Effects of aluminum substitution on the surface charge of colloidal goethite particles: experiments and MUSIC modeling.

Proton adsorption behavior on the surface of Al-substituted goethites as a function of pH and ionic strength was investigated and simulated with the multisite surface complexation (MUSIC) model. In addition, X-ray diffraction, X-ray photoelectron spectroscopy, and field emission scanning electron microscope were used to characterize the crystal structure, chemical composition, micromorphology, and surface properties of the Al-substituted goethite. Al substitution was found to affect the crystal structure and micromorphology of goethite. The morphological differences did not result in significant differences in PZC value but largely affected the surface charge values. Goethite surface charge capacity increased progressively with increasing amount of Al substitution, which was attributed to increases in the density of surface coordinated sites due to the increase in (021)/(110) face ratio. The optimization calculations enabled a satisfactory fitting of the titration data of both pure goethite and Al-substituted goethite, and the MUSIC model facilitated a more specific understanding of the charging behavior of Al-substituted goethite. The singly (≡FeOH + ≡AlOH) and triply coordinated (≡FeO + ≡AlFeO) surface groups were most likely responsible for the basic charging behavior of goethite in the pH range of 4-10. All results indicate that the MUSIC model has excellent performance in characterizing Al-substituted goethite, and the model has promising application prospect in other substituted metal (hydr)oxides.