EXTRACTION OF MERCURY(I1) FROM AQUEOUS THIOCYANATE SOLUTIONS WITH 5-(4-PYRIDYL)NONANE

IN BENZENE, AND ITS SUBSEQUENT ATOMIC-ABSORPTION SPECTROMETRIC DETERMINATION

SUHAIL AHMED. WASIM DIL, SHAMIM A. CHAUDHRI and M. EJAZ@

Nuclear Chemtstry Division. Pakistan Institute of Nuclear Sctence and Technology. P.O. Nilore. Rawalpindi. Pakistan

(Received 23 March 1978 Acceprrd 21 May 197X)

Summary-Tracer (- IO-‘MM) mercury(l1) can be quantttatively extracted with 5-(4-pyridyljnonane in benzene from aqueous thiocyanate solutions that are up to 6M in HCI. I M in H,SO, or 0.25M in HNOs, m a single extraction. Optimal conditions for the extraction are given. based on a critical study of the relevant factors such as the effects of the actds. thiocyanate. salting-out and complexmg agents and the reagent concentration. The mechamsm underlying these extractions is discussed on the basis of the results obtained from partttton and slope-analysis data. The extraction of the metal as Hg(PyN),(SCN), is Indicated. The extracted mercury can be stripped from the non-aqueous layer with various aqueous solutions. including nitrtc actd (>2M), sodmm citrate (2 I M) and sodium thiosul- phate (>O.lM). Common salts do not depress the extraction. Distributton coefficients and separation factors of several elements relative to mercury(I1) are reported for media that contam the optimal concentrations of the mineral acids and are m 0.2M m potassium thiocyanate. The data have been applied for the determmation of mercury m soil and water samples by atomic-absorption spectrometry.

The extraction and determination of traces of mer- cury have attracted considerable attention owing to the toxicity of mercury and its compounds. Numerous analytical methods for the extractive determination of mercury in various matrices have been described. Reviews have been published recently by Ure’ and Chilov.* A commonly used reagent for the extraction and preconcentration of mercury(H) is ammonium pyrrolidinedithiocarbamate (APDC) dissolved in methyl isobutyl ketone, (MIBK), which also co- extracts many other metal ions quantitatively. This paper is concerned with the use of S-(4-pyridyh- nonane dissolved in benzene as an extractant for mercury(I1) from various matrices. The method is rapid and fairly selective and the various parameters have wide tolerance: it has been adapted for the determination of the element by atomic-absorption spectrophotometry. The main purpose was to demon- strate the applicability of the procedure for the extrac- tion and possible preconcentration of the metal from spiked soil and water samples, without particular stress on low levels of detection. However, this extrac- tion procedure could well be used in combination with the cold vapour atomic-absorption technique for the determination of low levels of mercury.

EXPERIMENTAL

Reagents and tnstrutnentation

S(CPyridyl)nonane (PyN) was used as extractant. The characteristics of this compound are given elsewhere.3.4 Stock mercury(H) solution was made by dissolvmg the appropriate amount of analyttcal grade mercuric nitrate in 250 ml of distilled water and diluted as required to make

standards of low concentrattons. All solutions were pre- pared in doubly-disttlled demmerahzed water. lo3Hg tracer solutions were prepared by dissolvmg a weighed quantity of irradiated HgO m high-purity acids and diluted to volume with distilled water. The equipment used for the radiochemical assay was described prevtously.5,h Atomic- absorption analyses were done on a Carl-Zeiss FMD 3 Atomic Absorption Spectrophotometer equtpped with a deuterium lamp for automatic background compensation. a ICkm tttanmm universal burner usmg acetylene/air as gas mixture. and a Westinghouse mercury hollow-cathode lamp.

