TY - JOUR
T1 - Kinetic analysis of cation exchange in birnessite using time-resolved synchrotron X-ray diffraction
AU - Lopano, Christina L.
AU - Heaney, Peter J.
AU - Bandstra, Joel Z.
AU - Post, Jeffrey E.
AU - Brantley, Susan L.
N1 - Funding Information:
Funding for this research was provided by NSF grants EAR04-17741 , EAR07-45374 , and the Center for Environmental Kinetics Analysis (CEKA) , an NSF- and DOE-sponsored Environmental Molecular Science Institute ( NSF CHE-0431328 ). This research was carried out at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-98CH10886. This manuscript benefited greatly from the thoughtful feedback of three anonymous reviewers, the associate editor, Dr. Peggy O’Day, and Ms. Claire Fleeger at Penn State University.
PY - 2011/7/15
Y1 - 2011/7/15
N2 - In this study, we applied time-resolved synchrotron X-ray diffraction (TRXRD) to develop kinetic models that test a proposed two-stage reaction pathway for cation exchange in birnessite. These represent the first rate equations calculated for cation exchange in layered manganates. Our previous work has shown that the substitution of K, Cs, and Ba for interlayer Na in synthetic triclinic birnessite induces measurable changes in unit-cell parameters. New kinetic modeling of this crystallographic data supports our previously postulated two-stage reaction pathway for cation exchange, and we can correlate the kinetic steps with changes in crystal structure. In addition, the initial rates of cation exchange, R (Å3min-1), were determined from changes in unit-cell volume to follow these rate laws: R=1.75[K+(aq)]0.56, R=41.1[Cs+(aq)]1.10, R=1.15[Ba2+(aq)]0.50. Thus, the exchange rates for Na in triclinic birnessite decreased in the order: Cs≫K>Ba. These results are likely a function of hydration energy differences of the cations and the preference of the solution phase for the more readily hydrated cation.
AB - In this study, we applied time-resolved synchrotron X-ray diffraction (TRXRD) to develop kinetic models that test a proposed two-stage reaction pathway for cation exchange in birnessite. These represent the first rate equations calculated for cation exchange in layered manganates. Our previous work has shown that the substitution of K, Cs, and Ba for interlayer Na in synthetic triclinic birnessite induces measurable changes in unit-cell parameters. New kinetic modeling of this crystallographic data supports our previously postulated two-stage reaction pathway for cation exchange, and we can correlate the kinetic steps with changes in crystal structure. In addition, the initial rates of cation exchange, R (Å3min-1), were determined from changes in unit-cell volume to follow these rate laws: R=1.75[K+(aq)]0.56, R=41.1[Cs+(aq)]1.10, R=1.15[Ba2+(aq)]0.50. Thus, the exchange rates for Na in triclinic birnessite decreased in the order: Cs≫K>Ba. These results are likely a function of hydration energy differences of the cations and the preference of the solution phase for the more readily hydrated cation.
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U2 - 10.1016/j.gca.2011.04.021
DO - 10.1016/j.gca.2011.04.021
M3 - Article
AN - SCOPUS:79958845091
SN - 0016-7037
VL - 75
SP - 3973
EP - 3981
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 14
ER -