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Evaluation of the use of a sequential extraction procedure for the characterization and treatment of metal containing solid waste
Waste Management 21 (2001) 685–694 www.elsevier.com/locate/wasman
Evaluation of the use of a sequential extraction procedure for the characterization and treatment of metal containing solid waste Peter Van Herck, Carlo Vandecasteele * Department of Chemical Engineering, University of Leuven de Croylaan 46, B-3001 Heverlee, Belgium Received 8 September 2000; received in revised form 8 January 2001; accepted 18 January 2001
Abstract Metal containing wastes like MSWI fly ashes and blast furnace sludge form a major environmental problem as they are polluted with heavy metals. The ash has to be landfilled or can be used as a construction material, but a pretreatment is in general necessary. Washing of the ashes with water in order to dissolve soluble salts or extracting the heavy metals with chemicals are possibilities. Blast furnace sludge contains large quantities of iron and carbon and could be recycled in the blast furnace, if the zinc content were not that high. Using a hydrometallurgical process the zinc can be removed from the sludge particles. In order to evaluate such treatment methods knowledge of the leaching behaviour of the studied material is very important. One of the factors influencing the leaching behaviour is the composition and mineralogy of the solids. A sequential extraction procedure, whereby the material is sequentially leached with different leaching solutions, can be used as an aid to characterize the material and to determine which chemical conditions are needed to obtain a sufficient extraction efficiency. To verify the accuracy of the sequential extraction procedure, a method is tested on MSWI fly ash and evaluated by comparing the results with those of leaching experiments whereby the final pH of the leaching solutions is varied over a wide range. Based upon this evaluation some suggestions for the use of the sequential extraction procedure are made and an adapted procedure is suggested, and applied to a blast furnace sludge. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Heavy metals; Ash; Sequential extraction procedure; Leaching; Blast furnace sludge
1. Introduction In this laboratory we studied possibilities to remove heavy metals from municipal solid waste incineration fly ash, in view of landfilling of the fly ash and/or recovery of some of the metals [1, 2]. Moreover, a method for recycling and recovery of blast furnace sludge [3] was developed. Yearly, around 2.8 million tonnes of municipal waste is generated in Flanders, Belgium. From this total amount of MSW, 30% is selectively collected; 43% of the remaining fraction is processed in incinerators and 57% is landfilled [4]. Incineration of 1 tonne of such municipal waste leads to the formation of 10–50 kg of fly ash depending on the type of incinerator. The combustion residues in general, and fly ash in particular, * Corresponding author. Tel.: +32-16-32-27-27; fax: +32-16-3229-91. E-mail address: [email protected] (C. Vandecasteele).
form a major environmental problem. As the legal standards for the emission of contaminants are getting more stringent, the air pollution control systems of incinerators must be improved, resulting in an increase of the amount of residues. The fly ash is contaminated with heavy metals and polychlorinated dibenzodioxins and polychlorinated dibenzofurans, and must be considered as hazardous waste according to the Flemish environmental legislation [5]. Generally, dust particles are, for example, removed from the incinerator flue gas by means of an electrostatic precipitator. Wet scrubbers remove HCl and HF from the flue gas in a first stage and SO2 in a second stage. The first stage produces an acid wastewater mainly containing HCl and to a lesser extent HF. This acid solution can be used to leach the fly ash, in order to remove part of the heavy metals, as in the 3R process developed in the Karlsruhe Nuclear Research Centre [6]. Another possible treatment of fly ashes is washing the ashes with water to remove easily leachable salts before stabilization/solidification [7, 8].
0956-053X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(01)00011-3
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During the production of pig iron in a blast furnace, dust is generated and it leaves the furnace with the flue gas. This dust is removed from the flue gas in a dust bag and a cyclone. The smallest particles are removed with a wet scrubber resulting in a sludge. The particles in the sludge contain large quantities of carbon and iron and could therefore be regarded as a raw material for the blast furnace. However, the sludge has also a high concentration of zinc. The zinc input in the furnace has to be limited for technological reasons. Removal of the zinc from the sludge using a hydrometallurgical process is a possible solution for this problem [3]. For both types of waste a wet treatment method can be used, so knowledge about the leaching behaviour is very important. The composition of the ashes is an important factor influencing the leaching behaviour. The use of the total concentration of the metals in the fly ash with respect to leaching implies that all the forms of a given metal are equally soluble; such an assumption is untenable. It is possible that a large fraction of the metal is encapsulated in the silicate matrix structure or occurs as insoluble minerals. Therefore, the form in which the metals occur in the fly ash is important. It is difficult to obtain this information; however, a sequential extraction procedure can help to characterize the leaching behaviour by dividing the total concentration of a metal into fractions. To this end the fly ash is sequentially leached with different solutions, each selective for a given fraction. The use of sequential extractions provides information about the origin, mode of occurrence, biological and physicochemical availability, mobilization, and transport properties of trace metals [9]. Sequential extraction has been broadly used for soils and sediments where metals are present as traces, while the application to solid wastes with a high level of metals has been more limited. In our work we applied several extraction methods to obtain information on the leaching behavior of the metals of interest. Combination of the results obtained allowed us to evaluate the methods applied and to develop a modified extraction method which we believe to more accurately reflect the leaching behavior of the considered wastes. The results of this research will be discussed in this paper. The procedure applied at the start was based on that of Kirby and Rimstidt [10] which was the most recent one and was developed for materials similar to those considered in this work. Several other sequential extraction procedures which use different leaching solutions or use them in a different order were described earlier [11–13]. The used procedure divides the total amount of a given metal into seven different fractions, described as follows [9–13]: 1. Water soluble fraction: this fraction consists of the metal salts that are easily soluble, e.g. chlorides.
2. Ca-exchangeable fraction: this fraction gives an indication of the amount of metals bound to surface sites. The trace metals that are bound to those sites are replaced by calcium ions. This could occur in a landfill if a salt solution passes through the waste. 3. Acid soluble fraction: this fraction contains the metals that are released as acid soluble salts such as carbonates from the fly ash. Also, some of the oxides and hydroxides are dissolved. In the case of landfilling of fly ash, or use of the ashes as a construction material, this fraction can be important if the initially alkaline fly ash loses its buffering capacity over time. 4. Oxidizable fraction: this fraction corresponds to metals that are organically bound or occur as oxidisable minerals, e.g., sulfides. 5. Easily reducible fraction: the reducible fractions contain mainly the metals encapsulated in iron and manganese oxides, which may exist as nodules, concretions, or simply as a coating on particles. These oxides are excellent scavengers for trace metals and dissolve under reducing conditions. The easily reducible fraction consists mainly of amorphous oxides. 6. Moderately reducible fraction: this fraction consists of metals occuring in crystalline oxides. 7. Residue fraction: this fraction consists of metals that did not dissolve in the previous steps, for example, metals encapsulated in the silicate structure of the fly ash. For MSWI fly ash most attention was given to the matrix elements Al, Ca, K, Mg, and Na and to the heavy metals Cd, Cu, Pb, and Zn that occur in high concentrations and are toxic. For the blast furnace sludge the matrix elements Al, Ca, Fe, Mg, and Mn were considered as well as the heavy metals Pb and Zn that occur in high concentreations, are relatively toxic and of major interest (Zn) when recycling of the material in a blast furnace is aimed at. In the literature it is also pointed out that considerable redistribution of trace elements during the sequential extraction procedure can occur, as proven by experiments with artificial samples [14]. However, Tessier and Campbell [15] pointed out that tests with real samples showed no such important redistribution as found for the artificial samples. Another comment is about the physicochemical conditions used during the extraction procedure (strong reagents and fast kinetics), which cannot be extrapolated to naturally occurring processes (weak reagents and slow kinetics) [14]. However, when the sequential extraction procedure is used to evaluate a certain treatment method for metal containing wastes the conditions of the procedure can be extrapolated to the actual process, if the experimental
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conditions are the same. Yet, it must be clear that the sequential extraction procedure can be used only for a qualitative instead of a quantitative analysis.
2. Study of MSWI fly ash using the original procedure 2.1. Experimental 2.1.1. Material The procedure was first tested on fly ash from a municipal solid waste incineration facility. Samples of the MSWI fly ash studied here were obtained from the Houthalen Waste Incineration Facility (Regionale Milieuzorg, Houthalen, Belgium). Table 1 gives the total composition of the fly ash determined by total dissolution [2] on 4 samples taken as described later. High concentrations were obtained for elements like Al, Ca, K and Na and for anions like Cl , CO23 , PO34 and SO24 .For the heavy metals, the concentrations for Zn and Pb are especially high. 2.1.2. Sequential extraction procedure Table 2 summarizes the sequential extraction procedure. The waste material was sequentially leached with different leaching solutions: 0.5 g of waste material was Table 1 Total composition of the fly ash (g/kg) determined by total dissolutiona Component
Fly ash (g/kg)
Component
Fly ash (g/kg)
Ag Al Ca Cd Cl Co CO23 Cr Cu F Fe
0.0290.004 803 1104 0.240.01 54.0 0.02050.0008 22.1 0.500.08 0.780.03 0.6 11.60.4
K Mg Mn Na Ni Pb PO34 Sn SO24 Zn
45 1 12.6 0.5 0.87 0.05 30 1 0.0780.004 3.9 0.1 27.0 1.3 0.1 58.0 12.9 0.5
a
The results are given as the mean and standard deviation for four different samples of the same material. For anions only one sample was analysed.
