Seawater cadmium solubility

Thus the presence of 10" M Cl" has increased the total dissolved cadmium concentration even further. Although the increase is approxi mately 2 percent in this example because of the 01 concentration selected, quite significant increases in solubility are observed in solutions such as seawater, where the Cl- concentration is approximately 20 g/liter or 0.56 M. In seawater of pH 8 the solubility of cadmium hydroxide including all hydroxo and chlorocomplexes) is approximately 10+°-39 compared with lO- - M if no Cd(II) complexes were formed. This is an increase of 110 times in cadmium solubility. In seawater the major dissolved species is CdCla . The solubilization of metals such as cadmium and mercury by the formation of soluble chloro complexes has significance in relation to marine waste disposal. The discharge of freshwater streams containing these metals in suspension to a saline environment could well result in increased dissolved metal levels because of the formation of dissolved chloride complexes. 

Scarponi et al. [93] concluded that filtration of seawater through uncleaned membrane filters shows positive contamination by cadmium, lead, and copper. In the first filtrate fractions, the trace metal concentration maybe increased by a factor of two or three. During filtration, the soluble impurities are leached from the filter, which is progressively cleaned, and the metal concentration in the filtrate, after passage of 0.8 -11 of seawater, reaches a stable minimum value. Thus it is recommended that at least one litre of seawater at natural pH be passed through uncleaned filters before aliquots for analysis are taken... 

Zinc carbonate (0.6 to 5ppb seawater 5 to lOppb river water) has moderate solubility as a carbonate (21 mg/100 mL) (73). Zinc compounds are not particularly hazardous above certain limits they may be toxic and irritating. The maximum level allowed in sludge from wastewater is 3 g/kg. The predicted no effect concentration (PNEC) for ecotoxicological effects was 50 p g/L (5 pg/lOOmL) dissolved zinc (73). Current values are not a very serious risk. High zinc levels protect against cadmium intoxication and reduce lead absorption. Pockets of historical contamination exist (73). 

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