Separation of Zirconium (IV) and Hafnium (IV) by Extraction

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Separation of Zirconium(IV) and Hafnium(IV) by Extraction Chromatography Using Di(1-methylheptyl) Methyiphosphonate as a Stationary Phase Qiuquan WANG, Kin-ichi TsUNODAand Hideo AKAIWAt Department of Chemistry, Faculty of Engineering, Gunma

University,

Kiryu

376, Japan

The extraction chromatographic behavior of zirconium(IV) and hafnium(IV) was investigated using di(1-methylheptyl) methylphosphonate (DMHP) as a stationary phase in the media of hydrochloric acid-ammonium chloride by a batch method. Through a log-log analysis and the IR spectra the extractable species has been proposed to be (DMHPH+)2ZrC162-and (DMHPH+)2HfC162-. The quantitative separation of Zr(IV) and Hf(IV) with ratios of 1:1 to 100 :1 from each other has been achieved by DMHP-extraction chromatography. Keywords

Extraction chromatography, di(1-methylheptyl) methylphosphonate, zirconium(IV), hafnium(IV)

As a result of the lanthanoids contraction, zirconium and hafnium become the most typical pair of elements which have a similar chemical behavior. Thus, these two elements invariably exist with each other in nature. On the other hand, because of the different physical properties of the Zr and Hf nuclei, such as the absorption cross-section for thermal neutrons, these two elements are used for different purposes in a nuclear power reactor. Although a vast quantity of research on the mutual separation of Zr and Hf with solvent extraction and ion-exchange chromatographic methods has been done, the development of a more effective mutual separation method for these two elements still attracts the attention of many separation chemists today. The studies of the ion-exchange and solvent extraction as well as other chromatographic behaviors on Zr and Hf before 1986 were reviewed by Korkischl, Nandi et al.2, and Hala.3 Recently, Khopkar et al. used high-molecularweight amine, Amberlite LA-1 in citric acid solutions4 and Aliquat 336 in ascorbic acid solutions5,6 to separate Zr(IV) and Hf(IV) from each other as well as Zr(IV) or Hf(IV) from other similar elements; Mishra and his coworkers utilized the synergistic extraction of Aliquat336 or Alamine-336 with tributyl phosphate (TBP)' and Aliquat-336 with trioctyl phosphine oxide (TOPO)8 to extract Zr(IV) and to separate from Nb(V) and Hf(IV). Such methods are based on an association of the highmolecular-weight amine cation with the anionic metal complexes which are formed through the reaction between metal ions and inorganic or organic anions, such as chloride or citrate anions, and at the same time the t To whom

correspondence

should

be addressed

.

coordination from neutral extractants, such as TBP or TOPO to the metal ions. Pobi and Das9 introduced an N-benzoylphenyl hydroxylamine group onto styrenedivinyl benzene copolymer beads to obtain a chelating resin, and applied it for separating Zr(IV) and Hf(IV) from each other. Sulfoxides, such as dibutyl sulfoxide10 and petroleum sulfoxide (PSO)11, have also been applied to such a purpose; a quantitative mutual separation of Zr(IV) and Hf(IV) was achieved using PSO-extraction chromatography in an HCl-NH4SCN medium." Furthermore, many studies have been done on organophosphorous extractants in order to separate these metals. Shinde and his coworkers used triphenyl phosphine oxide (TPPO)12"3 and tris(2-ethylhexyl) phosphate (T2EHP)14 for the mutual separation of group-IV elements as well as lanthanides, thorium, zirconium and hafnium by solvent extraction. Di(2ethylhexyl) phosphoric acid (HDEHP)15 as a liquid ion exchanger was used to separate Zr(IV), Nb(V) and Hf(IV) from one another by an extraction chromatographic method with an H2504-oxalic acid-H202 solution as a mobile phase. Moreover, the mutual separation of Zr(IV) and Hf(IV) was also tried by a liquid-membrane method using TBP.16 Among the various separation techniques, such as solvent extraction and ion exchange, extraction chromatography can offer not only the high selectivity of solvent extraction, but also the high efficiency of chromatography at the same time. We have thus previously" applied the extraction chromatographic method using PSO for the mutual separation of Zr(IV) and Hf(IV). In this study, di(1-methylheptyl) methylphosphonate (DMHP) was used as an extractant for the extraction

