Page 280 - Materials Chemistry, Second Edition
P. 280

Groundwater Remediation                                          263



           usually insoluble. Lime or caustic soda addition is often made to precipitate
           the metals. The solubility of metal hydroxides is sensitive to pH, and the
           reaction can be expressed in a general form:

                                   M(OH)  ↔ M  n +  + nOH −               (6.37)
                                         n
           where M represents the heavy metal, OH  is the hydroxide ion, and n is the
                                                −
           valence of the metal.
             The equilibrium equation can be written as:

                                             +
                                                   − n
                                             n
                                        = [M ][OH ]                       (6.38)
                                     K sp
           where K  is the equilibrium constant (often called the solubility product),
                   sp
           [M ] is the molar concentration of the heavy metal, and [OH ] is the molar
              n+
                                                                  −
           concentration of hydroxide ions. For example, the K  values for Cr(OH) ,
                                                                              3
                                                           sp
           Fe(OH) , and Mg(OH)  at 25°C are 6 × 10 −31  M , 6 × 10 −36  M , and 9 × 10 −12  M ,
                                                    4
                                                               4
                                                                              3
                  3
                               2
           respectively.
           Example 6.20:  Chemical Precipitation for Magnesium Removal
           Sodium hydroxide is added to a continuous-flow stirred tank reactor (CFSTR) to
           remove magnesium ion from an extracted groundwater stream (Q = 150 gpm).
                                                                             2+
           The temperature of the reactor is kept at 25°C and pH = 11. The influent Mg
           concentration is 100 mg/L. If the solids are settled to 10% by weight, estimate
                   2+
              (a) Mg  concentration in the treated effluent (mg/L)
              (b) rate of Mg(OH)  produced (lb/day)
                              2
              (c) rate of sludge produced (lb sludge/day)
           (Note: The solubility product of Mg(OH)  is 9 ×10 −12  M  at 25°C; MW of
                                                               3
                                                  2
           Mg = 24.3.)
              Solution:
               (a)  Write the reaction of precipitation first:
                                                2+
                                          ↔ Mg +2OH     −
                                  Mg(OH) 2
                   At pH = 11, the hydroxide concentration [OH ] is equal to 10 .
                                                                          −3
                                                            −
                       Use the solubility product equation to determine the magne-
                       sium concentration as:
                                   +
                                                              −3 2
                                  2
                                                          +
                                        − 2
                                                         2
                             = [Mg][OH]    = ×910 −12  = [Mg ][10 ]
                         K sp
                          2+
                       [Mg ] = 9 × 10  M = (9 × 10  mole/L)(24.3 g/mole)
                                               −6
                                   −6
                             = 2.19 × 10  g/L = 0.22 mg/L
                                      −4
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