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Advisor(s)
Abstract(s)
A aplicação de técnicas electroquímicas na área do ambiente é cada vez mais uma
realidade.
Uma das classes de poluentes mais perigosos é a dos metais pesados. Existem vários
métodos para a separação e/ou recuperação de iões metálicos em solução para
posteriormente serem reutilizados, recuperando assim o seu valor comercial e minimizando
efeitos nocivos no ambiente.
Pretende-se, neste trabalho, recuperar electroquimicamente os metais em solução,
reduzindo-os, se possível, à forma metálica ou removendo-os noutra forma. Na primeira parte
deste estudo foram realizadas remoções de alguns iões metálicos partindo de soluções
modelo, contendo um só dos metais pesados e numa segunda parte a partir de uma solução
dos 4 metais em estudo: Cu2+, Pb2+, Cd2+ e Zn2+. As concentrações dos metais em solução
foram escolhidas tendo em conta os métodos de análise seleccionados, algumas limitadas pela
solubilidade dos sais dos metais. Quantificou-se a percentagem de recuperação do metal
consoante o potencial aplicado, quer por determinação da massa depositada sobre os
eléctrodos quer por medidas da concentração do metal em solução ao longo dos ensaios,
recorrendo à análise das soluções por espectroscopia de absorção atómica. Embora se
pretendesse remover o metal por redução catódica, esta nem sempre foi a única forma de
remoção. Usaram-se soluções de sulfatos e/ou cloretos dos metais, acidificadas a pH 3,5,
aplicando-se diferentes potenciais consoante o metal. Os ensaios tiveram a duração de 3
horas. Os potenciais a aplicar para a recuperação de cada metal foram previamente
seleccionados através de estudos voltamétricos. Os ensaios de recuperação dos metais em
solução foram realizados por cronoamperometrias, usando uma célula de um compartimento,
com uma placa de aço a funcionar como cátodo, entre 2 placas de platina, que funcionavam
como ânodos, e o eléctrodo de Ag/AgCl, KClsat, como eléctrodo de referência. Para o metal
chumbo realizaram-se também ensaios em células de 2 compartimentos.
O ião Cu2+ em solução foi recuperado por redução sobre os cátodos na forma de Cu2O
a partir da solução do ião e nas formas de cobre metálico e Cu2O a partir da solução da
mistura de iões, nas mesmas condições iniciais de concentração de cobre, potencial aplicado
e pH, em células de 1 compartimento.
O ião Pb2+ foi recuperado na forma metálica sobre o cátodo numa célula de 2
compartimentos (em que o ião Pb2+ era colocado só na solução catódica) e recuperado na
forma metálica sobre o cátodo e na forma de PbO2 sobre o ânodo, numa célula de um
compartimento.
Os iões metálicos Cd2+ e Zn2+ foram recuperados na forma metálica sobre o cátodo,
em células de 1 compartimento. Os melhores resultados de remoção obtidos para as soluções individuais dos metais
pesados, tendo como base os resultados das análises por espectrometria de absorção atómica
e para as condições experimentais estudadas foram: Cu2+ 99,51 % a E= -0,1 V; Pb2+ 99,61 % a
E= -0,8 V; Cd2+ 92,58 % a E= -0,9 V e Zn2+ 37,66 % a E= -1,3 V, todos obtidos em células de um
compartimento.
Na remoção de metais da solução mista verificou-se que o tempo dos ensaios foi
manifestamente insuficiente para obter uma remoção selectiva dos mesmos mas mostrou ser
possível atingir um elevado grau de remoção dos metais a partir da mistura.
