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A contaminação de solos por parte da indústria extrativa e de outras indústrias que recorrem a produtos químicos poluentes deixa esses terrenos incultos e abandonados, contaminados com metais pesados como Cádmio e Zinco. Isto constitui um grave problema ambiental que importa mitigar. Uma das potenciais soluções é a fitorremediação do solo contaminado, a qual apresenta a vantagem de ser um tratamento local. No entanto, no processo de descontaminação do solo, a biomassa que resulta do tratamento capta e retém nos seus diversos constituintes (raízes, caules, folhas, sementes) os metais pesados. Deste modo, há uma transferência dos poluentes para a biomassa, potencialmente aumentando aí a concentração em metais pesados. É importante dar um destino adequado à biomassa proveniente da Fitorremediação. Isto pode ser conseguido através da sua conversão a biocombustíveis líquidos (como o biodiesel ou o bioetanol), sendo que possuem a vantagem adicional de estes serem provenientes de fontes renováveis, não havendo assim competição com outras biomassas cultivadas para fins energéticos, promovendo a melhoria da qualidade do solo no seu crescimento. Assim, o objetivo deste trabalho é o de avaliar o potencial bioenergético da biomassa resultante da fitorremediação de um solo contaminado com metais pesados através da produção de bioetanol, analisando a qualidade deste biocombustível, nomeadamente no que respeita à presença de metais pesados e ao potencial impacto da presença de metais pesados no rendimento do processo. Deste modo, foi obtido etanol pela destilação de palha de milho a qual foi submetida a um pré-tratamento ácido, a uma hidrólise enzimática e fermentação. Foram determinadas as melhores combinações entre os ácidos e enzimas utilizado e submeteuse a palha de milho contaminada a essas mesmas condições. Para o pré-tratamento adicionou-se 150 ml dos ácidos H2SO4, HCl, HNO3 e CH3COOH com uma concentração de 3% (v/v) a 20 g de palha de milho contaminada e não contaminada, tendo as amostras sido digeridas em frascos num banho termostático a 85 oC por 48 horas. Na etapa de hidrólise enzimática, após neutralização do digerido até ao pH 5 nas amostras, adicionaram-se 2 ml das enzimas Accellerase ou Ultraflo e a hidrólise das amostras procedeu em banho termostático por 13 horas a 50 oC. Para a fermentação diluiu-se 40 g da levedura Saccharomyces cerevisiae em 200 mL de água desionizada e adicionou-se a cada amostra 25 mL dessa mistura. Deixou-se o hidrolisado de cada uma das amostras a fermentar em banho termostático a uma temperatura de 37 oC por 11 dias. A destilação do etanol deu-se a uma temperatura de banho de 60 oC e a uma pressão de 120 mbar. Analisaram-se as concentrações dos açúcares simples durante os diferentes estágios de produção de etanol pelo método do ácido 3,5-dinitrosalicílico e determinou-se a concentração de etanol pelo método refratométrico. Realizou-se ainda uma análise do álcool recuperado por espectrofotometria de absorção atómica por forma a determinar a concentração de contaminantes (Zn e Cd) presentes no etanol obtido. Foi obtido um rendimento médio em etanol para a palha de milho contaminada de 0,51 getanol/gpalha e 0,32 getanol/gpalha para a palha tratada com HCl e Accellerase e 0,39 getanol/gpalha e 0,27 getanol/gpalha para a palha tratada com HNO3 e Ultraflo. Para a palha não contaminada, o rendimento médio em etanol é de 0,44 getanol/gpalha e de 0,37 getanol/gpalha para tratamento com HCL e Accellerase e HNO3 e Ultraflo, respetivamente. Na análise da concentração de contaminantes verificou-se que a concentração destes era extremamente baixa, tendo sido os valores lidos inferiores aos usados na construção da curva de calibração (< ppm), sendo, portanto, os resultados inconclusivos.
