The influence of seasonalness on the structural characteristics of aquatic humic substances extracted from Negro River (Amazon State) waters: interactions with Hg(II)

Abstract

Neste trabalho, substâncias húmicas foram extraídas de amostras de água coletadas mensalmente na Bacia do Rio Negro-AM para estudar suas propriedades no ambiente amazônico e interações com íons mercúrio, considerando a influência da variação sazonal na sua formação. Os parâmetros razões atômicas C/H, C/N e C/O, grupos funcionais, radicais livres do tipo semiquinona, pH, índices pluviométricos e fluviométricos, concentrações de mercúrio foram interpretados por análise hierárquica de cluster (AHC) e análise de componentes principais (ACP). As análises estatísticas mostraram que, quanto maior o índice pluviométrico e menor o índice fluviométrico maior o grau de humificação das substâncias húmicas aquáticas. A seguinte ordem decrescente do grau de humificação das SHA coletadas mensalmente pode ser estabelecida: Nov/02 a Fev/03 > Mar/02 a Mai/02 > Jun/02 a Out/02. As maiores concentrações de mercúrio foram determinadas nas amostras mais humificadas, sugerindo que devido a rearranjos inter e/ou intramoleculares, quanto maior o grau de humificação maior a afinidade dos íons Hg(II) pelas substâncias húmicas aquáticas.In this work, humic substances were extracted from water samples collected monthly from the Negro River basin in the Amazon state (Brazil) to study their properties in the Amazonian environment and interactions with the mercury ion considering the influence of seasonalness in this formation. The C/H, C/N and C/O atomic ratio parameters, functional groups, concentration of semiquinone-type free radicals, pH, pluviometric and fluviometric indices, and mercury concentrations were interpreted using hierarchical cluster analysis (HCA) and principal component analysis (PCA). The statistical analyses showed that when the pluviometric index was greater and the fluviometric index was smaller, the degree of humification of aquatic substances was greater. The following decreasing order of the degree of humification of the AHS collected monthly was established: Nov/02 to Feb/03 > Mar/02 to May/02 > Jun/02 to Oct/02. The greatest concentrations of mercury were detected in more humidified samples. These results suggest that due to inter and/or intra-molecular rearrangements, the degree of humification of aquatic humic substances is related to its affinity for Hg(II) ions.Keywords: aquatic humic substance, seasonality, Negro River, Amazon, mercury IntroductionFresh water streams have an important function in the global carbon cycle in that they conduct organic matter (OM) of terrestrial origin and transfer it to the oceans. 1 The Amazonian region is home to the most extensive fluvial system which also has greatest liquid mass of the whole earth and it is covered by the largest pluvial tropical forest. According to Bluntschli, 2 the circulation of marine water through the atmospheric compartment above the forest and then from the forest across the fluvial plains again to the sea has a decisive 861 Oliveira et al. Vol. 18, No. 4, 2007 function in the Amazonian ecosystem. According to Sioli, 3 the forest and the regimes...

Influence of seasonality on the interaction of mercury with aquatic humic substances extracted from the Middle Negro River Basin (Amazon)

“…2 Statistical analysis was used to classify AHS into three groups according to the degree of humification, which corresponded to the periods in which they were collected: March- One sample of AHS from each group was selected for molecular size fractionation and analysis of the carbon content distribution and complexation capacity for mercury ions. The samples selected for fractionation were those collected in March, July and December 2002.…”

“…2 The nature of OM is influenced by the alternating flood and low water regime, 3 and its interaction with mercury influences the biogeochemical cycle of the metal in terms of its transport, speciation, solubility and bioavailability. 1,2,4,5 An important characteristic of the biogeochemical cycle of mercury is the variety of routes that it can take in the environment, including its release at the soil/atmosphere and water/atmosphere interfaces, atmospheric transport, and deposition to soil and water surfaces. In contact with soils and sediments, sorption of mercury in insoluble form may be followed by methylation/ demethylation.…”

Comparative characterization of humic substances extracted from freshwater and peat of different apparent molecular sizes

“…The O/C values for F1 and F2 fractions of HSP are slightly lower than the values for the same AHS fractions. The C/N ratios of the AHS fractions were considerably greater than those of the HSP, indicating that the largest contribution to the humification of AHS resulted from the decomposition of vascular plants (Rocha et al, 2007).…”

Major aspects of the mercury cycle in the Negro River Basin, Amazon

“…Mercury concentration in the water column was positively correlated with dissolved organic carbon (DOC), indicating metalorganic-bound species, especially involving humic and fulvic substances, which are the main form transported from watershed to aquatic systems through runoff. 63 In reaching white and black waters, mercury from terrestrial sources finds different redox conditions that vary according to the hydrological cycle and soil type in the drainage area. Black waters, with acid pH and high DOC level, and facing a dry period with no input of fresh labile organic matter from soil, creates a natural sunlit oxidative barrier that avoids DGM formation (thus mercury export), and degrades part of the methylmercury formed.…”