A new approach to evaluate toxic metal transport in a catchment.

Competitive sorption and desorption of Cd2+, Pb2+, and Hg2+ onto riverbank and sediment samples of an area impacted by pyritic residue in a Southern Brazilian catchment were evaluated. Although these ions are considered poorly mobile, a new approach has been proposed to assess their behavior and associated risk. In this sense, factorial design and three-dimensional surface methodology are proposed to describe the competitive sorption behavior of the metal ion in the environmental matrix, as well as an innovative mobilization factor (MF) to describe the desorption rate from the integration of the normalized difference of sorption-desorption fluorescence peaks. Sorption was carried out with a central composite factorial design (23) to estimate simultaneous effects of independent variables. Three-dimensional surface analysis indicated increasing Cd2+ equilibrium concentration (Ceq) with Hg2+ and Pb2+ initial concentration (Ci), showing synergistic effect and low Cd2+ affinity to the solid phase. Statistical analysis presented [Formula: see text] as a significant variable for cadmium and lead dynamics, although [Formula: see text] was also significant for Hg2+ releasing to the liquid phase. After integrating the sorption and desorption fluorescence peaks, the MF for Cd2+, Pb2+, and Hg2+ was around 0.2, 0.5, and 0.1 in riverbank sediment, and 0.3, 0.9, and 0.1 in sediment, respectively. Hence, consistent ion mobilization along the river was observed, with Pb2+ mobilizing 9 and 6 times more than Hg2+ and Cd2+, respectively. The transport of ions such as Pb2+ and Hg2+, usually considered immobile, has indeed occurred, causing contamination through the watershed and increasing environmental risk. Graphical Abstract A new approach to determine toxic metal mobilization factor in a river catchment.