The Role of Aluminum Substitution into Ferrihydrite Nanoparticles on Sulfate Adsorption.

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. In this study, we investigated the effect of Al substitution (12% and 24 % Al) on the interaction energy of chloride (Cl^-) and nitrate (NO₃^-), and adsorption of sulfate (SO₄^2-) onto ferrihydrite. 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 nitrate and the endothermic peak of chloride with increasing Al concentration: from – 1.980 mJ/mg in 0 % AlFh (pure Fh) to -5.83 mJ/mg in 24 % AlFh at pH 5.6 and from -5.548 mJ/mg in 0 % AlFh to -14.681 mJ/mg 24 % AlFh in pH 3.0. The reaction of sulfate with the Fh surface was exothermic, and it increased with increasing Al substitution ( -0.167 mJ/mg to -0.741 mJ/mg in 0 % and 24 % AlFh) and decreasing pH. Furthermore, results showed that while Cl^- and NO₃^- have an entirely reversible ion exchange reaction on different AlFhs, sulfate adsorption exhibited an initial irreversibility, although sulfate progressively desorbed, with the desorption rate decreasing with increasing Al in both pHs.