Atomic insights into the mechanisms of Al3+ or Cr3+ affecting ferrihydrite nucleation

Abstract

Various exotic metal cations commonly coexist with ferrihydrite nanoparticles in natural environments. Lack of knowledge about the metal cations effects on ferrihydrite nucleation and growth greatly blights the deep understanding of ferrihydrite mineralogical properties and reactivities, and thus the fates of associated nutrients, heavy metals/metalloids, and organic pollutants. Here, the nucleation processes and mechanisms of ferrihydrite nanoparticles in the presence of Al³⁺ or Cr³⁺ were studied by combining visible spectroscopy, in situ quick X-ray absorption fine structure spectroscopy and quantum chemical calculations. The formation of ferrihydrite can be divided into three stages. In stage 1, Fe(H₂O)₆³⁺ forms μ-oxo Fe dimers, with the gradual increase of Fe–O bond length (d_Fe–O) and disappearance of Fe–O multiple scattering. In stage 2, ferrihydrite particles begin to form and grow slowly, during which d_Fe–O continues to increase and edge- and corner-sharing Fe–Fe bonds appear. In stage 3, ferrihydrite growth rate increases significantly and continues to the end of the reaction, with the decreases of d_Fe–O. The presence of metal cations at a molar ratio of 0.1 to Fe hinders the formation of μ-oxo dimers by affecting the Fe³⁺ hydrolysis and polymerization at stage 1 and stage 2, but promotes the conversion of the μ-oxo dimer to the dihydroxo dimer with lower energy barrier and the creation of crystal growth sites and thus enhances the ferrihydrite nucleation and growth at stage 3.