Application of sequential extraction

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Microchemical Journal 72 (2002) 9–16

0026-265X/02/$ - see front matter 䊚 2002 Elsevier Science B.V All rights reserved.

PII: S 0 0 2 6 - 2 6 5 X Ž 0 1 0 0 1 4 3 - 6

Application of sequential extraction and the ICP-AES method for

study of the partitioning of metals in fly ashes

Agnieszka Smeda, Wieslaw Zyrnicki*

Wroclaw University of Technology, Chemistry Department, Institute of Inorganic Chemistry and Metallurgy of Rare Elements,

Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland

Received 11 June 2001; received in revised form 11 September 2001; accepted 14 September 2001

Abstract

In this work, the original BCR extraction scheme was modified and applied to study the partitioning of metals in fly ashes In the first step, the water-soluble fraction was investigated here The next metal fractions were acid-soluble, reducible, and oxidisable Two kinds of coal fly ash certified reference materials were analysed Metal concentrations in the extracts were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES) The efficiency of the extraction process for each step was examined The partitioning of metals between the individual fractions was investigated and is discussed.䊚 2002 Elsevier Science B.V All rights reserved

Keywords: Sequential extraction; Fly ash; BCR scheme; Metal partitioning; Inductively coupled plasma atomic emission

spectrometry (ICP-AES)

1 Introduction

The interest in using coal in power plants to

produce electricity has not decreased in recent

years Coal ash is the fossil-fuel combustion

resi-due from coal power plants Deposits of fuel ashes

are a serious problem as a source of inorganic

pollution

Knowledge of the chemical and physical

prop-erties of the ashes is important to assess the risk

of potential environmental mobility of toxic trace

metals The availability and mobility of elements

*Corresponding author Tel.: 71-320-2494; fax:

q48-71-328-4330.

E-mail address:

zyrnicki@ichn.ch.pwr.wroc.pl (W Zyrnicki).

occurring in fly ashes depend on the physicochem-ical forms of the elements

Extraction methods are the tool for examination

of the element speciation There are several extrac-tion procedures reported in the literature, based on different sequence schemes w1–7x and carried out under various operating conditions w8–11x The approach developed by Tessier in 1979 w12x and so-called the BCR procedure w13x (proposed in

1993 by the European Community’s Bureau of References — now The Standards, Measurements and Testing Programme) are the most popular

schemes The BCR scheme has been elaborated to harmonise methodology and to enable the compar-ison of results from different laboratories The BCR scheme has been widely applied to various

Table 1 Instrumental parameters and operating conditions for ICP-AES

Plasma

Generator frequency 40 MHz

Spray chamber Scott type

Monochromator

Gratings 4320 and 2400 grooves ymm

Argon flow rates

Plasma gas 13 dm 3 ymin

Sheath gas 0.2 dm 3 ymin

Nebuliser gas 0.3 dm 3 ymin

Sample uptake 1.0 cm 3 ymin

Elements and analytical lines (nm)

matrices, e.g sewage sludge w8,10,11,14x, different

soils w9,15–18x, and marine w6,19x and river

sedi-ments w2,3,17,20,21x

So far, sequential extraction has rarely been

used to analyse fly ash samples w5,17x Distribution

of Cd in fractions of the coal fly ash NBS 1633a

w17x, and Cd, Cr, Cu, Pb, Zn and V in the fractions

of a brown coal w5x were recently studied with the

aid of atomic absorption spectrometry

In the present study, the partitioning of metals

(Al, Ba, Ca, Cr, Cu, Fe, Mg, Mn, Ni, Sr, V and

Zn) and Bhas been investigated using a sequential

extraction procedure with the aid of inductively

coupled plasma atomic emission spectrometry The

BCR extraction protocol has been modified by the

introduction of leaching with deionized water as

the first step Fractionation of the elements in the

coal ashes has been examined and is discussed

2 Experimental

2.1 Instrumentation

The concentrations of metals in the extracts

were measured by inductively coupled plasma

atomic emission spectrometry(ICP-AES) A

Job-in-Yvon sequential ICP spectrometer (JY 38S)

was used for measurements The instrumental

operating conditions are shown in Table 1

For extraction, a horizontal, mechanical

water-bath shaker was employed A centrifuge was used

for separation of the solid phase from the

extrac-tion liquid

2.2 Samples

Two certified reference materials were examined

here: CTA-FFA-1 (Fine Fly Ash CTA-FFA-1;

