Use of a certified reference material for extractable trace metals to assess sources of uncertainty in the BCR three-stage sequential extraction procedure

Analytica Chimica Acta 382 (1999) 317±327

Use of a certi®ed reference material for extractable trace metals to assess sources of uncertainty in the BCR three-stage sequential extraction procedure A. Sahuquilloa, J.F. LoÂpez-SaÂncheza,*, R. Rubioa, G. Raureta, R.P. Thomasb, C.M. Davidsonb, A.M. Ureb a Departament de QuõÂmica AnalõÂtica, Universitat de Barcelona, Avd. Diagonal 647, E-08028, Barcelona, Spain Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, UK

b

Received 30 June 1998; received in revised form 30 September 1998; accepted 12 October 1998

Abstract Various potential sources of irreproducibility in the BCR three-stage sequential extraction procedure have been investigated using the lake sediment CRM 601. Of the variables considered, the pH of the hydroxylamine hydrochloride in Step 2 proved most important. Factors such as the type of acid used in pH adjustment, the temperature and duration of extraction, and working under nitrogen did not affect precision, although they did alter the amounts of metals extracted. Improved precision was obtained when the hydroxylamine hydrochloride concentration was increased from 0.1 to 0.5 mol lÿ1 and when the speed of the centrifugation was increased from 1500 to 3000  g. The addition of a MgCl2 wash between the steps of the extraction procedure gave rise to increased uncertainty. Although it did not adversely affect reproducibility, the use of ®ltration to separate solid and liquid phases was not recommended since it promoted dissolution of non-target phases. Neither ammonium hydrogen oxalate nor oxalic acid proved suitable alternatives in Step 2 owing to precipitation of insoluble lead salts, particularly in the presence of calcium. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Operational speciation; Certi®ed reference materials; Trace metals; Sequential extraction; Sediment

1. Introduction There is considerable interest in the certi®cation of reference materials for environmental analysis [1]. However, the usefulness of a certi®ed reference material (CRM) in validation of analytical methodology depends critically on how well the certi®ed values are established. When materials are prepared for certi®*Corresponding author. Tel.: +34-93-402-90-83; fax: +34-93402-12-33; e-mail: [email protected]

cation of total metal content it is generally relatively straightforward to specify recommended values and their associated uncertainties. Special dif®culties exist, however, when the species to be determined are isolated via an operationally-de®ned procedure. Small operational variations can lead to non-comparability; and it is vital that a suf®ciently detailed description of the analytical procedure is provided, and rigorously followed, if harmonious results are to be obtained from any certi®cation or other interlaboratory study.

0003-2670/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0003-2670(98)00754-5

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Extractable metal content in soils is usually determined using single extractants, whereas procedures such as sequential extraction are mainly applied to sediments. Results obtained by sequential extraction are particularly susceptible to irreproducibility since errors can easily be propagated between steps. CRMs certi®ed for metals extractable by single and sequential extraction procedures are amongst those produced under the auspices of the EU Standards, Measurement and Testing Programme (formerly the Community Bureau of Reference (BCR)) [2]. There has been recent interest in factors which in¯uence the quality of these materials, including particle size distribution [3] and stability [4]. In the framework of BCR, a three-step sequential extraction procedure has been developed [5] and a sediment reference material (CRM 601), has been produced which is certi®ed for metals (Cd, Cr, Cu, Ni, Pb and Zn) extractable by the scheme [4,6]. In the protocol, the sample is ®rst treated with acetic acid to liberate exchangeable/acid-extractable metals (Step 1), then metals associated with the reducible phase are solubilised using hydroxylamine hydrochloride (Step 2). In Step 3, metals released by oxidation with hydrogen peroxide are isolated in 1.0 mol lÿ1 ammonium acetate. In CRM 601, concentrations of extractable Cd, Cr, Ni, Pb and Zn are certi®ed in Step 1, but only Cd, Ni and Zn in Step 2, and Cd, Ni and Pb in Step 3. Indicative (but not certi®ed) values for extractable Cu in Step 1 and Pb in Step 2 are also given [6]. The remaining metals could not be certi®ed due to high variability between results obtained by different laboratories. The BCR procedure has been successfully applied to a variety of matrices, including lake [7], lagoon [8] and marine sediments [9±11], sewage sludge [12], soil [13] and industrially-contaminated made-up ground [14]. Some workers, however, reported dif®culties with the scheme, including lack of phase selectivity [15,16], redistribution of analytes between phases [17] and variability between operators, which was attributed to small variations in the pH of the hydroxylamine hydrochloride solution [18]. Such studies highlighted the need for re®nement of the sequential extraction procedure. This paper describes a systematic study of variables identi®ed as potential sources of uncertainty in use of the BCR sequential extraction scheme. The aim was to

