Mecanismo de fotodegradação de compostos orgânicos catalisada por TiO2

Conventional technology used in the treatment of wastewater has been pointed as a major environmental problem for sustainable development, since minimization is not addressed accordingly. Advanced oxidation processes (AOP), based on the formation of hydroxyl radical (•OH), a powerful oxidant agent, have been considered to be a potential technology for the destruction of many toxic compounds. Photocatalysis using solar light, an AOP, has been studied for nearly 20 years and recently attracted great interest as a clean-up technology. However, solar detoxification processes have not yet achieved commercial success. This article presents an overview of reaction mechanisms at the surface of semiconductors used as photocatalysts (specially TiO2), when heterogeneous photocatalysis is used to remove hazardous compounds from contaminated sites.

Cinética e mecanismo da reação de auto-oxidação de S(IV) catalisada por íons metálicos de transição

The oxidation process of sulfur (IV) species (SO2, HSO3- e SO32-) by oxygen, catalysed by trace metal ion and complexes, can play an important role in atmospheric, analytical and bioinorganic chemistry. An overview of the most important reactions in these fields is presented. A fascinating redox cycling of the metal ions and complexes during such autoxidation process was revealed by the combination of kinetics and coordination chemistry studies.

Construção e otimização de um reator de baixo custo para a fotodegradação da matéria orgânica em águas naturais e sua aplicação no estudo da especiação do cobre por voltametria

This work describes the construction of a home-made low-cost reactor, using easily available materials, capable of destroying efficiently dissolved organic matter. Just 30 minutes of irradiation were sufficient to destroy more than 99% of the humic acids present in a solution of 4 mg C L-1. Copper speciation was evaluated in natural waters of different salinities to test the reactor's efficiency in destroying organically complexed metal species. The effect of the organic matter concentration, salinity, dissolved oxygen and temperature in the photo-oxidation process is discussed.

Síntese e caracterização de novos compostos de coordenação de cobre (II) com ligantes não-simétricos N,O-doadores: contribuições para o sítio ativo da galactose oxidase

The reactions of four new unsymmetrical N,O-donor ligands, {H2BBPETEN= [N-(2-hydroxybenzyl) - N,N' - bis(2 methylpyridyl) -N'-(hydroxyethyl) ethylenodiamine], H3BPETEN=[N,N'- bis(2-hydroxybenzyl) -N- (2-methylpyridyl) -N'- (hydroxyethyl) ethylenodiamine], HTPETEN=[N,N,N'- tris(2-methylpyridyl) -N'- (hydroxyethyl) ethylenodiamine] and H3BIMETEN=[N,N'-(2-hydroxybenzyl)-N-(1-methylimidazol-2-il-methyl)-N'- (hydroxyethyl)ethylenodiamine]}, with Cu(II) salts afforded the following mononuclear compounds: [CuII(HBBPETEN)]ClO4, [CuII(H2BPETEN)]ClO4 , [CuII(HTPETEN)](PF6)2 and [CuII(H2BIMETEN)]ClO4 . All were characterized by EPR, electronic spectroscopy and electrochemistry. The four copper (II) compounds showed interesting electrochemistry properties. All presented an anodic wave that can be attributed to the Cu (I) oxide formation at the electrode surface, or to a Cu0 sediment at the same surface or yet, to Cu(I) -> Cu(II) oxidation process with coupled chemistry reaction, due to their irreversibility. Two of the complexes are described as interesting synthetic models for the active site of the metalloenzyme galactose oxidase.

Química atmosférica do enxofre (IV): emissões, reações em fase aquosa e impacto ambiental

The oxidation process of sulfur(IV) species by oxygen, ozone and nitrogen oxides, catalysed by trace metal ions, can play an important role in atmospheric chemistry processes like acid rain, visibility degradation and health hazard. An overview of the more relevant investigations on emissions sources, aqueous phase conversion process and environmental impact is presented.

Degradação de corantes reativos pelo sistema ferro metálico/peróxido de hidrogênio

In this work the degradation of aqueous solutions of reactive azo-dyes is reported using a combined reductive/advanced oxidative process based in the H2O2/zero-valent iron system. At optimized experimental conditions (pH 7, H2O2 100 mg L-1, iron 7 g L-1) and using a continuous system containing commercial iron wool, the process afforded almost total discolorization of aqueous solutions of three reactive azo-dyes (reactive orange 16, reactive black 5 and brilliant yellow 3G-P) at a hydraulic retention time of 2.5 min. At these conditions the hydrogen peroxide is almost totally consumed while the released total soluble iron reaches a concentration compatible with the current Brazilian legislation (15 mg L-1).

Análise de imagem em química analítica: empregando metodologias simples e didáticas para entender e prevenir o escurecimento de tecidos vegetais

A simple and didactic experiment was developed for image monitoring of the browning of fruit tissues caused by the enzyme polyphenol oxidase. The procedure, easy and inexpensive, is a valuable tool to teach and demonstrate the redox reaction between the enzyme and the natural polyphenols. To obtain the browning percentage for apple, pear and banana, digital photographs were employed, and the images were analyzed by means of Monte Carlo methods and digital analysis programs. The effects of several experimental conditions were studied, such as pH, light, temperature and the presence of oxygen or anti-oxidants. It was observed that each fruit presented a different condition that better minimized the oxidation process. The absence of oxygen and the application of a bissulphite solution were sufficient to keep the quality of all fruits tested.

