Significance of pH and Chloride Concentration on Behavior of Heavy Metal Pollutants: Mercury(II), Cadmium(II), Zinc(II), and Lead(II)

Abstract

Calculations were performed (i) to assess the degree to which Hg(II), Cd(II), Zn(II), and Pb(II) complex with hydroxyl and chloride ions and (ii) to evaluate the significance of such complexation in natural systems. Results indicate that both the hydroxy and chloride complexes may contribute to the mobilization of these heavy metal ions in the environment. Hydrolysis of Hg(II) becomes important at pH values above 1 whereas Pb(II), Zn(II), and Cd(ll) hydrolyze above pH 5, 7, and 8, respectively. Chlorides complex with Hg(II) at chloride concentrations above 10−9M (35 × 10−6 ppm). HgCl 2 forms above 10−7.5M Cl−(1.1 × 10−3 ppm), and HgCl 3− and HgCl 42‐ formation occurs above 10−2M Cl−(350 ppm). The MCl+ species of Zn(II), Cd(II), and Pb(II) appear at chloride concentrations above 10−3M (35 ppm), and MCl 2 complexes occur above 10−2M (350 ppm Cl−). The respective MCl 3− and MCl 42‐ species become important above 10−1M Cl−(3,500 ppm). Hydrolysis and chloride complexation of these heavy metal ions are important factors affecting the solubility of the sparingly soluble salts of these metal ions. This is most pronounced for mercuric salts. Intrinsic solubilities of the metal‐ion hydroxides allow for 160 ppm Zn(II) and 107 ppm Hg(II) to be soluble as complexed Zn(OH)2 and Hg(OH)2, respectively. These values are higher than calculated solubilities based on solubility products. An example of the competition between hydroxy and chloride complexes shows that at pH 8.5 and a chloride concentration range of 350–60,000 ppm, Hg(II) and Cd(II) are mainly complexed by chlorides. Zn(II) and Pb(II), under these conditions, are predominantly in the form of hydroxy complexes.