Trace elements (Mn, Cr, Pb, Se, Zn, Cd and Hg) in emissions from a pulverized coal boiler

Trace elements (Mn, Cr, Pb, Se, Zn, Cd and Hg) in emissions from a pulverized coal boiler

Fuel Processing Technology 86 (2004) 23 – 32 www.elsevier.com/locate/fuproc Trace elements (Mn, Cr, Pb, Se, Zn, Cd and Hg) in emissions from a pulver...

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Fuel Processing Technology 86 (2004) 23 – 32 www.elsevier.com/locate/fuproc

Trace elements (Mn, Cr, Pb, Se, Zn, Cd and Hg) in emissions from a pulverized coal boiler Yaji Huang a, Baosheng Jin a,*, Zhaoping Zhong a, Rui Xiao a, Zhiyong Tang a, Huifeng Ren b a Workgroup of Jin Baosheng, Education Ministry Key Laboratory on Clean Coal Power Generation and Combustion Technology, Southeast University, 2 Si Pai Lou, Nanjing 210096, People’s Republic of China b Thermoelectric Power Plant of Yangzi Petrochemical Co., Nanjing, China

Accepted 8 October 2003

Abstract The concentrations of seven trace elements (Mn, Cr, Pb, Se, Zn, Cd, Hg) in raw coal, bottom ash and fly ash were measured quantitatively in a 220 tons/h pulverized coal boiler. Factors affecting distribution of trace elements were investigated, including fly ash diameter, furnace temperature, oxygen concentration and trace elements’ characteristics. Modified enrichment factors show more directly element enrichment in combustion products. The studied elements may be classified into three groups according to their emission features: Group 1: Hg, which is very volatile. Group 2: Pb, Zn, Cd, which are partially volatile. Group 3: Mn, which is hardly volatile. Se may be located between groups 1 and 2. Cr has properties of both Groups 1 and 3. The smaller the diameter of fly ash, the higher is the relative enrichment of trace elements (except Mn). Fly ash shows different adsorption mechanisms of trace elements and the volatilization of trace elements rises with furnace temperature. Relative enrichments of trace elements (except Mn and Cr) in fly ash are larger than that in bottom ash. Low oxygen concentration will not always improve the volatilization of trace elements. Pb forms chloride more easily than Cd during coal combustion. D 2003 Published by Elsevier B.V. Keywords: Trace element; Occurrence; Volatility; Relative enrichment; Classification of elements; Coal ash

1. Introduction Coal is a heterogeneous fuel containing varying amounts of inorganic substances and combustible metamorphosed prehistoric vegetation. Presently, about 84 elements have * Corresponding author. Tel.: +86-25-3794744; fax: +86-25-7714489. E-mail address: [email protected] (B. Jin). 0378-3820/$ - see front matter D 2003 Published by Elsevier B.V. doi:10.1016/j.fuproc.2003.10.022

