Calorimetric study of the influence of aluminum substitution in ferrihydrite on sulfate adsorption and reversibility

Ferrihydrite (Fh) is a nanocrystalline iron (hydr)oxide pervasive in various surface environments. It has high specific surface areas and high density of reactive surface-sites, both of which properties impart a consequential role in determining the fate and transport of environmental nutrients and contaminants. In natural environments, Fh readily reacts with impurities, such as aluminum (Al) and has variable substituted chemical compositions and surface properties. This work examines the effect of aluminum (Al) incorporation (0%, 12% and 24 mol% Al) on the interaction energy of chloride (CV) and nitrate (NO3-), and adsorption/desorption of sulfate (SO42-) onto Fh. Microcalorimetry experiments were conducted at pHs 3.0 and 5.6, along with a detailed characterization of all samples. Results showed a significant increase in the energetics of the exothermic peak of NO3- and the endothermic peak of CV with increasing Al concentration and decreasing pH values. Furthermore, the exothermic heat of exchange, adsorption, irreversibility and fraction of inner-sphere complexes for sulfate interaction with Fh increased with more Al concentration and acidic pH. (C) 2019 Elsevier Inc. All rights reserved.