Incorporation of
economically or environmentally consequential polyvalent metals into
iron (oxyhydr)oxides has applications in environmental chemistry,
remediation, and materials science. A primary tool for characterizing
the local coordination environment of such metals, and therefore building
models to predict their behavior, is extended X-ray absorption fine
structure spectroscopy (EXAFS). Accurate structural information can
be lacking yet is required to constrain and inform data interpretation.
In this regard, ab initio molecular dynamics (AIMD) was used to calculate
the local coordination environment of minor amounts of U incorporated
in the structure of goethite (α-FeOOH). U oxidation states (VI,
V, and IV) and charge compensation schemes were varied. Simulated
trajectories were used to calculate the U L<sub>III</sub>-edge EXAFS
function and fit experimental EXAFS data for U incorporated into goethite
under reducing conditions. Calculations that closely matched the U
EXAFS of the well-characterized mineral uraninite (UO<sub>2</sub>),
and constrained the <i>S</i><sub>0</sub><sup>2</sup> parameter
to be 0.909, validated the approach. The results for the U-goethite
system indicated that U(V) substituted for structural Fe(III) in octahedral
uranate coordination. Charge balance was achieved by the loss of one
structural proton coupled to addition of one electron into the solid
(−1 H<sup>+</sup>, +1 e<sup>–</sup>). The ability of
AIMD to model higher energy states thermally accessible at room temperature
is particularly relevant for protonated systems such as goethite,
where proton transfers between adjacent octahedra had a dramatic effect
on the calculated EXAFS. Vibrational effects as a function of temperature
were also estimated using AIMD, allowing separate quantification of
thermal and configurational disorder. In summary, coupling AIMD structural
modeling and EXAFS experiments enables modeling of the redox behavior
of polyvalent metals that are incorporated in conductive materials
such as iron (oxyhydr)oxides, with applications over a broad swath
of chemistry and materials science
