Mechanism Of Hydrated Electron Generation By UV Photolysis Of Phenol And P-benzoquinone In Aqueous Solution

As an important part in natural waters,chromophoric dissolved organic matter?CDOM?can be the source of hydrated electron(e_aq^- )with the assistance of steady-state and laser flash photolysis techniques.The isolated e_aq^- from the photolysis of CDOM can be used to destroy hydrophobic chlorinated compounds,which indicates that the establishment of novel advanced reduction processes?ARPs?can be based on the photolysis of CDOM,without exogenous addition of other reductants.Moreover,as a common component of CDOM,phenolic compounds are well-known to eject e_aq^- q via the associated photolysis,where phenolic compounds act as the electron donors.While,the mechanism of the photolysis of phenolic compounds in aqueous solution is still a matter of debate,which warrants further studies.Firstly,a UV/C₆H₅OH process to generate e_aq^- was experimentally and theoretically studied,where monochloroacetic acid?MCAA?was selected as the probe of e_aq^- .It was demonstrated that the e_aq^- generation efficiency was dependent on the phenol concentration and p H.To interpret the dependence,a mechanism for the generation of e_aq^- from phenol was proposed and confirmed by the quantum chemical calculations.Theoretically,phenol could eject e_aq^- and phenoxyl radical?C₆H₅O^·?,followed by the addition of hydroxyl ion?OH^-?to C₆H₅O^·,and the simultaneous formation of phenol and p-hydroquinone was accomplished by hydrogen abstraction of the adduct with C6H5O^·as hydrogen acceptor?period I?.The generated p-hydroquinone could also release e_aq^- with p-benzoquinone?p-BQ?as the product?period II?.Totally,one mole of phenol could generate four moles of e_aq^- via two periods,and two moles were generated in period I and two moles were in period II.Experimentally,e_aq^- could be ejected from phenol and phenolate,and the molar ratios of the species were determined by p H.Kinetically,the energy barriers of the electron release from phenol and phenolate were 63.7 kcal·mol^-1and 62.3 kcal·mol^-1,respectively,which confirmed that the generation of e_aq^- from phenolate was much more efficient than that from phenol.Secondly,based on the discussion above,it's obvious that p-BQ was the primary product of the UV/C₆H₅OH process.Thus,a UV/p-BQ process to induce e_aq^- q generation was predicted by quantum chemical calculations and validated by experiment.Theoretically,the photolysis of p-BQ under UV irradiation at 253.7 nm could induce water to generate e_aq^- with a molar ratio of 1:2 via the direct triplet mechanism,in which 1,4-addition reaction of the first triplet state of p-BQ with water was the key step.Experimentally,MCAA(the probe of e_aq^- )was used to detect e_aq^- q generated in the UV/p-BQ process.The generation efficiency showed a positive linear dependence on the p-BQ concentration,which illustrated the crucial role of p-BQ on the generation of e_aq^- .During the photolysis,p-hydroquinone was the primary intermediate for the generation of e_aq^- .Kinetically,the energy barriers of the e_aq^- generation from p-HOC₆H₄OH,p-HOC₆H₄O^-and p-^-OC₆H₄O^-were 100.8 kcal·mol^-1,46.5 kcal·mol^-1and 5.6 kcal·mol^-1,respectively.Both the experimental and theoretical results show that the generation of e_aq^- was much more efficient from the anions than that from p-HOC₆H₄OH.Finally,the effect of alcohols as hydrogen donors on e_aq^- generation from p-BQ photolysis was investigated by quantum chemical calculations and experiments to further understand the underlying mechanisms associated within the UV/p-BQ process.Theoretically,the UV photolysis of p-BQ at 253.7 nm in the presence of H₂O could induce the formation of p-hydroquinone and hydroxy-p-benzoquinone,which was almost independent of the addition of CH3 OH.While,the first triplet state of hydroxy-p-benzoquinone preferred to react with CH3OH?rather than H₂O?to release the corresponding semiquinone radical and hydroxymethyl radical?^·CH₂OH?.These two radicals could induce the reduction of p-BQ to p-benzosemiquinone radical and thus enhanced the formation of p-hydroquinone as the precursor of e_aq^- .Experimentally,the detection of e_aq^- generated in the process was accomplished by the degradation of MCAA(the probe of e_aq^- ).With the addition of CH₃OH,the degradation of MCAA was accelerated in the UV/p-BQ process.A similar acceleration was also observed by the addition of ethanol or 2-propanol with the?-H.However,tert-butanol,which is without the?-H in the structure,didn't induce the acceleration.The findings in this study may therefore help to elucidate the mechanism of e_aq^- q generation from the natural phenolic compounds as well as quinones,which make it possible to find the environmentally benign compounds?e.g.,plant phenolics?to establish novel e_aq^- based ARPs.