X-RAY DIFFRACTION EVIDENCE FOR TRANSITION METAL CATION EXCHANGE IN ANIONIC CLAYS IN ROOM-TEMPERATURE AQUEOUS FLUIDS

In anionic clays, trivalent cations within brucite-like octahedral sheets generate a net positive charge that is electrostatically balanced by anionic groups in the interlayer region. Many anionic clays exhibit a heightened ability to swap their interlayer anions with negatively charged species in solution, such as cyanide ions, pesticides, and radioactive wastes. Based on measurements of changes in fluid chemistry, Komarneni et al. (1998) have argued that hydrotalcite-like anionic clays can remove dissolved transition metal cations (e.g., Co²⁺, Ni²⁺, Zn²⁺, and Cu²⁺) from solutions at room temperature by diadochic exchange reactions with octahedral Mg.

We have examined the reaction products of hydrotalcite-like clays in the presence of dissolved transition metals using synchrotron X-ray powder diffraction, and our Rietveld analyses provide direct support for the proposed exchange model. Commercially prepared manasseite {[Mg₆²⁺ Al₂³⁺ (OH)₁₆]^{2+ .} [CO₃ ^. 4H₂O]^2-} powders (0.05 gm) were equilibrated at room temperature in 25 ml solutions at pH 4-5 with 0.01 M of CuCl₂ and, separately, ZnCl₂ for 24 hr at room temperature. Synchrotron X-ray diffraction data for the dried powders were collected at Beamline X7B, National Synchrotron Light Source, Brookhaven National Laboratory using a Mar345 imaging plate. Rietveld analyses of the starting and reacted powders were well-behaved with final weighted residuals below 0.04. The Cu-exchanged manasseite experiment generated a significant amount (57 wt%) of paratacamite [Cu₂Cl(OH)₃] as a side-product with minor quantities of periclase. Occupancy factors for the octahedral sites in the cupric manasseite refined to 31% Cu and 69% Mg. Similarly, the Zn-exchanged manasseite yielded minor amounts of brucite, periclase, and zincite; octahedral occupancy factors for manasseite refined to 21% Zn and 79% Mg.

Our analyses also have offered the first high-resolution structure refinement for endmember manasseite with a=3.0534(1) Å and c=22.964(2) Å in space group R-3m (R_wp=0.0285). Moreover, our temperature-resolved synchrotron heating experiments revealed that Cu-substituted manasseite exhibits a thermal stability that is very similar to the end-member composition, with both structures transforming reconstructively via dehydration reactions at ~150 ^oC.