Table 2 Sequential extraction procedure Fraction
Leaching solution
1 Water soluble Distilled water, 3 h 2 Ca-exchangeable 0.5 M Ca(NO3)2, 3 h 3 Acid soluble 0.5 M CH3COOH, 3 h. 4 Oxidizable 0.1 M Na4P2O7, 3 h. 5 Easily reducible 0.175 M (NH4)2C2O4+0.1 M H2C2O4, 3 h. 6 Moderately reducible 0.1 M Na2EDTA+0.3 M NH2OH.HCI, 24 h. 7 Residue Total dissolution with HF, HCl and HClO4
used and sequentially extracted with 100 ml of each leaching solution. The 0.5 g was taken from a larger (2 kg) sample of the considered material. This large sample was thoroughly homogenised by shaking for several days. The material was taken at various locations of the homogenised sample in order to compose a 5 g sample, which was again homogenised by shaking. The final 0.5 g sample was taken from the 5 g sample. Comparison of four samples prepared in this way showed that the 0.5 g samples were representative of the 2 kg sample (see below, Tables 3, 5 and 6). The high L/S ratio (200 ml/g) was chosen in order to dissolve soluble materials to a large extent, avoiding as much as possible unexpected solubility limitations. The slurry was shaken mechanically for the time indicated in Table 2 and afterwards centrifuged. The 3 h and 24 h contact times come from the work of Kirby and Rimstidt [10], and it was verified in preliminary experiments that longer contact times did not make a significant difference. After the separation of the liquid from the solids, 100 ml of distilled water was added to the remaining fly ash in order to wash out the remaining leaching solution. The obtained slurry was again centrifuged and the liquid separated from the solids. The solution contained negligible amounts of trace elements, because the elements that would leach, were already leached in previous extraction steps. It appeared unnecessary to take the elements in the washTable 3 Comparison of the total composition of MSWI fly ash measured with the sequential extraction procedure and with total dissolutiona
Na K Al Ca Mg Zn Pb Cu Cd
Sequential extraction
Total dissolution
32.8 0.5 48.1 0.1 66.7 0.4 118 5 10.690.06 14.2 0.2 4.30 0.02 0.9740.007 0.2360.001
30 1 45 1 80 3 110 4 12.6 0.5 12.9 0.5 3.9 0.1 0.78 0.03 0.24 0.01
a The results are given in g/kg of fly ash. The results are the mean and standard deviation of four results.
Table 4 pH and redox potential (mV) in the different steps of the sequential extraction procedure for MSWI fly asha
Step 1 Step 2 Step 3 Step 4 Step 5 Step6 Step7 a
pH
Redox potential (mV)
10.5 9.3 3.1 10.1 3.2 – –
285 325 355 210 400 – –
The redox potential is relative to a saturated Ag/AgCl electrode.
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Table 5 Results of the sequential extraction procedure for Na, K, Al, Ca and Mg for MSWI fly asha
Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 a
Na
K
Al
Ca
Mg
54.6 0.3 0.79 0.03 29 1 – 10.7 0.9 – 5.3 0.1
71.1 0.2 1.41 0.04 13.4 0.1 – 5.2 0.1 – 8.92 0.08
3.030.01 2.330.01 47.50.4 3.390.03 17.50.3 1.990.03 24. 10.3
21.30.2 – 624 2.180.03 1.270.08 7.80.2 5.590.08
2.410.02 1.940.03 58.00.5 1.7500.009 10.90.2 0.5150.009 24.50.3
The results correspond to the relative percent extracted of each metal and are given as the mean and standard deviation of four results.
Table 6 Results of the sequential extraction procedure for Zn, Pb, Cu and Cd for MSWI fly asha
Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 a
Zn
Pb
Cu
Cd
1.0460.004 2.730.03 831 1.310.02 2.890.04 0.180.02 8.610.07
2.070.02 4.170.05 78.70.5 7.220.07 1.860.02 3.20.1 2.80.2
1.2680.006 3.400.04 80.20.3 1.690.03 5.390.06 1.00.6 7.070.07
0.930.04 6.30.1 89.80.4 – 0.34 0.70.2 1.820.08
The results correspond to the relative percent extracted of each metal and are given as the mean and standard deviation of four results.
ing solution into account in the mass balance of the fraction concerned (see also Table 3). The next extraction step was then carried out on the solid from the previous step. The pH and redox potential were measured for each leaching solution. The redox potential was measured with a platinum electrode relative to a saturated Ag/AgCl electrode. In order to deduce the redox potential relative to a standard hydrogen electrode, a value of 204 mV may be added. The metal concentrations in each leaching solution were measured by ICP-MS (Inductively Coupled Plasma Mass Spectrometry) using a VG PlasmaQuad PQ2Plus. During the measurements the high concentration of the leaching solutions can cause matrix effects which were for the measurement of the elements with high concentrations avoided by using dilution and by using In as an internal standard. To avoid interferences and matrix effects, the elements with low concentrations were separated from the matrix with ion exchange and then measured. For the ion exchange an automatic TRACECON system (Knapp Logistic Automation) was used, equipped with an EDTrA column. For the water soluble step, the chloride and sulphate concentrations were measured with CZA (Capillary Zone Electrophoresis, Waters Quanta 4000). The leaching solutions of the other steps were not analysed for anions, because of the high anion concentration of the original leaching solutions compared with the dissolved anions.
2.1.3. Leaching experiments at different pH values The results of the water soluble and acid soluble step can be compared with earlier leaching experiments at different pH [2]. During these experiments, HCl solution was added to the fly ash with a liquid to solid ratio of 10 1/kg. The concentration of the HCl-solution was varied in order to obtain a varying final pH. The leaching efficiency is defined as the ratio of the leached amount of a given element over the total amount of this element in the sample, and is expressed in percent. pH values higher than 10 were obtained using NaOH instead of HCl. Fig. 1 gives the leaching efficiency of several elements as a function of the final pH. 2.2. Results and discussion The sequential extraction procedure was performed four times and the results are always given as the mean of these four results together with the standard deviation. Table 3 compares the sum of the concentrations obtained during the sequential extraction procedure with those obtained with total dissolution. In general the results match rather well in spite of the possible errors that can be made during the sequential extraction procedure. Indeed, after each step the slurry is first centrifuged and then the supernatant is removed. It is possible that a small amount of solids is removed with the supernatant, which might lead to a lower metal
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concentration in the following steps. Table 4 gives the pH and redox potential during the different steps of the procedure. Tables 5 and 6 give the results of the sequential extraction procedure for Na, K, Al, Ca, Mg, Zn, Pb, Cu, and Cd. 2.2.1. Water soluble step Extraction in the water soluble step is high for Na (54.6%), K (71.1%) and Ca (21.3%). These metals occur as easily soluble salts in the fly ash. The other metals in Tables 5 and 6 have a water soluble fraction below 5%. Table 7 gives the percentage of chloride and sulphate leached in the water soluble step, relative to the amount measured by total dissolution of the fly ash sample. It is clear that most of the chloride and sulphate leach in the water soluble step. Probably Na, K and Ca occur as chloride and sulphate salts in the fly ash. High solubilities of chlorides and sulphates were also reported by Kirby and Rimstidt [16]. These highly soluble salts can contaminate groundwater near the disposal site and increase the solubility of heavy metals through complexation, or can promote corrosion when the ashes are used as a construction material. Eighmy et al. [17] gave an overview of five studies with X-ray diffraction. According to these studies Na occurs as NaCl or Na2SO4, K as KC1 and K2SO4 and Ca as CaSO4. Similar results were reported by Giordano et al. [18]. The final pH after leaching is 10.5 (Table 4). This indicates that part of the dissolved metals also occur as
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basic metal salts, oxides or hydroxides. It is possible that some of the heavy metals like Zn occur in the fly ash as very soluble salts, e.g. Cl-salts which dissolve immediately. Because of the high pH of the solution, hydroxides are formed and the metals precipitate. The conclusion is that with only the results of the sequential extraction procedure, it is hard to say which mineral of a certain metal occurs in the fly ash; only assumptions can be made. The results of the procedure explain only part of the interaction with the leaching solution and not the speciation of the material, although it will help to determine the speciation in combination with other techniques. In Van Herck et al. [1] computer modelling using the MINTEQA2 program was applied to calculate which minerals precipitate and to explain the curves giving the leaching efficiency as a function of pH and the results of the sequential extraction. The results of the leaching experiments given in Fig. 1 show that for a pH around 10, the leaching efficiency for most elements is very low. This is in agreement with the Table 7 Leaching of chloride and sulphate in the water soluble stepa % Cl SO4 a
854 901
The results are expressed as relative percent extracted and correspond to the mean and standard deviation of four results.
Fig. 1. Leaching efficiency (%) as a function of the final pH for MSWI fly ash.
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results of the water soluble step of the sequential extraction procedure.
final pH of the oxidizable step were the same as for the acid soluble step.
2.2.2. Ca exchangeable The Ca exchangeable fraction (final pH=9.3) is small for all the metals; only Cd reaches a value above 5%. In general, the Ca exchangeable fraction is too small to be significant, so that the second extraction step could be omitted.
