| Humic acids(HA),the major components of dissolved organic matter(DOM),are widely distributed in aquatic environment,and play an important role in photochemistry because they are one of the most important natural sunlight-absorbing components.HA can affect the transfer and transformation of persistent toxic substances(PTS) in natural water under sunlight irradiation.Iron,a ubiquitous element in natural water,can complex with HA to form HA-Fe(Ⅲ) complex which could affect the photodegradation of PTS.However,the relationship between photochemical activity and HA's structure hasn't been well established due to HA's complexity.Therefore,it is important to separate HA according to structure and functional group using novel and effective methods,and to study photolysis pathway of PTS in solutions containing HA fractions and Fe(Ⅲ).In this dissertation,the separation of HA and the effects of their fractions on the photodegradation of 2,4-dichlorophenoxyacetic acid (2,4-D) under simulated sunlight were investigated,and the main results are as following:(1) The interactions of HA with Fe(Ⅲ) were characterized by Fourier transform infrared spectroscopy(FTIR) spectra,ultraviolet-visible(UV-vis) absorption spectra and fluorescence spectra,indicating the formation of HA-Fe(Ⅲ) complex.Electron paramagnetic resonance (EPR) spectra show·OH radicals are generated with pulsed laser illumination atλ=355 nm in solutions containing HA,Fe(Ⅲ) and HA-Fe(Ⅲ) complex.Under the simulated sunlight irradiation(500 W Xe lamp,λ＞290 nm),the photodegradation of 2,4-D followed the pseudo-first-order reaction kinetics.The pseudo-first-order rate constant of 2,4-D photodegradation with the presence of only 2,4-D(2 mg/L) was 0.007 h-1.In the presence of HA(5 mg/L),Fe(Ⅲ)(0.2 mmol/L) and HA-Fe(Ⅲ) complex,the rate constants of 2,4-D degradation were 0.004,0.034,and 0.046 h-1,respectively.The results indicated that in the coexistence of HA and Fe(Ⅲ),the formed HA-Fe(Ⅲ) complex showed better increased effect on the photodegradation of 2,4-D and had better photo-catalytic activity.(2) Based on the different polarity,HA was separated into four fractions(FA,FB,FC,and FD) from weak to strong by silica gel chromatogram using different volume proportions of ethanol and water as the mobile phase.Elemental analysis,UV-vis absorption spectra and FTIR spectra showed that the fractions FC and FD possessed more aromatic C=C content.The influences of HA and its fractions on the photolysis were investigated by the photodegradation of 2,4-D solutions under 500 W Xe lamp irradiation.All the bulk HA and its fractions decreased the degradation rates of 2,4-D.The fractions of strong polarity FC and FD retarded the degradation rate more than the fractions of weak polarity FA and FB,indicating the photochemical activity of HA depended on its structure and functional group.(3) Copper phthalocyanine(CuPc) imprinted polymers(MIP) were synthesized successfully and employed to separate phthalocyanine-like(Pc-like) substances from HA using MIP technique effectively.EPR spectra indicated that more·OH radicals were generated in solution of the Pc-like fraction(Fmip-b) under light irradiation(λ=355 nm).Fmip-b presented better photochemical activity for degradation of 2,4-D when irradiated by simulated sunlight(Xe lamp phytotron,λ＞290 nm).The pseudo-first-order rate constant of 2,4-D photodegradation in the presence of Fmip-b was 2.5 times as high as that the presence of effluent fraction(Feff).(4) Chlorophyll-MIP,hemin-MIP,and meso-tetraphenylporphine-MIP(TPP-MIP) were employed to separate structure-like substances from HA,respectively.Furthermore,the integration of chlorophyll-MIP,hemin-MIP,and TPP-MIP was used to separate extendedπ-conjugated porphyrin-like substances from HA.Under the simulated sunlight irradiation (Xe lamp phytotron,λ＞290 nm),the porphyrin-like fraction(Fmip-b) in HA presented better photochemical activity for degradation of 2,4-D than effluent fraction(Feff).According to these results,separating HA using silica gel chromatogram or MIP technique was feasibile,and the fractions with better photochemical activity were separated from HA effectively.Our results also indicated the different fractions of HA affected geochemistry process of other coexisting organic compounds evidently.These works are helpful for understanding the mechanism of PTS photodegradation in the presence of HA in nature.
|
PDF Full Text Request