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Distribution and mode of occurrence of uranium in bottom ash derived from high-germanium coals
JES-00501; No of Pages 8 J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 5 ) XX X–XXX
Available online at www.sciencedirect.com
ScienceDirect www.journals.elsevier.com/journal-of-environmental-sciences
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Yinglong Sun1 , Guangxia Qi1,2 , Xuefei Lei1 , Hui Xu1 , Lei Li1 , Chao Yuan1 , Yi Wang1,⁎
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1. Department of Environmental Engineering, School of Environment, Tsinghua University, Beijing 100084, China. E–mail: [email protected] 2. Department of Environmental Science and Engineering, School of Food and Chemical Engineering, Beijing Technology and Business University, Beijing 100048, China
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Distribution and mode of occurrence of uranium in bottom ash derived from high-germanium coals
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AR TIC LE I NFO
ABSTR ACT
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Article history:
The radioactivity of uranium in radioactive coal bottom ash (CBA) may be a potential danger
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Received 25 May 2015
to the ambient environment and human health. Concerning the limited research on the
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Revised 16 July 2015
distribution and mode of occurrence of uranium in CBA, we herein report our investigations
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Accepted 17 July 2015
into this topic using a number of techniques including a five-step Tessier sequential
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Available online xxxx
extraction, hydrogen fluoride (HF) leaching, Siroquant (Rietveld) quantification, magnetic
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Keywords:
showed that the uranium in the residual and Fe–Mn oxide fractions was dominant (59.1%
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Coal bottom ash
and 34.9%, respectively). The former was mainly incorporated into aluminosilicates,
Uranium
retained with glass and cristobalite, whereas the latter was especially enriched in the
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Radioactivity
magnetic fraction, of which about 50% was present with magnetite (Fe3O4) and the rest in
Tessier sequential extraction
other iron oxides. In addition, the uranium in the magnetic fraction was 2.6 times that in
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Magnetic separation
the non-magnetic fraction. The experimental findings in this work may be important for
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separation, and electron probe microanalysis (EPMA). The Tessier sequential extraction
establishing an effective strategy to reduce radioactivity from CBA for the protection of our
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local environment.
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© 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.
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Introduction
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Lignite is often used as fuel for many small to middle scale pithead power plants, and in some cases, as a raw resource for Ge smelters (Papastefanou, 2010; Dai et al., 2014c). However, in Lincang, Yunnan province, China, the average radioactivity of uranium (U) in lignite can reach 87.1 Bq/kg, much higher than that of the other types of coal (such as low-rank coals, middle-rank coals and high-rank coals) (Xiong et al., 2007; Yu, 2007). After burning, the natural radioactivity level of coal combustion ash is 4–10 times higher than that of the feed coals (Bhangare et al., 2014; Tripathi et al., 2013), which may be extremely dangerous for the surrounding environment and
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Published by Elsevier B.V.
human health. For example, the enrichment and transformation of radionuclides in coal fly ash and bottom ash has already caused secondary pollution, and has negatively impacted the local environment and human health in Yunnan province, China (Yu, 2007). Some late Permian coals are highly enriched in uranium (Dai et al., 2008, 2013a, 2013b, 2015a). Unfortunately, the radioactivity of their combustion residue (e.g., bottom ash) has not yet been studied in a great detail. In comparison with the abundant use of fly ash in construction materials (Dai et al., 2012; Camilleri et al., 2006; Eze et al., 2013; Lima et al., 2012), the coal bottom ash (CBA) is still stocked in piles close to coal fields, and could generate negative impacts on the surrounding environment including air,
⁎ Corresponding author. E-mail: [email protected] (Yi Wang).
http://dx.doi.org/10.1016/j.jes.2015.07.009 1001-0742/© 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
Please cite this article as: Sun, Y., et al., Distribution and mode of occurrence of uranium in bottom ash derived from highgermanium coals, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.07.009
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1. Materials and methods
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1.1. Samples and reagents
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The CBA samples were obtained from two different germanium (Ge) smelters in Lincang, Yunnan Province, China (samples no. 1 and no. 2). The samples were crushed with a ball mill, and then passed through a 500-mesh standard sieve (< 25 μm in diameter). The fine powder samples were dried at 105°C in a forced air oven to constant weight and stored in a
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1.3. HF leaching of the coal bottom ash residue
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A total of 5 g of the residual Tessier fraction was placed into a 150 mL Teflon beaker, and 80 mL of an HF solution (10%, 8%, 6%, 4%, or 2%, V/V) was added. The resulting suspension was magnetically stirred at 500 r/min for 20 min at room temperature, and then the slurry was centrifuged. The supernatant was collected and the residue washed with 80 mL of distilled water. The supernatant and washing solution were mixed and diluted up to 500 mL, and then a 10 mL-aliquot was withdrawn for a uranium content analysis. The solid residue was further washed twice, dried, and the weight of the residue was recorded. The leaching experiment was performed in duplicate.
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1.4. Magnetic separation of the coal bottom ash
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A total of 20 g of CBA was added to 2.0 L of distilled water, and the slurry was stirred vigorously with a magnetic rod (3000 G). This procedure was repeated until no more magnetic fraction adhered to the magnet. Then the residual parts were further subjected to a wet-type high intensity magnetic separator with a magnetic field intensity of 15,000 to 20,000 G to collect the weakly magnetic fractions. The weakly magnetic, strongly
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The combined samples were analyzed below. Tessier sequential extraction procedures were used to fractionate the uranium in the CBA into five components: exchangeable, bound to carbonates, bound to iron and manganese oxides, bound to organic matter, and remaining in residue (Tessier et al., 1979). The experimental procedures were analogous to those described in the literature (Wan et al., 2006; Smeda and Zyrnicki, 2002; Landsberger et al., 1995; Bódog et al., 1996). Briefly, (1) the CBA was extracted at room temperature for 3 hr with a sodium acetate solution (1 mol/L CH3COONa, pH 8.2) under continuous agitation; (2) the residue from (1) was leached at 50°C with a 1 mol/L sodium acetate solution adjusted to pH 5.0 with acetic acid (CH3COOH). Continuous agitation was maintained for 5 hr; (3) the residue from (2) was extracted with a 0.04 mol/L NH2OH–HCl solution in 25% (V/V) acetic acid. The extraction occurred at 60°C under continuous agitation for 8 hr; (4) a solution of 0.02 mol/L HNO3 and 30% H2O2 adjusted to pH 2 with HNO3 was added to the residue from step (3), and the mixture was heated at 85°C for 2 hr under continuous agitation. NH4Ac was then added and the sample was heated again to 65°C for 6 hr under continuous agitation; and (5) the residue from (4) was digested with a mixture of HF, HNO3 and HClO4 for total metal analysis. Each step was repeated four times, and the leachate was collected separately to measure the concentration of uranium and other major metals.
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1.2. Tessier sequential extraction of uranium in the coal bottom ash
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desiccator until further use. The CBA was characterized as a uranium-rich (374 ppm) material with low-level radioactivity (gross alpha decay (α) of 3.08 Bq/g, and gross beta decay (β) of 11.83 Bq/g). The two samples had no significant differences in terms of components and characteristics, therefore, they were combined together for further investigation.
