A pH‐DEPENDENT MODEL FOR THE CHEMICAL SPECIATION OF COPPER, ZINC, CADMIUM, AND LEAD IN SEAWATER

A pH-dependent model for the speciation of divalent Cu, Zn, Cd, and Pb ions in seawater was constructed with available and estimated thermodynamic stability constants and individual ion activity coefficients. This model was used to calculate the degree of interaction bctwccn each of the metal ions and the anions Cl-, Sod’-, HCOX-, CO,‘-, and OH- as a function of pH. Interactions between a cation and an anion were assumed to result only in the formation of complexes with coordination numbers of 1 to 4; polynuclcar and mixedligand complexes wcrc not included in the model. The calculations showed the following: All four metals are complexed to a considerable extent in seawater; with the exception of Cd, the distributions of chemical species of the metals vary greatly with changes in pH; Cu interacts primarily with OH- and COs2-, Zn with OH-, Pb with COa2- and Cl-, and Cd with Cl-; complexes with high coordination numbers (i.e. 3 and 4) are not formed to any appreciable extent in seawater.