The Mobility of Mo during Microbially Mediated Ferrihydrite Phase Transformation

Molybdenum (Mo) is an essential nutrient for almost all organisms. However, at high concentrations, it can be toxic to animals and plants. This study investigated the interactions of Mo(VI) with iron oxyhydroxides during ferrihydrite bioreduction in the presence of Fe(III)-reducing <i>Geobacter sulfurreducens</i>. Here, we showed that Mo concentration controlled ferrihydrite phase transformation, leading to Mo release. With the biotic reduction of ferrihydrite and Fe(II) production, Mo(VI) reduction and Mo(IV)O₂ formation were observed for the first time, which further immobilized Mo after surface adsorption of Mo(VI). At low Mo levels (Mo/Fe molar ratios of 1-2%), sufficient Fe(II) adsorption onto ferrihydrite resulted in its transformation into magnetite nanoparticles (>80%, ∼25 nm), which catalyzed the reduction of Mo(VI) to form Mo(IV)O₂ and immobilized Mo. Contrastingly, at high Mo concentrations (Mo/Fe molar ratios of 5-10%), Mo(VI)O₄^2- adsorption onto ferrihydrite limited Fe(II) adsorption; subsequently, less magnetite (<8-12%) formed while more goethite (∼30-50%, width and length >15 and 100 nm, respectively) and siderite (∼20-30%, width and length >100 and 200 nm, respectively) with larger particle sizes formed instead, causing Mo(VI) release due to lower Mo adsorption. This study provides a comprehensive understanding of the interaction mechanisms among <i>Geobacter sulfurreducens</i>, Mo(VI), and iron oxyhydroxides, enabling predictions and controls of long-term Mo mobility and Fe mineral transformation under a variety of biogeochemical scenarios.