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soil, ground water and human health (Bartoňová and Klika, 2014; Lanzerstorfer, 2015; Liu et al., 2011). Therefore, it is essential to process the relatively highly radioactive CBA prior to its reutilization and to minimize its negative influence on the ambient environment. As there are many uranium-rich coals in China and other countries, reducing radioactivity is becoming important. The main method to extract uranium from coal bottom ash is acid leaching. The experimental results of Paul and Seferinoğlu indicated that nearly 80% of uranium in coal ashes was leached with sulfuric acid after 14 days (Seferinoğlu et al., 2003; Paul et al., 2006), due to its predominant occurrence in uranium-organic compounds in the original coal. El-Hamid et al. (2014) reported that more than 97.1% leaching of the uranium in petroleum ash could be achieved, using a high sulfuric acid concentration (200 g/L) with 6% vol.% MnO2 oxidant and 6 hr of agitation. However, direct acid leaching of uranium from many other coal bottom ashes is difficult. Lei et al. (2014) were only able to leach less than 20% of the uranium from their samples. Zielinski et al. (2007) compared the leaching conditions of uranium and arsenic in coal ash and found that leaching of arsenic with a carbonate buffer solution was rapid and efficient (the leaching rate was 49%). In contrast, U barely leached (7%) in 2 weeks. Most explanations for the low leaching efficiency of uranium in CBA involve the relative insolubility of uranium residing in particles within a glassy matrix (Zielinski et al., 2007; Zielinski and Budahn, 1998). Thus, extraction of uranium from coal ashes greatly differs with coals and regions, but a uniform standard extraction method has not been developed for uranium-rich bottom ash (Zhang et al., 2008). Different uranium extraction methods, which depend on the combustion conditions (e.g., combustion temperatures, categories of raw coal, furnace types) and modes of occurrence of uranium in raw coals, would lead to different leaching efficiencies. Therefore, to effectively extract uranium and reduce radioactivity from CBA, the distribution and mode of occurrence of uranium in bottom ash must be known. The purpose of this work is to investigate the distribution and mode of occurrence of high uranium bottom ash. The samples were supplied from Lincang, southwestern China, and a number of extraction and analytical techniques were utilized including a five-step Tessier sequential extraction, hydrogen fluoride (HF) leaching, Siroquant (Rietveld) quantification, magnetic separation, and electron probe microanalysis (EPMA). The experimental findings in this work are not only important for understanding the distribution of uranium in bottom ash, but also for establishing an effective strategy to reduce radioactivity in CBA and protect the local environment.
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J O U RN A L OF E N V I RO N ME N TA L S CIE N CE S X X (2 0 1 5 ) XXX –XXX
Please cite this article as: Sun, Y., et al., Distribution and mode of occurrence of uranium in bottom ash derived from highgermanium coals, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.07.009
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193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219
2. Results and discussion
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2.1. Characteristics of the coal bottom ash
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Analogous to most coal combustion ashes, the major components of the CBA samples were SiO2 and Al2O3, which accounted for 83–84 weight percent (wt.%) of the total mass (Table 1). However, the concentration of uranium (374 mg/kg) was comparatively high and close to that of some low-grade uranium ores. From the XRD pattern of the CBA provided in Fig. S1 of the Supplementary data, it can be seen that the solid particles were composed mainly of amorphous aluminosilicate glass (56.9 and 56.7 wt.%) and crystalline phases including quartz (18.7 and 19.7 wt.%), mullite (12.9 and 13.7 wt.%), K-feldspar (8.1 and 5.0 wt.%), and a trace amount of cristobalite (3.5 and 4.9 wt.%) (Table 2).
224 225 226 227 228 229 230 231 232 233 234
U
221 220
64.1 19.2 6.83 5.24 5.33 2.54 0.99 0.54 0.21 0.20 0.074 0.08 0.055 0.04 0.08 0.054 0.076 0.063 0.042 0.030 0.013 0.017
F
192
65.7 18.6 6.92 4.11 4.35 2.38 0.88 0.34 0.17 0.10 0.065 0.09 0.096 0.04 0.04 0.031 0.034 0.026 0.027 0.021 0.017 0.016
O
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SiO2 Al2O3 CaO Loss on ignition Fe2O3 K 2O MgO TiO2 Na2O MnO WO3 BaO P2O5 U3O8 SrO Rb2O Cr2O3 ZrO2 GeO2 ZnO NbO NiO
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190
Sample no. 2 (wt.%)
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Chemical compositions of the coal bottom ash were determined by an X-ray fluorescence spectrometer (XRF) (XRF-1800, Shimadzu Company, Japan). All of the major elemental results in the ash were listed as oxides. The CBA samples were crushed and ground with a pestle and mortar, and each powdered sample was subjected to X-ray diffraction (XRD) analysis (D8 Advance X-ray Diffractometer system, Bruker Company, Germany) with Cu Kα radiation. The uranium content in the CBA was determined by an inductively coupled plasma mass spectrometer (ICP-MS) (Xseries2, Thermo Scientific Company, USA). Samples for analysis were subjected to microwave digestion with a mixture of HNO3, HF, and HCl at a volume ratio of 3/1/1. The quantitative composition of mineralogical phases in the samples was obtained with Siroquant™ software (Commonwealth Scientific and Industrial Research Organisation, Sydney, Australia), which was developed by Taylor (1991) based on the principles for powder X-ray diffractogram profiling established by Rietveld (1969). Further details related to the use of this technique for coal combustion products were provided by Ward et al. (1999, 2001) and Dai et al. (2014b, 2015b). Metakaolin and tridymite were consistent in representing the amorphous or glassy phase in the fly ash in the Siroquant quantitative analysis (Ward and French, 2006). In this work, tridymite was chosen for glass interpretation. A representative coal combustion bottom ash sample was subjected to an electron probe micro-analyzer (EPMA) to determine its distribution of elements (U, Al, Si, Fe, and O) in CBA. Energy dispersive spectrometer (EDS) and back scattered electron (BSE) analysis results were both obtained using the same instrument as EPMA. Samples were analyzed on an EPMA analyzer (JXA-8230, JEOL Company, Japan). The accelerating voltage was 20 kV with beam current of 10− 7 A.
Sample no. 1 (wt.%)
t1:1 t1:2 t1:3 t1:4
t1:5 t1:6 t1:7 t1:8 t1:9 t1:10 t1:11 t1:12 t1:13 t1:14 t1:15 t1:16 t1:17 t1:18 t1:19 t1:20 t1:21 t1:22 t1:23 t1:24 t1:25 t1:26 t1:27
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Elements
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1.5. Analytical methods and characterization of the coal bottom ash
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Table 1 – Elemental abundances (oxides of major elements) in the coal bottom ash (CBA) from Lincang, Yunnan Province, China.
magnetic, and non-magnetic fractions were oven-dried to constant weight, and their mass was recorded. In addition, the total iron (TFeO, expressed as an iron oxide) and uranium contents in these three fractions were measured.
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2.2. Distribution of uranium in the coal bottom ash
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According to the Tessier sequential procedures, the uranium in the CBA samples was mainly present in the Fe–Mn oxide fraction and residual fraction (94%, Table 3). The exchangeable uranium was weakly adsorbed on the surface of the CBA particles, and the uranium bound to carbonates and organic matter was negligible. These results are consistent with the literature, in that uranium can be concentrated in the undissolved aluminosilicate matrix (Smeda and Zyrnicki, 2002). As reported by Duff et al. (2002), uranium usually remains in the residual fraction, since it tends to occur in the primary and secondary silicates and other stable minerals. Since the content of Mn in CBA was less than 0.1–0.2 wt.% (see XRF data in Table 1), the uranium should be mainly bound to Fe oxides due to its high affinity for Fe-oxide minerals (Duff et al., 2002). Most of the uranium (59.1% ± 2.4%) in CBA remained in the residual phase (Table 3). This fraction of uranium is considered to be unavailable for acid leaching because it is mostly entrapped in aluminosilicates in CBA and cannot be easily leached by most chemical reagents under general conditions. Therefore, it is not substantially extracted except by dissolving the aluminosilicates with HF. Hence, it is commonly concluded that this fraction of uranium will not chemically impact the ambient environment. However, this also leads to difficulty in extracting and possibly utilizing the vast amount of uranium in the residual fraction. The enrichment of uranium in Fe oxides has been confirmed by previous studies. Gieré et al. (2003) has demonstrated that in coal ash, the Fe-rich particles were considerably enriched in uranium, and the concentration was usually 2–3 times higher than that of Fe-poor particles. Zielinski and Budahn (1998) also found that the Fe-rich particles in coal ash had generally higher concentration of uranium than that of other particles.