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been found in coal, including a large number of trace elements (TEs). Though the concentrations of trace elements in coal are very low (generally less than 100 ppmw), harmful effects of coal combustion on health and environment have been clearly established [1,2]. In fact, one previous study suggested that the concentrations of trace elements in soil around power plants exceeded the allowable limits of the US Environment Protection Agency (EPA) [1]. Harmful health effects of fluoride and selenium due to coal combustion have been found in south and southwest China [2]. Pollution control of trace elements during coal combustion has recently received much attention. Studies on the mobility of trace elements during coal combustion have shown that their volatility depends on their affinities and concentrations in coal, on the physical changes and chemical reactions of these elements with mineral, on the combustion parameters (e.g. temperature, excessive air coefficient), boiler output, and so on [1,3 – 13,15 – 24]. It has been proposed that elements with an affinity to organic matter and sulphides are more easily volatilized than those ones with affinity to silicate minerals [3,4]. Some researchers thought most of trace elements were enriched in the fly ash and depleted in the bottom ash [5,6]. Only the most volatile trace elements (e.g. Hg, Se) nearly entered the flue gas [5]. Gibb and Cenni demonstrated that the mercury retention in fly ash was related to the carbon content of the fly ash [7,8]. Klika compared the partitioning of trace element during coal combustion at 40% and 100% boiler output, and draw a conclusion that concentrations of nearly all trace elements were somewhat higher in fly ash at 40% boiler output than that at 100% boiler output [9]. The relationship of metal emissions and the fly ash particle size has been studied [4 –6,10]. The result showed that concentrations of Cd, Se, As, Zn, Ni and Pb increased significantly with decreasing particle size. Cu, Cd, Pb, As, Se, Sb and V were enriched in the sub-micron particles. The major use of coal is for the generation of power by combustion on a large scale. This means that most trace elements will be released and redistributed into bottom ash, fly ash, and the gaseous phase. Several authors have studied the partitioning of trace elements between different combustion products and classified those elements into three groups [8,11 –13]. Group 1 is very volatile and remains in gaseous phase through the entire process. Group 2 is partially volatile and is emitted mostly in fly ash while flue gas cools down. Group 3 is hardly volatile and is equally distributed between bottom ash and fly ash. But the classification criterion is not entirely same and not all parameters have necessarily been considered. For example, Cenni and Rong both classified Cr to Group 3 [8,11], while Diaz-Somoano classified Cr to Group 2 [12]. But Clarke considered Cr to be located between Groups 2 and 3 [13]. Despite the above-listed numerous studies, the behavior of trace element during coal combustion are still uncertain or incompletely understood and additional investigations are required. The present study consists of a detailed investigation of the emission features of trace element in Xuzhou bitumite coal and of their behavior during coal combustion at Thermoelectric Power Plant of Yangzi Petrochemical. Concentrations of seven trace elements (Mn, Cr, Pb, Se, Zn, Cd, Hg) in raw coal, bottom ash and fly ash were measured quantitatively in a 220 tons/h pulverized coal boiler. The study analyses the factors, which affect mission features of trace elements including fly ash diameter, furnace temperature, oxygen concentration and trace element characteristics. The objectives also include an evaluation of distribution of trace elements in the combustion products of coal.

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2. Experimental 2.1. Description of the boiler and base characters of coal There are altogether eight boilers, whose Main Continuous Ratings (MCR) are all 220 tons/h, at Thermoelectric Power Plant of Yangzi Petrochemical. Each boiler is connected to a three-field electrostatic precipitator (ESP) with total removal efficiency about 99%. A representative boiler was chosen for this study. The boiler has eight primary air nozzles and 12 s air nozzles. About 29.7 tons of crude coal per hour are consumed. The coal used in this study is Xuzhou bitumite. Average coal diameter is 35.1 Am (50% aerodynamic cut off diameter) by using HYDAO2000 laser light particle. 2.2. Experimental condition Velocities of the primary air and the second air monitored by standard Pilot tube were controlled at the beginning of experiment. It was found that actual air velocities were about 3 m/s higher than those displayed on the rate meter. RAYNGER MX4TM optical apparatus was used to verify furnace temperature. Airflow rate and rotation speed of coal feeder were simultaneously adjusted to obtain uniform horizontal furnace temperature. The Zirconia Oxygen Analyzer located is also checked out between the convective superheater and the economizer by using an Orsat Analyzer. It was found that Zirconia Oxygen Analyzer’s readings were about 2.0% too low. Combustion parameters were varied during the experiment, including the excessive air coefficient (a) and furnace temperature. In order to get three different excessive air coefficients (a), air velocity and rotation speed of coal feeder were adjusted and furnace temperature approximately kept constant (typically within 20 jC). Next, air velocity and rotation speed of coal feeder were adjusted to obtain three different furnace temperatures, keeping the excessive air coefficient constant (typically, the fluctuation is within 0.01). During the experiments, boiler capacity was 200– 220 tons/h, and exhausted flue gas temperature was 139– 148 jC. 2.3. Sample collection and analysis A number of particle samples were collected from various sites, which were pulverized coal classifier, bottom of boiler and each stage of electrostatic precipitators. Each test lasted 2– 3 h. In order to keep samples unanimous, samples of raw coal, bottom ash and fly ash were collected at different times. Raw coal was sampled about 1 h before bottom ash and finally, after another hour, fly ash was sampled. The particle samples were decanted quantitatively into acid-washed, tight High-Density Polyethylene (HDPE) containers. Flue gas was not sampled. In order to determine the pathways of trace elements to the environment and their occurrence and volatility during each test, Mn, Cr, Pb, Se, Zn, Cd and Hg were analyzed. A graphite oven atomic absorption spectrometer (AAS) was used for Se and Cd. Hg was analyzed using the cold vapour AAS-techniques and Mn, Cr, Pb, Zn were analyzed using X-ray fluorescence (XRF). Simultaneously the contents of ash in particle samples were

