Interaction of transition metals with carbonate green rust: Environmental applications and insights into ancient oceans

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Abstract

Carbonate green rust (GRCO) is a mixed-valence double hydroxide mineral found in suboxic and anoxic natural and engineered environments. It is highly reactive toward dissolved metals, as shown in past studies of GRCO-metal interactions. However, such interactions under conditions representative of modern and ancient marine systems remains poorly constrained, as does the fate of metals during GRCO aging and transformation into stable Fe-bearing minerals. Constraining these processes is critical for predicting long-term metal mobility and retention in marine and groundwater systems and for understanding the biogeochemical cycles and reduction-oxidation dynamics of metals in the anoxic and ferruginous Precambrian ocean, and the supply of bioessential metals to ancient microbial communities.
To address this knowledge gap, we investigated GRCO co-precipitation with Cd, Co, Cu, Mn, Ni, and Zn (0.1–300 M) in seawater-analog solutions at pH 8.0 under anoxic conditions. In aging experiments at 1 M metal concentrations over 13 months, we assessed metal retention during GRCO transformation. After 3 days, metal uptake generally followed the Irving–Williams series (Cd Mn Ni Co Cu Zn), except Ni and Co, which were reversed due to Co oxidation. 

Over 13 months, all metals except Mn were taken up completely, while Mn uptake increased gradually but was not quantitative. Cu and Zn exhibited effectively irreversible uptake, likely through co-precipitation at Fe sites and, for Cu, also reduction to Cu and precipitation. Mn and Co were slowly oxidized to Mn and Co, respectively, while Ni and Cd uptake increased gradually, reflecting slow kinetics of adsorption or structural incorporation. These findings provide mechanistic insight into divalent metal uptake by GRCO and demonstrate how solution composition, aqueous speciation, mineral transformations, and aging govern metal retention in both modern and ancient suboxic and anoxic environments. In light of these findings, we highlight the complexity of interpreting proxy records preserved in Precambrian Fe-enriched sedimentary archives.

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