Reversed-Phase Column Extractive Separation of Gd(III) with Poly[dibenzo-18-crown-6]

A poly[dibenzo-18-crown-6] exhibits good chemical stability, reusability and faster rate equilibrium for the separation of Gd(III). Both uptake and stripping of metal ions were rapid, indicating a better accessibility of the complexing sites. The proposed method has been applied for chromatographic separation of Gd(III) by using picric acid as medium and poly[dibenzo-18crown-6] as stationary phase. The influences of picric acid concentration, different eluting agents, and so forth, were discussed and the optimum condition were established. The breakthrough capacity of poly[dibenzo-18-crown-6] for Gd(III) was 0.572 ± 0.01 mmolg−1 of crown polymer. The proposed method has been applied to sequential chromatographic separation of their binary and multicomponent mixtures. Gd(III) has been determined from real samples with good analytical reliability.


Introduction
In recent years the separation chemistry of rare earth elements (REEs) continues to receive a growing interest.The major reasons for this stems from the importance of rare earths not only in industrial application but also in energy generation activities and environmental mitigation.Gadolinium is useful in nuclear techniques, in fuel element fabrication and in ceramic industries and as control rod and as refractory material [1,2].In view of all the above applications the separation and purification of Gd(III) are important.
The main focus of the extensive research on chelating resins is the preparation of functionalized polymer that can provide more flexible working conditions together with good stability, selectivity, high concentrating ability, high capacity of metal ions, and simpler operation [3][4][5][6].
Crown ethers are effective extractants due to their ability to form stable complexes with metal ions.In recent years extraction chromatography has emerged as a versatile and effective method for analytical and preparative scale metal ion separation [7][8][9].Also crown ethers are used as sorbents in chromatographic techniques [10][11][12][13][14][15][16][17] like TLC, extraction chromatography-liquid chromatography on columns, and gas chromatography for actinides and lanthanides.
To our knowledge no successful attempts were reported in the literature for the separation of Gd(III) using poly[dibenzo-18-crown-6] in picric acid media and column chromatography.The present paper describes a simple and sensitive method for the determination of Gd(III) using poly[dibenzo-18-crown-6] as stationary phase in picric acid media.In our study we use picric acid as medium because picrate is bulkier anion with higher hydrophobicity.The proposed method has been successfully applied for the separation and determination of Gd(III) from real materials.

Experimental
2.1.Apparatus and Reagents.All absorbance measurements were carried out using a digital visible spectrophotometer (215 D, Thermo Electron LLS, India), a digital pH meter A stock solution of Gd(III) was prepared by dissolving 1.091 g of gadolinium nitrate (AnalR grade, BDH, Poole, UK) in 100 mL of distilled deionised water and standardized gravimetrically [24].A solution containing 100 µgmL −1 of Gd(III) was prepared by appropriate dilution of the standard stock solution.Picric acid solution (5 × 10 −2 moleL −1 ) was prepared by dissolving 2.846 g of picric acid in distilled deionised water and diluted to 250 mL.All aqueous solutions were prepared with double distilled deionised water.
Poly[dibenzo-18-crown-6] from Merck Darmstadt, Germany was used after screening to 100-200 mesh.A total of 0.5 g of polymer was slurred with distilled deionised water and poured into a Pyrex glass chromatographic column (20 × 0.8 cm i.d.).The column was used after preconditioning with picric acid solution.All chemicals used are of analytical grade.

General
Procedure.100 µg of Gd(III) was mixed with picric acid solution in concentration range of 1 × 10 −2 to 1 × 10 −7 moleL −1 in a total volume of 10 mL.Then, the solution was passed through poly[dibenzo-18-crown-6] column preconditioned with the same concentration of picric acid as that of the sample solution at flow rate of 0.5 mLmin −1 .Afterwards the column was washed with the same concentration of picric acid, and the sorbed Gd(III) was then eluted with different eluting agents at the flow rate of 0.5 mLmin −1 .5.0 mL fractions were collected and analyzed spectrophotometrically with Arsenazo(III) [25,26] at 650 nm.The concentration of Gd(III) was calculated from a calibration graph.

Sorption of Gadolinium(III) on Poly[dibenzo-18-crown-6]
as a Function of Picric Acid Concentration.Sorption studies of Gd(III) were carried out from picric acid medium using 100 µgmL −1 of Gd(III).The concentration of picric acid was varied from 1 × 10 −2 to 1 × 10 −7 moleL −1 .It was found that there is quantitative sorption of Gd(III) from 1 × 10 −2 to 1 × 10 −4 moleL −1 picric acid as shown in Table 1.Sorbed Gd(III) was eluted with 4.0 M HCl.With further decrease in the concentration of picric acid the sorption of gadolinium also decreases.The subsequent sorption studies of Gd(III) were carried out with 1 × 10 −3 moleL −1 picric acid.

