Effect Of Phosphate On Organic Molecular Fractionation In The Ferrihydrite-Humic Acid Coprecipitate Complex

Ferrihydrite and organic matter are pervasive in soil,sediment and aquatic systems,forming organic-mineral complexes by co-precipitation,which plays an important role in the preservation and stabilization of organic carbon.Phosphate(PO43-)has a strong affinity for ferrihydrite,and its adsorption on ferrihydrite can be of fundamental importance in the fixation and storage of soil organic matter(SOM).However,there is a lack of relevant studies on the stability of ferrihydrite-coprecipitated organic carbon(OM),and the molecular behavioral characteristics of co-precipitated OM under the addition of phosphorus have not yet received sufficient attention.Therefore,a series of ferrihydrite-humic acid complexes with C/Fe molar ratios were prepared by co-precipitation using humic acid as a representative of SOM.And the structure and selective binding characteristics of the complexes were elucidated by mineral characterization and molecular analysis techniques.On this basis,batch isothermal and kinetic adsorption experiments were carried out,and the structure,components and molecular weight of organic molecules in solution before and after the reaction were characterized by ultraviolet spectrophotometer(Uv-vis),three-dimensional fluorescence spectrometer(3D-EEM)and high performance liquid-volume exclusion chromatography(HPSEC)to reveal the influence of phosphate on the stability and molecular fractionation of organic carbon in the complexes at different pH The main results of the study are as follows:1.Ferrihydrite selectively coprecipitates large molecular weight aromatic humic acid-like and fulvic acid-like(0.65-20 k Da)organic molecules from humic acid,and less from protein-like and small molecular weight organic molecules(0.34-0.65 k Da).Infrared spectroscopy(FTIR)analysis showed that humic acid formed co-precipitation complexes through coordination exchange of carboxyl groups with hydroxyl groups of hydrite;X-ray diffraction(XRD)and scanning electron microscopy(SEM)results showed that humic acid did not change the weak crystalline structure and irregular irregular block morphology of ferrihydrite with increasing initial C/Fe molar ratio.2.The competition of P for adsorption sites through ligand exchange complexation and the electrostatic repulsion of negative charge influenced the desorption of organic molecules from the ferrihydrite-humic acid co-precipitation complex.Under the condition pH 6.5 and pH 8.5,the electrostatic repulsion of phosphate may dominate and increase the desorption of humic acid.The TOC concentration of the reaction solution increased by 49.75 and 3.36 % with increasing initial P concentration,respectively.And under pH 4.5,phosphate may also form a bridge-type structure of flocculation with naturally desorbed,negatively charged organic carbon in the clear liquor through the ion-bonded bridging effect of Fe3+,reducing the desorption concentration of humic acid,and the TOC of the clear liquor decreased by about 31.14 % with the increase of initial P concentration.3.Phosphate enhanced the desorption fractionation of organic molecules on the ferrihydrite-humic acid co-precipitation complex,which facilitated the desorption of organic molecules of larger molecular weight.The organic molecular weight distribution of the reaction solution at different pH and phosphate concentrations was measured by HPSEC,and the results showed that with the increasing of exogenous phosphorus concentration(0-155 mg/L P)and the increase of reaction time,the molecular weight distribution of organic molecules in the clear liquor shifted to the direction of larger molecular weight,and the number average molecular weight and heavy average molecular weight of organic molecules increased,indicating that the humic acid with larger relative molecular weight were desorbed down gradually.4.The effect of P on the molecular fractionation of co-precipitated humic acid depends on the pH of the reaction system.At the pH 4.5,P promoted the desorption of fulvic acid-like and aromatic amino acid(0.7-3.0 k Da)on the complex.At pH 6.5,P may favor the desorption of fulvic acid-like or aliphatic compounds(4.0-6.0 k Da)on the complex.And at pH 8.5,P mainly promoted the desorption of aromatic humic acid-like fractions of 4.0-20.0 k Da on the complex.