MIL-HDBK-1005/9A
level. Use staged operation for batch treatment to complete
neutralization in at least two steps. Use smaller pH increments
(one to two units) for strong wastes.
e) Process Control. Use self-cleaning pH probes and
control systems that allow feed forward, feed back,
proportional, and manual control options.
2.6.5.2
Precipitation. Chemical precipitation involves
alteration of the ionic equilibrium to produce insoluble
precipitates that can be removed by sedimentation or granular
media filtration. The process can be preceded by chemical
oxidation (as for copper (Cu+) removal) or chemical reduction (as
for chrome (Cr+6) removal) to change the oxidation state of the
metal ions to a form that can be precipitated. The principle
metal precipitates are metal hydroxides, metal sulfides, and
metal carbonates.
a) Metal Ion Solubility. Heavy metal ion solubility
depends on the specific metal, system pH, temperature, and
degree of chemical complexation with organic and inorganic ions.
The effect of pH on solubility of selected heavy metal
hydroxides and sulfides is shown in Figure 3. Solubility of
metal carbonates is not shown since it is dependent on
wastewater alkalinity. Listed metal sulfides are less soluble
than the metal hydroxide at the same pH.
b) Precipitation Chemicals. Use lime or caustic soda
for pH adjustment for metal hydroxide precipitation. Use
hydrogen sulfide, sodium sulfide, or sodium bisulfide for metal
sulfide precipitation. Use sodium hydroxide with carbon dioxide
for metal carbonate precipitation.
c) Co-Mingled Metal Systems. Wastewaters that
contain several metals in solution may not be treatable by
adjustment to a single pH with a single chemical (for example,
cadmium and zinc cannot be simultaneously precipitated at
optimum pH for minimum metal hydroxide solubility). If these
metals occur in the same wastewater, two-staged (or more)
treatment is required. Alternatively, sulfide precipitation
could be used at a selected pH to produce approximately equal
low soluble metal levels. The solubilities presented in Figure
3 should be used for preliminary guidance only. Actual metal
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