improve reproducibility. The variables studied were: extractant pH, extraction temperature, extraction time and extractant concentration, together with the effects of the type of acid used in pH adjustment, working under nitrogen, and the method used to separate liquid and solid phases. The use of alternative reagents was also considered. The study focused on Step 2 of the sequential extraction since previous work had indicated that this was a major source of uncertainty. 2. Experimental 2.1. Sample description The sediment used in this study was BCR CRM 601. The material came from Lake Flumendosa (Sardinia, Italy) and was prepared and homogenised at the Joint Research Centre, Ispra (Italy). The major components of the sediment (determined by X-ray ¯uorescence spectrometry) are shown in Table 1, together with other characteristics. Total Cd, Cu, Cr, Ni, Pb and Zn were determined by inductively-coupled plasma atomic emission spectrometry (ICP/AES) following total open, acidic attack in a sand bath using mixtures of HNO3/H2O2 and HF/HClO4. 2.2. Sequential extraction 2.2.1. Reagents Extractants were prepared according to the following procedure. All dissolutions and dilutions were performed with double deionised or distilled water. Solution A (acetic acid, 0.11 mol lÿ1): 25  0.2 ml of redistilled glacial acetic acid (or, for example, Merck AnalaR/Suprapur grade without distillation) was added, in a fume cupboard, to about 0.5 l of water in a 1 l polyethylene bottle and made up to exactly 1 l with further water. 250 ml of this solution (acetic acid 0.43 mol lÿ1) was diluted to 1 l to obtain an acetic acid concentration of 0.11 mol lÿ1. Solution B (hydroxylamine hydrochloride, 0.1 mol lÿ1): 6.95 g of hydroxylamine hydrochloride was dissolved in 900 ml of water. The solution was acidified with concentrated nitric acid to pH 2.0 and made up to 1 l. This solution was prepared on the same day as the extraction was carried out.

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Table 1 Characteristics of lake sediment BCR CRM 601 Compound a

SiO2 CaOa Al2O3a Fe2O3a MgOa P2O5a K2 O a MnOa TiO2a Na2Oa Nb Ctotalb Corganicb Cinorganicb Cdc Cuc Crc Nic Pbc Znc

Content 48.78 5.73 13.93 7.27 2.19 0.88 2.59 0.16 0.77 0.61 0.63 6.38 4.60 1.79 11 240 148 72 231 824

Standard deviation

% CV

Number of determinations

0.28 0.04 0.12 0.05 0.05 0.02 0.04 0.005 0.02 0.009 0.01 0.06 0.06 0.06 0.30 2.90 2.00 4.00 4.80 10.00

0.58 0.66 0.87 0.69 2.42 1.82 1.60 3.12 3.00 1.43 1.76 0.93 1.24 3.35 2.8 1.2 1.3 0.6 2.1 1.2

20 20 20 20 20 20 20 20 20 10 20 20 20 20 4 4 4 4 4 4

a

Determined by X-ray fluorescence spectrometry and given as percentage. Given as percentage. c Determined after total acidic digestion and given in mg kgÿ1 dry weight. b

Solution C (hydrogen peroxide, 8.8 mol lÿ1): hydrogen peroxide was used as supplied by the manufacturer i.e. acid-stabilised to pH 2.0±3.0. Solution D (ammonium acetate 1.0 mol lÿ1): 77.08 g of ammonium acetate was dissolved in 900 ml of water. The solution was acidified to pH 2.0 with concentrated nitric acid and made up to 1 l. 2.2.2. Sequential extraction procedure Before opening any bottle of sediment, it was manually shaken for 5 min to rehomogenise the contents. Samples were taken with a plastic spatula. The extraction was performed in 100 ml, precleaned borosilicate glass, PTFE or polypropylene centrifuge tubes using an end-over-end mechanical shaker operating at 30 rpm, in a room at 20  28C. The temperature of the room was measured at the start and at the end of the extraction procedures. The sequential extraction procedure is described below: Step 1: 40 ml of solution A was added to 1 g of sediment in a 100 ml centrifuge tube and shaken for 16 h at room temperature (overnight). No delay

occurred between the addition of the extractant and the shaking beginning. The extractant was separated from the solid residue by centrifugation (1500  g) and decantation of the supernatant liquid into a high density, polyethylene container. The container was stoppered and the extract either analysed immediately or stored at 48C. The residue was washed by adding 20 ml of water, shaking for 15 min and finally centrifuging the resulting suspension. The supernatant was decanted and discarded, taking care not to discard any of the solid residue. The `cake' obtained upon centrifugation was broken manually or by using a vibrating rod prior to the next step. Step 2: 40 ml of solution B was added to the residue from Step 1 in the centrifuge tube, and the extraction was performed as described above. Step 3: 10 ml of solution C was added carefully, in small aliquots to avoid losses due to violent reaction, to the residue from Step 2 in the centrifuge tube. The tube was covered with a watch glass and the contents digested at room temperature for 1 h with occasional manual shaking. The digestion was continued by

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heating the covered tube for 1 h at 858C in a water bath, then the cover was removed and the volume reduced to a few ml. A further aliquot (10 ml) of solution C was added. The tube was heated again (858C for 1 h) then the watch glass was removed and the volume reduced to a few ml. 50 ml of solution D was added to the cool residue, which was extracted as described above. Blank extractions i.e. without sediment, were carried through the complete procedure for each set of analysis and using the same reagents. For correction to dry mass, a separate, 1 g sample of sediment was dried in an oven at 105  28C for 2 h or until constant mass was achieved (i.e. successive weighings differed by less than 1 mg). 2.3. Metal determination The metal contents of the extracts were determined by atomic spectrometry. Flame atomic absorption spectrometry (FAAS), electrothermal atomic absorption spectrometry (ETAAS), and ICP/AES were used. FAAS was performed with either a Unicam PU 9100 spectrometer or a Perkin-Elmer 1100B system. Both instruments were equipped with deuterium background correction which was used for all analytes except Cr, since the deuterium lamp output was too low at 357.9 nm. A fuel-lean air±acetylene ¯ame was used, except for the determination of Cr which required a fuel-rich air±acetylene ¯ame or a nitrous oxide±acetylene ¯ame.

