Selective solvent extraction of chromium(VI) using 2-hexylpyridine

Journal o f Radioanalytieal Chemistry, Vol. 42 (1978) 313-321

S E L E C T I V E S O L V E N T E X T R A C T I O N O F C H R O M I U M ( V I )

U S I N G 2 - H E X Y L P Y R I D I N E .

SEPARATION FROM URANIUM, THORIUM, FISSION PRODUCTS AND FIRST TRANSITION SERIES ELEMENTS

M. IQBAL, M. EJAZ*

Nuclear Chemistry Division, Pakistan lnst~'tute o f Nuclear Science and Technology, P.O. Nilore, Rawalpindi (Pakistan)

(Received March 17, 1977)

The extraction behaviour of trace and macroamounts of chromium(VI) from different mineral acid solutions by 2-hexylpyridine in chloroform has been investigated. In the chloride system, the extracted species is apparently (HPyH§ (Cr 2 07)2- or HPy§ for macro and trace amounts of chromium(VI), respectively. Among the common anions chloride and sulphate have little effect on extraction up to 1M concentration, while in the case of nitrate there is a continuous decrease in the extraction with the increase of salt concentration in the aqueous phase. The effect of ascorbate, acetate, citrate, oxalate, thiosulphate, thiocyanate ions on the extraction from 1M HCI was also examined. Separation factors of several elements relative to chromium(VI) have been described and the separation of chromium(IV) from a large number of elements has been achieved.

In t roduct ion

In a previous s tudy on the extract ion of gold by 2-hexylpyridine from different

mineral acid solutions I we have found that gold can be separated from a large num-

ber o f metal ions in dilute hydrochlor ic acid media, and among the several elements

tested for extract ion only chromium(VI) showed interference. I t was our opinion

that the investigation on chromium(VI) extract ion can provide valuable data on the

selective separation o f this metal ion. The present work was therefore under taken

tG study the applicabil i ty of 2-hexylpyridine as an extractant for chromium(VI)

from different mineral acid solutions. This amine, like 4-(5-nonyl) pyridine 2 is in-

soluble in water and easily soluble in a large number o f organic solvents. Interest-

ingly, 2-hexylpyridine is much more selective for the extract ion o f chromium(VI) in

*Author to whom correspondence should be addressed.

J. Radioanal. Chem. 42 (1978) 313

M. IQBAL, M. EJAZ: SELECTIVE SOI_.VENT EXTRACTION

dilute (1-3M) hydrochloric acid media than 4-(5-nonyl)pyridine. z Tetrachloroferric

acid which is coextracted in this acidity range by the commonly used solvating reagents and liquid anion-exchangers including 4-(5-nonyl)pyridine is not extracted. A large number of metal ions which form anionic complexes in dilute and moderate hydrochloric acid solutions are also not extracted. The investigation thus offers a

unique extractant that can be employed for the highly selective separation and/or concentration of chromium in conjunction with modern analytical techniques.

Experimental

Reagents, tracers and equipment

0.1M solutions of 2-hexylpyridine (HPy) were used (except for the solvent effect). The characteristics of this compound have been reported) Chloroform and all other reagents used in this study were of highest purity grade.

SlCr tracer (as chromate) of high specific activity was prepared as reported. 3 All other tracers used in this work were obtained either from the Radiochemical Centre, Amersham, or were prepared locally by (n, 3,) reaction or by separation of the daughter nuclide from the parent without a carrier.

The equipment used has been described elsewhere) -3

Procedure

In these experiments the aqueous phase was normally prepared to contain cations (radioisotopes) in concentrations of the order of < 10-SM and varying concentrations of acid solutions. The organic phase contained 2-hexylpyridine in chloroform with concentration of 10-1M (except in slope analysis data) and was equilibrated prior

to use with the aqueous solutions from which extraction was to be carried out.

Equal volumes (2 ml) of aqueous and organic phases were then mixed in glass vials, shaken at room temperature (22-+ 3 ~ for 5 min, and centrifuged. Aliquots of both phases were then taken for the measurements of the radioactivity. The preci- sion attained in distribution coefficient was generally better than -+ 2%. The distribution coefficient is defined as:

D = counts/min / ml of organic phase

counts / min/ml of aqueous phase

314 J: Radioanal. Chem. 42 (1978)

M. IQBAL, M. EJAZ: SELECTIVE SOLVENT EXTRACTION

Ao

Fig. 1. Distribution ratio of trace and macro (0.05M) amounts of chromium(VI) as a function of various acid molarities in the aqueous phases. Organic phase 0.1M HPy in chloroform: Curves: �9 - HC1, �9 - H 2SO4, �9 - H N O 3 ; the hollow symbols are for trace concentration

Results and discussion

The organic diluent has a significant influence on the extract ion of macro amounts

(0.05M) o f chromium(VI) by 2-hexylpyridine. Aromatic hydrocarbons like benzene

and xylene extract the complex incompletely, with the formation of a precipitate

at the interface. In extract ion with alcohols, the complex passes to the organic layer

which becomes turbid and its volume increases. The extraction with chloroform goes

to complet ion with clear phases. Hence chloroform was used as a diluent.

