Defects induced by Al substitution enhance As(V) adsorption on ferrihydrites

An original rationale is proposed to explain the controversial role of aluminum, a common substitutive element in ferrihydrite (Fh), on arsenic adsorption. The adsorption of arsenic on synthetic Al-for-Fe substituted Fh (AlFh) with up to 20 mol% Al was investigated at pH 5 and 8. The reduced interplanar spacings observed by selected area electron diffraction show that all AlFh samples are isomorphically substituted up to 20 mol% Al. A 15 mol% Al incorporation increases the arsenic uptake by 28%. In contrast, the Langmuir binding constants decrease, suggesting weaker bonds. Arsenic uptake reduces by 50% as pH rises from 5 to 8. The Al-for-Fe substitution in ferrihydrite causes structural defects, proton-compensated by OH groups, as indicated by the Vegard rule deviation. X-ray photoelectron spectroscopy demonstrates the increase in the relative amount of surface M-OH sites (45% to 77%) with Al concentration (AlFh-0 to AlFh-20), respectively. The enhanced As(V) uptake was ascribed to the insertion of hydroxyls on the Fh structural defects. Fourier-transformed-infrared spectroscopy showed that the sites modified by Al introduction are involved in As adsorption. These findings help to understand aluminum's role in arsenic adsorption, fixation, and fate in the environment.