Adsorption of inorganic phosphorus of different valences on metal oxides: A first-principles study

Abstract

<p indent="0mm">Phosphorus is the key factor limiting eutrophication. Although the prevailing decontamination technologies for the removal of orthophosphate are relatively mature, the advanced treatment of hypophosphite and phosphite has not attracted sufficient attention. Metal oxides are commonly employed as adsorbents for phosphate removal and are of great significance to the investigation of the adsorption behaviors and mechanisms of inorganic phosphorus of different valences on metal oxides. In this study, bulk phase and slab models of crystalline and amorphous Fe₂O₃, ZrO₂, La₂O₃ were constructed, and the adsorption configurations and energies of hypophosphite, phosphite, and orthophosphate on the above substrates were determined using first-principles calculations. Results show that hypophosphite, phosphite, and orthophosphate are adsorbed on the metal oxides mainly via the formation of M–O–P bonds (M = a metal atom) and hydrogen bonds (O–H). Comparison of adsorption energies revealed that orthophosphate is mainly chemisorbed on the surface of metal oxides and its adsorption energies are generally 0.7‒6.3 times greater than those of hypophosphite/phosphite. This finding reflects the specific adsorption of orthophosphate on metal oxides. By contrast, the adsorption energies of hypophosphite and phosphite are relatively low, which implies weak adsorption ability due to the competitive interference induced by coexisting ions. The adsorption energies on amorphous metal oxides are, on average, 3.1 times greater than those on their crystalline analogues. The adsorption energies on the investigated metal oxides followed the order Fe₂O₃&lt; ZrO₂&lt; La₂O₃. The adsorption energies on La oxides were 2.0–47.5 times greater than those of iron/zirconium oxides, indicating strong bonding between La and PO_{<italic>x</italic>} groups. This study is helpful for the selection and development of water decontamination technologies for the in-depth removal of phosphorus.