Changes in Crystallinity and Tracer-Isotope Distribution of Goethite during Fe(II)-Accelerated Recrystallization

Goethite (α-FeOOH) is a source and sink for trace elements in surficial environments. Its elemental and isotopic composition may be perturbed during recrystallization, particularly when accelerated by aqueous Fe(II), but the factors that control such reactivity and the extent to which it occurs are poorly understood. Here we react goethite samples of varying crystallinity in ⁵⁷Fe-enriched Fe(II) solutions and detail the temporal distribution of the tracer isotope and the evolution of goethite crystallites. Consistent with earlier work, isotope exchange occurs between dissolved Fe(II) and goethite. By completely dissolving Fe(II)-reacted goethite in sequential steps while measuring the tracer isotope, we reconstructed the goethite recrystallized with time. Initially, the tracer isotope is enriched at the goethite surface. With continued reaction, however, the ⁵⁷Fe tracer-isotope becomes distributed throughout the bulk goethite with an isotopic composition equal to that of Fe(II) dissolved in solution. Crystallite size increased by 7-45% after a 30 day reaction period with the largest increase occurring for goethite samples with poor initial crystallinity, small particle sizes, and large specific surface areas. These results suggest that substantial exchange of Fe in goethite occurs in the presence of dissolved Fe(II) but such reactivity decreases with increasing crystallinity. Crystallite size may be a predictive feature of the potential reactivity of iron oxides in the environment, and hence the mobility of associated metal ions.