Adsorption and Precipitation of Aqueous Zn(II) on Hematite Nano- and Microparticles

As part of a study of the effect of particle size on reactivity of hematite to aqueous metal ions, the sorption of Zn(II) on hematite nanoparticles and microparticles was examined over a wide range of Zn(II) concentrations using Zn K-edge EXAFS. When reacted with nanoparticles at pH 5.5 and low Zn(II) sorption densities (0.04 < Γ < 2.76 μmol/m), Zn(II) formed five-coordinated or a mixture of fourand six-coordinated surface complexes with an average Zn-O distance of 2.04(±0.02)Å. At pH 5.5 and high Zn(II) sorption densities (2.76 < Γ < 3.70 μmol/m), formation of surface precipitates is suggested based on the presence of second-shell Zn and multiple scattering features in the Fourier transform (FT) of the EXAFS spectra. EXAFS fitting of these high Γ samples yielded an average first-shell Zn-O distance of 2.10(±0.02)Å, with second-shell Zn-Fe and Zn-Zn distances of 3.23(±0.03)Å and 3.31(±0.03)Å, respectively. Qualitative comparison between the EXAFS spectra of these sorption samples and that of amorphous zinc hydroxide and Zn-bearing hydrotalcite indicates the development of surface precipitates with increasing Γ. EXAFS spectra of Zn(II) sorbed on hematite microparticles under similar experimental conditions showed no evidence for surface precipitates even at the highest Zn surface coverage (Γ = 4 μmol/m). These results indicate that reactivities of hematite nanoparticles and macroparticles differ with respect to Zn(II)aq, depending on Zn(II) sorption density. We suggest that the degree of hematite crystallinity affects the reactivity of hematite surfaces toward Zn(II)aq and the formation of the Zn(II) surface complexes.