Analytica Chimica Acta, 159 (1984) 255-263 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

SEPARATION OF MOLYBDENUM FROM TUNGSTEN WITH DI(2-ETHYLHEXYL)PHOSPHORIC ACID AS EXTRACTANT

N. R. DAS, B. NAND1 and S. N. BHATTACHARYYA*

Nuclear Chemistry Division, Saha Institute of Nuclear Physics, 92 Acharya Prafulla Chandra Road, Calcutta - 700 009 (India)

(Received 7th February 1983)

SUMMARY

The use of di(2ethylhexyl)phosphoric acid (HDEHP) as an extractant for the separa- tion of molybdenum from tungsten was examined with the help of molybdenum-99 and tungsten-187 as radiotracers. Effective separation was obtained when the aqueous phase contained phosphoric acid at pH 0.8-2 or pH 3-3.5, depending on the amounts of metal. The method is applicable to both tracer and milligram amounts of molybdenum. The structure of the extracted species was examined by infrared spectroscopy.

Molybdenum and tungsten form one of the critical pairs of elements that have very close chemical similarities [l] ; thus their mutual separation has always been of interest. Liquid-liquid extraction is one of the most efficient general methods for complex separation. In a continuation of attempts to apply the well-known extractant, di(2-ethylhexyl)phosphoric acid (HDEHP) for the mutual separation of various congeneric pairs of elements [2,3], the separation of molybdenum and tungsten is discussed here. The extraction of molybdenum with HDEHP has been described from different mineral acid media [4-151, but the separation of molybdenum and tungsten with HDEHP [15--171 from such aqueous acidic media has received little atten- tion. Laskorin et al. [15] used a 0.1 M nitric acid medium with extraction into kerosene.

In the study reported here, it was found that the extraction of tungsten is less efficient than that of molybdenum in various mineral acid media, the extent of extraction of both elements being dependent on the acidity of the aqueous solution. However, the percentage extraction of tungsten was greatly decreased when orthophosphoric acid was added to the aqueous medium. Hence the present paper offers a detailed study of the extraction of molybdenum and tungsten (tracer to milligram levels) by HDEHP for the effective separation of molybdenum and tungsten.

EXPERIMENTAL

Reagents The solvents (carbon tetrachloride, benzene, n-heptane, cyclohexane,

chloroform) and other chemicals used were of analytical grade. The

0003-2670/84/$03.00 0 1984 Elsevier Science Publishers B.V.

256

di( 2ethylhexyl)phosphoric acid (ICN Pharmaceuticals, Plainview, NY) was purified as described by Peppard et al. [ 181; the procedure involves hydroly- sis of mono and pyro esters with hydrochloric acid, which is removed by washing with water, partitioning between diethyl ether and ethylene glycol, and recovery of HDEHP by evaporation of the ether fraction.

The radiotracers molybdenum-99 (as molybdate) and tungsten-187 (as tungstate) were obtained from BARC (Trombay, India).

General procedure The metal ions were extracted from 25-ml portions of aqueous ortho-

phosphoric acid solutions containing the ions as molybdate or tungstate at definite concentrations, along with the respective radiotracers in the same chemical form. The pH of the resulting solution was measured (pH meter). This aqueous solution was equilibrated with equal volumes of organic phase containing a defined concentration of HDEHP at room temperature. The activities were measured with a GM counter, so that the activity from the daughter 99Tc in the case of 99Mo was avoided [ 41. An Ortec Ge( Li) detector (25% efficiency) with an 8K multichannel analyzer was used to measure the y-spectra. A Beckman IR-20A spectrophotometer was used for infrared measurements.

RESULTS AND DISCUSSION

The extraction behaviour of molybdenum and tungsten at tracer levels from various acidic media with HDEHP is summarized in Table 1, which shows that the percentage extraction of the metal ions decreases as the acidity is increased. The distribution ratios of molybdenum are always higher than those of tungsten, and the general trend of decreasing extraction with increasing acidity is most pronounced for tungsten, when orthophos- phoric acid is present in the aqueous phase.