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Please cite this article as: Sun, Y., et al., Distribution and mode of occurrence of uranium in bottom ash derived from highgermanium coals, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.07.009
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Table 2 – Crystalline phases in CBA.
t2:4
Crystalline
Quartz (wt.%)
Mullite (wt.%)
K-feldspar (wt.%)
Cristobalite (wt.%)
Glass (wt.%)
t2:5 t2:6
Sample no. 1 Sample no. 2
18.7 19.7
12.9 13.7
8.1 5.0
3.5 4.9
56.9 56.7
t2:8 t2:7
CBA: coal bottom ash.
t3:1 t3:2
Table 3 – Percentages of various forms of uranium in CBA by Tessier sequential procedure in the combined sample.
Exchangeable Bound to carbonates Bound to Fe oxides Bound to organic matter Residual Total
t3:12 t3:11
CBA: coal bottom ash.
271 272 273 274 275 276 277 278 279 280
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The uranium bound to carbonates most likely exists in the form of precipitates or co-precipitates with carbonates in CBA, and is a loosely bound phase and liable to change with environmental conditions (Filgueiras et al., 2002). Uranium bound to organic matter in the Tessier procedure accounted for 2.6% of the total uranium (Table 3). Organic components in CBA are typically represented by slightly changed and coked coal particles (Vassilev and Vassileva, 1996). Given that the organic matter in CBA is predominantly unburned carbon, the uranium is mainly sorbed by electron donating acceptor complexation reactions at the edge sites (Yakout et al., 2013).
2.3. Distribution and mode of occurrence of uranium in Fe oxides
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2.3.1. Distribution of uranium in different magnetic fractions
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To further investigate the distribution of uranium bound to Fe oxides, the samples were weighed and the uranium contents were determined in various magnetic fractions (Fig. 1). The uranium content in the fraction increased from 263 to 1057 mg/kg with an increase in magnetism. However, 55.1%
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Uranium content Distribution of uranium
50 40 30
600
20 400 10
Distribution of uranium (%)
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U
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Uranium content (mg/kg)
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200 Strongly magnetic
Weakly magnetic 1
Weakly magnetic 2
Non-magnetic
0
Category of fraction Fig. 1 – Distribution of uranium in different magnetic fractions in the combined sample.
288 289 290 291 292 293 294 295 296 297 298 299 300 301 302
2.3.2. Mode of occurrence of uranium bound to Fe oxides
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The uranium fraction in the Tessier procedure is the reducible fraction (U6+) that coexists with Fe and Mn oxides. Heavy metal ions can be scavenged by Fe and Mn oxides through one or a combination of the following mechanisms: co-precipitation, adsorption, surface complex formation, ion exchange, and penetration of the lattice (Duff et al., 2002; Ma et al., 2012). In the present research, the mechanism for the coexistence of uranium with Fe–Mn oxides probably involved co-precipitation and adsorption, since high valence uranium (U6+) compounds have a high affinity for Fe-oxide minerals, and these species become less stable under reducing conditions. Therefore, by reacting with NH2OH–HCl, both high-valence uranium (U6+) and iron (Fe3+) were reduced to U4+ and Fe2+, and the extraction of Fe-oxide-bound uranium became more effective (Ma et al., 2012; Duff et al., 2002). However, in this study, the extraction efficiency of uranium bound to Fe–Mn oxide was 34.9%, indicating that only one third of the uranium was leachable by these chemical procedures. The remaining iron oxides may have been entrapped in silicate glass and crystalline phases in the CBA and could not be leached by these chemical procedures. The quantitative XRD analyses of the magnetic fractions are shown in Table 5, which indicate that in the magnetic fractions of CBA, crystalline phases (such as quartz, mullite, cristobalite and glass) still occur. However, a new mineral
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Table 4 – Mass ratio and uranium distribution in various magnetic fractions of CBA in the combined sample.
t4:1 t4:2
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0.1 0.2 1.4 0.7 2.4 3.2
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± ± ± ± ± ±
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2.7 0.5 34.9 2.6 59.1 99.8
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t3:5 t3:6 t3:7 t3:8 t3:9 t3:10
Percentage (%)
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Fractions
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of uranium still remained in the non-magnetic fraction due to its large mass proportion (79.3%) (Table 4). Although the mass ratio of the strongly magnetic fraction was only 8.2%, it contained 23.1% of the total uranium (Table 4). On the basis of a correlation analysis of uranium and the major chemical components in the fraction (Table S1 and Fig. S2), the uranium content was found to be significantly correlated to the Fe content (0.993 Pearson coefficient). Therefore, the uranium was especially enriched in Fe oxides in the CBA. In the Tessier procedures, the mass ratio of uranium bound to Fe oxides was 34.9%, lower than that in the magnetic fractions (45%), which are rich in Fe (TFeO 6%–11.8%). This indicates that the uranium would not be simply bound to free Fe oxides, but partially with Fe oxides that might be surrounded by vitreous glass.
Magnetic fraction
Mass ratio (%)
Distribution of uranium (%)
Strongly magnetic Weakly magnetic 1 Weakly magnetic 2 Non-magnetic
8.2 7.6 4.9 79.3
23.1 15.3 6.5 55.1
CBA: coal bottom ash.
Please cite this article as: Sun, Y., et al., Distribution and mode of occurrence of uranium in bottom ash derived from highgermanium coals, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.07.009
305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327
t4:3
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Mineral phase
Percentage (%)
t5:12 t5:11
CBA: coal bottom ash.
336 337 338 339 340 341 342 343
2.4. Mode of occurrence of uranium in residual fraction
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phase, magnetite, was detected, and its percentage was as high as 11.7%. This result is consistent with the mass fraction of Fe-oxide (11.8%) found by chemical analysis, meaning that in the strongly magnetic fraction, almost all Fe-oxide exists in the form of magnetite (the main component is Fe3O4) and this fraction contains 23.1% of the total uranium. In the raw CBA, the magnetite could not be detected by powder XRD. The data demonstrated that after magnetic separation, the magnetite was concentrated in the strongly magnetic fraction. Considering the high concentration and mass ratio of uranium in the strongly magnetic fraction, it is very clear that uranium coexists with magnetite (23.1%) and other Fe-oxides (21.9%). To further verify the co-existence of uranium with Fe-oxides, a magnetic fraction sample was subjected to EPMA–EDS analysis, and as shown in Fig. 2, the simultaneous presence of Fe and uranium was clearly identifiable.
C
334
55 31.1 10.1 3.7 –
a
E
333
66.6 13.8 13.6 2.3 Albite (3.3) Calcite (0.4)
R
332
73.8 5.3 17.4 1.6 Albite (1.9)
R
331
53.8 21.5 9.9 3.1 Magnetite (11.7)
N C O
330
Non-magnetic
60 µm
U
329
b Intensity (a.u.)