2.2.5. Reducible steps The two reducing steps show low fractions for the heavy metals. Only the moderately reducible fraction for Cu is above 5%. The fraction is higher for the matrix elements. The redox potential in the moderately reducible step was not measured. Again the pH of both steps differs from the previous steps and should be adjusted.
2.2.3. Acid soluble step The acid soluble fraction (final pH=3.1) is very high for the heavy metals, the values range between 79 and 90%. Probably oxides and carbonates of these metals dissolve under acidic conditions. Cd has the highest acid soluble fraction (89.8%). Na and K have a lower acid soluble fraction because a major portion of these elements has already been leached during the water soluble step. For Al, Ca and Mg the acid soluble fraction is the largest fraction, but the relative amount is lower than for the heavy metals Zn, Pb, Cu, and Cd. The leaching experiments with varying pH (Fig. 1) show that at a pH of 3.1, as obtained during the acid soluble step, the extraction is complete for Cd and almost complete for Zn. However, the available fraction for Al is only partly dissolved at a pH of 3.1. The leaching efficiency for Cu and Pb is still below 10% at a pH of 3.1 (Fig. 1). The results of the sequential extraction procedure (Table 6), however, show that ca. 80% of Cu and Pb dissolves. This can possibly be explained by the complexing capabilities of acetic acid used in the acid soluble step. Cu and Pb form stable acetate complexes while the complexation with chloride is less stable [19]. It is important to realise that the results of the sequential extraction procedure are influenced by the type of leaching solution used. When, in the acid soluble step, acetic acid is used to extract the metals, high leaching efficiencies for Pb and Cu are obtained (Table 6). When the fly ash is leached with HCl, the leaching efficiencies for the same pH (3.1) are much lower. 2.2.4. Oxidizable step The oxidizable fraction is below 5% for all the metals except for Pb (7.22%). The pH in this step is 10.1 and the redox potential 210 mV. This step gives no good indication of the amount of metals that dissolve under oxidizing conditions, because the redox potential is even lower than in the previous step. So, metals that oxidize at this redox potential would already have been oxidized in the previous, acid soluble step. The results of the oxidizing step are doubtful and therefore it would be better to use another reagent such as NaOCl in order to create stronger oxidizing conditions. The pH of this step is also completely different from the one in the acid soluble step, which can possibly lead to a misinterpretation of the results. It would be better if the
2.2.6. Residue For Al and Mg the fractions contained in the residue (24%) are highest. For Na, K, Ca, Zn (8.6%), and Cu (7.1%) and mainly for Pb (2.8%) and Cd (1.8%) they are much lower.
3. Modification of the sequential extraction procedure and study of the blast furnace sludge 3.1. Experimental 3.1.1. Modified sequential extraction procedure The experiment with the MSWI fly ash showed that the results of the extraction procedure used are not always optimal and may lead to wrong conclusions. Some recommendations for changes in the extraction procedure will therefore be made. First of all, the Caexchangeable step can be omitted, because the amount of metals leached in this step is very small for incineration residues. For the acid soluble step it was pointed out that the final pH of the leaching solution is too high. The pH should be around 2.5, if one wants to almost completely extract the acid leachable fraction of Al and Zn. At this pH all the metal carbonates are dissolved. In a new procedure a pH of 2.5 is obtained by leaching with acetic acid (0.5 M) and adjusting the pH to 2.5 during the reaction by adding 12 M HCl. Using acetic acid and hydrochloric acid together has the advantage that lower final pH values are reached, while acetic acid may still, to some extent, form complexes with heavy metals that are more stable than chloride complexes for Zn and Pb. There was also some doubt about the oxidizing capabilities of the oxidizable step. Instead of Na4P2O7, NaOCl should be used as a stronger oxidizing reagent. During the extraction the redox potential relative to a saturated Ag/AgCl electrode was measured and adjusted to around 1200 mV by adding extra NaOCl. During this step the pH was also maintained at ca. 2.5 by adding HCl. Maintaining the pH at 2.5 has the advantage that it becomes possible to compare the acid soluble and the oxidizable steps. When a metal only dissolves during the oxidizable step, it means that at pH 2.5 the metal needs to be oxidized before it dissolves.
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The pH of the easily reducible step is also maintained at pH 2.5 by adding hydrochloric acid to the solution. However, for the moderately reducible step this procedure was not possible, because the reaction must occur in the dark during 24 h. Measuring the pH and redox potential during the extraction might disturb the reaction conditions. The final step remains the same as in the previous procedure. Table 8 presents the adapted sequential extraction procedure. In the leaching solutions the concentration of Al, Ca, Fe, Mg, Mn, Pb, and Zn were measured along with the pH and the redox potential relative to a saturated Ag/ AgCl electrode. 3.1.2. Material The results of the sequential extraction procedure, performed on the MSWI fly ash, showed that the redox potential would only have a small influence, as most of the heavy metals have already been leached almost completely after the acid soluble step. Performing the new procedure on the fly ash would therefore show little difference, so that another waste material was tested, blast furnace sludge. Table 9 gives the total composition of the dry solids (DS) in the blast furnace sludge, obtained from a Table 8 Adapted sequential extraction procedure used for blast furnace sludge Fraction
Leaching solution
1 2
Water soluble Acid soluble
3
Oxidizable
4
Easily reducible
5
Moderately reducible Residue
6
Distilled water, 3 h 0.5 M CH3COOH and add 12 M HCl until pH=2.5, 3 h 1.9 M NaOCl until E=1200 mV and 12 M HCl until pH=2.5, 3 h 0.175 M (NH4)2C2O4+0.1 M H2C2O4 and add 12 M HCl until pH = 2.5, 3 h 0.l M Na2EDTA+0.3 M NH2OH.HCl, 24 h Total dissolution with HF, HCl and HClO4
metallurgical company (Sidmar N.V., Gent, Belgium). The concentration of iron is very high, so that the sludge could be regarded as a raw material that could be recycled in the blast furnace. The concentration of zinc and to a lesser extent lead is however too high to allow direct recovery in the blast furnace. 3.1.3. Leaching experiments at different pH values Similar leaching experiments as for the MSWI fly ash were performed on the blast furnace sludge. HCl-solutions were used to obtain the varying final pH. When FeCl3 is used instead of HCl the pH also varies, but the redox potential is increased. Fig. 2 gives the leaching efficiency for Zn as a function of the final pH for both experiments. 3.2. Results and discussion Table 10 gives a comparison between the total concentrations of metals leached during the complete sequential extraction procedure and the concentration measured by total dissolution of the material. The results are in rather good agreement. The agreement is comparable with the one with the earlier procedure applied to MSWI fly ash (Table 3). Table 11 gives the final pH and redox potential after reaction. The redox potential is relative to a saturated Ag/AgCl electrode.
Table 10 Comparison of the total composition of blast furnace sludge measured with the sequential extraction procedure and with total dissolution (% by weight)
Al Ca Fe Mg Mn Pb Zn
Sequential extraction
Total dissolution
1.26 0.94 8.62 0.35 0.04 0.99 7.83
1.40 0.92 9.28 0.40 0.05 1.28 8.44
Table 9 Total composition of the blast furnace sludgea Component
DS (g/kg)
Al Ca CO23 Fe Mg Mn Pb SiO2 Zn
102 5.90.8 239 867 3.80.2 0.470.03 82 40.7 3911
Table 11 Final pH and redox potential in the different steps of the sequential extraction procedure for blast furnace sludgea
Step 1 Step 2 Step3 Step 4 Step 5 Step 6
pH
E (mV)
6.2 2.4 2.2 2.2 3.2 –
106 360 1105 501 72 –
a
The results are given as the mean and standard deviation of four samples. For anions only one sample was analysed.
a
The redox potential is relative to a saturated Ag/AgCl electrode.
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The alkalinity of the blast furnace sludge is lower than that of the MSWI fly ash, as can be seen from the pH of the water soluble step (6.2 compared with 10.5). The pH’s in the following three steps are almost the same, so they can be compared with each other. The redox potential in the oxidizable step is sufficiently high. The redox potential of the reducible steps is lower, as should be the case. Table 12 gives the metal concentrations in the different steps. The results are given as the fraction of the total measured metal concentration that is extracted in a given step. 3.2.1. Water soluble step The fraction of the metals in the water soluble step (pH=6.2) is lower than 1%, except for Ca and Mg. 3.2.2. Acid soluble step The fraction of Al, Ca and Mg is intermediate, the values lie between 29.5 and 57.1%. Fe and Mn give an
acid soluble fraction of 14.2 and 20.7%, respectively. Pb and Zn have high acid soluble fractions (72.0 and 80.0%). The pH of this step is maintained at 2.4 to be sure that all the carbonates are dissolved. This result can be compared with the leaching experiments with HCl (Fig. 2). The efficiency for Zn at pH 2.4 is about 75%, in excellent agreement with the 80.0% of Zn in the acid soluble step. For the other metals a good agreement was also found. 3.2.3. Oxidizable fraction In the oxidising step the redox potential is high enough to oxidize iron, while the pH of the solution is almost the same as in the previous step. So, if a metal salt dissolves only in this step, it means that oxidizing conditions are necessary to dissolve the salt. The fractions are around 5% or lower for Ca, Fe, Mg and Mn. The fraction of Al is somewhat more elevated (9.1%). Zn and Pb have both a fraction between 15 and 20%. So a considerable amount of Zn and Pb present in the
Fig. 2. Leaching efficiency (%) for Zn as a function of the final pH for leaching with HCl or FeCl3 for blast furnace sludge.
