Energy & Fuels 2005, 19, 807-812
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Leaching of Toxic Elements from Lignite and Agroresidue Ashes in Cultivated Soils of Crete D. Vamvuka,* J. Hahladakis, and D. Pentari Department of Mineral Resources Engineering, Technical University of Crete, Greece Received July 31, 2004. Revised Manuscript Received January 31, 2005
The agricultural residues of Crete are considered to be of premium importance for local energy production, substituting a large part of conventional fuels. Future co-firing applications of local lignite and agroresidues may create problems related to the disposal of ashes, which contain harmful constituents. In this regard, the mobility of trace elements from selected lignite and biomass ashes, as well as mixtures of them, was investigated through batch and sequential extraction procedures. In these tests, leaching conditions were aimed at a close approximation to field conditions. The concentrations of heavy metals released in local soil of acidic nature were determined by atomic absorption spectroscopy and were compared to those in the original samples. The results demonstrated that toxic metal ions were released in low quantities, below the legislative limit values, with the exception of Cr extracted from specific lignite/biomass mixtures. This leaching behavior was attributed to the incorporation of the trace elements in the crystal lattice of the soil minerals. A small portion was also captured by carbonates and/or iron oxides of the soil or the ash residue. Biomass ash addition to the lignites’ ashes was beneficial, reducing the heavy metal contents of the soil leachates.
Brown coals constitute the major energy source in Greece. The annual lignite production is over 70 million tons, which makes Greece the second largest lignite producer in the European Community market.1 This amount covers ∼70% of the demand for electricity production; however, it is characterized by high ash contents (the average value on a dry basis is 28.2%, with minimum value 8.3% and maximum value 48.2%), which results in environmental pollution, as well as the consumption of higher amounts of feedstock by power plants. As energy demands increase, new energy technologies, which can offer improved efficiency and minimum environmental impact, are needed. Biomass combustion has the potential to play an increasing role in meeting these demands. In Greece, where the economics are largely based on agriculture and forestry, plant biomass presently available in the form of agricultural residues has been found to be equivalent to 40-50% of the gross energy consumption.2 Hence, co-utilization of these materials with local coals for power production may provide environmental, technical, and economical benefits. The most convenient use of energy from such a process is in places where it is formed naturally or where it is produced as a process refuse. The island of Crete in the eastern Mediterranean contains numerous lignite deposits, in the order of 2.5 × 106 tons, varying in age from Middle Pliocene
(Plakias) to Pleistocene (Kandanos).3 Also, there is a large amount of biomass in the form of forest and agricultural residues, such as olive kernel, citrus tree wood, etc. The annual production of olive kernel only is equivalent to 40-60 ktoe, which accounts for approximately 1/3 of the total electricity consumed. Several plans have been formulated to exploit this renewable potential, suggesting that biomass residues can substitute a large part of conventional fuels. Most of these technologies are mature, can be embodied in the Cretan energy system, and contribute to the local/ regional development.4 However, all power generating plants produce a number of residues (bottom ash, boiler ash, fly ash, etc.), the relative amount of each one depending on the nature of the feed, the power plant configuration, and the emission control devices.5 An important issue which needs to be faced by these industrial plants is the environmental impact, resulting from the release of volatile heavy metals and other inorganic compounds contained in solid wastes. These elements are absorbed by the fly ashes, uncontrolled disposal of which may pose a significant risk to the environment due to possible leaching of toxic trace elements into nearby underground and surface waters, where their fate is relevant to the health of plants, animals, and humans.6,7 In this setting, energy from coal/biomass mixtures will be a sustainable and clean technology only if the solid byproducts can also be integrated into the biosphere.
* Author to whom correspondence should be addressed. Tel: +30 2821 37603. Fax: +30 2821 69554. E-mail:
[email protected]. (1) Vamvuka, D.; Mistakidou, E.; Drakonaki, S.; Foscolos, A.; Kavouridis, K. Energy Fuels 2001, 15(5), 1181. (2) Tsoutsos, T.; Umealu, O.; Koukios, E.; Marton, G. Proceedings of the 6th E. C. Conference in Biomass for Energy, Industry and Environment, Elsevier: Athens, 1991; p 1021.
