Interaction of metal oxyions and phosphate with carbonate green rust: Insights into Earth’s modern and ancient environments

Abstract

Carbonate green rust (GRCO${}_3$) is often found in suboxic and anoxic environments, and is known to be highly reactive towards dissolved metals and other aqueous compounds. Though the reactivity of GRCO${}_3$ towards a variety of aqueous compounds has been determined in previous studies, its reactivity under conditions relevant to modern and ancient marine environments has not been systematically explored. Furthermore, the fate of the aqueous compounds upon aging of the GRCO${}_3$ remains almost completely unknown.

To inform the interaction of GRCO${}_3$ with oxyion-forming metals during the mineral’s precipitation and aging, we conducted a series of co-precipitation experiments of Cr${}^{\mathrm{VI}}$, Mo${}^{\mathrm{VI}}$, V${}^V$, U${}^{\mathrm{VI}}$, and PO${}_4^{3-}$ with GRCO${}_3$ 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 oxycation and oxyanions between aqueous solution and GRCO${}_3$. Additionally, aging experiments up to 13 months in duration were conducted at metal concentrations of 1 $\mu$M and under the same experimental conditions, to quantify the retention or release of the metals associated with GRCO${}_3$ transformation to thermodynamically stable phases. Uptake of both V${}^V$ and Cr${}^{\mathrm{VI}}$ was near-quantitative, and these metals were retained during aging. We identify reduction by the Fe${}^{\mathrm{II}}$ in GRCO${}_3$ as the predominant mechanism of Cr${}^{\mathrm{VI}}$ uptake, whereas in V${}^V$ both reduction and adsorption are implied by our results. Up to $\sim$20% of U${}^{\mathrm{VI}}$ was taken up by reduction and retained during aging, and uptake of Mo${}^{\mathrm{VI}}$ was negligible. Uptake of PO${}_4^{3-}$ by adsorption and possibly substitution for carbonate groups in the GRCO${}_3$ was quantitative at initial PO${}_4^{3-}$ concentrations $\le 1\mu$M and declined to $\sim$60%–70% at higher concentrations, potentially due saturation of adsorption sites. In experiments with 1 $\mu$M PO${}_4^{3-}$, where uptake was initially quantitative, we observe release of $\sim$30% of the PO${}_4^{3-}$, likely due to transformation of the GRCO${}_3$ to other phases with a lower affinity towards PO${}_4^{3-}$. We relate the uptake of the metals and PO${}_4^{3-}$ to the chemical composition of the seawater-analog experimental solutions, and discuss the mechanisms of uptake and retention/release in the context of mineral transformations during aging. We identify a role for dissolved silica in both the uptake and the aged mineral assemblage, and discuss the implications of our findings for modern and ancient natural environments and for environmental remediation.