Distribution of trace elements as electroactive species for environmental monitoring of estuarine waters in Pará river (AAmazonia, Brazil)

Mining activities have been recognized as having a high environmental impact in the Amazon region in the last decades. In addition, the growing urbanization and port activities in the region are also directly linked to environmental impacts, and the risk of bioavailability of toxic metals to aquatic ecosystems. This work presents a study of the distribution of Cd(II), Pb(II), Ni(II), Co(II), and Tl(I) in the dissolved (total and reactive, <0.2 μm) and particulate (>0.2 μm) fractions in water samples collected close to ore refining sites within the Pará River estuarine zone. As an analytical strategy, the sequential determination of the metals was performed by highly sensitive voltammetric methods. The speciation analysis of the elements was based on the size fractionation by filtration through 0.2 μm membrane filters to obtain the dissolved (<0.2 μm) and labile particulate (>0.2 μm) fractions. The metallic dissolved species were quantified in concentrations ranging from <0.023 to ∼0.25 μg L⁻¹ for Cd(II), Co(II), Tl(I), and <0.20–∼7.0 μg L⁻¹ for Ni(II) and Pb(II). For the total dissolved species, a distribution pattern was observed in both dry and rainy seasons, which indicates a possible local anthropic influence. Cadmium and thallium presented similar distribution patterns concerning the lower concentration of reactive species compared to the other metals. On the other side, reactive species of cobalt, lead and nickel, were found be distributed within the study area in concentrations close to total dissolved species, regardless of seasonality. The highest particulate species were detected in the reference points of the study area, indicating that the elements are predominantly associated with particles in the areas without anthropogenic influences. The speciation studies suggest that there is a dynamic transition between dissolved and particulate phases, possibly controlled by factors such as adsorption/desorption and interactions with organic matter in this estuarine region.