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Although water may contain excess inorganic compounds of many kinds, heavy metals are one of the more common problems. Remediation (removal) of the compounds is commonly done by pH control. That is, the pH is raised to a level at which the metal is insoluble. Then, the precipitated metal is made to flocculate so that it will settle out of the water column. After the metal settles, the water can then be brought back to a normal pH by adding an acid.

Details of the above processes are beyond the scope of this course and text. A review of the above process, plus other processes for removing metals (ion exchange, reverse osmosis, electrodialysis, and distillation) are given in Nyer (1985). The thermodynamic equilibrium basis for the solubility of metals (and nonmetals) as a function of pH and Eh is reviewed in Fetter (1993). A more complete description is given in Garrels and Christ (1965).

Figures 10.1 and 10.2 (adapted from Garrels and Christ, 1965, Figs. 7.5d and 7.5e) are examples of how the stability field of an inorganic species, iron in this case, can be described using Eh-pH plots. Figure 10.1 shows that the maximum amount (activity) of dissolved Fe+2 that can exist in solution decreases both with increasing Eh and increasing pH. Figure 10.2 shows that the activity of dissolved Fe+3 that can exist in solution is unaffected by Eh (in the hematite + water field) but decreases with increasing pH.Comparison of the two figures shows that Fe+3

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