328
Weakly magnetic 2
F
Glass Quartz Mullite Cristobalite Others
Weakly magnetic 1
O
t5:6 t5:7 t5:8 t5:9 t5:10
Strongly magnetic
0
1
2
Spectrum 1
3
5 4 Energy (keV)
6
7
345
Table 6 – Composition of mineral phases in the residue after leaching with HF at various concentrations in the combined sample (units: wt.%).
t6:1 t6:2 t6:3
Mineral phase
Original residue
2% HF residue
4% HF residue
8% HF residue
10% HF residue
Quartz Cristobalite Mullite Glass
21.7 4.9 16.7 56.7
40.5 6.5 20.5 32.5
41.2 6.6 21.9 30.3
45.1 8.6 28.5 17.8
54.8 8.7 17 19.5
8
Fig. 2 – (a) EPMA (electron probe micro-analyzer) image and (b) EDS (energy dispersive X-ray spectrometer) spectrum of typical iron-rich fractions in the combined sample.
344
Previous studies have confirmed that the HF solutions can dissolve vitreous phases from coal fly ash, but not the crystalline phases, e.g., mullite, quartz, and hematite (Fernández-Jimenez et al., 2006). Palomo et al. (2004) has also demonstrated the determination of the vitreous content in the coal fly ash using HF. Dai et al. (2010) and Hulett et al. (1980) used 4% and 1% HF, respectively, to dissolve amorphous glasses in coal fly ash. Using diluted HF to dissolve amorphous silicate is based on the fact that the glass phase can dissolve rapidly in diluted HF, whereas the dissolution of crystalline mullite and quartz is much slower. In this study, HF leaching of the residual phase was also used to identify whether uranium is entrapped in the glass phase in the residue fraction. Each HF-leached residue was weighed, and the composition of each mineral phase was quantified by Siroquant, then the dissolution ratio of each mineral phase was calculated. As indicated by the data in Table 6, the glass in the residual fraction was efficiently dissolved (dissolution ratio of 80.1%–93.2%) via 2%–10% of HF leaching in comparison with the crystalline phase, e.g., quartz (dissolution ratio of 35.1%–55.3%), and this is consistent with research reported earlier (Fernández-Jimenez et al., 2006) (Table 7). The leaching rate of uranium significantly correlated with that of glass and cristobalite (Pearson coefficients 0.989 and 0.973, respectively) (Table S2 and Fig. S3). Therefore, uranium in the residual fraction is mainly retained with the glass and cristobalite phases, most probably borne on the surface of these two phases (Glagolev, 1962). The leaching of uranium was also significantly correlated with the leaching of silicon, aluminum and iron, with Pearson coefficients of 0.995, 0.984 and 0.998, respectively (Table S3 and Fig. S4). In this case, uranium is expected to exist in the Fe/Al/Si/ O-rich phases in the residual fraction (Fig. 3). Although there are very few studies on the distribution and modes of occurrence of uranium in bottom ash, a number of
R O
t5:5
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t5:4
Table 5 – Composition of mineral phases by Rietveld Siroquant in various magnetic fractions of CBA in the combined sample.
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t5:3 t5:2
E
t5:1
HF: hydrogen fluoride.
Please cite this article as: Sun, Y., et al., Distribution and mode of occurrence of uranium in bottom ash derived from highgermanium coals, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.07.009
346 347 Q2 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377
t6:4
t6:5 t6:6 t6:7 t6:8 t6:9 t6:10 t6:11
6
t7:4
HF concentration
t7:5 t7:6 t7:7 t7:8
10% 8% 4% 2%
t7:10 t7:9
HF: hydrogen fluoride.
Dissolution ratio of glass (wt.%)
Dissolution ratio of quartz (wt.%)
Dissolution ratio of mullite (wt.%)
Dissolution ratio of cristobalite (wt.%)
95.0 93.0 85.6 81.2
93.2 92.3 83.7 80.1
55.3 50.1 42.1 35.1
79.9 63.3 60.0 57.3
64.9 61.8 58.9 53.8
Undoubtedly, the recovery of uranium and the reduction of the radioactivity of CBA are essential before the ashes can be
391
395
C
390
E
389
O
U
Al
Si
R
388
R
387
O
386
C
385
Fe
N
384
U
383
F
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further utilized (e.g., as construction materials). The first step is to dissolve the vitreous phases and expose the Fe-oxides to chemical reagents. Then, on leaching with chemical reagents, uranium would dissolve. Based on these findings, we discussed a method in one of our previous studies for the removal of uranium from CBA by calcination with CaCl2 and leaching with HNO3 (Lei et al., 2014). Considering the continuous accumulation of CBA in the world (particularly in China), the total content of uranium will be huge. As a typical example, in Xiaolongtan power station, approximately 900,000 tons of fly and bottom ash is produced annually, and more than 5 million tons of recoverable ash has been stockpiled. The average content of uranium is 200 ppm, and hence this station could generate 180 tons of uranium every year. The recovery of uranium from CBA is becoming more and more practical to meet fuel demands for nuclear power. In addition, CBA is often enriched in Ge, W, U, Sr, Rb, Nb and some trace platinum metals (Dai et al., 2014a, 2014c; Seredin and Dai, 2014). It is therefore important to recover these elements, especially uranium, for sustainable development.
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research groups have reported that in coal, uranium is mostly associated with aluminosilicate and organic matter (Yang, 2009). For example, using a six-step sequential chemical extraction procedure, Dai et al. (2004) studied uranium in Late Paleozoic coal in the Ordos Basin of China. Their results indicate that most uranium (67%) occurred in association with aluminosilicate and organics. In addition, in coal fly ash, uranium acts as a lithophile, meaning that uranium is mainly associated with silicates and other oxysalts (Dai et al., 2010). Zielinski et al. (2007) compared the existence modes of uranium and arsenic, and found that the uranium mainly resided within the relatively insoluble glassy matrix of fly ash particles. In Zielinski and Finkelman (1997), the glassy components in fly ash were identified as the main host of uranium, and the distribution of uranium in fly ash particles was fairly uniform throughout the glassy matrix.
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Table 7 – Dissolution ratio of mineral phases and leaching of uranium after leaching with HF at various concentrations in the combined sample.
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Fig. 3 – BSE (back scattered electron) (the gray scale image) and EPMA images of the residual fraction showing the distribution of related elements in the combined sample. Please cite this article as: Sun, Y., et al., Distribution and mode of occurrence of uranium in bottom ash derived from highgermanium coals, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.07.009
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Acknowledgments
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The authors are sincerely grateful for the financial support from the Talent Support Fund of Tsinghua University (No. 413405001).
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.jes.2015.07.009.
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The distribution and mode of occurrence of uranium in a coal combustion bottom ash were studied in this research. In the studied CBA, the uranium coexists with Fe-oxides and the glass phase. Most of the uranium-containing Fe-oxides were entrapped in vitreous aluminosilicate, and only one third existed on the surface of the CBA particles. Uranium was also enriched in the magnetic fraction (45%), in which 23.1% of total uranium coexisted with magnetite, and the remainder (21.9%) co-existed with other Fe oxides or was surrounded by the vitreous glass phase. In the residual fraction, uranium was expected to be retained in the Fe/Al/Si/O-rich phases. To recover uranium from CBA, the first and most important step is to destroy the vitreous materials surrounding the uranium-Fe-oxide so that these grains are exposed to chemical reagents. By thoroughly understanding the distribution of uranium in CBA, it will be possible to establish effective strategies to reduce radioactivity in CBA for environmental safety, and to recover the uranium from CBA for sustainable development.