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analyzed. Results are shown in Table 2. Average concentrations of trace elements in the earth’s crust are also listed.

3. Results and discussion 3.1. Results of analysis Proximate analysis and ultimate analysis of coal are shown in Table 1. The concentrations of trace elements in raw coal, bottom ash and fly ash are shown in Table 2. Average concentrations of trace elements in the earth’s crust are also listed. 3.2. Trace elements in fly ash Some elements were highly enriched in fly ash while their concentrations in earth’s crust are very low (cf. Table 2). So fly ash does not only represent a significant potential source of toxic emissions to the environment, but ash from electrostatic precipitators may be a source of some useful trace elements (particularly Se). A method in which soluble salts and trace elements are extracted in an acidic medium is an attractive possibility [14]. Se is produced industrially as a by-product of Cu refining [15], and Se concentration of f 8 ppmw reported for an Arizona Cu ore deposit [15] is within the range of the 4.8 –12 ppmw found in the fly ash of this study. There have been progresses in this direction especially in Japan. But most of the methods are still under development in laboratories [14]. 3.3. Relative enrichment factors (REs) In order to investigate the change in the concentration of each trace element in the combustion products, relative enrichment factors (REs) are calculated. Different RE definitions have been proposed, one example being the Bottom Relative Enrichment coefficient proposed by Cenni et al. [8]: BRE=(Cif  Cib)/Cib. Cif represents the concentration of trace element i in fly ash and Cib represents concentration of trace element i in bottom ash. This factor has the advantage of including only ash samples, and it is therefore not influenced by analytical uncertainties due to interferences with carbon in the sample matrix. The basic assumption in this factor is that the relative concentration of some Table 1 Proximate analysis and ultimate analysis of coal

Moisture Volatile matter Fixed carbon Ash Higher caloric heat value Lower caloric heat value

Proximate analysis

Ultimate analysis

Mt Vad Cad Aad Qg Qd

C H S N O

4.9% 25.74% 38.53% 33.49% 20491 kJ/kg 19718 kJ/kg

Cad Had Sad Nad Oad

51.82% 3.57% 0.43% 0.9% 7.55%

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Table 2 The concentrations of trace elements in raw coal, bottom ash and fly ash No.

Furnace Excessive air The concentrations of trace elements (Ag/g or ppmw) Ash temperature coefficient content Mn Cr Pb Se Zn Cd Hg (%)

Coal 0–1 Bottom ash 1 – 1 1417 1.34 1 – 2 1437 1.37 1 – 3 1412 1.47 1 – 4 1363 1.48 1 – 5 1306 1.47 Fly ash 2 – 1 1417 1.34 2 – 2 1437 1.37 2 – 3 1412 1.47 2 – 4 1363 1.48 2 – 5 1306 1.47 1st-field 3 – 1 1390 1.49 2nd-field 3–2 3rd-field 3–3 Concentrations in the earth’s crust

109 288 274 260 253 256 229 254 271 276 274 262 192 178 950

26 44 36 41 41 43 24 25 30 27 23 26 41 50 100

16 7 15 15 21 18 45 55 43 52 54 48 80 86 12.5

3.5 0.8 1.0 0.9 1.8 1.2 9.2 10 5.2 5.4 6.7 4.8 7.0 12 0.05

15 15 21 36 22 21 38 48 52 52 45 50 64 88 70

0.020 0.068 0.039 0.086 0.052 0.060 0.045 0.052 0.063 0.064 0.060 0.060 0.112 0.166 0.2

