The solubility of several chemical pollutants is pH dependent. Thus, adjusting the pH to minimum solubility often allows efficient precipitation of these pollutants. Figure 7.32.1 shows the following three fundamental types of pH responses:
Case I shows the occurrence of minimum solubility or maximum solids precipitation at optimum pH value. Typical wastes producing this type of response include iron contained in acid mine drainage and fluoride and arsenic-bearing wastes. Case II shows a neutral or acidic waste containing certain heavy metals. The optimum pH is at the knee of the precipitated solids generation curve. Lime treatment of metal finishing waste is typical of a Case II response. Case III is the inverse of Case II; the chemical pollutant is precipitated due to pH depression, usually with sulfu-ric acid. A latex-bearing waste is typical of a Case III response.
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