Heavy Metals Precipitation

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Heavy metals precipitation

The principle technology to remove metals pollutants from wastewater is by chemical precipitation. Chemical precipitation includes two secondary removal mechanisms, coprecipitation and adsorption . Precipitation processes are characterized by the solubility of the metal to be removed. They are generally designed to precipitate trace metals to their solubility limits and obtain additional removal by coprecipitation and adsorption during the precipitation reaction.

There are many different treatment variables that affect these processes. They include the optimum pH , the type of chemical treatments used, and the number of treatment stages, as well as the temperature and volume of wastewater, and the chemical specifications of the pollutants to be removed. Each of these variables directly influences treatment objectives and costs. Treatability studies must be performed to optimize the relevant variables, so that goals are met and costs minimized.

In theory, the precipitation process has two steps, nucleation followed by particle growth. Nucleation is represented by the appearance of very small particle seeds which are generally composed of 10–100 molecules. Particle growth involves the addition of more atoms or molecules into this particle structure. The rate and extent of this process is dependent upon the temperature and chemical characteristics of the wastewater, such as the concentration of metal initially present and other ionic species present, which can compete with or form soluble complexes with the target metal species.

Heavy metals are present in many industrial wastewaters. Examples of such metals are cadmium , copper , lead , mercury , nickel , and zinc. In general, these metals can be complexed to insoluble species by adding sulfide, hydroxide, and carbonate ions to a solution. For example, the precipitation of copper (Cu) hydroxide is accomplished by adjusting the pH of the water to above 8, using precipitant chemicals such as lime (Ca(OH)₂) or sodium hydroxide (N_aOH). Precipitation of metallic carbonate and sulfide species can be accomplished by the addition of calcium carbonate or sodium sulfide. The removal of coprecipitive metals during precipitation of the soluble metals is aided by the presence of solid ferric oxide, which acts as an adsorbent during the precipitation reaction. For example, hydroxide precipitation of ferric chloride can be used as the source of ferric oxide for coprecipitation and adsorption reactions. Precipitation, coprecipitation, and adsorption reactions generate suspended solids which must be separated from the wastewater. Flocculation and clarification are again employed to assist in solids separation. The treatment is an important variable which must be optimized to effect the maximum metal removal possible.

Determining the optimal pH range to facilitate the maximum precipitation of metal is a difficult task. It is typically accomplished by laboratory studies, such as by-jar tests rather than theoretical calculations. Often the actual wastestream behaves differently, and the theoretical metal solubilities and corresponding optimal pH ranges can vary considerably from theoretical values.

[James W. Patterson ]