(3) Gentzis, T.; Goodarzi, F.; Koukouzas, C.; Foscolos, A. Int. J. Coal Geol. 1996, 30, 131. (4) Vamvuka, D.; Sdrolias, T.; Christou, M.; Tsoutsos, T. Proceedings of the 1st World Conference on Biomass for Energy and Industry, James & James Ltd.: Sevilla, 2000; p 991. (5) Seames, W. S.; Sooroshian, J.; Wendt, J. O. L. Aerosol Sci. 2002, 33, 77.
Introduction
10.1021/ef040067y CCC: $30.25 © 2005 American Chemical Society Published on Web 03/18/2005
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The concentration of trace elements in coal and to a lesser extent in biomass ashes, as well as its effect on soils and groundwaters, has been the subject of numerous scientists.8-15 However, the solubility of these elements is a function of many factors, such as mode of occurrence, pH, oxidation-reduction conditions, etc.10,16 Furthermore, the combined use of coal and biomass ashes for land application could provide important information for environmentally and agronomically sound management practices. The objective of this study was to assess the leaching behavior of different elements of local lignite and agroresidue ashes, as well as mixtures of them, by means of standard leaching tests. Chemical characterization of these materials was also included in the aim of this work, as a prerequisite to study the leaching characteristics and the consequent environmental aspect of ash disposal. The obtained data will help guide future studies aimed at evaluating the above blends under more likely environmental conditions in the context of co-firing applications of these fuels for power production. Experimental Section Materials. Two lignite samples, one from the Ptolemais basin in North Greece and one from Kandanos deposit in South Crete, as well as three biomass materials, as being the most abundant at the island of Crete, namely olive kernel, citrus tree wood, and forest residues, were used in this work. One soil sample was collected from the area of Kandanos. Characterization of Coal and Biomass Samples. After air-drying, homogenization, milling, and riffling, coal samples were ground to -250 µm and biomass samples to -425 µm. Each sample was then characterized in terms of proximate analysis, according to the ASTM standards (D3172-89, E871, D1102-84) using programmable laboratory furnaces, the ultimate analysis using a Leco type analyzer CHN-600, S532-500 (ASTM D3176-93, D3177-33), mineralogical analysis of ash using an X-ray diffractometer (XRD) type D-500 of Siemens, and chemical analysis of major elements in ash using an X-ray fluorescence spectrometer (XRF) type SRS-303 of Siemens (ASTM D4326-94). Trace elements were determined by atomic absorption spectroscopy (AAS), using a Perkin-Elmer model Analyst-100 spectrometer with a graphite furnace assembly (model HGA 800) and a deuterium arc lamp background correction system. Ammonium phosphate (NH4H2PO4) was used as a matrix modifier for Pb, while magnesium nitrate (MgNO3) was used in the case of Co, Cr, Ni, and Zn. The NIST standard reference material 16336 was analyzed along with the samples to check the correctness of the analytical proce(6) Clarke, L. B.; Sloss, L. S. Trace Elements Emissions from Coal Combustion and Gasification; IEA Coal Research, IEACR/49: London, 1992. (7) Lee, S.; Spears, D. A. Groundwater Contaminants and their Migration; Geol. Soc. Spec. Pub.: London, 1998; p 368. (8) Querol, X.; Juan, R.; Lopez-Soler, A.; Fernandez-Turiel, J.; Ruiz, C. R. Fuel 1996, 75(7), 821. (9) Fytianos, K.; Tsanikidi, B.; Voudrias, E. Environ. Int. 1998, 24(4), 477. (10) Swaine, D. J. Fuel Process. Technol. 2000, 65-66, 21. (11) Senior, C. L.; Helble, J. J.; Sarofim, Fuel Process. Technol. 2000, 65-66, 263. (12) Dare, P.; Gifford, J.; Hooper, R. J.; Clemens, A. H.; Damiano, L. F.; Gong, D.; Matheson, T. W. Biomass Bioenergy 2001, 21, 277. (13) Georgakopoulos, A.; Filippidis, A.; Kassoli, A.; FernandezTuriel, J.; Llorens, J.; Mousty, F. Energy Sources 2002, 24, 103. (14) Seferinogˇlu, M.; Paul, M.; Sandstro¨m, Ko¨ker, A.; Toprak, S.; Paul, J. Fuel 2003, 82, 1721. (15) Ciccu, R.; Ghiani, M.; Serci, A.; Fadda, S.; Peretti, R.; Zucca, A. Miner. Eng. 2003, 16, 187. (16) Van der Sloot, H. A.; Heasman, L.; Quevauviller, P. Harmonization of Leaching Extraction Tests; Elsevier: Amsterdam, 1997.
