Complexation mechanism of Pb2+ at the ferrihydrite-water interface: The role of Al-substitution.

Ferrihydrite is a poorly crystalline iron (hydr)oxide and highly efficient adsorbent for heavy metals. Al-substitution in ferrihydrite is ubiquitous in nature. However, the effect of Al-substitution on the surface reactivity of ferrihydrite remains unclear due to its low crystallinity. The present study aims to clarify the microstructure and interfacial reaction of Al-substituted ferrihydrite. Al-substitution had little effect on the morphology and surface site density of ferrihydrite, while the presence of ≡AlOH-0.5 sites resulted in higher proton affinity and surface positive charge of ferrihydrite. Besides, the affinity constant of Pb2+ adsorption on the surface of ferrihydrite decreased at higher Al content, which further decreased the adsorption performance of ferrihydrite for Pb2+. The modeling results revealed that bidentate complex was the dominant Pb complexation species on the surface of ferrihydrite, which was less affected by Al-substitution. The present study provides important insights into the effect of Al-substitution on the interfacial reaction at the ferrihydrite-water interface. The obtained parameters may facilitate the future advance of surface complexation model.