Prepared By Michigan Department of Environmental Quality Operator Training and Certification Unit

Some things (chemists use term compounds) dissolve in water – these are said to be soluble.
Some things (compounds) don’t dissolve in water – these are said to be insoluble.

A precipitation reaction is a reaction in which soluble ions in separate solutions are mixed together to form an insoluble compound that settles out of solution as a solid. That insoluble compound is called a precipitate .

soluble insoluble

Metals Precipitation Transforms dissolved contaminants into an insoluble solid, facilitating the contaminant's subsequent removal from the liquid phase by sedimentation and / or filtration. Usually accomplished by:
•
pH Adjustment
•
Chemical Coagulation
•
Flocculation
•
Separation

Hydroxide Precipitation
Most Soluble Heavy Metals Will Precipitate When pH is Raised to a Given Point pH is Raised by Addition of Hydroxide Cu(NO 3 ) 2 (aq) + 2 NaOH(aq) Cu(OH) 2 (s) + 2 NaNO 3 (aq)

Hydroxide Precipitation
Sodium Hydroxide, NaOH Caustic Soda Calcium Hydroxide, Ca(OH) 2 Hydrated Lime

Sodium Hydroxide Caustic Soda, Soda Lye Available in Liquid or Dry Form Liquid: Commercial Strength 50 % , 73 % NaOH Often Stored at 20 % 50 % Crystallizes at 53 Deg. F Dry: 99.9 % NaOH Safety Bag, Drum, or Bulk Caustic Poison Dangerous to Handle, Corrosive Prevent All Body Contact Protect Eyes

Calcium Hydroxide Ca(OH) 2 Hydrated Lime, Slaked Lime Available in Dry or Liquid Form Dry Form Available in Bags or Bulk Commercial Strength 82 - 95 % Safety Caustic, Irritant, Dusty Avoid Contact with Eyes, Nose, Respiratory System

Hydroxide Precipitation
Sodium Hydroxide VS Hydrated Lime Less Expensive Solids Settle Faster Higher Sludge Solids Conc Sludge Easier to Dewater Takes Longer to Neutralize More Complicated Feed System A Lot More Sludge

Process Water Chemical Treatment .. Equalization Tank Treatment Tank Treated Water Back To Equalization Tank Filter Press Sludge Bin Clarifier Sludge Tank

Cu(NO 3 ) 2 (aq) + 2 NaOH(aq) --> Cu(OH) 2 (s) + 2 NaNO 3 (aq) Ni(NO 3 ) 2 (aq) + 2 NaOH(aq) --> Ni(OH) 2 (s) + 2 NaNO 3 (aq) 2 AgNO 3 (aq) + 2 NaOH(aq) --> Ag 2 O(s) + 2 NaNO 3 (aq) + H 2 O(l)

Precipitation of Metals from Wastewater
The Solubility of Metals is Dependent on pH

Hydroxide Precipitation
From Chart (“Book Value”) Minimum Solubility (Best Removal) Value for Some Metals Cadmium pH 11.0
Copper pH 8.1
Chromium pH 7.5
Nickel pH 10.8
Zinc pH 10.1

Hydroxide Precipitation
Need to Determine Optimum pH for Waste May or May Not be Close to Book Value Metals Precipitate at Various pH Levels Depending On: The Metal Being Precipitated The Chemistry of the Wastewater Other Metals Present Presence of Chelates

Metal Precipitation Precipitation of more than one metal from the same solution: Adjust for Optimum pH for One Metal (With Lowest Discharge Permit Limit) Adjust for Compromise pH Operate in Stages Adjust pH for Each Metal in Stages Draw Off Sludge, or Transfer Supernatant Between Stages

Hydroxide Precipitation Expected Effluent Quality (mg/L) for Total Metals
Cr Cu Ni Cd Zn Precip + Settling 0.1 to 1.0
0.2 to 1.5
0.5 to 2.5
0.2 to 1.5
0.2 to 1.5
Precip + Settling + Filtration 0.05 to 0.5
0.05 to 0.5
0.25 to 1.5
0.05 to 1.0
0.05 to 1.0
NaOH Mix Tank Clarifier Filter

Hydroxide Precipitation of Soluble Metals Things that Make Life More Difficult Many Metals are Amphoteric Metal hydroxides that re-solubilize if the pH increases or decreases from the optimum precipitation value

Hydroxide Precipitation of Soluble Metals Things that Make Life More Difficult Many Metals are Amphoteric Double Solubility Curve pH Must be Carefully Controlled

