Adsorption mechanisms of Zn on hectorite as a function of time, pH, and ionic strength

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ID: 295594
2001
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Abstract
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 NaNO~3~, and with a total Zn concentration of 100 μM. In 0.01 M NaNO~3~, 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 NaNO~3~, 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 NaNO~3~ 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 NaNO~3~ are similar, con.rming the IS uptake mechanism operated under fully wet conditions at high ionic strength. In 0.01 M NaNO~3~, 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 α: angle between the X-ray polarization vector and the phyllosilicate plane β~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 χ^(k)^: EXAFS function χ~j~^α^: EXAFS contribution of a j shell at the α angle χ~j~^iso^: isotropic EXAFS contribution of a j shell at the magic angle N~j~^α^: apparent number of backscatterers in the j shell at the α 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 σ: Debye-Waller term in EXAFS spectroscopy S~0~^2^: 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 Zn~3~Si~4~O~10~(OH)~2~·nH~2~O ZnKer070: Zn-containing kerolite Zn~0.7~Mg~2.3~Si~4~O~10~(OH)~2~·nH~2~O ZnKer003: Zn-containing kerolite Zn~0.03~Mg~2.97~Si~4~O~10~(OH)~2~·nH~2~O OS: outer sphere IS: inner sphere E linkage: edge-sharing linkage C linkage: corner-sharing linkage I: ionic strength Zn~(aq)~^2+^: fully solvated Zn^2+^ 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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Authors Michel L. Schlegel
Journal american journal of science
Year 2001
DOI
10.2475/ajs.301.9.798
URL
https://ajs.scholasticahq.com/article/61664.pdf
Keywords Keywords not found

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