Heavy Metal Precipitation

Metals in low concentrations are usually essential for life and can be extremely toxic in high concentrations. They are naturally in many aquifers and in many industrial applications. In several areas compounds are used that have heavy metals such as raw material for production process. They are usually found in tanneries, electroplating,  metallurgic industry, mining, automobile industries, among others. These compounds are very important for their high toxicity, high persistence and their capacity of accumulation (low biodegradable).  Also, in water courses they can be found in their soluble, colloidal or suspended forms.

TAERSA develops treatment systems that depending on their scale, they can be compact and built in our workshop or in situ in reinforced concrete. We perform the study on characterisation of the effluents in order to determine with precision the metals present in water and thus develop a proper treatment.

There are several available processes for heavy metals removal in an industrial effluent or process/ drinking water. These are the following:

  • Traditional chemical precipitation (co-precipitation, sulphide, hydroxyl or carbonate precipitation)
  • Improved chemical precipitation (DTC)
  • Other methods such as oxidation-reduction, ionic exchange, adsorption, bioadsorption, membranes

Usually the more used is the first one (traditional chemical precipitation) for the majority of heavy metals. Inside heavy metals category we can include: arsenic, cadmium, chromium, copper, lead, mercury, nickel, zinc, among
others.

In such cases it is convenient that the flow with heavy metals should be minimised in the source and segregated from other flows that might exist in productive processes in order to reduce the required physiochemical treatment.

Common precipitants include hydroxyls (OH-), sulphides (s2-) and carbonates (CO3-). Metals are precipitated in
hydroxide form by the addition of lime or sodium hydroxide as a mean to increase pH up to the point of minimum
solubility. The pH with less solubility varies with the metal involved.

Heavy metal can also precipitate as sulphides or carbonates to a lesser extent.

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Heavy metal removal can be done to avoid inhibition in a later biological treatment, or previous to a traditional water  treatment for human consumption, or to attain the current legislation for dumping effluents.

In the industrial effluent treatment with heavy metals, it is common the need for pre-treatment of those effluents removing the compounds that restrict their removal by precipitation. For example, cyanides and ammonia form complexes with several heavy metals that restrict their removal by precipitation. In these cases ammonia can be removed from the liquid phase by stripping or chlorination at the break point.

For some metals such as As and Cd, co-precipitation with iron or aluminium is highly effective to remove low residual heavy metals concentrations. In these cases, metals are adsorbed to the formed floc. In order to achieve lower concentrations, it can be necessary a filtering phase later, to retain flocs.

Carbamate salts (DTC) are used in the “improved” precipitation. Due to their cost, this type of precipitation is reserved for units of final polishing after a traditional chemical precipitation.

Heavy metals can also be removed by other adsorption processes, but with an operative cost much higher than the ones mentioned before.  Activated carbon, aluminium oxide or non-regenerable synthetic materials are used.  In these processes a high pH favours cations adsorption and an acid pH favours the anions.

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