The extract ion of trace and macro amounts of Cr(VI) from hydrochloric, nitric

arid sulphuric acid solutions was studied using 0.1M HPy in chloroform. The results

presented in Fig 1 show that in all the three acids the extract ion is enhanced by an

increase in their concentrat ion in the aqueous phases and the extract ion in hydro-

chloric acid system is much bet ter than extractions from other mineral acids and was

therefore preferred for further studies. As is evident from Fig. 1, the curves for macro

amounts of Cr(VI) are higher than for trace concentrations. Probably this is be-



cause of the formation of condensation species of the metal. In the chloride and

nitrate system there is a sharp rise in D values as the concentration of the acids in-

creases from 0.01M to 0.5M. This can be explained as due to a decrease in the water

activity with the increase of the concentration of the acids in the aqueous phases. A part of the reason may be the extraction of metal oxy anions by ion-association mechanism with amine cations which are formed preferentialy with increasing acid

concentration. However, the former explanation appears more plausible since the per- manganate ions, presumably due to low aqueous hydration, are extracted quantita- tively from 0.01M acids. 4 The decrease in extraction with the increase of acidity in

Q

1 -7- o o ~ r x , . 7 - - - x

-2 -1 0

Ig [Anion] ,~M

Fig. 2. Effect of neutral chloride, sulphate and nitrate ions on the extraction of macro amounts of chromium(VI) for extraction by 0.1M HPy in chloroform from 1M hydrochloric acid. Curves: o - chloride, • - sulphate, �9 - nitrate

the aqueous phases may be due to the competition of the acids for the pyridine. In concentrated hydrochloric and sulphuric acid solutions, the decrease in extraction

can also be attributed to the reduction of chromium(VI) to inextractable chromium(II-I).

The effects of nitrate, chloride and sulphate on the extraction of chromium(VI) from 1M hydrochloric acid is shown in Fig. 2.The addition of neutral chloride and sulphate ions has little effect on the extraction, while the addition of nitrate ions gives rise to low distribution coefficients.

A graph of lg D vs lg[Ht~] at constant acidity (1M) of hydrochloric acid derived from the results indicates a linear relationship for trace concentration of the metal up to 0.1M concentration of the reagent (Fig. 3) and consequently a constant number of HPy molecules per chromium atom in the extracted complex. The slope obtained indicates the extraction of HCrO4 since the formation of HCr20~" and Cr2072 at trace concentration is unlikely to be important and the extraction reaction may be expressed as:

HPy + HC1 ~ (HPyH*)C1- (1)

(HPyH§ - + nCrOg ~ (I-IPyH§ + CI- (2)

316 J. Radioanal. Chem. 42 (1978)


_lg [HPg], M -

- -2

!

| Q

Fig. 3. Variation of the distribution coefficient of chromium(VI) as a function of the concentration of HPy in chloroform. Curves: o - tracer Cr(VI) �9 - 0.05M Cr(VI)

The increase in concentration of the reagent beyond 0.1M does not give a straight line and the D value decreases instead. This could be due to the aggrega- tion of the pyridine hydrochlorides. ! However, when the original acid concentration was lowered to 0.25M there was no deviation from linearity. Similar results were obtained in the corresponding 4-(5-nonyl)pyridine- HC1 system 2 and the

phenomenon has been explained in the corresponding gold system, x The plot of lg D vs lg [HPy] for 0.05M concentration of chromium(VI) from

1M HC1 shows a slope of unity at low reagent concentrations, and two at relative- ly high concentrations. HCrO2/HCr20~ may be preferably extracted at low reagent concentrations by ion pair mechanism expressed by the Eqs (1) and (2), or as

HPy + HC1 g (I-LPyH +) C1-

2(I-IPyH)+C1 - + Cr20-~- ~ (HPyH)~'Cr202- + 2C1- (3)

When at constant acid strength the concentration of the reagent increases, the extraction may be represented as

I-Wy + HC1 ~- ( H P y . . . H . . . Hay)*Cl-

2(I-IPy. �9 H . . . PyH)+C1 - + Ct202- ~ ( H I ~ . . . H . . . PyH)~'Cr20~- + 2C1- (4)



cn >

t.a

aq Ccr(vl)

I

Fig. 4. Extrac t ion i so therms in the sys tem 0 .1M H P y / c h l o r o f o r m , IM, 3M HC1 + variable Cr(VI)

-2

Ig [An ion ] ~Id

i a

Fig. 5. Effect o f various an ions o n the ex trac t ion o f c h r o m i u m ( V I ) (0 .05M) by 0 .1M HPy/chloro-

form. Curves: o - ascorbate , �9 - acetate, D - citrate, �9 - oxalate , A _ th iosulphate ,

�9 - t h i o c y a n a t e

318 J. Radioanal. Chem. 42 {1978)

M. I Q B A L , M. E J A Z : S E L E C T I V E S O L V E N T E X T R A C T I O N

c_ E

m E

s 10 6 ~-

10 5

i0 z.

