André M. Scheidegger

The kinetics of mixed Ni-Al hydroxide formation on clay and aluminum oxide minerals: a time-resolved XAFS study

Abstract In this study kinetic investigations were combined with X-ray Absorption Fine Structure (XAFS) measurements to determine Ni sorption processes on pyrophyllite, gibbsite, and montmorillonite over extended time periods (min-months). The kinetic investigations revealed that Ni sorption reactions (pH = 7.5, [Ni] initial = 3 mM, I = 0.1 M (NaNO 3 )) were initially fast (8–35% of the initial Ni was removed within the first 40 min). Thereafter, the rate of sorption decreased significantly and depended on the type of mineral surface. For the Ni/pyrophyllite system Ni removal was almost complete after a reaction time of 24h while for the Ni/gibbsite and Ni/montmorillonite systems metal sorption continued up to ≈2 months. XAFS data revealed the presence of a mixed Ni/Al phase in the Ni/pyrophyllite and Ni/gibbsite systems after a reaction time of minutes. These results suggest that adsorption and nucleation processes (mixed Ni/Al phase formation) can occur simultaneously over time scales of only minutes. However, our finding of a fast growing mixed Ni/Al phase cannot be extrapolated to other sorption systems. A reaction time of 48 h was required for the presence of a mixed Ni/Al phase in the Ni/mormorillonite system. As reaction time progressed, the number of second neighbor Ni atoms (N Ni-Ni ) at a distance of ≈3.05 A increased in all sorption systems, suggesting further growth of a mixed Ni/Al phase with increasing reaction time. Our study suggests that three phenomena occur at the mineral/liquid interface: (1) nonspecific (i.e, outer- sphere complexation) and/or specific adsorption (i.e., inner-sphere complexation), (2) dissolution of Al, and (3) nucleation of a mixed Ni/Al phase. The rate-limiting step is the dissolution of Al from the surface, which depends on the mineral substrate. Using the Ni linear sorption rates observed in the Ni/gibbsite and Ni/montmorillonite systems and assuming the Ni/Al ratios in our sorption samples are within the range of Ni/Al ratios provided in the literature (1.3–5.6), one can estimate an average Al dissolution rate which seems to be enhanced compared to the Al dissolution rates of the minerals alone. This finding indicates that the dissolution of clay and aluminum oxide minerals can be promoted by metal ions such as Ni(II) through the formation of a mixed Ni/Al phase.

A critical assessment of sorption-desorption mechanisms at the soil mineral/water interface

Sorption is one of the most important chemical processes in soil. It affects the fate and mobility of nutrients and contaminants in soils and waters greatly. This paper critically reviews the mechanisms of sorption/desorption phenomena at the mineral/water interface. A brief discussion of macroscopic, equilibrium approaches for describing sorption processes is provided. However, emphasis will be placed on the importance of understanding the rates of sorption/desorption processes and coupling the kinetic investigations with in-situ atomic/molecular resolution surface techniques. The use of X-ray absorption fine structure (XAFS) spectroscopy and scanning force microscopy (SFM) will be emphasized.

Structural evidence for the sorption of Ni(II) atoms on the edges of montmorillonite clay minerals: A polarized X-ray absorption fine structure study

The nature of surface complexes formed on Ni uptake onto montmorillonite (a dioctahedral smectite) has been investigated over an extended time period by polarized extended X-ray absorption fine structure (P-EXAFS) spectroscopy. Self-supporting films of Ni-sorbed montmorillonite were prepared by contacting Ni and montmorillonite at pH 7.2, high ionic strength (0.3 M NaClO4), and low Ni concentration ([Ni]initial = 19.9 μM) for 14- and 360-d reaction time. The resulting Ni concentration on the clay varied from 4 to 7 μmol/g. Quantitative texture analysis indicates that the montmorillonite particles were well orientated with respect to the plane of the film. The full width at half maximum of the orientation distribution of the c* axes of individual clay platelets about the normal to the film plane was 44.3° (14-d reaction time) and 47.1° (360-d reaction time). These values were used to correct the coordination numbers determined by P-EXAFS for texture effects. Ni K-edge P-EXAFS spectra were recorded at angles between the incident beam and the film normal equal to 10, 35, 55, and 80°. Spectral analysis led to the identification of three nearest cationic subshells containing 2.0 ± 0.5 Al at 3.0 A and 2.0 ± 0.5 Si at 3.12 A and 4.0 ± 0.5 Si at 3.26 A. These distances are characteristic of edge-sharing linkages between Al and Ni octahedra and of corner-sharing linkages between Ni octahedra and Si tetrahedra, as in clay structures. The angular dependence of the Ni-Al and Ni-Si contributions indicates that Ni-Al pairs are oriented parallel to the film plane, whereas Ni-Si pairs are not. The study reveals the formation of Ni inner-sphere mononuclear surface complexes located at the edges of montmorillonite platelets and thus that heavy metals binding to edge sites is a possible sorption mechanism for dioctahedral smectites. Data analysis further suggests that either the number of neighboring Al atoms slightly increases from 1.6 to 2 or that the structural order of the observed surface complexes increases from 0.01 A2 to 0.005 A2 with increasing reaction time. On the basis of the low Ni-Al coordination numbers, it appears that over an extended reaction time period of 1 yr the diffusion of Ni atoms in the octahedral layer is not the major uptake mechanism of Ni onto montmorillonite.

