Kapil Dev, Rita Pathak, G.N. Rao *

Transcription

1 Sorption behaviour of lanthanum(iii), neodymium(iii), terbium(iii), thorium(iv) and uranium(vi) on Amberlite XAD-4 resin functionalized with bicine ligands Kapil Dev, Rita Pathak, G.N. Rao * Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 1 016, India Received 26 February 1998; received in revised form August 1998; accepted 13 August 1998 Abstract The complexing properties (capacity, ph effect, breakthrough curve) of a chelating resin, containing bicine ligands, were investigated for La(III), Nd(III), Tb(III), Th(IV) and U(VI). Trace amounts of these metal ions were quantitatively retained on the resin and recovered by eluting with 1 M hydrochloric acid. The capacity of the resin for La(III), Nd(III), Tb(III), Th(IV) and U(VI) was found to be 0.35, 0.40, 0.42, 0.25 and 0.38 mmol g" 1, respectively. Separation of U(VI) and Th(IV) from Ni(II), Zn(II), Co(II) and Cu(II) in a synthetic solution was carried out. Keywords: Sorption behaviour; Bicine ligands; Chelation; Complexing 1. Introduction Chelating resins are frequently used in analytical chemistry for preconcentration of metal ions and their separation from interfering constituents prior to their determination by an instrumental method. A number of chelating resins have been prepared by incorporating different functional groups (e.g. ethylenediamine tetraacetic acid [1], iminodiacetic acid [2-4], 8-hydroxyquinoline [5], etc.) and their analytical properties investigated. The metal binding capacity, the metal binding strength and selectivity are important characteristics of a chelating resin. A high capacity is usually an advantage, as small amounts of chelating resin are sufficient to concentrate metal ions from a large sample volume, whereas strong metal binding can be disadvantageous in the elution step [6]. The selectivity of chelating resin is often related to that of the monomeric compound corresponding to the functional group. In our earlier communication, we have reported the synthesis of a polystyrenedivinylbenzene (XAD-4) based chelating resin containing bicine (n,n-bis(2-hydroxyethyl) glycine) groups and its analytical properties for some transition metal ions [7]. The resin has shown greater selectivity for some metal ions than that attainable with strongly binding resin containing aminopolycarboxylic acid groups. These studies are now extended to the

2 580 K. Dev et al. /Talanta 48 (1999) sorption behaviour of La(III), Nd(III), Tb(III), Th(IV) and U(VI). Rare earth elements (REEs), uranium and thorium are important elements not only in industrial applications but also in energy and environmental problems. Trivalent REEs and thorium are chelated by resins such as chelex-0 [8], but this chelating resin also has an affinity for alkali and alkaline earth elements, which limits the concentration factor achievable, necessitating additional separation steps [9,]. Myazaki and Barnes applied a poly(dithiocarbamate) chelating resin for concentration of some REEs and thorium, but the elution is tedious and requires resin digestion with strong mineral acids for quantitative elution [11]. Horvath and Barnes prepared a carboxymethylated polyethylenimine-polymethylene polyphenylene isocyanate chelating ion-exchange resin and applied it for concentration of transition metals, rare earths, uranium and thorium [12]. Masi and Olsina immobilized 8-quinolinol on Amberlite XAD-4, and used it for preconcentration of some REEs and subsequent determination by X-ray fluorescence [13]. In this paper, we report sorption behaviour of a XAD-4-bicine resin for thorium and uranium. XAD-4-Bicine resin shows higher selectivity for these elements and their separation from some transition metal ions (Ni(II), Zn(II), Co(II) and Cu(II)) is feasible on the resin columns. 2. Experimental 2.1. Reagents Polystyrene divinylbenezene resin (Amberlite XAD-4) and bicine were obtained from Fluka (Buchs, Switzerland). The stock metal ion solu- 0 CH 2 CH 2 OH C-OCH 2 CH 2 N > \ Fig. 1. XAD-4-Bicine resin. CH 2 COOH al Ol c Loadi U A 2 0 _o o- Th -* *- Tb a D- La / ffiit^-g^^p - ""- 0 " Nd /^^^\N // n 111? Ill HI/,, ^ pp Fig. 2. Effect of ph on metal sorption with bicine resin. tions were prepared by dissolving the following metal salts: La(III)/La(NO 3 ) 3-6H 2 O, U(VI)/ UO 2-6H 2 O, Th(IV)/Th(NO 3 ) 4-4H 2 O, Nd(III)/ NdCl 3-6H 2 O and Tb(III)/TbCl 3 6H 2 O (99.9% purity, BDH, India). The following buffers were used to control the ph of the solutions: hydrochloric acid-glycine (ph 1-3), sodium acetate-acetic acid (ph 3-6), ammonium acetate-ammonia (ph 6-8), and ammonium chloride-ammonia (ph 8-9) Instrumentation A digital visible spectrophotometer (Perkin- Elmer, Lambda 3B), A sequential inductively coupled plasma spectrometry-auger electron spectrometry (ICP-AES) spectrometer (Perkin- Elmer, P-40) and an ECIL (Hyderabad, India) AA 4139 flame atomic absorption spectrometry (FAAS) spectrometer for determining metal concentrations, an Elico (Hyderabad, India) digital ph-meter (LI-120) for ph measurements and a mechanical shaker with incubator (Scientific, New Delhi, India) having a speed of 200 strokes min^1 was used for carrying out equilibrium studies.

