Surface Energies,\nAl-Substitution Effects, and Water\nAdsorption on Ferrihydrite: Insights from Density Functional Theory\nCalculations

Ferrihydrite (Fe<sub>5</sub>O<sub>7</sub>(OH), <b>Fh</b>)\nplays a crucial role in various environmental processes involving\niron oxyhydroxides. Its poorly crystalline nature, high surface area,\nand numerous reactive sites make it significant in the formation of\ndifferent oxyhydroxides, as well as in the adsorption and reactive\nprocesses. To understand its surface properties, we performed density\nfunctional/plane wave (density functional theory/PW) calculations\nto estimate the cleavage surface energies along directions with low\nMiller indices. We investigated 25 cleavage surfaces in five different\ndirections. Among these, the (110) direction with the MO termination\nis the most favorable, with a cleavage energy of 2.61 J m<sup>–2</sup>, followed by the (100)-MO and (010)-MO cleavages, with energies\nof 2.84 and 2.96 J m<sup>–2</sup>, respectively. The surface\nenergy of the (110) surface was estimated to be 1.65 J m<sup>–2</sup>. We also examined the effect of Al-substitution at the Fe sites.\nCalculations for a ferrihydrite with 20% Al content (<b>Fh20</b>) indicated an average increase in the cleavage energy by 0.10 J\nm<sup>–2</sup>. The surface energy of the <b>Fh20</b> (110) surface was estimated to be 1.99 J m<sup>–2</sup>.\nWater adsorption on these surfaces was found to occur preferentially\nin a dissociative manner, with an adsorption energy of 122.7 kJ mol<sup>–1</sup>. Adsorption sites for Al on the <b>Fh20</b> surface exhibited a slightly higher adsorption energy of 124.6 kJ\nmol<sup>–1</sup>. Using the Wulff theorem, we determined the\nmineral habit of Fh, which revealed a trade-off: the predominance\nof the most stable (110) surface implies a larger, less stable (001)\nsurface. These findings suggest that the (001) surface is likely the\nmost reactive, playing a significant role in many chemical processes\non <b>Fh</b> surfaces.