Cation Exchange

Many different components of sediments are capable of adsorbing cations from solution and releasing equivalent amounts of previous retained cations back into solution by ion exchange. The ability of a given soil or sediment to retain cations can be measured and is referred to as its cation exchange capacity (CEC). Cation exchange occurs via electrostatic interactions between cations in the sediment porewaters and negatively charged sites on sediment particles or organic matter. A cation will be retained by the negatively charged adsorption site in the sediment if it outcompetes and replaces one of the preexisting cations. Since cations of trace metals (e.g., Cd, Cr, Cu, Hg, Pb, and Zn) generally displace hydrogen ions (H+) and other major cations (Ca2+, Mg2+, Na+, and K+) from exchange sites on sediments, trace metal concentrations are often enriched in sediments relative to their average crustal abundance (i.e., EF > 1).

Conversely, increasing ionic strength and concentrations of competing cations in solution decreases the adsorption of metals by neutralizing the negative surface charge and increasing competition for exchange sites. As previously noted, that process occurs in lakes with low acid-neutralizing capacity which are impacted by acid deposition. in those areas, base cations and metals (e.g., Al3+) are solubilized as the lakes become acidified and cation exchange sites are filled with hydrogen ions (H+), causing toxic conditions for the aquatic biota.

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