Environmental Engineering Reference
In-Depth Information
Precipitation region
MOH
MO +
soluble
MO -
soluble
pH
FIGURE 10.8
Solubility-precipitation chart for a metal hydroxide complex.
350
Zn
300
250
Pb
200
Pb, Cu, Zn
mixture
150
Zn
Cu
100
50
Cu
Pb
0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
pH
FIGURE 10.9
Precipitation of heavy metals Pb, Cu, and Zn in aqueous solution. Bottom curves are precipitation of individual
metal from a mixture of Pb, Cu, and Zn in equal proportions of metal nitrate solution (100 meq each). Top curves
are for single solutions of individual metals at 300 meq/l concentration. (Data from MacDonald, E., Aspects of
Competitive Adsorption and Precipitation of Heavy Metals by a Clay Soil , MEng thesis, McGill University, Montreal,
Ca nada, 1994.)
boundaries are not distinct separation lines, and that transition between the two regions
or zones occurs near the boundaries throughout the entire pH range.
The role of pH in the soil-water system is important because of the various complexes
formed in relation to pH. For example, when a heavy metal contaminant solution such as
a PbCl 2 salt enters a soil-water system at pH values below the precipitation pH of Pb, a
portion of the metals will be adsorbed by the soil particles. The ions remaining in solution
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