Surface complexation and precipitate geometry for aqueous Zn(II) sorption on ferrihydrite I: X-ray absorption extended fine structure spectroscopy analysis

“Two-line” ferrihydrite samples precipitated and then exposed to a range of aqueous Zn solutions (10⁻⁵ to 10⁻³ M), and also coprecipitated in similar Zn solutions (pH 6.5), have been examined by Zn and Fe K-edge X-ray absorption spectroscopy. Typical Zn complexes on the surface have Zn-O distances of 1.97(.02) Å and coordination numbers of about 4.0(0.5), consistent with tetrahedral oxygen coordination. This contrasts with Zn-O distances of 2.11(.02) Å and coordination numbers of 6 to 7 in the aqueous Zn solutions used in sample preparation. X-ray absorption extended fine structure spectroscopy (EXAFS) fits to the second shell of cation neighbors indicate as many as 4 Zn-Fe neighbors at 3.44(.04) Å in coprecipitated samples, and about two Zn-Fe neighbors at the same distance in adsorptionsamples. In both sets of samples, the fitted coordination number of second shell cations decreases as sorption density increases, indicating changes in the number and type of available complexing sites or the onset of competitive precipitation processes. Comparison of our results with the possible geometries for surface complexes and precipitates suggests that the Zn sorption complexes are inner sphere and at lowest adsorption densities are bidentate, sharing apical oxygens with adjacent edge-sharing Fe(O,OH)₆ octahedra. Coprecipitation samples have complexes with similar geometry, but these are polydentate, sharing apices with more than two adjacent edge-sharing Fe(O,OH)₆ polyhedra. The results are inconsistent with Zn entering the ferrihydrite structure (i.e., solid solution formation) or formation of other Zn-Fe precipitates. The fitted Zn-Fe coordination numbers drop with increasing Zn density with a minimum of about 0.8(.2) at Zn/(Zn + Fe) of 0.08 or more. This change appears to be attributable to the onset of precipitation of zinc hydroxide polymers with mainly tetrahedral Zn coordination. At the highest loadings studied, the nature of the complexes changes further, and a second type of precipitate forms. This has a structure based on a brucite layer topology, with mainly octahedral Zn coordination. Amorphous zinc hydroxide samples prepared for comparison had a closely similar local structure. Analysis of the Fe K-edge EXAFS is consistent with surface complexation reactions and surface precipitation at high Zn loadings with little or no Fe-Zn solid solution formation. The formation of Zn-containing precipitates at solution conditions two or more orders of magnitude below their solubility limit is compared with other sorption and spectroscopic studies that describe similar behavior.