RESUMO A caracterização geoquímica de águas salinas em reservatórios localizados em regiões semiáridas é uma questão importante a ser abordada, pois permite compreender a deterioração da qualidade da água causada pela evaporação. Neste estudo, o diagrama de Gibbs, razões iônicas e modelagem geoquímica foram empregados para decifrar os processos geoquímicos que afetam a evolução química da água de três reservatórios salinos localizados no semiárido do estado de Sergipe, Nordeste do Brasil. Os principais processos geoquímicos envolvidos na composição química da água dos reservatórios foram a dissolução do sal marinho, intemperismo dos silicatos, troca iônica, com uma contribuição limitada da dissolução de carbonatos. A modelagem geoquímica confirmou que a evaporação-cristalização é o principal mecanismo que controla a composição química da água, levando ao aumento das concentrações de Na+, Mg2+, Ca2+ e Cl-, e redução do HCO3- pela precipitação de calcita e dolomita. Além disso, os modelos simulados reproduziram a tendência observada nos dados hidroquímicos reais e indicaram excelente concordância entre as concentrações iônicas simuladas e as concentrações iônicas reais, para a maioria dos íons maiores. O maior desvio foi observado para o HCO3-, cujas concentrações reais foram muito mais elevadas que as previstas pela modelagem e isso tem sido atribuído a restrições cinéticas na precipitação da calcita.

ABSTRACT The geochemical characterization of saline water in reservoirs located in semi-arid regions is an important issue to be addressed, as it allows us to understand the deterioration of water quality caused by evaporation. In this study, the Gibbs diagram, ionic ratios and geochemical modeling were employed to decipher the geochemical processes that affect the chemical water evolution of three saline reservoirs located in the semi-arid region of Sergipe state, Northeastern Brazil. The reservoirs geochemical processes mainly include sea salt dissolution, silicate weathering, ion exchange, with a limited contribution from the dissolution of carbonates. Geochemical modeling confirmed that evaporation-crystallization is the main mechanism that controls the chemical composition of water, leading to increased concentrations of Na+, Mg2+, Ca2+ and Cl-, and reduction of HCO3- by precipitation of calcite and dolomite. Furthermore, the simulated models reproduced the trend observed in the real hydrochemical data and indicated excellent agreement between the simulated ion concentrations and the real ion concentrations for most of the larger ions. The highest deviation was observed for HCO3 whose actual concentrations were much higher than those predicted by the modeling, attributed to kinetic restrictions concerning calcite precipitation.

RESUMO Os processos geoquímicos que potencialmente controlam as características químicas das águas de três reservatórios, situados no estado de Sergipe, Nordeste do Brasil, foram avaliados usando o diagrama de Gibbs, análise de correlação, razões iônicas e modelagem geoquímica. Os resultados da modelagem geoquímica inversa, aplicada entre duas amostras de água de cada reservatório, foram concordantes com as observações da análise de correlação e razões iônicas, que indicaram a dissolução do sal marinho, o intemperismo dos silicatos e em menor proporção a dissolução dos carbonatos, como os principais processos geoquímicos responsáveis pelas características químicas das águas dos três reservatórios, inclusive confirmaram a importância da troca iônica para os reservatórios Macela e Ribeirópolis. Os resultados sugerem, portanto, que o crescente processo de salinização dos reservatórios é controlado principalmente pela interação água – rocha, podendo ter também contribuição da evaporação e de aportes antropogênicos.

ABSTRACT The geochemical processes that potentially control the chemical characteristics of the waters of three reservoirs located in the state of Sergipe, Northeastern Brazil, were evaluated using the Gibbs diagram, correlation analysis, ionic ratios and geochemical modeling. The results of the reverse geochemical modeling applied between two water samples from each reservoir were in agreement with the observations of the correlation analysis and ionic ratios, which indicated sea salt dissolution, silicate weathering and, to a lesser extent, carbonate dissolution, as the main geochemical processes responsible for the chemical characteristics of the waters of the three reservoirs, also confirming the importance of ion exchange for the Macela and Ribeirópolis reservoirs. These findings therefore suggest that the growing salinization process of the reservoirs is mainly controlled by water-rock interactions and may also be influenced by contributions from evaporation and anthropogenic inputs.

