Kinetic analysis of cation exchange in birnessite using time-resolved synchrotron X-ray diffraction

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 (Å3 min−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.