Antz?_vtica Chimica Acta, 91 (1977) 275-281 6 Eketier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

A SIMPLE DECOMPOSITION AND CHELATING RESIN SEPARATION FOR THZ DETERMINATION OF HEAVY METALS IN SILICATES BY ATOMX ABSORPTIOX SPECTROMETRY

TETSUO UCHIDA, MASAYUKI NAGASE, IS.40 KOJIMA, and CHUZO 1X3.4

Laboratory of Andytical Chemistry. Department of Engineering Sciences. h’ogoya institute of Technology, Showa-ku. Nagoya 466 (Japan)

(Received 1st June 1977)

SunlhIARY

A simple method for the determination of copper, nickel. zinc, and cadmium in silicate rock sanxpfes is reported. Silicates are decomposed with hydrofiuoric acid and aqua regia in a se&ed Teflon vessel. After centrifugation and addition of malonic acid, the supernatant liquid is passed through a small column of Chelex 100. The metals are efuted with 2 hi nitric acid, and determined by atomic absorption spectrometry. The method is rapid, simple, and free from contamination. The results obtained for 14 standard reference rock samples (USGS, GSJ, and CSRM) agree with literature data; the recovery, reproducibility, and accuracy of the proposed method are satisfactory.

For the d~~~~ation’of major [l, 21 and minor [2,3] elements in silicates by atomic absorption spectrornetry, the decomposition of silicates with a mixture of hydrofluoric acid and aqua regia in a sealed Teflon vessel has been reported. For accurate and precise determinations, minor and trace elements should preferably be separated from the sample solution to minimize interferences from the major elements. Recently, chelating resins have been applied for the separation and preconcenfxation of trace elements from large amounts of natural and sea waters [4--G] and from solutions containing high salt contents [7, 81. In comparison with solvent extraction, t.his technique is convenient and free from contamination. The determination of copper in silicates by ordinary atomic absorption spectrometry after decomposition in a sealed Teflon vessel and separation on a chelating resin has already been described [ 91..

The present paper deals ttith separation on a chelating resin and the atomic absorption spectrometry of copper, nickel, zinc, and cadmium after decom- position of silicates in a sealed Teffon vessel. The method has been applied successfully to the determination of these metals in standard silicate samples.

276

EXPEHWESTAL

Apparatus and eqwpmcnl

A Hitachi atomic absorption spectrophotomcter, Model 518, with an acetylene-air flame was used for copper, nickel. and zinc, and an atomic absorption spectrophotometer having a long absorption tube f 10) witir a hydrogen qir flame was used for cadmium‘ The decomposition of sample w&q ctilrd out in a scaled Teflon vessel of about 8.mI volume (Fig. 1). The chelating resin wdas used in a polypropylene column (BioRnd. Cat. No. 7311110).

Chebling resin Chelcx 100 (BioRad, 100.200 mesh, tiium form) was used. Before use,

the resin was rc;flatediy treated with 4 M hydrochloric acid and 4 M ammonia solution. The resin in the ammonium form was packed into the column (7 mm X 30 mm. 0.4 g dry weight) and finally conditioned with 0.1 M mnlonic acid solution of pli 6. The exchange capacity of the resin was 1.73 mhl Cu 8“ (dry wcigbt), determine-d by compIcximctric titration of copper.

Sit&, hydrochloric, and malonic acids and aqueous ammonia were obtain- ed from Wake Pure Chemicals Co., and hydrofluoric acid from Toshtba. All rcqcnts used were of super-pure grade except for mdonic and hydrofluoric acids which were of reagent grade. Standnrd solutions (1000 ppm) were pre- p.ared by dissolving copper, tine. and cadmium met&s (99.99955, hfitsuwa Chhemicals Co.,) and nickel oxide (Johnson Matthcy, Specpure) in hydra chloric or nitric acids. Working solutions were obtained by diluting the standard solutions with 0.1 M hydrochloric arid.

277

Into the Teflon vessef, place 0.5 g of powdered sample, 1 ml of aqua regia end 5 ml of 46% hydrofluoric acid. After sealing and standing for 36 h at 25”C, add 2 ml of 2.5 M malonic acid solution, and transfer the contents to a Teflon beaker with distilled water_ Without removal of any residue, adjust the pH of the sob&ion to 6 with concentrated ammonia liquor and dilute to ca. 50 ml with distilled water. After centrifugation, wash the residue with 5 m! of 0.05 M malonic acid solution of pH 6 containing about 1.3 M fluoride. Pass the supematant liquid and the washings tbrougb the column at a flow rate of 3 ml min-‘, wash the column with 5 ml of distilled water, and elute the trace elements with three IO-ml portions of 2 M nitric acid. Evaporate the combined eluents to dryness, dissolve in 10 mf of 0.1 M hydrochloric acid, end determine the concentration of metals in the solution by atomic absorption spectrometry with calibration curves.

