Local Crystal Structure of Ni2+ and Zn2+ Doped Saponite Determined by...

Due to their small particle size and high surface reactivity, clay minerals control geochemical conditions in soils. Particularly important are the sorption phenomena. In this work, the structure and speciation of the most stable edge surfaces of synthetic trioctahedral smectite saponite are measured by atomic force microscopy and modeled using molecular simulations based on density functional theory. The calculations predict that (110) and (010) are the most stable edge sites of saponite and dominate the particle shape at different degrees of hydration. The obtained theoretical results on the surface stability agree well with experimental observations and provide a basis for the further development of theoretical sorption complexation models. To address the thermodynamic driving force for the incorporation of trace metals into the structure of trioctahedral phyllosilicates, the free energy of Zn and Ni exchange reactions between the octahedral crystallographic site of saponite and the aqueous phase was obtained using the thermodynamic integration method. The obtained ab initio molecular dynamics trajectories were further used to derive theoretical X-ray absorption spectra of Zn and Ni incorporated into the saponite structure, which were compared with experimental X-ray absorption spectra of synthesized Zn- and Ni-doped saponite to validate the input parameters of the FEFF model.

Read more: https://doi.org/10.1021/acs.jpcc.5c01314