A Computational Approach to Understanding Layered Double Hydroxide Formation and Nickel Sorption On Iron Oxides.

Spectroscopic techniques using synchrotron radiation (e.g., (µ-)EXAFS, µ-XRF) are powerful tools to determine in situ metal speciation in soils.  These techniques accurately describe the inter-atomic distances of nearest neighboring atoms to elements of interest and provide elemental distribution data at the micron scale.  Synchrotron spectroscopies show clear evidence of metal precipitate formation in soils (e.g. Ni/Al layered double hydroxides) and describe sorption mechanisms of transition metals on oxide surfaces (e.g., nickel sorbed to iron oxides).  Formation of LDH precipitates increases metal stability in soil.  Thermodynamic data show that LDH compounds become more stable over time.  Though it is known that these compounds form in soils, it is not understood why they form.  The reason LDH formation is not understood is because of the difficulty in obtaining concomitant, real time thermodynamic and structural information.  Energy minimization calculations of clusters that imitate LDH formation and metal sorption on oxide surfaces can be used to obtain thermodynamic and structural information.  Energy minimization data is directly related to the stability and thermodynamic properties of the cluster.  The calculated inter-atomic distances can be compared to those obtained via EXAFS.  In this study, quantum chemical calculations were carried out on Ni/Al and Ni/Fe clusters to examine the thermodynamic properties and inter-atomic distances during various stages of Ni/Al LDH formation and Ni sorption to iron oxide. To mimic the LDH structure, single sheets of Ni-for-Mg substituted hydrotalcite were used as starting clusters for the calculations.  Ni sorbed to iron oxide clusters were created using a polyhedral approach to compare bond distance differences between monodentate mononuclear, bidentate binuclear, bidentate mononuclear, and tridentate mononuclear complexes.