ETAAS was performed with a Perkin-Elmer 4100ZL system with longitudinal Zeeman effect background correction, transverse heating and pyrolytic graphite-coated tubes with a built-in L'vov platform. An AS-70 autosampler was used for sample introduction. The instrumental conditions were optimised (pyrolysis and atomisation curves) for both standard solutions and sediment extracts (Table 2). ICP/AES was performed with a Perkin-Elmer Plasma II system equipped with a cross-¯ow nebuliser and Scott-type dual pass spray chamber. The operating parameters were as follows: RF power 1.2 kW (27 MHz); observation height 9 mm; sample uptake rate 1.0 ml minÿ1; gas ¯ow rates: nebuliser 1.0 l minÿ1, coolant 15 l minÿ1, auxiliary 1.0 l minÿ1. The analytical wavelengths (nm) used were: Cd 214.4 (ICP/AES) and 228.8 (AAS); Cr 267.7 (ICP/ AES) and 357.9 (AAS); Cu 231.6 (ICP/AES) and 324.8 (AAS); Ni 232.0; Pb 220.3 (ICP/AES) and 217.0 (AAS); Zn 213.9. 3. Results and discussion 3.1. Effect of the pH of the extractant Ten solutions of 0.1 mol lÿ1 hydroxylamine hydrochloride (NH2OH
HCl) were prepared and adjusted to different pH values, within the range pH 1.0±3.0, with concentrated HNO3. These were used to extract

Table 2 Measurement conditions for ETAAS Instrumental conditions (Step 2: 0.1 mol lÿ1 NH2OH)

Cd

Cr

Cu

Ni

Pb

Wavelength (nm) Lamp intensity (mA) Slit-width (nm) Type of signal

228.8 6 0.7 Peak area

357.9 25 0.7 Peak area

324.8 15 0.7 Peak area

232.0 25 0.2 Peak area

283.3 10 0.7 Peak height

Programme of temperatures Drying 1 Drying 2 Pyrolysis Atomisation Cleaning Matrix modifier

110/1/20a 130/5/30 500/10/20 1200/0/5 2400/1/2 Pd 50 mg lÿ1

110/1/25 130/15/30 1300/10/20 2100/0/4 2400/1/2 No

110/1/20 130/5/30 1000/10/20 1900/0/5 2400/1/2 No

110/1/20 130/5/30 1000/10/20 2300/0/5 2400/1/2 No

110/1/20 130/5/30 400/10/20 1500/0/5 2400/1/2 No

a

Temperature (8C)/ramp time (s)/hold time (s).

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Fig. 1. Effect of the pH of the hydroxylamine hydrochloride solution on the extractability of Cd (^ pH of extractant; & pH of extract).

replicate portions of residues obtained following Step 1 of the sequential extraction of CRM 601. Cd, Ni and Zn were found to be less sensitive to pH variations than Cr, Cu and Pb, which showed a dramatic decrease in both extractability and reproducibility as pH increased. Figs. 1 and 2 illustrate the behaviour for one metal representative of each group. Cd extractability decreased by some 35% over the pH range studied, with a coef®cient of variation (CV) of less than 8% for n ˆ 4 measurements at each pH value. For Ni and Zn, a decrease in extractability of about 65% was observed with increasing pH. The CV values were generally <10%, although a few higher values (up to 19%) were obtained for Ni. Almost no Cu, Cr or Pb was removed from the sediment when the pH of the extractant was greater than 1.8, and the CV values were high (25±53%).

For this sediment, there were some pronounced variations in metal extractability around pH 2.0, the value recommended in the BCR procedure. This variability decreased if the pH of the extractant was lowered to pH 1.5. This was partly due to the increased amounts of metals extracted, but also to a decrease in the ability of the sediment matrix to alter the pH of the liquid phase during the extraction. This `pH shift' was always apparent and meant that the pH of the ®nal sediment extract was signi®cantly greater than that of the extractant added (see Figs. 1 and 2). When the initial extractant pH was around 1.0, the shift was only about 0.5 pH, but around pH 2.0 the pH shift increased by up to 1.5. This effect was probably due to the increased buffering capacity of the extractant at lower pH. It should be stressed that the changes of pH for other samples will be dependent on their composition,

Fig. 2. Effect of the pH of the hydroxylamine hydrochloride solution on the extractability of Cr (^ pH of extractant; & pH of extract).

