Geochemical Decoupling\nof Iron and Zinc during Transformation\nof Zn-Bearing Ferrihydrite in Reducing Sediments

The transformation of the mineral ferrihydrite in reducing\nenvironments,\nand its impact on the mobility of incorporated trace metals, has been\ninvestigated in model laboratory studies, but studies using complex\nsoil or sediment matrices are lacking. Here, we studied the transformation\nof zinc (Zn)-bearing ferrihydrite labeled with <sup>57</sup>Fe and\nmixed with natural sediments, incubated in reducing conditions for\nup to six months. We tracked the evolution of Fe and Zn speciation\nwith <sup>57</sup>Fe Mössbauer spectroscopy and with bulk and\nmicro-X-ray absorption spectroscopy. We show that Fe was readily reduced\nand incorporated into a poorly crystalline mixed-valence Fe(II)–Fe(III)\nphase resembling green rust. In parallel, Zn was released in the surrounding\nporewater and scavenged by precipitation with available ligands, particularly\nas zinc sulfide (ZnS) or Zn-carbonates. Early in the mineral transformation\nprocess, the chemical behavior of Fe was decoupled from Zn, suppressing\nthe impact of Zn on the rates and products of the ferrihydrite transformation.\nOur results underline the discrepancy between model experiments and\ncomplex field-like conditions and highlight the importance of sediment\nand soil geochemistry and ligand competition on the fate of divalent\nmetal contaminants in the environment.