3 K. Dev et al. /Talanta 48 (1999) Table 1 Metal uptake capacities Metal ion Optimum ph Capacity (S.D.) (mmol g ' resin) CPPI resin a Poly(dithiocarbamate) resin b La(III) Nd(III) Tb(III) Th(IV) U(VI) (0.02) 0.40(0.01) 0.42(0.01) 0.25(0.02) 0.38(0.01) a From Ref. [12]. b From Ref. [11] Preparation of XAD-4-bicine resin Preparation and characterization of XAD-4- bicine resin (Fig. 1) has been described in a previous paper [7] Optimum ph of metal ion uptake The optimum ph of metal ion uptake was determined by batch equilibrium techniques. Excess metal ion (50 ml, 50 ^g ml^1) was shaken with 0 mg of resin for 1 h. The ph of the metal ion solution was adjusted prior to equilibration over a range of 2-9 with buffer solutions. After the equilibration, the sorbed metal ions were eluted with 1 M hydrochloric acid. The concentration of the metal ions in the solution was determined spectrophotometrically using Alizarin Red-S [14] as the colorimetric reagent for La(III), Nd(III) and Tb(III), Thoron [15] for Th(IV), and dibenzoylmethane [16] for U(VI) Resin capacity The capacity of the resin was determined by shaking the excess metal ions (50 ml, 0 ^g ml^1) with 50 mg resin for 6 h at optimum sorption ph. The resin was filtered off and the concentration of the sorbed metal ion was determined spectrophotometrically after eluting it with 1 M hydrochloric acid ph dependence of trace metal ion uptake A 50 ml volume of a buffered solution containing 5 \ig ml ~ 1 of metal ion was shaken with 0 mg of resin for 1 h in a glass stoppered bottle. The resin was filtered off and the concentration of the sorbed metal ions was determined after eluting with 1 M HCl ( ml) and diluting the resultant sample solution to 50 ml with distilled water Equilibrium time Fig. 3. ph dependence of the uptake of trace metals: (a) La(III), (b) Nd(III), (c) Tb(III), (d) Th(IV), (e) U(VI). To determine the time of equilibrium for the metal under investigation, the metal ion solution (50 ml, ig ml^1) at a constant ph was sampled in six bottles. These bottles were removed from the shaker at regular intervals of time and the concentration of the sorbed metal ions was determined. The duplicate values agreed with a precision of + 2%.

