| Summary: | <p/> <p>The form or speciation of a metal in natural waters can change its kinetic and thermodynamic properties. For example, Cu(II) in the free ionic form is toxic to phytoplankton, while copper complexed to organic ligands is not toxic. The form of a metal in solution can also change its solubility. For example, Fe(II) is soluble in aqueous solutions while Fe(III) is nearly insoluble. Natural organic ligands interactions with Fe(III) can increase the solubility by 20-fold in seawater. Ionic interaction models that can be used to determine the activity and speciation of divalent and trivalent metals in seawater and other natural elements will be discussed. The model is able to consider the interactions of metals with the major (Cl<sup>-</sup>, SO<sub>4</sub><sup>2-</sup>, HCO<sub>3</sub><sup>-</sup>, CO<sub>3</sub><sup>2-</sup>, Br<sup>-</sup>, F<sup>-</sup>) and minor (OH<sup>-</sup>, H<sub>2</sub>PO<sub>4</sub><sup>-</sup>, HPO<sub>4</sub><sup>2-</sup>, PO<sub>4</sub><sup>3-</sup>, HS<sup>-</sup>) anions as a function of temperature (0 to 50 °C), ionic strength [0 to 6 m (m = mol kg-<sup>1</sup>)] and pH (1 to 13). Recently, it has been shown that many divalent metals are complexed with organic ligands. Although the composition of these ligands is not known, a number of workers have used voltammetry to determine the concentration of the ligand [L<sup><it>n</it></sup>] and the stability constant (<it>K</it><sub>ML</sub>) for the formation of the complex</p> <p>M<sup>2+ </sup>+ L<sup><it>n </it></sup>→ ML<sup><it>n</it>+2 </sup><it>K</it><sub>ML</sub>= [ML<sup><it>n</it>+2</sup>]/[M<sup>2+</sup>] [L<sup><it>n</it></sup>]</p> <p>We have added the experimental values of <it>K</it><sub>ML </sub>for the formation of complexes of natural organics in seawater of known concentration ([L<sup><it>n</it></sup>]) with Cu<sup>2+</sup>, Zn<sup>2+</sup>, Cd<sup>2+</sup>, Co<sup>2+</sup>, and Fe<sup>3+ </sup>. The model can be used to examine the competition of inorganic and organic ligands for divalent metals as a function of ionic strength. The importance of organic ligands in controlling the solubility of Fe(III) in seawater will be discussed. New experimental studies are needed to extend the model to higher temperatures and ionic strength.</p>
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