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chromatographic separation of these metals. When compared with the most typical phosphorous extractant, TBP, DMHP has a stronger extractive ability owing to the substitution of a methyl group for one alkoxyl group (CH3CH2CH2CH2O-) of TBP, which leads to a higher electron density on the oxygen atom of P=0 in the DMHP molecule than that in TBP. Moreover, its solubility to an aqueous phase is smaller than that of TBP, which is a suitable nature for an extractant used in extraction chromatography. Although DMHP was used in the extraction of rare-earth elements 17,scandium and thorium18 as well as iron19, it has never been applied to the mutual separation of Zr(IV) and Hf(IV). Thus, the extraction chromatographic behavior of Zr(IV) and Hf(IV) using DMHP-resin as a stationary phase in an HCl-NH4C1 medium has been investigated by the batch method, which is usually considered to be one plate of a chromatographic column.2o,21 The extractable species was proposed to be (DMHPH+)2MC162- [M=Zr(IV) or Hf(IV)] through a log-log analysis and the IR spectra. Moreover, the influences of the acidity and salting-out reagent on the distribution ratios of Zr(IV) and Hf(IV) were considered. Finally, the quantitative separation of Zr(IV) and Hf(IV) was achieved by DMHP-extraction chromatography.

Experimental

Materials and chemicals Di(1-methylheptyl) methylphosphonate (DMHP) was offered by Dr. Yenkun Hu of the Institute of Yuelong Chemical Plant, China; the purity was more than 99%, and it was used without further purification. DMHP resin was prepared as follows. Appropriate amounts of styrene and divinyl benzene with a ratio of 5 :1 as well as benzoyl peroxide (1% weight) was added into a threenecked flask, respectively; then, an appropriate amount of deionized water (5-times the organic volume) containing 1%(w/v) polyvinyl alcohol was added. Before the polymerization reaction of styrene and divinyl benzene started, an appropriate amount of DMHP was added into the mixture. The reaction mixture was stirred mechanically at 80° C to obtain DMHP resin; the resin was then sieved into 100 - 200 mesh. Resins containing 5 to 55 wt.% of DMHP were prepared. Zr(IV) and Hf(IV) standard solutions (Aldrich Chemical Company, Inc.) were 1000 µg cm3 (5 wt.% HCl) and 990 µg cm 3(1 wt.% HC1), respectively. All other chemicals used were of guaranteed grade; deionized water was used throughout.

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Glass Co., Ltd.) was used to collect the effluent from the column; an Iwaki KM Shaker was used to shake a separatory funnel; a Shimadzu ICPS-1000III sequential ICP atomic emission spectrometer was employed for determining the content of Zr(IV) or Hf(IV). A JASCO A-102 infrared spectrometer was used to measure the IR spectra of DMHP and the extracted complexes.

Procedure Determination of distribution ratios of Zr(IV) and Hf(IV) in the medium of DMHP-HCl with the batch method An appropriate amount of HCI, Zr(IV), Hf(IV), as well as NH4C1 for controlling the Cl-concentration and ionic strength, was added in a separatory funnel, and the solution volume was adjusted to 20.0 cm3 with deionized water. The separatory funnel was shaken for 1 h after the addition of 0.50 g DMHP-resin in spite of the equilibrium being achieved within 5 min. After filtration, the concentrations of Zr(IV) and Hf(IV) in the aqueous phase were determined by ICP-AES. The distribution ratio (D) of Zr(IV) or Hf(IV) can be calculated using D =[(C0 - C)/C](V/m),

(1)

where Co and C denote the initial and equilibrium concentrations of Zr(IV) or Hf(IV) in the aqueous phase, respectively, V is the volume of the aqueous phase (cm3), and m is the mass of DMHP-resin (g). Mutual separation of Zr(I V) and Hf(I V) by DMHP extraction chromatographic method A sample solution (0.5 cm3) containing Zr(IV) and Hf(IV) with a different ratio (100:1 to 1:1) was applied to the top of 46X250 mm extraction chromatographic column containing a 2.5 g DMHP-resin stationary phase after the column was equilibrated with a suitable mobile phase. Then, Zr(IV) and Hf(IV) were eluted by a mixture of HCl and NH4Cl used as a mobile phase, and the effluent from the column was collected in fractions of 8 cm3 each. The contents of Zr(IV) and Hf(IV) in the fractions were measured by ICP-AES.