The distribution ratios for the extraction of the metal ions at tracer levels from orthophosphoric acid medium by 0.1 M HDEHP in n-heptane over a wide pH range are shown in Fig. 1. The distribution ratio of molybdenum

TABLE 1

Extraction (96) of molybdenum end tungsten at tracer levels from different acids with 0 .l M HDEHP in n-heptane

Acid HCl HNO, H,SO, HClO, H,P 0, (N) MO W MO W MO W MO W MO W

0.1 98.0 68.8 95.2 72.7 92.7 56.7 98.6 67.2 94.5 29.1 1.0 84.0 62.3 94.3 70.0 91.0 54.2 97.2 62.6 89.6 10.1

5.0 78.0 57.9 93.3 69.0 90.3 50.4 97.2 61.2 15.0 _a

257

IOC

IO

;;

2 z 5 I

i D

;

7.i 6 0.1

0.01

0.001

1

102 5

I E - = 4 ? :

(8)

101

PH

J 8 0 40 80 120 160 200 :

Channel number

Fig. 1. Effect of pH on the distribution ratios for molybdenum and tungsten extractions: (A) 99Mo tracer in H,PO, medium with HDEHP in n-heptane; (B) P9Mo tracer in H,PO, medium with HDEHP in chloroform; (C) 99M~ tracer in H,AsO, medium with HDEHP in n-heptane; (D) “‘W tracer in H,PO, medium with HDEHP in n-heptane; (E) l”W tracer in H,AsO, medium with HDEHP in n-heptane. A 0.1 M solution of HDEHP was used in all cases.

Fig. 2. -ySpectra: (A) 99Mo and 18’W mixture; (B) aqueous fraction left after extraction of e9Mo; (C) the HDEHP extract.

increases linearly as the pH increases to about 2 and then decreases above pH 3.2 (Fig. 1, curve A). In this pH range, molybdenum(V1) mainly exists in the cationic form, such as MOO?, MoO(OH)r or Mo~O(OH)~*H~O~+ [ 11. In a strongly acidic solution, molybdenum(V1) is mainly present as MoOi species [ 1,191 which can exchange with HDEHP,

MOO:’ + 2HDEHP = MoO~(DEHP)~ + 2H+ (1)

Therefore, a linear increase in the distribution ratio with pH is expected. The straight line observed in the pH range 0.5-2.0 (Fig. 1, curve A) has a

258

slope of 2, which agrees with that required by Eqn. 1. As the pH is increased above 3.2, the molybdenum species in the aqueous phase change. In the pH range 3.2-7, the decreasing distribution ratios for molybdenum are consist- ent with the aggregation of molybdenum to form the heteropoly anion, H3PM~12040 [l] . In alkaline solution, of course, the MoOz- ion present is not extracted with HDEHP.

Tungsten shows a similar trend of extraction (Fig. 1, curve D). The WO: ion prevails in the aqueous solution and the distribution ratios increase upto pH 1.4, the slope being about 2. Further increase of pH decreases the distri- bution ratio, probably because of gradual formation of H2W12040.xHz0 [20]. However, in the pH range 4-7, the distribution ratio for tungsten again increases; in this pH range, tungsten exists as paratungstate, [HW6021]5- [21]. This anionic form should not be extracted with HDEHP, but the increasing distribution ratios indicate that some interaction does occur, despite the high charge on the anion. It could be argued that there is an ion-dipole interaction between paratungstate and an -OH group of HDEHP in the pH range 4-7, though it is then difficult to see why the hetero poly- tungstate anion undergoes no such interaction.

In addition to the characteristic variation in the distribution ratios with pH, it should be noted that the distribution ratios for molybdenum are much larger than those for tungsten, probably because the heteropoly anion of tungsten is more stable than that of molybdenum [ 11. That this hetero- poly anion formation plays an important role in these extractions is clear from curves C and E in Fig. 1, which shows the distribution ratios when the arsenate ion replaces phosphate in the solution, forming the corresponding heteropoly anion. Molybdophosphoric acid is well known to be more stable than the corresponding arsenic acid [l] which agrees well with the present results that the distribution ratios for both elements are higher in arsenic acid than in orthophosphoric acid medium. Incidentally, it was shown that phosphorus in the aqueous phase does not exchange with that in the organic phase, by making appropriate extractions with radioactive “P in the aqueous phase; no 32P was detected in the extract with HDEHP.

