extracción secuencial-Tack_1999

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INTRODUCTION

Sequential extraction is frequently applied for the frac-tionation of solid phase associated elements in soils andsediments (Pickering, 1981; Lund, 1990; Ure, 1990a;

Tack and Verloo, 1995). Apart from conceptual prob-lems associated with the use of sequential extraction(e.g., non-selectivity of the extractants, redistributionof trace element among phases during extraction),proper sample handling and preparation (e.g., freezedrying, oven drying) remains a major practical prob-lem, because it critically influences the results of asequential extraction (Rapin et al., 1986). For anoxicsediments, the necessity of maintaining anoxic condi-tions during sampling, sample treatment and extractionhas been clearly evidenced (Rapin et al., 1986;Wallmann et al., 1993).

An approach where the same information is gainedfrom separate, single extractions can be advantageous:sample preservation is critical only before adding thereagent and during one extraction, there is no risk forsample losses during consecutive phase separations and

washing steps, not all work is lost when something goeswrong during an extraction step, and finally, results canbe obtained faster because the extractions can becarried out simultaneously. Drawbacks include thatmore sample is needed and that errors may be intro-duced due to sample heterogeneity.

Gupta et al. (1990) developed a scheme consistingof several single extraction to substitute for sequentialextraction of metals in sewage sludge by the procedureof Stover et al. (1976). They found a good agreementin the metal fractionation patterns, obtained by the twomethods. Bendell-Young et al. (1992) compared a

Chemical Speciation and Bioavailability, (1999), 11(2) 43

Single extractions versus sequential extraction for

the estimation of heavy metal fractions in reduced

and oxidised dredged sediments

Filip M.G. Tack* and Marc G. Verloo

Laboratory of Analytical Chemistry and Applied Ecochemistry, University of Gent, Coupure Links 653, B-9000Gent, Belgium

ABSTRACT

Sequential extraction is applied to estimate the chemical association of trace elements in soils and sediments.An approach where the same information is gained from single extractions would be advantageous: samplepreservation is critical only before and during one extraction, there is no risk for sample losses during consecutivesteps and results can be obtained faster. The drawbacks are that more sample is needed and that sampleheterogeneity may introduce errors. Five soil or sediment samples were subjected to sequential extractionaccording to Tessier et al. and to single extractions, using the reacting conditions of the sequential steps.Estimates of the acid extractable, reducible and residual fractions from single extractions generally agreed withthese determined by sequential extraction. The oxidisable fraction should be determined by extraction of theresidue of a hydroxylaminehydrochloride extraction. The differences observed between determined andestimated fractionation would not seriously affect interpretation of the results. To estimate metal fractions in soilsand sediments, the use of single extractions rather than sequential extraction according to Tessier et al. couldbe justified for practical purposes.

Keywords: Heavy metals, sequential extraction, single extraction, fractionation, speciation, sediment.

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simultaneous extraction procedure for low carbonatesediments with the method of Tessier et al. (1979). Wecompared sequential extraction according to Tessier et

al. (1979) of a high alkalinity reduced dredged sedi-ment with the results, obtained from single extractionsthat used reacting conditions similar to these of thesequential steps (Tack et al., 1996). The estimates ofthe exchangeable, acid extractable and reducible frac-tions were in good agreement with the fractionsobtained using sequential extraction, while the oxidis-able fraction should be determined by extraction of theresidue of a hydroxilamine-hydrochloride extraction.In this report, results are presented for one morereduced sediment and three oxidised dredged sedimentderived surface soils, in order to support our previousfindings (Tack et al., 1996).

MATERIALS AND METHODS

Sampling and sample preparationSamples MG 1 and MG 2 were dredged materialderived surface soils. These were sampled to a depth of30 cm at confined disposal sites situated on the left bankof the deviation canal from the river Leie, at Meigem inBelgium. The site denoted as MG 1 is 10 years old.Experimental forestry plantations of ash tree (Fraxinussp.), Canadian flea-bane (Erigeron canadensis L.),water-willow (Salix caprea L.), great bindweed(Calystegia sepium R.Br.), and poplar (Populus sp.)were growing at the sampled area. Disposal site MG 3

received sediments 3 years before our sampling and wasdensely covered by stinging nettle (Urtica dioica L.).Two samples, GH 1 and GH 2, were taken at the

Geuzenhoek disposal site to a depth of 30 cm. SedimentGH 1 was disposed one year before our sampling. Asuperficial layer of approx. 3040 cm was partiallydried and oxidised and sparingly covered with weeds.Sample GH 2 was from a compartment where thesediment still was covered with 3050 cm water andwas in a reduced state. Sample K 1 was sampled dur-ing dredging operations from the Scheldt estuary. Theseresults were presented in our previous study (Tack etal., 1996) and are included here for comparison.