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Available online at www.sciencedirect.com
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Yinglong Sun1 , Guangxia Qi1,2 , Xuefei Lei1 , Hui Xu1 , Lei Li1 , Chao Yuan1 , Yi Wang1,⁎
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1. Department of Environmental Engineering, School of Environment, Tsinghua University, Beijing 100084, China. E–mail: [email protected] 2. Department of Environmental Science and Engineering, School of Food and Chemical Engineering, Beijing Technology and Business University, Beijing 100048, China
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Distribution and mode of occurrence of uranium in bottom ash derived from high-germanium coals
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Article history:
The radioactivity of uranium in radioactive coal bottom ash (CBA) may be a potential danger
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Received 25 May 2015
to the ambient environment and human health. Concerning the limited research on the
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Revised 16 July 2015
distribution and mode of occurrence of uranium in CBA, we herein report our investigations
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Accepted 17 July 2015
into this topic using a number of techniques including a five-step Tessier sequential
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Available online xxxx
extraction, hydrogen fluoride (HF) leaching, Siroquant (Rietveld) quantification, magnetic
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Keywords:
showed that the uranium in the residual and Fe–Mn oxide fractions was dominant (59.1%
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Coal bottom ash
and 34.9%, respectively). The former was mainly incorporated into aluminosilicates,
Uranium
retained with glass and cristobalite, whereas the latter was especially enriched in the
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Radioactivity
magnetic fraction, of which about 50% was present with magnetite (Fe3O4) and the rest in
Tessier sequential extraction
other iron oxides. In addition, the uranium in the magnetic fraction was 2.6 times that in
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Magnetic separation
the non-magnetic fraction. The experimental findings in this work may be important for
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separation, and electron probe microanalysis (EPMA). The Tessier sequential extraction
establishing an effective strategy to reduce radioactivity from CBA for the protection of our
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local environment.
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© 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.
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Introduction
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Lignite is often used as fuel for many small to middle scale pithead power plants, and in some cases, as a raw resource for Ge smelters (Papastefanou, 2010; Dai et al., 2014c). However, in Lincang, Yunnan province, China, the average radioactivity of uranium (U) in lignite can reach 87.1 Bq/kg, much higher than that of the other types of coal (such as low-rank coals, middle-rank coals and high-rank coals) (Xiong et al., 2007; Yu, 2007). After burning, the natural radioactivity level of coal combustion ash is 4–10 times higher than that of the feed coals (Bhangare et al., 2014; Tripathi et al., 2013), which may be extremely dangerous for the surrounding environment and
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Published by Elsevier B.V.
human health. For example, the enrichment and transformation of radionuclides in coal fly ash and bottom ash has already caused secondary pollution, and has negatively impacted the local environment and human health in Yunnan province, China (Yu, 2007). Some late Permian coals are highly enriched in uranium (Dai et al., 2008, 2013a, 2013b, 2015a). Unfortunately, the radioactivity of their combustion residue (e.g., bottom ash) has not yet been studied in a great detail. In comparison with the abundant use of fly ash in construction materials (Dai et al., 2012; Camilleri et al., 2006; Eze et al., 2013; Lima et al., 2012), the coal bottom ash (CBA) is still stocked in piles close to coal fields, and could generate negative impacts on the surrounding environment including air,
⁎ Corresponding author. E-mail: [email protected] (Yi Wang).
http://dx.doi.org/10.1016/j.jes.2015.07.009 1001-0742/© 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
Please cite this article as: Sun, Y., et al., Distribution and mode of occurrence of uranium in bottom ash derived from highgermanium coals, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.07.009
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1. Materials and methods
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1.1. Samples and reagents
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The CBA samples were obtained from two different germanium (Ge) smelters in Lincang, Yunnan Province, China (samples no. 1 and no. 2). The samples were crushed with a ball mill, and then passed through a 500-mesh standard sieve (< 25 μm in diameter). The fine powder samples were dried at 105°C in a forced air oven to constant weight and stored in a
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1.3. HF leaching of the coal bottom ash residue
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A total of 5 g of the residual Tessier fraction was placed into a 150 mL Teflon beaker, and 80 mL of an HF solution (10%, 8%, 6%, 4%, or 2%, V/V) was added. The resulting suspension was magnetically stirred at 500 r/min for 20 min at room temperature, and then the slurry was centrifuged. The supernatant was collected and the residue washed with 80 mL of distilled water. The supernatant and washing solution were mixed and diluted up to 500 mL, and then a 10 mL-aliquot was withdrawn for a uranium content analysis. The solid residue was further washed twice, dried, and the weight of the residue was recorded. The leaching experiment was performed in duplicate.
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1.4. Magnetic separation of the coal bottom ash
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A total of 20 g of CBA was added to 2.0 L of distilled water, and the slurry was stirred vigorously with a magnetic rod (3000 G). This procedure was repeated until no more magnetic fraction adhered to the magnet. Then the residual parts were further subjected to a wet-type high intensity magnetic separator with a magnetic field intensity of 15,000 to 20,000 G to collect the weakly magnetic fractions. The weakly magnetic, strongly
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The combined samples were analyzed below. Tessier sequential extraction procedures were used to fractionate the uranium in the CBA into five components: exchangeable, bound to carbonates, bound to iron and manganese oxides, bound to organic matter, and remaining in residue (Tessier et al., 1979). The experimental procedures were analogous to those described in the literature (Wan et al., 2006; Smeda and Zyrnicki, 2002; Landsberger et al., 1995; Bódog et al., 1996). Briefly, (1) the CBA was extracted at room temperature for 3 hr with a sodium acetate solution (1 mol/L CH3COONa, pH 8.2) under continuous agitation; (2) the residue from (1) was leached at 50°C with a 1 mol/L sodium acetate solution adjusted to pH 5.0 with acetic acid (CH3COOH). Continuous agitation was maintained for 5 hr; (3) the residue from (2) was extracted with a 0.04 mol/L NH2OH–HCl solution in 25% (V/V) acetic acid. The extraction occurred at 60°C under continuous agitation for 8 hr; (4) a solution of 0.02 mol/L HNO3 and 30% H2O2 adjusted to pH 2 with HNO3 was added to the residue from step (3), and the mixture was heated at 85°C for 2 hr under continuous agitation. NH4Ac was then added and the sample was heated again to 65°C for 6 hr under continuous agitation; and (5) the residue from (4) was digested with a mixture of HF, HNO3 and HClO4 for total metal analysis. Each step was repeated four times, and the leachate was collected separately to measure the concentration of uranium and other major metals.
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desiccator until further use. The CBA was characterized as a uranium-rich (374 ppm) material with low-level radioactivity (gross alpha decay (α) of 3.08 Bq/g, and gross beta decay (β) of 11.83 Bq/g). The two samples had no significant differences in terms of components and characteristics, therefore, they were combined together for further investigation.
