Eucler B. Paniago

Bioleaching of zinc and nickel from silicates using Aspergillus niger cultures

In this work, we investigated the role of bacteria from the genera Bacillus and Pseudomonas and fungi from the genera Aspergillus and Penicillium in the leaching process of two different silicates (calamine and garnierite). Since the results obtained with A. niger were better than those with different bacteria, a more detailed investigation of the leaching process with this microorganism was conducted. Moreover, although it is clear that the citric acid generated by fungi could be an important leaching agent acting in the solubilization of the used silicates, other products of metabolism could be involved. Related to this, the results obtained with chemical leaching using low concentrations of citric acid (lower than 10 mM) showed, for both calamine and garnierite, that the respective dissolution of zinc and nickel was much lower when compared to those processes in which cultures or supernatant liquor of A. niger cultures were used and in which the maximum concentration of citric acid was 8 mM. The results obtained also suggest that the type of mineral (and/or the metal present in it) presents a different susceptibility to the bioleaching process and also demonstrate that depending of the situation, the presence of the fungi cells seem to improve the leaching process. From a practical point of view, the high yield rate of extracting metals from silicates obtained by using for example, supernatant liquors of A. niger cultures, is noteworthy. This bioleaching process present two advantages as compared to conventional chemical leaching processes: (a) the very low concentrations of organic compounds present in such a situation represent a lower ecological risk; and (b) even with a lower final yield, the economical cost of a such process. Both characteristics could facilitate its industrial application.

Equilibrium and kinetic studies of iron(II) and iron(III) complexes of some α(N)-heterocyclic thiosemicarbazones. Reduction of the iron(III) complexes of 2-formylpyridine thiosemicarbazone and 2-acetylpyridine thiosemicarbazone by cellular thiol-like reducing agents

alpha (N)-heterocyclic thiosemicarbazones have been shown to possess antitumor properties in mammalian cells through the inhibition of DNA synthesis; the ability to provide the inhibitory action is probably due to coordination of iron. This paper deals with equilibrium and kinetic studies involving 2-formylpyridine thiosemicarbazone (HFPT) and 2-acetylpyridine thiosemicarbazone (HAPT) coordinated to Fe(II) and Fe(III) cations in aqueous solution. The formation constants of all species present in equilibrium were determined. Kinetic measurements of the reduction of the Fe(III) complex of both ligands by thiolic reducing agents, that can act as structural models of cellular thiols, i.e., N-acetyl-L-cysteine (ACCIS) and dithiothreitol (DTT), were carried out. The experimental data lead to a rate law of the type v = k1[A] + k2[A] [B], where [A] represents the concentration of the complex and [B] that of the reducing agent, indicating the coexistence of two reaction pathways. One pathway depends only on the complex concentration and occurs even in the absence of the reducing agent, and the other involves both the complex and ACCIS or DTT.

Design, structural and spectroscopic elucidation, and the in vitro biological activities of new diorganotin dithiocarbamates.

The reaction of 2,2-dimethoxy-N-methylethyllamine or 2-methyl-1,3-dioxolane with CS(2) in alkaline media produced two novel dithiocarbamate salts. Subsequent reactions with organotin halides yielded six new complexes: [SnMe(2){S(2)CNR(R(1))(2)}(2)] (1), [Sn(n-Bu)(2){S(2)CNR(R(1))(2)}(2)] (2), [SnPh(2){S(2)CNR(R(1))(2)}(2)] (3), [SnMe(2){S(2)CNR(R(2))(2)}(2)] (4), [Sn(n-Bu)(2){S(2)CNR(R(2))(2)}(2)] (5), [SnPh(2){S(2)CNR(R(2))(2)}(2)] (6), where R = methyl, R(1) = CH(2)CH(OMe)(2), and R(2) = 2-methyl-1,3-dioxolane. All compounds were identified in terms of infrared, (1)H and (13)C NMR, and the complexes were also characterized using (119)Sn NMR, (119)Sn Mössbauer and X-ray crystallography. The biological activity of all derivatives has been screened in terms of IC(90) and IC(50) against Aspergillus flavus, Aspergillus niger, Aspergillus parasiticus, Penicillium citrinum, Curvularia senegalensis, Staphylococcus aureus, Listeria monocytogenes, Bacillus cereus, Streptococcus sanguinis, Escherichia coli, Citrobacter freundii, Salmonella typhimurium, and Pseudomonas aeruginosa and the results correlated well with a performed study of structure-activity relationship (SAR). Complexes (3), (5) and (6) displayed the best IC(90) and IC(50) in the presence of the fungi, greater than that of miconazole, used as control drug.

The formation constants and structures of copper(II)-glycinehydroxamic acid complexes

Abstract Formation constants for the complexes formed between copper(II) and glycinehydroxamic acid (H 2 NCH 2 CONHOH) have been measured potentiometrically at 25.0 °C and I = 0.10 M (NaClO 4 ). The existence of a dimeric copper(II) species has been shown from the line broadening of the epr spectrum of a solution at room temperature. The structures of the complexes are discussed and conclusions are drawn based on absorption and epr spectra.

A PHYSICOCHEMICAL STUDY OF THE TETRACYCLINE COORDINATION TO OXOVANADIUM(IV)

The interaction of tetracycline and oxovanadium(IV) in aqueous solution was studied by potentiometric and spectrophotometric methods. Oxovanadium(IV) ions form both a positively charged 1:1 and a neutral 2:1 metal-ligand complex with tetracycline. When a 1:1 ligand-to-metal ratio mixture is used at about pH 4.5 the 1:1 species predominates, being replaced at pH 6 by the binuclear complex. The binuclear complex has been isolated and fully characterised. Infrared and EPR studies suggest the existence of two distinct vanadyl binding sites. Our results indicate that the first vanadium coordinates to the BCD-ring system and the second one to the A-ring. Biological implications of the existence of a neutral complex at physiological pH are briefly discussed.

