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.
        Precipitate Formation
                 soluble            insoluble 
  Precipitation of Metals from Wastewater
  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(NO3)2(aq)       +     2 NaOH(aq)                          Cu(OH)2(s) 
                                                                                + 2 NaNO3(aq)