Abstract
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, geometries, 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 have 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 thus 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 the introduction of H2O/OH species, which coordinate 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.
Highlights
Water is a fundamental resource for the sustainment of life, and so it is essential that access to clean water for drinking, sanitation, and other uses is feasible globally
The features of the partial density of states (PDOS) in the present study are quite comparable to those obtained by Das et al [28] and Hou et al [30] with generalised gradient approximation (GGA)+U methods, they observed smaller bandgaps estimated at 1.14 eV and 0.72 eV, respectively
H3 AsO3 tended towards deprotonation, which can be attributed to the fact that less energy is required to break a weak O-H bond in H3 AsO3 than what is released upon forming an Osurf -H bond
Summary
Water is a fundamental resource for the sustainment of life, and so it is essential that access to clean water for drinking, sanitation, and other uses is feasible globally. Arsenical compounds occur naturally in water due to volcanic activity, weathering of rocks, solubilisation, and transport of sediment. They are becoming increasingly present in the environment as a result of human activities and the use of herbicides, pesticides, and waste products, accelerated by erosion of land and combustion of fossil fuels [2]. The adsorption of arsenic species onto iron oxide and iron oxyhydroxide mineral surfaces has been at the centre of many studies [4,5,6,7]. Molecular-level insight into the interactions of arsenic species with different iron oxide mineral surfaces is essential to understanding the adsorption mechanisms
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