Difusão e cinética de decomposição do ozônio no processo de fumigação de grãos de milho (Zea mays)

• Autor(a) principal: Santos, Joseane Erbice dos
• Data de Publicação: 2008
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: LOCUS Repositório Institucional da UFV
• Texto Completo: http://locus.ufv.br/handle/123456789/644
• topic: Difusão do gás ôzônio; Taxa de decomposição do gás ozônio; Coeficiente efetivo de difusão; Ozone gas diffusion; Ozone gas decomposition rate; Effective diffusion coefficient; CNPQ::CIENCIAS AGRARIAS::ENGENHARIA AGRICOLA::ENGENHARIA DE PROCESSAMENTO DE PRODUTOS AGRICOLAS::PRE-PROCESSAMENTO DE PRODUTOS AGRICOLAS

The need to find alternatives for insect pest control promotes research seeking to develop new methods and techniques for maintenance and preservation of product quality. One alternative is the use of modified atmosphere, using ozone gas (O3) as fumigant. Ozone is a strong oxidizing and biocide agent produced by electric discharge, without the need for handling, storage or packaging disposal. The objectives of this work were to to analyze the gas residual concentration after grain fumigation; determine the time for saturation and model the gas decomposition kinetics in the grain mass; evaluate the effect of saturation on grain physiological characteristics and determine the effective diffusion coefficient, considering the diffusion and decomposition of O3. Ozone was obtained from a generator developed by the Physics Department of the Technological Institute of Aeronautics (ITA). To determine the decomposition kinetics, three samples of 1 kg of maize grains with 12.8% wb average moisture . Each sample was stored in a 3.25 L glass container and subjected to atmosphere modified with 100 ppm of ozone gas at 4.6 L min-1 flow rate. O3 residual concentration was measured at regular intervals until steady rate, which indicated the saturation of the O3- maize system. Decomposition kinetics was determined from the saturation of the grain porous mass by the ozone, following the change in concentration with time. The O3 kinetics decomposition for previously saturated grain was studied by testing zero-, first-, second- and two-third order kinetics models. Grain moisture content was determined, and to evaluate the physiological characteristics, germination and electrical conductivity tests were carried out. The effective diffusion coefficient was determined, using three samples of 1 kg of grains packed in a stainless steel cylindrical column attached to a gas chamber, which was an adaptation of the diaphragm cell. Subsequently, a diffusion model was developed, considering the steady state in porous medium. To determine the ozone residual concentration, both for the decomposition kinetics and the effective diffusion coefficient, the gas was absorbed in an acidified potassium iodide solution, and the released iodine was titrated with sodium thiosulfate, using starch suspension as indicator. An exponential function was adjusted to data on ozone residual concentration and the first order model gave the best fit to the decomposition kinetics data. The decomposition rate constant (k), which is given by the slope of the line, was 0.1243 min-1, and the half-life time, which confirms the interaction of the O3-maize system was 5.57 min. The results led to the conclusion that O3 gets decomposed, during maize fumigation to two distinct phases: an initial and rapid decomposition due to the saturation of active sites in the first 70 min, followed by a slower decomposition. Maize grains showed no changes in physiological characteristics after saturation with ozone. The coefficient of effective diffusion for ozone through the grain mass is 1.29 x 10-2 cm2 s-1 and, for validation and comparison purposes, the ozone effective diffusion coefficient in maize grain, calculated by the Brokaw s theoretical model, was 1.31 x 10-2 cm2 s-1 for previously porosity of 0.40.