Surface Complexation Modeling for Chromate Adsorption on Al – Ferrihydrite

Ferrihydrite (FH) is a poorly crystalline oxyhydroxide with high surface reactivityand,therefore,has an important role in thetransport of contaminantsin the subsurface. Although naturally occuring FHoftencontainsimpuritiessuch as Al substitutions, however the impact of theseon surface propertiesare not yet fully understood.True substitution of Al occurs only up to ~20% molar basis[1], however sorption studies typically focused on higher levels that leadto distinct Fe-Al assemblages [2]. A recent study of low-substitute Al-Fh indicated that the speciation of chromate, selenate and sulfate on the surface changed substantially with increased Al content [3].In this study,synthetic Al –substituted FH (0, 6 and 18 % mol Al/(Al+Fe))wascharacterized with transmission electron microscopy (TEM)to examinedifferences in morphologyand particle size.Batch adsorption studies for chromate were performed and indicated that while the maximum adsorption capacity was similar, the slope around neutral pH increased with increased Al substitution.The SCM modeling efforts employed a CD-MUSIC approach tied with aBasic Stern electrostatic model, utilizing distinct sites for Fe and Al binding of chromate and taking into account the different species present. To our knowledge, thisisthe firstsurface complexationmodelingstudy forAl-FH. Sensitivity analysis for surface complexation andspecific adsorption equilibrium constants was performed.