Process for removing heavy metals from water and wastewater

The choice of treatment technology depends on the concentration, the type of heavy metal and the water quality requirements. CP systems are often combined with ion exchangers or activated carbon filters.

1. precipitation and flocculation in CP plants

Chemical precipitation is a key process in water and wastewater treatment for the removal of heavy metals. The process is based on the chemical conversion of dissolved heavy metal ions into poorly soluble compounds, which can then be removed from the water by sedimentation, flotation or filtration.

Mechanism of precipitation

• Reaction with hydroxide ions (hydroxide precipitation):

  • The addition of alkalis such as milk of lime (Ca(OH)₂) or caustic soda (NaOH) leads to an increase in the pH value, causing heavy metal ions to precipitate in the form of sparingly soluble metal hydroxides.
  • This produces metal hydroxide precipitates, which can be separated from the water due to their low solubility.

• Reaction with sulphides (sulphide precipitation):

  • For heavy metals with particularly low solubility as sulphide (e.g. mercury, lead), sulphide ions are used, typically from sodium sulphide (Na₂S).
  • Sulphides have the advantage that they form stable precipitates even at slightly acidic pH values.

• Precipitation with complexing agents:

  • In cases where heavy metals are present in stable complexes (e.g. due to chelating agents such as EDTA), special reagents may be required to break down the complexes before precipitation takes place.

Factors influencing the precipitation

The effectiveness of precipitation depends on various chemical and physical factors that must be carefully controlled:

1. pH value:

   • The pH value is the most important parameter, as the solubility of most heavy metal hydroxides is strongly dependent on the pH value.
   • Examples:
     • Zinc hydroxide precipitates optimally at pH 9-10.
     • Iron(III) hydroxide precipitates at a pH value of 6-8.
   • Precise control of the pH value is necessary, as other metals (e.g. aluminum) can become soluble again if the pH value is too high.

2. Concentration of the reagents:

   • A sufficient amount of precipitant is required to bind all heavy metal ions. However, overdosing can lead to increased operating costs and additional chemical consumption.

3. Temperature:

   • The reaction rate and the solubility of the precipitation products are temperature-dependent. Higher temperatures generally promote the reaction rate.

4. Stirring intensity and contact time:

   • Good mixing ensures intensive contact between the reagents and the heavy metal ions.
   • The contact time must be long enough for the chemical reactions to take place completely.

5. Interference from other ions:

   • Anions such as chloride or sulphate can increase the solubility of certain heavy metal compounds and thus reduce the efficiency of precipitation.
   • In such cases, additional precipitation reagents or pre-treatment steps are required.

Flocculation: stabilization and separation of precipitation products

After precipitation, the precipitation products produced often remain dispersed in the water as fine particles. Flocculation is used to combine these particles into larger aggregates (flocs), which are easier to settle or separate.

Mechanism of flocculation

• Coagulation:
  • By adding coagulants such as iron(III) or aluminum salts, the electrostatic repulsive forces between the particles are reduced, which facilitates their agglomeration.
• Flocculation:
  • Flocculants such as polymers (e.g. polyacrylamides) increase particle formation by binding the flocs together through bridging.

Limits of precipitation and flocculation

1. Complexed heavy metals:

   • Heavy metals that are present in stable organic or inorganic complexes are difficult to precipitate. Prior splitting of the complexes (e.g. by oxidation or reduction) is necessary.

2. Residual concentrations:

   • Die chemische Fällung erreicht häufig nicht die extrem niedrigen Restkonzentrationen (< 0,01 mg/l) , die in manchen Industrien erforderlich sind. In solchen Fällen sind ergänzende Verfahren wie Adsorption oder Ionenaustausch erforderlich.

3. Sludge production:

   • Precipitation generates large quantities of precipitation sludge, which must be treated and disposed of as hazardous waste. The disposal costs can be considerable.

4. Chromium(VI):

   • Heavy metals such as chromium(VI) must be reduced to the less toxic form chromium(III) before precipitation, for example by adding sodium bisulphite.