The removal of toxic and heavy metal contaminants from aqueous waste streams and industrial effluents is one of the most important environmental issues being faced the world over. In order to combat this problem, the commonly used procedures for removing metal ions from dilute aqueous streams include chemical precipitation, ion exchange, reverse osmosis and solvent extraction. However, these techniques have certain disadvantages such as incomplete metal removal, high reagent and energy requirements, generation of toxic sludge or other waste products that require disposal. The hazardous wastes generated from metal mining and smelting operations also need to be decontaminated before entering the ecosystem. Chromium contamination of soil and ground water is a significant problem worldwide. The extensive distribution of this pollutant is due to its numerous industrial applications such as metal plating, alloying, leather tanning and wood industry. Cr (VI) is toxic and carcinogenic in nature while Cr (III) is innocuous. Conventional chromium removal techniques involve reduction to the Cr (III) form and subsequent precipitation as its hydroxide. However, disposal of the solid sludge remains a problem. The search for alternative and innovative treatment techniques has focussed attention on the metal uptake capacities of various microorganisms such as yeast, algae, fungi and bacteria. It is well documented that microbial biomass is capable of adsorbing metal ions from aqueous solution even when the cells have been killed. In the present investigation, the potential of utilising a gram positive, neutrophilic, facultative anaerobe like Bacillus polymyxa, in the bioremoval of Cr (VI) and Cr (III), has been assessed under different conditions. The growth of Bacillus polymyxa has been studied in the presence of varying concentrations of chromium ions. Subsequently, adaptation of the bacteria to Cr (VI) and Cr (III) has also been carried out. The biological reduction of Cr (VI) and its biosorption have been monitored during the growth of the unadapted and 2 ppm Cr (VI) adapted strains. The bioremoval of Cr (VI) and Cr (III) has also been assessed using the metabolic products obtained during bacterial growth. Detailed investigations have been carried out to determine the bioremoval efficiencies of both living and non-living cells of Bacillus polymyxa, with respect to Cr (III) and Cr (VI). The various parameters influencing the bioremoval of chromium by the cells, such as time, pH, wet biomass loading and initial metal concentration, have been studied. Electrokinetic studies on the bacterial cells, before and after interaction with Cr (VI) and Cr(III)have been carried out. The morphological changes induced in the bacterial strains consequent to interaction with Cr (III) and Cr (VI) have been examined by scanning electron microscopy. The results of the present investigation revealed that bioreduction of Cr (VI) was feasible during the growth of both adapted and unadapted bacteria. The time taken for 90% bioremoval was 72 h in the case of the unadapted strain, whereas with the adapted strain only around 48 h were required to achieve comparable results. The metabolic products obtained by enzymatic bacterial action were also found to be efficient in bringing about the bioremoval of Cr (VI). The bioremoval efficiency was marginally better when a lower concentration of Cr (VI) was used. Over 80% bioremoval was achieved in about 10 h using 2 ppm Cr (VI) while almost 48 h were necessary for a similar amount of removal to be effected using 5 ppm Cr (VI). In the case of the metabolite obtained from the adapted strain, complete removal of 2 ppm Cr (VI) was possible in 24 h. The living cells of Bacillus polymyxa were not only able to accumulate Cr (VI) but were also capable of bioreduction to the Cr (III) form, when the pH was in the range of 1.5 to 4. The maximum bioremoval of about 75% of Cr (VI) was observed at pH 2, with 45% being attributed to bioreduction, with an equilibration time of 48 h. In the case of Cr (III) nearly 90% uptake could be achieved at a natural pH of 5.5, equilibration time of 24 h and using 1 g of wet biomass. Biosorption was the only method of removal present in the non-living system. In the case of nonliving biomass, the optimum conditions for maximum Cr (VI) removal (65%) were pH 2, equilibration time of 12 h and a biomass loading of 1 g, whereas for Cr (III), the maximum uptake of about 97% occurred at an initial pH of 5, equilibration time of 12 h and 0.4 g wet biomass. The non-living cells showed a better efficiency in removing Cr (III), while the living cells exhibited a greater tendency towards the bioremoval of Cr (VI) than the non-living ones. Electrokinetic measurements revealed that consequent to interaction with Cr (VI) or Cr (III), significant surface modification was brought about on the cells of Bacillus polymyxa. Further, the isoelectric point was found to be shifted towards less acidic values after interaction with Cr (III) or Cr (VI). The probable mechanisms of the bioremoval processes are highlighted.

