Metals in the aquatic environment – Interactions and implications for the speciation

Metals when entering in natural aquatic environments are rapidly complexed by biotic and abiotic ligands, and, therefore, their bioavailability is influenced by their speciation. On the other hand, metals speciation is influenced by the interactions with various components present in the environmental matrix, including dissolved and particulate organic matter, and inorganic materials including hydrous ferric and manganese oxides, clays and nanoparticles. Factually accepted but so far neglected, is the evidence that some of these components undergo kinetic physicochemical transformations largely modifying metals dynamic speciation and, thus, their fate, transport and bioavailability. Environmental systems are by nature not at equilibrium; thus, the use of equilibrium models is highly oversimplified and prevents us from making more than semi-empirical predictions as to the fate of chemical pollutants, including metals. Hence, the environmental risk assessment of metal ions depends on modeling their kinetic fate and mobility in presence of an environmental relevant media. Several key questions that underlie the effort to understand how metals, in particular Cu, Cd, Pb and Zn, behave in freshwater systems and how they may impact the environment, have being addressed: (i) How does the surrounding matrix affect the metal dynamic speciation, and, thus, the bioavailable metal species? Specifically, the role of colloids had been looked; (ii) To what extent can the interactions with the surrounding matrix result in more mobile and bioavailable metal species contributing to the uptake by the organism? The achievement of these goals has been performed by mainly using electroanalytical techniques: i) Scanned Stripping Chronopotentiometry, which allows the quantification of the kinetic rate parameters and of the diffusion coefficient of the colloids and particles, and ii) Absence of Gradients and Nernstian Equilibrium Stripping that permits the quantification of the free metal ion concentration with the lowest detection limit known today allowing to work at environmental relevant concentrations (ca. 10-9-10-8 M).