The Mobility of\nMo during Microbially Mediated Ferrihydrite\nPhase Transformation

Molybdenum (Mo) is an essential nutrient for almost all\norganisms.\nHowever, at high concentrations, it can be toxic to animals and plants.\nThis study investigated the interactions of Mo(VI) with iron oxyhydroxides\nduring ferrihydrite bioreduction in the presence of Fe(III)-reducing Geobacter sulfurreducens. Here, we showed that Mo\nconcentration controlled ferrihydrite phase transformation, leading\nto Mo release. With the biotic reduction of ferrihydrite and Fe(II)\nproduction, Mo(VI) reduction and Mo(IV)O<sub>2</sub> formation were\nobserved for the first time, which further immobilized Mo after surface\nadsorption of Mo(VI). At low Mo levels (Mo/Fe molar ratios of 1–2%),\nsufficient Fe(II) adsorption onto ferrihydrite resulted in its transformation\ninto magnetite nanoparticles (>80%, ∼25 nm), which catalyzed\nthe reduction of Mo(VI) to form Mo(IV)O<sub>2</sub> and immobilized\nMo. Contrastingly, at high Mo concentrations (Mo/Fe molar ratios of\n5–10%), Mo(VI)O<sub>4</sub><sup>2–</sup> adsorption\nonto ferrihydrite limited Fe(II) adsorption; subsequently, less magnetite\n(<8–12%) formed while more goethite (∼30–50%,\nwidth and length >15 and 100 nm, respectively) and siderite (∼20–30%,\nwidth and length >100 and 200 nm, respectively) with larger particle\nsizes formed instead, causing Mo(VI) release due to lower Mo adsorption.\nThis study provides a comprehensive understanding of the interaction\nmechanisms among Geobacter sulfurreducens, Mo(VI), and iron oxyhydroxides, enabling predictions and controls\nof long-term Mo mobility and Fe mineral transformation under a variety\nof biogeochemical scenarios.