Effect of Solution and Solid-Phase Conditions on the Fe(II)-Accelerated Transformation of Ferrihydrite to Lepidocrocite and Goethite

Abstract

Aqueous ferrous iron (Fe(II)) accelerates the transformation of ferrihydrite into secondary, more crystalline minerals however the factors controlling the rate and, indeed, the underlying mechanism of this transformation process remain unclear. Here, we present the first detailed study of the kinetics of the Fe(II)-accelerated transformation of ferrihydrite to goethite, via lepidocrocite, for a range of pH and Fe(II) concentrations and, from the results obtained, provide insight into the factors controlling the transformation rate and the processes responsible for transformation. A reaction scheme for the Fe(II)-accelerated secondary mineralization of ferrihydrite is developed in which an Fe(II) atom attaches to the ferrihydrite surface where it is immediately oxidized to Fe(III) with the resultant electron transferred, sequentially, to other iron oxyhydroxide Fe(III) atoms before release to solution as Fe(II). 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 particular 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.

Aqueous Fe(II)-Induced Phase Transformation of Ferrihydrite Coupled Adsorption/Immobilization of Rare Earth Elements

“…The samples displayed remarkably different appearances under the microscope. Based on the comparison, the star-like particles, indicating the formation of goethite and magnetite [19,30], could be recognized from CK and Ho(III)-added samples, while lath-like lepidocrocite [30] was detected in the La(III)-added sample. The TEM images for the final solids obtained at 60 days confirmed the significant differences among the structural forms of the solid products from Fe(II) aq -induced ferrihydrite under conditions without the effects of REEs (CK) and with the effects of La(III) and Ho(III) (Figure 3).…”

“…The samples displayed remarkably different appearances under the microscope. Based on the comparison, the star-like particles, indicating the formation of goethite and magnetite [19,30], could be recognized from CK and Ho(III)-added samples, while lath-like lepidocrocite [30] was detected in the La(III)-added sample.…”

“…Ferrihydrite was prepared in the laboratory according to previous studies [18,19]. Firstly, 20 g of Fe(NO 3 ) 3 •9H 2 O was dissolved in 250 mL of double distilled de-ionized (DDI) water.…”

“…Environmental factors including pH, temperature, Fe(II) aq concentration, and ionic strength were reported to influence the Fe(II) aq -induced phase transformation of ferrihydrite [19,25]. Previous studies stated that metal-substitution in iron (hydr)oxides slowed down the phase transformation [31,32].…”

Enrofloxacin Transformation on Shewanella oneidensis MR-1 Reduced Goethite during Anaerobic–Aerobic Transition

“…28 On the contrary, the highest ENR degradation on bioreduced goethite was obtained at neutral pH ( Figure S5). This observation can be explained by the fact that the acidic condition inhibits the adsorption of bioreduced Fe(II) on goethite due to charge repulsion, 29 whereas the alkaline condition promotes the oxidation of bioreduced Fe(II) by dissolved O 2 . 30 Thus, only a neutral pH facilitates the formation of the Fe(II)/Fe(III) complex, and this neutral pH adaptation endows the microbial mediated redox process a crucial one in natural environments.…”