RESUMO
Ferrihydrite is widespread in clays, soils, and living organisms and was found on Mars. This iron-mineral could be found on the prebiotic Earth, which also contained simple monomeric amino acids. For prebiotic chemistry, it is important to understand how amino acids have an effect on the process of iron oxide formations. There are three important results in this work: (a) preconcentration of cysteine and aspartic acid, (b) formation of cystine and probably the cysteine peptide occurred during ferrihydrite syntheses, and (c) amino acids have an effect on iron oxide synthesis. For samples containing aspartic acid and cysteine, their presence on the surface or mineral structure can be confirmed by FT-IR spectra. Surface charge analysis showed a relatively high decrease for samples synthesized with cysteine. Scanning electron microscopy did not show marked morphological differences among the samples, except for the seawater sample containing cysteine, which had a lamina-shaped morphology surrounded by circular iron particles, indicating the possible formation of a cysteine structure involving iron oxide particles. The thermogravimetric analysis of the samples indicates that the presence of salts and amino acids in the synthesis of ferrihydrite has an effect on the thermal behavior of the iron oxide/amino acids and modifying the water-loss temperature. The heating of the cysteine samples, synthesized in distilled water and artificial seawater, showed several peaks of degradation of cysteine. In addition, heating of the aspartic acid samples produced the polymerization of this amino acid and peaks of degradation of it. FTIR spectra and XRD patterns did not indicate the precipitation of methionine, 2-aminoisobutyric acid, lysine, or glycine with the iron oxide formations. However, the heating of the glycine, methionine and lysine samples, synthesized in artificial seawater, showed peaks that could be attributed to the degradation of them. Then this could be an indication that these amino acids precipitate with the minerals during the syntheses. Also, the dissolution of these amino acids in artificial seawater prevents the formation of ferrihydrite.
Assuntos
Aminoácidos , Cisteína , Aminoácidos/química , Cisteína/química , Ácido Aspártico , Lisina , Espectroscopia de Infravermelho com Transformada de Fourier , Compostos Férricos/química , Minerais/química , Ferro , Água do Mar/química , Metionina , Água , Glicina , AdsorçãoRESUMO
Magnetite is an iron oxide mineral component of primitive Earth. It is naturally synthesized in different ways, such as magma cooling as well as olivine decomposition under hydrothermal conditions. It is probable magnetite played a significant role in biogenesis. The seawater used in the current work contained high Mg2+, Ca2+ and SO42- concentrations, unlike the seawater of today that has high Na+ and Cl- concentrations. It is likely that this seawater better resembled the ion composition of the seas of the Earth from 4 billion years ago. Cyanide and thiocyanate were common molecules in prebiotic Earth, and especially in primitive oceans, where they could act on the magnetite mechanism synthesis via Fe2+ interaction. In this research, magnetite samples that were synthesized under prebiotic conditions in the presence of cyanide or thiocyanate, (both with and without artificial seawater), showed that, besides magnetite, goethite and ferrihydrite can be produced through different Fe2+-ion interactions. Cyanide apparently acts as a protective agent for magnetite production; however, thiocyanate and seawater 4.0 Gy ions produced goethite and ferrihydrite at different ratios. These results validate that Fe3+ oxides/hydroxides were possibly present in primitive Earth, even under anoxic conditions or in the absence of UV radiation. In addition, the results show that the composition of water in early oceans should not be neglected in prebiotic chemistry experiments, since this composition directly influences mineral formation.
RESUMO
Fenton's process has proven to be a viable alternative for the oxidation of organic pollutants in mixed waste chemicals, with a high efficiency in the removal of heavy metals from the effluent as a side effect. However, a solid residue loaded with metals is obtained as a by-product. In this paper, the solids originating from the Fenton's oxidation of mixed waste chemicals from an academic laboratory were studied. The process was applied to different wastewater cases (two composite samples generated in different periods) in order to study the effect of their characteristics on the residues composition. Residues were evaluated and classified according to their leaching properties. In addition, a most comprehensive assessment of metal mobility in the residues was obtained by chemical speciation. The Fenton's residues were classified as hazardous according to Brazilian waste regulations. The application of the sequential dissolution procedure indicated that the metals in the Fenton's residues are mainly constituted of amorphous material (over 80%). Furthermore, the reactive fractions of the residues (exchangeable and amorphous iron oxide fractions) retain most of remaining metals. Therefore, the Fenton's residues present great potential for environmental contamination, and require an administration system and control of their final disposal.