Contamination of soils by the extractive industry and other industries that use polluting chemical products leaves these uncultivated and abandoned lands contaminated with heavy metals such as Cadmium and Zinc. This is a serious environmental problem which needs to be mitigated. One of the potential solutions is phytoremediation of contaminated soil, which has the advantage of being a local treatment. However, in the process of soil decontamination, the biomass that results from the treatment captures and retains heavy metals in its various constituents (roots, stems, leaves, seeds). Therefore, there is a transfer of the pollutants to the biomass, potentially increasing the concentration in heavy metals. It is important to give a suitable destination to biomass from phytoremediation. This can be achieved by converting it to liquid biofuels (such as biodiesel or bioethanol), that have the additional advantage of being produced from renewable sources, so there is no competition with other biomasses grown for energy purposes, promoting the improvement of soil quality in its growth. Therefore, the aim of this work is to evaluate the biomass resulting from the phytoremediation of a soil contaminated with heavy metals through the production of bioethanol, analyzing the quality of this biofuel, namely in what concerns the presence of heavy metals and the potential impact of the presence of heavy metals on the yield of the process. Thus, we obtained ethanol by submitting the corn stover to an acid pre-treatment, followed by an enzymatic hydrolysis and fermentation. The fermentation broth was then distilled to recover ethanol. The best conditions where determined choosing the combination of the acid and enzyme that promoted the best results, and the corn stover produced in the contaminated soil was submitted to the best conditions. For the pre-treatment, 150 mL of acid H2SO4, HCl, HNO3 and CH3COOH with a concentration of 3% (v/v) were added to 20 g of corn stover produced in the contaminated and non-contaminated soil, in closed cap flasks in a thermostatic bath at 85 oC for 48 hours. In the enzymatic hydrolysis, after neutralizing the digested samples to a pH of 5, it was added 2 ml of the enzymes Accellerase or Ultraflo and once more the samples in the closed flasks where submitted to a constant temperature of 50 oC in a thermostatic bath for 13 hours. For the fermentation, 40 g of yeast Saccharomyces cerevisiae was diluted in 200 ml of deionized water and 25 ml of the mixture was added to each sample flask. The simple sugars in each sample flask where then fermented at 37 oC for 11 days. For the distillation of the ethanol, the bath temperature was 60 oC and the pressure was 120 mbar. It was analyzed the concentration of simple sugars (as glucose) during the different stages of the ethanol production using the method of 3,5-dinitrosalicilic acid and the concentration of ethanol was determined with refractometer. It was also made analyzed the concentration of heavy metals (Cd and Zn) in the recovered ethanol, using atomic absorption spectrophotometry. The average yield in ethanol obtained for the corn stover produced in the two contaminated soils was 0.51 gethanol/gstover and 0.32 gethanol/gstover for the corn stover treated with HCl and Accellerase and 0.39 gehtanol/gstover and 0.27 gethanol/gstover for the corn stover treated with HNO3 and Ultraflo, respectively. For the corn stover produced in the control soil, the yield in ethanol was 0.44 gethanol/gstover and 0.37 gethanol/gstover for the treatment with HCl and Accellerase and HNO3 and Ultraflo, respectively. The results of the analysis of the concentration of contaminants in the recovered ethanol revealed too low concentration, lower than the values used to build the calibration curve (< ppm), therefore the results are inconclusive.
Contamination of soils by the extractive industry and other industries that use polluting chemical products leaves these uncultivated and abandoned lands contaminated with heavy metals such as Cadmium and Zinc. This is a serious environmental problem which needs to be mitigated. One of the potential solutions is phytoremediation of contaminated soil, which has the advantage of being a local treatment. However, in the process of soil decontamination, the biomass that results from the treatment captures and retains heavy metals in its various constituents (roots, stems, leaves, seeds). Therefore, there is a transfer of the pollutants to the biomass, potentially increasing the concentration in heavy metals. It is important to give a suitable destination to biomass from phytoremediation. This can be achieved by converting it to liquid biofuels (such as biodiesel or bioethanol), that have the additional advantage of being produced from renewable sources, so there is no competition with other biomasses grown for energy purposes, promoting the improvement of soil quality in its growth. Therefore, the aim of this work is to evaluate the biomass resulting from the phytoremediation of a soil contaminated with heavy metals through the production of bioethanol, analyzing the quality of this biofuel, namely in what concerns the presence of heavy metals and the potential impact of the presence of heavy metals on the yield of the process. Thus, we obtained ethanol by submitting the corn stover to an acid pre-treatment, followed by an enzymatic hydrolysis and fermentation. The fermentation broth was then distilled to recover ethanol. The best conditions where determined choosing the combination of the acid and enzyme that promoted the best results, and the corn stover produced in the contaminated soil was submitted to the best conditions. For the pre-treatment, 150 mL of acid H2SO4, HCl, HNO3 and CH3COOH with a concentration of 3% (v/v) were added to 20 g of corn stover produced in the contaminated and non-contaminated soil, in closed cap flasks in a thermostatic bath at 85 oC for 48 hours. In the enzymatic hydrolysis, after neutralizing the digested samples to a pH of 5, it was added 2 ml of the enzymes Accellerase or Ultraflo and once more the samples in the closed flasks where submitted to a constant temperature of 50 oC in a thermostatic bath for 13 hours. For the fermentation, 40 g of yeast Saccharomyces cerevisiae was diluted in 200 ml of deionized water and 25 ml of the mixture was added to each sample flask. The simple sugars in each sample flask where then fermented at 37 oC for 11 days. For the distillation of the ethanol, the bath temperature was 60 oC and the pressure was 120 mbar. It was analyzed the concentration of simple sugars (as glucose) during the different stages of the ethanol production using the method of 3,5-dinitrosalicilic acid and the concentration of ethanol was determined with refractometer. It was also made analyzed the concentration of heavy metals (Cd and Zn) in the recovered ethanol, using atomic absorption spectrophotometry. The average yield in ethanol obtained for the corn stover produced in the two contaminated soils was 0.51 gethanol/gstover and 0.32 gethanol/gstover for the corn stover treated with HCl and Accellerase and 0.39 gehtanol/gstover and 0.27 gethanol/gstover for the corn stover treated with HNO3 and Ultraflo, respectively. For the corn stover produced in the control soil, the yield in ethanol was 0.44 gethanol/gstover and 0.37 gethanol/gstover for the treatment with HCl and Accellerase and HNO3 and Ultraflo, respectively. The results of the analysis of the concentration of contaminants in the recovered ethanol revealed too low concentration, lower than the values used to build the calibration curve (< ppm), therefore the results are inconclusive.
Description
Keywords
Bioetanol Contaminantes Descontaminação Fitorremediação Milho Bioethanol Contaminants Corn Decontamination Phytoremediation