RESULTS AND DISCUSSION

l.kmr2positbn of silicates The powdered silicates were completely decomposed with smah amounts

of aqua regia and hydrofluoric acid in the vessel at 25°C after 16 h; the decomposition time of about 36 h is recommended for security. This oper- ation is very simple and free from airborne contamination.

Effect of pH and masking agents on adsorption of m&&s Of the metals in silicates, copper, nickel, zinc, cadmium, cobalt, and lead

are of particular interest. Preliminary experiments showed that these metals are completely adsorbed on the resin from Michaelis buffer or simple solutions in the pH range 3-9 for copper and nickel and 4-9 for lead, zinc, cadmium, and cobalt (Fig. 2). The order, Cu > Ni - Pb > Zn - Co - Cd, is consistent

20-

t I I I , I

3 4 5 6 7 6

PH

1

Fig. 2. Aci~orption curves of metals fz-om Michaelis buffer or simple solutions. TOW

voIume,50ml.OCu;oZn;~M;DCd;~Co;~Pb.

with that of the condition;?i stability constants. /(MY*. of the mdds with ~.(pheny~methy~)im~~~~tic acid, which is tbe function&I goup of the resin. These metals adsorbed on the r&n are completely &ted wit!! two 10.~1 portions of 2 M nitric acid.

Of the major elements in ~ilicatcs, iron and aluminum interfere wkh the actsorption of trace elements for two rasons: the Iarge conditional stability co;umt of the iron(Ilf) complex with ~-(pheny~me~yl)imin~ia~etic acid. and the 0351? of hydr:*lysis of iron and aluminum at ?L mthpr low pH range, which causes coprecipitation of the trace elements with these hydroxides. Masking is therefore necessary. Of the masking agents studied, i.e. glycine. triethanoiamine, malonic, WC, and citric acids, malonic acid is the best. Previously [S J. citric acid was used as the masking agent and/or auiliary complexing agent for iron and aluminum during the adsorption of copper at pH 4. Copper was completely adsorbed at pH 3--9, nickc*i, rint, and cobalt rrt pH 7-9, cadmium at pli 7-8, and lead at plf 8-43 from 0.1 l citrak solution containing 100 mg of Fe and 1.8 g of NH,F in 50 ml (Fig. 31. How. cvcr, in practice, most of these? metals were only partly adsorbed even at pH 7-8 from the dig&cd solution of silicates. From 0.1 hf maionate solution containing 100 mg of Fe and 1.8 p of N&F in 50 ml. thesr! met& WPCM com- pl~bIy adsorbed on the resin at pH values above 5.5 (F&. 4). In both cases, lead was adsorbed onfy at higher pti vahcs in tic presence of fluoride ion. By taking the side-reaction coefficients for &he metal complexes with malonatc into account, the order, Cu > Ni > Zn > Co, is consistent with that of the condition& stnbility constants for the N-(yhenylmethyf)iminodiacetic acid ccmplexcs (Table 1). At pH 7, hydrolysis of iron and aluminum does occur. Thus tie optimum pH range for adsorption of metals is 55-6.6 (Fig. 4).

In the supemntant solution obtained after the decomposition of silicates, the cobalt ion seems to be present in the divalent and trivalent states. C)nfy

,_ _ _ . . . _ - _.---.-.- __-_

279

20 - ::, I= I

i I , , I *

3 c 5 6 7 PH

Fig. 4. Adsorption curves of metals from 0.1 M malonate sotution. Conditions and symbols as for Fig. 2. Above pH 7, hydrated iron oxiide precipitates.

Canditional stability constants (fog FL&‘) for the copper, nickel, zinc, snd cobalt complexes with IV-(phenyfmethyI)immodiacetie acid in the presence of 0.1 M mrrtonic acid

pH Cu Ni Zn co

3 2.51 1.4s 0.70 -0.13 4 2.95 2.3s 1.72 Lt.11

z 2.75 3.25 2.82 2.30 I.69 2.22 1.44 2.07 7 4.26 3.82 3.23 3.08 8 5.13 4.69 4.10 3.35 9 5.69 5.25 4.66 4.52

“Kssy = ~h¶Y~/~M’f~Y’] = K~~p&u~~., The stability constants rrsed are taken from ref. II.

about 75% of the cobalt, presumably divalenf was adsorbed on the resin at pH 6 from malonate solution; cobalt(II1) is not adsorbed. Cobalt can be completely adsorbed if the supernatant liquid is evaporated t9 dryness and dissoived in hydrochforic acid. Lezd in siIicates is included completely in the residue obtained during the decomposition; for lead fluoride is coprecipitzted . with calcium and alurnimmxl fluorides. From these reasons and for simplicity, only copper, nickel, zinc, and ca.dmium were determined in this study.

Recovery, reproducibility, and accuracy for four metals Tabfe 2 shows the reproducibility and accmcy obtained for the four

meta& in the standard rock JB-1. The recoveries obtained by addition of stvldard solutions to this sample are shown in Table 3.

260

TABLE 2

Hrproducibtlity and accurrcy (n - 20) for lht rundud rock J&l (SOO.mg samples) -.,- .-_ -_---_ ._ - - --_-

EJrmont hltan - s Htported r~ur RtW*tw (PPm) (ppm) (S)

_ . _. _- _-_ _ __ __. ._._-_-_- ._._ _ cu 52.3 : 2.1 55.7 95 Cd 0.13 - 0.02 0.11 114 Ni 131-5 135 97 ir? HO.6 9 3.6 HI 96 _____ - - ._ -- -- __- -. - .- --.----

TABLE 3

itecovrry of copper. odmium. nickel. and ttnc (n - 5) added to umplcs (500 mg) of standard rock JR-1 - . - - -- -- -._----A---- --

q cmcot Rewnl Addrd Found RItOWry (WI) (rc) (a) (96)

-- .- - -. - .- ._ __._ _ _. - - _... -_ -- cu 26.4 25.0 51.6 100 a 0.065 0.050 0.11 97 Ni 65.5 60.0 127 101 Zn 40.3 40.0 79.3 93 ,_-.- ..- _ _. --__ ___-_-_- -

TARLE 4

:tt~~lca for trace metals in rmdard rthmte rocks (All rrrulU arc gtrcn In ppm)

- .-_ . -____---_ ---_ - -_ SIliCdle CU

--__ -- -.-_- Ni Cd zn ---._-

Found Rtporttd Found Rtportid Found Reported Found Reporta --_-__ _-.-_- ____ --_ .- .-.-_---._-- -_-

0.w JG.1 2.3 3.9 0.04 0.046 0.4 9.2 39.6 JR.1 62.9 53.7 0.13 0.114 131 135 60 6 t:

.a2 JA.lf 41.0 -’ 0.09 0.6 - 83.6 -

261 - 0.13 - 12.0 - 104 -

L’SGS G.2 9.1 11.7 0.02 O.OJ9 1.6 5.1 60.1 6b CSP,l 31.a J3.J 0.03 0.06 6.0 121 97.6 98 AGV.1 37.6 39.7 0.07 0.09 13.9 16-6 83.6 84 W-l 111 110 0.17 O.lb 74.8 76 92.3 66 DCR.1 1 b.8 16.4 0.14 0.12 9.9 10.8 119 120 DTS-1 69 7.0 O.OJ 0.12 2260 2269 28.4 4s KG1 9.s Il.3 0.04 0.1 1340 WJ9 ao.2 36

WRbl SY-2 2.9 6.6 0.23 - 0.7 11 226 247 SY.3 17.3 la 0.24 - 7.2 11 22b 24b MRG-1 134 13s 0.19 -- 190 1*x 174 167

-B---m - --- - - --_ _-___ _ --- -. ._-__ _- -- .

l .%Tv armpIe.

281

R.esults for copper, nickel, zinc, and cadmium in standard silicate samples of USGS, GS3, and CSRM are summarized in Table 4. These values are in reasonably good qeement with the data reported by Flanagan [12], dndo et al. [13J, and Abbey et al. fl4].

The proposed method of decomposition of silicates at room temperature is simple, free from airborne contamination, and excellent for separating trace elements from major elements without using !arge amounts of reagents.

REFERENCES

I B. Bernas, Anal. Chem., 80 (1968) 1682. 2 R. T. T. Rantala and D. l-l, rLoring, At. Abs. New& 12 (1973) 97, 3 D. E. Buckley and R. E. Cram&one, Chem. Geol., 7 (29il) 275. 4 D. G. Biechler, AnaI. Chem., 37 (1965) 1054. 5 J. P. Riley and D. Taylor, Anal. Chim. Aeta, 40 (1968) 479. 6 hl. 1. Abdullah and L G. Royle, Anal. Chim. Acta, 58 (1972) 283. 7 l-l. lmoto, Bunseki Kagsku. 10 (1961) 1334. 8 R. Kawabuchi, M. Kanke, T. Muraoka, and M. Yamauchi, Bunscki Kagaku, 25 (1976)

213. 9 T. Uchida, M. Nagase, and C. Iida. Anal. Lett., 8 (X975} S25.

10 T. Uchida and C. Iida, -4~~1. Spcctrosc.. 29 (1975) 57_ 11 L. G. Sillen and k E Martell, Stability Constants of Metal-Ion Complexes, The

Chemical Society, London, 1964, Supplement No. 1. 1971. 12 F. J. Flanagan, Geochim. Cosmochim. Acta, 37 (1973) 1169. 13 A. Ando, -y1 Kurasawa, T. Ohmori, and E. Takeda, Geochem. J., 8 (1974) 175. 14 S. Abbey, A. H. Gill&on, and G. Perrauit. A Report on the Collaborative Analysis of

Three Canadian Rock Samples for Use as Certified Reference Materials, 1975.