The role of microbial activities on the transformation of chromium (Cr) remediation products has generally been overlooked. This study investigated the stability of Cr(III)-Fe(III)-(oxy)hydroxides, common Cr(VI) remediation products, with a range of compositions in the presence of common microbial exudates, siderophores and small organic acids. In the presence of a representative siderophore, desferrioxamine B (DFOB), iron (Fe) was released at higher rates and to greater extents relative to Cr from all solid phases. The presence of oxalate alone caused the release of Cr, but not of Fe, from all solid phases. In the presence of both DFOB and oxalate, oxalate acted synergistically with DFOB to increase the Fe, but not the Cr, release rate. Upon reaction with DFOB or DFOB + oxalate, the remaining solids became enriched in Cr relative to Fe. Such incongruent dissolution led to solid phases with different compositions and increased solubility relative to the initial solid phases. Thus, the presence of microbial exudates can promote the release of Cr(III) from remediation products via both ligand complexation and increased solid solubility. Understanding the potential reaction kinetics and pathways of Cr(VI) remediation products in the presence of microbial activities is necessary to assess their long-term stability.

The chromium (Cr) isotope system has emerged as a potential proxy for tracing the Earth’s atmospheric evolution based on a redox-dependent framework for Cr mobilization and isotope fractionation. Although studies have demonstrated that redox-independent pathways can also mobilize Cr, no quantitative constraints exist on the associated isotope fractionations. Here we survey the effects of common environmental ligands on the dissolution of Cr(III)-(oxy)hydroxide solids and associated Cr isotope fractionation. For a variety of organic acids and siderophores, δ53Cr values of dissolved Cr(III) are −0.27 to 1.23‰, within the range of previously observed Cr isotope signatures in rock records linked to Cr redox cycling. Thus, ligand-promoted dissolution of Cr-containing solids, a redox-independent process, must be taken into account when using sedimentary Cr isotope signatures to diagnose atmospheric oxygen levels. This work provides a step towards establishing a more robust framework for using Cr isotopes to track the evolution of the Earth’s atmosphere.

No abstract

No abstract

No abstract