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soil, ground water and human health (Bartoňová and Klika, 2014; Lanzerstorfer, 2015; Liu et al., 2011). Therefore, it is essential to process the relatively highly radioactive CBA prior to its reutilization and to minimize its negative influence on the ambient environment. As there are many uranium-rich coals in China and other countries, reducing radioactivity is becoming important. The main method to extract uranium from coal bottom ash is acid leaching. The experimental results of Paul and Seferinoğlu indicated that nearly 80% of uranium in coal ashes was leached with sulfuric acid after 14 days (Seferinoğlu et al., 2003; Paul et al., 2006), due to its predominant occurrence in uranium-organic compounds in the original coal. El-Hamid et al. (2014) reported that more than 97.1% leaching of the uranium in petroleum ash could be achieved, using a high sulfuric acid concentration (200 g/L) with 6% vol.% MnO2 oxidant and 6 hr of agitation. However, direct acid leaching of uranium from many other coal bottom ashes is difficult. Lei et al. (2014) were only able to leach less than 20% of the uranium from their samples. Zielinski et al. (2007) compared the leaching conditions of uranium and arsenic in coal ash and found that leaching of arsenic with a carbonate buffer solution was rapid and efficient (the leaching rate was 49%). In contrast, U barely leached (7%) in 2 weeks. Most explanations for the low leaching efficiency of uranium in CBA involve the relative insolubility of uranium residing in particles within a glassy matrix (Zielinski et al., 2007; Zielinski and Budahn, 1998). Thus, extraction of uranium from coal ashes greatly differs with coals and regions, but a uniform standard extraction method has not been developed for uranium-rich bottom ash (Zhang et al., 2008). Different uranium extraction methods, which depend on the combustion conditions (e.g., combustion temperatures, categories of raw coal, furnace types) and modes of occurrence of uranium in raw coals, would lead to different leaching efficiencies. Therefore, to effectively extract uranium and reduce radioactivity from CBA, the distribution and mode of occurrence of uranium in bottom ash must be known. The purpose of this work is to investigate the distribution and mode of occurrence of high uranium bottom ash. The samples were supplied from Lincang, southwestern China, and a number of extraction and analytical techniques were utilized including a five-step Tessier sequential extraction, hydrogen fluoride (HF) leaching, Siroquant (Rietveld) quantification, magnetic separation, and electron probe microanalysis (EPMA). The experimental findings in this work are not only important for understanding the distribution of uranium in bottom ash, but also for establishing an effective strategy to reduce radioactivity in CBA and protect the local environment.
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2. Results and discussion
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2.1. Characteristics of the coal bottom ash
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Analogous to most coal combustion ashes, the major components of the CBA samples were SiO2 and Al2O3, which accounted for 83–84 weight percent (wt.%) of the total mass (Table 1). However, the concentration of uranium (374 mg/kg) was comparatively high and close to that of some low-grade uranium ores. From the XRD pattern of the CBA provided in Fig. S1 of the Supplementary data, it can be seen that the solid particles were composed mainly of amorphous aluminosilicate glass (56.9 and 56.7 wt.%) and crystalline phases including quartz (18.7 and 19.7 wt.%), mullite (12.9 and 13.7 wt.%), K-feldspar (8.1 and 5.0 wt.%), and a trace amount of cristobalite (3.5 and 4.9 wt.%) (Table 2).
224 225 226 227 228 229 230 231 232 233 234
U
221 220
64.1 19.2 6.83 5.24 5.33 2.54 0.99 0.54 0.21 0.20 0.074 0.08 0.055 0.04 0.08 0.054 0.076 0.063 0.042 0.030 0.013 0.017
F
192
65.7 18.6 6.92 4.11 4.35 2.38 0.88 0.34 0.17 0.10 0.065 0.09 0.096 0.04 0.04 0.031 0.034 0.026 0.027 0.021 0.017 0.016
O
191
SiO2 Al2O3 CaO Loss on ignition Fe2O3 K 2O MgO TiO2 Na2O MnO WO3 BaO P2O5 U3O8 SrO Rb2O Cr2O3 ZrO2 GeO2 ZnO NbO NiO
R O
190
Sample no. 2 (wt.%)
P
189
Chemical compositions of the coal bottom ash were determined by an X-ray fluorescence spectrometer (XRF) (XRF-1800, Shimadzu Company, Japan). All of the major elemental results in the ash were listed as oxides. The CBA samples were crushed and ground with a pestle and mortar, and each powdered sample was subjected to X-ray diffraction (XRD) analysis (D8 Advance X-ray Diffractometer system, Bruker Company, Germany) with Cu Kα radiation. The uranium content in the CBA was determined by an inductively coupled plasma mass spectrometer (ICP-MS) (Xseries2, Thermo Scientific Company, USA). Samples for analysis were subjected to microwave digestion with a mixture of HNO3, HF, and HCl at a volume ratio of 3/1/1. The quantitative composition of mineralogical phases in the samples was obtained with Siroquant™ software (Commonwealth Scientific and Industrial Research Organisation, Sydney, Australia), which was developed by Taylor (1991) based on the principles for powder X-ray diffractogram profiling established by Rietveld (1969). Further details related to the use of this technique for coal combustion products were provided by Ward et al. (1999, 2001) and Dai et al. (2014b, 2015b). Metakaolin and tridymite were consistent in representing the amorphous or glassy phase in the fly ash in the Siroquant quantitative analysis (Ward and French, 2006). In this work, tridymite was chosen for glass interpretation. A representative coal combustion bottom ash sample was subjected to an electron probe micro-analyzer (EPMA) to determine its distribution of elements (U, Al, Si, Fe, and O) in CBA. Energy dispersive spectrometer (EDS) and back scattered electron (BSE) analysis results were both obtained using the same instrument as EPMA. Samples were analyzed on an EPMA analyzer (JXA-8230, JEOL Company, Japan). The accelerating voltage was 20 kV with beam current of 10− 7 A.
Sample no. 1 (wt.%)
t1:1 t1:2 t1:3 t1:4
t1:5 t1:6 t1:7 t1:8 t1:9 t1:10 t1:11 t1:12 t1:13 t1:14 t1:15 t1:16 t1:17 t1:18 t1:19 t1:20 t1:21 t1:22 t1:23 t1:24 t1:25 t1:26 t1:27
D
188
Elements
E
187
T
186
1.5. Analytical methods and characterization of the coal bottom ash
C
185
E
184
R
183
Table 1 – Elemental abundances (oxides of major elements) in the coal bottom ash (CBA) from Lincang, Yunnan Province, China.
magnetic, and non-magnetic fractions were oven-dried to constant weight, and their mass was recorded. In addition, the total iron (TFeO, expressed as an iron oxide) and uranium contents in these three fractions were measured.
R
182
N C O
181
2.2. Distribution of uranium in the coal bottom ash
235
According to the Tessier sequential procedures, the uranium in the CBA samples was mainly present in the Fe–Mn oxide fraction and residual fraction (94%, Table 3). The exchangeable uranium was weakly adsorbed on the surface of the CBA particles, and the uranium bound to carbonates and organic matter was negligible. These results are consistent with the literature, in that uranium can be concentrated in the undissolved aluminosilicate matrix (Smeda and Zyrnicki, 2002). As reported by Duff et al. (2002), uranium usually remains in the residual fraction, since it tends to occur in the primary and secondary silicates and other stable minerals. Since the content of Mn in CBA was less than 0.1–0.2 wt.% (see XRF data in Table 1), the uranium should be mainly bound to Fe oxides due to its high affinity for Fe-oxide minerals (Duff et al., 2002). Most of the uranium (59.1% ± 2.4%) in CBA remained in the residual phase (Table 3). This fraction of uranium is considered to be unavailable for acid leaching because it is mostly entrapped in aluminosilicates in CBA and cannot be easily leached by most chemical reagents under general conditions. Therefore, it is not substantially extracted except by dissolving the aluminosilicates with HF. Hence, it is commonly concluded that this fraction of uranium will not chemically impact the ambient environment. However, this also leads to difficulty in extracting and possibly utilizing the vast amount of uranium in the residual fraction. The enrichment of uranium in Fe oxides has been confirmed by previous studies. Gieré et al. (2003) has demonstrated that in coal ash, the Fe-rich particles were considerably enriched in uranium, and the concentration was usually 2–3 times higher than that of Fe-poor particles. Zielinski and Budahn (1998) also found that the Fe-rich particles in coal ash had generally higher concentration of uranium than that of other particles.