The reduced sediments (GH 2 and K 1) were stored

in closed containers under water and kept at ambienttemperature. The oxidised soils/sediments (MG 1,MG 3 and GH 1) were air dried during several days,hand-crushed using a mortar and sieved (2 mm).Selected physical and chemical properties of the sedi-ments studied are presented in Table 1.

Chemical analysisSediment pH was measured potentiometrically in asuspension of 10 g sediment in 50 mL distilled waterafter 12 hrs. Carbonate content was determined by backtitration of an excess of 0.5 mol L1 HCl added to 1 g

oven dried and 2-mm sieved sample with 0.5 mol L1

NaOH (Nelson, 1982). Organic carbon was estimatedby the Walkley-Black method (Nelson and Sommers,

1982). Particle size fractionation was performed withdry and wet sieving techniques (Gee and Bauder, 1986).

Sequential extractionThe sediments were sequentially extracted accordingto the Tessier method (Tessier et al., 1979). The resid-ual fraction was determined after aqua regia destruc-tion (Ure, 1990b). Aliquots of the sample were sub-jected to single extractions using similar reactingconditions as the steps of the sequential extraction.

The extracting conditions applied are outlined inTable 2. The extractions were performed on the equiv-alent of 3.00 g sediment dry matter in 250 mL poly-ethylene centrifuge tubes. The sediment dry weight was

determined on a separate portion by drying at 120Cuntil constant weight. To prevent oxidation duringextraction of the reduced sediments, the tube wassealed with laboratory film after flushing the head spacewith N2-gas. This precaution was taken during the firstthree steps of the sequential extraction procedure only.After each extraction step, the suspension was centri-fuged (1,500 g during 20 min) and the supernatant solu-tion removed using a syringe. The remaining solidswere suspended in 24 mL deionised water, that wasseparated in a similar way and discarded.

Metal concentrations in the supernatant solutionwere determined with flame atomic absorption (Varian

AA-1475 or SPECTRAA-10). Calcium was measuredwith flame emission (Eppendorf ELEX 631). For eachextract, external standards, prepared in the corre-sponding extraction solution, were used for calibration.The exact volume of the extract after each extractionstep was determined by weighing the centrifuge tubejust before sampling the supernatant liquid. Extractionswere replicated three times for the oxidised sedimentsMG 1, MG 3 and GH 1, and repeated four times forGH 1 and K 1.

RESULTS AND DISCUSSION

According to the fractionation scheme of Tessier et al.

(1979), metals are categorised as exchangeable,bound to carbonates, bound to iron and manganeseoxides, bound to organic matter and residual. Theextractions, however, are not so specific as previouslystated (Tack and Verloo, 1995). The correspondingfractions will therefore be referred to as exchange-able, acid extractable, reducible, oxidisable andresidual, respectively.

Exchangeable fractionThe exchangeable fraction consists of metals that arereadily leached by a neutral salt. In soils and sediments,

Single extractions versus sequential extraction for the estimation of heavy metal fractions44

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this metal fraction is usually low and for many metals,concentrations in the extracts were below the detectionlimit of flame atomic absorption (Table 3). In the oxi-dised sediments GH 1, MG 1 and MG 2, as opposed tothe reduced sediments, the exchangeable fractions of Cd,Ni, and Zn were significant, illustrating that metals tend

to be mobilised when sediments are brought in uplandconditions (Gambrell, 1994).Acid extractable fractionThe acid extractable fraction is obtained by extract-

ing the soil or sediment in an acetic acid/acetate bufferat pH 5 (NaOAc-extraction). In principle, this stepshould selectively dissolve carbonates and therebyrelease associated metals (Tessier et al., 1979). Metalsreleased in that step may also include species that aremore strongly adsorbed on different solid phases of thesystem and hence were not released at pH 7 during theprevious step (Kim and Fergusson, 1991).