0.096 n.d. n.d. n.d. n.d. n.d. 0.096 0.085 0.074 0.056 0.036 0.019 0.054 0.066 0.043

33.49 86.40 89.40 90.50 93.57 95.90 92.14 96.75 98.04 98.34 99.08 98.08 98.98 99.04

n.d. = Not determined.

elements (like Mn), which does not vaporize, must have the same concentration in fly ash as that in bottom ash. Meij defined REs as the ratio between the concentration in fuel and the concentration in combustion products [16]. The mathematical expression for this factor is: REs=(Cin/ Cic)  [(Aad)c]/[100]. Here, Cin represents concentration of trace element i in different combustion products and Cic represents concentration of trace element i in raw coal. (Aad)c represents content of ash in raw coal in percent. This factor only takes into account the influence of ash content in raw coal on RE, but does not include the influence of ash content in fly ash and bottom ash. Now, the Meij factor is improved, defining REs=(Cin/Cic)  [(Aad)c]/[(Aad)n]. (Aad)n represents the concentration of ash in different combustion products. RE describes the enrichment of trace metals in different products normalized to the ash content of the fuel. The improved factor of Meij can more directly show element enrichment in combustion products. If REs equals 1, trace element i is not enriched nor depleted in the particular ash. If REs is larger or smaller than 1, trace element i is enriched or depleted respectively. REs of trace elements are listed in Table 3. 3.4. Influence of particle diameter on REs of trace elements From Fig. 1, it can be seen that most of the trace elements tend to deposit on the smaller particles. The smaller the diameter of fly ash, the higher the relative enrichment of trace elements (except Mn). Compared with other trace elements, Mn shows an opposite trend, i.e. the smaller the diameter of fly ash, the less the relative enrichment. It is concluded that Mn in raw coal in our experiment exists in some steady and high melting point coarse phase.

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Table 3 Relative enrichment facts of trace elements

Bottom REb

Fly ash REf

1st-fieldERf 2nd-fieldERf 3rd-fieldERf

No.

Furnace temperature

Excessive air coefficient

Relative enrichment factors (REs) Mn

Cr

Pb

Se

Zn

Cd

Hg

1–1 1–2 1–3 1–4 1–5 2–1 2–2 2–3 2–4 2–5 3–1 3–2 3–3

1417 1437 1412 1363 1306 1417 1437 1412 1363 1306 1390

1.34 1.37 1.47 1.48 1.47 1.34 1.37 1.47 1.48 1.47 1.49

1.024 0.942 0.883 0.831 0.820 0.764 0.807 0.849 0.862 0.850 0.821 0.596 0.552

0.656 0.519 0.584 0.564 0.578 0.336 0.333 0.394 0.354 0.299 0.341 0.534 0.650

0.170 0.351 0.347 0.470 0.393 1.022 1.190 0.918 1.107 1.141 1.024 1.692 1.818

0.089 0.107 0.095 0.184 0.120 0.955 0.989 0.508 0.525 0.647 0.468 0.677 1.159

0.388 0.524 0.888 0.525 0.489 0.921 1.108 1.184 1.181 1.014 1.138 1.444 1.984

1.317 0.730 1.591 0.931 1.048 0.818 0.900 1.076 1.090 1.014 1.024 1.895 2.807

0 0 0 0 0 0.363 0.306 0.263 0.199 0.127 0.068 0.190 0.232

From the first-field of electrostatic precipitator to the third-field, the diameter of ash particles becomes smaller and the surface area of ash becomes greater. Metals vaporize from coal during the combustion process and re-condense in the post-combustion zone. Ash particles with diameters in the range of 0.01– 0.1 Am will be primarily formed via vaporization and condensation of major elements in coal such as silicon and iron [17]. The high surface area of sub-micron aerosol can cause a higher proportion of gaseous trace metal to deposit on the sub-micron ash as the flue gas cools, and results in an enrichment of certain element in the particles. Particles with diameters of approximately 0.1 Am are collected with the least efficiency in particulate collection devices such as electrostatic

Fig. 1. Relationship between REs of trace elements in fly ash and order of fields of ESP.

