Adsorption mechanisms of metal ions on the potassium dihydrogen phosphate (1 0 0) surface: A density functional theory-based investigation

Abstract

The adsorption of metal ions (K+, Na+, Ca2+, Cu2+, Al3+, Cr3+) on the (1 0 0) surface of potassium dihydrogen phosphate (KDP) has been studied using density functional theory (DFT). Calculation results show that all the investigated metal ions can be spontaneously adsorbed on the surface with negative adsorption energies. The adsorption stability increases in the order of Na+ < K+ < Cu2+ < Ca2+ < Al3+ < Cr3+, and shows a consistent trend as the adsorbed metal ion valence (monovalent < divalent < trivalent). Three types of stable adsorption configurations are observed, corresponding to three different bonding mechanisms. Na+, K+ and Ca2+ ions with a large radius can form two ionic bonds and one weak covalent bond with the O and H atoms respectively. In addition, the medium-sized ion of Cu2+ forms two covalent bonds with the O and H atoms. Furthermore, Al3+ and Cr3+ ions with the smallest radius form two metal-oxygen and one metal-hydrogen covalent bonds with the surface, making one H–O bond broken. Compared with other metal ions, Al3+ and Cr3+ have the strongest interactions with the surface, which can be explained by the significant electron transfer and more stable covalent bond formations between these two ions and the surface.