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i.e. calcareous sediments will produce pH shifts greater than the siliceous ones. This work demonstrated that pH is of paramount importance. Hence, an incorrect pH adjustment (as might be obtained with an improperly calibrated pH meter) could be a major source of irreproducibility, particularly between laboratory variance, in the BCR procedure. An alternative method of pH adjustment, in which a ®xed volume of dilute (2 mol lÿ1) nitric acid was used, was tested at both pH 1.5 and pH 2.0. The variability observed in the pH values obtained by different laboratories, on different days and with different operators, was very small (<0.03 pH). Thus, the addition of a ®xed volume of dilute nitric acid to adjust the pH of the NH2OH
HCl was proposed. 3.2. Effect of the type of acid used in pH adjustment The use of concentrated nitric and hydrochloric acids to adjust the pH of the NH2OH
HCl was compared. No signi®cant difference in reproducibility was observed at either pH 1.5 or pH 2.0, but the use of HCl caused an increase of up to 100% in the amounts of copper and nickel extracted at pH 1.5. This may be due to the formation of soluble chloride complexes. Since the extent of complex formation could differ between sediments, and could not be controlled, the use of nitric acid was preferred. 3.3. Effect of extraction temperature Six replicates of the residues remaining at the end of Step 1 of the sequential extraction were extracted at three different temperatures: room temperature (208C), 268C and 408C. Temperatures of 268C and 408C were maintained by submerging the shaker assembly in a water bath controlled by a thermostat.

The amounts of metals extracted are shown in Table 3. Results were subjected to statistical analysis (one-way ANOVA test at 95% con®dence level). No signi®cant differences were found between the mean concentrations of Cd and Cr released at the three temperatures, but the effect of temperature was signi®cant for the other analytes. Three different statistical tests (Cochran's, Bartlett's and Hartley's tests) [19] were applied to investigate possible differences between the standard deviations obtained at different temperatures. The uncertainties were found to be insigni®cantly different for Cd, Cr, Cu, Ni and Zn, but signi®cantly different for Pb, which showed the highest % CV at 268C. It was concluded that temperature did not markedly affect the overall reproducibility, but did in¯uence the amounts of metals extracted. In general, as might be expected, greater amounts were extracted at the highest temperature, 408C. 3.4. Effect of extraction time Two Step-1 residues were shaken with 0.1 mol lÿ1 NH2OH
HCl for 2, 4, 8, 12, 16 and 24 h. No signi®cant trends in metal extractability with time were observed, except for Cu. The amount of Cu extracted increased up to 8 h, then remained constant for longer shaking periods. The dispersions obtained with different extraction times were all visually similar. It, therefore, appeared that extraction time did not have a pronounced effect on release of metals from the reference sediment, and that the 16 h extraction was adequate. 3.5. Effect of an inert atmosphere It is conceivable that the presence of atmospheric oxygen may interfere with the reduction step (Step 2)

Table 3 Effect of temperature on reproducibility and metal extractability Metal (mg kgÿ1)a

T ˆ 208C

% CV

T ˆ 268C

% CV

T ˆ 408C

% CV

Cd Cr Cu Ni Pb Zn

2.18  0.08 0.27  0.06 0.84  0.14 3.29  0.28 8.57  1.08 136  4.37

3.5 24 17 8.6 13 3.2

2.06  0.04 0.20  0.07 0.54  0.12 2.73  0.32 5.96  1.65 131  8.87

2.1 35 23 12 28 6.8

2.10  0.13 0.32  0.10 0.95  0.13 4.55  0.58 33.4  4.60 198  6.3

6.1 32 14 13 14 3.2

a

Results are expressed as mean values  one standard deviation, for n ˆ 6.

