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Authors
Abstract(s)
Num passado não muito distante, os herbicidas foram excessivamente utilizados. Um
maior conhecimento dos perigos da sua utilização tem levado a um consumo mais
moderado, até porque a maioria dos herbicidas, como é o caso do ácido cianúrico, são
considerados poluentes orgânicos persistentes, devido à sua persistência nos
ecossistemas, mesmo que em pequenas concentrações, o que representa um perigo para
a saúde humana e para o meio ambiente. A necessidade de evitar as contaminações do
meio ambiente tem incentivado a procura de novas tecnologias que permitam a
eliminação destes poluentes, como é o caso dos processos eletroquímicos.
Este trabalho teve como objetivo estudar a degradação do ácido cianúrico, por
oxidação anódica num ânodo de BDD, testando diferentes condições experimentais.
Foram realizados ensaios laboratoriais a densidades de corrente entre 10 e 70 mA cm-2 e
utilizando dois eletrólitos suporte – sulfato e cloreto de sódio. A eficiência da degradação
eletroquímica dos ensaios foi monitorizada através da determinação de diferentes formas
de carbono e de azoto, por cromatografia líquida de alta eficiência, espetrofotometria
de absorção no UV-Vis, e por voltametria cíclica.
Em ensaios preliminares verificou-se que para densidades de corrente inferiores a 30
mA cm-2 a remoção da carga orgânica era mínima, e para valores superiores a 50 mA cm2 não havia melhoria, aumentando apenas o consumo energético, com a consequente
redução de eficiência. Assim, o estudo foi efetuado apenas para as densidades de corrente
de 30 e 50 mA cm-2
.
Apesar de se saber que a carência química de oxigénio de soluções aquosas de ácido
cianúrico é zero, esperava-se que ao longo do ensaio se fossem formando outros
compostos orgânicos que dessem origem a valores detetáveis de COD, o que não
aconteceu, dando a indicação que, a haver intermediários, eles devem ser essencialmente
biureto e ureia. Observaram-se remoções de carbono muito baixas, inferiores a 10%, com
exceção dos ensaios realizados com o sulfato de sódio como eletrólito a 50 mA cm-2
,
tendo-se verificado que a remoção de carbono orgânico aumenta com a densidade de
corrente. Para a densidade de corrente inferior, há maior mineralização na presença de
cloretos.
Quanto à remoção de azoto total na presença de sulfato, à densidade de corrente
inferior é praticamente nula. No entanto, quando se usa o cloreto de sódio, a redução de
azoto total é superior a 50% e a redução de espécies carbonadas é cerca de 40%, havendo
formação de carbonato, cuja concentração se mantém aproximadamente constante na
parte final dos ensaios. Em relação à informação obtida por cromatografia líquida de alta eficiência, os
resultados não foram satisfatórios, pois não foi possível determinar com segurança a
variação da concentração do ácido cianúrico em nenhum dos eletrólitos testados. A
informação recolhida por voltametria cíclica também não foi relevante, pois,
aparentemente, a oxidação do ácido cianúrico só acontece na zona de evolução do
oxigénio.
