First-Principles Density Functional Theory Characterisation of the Adsorption Complexes of H3AsO3 on Cobalt Ferrite (Fe2CoO4) Surfaces - data

The mobility of arsenic in aqueous systems can be controlled by its adsorption onto the surfaces of iron oxide minerals such as cobalt ferrite (Fe2CoO4). In this work, the adsorption energies, ge-ometries, and vibrational properties of the most common form of As(III), arsenous acid (H3AsO3), onto the low-index (001), (110) and (111) surfaces of Fe2CoO4 has been investigated under dry and aqueous conditions using periodic density functional theory (DFT) calculations. The dry and hydroxylated surfaces of Fe2CoO4 steadily followed an order of increasing surface energy, and therefore decreasing stability, of (001) < (111) < (110). Consequently, the favourability of H3AsO3 adsorption increased in the same order, favouring the least stable (110) surface. However, by analysis of the equilibrium crystal morphologies, this surface is unlikely to occur naturally. The surfaces were demonstrated to be further stabilised by introducing H2O/OH species, which co-ordinate the surface cations, providing a closer match to the bulk coordination of the surface species. The adsorption complexes of H3AsO3 on the hydroxylated Fe2CoO4 surfaces with the inclusion of explicit solvation molecules are found to be generally more stable than on the dry surfaces, demonstrating the importance of hydrogen-bonded interactions. Inner-sphere complexes involving bonds between the surface cations and molecular O atoms were strongly favoured over outer-sphere complexes. On the dry surfaces, deprotonated bidentate binuclear configurations were most thermodynamically favoured, whereas monodentate mononuclear configurations were typically more prevalent on the hydroxylated surfaces. Vibrational frequencies were analysed to ascertain the stabilities of the different adsorption complexes and to assign the As–O and O–H stretching modes of the adsorbed arsenic species. Our results highlight the importance of cobalt as a potential adsorbent for arsenic-contaminated water treatment.

Data underpinning the work are available in the .xlsx format (can be viewed either by MS Office or Libre Office) comprising 5 datasheets named according to their contents or Figure numbers as they appear in the manuscript. Data for all optimized structures are available in CONTCAR format of the VASP simulation program and can be visualized using the VESTA software.  The DOS data has two columns (Energy on the x-axis, and DOS on the y-axis).

Research results based upon these data are published at https://doi.org/10.3390/min11020195