Interaction of divalent transition metals with carbonate green rust: Insights into modern and ancient environments

Description

Carbonate green rust (GRCO₃), which has been reported in suboxic and anoxic environments, is highly reactive towards dissolved metals and other aqueous compounds. The interaction of GRCO₃ towards a variety of aqueous compounds has been the topic of past studies, but its reactivity under conditions relevant to modern and ancient marine environments has not been systematically explored. Furthermore, the fate of the aqueous compounds as the GRCO₃ ages and transforms to stable iron-bearing minerals has rarely been examined. We bridge these knowledge gaps in a series of co-precipitation experiments of the divalent metals Cd(II), Co(II), Cu(II), Mn(II), Ni(II), and Zn(II) with GRCO₃ in seawater-analog solutions, at pH 8.0, 25˚C, and under anoxic conditions. The experimental results are provided as uptake percentages at different metal:Fe ratios and as a series of partition coefficients of the studied divalent metals between aqueous solution and GRCO₃. Additionally, aging experiments up to 13 months in duration were conducted at metal concentrations of 1µM and under the same experimental conditions, to quantify the retention or release of the metals associated with GRCO₃ maturation and its transformation to thermodynamically stable phases. Uptake of Zn(II) by isomorphic substitution for iron or adsorption to the GRCO₃ surface was quantitative, and most of the Zn(II) was retained during partial transformation of the GRCO₃ to goethite. Uptake of Cu(II) by reduction to metallic Cu and possibly isomorphic substitution was also quantitative and apparently irreversible upon aging under anoxic conditions. Uptake of Ni(II) was ~90% and increased to ~100% within 3 to 13 months. About 80% of dissolved Co(II) was taken up by GRCO₃ within three days and an increase to 100% was observed over the course of 6 to 13 months and likely related to partial oxidation of the Co(II) to Co(III). Uptake of Cd(II) was initially low (~10%), but became quantitative within 3 months of aging, possibly due to sluggish kinetics of adsorption, the existence of strong aqueous complexes, or gradual substitution for Fe(II) in the GRCO₃ structure. A marked increase in Mn(II) uptake during aging may be the result of oxidation to Mn(III/IV) by the Fe(III) in the GRCO₃. We discuss the mechanisms of divalent metal uptake by GRCO₃, the effects of solution composition on uptake, the effects of mineral transformation during aging, similarities and differences between interaction of the metals with GRCO₃ and ferrihydrite, and the implications of our findings for modern and ancient natural environments and for environmental remediation.

Institutions

Categories

Funding

Licence