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

Abstract Using a novel combination of in situ potentiometric experiments and quick-scanning XAS we present Fe K-edge XAS spectra (to k  = 12 A −1 ) during Fe III hydrolysis and precipitation in 0.33 M Fe(ClO 4 ) 3 , Fe(NO 3 ) 3 , FeCl 3 and Fe 2 (SO 4 ) 3 solutions up to pH 4.8. Edge-sharing Fe III polymers appeared almost immediately upon hydrolysis with strong evidence for a μ-oxo dimer species forming in the Fe(ClO 4 ) 3 , Fe(NO 3 ) 3 and FeCl 3 solutions. The effects of SO 4 on hydrolysis and polymerization pathways included inhibition of both the formation of the μ-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 ∼3.43 A 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.