Submitted by MARCIA ROVADOSCHI (marciar@unifra.br) on 2018-08-17T19:22:30Z No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Dissertacao_BibianaCulauLopes.pdf: 2194937 bytes, checksum: 262e67bf2660f0e0bc2210c458d84e70 (MD5) / Made available in DSpace on 2018-08-17T19:22:30Z (GMT). No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Dissertacao_BibianaCulauLopes.pdf: 2194937 bytes, checksum: 262e67bf2660f0e0bc2210c458d84e70 (MD5) Previous issue date: 2017-03-31 / One of the most common methods applied to heavy metal removal from contaminated aqueous solutions is adsorption through materials like activated carbon, and silica. In general, those materials are efficient and possible to reuse, although after their lifespan they become an environment liability, especially if inappropriately discharged. So that, it is seeking for materials that show more efficiency, that are easily regenerated, reusable and that generate less environmental impact. It is promising the use of graphene oxide (GO) as an adsorbent for environmental pollutants. GO with metallic nanoparticles attached to it, they confer magnetic features to GO, which makes it separation from the aqueous solution easy. In this study it was synthesized graphene oxide through Hummers & Offeman (1958) adapted method. Synthesized GO as well as commercial graphene oxide (Sigma) were submitted to hydrothermal process for insertion of ferrite (NiFe3O4) in its structure, then forming a nanocomposite NiFe2O4/GO. Nanomaterials were characterized by DRX, FTIR, Raman spectroscopy, MEV, BET and magnetization test, which all prove the formation and magnetization of the material. A solution of 100 mg.L-1 concentration of potassium dichromate (K2Cr2O7), was used as Cr(VI) source, for adsorption tests. The results show great efficiency of adsorption, which was stable in 150 min of stirring for more concentrated solutions of Cr(VI). It was used 500 mg.L-1 concentration of potassium dichromate in the photocatalytic process, which resulted in significant decrease of Cr(VI) and release of Cr(III) in the solution. The results obtained show high efficiency in the adsorption and the removal mechanism defined was through electrostatic attraction, due to the difference between charges of adsorbent and pollutant. A solution with 0.03 g of commercial graphene oxide decorated with ferrite and potassium dichromate in a concentration of 80 mg.L-1, adsorption reaction achieved the equilibrium, approximately, 20 minutes after the reaction started. When tested concentrations of 100, 80 and 40 mg.L-1 of potassium dichromate, the reaction achieved the equilibrium in 30 minutes. Adsorption reactions in concentrations of 300, 250 and 150 mg.L-1 of potassium dichromate stabilized 150 minutes after the reaction started. All tests were done with a fixed mass of 0.30 g of commercial graphene oxide decorated with ferrite. The results indicated that the major efficiency, when higher concentrations of Cr(VI) 9 source, depends on the adjustment of adsorbent added. Photocatalytic reaction promoted the reduction of Cr(VI) to Cr(III), which was proved through the difference in absorbance solution. The removal mechanism was through difference between charges, photocatalytic reaction also promoted the desorption of Cr(III) from the adsorbent surface, as it has negatively charged, they naturally repel. / Um dos métodos mais comuns aplicado para remoção de metais pesados de soluções aquosas contaminadas é por adsorção através de materiais, como carvão ativado e sílica. Em geral esses materiais são eficientes e possibilitam a reutilização, porém ao final de sua vida útil se tornam passivo ambiental, principalmente se não forem descartados de forma correta. Desta forma busca-se por materiais que apresentem maior eficiência, facilidade de regeneração, de reutilização e que gerem menor impacto ambiental. O uso do óxido de grafeno como adsorvente, para remoção de poluentes de amostras ambientais, tem se mostrado promissor, principalmente se tratando de íons de metais. Quando o óxido de grafeno é agregado a nanopartículas metálicas, isso promove um caráter magnético, o que facilita a separação deste da solução aquosa. No presente estudo foi sintetizado o óxido de grafeno através do método adaptado de Hummers, Offeman (1958). O óxido de grafeno sintetizado, assim como óxido de grafeno comercial (Sigma), foram submetidos a um processo para inserção de ferrita (NiFe3O4) em sua estrutura via processo hidrotermal, formando o nanocompósito NiFe2O4/GO. Os nanomateriais obtidos foram caracterizados por difração de raios-x, espectroscopia no infrevermelho, espectroscopia Raman, microscopia eletrônica de varredura, teste de Brunauer-Emmett-Taller e teste de magnetização, os quais comprovam formação do óxido de grafeno e da magnetização do mesmo. Para os testes de adsorção e fotocatálise, utilizou-se solução de dicromato de potássio (K2Cr2O7) como fonte de Cr(VI). Os resultados obtidos demonstram alta eficiência de adsorção e o mecanismo de remoção definido foi por atração eletrostática, devido a diferença de cargas entre o adsorvente e o poluente. Para uma solução contendo 0,30 g de óxido de grafeno comercial decorado com ferrita e com uma concentração de 80 mg.L-1 de dicromato de potássio, a reação de adsorção atingiu o equilíbrio aproximadamente 20 minutos depois do inicio do processo. Quando testadas as concentrações de 100, 80 e 40 mg.L-1 de dicromato de potássio, a reação atingiu o equilíbrio em 30 minutos. As reações de adsorção em concentrações de 300, 250 e 150 mg.L-1 de dicromato de potássio estabilizaram depois de 150 minutos depois do inicio da reação. Todos os testes foram realizados com a quantidade fixa de 0,30 g de óxido de grafeno comercial decorado com ferrita. Os resultados indicam que a maior eficiência, 7 quando em maiores concentrações de fonte de Cr(VI), depende do ajuste na quantidade de adsorvente adicionado. A reação de fotocatálise promoveu a redução do Cr(VI) para Cr(III), o que foi comprovado através da diferença medida nas absorbâncias da solução. O mecanismo de remoção foi por por diferença de cargas, a reação de fotocatálise promove também a dessorção do Cr(III) da superfície do adsorvente, já que esse é carregado negativamente, eles se repelem naturalmente.