TY - JOUR
T1 - Mineralogical and geochemical constraints on chromium oxidation induced by birnessite
AU - Kong, Kyeong Pil
AU - Fischer, Timothy B.
AU - Heaney, Peter J.
AU - Post, Jeffrey E.
AU - Stubbs, Joanne E.
AU - Eng, Peter J.
N1 - Funding Information:
Funding for this work was provided by NSF grant EAR-1552211, and The Pennsylvania State University. X-ray diffraction and spectroscopy experiments were conducted at GeoSoilEnviroCARS (The University of Chicago, Sector 13) at beamlines 13-BM-C and 13-ID-C at the Advanced Photon Source (APS), Argonne National Laboratory. GSECARS is supported by the National Science Foundation - Earth Sciences (EAR - 1634415) and Department of Energy- GeoSciences (DE-FG02-94ER14466). Spectrophotometry analyses following synchrotron TRXRD were performed in the Center for Nanoscale Materials (CNM). Both APS and CNM are U.S. Department of Energy Office of Science User Facilities operated by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Special thanks to the staff members at GSECARS and CNM who were critical to our experimentation and data analysis processes: Antonio Lanzirotti, Matthew Newville, Nancy Lazarz, and David Gosztola. In addition, we thank Lee Kump for the use of his spectrophotometer at Penn State University
Funding Information:
Funding for this work was provided by NSF grant EAR-1552211 , and The Pennsylvania State University. X-ray diffraction and spectroscopy experiments were conducted at GeoSoilEnviroCARS (The University of Chicago, Sector 13) at beamlines 13-BM-C and 13-ID-C at the Advanced Photon Source (APS), Argonne National Laboratory. GSECARS is supported by the National Science Foundation - Earth Sciences ( EAR - 1634415 ) and Department of Energy- GeoSciences ( DE-FG02-94ER14466 ). Spectrophotometry analyses following synchrotron TRXRD were performed in the Center for Nanoscale Materials (CNM). Both APS and CNM are U.S. Department of Energy Office of Science User Facilities operated by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Special thanks to the staff members at GSECARS and CNM who were critical to our experimentation and data analysis processes: Antonio Lanzirotti, Matthew Newville, Nancy Lazarz, and David Gosztola. In addition, we thank Lee Kump for the use of his spectrophotometer at Penn State University
Publisher Copyright:
© 2019 Elsevier Ltd
PY - 2019/9
Y1 - 2019/9
N2 - We have explored redox reactions between dissolved Cr and the phyllomanganate birnessite with high resolution through simultaneous synchrotron X-ray diffraction, X-ray spectroscopy, and fluid analysis at different concentrations of solution pH. Specifically, we collected time-resolved synchrotron X-ray diffraction patterns and X-ray absorption near edge structure (XANES) spectra from triclinic Na-birnessite every 15 min while passing pH controlled 1.0 mM Cr(III) nitrate solutions through a capillary cell. In addition, we quantified Cr(VI) concentrations of the eluate solution every 15 min using spectrophotometry. Consistent with previous studies, we observed an increased rate of Cr(VI) production with decreasing pH. We attribute the comparatively slow kinetics of Cr(III) oxidation at pH 5.0 and 4.0 to a transformation from triclinic to hexagonal birnessite. This solid-state transition reproducibly coincided with a ∼10-fold decline in the extent of oxidation of aqueous Cr(III). Control experiments without Cr(III) revealed no evidence for birnessite transformation within the same time frame, and experiments with hexagonal birnessite as the starting material generated solutions with low fractions (∼3 mol%) of dissolved Cr(VI) from start to finish. At pH 3.0 and 2.0, however, production of Cr(VI) was consistently higher than was observed at pH 5.0 and 4.0. Specifically, at pH 2.0, 80 mol% of the influent Cr(III) was oxidized to Cr(VI) during the experiment compared to 20 mol% at pH 5.0. XANES analyses showed evidence for both Cr(III) and Cr(VI) adsorbing onto the surface of birnessite at all pH values. We propose that Cr(III) is oxidized to Cr(VI) by an electron exchange that reduces Mn(III) in birnessite to Mn(II). At pH 3.5 and higher, the structure of birnessite consequently transforms to hexagonal birnessite. By this pathway, the birnessite crystal structure critically controls the oxidation of dissolved Cr(III) due to the accessibility of reactive Mn(III) in triclinic birnessite relative to hexagonal birnessite. Below pH 3.5, however, birnessite dissolution systematically exposes reactive sites that enable the continuous oxidation of Cr(III) to Cr(VI).
AB - We have explored redox reactions between dissolved Cr and the phyllomanganate birnessite with high resolution through simultaneous synchrotron X-ray diffraction, X-ray spectroscopy, and fluid analysis at different concentrations of solution pH. Specifically, we collected time-resolved synchrotron X-ray diffraction patterns and X-ray absorption near edge structure (XANES) spectra from triclinic Na-birnessite every 15 min while passing pH controlled 1.0 mM Cr(III) nitrate solutions through a capillary cell. In addition, we quantified Cr(VI) concentrations of the eluate solution every 15 min using spectrophotometry. Consistent with previous studies, we observed an increased rate of Cr(VI) production with decreasing pH. We attribute the comparatively slow kinetics of Cr(III) oxidation at pH 5.0 and 4.0 to a transformation from triclinic to hexagonal birnessite. This solid-state transition reproducibly coincided with a ∼10-fold decline in the extent of oxidation of aqueous Cr(III). Control experiments without Cr(III) revealed no evidence for birnessite transformation within the same time frame, and experiments with hexagonal birnessite as the starting material generated solutions with low fractions (∼3 mol%) of dissolved Cr(VI) from start to finish. At pH 3.0 and 2.0, however, production of Cr(VI) was consistently higher than was observed at pH 5.0 and 4.0. Specifically, at pH 2.0, 80 mol% of the influent Cr(III) was oxidized to Cr(VI) during the experiment compared to 20 mol% at pH 5.0. XANES analyses showed evidence for both Cr(III) and Cr(VI) adsorbing onto the surface of birnessite at all pH values. We propose that Cr(III) is oxidized to Cr(VI) by an electron exchange that reduces Mn(III) in birnessite to Mn(II). At pH 3.5 and higher, the structure of birnessite consequently transforms to hexagonal birnessite. By this pathway, the birnessite crystal structure critically controls the oxidation of dissolved Cr(III) due to the accessibility of reactive Mn(III) in triclinic birnessite relative to hexagonal birnessite. Below pH 3.5, however, birnessite dissolution systematically exposes reactive sites that enable the continuous oxidation of Cr(III) to Cr(VI).
UR - http://www.scopus.com/inward/record.url?scp=85067844504&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85067844504&partnerID=8YFLogxK
U2 - 10.1016/j.apgeochem.2019.104365
DO - 10.1016/j.apgeochem.2019.104365
M3 - Article
AN - SCOPUS:85067844504
VL - 108
JO - Applied Geochemistry
JF - Applied Geochemistry
SN - 0883-2927
M1 - 104365
ER -