Polish Certified Reference Material for

multiele-ment trace analysis) and ENO No.12-1-01 (major

and trace elements in brown coal fly ash ENO

No.12-1-01; Slovak Certified Reference Material)

2.3 Reagents

Standard solutions were prepared by dilution of

a multielement standard solution (Merck, 1000

mgycm3) All reagents were at least of analytically

pure grade Hydroxylamine hydrochloride solution

(PPH-POCh, Gliwice, Poland) was prepared prior

to use For pH adjustments, nitric acid (65%,

Merck, Germany) was used All solutions were

prepared with deionized water (18.3 MV cm

resistivity, Barnstead Easy pure RF series 703)

Glass- and plasticware were cleaned in 10% HNO in an ultrasonic bath and then rinsed a few3

times with deionized water

2.4 Procedure

The four-step extraction procedure shown in Fig

1 was used here The following metal fractions were investigated: water-soluble forms removed by water (deionized); acid-soluble forms associated

with carbonates; reducible forms associated with oxides and hydroxides of Al, Mn and Fe; and oxidisable forms associated with organic matter or

A Smeda, W Zyrnicki / Microchemical Journal 72 (2002) 9–16

Fig 1 Schematic diagram of the sequential extraction

procedure.

sulfides(for more details see w19x) Five samples

(1 g) of each ash were placed in separate 50-ml

polypropylene tubes For each step of the

extrac-tion, a blank sample (without ash) was carried

out There was no delay between adding the

extractants and beginning the shaking The extracts

were stored in polypropylene bottles and kept at 4

8C before measurement

Between each stage, the residues were washed

with 20 ml of deionized water, followed by shaking

for 20 min and centrifugation The supernatant

(washing solution) was discarded, taking care not

to lose any of the solid residue

Digestion of the residue is not specified in the BCR protocol, so the residual fraction was calcu-lated as the difference between the total element concentration and the sum of all previous steps In this part of our study, we strictly followed the original BCR procedure

3 Results and discussion

The main elements in the brown coal ash(ENO

No.12-1-01) with certified concentration above 1%

were Mg (1.2%), K (1.7%), Ca (3.4%), Fe (7.5%), Al (10.8%) and Si (25.7%) Certified

values for the concentration of the main elements

in bituminous coal ash were: 1.6% Mg; 2.2% K; 2.2% Na; 2.3% Ca; 4.9% Fe; 14.8% Al; and 22.5% Si No information on carbon and oxygen contents was available The total concentrations of sulfur and boron (0.25% and 291 mgyg,

respec-tively) were known only for the brown coal ash

X-Ray diffraction analysis was employed to determine the crystalline compounds in fly ashes The proportion of crystalline components in both samples varied, depending on the type of coal The main crystalline phases in the brown coal ash were anhydrite, quartz, hematite, labradorite and magnesioferrite Small amounts of mullite and aragonite were also found The bituminous coal fly ash contained mainly mullite, quartz and crys-talline Ca Al O Anhydrite, magnesioferrite, lab-3 2 6

radorite, aragonite, hematite, lime and periclase were also identified Large amounts of amorphous phases were found in both ashes

Results of the modified BCR extraction proce-dure for the main and trace elements are presented

in Table 2 and Figs 2 and 3 For each material and extraction, five samples were simultaneously analysed to determine the precision of the meas-urements The RSD values varied over a wide range and depended on the element and the extrac-tion stage Very good precision, usually in the range 1–15%, was achieved for most of the elements analysed here In a few cases, the RSD values obtained were approximately 60% Such low precision was observed for measurements of the water-soluble fraction, in which some metal

Table 2

Metal partitioning obtained by sequential extraction

Element Step 1 (mgykg) Step 2 (mgykg) Step 3 (mgykg) Step 4 (mgykg) Total content (mgykg)

Al 23"11 127"41 720"27 692"55 1228"119 893"110 33"18 36"13 14.87"0.39 a 10.8"0.3 a

(1.5=10 ) y 2 (1.2=10 ) y 1 (4.8=10 ) y 1 (6.4=10 ) y 1 (8.3=10 ) y 1 (8.3=10 ) y 1 (2.2=10 ) y 2 (3.3=10 ) y 1