Table 12 Metal fractions (%) in the different steps of the sequential extraction procedure for blast furnace sludge
Step 1 Step 2 Step 3 Step 4 Step 5 Step 6
Al
Ca
Fe
Mg
Mn
Pb
Zn
0.04 57.1 9.11 8.15 0.21 25.4
18.1 44.9 4.49 5.26 10.7 16.5
0.02 14.2 5.27 39.0 0.70 40.8
12.2 29.5 2.05 12.8 1.89 41.5
1.16 20.7 3.95 26.7 4.66 42.8
0.05 72.0 18.0 3.44 4.72 1.79
0.06 80.0 16.6 2.20 0.39 0.89
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solids of the sludge can only dissolve under oxidizing conditions. The redox potential has a substantial influence on the leaching behaviour of Zn and Pb. The redox potential in this step was 1105 mV. At pH 2.4 about 95% of the total amount of Zn is dissolved (Fig. 2) when FeCl3 is used. When only HCl is used to leach the sludge, the leaching efficiency is 75%. The difference between these two leaching efficiencies, 20%, is the amount of Zn present as an oxidizable mineral, presumably sfalerite, ZnS, in the sludge particles. The oxidizable fraction in the sludge, determined with the sequential extraction procedure, gives 16.6%, in good agreement with the previous value. 3.2.4. Easily reducible step In this step the redox potential is lower than in the previous step. The results show that 39% of the iron present in the sludge dissolves during this step. Theoretically all the Fe(II) will dissolve at pH=2.5 in the acid soluble step, whereas at pH=2.5 (acid soluble and oxidisable steps) Fe(III) remains precipitated. In the easily reducible step, the redox potential is low enough to reduce, in the absence of Fe(II), most of the Fe(III) to Fe(II) at pH 2.5, which will dissolve at this pH. Although the dissolution of Fe(II) in the acid soluble step (and water soluble step), leads to lower redox potentials than in the reducible step, obviously, when a very large excess of Fe(II) is present no reduction of Fe(III) to Fe(II) takes place. The extracted fraction for Mn is 26.7% The research literature shows that iron and manganese often occur together as oxides, which possibly encapsulate other elements [9,10,12] . Pb leaches only for 3.4% and Zn for 2.2%. The fractions for Al, Ca and Mg are between 5 and 13%. 3.2.5. Moderately reducible fraction Although the redox potential is still lower, almost no additional Fe is leached during this step and only 4.6% of the Mn is leached. All the iron that could be reduced was already reduced in the easily reducible step. The fractions of the other metals are below 5%, except for Ca (10.7%). 3.2.6. Residue In the residue of the sludge, considerable amounts of Al, Ca, Fe, Mg and Mn are still found. These elements, together with Si, form the matrix of the particles or are encapsulated in matrix. Only 1.79% of Pb and 0.89% of Zn are present in the residue. The two metals are almost completely dissolved in the previous steps.
4. Conclusions The sequential extraction procedure can be a help in understanding which form the different metals take in
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the fly ash, giving an indication of the mineralogy of the element. It is however uncertain if the accuracy of the method is high enough to use the results for a quantitative partitioning of the metals into the fractions described. In the water soluble step, chlorides and sulphates dissolve together with Na, K and part of the Ca. The Ca exchangeable fraction is low for the metals and can be omitted for testing waste material. The largest fraction for the heavy metals is the acid soluble fraction. Probably oxides and carbonates of these metals dissolve. However, the pH should be adjusted to 2.5 with HCl, for example, to ensure complete dissolution of carbonates and oxides. The results of the oxidizing step in the original procedure are doubtful because of the low redox potential compared with the redox potential of the acid soluble step. Therefore, it would be better to use another reagent such as NaOCI in order to obtain stronger oxidizing conditions. The pH should be set to 2.5 to make a comparison with the other steps possible. This comment is also valid for the reducible steps. Therefore, a modified procedure is proposed. It can be concluded that acid leaching will be very important for the treatment of fly ashes while the influence of the redox potential is small because most of the heavy metals have already leached almost completely after the acid soluble step. The results of the modified sequential extraction procedure, used on blast furnace sludge, show that Zn and Pb dissolve in acid while the leachability of iron in acid is rather low. Also, the influence of the redox potential on the teachability is obvious. Oxidizing conditions are necessary to leach an additional amount of about 15% of the zinc present. It can be concluded that the performance of the modified sequential extraction procedure is much better than that of the previous procedure, as shown by the agreement with the leaching experiments at varying pH and redox potential. The different steps now give a better understanding of the leaching behaviour in view of recycling the waste material. The results, however, must be evaluated carefully. It is never possible to reach complete selectivity in each step as is also pointed out in literature. The interpretation of the results is only a qualitative one. It can be deduced whether a large fraction of the material dissolves in water or acid or whether oxidizing or reducing conditions are necessary. From this research it appeared that an oxidizing environment was necessary in order to reach sufficient removal of Zn from the sludge. The selectivity of the method is not good enough to investigate the speciation of the metals in the waste material. It only gives an indication of the equilibrium between solid material and leaching solution and the results must be compared with e.g. results of XRD to learn more about the speciation of the elements. For the blast furnace sludge this was carried out in by Van Herck et al. [3] and it was shown that after
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leaching with acid zinc occurred mainly as sfalerite (ZnS) and partly as Franklinite (ZnO.Fe2O3). Comparison of the leaching behaviour of the sludge with that of sfalerite and Franlinite geochemical standards confirmed this and showed that sfalerite dissolves in acid oxidising conditions, whereas Franklinite practically does not dissolve.
Acknowledgements Grateful acknowledgement is made to SIDMAR N.V. and V.L.I.M. (Vlaams Impulsprogramma voor Milieuonderzoek) for their support to this study. References [1] Van Herck P, Yan der Bruggen B, Vogels G, Vandecasteele C. Application of computer modelling to predict the leaching behaviour of heavy metals from MSWI fly ash and comparision with a sequential extraction method. Waste Management 2000;20:203. [2] Van der Bruggen B, Vogels G, Van Herck P, Vandecasteele C. Simulation of acid washing of municipal solid waste incineration fly ashes in order to remove heavy metals. Journal of Hazardous Materials 1998;57:127. [3] Van Herck P, Vandecasteele C, Swennen R, Mortier R. Zinc and lead removal from blast furnace sludge with a hydrometallurgical process. Environ Sci Technol 2000;34:3802. [4] Wille D. and De Boeck G., Inventarisatie Huishoudelijke Afvalstoffen in Vlaanderen in 1994, Productie, Inzameling en Verwerking, Openbare Afvalstoffenmaatschappij voor het Vlaamse Gewest, Publicatie nr. D/1996/5024/4, Mechelen, Belgium, (1996). [5] Senelle R, Dujardin J, van Damme M. VLAREM II. Brugge, Belgium: Die Keure La Charte, 1995.
[6] Vehlow J, Brown H, Horch K, Merz A, Schneider J, Stieglitz L, et al. Semi-technical demonstration of the 3R-process. Waste Management and Research 1990;8:461. [7] Mulder E. Pretreatment of MSWI fly ash for useful application. Waste Management 1996;16:181. [8] Tobler HP. Behandlung und verfestigung von ruecksta¨nden und muellverbrennungsanlagen in der Schweiz. Entsorgungspraxis Special 1988;1988:188. [9] Tessier A, Campbell PGC, Bisson M. Sequential extraction procedure for the speciation of particulate trace metals. Analytical Chemistry 1979;7:844. [10] Kirby CS, Rimstidt JD. Mineralogy and surface properties of municipal solid waste ash. Environ Sci Technol 1993;27:652. [11] Kersten M, Fo¨rstner U. Chemical fractionation of heavy metals in anoxic estuarine and coastal sediments. Wat Sci Tech 1986;18:121. [12] Theis TL, Padgett LE. Factors affecting the release of trace metals from municipal sludge ashes. Journal WCPF 1983;55:1271. [13] Francis AJ, Dodge CJ, Rose AW, Ramirez AJ. Aerobic and anaerobic microbial dissolution of toxic metals from coal wastes: mechanisms of action. Environ Sci Technol 1989;23:435. [14] Nirel PMV, Morel FMM. Technical note: Pitfalls of sequential extractions. Wat Res 1990;24:1055. [15] Tessier A, Campbell PGC. Comment on ‘‘Pitfalls of sequential extractions’’. WatRes 1991;25:115. [16] Kirby CS, Rimstidt JD. Interaction of municipal solid waste ash with water. Environmental Science and Technology 1994;28:443. [17] Eighmy TT, Eusden JD, Krzanowski JE, Domingo DS, Stampfli D, Martin JR, et al. Comprehensive approach toward understanding element speciation and leaching behaviour in municipal solid waste incineration electrostatic precipitator ash. Environmental Science and Technology 1995;29:629. [18] Giordano PM, Behel AD, Lawrence JE, Solleau JJ, Bradford BN. Mobility in soil and plant availability of metals derived from incinerated municipal refuse. Environ Sci Technol 1983;17:193. [19] Sille´n LG, Martell AE. Stability constants of metal-ion complexes. Burlington House, London, England: The Chemical Society, 1964.
