Structural study of selenium(IV) substitutions in calcite

In this work we present the results of a crystallographic study of synthetic calcite co-precipitated with selenium. We investigated both at a macroscopic and nanoscopic level, whether Se(IV) can be incorporated to bulk calcite by substitution at carbon sites. Neutron diffraction experiments, EXAFS spectroscopy and a theoretical modeling of the crystallographic structure using VASP are reported. We have found that the calcite unit cell volume obtained from the VASP simulations increases linearly with Se content in the structure. This allows estimating the Se content effectively incorporated into calcite based on volume measurements. Diffraction results are combined with VASP modeling to estimate a concentration of Se(IV) co-precipitated with calcite of 30-75 mmol/kg solid. EXAFS spectroscopy, in combination with our theoretical model of the local structure surrounding the Se atom, confirms the possibility of selenite ion substituting for carbonate anion in the calcite structure. (C) 2009 Elsevier B.V. All rights reserved.