Transformation of various kinds of goethite into magnetite: Effect of chemical and surface properties

Fe^II-induced mineralogical transformations of three goethites (G1, G2 and G3), having different crystal habits, morphologies and surface properties, into magnetite were studied. Experimental data revealed that all estimated constants including kinetic rate constant of magnetite formation, first order rate constant of goethite reductive dissolution and Langmuir sorption capacity of Fe^II onto goethite followed the order G1>G2~G3, which is in agreement with the calculated site density. The high intrinsic reactivity of G1 emphasizes the importance of crystal faces reactivity for the transformation process. The presence of specific crystal faces (e.g. (021) or (121)) may favor the formation of inner-sphere complexes and therefore the electron transfer. Spectroscopic investigations suggested that partial conversion of goethite to magnetite may proceed via solid-state reaction, but dissolution/re-crystallization process cannot be fully excluded. The coordination of Fe^II at goethite surface via the formation of bidentate or tridentate complex is a prerequisite to electron transfer, a key step to initiate solid-state transformation process or reductive dissolution reaction. The presence of strong chelating ligand (e.g. phosphate) is expected to hinder the adsorption sites and therefore affect both topotactic and reconstructive processes. Microscopic level characterizations such as crystal structure and surface site density are therefore required to describe the transformation extent of iron oxides.