Coexisting Goethite\nPromotes Fe(II)-Catalyzed Transformation\nof Ferrihydrite to Goethite

Abstract

In redox-affected soil environments, electron transfer\nbetween\naqueous Fe­(II) and solid-phase Fe­(III) catalyzes mineral transformation\nand recrystallization processes. While these processes have been studied\nextensively as independent systems, the coexistence of iron minerals\nis common in nature. Yet it remains unclear how coexisting goethite\ninfluences ferrihydrite transformation. Here, we reacted ferrihydrite\nand goethite mixtures with Fe­(II) for 24 h. Our results demonstrate\nthat with more goethite initially present in the mixture more ferrihydrite\nturned into goethite. We further used stable Fe isotopes to label\ndifferent Fe pools and probed ferrihydrite transformation in the presence\nof goethite using ⁵⁷Fe Mössbauer spectroscopy and\nchanges in the isotopic composition of solid and aqueous phases. When\nferrihydrite alone underwent Fe­(II)-catalyzed transformation, Fe atoms\ninitially in the aqueous phase mostly formed lepidocrocite, while\nthose from ferrihydrite mostly formed goethite. When goethite was\ninitially present, more goethite was formed from atoms initially in\nthe aqueous phase, and nanogoethite formed from atoms initially in\nferrihydrite. Our results suggest that coexisting goethite promotes\nformation of more goethite via Fe­(II)–goethite electron transfer\nand template-directed nucleation and growth. We further hypothesize\nthat electron transfer onto goethite followed by electron hopping\nonto ferrihydrite is another possible pathway to goethite formation.\nOur findings demonstrate that mineral transformation is strongly influenced\nby the composition of soil solid phases.