Environmental Engineering Reference
In-Depth Information
Ion exchange is becoming used more extensively in water and wastewater treatment.
Ion exchange is primarily used for water softening (Ca 2 + and Mg 2 + ) and for water dem-
ineralization. For water softening, Ca 2 + and Mg 2 + are replaced with Na + to prevent
scale formation. For complete water demineralization, all cations and anions are replaced
with H + and OH , respectively. This approach is also used in wastewater treatment. It
is important to note that not all dissolved ions are removed equally and/or completely.
Ions that are low on the selectivity preference order (described in Section 8.5) may not be
completely removed.
8.3
Environmental applications
The treatment of mine drainage water, removal of ammonia and nitrates from groundwater,
and the treatment of nuclear waste solutions are some examples of environmental appli-
cations. Ion-exchange resins can also be used for pollutant removal from gas streams. For
example, H 2 S and NH 3 have been removed using macroreticular carboxylic acid resins
and quaternary ammonium anion-exchange resins, respectively. The selective removal of
these two impurities in hydrogen-cycle gas streams from oil refinery processes and their
subsequent recovery by thermal elution using an inert gas are one important application
[1]. For these applications, additional processing steps are usually required to obtain the
material in pure form or reduce the volume prior to disposal (water removal, ion separation,
precipitation, etc.).
The efficiency of waste treatment is strongly dependent on the regenerant consumption.
Success is likely if the process fluid phase is either acidic or basic since this will affect the
initial ion exchange. Actual process design depends on the waste to be treated, pollutant
concentration, flowrate, and other operating conditions.
8.4
Ion-exchange mechanisms
All ion exchangers, whether natural or synthetic, have fixed ionic groups that are bal-
anced by counterions to maintain electroneutrality. The counterions exchange with ions
in solution. As an example, consider the schematic cation-exchange resin shown in
Figure 8.1.
The resin containing cation B + is placed in a solution containing cation A + . The cations
A + and B + will diffuse due to a concentration gradient between the resin and solution.
The chemical equation for this particular exchange reaction within the ion-exchange
resin is:
A + +
( R ) B +
B + +
( R ) A + ,
(8.1)
where R represents the negatively charged functional group of the resin.
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