Modelling phosphate and arsenate adsorption on cerium dioxide: A density functional theory study

Phosphate and arsenate species provide challenging environmental problems necessitating the search for efficient removal mechanisms from natural waters. Arsenic and phosphorous compounds have a high affinity for metal (hydr)oxide surfaces, and since phosphate and arsenate are isostructural, they have similar adsorption behaviour. This study provides results on arsenate and phosphate adsorption onto cerium dioxide using density functional theory, which is a promising adsorbent for the removal of these species. Ceria was modelled as the {100}, {110}, and {111} faces representative of ceria nanoparticles, assuming compositions from fully oxidised to fully reduced surfaces. Phosphate is generally more stable than arsenate adsorption, in agreement with macroscopic experimental studies. However, in some of the probed surfaces and adsorption densities, the relative stability flipped, which is an important finding for the understanding of the competitive behaviour between the species, especially in systems controlled by fast initial adsorption kinetics. The strength of the adsorption is generally stronger as the concentration of surface Ce³⁺ increases, while remaining stable as the concentration of adsorbates increases. As the concentration of adsorbates increases, a complex hydrogen-bond network forms, but is not sufficient to stabilize the adsorption further as the adsorbates compete for surface anchoring sites disrupting the adsorption process.