B407"45 23.9"3.1 120"10 55.5"1.6 15.0"2.2 12.6"1.8 6.11"1.32 5.25"0.43 – 291"39

Ca 4910"400 2450"70 4960"390 6590"160 694"92 1650"220 483"150 590"80 2.29 a 3.42"0.24 a

Fe 0.15"0.12 1.49"0.56 9.86"1.32 30.8"3.3 770"68 768"77 9.49"2.15 10.1"6.8 4.89"0.14 a 7.49"0.11 a

(2.5=10 ) y 4 (2.0=10 ) y 3 (2.0=10 ) y 2 (4.1=10 ) y 2 (1.6) (1.0) (1.9=10 ) y 2 (1.3=10 ) y 2

Mg 53"15 190"9 5400"80 1020"30 527"84 229"36 108"16 72.7"5.7 1.55 a 1.17"0.05 a

Values in parentheses represent distributions in % FFA, Fine Fly Ash Reference Material CTA-FFA-1; BCA, Brown Coal Ash Reference Material ENO No 12-1-01.

Values in wt.%.

a

A Smeda, W Zyrnicki / Microchemical Journal 72 (2002) 9–16

Fig 2 Comparison of metal distributions for the different fractions (in %): s1, water-soluble; s2 acid-soluble; s3, reducible; and

s4, oxidisable fractions.

concentrations were very low or close to their

detection limits

For most elements, the distribution of metals in

the extracts was similar for both ashes Notable

differences observed for some elements were

con-nected with the nature of the materials

Analysis of the content of the major elements

reported by the supplier of the ashes indicates that

Ca and Mg should be in silicate and

aluminosili-cate forms X-Ray diffraction spectra suggest that

the Ca and Mg silicates are amorphous Significant

differences appear in the case of Ca and Mg for

the ashes analysed The extraction efficiency of

Ca was considerably higher for bituminous than for brown coal ash The quantity of calcium in the water-soluble fraction was approximately one order

of magnitude higher than the magnesium content

In the second fraction(acid-soluble and associated

with carbonates), the Ca and Mg concentrations

were meaningful and similar More than 60% of the Ca and Mg was in the residue Of the other major elements, Al and Fe behaved very similarly and remained in the deposits after extraction For both ashes, nearly identical results were observed More than 98% of the aluminium and iron were identified in the residual fraction Al was present

Fig 3 Comparison of metal distributions for the different fractions (in %): s1, water-soluble; s2, acid-soluble; s3, reducible; and

s4, oxidisable fractions.

in different forms, mainly in compounds with

silicon as crystalline mullite and with Ca as

Ca Al O Fe occurred in the ashes in the form of3 2 6

oxides, such as magnetite or hematite It is very

likely that Fe is also present in amorphous forms

Boron, which remained in significant quantities

in the residue (brown coal ash), can be both in

borate and boride forms

Of the trace elements, chromium, boron and

strontium were relatively easily extracted by

deion-ized water Zinc and titanium in the bituminous

coal ash were found in forms that are not easily

soluble in water Their concentrations in the water extracts were below or very close to their detection limits For brown coal ash, Ti and Zn were also practically not extracted — this fact indicates that these elements are not released under typical envi-ronmental conditions In the acid-soluble fraction, nearly 10% of chromium, copper and strontium were found to be associated with carbonates The reducible fraction contained a considerable amount

of vanadium (17% for bituminous coal ash and

9% for brown coal ash) The extraction efficiency

for other elements did not exceed 5% of their total

A Smeda, W Zyrnicki / Microchemical Journal 72 (2002) 9–16

concentrations For chromium and vanadium, high

values were found in the oxidisable fractions —

they were extracted in 8–9% for Cr and 5–7% for

V A large portion of the elements analysed was

found in the residual fraction For titanium, the

metal was practically not extracted Less than 2%

of its total content was found in fractions 1–4

Barium, copper and nickel were also hardly

extracted from the ashes(85–93% in the residue)