Trends and strategies of ozone application in environmental problems

Efforts presented by the scientific community in recent years towards the development of numerous green chemical processes and wastewater treatment technologies are presented and discussed. In the light of these approaches, environmentally friendly technologies, as well as the key role played by the well-known advanced oxidation processes, are discussed, giving special attention to the ones comprising ozone applications. Fundamentals and applied aspects dealing with ozone technology and its application are also presented.

Avaliação do potencial de processos oxidativos avançados para remediação de águas contaminadas com geosmina e 2-MIB

In this work, the efficiency of some homogeneous advanced oxidation processes (UVC/H2O2, Fe2+/H2O2, UVC/Fe2+/H2O2, UVA/Fe2+/H2O2, solar/Fe2+/H2O2) was investigated toward the degradation of geosmin and 2-methylisoborneol (2-MIB). The effect of relevant experimental parameters (ie. pH, Fe2+ and H2O2 concentration) was first investigated by factorial design, using camphor as a model substrate. In the geosmin and 2-MIB degradation studies the Fenton processes assisted by solar and UVA radiation offered the most promising results, mainly on account of high degradation capacity (higher than 80% at a reaction time of 60 min), high operational simplicity and low cost.

Degradação de fármacos residuais por processos oxidativos avançados

The concern about aquatic ecosystems and the potential risk of drinking water contamination by pharmaceuticals have stimulated the study of processes for the efficient degradation of these contaminants, since the conventional treatment have been inefficient on that purpose. The advanced oxidation processes (AOPs) appear as viable alternatives due to their efficiency on the degradation of different classes of organic contaminants. This review presents an overview of the main AOP (O3, H2O2/UV, TiO2/UV, Fenton and photo-Fenton) which have been applied to the degradation of different pharmaceuticals. The main results obtained, intermediates identified and toxicity data are presented.

Nióbia sintética modificada como catalisador na oxidação de corante orgânico: utilização de H2O2 e O2 atmosférico como oxidantes

In this work synthetic niobia was used to promote the oxidation of methylene blue dye in aqueous medium. The niobia was characterized by N2 adsorption/desorption, XRD and TG measurements. The presence of reactive species on the niobia surface strongly increased the oxidation rate of the methylene blue dye. The reaction mechanism was studied by ESI-MS suggesting that the oxidation of the organic dye involve oxidizing species generated mainly after previous treatment with H2O2. It can be observed that the catalyst is a good material in the activation of gas (atmospheric oxygen) or liquid (hydrogen peroxide) oxidant agent with a total discoloration of the dye solution after only 1 h of reaction.

Remoção de compostos orgânicos em água empregando carvão ativado impregnado com óxido de ferro: ação combinada de adsorção e oxidação em presença de H2O2

In this work, composites based on activated carbon/iron oxide (AC/Fe) were prepared in two different proportions (AC/Fe 5/1 and 1/1) and evaluated in the removal of the organic dye methylene blue (MB). Physical-chemical properties of the composites were determined by X-ray diffraction (XRD), adsorption/dessorption of N2 isotherm, temperature programmed reduction (TPR) and scanning electron microscopy (SEM). Results showed that goethite (α- FeOOH), with nanometer particle size, was formed over carbon surface for both composites. These materials showed high efficiency to remove MB from solution by combined adsorption and oxidation process. The AC/Fe 1/1 showed to be more active in (MB) oxidation then AC/Fe 5/1.

Aumento del contenido de ácidos húmicos en un carbón de bajo rango a través de la oxidación con aire y con peróxido de hidrogeno o ácido nítrico

Low-rank coals are an important source of humic acids, which are important in retention processes of water and nutrients in plants. In this study coal samples of Montelibano, Colombia, were oxidized with air at different temperatures and subsequently with H2O2 and HNO3. The materials were characterized by FTIR, proximate and elemental analysis, and quantification of humic acids. The oxidation process led to an increased content of oxygenated groups and humic acids in the carbonaceous structure. The solid oxidized with air at 200 ºC for 12 h and re-oxidized with HNO3 for 12 h showed the highest percentage of humic acids (85.3%).

Avaliação da estabilidade oxidativa do óleo bruto de açaí (Euterpe oleracea) na presença de compostos fenólicos puros ou de extratos vegetais amazônicos

A final 241 µM of ascorbyl palmitate and 555 µM of the following antioxidants separately: BHA, myricetin and quercetin standards, and extracts of Byrsonima crassifolia, Inga edulis or Euterpe oleracea, were added to crude açai oil and submitted to the oxidation process at 60 ºC for 11 days. Among the antioxidants used, only the myricetin standard showed the ability to defer the oxidation process until the third day of treatment. B. crassifolia, I. edulis and E. oleracea extracts showed no preventive capacity against the oxidation process, despite their high concentration phenolic compounds and antioxidant activities.

Ascorbic acid determination using a carbon paste electrode modified with iron(III) ions adsorbed on humic acid

An amperometric sensor was constructed, by using humic acids to immobilize Fe3+ ions on a carbon paste electrode (CPE-HA-Fe), and used for ascorbic acid (H2A) determination. The cyclic voltammogram of the electrode showed electrochemical response due to the Fe3+/Fe2+ couple at E1/2=+0.78 V vs SCE, using 0.5 mol L-1 KCl and 0.2 mol L-1 acetate/0.020 mol L-1 phosphate buffer, at pH = 5.4, as supporting electrolyte. When H2A is added to the electrolyte solution it is observed an oxidation process. The oxidation current, obtained by chronoamperommetry at +0.87 V vs SCE, is proportional to the concentration, represented by the equation I(µA) = 7.6286 [H2A] (mmol L-1) + 1.9583, r = 0.9996, for concentrations between 0.0 and 1.4 mmol L-1. The electrode showed high stability and was used for H2A determination in a natural orange juice.