Evaluation of the use of a sequential extraction procedure for the characterization and treatment of metal containing solid waste Peter Van Herck, Carlo Vandecasteele * Department of Chemical Engineering, University of Leuven de Croylaan 46, B-3001 Heverlee, Belgium Received 8 September 2000; received in revised form 8 January 2001; accepted 18 January 2001
Abstract Metal containing wastes like MSWI fly ashes and blast furnace sludge form a major environmental problem as they are polluted with heavy metals. The ash has to be landfilled or can be used as a construction material, but a pretreatment is in general necessary. Washing of the ashes with water in order to dissolve soluble salts or extracting the heavy metals with chemicals are possibilities. Blast furnace sludge contains large quantities of iron and carbon and could be recycled in the blast furnace, if the zinc content were not that high. Using a hydrometallurgical process the zinc can be removed from the sludge particles. In order to evaluate such treatment methods knowledge of the leaching behaviour of the studied material is very important. One of the factors influencing the leaching behaviour is the composition and mineralogy of the solids. A sequential extraction procedure, whereby the material is sequentially leached with different leaching solutions, can be used as an aid to characterize the material and to determine which chemical conditions are needed to obtain a sufficient extraction efficiency. To verify the accuracy of the sequential extraction procedure, a method is tested on MSWI fly ash and evaluated by comparing the results with those of leaching experiments whereby the final pH of the leaching solutions is varied over a wide range. Based upon this evaluation some suggestions for the use of the sequential extraction procedure are made and an adapted procedure is suggested, and applied to a blast furnace sludge. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Heavy metals; Ash; Sequential extraction procedure; Leaching; Blast furnace sludge
1. Introduction In this laboratory we studied possibilities to remove heavy metals from municipal solid waste incineration fly ash, in view of landfilling of the fly ash and/or recovery of some of the metals [1, 2]. Moreover, a method for recycling and recovery of blast furnace sludge [3] was developed. Yearly, around 2.8 million tonnes of municipal waste is generated in Flanders, Belgium. From this total amount of MSW, 30% is selectively collected; 43% of the remaining fraction is processed in incinerators and 57% is landfilled [4]. Incineration of 1 tonne of such municipal waste leads to the formation of 10–50 kg of fly ash depending on the type of incinerator. The combustion residues in general, and fly ash in particular, * Corresponding author. Tel.: +32-16-32-27-27; fax: +32-16-3229-91. E-mail address: [email protected] (C. Vandecasteele).
form a major environmental problem. As the legal standards for the emission of contaminants are getting more stringent, the air pollution control systems of incinerators must be improved, resulting in an increase of the amount of residues. The fly ash is contaminated with heavy metals and polychlorinated dibenzodioxins and polychlorinated dibenzofurans, and must be considered as hazardous waste according to the Flemish environmental legislation [5]. Generally, dust particles are, for example, removed from the incinerator flue gas by means of an electrostatic precipitator. Wet scrubbers remove HCl and HF from the flue gas in a first stage and SO2 in a second stage. The first stage produces an acid wastewater mainly containing HCl and to a lesser extent HF. This acid solution can be used to leach the fly ash, in order to remove part of the heavy metals, as in the 3R process developed in the Karlsruhe Nuclear Research Centre [6]. Another possible treatment of fly ashes is washing the ashes with water to remove easily leachable salts before stabilization/solidification [7, 8].
0956-053X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(01)00011-3
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During the production of pig iron in a blast furnace, dust is generated and it leaves the furnace with the flue gas. This dust is removed from the flue gas in a dust bag and a cyclone. The smallest particles are removed with a wet scrubber resulting in a sludge. The particles in the sludge contain large quantities of carbon and iron and could therefore be regarded as a raw material for the blast furnace. However, the sludge has also a high concentration of zinc. The zinc input in the furnace has to be limited for technological reasons. Removal of the zinc from the sludge using a hydrometallurgical process is a possible solution for this problem [3]. For both types of waste a wet treatment method can be used, so knowledge about the leaching behaviour is very important. The composition of the ashes is an important factor influencing the leaching behaviour. The use of the total concentration of the metals in the fly ash with respect to leaching implies that all the forms of a given metal are equally soluble; such an assumption is untenable. It is possible that a large fraction of the metal is encapsulated in the silicate matrix structure or occurs as insoluble minerals. Therefore, the form in which the metals occur in the fly ash is important. It is difficult to obtain this information; however, a sequential extraction procedure can help to characterize the leaching behaviour by dividing the total concentration of a metal into fractions. To this end the fly ash is sequentially leached with different solutions, each selective for a given fraction. The use of sequential extractions provides information about the origin, mode of occurrence, biological and physicochemical availability, mobilization, and transport properties of trace metals [9]. Sequential extraction has been broadly used for soils and sediments where metals are present as traces, while the application to solid wastes with a high level of metals has been more limited. In our work we applied several extraction methods to obtain information on the leaching behavior of the metals of interest. Combination of the results obtained allowed us to evaluate the methods applied and to develop a modified extraction method which we believe to more accurately reflect the leaching behavior of the considered wastes. The results of this research will be discussed in this paper. The procedure applied at the start was based on that of Kirby and Rimstidt [10] which was the most recent one and was developed for materials similar to those considered in this work. Several other sequential extraction procedures which use different leaching solutions or use them in a different order were described earlier [11–13]. The used procedure divides the total amount of a given metal into seven different fractions, described as follows [9–13]: 1. Water soluble fraction: this fraction consists of the metal salts that are easily soluble, e.g. chlorides.
2. Ca-exchangeable fraction: this fraction gives an indication of the amount of metals bound to surface sites. The trace metals that are bound to those sites are replaced by calcium ions. This could occur in a landfill if a salt solution passes through the waste. 3. Acid soluble fraction: this fraction contains the metals that are released as acid soluble salts such as carbonates from the fly ash. Also, some of the oxides and hydroxides are dissolved. In the case of landfilling of fly ash, or use of the ashes as a construction material, this fraction can be important if the initially alkaline fly ash loses its buffering capacity over time. 4. Oxidizable fraction: this fraction corresponds to metals that are organically bound or occur as oxidisable minerals, e.g., sulfides. 5. Easily reducible fraction: the reducible fractions contain mainly the metals encapsulated in iron and manganese oxides, which may exist as nodules, concretions, or simply as a coating on particles. These oxides are excellent scavengers for trace metals and dissolve under reducing conditions. The easily reducible fraction consists mainly of amorphous oxides. 6. Moderately reducible fraction: this fraction consists of metals occuring in crystalline oxides. 7. Residue fraction: this fraction consists of metals that did not dissolve in the previous steps, for example, metals encapsulated in the silicate structure of the fly ash. For MSWI fly ash most attention was given to the matrix elements Al, Ca, K, Mg, and Na and to the heavy metals Cd, Cu, Pb, and Zn that occur in high concentrations and are toxic. For the blast furnace sludge the matrix elements Al, Ca, Fe, Mg, and Mn were considered as well as the heavy metals Pb and Zn that occur in high concentreations, are relatively toxic and of major interest (Zn) when recycling of the material in a blast furnace is aimed at. In the literature it is also pointed out that considerable redistribution of trace elements during the sequential extraction procedure can occur, as proven by experiments with artificial samples [14]. However, Tessier and Campbell [15] pointed out that tests with real samples showed no such important redistribution as found for the artificial samples. Another comment is about the physicochemical conditions used during the extraction procedure (strong reagents and fast kinetics), which cannot be extrapolated to naturally occurring processes (weak reagents and slow kinetics) [14]. However, when the sequential extraction procedure is used to evaluate a certain treatment method for metal containing wastes the conditions of the procedure can be extrapolated to the actual process, if the experimental
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conditions are the same. Yet, it must be clear that the sequential extraction procedure can be used only for a qualitative instead of a quantitative analysis.
2. Study of MSWI fly ash using the original procedure 2.1. Experimental 2.1.1. Material The procedure was first tested on fly ash from a municipal solid waste incineration facility. Samples of the MSWI fly ash studied here were obtained from the Houthalen Waste Incineration Facility (Regionale Milieuzorg, Houthalen, Belgium). Table 1 gives the total composition of the fly ash determined by total dissolution [2] on 4 samples taken as described later. High concentrations were obtained for elements like Al, Ca, K and Na and for anions like Cl , CO23 , PO34 and SO24 .For the heavy metals, the concentrations for Zn and Pb are especially high. 2.1.2. Sequential extraction procedure Table 2 summarizes the sequential extraction procedure. The waste material was sequentially leached with different leaching solutions: 0.5 g of waste material was Table 1 Total composition of the fly ash (g/kg) determined by total dissolutiona Component
Fly ash (g/kg)
Component
Fly ash (g/kg)
Ag Al Ca Cd Cl Co CO23 Cr Cu F Fe
0.0290.004 803 1104 0.240.01 54.0 0.02050.0008 22.1 0.500.08 0.780.03 0.6 11.60.4
K Mg Mn Na Ni Pb PO34 Sn SO24 Zn
45 1 12.6 0.5 0.87 0.05 30 1 0.0780.004 3.9 0.1 27.0 1.3 0.1 58.0 12.9 0.5
a
The results are given as the mean and standard deviation for four different samples of the same material. For anions only one sample was analysed.