The remarkable decrease in the distribution ratios for tungsten in ortho- phosphoric acid medium is very important for the separation. Molybdenum is extracted at the tracer level almost free from tungsten at pH 3-3.5 from orthophosphoric acid medium with 0.1 M HDEHP in n-heptane. The decon- tamination factor, calculated as DMo/Dw (D = distribution ratio) [22] was >500 in the pH range 3-3.5 (curves A and C, Fig. 1). The absence of sig- nificant contamination from ‘*‘W in the organic phase was verified by y-ray spectrometry (Fig. 2). As can be seen, the spectrum of the organic layer (curve C) shows no significant amount of l*‘W, while the aqueous phase (curve B) is free from “MO [ 221.

This separation was extended to extractions of milligram amounts of the two elements. It was found that the distribution ratios decreased as the con- centration of the metal ions increased throughout the pH range studied

259

(Fig. 3). The distribution ratio for 0.0025 M molybdenum(V1) in the aqueous solution increases initially upto about pH 2, then decreases; for 0.02 M molybdenum(VI), this ratio increases only upto pH 1.1. The point of maxi- mum extraction shifts towards lower pH with increasing metal ion con- centration in the aqueous solution (Figs. 1 and 3). This observed shift can be explained if one considers the chemistry of formation of hydrolytic poly- meric species of molybdenum(V1) [23, 241; the consumption of hydrogen ions naturally increases with increasing molybdenum concentration, to provide the MoOZ,’ ions needed for the extraction by HDEHP.

The case of tungsten(V1) at milligram levels is shown in Fig. 3, curve C. The formation of WO’,+ occurs at pH <l [20] ; at about pH 1, tungsten is mainly present as hydrated tungstic oxide, WOJ*2Hz0 and WOB-H,O [l]. Therefore, extraction of 0.0014 M tungsten(V1) at pH l--2.5 may be con- sidered as being due to dipole-dipole interaction with an OH group of the HDEHP molecule rather than to WOP formation as on the tracer scale. Above pH 2.5, heteropolyacid aggregation becomes pronounced and so extraction decreases; the later increases in extraction at pH 6 may be explained in the same way as curve D in Fig. 1. In this larger-scale work, the efficiency of separation was estimated by measuring, by r-ray spectrometry, the extent of coextraction of the two metal ions. The results (Table 2) indicate that con- tamination by tungsten in the separation of molybdenum does not exceed about 1%.

IO

0.01

100

HDEHP (Ml PH

Fig. 3. Variation of distribution ratios with pH in the H,PO,/O.l M HDEHP/n-heptane system. Concentration of metal ion: (A) 2.6 x lo* M Mo(V1); (B) 2.0 x 10” M Mo(V1); (C) 1.4 x 1O-3 M W(V1).

Fig. 4. Dependence of the distribution ratios for MO and W in H,PO, medium on HDEHP concentration. Extractions: (A) 99M~ tracer at pH l-1.2, (B) 5 X 10” M Mo at pH 2.1, (C) 2.5 X 10” M MO at pH 2.8; (D) *MO tracer at pH 1.2; (E) l”W tracer at pH 1.2; (F) 1.4 x 10” M W at pH 2.8. The HDEHP solution was in n-heptane, except for curve D, for which chloroform was used.

260

TABLE 2

Contamination by tungsten in the extraction of 2.5 X lo-’ M molybdenum in phosphoric acid media by 0.1 M HDEHP at pH 1.4 and pH 2.1

Tungsten cont. (x lo-$ M)

Distribution ratios

DErlo/pH 1.4 Dw/pH 1.4 D&pH 2.1 Dw/pH 2.1

0.3 2.27 0.026 10.0 0.025 0.6 2.45 0.004 10.0 0.02 0.9 2.50 0.004 10.0 0.023 1.2 2.49 0.004 10.0 0.02 1.7 2.50 0.004 10.0 0.02

The effect of the HDEHP concentration on the extractibility of the metal ions was examined (Fig. 4). The distribution ratio for molybdenum increases with increasing HDEHP concentration, starting from 0.01 M HDEHP in n- heptane, and tends to level off at different values, depending on the amount of molybdenum. Extraction of tracer tungsten(V1) starts only at a higher concentration of HDEHP (0.1 M) and steadily increases with increase in the concentration of HDEHP.

