Application of sequential extraction

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Microchemical Journal 72 (2002) 9–16 0026-265X/02/$ - see front matter ᮊ 2002 Elsevier Science B.V. All rights reserved. PII: S0026-265X Ž 01 . 00143-6 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. ᮊ 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.: q48-71-320-2494; 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 w 1–7 x and carried out under various operating conditions w 8–11 x . The approach developed by Tessier in 1979 w 12 x and so-called the BCR procedure w 13 x (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 10 A. Smeda, W. Zyrnicki / Microchemical Journal 72 (2002) 9–16 Table 1 Instrumental parameters and operating conditions for ICP-AES Plasma Generator frequency 40 MHz RF power 1.0 kW Nebuliser Cross-flow Spray chamber Scott type Monochromator Type Czerny–Turner HR 1000 Focal length 1 m Gratings 4320 and 2400 groovesymm Argon flow rates Plasma gas 13 dm ymin 3 Sheath gas 0.2 dm ymin 3 Nebuliser gas 0.3 dm ymin 3 Sample uptake 1.0 cm ymin 3 Elements and analytical lines (nm) Al 396.152 Ba 233.527 Ca 317.933 Cr 267.716 Cu 324.754 Fe 259.924 Mg 285.213 Mn 259.373 Ni 221.647 Sr 407.771 Ti 334.941 V 292.402 Zn 202.548 matrices, e.g. sewage sludge w 8,10,11,14 x , different soils w 9,15–18 x , and marine w 6,19 x and river sedi- ments w 2,3,17,20,21 x . So far, sequential extraction has rarely been used to analyse fly ash samples w 5,17 x . Distribution of Cd in fractions of the coal fly ash NBS 1633a w 17 x , and Cd, Cr, Cu, Pb, Zn and V in the fractions of a brown coal w 5 x 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 B has 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 mgycm ). All reagents were at least of analytically 3 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 few 3 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 11A. Smeda, W. Zyrnicki / Microchemical Journal 72 (2002) 9–16 Fig. 1. Schematic diagram of the sequential extraction procedure. sulfides (for more details see w 19 x ). Five samples (1g) 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- 326 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 12 A. Smeda, W. Zyrnicki / Microchemical Journal 72 (2002) 9–16 Table 2 Metal partitioning obtained by sequential extraction Element Step 1 (mgykg) Step 2 (mgykg) Step 3 (mgykg) Step 4 (mgykg) Total content (mgykg) FFA BCA FFA BCA FFA BCA FFA BCA FFA BCA 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 ) y2 (1.2=10 ) y1 (4.8=10 ) y1 (6.4=10 ) y1 (8.3=10 ) y1 (8.3=10 ) y1 (2.2=10 ) y2 (3.3=10 ) y1 B 407"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 (8.2)(19)(4.3)(1.8) 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 (21)(7.2)(22)(19)(3.0)(4.8)(2.1)(1.7) 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 ) y4 (2.0=10 ) y3 (2.0=10 ) y2 (4.1=10 ) y2 (1.6)(1.0)(1.9=10 ) y2 (1.3=10 ) y2 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 (3.4=10 ) y1 (1.6)(35)(7.8)(3.4)(2.0)(6.9=10 ) y1 (6.2=10 ) y1 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 13A. 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 14 A. Smeda, W. Zyrnicki / Microchemical Journal 72 (2002) 9–16 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 of 326 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 15A. 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. w 5 x 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 obtained 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 ZnS) would be expected in fly ashes. 23 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. 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