Aqueous Fe(II) induces the transformation of ferrihydrite to more crystalline minerals such as lepidocrocite or goethite through a thermodynamically-driven dissolution-precipitation reaction. Although this reaction is well constrained, repartitioning of adsorbed or co-precipitated metals during these transformations is not well understood. We conducted column experiments to examine the fate of adsorbed Mo during Fe(II)-induced 2-line ferrihydrite transformation. We passed artificial groundwater containing 0.1 mM MoO42− through columns of ferrihydrite-coated sand until breakthrough (C/C0 = 1). We destructively sampled one replicate column to determine initial Mo retention mechanisms. Raman spectroscopy and X-ray absorption spectroscopy (XAS) revealed that Mo retention initially results from inner-sphere complexation of MoO4 tetrahedra at ferrihydrite surfaces. To induce ferrihydrite transformation, we then passed artificial groundwater containing either 0.2 or 2.0 mM Fe(II) through the remaining columns. Iron(II) introduction initially mobilized 52% and 16% of adsorbed Mo with the low and high Fe(II) inputs, respectively. Long-term Mo release was limited due to structural incorporation of MoO6 octahedra into neoformed phases, which included lepidocrocite [γ-FeOOH] and, potentially, goethite [α-FeOOH]. Questions remained about the mechanism of Mo incorporation and, particularly, the observed Mo coordination change. We subsequently conducted a complimentary series of batch experiments, which examined the impact of pH, surface coverage, and Fe(II) concentration on Mo repartitioning behaviour. Preliminary XAS results suggest that a pH change related to Fe(III) hydrolysis may promote a Mo coordination change. We hypothesize that this coordination change facilitates Mo incorporation into neoformed Fe(III) (oxyhydr)oxide transformation products and may represent an important Mo sequestration mechanism in Fe-rich environments.

Structural incorporation of adsorbed Mo(VI) during Fe(II)-induced ferrihydrite transformation

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.