D. SERDOLIT® Chelating Resins
SERDOLIT® Chelating Resins
A special kind of cation exchangers are our CHELITES: these are resins which have the capacity to selectively bind polyvalent cations via complexing anchor groups. Removal of heavy metal ions from solution is fast and complete. In addition, chelites have ion exchanging properties. Therefore it is necessary to equilibrate the chelating resins with the working buffer solution prior to the specific application, in order to avoid changes in the overall ionic composition of the surrounding medium.
SERDOLIT® Chelite CHE is a macroporous (macroreticular) polystyrene based resin, crosslinked with DVB and derivatized with iminodiacetic acid groups. These have a high affinity for heavy metal cations over alkali or alkaline earth metals. The macroporous structure provides high resistance to osmotic shock and short ion diffusion paths resulting in improved kinetics. The apparent selectivity for a given metal depends upon concentration, the presence of other species, and pH. In the table below, some findings at different pH values are shown. These data provide a guideline of relative selectivities.
The affinity of the resin for a given metal can be increased or decreased by adjusting the pH. Many feed streams contain particular matter which can clog the exchanger bed. Backwashing is necessary at regular intervals. If glass columns are used, the swelling (hydraulic expansion) of the resin must be considered. The resin swells about 35 % during change from the hydrogen to monovalent salt form. Thus regeneration is best done in a funnel, in the following order: 10 % acid, water, 10% caustic and finally water to neutrality of the eluent.
To obtain the desired pH, buffer solutions are preferably used. As an example for pH 4, 10 bed volumes of a 0.5 M solution of NaH2PO4/Na2HPO4 adjusted to pH 4 and finally 10 volumes of water are percolated.
As SERDOLIT® Chelite CHE has an extremely high affinity for polyvalent cations, the amount of regenerant is therefore higher than required for conventional weakly acidic cation exchange resins.
Table 7: Selectivities of Chelite CHE for cations at different pH values
|pH||0.5 ||2 ||2.5 ||3||3.5||4 ||5||5.5||7 |
The relative selectivity of this resin for various cations in a diluted neutral solution decreases in the following order:
Hg2+ > Cu2+ > Pb2+ > Ni2+ > Zn2+ > Cd2+ > Co2+ > Fe2+ > Be2+ > Mn2+ > Ca2+ > Sr2+ > Ba2+ >> Na+, K+
SERDOLIT® Chelite P is a macroporous polystyrene based resin, crosslinked with DVB and derivatized with aminomethylphosphonic groups. It can be distinguished from SERDOLIT® Chelite CHE by its greater affinity for cations of low atomic mass, e.g. Mg2+ or Ca2+. It has high selectivity for transition and heavy metals (Pb, Cu and Zn in particular). The resin swells about 50% during its transformation from the hydrogen to the sodium form, so the same precautions as described above must be applied.
Operating pH range
The resin can operate in a neutral, acidic or alkaline medium, but since its capacity depends on the pH, we recommend the following minimum pH values:
Table 8: Selectivities of Chelite P for cations at different pH values
| pH ||2 ||2.5||3||4.5|
Sr2+ , Co2+
The relative affinity of this resin for the various cations in a diluted neutral solution decreases in the order shown below:
Fe3+ > Pb2+ > Cu2+ > Al3+ > Zn2+ > Mg2+ > Ca2+ >> Cd2+ > Ni2+ > Co2+ > Sr2+ > Ba2+ > Na+