Table 2 Sequential extraction procedure Fraction
Leaching solution
1 Water soluble Distilled water, 3 h 2 Ca-exchangeable 0.5 M Ca(NO3)2, 3 h 3 Acid soluble 0.5 M CH3COOH, 3 h. 4 Oxidizable 0.1 M Na4P2O7, 3 h. 5 Easily reducible 0.175 M (NH4)2C2O4+0.1 M H2C2O4, 3 h. 6 Moderately reducible 0.1 M Na2EDTA+0.3 M NH2OH.HCI, 24 h. 7 Residue Total dissolution with HF, HCl and HClO4
used and sequentially extracted with 100 ml of each leaching solution. The 0.5 g was taken from a larger (2 kg) sample of the considered material. This large sample was thoroughly homogenised by shaking for several days. The material was taken at various locations of the homogenised sample in order to compose a 5 g sample, which was again homogenised by shaking. The final 0.5 g sample was taken from the 5 g sample. Comparison of four samples prepared in this way showed that the 0.5 g samples were representative of the 2 kg sample (see below, Tables 3, 5 and 6). The high L/S ratio (200 ml/g) was chosen in order to dissolve soluble materials to a large extent, avoiding as much as possible unexpected solubility limitations. The slurry was shaken mechanically for the time indicated in Table 2 and afterwards centrifuged. The 3 h and 24 h contact times come from the work of Kirby and Rimstidt [10], and it was verified in preliminary experiments that longer contact times did not make a significant difference. After the separation of the liquid from the solids, 100 ml of distilled water was added to the remaining fly ash in order to wash out the remaining leaching solution. The obtained slurry was again centrifuged and the liquid separated from the solids. The solution contained negligible amounts of trace elements, because the elements that would leach, were already leached in previous extraction steps. It appeared unnecessary to take the elements in the washTable 3 Comparison of the total composition of MSWI fly ash measured with the sequential extraction procedure and with total dissolutiona
Na K Al Ca Mg Zn Pb Cu Cd
Sequential extraction
Total dissolution
32.8 0.5 48.1 0.1 66.7 0.4 118 5 10.690.06 14.2 0.2 4.30 0.02 0.9740.007 0.2360.001
30 1 45 1 80 3 110 4 12.6 0.5 12.9 0.5 3.9 0.1 0.78 0.03 0.24 0.01
a The results are given in g/kg of fly ash. The results are the mean and standard deviation of four results.
Table 4 pH and redox potential (mV) in the different steps of the sequential extraction procedure for MSWI fly asha
Step 1 Step 2 Step 3 Step 4 Step 5 Step6 Step7 a
pH
Redox potential (mV)
10.5 9.3 3.1 10.1 3.2 – –
285 325 355 210 400 – –
The redox potential is relative to a saturated Ag/AgCl electrode.
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Table 5 Results of the sequential extraction procedure for Na, K, Al, Ca and Mg for MSWI fly asha
Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 a
Na
K
Al
Ca
Mg
54.6 0.3 0.79 0.03 29 1 – 10.7 0.9 – 5.3 0.1
71.1 0.2 1.41 0.04 13.4 0.1 – 5.2 0.1 – 8.92 0.08
3.030.01 2.330.01 47.50.4 3.390.03 17.50.3 1.990.03 24. 10.3
21.30.2 – 624 2.180.03 1.270.08 7.80.2 5.590.08
2.410.02 1.940.03 58.00.5 1.7500.009 10.90.2 0.5150.009 24.50.3
The results correspond to the relative percent extracted of each metal and are given as the mean and standard deviation of four results.
Table 6 Results of the sequential extraction procedure for Zn, Pb, Cu and Cd for MSWI fly asha
Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 a
Zn
Pb
Cu
Cd
1.0460.004 2.730.03 831 1.310.02 2.890.04 0.180.02 8.610.07
2.070.02 4.170.05 78.70.5 7.220.07 1.860.02 3.20.1 2.80.2
1.2680.006 3.400.04 80.20.3 1.690.03 5.390.06 1.00.6 7.070.07
0.930.04 6.30.1 89.80.4 – 0.34 0.70.2 1.820.08
The results correspond to the relative percent extracted of each metal and are given as the mean and standard deviation of four results.
ing solution into account in the mass balance of the fraction concerned (see also Table 3). The next extraction step was then carried out on the solid from the previous step. The pH and redox potential were measured for each leaching solution. The redox potential was measured with a platinum electrode relative to a saturated Ag/AgCl electrode. In order to deduce the redox potential relative to a standard hydrogen electrode, a value of 204 mV may be added. The metal concentrations in each leaching solution were measured by ICP-MS (Inductively Coupled Plasma Mass Spectrometry) using a VG PlasmaQuad PQ2Plus. During the measurements the high concentration of the leaching solutions can cause matrix effects which were for the measurement of the elements with high concentrations avoided by using dilution and by using In as an internal standard. To avoid interferences and matrix effects, the elements with low concentrations were separated from the matrix with ion exchange and then measured. For the ion exchange an automatic TRACECON system (Knapp Logistic Automation) was used, equipped with an EDTrA column. For the water soluble step, the chloride and sulphate concentrations were measured with CZA (Capillary Zone Electrophoresis, Waters Quanta 4000). The leaching solutions of the other steps were not analysed for anions, because of the high anion concentration of the original leaching solutions compared with the dissolved anions.
2.1.3. Leaching experiments at different pH values The results of the water soluble and acid soluble step can be compared with earlier leaching experiments at different pH [2]. During these experiments, HCl solution was added to the fly ash with a liquid to solid ratio of 10 1/kg. The concentration of the HCl-solution was varied in order to obtain a varying final pH. The leaching efficiency is defined as the ratio of the leached amount of a given element over the total amount of this element in the sample, and is expressed in percent. pH values higher than 10 were obtained using NaOH instead of HCl. Fig. 1 gives the leaching efficiency of several elements as a function of the final pH. 2.2. Results and discussion The sequential extraction procedure was performed four times and the results are always given as the mean of these four results together with the standard deviation. Table 3 compares the sum of the concentrations obtained during the sequential extraction procedure with those obtained with total dissolution. In general the results match rather well in spite of the possible errors that can be made during the sequential extraction procedure. Indeed, after each step the slurry is first centrifuged and then the supernatant is removed. It is possible that a small amount of solids is removed with the supernatant, which might lead to a lower metal
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concentration in the following steps. Table 4 gives the pH and redox potential during the different steps of the procedure. Tables 5 and 6 give the results of the sequential extraction procedure for Na, K, Al, Ca, Mg, Zn, Pb, Cu, and Cd. 2.2.1. Water soluble step Extraction in the water soluble step is high for Na (54.6%), K (71.1%) and Ca (21.3%). These metals occur as easily soluble salts in the fly ash. The other metals in Tables 5 and 6 have a water soluble fraction below 5%. Table 7 gives the percentage of chloride and sulphate leached in the water soluble step, relative to the amount measured by total dissolution of the fly ash sample. It is clear that most of the chloride and sulphate leach in the water soluble step. Probably Na, K and Ca occur as chloride and sulphate salts in the fly ash. High solubilities of chlorides and sulphates were also reported by Kirby and Rimstidt [16]. These highly soluble salts can contaminate groundwater near the disposal site and increase the solubility of heavy metals through complexation, or can promote corrosion when the ashes are used as a construction material. Eighmy et al. [17] gave an overview of five studies with X-ray diffraction. According to these studies Na occurs as NaCl or Na2SO4, K as KC1 and K2SO4 and Ca as CaSO4. Similar results were reported by Giordano et al. [18]. The final pH after leaching is 10.5 (Table 4). This indicates that part of the dissolved metals also occur as
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basic metal salts, oxides or hydroxides. It is possible that some of the heavy metals like Zn occur in the fly ash as very soluble salts, e.g. Cl-salts which dissolve immediately. Because of the high pH of the solution, hydroxides are formed and the metals precipitate. The conclusion is that with only the results of the sequential extraction procedure, it is hard to say which mineral of a certain metal occurs in the fly ash; only assumptions can be made. The results of the procedure explain only part of the interaction with the leaching solution and not the speciation of the material, although it will help to determine the speciation in combination with other techniques. In Van Herck et al. [1] computer modelling using the MINTEQA2 program was applied to calculate which minerals precipitate and to explain the curves giving the leaching efficiency as a function of pH and the results of the sequential extraction. The results of the leaching experiments given in Fig. 1 show that for a pH around 10, the leaching efficiency for most elements is very low. This is in agreement with the Table 7 Leaching of chloride and sulphate in the water soluble stepa % Cl SO4 a
854 901
The results are expressed as relative percent extracted and correspond to the mean and standard deviation of four results.
Fig. 1. Leaching efficiency (%) as a function of the final pH for MSWI fly ash.
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results of the water soluble step of the sequential extraction procedure.
final pH of the oxidizable step were the same as for the acid soluble step.
2.2.2. Ca exchangeable The Ca exchangeable fraction (final pH=9.3) is small for all the metals; only Cd reaches a value above 5%. In general, the Ca exchangeable fraction is too small to be significant, so that the second extraction step could be omitted.