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precipitators and bag-houses [18]. Thus fly ash enriched in the fine particles represents a significant potential source of toxic emission to the air from coal-fired power plants. Jyh-Cherng studied the adsorption mechanism of trace elements during incineration, and concluded that adsorption mechanisms of trace elements were related to the volatility of heavy metal species formed at high temperatures [19]. For less volatile trace elements, chemical reaction is the major adsorption mechanism. But more volatile trace elements are easily released at high temperatures and hardly react with ash to form thermostable compounds. Thus physical processes such as condensation, coagulation, and sedimentation are the important mechanisms for more volatile elements such as Cd, Zn, and Se. Cr concentration in fly ash shows a less clear trend with particle diameter than most of the other studies elements, which indicates that Cr adsorption on fly ash is mainly chemical. The main Cr species at low temperature 500 K is Cr2O3, which may react with CaO in fly ash: Cr2O3.CaO ! CaO.Cr2O3 [20]. For Pb, both chemical adsorption and physical reactions are important. When flue gas temperature is below 800 K, dominant species of Pb is PbCl2 [17,21], which may reach with Al2O3 and SiO2 in fly ash. The reaction is: PbCl2 + Al2O3 + 2 SiO2 + H2O ! PbO.Al2O3.2 SiO2 + 2 HCl [17]. At the combustion temperature (about 1350 jC), mercury completely releases from coal. This holds true also for most of the length of the cooling pipes. Its concentration in the bottom ash is not detectable (cf. Table 2). The concentration of Hg in fly ash contributes to the high surface area of fly ash and the unburned components (char) in ash [7,8]. The more carbon content in fly ash, the more REs of Hg. Since Hg species at outlet gas temperature (about 140 jC) are mostly Hg, Hg2Cl2 and HgO, and these species’ partial vapour pressures are below atmospheric pressure, Hg is mainly in the vapour phase. 3.5. Influence of furnace temperature on REs of trace elements The volatilization of Cr, Zn, and Hg rises with furnace temperature when the excessive air coefficient between superheater and economizer is approximately constant (comparing 2 –3, 2– 4, 2 –5 sample). Furnace temperature changes in the experiment were too small to affect REs of trace elements in bottom ash significantly. 3.6. Influence of excessive air coefficient on REs of trace elements Comparing 2– 1, 2 – 2 sample with 2– 3 sample from Table 3, it is can be found that the reducing oxygen increases the concentrations of some trace elements like Se, Hg in fly ash under approximately constant furnace temperature. But other elements don’t abide the rule. Rong also considered that reducing conditions did not always enhance the trace elements volatilization, which depended on characteristics of both trace elements and coal [11]. 3.7. Influence of character of trace elements on REs From Table 3, it can be concluded that the REs of most of trace elements in fly ash are larger than those in bottom ash (except Mn, Cr). Some minerals like K, Na compounds containing trace elements, which volatilise from raw coal at high temperature re-

