Chapter 13: Cadmium-Binding Properties of Surface Waters: Role of Dissolved and Suspended Matter

MINABERRY YANINA; BARRAL, EZEQUIEL; RAMÍREZ S.; GORDILLO GABRIEL J.

Cadmium: Characteristics, Sources of Exposure, Health and Environmental Effects.

Nova Science Publishers

Lugar: New York; Año: 2013; p. 295 - 310

Abstract Auto-depuration of an aquatic system is a set of physico-chemical and biological processes that naturally occur and that balance natural and/or anthropogenic perturbations. In the case of metals, which can not be degraded, they are present in an aquatic environment in a variety of more or less toxic forms. Metals such as cadmium and lead in the dissolved phase are distributed among hydrated ions, inorganic complexes with ions (like chloride, sulphate, etc) and organic complexes with dissolved organic matter (DOM). Suspended particulate matter (SPM) are also capable of binding metal ions on to their surface. Understanding such a complex scenario is relevant since the binding capacity of these components determines the characteristics of the aquatic environment as a whole. To this end, the binding capacity of a natural clay (mainly illita; chlorite and montmorillonite are minor components) towards cadmium and lead was studied. To do so, a natural system with high content of clay was selected, as a representative of several natural water systems in the planet. In fact, surface water samples were taken in the superior part of the Rio de La Plata estuary with an average SPM concentration (169 mg dm-3), similar to other important rivers as Orinoco (132 mg dm-3), Amazonas (162 mg dm-3) and Mississippi (200 mg dm-3). Due to the simultaneous presence of organic matter it was decided to evaluate the binding behavior of both DOM and SPM. Conditional binding constants (for DOM and SPM), ligand concentration (for DOM) and surface sites concentration (for SPM) are the binding parameters that characterize the complexing capacity (CC) of the system, which were obtained from electrochemically monitored titration curves. Results show SPM has a key role in cadmium and lead binding, thus immobilizing metals and removing them from the aqueous phase. However, addition of lead ion to a system containing adsorbed cadmium can displace this metal from the surface sites. In the dissolved phase, binding of lead to DOM has been detected. Binding of cadmium to DOM was not observed under the experimental conditions. This study indicates that in the case of a water course with a high concentration of SPM, like Rio de la Plata, it plays a major role in binding dissolved lead and cadmium, thus contributing to the auto-depuration of the aqueous phase.