EXAFS Study of Zn Sorption Mechanisms on Montmorillonite

Abstract

Extended X-ray absorption fine structure (EXAFS) analysis of zinc sorption on montmorillonite showed that different types of surface complexes or surface precipitates were formed depending on the reaction time. With an initial zinc concentration of 10(-3) M at neutral pH, zinc remained octahedrally coordinated with about six oxygen atoms at Zn-O bond distances of 2.02-2.07 A for up to six months. For samples aged up to 11 days, the Zn-Zn contribution in the second shell suggested formation of multinuclear surface complexes or surface precipitates. For samples aged 20 days and more, Zn-Zn and Zn-Si contributions in the second shell suggested formation of mixed metal coprecipitates such as a Zn phyllosilicate-like phase. Formation of these mixed metal solids probably accounts for the slow continuous sorption reaction at aging times exceeding 20 days. Sequestration of Zn in mixed metal precipitates and the stability of these phases can reduce the concentration, mobility, and toxicity of Zn in soils or sediments.

Biogenic nanoparticulate UO2: Synthesis, characterization, and factors affecting surface reactivity

“…However, as the samples were allowed to dry (increasing the Zn(II) sorption loadings), sixfold Zn(II) polynuclear species were observed. In contrast, only polynuclear Zn(II) inner-sphere sorption complexes were observed on c-MnOOH (Bochatay and Persson, 2000) and montmorillonite (Lee et al, 2004), independent of the Zn(II) sorption loadings. In these studies, Zn-Zn distances were observed at 3.33 Å (c-MnOOH) and 3.1 Å (montmorillonite).…”

EXAFS studies on adsorption irreversibility of Zn(II) on TiO2: Temperature dependence

“…In recent years, the EXAFS technique has been increasingly employed for providing structural and compositional information of surface complexes [19,20], which often offers critical insights into the underlying adsorption/desorption mechanisms [21][22][23]. This facilitates our understanding of commonly observed phenomena such as sorption hysteresis and slow sorption/desorption processes.…”

Evidence for the nucleation and epitaxial growth of Zn phyllosilicate on montmorillonite

“…These (bio)geochemical enrichments generally persist as a result of element immobilization by inorganic solids. For example, field studies and batch experiments on bentonite powders outlined the importance of Zn retention by phyllosilicates, either by sorption on the surface or by incorporation in a clay structure (Ross, 1946;Manceau et al, 2000Manceau et al, , 2002aManceau et al, , 2004Scheinost et al, 2002;Lee et al, 2004;Isaure et al, 2005;Panfili et al, 2005). To better understand the physico-chemical conditions favoring either of these two retention mechanisms, characterization of the reactivity of soil phyllosilicates toward Zn and other trace elements is needed.…”