In the water treatment community, precipitation occurs when dissolved materials in water are converted to solids through adjustments to the pH or solubility of the solution.
In other words, precipitation works either by modifying the target substance to make it insoluble or by decreasing the solubility of the solution. Once in solid particle form, the contaminants can be processed to allow their extraction.
Adding the Precipitant
Precipitants are ionic solutions used to agglomerate molecules. For example, adding alum as a precipitant to a solution containing calcium bicarbonate and magnesium will cause these three molecules to agglomerate and form an aluminum hydroxide precipitate.
Ca(HCO3)2 + Mg + KAl(SO4)2 12H2O -> AI(OH)3
The precipitate
As mentioned above, the precipitate is the result of the addition of a precipitant to an aqueous solution. It consists of the solids formed during precipitation. To take the example above, the precipitate would be aluminum hydroxide (Al(OH)3)
More specifically, the precipitate is an ionic and insoluble agglomeration that is formed by the combination of cations or anions present in the aqueous solution.
In parenthesis, the solution remaining after the formation of the precipitate is called the supernatant liquid.
The choice of the precipitant
Since not all precipitants react in the same way with the various contaminants, it is important to select the type of product to use. To do this, it is necessary to rely on the solubility rules and the net ionic equation.
Coagulation and precipitation
Following precipitation, it is common to proceed to coagulation in order to increase the mass of the molecules.
The coagulant is the chemical agent at the center of the coagulation process. It is an agent that forces dissolved solids and suspended matter to clump together to form more massive molecules.
Whether cationic, anionic or nonionic, coagulants are generally polymers since their shape and ionic charge give them the ability to "bridge" dissolved and suspended solids together.
It is this step that activates the formation of small floc particles, which is useful for the flocculation step.
Coagulation, flocculation or precipitation?
These terms are very often interchanged with each other and yet each of them has its own definition. In fact, although they are all interrelated, they are not synonyms!
As we have explained, precipitation is a chemical reaction that affects the pH or solubility of water to allow certain contaminants to clump together.
Coagulation is an additional step aimed at forming more massive particles to allow their extraction. It is the addition of the coagulant that will allow the formation of the floc particles allowing the flocculant to work.
A little under the same principle as the two preceding treatments, the flocculation allows the agglomeration of certain particles in order to increase their mass and simplify their extraction.
In short, for more details on coagulation and flocculation, visit this article: