Characteristics of Phosphate Adsorption-Desorption Onto Ferrihydrite

The adsorption-desorption behavior of phosphate on ferrihydrite, goethite, and hematite and underlying mechanisms were comparatively investigated with batch kinetics, solution equilibrium, and mineral characterization. The phosphate adsorption kinetics for all Fe (hydr)oxides are described by a fast initial adsorption followed by a slow reaction, both of which comply with pseudo first-order kinetics. Ferrihydrite shows a higher amount of phosphate adsorbed and OH− release, a faster slow reaction, and an extra long diffusion reaction. Analysis of OH− release kinetics reveals that the adsorption process includes two steps: phosphate preferentially exchanges with two groups of Fe-OH21/2+: it occurs in an extremely short time for ferrihydrite and goethite, yet more time for hematite; phosphate exchange with Fe-OH21/2+ and Fe-OH1/2−: the amount of OH− released and phosphate adsorbed shows a linear correlation. Isotherms of the three minerals show better conformity to the Freundlich model than the Langmuir model because of their surface heterogeneity. The desorption percentage of adsorbed phosphate using KCl in order are hematite (12.5%) > goethite (10%) > ferrihydrite (8.5%). The lower desorption for ferrihydrite indicates a higher percentage of specific adsorption. On the other hand, citric acid promotes desorption on ferrihydrite more than on goethite and hematite via a dissolution mechanism. Ferrihydrite possesses poor crystallinity, higher surface site density and micropore volume, leading to its larger phosphate adsorption, higher percentage of specific adsorption, faster adsorption rate, and slow diffusion reaction stage.