TY - JOUR
T1 - Transformations from triclinic to hexagonal birnessite at circumneutral pH induced through pH control by common biological buffers
AU - Ling, Florence T.
AU - Heaney, Peter J.
AU - Post, Jeffrey E.
AU - Gao, Xiang
N1 - Funding Information:
Funding was provided by NSF grant EAR-1147728 . Portions of this work were performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation — Earth Sciences ( EAR-1128799 ) and Department of Energy — GeoSciences ( DE-FG02-94ER14466 ). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
Publisher Copyright:
© 2015 Elsevier B.V.
PY - 2015/11/27
Y1 - 2015/11/27
N2 - Laboratory experiments that explore the bioprecipitation or redox transformations of layered Mn oxides commonly employ buffers, such as the HEPES and MES buffers, to maintain solution pH to near neutrality. The assumption is that holding solution pH constant does not serve as the primary control for the variety of Mn oxide produced. To test this assumption, synthetic triclinic Na-birnessite was reacted in batch experiments with a pH 7 HEPES buffer, a pH 7 MES buffer, and an unbuffered pH 7 solution for 14 days in total darkness. At the end of the experimental run, the Mn oxide solids were analyzed by conventional and synchrotron X-ray powder diffraction. These assays revealed that in the presence of the HEPES buffer, triclinic Na-birnessite completely transformed into highly crystalline hexagonal H-birnessite. In unbuffered solutions starting at pH7 and in the presence of MES, which offers a lower buffering capacity than does HEPES, triclinic Na-birnessite partially transformed to poorly crystalline hexagonal H-birnessite. The unbuffered pH7 solution exhibited an increase in pH to 8.03. We interpret the results to indicate that: 1) buffers can indirectly promote the transformation of triclinic Na-birnessite to hexagonal H-birnessite by serving as a source of H+, even at circumneutral pH; 2) triclinic Na-birnessite alone can stimulate hydrolysis, which in turn induces an exchange of H+ for interlayer Na+; and 3) H-birnessite and Na-birnessite operate as an acid-conjugate base pair.
AB - Laboratory experiments that explore the bioprecipitation or redox transformations of layered Mn oxides commonly employ buffers, such as the HEPES and MES buffers, to maintain solution pH to near neutrality. The assumption is that holding solution pH constant does not serve as the primary control for the variety of Mn oxide produced. To test this assumption, synthetic triclinic Na-birnessite was reacted in batch experiments with a pH 7 HEPES buffer, a pH 7 MES buffer, and an unbuffered pH 7 solution for 14 days in total darkness. At the end of the experimental run, the Mn oxide solids were analyzed by conventional and synchrotron X-ray powder diffraction. These assays revealed that in the presence of the HEPES buffer, triclinic Na-birnessite completely transformed into highly crystalline hexagonal H-birnessite. In unbuffered solutions starting at pH7 and in the presence of MES, which offers a lower buffering capacity than does HEPES, triclinic Na-birnessite partially transformed to poorly crystalline hexagonal H-birnessite. The unbuffered pH7 solution exhibited an increase in pH to 8.03. We interpret the results to indicate that: 1) buffers can indirectly promote the transformation of triclinic Na-birnessite to hexagonal H-birnessite by serving as a source of H+, even at circumneutral pH; 2) triclinic Na-birnessite alone can stimulate hydrolysis, which in turn induces an exchange of H+ for interlayer Na+; and 3) H-birnessite and Na-birnessite operate as an acid-conjugate base pair.
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U2 - 10.1016/j.chemgeo.2015.10.007
DO - 10.1016/j.chemgeo.2015.10.007
M3 - Article
AN - SCOPUS:84945180059
SN - 0009-2541
VL - 416
SP - 1
EP - 10
JO - Chemical Geology
JF - Chemical Geology
ER -