No presente estudo, um novo material compósito (SiO2/TiO2/ZnO) foi avaliado para pré-concentração de cobre em amostras de água e alimentos. O sorvente foi caracterizado por microscopia eletrônica de varredura (SEM), infravermelho com transformada de Fourier (FT-IR), análise de dados texturais, fluorescência de raios X por energia dispersiva (EDXRF) e difratometria de raios X (XRD). A pré-concentração em linha e determinação espectrofotométrica na região visível foi baseada na pré-concentração de 20,0 mL de amostra em pH 7,0 através de 50 mg do sorvente, numa vazão de 6,4 mL min-1. Um volume fixo de 0,3 mL de HNO3 0,5 mol L-1 elui os íons cobre que reagem com dietilditiocarbamato (DDTC) 0,3% (m/m) cujo complexo formado (Cu(DDTC)2) é monitorado a 452 nm. A seletividade do sorvente foi examinada avaliando o efeito de íons interferentes. O método proposto forneceu linearidade variando de 20,0 a 230,0 mg L-1 (r = 0,998), fator de pré-concentração de 35,9 e limite de detecção de 5,6 mg L-1. Amostras de água e alimentos foram analisadas com sucesso pelo método proposto e a exatidão foi checada por meio de análises de materiais certificados de referência de proteína e fígado de peixe.

In the present study, a new composite material (SiO2/TiO2/ZnO) for copper preconcentration in water and food samples was evaluated. The sorbent was characterized by scanning electronic microscopy (SEM), Fourier transform infrared spectrophotometry (FT-IR), textural data analyses, energy dispersive X-ray fluorescence (EDXRF) and X-ray diffractometry (XRD). The on-line preconcentration and visible spectrophotometric determination was based on loading 20.0 mL of sample at pH 7.0 through 50 mg of sorbent at a flow rate of 6.4 mL min-1. A fixed volume of 0.3 mL of 0.5 mol L-1 HNO3 elutes copper ions, in which react with 0.3% (m/m) diethyldithiocarbamate (DDTC) whose complex formed (Cu(DDTC)2) is monitored at 452 nm. The selectivity of sorbent has been examined by assessing the effect of interfering ions. The proposed method provided linearity ranging from 20.0 to 230.0 mg L-1 (r = 0.998), preconcentration factor of 35.9 and limit of detection of 5.6 mg L-1. Water and food samples were successfully analyzed by proposed method and the accuracy was checked by analysis of certified reference materials of protein and liver of fish.

Second law characteristics of heat transfer and fluid flow due to free convection of non-Newtonian fluids over a horizontal plate with prescribed surface heat flux in a porous medium are analyzed. Velocity and temperature fields are obtained numerically using an implicit finite difference method under the similarity assumption and these results are used to compute the entropy generation rate Ns, irreversibility ratio φ and the Bejan number Be for both Newtonian and non-Newtonian fluids. The effects of power-law index n, heat flux variation parameter λ , and modified duty parameter, G on the dimensionless entropy generation rate Ns, and the Bejan number Be are investigated and presented graphically.