Siderophore and Organic Acid Promoted Dissolution and Transformation of Cr(III)-Fe(III)-(oxy)hydroxides Emily M. Saad, † Jingying Sun, ‡ Shuo Chen, ‡ Olaf J. Borkiewicz, § Mengqiang Zhu, ∥ Owen W. Duckworth, ⊥ and Yuanzhi Tang* ,† † School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332-0340, United States ‡ Department of Physics and Texas Center for Superconductivity (TCSUH), University of Houston, Houston, Texas 77204, United States § Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States ∥ Department of Ecosystem Science and Management, University of Wyoming, Laramie, Wyoming 82071, United States ⊥ Department of Crop and Soil Science, North Carolina State University, Raleigh, North Carolina 27695, United States * S Supporting Information ABSTRACT: The role of microbial activities on the trans- formation 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 prod- ucts, 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. 1. INTRODUCTION Chromium (Cr) is a significant contaminant in the United States 1 and around the globe. The most common valence states of Cr in soils and natural waters are Cr(III) and Cr(VI). 2 Under oxic conditions, Cr speciation is dominated by the highly soluble and mobile Cr(VI) species, chromate (CrO 4 2‑ ) and bichromate (HCrO 4 2‑ ). 3 Due to its higher bioavailability and strong oxidizing capability, Cr(VI) exhibits a toxicity threat that is 100 times greater than that of Cr(III). 4 Contrastingly, under circumneutral pH conditions, Cr(III) is traditionally considered insoluble and stable. 3 A wide range of reactants are capable of reducing Cr(VI) to Cr(III), including biotic (e.g., dissimilatory metal reducing microbes) 5−8 and abiotic (e.g., sulfide, Fe(0), Fe(II), and organic compounds) 9−13 pathways. The common reduction products are either pure Cr(III)-(oxy)hydroxides or a mixed Cr(III)-Fe(III)-(oxy)hydroxide solid solution series in the presence of iron (Fe) and due to the structural similarities between Cr(III) and Fe(III). 12,14,15 These (oxy)hydroxides are sparingly soluble 14 and are typically considered as the ultimate sink of Cr in subsurface environments. 2 Cr(III) can be oxidized by ubiquitous manganese (Mn) oxides to Cr(VI), which leads to contamination. 16 Studies have shown that the oxidation kinetics of Cr(III) by Mn oxides are affected by Cr speciation (e.g., soluble Cr(III) or Cr(III)− organic complex vs solid Cr(OH) 3 ). 16−21 Thus, understanding the potential solubilization mechanisms of Cr(III)-containing solids under environmental conditions and their subsequent oxidation efficiency by Mn oxides is critical for constraining the fate of Cr in the subsurface 22 and evaluating the long-term efficiency of biotic/abiotic Cr(VI) remediation techniques. Siderophores and small organic acids are both ubiquitous organic molecules produced by a wide range of microorganisms (e.g., bacteria and fungi) and plants 23,24 and are often coexuded. 25,26 Siderophores are organic chelating agents with a high affinity for Fe(III) and can facilitate Fe solubilization and bioavailability from low solubility Fe(III)-containing mineral Received: October 25, 2016 Revised: February 15, 2017 Accepted: February 20, 2017 Published: February 20, 2017 Article pubs.acs.org/est © 2017 American Chemical Society 3223 DOI: 10.1021/acs.est.6b05408 Environ. Sci. Technol. 2017, 51, 3223−3232