Results

and

Discussion

Mechanism in the process of Zr(IV) and Hf(IV) transferring from the mobile phase (HCl-NH4Cl) to the stationary phase (DMHP) In the DMHP-HCI-NH4Cl system, the distribution process of Zr(IV) and Hf(IV) may be assumed to be CH3

Apparatus Extraction chromatographic column: a glass column with a water jacket, ~6 X 220 mm, was used for separation. The column temperature was maintained at 295 K with a waterbath (T-80; Tokyo Rikakikai Co., Ltd.) and a pump (CP08-PPRV-10; Nikkiso Eiko Co., Ltd.); an automatic fraction collector (FRC-2100; Iwaki

nR0-P=0

+ MCli"-i

+ (i-4)H+±

RO

CH3 (RO-P=O) RO

nH(i-4)MCI, t

(2)

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where M denotes Zr(IV) or Hf(IV); R is CH3CH(CH2)5CH3. Thus, the equilibrium constant (K) can be expressed as K=

[[(RO)2CH3PO]fH(i-4)MCIi]Sy

[(RO)2CH3PO)]sn yin[MCIi(4-i)]yMCli(4-i) [H+](i-4) yH+(i-4) , (3)

where y, yo,YMCli(4-i) and yH+are the activity coefficients of [(RO)2CH3PO]nH(i-4)MCIi,(RO)2CH3PO, MCIi(4-i)and H+, respectively, and subscript s denotes the stationary phase. Because the ionic strength was kept constant in the mobile phase by adding HCl-NH4C1 and the temperature was fixed at 298 K, the change in the activity coefficient may be neglected. Therefore, K can be expressed as K' [[(RO)2CH3PO]fH(i-4)MCIi]S [(RO)2CH3PO]S [MCl

][

]

(4)

Fig. 1 log D vs. log[H+]. DMHP-resin.

CHCI+CNH4Cl,

9 mol dm-3;

55%

where K'=y/yo yMCli(4-') yH+(i-4), which can be considered to be a constant here. Assuming that Zr(IV) and Hf(IV) are dominantly in the form of MCIi(4-i)in the mobile phase, the distribution ratio (D) can be written as [[(RO)2CH3PO]nH(i-4)MCIi]S D=

[MCIi(4-i)] = [(RO)2CH3PO]Sn[H+](i-4)K/K' .

(5)

Thus,

log D = log K/K' -+ nlog[(RO)2CH3PO]S + (i-4)log[H+].

(6)

Changing the concentration of H+ (or DMHP) at the constant concentration of DMHP (or H+), the distribution ratios of Zr(IV) and Hf(IV) were measured. Plots of log D vs. log[H] and log D vs. log[DMHP] are shown in Figs. 1 and 2. In the concentration range studied, the log D increases along with an increase in log[H+] or log[DMHP], and straight lines having a slope of 2 were obtained for both metals. Since the extractable species was therefore proposed to be [(RO)2CH3POH+]2MC162-,the distribution process of Zr(IV) and Hf(IV) to the stationary phase(DMHP) is also proposed to be as follows: CH3 CH3 2R0-P=0 + MC162-+ 2H+ (RO-P=OH+)2MC162RO RO (7) Study of the IR spectra of extracted complexes A Zr(IV) or Hf(IV) solution (1.00 mg cm 3) containing an appropriate amount of HCl and NH4Cl was added into a 2 g DMHP-resin column until the concentration of Zr(IV) or Hf(IV) in the effluent reached 1.0 mg cm-3. The column was smaller than that described in the experimental section, but was prepared using the same

Fig.

2

log D vs. log[DMHP].

