-,the ultimate degradation products are CO₂ and F^-,As a result,PFPA was decomposed step by step.It can effectively promote the degradation of PFPA after adding Fe³⁺with activation of the C-F bond and reduction of activation energy.Other metal ions such as Zn²⁺,Mg²⁺,Cu²⁺,Ni²⁺,Co²⁺activate the C-F bond in varying degrees.Fe³⁺and Cu²⁺have best catalytic activity,which have significant effect on defluorinated degradation of PFPA.Co²⁺and Ni²⁺increase the degradation rate of PFPA to a certain extent.The effects of Mg²⁺and Zn²⁺are not very good.?3?Theoretical study on the optoelectronic properties of PFOA and metal complexes was investigated by using time-dependent density functional theory DFT/TD-DFT.The electrostatic potential,frontier molecular orbital,energy gap,ionization energy,electron affinity,excitation energy and absorption spectra of PFOA and[C₇F₁₅COO-Fe]²⁺were systematically calculated.The results show that the energy gap of[C₇F₁₅COO-Fe]²⁺complexes is significantly reduced from 7.08 eV of the original PFOA to6.53eV of[C₇F₁₅COO-Fe]²⁺under the influence of Fe³⁺.The reduction of required transition energy leads to the increasing of the absorption wavelength of[C₇F₁₅COO-Fe]²⁺compared with PFOA,resulting in a characteristic absorption peak of 286 nm.Fe³⁺is significantly effective on the photocatalytic degradation of PFOA.The effects of Cu²⁺,Co²⁺and Ni²⁺follow.The absorption peaks of Zn²⁺,Mg²⁺and Mn²⁺locate near 200 nm,which are ineffective on the degradation of PFOA.The degradation effect of Ti⁴⁺is not obvious.