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Adsorption Mechanisms of Zn on Hectorite as a Function of Time, pH, and Ionic Strength

Environmental Geochemistry Group, LGIT-IRIGM, University of Grenoble and CNRS, BP 53 F-38 041 Grenoble cedex 9, France

The mechanisms of Zn uptake in dilute suspensions ( 2 g L^–-1) of hectorite were investigated by kinetics experiments and extended X-ray absorption .ne structure (EXAFS) spectroscopy on wet pastes and self-supporting films of Zn-sorbed hectorite. Kinetics experiments were performed at pH 4 and 6.5, in 0.3 and 0.01 M NaNO₃, and with a total Zn concentration of 100 µM.

In 0.01 M NaNO₃, signi.cant amounts of Zn were sorbed within the first 5 min of reaction at both pHs. This rapid uptake is consistent with Zn adsorption on exchange sites located on hectorite basal planes. After this fast sorption, the amount of sorbed Zn slowly increased at pH 6.5 but decreased at pH 4. In 0.3 M NaNO₃, Zn uptake was less rapid within the .rst 5min, and the amount of sorbed Zn subsequently increased at both pHs, with more Zn sorbing at the higher pH. This behavior is consistent with Zn adsorption on pH-dependent sites. The dissolution of hectorite was monitored during Zn sorption. In 0.3 M NaNO₃ at pH 6.5, initial Zn uptake was correlated with an excess release of Mg as compared to a hectorite suspension without added Zn. In contrast, Si release was inhibited initially by Zn addition. At pH 4, Zn addition did not affect the dissolution rate of hectorite.

At pH 6.5, polarized-EXAFS (P-EXAFS) spectra obtained on self-supporting films for the two ionic strengths at reaction times between 6 and 120 h show similar crystallochemical environments for Zn. Four atomic shells were identi.ed: a nearest O shell at an interatomic distance R_Zn-O = 2.06 ± 0.01 Å, a Mg shell at R_Zn-Mg = 3.06-3.09 ± 0.03 Å, a Si shell at R_Zn-Si = 3.23-3.26 ± 0.03 Å, and a next-nearest O shell at R_Zn-O = 3.69-3.74 ± 0.06 Å. R_Zn-Mg and R_Zn-Si values are characteristic of edge-shared Zn and Mg octahedra and of corner-shared Zn octahedra and Si tetrahedra, respectively. The angular dependencies of the Zn-Mg and Zn-Si contributions indicated that Zn-Mg pairs were oriented parallel to the film plane, whereas Zn-Si pairs were not. These results indicate that inner-sphere (IS) Zn surface complexes formed at layer edges of hectorite platelets, in continuity to octahedral sheets. Magic-angle EXAFS spectra of wet pastes and self-supporting films obtained at pH 6.5 in 0.3 M NaNO₃ are similar, con.rming the IS uptake mechanism operated under fully wet conditions at high ionic strength. In 0.01 M NaNO₃, a continuous evolution with increasing reaction time from predominantly outer-sphere (OS) to predominantly IS complexes was observed for wet pastes, suggesting that Zn initially sorbed as exchangeable OS complexes on interlayer sites, and then migrated to layer edges to form IS surface complexes. Based on previous work, Zn has a higher af.nity than Co for the hectorite surface, in keeping with the higher stability of Zn phyllosilicates.

SYMBOLS:

EXAFS

extended X-ray absorption fine structure

P-EXAFS

polarized extended X-ray absorption fine structure

FT

Fourier transform of the EXAFS spectrum

RSF

radial structure function

angle between the X-ray polarization vector and the phyllosilicate plane

ß_j

angle between the c* axis of the phyllosilicate layer and the vectors connecting the X-ray absorbing atom to backscattering atoms in the j shell

k

modulus of the wavevector in EXAFS spectroscopy

^(k)

EXAFS function

_j

EXAFS contribution of a j shell at the angle

_j^iso

isotropic EXAFS contribution of a j shell at the magic angle

N_j

apparent number of backscatterers in the j shell at the angle

N_j^35°

structural number of backscatterers in the j shell

R_{Zn – j}^EXAFS

EXAFS-derived interatomic distance between Zn absorber and backscattering atoms in the j shell

Debye-Waller term in EXAFS spectroscopy

S₀²

amplitude reduction factor in EXAFS spectroscopy

R_P

reliability factor used to adjust model to sample EXAFS spectra

ZnKer

Zn-containing kerolite

ZnKer300

Zn-kerolite Zn₃Si₄O₁₀(OH)₂·nH₂O

ZnKer070

Zn-containing kerolite Zn_0.7Mg_2.3Si₄O₁₀(OH)₂·nH₂O

ZnKer003

Zn-containing kerolite Zn_0.03Mg_2.97Si₄O₁₀(OH)₂·nH₂O

OS

outer sphere

IS

inner sphere

E linkage

edge-sharing linkage

C linkage

corner-sharing linkage

I

ionic strength

Zn_(aq)²⁺

fully solvated Zn²⁺ cation

[Zn]_aq

concentration of Zn in the supernatant

[Si]_aq

concentration of Si in the supernatant

[Mg]_aq

concentration of Mg in the supernatant

Zn_T

total Zn concentration in the suspension

Co_T

total Co concentration in the suspension

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