No abstract

Recebido em 7/4/03; aceito em 12/11/03; publicado na web em 27/05/04 BEHAVIOR OF METHYLMERCURY IN THE ENVIRONMENT. Methylmercury is the most hazardous mercury species known. Due to its high stability, lipid solubility, and ionic properties, this compound shows a high ability to cross membranes in living organisms, damaging the central nervous system, mainly the brain, and the effects of chronic poisoning are progressive. In this paper some aspects related to the toxicity and the cases of methylmercury poisoning are described. Other aspects related to the behavior of methylmercury and the environmental factors that influence the transformation of mercury in the water and sediment, with emphasis on the methylation/demethylation reactions and the mercury cycle are discussed.Keywords: human health; methylation; demethylation. INTRODUÇÃOO metilHg é a espécie mais tóxica do mercúrio e o interesse em seu estudo deve-se principalmente à capacidade de ser bioacumulado através da adsorção em corpos superficiais, na ingestão de alimentos, principalmente de peixes, bem como sua entrada antrópica no ambiente, em até um milhão de vezes ao longo da cadeia alimentar aquática 1,2 . Nos sedimentos o metilHg é consideravelmente mais tóxico que o mercúrio inorgânico, apesar de representar, em média, apenas cerca de 1,5% do mercúrio total. Particulados ricos em Hg 2+ são transportados para o sedimento, onde o metal pode ser metilado por bactérias sulfato-redutoras. Em adição à metilação, as bactérias presentes no sedimento podem também desmetilar o metilmercúrio, via reação reversa. O balanço das reações de metilação e desmetilação determina se um ambiente atuará como fonte ou sumidouro de metilmercúrio 1 . A dinâmica do metilHg no ambiente não está totalmente esclarecida, muito embora haja diversos trabalhos na literatura centrados na dinâmica de geração e consumo do metilHg em variadas matrizes ambientais. Neste sentido, este trabalho tem como principal objetivo apresentar um levantamento das transformações do metilHg na natureza, trazendo ao conhecimento da comunidade científica aspectos relevantes que determinam o comportamento desta substância tóxica persistente no ambiente. PROPRIEDADES FÍSICO-QUÍMICAS DO METILHgA literatura apresenta poucos dados a respeito das propriedades físico-químicas do metilHg 3-5 , sendo que algumas destas encontramse na Tabela 1. A temperatura de decomposição é de 700 ºC, porém a temperatura de volatilização ainda não é conhecida. A substância mais empregada como padrão analítico em trabalhos rotineiros de quantificação do metilHg é o cloreto de metilHg, que é relativamente estável em água de chuva, na neve e em águas de rios 3 . Com relação à estabilidade do metilHg, nota-se que esta é dependente de fatores como o tipo de recipiente empregado para o armazenamento, a temperatura e, também, o meio em que este se encontra dissolvido [6][7][8][9] . Lansens e colaboradores 10 demonstraram que uma solução contendo 10 µg/L de metilHg é estável em água ultrapura por 8 semanas, quando estocada a 5 ºC. No ent...

Recebido em 12/4/04; aceito em 13/10/04; publicado na web em 2/2/05 FACTORS AFFECTING Hg (II) ADSORTION IN SOILS FROM THE RIO NEGRO BASIN (AMAZON). Mercury (II) adsorption studies in top soils (top 10 cm) from the Rio Negro basin show this process depends strongly on some selected parameters of the aqueous phase in contact with the soils. Maximum adsorption occurred in the pH range 3.0-5.0 (>90%). Dissolved organic matter shows an inhibitory effect on the availability of Hg (II) to be adsorbed by the soils, whereas a higher chloride content of the solution resulted in a lower adsorption of Hg (II) at pH 5.0. Soils with higher organic matter content were less affected by changes in the salinity. An increase in the initial Hg (II) concentration increased the amount of Hg (II) adsorbed by the soil and decreased the time needed to reach equilibrium. A Freundlich isotherm provided a good model for Hg (II) adsorption in the two types of soil studied. The kinetics of Hg (II) adsorption on Amazonian soils showed to be very fast and followed pseudo-second order kinetics. An environmental implication of these results is discussed under the real scenario present in the Negro River basin, where acidic waters are in contact with a soil naturally rich in mercury.

In this study, nutrients were immobilized on the hydrochars obtained by hydrothermal carbonization (HTC) of a vinasse and sugarcane bagasse mixture, in the presence of acid, base and salt additives at temperatures of 150, 190 and 230°C. The increase in temperature caused higher immobilization of Ca, Mg, K, N, Cu, Mn, Zn, B, P and Fe in all hydrochars produced. HPO and NaOH immobilized higher amounts of P, Mg and Mn, while Ca was immobilized in higher quantities in the presence of HPO and (NH)SO. The addition of HSO, HPO and (NH)SO was responsible for an increased immobilization of P, N, Ca, Mg and K. The immobilization of B, not present in the starting raw material, was possible with the addition of HBO. The results showed that it is possible to alter the reaction medium to immobilize nutrients on hydrochars produced from vinasse and sugarcane bagasse, for agricultural applications.

No abstract

No abstract

Purpose Sugarcane bagasse and vinasse are wastes generated at large scales by the Brazilian sugarcane industry. Therefore, new waste treatment and management practices are essential for a sustainable industrial growth and here we purpose the hydrothermal carbonization (HTC) to converts wet biomass into carbon-based solids. Methods HTC of a mixture of sugarcane bagasse and vinasse was conducted at different temperatures, reaction times and phosphoric acid percentages. The chemical, structural and morphological properties of the hydrochars were evaluated by elemental analysis (CHNS), nutrient quantification (P, Ca, Mg, K), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results In the presence of phosphoric acid, the hydrochar yield increased as the ash content increased due to phosphate precipitates, as observed by XRD. The yield of the hydrochar decreased and the carbon and nitrogen content increased when the temperature increased from 180 to 230°C. Hydrochars are amorphous and compositionally similar to lignites. The FTIR spectra showed bands at approximately 1700 and 1600 cm-1 in the hydrochar due to carboxylation and aromatization of the products, respectively. The presence of carboxylic acids is important due to their ability to interact with cations and hydrophilic molecules. Additionally, nutrients such as P, N, K, Ca, and Mg were concentrated in the hydrochar as inorganic phases. Conclusions HTC applied to sugarcane bagasse and vinasse wastes produces hydrochars primarily containing carbon, nitrogen, and other nutrients as inorganic phases. Hydrochars could potentially be used as an agricultural fertilizer.

No abstract