Distribuição e fracionamento geoquímico de metais em sedimentos estuarinos do semiárido brasileiro sob diferentes estações climáticas

Abstract

Estuaries are dynamic systems that play a key role in the accumulation and transport of contaminants, such as metals, from mainland to the ocean. The distribution, mobility and bioavailability of metals in estuarine sediments are dependent on physicochemical factors such as pH, redox potential, grain size, carbonate, organic matter and Fe-Mn oxides-hydroxides, and resuspension events. This study investigated the distribution and geochemical partitioning of metals (Cu, Zn, Al, Ba) in the surface sediments of the Jaguaribe River estuary, located in the Brazilian northeastern semi-arid, in two climatic seasons, in order to determine the origin and influence of the geochemical carriers on the mobility of these elements. For the fractionation, it was applied and adapted geochemical sequential extraction method from Tessier extracting five geochemical forms: exchangeable, bound to carbonates, bound to Fe and Mn oxides, bound to organic matter and residual fraction. Metals were quantified by atomic absorption spectrometer (AAS). The total organic carbon (TOC) was determined by the wet oxidation method and total nitrogen by the Kjedahl method. The Cu, Zn, Al showed concentrations decreasing towards the mouth of the estuary, whereas Ba showed a contrary trend in the dry season. The geochemical correlations indicated that Cu, Zn, Al presented a similar origin and transport mechanisms along the estuary, which was confirmed by the predominance of the residual fraction in these elements and the enrichment fators (EF) that indicated no or little enrichment. The lower proportion of labile fractions (carbonate, reducible and oxidizable) indicates a small anthropogenic contribution, low mobility, bioavailability and toxicity at the metals in the estuary, and were ratified by the use of the RAC index. Ba, in turn, presented origin and transport mechanisms distinct from the other metals and was mainly associated with the carbonate fraction, being the element whose concentrations varied between sampling points and climatic seasons.