Competing Fe(II)-Induced Mineralization Pathways of Ferrihydrite

Owing to its high surface area and intrinsic reactivity, ferrihydrite serves as a dominant sink for numerous metals and nutrients in surface environments and is a potentially important terminal electron acceptor for microbial respiration. Introduction of Fe(II), by reductive dissolution of Fe(III) minerals, for example, converts ferrihydrite to Fe phases varying in their retention and reducing capacity. While Fe(II) concentration is the master variable dictating secondary mineralization pathways of ferrihydrite, here we reveal that the kinetics of conversion and ultimate mineral assemblage are a function of competing mineralization pathways influenced by pH and stabilizing ligands. Reaction of Fe(II) with ferrihydrite results in the precipitation of goethite, lepidocrocite, and magnetite. The three phases vary in their precipitation extent, rate, and residence time, all of which are primarily a function of Fe(II) concentration and ligand type (Cl, SO4, CO3). While lepidocrocite and goethite precipitate over a large Fe(II) concentration range, magnetite accumulation is only observed at surface loadings greater than 1.0 mmol Fe(II)/g ferrihydrite (in the absence of bicarbonate). Precipitation of magnetite induces the dissolution of lepidocrocite (presence of Cl) or goethite (presence of SO4), allowing for Fe(III)-dependent crystal growth. The rate of magnetite precipitation is a function of the relative proportions of goethite to lepidocrocite; the lower solubility of the former Fe (hydr)oxide slows magnetite precipitation. A one unit pH deviation from 7, however, either impedes (pH 6) or enhances (pH 8) magnetite precipitation. In the absence of magnetite nucleation, lepidocrocite and goethite continue to precipitate at the expense of ferrihydrite with near complete conversion within hours, the relative proportions of the two hydroxides dependent upon the ligand present. Goethite also continues to precipitate at the expense of lepidocrocite in the absence of chloride. In fact, the rate and extent of both goethite and magnetite precipitation are influenced by conditions conducive to the production and stability of lepidocrocite. Thus, predicting the secondary mineralization of ferrihydrite, a process having sweeping influences on contaminant/nutrient dynamics, will need to take into consideration kinetic restraints and transient precursor phases (e.g., lepidocrocite) that influence ensuing reaction pathways.