Iron oxyhydroxide polytype (γ-, δ- and β-FeOOH) structures govern Zn mobility

Abstract

Iron oxyhydroxide polytype minerals, such as goethite (α-FeOOH), lepidocrocite (γ-FeOOH), feroxyhyte (δ-FeOOH) and akaganeite (β-FeOOH), widely exist in terrestrial environments, and play important roles in mediating heavy metal mobility. Previous studies mainly focus on goethite, while the other three polytypes have been rarely explored. In the present work, Zn retention contents and mechanisms in Zn-coprecipitated γ-, δ- and β-FeOOH minerals were thoroughly investigated. Additionally, Zn mobility during anoxic incubation of Zn-containing lepidocrocite and feroxyhyte samples was monitored. It demonstrates that, feroxyhyte and lepidocrocite can accommodate relatively higher amounts of Zn (7.71 ± 0.20 mol% and 8.56 ± 0.22 mol% respectively) than akaganeite that retains almost no Zn (0.054 ± 0.002 mol%). In contrast, Zn has the most significant effect on akaganeite morphology. Part of total Zn is incorporated into the lepidocrocite or feroxyhyte structure by substitution for lattice Fe while the remaining Zn is adsorbed on the mineral surfaces. The different Zn substitution levels in these FeOOH minerals are greatly related with the mineral structures, that is, the ratio of edge-sharing to corner-sharing [FeO6] and thus the spatial freedom, and the pHs of the mineral synthetic solutions. During anoxic incubation in the presence of 1 mM Fe2+ at pH 6 ± 0.05 for 60 d, both Zn-containing lepidocrocite and feroxyhyte keep stable, while the dissolved Zn concentrations rapidly increase at initial stage, then decrease after reaction for 3 d, and increase again from 10 d till to the end. The final Zn release from feroxyhyte (∼98%) is much higher than that from lepidocrocite (∼30%), probably owing to the higher Fe2+-induced reductive dissolution and desorption rates, and larger amounts of Zn adsorbed on the surfaces of the former. These results help to understand the effects of Zn on the physicochemical properties of FeOOH polymorphs and the geochemical behaviors of Zn associated with Fe oxyhydroxide minerals.