The application of electrochemical techniques in the environment is an increasing reality. One of the most dangerous classes of pollutants is the heavy metals. There are several methods for the separation and recovery of metal ions in solution for later reuse, recover their commercial value and minimize environmental damage. This work intends to recover the metals in solution electrochemically, reducing them, if possible, in metallic state or removing them in another form. In the first part of this study removals of some metal ions from model solutions were made, containing only one of the heavy metals and, in a second part, from a solution of the four metals under study: Cu2+ , Pb2+, Cd2+ and Zn2+. The metal concentrations in solution were chosen attending the analytical methods selected, some of them limited by the solubility of the metallic salts. The percentage of the recovery of metal was quantified attending the potential applied, by determining the mass deposited on the electrodes and by measurements of metal concentration in solution during the tests, analyzing the solutions by atomic absorption spectroscopy. Although the objective was to remove the metal by cathodic reduction, this was not, the only way to do it. In the assays were used solutions of sulfates and/or chlorides of metals, acidified to pH 3.5, applied different potentials, depending on the metal. Each process took 3 hours. The potentials to apply on the recovery of each metal were selected previously by voltammetric studies. Tests of the metals in solution were performed by chronoamperometry using a cell from one compartment with a steel plate to act as cathode, between two plates of platinum, which functioned as anode electrode and Ag/AgCl, KClsat electrode, as reference electrode. The assays with lead also took place in cells of two compartments. The Cu2+ ion in solution was recovered by reduction on the cathodes in the form of Cu2O from the solution of that ion and in the form of metallic copper and copper oxide (I) from the solution of the mixture of ions, under the same initial conditions of copper concentration, applied potential and pH, in just one cell compartment. The Pb2+ ion was recovered in metallic form on the cathode in a cell of two compartments (in which the Pb2+ ion was placed only in the cathodic solution) and, recovered in the metallic form on the cathode and in the form of PbO2 on the anode (in a one cell compartment. Cd2+ and Zn2+ metal ions were recovered in metallic form on the cathode, in cells of one compartment. The best removal results obtained in the individual solutions of heavy metals for the experimental conditions studied, were based on the results of analysis by atomic absorption spectrometry: Cu2+ 99.51% at E = -0.1 V; Pb2+ 99.61% at E = -0.8 V; Cd2+ 92.58% at E = -0.8 V and Zn2+ 37.66% at E = -1.3 V, which were obtained in just one cell compartment. In the removal of metals from the mixed solution, the conclusion was that the duration of the assays was clearly inadequate for a selective recovery of metals, but most of the metals in solution were removed.
The application of electrochemical techniques in the environment is an increasing reality. One of the most dangerous classes of pollutants is the heavy metals. There are several methods for the separation and recovery of metal ions in solution for later reuse, recover their commercial value and minimize environmental damage. This work intends to recover the metals in solution electrochemically, reducing them, if possible, in metallic state or removing them in another form. In the first part of this study removals of some metal ions from model solutions were made, containing only one of the heavy metals and, in a second part, from a solution of the four metals under study: Cu2+ , Pb2+, Cd2+ and Zn2+. The metal concentrations in solution were chosen attending the analytical methods selected, some of them limited by the solubility of the metallic salts. The percentage of the recovery of metal was quantified attending the potential applied, by determining the mass deposited on the electrodes and by measurements of metal concentration in solution during the tests, analyzing the solutions by atomic absorption spectroscopy. Although the objective was to remove the metal by cathodic reduction, this was not, the only way to do it. In the assays were used solutions of sulfates and/or chlorides of metals, acidified to pH 3.5, applied different potentials, depending on the metal. Each process took 3 hours. The potentials to apply on the recovery of each metal were selected previously by voltammetric studies. Tests of the metals in solution were performed by chronoamperometry using a cell from one compartment with a steel plate to act as cathode, between two plates of platinum, which functioned as anode electrode and Ag/AgCl, KClsat electrode, as reference electrode. The assays with lead also took place in cells of two compartments. The Cu2+ ion in solution was recovered by reduction on the cathodes in the form of Cu2O from the solution of that ion and in the form of metallic copper and copper oxide (I) from the solution of the mixture of ions, under the same initial conditions of copper concentration, applied potential and pH, in just one cell compartment. The Pb2+ ion was recovered in metallic form on the cathode in a cell of two compartments (in which the Pb2+ ion was placed only in the cathodic solution) and, recovered in the metallic form on the cathode and in the form of PbO2 on the anode (in a one cell compartment. Cd2+ and Zn2+ metal ions were recovered in metallic form on the cathode, in cells of one compartment. The best removal results obtained in the individual solutions of heavy metals for the experimental conditions studied, were based on the results of analysis by atomic absorption spectrometry: Cu2+ 99.51% at E = -0.1 V; Pb2+ 99.61% at E = -0.8 V; Cd2+ 92.58% at E = -0.8 V and Zn2+ 37.66% at E = -1.3 V, which were obtained in just one cell compartment. In the removal of metals from the mixed solution, the conclusion was that the duration of the assays was clearly inadequate for a selective recovery of metals, but most of the metals in solution were removed.
Description
Keywords
Metais pesados Metais pesados - Recuperação catódica Metais pesados - Técnicas electroquímicas Metais pesados - Electrodeposição Recuperação de metais - Reagentes