2.2.5. Reducible steps The two reducing steps show low fractions for the heavy metals. Only the moderately reducible fraction for Cu is above 5%. The fraction is higher for the matrix elements. The redox potential in the moderately reducible step was not measured. Again the pH of both steps differs from the previous steps and should be adjusted.
2.2.3. Acid soluble step The acid soluble fraction (final pH=3.1) is very high for the heavy metals, the values range between 79 and 90%. Probably oxides and carbonates of these metals dissolve under acidic conditions. Cd has the highest acid soluble fraction (89.8%). Na and K have a lower acid soluble fraction because a major portion of these elements has already been leached during the water soluble step. For Al, Ca and Mg the acid soluble fraction is the largest fraction, but the relative amount is lower than for the heavy metals Zn, Pb, Cu, and Cd. The leaching experiments with varying pH (Fig. 1) show that at a pH of 3.1, as obtained during the acid soluble step, the extraction is complete for Cd and almost complete for Zn. However, the available fraction for Al is only partly dissolved at a pH of 3.1. The leaching efficiency for Cu and Pb is still below 10% at a pH of 3.1 (Fig. 1). The results of the sequential extraction procedure (Table 6), however, show that ca. 80% of Cu and Pb dissolves. This can possibly be explained by the complexing capabilities of acetic acid used in the acid soluble step. Cu and Pb form stable acetate complexes while the complexation with chloride is less stable [19]. It is important to realise that the results of the sequential extraction procedure are influenced by the type of leaching solution used. When, in the acid soluble step, acetic acid is used to extract the metals, high leaching efficiencies for Pb and Cu are obtained (Table 6). When the fly ash is leached with HCl, the leaching efficiencies for the same pH (3.1) are much lower. 2.2.4. Oxidizable step The oxidizable fraction is below 5% for all the metals except for Pb (7.22%). The pH in this step is 10.1 and the redox potential 210 mV. This step gives no good indication of the amount of metals that dissolve under oxidizing conditions, because the redox potential is even lower than in the previous step. So, metals that oxidize at this redox potential would already have been oxidized in the previous, acid soluble step. The results of the oxidizing step are doubtful and therefore it would be better to use another reagent such as NaOCl in order to create stronger oxidizing conditions. The pH of this step is also completely different from the one in the acid soluble step, which can possibly lead to a misinterpretation of the results. It would be better if the
2.2.6. Residue For Al and Mg the fractions contained in the residue (24%) are highest. For Na, K, Ca, Zn (8.6%), and Cu (7.1%) and mainly for Pb (2.8%) and Cd (1.8%) they are much lower.
3. Modification of the sequential extraction procedure and study of the blast furnace sludge 3.1. Experimental 3.1.1. Modified sequential extraction procedure The experiment with the MSWI fly ash showed that the results of the extraction procedure used are not always optimal and may lead to wrong conclusions. Some recommendations for changes in the extraction procedure will therefore be made. First of all, the Caexchangeable step can be omitted, because the amount of metals leached in this step is very small for incineration residues. For the acid soluble step it was pointed out that the final pH of the leaching solution is too high. The pH should be around 2.5, if one wants to almost completely extract the acid leachable fraction of Al and Zn. At this pH all the metal carbonates are dissolved. In a new procedure a pH of 2.5 is obtained by leaching with acetic acid (0.5 M) and adjusting the pH to 2.5 during the reaction by adding 12 M HCl. Using acetic acid and hydrochloric acid together has the advantage that lower final pH values are reached, while acetic acid may still, to some extent, form complexes with heavy metals that are more stable than chloride complexes for Zn and Pb. There was also some doubt about the oxidizing capabilities of the oxidizable step. Instead of Na4P2O7, NaOCl should be used as a stronger oxidizing reagent. During the extraction the redox potential relative to a saturated Ag/AgCl electrode was measured and adjusted to around 1200 mV by adding extra NaOCl. During this step the pH was also maintained at ca. 2.5 by adding HCl. Maintaining the pH at 2.5 has the advantage that it becomes possible to compare the acid soluble and the oxidizable steps. When a metal only dissolves during the oxidizable step, it means that at pH 2.5 the metal needs to be oxidized before it dissolves.
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The pH of the easily reducible step is also maintained at pH 2.5 by adding hydrochloric acid to the solution. However, for the moderately reducible step this procedure was not possible, because the reaction must occur in the dark during 24 h. Measuring the pH and redox potential during the extraction might disturb the reaction conditions. The final step remains the same as in the previous procedure. Table 8 presents the adapted sequential extraction procedure. In the leaching solutions the concentration of Al, Ca, Fe, Mg, Mn, Pb, and Zn were measured along with the pH and the redox potential relative to a saturated Ag/ AgCl electrode. 3.1.2. Material The results of the sequential extraction procedure, performed on the MSWI fly ash, showed that the redox potential would only have a small influence, as most of the heavy metals have already been leached almost completely after the acid soluble step. Performing the new procedure on the fly ash would therefore show little difference, so that another waste material was tested, blast furnace sludge. Table 9 gives the total composition of the dry solids (DS) in the blast furnace sludge, obtained from a Table 8 Adapted sequential extraction procedure used for blast furnace sludge Fraction
Leaching solution
1 2
Water soluble Acid soluble
3
Oxidizable
4
Easily reducible
5
Moderately reducible Residue
6
Distilled water, 3 h 0.5 M CH3COOH and add 12 M HCl until pH=2.5, 3 h 1.9 M NaOCl until E=1200 mV and 12 M HCl until pH=2.5, 3 h 0.175 M (NH4)2C2O4+0.1 M H2C2O4 and add 12 M HCl until pH = 2.5, 3 h 0.l M Na2EDTA+0.3 M NH2OH.HCl, 24 h Total dissolution with HF, HCl and HClO4
metallurgical company (Sidmar N.V., Gent, Belgium). The concentration of iron is very high, so that the sludge could be regarded as a raw material that could be recycled in the blast furnace. The concentration of zinc and to a lesser extent lead is however too high to allow direct recovery in the blast furnace. 3.1.3. Leaching experiments at different pH values Similar leaching experiments as for the MSWI fly ash were performed on the blast furnace sludge. HCl-solutions were used to obtain the varying final pH. When FeCl3 is used instead of HCl the pH also varies, but the redox potential is increased. Fig. 2 gives the leaching efficiency for Zn as a function of the final pH for both experiments. 3.2. Results and discussion Table 10 gives a comparison between the total concentrations of metals leached during the complete sequential extraction procedure and the concentration measured by total dissolution of the material. The results are in rather good agreement. The agreement is comparable with the one with the earlier procedure applied to MSWI fly ash (Table 3). Table 11 gives the final pH and redox potential after reaction. The redox potential is relative to a saturated Ag/AgCl electrode.
Table 10 Comparison of the total composition of blast furnace sludge measured with the sequential extraction procedure and with total dissolution (% by weight)
Al Ca Fe Mg Mn Pb Zn
Sequential extraction
Total dissolution
1.26 0.94 8.62 0.35 0.04 0.99 7.83
1.40 0.92 9.28 0.40 0.05 1.28 8.44
Table 9 Total composition of the blast furnace sludgea Component
DS (g/kg)
Al Ca CO23 Fe Mg Mn Pb SiO2 Zn
102 5.90.8 239 867 3.80.2 0.470.03 82 40.7 3911
Table 11 Final pH and redox potential in the different steps of the sequential extraction procedure for blast furnace sludgea
Step 1 Step 2 Step3 Step 4 Step 5 Step 6
pH
E (mV)
6.2 2.4 2.2 2.2 3.2 –
106 360 1105 501 72 –
a
The results are given as the mean and standard deviation of four samples. For anions only one sample was analysed.
a
The redox potential is relative to a saturated Ag/AgCl electrode.
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The alkalinity of the blast furnace sludge is lower than that of the MSWI fly ash, as can be seen from the pH of the water soluble step (6.2 compared with 10.5). The pH’s in the following three steps are almost the same, so they can be compared with each other. The redox potential in the oxidizable step is sufficiently high. The redox potential of the reducible steps is lower, as should be the case. Table 12 gives the metal concentrations in the different steps. The results are given as the fraction of the total measured metal concentration that is extracted in a given step. 3.2.1. Water soluble step The fraction of the metals in the water soluble step (pH=6.2) is lower than 1%, except for Ca and Mg. 3.2.2. Acid soluble step The fraction of Al, Ca and Mg is intermediate, the values lie between 29.5 and 57.1%. Fe and Mn give an
acid soluble fraction of 14.2 and 20.7%, respectively. Pb and Zn have high acid soluble fractions (72.0 and 80.0%). The pH of this step is maintained at 2.4 to be sure that all the carbonates are dissolved. This result can be compared with the leaching experiments with HCl (Fig. 2). The efficiency for Zn at pH 2.4 is about 75%, in excellent agreement with the 80.0% of Zn in the acid soluble step. For the other metals a good agreement was also found. 3.2.3. Oxidizable fraction In the oxidising step the redox potential is high enough to oxidize iron, while the pH of the solution is almost the same as in the previous step. So, if a metal salt dissolves only in this step, it means that oxidizing conditions are necessary to dissolve the salt. The fractions are around 5% or lower for Ca, Fe, Mg and Mn. The fraction of Al is somewhat more elevated (9.1%). Zn and Pb have both a fraction between 15 and 20%. So a considerable amount of Zn and Pb present in the
Fig. 2. Leaching efficiency (%) for Zn as a function of the final pH for leaching with HCl or FeCl3 for blast furnace sludge.
