Prediction of the bioavailability of potentially toxic elements in freshwaters. Comparison between speciation models and passive samplers

The aim of this work is to predict the bioavailability of the Potentially Toxic Elements (PTEs) Cd, Pb, Hg, Ni, Cu, Zn, As, Cr and Se in 6 sites within the Ebro River basin. In situ Diffusive gradient in thin-films (DGTs) and classical sampling have been used and compared. The potentially bioavailable fractions of each PTE was estimated by modelling their chemical speciation using three programs (WHAM 7.0, Visual MINTEQ 3.1 and Bio-met), following the suggestions published in recent European regulations. Results of the equilibrium-based models WHAM 7.0 and Visual MINTEQ 3.1 indicate that As, Cd, Ni, Se and Zn, predominate as free metals ions or forming inorganic soluble complexes. Copper, Pb and Hg bioavailability is conditioned by their affinity to dissolved humic substances. According to Visual MINTEQ 3.1, Cr is subjected to redox reactions, being Cr (VI) present (at low concentrations) in the studied rivers. According to Bio-met model, the bioavailability of Cu and Zn is highly influenced by soluble organic matter and water hardness, respectively. For most PTEs, the bioavailability estimated by deploying DGTs in river waters tends to be slightly lower than the estimation obtained with speciation models, since in real conditions more environmental factors take place comparing to the finite number of parameters considered in models.