E.utraction procedure

Partition coefficients were determmed by a procedure similar to the one used for the corresponding zinc system 4 The period of equilibration was varied from 0 5 to IS mm. The extraction was quantitative with 1 mm equilibration or more. A 5-min equilibratton time was used. The concen- tration of mercury(I1) in the origmal aqueous phases was IO-‘M

Procedure for opening out soil 5atnple.c

Of the various methods used’ for opening out SOIIS for mercury determmation. the one recommended by Iskandar et a1.s was adopted for our purpose. (Procedures requirmg hydrochloric actd m the mitral acid mixtures were avotded. because although it increases the sensittvity of mercury determination,9 the varying amounts of the acid left after evaporation caused a large fluctuation in the atomic- absorption signals; hydrochloric actd was added to the final solution before aspiration mto the flame, to make use of the increase in sensitivity.) The container with I g of soil sample (as received or spiked with Hg) was placed in an ice-water bath and I5 ml of 2: 1 sulphuric acid-mtric acid mixture were added slowly. After expulsion of nitrous fumes the mixture was digested for 2-3 hr on a water-bath at 5&60’. The solutton was filtered through a porosity-3 sintered-glass disc, neutralized with cont. sodium hydrox- ide solution and adlusted to give IOOml of solution 0.1 M

563

SUHAIL AHMED et al.

log [Acld]. M log [Ami]. M

Fig. I. Distribution of H&II) between O.IM PyN/benzene and aqueous mineral acid solutions (A) without and (B) with thiocyanate (O.OZM).

in nitric acid and 0.2M in thiocyanate, before extraction by the described procedure. Blanks and standard mercury- (II) solution were treated in the same manner.

RESULTS AND DISCUSSION

In Fig. IA log D is plotted against the concen- tration of hydrochloric, nitric and sulphuric acid in the aqueous phase for the extraction of trace amounts (z IO-*M) of mercury(I1) by O.lM PyN in benzene. In the hydrochloric acid system, the extraction from dilute acid solutions can be ascribed to the hydro- lysed species while the extraction from moderately concentrated acid solutions is evidently due to the extraction of anionic chloride complexes of the metal; these are obviously present since trioctylamine extrac- tion” in this region is high. The decrease of the parti- tion coefficient with increase of acidity in the aqueous phase can be attributed to the fact that the acid itself is extracted by the organic reagent and much less of the reagent is available for extraction of the complex. In nitric and sulphuric acid solutions, appre- ciable extraction is observed from the dilute solutions, probably of hydrolysed metal species of the type Hg(OH)(A) where A is the anion of the supporting acid. The decrease in extraction at high acidities can be explained as due to the competition of the acids for association with the free base. In the sulphuric acid system. an intermediate third phase is formed at acid concentrations above 1M as in the corre- sponding zinc system4 and can be explained simi- larly.

Figure 1B shows the effect of thiocyanate (0.02M) on the extraction of mercury(I1) by 0.1 M PyN in ben-

zene from various concentrations of the mineral acids. The addition of thiocyanate promotes extraction in all three systems probably because thiocyanate ions give larger and less hydrated complexes. In the hydro- chloric and nitric acid systems, th: distribution coeffi- cients for mercury(H) first rise with aqueous thio- cyanate concentration, pass through maxima at initial thiocyanate concentrations of 0.1-1M and then fall again. In the sulphuric acid system, the shape of the extraction isotherm is almost similar to that for the extraction from the acid alone, but the distribution ratios are greater by an order of masitude.

The extraction of trace amounts of mercury(I1) by O.lM PyN in benzene was also investigated as a func- tion of thiocyanate concentration at several concen- trations of mineral acid (Fig. 2). All the curves show maxima at around O.I-OSM potassium thiocyanate in the initial aqueous phase. The extraction is quanti-

tative over the whole region of thiocyanate concen- tration investigated except when the concentration of nitric and sulphuric acid solutions is too low or that of hydrochloric acid is too high. The variation of the distribution coefficient may be attributed to the for- mation of metal ion-thiocyanate complex and to the competition between metal ion and thiocyanate for association with the pyridine.