Hydroxide Precipitation of Soluble Metals Things that Make Life More Difficult Many Metals are Amphoteric Double Solubility Curve pH Must be Carefully Controlled Presence of Chelating Agents will Interfere with Ability to Precipitate Metals

Can’t Live With ‘em, Can’t Do Without ‘em An agent that will form a compound with a heavy metal ion, with the intended purpose of keeping the metal in solution.
Highly Desired in Plating Solutions A
Problem in Wastewater Treatment

3

Methods of Addressing the Problem 1. Precipitate at high pH pH > 10 (may also add CaCl 2 ) 2. Add a material which the chelate prefers over the metal to be precipitated.
Ferrous Sulfate 3. Use a reducing agent to convert the heavy metal from the ion back to the metal Sodium Borohydride
• •
4. Use a precipitating chemical that steals the metal from the chelate Sulfide Sodium Dimethyldithiocarbamate (DTC)

Extremely Low Metal Solubilities Often Preferred for Wastewaters Containing Chelating Agents More Negative Environmental Impacts More Costly (About Twice $ as for OH) May Produce H 2 S in Lethal Concentrations

Cu(NO3) 2 (aq) + Na 2 S(aq) --> CuS(s) + 2 NaNO 3 (aq) CdCl 2 (aq) + Na 2 S(aq) --> CdS(s) + 2 NaCl(aq) Ni(NO 3 ) 2 (aq) + Na 2 S(aq) --> NiS(s) + 2 NaNO 3 (aq)

Ca(OH) 2 FeS
Mix Tank Mix Tank
Cr Cu Ni Cd Zn OH Precip + Settling + Filtration 0.05 to 0.5
0.05 to 0.5
0.25 to 1.5
0.05 to 1.0
0.05 to 1.0
Clarifier Filter
OH + S Precip + Settling + Filtration 0.01 to 0.20
0.01 to 0.25
0.05 to 0.50
0.01 to 0.25
0.01 to 0.25

Sodium Dimethyldithiocarbamate (DTC or SDDC) Organic sulfur compound Controls chelated waste streams Not pH sensitive – any pH above 3 Non amphoteric Very toxic Agricultural insecticide Industrial biocide

Sodium polythiocarbonate (PTC) Thio-Red Much less toxic than DTC 50% less sludge than DTC, lime or iron chemistries; Requires lower dosing of flocculent chemistry for liquid/solid separation Treated waters pass toxicity tests for National Pollutant Discharge Elimination System (NPDES) discharges

Hydroxide Precipitation of Soluble Metals Things that Make Life More Difficult Metals are Amphoteric Double Solubility Curve pH Must be Carefully Controlled Presence of Chelating Agents will Interfere with Ability to Precipitate Metals Hydroxide Sludges May be Difficult to Dewater Metal Finishing Sludge is Classified as a Hazardous Waste by Federal & State Law (RCRA)

Batch Treatment Usually Preferred for Small Flows Either a Holding Tank and Reaction Tank, or Two Reaction Tanks Reaction Tanks Serve Also as Clarifiers Allows for Maximum Control of Effluent Quality Influent Wastewater Effluent Sludge Effluent Sludge

Continuous (Flow-Through) Treatment Usually Used For Larger Flows Addition of OH Controlled by pH Meter / Controller NaOH pH React Storage Clarifier Flocculation

Sludge Recirculation Precipitated Solids Act as Seed Structure for Forming Precipitate Batch Treatment Portion of Settled Sludge Left on Tank Bottom that Re-suspends When Mixing For New Batch Begins

Continuous Treatment Sludge is Recirculated from Sludge Hopper of Clarifier to the Influent or Reaction Tank

Review – Metal Precipitation 1 Sodium Hydroxide is hazardous to handle in that 2. List three options for precipitating two metals with different optimum pH.
a.
Optimize for One Metal b.
Find a Compromise pH c.
Operate as a Two-Stage System

3. Metal hydroxides that resolubilize if the pH increases or decreases from the optimum precipitation value are said to be 4. Ammonia, phosphates, and EDTA are solutions. 5. Describe the problem that may occur when precipitating metals from wastewater where the materials described in #4 are present.
Chelates hold metals in solution, not allowing them to precipitate.

6. List 3 ways a wastewater treatment plant operator may be able to overcome the problem in #5 that is caused by the materials listed in #4.
a.
Precipitate at high pH b.
Add a metal to tie up chelate c.
Add a chemical that steals metal from the chelate (Sulfide) 7. Recirculation of _________________ may be beneficial in that it provides “seed” material for the precipitation process.

Prepared By Michigan Department of Environmental Quality Operator Training and Certification Unit