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Table 4 Effect of working under an inert atmosphere (N2) Metal (mg kgÿ1)a

Normal procedure

% CV

Inert atmosphere

% CV

Cd Cr Cu Ni Pb Zn

2.18  0.08 0.27  0.06 0.84  0.14 3.29  0.28 8.57  1.08 136  4.37

3.5 24 17 8.6 13 3.2

2.15  0.16 0.29  0.08 0.61  0.23 3.07  0.53 8.22  2.11 144  11

7.4 28 37 17 25 7.6

a

Results are expressed as mean values  one standard deviation, for n ˆ 5.

in the BCR extraction scheme and hence give rise to irreproducibility. To investigate this, sample manipulation was carried out inside a glove-box ®lled with nitrogen. The residue from Step 1 of the extraction was stored in the glove-box prior to addition of the reductant, and the NH2OH
HCl was prepared under nitrogen. Equivalent sets of extractions (®ve replicates of each) were performed, at the same time, under nitrogen and in normal laboratory air. Results are shown in Table 4. Statistical tests indicated no signi®cant difference between either the amounts of metals extracted, or the standard deviations in the amounts, under the two sets of conditions. Since it gave no improvement in either accuracy or precision, but increased the complexity of the experimental procedure, the use of nitrogen was not recommended. 3.6. Effect of the method used to separate liquid and solid phases after extraction Different options for isolating the sediment extract were considered. These included the addition of a ®ltration step after centrifugation, altering the speed and time of centrifugation, and the use of a MgCl2 washing solution between steps. A ®ltration stage, and optimisation of the centrifugation procedure, were studied to determine whether losses of particulate material between the steps of the sequential extraction contributed to irreproducibility. The MgCl2 was investigated as a more vigorous alternative to the water wash which is used in the BCR procedure to minimise carry-over of metals released during one step of the extraction into the next step. Vacuum ®ltration was assessed using two types of ®lter, Whatman type 542 ®lter paper and Whatman

`Cyclopore' polycarbonate membrane ®lters (0.4 mm pore size). In each case, the ®lter used was added to the centrifuge tube and extracted, along with the residue, in the subsequent stage of the sequential extraction. When results obtained were compared with those from the conventional procedure (centrifugation only) the recoveries were close to 100% for Steps 1 and 2 (Table 5). However, signi®cantly higher concentrations of most analytes were extracted in Step 3. This was apparent for both types of ®lter, but more pronounced for the ®lter paper. A possible explanation is a catalytic effect of Fe(II) in the presence of carbon (derived from the ®lter) which promotes attack on more resistant phases of the sediment [20]. Since the ®ltration stage did not improve the precision of the procedure (i.e. similar % CV values were obtained in all steps) the use of centrifugation alone was recommended. An improvement in precision was obtained when the centrifugation speed was increased from 1500 to 5000  g due to easier separation of the phases and reduced loss of residue between steps. However, the inclusion of a MgCl2 washing step degraded the precision, especially for Cu and Pb (Fig. 3). Since many laboratories do not have access to a high speed centrifuge, the effect of increasing the centrifugation time from 10 to 20 and 30 min, at an intermediate and more widely available speed (3000  g) was assessed. A slight improvement in precision, over six replicate samples, was obtained for times greater than 10 min. 3.7. Effect of increasing the extractant concentration The effects of increasing the NH2OH
HCl concentration from 0.1 to 0.5 and 1.0 mol lÿ1 were

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Table 5 Effect of filtration of the extract after centrifugation Metal