Table 12 Metal fractions (%) in the different steps of the sequential extraction procedure for blast furnace sludge
Step 1 Step 2 Step 3 Step 4 Step 5 Step 6
Al
Ca
Fe
Mg
Mn
Pb
Zn
0.04 57.1 9.11 8.15 0.21 25.4
18.1 44.9 4.49 5.26 10.7 16.5
0.02 14.2 5.27 39.0 0.70 40.8
12.2 29.5 2.05 12.8 1.89 41.5
1.16 20.7 3.95 26.7 4.66 42.8
0.05 72.0 18.0 3.44 4.72 1.79
0.06 80.0 16.6 2.20 0.39 0.89
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solids of the sludge can only dissolve under oxidizing conditions. The redox potential has a substantial influence on the leaching behaviour of Zn and Pb. The redox potential in this step was 1105 mV. At pH 2.4 about 95% of the total amount of Zn is dissolved (Fig. 2) when FeCl3 is used. When only HCl is used to leach the sludge, the leaching efficiency is 75%. The difference between these two leaching efficiencies, 20%, is the amount of Zn present as an oxidizable mineral, presumably sfalerite, ZnS, in the sludge particles. The oxidizable fraction in the sludge, determined with the sequential extraction procedure, gives 16.6%, in good agreement with the previous value. 3.2.4. Easily reducible step In this step the redox potential is lower than in the previous step. The results show that 39% of the iron present in the sludge dissolves during this step. Theoretically all the Fe(II) will dissolve at pH=2.5 in the acid soluble step, whereas at pH=2.5 (acid soluble and oxidisable steps) Fe(III) remains precipitated. In the easily reducible step, the redox potential is low enough to reduce, in the absence of Fe(II), most of the Fe(III) to Fe(II) at pH 2.5, which will dissolve at this pH. Although the dissolution of Fe(II) in the acid soluble step (and water soluble step), leads to lower redox potentials than in the reducible step, obviously, when a very large excess of Fe(II) is present no reduction of Fe(III) to Fe(II) takes place. The extracted fraction for Mn is 26.7% The research literature shows that iron and manganese often occur together as oxides, which possibly encapsulate other elements [9,10,12] . Pb leaches only for 3.4% and Zn for 2.2%. The fractions for Al, Ca and Mg are between 5 and 13%. 3.2.5. Moderately reducible fraction Although the redox potential is still lower, almost no additional Fe is leached during this step and only 4.6% of the Mn is leached. All the iron that could be reduced was already reduced in the easily reducible step. The fractions of the other metals are below 5%, except for Ca (10.7%). 3.2.6. Residue In the residue of the sludge, considerable amounts of Al, Ca, Fe, Mg and Mn are still found. These elements, together with Si, form the matrix of the particles or are encapsulated in matrix. Only 1.79% of Pb and 0.89% of Zn are present in the residue. The two metals are almost completely dissolved in the previous steps.
4. Conclusions The sequential extraction procedure can be a help in understanding which form the different metals take in
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the fly ash, giving an indication of the mineralogy of the element. It is however uncertain if the accuracy of the method is high enough to use the results for a quantitative partitioning of the metals into the fractions described. In the water soluble step, chlorides and sulphates dissolve together with Na, K and part of the Ca. The Ca exchangeable fraction is low for the metals and can be omitted for testing waste material. The largest fraction for the heavy metals is the acid soluble fraction. Probably oxides and carbonates of these metals dissolve. However, the pH should be adjusted to 2.5 with HCl, for example, to ensure complete dissolution of carbonates and oxides. The results of the oxidizing step in the original procedure are doubtful because of the low redox potential compared with the redox potential of the acid soluble step. Therefore, it would be better to use another reagent such as NaOCI in order to obtain stronger oxidizing conditions. The pH should be set to 2.5 to make a comparison with the other steps possible. This comment is also valid for the reducible steps. Therefore, a modified procedure is proposed. It can be concluded that acid leaching will be very important for the treatment of fly ashes while the influence of the redox potential is small because most of the heavy metals have already leached almost completely after the acid soluble step. The results of the modified sequential extraction procedure, used on blast furnace sludge, show that Zn and Pb dissolve in acid while the leachability of iron in acid is rather low. Also, the influence of the redox potential on the teachability is obvious. Oxidizing conditions are necessary to leach an additional amount of about 15% of the zinc present. It can be concluded that the performance of the modified sequential extraction procedure is much better than that of the previous procedure, as shown by the agreement with the leaching experiments at varying pH and redox potential. The different steps now give a better understanding of the leaching behaviour in view of recycling the waste material. The results, however, must be evaluated carefully. It is never possible to reach complete selectivity in each step as is also pointed out in literature. The interpretation of the results is only a qualitative one. It can be deduced whether a large fraction of the material dissolves in water or acid or whether oxidizing or reducing conditions are necessary. From this research it appeared that an oxidizing environment was necessary in order to reach sufficient removal of Zn from the sludge. The selectivity of the method is not good enough to investigate the speciation of the metals in the waste material. It only gives an indication of the equilibrium between solid material and leaching solution and the results must be compared with e.g. results of XRD to learn more about the speciation of the elements. For the blast furnace sludge this was carried out in by Van Herck et al. [3] and it was shown that after
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leaching with acid zinc occurred mainly as sfalerite (ZnS) and partly as Franklinite (ZnO.Fe2O3). Comparison of the leaching behaviour of the sludge with that of sfalerite and Franlinite geochemical standards confirmed this and showed that sfalerite dissolves in acid oxidising conditions, whereas Franklinite practically does not dissolve.
Acknowledgements Grateful acknowledgement is made to SIDMAR N.V. and V.L.I.M. (Vlaams Impulsprogramma voor Milieuonderzoek) for their support to this study. References [1] Van Herck P, Yan der Bruggen B, Vogels G, Vandecasteele C. Application of computer modelling to predict the leaching behaviour of heavy metals from MSWI fly ash and comparision with a sequential extraction method. Waste Management 2000;20:203. [2] Van der Bruggen B, Vogels G, Van Herck P, Vandecasteele C. Simulation of acid washing of municipal solid waste incineration fly ashes in order to remove heavy metals. Journal of Hazardous Materials 1998;57:127. [3] Van Herck P, Vandecasteele C, Swennen R, Mortier R. Zinc and lead removal from blast furnace sludge with a hydrometallurgical process. Environ Sci Technol 2000;34:3802. [4] Wille D. and De Boeck G., Inventarisatie Huishoudelijke Afvalstoffen in Vlaanderen in 1994, Productie, Inzameling en Verwerking, Openbare Afvalstoffenmaatschappij voor het Vlaamse Gewest, Publicatie nr. D/1996/5024/4, Mechelen, Belgium, (1996). [5] Senelle R, Dujardin J, van Damme M. VLAREM II. Brugge, Belgium: Die Keure La Charte, 1995.
[6] Vehlow J, Brown H, Horch K, Merz A, Schneider J, Stieglitz L, et al. Semi-technical demonstration of the 3R-process. Waste Management and Research 1990;8:461. [7] Mulder E. Pretreatment of MSWI fly ash for useful application. Waste Management 1996;16:181. [8] Tobler HP. Behandlung und verfestigung von ruecksta¨nden und muellverbrennungsanlagen in der Schweiz. Entsorgungspraxis Special 1988;1988:188. [9] Tessier A, Campbell PGC, Bisson M. Sequential extraction procedure for the speciation of particulate trace metals. Analytical Chemistry 1979;7:844. [10] Kirby CS, Rimstidt JD. Mineralogy and surface properties of municipal solid waste ash. Environ Sci Technol 1993;27:652. [11] Kersten M, Fo¨rstner U. Chemical fractionation of heavy metals in anoxic estuarine and coastal sediments. Wat Sci Tech 1986;18:121. [12] Theis TL, Padgett LE. Factors affecting the release of trace metals from municipal sludge ashes. Journal WCPF 1983;55:1271. [13] Francis AJ, Dodge CJ, Rose AW, Ramirez AJ. Aerobic and anaerobic microbial dissolution of toxic metals from coal wastes: mechanisms of action. Environ Sci Technol 1989;23:435. [14] Nirel PMV, Morel FMM. Technical note: Pitfalls of sequential extractions. Wat Res 1990;24:1055. [15] Tessier A, Campbell PGC. Comment on ‘‘Pitfalls of sequential extractions’’. WatRes 1991;25:115. [16] Kirby CS, Rimstidt JD. Interaction of municipal solid waste ash with water. Environmental Science and Technology 1994;28:443. [17] Eighmy TT, Eusden JD, Krzanowski JE, Domingo DS, Stampfli D, Martin JR, et al. Comprehensive approach toward understanding element speciation and leaching behaviour in municipal solid waste incineration electrostatic precipitator ash. Environmental Science and Technology 1995;29:629. [18] Giordano PM, Behel AD, Lawrence JE, Solleau JJ, Bradford BN. Mobility in soil and plant availability of metals derived from incinerated municipal refuse. Environ Sci Technol 1983;17:193. [19] Sille´n LG, Martell AE. Stability constants of metal-ion complexes. Burlington House, London, England: The Chemical Society, 1964.