Investigation of Ni Sorption on Pyrophyllite:  An XAFS Study

Abstract

Sorption reactions at the solid-water interface decrease solute mobility and often control the fate, bioavailability, and transport of metal ions in soils and groundwaters. A thorough understanding of the structural environment of metals at the solid-water interface is therefore of fundamental importance. In this study, X-ray absorption fine structure (XAFS) spectroscopy was used to discern the local atomic structure of Ni(II) sorbed onto pyrophyllite. The first coordination shell consists of 6 O atoms at 2.02-2.04 Å. For the second shell, XAFS data suggest a single Ni-Al/Si distance (2.96-3.03 Å), indicative of edge sharing of Ni and Al octahedra and possibly the presence of mixed nickel-aluminum hydroxides. As Ni surface loading on pyrophyllite increased, the number of Ni second-neighbor atoms at a distance of 2.99-3.00 Å increased from N ≈ 1 to N ≈ 5. The presence of multinuclear surface complexes was depicted at low surface loading and at reaction conditions undersaturated with respect to the formation of Ni(OH) 2 (s). This observation suggests that the total coverage of surface sites is not necessary for the formation of multinuclear surface complexes and implies that the pyrophyllite surface promotes hydrolysis and multinuclear complex formation.

Translational diffusion of water and its dependence on temperature in charged and uncharged clays: A neutron scattering study

“…For montmorillonite the CEC used was 99 milliequivalents/100 g, 33 and for illite, it was 26.6 milliequiavalents/100 g. 31 The last column in Table I shows the average number of water layers between two clay surfaces calculated as given by González Sánchez et al 32 for Na-and Ca-montmorillonite, Na-and Ca-illite, and kaolinite. Pyrophyllite ͑with 30% porosity͒ was calculated in the same way using an S BET ͑external surface area͒ of 6.93 m 2 / g, considering S BET Ϸ S EGME ͑total surface area͒ as stated by Scheidegger et al 34 For montmorillonite in the f.h form we have an average of about two layers of water between the surfaces, in the h.h about a single layer, and in the quarter-hydrated ͑q.h͒ statistically half a water layer ͑see Table I͒. These numbers agree well with x-ray diffraction measurements because of the stepwise change in the d spacing.…”

Spectroscopic Evidence for the Formation of Mixed-Cation Hydroxide Phases upon Metal Sorption on Clays and Aluminum Oxides

“…9) (15,19,20). Kaolinite and pyrophyllite were selected The remaining wet pastes were washed with excess highbecause they represent 1:1 and 2:1 clays which have no purity water to adequately remove entrained electrolyte and structural charge in their pure stoichiometric form (Al 2 Si 2 -then centrifuged again.…”

XAFS Spectroscopy Study of Cu(II) Sorption on Amorphous SiO2and γ-Al2O3: Effect of Substrate and Time on Sorption Complexes

“…Formation of multimeric surface complexes, clusters, and precipitates is, by analogy with homogeneous systems, likely to be time dependent. There are hints of this in interpretation of the commonly observed two-stage time dependence of ion uptake, and there is XAFS spectroscopic evidence as well involving Ni(II) uptake by pyrophyllite (7,30) and Co(II) uptake by kaolinite (31). To more accurately predict possible changes in polymerization of metal-ion complexes at oxide-aqueous solution interfaces, including the formation of precipitates, as a function of surface coverage and time, it is necessary to understand the effects of different substrates in promoting these changes.…”