Abstract
In anaerobic environments, dissolved Fe(II) can accelerate the transformation of metastable iron oxides, such as ferrihydrite and lepidocrocite, to more thermodynamically stable mineral phases. As one of the widely existed substances in the environment, organic matters can strongly interact with Fe(II) ions and iron oxides through different functional groups, and then change the type and relative proportion of secondary minerals in metastable iron minerals phase transformation. Based on the theory of reactive intermediate Fe(III) (labile Fe(III)), some low molecular weight organic ligands were selected as the models to investigate their role in the Fe(II)-catalyzed ferrihydrite transformation process, the results showed that organic ligands had an impact at multiple stages throughout the whole transformation process: i) changing the reductive potential of Fe(II) ions by complexation, and occupying the adsorption sites on the surface of ferrihydrite to affect the electron transfer process of Fe(II)-ferrihydrite; ii) complexing with the intermediate labile Fe(III) and reducing the effective supersaturation of the system, which is not conducive to the nucleation of the secondary mineral; iii) adsorbing on the surface of secondary mineral crystal nuclei, changing the surface energy and nucleation barrier. These effects of organic ligands are directly related to the number of carrying functional groups and the adsorption structures on the surface of secondary minerals. The results provide a new perspective and method for understanding the metastable iron (oxyhydr)oxides transformation in the more complex natural environment.
