The Stability and Structure of Amorphous Ferric Oxyhydroxides in the Presence of Environmentally Revelant Ligands

Abstract

Fe-oxyhydroxides play a vital role in Fe bioavailability and heavy metal adsorption in the environment. Amorphous Fe-oxyhydroxides (AFHs) are more bioavailabile and have a higher sorption capacity than crystalline phases, but the stability of AFHs under environmental conditions (pH 5-9, 25 ˚C) is not known. Ligands associated with AFHs have the potential to influence the stability and structure of AFHs. The stability, structure, and ligands interactions with AFHs were monitored using IR and XAS techniques at pH 5.5 and 8.0 in the presence of environmentally relevant ligands.

The AFH structure is similar to the structure of ferrihydrite. The only deviation to the initial AFH structure was for Cl- and SO4-AFHs hydrolyzed with HCO3<super>-</super>, which showed similarities to akaganéite and schwertmannite, respectively. The AFHs are primarily composed of Fe-Fe edge-sharing octahedra in the initial stages of AFH formation, and transform by forming more corner-sharing Fe-Fe linkages.

The stability of AFHs formed in the presence of Cl<super>-</super>, NO3<super>-</super>, SO4<super>2-</super> varied significantly. Cl-AFHs took the longest to form goethite followed by NO3- and SO4-AFHs at pH 5.5. At pH 8.0, SO4-AFHs took the longest to form goethite with NO3- and Cl-AFHs forming goethite quickly due to the loss of ligand interactions. The rate of goethite formation at both pHs also varied with ligand. The addition of dissolved SiO4<super>4-</super> did not influence the stability of AFHs significantly unless it was added after the AFHs had formed. Over time, ligand concentrations in AFHs decreased, indicating that ligand expulsion is closely tied to the structural reorganization of AFHs. Finally, calorimetric data revealed that AFH phases formed in the presence of different ligands have different enthalpies of dissolution at pH 5.5, indicating that the associated ligands have a direct role in the stability of AFHs. Therefore, the interactions of ligands with AFHs must be taken into account if the role of AFHs in biological metal acquisition and heavy metal storage in the environment is to be fully understood.