Amberlite XAD-2 functionalized with phenolic ligands as metal ion extractants: A Review
Manjeet Kumar1 and D. P. S. Rathore2
Chemical Laboratory, Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, West Block-VII, R. K. Puram, New Delhi1 & Nagpur2; E-mail:
EXTENDED ABSTRACT
The methods commonly used for analyte separation/ preconcentration include: evaporation, co-precipitation, liquid-liquid extraction [solvent extraction], and solid-liquid extraction [ion exchange, and chelating ion exchange resins]. The estimation of metal ions at a very low concentration level is considered very important in the context of geochemical explorations, environmental monitoring, biosorption, clinical and forensic analysis, high purity material designing and other miscellaneous applications.
On evaporation, matrix effects (total dissolved salts) are increased, thereby creating problem during nebulization of the solution, while in co-precipitation, several factors may contribute to lack of quantitative recovery in addition to the solubility of the precipitate.
Although solvent extraction is one of the most popular techniques for the separation of metal ions but has limited application in enriching trace amounts of ions present in dilute solutions. This is because the degree of extraction decreases as the aqueous-to-organic phase ratio is increased, effectively limiting the sample size used and the preconcentration factor achieved. The attainment of equilibrium may take several hours. The mutual solubility of two phases, emulsion formation, toxic nature of organic solvents and handling of large volume of organic solvent, which is inflammable, are other disadvantages. Many times multi-step extraction is also necessary for complete recovery.
On the other hand, ion exchangers are insoluble solid materials, which contain exchangeable cations or anions. These ions can be exchanged for a stoichiometrically equivalent amount of other ions initially present in an electrolyte solution when an ion exchanger is brought into contact with it. While chelating ion exchange resins represent an important category of synthetic resins frequently employed to selectively preconcentrate metal ions from large sample volume, followed by determination with an element specific detectors. Ion exchange chromatography and chelating ion exchangers both have been used extensively for preconcentration and separation of metal ions. These resins do not work in the same way as conventional cationic or anionic resins.
CHELATING POLYMERIC RESINS
Chelating ion exchange resins have received considerable attention owing to their inherent advantages over simple ion exchangers, i.e., their greater selectivity to bind metal ions. They are also designated as “functionalized polymers”, “chelating sorbents”, “chelating resins” and “chelating polymeric resins”. They are generally prepared by immobilizing a chelating ligand onto a support matrix through physical sorption or chemical spacer/ coupler. The kind of metal ion and the ligand strictly determine the bond length and angle between the central metal ion and the coordination ligands in metal complexes.
Chelating resins sorb metal ions through chelation. The term chelate effect refers to the fact that a chelated complex, i.e., one formed by a bidentate or a multidentate ligand, is more stable than the corresponding complex with monodentate ligands. The greater the number of points of attachment of ligand to the metal ion, the greater is the stability of the complex. Chelating/ functionalized groups are usually capable of interecting with a large number of metals forming a five / six membered chelate ring, but the stability of the formed complexes differs and depends on sorption conditions.
SYNTHESIS OF CHELATING POLYMERIC RESINS
Chelating resins based on organic polymers may either have functional groups in their polymeric skeleton or modified by chelating ligand that is immobilized by physical adsorption onto them via p -p dispersion forces and/ or ion exchange, or chemical bonding. The chelating resins developed through physical adsorption on polymer matrices suffer from ligand leaching problem and offers restricted reusability of the resin. Nevertheless, their easy preparation is definitely an advantage. The anchoring of ligands on these polymers through covalent bonding is attained, either by covalently immobilized on them directly or via chemical spacer/ coupler. The methylene (--CH2--) spacer and azo (--N=N--) spacer are commonly used. The immobilization of ligands through covalent linkage on the polymer matrix as a pendant group with or without spacer offers much wider ranges of possibilities in fabricating chelating resins and consequently their tailoring for applications (such as a preconcentrating matrix for metal ions) becomes possible. The methylene (--CH2--) spacer is generated through the chloromethylation of ligand or polymer matrix followed by Friedel Crafts alkylation with an appropriate moiety. The azo (--N=N--) spacer is created when Amberlite XAD-2, 4 or 16 [having polystyrene divinylbenzene skeleton] after cleansing was nitrated and thereafter the nitro groups were reduced to get the amino polymer. It was then diazotized and resulting diazonium salt was coupled with phenolic ligands in weakly alkaline media or amino ligands in weakly acidic media.
CHARACTERISATION OF CHELATING RESINS AND THEIR METAL COMPLEXES
The characterizations of newly synthesized chelating resins involve use of analytical and physico-chemical techniques. The physical methods are of greater importance as the selectivity is considered to depend on the chelating functional groups, which determines the coordination behaviour of these groups towards the metal ions and the geometry around the metal ions.
ANALYTICAL APPLICATIONS
The chelating resins have been found useful for separation and preconcentration of not only transition, alkaline earth, rare-earth, and uranium metal ions but are also useful in organic synthesis, polymer drug graft, and catalysis. A number of review articles1-13 detailing with the separation/ preconcentration of metals ions by chelating resins have appeared in the last two decades. In the present paper, the promising chelating resins, based on Amberlite XAD-2 (Table), synthesized in the author’s laboratory14,15 are discussed.