The contamination of natural environments by trace metals has several adverse effects on human health, which is mainly due to the high toxicity, persistence and reactivity of these species in the environment. To understand the bioavailability of trace metals, it is necessary to know the speciation in equilibrium and the dynamic behavior of the system. In this context, the present work aims to present a bibliographic review about the potentialities of the Stripping Chronopotentiometry (SCP) and Stripping Chronopotentiometry at Scanned Deposition Potential (SSCP) in the study of dynamic trace metal speciation. These techniques have low detection limits and do not suffer interference due to the adsorption of organic matter on the surface of the working electrode. Coupling these techniques with the Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) enables simultaneous determination of free ions, complexation equilibrium constants and diffusion coefficients of the complexes, thus providing information on the dynamic and heterogeneity of the metal complexes in the presence of macromolecules or particles. In this paper we will present the theory of dynamic speciation of trace metals, a detailed description of how to use the techniques to obtain this information, and examples of application in model systems and natural samples.

This work presents a critical overview of the NICA-Donnan model for trace metal thermodynamic speciation with humic substances obtained from natural organic matter in soils, natural waters and anthropogenic sources. A first part covers the theoretical aspects of the model, how to obtain good experimental data and perform the modelling. A second part deals with the developments in the last decade namely the use of the generic parameters, criticism and alternatives to the electrostatic modeling, the understanding of the NICA equation and an analysis of the published data, ending with the needs and opportunities for model development.

A espectroscopia de fluorescência e o carbono orgânico dissolvido (COD) foram utilizados para caracterização da matéria orgânica dissolvida (MOD) no estuário do Rio Piauí, um estuário tropical situado no Estado de Sergipe, nordeste do Brasil. As intensidades de fluorescência foram maiores no período chuvoso e menores no período seco, evidenciando que a variabilidade das propriedades fluorescentes da MOD é ditada, principalmente, pela descarga fluvial e pelo processo de diluição no estuário. No período chuvoso, os compostos tipo triptofano e húmicos predominaram na composição da MOD, enquanto no período seco ocorreu um aumento significativo dos constituintes tipo proteínas. As relações das intensidades de fluorescência com a salinidade sugerem um comportamento não conservativo para a MOD fluorescente. O COD mostrou um comportamento conservativo no período seco e não conservativo no período chuvoso, com produção de COD na parte superior do estuário. A complexação da MOD com o Cu(II), estudada usando a técnica de supressão da fluorescência, evidenciou o potencial de complexação dos fluoróforos tipo proteínas, tripofano e húmico.

Fluorescence spectroscopy and dissolved organic carbon (DOC) measurements were used to characterize the dissolved organic matter (DOM) in the Piauí River estuary, a tropical estuary situated in the State of Sergipe, Northeast Brazil. Fluorescence intensities were greater during the rainy period than the dry period, demonstrating that variability in the fluorescence properties of the DOM was determined mainly by the fluvial discharge and the dilution processes occurring in the estuary. During the rainy period, tryptophan-like and humic substances were the main components of the DOM, while during the dry period there was a significant increase of protein-like constituents. Relationships identified between fluorescence intensities and salinity were indicative of non-conservative behavior of the fluorescent DOM. DOC showed behavior that was conservative during the dry period, and non-conservative during the rainy period, with production of DOC in the upper reaches of the estuary. The complexation of DOM with Cu(II), studied using the fluorescence suppression technique, showed the complexation potentials of protein, tryptophan and humic type fluorophores.

Surface sediments from the River São Francisco were analyzed to investigate the impact, due to the presence of metals (Cd, Cu, Pb, Zn) in wastes from a metallurgical industry in the city of Três Marias/MG, Brazil. The concentrations and geochemical associations of Pb, Zn and trace metals associated with the minerals employed in zinc production were measured. Sediments close to discharge locations were highly contaminated with Pb (332-512 μg g-1) and Zn (7872-10780 μg g-1), with values decreasing rapidly due to dilution and hydraulic sorting. Evaluation of toxicity according to the Consensus-based Sediment Quality Guidelines indicated for Cd, Pb and Zn a high probability of adverse effects on aquatic biota at these sites.