Role of complexation processes in cadmium mobilization during estuarine mixing

Abstract

Complexation reactions with ligands in solutions¹ and on solid surfaces^2–4 are essential features of the biogeochemical cycling of trace metals such as cadmium⁵. Field studies^6–8 and laboratory experiments^8–11 show that cadmium can be mobilized from riverine participate matter when river water mixes with sea water. Although this mobilization is often attributed to the formation of cadmium chloro complexes in solution^6,9,11, the role of these competitive complexation processes has not been quantified. Moreover, there is controversy concerning the reversibility of the interaction of cadmium with particulates^11–13, which is an essential factor controlling the amount of cadmium that can be mobilized. Here we report the results of laboratory experiments and equilibrium calculations which demonstrate that cadmium partition between suspended particles and solution is completely reversible, and that complexation reactions in solution can completely account for the amount of cadmium released from suspended particles. Apparently, this toxic metal is not irreversibly fixed on particles, but can be released in response to changes in the aquatic environment. This may have important implications for the bioavailability of particle-bound cadmium.