Evaluation of the bioavailability of potentially toxic metals in surface sediments collected from a tropical river near an urban area

The objective of this study was to evaluate the bioavailability of the metals cadmium, copper, lead, nickel, and zinc from sediment samples collected in the Paraguaçu river between the cities of Cachoeira and São Félix in the state of Bahia in Brazil. The method used was the acid extraction of volatile sulfides and metals extracted simultaneously (AVS-SEM) as described in the literature. For the extraction of sulfide, an argon drag system was used after acidification of the samples with HCl, releasing the volatile sulfides collected in the basic solution. Its determination was made by molecular absorption spectrophotometry using the methylene blue method. For quantification of the metals, the sediment with acid was filtered and the residual solution was analyzed by inductively coupled plasma mass spectrometry (ICP MS) in which the elements that were associated with the sulfide were released in solution. The bioavailability evaluation was done by the relation between the sum of the concentrations of the metals in solution (SEM) and the concentration of sulfide in each sample (AVS). When the ∑SEM/AVS ratio is >1, the medium is considered contaminated by the high bioavailability of the metals to other environmental compartments. In the samples collected in the Paraguaçu river in two campaigns, with an ∑SEM/AVS ratio of 0.007 to 1.082, the last value being the only critical>1, while the other quantities in the metallic phase were not available for a biota. By comparing the metal concentration data with the internationally established tolerance limits, all the values found were below the critical values, presenting no toxic risks to the surrounding ecosystem. •The concentrations SEM-AVS in sediments were determined.•Core sediment samples were collected along Paraguaçu.•Heavy metal accumulation is enhanced by the fine sediment granulometry and enriched organic carbon content.•For both sampling campaigns, all the values found were below the internationally established values.•Formation of metal sulphides is controlling the dispersal of metals (Pb, Cd, Cu, Zn, and Ni).