Local structure analysis of strontium sorption to hydrous manganese oxide

To develop mechanistic models of contaminant distribution processes, we conducted an X-ray absorption fine structure analysis of strontium sorption to hydrous manganese oxide (HMO). Sr K-edge measurements were performed at 298, 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 2.58 Å. 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 Å. Furthermore, the temperature dependence of the strontium-oxygen bond also suggests physical adsorption due to the large contribution of the dynamic component of the Debye Waller factor. Although second-shell data are consistent with either 3 manganese atoms at 4.12 Å or 6 strontium atoms at 3.88 Å, both the near- edge and fine structure data for the manganese K-edge indicate 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.