Transformation of 2-line ferrihydrite to goethite at alkaline pH

TL;DR

This study investigates how 2-line ferrihydrite transforms into goethite at high alkaline pH, revealing rapid transformation rates and providing a model to estimate this process in environmental and industrial contexts.

Contribution

It presents detailed kinetic data and a mathematical model describing ferrihydrite to goethite transformation at alkaline pH, using advanced synchrotron techniques.

Findings

Transformation half-life decreases from months to hours at high pH.

Transformation rate constants increase exponentially with pH.

Goethite likely precipitates directly from solution, controlled by ferrihydrite re-dissolution.

Abstract

The transformation of 2-line ferrihydrite to goethite from supersaturated solutions at alkaline pH >= 13.0 was studied using a combination of benchtop and advanced synchrotron techniques such as X-ray diffraction, thermogravimetric analysis and X-ray absorption spectroscopy. In comparison to the transformation rates at acidic to mildly alkaline environments, the half-life,t_1/2, of 2-line ferrihydrite reduces from several months at pH = 2.0, and approximately 15 days at pH = 10.0, to just under 5 hours at pH = 14.0. Calculated first order rate constants of transformation, k, increase exponentially with respect to the pH and follow the progression log_10 k = log_10 k_0 + a*pH^E3. Simultaneous monitoring of the aqueous Fe(III) concentration via inductively coupled plasma optical emission spectroscopy demonstrates that (i) goethite likely precipitates from solution and (ii) its formation is rate-limited by the comparatively slow re-dissolution of 2-line ferrihydrite. The analysis presented can be used to estimate the transformation rate of naturally occurring 2-line ferrihydrite in aqueous electrolytes characteristic to mine and radioactive waste tailings as well as the formation of corrosion products in cementitious pore solutions.