Facet-Dependent Adsorption of Phosphate on Hematite Nanoparticles: Role of Singly Coordinated Hydroxyl

Abstract

Phosphorus is an essential nutrient for plant and animal growth, while excessive phosphorus discharges can cause eutrophication of water bodies, leading to deterioration of water quality and posing serious risks to biodiversity and human health. Hematite is abundant on the Earth’s surface and plays a key role in phosphorus cycle. In particular, hematite nanoparticles may adsorb organic or inorganic phosphorus, consequently affecting phosphorus fate and effects. Yet, how the intrinsic properties of hematite (e.g., crystal facet) affect its adsorption process of phosphorus remains unclear. Here, three hematite nanoparticles with different exposed crystal facets were controllably synthesized to investigate their adsorption of phosphate under different pH conditions. The results revealed that the efficiency of hematite for adsorbing the phosphate depends on the crystal facets of hematite in the order of {104} > {110} > {001}. The phosphate adsorption on the hematite surface involves inner-sphere complexation between the phosphate and surface hydroxyl groups of hematite. The facet-dependent adsorption affinity of phosphate to hematite is mainly determined by the content of singly coordinated hydroxyl groups of hematite. These findings are useful to evaluate the potential environmental risks of iron oxide nanoparticles as a medium to the biochemical cycle of P element.