Kinetic and mechanism of vesicle lipoperoxide decomposition by Fe(II)

Abstract Small unilamellar vesicles of l -α-phosphatidylcholine have been subjected to peroxidation and the initial kinetic rate of decomposition of these peroxides (ROOH) induced by the presence of Fe(II) or alternatively the oxidation of Fe(II) to Fe(III) by these peroxides has been determined. The oxidation of Fe(II) exhibits saturation kinetics. A mechanism is then proposed involving, as a first step, the fixation of Fe(II) to the membrane, and then the decomposition of the peroxide by the complex thus formed. The disruption of the RO-OH bond follows a second-order rate law with k 2 = 1500 ± 500 M −1 · s −1 at 25°C. Moreover, these data suggest that the ROOH groups are localized at the aqueous interface.

Crystal and molecular structure of a copper(II)-glycinehydroxamic acid complex

Abstract Crystals of trans -bis(glycinehydroxamato)copper- (II) dihydrate (CuC 4 H 10 N 4 O 4 ·2H 2 O) were obtained by mixing concentrated solutions of copper(II) perchlorate and glycinehydroxamic acid (H 2 NCH 2 - CONHOH) in a 1:2 molar ratio and adjusting the pH to about 8. X-ray crystallographic data show the crystals to be orthorhombic: Pbca , a = 5.052(3), b = 11.152(2), c = 17.876(2) A, V = 1007 A 3 , Z = 4. The copper ion lies on an inversion center, in which it is octahedrally coordinated by four nitrogens and two oxygens from neighboring molecules. IR spectra of the complex confirm these crystallographic results. The ESR powder and dilute solution spectra are characteristic of axial symmetry with respectively g ‖ = 2.198, g ⊥= 2.083 and g ‖ = 2.196, g⊥= 2.055 and A Cu ‖ = 202 gauss, A Cu ⊥= 30.2 gauss and A N ‖ = 15 gauss.

Copper(II) mixed ligands complexes of hydroxamic acids with glycine, histamine and histidine

Um estudo potenciometrico e espectrofotometrico de complexos mistos de CuII, fisiologicamente interessantes, foi realizado envolvendo acidos hidroxâmicos como ligantes primarios (A) e ligantes secundarios (B) representados por histamina ou pelos aminoacidos glicina e histidina. Todos sao potencialmente capazes de formar quelatos com aneis de cinco ou seis membros. A constante de formacao e o espectro de absorcao no visivel sao calculados para cada uma das especies identificadas, tanto binarias quanto ternarias. As estruturas mais provaveis das especies mistas sao propostas com base nas constantes de formacao calculadas, na estabilizacao em relacao as duas especies binarias correspondentes, bem como nos espectros de absorcao no visivel. Ficou comprovado que os complexos de Cu-hidroxamato nao favorecem significativamente a formacao de complexos ternarios com aminoacidos que possuem apenas os grupos amino e carboxilato; contudo, a formacao de complexos ternarios com a histidina e fortemente favorecida, devido a presenca do grupo imidazol.

Interaction of N-hydroxyacetamide with vanadate: A density functional study

Abstract The interaction between N -hydroxyacetamide (HL) and vanadate (VO 3 − ) has been theoretically studied using density functional theory. All possible tautomers and conformations of two complexes formed have been fully optimized and vibrational analysis performed. From reported experimental results these two complexes have been shown to be in equilibrium in acidic aqueous solution: VO 2 LH 2 O and VO 2 (HL)L. The pentacoordinated VO 2 LH 2 O species having an intramolecular proton transferred from the coordinating H 2 O ligand to the oxo group, is the most stable. Seemingly, the most stable hexacoordinated VO 2 (HL)L species also has an oxo group protonated. Based on the analysis of the dipole moments of the species, the solvent effects within the continuum model are unlikely to change the relative stabilities of the different tautomers and conformers. The experimental infra-red spectrum of the VO 2 LH 2 O species has been measured and compared directly to the calculated frequencies. The most important peaks have been assigned to the corresponding normal modes. From the Mulliken population analysis, it is shown that the net charge on the vanadium atom and the oxygens surrounding the metal center are similar in the two species. The different coordination numbers may explain the differences of the reported 51 V NMR chemical shifts exhibited by these two species.

Influence of the Sol-Gel pH Process and Compact Film on the Efficiency of TiO2-Based Dye-Sensitized Solar Cells

The influence of pH during hydrolysis of titanium(IV) isopropoxide on the morphological and electronic properties of TiO2 nanoparticles prepared by the sol-gel method is investigated and correlated to the photoelectrochemical parameters of dye-sensitized solar cells (DSCs) based on TiO2 films. Nanoparticles prepared under acid pH exhibit smaller particle size and higher surface area, which result in higher dye loadings and better short-circuit current densities than DSCs based on alkaline TiO2-processed films. On the other hand, the product of charge collection and separation quantum yields in films with TiO2 obtained by alkaline hydrolysis is c.a. 27% higher than for the acid TiO2 films. The combination of acid and alkaline TiO2 nanoparticles as mesoporous layer in DSCs results in a synergic effect with overall efficiencies up to 6.3%, which is better than the results found for devices employing one of the nanoparticles separately. These distinct nanoparticles can be also combined by using the layer-by-layer technique (LbL) to prepare compact TiO2 films applied before the mesoporous layer. DSCs employing photoanodes with 30 TiO2 bilayers have shown efficiencies up to 12% higher than the nontreated photoanode ones. These results can be conveniently used to develop optimized synthetic procedures of TiO2 nanoparticles for several dye-sensitized solar cell applications.