Pollution and hard water treatment by precipitation

In this lesson, we will learn:

• How precipitation is used to treat water pollution and hard water.
• To recall the meaning of the terms suspension and supernatant.
• How to use K_sp and ion concentrations to solve problems related to hard water and pollution.

• Being able to precipitate ions out of solution is very important in chemistry. Removing impurities or ‘treating’ a water sample is similar to cleaning a work space.
  • Some dirt/stains are very light and don’t interfere with general working. Dust builds up but can be removed quickly too, so a general dry wipe across a surface is enough.
    In chemistry, light metal ions such as Na⁺ or K⁺ are fairly safe in living environments. They are very soluble in water but are carried around quite easily too, and they do not build up to dangerous levels very often. Many living organisms actually need these lighter ions for living processes.
  • On the other hand, some dirt/stains build up slowly but they are harder to remove and can make workspaces harder to use. Sometimes we need to use chemicals specifically designed to remove dirt.
    Transition metal ions and heavy metal ions such as Pb²⁺ and Cd²⁺ have high mass, build up where they are produced and interfere with living processes. These properties make them harmful to living organisms in rivers or other aquatic environments.
  
  
• Although they are harmful, heavy metals can be precipitated out of solution. When we precipitate the harmful heavy metal ions, they are converted from the aqueous state (where they cause harm), to the solid state which can be trapped and filtered out of the solution. There are three stages we need to describe in this process:
  • The solution, as we already know is when the ions are still dissolved.
  • The suspension, where the aqueous ions have been precipitated but have not settled in the container/system.
  • The precipitate, where the ions have settled at the bottom of the container. The remaining solution without the precipitate is called the supernatant.
  Precipitating pollutant ions out of solution can be done using the principles from Predicting a precipitate and Solubility product.
  
  
• Worked example:
  
  A sample of waste water has a Pb²⁺ concentration of 5*10^-4 M, and ‘safe levels’ of lead ions in water can be said to be a maximum of 7.25*10^-8 M.
  
  What amount of OH^- ions need to be added to precipitate lead ions, so that [Pb²⁺] drops to the safe level at most? The K_sp for Pb(OH)₂ is 1.43*10^-20 M.¹
  
  First, write the equilibrium that will occur.
  
  Pb(OH)_{2 (s)} $\, \rightleftharpoons \enspace$Pb²⁺_(aq) + 2OH^-_(aq)
  Now write the K_sp expression for this using the value above:
  
  K_sp = 1.43 * 10^-20 = [Pb²⁺][OH^-]²
  [OH^-]² = $\large \frac{1.43 * 10^{-20}}{7.25 * 10^{-8}}$ = 1.97 * 10^-13 M
  [OH^-] = $\sqrt{1.97 * 10^{-13}}$ = 4.44 * 10^-7 M
  
  The calculation above is finding the hydroxide ion concentration that causes the equilibrium with Pb(OH)_{2 (s)} to be established, so if [Pb²⁺] greater than the 7.25*10^-8 M safe level was reached, the equilibrium would simply shift to make more precipitate.
  
  
• Treating hard water involves precipitation as well. Hard water is caused by calcium (Ca²⁺) and magnesium (Mg²⁺) ions dissolved in water when limestone (CaCO₃) and MgCO₃ in the ground reacts with rain water (which is slightly acidic) and gets into the water supply.
  
  CaCo_{3 (s)} $\enspace \rightleftharpoons \enspace$ Ca²⁺_(aq) + CO_{2 (g)} + H₂O_(l)
  
  Hard water has consequences in daily life:
  • It gives drinking water a bad smell/taste.
  • The dissolved ions can precipitate in piping and other places where it is heated, which can reduce the flow of pipes and make some appliances less useful. 
  • It makes soap less effective because the active ingredient in soap forms a precipitate with the Ca²⁺ and Mg²⁺ ions instead of cleaning the dirt you are using the soap to clean!
  
  
• Hard water is treated by using Na₂CO₃. This will precipitate CaCO₃ again by the original equilibrium. Both CaCO₃ and MgCO₃ have low solubility.
  • Permanent hard water is when the water does not contain any HCO₃^-$\,$ so it must be added.
  • Temporary hard water already contains HCO₃^- $\,$ so heating the water can remove hardness.