Vamvuka et al. dure. Prior to AAS analysis, all samples were dissolved by digestion with HCl, HF, and HNO3 acids in Teflon beakers heated in a water bath at 80 °C. Characterization of Soil. The soil sample, after passing through a 2-mm sieve, was subjected to particle size analysis and determination of the sand, silt, and clay proportions via the hydrometer method.17 Elemental composition was ascertained by means of the elemental analyzers Leco HF10 and Gasometric Carbon analyzer 572-100, while mineralogical and chemical analysis was performed using the same analytical methods as for the coal and biomass samples. Soil pH was measured with an electrode from 1:1 water to solid slurries, whereas cation-exchange capacity was measured by applying the ammonium acetate method.18 Leaching Procedures. Prior to the leaching tests, hightemperature ashes from the coal and biomass samples were prepared by heating at 780 °C to constant weight in a laboratory muffle. Blends of coal/biomass ashes were also prepared at proportions of 90:10. These were generated under the same experimental conditions from the combined combustion of coal and biomass samples. A batch leaching procedure in five steps using deionized water as the leaching medium was adopted in this work, as preliminary continuous column leaching experiments, simulating the release of components from a soil-ash mixture to a water phase, proved to be extremely slow. In this serial batch test, the same quantity of material was extracted several times with fresh deionized water, to get an estimate of long-term leaching behavior. This is a simple and fast way to obtain information about the order of magnitude of the relative element mass that can be leached from an ash residue. To maintain compatibility with field conditions, soil-ash mixtures were prepared at a ratio of 95:5 and no extraction with strong acids was applied. For each leaching step, 5 g of dry sample (soil-ash mixture) was put into a 100-mL centrifuge tube, where 50 mL of deionized water was added (solid-to-liquid ratio S/L ) 1:1019). After centrifugation (Biofuge 22R by Heraeus) at 9000 rpm for 15 min, the supernatant was decanted and filtered through a 0.45-µm micropore membrane filter and the pH of the extract was measured. This process was repeated five times. The filtered leachates were transferred to 50-mL plastic vials and analyzed for trace element concentrations by AAS. For comparison reasons, trace elements were also determined in the soil-water extracts. To investigate the type of bonding of trace elements in the samples and hence their potential dissolution under various conditions, it was necessary to carry out some sequential extraction tests with various reagents, as proposed by Tessier et al.,20 Schuman,21 and Sims.22 Thus, 1 N NaCl at pH 7 was first used to obtain exchangeable ions for a S/L ) 1:4. Subsequently, the same sample was treated with 1 M CH3COONa at pH 4.5-5 for S/L ) 1:4 to determine the elements which were bound to carbonates. Then, a mixture of 0.2 M (NH4)2C2O4, 0.2 M H2C2O4 (pH 3), and 0.1 M C6H8O6 for S/L ) 1:10 was used to determine the elements which were bound to Fe/Al oxides followed by an attack with aqua regia (S/L ) 3:1) to study the elements tied up in the crystal lattice of the minerals. The trace element contents of the four leachates were determined using the same analytical method as before.
