Cleaning and Neutralizing Spills of Aqueous Solutions Containing Heavy Metals

This procedures was developed by Margaret-Ann Armour, Assistant Chair of the Chemistry Department at the University of Alberta.

Aqueous solutions containing heavy metal salts can be difficult to dispose. The objective for this procedure is to remove the heavy metal ions for disposal as hazardous waste. The divalent metal ions (M2+) will combine with sodium metasilicate to form insoluble metal silicate by the following reaction:

M2+ + Na2SiO3 ==> MSiO3 + 2Na+

Materials Needed

• spill mix comprised of equal volumes of sodium bicarbonate, bentonite (clay cat litter) and dry sand
• aqueous solution of sodium metasilicate at a concentration of Na2SiO3.0.5H2O, 12.5 g per 100 ml water
• 2M sulfuric acid prepared by slowly adding 10 ml of concentrated acid to 50 ml of cold water. Always add acid to water, never the other way around.

Procedure

1. Cover the spill with enough spill mix to absorb the liquid.
   
2. Scoop the mixture into a container and place in a fume hood.
   
3. Add enough water to dissolve the sodium bicarbonate in the spill mix.
   
4. Allow the solids to settle and decant the supernatant into another container.
   
5. Wash the solids with water and allow settling. Add the washings to the decanted liquid.
   
6. Stir in the sodium metasilicate solution until there is no further precipitation of solids.
   
7. Using the 2M sulfuric acid, adjust the pH of the solution to the pH shown in Table 1 for the metal ion that was present in the spilled solution.
   
8. Allow the mixture to stand for 12 hours to ensure complete precipitation.
   
9. Collect the solid by filtration or by evaporation. Pour the liquid down the drain.
   
10. Package and label the solid for disposal as hazardous waste.

Table 1: Optimum pH for the precipitation of metal ions as silicates

If you have any questions about these procedures or where to get the appropriate supplies, please contact James Boehlert at 258-7882.