Recovery with respect to normal procedure

Step 1a

% CV

Step 2

Cd

Cellulose paper Polycarbonate

0.77  0.15 1.01  0.19

19.5 18.8

±b 1.05  0.08

Cr

Cellulose paper Polycarbonate

± 1.01  0.23

± 22.8

± 0.97  0.15

Cu

Cellulose paper Polycarbonate

0.98  0.05 1.01  0.01

5.10 0.99

1.27  0.20 0.99  0.13

Ni

Cellulose paper Polycarbonate

0.89  0.20 1.00  0.08

22.5 8.00

± 0.94  0.17

Pb

Cellulose paper Polycarbonate

± 1.05  0.47

± 44.8

Zn

Cellulose paper Polycarbonate

0.98  0.02 1.07  0.12

Fe

Cellulose paper Polycarbonate

Mn

Cellulose paper Polycarbonate

a

% CV ± 7.62

Step 3

% CV

± 1.14  0.09

± 7.89

± 15.5

1.11  0.05 1.20  0.14

4.50 11.7

15.7 13.1

1.24  0.05 1.17  0.08

4.03 6.84

± 1.81

1.83  0.36 1.19  0.06

19.7 5.04

1.08  0.12 1.01  0.10

11.1 9.90

1.30  0.05 1.02  0.31

3.85 30.4

2.04 11.2

1.01  0.02 1.00  0.05

1.98 5.00

1.63  0.10 1.13  0.10

6.13 8.85

1.11  0.08 0.99  0.04

7.21 4.04

1.04  0.03 0.99  0.03

2.88 3.03

3.70  0.20 1.68  0.87

5.40 51.8

0.96  0.03 1.00  0.02

3.12 2.00

0.98  0.04 1.00  0.04

4.08 4.00

2.41  0.13 1.13  0.04

5.34 3.54

Results are expressed as mean values  one standard deviation, for n ˆ 5. Results lower than detection limit for FAAS or ICP-AES.

b

Fig. 3. Effect of centrifugation speed and inclusion of a MgCl2 washing step.

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Table 6 Effect of increasing the hydroxylamine hydrochloride concentration in Step 2 Metal Step 1a (mg kgÿ1) Batch Ac

Batch B

Batch C

Batch A

Batch B

Cd Cr Cu Fe Mn Ni Pb Zn

4.45  0.14 0.34  0.03 10.2  0.2 51.5  2.3 312  10 8.06  0.28 3.32  0.60 262  8

4.42  0.03 0.25  0.05 10.4  0.2 31.8  1.6 287  3 7.74  0.54 2.23  0.36 266  3

2.73  0.11 0.42  0.06 2.70  0.29 941  25 127  4 4.72  0.45 12.4  0.8 151  6

3.09  0.04 3.03  0.13 0.87  0.09 1.28  0.10 6.04  0.19 12.8  0.6 1393  28 1356  84 136  1 142  3 5.36  0.41 5.38  0.2 65.7  2.6 118  5 176  2 165  9

4.20  0.55 0.27  0.05 10.4  0.06 50.9  1.5 301  4 8.43  0.44 2.90  1.00 275  7

Step 2b

Step 3b Batch C

Batch A

Batch B

Batch C

1.66  0.09 17.9  2.0 113  7 526  246 46.5  1.3 6.27  0.31 91.9  23.4 133  9

1.62  0.06 2.16  0.08 20.3  0.5 24.1  0.3 136  4 145  3 1412  16 1958  30 54.7  0.8 87.1  2.1 8.01  0.25 9.74  0.20 82.0  2.4 64.7  1.4 155  3 197  4

a

Results are the mean values of 15 independent determinations. Results are the mean values of five independent determinations. c Hydroxylamine hydrochloride concentration: batch A, 0.1 mol lÿ1; batch B, 0.5 mol lÿ1; batch C, 1.0 mol lÿ1. b

investigated (Table 6). Greater amounts of all the analytes were extracted when the concentration was increased to 0.5 mol lÿ1, but only Cr, Cu and Pb showed a further increase in extractability at 1.0 mol lÿ1. The increase in amounts of metal extracted at higher reductant concentrations may be due to more effective attack on the more refractory, crystalline oxyhydroxides as well as the amorphous forms. It should be noted that the use of a more concentrated reductant also increased the amounts of metals extracted in Step 3, except for Pb which showed a decrease. With respect to reproducibility, lower % CV values were obtained for both 0.5 and 1.0 mol lÿ1 NH2OH
HCl than for 0.1 mol lÿ1, but

there was no improvement at 1.0 mol lÿ1 with respect to 0.5 mol lÿ1. 3.8. Investigation of alternative reagents A large number of sequential extraction procedures exists (see [21] and references therein) and various alternative reagents have been used to isolate the reducible fraction of sediment samples. Two popular alternatives to hydroxylamine hydrochloride are ammonium hydrogen oxalate and oxalic acid. A potential limitation to the use of these reagents, however, is the precipitation of Cd and Pb from oxalate solutions, particularly in the presence of calcium.

