Basic Properties of MgAl-Mixed Oxides in CO2 Adsorption at High Temperature.

The increase of consciousness towards global warming and the need to reduce greenhouse gas emissions lead to the necessity of finding alternative applications based on easy-to-use materials in order to control and reduce global CO2 emissions. Layered Double Hydroxides (LDHs) and LDH-derived materials are potentially good adsorbents for CO2, thanks to their low cost, easy synthesis, high sorption capacity, and surface basicity. They have been intensively studied in CO2 capture at high temperature, presenting variable sorption capacities for MgAl LDHs with the same composition, but prepared under different synthesis conditions. The ambient temperature coprecipitation synthesis method is an attractive one-step procedure to synthesize LDHs under mild conditions, with low energy consumption and short synthesis time. The present study is based on the synthesis and characterization of hydrotalcites by a mild-conditions coprecipitation process and the production of derived mixed oxides to be used as CO2 adsorbents. A critical comparison to similar materials is reported. Moreover, the effect of the surface basicity of the derived mixed oxides (measured by adsorption calorimetry) and the CO2 sorption capacity are discussed, showing a linear correlation between the amount of weak and very strong basic sites and the CO2 adsorption behavior.

Enhanced interlayer trapping of a tetracycline antibiotic within montmorillonite layers in the presence of Ca and Mg.

The formation of a ternary antibiotic-metal-clay complex is hypothesized as the primary adsorption mechanism responsible for the increased adsorption of tetracycline antibiotics on smectites in the presence of divalent metal cations under circumneutral and higher pH conditions. To evaluate this hypothesis, we conducted a spectroscopic investigation of oxytetracycline (OTC) interacting with Na-montmorillonite in the presence and absence of Ca or Mg salts at pH 6 and pH 8. Despite a two-fold increase in OTC adsorbed in the presence of Ca or Mg, both solid-state nuclear magnetic resonance and infrared signatures of the OTC functional groups involved in metal complexation implied that the formation of an inner-sphere ternary complexation was not significant in stabilizing the adsorbate structures. The spectroscopic data further indicated that the positively-charged amino group mediated the OTC adsorption both in the absence and presence of the divalent metal cations. Focusing on the experiments with Mg, X-ray diffraction analysis revealed that the metal-promoted adsorption was coupled with an increased intercalation of OTC within the montmorillonite layers. The resulting interstratified clay layers were characterized by simulating X-ray diffraction of theoretical stacking compositions using molecular dynamics-optimized montmorillonite layers with and without OTC. The simulations uncovered the evolution of segregated interstratification patterns that demonstrated how increased access to smectite interlayers in the presence of the divalent metal cations enhanced adsorption of OTC. Our findings suggest that specific aqueous structures of the clay crystallites in response to the co-presence of Mg and OTC in solution served as precursors to the interlayer trapping of the antibiotic species. Elucidation of these structures is needed for further insights on how aqueous chemistry influences the role of smectite clay minerals in trapping organic molecules in natural and engineered soil particles.

Competitive Fe(II)-Zn(II) uptake on a synthetic montmorillonite.

The interaction of Fe(II) with clay minerals is of particular relevance in global geochemical processes controlling metal and nutrient cycles and the fate of contaminants. In this context, the influence of competitive sorption effects between Fe(II) and other relevant transition metals on their uptake characteristics and mobility remains an important issue. Macroscopic sorption experiments combined with surface complexation modeling and extended X-ray absorption fine structure (EXAFS) spectroscopy were applied to elucidate competitive sorption processes between divalent Fe and Zn at the clay mineral-water interface. Sorption isotherms were measured on a synthetic iron-free montmorillonite (IFM) under anoxic conditions (O2 <0.1 ppm) for the combinations of Zn(II)/Fe(II) and Fe(II)/Zn(II), where the former metal in each pair represents the trace metal (<10(-7) M) and the latter the competing metal at higher concentrations (10(-7) to 10(-3) M). Results of the batch sorption and EXAFS measurements indicated that Fe(II) is competing with trace Zn(II) for the same type of strong sites if Fe(II) is present in excess, whereas no competition between trace Fe(II) and Zn(II) was observed if Zn(II) is present at high concentrations. The noncompetitive behavior suggests the existence of sorption sites which have a higher affinity for Fe(III), where surface-induced oxidation of the sorbed Fe(II) to Fe(III) occurred, and which are not accessible for Zn(II). The understanding of this competitive uptake mechanism between Fe(II) and Zn(II) is of great importance to assess the bioavailability and mobility of transition metals in the natural environment.

Fe(II) sorption on a synthetic montmorillonite. A combined macroscopic and spectroscopic study.

