ABSTRACT
It is well recognized that the sorption affinity toward target solutes can be greatly enhanced by modifying and tailoring the interfacial chemistry of the sorbent. To this end, Helfferich [1,2] was the first to conceive the use of Cu(II)- or Ni(II)-loaded weak-acid cation exchange resins for ligands sorption through Lewis acid-base interaction. In such ligandexchange processes, the water molecules (weak ligands) present at the coordination spheres of immobilized Cu(II) and Ni(II) in the cation exchange resins are replaced by relatively strong ligands, such as ammonia or ethylenediamine. The following provides a typical ligand-exchange reaction with ammonia, where M represents a divalent metal ion like Ni(II) or Cu(II) with strong Lewis acid properties and the overbar denotes the exchanger phase:
(RCOO-)2M 2+(H20)w + rcNH3
^ (RCOO-)2M 2+(NH3)„ + nH20 (1)
2 2 9
The metal-loaded weak-acid cation exchange resins are electrically neutral and do not have any anion-exchange capacity, and, also, the negatively charged fixed co-ions (carboxylates in this case) of the polymer will not allow uptake of any anions in accordance with the Donnan co-ion exclusion principle. Thus, in spite of being strong ligands, the anions in Eq. (2) cannot displace water molecules (much weaker ligands) from the coor-
In addition to providing a quantitative approach toward determining ligand-exchange capacity of metal-loaded cation exchangers, Helfferich [2] also unveiled a striking similarity between ion-exchange and ligandexchange processes. In heterovalent ion exchange, it is well known that with an increase in electrolyte concentration, the affinity (or binary separation factor) of the counterion with lower valence increases over the counterion with higher valence. In the realm of ion exchange, this phenomenon is popularly known as electroselectivity reversal In a similar vein, Helfferich showed that the affinity of a monodentate ligand (ammonia) toward the exchanger in ligand-exchange processes is enhanced over a bidentate ligand (1,3-diaminopropanol) as the total ligand concentration is increased. This observation provided the basis for efficient regeneration of the bed at the end of the ligand-exchange process. During the last thirty years, much work has been done in applying the concept of ligand exchange in the areas of analytical chemistry, separation technology, and pollution control processes with varied amount of success [3-8].
