Estuaries encompass the river/ocean interface, a physically and chemically dynamic region where biogeochemical processes radically modify the composition of river waters. Pronounced chemical reactivity results from the mixing of fresh water and sea-water, which gives rise to sharp gradients in the estuarine master variables of salinity, temperature, dissolved, O2, pH and the type and concentration of particles in suspension. The rapidly changing reaction conditions induce sorption, flocculation and redox cycling of trace metals and because particle–water interactions are time-dependent, trace elements are transported in a complex manner. A knowledge of the kinetics, mechanisms and equilibria associated with particle–water interactions is essential in the development of biogeochemical models capable of accurately predicting the fate of trace elements, particularly toxic metals from anthropogenic sources. This critical review examines the key processes controlling the speciation of trace metals based on field observations, controlled laboratory experiments and biogeochemical models, using European estuaries as exemplars.
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