4 582 K. Dev et al. /Talanta 48 (1999) Shaking time ( min.) Fig. 4. Rate of uptake of metal ions Effect of diverse ions Standard solutions of U(VI) and Th(IV) (50 ml, ng ml" 1 ) containing 0,, 00 and 1 Hg ml- 1 of Cu(II), Ni(II), Pb(II), Cd(II), Zn(II), Hg(II), Mn(II), Fe(III) and Cr(III); and 00 ng ml- 1 of Na(I), K(I), Ca(II) and Mg(II); and 00 \ig ml" 1 of Cl", NO 3 -, CH3COO- and SO^- were analysed. The eluate was checked for U(VI) and Th(IV) by ICP-AES and transition metal ions by FAAS. Table 2 Effect of diverse ions Tolerance limit [ion]/[th(iv)] or [U(VI)] >0 > Diverse ion C1-, NO3-, SO;;-, CH3COO- Na(I), K(I), Ca(II), Mg(II) Cu(II), Ni(II), Co(II), Zn(II) Cd(II), Cr(III) Fe(III), Pb(II) a Performed at ph 4.5 (amount of Th(IV) or U(VI) taken, ug ml" 1 ) Concentration and separation of metal ions The batch equilibration technique was used to concentrate the trace metal ions. A sample solution ( ml) containing 0.1 (xg ml- 1 metal ions was adjusted at optimum ph of sorption with a buffer solution and shaken with 0 mg of resin for min. Sorbed metal ions were eluted with 1 M HCl ( ml) and the concentration of the metal ions in the eluent was determined spectrophotometrically. Separation of uranium and thorium from various transition metal ions was carried out on the XAD-4-bicine resin. A 50 ml solution containing uranium or thorium (20 (xg ml- 1 ) and another Table 3 Preconcentration of metal ion (amount of metal ion taken, 0.1 ug ml- 1 ) Metal La(III) Nd(III) Tb(III) Th(IV) U(VI) Sample volume (ml) Metal found a (ug ml Values agreed with a precision of + 1%. l )

5 K. Dev et al. /Talanta 48 (1999) Table 4 Separation of U(VI) from other metal ions performed at ph 4.5 (amount of U(VI) taken, 20 Lig Metal ion Amount of metal added (Lig ml ') Amount of U(VI) found a (Lig ml ') Metal ion found a (Lig ml ') Ni(II) Co(II) Zn(II) Cu(II) a Values agree with a precision of + 1%. Table 5 Separation of Th(IV) from other metal ions performed at ph 4.5 (amount of Th(IV) taken, 20 Lig ml^1) with the batch method Metal ion Amount of metal added (Lig ml^1) Amount of Th(IV) found a (Lig ml Metal ) a (Lig ml^1) Ni(II) Zn(II) Co(II) Cu(II) 1 Values agreed within + 1% metal ion was shaken with 0 mg of resin at ph 4.5. Uranium or thorium retained on the resin was eluted with 1 M hydrochloric acid ( ml). The concentration of nickel, cobalt, zinc or copper was determined in the effluent by AAS. 3. Results and discussion In a preliminary experiment, the sorption behaviour of La, Nd, Tb, Th, and U on XAD-4- bicine resin at different ph values was examined by the batch method and the results are presented in Fig. 2. Sorption begins in every case at ph and a limiting value is attained near ph 6. A slight decrease in the sorption in observed beyond ph for most of the metal ions. The capacity of the resin is an important factor to determine how much resin is required to quantitatively remove a specific metal ion from the solution. The capacity of each metal ion is reported in Table 1 along with the standard deviation (S.D.). The theoretical value of the capacity should be equivalent to the ligand loading which is 0.44 mmol g~ : of resin for XAD-4-bicine resin (calculated from the nitrogen content). The lower values could be due to the rigidity of the polymeric matrix and the inability of all bicine groups participating in chelation due to steric restriction. The ph dependence of trace metal ion uptake was carried out to determine if the uptake of trace quantities of the metal ion is possible at ph values other than the optimum ph for chelation. The results are shown in Fig. 3. The complete sorption for the trace metal ions is in the region of ph for La(III), for Nd(III), 5-9 for Tb(III), for Th(IV) and for U(VI). A special feature in these breakthrough curves is the complete sorption of these metal ions in the ph range This should enable their separation from many transition metal ions, which are not retained at all in this ph region [7]. Interaction of the resin with the metal ion is sufficiently rapid, allowing the resin to also be used in a packed column. Fig. 4 shows the rate of uptake of La(III), Th(IV) and U(VI) determined at optimum ph of sorption. More than 90% of the metal ion is extracted within 5 min of its interaction with the resin. A similar behaviour is observed for other metal ions. The effect of diverse ions on the sorption of U(VI) and Th(IV) by XAD-4-bicine resin was investigated and results are shown in Table 2. The

