Transformation of 2-Line Ferrihydrite to Goethite at Alkaline pH.

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, t1/2, of 2-line ferrihydrite reduces from several months at pH = 2.0, and approximately 15 days at pH = 10.0, to just under 5 h at pH = 14.0. The calculated-first order rate constants of transformation, k, increase exponentially with respect to the pH and follow the progression log10 k = log10 k0 + a·pH3. 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 redissolution 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.

Aerobic and anaerobic oxidation of ferrous ions in near-neutral solutions.

Whilst the oxidation of Fe(II) in aerobic conditions has been studied thoroughly, an in-depth knowhow on the fate or stability of Fe(II) in solutions with near-neutral pH under anaerobic conditions is still lacking. Here, we experimentally investigated the kinetics of Fe(II) oxidation in solutions with pH ranging between ∼5 and 9, under aerobic (when solutions were in equilibrium with atmospheric oxygen) and anaerobic conditions (when the dissolved oxygen concentration was ∼10-10 mol/L), by colorimetric means. Experimental results and thermodynamic considerations presented here, show that Fe(II) oxidation in anaerobic conditions is first-order w.r.t. [Fe(II)], and proceeds with set of parallel reactions involving different hydrolysed and non-hydrolysed Fe(II) and Fe(III) species, similar to that observed in aerobic conditions. However, in the absence of oxygen, the cathodic reaction accompanying the anodic oxidation of Fe(II), is the reduction of H2O (l) releasing H2 (g). Hydrolysed Fe(II) species oxidise much faster than Fe2+ and their concentrations increases with pH, leading to increased Fe(II) oxidation rates. Additionally, we also show the importance of the type of buffer used to study Fe(II) oxidation. Therefore, for the oxidation of Fe(II) in near-neutral solutions, the speciation of Fe(II) and Fe(III), the presence of other anions and the pH of the solution are critical parameters that must be considered. We anticipate that our results and hypothesis will find use in reactive-transport models simulating different processes occurring in anaerobic conditions such as corrosion of the steel in concrete structures, or in nuclear waste repositories.