ACKNOWLEDGEMENT
The authors wish to thank Shri P. S. Parihar, Director, Additional Director (OP-I), Shri C. L. Bhairam, Regional Director, NR and Dr. G. Chakrapani, Head, Chemistry Group and Shri Adarsh Kumar, Incharge, Chemistry Lab., NR, AMD for kind permission to present.
REFERENCES
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7. C. Kantipuly, S. Katragadda, A. Chow, H. D. Gresser, Talanta 37 (1990) 491.
8. D. Bilba, D. Bejan, L. Tofan, Croatia Chim. Acta 71 (1998) 155.
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10. C. Y. Liu, K.L. Cheng, Chelating polymers, in “Facets of Coordination Chemistry”, (eds.) B. V. Agarwala and K. N. Munshi, World Scientific, Singapur, 1993, Ch.10, pp. 123-135.
11. D. E. Leyden, W. Wegscheider, Anal. Chem. 53 (1981) 1059A.
12. G. Schmuckler, Talanta 12 (1965) 281.
13. M. Kumar, Ph. D. thesis entitled “Amberlite XAD-2 Functionalized with Phenolic Ligands as Metal Ion Extractants” submitted to Indian Institute of Technology, Delhi in July, 2001.
14. M. Kumar, D. P. S. Rathore, A. K. Singh, Analyst 125 (2000) 1221; Talanta 51 (2000) 1187; Mikrochimica Acta 137 (2001) 127; Fresenius J. Anal. Chem. 370 (2001) 377.
15. M. Kumar (2007), Exploration and Research for Atomic Minerals, Vol.17, pp.15-20.
Table: Parameters of Chelating Resins for Preconcentration of Metal IonsChelating Resin
(stationary phase) / Metal / Opti-
mum
pH / Mobile phase
(eluent)
M HNO3 / Sorption capacity
(mg g-1) / Precon.
factor / Loading half time
t1/2, (min.) / Precon. limit / % Reco-
very
o-Aminophenol functionalized Amberlite XAD-2 / Cu
Cd
Co
Ni
Pb
Zn / 6.2-7.4
5.6-7.2
5.6-9.0
6.0-9.0
5.0-9.0
5.7-7.0 / 1-4
1-4
3-4
2-4
1-4
1-4 / 3.37
3.42
3.29
3.24
2.94
3.32 / 50
50
100
65
40
40 / 14
8
10
15
18
11 / 20 ppb
10 ppb
10 ppb
20 ppb
25 ppb
10 ppb / 98
99
98
97
91
98
Tiron functionalized Amberlite XAD-2 / Cu
Cd
Co
Ni
Pb
Zn
Mn
Fe
U (VI) / 4-6
5-6
5-7
5-6
4-5.5
5-6
6.5-7.5
5-6
4.5-5.5 / 2-3
2-3
3
3-4
2-3
2-3
2-3
2-3
4 / 14.0
9.5
6.5
12.6
12.6
11.1
10.0
5.6
7.7 / 200
48
56
150
25
180
64
80
150 / 2.9
3.6
2.9
3.8
4.0
2.9
3.2
2.8
3.6 / 20 ppb
15 ppb
20 ppb
10 ppb
25 ppb
5 ppb
20 ppb
10 ppb
3 ppb / 99
97
98
98
91
98
95
99
95
Pyrogallol Immobilized Amberlite XAD-2 / Cu
Cd
Co
Ni
Pb
Zn
Mn
Fe
U (VI) / 5.5-6.5
5.5-7.5
5.5-7.0
5.5-7.0
5.5-6.5
5.5-6.5
5.5-8.0
5.5-6.2
5.5-6.2 / 2-4
3-4
3-4
4
4
3-4
4
3-4
2-4 / 4.53
5.22
4.10
4.10
6.71
4.54
4.51
4.62
4.50 / 64
40
56
120
25
160
120
140
70 / 2.6
2.3
2.7
3.0
3.3
2.1
2.5
2.0
2.8 / 10 ppb
20 ppb
10 ppb
10 ppb
25 ppb
5 ppb
10 ppb
10 ppb
3 ppb / 97
99
96
95
90
99
93
99
95
Quinalizarin Anchored on Amberlite XAD-2 / Cu
Cd
Co
Pb
Zn
Mn
U (VI) / 5.0-6.5
5.5-7.0
5.5-6.5
5.5-6.5
5.5-6.5
5.5-7.0
5.0-6.0 / 2-4
4
4
4
2-4
4
2-4 / 3.15
1.70
1.62
5.28
1.42
1.00
2.18 / 100
50
40
50
100
66
66 / 8.5
5.3
10.2
15.0
7.4
14.1
6.3 / 10 ppb
20 ppb
15 ppb
25 ppb
10 ppb
10 ppb
3 ppb / 98
98
93
91
96
93
95
Preconcentration Factor: It is a factor, which denotes how many fold; the analyte can be preconcentrated from the original concentration.
Preconcentration Limit: It is a limit, which denotes what is the lowest concentration that can be preconcentrated quantitatively from the original solution.
Sorption Capacity: This is the amount of analyte (in mg) which can be sorbed per gram of chelating ion exchange resin.
Loading half time, t1/2 : The equilibration time in which resin becomes 50% saturated with a metal ion (i.e. when amount of metal ion sorbed on the resin is half of its sorption capacity)