Sorption mechanisms of antimonate on Mg-Fe layered double hydroxides

Barbora Hudcováa, M. Erbenb, M. Vítkováa and M. Komáreka

a Department of Environmental Geosciences, Czech University of Life Sciences Prague, Czech Republic

b Department of General and Inorganic Chemistry, University of Pardubice, Czech Republic

hudcovab@fzp.czu.cz

Layered double hydroxides (LDHs) have been proposed as effective sorbents for Sb(V), but studies investigating sorption mechanisms usually lack a comprehensive mechanistic/modeling approach. Moreover, thermodynamically based surface complexation models (SCMs) describing Sb(V) sorption on LDHs have not yet been examined and the number of other studies dealing with the SCMs investigating Sb(V) sorption on different solid materials is also limited. Therefore, we propose coupling SCMs with various spectroscopic and microscopic techniques to investigate the Sb(V) binding arrangements on the Mg-Fe LDH surface. The Sb(V) sorption on Mg-Fe LDHs was performed at different initial Sb(V) concentrations, ionic strengths and pH values. The removal rate (described by pseudo-first order kinetics) and the maximal (ad)sorbed amount (described by Langmuir model) increased with decreasing pH values. The pH-dependent sorption data at different ionic strengths and Sb(V) concentrations were further modeled by the diffuse layer model (DLM) which predicted preferable formation of monodentate mononuclear complexes on the Mg-Fe LDH surface. Spectroscopic (XRD, FTIR-ATR, XPS) and microscopic (SEM/EDX with elemental mapping) techniques were used to further specify the sorption mechanisms. The influence of chemical adsorption, surface-induced precipitation of brandholzite, formation of brandholzite-like phases and/or anion exchange was observed depending on the Sb(V) concentrations and pH values. At lower pH values, the negatively charged Sb(V) surface complexes adsorbed on the Mg-Fe LDH surface provided binding sites for Mg(II) that is commonly leached from Mg-Fe LDHs. This process resulted in the formation of individual layers corresponding to the brandholzite-like phases. Nevertheless, the surface-induced precipitation of crystalline brandholzite occurred only at higher pH values. The binding of Sb(V) to both Fe(III) and Mg(II) in the Mg-Fe LDH structure was confirmed by spectroscopic techniques suggesting the proposed sorption mechanisms. At all pH values, Sb(V) was nonhomogeneously distributed on the Mg-Fe LDH surface (especially on smaller particles) showing the presence of different (ad)sorption sites, i.e., monolayer/multilayer adsorption as well as surface-induced precipitation. The surface complexation modeling supported by solid-state analyses provided a strong tool to investigate the binding arrangements of Sb(V) on the Mg-Fe LDH surface. Such a comprehensive mechanistic/modeling approach has not previously been presented and predicted the Sb(V) sorption behavior on Mg-Fe LDHs under different conditions. Moreover, the presented mechanistic/modeling approach can be useful for newly synthesized materials to evaluate their Sb(V) sorption properties.

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