6 584 K. Dev et al. /Talanta 48 (1999) tolerance limit is the maximum ratio of the concentration of diverse ion to that of U(VI) or Th(IV) at which the concentration of diverse ion in the eluate (1 M HCl) is negligible and the concentration of U(VI) or Th(IV) is the same as taken initially. The resin has shown a high tolerance limit for alkali and alkaline earth metals. This is particularly useful for the analysis of U(VI) and Th(IV) in natural samples; for example, sea water, which contains large amount of alkali and alkaline earth metals. Preconcentration of the metal ions on the resin was carried out and the results are shown in Table 3. Metal ions could be enriched up to 50 times with XAD-4-bicine resin. The chelated metal ions can be eluted from the XAD-4-bicine with dilute acids and the resin can be used repeatedly. In contrast, metals can be recovered effectively from the poly(dithiocarbamate) resin generally after the digestion of the resin [11]. The separation of trace amounts of uranium in the presence of diverse metal ions was carried out by the batch method (Table 4). Uranium is separable from nickel, copper, cobalt and zinc as they are not retained on the resin at ph 4.5. Similarly, separation of thorium was carried out from binary mixtures containing copper, nickel, cobalt or zinc (Table 5). 4. Conclusion XAD-4-Bicine resin has a good potential for enrichment of trace amounts of La, Nd, Tb, Th and U from large sample volumes. The resin can be applied over a wide ph range (4.5-9) for collection of trace metals. The resin shows selectivity for these metal ions over some transition metal ions and permits their separation from the latter. The capacity of the resin is sufficiently high to preconcentrate more than one metal ion simultaneously. Acknowledgements The authors thank BRNS (Bhabha Atomic Research Centre, India) for financial assistance. References [1] E.M. Moyers, J.S. Fritz, Anal. Chem. 49 (1977) 418. [2] M. Marhol, K.L. Cheng, Talanta 21 (1974) 751. [3] S. Tomoshige, M. Hirai, H. Ueshima, Anal. Chim. Acta 115 (1980) 285. [4] P. Figura, B. McDuffie, Anal. Chem (1950) 49. [5] W.M. Landing, C. Haraldsson, N. Paxeus, Anal. Chem. 58 (1986) [6] S. Blain, P. Appriov, H. Handel, Analyst 116 (1991) 815. [7] K. Dev, G.N. Rao, Talanta 42 (1995) 591. [8] Bulletin 2020, Bio-Rad Laboratories Product Information, Richmond, CA, USA, [9] S.S. Berman, J.W. McLaren, S.N. Willie, Anal. Chem. 52 (1980) 488. [] R.E. Sturgeon, S.S. Berman, J.A.H. Desavniers, A.P. Mykytivk, J.W. McLaren, D.S. Russell, Anal. Chem. 52 (1980) [11] A. Myazaki, R.M. Barnes, Anal. Chem. 53 (1981) 299. [12] Zs. Horvath, R.M. Barnes, Anal. Chem. 58 (1986) [13] A.N. Masi, R.A. Olsina, Talanta 40 (1993) 931. [14] R.W. Rinehart, Anal. Chem. 26 (1954) [15] A. Mayer, G. Bradshaw, Analyst 77 (1952) 154. [16] J.H. Yoe, F. Will III, R.A. Black, Anal. Chem. 25 (1953) 1200.