This indicates that these elements are concentrated

in the undissolved aluminosilicate matrix

Due to existence of many different sequential

extraction procedures, it is very difficult to

com-pare the results obtained by various authors On

the other hand, not many such studies have been

reported so far for coal fly ashes Comparison of

our results for Cr, Zn and V with those obtained

by Bodog et al w5x shows that the content of these´

metals in the residual ash fraction could be

signif-icantly different On the other hand, the Cr, Zn

and V distributions in the reducible fractions and

bound to MnyFe oxides of various ashes are

comparable

So far, only one reference material (CRM 601,

lake sediment) certified for metals extractable by

the BCR procedure has been produced Only Cd,

Cr, Cu, Ni, Zn and Pb contents have been certified

No such a reference material is available for fly

ashes Thus, a reference material for the sequential

extraction has not been used here

4 Conclusions

The ICP-AES method is very applicable to study

of a multielement extraction process and metal

partitioning in materials such as fly ashes

The sequential extraction procedure reveals

much more information about elements

investigat-ed than data obtaininvestigat-ed from measurements of their

total concentrations Development of the BCR

procedure (with water extraction as the first step)

is recommended, because the extraction of

water-soluble species yields very important information

necessary to evaluate the risk of environmental

pollution by dumps of coal ashes

The BCR procedure enabled comparison of

metal partitioning in various types of

environmen-tal samples (soil, sediments, and sewage sludge)

However, for better understanding of the speciation and partitioning of metals in specific materials, such as fly ashes, more advanced tests should be carried out For example, the presence of sulfur as various metal sulfides (such Cu S, CuS, FeS,2

Fe S and ZnS2 3 ) would be expected in fly ashes

Therefore, it seems to be necessary to add more extraction steps with properly selected extractants, including more aggressive reagents

At the present state of knowledge, it is very difficult to explain in detail both the distribution and speciation of metals in materials such as fly ashes Coal ashes are generated in a very aggres-sive combustion process and comparison of the ashes with other materials, such as soil and sedi-ments, shows that heavy metals are not removed

as easily as from these materials

References

w1x Z Mester, C Cremisini, E Ghiara, R Morabito, Anal.

Chim Acta 359 (1998) 133–142.

w2x P Pardo, J.F Lopez-Sanchez, G Rauret, Anal Chim ´ ´ Acta 376 (1998) 183–195.

w3x K Polyak, J Hlavay, Fresenius J Anal Chem 363 ´

(1999) 587–593.

w4x R Zufiaurre, A Olivara, P Chamorro, C Nerın, A ´ Callizo, Analyst 123 (1998) 255–259.

w5x I Bodog, K Polyak, Z Csikos-Hartyanyi, J Hlavay, ´ ´ ´ ´ Microchem J 54 (1996) 320–330.

w6x J Usero, M Gamero, J Morillo, I Gracia, Environ Int.

24 (1998) 487–496.

w7x M.B Alvarez, M.E Malla, D.A Batistoni, Fresenius J.

Anal Chem 369 (2001) 81–90.

w8x B Perez-Cid, I Lavilla, C Bendicho, Fresenius J Anal ´ Chem 363 (1999) 667–672.

w9x E Campos, E Barahona, M Lachica, M.D Mingorance,

Anal Chim Acta 369 (1998) 235–243.

w10x B Perez-Cid, I Lavilla, C Bendicho, Anal Chim Acta ´

360 (1998) 35–41.

w11x B Perez-Cid, I Lavilla, C Bendicho, Anal Chim Acta ´

378 (1999) 201–210.

w12x A Tessier, P.G.C Campbell, M Bisson, Anal Chem.

51 (1979) 844–851.

w13x A.M Ure, P Quevauviller, H Muntau, B Griepink, Int.

J Environ Anal Chem 51 (1993) 135–151.

w14x B Perez-Cid, I Lavilla, C Bendicho, Analyst 121 ´

(1996) 1479–1484.

w15x B Chen, X Shan, D.-Q Shen, S.-F Mou, Fresenius J.

Anal Chem 357 (2001) 941–945.

w16x C.M Davidson, A.L Duncan, D Littlejohn, A.M Ure,

L.M Garden, Anal Chim Acta 363 (1998) 45–55.

w17x M.D Petit, M.I Rucandio, Anal Chim Acta 401

(1999) 283–291.

w18x J Szakova, P Tlustos, J Balık, D Pavlıkova, V Vanek, ´ ´ ˇ ´ ´ ´

Fresenius J Anal Chem 363 (1999) 594–595.

w19x B Marin, M Valladon, M Polve, A Monaco, Anal.

Chim Acta 342 (1997) 91–112.

w20x H.D Fiedler, J-F Lopez-Sanchez, R Rubio, et al., ´ ´ Analyst 119 (1994) 1109–1114.

w21x A Belazi, C.M Davidson, G.E Keating, D Littlejohn,

M McCartney, J Anal At Spectrosc 10 (1995)

233–240.