**2.1. Covalent linkages**

Lindoy and Eaglen [14] noted that metal ions were removed by the material produced and noted in Figure 1. A special feature was that due to the spacing established by the three-carbon bridge or linkage " the material maintains a high ion-complexation capacity." These workers also demonstrated metal-ion removal capacity for representative metals (transition metal ions, alkali metal ions, etc). The material under dynamic conditions removed 50 mg Cu(II) /g composite. The metal absorbing ability followed the order Cu2+ > Cd2+ > Zn2+.

The system was effective as a solid-phase preconcentration agent for cobalt(II), nickel(II), and copper(II) at pH 5.5 using column chromatography. A especially desirable feature was that

Another covalent approach is condensation of a potential ligand of the type HO-Y- Z where Y is a hydrocarbon moiety and Z is a coordinating group NH or NH2 or SH. The mixture of solid substrate (clay, silica)was suspendedintoluenewiththepotentialligands andadropof sulfuric acidinaflaskattachedtoaDean-Starktube,whichisattachedtoacondenser.Thereactionoccurs between OH on the potential ligand and OH group on silica with the elimination of water using azeotropic distillation. Water produced in the reaction flask (2, Figure 2) distills as a constantboiling mixture (3), that is cooled in a condenser (5), and separates into two components, which drop into a calibrated Dean-Stark tube (8). The water drops to the bottom of the tube; toluene returns to the reaction flask (2). The progress of the reaction can be measured, e.g., condensa‐ tionof 0.1mole ofHO-Y-Zshouldgive 1.8milliliters ofwater.The reactionis allowedtoproceed

Examples are available from a number of studies. The authors demonstrated that the use of ion-exchange resins as supports for chelators that can be derivatized or converted to ions is a versatile technique [22]. Examples involving two different ion-exchange resins help indicate

An *anion-exchange resin* in the hydroxide form RNHOH (e.g., IR-120), can be treated with other

A *cation-exchange resin* exists as a polyalkylsulfonic acid, RSO3H and can react with a chelating

Lee and coworkers [23] used the technique shown (Eqn 2) to load a number of chelating agents, among them chromotropic acid, onto the anion exchange resin Dowex® 1-X8 (chloride form). These composites are easy to prepare, and loading of metal ions on the chromatography

An anion exchange resin (e.g. Amberlite® R-120) was treated with protonated dithiooxamine, H2NC(S)C(S)NH2. Using the supported ligand, quantitative removal of copper, cadmium, and lead ion solutions at neutral or slightly alkaline solutions of deionized or tap water, but poor

+ -

Chromatographic Separations with Selected Supported Chelating Agents

http://dx.doi.org/10.5772/55521

127

4 4 RNH OH + NaCh RNH Ch + Na + OH ® (2)

+ RSO H + HCh RSO H Ch + H+ 3 3 ® (3)

the three metals could be cleanly separated by adjusting the pH of the eluent [16].

until completion [17, 20]. Clays were also used as supports [18, 19].

chelating agents HCh converted to the anionic forms, NaCh (Eqn. 2).

(Eqn. 3).

columns showed the metal ion loading as a color change.

results were obtained with sea water [22].

**2.2. Ionic linkages**

the range of the technique.

agent in a protonated form HCh+

**Figure 1.** Flow chart for the synthesis of a supported chelating agent. BPEI consists of branched ethylenediimino moi‐ eties [14]

A similar technique was used by Soliman, who also used a silica gel matrix and a covalent linker to tie to a series of amines, mono-, di-, tri-, and tetra-amine [15]. Using a batch equili‐ brium technique, he measured the removal capacities (mmole/g) for divalent forms of cobalt, nickel, copper, zinc, cadmium, and lead. In general maximum removal values (at optimum pH values) were obtained for the tetra-amine species.

El Ashgar used a variation on the technique [16]. Specifically, 3-chloropropyltrimethoxysilane was used to alkylate diethylenetriamine, i.e, reaction with the halide end to produce a precursor of a polymer (I, Eqn. 1). The reaction of I with tetraethylorthosilicate resulted in a "diethylenetriamine polysiloxane immobilized ligand system."

$$\begin{aligned} \text{H}\_{3}\text{O}\_{3}\text{Si}(\text{CH}\_{2})\_{3}\text{NH}(\text{CH}\_{2})\_{2}\text{NH}\left(\text{CH}\_{2}\right)\_{2}\text{NH}\_{2} &+ & 2\left(\text{EtO}\right)\_{4}\text{Si} \rightarrow \text{Polymer} \\ \text{I} \end{aligned} \tag{1}$$

The system was effective as a solid-phase preconcentration agent for cobalt(II), nickel(II), and copper(II) at pH 5.5 using column chromatography. A especially desirable feature was that the three metals could be cleanly separated by adjusting the pH of the eluent [16].

Another covalent approach is condensation of a potential ligand of the type HO-Y- Z where Y is a hydrocarbon moiety and Z is a coordinating group NH or NH2 or SH. The mixture of solid substrate (clay, silica)was suspendedintoluenewiththepotentialligands andadropof sulfuric acidinaflaskattachedtoaDean-Starktube,whichisattachedtoacondenser.Thereactionoccurs between OH on the potential ligand and OH group on silica with the elimination of water using azeotropic distillation. Water produced in the reaction flask (2, Figure 2) distills as a constantboiling mixture (3), that is cooled in a condenser (5), and separates into two components, which drop into a calibrated Dean-Stark tube (8). The water drops to the bottom of the tube; toluene returns to the reaction flask (2). The progress of the reaction can be measured, e.g., condensa‐ tionof 0.1mole ofHO-Y-Zshouldgive 1.8milliliters ofwater.The reactionis allowedtoproceed until completion [17, 20]. Clays were also used as supports [18, 19].
