Effects Of Phosphate On Organic Molecular Fractionation In Goethite-humic Acid Complex

Iron（hydrogen）oxide in soil can immobilize soil organic carbon by adsorption,thus forming mineral-organic complexes.The application of phosphate and organic fertilizers in agricultural activities greatly increases the input of phosphorus in the biosphere,and the coordination exchange between phosphate and goethite competes for adsorption sites of organic matter,which was not conducive to soil organic carbon retention.Therefore,goethite-humic acid complexes were prepared in this study and characterized by X-ray diffraction（XRD）,scanning electron microscope（SEM）,and fourier transform infrared spectroscopy（FTIR）techniques to analyze the influence of the initial C/Fe mole ratio on the phase and microstructure of the complexes.In this study,goethite-humic acid with high OC loading capacity were selected for kinetic experiments and isothermal adsorption experiments.The effects of phosphate on the stability and molecular characteristics of adsorbed organic carbon at different p H values were investigated by TOC total organic carbon analyzer,UV-VIS spectrometry（Uv-vis）,high performance size exclusion chromatography（HPSEC）and three dimensional excitation-emission matrix spectra（3D-EEM）.It provides a theoretical basis for the balance between long-term stability of soil organic carbon and availability of soil phosphorus.The main resultss obtained as follows:（1）The C/Fe molar ratio of the complex increases linearly with the increase of the initial C/Fe molar ratio of the system.When the initial C/Fe molar ratio of the system is 2.0,the C/Fe molar ratio of the correspongding complex is 0.98,and the adsorption site of goethite is saturated.The hydroxyl group on the surface of goethite reacts with the carboxyl group of humic acid to form goethite-humic acid complex,and the presence of humic acid does not change the crystal structure of goethite.The morphology of goethite is needle-like,and the morphology of the complex is massive.With the increase of the initial C/Fe mole ratio,the agglomeration of the complex becomes more obvious.The organic molecules of goethite-humic acid complex are aromatic,protein-like,fulvic acid and humic acid components in humic acid.（2）The addition of exogenous P accelerated the desorption rate of organic molecules on goethite-humic acid complex through hydrophobic distribution,surface complexation,competition for adsorption sites and electrostatic repulsion with negative charge.With the increase of P concentration and the prolongation of reaction time,the distribution of organic molecular weight in the clear liquid shifted to a larger molecular weight,indicating that P promoted the desorption of organic molecules with molecular weight ranging from 3.86 to 12.91 k Da in the complex.The main components of organic molecules desorbed in the complex are protein-like components formed by tyrosine,fulvic acid like components and humic acid-like components.（3）The effect of exogenous P on the desorption strength of organic molecules in goethite-humic acid complex depends on the p H value of the reaction system.PH influences the molecular fractionation of adsorbed organic molecules of goethite by P desorption by influencing the solubility,molecular structure,functional group properties of humic acid and surface charge properties of goethite.The Freundlich isothermal adsorption equation can well describe the adsorption process of P on goethite-humic acid complex.The desorption process of organic carbon on goethite-humic acid complex conforms to first-order kinetic model.The desorption process of OC is divided into two stages:fast reaction and slow reaction.（4）In p H gradient kinetics experiment（reaction time 90～300 min）and p H 4.5isothermal adsorption experiment(initial P concentration 2.5～300 mg·L^-1),the amount of phosphate desorption OC decreased,indicating that P may be affected by ionic bond bridge of Fe（Ⅲ）.The flocculant with bridge structure was formed with negative organic molecules in the natural desorption liquid,which reduced the desorption capacity of humic acid.（5）The p H of the reaction system has a profound influence on the stability of organic components in goethite-humic acid complex.With the increase of p H,exogenous P promotes the desorption of fulvic acid,protein-like and humic acid components in the complex,while the aromatic histone components in the complex are less desorption by P.Exogenous P promoted the desorption of organic moleculeswith molecular weights ranging from 0.26～5.08 k Da,0.16～12.03 k Da and0.13～12.91 k Da at p H 4.5,6.5 and 8.5,respectively.