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A modified carbon paste electrode with SiO 2 /SnO 2 /Phosphate/Meldolas blue, SSPMelB, was used to study the electrocatalytic oxidation of ascorbic acid by cyclic voltammetry and chronoamperometry. The adsorbed dye mediates ascorbic acid oxidation at an anodic potential of 0.04 V vs. saturated calomel electrode (SCE) at pH 7.0, in 0.5 mol L À1 solution. The linear range of the sensor is between 4.0 Â 10 À7 and 2.0 Â 10 À3 mol L À1, with a limit of detection of 4.0 Â 10 À7 mol L À1. This novel electrode shows good analytical performance for determination of ascorbic acid in medicine and commercial fruit juice.

The natural cotton fiber was used to synthesize an anion exchange, containing ZrO 2 film on its surface, NCFZC (natural cotton fiber/ZrO 2 composite). This anion exchanger was produced by the reaction of the zirconium oxychloride and hydroxyl groups on surface of the natural cotton fiber. The material was used for Cr(VI) ions adsorption studies. Adsorption equilibrium time and optimum pH for Cr(VI) adsorption were found to be 6 h and 4.0, respectively. The Langmuir and Temkin isotherms were used to models adsorption equilibrium data. The adsorption capacity of NCFZC was found to be 1.33 mmol/g. Kinetic studies showed that the rate of adsorption of Cr(VI) on NCFZC obeyed a pseudo-second-order kinetic model.

A B S T R A C TAdsorption experiments were carried out using waste Mexerica mandarin "Citrus nobilis" peel as a new biosorbent to adsorb Cu(II), Cd(II), and Pb(II) ions from aqueous solutions at room temperature. To achieve the best adsorption conditions, the influences of pH and contact time were investigated. The isotherms of adsorption were fitted to the Langmuir and Freundlich equations. Based on the experimental data the adsorption order was Pb(II)>Cd(II) >Cu(II) (in mg g À1 ) on the new biosorbent. This quick adsorption process reached the equilibrium before 20 min, with maximum adsorptions at pH 5.0. Kinetic studies were carried out using the pseudo-first-order and pseudo-second-order models. Thermodynamic aspects of the adsorption process were investigated. The new biosorbent was used in columns for the removal of Cu, Cd, and Pb ions of the samples of industrial effluent and its efficiency and regeneration potential were studied.

ResumoO compósito SiO 2 /SnO 2 /Fosfato, SSF, foi preparado pelo processo sol-gel. O corante redox azul de metileno foi imobilizado na superfície do SSF. Esse novo material foi utilizado na preparação de um eletrodo de pasta de carbono modificado (SSFAM) e suas características eletroquímicas foram estudadas pela técnica da voltametria cíclica. O sensor SSFAM apresentou um potencial formal de -90 mV vs ECS, uma excelente estabilidade quanto ao número de ciclos e quanto ao pH da solução de eletrólito suporte na faixa de 2,0 a 8,0. O sensor SSFAM foi utilizado para a determinação de ácido ascórbico (Vitamina C), com a utilização da técnica da cronoamperometria, e apresentou um tempo de resposta de 1,5 s, uma faixa linear de resposta entre 2,0 x 10 -4 e 3,0 x 10 -3 mol L , considerando uma relação sinal/ ruído = 3. Os resultados obtidos para amostras reais pelo sensor SSFAM foram comparados com os do método padrão de determinação de ácido ascórbico pelo reagente 2,6-diclorofenilindofenol. A comparação dos resultados obtidos pelos dois métodos demonstrou que o sensor SSFAM possui grande potencial de utilização. Palavras-chave: Sensor. Sol-gel. Ácido ascórbico. AbstractThe SiO 2 /SnO 2 /Phosphate composite, SSF, was prepared by the sol-gel process. The redox dye methylene blue was immobilized on the SSF surface. This new material was utilized in the preparation of a modified carbon paste electrode (SSFAM) and its electrochemical characteristics were studied by the cyclic voltammetry technique. The SSFAM sensor presented a formal potential of -90 mV vs SCE, excellent stability regarding the number of cycles and regarding the pH of electrolyte support solution ranging from 2.0 to 8.0. The SSFAM sensor was utilized for the determination of ascorbic acid (Vitamin C), utilizing the chronoamperometry technique, and presented a response time of 1.5 s, a linear response range between 2.0 x 10 -4 and 3.0 x 10 -3 mol L -1 , with a detection limit of 7.8 x 10 -6 mol L -1, considering a relation signal/noise = 3. The results obtained for real samples by the SSFAM sensor were compared with those from the standard method for determination of ascorbic acid by the 2,6-dichlorophenilindophenol. The comparison of the obtained results by the two methods demonstrated that the SSFAM sensor has great utilization potential.

As fases LiMn2O4 e LiCr0,2Mn1,8O4 em pó foram preparadas por processo Pechini, modificado de maneira a diminuir o número de etapas e trabalhar com temperaturas inferiores ou iguais a 400oC. As 2 fases cristalizam no mesmo arranjo espinélio, com parâmetros de rede iguais a 8,21 e 8,22 Å respectivamente. Embora com temperatura de recozimento baixa, elas apresentaram boa qualidade cristalina e ausência de contaminação por resíduos orgânicos.

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