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12.6                      CHAPTER TWELVE


         space between the ion exchange groups, which in turn reduces  change repulsion and low-
         ers  the resistance  to penetration  by the hypochlorite ion into the resin.  Therefore, lower-
         ing the DVB  cross-linker level increases  susceptibility to attack by chlorine in two ways.
         The life expectancy  under  oxidizing conditions  of a  water softening resin is proportional
         to  the  square  of the  cross-linking  level.  Reducing  the  DVB  level by  50%,  for instance,
         will decrease resin life due to oxidation by chlorine by a factor of 4.  Most domestic  soft-
         ener manufacturers  use  cation resins  with  8.0%  DVB.
           Anion  resins  are  similarly  affected  by  chlorine  except  that  the  negatively  charged
         hypochlorite ion is not excluded from the polymer.  Therefore,  anion resins  are more sus-
         ceptible  to  chlorine  attack.  Since  it  is  an  anion,  the  hypochlorite  ion  is  exchanged  onto
         the  anion  exchange resin.  The  amine  group  of the  anion resin  is the primary  point of at-
         tack  by  the  hypochlorite  ion.  Although  de-cross-linking  does  occur,  the  primary  indica-
         tor of degradation  is loss of functionality.
           Chlorine attack  on the amine  groups  of an  anion exchange resin can cause either par-
         tial  oxidation  of  the  amine  groups,  which  lowers  the  basicity  (more  weakly  basic),  or
         cleavage of the entire amine group from the resin, which destroys the ion exchange group.
         In either case  the  result  is  lower water  retention  values.  This  tends  to  counter the  effect
         of DVB  cleavage.  Because  of the  double  route  that  oxidation  takes  in  the  anion  resins,
         the  water  retention  may  not  immediately  be  affected  by  chlorine  attack.  Therefore,  the
         best  way  to  measure  anion  resin  degradation  from  oxidation  is  to  track  the  changes  in
         strongly  basic,  weakly  basic,  and  total  capacities.

         Chemical Degradation.   The  ion  exchange  groups  can  become  impaired  by  an  ion ex-
         change  reaction  that  is not easily reversible,  or by  surface  clogging,  which  occurs  when
         a resin  becomes  fouled with oil, rust,  or biological slime.  When  any  of this  happens,  the
         resin is referred to as being fouled. Fouling due to coating by water-soluble polymers from
         clarification upsets  also  occurs.
           The most common form of cation resin fouling is due to chemical precipitants  such as
         iron  or barium.  These  can either coat  the  resins  or form clumps  of precipitated  salts  that
         cannot  be removed by  the  normal  regeneration  process.
           Anion  resins  are  commonly  fouled  by  naturally  occurring  organic  substances  which,
         though  exchangeable  as  ions,  have  such  slow  diffusion  rates  that  they  are  not fully re-
         moved  by  the  normal  regeneration  process  unless  special  resins  or  special  regeneration
         procedures  are used.  This often makes  their exchange onto the resins  an irreversible pro-
         cess.  When  this  continues  over  a  long  time  the  resin  will  become  impaired  and  perfor-
         mance  will drop  off. This  condition  is commonly  referred  to as organic fouling.
           Proper  regeneration  techniques  and  chemical  procedures  reduce  the tendency  to foul.
         Some types of resins are more resistant to fouling than others,  but usually there is a trade-
         off in other properties  so that  selection of the type  of anion  resin  for a  given application
         depends  on  several  factors,  such  as  water  temperature  ranges,  operating  efficiency ca-
         pacity  requirements,  regeneration  temperatures,  and  required quality  levels.


         THE ION EXCHANGE  PROCESS

         It is important to keep in mind that the ion exchange process only works with ions. Sub-
         stances  that  do  not ionize  in  water  are  not removed  by  ion  exchange.  Each  type  of ion
         exchange resin exhibits  an order of preference for various ions.  This can be stated quan-
        titatively through  selectivity coefficients. These  are similar to the equilibrium constants
         in  ionic  equilibria.  In  a  similar  manner,  the  equilibrium  concentrations  of  ions  in  the
        resin  phase  and  in  the  water  phase  can  be  calculated  from  the  selectivity  coefficient.
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