Maria Sylvia Silva Dantas

A comparative study of As(III) and As(V) in aqueous solutions and adsorbed on iron oxy-hydroxides by Raman spectroscopy

The sorption of the arsenite (AsO(3)(3-)) and the arsenate (AsO(4)(3-)) ions and their conjugate acids onto iron oxides is one of main processes controlling the distribution of arsenic in the environment. The present work intends to provide a large vibrational spectroscopic database for comparison of As(III) and As(V) speciation in aqueous solutions and at the iron oxide - solution interface. With this purpose, ferrihydrite, feroxyhyte, goethite and hematite were firstly synthesized, characterized in detail and used for adsorption experiments. Raman spectra were recorded from As(III) and As(V) aqueous solutions at various pH conditions selected in order to highlight arsenic speciation. Raman Scattering and Diffuse Reflectance Infrared Fourier Transform (DRIFT) studies were carried out to examine the respective As-bonding mechanisms. The collected data were curve-fitted and discussed according to molecular symmetry concepts. X-ray Absorption Near Edge Spectroscopy (XANES) was applied to confirm the oxidation state of the sorbed species. The comprehensive spectroscopic investigation contributes to a better understanding of arsenic complexation by iron oxides.

Molecular modeling of iron and arsenic interactions with carboxy groups in natural biomass

Environmental context. Arsenic has been considered one of the most important global environmental pollutants. Its occurrence in water systems is a result of natural processes and anthropogenic activities. In view of their high toxicity and the consequent health problems associated with human exposure to contaminated waters and food, there is an increasing interest in the study of the specific interactions of arsenic species with organic matter. Here, specific interactions among arsenic, iron and a vegetable biomass are investigated with a view to demonstrate how these interactions can affect arsenic mobility in the environment. Abstract. The interaction of iron and arsenic with dried lettuce leaves was investigated using a combination of spectroscopic techniques. Iron binding to carboxy groups is indicated by a decrease of 84% in iron loading after esterification. According to extended X-ray absorption fine structure (EXAFS) analysis, FeIII is coordinated by six oxygen atoms (Fe–O distance of 1.98 A), two carbon atoms (Fe–C distance of 2.85 A) in a bidentate mononuclear form, and 0.5 or 1 arsenic atoms (Fe–As distance of 2.93–2.94 A). Arsenic is sorbed only when the biomass has been previously loaded with iron. AsV is coordinated by four oxygen atoms (As–O distance of 1.71 A) and one iron atom in a bidentate mononuclear form or two iron atoms (As–Fe distance of 2.93–2.94 A) in a bidentate binuclear form. In conclusion, the results demonstrate that carboxylic acid groups can affect AsV mobility in the environment so long as iron is available for bridging.

Raman and IR spectroscopic investigation of As adsorbed on Mn3O4 magnetic composites.

Raman and IR spectra were recorded of the As-loaded Mn(3)O(4) magnetic composites obtained from the adsorption studies performed with As(III). XANES results for the composite after As(III) removal tests show that the As adsorbed is at the oxidized arsenic form, As(V). Monodentate and bidentate surface complexes are suggested for arsenic adsorption onto the composite (5-16 mg/g). Precipitation of manganese arsenate is observed for high As loading (35 mg/g).

Magnetized manganese oxide nanocomposite for effective decontamination of Cd(II) from wastewaters

In this study, a composite with magnetic properties has been successfully synthesized by a novel and environmentally friendly route and is applied to Cd(II) adsorption for water decontamination. The quantification of the phases obtained by Rietveld refinement has shown the presence of 84% of Mn3O4 and 16% of Fe3O4. Transmission electron microscopy image shows an aggregate of Mn3O4 nanoparticles without specific orientation and the predominance of octahedral morphology with nanoparticles size estimated around 25-30 nm. The Cd(II) adsorption isotherm is fitted using the Langmuir-Freundlich model. The estimated maximum adsorption capacities of Cd(II) at pH 6 and 7 are similar (0.28 ± 0.02 and 0.31 ± 0.02 mg/m2, respectively). The kinetic results show that the studied system follows the pseudo-second-order model. The Raman results indicate that Cd is being specifically adsorbed by the Mn3O4 in the composite. The hysteresis curve of the composite Mn3O4/Fe3O4 has changed when compared to the pure magnetite; however, the coercive field after the addition of manganese oxide remains unaltered and does not change with a value around 158 Oe. The turbidity tests showed that the magnetic sedimentation was efficient and promising for wastewater treatment in large scale. These materials can be conveniently recovered by magnetic separation, avoiding the filtration steps, which will make easier the solid-liquid separation operation that follows the adsorption process.

Interactions between a collagen-binding adhesive and dental substrate

The adhesion between polymer adhesives and dentin involves the removal of minerals, by acid etching the dentin surface, to allow for monomer infiltration among collagen fibrils. However, instability within this hybrid layer has been observed, which may partially be attributed to a slow degradation of the collagen not protected by the adhesive. The objective of this work was to improve the stability of the hybrid layer by increasing the interactions between adhesives and collagen through the incorporation of a monomer that behaves reactively toward collagen in the adhesive. In this work, a control adhesive was prepared based on a chemical formulation containing Bis-GMA and HEMA, and was modified by incorporating different concentrations (5, 10, and 20%) of NSA (N-acryloxysuccinimide), which contains an ester group capable of interacting with primary amine. These experimental adhesives were applied to bovine dentin for the in vitro production of the interfacial layer, which was analyzed by Raman spectroscopy and scanning electron microscopy. The obtained results showed that it was possible to incorporate the NSA monomer into the adhesive up to a maximum concentration of 10%. The presence of amide bonds in the Raman spectra, as well as the presence of a large amount of experimental adhesive within the interfacial layer, as indicated in the scanning electron microscopy images, may suggest the occurrence of chemical interactions between polymers and dentin collagen fibrils. These interfacial chemical interactions involving dental adhesives and dentin can be an important tool in protecting the hybrid layer from degradation.