Dr. Dmitri Kopeliovich
Ion exchange resin is a cross-linked porous polymer substance containing functional (ionogenic) groups with mobile ions, which may be replaced with ions of the same charge dissolved in the surrounding liquid media.
The polymer base of most of ion exchange resins is styrene-divinylbenzene copolymer.
The molecular structure of the copolymer presents a network of high-weight polystyrene molecules cross-linked by the molecules of divinylbenzene (DVB).
Cross-linking provides toughness and insolubility of the resin.
The amount of the cross-linking agent (divinylbenzene) may vary within the range 2%-16%.
The functional groups are attached to the styrene rings.
The chemical formulas of some ion exchanging resins:
Ion exchange resins may differ in their porous structure: microporous (gel) resins and macroporous resins.
Gel resins are most widely used. Gel resins have higher ion capacity.
Macroporous resins are used for treating severely contaminated solutions due to their ability to elute foulants.
The general chemical reaction of ion exchange:
nRA + Bn+ = RnBn+ + nA+
nRA + Bn- = RnBn- + nA-
R - ionized resin molecule with an attached functional group;
A+ / A- - exchangeable ion;
Bn+ / Bn- - cation/anion dissolved in the liquid;
n+ /n- - electrical charge of the ion B.
The equilibrium constant of the reaction (selectivity coefficient):
K = [A]n[RnB]/[RA]n[B]
Where [A], [RnB], [RA], [B] - concentrations of A, RnB, RA and B.
Selectivity coefficient indicates the degree of the ions (A and B) exchanging. The higher the selectivity coefficient the higher fraction of the ion B is replaced with the ion A.
Selectivity coefficient also determines a preference of an ion to another ion. The ions with higher selectivity coefficient will replace the ions with lower selectivity coefficient in the resin molecules.
The values of the selectivity coefficients for various cations in strong cation exchangers are arranged as follows (decreasing order):
Fe3+, Al3+, Pb2+, Sr2+, Ca2+, Co2+, Ni2+, Cu2+, Zn2+, Mg2+, Mn2+, Ag+, Cs+, Cd+, K+, NH4+, Na+, H+, Li+, Hg2+
The values of the selectivity coefficients for various anions in amino base anion exchangers are arranged as follows (decreasing order):
SO42-, CrO42-, PO43-, HSO4-, NO3-, Br-, Cl-, HCO3-, F-, OH-
Strong Acid Cation (SAC) resins behave similar to strong acids.
Strong Acid Cation resins are available in two forms: hydrogen (R-SO3H) or sodium (R-SO3Na).
The typical strong acid cation exchange reaction:
2(R-SO3Na) + CaCl2 = (R-SO3)2Ca + 2NaCl
Cross-linking level of the Strong Acid Cation resins is 8-10%.
The ion exchange capacity of Strong Acid Cation resins does not depend on the solution PH.
Strong Acid Cation resins are used for water softening and demineralization.
The exhausted Strong Acid Cation resins may be regenerated.
Regeneration in hydrogen (acid) form is performed by a strong acid (e.g., HCL). Regeneration in sodium (salt) form is performed by sodium chloride solution (NaCl).
Weak Acid Cation (WAC) resins behave similar to weak acids.
Weak Acid Cation resins are available in hydrogen form (R-COOH).
Weak Acid Cation resins have high affinity for hydrogen ions therefore they are easily regenerated by stoichiometric amount of acid.
The ion exchange capacity of Weak Acid Cation resins increases with an increase of the solution PH. WAC resins are not used for treatment acidic (PH<6) solutions.
Weak Acid Cation resins are used for demineralization and dealkalization of water.
Strong Base Anion (SBA) resins behave similar to strong bases.
Strong Base Anion resins are available in hydroxide form: (R-NH3OH).
The typical strong base anion exchange reaction:
R-NH3OH + HNO3 = R-NH3NO3 + H2O
Strong Base Anion resins are used for demineralization and dealkalization of water.
The exhausted Strong Base Anion resins may be regenerated by concentrated sodium hydroxide (NaOH).
Weak Base Anion (WBA)resins behave similar to weak bases.
The typical weak base anion exchange reaction:
R-NH2 + HNO3 = R-NH3NO3
The ion exchange capacity of Weak Base Anion resins increases with a decrease of the solution PH. WBA resins are not used for treatment basic (PH>6) solutions.
Weak Base Anion resins sorb only anions of strong acids (chlorides, nitrates, sulfates).
Weak Base Anion are easily regenerated by small amounts of weak bases (such as ammonia or sodium carbonate), which neutralize the acid taken up by the resin.
Ion exchange resins are used for two types of water treatment: