574 Analyst, July, 1976, Vol. 101, fip. 574-578

Simultaneous Extraction and Concentration of Cadmium and Zinc from Soil Extracts

A. H. C. Roberts, M. A. Turner and J. K. Syers Department of Soil Science, Massey University, Palmerston North, New Zealand

A method for the simultaneous extraction and concentration of cadmium and zinc from soil extracts is described and evaluated. The procedure, using dithizone - carbon tetrachloride extraction a t pH 4.5, is simple and reliable, giving an essentially quantitative recovery of cadmium and zinc added to calcium chloride extracts of several contrasting soils. The method was used to evaluate the effect of long-term superphosphate fertiliser addition on the cadmium and zinc contents of soil.

Current interest in health-related aspects of cadmium in the environment1-6 and evidence that soil and fertiliser materials can act as potential sources of cadmium in human foods728 have focused attention on the paucity of information on the reactions of cadmium in soil - water systems. Relatedly, the antagonistic effects of cadmium and zinc in certain enzyme reactions,g and the natural association of these two elements,loJ1 point to the importance of understanding the interactions of cadmium and zinc in soil - water systems.

Attempts have been made to describe the reactions of cadmium and zinc with soils by using a sorption isotherm approach.12-14 In most previous studies, the solution concentrations of cadmium and zinc have usually been much higher than those encountered in extracts of soils. In order to determine the metals when present a t such low concentrations, pre- concentration procedures followed by atomic-absorption spectrophotometric analysis are required. Commonly, concentration procedures for cadmium and zinc involve chelating agents together with appropriate organic so1vents.16J6 Concentration and extraction under alkaline conditions have been used for c a d m i ~ m , l ~ * ~ ~ whereas concentration and extraction under acidic conditions have been used for z i n ~ . ~ ~ ~ ~ * Although 1,5-diphenylthiocarbazone (dithizone) has commonly been recommended for the colorimetric determination of both cadmium and zinc,l9 its use as a concentrating reagent prior to atomic-absorption analysis is less well documented.

This paper describes and evaluates a simple and effective procedure for the simultaneous extraction and concentration of low amounts of cadmium and zinc from soil extracts for routine analysis by atomic-absorption spectrophotometry. The procedure was used to investigate the possible accumulation of cadmium and zinc in a soil with a known, long-term history of superphosphate application.

Experimental Reagents

with three successive 20-ml volumes of 0.01 M hydrochloric acid. Dithizone solution, 0.01 yo m/V in carbon tetrachloride. Purified by shaking 500-ml volumes

Hydroxylammonium chloride solution, 10% m/V. Ammonia solution, 25% VlV. Hydrochloric acid, redistilled, 3.1 M. Stock cadmium solzction, 8.9 MM cadmium as cadmium chloride in 0.1 M hydrochLoric acid. Stock zinc solution, 15.3 mM zinc as zinc acetate in 1 M acetic acid. Sodium acetate bufer solution, 0.5 M. Adjusted to pH 5.0 with redistilled hydrochloric acid

Calcium chloride solution, 0.01 M. Combined working standards for cadmium and zinc were prepared from the stock solutions

All glassware was soaked in hydrochloric acid and thoroughly rinsed with double-distilled,

and extracted with five successive 10-ml aliquots of the purified dithizone solution.

to cover the concentration range 0-1 pg ml-l.

de-ionised water prior to use.

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ROBERTS, TURNER AND SYERS 575 Procedure

The procedure is designed to make a 10-fold concentration of cadmium and zinc, simul- taneously, from aqueous solutions or soil extracts.

To a 50-ml aliquot of sample extract in a 100-ml beaker, add 10 ml of 0.5 M sodium acetate buffer and 1 ml of 10% hydroxylammonium chloride solution. The hydroxylammonium chloride is used to prevent the oxidation of dithizone during e~tract i0n. l~ Adjust the pH to 4.5 with redistilled hydrochloric acid or ammonia solution using a pH meter. Rinse the pH electrodes with distilled, deiionised water and transfer the solution quantitatively into a 250-ml separating funnel. Extract twice with 5-ml aliquots of dithizone solution. During each extraction, shake the separating funnel vigorously by hand for 1 min. Allow the aqueous and organic phases to separate and transfer the organic phase to a second separating funnel. To the combined organic phases add 5 ml of 0.01 M hydrochloric acid to back-extract cadmium and zinc into the aqueous phase. Shake for 1 min, discard the organic phase and determine the cadmium and zinc concentrations in the aqueous phase using atomic-absorption spectro- photometry. Prepare appropriate standards of cadmium and zinc by using the same procedure.

Evaluation of Procedure $H of extraction

The effect of extraction pH on the recovery of small amounts of cadmium and zinc from de-ionised water was evaluated for unit pH increases over the pH range from 3.0 to 11.0. At each pH step the recovery of 2.5 pg each of added cadmium and zinc was determined. The buffers used to maintain the desired pH were: 0.5 M sodium acetate (pH 3.0-7.0), 10% sodium citrate (pH 8.0-9.0) and 17% potassium sodium tartrate (pH 10.0-11.0). Extractions at each pH value were replicated five times. The recoveries of cadmium and zinc were deter- mined by comparison with standards prepared in de-ionised water.

Recovery from water The simultaneous recovery of cadmium and zinc from de-ionised water was determined

by extracting and concentrating the metals from solutions containing 0.25, 0.50 and 1.00 pg each of cadmium and zinc using the developed procedure. Three replicates were used and the recoveries determined as above.

Recovery from soil extracts Four surface soils, sampled at 0-10 cm depth, were selected so as to provide a range of

soil components of potential importance in cation sorption : Egmont black loam (allophane), Okaihau gravelly clay (crystalline and short-range order iron oxides), Makerua peaty silt loam (organic matter) and Tokomaru silt loam (crystalline layer silicates).

Extracts were obtained by shaking each soil with 0.01 M calcium chloride solution for 2 h at 20 "C, using a soil to extractant ratio of 1: 10. The suspensions were centrifuged at 12 500 rev min-l for 10 min at 20 "C and the supernatant liquids passed through a Millipore filter ((0.45 pm). Aliquots (50 ml) of the soil extracts were then extracted and concentrated using the developed procedure. The recoveries of added cadmium and zinc were determined for amounts of 1.0 and 2.5 pg, added to the soil immediately prior to the extraction and concentration step. All determinations were made in triplicate and the recoveries determined as the difference between the cadmium and zinc contents of the soil extracts in the presence and absence of the added metals.

Variability study The variability of the procedure when employed for extracting and concentrating cadmium

and zinc from calcium chloride extracts of Makerua peaty silt loam and New Plymouth black loam was assessed. The within-day variability of the method was investigated using 18 replicate determinations of the amounts of extractable cadmium and zinc. For between- day variability, triplicate determinations were made on each of five consecutive days. In these studies the conditions of extraction were as before. The results were analysed statis- tically using a one-way analysis of variance.

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576 ROBERTS et al. : SIMULTANEOUS EXTRACTION AND CONCENTRATION Analyst , VoZ. 101

Efect of fertiliser addition on cadmium and zinc contents of soil The method was used to assess the change in cadmium and zinc status of Papatoetoe

silt loam that had received regular top-dressing of 36 kg ha-l of phosphorus as superphosphate fertiliser for a period of 30 years. The fertilised soils, together with an adjacent unfertilised control area, were sampled at depths of 0-5 and 5-10 cm. The amounts of extractable cad- mium and zinc were determined using a 2-h extraction time at 20 "C and a soil to extractant ratio of 1 : 10. Total cadmium and zinc were determined by digesting 0.3 g of soil with 10 ml of a 1 + 1 mixture of hydrofluoric and nitric acids and evaporating the solution to dryness on a water-bath. The digestion was repeated three times, after which the soil residue was dissolved in 50 ml of 0.31 M hydrochloric acid. Total zinc was determined directly on the digest. For determination of cadmium, the solution was concentrated five-fold by extracting 30 ml of the digestion solution with four 10-ml aliquots of the dithizone - carbon tetrachloride extracting mixture. The combined organic extracts were back-extracted into 6 ml of 0.01 M hydrochloric acid, as described previously.

Results and Discussion The simultaneous recovery of cadmium and zinc (both in 2.5-pg amounts) from a de-

ionised water system was essentially quantitative over the pH range 4.0-7.0, values ranging from 92 to 102% (Table I). Based on these results, an extraction pH of 4.5 was used for all subsequent studies. The use of a common pH value for the simultaneous extraction and determination of cadmium and zinc contrasts with previous reports that have recommended an alkaline extraction for c a d m i ~ m ~ ~ J ~ and an acidic extraction for z i n ~ . ~ ~ J ~ An essentially complete recovery of small amounts of cadmium and zinc added to de-ionised water was obtained using the developed procedure (Table 11). Recoveries ranged from 90 to lOOyo at the three levels of addition used.

Adeau

TABLE I EFFECT OF pH ON EXTRACTION AND RECOVERY OF CADMIUM AND ZINC

FROM DE-IONISED WATER

PH 3.0 4.0 5.0 6.0 7.0 8.0 9.0

10.0 11.0

Recovery, yo Cadmium -

56 65 102 102 98 98 98 92 98 98 89 102 88 95 87 89 2 2

te recoveries were also obtained when cadmium and zin were add d"to calcium chloridk extracts of four contrasting soils and simultaneously extracted (Table 111). There was no indication from these results that soluble constituents from the soils interfered with the extraction and concentration of either cadmium or zinc. The results obtained in the variability study (Table IV) indicate that the developed procedure gave acceptable results for the within- and between-day extractability of cadmium and zinc from two soils.

TABLE I1 RECOVERY OF CADMIUM AND ZINC FROM DE-IONISED WATER

Addedlpg Recoveredlpg Recovery, % Addedlpg Recoveredlpg Recove;, % Cadmium Zinc

A A f \ r

0.25 0.25 100 0.25 0.25 100 0.50 0.46 92 0.50 0.45 90 1.00 0.94 94 1.00 0.94 94

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July, 1976

RECOVERY

Soil

O F CADMIUM AND ZINC FROM SOIL EXTRACTS

TABLE I11 577

O F CADMIUM AND ZINC ADDED TO CALCIUM CHLORIDE EXTRACTS OF SOILS

Cadmium Ziiic - 1 Added/pg Recovered/pg Recovery, yo Recovered/ pg Recovery,

Egmont black loam 0 0.05 - 0.75 - 1.00 1.15 110 1.85 110 2.50 2.80 110 3.25 100

Makerua peaty silt loam 0 0.05 1.00 1.10 2.50 2.65

Tokomaru silt loam 0 0.01 1.00 1.00 2.50 2.60

Okaihau gravelly clay 0 0.84 1 .oo 1.10 2.50 2.65

- 1.00 - 105 1.95 95 104 3.45 98 - 1.15 - 100 2.10 95 104 3.40 90 - 3.10 - 102 4.10 100 103 5.40 92

The regular and long-term application of superphosphate fertiliser to Papatoetoe silt loam did not produce a measurable change in the total cadmium content of the soil (Table V). There was a suggestion of an increase in the amount of extractable cadmium resulting from fertiliser addition. In contrast, total zinc showed an increase in the fertilised soil but this was not paralleled by a corresponding increase in extractable zinc. These results are a t variance with previous reports,'~~ which indicated that superphosphate fertiliser addition substantially increased the cadmium content of soils. For both fertilised and unfertilised Papatoetoe silt loam, the amounts of cadmium extracted by calcium chloride solution were generally much lower than those recorded for cadmium extracted by ammonium chloride in a range of unfertilised Australian soils.8

TABLE IV WITHIN- AND BETWEEN-DAY VARIABILITY IN AMOUNTS OF CADMIUM AND ZINC

EXTRACTED FROM SOILS BY CALCIUM CHLORIDE

Cadmium Zinc -7 -

Source of A.V. A.V. Soil variation CLg g-l results* p g 8-l results*

Makerua peaty silt loam Within-day 0.022 0.004 - 0.249 & 0.020 - Between-day 0.030 f 0.009 n.s. 0.238 0.010 t

New Plymouth black loam Within-day 0.027 & 0.005 - 0.891 & 0.060 - Between-day 0.029 & 0.003 n.s. 0.945 & 0.050 n.s.

* A.V. = analysis of variance; n.s. = not significant. t P < 0.05.

Evaluation of the developed procedure indicates that cadmium and zinc can be extracted and concentrated simultaneously from de-ionised water and from aqueous extracts of soil. The procedure is simple and reliable, and up to 30 samples can be fully processed for cadmium and zinc determination in an average working day. The sensitivity of the procedure is essen- tially that of the atomic-absorption spectrophotometer. With the use of a 1: 10 soil to

TABLE V EXTRACTABLE AND TOTAL CADMIUM AND ZINC CONTENTS OF PAPATOETOE SILT LOAM

Cadmiumlpg 8-1 Zinc/pg g-l Depth of - &

Soil sample/cm Extractable Total Extractable Total Unfertilised 0-5 0.004 0.60 0.080 143.2

5-10 0.004 0.20 0.082 136.5 Fertilised 0-5 0.006 0.60 0.080 158.2

5-10 0.006 0.40 0.081 158.2

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578 ROBERTS, TURNER AND SYERS

extractant ratio and a 50-ml aliquot of extract, it is possible to determine the very small amounts of cadmium and zinc present in the equilibrium solutions of soils.

This research was supported by the Faculty of Agricultural and Horticultural Sciences, Massey University, and by the University Grants Committee in the form of a scholarship to A.H.C.R. The assistance of Dr. J. Rogers and Mr. F. Muller, New Zealand Fertilizer Manufac- turers’ Research Association, in providing the samples of Papatoetoe silt loam is appreciated.

1. 2. 3. 4.

5. 6. 7. 8. 9.

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12. 13. 14. 15.

16. 17. 18. 19.

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American Association for the Advancement of Science Publication No. 85, 1967, p. 343. Schroeder, H. A., J . Chron. Dis., 1965, 18, 647. Schroeder, H. A., Nason, A. P., Tipton, I. F., and Balassa, J. J., J . Chron. Dis., 1967, 20, 179. Schroeder, H. A., and Balassa, J . J., Science, N.Y., 1963, 140, 819. Williams, C. H., and David, D. J., Aust. J . Soil Res., 1973, 11, 43. Vallee, B. L., and Ulmer, D. D., A . Rev. Biochem., 1972, 41, 91. Sandstead, H. H., in “Geochemistry and the Environment,” National Academy of Sciences, Wash-

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John, M. K., Can. J . Soil Sci., 1972, 52, 343. Shuman, L. M., Proc. Soil Sci. Soc. Am., 1975, 39, 454. Tiller, I<. G., Honeysett. J. L., and De Vries, M. P. C., Aust. J . Soil Res., 1972, 10, 165. Morrison, G. H., and Freiser, H., “Solvent Extraction in Analytical Chemistry,” John Wiley &

Mulford, C. E., Atom. Absorption Newsl., 1966, 5, 88. Williams, C. H., David, D. J., and Iismaa, O., Commun. Soil Sci. Plant Analysis, 1972, 3, 399. Westoo, G., AnaZyst, 1963, 88, 287. Sandell, E. B., “Colorimetric Determination of Traces of Metals,” Interscience Publishers, New

Received December 30th, 1975 Accepted January 27th, 1976

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