Marine chemistry of cadmium: A comparison of theoretical and field observations

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Abstract
Thermodynamic models showed that CdCO3(c) and Cd3(PO4)2(c) were the most stable Cd minerals in oxic and moderately oxic marine environments. A comparison between the published data on dissolved Cd in seawater and the thermodynamic observations, however, supported the formation of Cd phosphate. It was, therefore, proposed that formation/dissolution of Cd phosphate could control dissolved Cd in interstitial waters and seawaters. A mechanism for Cd phosphate formation is also proposed. Chloro‐complexes were the most abundant Cd species in standard seawater with CdCl2° (57 %) as the most important species, followed by CdCl+ (39 %) and CdCl3 − (11 %). Increase in salinity enhanced concentrations of high order Cd‐chloro complexes and vice versa. The concentration of ionic Cd2+ was found to be about 1 % of the total dissolved Cd. In anoxic environment (pe+pH < 4.5), Cd sulfide was the most stable mineral and Cd‐sulfide complexes were the most abundant chemical forms of Cd in seawater. Reports on the adsorption of Cd on solid surfaces in seawater and sediments were conflicting and suggested that adsorption phenomenon was not important in controlling dissolved Cd in seawater.