Experiments were performed to ascertain the com- position of the extractable species by extracting mercury(I1) at fixed acid concentration with varying concentrations of PyN, with benzene as diluent. The experiments were carried out with and without thio- cyanate ions present. Figure 3A shows the plots of log D us. log[PyN-J for O.lM solutions of the mineral acids without thiocyanate. For the hydrochloric acid

Extraction of mercury(H)

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log lscR1, M log [SCNI, M log [SCisl, M

Fig. 2. Variation of the distribution coefficients of Hg(II) as a function of thiocyanate concentration in constant acidity solutions; extractant O.lM PyN/benzene. (A) HCI system: (I) 0.01 M; (2) 0.1 M; (3) 0.5M; (4) l.OM; (5) 3.OM. (B) HNO, system: (1) O.OlM; (2) O.IM; (3) 0.5M. (C) H2S04 system:

(1) 0.01M; (2) O.lM; (3) 0.5M; (4) 1.OM.

565

system the slope was around 2, whereas for the nitric molecules of the extractant. Figure 3B shows the and sulphuric acid systems, the slope of the linear dependence of log D on the reagent concentration, part of the curves was _ 1.5, suggesting the extraction for 0.1 M mineral acid solutions 0.2M in thiocyanate. of a mixture of compounds containing one and two The linear parts of the curves below O.OlM concen-

A

- log [WY], M

-e----- log [N:Npy]. M

Fig. 3. Influence of PyN concentration on the extraction coefficient of Hg(I1) from constant acidity solutions (A) without thiocyanate and (B) 0.2M in KSCN. (1) O.lM HCI; (2) O.lM HNO,; (3) O.lM

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Extraction of mercury(II) 567

Table 1. Distribution coefficients and separation factors of different metal tons from O.IM PyN/bcnzenc-mmcral acid (0.2M with respect to KSCN) extraction systems

Distribution coefficients Separation factors

Metal ion Concentration. M 0 IM HCI O.IM HNO, 0. I M H2S04 O.IM HCI O.IM HNO, 0. I M H2S0,

Hg(W Tc(VII) U(VIl Cr(VI) Mo(V1) MO(V) Ru(IV) Hf(lV) Cr(IIIl Y(III) Ce(III) Nd(III) Fe(II1) La(II1) Pm(II1) Ir(II1) Sr(I1) Zn(I1) Co(II) Cu(I1) Cs(Il Ag(Il Br-

10-S C.F* lo-’ C.F lo-’ 10-s IO-h 10-l C.F 1O-8 IO-@ IO-* lo-” IO-h IO-’ lo-’ lo-* IO-’ lo-9 lo-” lO-8 10-s lo-4

III3 839 6.2 4.1

21.8 26.3 2.3 2.6 0.23 I.0

I88 110 0. I 0.47 0.3 1.2 0.04 0.1 I I.1 I 03 0.00 0.00 0. I4 0.0 I 7 2.04 005 0.05 0.00 0.00 0.06 002 0.01 000

1197 243 204 359 47 39

0.00 0.00 189 239

0.18 0.25

C.F. = carrier-free

900 5.6

15.3 0.35 4.7

15.9 0.07 1 43 0.12 1.17 000 0.02

17 0.27 0.00 0.03 0.00

I046 I I9 30 0.00

144 0.24

tration of the reagent have a slope of about 2. The

linearity breaks down above O.OlM concentration of the reagent and the slope decreases, probably because of aggregation of the pyridine salts.

The effect of common salts on the extraction of mercury was investigated at constant acidity with and without thiocyanate. The results are presented in Fig. 4. The addition of sodium chloride (Fig. 4A) to the (0.1 or OSM) hydrochloric acid system appears to have a depressant effect on extraction up O.IM concentration of added chloride. This is possibly due to a shift in the equilibrium. favourmg formation of relatively less extractable neutral chloride complexes of mercury(I1). With 1 M hydrochloric acid, the D value decreases with added chloride concentration up to I M. This IS probably because the comparatively

less extractable neutral chloride complexes are more readily formed at relatively high acid concentrations. A subsequent increase in extraction at salt concen- tration above 1M can be ascribed to the salting-out effect and/or to the formation of anionic chloride

1.8 X IO’ 1.X X I02 51 32

4.x X IO’ 3.2 X IO’ 4.8 x IO’ 8.4 X IO’

59 76 I.1 x IO4 I.8 x IO3 3.7 X 103 6.9 x IO’ 2.x X IO4 7.6 x IO3 1.0 x IO3 8.1 x IO”