Results and Discussion Chemical Analyses of the Samples and Soil. The proximate and ultimate analysis results of the samples (17) Bouyoukos, G. H. Agr. J. 1951, 43, 434. (18) Schollenberger, C. J.; Simon, R. H. Soil Sci. 1945, 59, 13. (19) European Community Council Directive 91/96/WC. Hazardous Wastes EWC Codes 1001 and 1901, CWC Codes 1001 and 1901, Council decision 22/12/94 (94/904/EC), 1994. (20) Tessier, A.; Cambell, P. G. C.; Bisson, M. Anal. Chem. 1979, 51, 844. (21) Schuman, L. M. Soil Sci. 1986, 140, 11. (22) Sims, J. T. Soil Sci. Soc. Am. J. 1986, 50, 367.
Toxic Leaching from Lignite and Agrogresidue Ashes
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Table 1. Proximate and Ultimate Analyses of the Samples (% Dry Weight)
minerals, identified in the mixtures, were identical with those identified in the lignites only due to the small percentage of biomass in them. However, in the case of Ptolemais lignite/biomass blends, a new phase, monticellite, was formed from quartz and periclase, which were present in the original lignite. The XRD pattern of the soil in Figure 1 reflects the origin of the parent rock of the surrounding area, which is phyllitic and quarzitic.23 Some phosphorus minerals were also identified, the presence of which can be explained by the fact that the soil was sampled under olive trees during the fertilizing period. From the chemical analysis of the ashes in major oxides (Table 3), it is obvious that some elements fingerprint the different regional geological setting between the Ptolemais and Kandanos lignite deposits. Thus, the ash of Kandanos showed remarkably higher concentrations of Si, Al, Fe, K, and Na oxides, with lower concentration of Ca oxide than the ash produced by the combustion of lignite from Ptolemais basin. Kandanos ash had the highest amount of Si (40%) and the lowest amount of Ca (0.7%) among all samples. Biomass ashes were rich in Ca and Si oxides and contained much more K, Na, and P (except forest residue) than lignitic ashes. It is noteworthy that the percentage of Ca in the citrus tree wood ash was 53%, whereas that of K in the forest residue ash was over 16%. The effect of biomass addition to the two lignites is practically apparent only in the case of lime, the concentration of which in the ashes was abundant. The concentrations of all other oxides in the mixtures were close to the lignites’ values. As concerns the chemical analysis of the soil, the high concentrations of Al, Si, and Fe, determined by AAS, are in accordance with the results obtained by the hydrometer method in Table 4 (i.e., sand, 42.5%; clays, 16.1%; silt, 41.4%). The determination of trace elements of ashes by AAS is presented in Figures 2 and 3. In all samples, the elements Co, Zn, Cr, Cu, Ni, and Mn were enriched, while Se and Pb ranged from 0 to 62 and 9 to 83ppb only and were omitted from the graphs. The concentrations of trace elements in the biomass ashes, especially in forest residue ash, were higher than those determined for the two lignites. Forest residue ash was very rich in
sample ptolemais lignite kandanos lignite forest residue olive kernel citrus tree wood pt. lignite/forest residue pt. lignite/olive kernel kd. lignite/olive kernel kd. lignite/citrus tree wood soil a
volatile fixed matter carbon ash
C
H
N
O
S
55.0 48.3 53.2 51.2 46.0 55.0
5.3 3.1 6.2 6.1 5.9 5.4
1.9 1.0 0.3 0.8 1.0 1.5
24.1 21.3 40.0 39.3 41.2 26.3
0.65 0.71 0.09 0.09 0.03 0.44
47.7 47.5 79.8 72.6 76.1 49.2
39.3 27.0 20.0 24.8 18.0 39.5
13.0 25.5 0.2 2.6 5.9 11.3
49.1
39.5
11.4 55.0
5.4 1.6 26.2 0.44
49.8
27.7
22.5 48.5
3.2 1.0 23.2 0.69
49.9
26.4
23.7 48.7
3.2 0.9 23.5 0.55
2.2a
0.08
Corg, 1.8; Cinorg, 0.4.