From the above studies, the whole process of extraction by HDEHP may be postulated as follows. First, the metal ion interacts with HDEHP at the interface of the organic and aqueous layers, where the monomeric HDEHP molecules are orientated with their polar end towards the aqueous phase, the selectivity of the interaction depending on the formation constant of the complex produced. At 0.001-0.1 M HDEHP, the slopes of the curves are about 2, which indicates that two molecules of HDEHP are associated with the metal ion complex. On further increase of the concentration of HDEHP, these slopes decrease (Fig. 4), possibly because of aggregation; aggregation of the extractant depends on such factors as HDEHP concentration, nature of the metallic species and type of the apolar diluent used [ 251. The distribution ratio values were found to decrease with increase in the dielectric constant of the diluents used (Table 3).

Infrared studies The infrared spectra were examined over two regions covering 700-

3000 cm-‘. The spectrum of an extract of 0.02 M Mo(V1) from aqueous orthophosphoric acid into 0.1 M HDEHP in carbon tetrachloride is com- pared with the spectrum of free HDEHP in the same solvent in Fig. 5. Carbon tetrachloride was chosen because it has no alkyl group. With HDEHP alone, the broad intense band round 1000 cm-’ can be ascribed to the superimposition of P-O(C), C-O(P), (P)-OH, P-O--(H) groups [26]. In the molybdenum complex, this band is split up and the symmetric and antisymmetric stretching vibrations of the MOO, group [27] were observed at 960 cm-‘.

261

TABLE 3

Distribution ratios (D) for molybdenum or tungsten on extraction from aqueous 0.1 M phosphoric acid at pH 2.37 by 0.1 M HDEHP in different diluents

Diluent Dielectric constant

n-Heptane 1.9 19.01 0.700 11.00 0.020 Cyclohexane 2.052 18.06 0.200 10.05 0.020 Carbon 2.240 10.62 0.060 0.13 0.008 tetrachloride Benzene 2.283 8.02 0.054 0.12 0.008 Toluene 2.387 7.09 0.040 0.12 0.008 Chloroform 5.06 4.60 0.005 0.11 0.005

aFor lo* M Mo(V1). bFor lo-* M W(V1). cFor 2.5 x lo-’ M Mo(V1). dFor 1.4 X lo-” M W(V1).

In the spectrum of HDEHP, the P=O stretching vibration is observed at 1220 cm-’ [ 281 whereas the spectrum of the complex exhibits a strong band around 1170 cm-‘, which may be ascribed to the involvement of P=O group in forming a coordinate bond with MOO 22+, as suggested by Kolarik [ 61. The maximum at 1360 cm-’ is ascribed to the deformation mode of hydrogen atoms in the -CHS group and that at 1465 cm-’ is due to the -CHz group

II. 11 I. I. I1 I. 3000 2600 1800 1600 1400 1200 1000 80

Wavenumber (cm-‘)

Fig. 5. Infrared spectra of samples in carbon tetrachloride (0.1 mm cells): (A) 0.1 M HDEHP; (b) MO-HDEHP complex ; (C) deuterated HDEHP.

262

of HDEHP [28]. These bands shift only slightly in the complex. The absorp- tion band at 1’710 cm-’ for free HDEHP is ascribed to the deformed -OH bond [ 281, which is known to be affected by hydrogen bonding. In the metal complex, this band appears as a broad shallow band around 1620 cm-’ [28] ; deuteration of the free ligand shifts this deformation mode to 1555 cm-‘. From this, it can be concluded that the --OH group remains hydrogen-bonded in the metal complex, although this does not agree with the structure proposed by Zelikman and Nerezov [7] . The P-OH stretching vibration around 2840 cm-’ arises from a dimeric structure (hydrogen bonding) [28] . The band from free HDEHP is less intense from the metal complex, suggest- ing that the metal cation replaces hydrogen in the complex, breaking the hydrogen-bonded structure and affecting the phosphoryl oxygen.

From these observations, it appears that the molybdenum-HDEHP complex extracted from aqueous phosphoric acid solution, has a closed ring structure:

““\ P” “O\ /“” oAP+y

This structure does not agree with that reported by Zelikman and Nerezov [7] but closely agrees with that put forward by Kolarik [6].

