Metals Removal by pH Adjustment

This demonstration shows how dissolved metals are removed from wastewater by raising pH until the metal forms an insoluble complex. Two common metals are used in this demonstration; Aluminum (found in alum) and iron (found in ferric chloride). Alum and ferric chloride are used at wastewater treatment plants for phosphorus removal. Alum crystals can be easily found at grocery stores in the spice section. Ferric Chloride crystals can be obtained from laboratory chemical suppliers.

Materials:

  • 500 ml beakers, jars, or other glassware
  • Stir spoon
  • A 10 to 20 ml pipette, or any other measuring device
  • A base solution: Either household ammonia or 4% sodium hydroxide (20gms NaOH to 500mls water)
  • A metal salt to precipitate: Either/or 1 level teaspoon crystals or 1 ml liquid FeCl3 or Alum


Procedure:

Label beakers for the metal salt. Add 500mls water to each beaker. Then add ferric chloride and/or alum to the respective beaker. Mix. Allow time for the crystals to dissolve. Add a base to raise pH allowing an insoluble precipitate to form. The amount will depend on your water's alkalinity and the acidity of the metal salt you use. (Eight to ten mls is what I used.) Be sure to stir the water while adding the base to mix completely. Use a circular motion, continuing for 5-10 sec. after the base addition. Slow the stirring, remove the spoon, and as the water slows on its own it provides the correct conditions to bring the metal precipitate together. Let stand and the metal precipitate will settle.


Additional Demonstration:

The treated water can be filtered through a coffee filter placed in its holder. If it plugs, try putting a half-inch sand in the filter first. Pour the mixture through.

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