Question

Identify how precipitation reactions can be used in the water treatment and purification processes to help maintain a safe municipal water supply

Answer 1

Water also contains many molecules and ions in solution. Many of the ions in solution can be removed by precipitation: reacting the ions (to be removed) with other ions to produce insoluble solids that can be removed by sedimentation or filtration.

A typical precipitation reaction used to remove ions in water treatment follows the reaction between ions from two aqueous solutions produced a solid precipitate.

A(aq) + B(aq) $\rightarrow$ C(s)

ion to be removed added ion precipitate

Many inorganic compounds can form very insoluble compounds, and often some toxic inorganic compounds that may be in the water supply, such as arsenic and chromium are soluble, but when the proper compound is added to the water, there is a displacement reaction, and suddenly you have formed a very insoluble inorganic compound that has removed the toxic compound.

For instance, a mixture of iron filings and sand can remove arsenic from the water, and is being used in places like Bangladesh.

Two major classes of ions are typically removed via precipitation:

Calcium (Ca²⁺) ion and magnesium (Mg²⁺) ion that have been leached from minerals in the ground cause the condition known as "water hardness". These ions do not pose any health threat, but they can engage in reactions that leave insoluble mineral deposits, such as scum rings on bathtubs and cooking vessels, or scale on industrial boilers, which decreases the boilers' efficiency. These deposits can make hard water unsuitable for many uses.

Iron (Fe²⁺) ion and manganese (Mn²⁺) ion can stain plumbing fixtures and laundered clothes. These ions may also promote the growth of certain bacteria, which give foul tastes and odors to the water.

In addition, ion exchange resins and other materials can be used.

In water treatment, the method is usually referred to as using flocculants.