Structure of uranium sorption complexes at montmorillonite edge sites

Summary Extended X-ray absorption fine structure (EXAFS) spectroscopy at the uranium LIII-edge was used for determining the structural environment of aqueous uranyl sorbed onto montmorillonite. The study reveals that uranyl uptake at pH ∼5-∼7 and at an initial uranyl concentration of 5×10−5 M takes place at amphoteric surface hydroxyl sites as inner-sphere complex. The measured bond distances between uranium and the equatorial oxygen atoms are in the range of 2.34 Å and 2.37 Å indicating an inner-sphere coordination. At ∼3.4 Å the presence of a U-Al backscattering pair was determined. This backscattering pair indicates that the binding of the uranyl unit to amphoteric surface hydroxyl sites occurs preferred as a bidentate inner-sphere complex on aluminol groups.

Neoformation of Ni phyllosilicate upon Ni uptake on montmorillonite: A kinetics study by powder and polarized extended X-ray absorption fine structure spectroscopy

Abstract Wet chemistry kinetics and powder and polarized extended X-ray absorption fine structure (EXAFS and P-EXAFS) spectroscopy were combined to investigate the mechanism of Ni uptake on montmorillonite, at pH 8, high ionic strength (0.2 M Ca(NO3)2), initial Ni concentration of 660 μM, and solid concentration of 5.3 g/L. Approximately 20% of Ni sorbed within the first 24 h; thereafter, the Ni uptake rate slowed, and 12% of the initial Ni concentration remained in solution after 206 d of reaction time. Powder EXAFS spectra collected on wet pastes at 1, 14, 90, and 206 d showed the presence of Ni-Ni pairs at ∼3.08 A in an amount that gradually increased with time. Results were interpreted by the nucleation of a Ni phase having either an α-Ni-hydroxide– or a Ni-phyllosilicate–like local structure. The latter possibility was confirmed by recording P-EXAFS spectra of a highly textured, self-supporting montmorillonite film prepared in the same conditions as the wet samples and equilibrated for 14 d. The orientation distribution of the c*-axes of individual clay particles off the film plane, as measured by quantitative texture analysis, was 32.8° full width at half maximum, and this value was used to correct from texture effect the effective numbers of Ni and Si nearest neighbors determined by P-EXAFS. Ni atoms were found to be surrounded by 2.6 ± 0.5 Ni atoms at 3.08 A in the in-plane direction and by 4.2 ± 0.5 Si atoms at 3.26 A in the out-of-plane direction. These structural parameters, but also the orientation and angular dependence of the Ni and Si shells, strongly support the formation of a Ni phyllosilicate having its layers parallel to the montmorillonite layers. The neoformation of a phyllosilicate on metal uptake on montmorillonite, documented herein for the first time, has important geochemical implications because this dioctahedral smectite is overwhelmingly present in the environment. The resulting sequestration of sorbed trace metals in sparingly soluble phyllosilicate structure may durably decrease their migration and bioavailability at the Earth’s surface and near surface.

KINETICS OF THE FORMATION AND THE DISSOLUTION OF NICKEL SURFACE PRECIPITATES ON PYROPHYLLITE

The kinetics of the formation and the dissolution of Ni surface precipitates on pyrophyllite was studied in order to gain an understanding of the dynamics of heavy metal ion reactions in soils. Ni sorption at pH = 7.5 was initially fast. 25% of the initial Ni was removed within minutes. Thereafter, a gradual decrease in sorption was observed. Based on previous spectroscopic evidence, we attribute the fast reaction stage to adsorption phenomena and the slow reaction stage to nucleation processes on the pyrophyllite surface. The detachment of Ni from surface precipitates at pH = 4 and pH = 6 involves a small amount of Ni (< 10%) being desorbed relatively fast. The desorption of specifically adsorbed, mononuclear bound Ni may account for this rapid Ni release. Thereafter, Ni detachment was extremely slow, and the rate depended strongly on the experimental desorption method. Utilizing a conventional batch technique, further Ni release became negligible. The non-removal of reaction products may have caused the formation of secondary precipitates. Under steady-state conditions a constant Ni detachment rate was observed which we attribute to the dissolution of Ni surface precipitates. Compared to the dissolution of crystalline Ni(OH)2, Ni detachment from pyrophyllite was slow. We hypothesize that the slow Ni detachment is due to the dissolution of mixed NiAl-hydroxides which formed prior to the desorption experiment and which have a lower solubility.

