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What is chemical precipitation?

The chemical precipitation is a process of wastewater treatment in which chemicals are added to transform contaminant ions dissolved solid particles. In other words, it consists of generating sedimentable particles in wastewater by the addition of chemical substances.

The main objective of chemical precipitation is to remove contaminating ions such as heavy metal cations by the addition of counter ions (ions of opposite charge) that form less soluble compounds with these metals.

Although most of the contaminants to be removed are metals such as cobalt, cesium or mercury that form cations (positively charged ions), it is also used to remove anions (negatively charged ions) such as fluoride, cyanide and phosphate.

Once the solid precipitates, it is necessary to carry out a series of steps in order to separate it from the rest of the water. These steps include flocculation, sedimentation, and solid-liquid separation.

The chemical precipitation technique is based on the solubility balance of ionic compounds and the solubility product constant. In most cases, the solids that precipitate are poorly soluble hydroxides, although other types of ions also precipitate.

In all cases of chemical precipitation, the solid that is formed is a poorly soluble ionic compound that is in equilibrium with the ions in solution according to the general reaction:

The final concentration of the metal, M, that remains in the solution is determined by the equilibrium constant of this reaction, as well as by the concentration of the anion, A, that is added.

For example , if it is a hydroxide, the final concentration of the metal will depend on the concentration of hydroxide ions in the solution and, therefore, on the pH.

Step-by-step chemical precipitation process

The chemical precipitation process consists of four basic steps:

Step 1: Adding the precipitating agent and adjusting the pH.

This is the initial stage in which the precipitating agent of your choice is added (in most cases it is calcium hydroxide). The pH is adjusted to maximize precipitation and optimize the final purity of the water.

Step 2: Flocculation.

This stage consists of allowing the small solids that are formed in the first stage of precipitation to join together to form larger particles or “flocs” (hence its name).

These will settle more quickly in the next step. To promote the formation of these particles without breaking them, the agitation is done more slowly.

Step 3: Sedimentation.

It consists of letting the freshly treated water mixture rest in a tank to let all the solid particles go to the bottom (settle).

Step 4: Solid-liquid separation.

In most cases, the solid-liquid separation consists of a simple decantation of the water that remains on top of the sediment. In other cases, separation is achieved by filtering or even centrifuging the treated water, depending on how dangerous the contaminants are.

Types of chemical precipitation

Hydroxide precipitation

This is the most common type of chemical precipitation and consists of increasing the pH by adding an alkaline compound that is almost always calcium hydroxide (Ca (OH) 2 ).

The chemical reaction that occurs is:

In the above reaction, M represents any cation of the metal to be removed. The solid that forms in this case is a very poorly soluble hydroxide.

In the case of hydroxide precipitation, the general rule is that the higher the OH  concentration (the higher the pH), the lower the concentration of the metal that remains in the water, after treatment. In other words, the higher the pH, the higher the final purity of the water.

Sulfide precipitation

This type of chemical precipitation is used mainly for the removal of heavy metals, such as lead or mercury, in the form of very poorly soluble sulfides. To achieve this, both soluble and insoluble sulfides can be added to the water to be treated.

The most commonly used sulfides are soluble ones that include hydrogen sulfide (H 2 S) and sodium sulfide (Na 2 S). The general precipitation reaction is:

Ferrous sulfide can also be added which is insoluble, but contributes enough sulfide ions to the solution to precipitate most heavy metals.

Carbonate precipitation

Carbonates of many metals are less soluble even than hydroxides and can be precipitated from wastewater by adding calcium carbonate, or by converting hydroxides to carbonates by bubbling carbon dioxide into the alkalized mixture.

Cyanide precipitation

Unlike the previous cases, here we seek to precipitate cyanide, which is an anion, instead of a metal. Cyanide is a very dangerous poisonous pollutant and can be removed from wastewater by adding zinc sulfate or ferrous sulfate, as it forms complexes with these precipitating metals.

Co-precipitation

Coprecipitation occurs when one solute precipitates along with another. In these cases, the first solute is trapped in the solid when a precipitate of another compound forms.

It is as if the solid being formed left the first solute caged and dragged it out of the solution. This phenomenon is used to eliminate some contaminants that are difficult to precipitate on their own.

Applications

Chemical precipitation is used in countless industries in order to meet environmental wastewater quality requirements.

Hydroxide precipitation is used in the industries of:

  • Metallic finishes
  • Inorganic compound manufacturing
  • Foundries
  • Manufacture of fireworks and explosives
  • Porcelain enameling
  • Coal mining
  • The pharmaceutical industry, among others.

On the other hand, sulfide precipitation is used in industry to remove contaminants such as mercury, lead, and silver. This has application in the following industries:

  • Textile industry
  • Mining
  • Photographic equipment and supplies
  • Non-ferrous metal manufacturing, among others

Besides these industries, precipitation is the most important technique for the purification of the cooling water of nuclear reactors . In this industry, it is used to eliminate radioactive cations such as plutonium, américo, cesium and others.

Examples of chemical precipitation

Strontium precipitation with calcium carbonate

The precipitation reaction in this case is:

The reaction is carried out at an optimum pH of 10.5 and achieves concentrations of this metal up to 100 times lower than the initial one.

Antimony hydroxide precipitation

To remove antimony from wastewater, titanium hydroxide can be used as a precipitating reagent. The reaction is:

This is carried out in a pH range that goes from 5 to 8.5 and it also manages to reduce the concentration to a value 100 times lower than the initial one.

Cesium precipitation with tetraphenylborate

This is a special case that allows the precipitation of cesium, an alkali metal difficult to precipitate. The reaction is:

Using tetraphenylborate brings two great benefits: first, that it works at practically any pH (from pH 1 to 13) and second, that the low solubility of the salt that is formed allows the final concentration of the contaminant to be reduced by a factor of a thousand.

Precipitation of copper with sodium sulfide

The precipitation reaction in this case is:

Copper removal from wastewater by this method is carried out at pH values ​​above 8 and its concentration is reduced by more than 99%.

Coprecipitation of Chromium, Lead and Zinc with Ferric Hydroxide and Aluminum Hydroxide

In this case, sodium hydroxide is added to the water to precipitate iron and aluminum which is present in the form of ferric hydroxide (Fe (OH) 3 ) and aluminum hydroxide (Al (OH) 3 ). When these two hydroxides precipitate, they carry away part of the chromium, lead and zinc ions. Hydrogen sulfide post-treatment removes almost all of these three metals.

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