Fig. 4. Comparison of metal solubilities in ammonium oxalate. aRatio of metal content in oxalate solution with respect to the control solution. Solution 1: 25 ml of metal solution ‡ 25 ml of ammonium hydrogen oxalate (pH ˆ 0.9). Solution 2: after solution 1 adjusted to pH 2.5. Solution 3: same as solution 1 except with 160 mg mlÿ1 Ca present. Solution 4: after solution 3 adjusted to pH 2.5.

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Calcium carbonate is a nominal target phase for Step 1 of the BCR scheme. However, in tests, calcium was still present in the extract obtained in Step 2 following three successive extractions with the Step 1 reagent (0.11 mol lÿ1 acetic acid). A solubility test was performed for Cu, Cd and Pb by adding 25 ml of a solution containing 120 mg lÿ1 of each metal (in dilute nitric acid) to 25 ml of a 0.2 mol lÿ1 solution of ammonium hydrogen oxalate or oxalic acid (pH 2.5) to give a mixture (pH 0.9). After 2 h, a precipitate had formed. This was ®ltered off and an aliquot (1 ml) of the ®ltrate analysed. The pH was then adjusted to 2.5 with ammonia and the solution left to stand overnight. It was ®ltered again and the ®ltrate analysed as before. The experiment was repeated in the presence of 160 mg lÿ1 calcium. The results obtained were normalised with respect to those obtained for a control solution, subjected to the same procedure, which contained metal ions but no reagent. As shown in Figs. 4 and 5, Cu solubility was virtually unaffected by the alteration in pH or the presence of calcium. Cd solubility was slightly reduced at pH 2.5 in both reagents and at pH 0.9 by the presence of ammonium oxalate and calcium. The greatest reduction occurred when both ammonium hydrogen oxalate and calcium were present at pH 2.5. A signi®cant amount of Pb was precipitated by both reagents under all conditions. It can thus be concluded that neither ammonium hydrogen oxalate nor oxalic acid are suitable for the replacement of

hydroxylamine hydrochloride in Step 2 of the BCR scheme. 4. Conclusions This study has revealed some sources of uncertainty in application of the BCR three-stage sequential extraction procedure to sediment samples and has indicated where modi®cations to the protocol are necessary. Of the variables considered, the pH of the extractant proved to be of paramount importance. The type of acid used for pH adjustment, the temperature and duration of the extraction, and working under nitrogen did not strongly in¯uence the precision of the results obtained in Step 2, nor did the incorporation of additional ®ltration or washing steps in the procedure. Some of these variables, however, did in¯uence metal extractability. In order to improve the reproducibility of the BCR scheme, Step 2 should be performed with 0.5 mol lÿ1 hydroxylamine hydrochloride adjusted to pH 1.5 by the addition of a ®xed volume of dilute nitric acid. The speed of centrifugation should be increased from 1500 to 3000  g. Acknowledgements AS and RPT acknowledge the ®nancial support of the EU, Standards, Measurement and Testing Pro-

Fig. 5. Comparison of metal solubilities in oxalic acid. aRatio of metal content in oxalic solution with respect to the control solution. Solution 1: 25 ml of metal solution ‡ 25 ml of oxalic acid (pH ˆ 0.9). Solution 2: after solution 1 adjusted to pH 2.5. Solution 3: same as solution 1 except with 160 mg mlÿ1 Ca present. Solution 4: after solution 3 adjusted to pH 2.5.

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