236
Please cite this article as: Sun, Y., et al., Distribution and mode of occurrence of uranium in bottom ash derived from highgermanium coals, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.07.009
237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267
4 t2:1 t2:3 t2:2
J O U RN A L OF E N V I RO N ME N TA L S CIE N CE S X X (2 0 1 5 ) XXX –XXX
Table 2 – Crystalline phases in CBA.
t2:4
Crystalline
Quartz (wt.%)
Mullite (wt.%)
K-feldspar (wt.%)
Cristobalite (wt.%)
Glass (wt.%)
t2:5 t2:6
Sample no. 1 Sample no. 2
18.7 19.7
12.9 13.7
8.1 5.0
3.5 4.9
56.9 56.7
t2:8 t2:7
CBA: coal bottom ash.
t3:1 t3:2
Table 3 – Percentages of various forms of uranium in CBA by Tessier sequential procedure in the combined sample.
Exchangeable Bound to carbonates Bound to Fe oxides Bound to organic matter Residual Total
t3:12 t3:11
CBA: coal bottom ash.
271 272 273 274 275 276 277 278 279 280
F
O
The uranium bound to carbonates most likely exists in the form of precipitates or co-precipitates with carbonates in CBA, and is a loosely bound phase and liable to change with environmental conditions (Filgueiras et al., 2002). Uranium bound to organic matter in the Tessier procedure accounted for 2.6% of the total uranium (Table 3). Organic components in CBA are typically represented by slightly changed and coked coal particles (Vassilev and Vassileva, 1996). Given that the organic matter in CBA is predominantly unburned carbon, the uranium is mainly sorbed by electron donating acceptor complexation reactions at the edge sites (Yakout et al., 2013).
2.3. Distribution and mode of occurrence of uranium in Fe oxides
282
2.3.1. Distribution of uranium in different magnetic fractions
283
To further investigate the distribution of uranium bound to Fe oxides, the samples were weighed and the uranium contents were determined in various magnetic fractions (Fig. 1). The uranium content in the fraction increased from 263 to 1057 mg/kg with an increase in magnetism. However, 55.1%
R
O
1000 800
60
Uranium content Distribution of uranium
50 40 30
600
20 400 10
Distribution of uranium (%)
1200
C
287
N
286
U
285
Uranium content (mg/kg)
284
R
281
200 Strongly magnetic
Weakly magnetic 1
Weakly magnetic 2
Non-magnetic
0
Category of fraction Fig. 1 – Distribution of uranium in different magnetic fractions in the combined sample.
288 289 290 291 292 293 294 295 296 297 298 299 300 301 302
2.3.2. Mode of occurrence of uranium bound to Fe oxides
303
The uranium fraction in the Tessier procedure is the reducible fraction (U6+) that coexists with Fe and Mn oxides. Heavy metal ions can be scavenged by Fe and Mn oxides through one or a combination of the following mechanisms: co-precipitation, adsorption, surface complex formation, ion exchange, and penetration of the lattice (Duff et al., 2002; Ma et al., 2012). In the present research, the mechanism for the coexistence of uranium with Fe–Mn oxides probably involved co-precipitation and adsorption, since high valence uranium (U6+) compounds have a high affinity for Fe-oxide minerals, and these species become less stable under reducing conditions. Therefore, by reacting with NH2OH–HCl, both high-valence uranium (U6+) and iron (Fe3+) were reduced to U4+ and Fe2+, and the extraction of Fe-oxide-bound uranium became more effective (Ma et al., 2012; Duff et al., 2002). However, in this study, the extraction efficiency of uranium bound to Fe–Mn oxide was 34.9%, indicating that only one third of the uranium was leachable by these chemical procedures. The remaining iron oxides may have been entrapped in silicate glass and crystalline phases in the CBA and could not be leached by these chemical procedures. The quantitative XRD analyses of the magnetic fractions are shown in Table 5, which indicate that in the magnetic fractions of CBA, crystalline phases (such as quartz, mullite, cristobalite and glass) still occur. However, a new mineral
304
Table 4 – Mass ratio and uranium distribution in various magnetic fractions of CBA in the combined sample.
t4:1 t4:2
T
270
0.1 0.2 1.4 0.7 2.4 3.2
C
269
± ± ± ± ± ±
E
268
2.7 0.5 34.9 2.6 59.1 99.8
R O
t3:5 t3:6 t3:7 t3:8 t3:9 t3:10
Percentage (%)
P
Fractions
D
t3:4
E
t3:3
of uranium still remained in the non-magnetic fraction due to its large mass proportion (79.3%) (Table 4). Although the mass ratio of the strongly magnetic fraction was only 8.2%, it contained 23.1% of the total uranium (Table 4). On the basis of a correlation analysis of uranium and the major chemical components in the fraction (Table S1 and Fig. S2), the uranium content was found to be significantly correlated to the Fe content (0.993 Pearson coefficient). Therefore, the uranium was especially enriched in Fe oxides in the CBA. In the Tessier procedures, the mass ratio of uranium bound to Fe oxides was 34.9%, lower than that in the magnetic fractions (45%), which are rich in Fe (TFeO 6%–11.8%). This indicates that the uranium would not be simply bound to free Fe oxides, but partially with Fe oxides that might be surrounded by vitreous glass.
Magnetic fraction
Mass ratio (%)
Distribution of uranium (%)
Strongly magnetic Weakly magnetic 1 Weakly magnetic 2 Non-magnetic
8.2 7.6 4.9 79.3
23.1 15.3 6.5 55.1
CBA: coal bottom ash.
Please cite this article as: Sun, Y., et al., Distribution and mode of occurrence of uranium in bottom ash derived from highgermanium coals, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.07.009
305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327
t4:3
t4:4 t4:5 t4:6 t4:7 t4:8 t4:9 t4:10
5
J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 5 ) XX X–XXX
Mineral phase
Percentage (%)
t5:12 t5:11
CBA: coal bottom ash.
336 337 338 339 340 341 342 343
2.4. Mode of occurrence of uranium in residual fraction
T
335
phase, magnetite, was detected, and its percentage was as high as 11.7%. This result is consistent with the mass fraction of Fe-oxide (11.8%) found by chemical analysis, meaning that in the strongly magnetic fraction, almost all Fe-oxide exists in the form of magnetite (the main component is Fe3O4) and this fraction contains 23.1% of the total uranium. In the raw CBA, the magnetite could not be detected by powder XRD. The data demonstrated that after magnetic separation, the magnetite was concentrated in the strongly magnetic fraction. Considering the high concentration and mass ratio of uranium in the strongly magnetic fraction, it is very clear that uranium coexists with magnetite (23.1%) and other Fe-oxides (21.9%). To further verify the co-existence of uranium with Fe-oxides, a magnetic fraction sample was subjected to EPMA–EDS analysis, and as shown in Fig. 2, the simultaneous presence of Fe and uranium was clearly identifiable.
C
334
55 31.1 10.1 3.7 –
a
E
333
66.6 13.8 13.6 2.3 Albite (3.3) Calcite (0.4)
R
332
73.8 5.3 17.4 1.6 Albite (1.9)
R
331
53.8 21.5 9.9 3.1 Magnetite (11.7)
N C O
330
Non-magnetic
60 µm
U
329
b Intensity (a.u.)
328
Weakly magnetic 2
F
Glass Quartz Mullite Cristobalite Others
Weakly magnetic 1
O
t5:6 t5:7 t5:8 t5:9 t5:10
Strongly magnetic
0
1
2
Spectrum 1
3
5 4 Energy (keV)
6
7
345
Table 6 – Composition of mineral phases in the residue after leaching with HF at various concentrations in the combined sample (units: wt.%).
t6:1 t6:2 t6:3
Mineral phase
Original residue
2% HF residue
4% HF residue
8% HF residue
10% HF residue
Quartz Cristobalite Mullite Glass
21.7 4.9 16.7 56.7
40.5 6.5 20.5 32.5
41.2 6.6 21.9 30.3
45.1 8.6 28.5 17.8
54.8 8.7 17 19.5
8
Fig. 2 – (a) EPMA (electron probe micro-analyzer) image and (b) EDS (energy dispersive X-ray spectrometer) spectrum of typical iron-rich fractions in the combined sample.
