Characterization Of Soil Fulvic Acid Structure And Its Interaction Mechanism With Copper Ions

Soil fulvic acid(FA)is a heterogeneous organic mixture containing various functional groups such as carboxyl,phenolic hydroxyl and amino groups.As one of the important components in soil organic matters,FA not only provides nutrients for soil organisms,but also can bind with environmental pollutants to form complexes.Thus,FA can affect the migration,transformation,and bioavailability of pollutants in environments.Therefore,a comprehensive understanding of the FA structures and their binding mechanisms with specific heavy metals can provide theoretical basis for effectively investigating the soil pollution remediation and the potential risks of environmental pollutants.The FA was successfully divided into five sub-fractions using XAD-8 resin combined with the sodium pyrophosphate buffer solutions with different pH values,which reduced the heterogeneity of FA.The thermogravimetric analysis-Fourier transform infrared spectroscopy-mass spectrometry(TG-FTIR-MS),ultraviolet visible(UV-Vis),synchronous fluorescence spectroscopy(SFS),and excitation-emission matrix(EEM)spectroscopy combined with differential spectroscopy,Gaussian peak model,and two-dimensional correlation spectroscopy(2D COS)were used to investigate the structures and thermal stability of FA,as well as the differences of FA sub-fractions at the perspective of chromophores and fluorophores.Based on the data from the copper titration experiments,the binding properties and mechanisms of FA sub-fractions with copper were investigated.The main conclusions are as follows:1)The pyrolysis components in FA and HA were similar.Compared with HA,FA contained more carboxyl and phenolic hydroxyl and easily degradable components(hemicellulose-like and cellulose-like components),and less refractory components(lignin-like components).Therefore,FA had higher reactivity and provided more abundant active sites for binding with pollutants in environmental media.FA is a component of soil organic matter that is more likely to affect the migration,transformation and bioavailability of pollutants.2)With the elution order of FA sub-fractions,the content of salicylic-like and carboxyl decreased,while the content of phenolic hydroxyl increased.The alkaline buffers could elute more tyrosine-like and tryptophan-like components,while the contents of fulvic-like components decreased continuously with the elution order.3)With increases of copper concentration,the sequential response of fluorophores in acidic-eluting components,neutral-eluting components,and basic-eluting components followed by humic acid-like→fulvic acid-like→tryptophan-like;humic acid-like→fulvic acid-like;and humic acid-like→tryptophan-like→fulvic acid-like→tyrosine-like,respectively.The conditional stability constants(log KCu)of fluorophores in FA sub-fractions were ranged from 4.64 to 5.56,and the order of log KCu values of different fluorophores was consistent with the sequential responses from 2D COS analysis.