Local structure analysis of strontium sorption to hydrous manganese oxide

To develop mechanistic models of contaminant distribution processes, we con ducted an X-ray absorption fine structure analysis of strontium sorption to hydrous manganese oxide (HMO). Sr K-edge measurements were performed at 29 8, 220, and 77 K, and at sample loadings from 10(-4) to 10(-2) mol Sr/g HMO . Results from fitting the first shell in the sorbed samples indicate that strontium is surrounded by 10-12 oxygen atoms at an average distance of 2.5 8 Angstrom. This coordination environment is consistent with the strontium atom remaining hydrated upon sorption to the oxide, where in water hydrated strontium has approximately 9 atoms of oxygen at 2.62 Angstrom. Furthermor e, the temperature dependence of the strontium-oxygen bond also suggests ph ysical adsorption due to the large contribution of the dynamic component of the Debye Waller factor. Although second-shell data are consistent with ei ther 3 manganese atoms at 4.12 Angstrom or 6 strontium atoms at 3.88 Angstr om, both the near-edge and fine structure data for the manganese K-edge ind icate that the local coordination environment of the manganese ion remains intact as a function of time or strontium sorption. Furthermore, the local structure of amorphous manganese oxide is highly ordered, (C) 2000 Academic Press.