Extended X-ray absorption fine structure (EXAFS) and Mössbauer spectroscopy combined with macroscopic sorption experiments were employed to investigate the sorption mechanism of Fe(II) on an iron-free synthetic montmorillonite (Na-IFM). Batch sorption experiments were performed to measure the Fe(II) uptake on Na-IFM at trace concentrations as a function of pH and as a function of sorbate concentration at pH 6.2 and 6.7 under anoxic conditions (O2 < 0.1 ppm). A two-site protolysis nonelectrostatic surface complexation and cation exchange sorption model was used to quantitatively describe the uptake of Fe(II) on Na-IFM. Two types of clay surface binding sites were required to model the Fe(II) sorption, the so-called strong (≡S(S)OH) and weak (≡S(W)OH) sites. EXAFS data show spectroscopic differences between Fe sorbed at low and medium absorber concentrations that were chosen to be characteristic for sorption on strong and weak sites, respectively. Data analysis indicates that Fe is located in the continuity of the octahedral sheet at trans-symmetric sites. Mössbauer spectroscopy measurements confirmed that iron sorbed on the weak edge sites is predominantly present as Fe(II), whereas a significant part of surface-bound Fe(III) was produced on the strong sites (∼12% vs ∼37% Fe(III) species to total sorbed Fe).

Interactions of oxytetracycline with a smectite clay: a spectroscopic study with molecular simulations.

Binding of antibiotics to clay minerals can decrease both their physical and biological availability in soils. To elucidate the binding mechanisms of tetracycline antibiotics on smectite clays as a function of pH, we probed the interactions of oxytetracycline (OTC) with Na-montmorillonite (MONT) using X-ray diffraction (XRD), infrared (IR), and solid-state nuclear magnetic resonance (NMR) spectroscopies, and Monte Carlo molecular simulations. The XRD patterns demonstrate the presence of OTC in the MONT interlayer space at acidic pH whereas complexation of OTC by external basal and edge sites seems to prevail at pH 8. At both pH, the (1)H-(13)C NMR profile indicates restricted mobility of the adsorbed OTC species; and, -CH(3) deformation and C-N stretching IR vibration bands confirm a binding mechanism involving the protonated dimethylamino group of OTC. Changes in the (23)Na NMR environments are consistent with cation-exchange and cation complexation reactions at the different sites of adsorption. Molecular simulations indicate that MONT interlayer spacing and structural charge localization dictate favorable binding conformations of the intercalated OTC, facilitating multiple interactions in agreement with the spectroscopic data. Our results present complementary insights into the mechanisms of adsorption of TETs on smectites important for their retention in natural and engineered soil environments.

Solid-state NMR investigation on the interactions between a synthetic montmorillonite and two homopolypeptides.

Interactions of two homopolypeptides (polylysine and polyglutamic acid) with a synthetic montmorillonite were studied by 1H MAS, 1H-27Al HETCOR and 1H-13C CP-MAS NMR experiments. 1H-27Al HETCOR with 1H spin-diffusion NMR appears to be a powerful probe for the identification of the polypeptide fragments, which interact with the montmorillonite interlayer surfaces. In particular, selective interactions were observed between the polypeptide side-chains and the montmorillonite octahedral aluminum atoms. 1H-13C CP-MAS NMR experiments were used to assess the dynamics of the two polypeptides through the measurement of the t(1/2) characteristic time of selected carbons. Results indicate that the local mobility of the side chains and their interaction with the montmorillonite layers depend on the nature of the adsorbed polypeptides.

Analysis of two bentonites of enological interest before and after commercial activation by solid Na(2)CO(3).

Two natural calcium-rich bentonites used for the removal of wine proteins, originating from Greece and Turkey, were studied by X-ray diffraction, thermal analysis, and solid state NMR, before and after activation by solid Na(2)CO(3). Exchange of Ca(2+) by Na(+) mainly occurs for cations located at the edge of layers and only weakly for interlayer cations. This Na(2)CO(3) activation process leads to an increased efficiency in the adsorption process of bovine serum albumin (BSA) used as a model protein for both bentonites. A direct correlation is observed between the extent of Ca(2+)/Na(+) exchange, the strength of adsorption of BSA, and the extent of unfolding of BSA backbone.

Catalytic wet peroxide oxidation of phenol over Fe-exchanged pillared beidellite.

This study presents an evaluation of the catalytic performances of a Fe-exchanged Al-pillared synthetic beidellite for the wet hydrogen peroxide oxidation of phenolic aqueous wastes. The catalyst was prepared by a cation doping technique, its properties being determined by DRX, BET and chemical analysis techniques. All the tests were performed on a laboratory scale set-up. Important factors affecting catalyst activity and phenol removal efficiencies were studied, i.e. the effect of pH, temperature, catalyst concentration and the stability of the catalyst. The experimental results indicate that the use of this catalyst allows a total elimination of phenol and a significant removal of chemical oxygen demand, without significant leaching of Fe ions. Thus, considering the lowest Fe concentrations in solution after oxidation, at pH=5, 50 degrees C, and 180 min. COD removal efficiency of 87.9% was obtained. It was also observed that by using this catalyst, it is possible to extend the range of pH values for which Fenton-type oxidations can occur.