CHCI, 6 mol dm

3.

procedure. The solution was then flowed out from the column. After the extracted complex was stripped from the column with diethyl ether, diethyl ether was evaporated under an IR lamp to obtain a sample for IR measurements. The IR data of the extracted complexes and DMHP are listed in Table 1. The vibration peak of vOHat 3400 cm 1 for DMHP indicates that there is water molecule associated with DMHP through a hydrogen bond. In the IR spectra of the Zr(IV) and Hf(IV) extracted complexes, the vibration peak of vP=o at 1250 cm 1 (in DMHP) shifted to a lower frequency at 1110 cm 1 of (DMHPH+)2ZrC162- or at 1 130 cm 1 of (DMHPH+)2HfC162-. This may have been due to the fact that the oxygen in the group of P=0 is protonated in a higher HCl concentration medium, and the protonated P=OP group associates with ZrCl62- or HfCl62- by an electrostatic force.

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Table 1 IR data (wavenumber/cm-1) of extracted complexes

Table

2

Effect

mobile

phase'

of adding

a salting-out

DMHP

agent,

and

NH4C1,

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the

to the

Fig. 3 Extraction chromatogram of Zr(IV) and Hf(IV). Stationary phase, 55% DMHP-resin; mobile phase, 5 mol dm 3 HCI+I mol dm 3 NH4C1; column, 6X220 mm; column temperature, 298 K; sample, 200 µg Zr(IV)+2 µg Hf(IV); flow rate, 1 cm3 cm-2 min-1.

separation of Zr(IV) and Hf(IV) from each other. This system may be useful for the mutual separation of other elements, such as lanthanoids and actin.oids.

Separation of Zr(IV) and Hf(IV) with DMHP extraction chromatography On the column of ~6X220 mm containing 2.5 g 55% DMHP-resin at 298 K, a different HC1 concentration in the mobile phase without NH4Cl was tested for a quantitative mutual separation of Zr(IV) and Hf(IV); the results indicated that more than 6 mol dm-3 HC1 was needed to realize the quantitative mutual separation of Zr(IV) and Hf(IV). Usually a lower acid concentration in the mobile phase is preferred in extraction chromatography, because such a condition can prolong the lifetime of the resin. Thus, the salting-out reagent NH4Cl was used to make the acidity lower and to keep the Cl- concentration in the mobile phase sufficiently high. Table 2 shows that the addition of NH4C1 not only increases the distribution ratios of Zr(IV) and Hf(IV), but also improves the separation factor for Zr(IV) and Hf(IV). From further experiments, 5 mol dm 3 HCI with 1 mol dm 3NH4C1in the mobile phase was found to be the most suitable for the quantitative separation of Zr(IV) and Hf(IV) under the condition of ~6X220 mm column containing 2.5 g 55% DMHP-resin at 298 K. When compared with our previous work concerning the petroleum sulfoxide (PSO) systemll, the separation factor (/3zr/Hf),which was more than 4.5 in the present study, is greater than that of the PSO system (azr/Hf=3.6) (see Table 2). Zr(IV) and Hf(IV) at a ratio of 100 :1 to 1:1 can be quantitatively separated from each other by this method. A typical separation chromatogram is shown in Fig. 3. Conclusively, extraction DMHP can be successfully

chromatography using applied to the quantitative

The

authors

assistance. and

Dr.

thank

Mr.

The authors T. Suzuki

Yenkun

Hu

of the

offering

DMHP.

for

K.

Hoshino

also thank their

Institute

for

his experimental

Dr. S. Aizawa,

valuable of Yuelong

Dr. T. Aiba

discussions,

and

Dr.

Chemical

Plant

for

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(1995). N. R. Das and S. Lahiri, J. Radioanal. Nucl. Chem., 163, 213 (1992). M. A. Chaudry and B. Ahmed, Sep. Sci. Technol.,27,199 (1992). J. Ni, Q. Su, K. Yao and C. Peng, Huaxue Tongbao, 6, 1 (1979). Y. Chen, Y. Li and G. Chen, Xiyou Jinsu, 5(6), 82 (1981).

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Y. Chen, Xiyou Jinsu, 6(5), 12 (1982). T. Suzuki, K. Tsunoda and H. Akaiwa, Anal. Sci., 11, 39 (1995). Q. Wang, K. Tsunoda, H. Akaiwa, L. Yang, H. Ma, Y. Li and C. Peng, Anal. Sc., 12, 231 (1996). (Received October 14, 1996) (Accepted November 25, 1996)