I03 I-

10 z

10

Ce,Fe / % Zr - Nb

I ' " ] ~ ~Mn Fe,Co, Cu

j 2

] i : l 0.2 0.4 0.6 08 t o

; 1 i : t - ]2 t 4 t6 t.8 2.0~ Energ 9 , MeV

Fig. 6. Gamma-spectra of the sample and the separated Cr(VI) fraction. Curve 1 - gamma-spectrum of the mixture, curve 2 - gamma-spectrum of separated st C r ( V I )

The loading isotherms for Cr(VI) are plotted in Fig. 4 for two concentrations (1.0 and 3M) of hydrochloric acid. The maximal obtainable concentration in either case is close to 5 g/1 indicating the formation of 1:1 complex, possibly as (NPyH); Cr20~-. In the case of 1.0M acid concentration the loading decreases sharply when the chromium(VI) concentration in the original aqueous solutions exceeds 20 g/l. The explanation for this may be the same as already advanced in the case of 4-(5-nonyl)pyridine system 2 viz a decrease in the concentration of H + ions with condensation of chromium species in the aqueous phase and subse- quent decrease in the stability of the amine cations.

The effect of ascorbate, acetate, citrate, oxalate, thiosulphate and thiocyanate ions on the extraction of Cr(VI) from 1.OM HCI was investigated by 0.1M HPy/chlo-

J. Radioanal. Chem. 42 (1978) 319 2


Table 1 Separation factors of different metal ions relative to chromium(VI)

in 0.1M HPy/chloroform- 1M HCI extraction system

Metal Separation factors > DCr(VI)/D M

U(VI)

Nb(V)

Zr(IV)

Th(IV) Fe(III) Y(III) La(III) Ce(III)

Pm(III) Tb(III)

Ba(II) Sr(II) Ca(II)

Mn(II) Ni(II)

Co(II) Mg(II) Cu(II)

Zn(II)

Cs

Na

Concentration, mol/1

10-3

C.F.* 107

C.F. I0-7

C.F. C.F. 10-8

i0 -g

I0-8 10-8

10 -9 10-8

tO-S 10-7 10-8

lO-S lO-S 10-7

10-s

10-~

10 4

10 6

10 ,~ 10 6

lO s 10 6

10 6

10 ~ 10 6

10 ~ 10 6 10 6

10 ~ 10 6

10 6

10 ,~ 10 s 10 s lO s

10 ,~ 10 6

*C.F. - carrier-free.

roform. There is a regular decrease in the extraction of chromium(VI) with increas-

ing concentration of these ions in the aqueous phase except in the case of acetate

ions where the distribution ratio remains fairly constant up to 0.25M concentration

and then decreases. It is seen that the addition of 1M concentration of these ions

in the original aqueous solutions reduces the extraction drastically and therefore

can be used as the reagents for back extraction.

With a view to find the selectivity of the extraction process in 1.0M hydrochloric

ac id , the behaviour of several metal ions was examined. The data presented in Ta-

ble 1 show that the extraction of chromium is very selective. Tetrachloroferric acid

which is coextracted at this acid concentration by the commonly used extractants

is not extracted. The extraction of uranium(VI) which forms fairly strong anionic

320 J. Radioanal. Chem. 42 (1978)


complexes at this acidity is also not extracted. Such a highly selective extraction

of chromium(VI) is very difficult to achieve by the commonly used solvent extrac-

tion reagents. The data was tested by carrying out the separation of chromium(VI) (0.05M) from a test solution containing known concentrations of the test compo- nents (9SZr, 9SNb, 144Ce, 51Cr, 14~ 14~ 64Cu, 65Zn, S4Mn, 6~ indicator

amounts, mg amounts of Fe(III) 10 mg/ml) in two equilibrations followed by one scrub stage. Very clean separation was achieved. This is illustrated by the gamma

spectra of the mixture and the separated Cr(VI) fraction in Fig. 6.

References

1. M. IQBAL, M. EJAZ, S. A. CHAUDHRI, M. R. AHMED, Separation Science, 11 (1976) 255. 2. M. IQBAL, M. EJAZ, Talanta, 22 (1975) 113. 3. M. IQBAL, M. EJAZ, Radiochim. Acta, 22 (1975) 49. 4. M. EJAZ, (unpibiished data).

J. Radioanal. Chem. 42 (1978) 321 2*

Documents

Selective solvent extraction of chromium(VI) using 2-hexylpyridine