When single NaOAc-extraction is applied to a soilor sediment, reaction conditions are such that metals of

the exchangeable fraction will also be released. Theacid extractable fraction hence can be estimated fromthe difference between NaOAc-extraction and MgCl2-extraction. This is supported by the overall good agree-ment between the acid extractable fraction determinedby sequential extraction and that, estimated from

single extractions (Table 4). For MG 1 and MG 2,standard errors of the estimation of acid extractable Cdwere large compared to that of its determination. Thestandard error of the exchangeable fraction in thesesediments (Table 3) affected the precision of the esti-mate of the relative small acid extractable fraction fromsingle extractions.

In sediment GH 1, acid extractable Pb determined bysequential extraction was only half the amount esti-mated by single extractions (Table 4). This was perhapsrelated to the sediment being only partially oxidised.Iron hydroxides may have formed during the MgCl2-extraction. Lead can then sorb onto these hydroxides

Filip M.G. Tack and Marc G. Verloo 45

Table 1 Characteristics of the sediments studied

GH 1 G H 2 K 1 MG 1 MG 2

Granulometry (%)0-2 m 29 15 34 37 472-50 m 57 31 57 50 51> 50 m 14 54 9 13 2

pH-H2O (1:5) 7.3 7.5 7.3 7.4 7.2

Organic carbon (%) 5.0 0.3 3.9 0.3 5.4 0.1 3.7 0.4 4.9 0.4CaCO3

(%) 10.7 0.7 11.0 0.1 15.3 0.3 9.1 0.5 7.7 0.6

Mean SD of 3 replicates

Table 2Experimental extraction conditions used per gram reduced sediment

MgCl2-extraction 8 mL 1 mol L1 MgCl2 (pH 7), 1 h, room temperature, continuous shaking

NaOAc-extraction 8 mL 1 mol L1

NaOAc + HOAc (pH 5), 5 h, room temperature, continuous shakingNH2OH.HCl-extraction 20 mL 0.04 mol L1 NH2OH.HCl in 25% HOAc, 6 h, 95C, intermittent shaking

H2O2-extraction 3 mL 0.02 HNO3 + 2 mL 30% H2O2 (pH 2), 2 h, 85C, intermittent shaking; 3 mL 30% H2O2 (pH 2),3 h, 85C, intermittent shaking; 5 mL 3.2 mol L1 NH4OAc in 20% HNO3 + 7 mL H2O, 30 min, roomtemperature

Aqua regia destruction 7.5 mL 37% HCl + 2.5 mL 65% HNO3, overnight at room temperature, 2 h heating under reflux

Table 3Exchangeable fraction determined by MgCl2-extraction (mg kg1 dry matter SD)

Metal GH 1 GH 2 K 1 MG 1 MG 2

Ca 5200 140 3258 190 4140 150 5100 360 10300 690Cd 2.5 0.4 < 0.07 < 0.07 4.6 1.3 7.2 1.8Co 3.1 0.2 < 0.3 < 0.3 < 0.3 < 0.3Cu < 0.2 < 0.2 < 0.2 < 0.2 < 0.2Fe 18 8 370 15 10 1.5 9 5 11 7Mn 111 18 138 15 37 1 6 1 7 3Ni 1.6 0.3 < 0.3 < 0.3 2.2 0.3 3.5 0.7Pb 3.5 0.2 < 0.6 < 0.6 < 0.6 < 0.6Zn 37 4 < 0.04 < 0.04 25 2 30 4

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and be released less effectively by NaOAc duringsequential extraction. This may be supported by com-paring Fe and Mn concentrations in the exchangeablefraction of reduced and oxidised sediments (Table 3).While Mn in the exchangeable fraction was two tothreefold higher than in the fully oxidised sediments

MG 1 and MG 2, Fe was in the same range. On the otherhand, both Fe and Mn were high in the reduced sedi-ments GH 2 and K 1.

Reducible fractionThe reducible fraction is determined after extractionwith acidic NH2OH.HCl. Conceptually, this reagent isdesigned to release amorphous ironmanganese oxidesand associated metals (Tessier et al., 1979). If appliedas a single reagent, it will, because of the low reagentpH, release adsorbed and carbonate associated metalsas well. The reducible fraction hence could be esti-mated by the difference of the NH2OH.HCl-extractableand the NaOAc-extractable amounts (Table 5).

