Stability of Iron in Clays Under Different Leaching Conditions

The iron chemistry of aluminosilicates can markedly affect their adsorption properties due to possible changes in surface charge upon exposure to a variety of processes in the environment. One of these processes is chemical leaching, but to date little has been reported on the susceptibility of structural Fe to chemical leaching. The purpose of the current study was to determine the effects of solution pH on the stability of structural Fe in kaolinites, illite, and bentonite and the potential for formation of ancillary (oxyhydr)oxides. Structurally bound Fe does not participate in sorption properties but Fe that is released and phase transformed during leaching could take part in adsorption processes and form complexes and/or covalent bonds via Fe ions. Five different Fe-bearing clay minerals were treated in 0.5 M and 2 M HCl, distilled H2O, 0.1MKCl, and 0.5MKHCO3 for 24 h. The amount of Fe leached varied from 10 μg g-1 (for 0.1 M KCl) to 104 μg g-1 (for 2 M HCl) depending on the leaching agents. Acidic and water treatments indicated a relative independence of leached Fe on the initial Fe content in the clay and, conversely, a heavy dependence on the crystallinity of initial Fe phases. Well crystallized Fe(III) was stable during the leaching process, while poorly crystallized and amorphous Fe(III) phases were less stable, forming new ion-exchangeable Fe3+ particles. Under alkaline conditions, no relation between Fe crystallinity and mobility was found. The structural and surface changes resulting from leaching processes were identified by equilibrium adsorption isotherms. In kaolinite, the specific surface area (SBET) and porosity changed independently of Fe leaching due to the stability and crystallinity of Fe. In bentonite, the number of micropores was reduced by their partial saturation with Fe3+ particles caused by poorly crystallized and more reactive Fe forms during the leaching process. Potential phase transformations of Fe were characterized by the voltammetry of microparticles; well crystallized Fe(III) oxides remained stable under leaching conditions, while poorly crystallized and amorphous Fe(III) phases were partially dissolved and transformed to reactive Fe3+ forms.