An in situ XAS study of ferric iron hydrolysis and precipitation in the presence of perchlorate, nitrate, chloride and sulfate

Using a novel combination of in situ potentiometric experiments and quick-scanning XAS we present Fe K-edge XAS spectra (to k = 12 angstrom(-1)) during Fe-III hydrolysis and precipitation in 0.33M Fe(ClO4)(3), Fe(NO3)(3), FeCl3 and Fe-2(SO4)(3) solutions up to pH 4.8. Edge-sharing Fe-III polymers appeared almost immediately upon hydrolysis with strong evidence for a mu-oxo dimer species forming in the Fe(ClO4)(3), Fe(NO3)(3) and FeCl3 solutions. The effects of SO4 on hydrolysis and polymerization pathways included inhibition of both the formation of the mu-oxo dimer and double corner Fe-III bonding, ultimately resulting in the precipitation of schwertmannite. As such, under these experimental conditions, double corner Fe-III bonding appears to be critical to the formation of ferrihydrite. The spectral trends indicated that the decomposition/transformation of the dimer was sudden and broadly coincident with shortening average Fe-O bond distances, increased Fe neighbors at similar to 3.43 angstrom and a pre-edge energy transformation suggestive of decreased ligand field strength as well as increasing proportions of tetrahedral Fe-III. This result suggests that the incorporation of tetrahedral Fe-III into ferrihydrite occurs only at the latter stages of extended polymerization. (C) 2016 Elsevier Ltd. All rights reserved.