Estimates of the reducible fraction agreed in mostcases with the determination using sequential extrac-tion. Significant differences occurred primarily forthe soils MG 1 and MG 2. These have the highest claycontent among the sediments studied (Table 1).

If Ca is considered indicative for the amounts ofcarbonates dissolved, then it may be concluded thatsimilar amounts of carbonates were released by sequen-tial extraction compared to the single approach.Differences in dissolved Fe between the reducible frac-tion determined by sequential extraction and that esti-mated from single extractions indicate that the extrac-

tion efficiency of iron oxides was affected by the pro-cedure used.

For soils MG1 and MG 2, significantly more Fe wasreleased in the reducible fraction of sequential extrac-tion than estimated from single extractions. For K 1, onthe other hand, Fe was significantly lower, while

differences in Fe were not significant for sediments GH1 and GH 2. This behaviour is not readily explained asseveral factors may influence the extraction efficiencyof iron oxides. The final pH during single NH2OH.HCl-extraction will usually be higher than in sequentialNH2OH.HCl-extraction because more acid is needed todissolve carbonates that, in the sequential approach,have been dissolved during the preceding NaOAc-extraction. The final pH of the single extraction withNH2OH.HCl was 2.81 and 2.72 for soils MG 1 andMG 2, respectively, as compared to 2.39 and 2.43 forthe reducible step in sequential extraction. Duringsingle extraction, an undisturbed sediment matrix iscontacted with NH2OH.HCl. Iron oxides may, for

example, be coated by carbonate phases and hence bedissolved more slowly than in sequential extraction.These factors may result in a lower extractability ofFe during single extraction than during sequentialextraction. On the other hand, effects of oxidation dur-ing sequential extraction of reduced sediments mayresult in a lower Fe extractability during the reduciblestep of sequential extraction compared to that estimatedfrom single extractions. In particular for stronglyreduced sediments such as K 1, much stricterprecautions than those we took may be required to pre-vent any oxidation from occurring during the subse-


Table 4Acid extractable fraction determined (D) by sequential extraction and estimated (E) by subtraction of NaOAc-extractable andMgCl2-extractable amounts (mg kg

1 dry matter SD)


Ca D 31,500 860 21,900 2,400 37,000 4,600 30,100 480 26,900 240E 34,400 1,400 24,500 2,100 33,100 2,500 31,100 1,700 28,200 2,000

Cd D 5.2 0.3 < 0.7 < 0.7 3 0.9 2.8 0.1E 5.4 0.6 < 1 < 1 1.7 3.0 1.7 2.9

Co D 6.5 0.4** 2.4 0.5 4.2 0.3* 2.1 0.1 2.9 0.2E 3.3 0.6 2.2 0.4 3.4 0.2 3 1 3.6 1

Cu D 7.0 2.5 < 0.2 < 0.2 12.0 0.7 23.8 0.7**E 7.7 0.5 < 0.3 < 0.3 12.6 2.4 27.5 1.4

Fe D 33 1 7,100 80 4,970 1,100 21 2 32 2E 31 9 7,400 440 3,400 720 16 6 31 9

Mn D 260 4 890 99 510 45 97 1 118 1*E 260 26 800 71 460 31 100 6 89 9

Ni D 9.3 0.6 5.6 0.6* 6.8 0.6 4.9 0.5 16.9 0.4E 8.8 0.9 7.1 0.8 7.4 0.5 4.4 2.1 14.1 2.0

Pb D 17 2** < 0.6 11 2 15 2 20 2E 28 2 < 0.9 13 2 13 4 20 2

Zn D 740 27 160 23* 50 15 410 3* 580 16E 725 33 200 17 55 9 340 36 510 38

The significance of the difference between determined and estimated amounts was evaluated using an independent sample Ttest at the95% (*) or the 99% (**) significance level.

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quent extraction and washing steps of the sequentialprocedure.

For the other metals there generally was no signifi-cant difference between the determined reducible frac-tion and that, estimated from single extractions. Adifference in Pb for sample K 1 was slightly significant.