344
Previous studies have confirmed that the HF solutions can dissolve vitreous phases from coal fly ash, but not the crystalline phases, e.g., mullite, quartz, and hematite (Fernández-Jimenez et al., 2006). Palomo et al. (2004) has also demonstrated the determination of the vitreous content in the coal fly ash using HF. Dai et al. (2010) and Hulett et al. (1980) used 4% and 1% HF, respectively, to dissolve amorphous glasses in coal fly ash. Using diluted HF to dissolve amorphous silicate is based on the fact that the glass phase can dissolve rapidly in diluted HF, whereas the dissolution of crystalline mullite and quartz is much slower. In this study, HF leaching of the residual phase was also used to identify whether uranium is entrapped in the glass phase in the residue fraction. Each HF-leached residue was weighed, and the composition of each mineral phase was quantified by Siroquant, then the dissolution ratio of each mineral phase was calculated. As indicated by the data in Table 6, the glass in the residual fraction was efficiently dissolved (dissolution ratio of 80.1%–93.2%) via 2%–10% of HF leaching in comparison with the crystalline phase, e.g., quartz (dissolution ratio of 35.1%–55.3%), and this is consistent with research reported earlier (Fernández-Jimenez et al., 2006) (Table 7). The leaching rate of uranium significantly correlated with that of glass and cristobalite (Pearson coefficients 0.989 and 0.973, respectively) (Table S2 and Fig. S3). Therefore, uranium in the residual fraction is mainly retained with the glass and cristobalite phases, most probably borne on the surface of these two phases (Glagolev, 1962). The leaching of uranium was also significantly correlated with the leaching of silicon, aluminum and iron, with Pearson coefficients of 0.995, 0.984 and 0.998, respectively (Table S3 and Fig. S4). In this case, uranium is expected to exist in the Fe/Al/Si/ O-rich phases in the residual fraction (Fig. 3). Although there are very few studies on the distribution and modes of occurrence of uranium in bottom ash, a number of
R O
t5:5
P
t5:4
Table 5 – Composition of mineral phases by Rietveld Siroquant in various magnetic fractions of CBA in the combined sample.
D
t5:3 t5:2
E
t5:1
HF: hydrogen fluoride.
Please cite this article as: Sun, Y., et al., Distribution and mode of occurrence of uranium in bottom ash derived from highgermanium coals, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.07.009
346 347 Q2 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377
t6:4
t6:5 t6:6 t6:7 t6:8 t6:9 t6:10 t6:11
6
t7:4
HF concentration
t7:5 t7:6 t7:7 t7:8
10% 8% 4% 2%
t7:10 t7:9
HF: hydrogen fluoride.
Dissolution ratio of glass (wt.%)
Dissolution ratio of quartz (wt.%)
Dissolution ratio of mullite (wt.%)
Dissolution ratio of cristobalite (wt.%)
95.0 93.0 85.6 81.2
93.2 92.3 83.7 80.1
55.3 50.1 42.1 35.1
79.9 63.3 60.0 57.3
64.9 61.8 58.9 53.8
Undoubtedly, the recovery of uranium and the reduction of the radioactivity of CBA are essential before the ashes can be
391
395
C
390
E
389
O
U
Al
Si
R
388
R
387
O
386
C
385
Fe
N
384
U
383
F
394
382
O
2.5. Strategy for uranium removal from the coal bottom ash
381
R O
393
380
further utilized (e.g., as construction materials). The first step is to dissolve the vitreous phases and expose the Fe-oxides to chemical reagents. Then, on leaching with chemical reagents, uranium would dissolve. Based on these findings, we discussed a method in one of our previous studies for the removal of uranium from CBA by calcination with CaCl2 and leaching with HNO3 (Lei et al., 2014). Considering the continuous accumulation of CBA in the world (particularly in China), the total content of uranium will be huge. As a typical example, in Xiaolongtan power station, approximately 900,000 tons of fly and bottom ash is produced annually, and more than 5 million tons of recoverable ash has been stockpiled. The average content of uranium is 200 ppm, and hence this station could generate 180 tons of uranium every year. The recovery of uranium from CBA is becoming more and more practical to meet fuel demands for nuclear power. In addition, CBA is often enriched in Ge, W, U, Sr, Rb, Nb and some trace platinum metals (Dai et al., 2014a, 2014c; Seredin and Dai, 2014). It is therefore important to recover these elements, especially uranium, for sustainable development.
T
392
research groups have reported that in coal, uranium is mostly associated with aluminosilicate and organic matter (Yang, 2009). For example, using a six-step sequential chemical extraction procedure, Dai et al. (2004) studied uranium in Late Paleozoic coal in the Ordos Basin of China. Their results indicate that most uranium (67%) occurred in association with aluminosilicate and organics. In addition, in coal fly ash, uranium acts as a lithophile, meaning that uranium is mainly associated with silicates and other oxysalts (Dai et al., 2010). Zielinski et al. (2007) compared the existence modes of uranium and arsenic, and found that the uranium mainly resided within the relatively insoluble glassy matrix of fly ash particles. In Zielinski and Finkelman (1997), the glassy components in fly ash were identified as the main host of uranium, and the distribution of uranium in fly ash particles was fairly uniform throughout the glassy matrix.
379
P
378
Leaching of U (wt. %)
D
t7:3
Table 7 – Dissolution ratio of mineral phases and leaching of uranium after leaching with HF at various concentrations in the combined sample.
E
t7:1 t7:2
J O U RN A L OF E N V I RO N ME N TA L S CIE N CE S X X (2 0 1 5 ) XXX –XXX
Fig. 3 – BSE (back scattered electron) (the gray scale image) and EPMA images of the residual fraction showing the distribution of related elements in the combined sample. Please cite this article as: Sun, Y., et al., Distribution and mode of occurrence of uranium in bottom ash derived from highgermanium coals, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.07.009
396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415
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J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 5 ) XX X–XXX
Acknowledgments
439 441
The authors are sincerely grateful for the financial support from the Talent Support Fund of Tsinghua University (No. 413405001).
443 442
Appendix A. Supplementary data
444 445
Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.jes.2015.07.009.
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The distribution and mode of occurrence of uranium in a coal combustion bottom ash were studied in this research. In the studied CBA, the uranium coexists with Fe-oxides and the glass phase. Most of the uranium-containing Fe-oxides were entrapped in vitreous aluminosilicate, and only one third existed on the surface of the CBA particles. Uranium was also enriched in the magnetic fraction (45%), in which 23.1% of total uranium coexisted with magnetite, and the remainder (21.9%) co-existed with other Fe oxides or was surrounded by the vitreous glass phase. In the residual fraction, uranium was expected to be retained in the Fe/Al/Si/O-rich phases. To recover uranium from CBA, the first and most important step is to destroy the vitreous materials surrounding the uranium-Fe-oxide so that these grains are exposed to chemical reagents. By thoroughly understanding the distribution of uranium in CBA, it will be possible to establish effective strategies to reduce radioactivity in CBA for environmental safety, and to recover the uranium from CBA for sustainable development.
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Please cite this article as: Sun, Y., et al., Distribution and mode of occurrence of uranium in bottom ash derived from highgermanium coals, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.07.009
471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539
8
E
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Please cite this article as: Sun, Y., et al., Distribution and mode of occurrence of uranium in bottom ash derived from highgermanium coals, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.07.009
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