Carina Luengo

Arsenate adsorption and desorption kinetics on a Fe(III)-modified montmorillonite

The adsorption-desorption kinetics of arsenate on a Fe(III)-modified montmorillonite (Fe-M) was studied at different arsenate concentrations, pH and stirring rates. The synthesized solid was a porous sample with Fe(III) present as a mix of monomeric and polymeric Fe(III) species in the interlayer and on the external surface. Adsorption took place in a two-step mechanism, with an initial fast binding of arsenate to Fe(III) species at the external surface (half-lives of 1 min or shorter) followed by a slower binding to less accessible Fe(III) species in pores and the interlayer (half-lives of around 1 h). Desorption kinetics also reflected the presence of externally and internally adsorbed arsenate. At pH 6 the maximum adsorbed arsenate was 52 μmol/g, a value that is low as compared to adsorption on ferrihydrite (700 μmol/g) and goethite (192-220 μmol/g). However, since the Fe(III) content of Fe-M is much lower than that of ferrihydrite and goethite, Fe(III) species in Fe-M are more efficient in binding arsenate than in ferrihydrite or goethite (one As atom is attached every 8.95 iron atoms). This high binding efficiency indicates that Fe(III) species are well spread on montmorillonite, forming small oligomeric species or surface clusters containing just a few iron atoms.

Aggregate Structural Transitions Noticed for the Didodecyldimetilammonium Bromide-Sodium Dehydrocholate Catanionic Mixed System at Low Concentration

The phase behavior for the very diluted DDAB-NaDHC-water system was investigated at 25°C. Fluorescence, conductivity, and surface tension experiments were carried out to determine the cac values. Comparison between experimental and calculated values shows the suitability of the regular theory to predict the critical aggregation concentrations for the tested systems. The interaction between both amphiphiles inside aggregates is not ideal, showing a large synergism. Such fact is reflected by the activity of coefficients values. The aggregates are mainly comprised by DDAB (18–30 % mol NaDHC) regardless of the NaDHC solution molar fraction; these molecules act as a good solvent of bile salt type molecule. Nevertheless, the gradual inclusion of NaDHC molecules inside DDAB bilayers leads to structural transformations. These facts are supposed to be due the combination of two effects: (i) the reduction of head groups hydration and (ii) the increment of chain repulsion.

Quantum chemical study on surface complex structures of phosphate on gibbsite

Quantum mechanics calculations based on the density functional theory (DFT) were used to identify phosphate surface complexes on gibbsite at low and high pH. The different phosphate species were represented using the Al₆(OH)₁₈(H₂O)₆ cluster model considering four different geometries: monodentate mononuclear (Pmm), monodentate binuclear (Pmb), bidentate mononuclear (Pbm) and bidentate binuclear (Pbb). The corresponding adsorption reactions were modelled via ligand exchange between phosphate species and surface functional groups (hydroxyls and protonated hydroxyls at high and low pH, respectively). The theoretical results indicate that phosphate surface complexes are thermodynamically more favored at acid pH, in agreement with experimental evidences. The first step in these reactions, i.e., the generation of required aluminum vacant sites, was predicted to be particularly favorable when singly coordinated aquo groups are released. Stretching and bending vibrational frequencies associated with the different surface structures were calculated at both pH conditions. The corresponding values at low pH were found to be shifted to higher frequencies with respect to those ones at high pH. ATR-FTIR studies were also carried out. The resulting spectra are dominated by a strong band within the 800-840 cm(-1) interval due to P-OH stretching modes. The corresponding peak appearing around 820 cm(-1) at high pH is shifted to lower frequencies with respect to the position at low pH, a tendency well predicted by DFT calculations.