The Characteristics Of Phosphate Adsorption On Aluminum-substituted Ferrihydrite And CD-MUSIC Modeling

Ferrihydrite is a typical amorphous iron oxide in the soil,which can strongly adsorb various pollutants and nutrients in the environment.It played critical roles in the migration of nutrients,trace elements and pollutants in the environment.Aluminum substitution in ferrihydrite is a common phenomenon in soil,and it led to changes in the physical and chemical properties of ferrihydrite and adsorption characteristics of environmental anions.Phosphorus is an essential macronutrient for plant growth,whereas little phosphorus is available in most soils.Because most available phosphorus was fixed by soil active components such as iron oxides,resulting in a high content of total phosphorus but a lack of available phosphorus.Therefore,it is significant to study the adsorption behavior and mechanism of phosphorus on the aluminum-substituted ferrihydrite.The CD-MUSIC model is widely used to describe the adsorption behavior of ions on the surface of iron oxides,while the parameters of model need to be adjusted when the structure of ferrihydrite is doped with aluminum.In this study,four types of aluminum-substituted ferrihydrite were synthesized in the laboratory,and the structure and chemical composition of the aluminum-substituted ferrihydrite were characterized by X-ray diffraction(XRD)?transmission electron microscopy(TEM)and inductively coupled plasma-optical emission spectrophotometry(ICP-OES)techniques;the charging curve is performed by the potentiometric titration technique;phosphate adsorption on the aluminumsubstituted ferrihydrite at different p H values is conducted via batch adsorption experiments;CD-MUSIC model was used to fit proton adsorption and phosphorus adsorption data,and obtained the model parameters and the morphological distribution of surface complexes;phosphate speciation on the surface of ferrihydrite was obtained through in situ Fourier transform infrared(ATR-FTIR)spectroscopy,and the morphological type and quantification of surface phosphate complexes are obtained by peak fitting and two-dimensional correlation spectroscopy(2D-COS)analysis.The main results of this study are summarized as follows:1.Aluminum-substituted ferrihydrite(F-0,F-10,F-20 and F-30)were synthesized by uniform precipitation method,and the amount of aluminum-substituted ferrihydrite measured by dissolution experiment was 0,11.6,19.1 and 25.6 mol%,respectively.TEM showed that aluminum substitution hardly changed the morphology of ferrihydrite particles,and it still existed as weak crystals and strong aggregates.Aluminum substitution reduced the crystal size of ferrihydrite nanoparticles and increased the surface charge density and point of zero charge(PZC).2.Phosphate adsorption capacity onto aluminum-substituted ferrihydrite decreased with the increase of p H.Aluminum substitution increased the phosphate adsorption capacity.There was no significant difference for the phosphate adsorption capacity onto F-0,F-10 and F-20,while 30% aluminum substitution significantly increased phosphate adsorption capacity.3.Monodentate,nonprotonated bidentate and protonated bidentate were main phosphate speciation on the surface of ferrihydrite.Phosphorus concentration,p H and aluminum substitution affected the morphological distribution of phosphate complexes on the surface of ferrihydrite.At relatively high p H values,the bidentate complexes undergo a deprotonation reaction,and they gradually transformed into monodentate complexes as p H increased.Lower phosphate adsorption is not favored the protonation reaction of bidentate complexes.Aluminum substitution did not change the type of phosphate complexes on the surface of ferrihydrite,but increased the ratio of bidentate complexes and reduced the ratio of monodentate complexes.