For Cu, amounts extracted during sequential extractionwere significantly higher. However, the amountsreleased in that fraction are low compared to theamount associated with the subsequent, oxidisable frac-tion and will therefore not influence the overall distri-bution pattern of Cu.

Oxidisable fractionThe oxidisable fraction is determined after destructionof organic matter with hydrogen peroxide at pH 2(Table 6). Because of the acidity, adsorbed metals andcarbonate associated metals can be expected to bereleased when H2O2-extraction is applied to the freshsediment (Table 7). A comparison of Ca extracted with

H2O2 and acid extractable Ca (Table 4) suggests thatthe efficiency in releasing carbonates was at leastsimilar to NaOAc-extraction.

Because reacting conditions are oxidising rather thanreducing, only approx. 10% of iron that may bereleased with NH2OH.HCl is dissolved by single H2O2-extraction (Table 7). Sediment K 1, which was stronglyreducing, was an exception. Iron sulphides present mayexplain the high levels of Fe extracted by H2O2 (Tessieret al., 1979; Shanon and White, 1991). Metals areextracted with a varying efficiency and may be as lowas the sum of the exchangeable and acid extractable

fractions (e.g. Zn in GH 1), or higher than the sum ofthe exchangeable , acid extractable and reducible frac-tions (e.g. Pb in K 1). For example, single H2O2-extrac-tion is able to efficiently release metal sulphides includ-ing the portion that may also be released byNH2OH.HCl (Shanon and White, 1991). In contrast,

metals bound by iron oxides are not released by singleH2O2-extraction, although they are efficiently releasedby NH2OH.HCl. Estimation of the oxidisable step fromsingle extractions is therefore not possible. The oxidis-able fraction may, however, be determined on theresidue of the hydroxylamine hydrochloride extraction.

Residual fractionThe residual fraction may be estimated by subtractingthe NH2OH.HCl-extractable amounts and the oxidis-able fraction from the total content (Table 8). In mostcases, there was no significant difference between thedetermined residual content and the contents evaluatedby difference. The standard deviations of the estimates

by single extraction moreover were in the range of thestandard deviations of the sequential extraction. For Zn,larger standard deviations on the estimates resultedfrom the standard deviations on the high acid extract-able and reducible fractions.

Metal fractionationThe fractionations of selected metals, determined usingsequential extraction and estimated from single extrac-tions, are compared in Figure 1. Determined and esti-mated fractionation agreed fairly. Although differenceswere in several occasions statistically significant, these


Table 5Reducible fraction determined (D) by sequential extraction and estimated (E) by subtraction of NH2OH.HCl-extractable andNaOAc-extractable amounts (mg kg1 dry matter SD)


Ca D 6,750 190 13,440 2,300 10,700 3,000 5,100 42 4,904 67*E 4,600 1,500 10,100 4,600 14,800 3,600 2,380 1,800 < 2,000

Cd D 3.3 0.5 3.7 0.3* 9.6 0.3 4.5 1.3 3.4 0.2E 2.7 0.6 4.3 0.3 10.1 0.4 6.5 2.9 2.7 2.3

Co D 5.8 0.7** < 0.9 8.2 0.8 5.5 0.1 4.6 0.1*E 10.7 1.1 1.2 0.7 7.6 1.1 11.0 3.8 12.0 2.4

Cu D < 0.5 < 0.5 21.6 1.3** 17.4 2.3* 14.8 2.5**E 0 0.9 < 0.5 11.6 0.6 1.4 4.3 0 1.8

Fe D 7,700 100 7,400 1,300 13,600 930** 7,860 200** 9,500 320**E 7,330 250 9,800 7,900 19,600 720 5,800 44 6,800 470

Mn D 306 10* 600 62 366 36 527 14** 483 16**E 378 21 940 360 376 450 166 42 157 62

Ni D 17.6 0.8 9.3 0.4 9.9 0.8 16.9 1.0 45.5 3.0E 15.7 1.1 9.7 2.2 11.0 0.7 15.1 2.3 41.9 3.8

Pb D 50 4 62 6 64 8* 47 4 40 4E 44 5 55 6 79 2 52 6 49 5

Zn D 670 13 1,400 49 435 26 760 20 895 9E 670 42 1,480 150 440 15 770 120 820 120


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would not seriously affect interpretation of the results.As discussed before, most of these differences wererelated to the propagation of errors in the fraction esti-mates rather than to systematic differences in extrac-tion efficiency between the sequential and the singleextraction approach.