Iodine species uptake by cement and CSH studied by I K-edge X-ray absorption spectroscopy

Summary The uptake of iodine species (I−/IO3−) by HCP (hardened cement paste) and a CSH (calcium silicate hydrate) phase under highly alkaline conditions has been investigated using X-ray absorption spectroscopy (XAS). The study was performed at the I K-edge (33.169 keV) instead of the I L3-edge (4.557 keV) to avoid interference with Ca (K-edge=4.038 keV), a major element in HCP and CSH phases. The XANES (X-rays absorption near-edge structure) spectra revealed no changes in the formal oxidation state of iodide (I(-I)−) and iodate (I(V)O3−) upon uptake by HCP and CSH. The EXAFS (extended X-ray absorption fine structure) oscillations from I− treated HCP and CSH samples were found to be extremely weak, limiting interpretation of the EXAFS data. The IO3− EXAFS spectra showed that the IO3− entity consisting of three oxygen atoms with a characteristic I-O distance (∼1.78 Å) is maintained upon uptake by HCP and CSH. XANES further indicated that CSH is not the uptake-controlling phase in HCP.

Ni clay neoformation on montmorillonite surface.

Polarized extended X-ray absorption fine structure spectroscopy (P-EXAFS) was used to study the sorption mechanism of Ni on the aluminous hydrous silicate montmorillonite at high ionic strength (0.3 M NaClO4), pH 8 and a Ni concentration of 0.66 mM. Highly textured self-supporting clay films were obtained by slowly filtrating a clay suspension after a reaction time of 14 days. P-EXAFS results indicate that sorbed Ni has a Ni clay-like structural environment with the same crystallographic orientation as montmorillonite layers.

Ni phases formed in cement and cement systems under highly alkaline conditions: an XAFS study

X-ray absorption fine structure (XAFS) spectroscopy was applied to assess the solubility-limiting phase of Ni in cement and cement minerals. The study reveals the formation Ni and Al containing hydrotalcite-like layered double hydroxides (Ni-Al LDHs) when cement material (a complex mixture of CaO, SiO2, Al2O3, Fe2O3 and SO3) was treated with Ni in artificial cement pore water under highly alkaline conditions (pH = 13.3). This finding indicates that Ni-Al LDHs and not Ni-hydroxides determine the solubility of Ni in cement materials.

IDENTIFICATION OF NEOFORMED Ni-PHYLLOSILICATES UPON Ni UPTAKE IN MONTMORILLONITE: A TRANSMISSION ELECTRON MICROSCOPY AND EXTENDED X-RAY ABSORPTION FINE STRUCTURE STUDY

The aim of this work was to investigate whether neoformed Ni-phyllosilicates can be observed and identified using transmission electron microscopy (TEM) in combination with energy dispersive spectroscopy (EDS). The investigations focused on Ni-phyllosilicates formed from Ni-doped montmorillonite. The reaction conditions (pH 8, [Ni]initial = 660 and 3300 µM, 0.2 M Ca(NO3)2) employed were similar to those used in previous polarized extended X-ray absorption fine structure (P-EXAFS) investigations of neoformed Ni-phyllosilicates in a Ni-montmorillonite system.The TEM investigations of Ni-doped montmorillonite revealed the presence of small, thin particles consisting of coherent stacks that yielded only three to five lattice fringes with spacings consistent with smectites. These small particles were neoformed phyllosilicates, based on the fact that the small particles were only observed in Ni-doped samples and their Ni content, as determined from EDS analysis, was high (up to 10 wt.% NiO). Furthermore, the particles did not possess the characteristic properties of montmorillonite particles, such as a 2:1 Si to Al ratio; instead these particles were rich in Si (up to 75 wt.% SiO2). Unlike montmorillonite, these particles did not contain any Fe. The particles were also significantly more resistant to electron beam damage than montmorillonite particles, and EXAFS measurements confirmed the presence of neoformed Ni-phyllosilicates.The TEM study further indicates the presence of a variety of additional minerals (e.g. cristobalite, halloysite) and an X-ray amorphous Si-rich phase. A Ni signal could only be detected in the latter phase at high Ni loadings (403 µmol/g), suggesting that Ni uptake at low metal loadings (<90 µmol/g) is mainly controlled by the neoformation of phyllosilicates.