> IO5 > IO5 7.9 X IO3 x.4 X IO4 16 x IO’ 41 x 101 2.2 x IO4 I.7 X IO4

>105 > IOh 1.8 x IOJ 4.2 x IO4

> IO5 1 IO’ 0.9 3.5 55 2.3

24 22 > IOh > IOh

59 35 6.2 x IO3 3.4 x IO’

16x IO’ 59

2.6 x IO3 1.9 X IO’

57 1.3 x IO4 6.3 x IO’ 7.5 x IO’ 1.7 X IO’

> IO< 4.5 x to4

52 3.3 x IO-’

> IO9 3.0 x IO4

> IO< 0.9 76

30 > IO”

63 3.7 x 10’

species. However. over the whole range of added salt concemration. the extraction IS almost quantitative. In the nitric and sulphuric acid systems (Figs. 4B and 4C). the corresponding metal salts do not appear to have much salting out and/or complexing effect on the extraction, which remains more or less constant.

We also recorded the effect of neutral chloride, nitrate and sulphate salts on the extraction of mer- cury(H) from 0.1 M solutions of the corresponding mineral acids (OX4 in thiocyanate). The results pre- sented in Fig. 4D show that in these systems the extraction is not very much influenced by the pres- ence of neutral salts in the aqueous phase.

The extractlon of mercury(l1) was also studied as a function of the concentration of different complex- ing agents added to 0.1 M aqueous solutions of the mineral acids (0.2M in thiocyanatekFig. 5. In the thiosulphate and acetate systems, the extractlon de- creases with increasing concentration of these ions in the original aqueous solutions but the decrease is not very pronounced. Oxalate and citrate almost com-

Table 2.

Hg found by extraction and AAS determination. pprn

Hg added. Water + 0.25M

PP” so11 Water K,SOI

25 24& I 50 48.5 f 2 48.5 L- I.5 51.0 + 2

100 98.8 k I 98.5 + I 99.5 * I.5

568 SUHAIL AHMED er al.

pletely suppress the extraction when present in above 0.1 M concentration.

To find the selectivity the behaviour of a number of metal ions, including other inorganic pollutants, was examined. The data presented in Table 1 show that it is possible to separate mercury(II) from a number of elements in a single extraction followed by a few scrub stages. Zinc, cobalt and copper, however, are quantitatively co-extracted. indicating that the reagent has potential for the preconcen- tration of toxic metal ions from solutions.

For quantitative stripping of the metal into the

aqueous phase, the organic phase containing mer- cury(U) was equilibrated with an aqueous phase 2M

in nitric acid 0.2M in thiocyanate and containing 1% of hydrochloric acid. Table 2 shows the recoveries of added Hg after extraction and stripping steps, as determined by atomic-absorption spectrophotometry. Nitric acid (2M) without thiocyanate also strips mer- cury but less efficiently. Moderately concentrated per-

chloric acid was also found to strip mercury. Oxalate and citrate also act as stripping agents although but there is a loss of mercury activity, the cause of which was not elucidated.

REFERENCES

I. A. M. Ure, A&. Chim Acta. 1975. 76. I. 2. S. Chilov. Talanra. 1975. 22. 205. 3. M. Iqbal and M. Ejaz. Anal. Chew.. 1975. 47. 936. 4. M. Iqbal. A. Qureshi and M. Ejaz. Talanta. 1978. 25.

371. 5. M. Ejaz. Sep. SCI. 1975. IO, 425. 6. M. Iqbal. M. Ejaz, S. A. Chaudhri and R Ahmed. rhid..

1976, 11. 255. 7. H. Agemian and A. S. Y. Chau. Ana/.wr. 1976. 101.

91. 8. 1. K. Iskandar. J. K. Syers, L. W. Jacobs. D. R. Keeney

and J. T. Gilmour. ibid.. 1972. 97. 388. 9. J. W. Owens and E. S. Gladney. Anal. Chem. 1976.

48. 787. IO. M. Y. Mirza and M. Ejaz. Anal. Chirw Acta 1967. 37.

402.