Synthesis, Characterization and Decomposition of Potassium Jarosite for Adsorptive As(V) Removal in Contaminated Water: Preliminary Study.

Jarosite-type compounds precipitated in the zinc industry for iron control can also incorporate arsenic and can be used for wastewater treatment for As elimination. According with the last, this work is related to arsenic incorporation at room temperature in decomposed potassium jarosite. The work began with the synthesis of the compound at 75 °C for 9 h using Fe2(SO4)3 and K2SO4 at a pH of 1.1. Once jarosite was obtained, solids were subjected to an alkaline decomposition using NaOH at pH 10 for 30 min, and then As was added to the solution as HAsNaO4 and the pH modified by adding HNO3 until it reached a value of 1.1. The initial, intermediate, and final products were wholly characterized by scanning electron microscopy (SEM) in conjunction with energy dispersive spectrometry (EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (RS), and X-ray photoelectron spectrometry (XPS). The obtained results show that As(V) can be adsorbed by ionic exchange in the amorphous FeOH structure of decomposed jarosite and when pH decreased to 1.1, the compound recrystallized, incorporating up to 6% As on average, which is indicative that this process can be used to reduce As in contaminated waters.

A theoretical study on the electronic, structural and optical properties of armchair, zigzag and chiral silicon-germanium nanotubes.

In this work we have studied infinite size silicon-germanium alloy nanotubes of several types, armchair, zigzag and chiral, by theoretical analysis based on density functional theory as implemented in the SIESTA code, which utilizes a linear combination of atomic orbitals and a generalized gradient approximation proposed by Perdew, Burke and Ernzerhof (GGA-PBE) for the exchange and correlation energy. The structures were relaxed until the atomic forces were less than 0.0001 eV Å-1. The electronic band structure, density of states and cohesive energy were then computed; the optical calculation was run in between 0 and 6 eV, with a broadening of 0.05 eV. The obtained results exhibit the deformation of the structure on the surface, which seems to be related to its stability. The armchair and zigzag tubes are direct band gap semiconductor materials, while chiral nanotubes shift from indirect to direct bandgap semiconductors, depending on their diameter size. Likewise, the bandgap depends on the diameter of the SiGe nanotubes (SiGeNTs). We have associated the absorption curves and the density of states through Van Hove singularities. In summary, our results on the structural and electronic properties of SiGeNTs elucidate their possible applications in thermoelectrics, photovoltaics and nanoelectronics, while the possibility of associating the absorption curves with the density of states provides a method of characterization.

Supramolecular polymerization of a prebiotic nucleoside provides insights into the creation of sequence-controlled polymers.

Self-assembly of a nucleoside on Au(111) was studied to ascertain whether polymerization on well-defined substrates constitutes a promising approach for making sequence-controlled polymers. Scanning tunneling microscopy and density functional theory were used to investigate the self-assembly on Au(111) of (RS)-N(9)-(2,3-dihydroxypropyl)adenine (DHPA), a plausibly prebiotic nucleoside analog of adenosine. It is found that DHPA molecules self-assemble into a hydrogen-bonded polymer that grows almost exclusively along the herringbone reconstruction pattern, has a two component sequence that is repeated over hundreds of nanometers, and is erasable with electron-induced excitation. Although the sequence is simple, more complicated ones are envisioned if two or more nucleoside types are combined. Because polymerization occurs on a substrate in a dry environment, the success of each combination can be gauged with high-resolution imaging and accurate modeling techniques. These characteristics make nucleoside self-assembly on a substrate an attractive approach for designing sequence-controlled polymers. Further, by choosing plausibly prebiotic nucleosides, insights may be provided into how nature created the first sequence-controlled polymers capable of storing information. Such insights, in turn, can inspire new ways of synthesizing sequence-controlled polymers.

Structures and chiroptical properties of the BINAS-monosubstituted Au38(SCH3)24 cluster.

The structure and optical properties of a set of R-1,1'-binaphthyl-2,2'-dithiol (R-BINAS) monosubstituted A-Au38(SCH3)24 clusters are studied by means of time dependent density functional theory (TD-DFT). While it was proposed earlier that BINAS selectively binds to monomer motifs (SR-Au-SR) covering the Au23 core, our calculations suggest a binding mode that bridges two dimer (SR-Au-SR-Au-RS) motifs. The more stable isomers show a negligible distortion induced by BINAS adsorption on the Au38(SCH3)24 cluster which is reflected by similar optical and Circular Dichroism (CD) spectra to those found for the parent cluster. The results furthermore show that BINAS adsorption does not enhance the CD signals of the Au38(SCH3)24 cluster.

On the origin of the optical activity displayed by chiral-ligand-protected metallic nanoclusters.

Ligand-protected metallic clusters exhibit optical activity when chiral molecules are used as protecting units. Various mechanisms, such as the inherently chiral metallic cluster core, the dissymmetric field effect, and the chiral footprint model, have been proposed as possible explanations of the nonzero circular dichroism (CD) spectra found for these nanoscale materials. This communication presents a first-principles theoretical study of the CD spectrum of the [Au(25)(SR)(18)](-) cluster that was undertaken to gain insight into the physicochemical origin of the optical activity measured for the glutathione-protected [Au(25)(SG)(18)](-) cluster. The calculated CD spectrum of the cysteine-protected cluster, with R(cys) = C(beta)H(2)-C(alpha)H(NH(2))-COOH, shows good agreement with the experimental data obtained for the glutathione-protected cluster. Analysis of the calculated CD spectra of the peculiar two-shell metallic core and the two distinct thiolate-Au binding modes existing in the [Au(25)(SR(cys))(18)](-) cluster showed that the weak CD signal due to the slight distortion of cluster core is enhanced by the dissymmetric location of the ligands forming the Au-S binding modes. This result shows that the mechanisms proposed to explain the optical activity of chiral-ligand-protected metallic clusters cannot be differentiated but are acting concurrently. It is also predicted that the CD line shape should be highly sensitive to the orientation of the thiolate ligands forming the cluster protecting layer and to the stability of the thiolate-Au binding modes.