CONCLUSIONS

The data confirmed that the estimation of the exchange-able, acid extractable, reducible and residual fractionsfrom single extractions on separate subsamples wasin general equivalent to their determination using


Table 6 Oxidisable fraction determined by H2O2-extraction (mg kg1 dry matter SD)


Ca 353 10 1375 220 670 51 459 48 399 48Cd < 0.2 < 0.2 < 0.2 < 0.2 < 0.2Co < 0.9 < 0.9 10 0.1 < 0.9 < 0.9Cu 105 5 51 4 115 3 67 1 114 3Fe 950 28 80 20 9500 1200 1300 63 1760 44Mn 43 2 58 5 49 1 35 2 38 2Ni 9.3 0.4 < 0.8 14.7 0.3 6.0 0.3 14.0 0.2Pb 21 4 28 4 92 10 10 1 < 1.4Zn 154 3 205 8 90 2 121 12 143 9

Table 7Metals released during single reagent extraction with H2O2 (mg kg1 dry matter SD)


Ca 31,200 990 30,000 860 53,900 720 27,800 660 26,000 550Cd < 0.2 < 0.2 11.0 0.3 7.1 0.7 8.0 1.2Co 5.3 0.2 < 0.9 15.0 0.1 5.9 0.5 7.2 0.1Cu 59 4 32 1 104 1 49 2 87 1Fe 120 6 68 10 24,000 280 131 13 132 10Mn 489 16 942 54 940 4 182 58 144 8Ni 11.6 0.5 9.3 0.5 18.5 1.7 9.4 0.5 20.9 0.6Pb 17 1 27 1 165 1 19 3 18 2Zn 840 28 1,150 25 505 5 470 50 620 60

Table 8Residual fraction determined (D) by sequential extraction and estimated (E) by subtraction of NH2OH.HCl-extractable amountsand the oxidisable fraction from aqua regia extractable amounts (mg kg1 dry matter SD)


Ca D 274 13 261 34 328 10 249 43 269 47E < 2000 < 4000 < 3000 < 3000 < 2500

Cd D < 0.9 < 0.9 < 0.9 < 0.9 < 0.9E < 1.2 1.1 1.1 < 0.4 < 1.1 < 1.4

Co D 5.7 0.5* < 4 11.3 1.3** 7.6 3.1 12.4 2.1E 2.4 0.9 6.0 1.1 4.0 1.5 7.1 5.6 9.5 5.2

Cu D 44 3 23 4 27 1** 47 8 66 6E 54 5 23 5 < 5.0 57 10 95 17

Fe D 24,900 1,100 23,400 8,100 30,700 680* 3,750 2,500* 41,000 3,000E 22,700 1,600 15,800 7,900 26,500 2,600 25,000 6,700 28,200 10,000

Mn D 83 2** 150 44 103 4 82 6 84 4E < 16 < 370 150 440 80 82 120 64

Ni D 19 1** 12 3 28 3* 22 3 26 5E 11 1 13 2 19 3 20 4 22 9

Pb D 245 11 420 33 23 1* 169 2 190 21E 245 8 380 36 < 10 180 19 200 18

Zn D 76 5* 100 32 66 2 166 42 146 26E < 35 190 160 45 17 < 160 30 190


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sequential extraction for both reduced and oxidisedsediments containing high levels of carbonates andorganic carbon. The oxidisable fraction should be esti-mated from a H2O2-extraction on the residue of theNH2OH.HCl-extract.

Fractions estimated from single extractions agreedin most cases with these determined using sequentialextraction. Significant differences that were observedin some cases would not seriously affect interpretationof the results. The use of single extractions instead of


Figure 1Metal fractionation of Cd, Co, Cu. Ni, Pb and Zn as a percent of total metal content, determined by sequentialextraction (D) and estimated by single extractions (E).

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sequential extraction to estimate metal fractions in soilsand sediments hence can be justified for practicalpurposes.

ACKNOWLEDGMENTS

We wish to thank Herwig Vossius and Nico THooft fortheir valuable contributions to the experimental workand for the clarifying discussions.

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Paper received: 12 August 1997; accepted 22 September1997.


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