Enhanced immobilization of sorbed Pb and Cr during the Fe(ii)-catalyzed transformation of ferrihydrite

Abstract

The secondary mineralization of metastable iron (oxyhydr)oxides into thermodynamically stable ones occurring in soil–water interface and sediment mediates the transport of pre-associated trace metal elements (and of other pollutants) in surficial environments. However, the fate of these associated metals and their impact during/on this mineralogical process remain incompletely understood. To address this question, we investigated the transformation of typical trace metal cation [Pb(II)] or anion [Cr(VI)O₄²⁻] sorbed nanoscale ferrihydrite upon exposure to aqueous Fe(II). After 14 days of interaction, pre-sorbed Pb(II) modified marginally the extent of ferrihydrite conversion (∼93% vs. ∼94%), but did retard the formation of goethite (∼9% vs. ∼29%) by incorporating it into intermediates. In contrast, the transformation of Cr(VI)O₄²⁻ pre-sorbed ferrihydrite was essentially hindered (∼11%) owing to the formation of (FeO)₂CrO₂⁻ complex and less conductive Fe_xCr_1−x(OH)₃ at the ferrihydrite surface. In all cases, the secondary mineralization of ferrihydrite significantly enhanced immobilization of sorbed metals (Pb/Cr release rate constant decreased by 99%/37%) owing to the formation of more stable iron (oxyhydr)oxides (i.e., hematite, goethite, and lepidocrocite) partially incorporating pre-sorbed Pb(II) (>24%), or to the formation of Fe_xCr_1−x(OH)₃ and Cr₂O₃ that are more resistant to acid corrosion than sorbed species. These findings offer valuable insights into the fate of typical trace metal elements during ferrihydrite transformation and contribute to the development of remediation strategies for metal contaminants.