The kinetics of the Fe(II)-accelerated transformation of ferrihydrite to goethite have been investigated for a range of pH and Fe(II) concentrations in order to ascertain the factors controlling the transformation rate. Psuedo-first order rate constants were determined for a reaction scheme whereby ferrihydrite was rendered reactive before transforming either directly to goethite or to goethite indirectly via transient formation of lepidocrocite. Examination of these rate constants revealed that lepidocrocite was most prevalent when solid-associated Fe(II) concentrations were low. Direct goethite production dominated at higher pH, and the rate of its production appeared to be controlled by both the proportion of solid-associated Fe(II) as well as the rate of continuous transport of Fe(II) from solution to the oxyhydroxide surface. A reaction scheme for the Fe(II)-accelerated secondary mineralization of ferrihydrite is developed in which Fe(II) attaches to the ferrihydrite surface where it is immediately oxidized to Fe(III) and the transferred electron is conducted through the oxyhydroxide. This freshly precipitated Fe(III) forms the nuclei for the formation of secondary minerals and also facilitates the ongoing uptake of Fe(II) from solution by creation of fresh surface sites. The concentration of solid-associated Fe(II) and the rate of transport of Fe(II) to the oxyhydroxide surface appear to determine which secondary minerals form and their rates of formation. Lepidocrocite growth is enhanced at lower solid-associated Fe(II) concentrations while conditions leading to more rapid uptake of Fe(II) from solution lead to higher goethite growth rates.<br/><br/>Boland, D.D., Collins, R.N., Miller, C.J., Glover, C.J. and Waite, T.D. (2014). Factors controlling the Fe(II)-accelerated transformation of ferrihydrite to lepidocrocite and goethite. Environ. Sci. Technol. 48, 5477 – 5485.

Effect of solution and solid-phase conditions on the Fe(II)-accelerated transformation of ferrihydrite to lepidocrocite and goethite

249th National Meeting (2015)

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