Adsorption of Cr(VI) on Al-substituted hematites and its reduction and retention in the presence of Fe2+ under conditions similar to subsurface soil environments.

Aluminum substitution is common in iron (hydr)oxides in subsurface environments, and can significantly modify mineral interactions with contaminants. However, few studies investigate Cr(VI) adsorption and its subsequent mobility on Al-substituted iron (hydr)oxide surfaces. Here shows that Al substitution gradually modifies hematite crystals from {101}, {112}, {110} and {104} faceted rhombohedra to {001} faceted plates, resulting in a general decrease in Cr(VI) adsorption density and favoring of monodentate mononuclear over bidentate binuclear Cr(VI) adsorption complexes. Consequently, the mobility of Cr(VI) might be increased in environments with an abundance of Al-containing iron (hydr)oxides. However, pre-adsorption of Fe2+ on hematite promotes Cr(VI) adsorption, reduction and fixation, and Al-substituted hematite removes more Cr(VI) than pure hematite. Similarly, although addition of Fe2+ to Cr(VI)-adsorbed hematite remobilizes a small proportion of Cr, it greatly increases the proportion of Cr fixed. As the coexistence of Fe2+ and iron (hydr)oxides is common in subsurface environments, Al-containing iron (hydr)oxides will promote Cr(VI) uptake and retention, with a significant proportion fixed as Cr(III), limiting Cr mobility and toxicity. These results offer new insights into how iron (hydr)oxides might control the behaviors of other high-valence redox-sensitive contaminants, and provide a platform for modeling such processes in complex soil and sediment systems.