Mechanistic and thermodynamic interpretations of zinc sorption onto ferrihydrite

Elucidating the reaction mechanisms and estimating the associated transport and thermodynamic parameters are important for an accurate description of the fate of toxic metal pollutants, such as Zn(II), in soils and aquatic ecosystems rich in iron oxides. Consequently, sorption of Zn(II) ions onto ferrihydrite was investigated with macroscopic and spectroscopic studies as a function of pH (4.0-8.0), ionic strength (10(-3)-10(-1) M NaNO(3)), aqueous Zn(II) concentration (10(-8)-10(-2) M), and temperature (4-25 degrees C). Present findings suggest that, for a given set of pH and temperature conditions, Zn sorption onto ferrihydrite can best be described by one average reaction mechanism below the saturation limits. Thermodynamic analyses reveal that the Zn(II) ions sorb onto the ferrihydrite surfaces via strong endothermic chemical reactions. Consistently, X-ray absorption spectroscopic (XAS) analyses confirm that, at pH < 6.5, for all Zn loadings, Zn(II) ions form corner-sharing, mononuclear, bidentate inner-sphere complexes with ferrihydrite, where R(Zn-O) approximately 1.97 A and R(Zn-Fe) approximately 3.48 A. For pH >/=6.5, similar sorption complexes were observed at lower sorption densities. Then again, for pH >/=6.5 and at higher sorption densities, Zn(II) ions may begin to form zinc-hydroxide-like polynuclear sorption complexes on the surfaces of the ferrihydrite, where R(Zn-Zn) approximately 3.53 A. Surprisingly, small changes in temperature had a significant impact on the affinity of zinc for the ferrihydrite surface; equilibrium sorption capacity decreased by 3-4 orders of magnitude as temperature fell from 25 to 4 degrees C for all pH. Zinc sorption onto ferrihydrite, therefore, is governed by pH as well as by temperature and sorbate/sorbent ratio.