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Comportamento geoquímico do arsénio em águas subterrâneas e superficiais de diferentes contextos geológicos
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Resumo(s)
Esta dissertação tem como objetivo a análise dos fatores que condicionam a concentração,
mobilidade, adsorção e precipitação de arsénio (As) em águas subterrâneas e superficiais, através do
estudo comparativo entre águas de contextos hidrológicos distintos: Vulcão das Furnas, Açores
(ambiente vulcânico) e Ribeira de Água-Forte, Aljustrel (Faixa Piritosa Ibérica).
Tratamento estatístico, cálculo do potencial redox das soluções, estudo das condições de
solubilidade e estabilidade, especiação e cálculo de equilíbrios água – rocha são metodologias utilizadas
para a concretização dos objetivos.
As águas do Vulcão Furnas são termais, redutoras, de fácies predominantemente bicarbonatada
sódica, ricas em voláteis de origem vulcânica (CO2 e H2S), apresentando uma vasta gama de valores de
pH. As concentrações de arsénio variam entre 15 e 1590 µg/L, predominando a espécie As(III), cuja
solubilidade está condicionada pela temperatura, pH, Eh e pela presença de espécies reduzidas de S.
As águas da Ribeira de Água Forte, afetadas por lixiviação de sulfuretos, caracterizam-se por
forte acidez, ambiente oxidante, com concentrações de arsénio entre 5,80 e 29 000 µg/L, onde prevalece
o As(V), cuja mobilidade é controlada pela ocorrência de outros metais, como Fe e Mn e processos de
oxidação. O arsénio é gradualmente adsorvido à medida que o Fe dissolvido precipita sob a forma de
jarosite e Fe(OH)3, em função do pH e Eh.
A análise conjunta das amostras dos dois casos de estudo colocou em evidência uma forte
correlação Eh – pH, validada por elevados coeficiente correlação linear (R2>0,99). A relação linear
obtida permite estimar valores de Eh em amostras que careciam de medições redox, a partir do pH.
Os resultados obtidos permitem um conhecimento aprofundado dos mecanismos geoquímicos
responsáveis pela distribuição do arsénio em diferentes ambientes hidrogeológicos, contribuindo para
uma gestão mais eficaz e sustentável de recursos hídricos em regiões suscetíveis à contaminação por
arsénio.
This dissertation aims to analyse the factors that condition the concentration, mobility, adsorption, and precipitation of arsenic (As) in groundwater and surface water by comparing waters from different hydrological contexts: Furnas Volcano, Azores (volcanic environment) and Ribeira de Água-Forte, Aljustrel (Iberian Pyrite Belt). Statistical treatment, calculation of the redox potential of solutions, study of solubility and stability conditions, speciation, and calculation of water-rock equilibria are the methodologies used to achieve the objectives. The waters of the Furnas Volcano are thermal, reductive, predominantly sodium bicarbonate, rich in volatiles of volcanic origin (CO2 and H2S), with a wide range of pH values. Arsenic concentrations vary between 15 and 1590 µg/L, predominantly the As(III) species, whose solubility is conditioned by temperature, pH, Eh, and the presence of reduced S species. The waters of the Água-Forte stream, affected by sulphide leaching, are characterised by strong acidity, an oxidising environment, with arsenic concentrations between 5.80 and 29,000 µg/L, where As(V) prevails, whose mobility is controlled by the occurrence of other metals such as Fe and Mn and oxidation processes. Arsenic is gradually adsorbed as dissolved Fe precipitates in the form of jarosite and Fe(OH)3, depending on pH and Eh. The joint analysis of the samples from the two case studies showed a strong Eh ̶ pH correlation, validated by high linear correlation coefficients (R2>0,99). The linear relationship obtained makes it possible to estimate Eh values from pH in samples that required redox measurements. The results obtained provide in-depth knowledge of the geochemical mechanisms responsible for the distribution of arsenic in different hydrogeological environments, contributing to more effective and sustainable management of water resources in regions susceptible to arsenic contamination.
This dissertation aims to analyse the factors that condition the concentration, mobility, adsorption, and precipitation of arsenic (As) in groundwater and surface water by comparing waters from different hydrological contexts: Furnas Volcano, Azores (volcanic environment) and Ribeira de Água-Forte, Aljustrel (Iberian Pyrite Belt). Statistical treatment, calculation of the redox potential of solutions, study of solubility and stability conditions, speciation, and calculation of water-rock equilibria are the methodologies used to achieve the objectives. The waters of the Furnas Volcano are thermal, reductive, predominantly sodium bicarbonate, rich in volatiles of volcanic origin (CO2 and H2S), with a wide range of pH values. Arsenic concentrations vary between 15 and 1590 µg/L, predominantly the As(III) species, whose solubility is conditioned by temperature, pH, Eh, and the presence of reduced S species. The waters of the Água-Forte stream, affected by sulphide leaching, are characterised by strong acidity, an oxidising environment, with arsenic concentrations between 5.80 and 29,000 µg/L, where As(V) prevails, whose mobility is controlled by the occurrence of other metals such as Fe and Mn and oxidation processes. Arsenic is gradually adsorbed as dissolved Fe precipitates in the form of jarosite and Fe(OH)3, depending on pH and Eh. The joint analysis of the samples from the two case studies showed a strong Eh ̶ pH correlation, validated by high linear correlation coefficients (R2>0,99). The linear relationship obtained makes it possible to estimate Eh values from pH in samples that required redox measurements. The results obtained provide in-depth knowledge of the geochemical mechanisms responsible for the distribution of arsenic in different hydrogeological environments, contributing to more effective and sustainable management of water resources in regions susceptible to arsenic contamination.
Descrição
Tese de mestrado, Geologia (Geodinâmica e Recursos Geológicos), 2024, Universidade de Lisboa, Faculdade de Ciências
Palavras-chave
Arsénio pE ̶ pH mobilidade precipitação distribuição Teses de mestrado - 2024