| Goethite(α-Fe OOH) is one of the common, naturally occurring mineral materials and most important crystalline iron minerals found in soils. It is a highly reactive mineral and plays an important role in environmental processes due to its large surface area, high charge capacity, strong affinity toward different nutrients and pollutants contaminants. Therefore, the surface chemistry of goethite characteristics of soil science is an important research direction. Several decades of research have shown goethite rarely exist as pure phases in natural environments. Usually, goethite often contains appreciable aluminium as an isomorphous substitution. As a result of Al3+ substitute Fe3+ in the octahedral sites, aluminum substitution in goethite can significantly change its crystal structure, micromorphologies and surface chemical properties, leading to the classical parameter settings of goethite are not suitable for CD-MUSIC(Charge Distrubution and Multisite Surface Complexation) with simulation and forecasting. In our studies, a series of Al-substituted goethite were synthesized using a homogenous precipitation procedure. Powder X-ray diffraction(XRD), Inductively Coupled Plasma(ICP), X-ray photoelectron spectroscopy(XPS) and scanning electron microscope(SEM) i.e. were used to confirm the successful preparation of Al-doped goethite and characterize the crystal structure, chemical composition, micromorphologies and surface properties of the aluminum substituted goethite with different Al content. Proton adsorption behavior for different amounts of aluminum substitution into goethites as a function of p H and ionic strength was investigated by acid-base titrations. In addition, the experimental charging curves were described quantitatively by a multisite surface complexation(MUSIC) model and distinguished the most sensitive modeling parameter. Extended X-ray absorption fine structure sprctroscopy(EXAFS) and isothermal sorption batch experiments were used to investigate the surface complex species and sorption density of Pb(II) adsorbed on Al-doped goethite. Furtheremore, using the CD-MUSIC approach with the optimal parameters from the MUSIC to fit the adsorption data. Based on the results of model fitting, the surface speciation of Pb(II) has been predicted. The main results were listed as following:1. The homogenous co-precipitation method provided a better way to obtain the good crystallinity Al-goethite(G-0, G-10 and G-15), and the Al substitution amount for G-10 and G-15 are 6.97 and 9.26 mol% on the basis of the results of ICP. Samples characterization results indicate that Al dopants are distributed on the Fe lattice. Aluminum substituted goethite had effect on its crystal structure and micromorphology. The Unit cell parameters and the ratio between length and width of goethite experienced a decrease with the increase of Al substitution amount, which means the fraction of ratio between the(110) faces and the(021) faces decrease. Goethite dissolved in the 6 mol/L HCl solution impeded by the presence of Al dopants, indicate that the crystal structure of Al-goethite more stable than pure goethite.2. Differences in crystal morphology of the Al-goethite, which results in slightly different PZC values: 9.3 for G-0, 9.2 for G-10 and G-15, are more expressed in surface charge density than in the PZC values. With aluminum amount substitution increases, surface charge density increases progressively, which demonstrated surface coordinated sites density increasing. The relative content of three species of oxygen lattice oxygen(O2-), hydroxide oxygen(OH-) and oxygen in molecular water H2 O were fitted.These XPS analyze results illustrate that the abundance of OH showed an upward tendency in the spectra of the Al substituted goethite systems, which are consistent with the results of potentiometric titration.3. Isotherms results showed that the Pb(II) adsorption density on the surface of goethite significantly reduced with the amounts of Al increased. This is due to the charge density increased with the Al doped amount increasing, leading to the increased electrostatic repulsion from a dominantly positive surface and Pb(II) in solution, so it is diffcult for Pb(II) adsorpted on Al-goethite. EXAFS analyed the main surface complex speacies for Pb(II) bind on the samples at different p H. The modeled results illustrat that p H value influences the Pb(II) complexation of pure goethite more than Al-goethie.At high p H conditions, the mainly complex species of pure goethite are tridentate and its hydrolyzed ones, bidentate and its hydrolyzed ones exsited at low p H conditions.For Al-goethite, the mianly complex species are bidentate and its hydrolyzed ones only at different p H value.4. Considering a progressing increasing proportion of crystal face((021)/(110)), the primary proton adsorption data were fitted by MUSIC. The satisfactorily fitting results demonstrated that the MUSIC model has improved the understanding of the difference in charging behavior of Al substituted goethite. The optimization calculations for the proton binding results demonstrated that the predictions are sensitive to the capacitance of the Stern Layer. The singly and triply coordinated surface groups are most likely responsible for the basic charging behavior of goethite in the p H 4–10. The optimal parameters of Stern layer capacitance and the total site density became lager when Al substitution amount of goethite has an increase, specific numerical performance: CG-0(1.35 F/m2) < CG-10(2.8 F/m2) < CG-15(3.5 F/m2), NG-0(6.15 site/nm2) < NG-10(6.3 site/nm2) < NG-15(6.5 site/nm2). 5. The Pb(II) adsorption data for the samples were calculated using CD-MUSIC, which assumed two types of active coordination sites, namely singly(≡Fe OH-0.5+≡Al OH-0.5) and triply(≡Fe3O-0.5+ ≡Al3O-0.5) surface groups. Four relevant surface species that are present as the bidentate complexs([Fe(Al)OH]2Pb and [Fe(Al)OH]2Pb OH) and tridentate complexs([Fe(Al)OH]2Pb[Fe(Al)3O] and [Fe(Al)OH]2Pb OH[Fe(Al)3O]) are resolved in the fit. The charge distribution factor f and Pb(II) affinity constant Log KPb have to be adjusted. All the modeling results indicated that CD-MUSIC displayed an excellent performance in characterizing adsorption data. Furtheremore, the surface speciation distribution will be evaluated at cover large ranges of equilibrium concentrations, two different of p H values by optimal parameters. The results indicated tridentate complexs just exsited at high p H conditions for the G-0, under other conditions are bidentate complexes. Moreover, dominant surface species distribution are in agreement with the surface complexes derived from EXAFS data, which is further evidence of the reliability of the CD-MUSIC fitting results. |
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