Breakthrough Capacity of Poly[dibenzo-18-crown-6].
The breakthrough capacity of Gd(III) was carried out on 1.0 g of poly[dibenzo-18-crown-6] with 1 × 10 −3 moleL −1 picric acid.The volume of Gd(III) sample solution employed was 10.0 mL.The concentration of Gd(III) was varied from 100 to 1100 µg of Gd(III) 10 mL −1 of solution.After sorption, the elution of Gd(III) was carried out with 0.1 M HBr at a flow rate of 0.5 mL min −1 .Sorption of Gd(III) was quantitative up to 900 µg10 mL −1 .The extent of sorption of Gd(III) is decreased with the increase in the concentration of Gd(III) as shown in Figure 2. The breakthrough capacity of poly[dibenzo-18-crown-6] for Gd(III) was found to be 0.57 mmolg −1 of crown polymer.

Diverse Ion Effect.
To assess the usefulness of this method, the effects of foreign ions, which are likely to interfere with the proposed method for determination of Gd(III) were studied.An aliquot of solution containing 100 µg of Gd(III) was mixed with foreign ions and picric acid was added so that its concentration was 1 × 10 −3 moleL −1 in total volume of 10 mL.The tolerance limit was set not to cause more than ±2% deviation in the recovery of Gd(III).The column was equilibrated with 1 × 10 −3 moleL −1 picric acid, and binary mixture solution was passed through a poly [dibenzo-18-crown-6] column at flow rate of 0.5 mLmin −1 .Subsequently, the column was washed with 15 mL of 1 × 10 −3 moleL −1 picric acid to remove unsorbed metal ions.Various foreign ions were not sorbed and hence passed through the column.The effluent was collected and analyzed for foreign ion content.It was found that most of alkali metals show high tolerance limits.The alkaline earth metals were sorbed quantitatively.Most of the p-block and dblock elements showed low tolerance limit.Amongst the inner transition elements U (VI) was sorbed along with Gd(III) and its separation is carried out in our subsequent study employing multicomponent mixtures.The anions of inorganic and organic acids have shown high tolerance limit except SO 4 2− .The tolerance limit of various foreign ions is shown in Table 2.

Separation of Gadolinium(III) from Multicomponent
Mixtures.A mixture of Li(I)/Fe(III)/Mo(VI)/Sr(II)/Ca(II), U(VI), Gd(III), and Ba(II) was resolved by following the proposed procedure of Gd(III).The mixture was passed through the poly[dibenzo-18-crown-6] column under optimum condition of Gd(III), where Li(I)/Fe(III)/Mo(VI)/Ca(II)/Sr(II) was not sorbed and passed through column which was then determined by standard procedure [24,26].U(VI) and Ba(II) were quantitatively sorbed on column along with Gd(III).The separation of Gd(III) from U(VI) can be achieved by the use of 0.2 M LiOH as a eluting agent; at this condition Gd(III) and Ba(II) was not eluted and remained on column.Gd(III) was eluted with 2.0 M perchloric acid, and the remaining Ba(II) on column was eluted with 4.0 M hydrochloric acid and determined by standard methods [24,26].The results are shown in Table 3 and chromatograms of all metal ions in Figure 3.
3.6.Reproducibility of the Method.Reproducibility of the method was checked by thirty replicate analyses of a standard Gd(III) solution.The results indicate the method to be fairly reproducible.The poly[dibenzo-18-crown-6] could be recycled many times without affecting its sorption capacity.The reusability of poly[dibenzo-18-crown-6] with standard deviations is shown in Table 4.

Determination of Gadolinium(III) from Real Samples.
The composite material sample (GdSrO 2 ) after acid treatment is subjected to the proposed method for the determination of Gd(III).The percentage of Gd(III) found after triplicate analysis is 9.89 (±0.11) as against the reported value of 10.0%.

Conclusion
The proposed method affords an attractive feature as compared to the solvent extraction technique, that is, it is free from any organic diluents, leading to potential green chemistry applications.It permits the separation of Gd(III) from
Various Eluting Agents.Gd(III) was eluted out from the column with different strengths of acids such as HCl, H 2 SO 4 , HClO 4 , CH 3 COOH, and HBr.The concentrations of the eluting agents were varied from 0.1 to 8.0 M. Gadolinium(III) was eluted quantitatively with 2.0-8.0M HCl, 0.1-0.5 M H 2 SO 4 , 1.0-8.0M HClO 4 , 7.0-8.0M CH 3 COOH, and 0.1-5.0M HBr.Further, the elution study of Gd(III) in this work was carried out with 0.1 M HBr.The elution profile of Gd(III) with different eluting agents is shown in Figure 1.

Figure 2 :
Figure 2: Effect of varying concentrations of Gd(III).The concentration of Gd(III) was varied from 100 to 1100 µg of Gd(III) 10 mL −1 of solution.Sorption of Gd(III) was quantitative up to 900 µg10mL −1 .
* Average of triplicate analysis.* * No sorption passing through the column.

Table 4 :
Reproducibility of the method.