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Chemical Treatment

This section covers fundamental concepts of chemical treatments in water treatment processes, focusing on precipitation and redox reactions. Key principles include solubility and equilibrium equations, as well as determining the concentration of various ions in solutions. Activities involve calculating equilibrium concentrations after precipitation, and examples illustrate crucial concepts such as the effect of additives like sodium chloride on metal ion levels. Practical applications are highlighted, including the design and pH control for effective metal ion removal in wastewater treatment.

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Chemical Treatment

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  1. Chemical Treatment Precipitation and Reduction/Oxidation (Section 9-4)

  2. Solubility and Equilibrium AmBn (s)  mAn+ + nBm- Ksp = [An+]m [Bm-]n

  3. Example: Solubility Equation What concentration of calcium would be in solution at equilibrium with phosphate?

  4. Activity – Team If 50.0 mg of carbonate and 50.0 mg of calcium are present in 1 L of water, what will be the final (equilibrium) concentration of calcium? (Remember that the solubility equation uses the final (equilibrium) concentration, i.e., after precipitation has occurred.)

  5. Precipitants and Solubility • Hydroxides • Sulfides • Carbonates • Sulfates • Chlorides

  6. Activity – Team Given a saturated solution of calcium carbonate, how many moles of calcium ion will remain in solution after the addition of 3.16 x 10-4 M Na2CO3 at 25 oC?

  7. Example: Precipitant Dose Simple System Given a solution with an aqueous silver concentration of 100 mg/L, determine the mass of sodium chloride that must be added to reduce the aqueous silver concentration to 1 mg/L. The total solution volume is 55 gals (208.2 L).

  8. Example: Precipitant Dose Complexation Consider a cadmium-containing wastewater. It is to be precipitated with hydroxide to a free cadmium concentration of 1 mg/L (8.896 x 10-6 M). Determine the required equilibrium pH and the total soluble-phase cadmium concentration.

  9. Example continued Soluble Cadmium Species Cd2+ + H2O  CdOH+ + H+ Ksp = 10-10.1 Cd2+ + 2H2O  Cd(OH)2 + 2H+ Ksp = 10-14.35 Cd2+ + 3H2O  Cd(OH)3- + 3H+ Ksp = 10-33.3 Cd2+ + 4H2O  Cd(OH)42- + 4H+ Ksp = 10-47.4 Cd(OH)2(s) + H+ Cd2+ + 2H2O Ksp = 10+13.65

  10. Coagulation/Flocculation/Settling Successful precipitation requires effective separation.

  11. Example: Jar Test

  12. Process

  13. Design of Precipitation Process

  14. Activity – Team A metal plating firm is installing a precipitation system to remove zinc. They plan to use a pH meter to control the feed of hydroxide solution to the mixing tank. What pH should the controller be set at to achieve a zinc effluent concentration of 0.80 mg/L?

  15. Redox Reactions

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