Figure 4 Metal hydroxide solubilities From

complexes CdOH+ and Cd(OH)ias the pH approaches to optimum pH. Cd(OH)°is as mentioned above, independent of pH and constitutes a limit to minimum solubility at all points near the optimum pH.

Beyond the optimum pH of 11.2, solubility increases due to increasing concentration of negatively charged complexes and is determined to a great extent by Cd(OH)j an(j k i:ii);]:.-. . Optimum pH is quite important for many of the metals because their solubility changes abruptly around the optimum pH. In Figure 4, the optimum pH of zinc is read as 9.2 and the solubility at this pH is lower than 0.1 mg/L, which is a low enough value for direct discharge to many receiving bodies. However, one unit decrease in pH increases the solubility nearly a hundredfold and zinc concentration reaches 9 mg/L at pH 8.2. However, some metals have a rather flat shape around optimum pH and solubility does not exhibit radical changes over pH ranges of one or even two units. Trivalent chromium is a good example for this case (Fig. 5) [8]. Figure 5 is obtained from the data given in the literature [9]. The shape of the solubility curve determines the precipitation strategy and minimum concentration obtainable by the hydroxide precipitation













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