An in situ quick-EXAFS and redox potential study of the Fe(II)-catalysed transformation of ferrihydrite

In this study we examine the kinetics of the Fe(II)-accelerated conversion of ferrihydrite to goethite using quick-scanning X-ray absorption spectroscopy (XAS) and measure the associated changes in oxidation–reduction potential (ORP). The quick-scanning method, where full scans up to k = 15 Å−1 were achieved in around 20 s, provided improvements over traditional XAS methods, particularly with regard to the ability to conduct in situ experiments. The method allowed us to quantify the kinetics of the conversion of ferrihydrite to goethite at two different Fe(II) concentrations (0.25 M and 0.05 M) and at two different pH values (5.85 and 6.20). The higher Fe(II) concentration resulted in a five-fold higher first order rate constant for the conversion of ferrihydrite to goethite than at the lower Fe(II) concentration despite surface complexation modelling indicating that similar concentrations of surface-adsorbed Fe(II) species were present under both conditions. These results suggest that bulk solution Fe(II) concentrations rather than surface-adsorbed Fe(II) concentrations are controlling the crystallisation kinetics of ferrihydrite to goethite under the conditions used in these quick-XAS studies, most likely through direct injection of electrons from aqueous Fe(II) into the conduction band of ferrihydrite. The ORP decreased monotonically over time with this behaviour reasonably described using a simple model in which bulk solution Fe3+ activity was assumed to be determined by the extent of transformation of ferrihydrite to goethite and the associated Fe3+ activity in local equilibrium with each iron oxide phase.