In a recent past, herbicides were overused. The knowledge of the dangers of their use has led to a more moderate consumption, because most herbicides, such as cyanuric acid, are considered persistent organic pollutants, due to their persistence in ecosystems, even in small concentrations, which poses a danger to human health and to the environment. To avoid environmental contamination, it is necessary to search for new technologies that allow the elimination of these pollutants. Among these new technologies, electrochemical processes have received the attention of many researchers. This work aimed to study the degradation of cyanuric acid, by anodic oxidation at a BDD anode, testing different experimental conditions. Laboratory tests were carried out at current densities between 10 and 70 mA cm-2 and using two supporting electrolytes – sodium sulfate and sodium chloride. The electrochemical degradation efficiency of the assays was monitored through the determination of different forms of carbon and nitrogen, by high performance liquid chromatography, UV-Vis absorption spectrophotometry, and by cyclic voltammetry. It was found that for current densities below 30 mA cm-2 the removal of organic load was minimal, and for values above 50 mA cm-2 there was no improvement, only increasing energy consumption, with a consequent reduction in efficiency. Thus, the study was performed only for current densities of 30 and 50 mA cm-2 . Although it is known that the chemical oxygen demand of aqueous solutions of cyanuric acid is zero, it was expected that some compounds formed during the degradation assays would give rise to detectable values of chemical oxygen demand. This didn’t happen, indicating that if there are intermediates, they must be mainly biuret and urea that also present a null chemical oxygen demand. Very low carbon removals, less than 10%, were observed, with the exception of the tests carried out with sodium sulfate as electrolyte at 50 mA cm-2 , which showed that the removal of organic carbon increases with the current density. For the lowest current density, there is higher mineralization in the presence of chlorides. As for total nitrogen removal in the presence of sulfate, at the lowest current density it is practically null. However, when sodium chloride is used, the reductions in total nitrogen are higher than 50% and those of carbonaceous species around 40%, with the formation of carbonate, whose concentration remains approximately constant in the final part of the degradation assays. Regarding the information obtained by high performance liquid chromatography, the results were not satisfactory, as it was not possible to reliably determine the variation in the concentration of cyanuric acid in any of the tested electrolytes. The information collected by cyclic voltammetry was also not relevant, since, apparently, the oxidation of cyanuric acid only takes place in the oxygen evolution region.
In a recent past, herbicides were overused. The knowledge of the dangers of their use has led to a more moderate consumption, because most herbicides, such as cyanuric acid, are considered persistent organic pollutants, due to their persistence in ecosystems, even in small concentrations, which poses a danger to human health and to the environment. To avoid environmental contamination, it is necessary to search for new technologies that allow the elimination of these pollutants. Among these new technologies, electrochemical processes have received the attention of many researchers. This work aimed to study the degradation of cyanuric acid, by anodic oxidation at a BDD anode, testing different experimental conditions. Laboratory tests were carried out at current densities between 10 and 70 mA cm-2 and using two supporting electrolytes – sodium sulfate and sodium chloride. The electrochemical degradation efficiency of the assays was monitored through the determination of different forms of carbon and nitrogen, by high performance liquid chromatography, UV-Vis absorption spectrophotometry, and by cyclic voltammetry. It was found that for current densities below 30 mA cm-2 the removal of organic load was minimal, and for values above 50 mA cm-2 there was no improvement, only increasing energy consumption, with a consequent reduction in efficiency. Thus, the study was performed only for current densities of 30 and 50 mA cm-2 . Although it is known that the chemical oxygen demand of aqueous solutions of cyanuric acid is zero, it was expected that some compounds formed during the degradation assays would give rise to detectable values of chemical oxygen demand. This didn’t happen, indicating that if there are intermediates, they must be mainly biuret and urea that also present a null chemical oxygen demand. Very low carbon removals, less than 10%, were observed, with the exception of the tests carried out with sodium sulfate as electrolyte at 50 mA cm-2 , which showed that the removal of organic carbon increases with the current density. For the lowest current density, there is higher mineralization in the presence of chlorides. As for total nitrogen removal in the presence of sulfate, at the lowest current density it is practically null. However, when sodium chloride is used, the reductions in total nitrogen are higher than 50% and those of carbonaceous species around 40%, with the formation of carbonate, whose concentration remains approximately constant in the final part of the degradation assays. Regarding the information obtained by high performance liquid chromatography, the results were not satisfactory, as it was not possible to reliably determine the variation in the concentration of cyanuric acid in any of the tested electrolytes. The information collected by cyclic voltammetry was also not relevant, since, apparently, the oxidation of cyanuric acid only takes place in the oxygen evolution region.
Description
Keywords
Ácido Cianúrico Bdd Degradação Eletroquímica Ureia Voltametria Cíclica