Grateful acknowledgement is due to Dr. S. Ghosh, Reader, Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta-700 032, for his help with i.r. spectroscopy.

REFERENCES

1 Comprehensive Inorganic Chemistry, J. C. BaiIar, H. J. Emeleus, Sir Ronald Nyholm and A. F. TrotmanDickenson (Eds.), Pergamon Press, Oxford, 1973, Vol. 3, Ch. 36; Vol. 4, Ch. 51.

2 N. R. Das, B. Nandi and S. N. Bhattacharyya,Int. J. Appl. Radiat. Isot., 32 (1981) 205. 3 N. R. Das, B. Nandi and S. N. Bhattacharyya, Radioanal. Chem., 62 (1981) 53. 4 T. S. Urban’ski, M. Chojecki and R. Kaczyn’ska, Radioanal. Chem., 30 (1976) 369. 5 A. A. Palant and V. A. Rejnickenko, J. Appl. Chem. USSR, 46 (1973) 1124; Zh. Prikl.

Khim. (Leningrad), 46 (1973) 1062. 6 Z. Kolarik, J. Inorg. Nucl. Chem., 35 (1973) 2026. 7 A. N. Zehkman and V. M. Nerezov, Zh. Neorg. Khim., 14 (1969) 1307. 8 T. D. Titkova and V. I. Levin, Radiokhimiya, 17 (1975) 55. 9 V. L. Bychovtsov, Radiokhimiya, 13 (1971) 667.

10 Yu. B. Kletnik,I.A.Bykhovskayaand L.V. Sekretova,Zh.AnaI.Khim., 24 (1969) 707. 11 F. Cerrai and G. Ghersini, J. Chromatogr., 24 (1966) 383. 12 A. N. Zelikman, Molybdenum (in Russian), Atomizdat, Moscow, 1972. 13 I. L. Jenkins and A. G. Wain, J. Appl. Chem., 14 (1964) 449. 14 S. M. Karpacheva, V. S. Smelov, Yu. I. Vereshehagin, I. V. Davydov, V. T. MoskaIeva,

M. N. Ryzhov and A. V. Strakhova, Zh. Neorg. Khim., 12 (1967) 1926. 15 B. N. Laskorin, V. S. UI’yanova and R. A. Sviridova, Zh. PrikI. Khim. (Leningrad), 36

(1962) 2409.

263

16 G. M. Vol’dman, A. N. Zelikman and I. Sh.Khutoretskaya, Izv. Vyssh. Uchebn. Zaved. Tsvet. MetaIl., No. 2 (1974) 97.

17 F. Esnault, M. Robaglia, J. M. Latard and J. M. Demarthe, International Solvent Extraction Conference, Lyon, France, Sep. 1974.

18 D. F. Peppard, G. W. Mason, I. L. Maier and W. J. DriscoII, J. Inorg. Null. Chem., 4 (1957) 334.

19 Stephen J. Lippard (Ed.), Progress in Inorganic Chemistry, Vol. 22, Interscience, NY, 1977, p. 35.

20 H. J. Emeleus and J. S. Anderson, Modern Aspects of Inorganic Chemistry, 2nd edn., Routledge & Kegan Paul, London, 1954, p. 212.

21 J. F. Duncan and D. L. Kepert, J. Chem. Sot., (1961) 5317;(1962) 205. 22 F. Adams and R. Dams, Applied Gamma-ray Spectrometry, Pergsmon Press, Oxford,

1972, pp. 549,627. 23 M. L. Freedman, J. Inorg. NucI. Chem., 25 (1963) 575. 24K. B. Yatsimirskii and I. I. Alekseeva, Izv. Vyssh. Uchebn. Zaved. Khim. Khim.

Tekhnol., l(l958) 53. 25 J. H. Fendler, Act. Chem. Res., 9 (1976) 153. 26 V. I. Spiryakov, P. G. Kurtikov, V. M. Barinov, A. S. Solovkin and G. N. Yakovlev,

Radiokhimiya, 14 (1972) 574. 27 F. A. Cotton and R. M. Wing, Inorg. Chem., 4 (1965) 867. 28 L. J. Bellamy, The Infrared Spectra of Complex Molecules, Methuen, London, 1959,

pp. 21-23,312,319.