under study are compared in Table 1. The biomass materials are characterized by a low ash content, a very low sulfur content, and a high combustibles content, indicating the much better quality of these fuels than that of the two lignites. Low carbon and sulfur contents were also obtained for the soil. Furthermore, it can be seen that when the lignites were mixed with the biomass samples, these properties varied in proportion to the contribution of each component in the blend. Table 2 lists the crystalline mineral species in the samples, which were identified by XRD, to investigate their correlation with the leachability of the ashes. The main characteristic of these ash materials is that they are dominated by Ca-based minerals in the form of calcite, anhydrite, and portlandite. Anhydrite was formed by fixing sulfur dioxide with calcium oxide (derived from the decomposition of calcite) and/or dehydration of gypsum, while portlandite was formed by absorption of moisture by calcium oxide after ashing. Lignite ashes contained a significant quantity of lime as well. Moreover, all samples with the exception of citrus tree wood were rich in quartz and iron-based minerals. The speciation of K, which was abundant in the biomass samples, was dominated by carbonates (fairchildite), sulfates (arkanite), and K-feldspars. The presence of hydroxylapatite in olive kernel ash is most probably associated with the use of fertilizers in agriculture. The
Table 2. Mineralogical Analysis of Ash Samplesa sample mineral phases
ptolemais lignite
kandanos lignite
forest residue
olive kernel
citrus tree wood
pt. lignite/ forest residue
pt. lignite/ olive kernel
kd. lignite/ olive kernel
kd. lignite/citrus tree wood
quartz calcite anhydrite feldspars periclase hematite gehlenite goethite calcium oxide siderite hedenbergite dolomite fairhildite hydroxylapatite portlandite monticellite arcanite
++ + ++
+++
+ +++ + + +
+ ++ +
++ +++ ++
++ +++ ++
+++
+++
++
+ + +
+ + +
+++ +
++ +
+
+
+
+
+
+
a
+ + ++
++ ++
++
++
+
+ + + + ++ + +++ + +++ ++
+, low intensity; ++, medium intensity; +++, high intensity.
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Figure 1. XRD diagram of Kandanos soil. Table 3. Chemical Analysis of Ashes (1050 °C) in Major Oxides (%wt) sample
SiO2
Al2O3
Fe2O3
MnO
TiO2
CaO
K2O
MgO
P2O5
Na2O
SO3
loss of ignition (%)
ptolemais lignite kandanos lignite forest residue olive kernel citrus tree wood pt. lignite/forest residue pt. lignite/olive kernel kd. lignite/olive kernel kd. lignite/citrus tree wood soil
20.91 40.10 17.25 29.09 6.0 22.42 22.81 40.60 40.40 60.40
9.45 18.70 5.56 5.39 1.70 10.21 10.16 18.60 18.90 19.70
5.96 22.10 8.44 1.84 1.30 5.63 5.51 22.30 22.50 7.03
0.03 0.06 0.05 0.08 0.03 0.03 0.03 0.02 0.09
0.40 1.64 0.11 0.20 0.30 0.29 1.63 1.65 1.31
47.29 0.71 32.66 37.23 53.20 39.15 39.39 1.17 1.28 0.38
0.20 3.37 16.32 10.48 5.10 0.46 1.12 3.55 3.16 2.50
3.34 2.53 7.89 2.26 4.80 3.71 3.20 2.59 2.68 1.63
0.19 0.17 3.46 6.80 0.19 0.26 0.31 0.27 0.12
0.17 3.21 2.02 1.69 2.20 0.01 0.01 3.01 3.12 2.01
10.04 5.49 16.0 3.82 2.40 10.40 9.42 4.32 3.74 2.80
1.88 1.92 4.40 15.90 8.0 8.0 1.90 2.18 1.88
Table 4. Physical and Chemical Properties of Kandanos Soil sand (%) clays (%) silt (%) pH cation exchange capacity (mequiv/100 g) heavy metal content (ppm) Co Zn Cr Cu Ni Mn Se Pb
42.5 16.1 41.4 5.1 7.9 14.1 70.0 54.0 28.4 38.4 414.0
