Interfacial Properties of Al-Ferrihydrites: Surface Complexation Modeling as a Probe of Surface Structure

Surface reactivity of minerals is impacted by structural impurities in minerals that are prevalent in nature. In this study, we investigate the impact of Al substitution on ferrihydrite's surface reactivity through surface complexation modeling. Al-bearing ferrihydrites were prepared with various amounts of Al, including 6, 12, and 18 mol % Al. Potentiometric titrations were simulated with a 1-pK charge distribution multisite complexation (CD-MUSIC) model and the MUlti-start optimization algorithm for surface complexation equilibrium parameters (MUSE). The surface structure model for Al-ferrihydrite was based on the assumptions that the total surface site density of Alferrihydrite is similar to pure ferrihydrite, the mol % Al corresponds to the respective percentage of the total site density of the mineral surface, and Al is substituting singly coordinated surface hydroxyls in Fe1 octahedral sites. Sensitivity analysis on the protonation constant and site density of Al sites revealed a nonuniform distribution of Al compared to the bulk composition. The results indicate a relative increase of AlOH sites on the Al-Ferrihydrite surface by 21% for 6 Al-Fh, 25% for 12 Al-Fh, and 30% for 18 Al-Fh, which suggests that the entire surface will contain only singly coordinated AlOH groups at 22 mol % Al, instead of the maximum of 29% based on a uniform distribution between bulk and surface. Overall, the findings of this study indicate that the influence of Al is higher than anticipated based on the molar ratio and provide insight into the distribution of impurities within the structure of ferrihydrite.