Study On The Photochemical Behavior Of Fulvic Acid And Sub-fractions And Humic Acid

Fulvic acid(FA)and humic acid(HA)are soluble organic matter fraction of hunmic substances,which are rich in different functional groups,such as carboxylic and phenolic groups,and has strong abilities of migration,transformation,complexation,adsorption and reduction-oxidation.Photodegradation affects the properties of humic substances such as functional group morphology,chemical composition,molecular structure and conformational conformation,which in turn promote or constrain environmental behaviors such as migration,transformation and bioavailability of organic pollutants and heavy metals in the environment.In this study,FA and HA from forest soil were extracted and purified using the XAD-8 resin adsorption technique,and five FA sub-fractions(marked as FA3,FA5,FA7,FA9 and FA13)were separated using the XAD-8 resin combined with the pyrophosphate stepwise elution technique.Based on UV-Vis absorption spectra and excited emission matrix fluorescence,the functional group morphology,structural composition and reaction properties of FA,HA and FA subfractions during photodegradation were explored by combining differential absorption method,Gaussian peak fitting model,fluorescence region integration method,parallel factor analysis and kinetic model.The major research contents were as follows:1)FA and HA exhibit different spectral changes during photodegradation due to the differences in their molecular structures,functional groups and other properties.The differential absorption spectra combined with the Gaussian peak-fitting model had the advantages to highlight the small dynamic changes in the absorption spectra before and after FA and HA irradiation,and the fitted Gaussian characteristic peaks were related to phenolic hydroxyl,carboxyl and phenolic hydroxyl chromophores.The fluorescence spectra of FA and HA showed that different types of fluorescence peaks underwent different degrees of red shift during photodegradation,and the fluorescence peak of HA underwent splitting during irradiation.Three-dimensional fluorescence spectra combined with the PARAFAC model identified four fluorescent components(C1-C4)in FA and HA,which were classified as fulvic-like components,terrestrial humic-like components,microbial humic-like components and protein-like components,respectively.During photodegradation,the F_max of C1-C3 components of FA and C3 components of HA decreased with increasing irradiation time,while the F_max of C1 and C2 components of HA increased with increasing irradiation time.During the photodegradation process of FA and HA,the degradation rate constants obtained from the first-order kinetic indicated that the fulvic-like components degraded at a higher rate than the microbial humic-like components and terrestrial humic-like components.The half-life of each fluorescent components in FA and HA were less than 12 hours,and the stability of each component was terrestrial humic-like components>microbial humic-like components>fulvic-llike components in order.2)There are significant differences in the fluorescence components and physicochemical properties of FA sub-fractions,and the photodegradation process has different degrees of influence on the fluorescence components of FA sub-fractions.Five fluorescence peaks were observed in FA sub-fractions,Peak A and B for humic-like materials,Peak C for fulvic acid-like materials,and Peak D and E for protein-like materials.two protein-like peaks,Peak D and Peak E,which were not found in other sub-fractions,were observed in FA9 and FA13.The positions of the fluorescence peaks in the FA sub-fractions underwent different degrees of red/blue shifts during irradiation.The regional fluorescence response values of the fluorescence regional integration method indicated that fulvic-like and humic-like materials were the major materials of the FA sub-fractions.During photodegradation,the regional fluorescence intensity of FA subfractions representing tryptophan-like,tyrosine-like,humic-like and fulvic-like materials changed to different degrees.Parallel factor analysis identified five fluorescent components in the FA sub-fractions,among which C1 was classified as humic-like components,C2 and C3were classified as fulvic-like components,and C4 and C5 were classified as protein-like components.During the photodegradation process,the F_max of all five fluorescent components decreased to different degrees,and the degradation of fulvic-like and humic-like components was higher than that of protein-like components.During the photodegradation of FA sub-fractions,different types of fluorescent components showed differences in photoreactivity,and the photodegradation rate constants and half-life indicated that the fulvic-like and humic-like components had higher photosensitivity than the protein-like components.