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coagulates on fly ash during flue gas cooling. Mn and in particular Cr show some enrichment in bottom ash compared with fly ash. However, while RE of Mn approaches 1, RE of Cr is far less than 1. We suggest that in this experiment some Cr is associated with the organic part of the coal. This part of Cr volatilises at the beginning of coal burning, which causes concentration of Cr in bottom ash to decrease. In the post-combustion zone, organic Cr may forms sub-micron particles that are emitted from the chimney. Senior et al. [5] and Vassilev et al. [22] also concluded that part of Cr was associated with the organics. Table 3 also shows that though boiling point of Pb (2032K) is higher than that of Cd (1042 K), REs of Pb in fly ash is slightly higher than that of Cd. The explanation is probably that Pb has a stronger affinity to Cl in flue gas than Cd, and the boiling point of PbCl2 (1223 jC) is far less than that of CdO (1773 jC). These results are in agreement with previously published results [11,22]. 3.8. Classification of trace elements Combustion products are divided into three parts: bottom ash, fly ash and flue gas. For each element mass conservation implies that: Mic + Mif + Mib + Mia. Mic represents total mass of trace element i in coal, Mif represents total mass of trace element i in fly ash. Mib represents total mass of trace element i in bottom ash, and Mia represents total mass of trace element i in flue gas. Because it is difficult to obtain the distribution of ash in big power plants, we assume that ash distribution among bottom ash, fly ash and flue gas is 10:89:1 according to empirical data and the original parameters of electrostatic precipitator [5,23,24]. The percents of trace elements in flue gas have been estimated using the following assumption. For a given trace element: Ria = 1  Rif  Rib, Ria(Mia/ Mic) represents the percent of trace element i in gas. Rif(Mif/Mic) represents the percent of trace element i in fly ash and Rib(Mib/Mic) represents the fraction of trace element i in bottom ash. Now, the representative samples 0 – 1, 1 – 4, 2 – 4 were chosen to calculate the distributions of trace elements in combustion products. Results are shown in Fig. 2. For some elements total percent exceeds 100. There are at least two reasons for this. Firstly, there are inevitable errors in the measurements. Secondly, in the calculations all fly ash is assumed to be from the electrostatic precipitator. Actually there are also minor contributions of other ash (e.g. economizer bottom ash, sediment ash in equipment wall, sedimentation ash in chimney, and so on). Most of trace elements appear to enter fly ash from electrostatic precipitator and flue gas. Because mass of fly ash is far higher than that of bottom ash, the fraction of the trace elements in bottom ash is very low compared with that in fly ash. The elements can be divided into three groups according to their occurrence and volatility: Group 1: Hg, which is very volatile with REfHREb and is emitted mainly in the vapour phase (Ra>80%). Group 2: Pb, Zn and Cd, which are vaporized at intermediate temperatures and are emitted mostly in fly ash (REfHREb and Rf>80%). Group 3: Mn, which is hardly vaporized and is found in approximately equal concentrations in bottom ash and fly ash (REf < REb). In addition, Se is located between Groups 1 and 2. Because some of Cr is associated with organics in coal, Cr possesses characteristics of both Groups 1 and 3 during combustion. Cenni and Rong both classified Cr to Group 3 [8,11]. We

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Fig. 2. Percent of trace elements in combustion products.

consider that if most of the Cr in coal is associated with minerals, Cr belongs to Group 3. If most of Cr in coal is associated with organics, Cr constitutes a special group.

4. Conclusions The results lead to the following conclusions. (1) The improved coefficient of Meij shows more directly element enrichment in combustion products. Most of trace elements tend to deposit on the smaller particles. The smaller the diameter of fly ash, the more relative enrichment of trace elements (except Mn). Concentrations of some trace elements (especially Pb, Se) in fly ash are far greater than their average concentrations in the earth crust, which implies both, a significant potential source of toxic emissions to the environment and a source for extracting some useful metals. (2) Cd, Zn and Se adsorption on fly ash is physical and Cr adsorption on fly ash is mainly chemical, while chemical adsorption and physical adsorption are both important for Pb. (3) The volatilisations of Cr, Zn, Hg rises with furnace temperature when excessive air coefficient (a) between superheater and economizer is approximately constant. Reducing oxygen increases the volatilization of some trace elements like Se, and Hg under the condition of approximately constant furnace temperature. But low oxygen concentration can not always increase the volatilisation of trace elements. (4) Because some minerals like K, Na compounds which volatilize from raw coal at high temperature re-coagulates on fly ash during flue gas cooling, Mn and Cr have slight enrichment in bottom compared with fly ash. But the REs of Mn are different from that of Cr.

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(5) Though the boiling point of Pb is less than that of Cd, REs of Pb in fly ash is generally higher than that of Cd and REs of Pb in bottom ash is less than that of Cd, Since Pb forms more easily than Cd chloride during coal combustion. Trace elements may be classified into three Groups according to their occurrence and volatility: Group 1: Hg. Group 2: Pb, Zn, Cd. Group 3:Mn. Se may be located between Groups 1 and 2.Cr has properties of both Group 1 and Group 3.

Acknowledgements The authors would like to acknowledge the financial support of Education Ministry through contract K980026 and State Key Project for Fundamental Research through contract G19990221053. We thank the technical and administrative assistance provided by Thermoelectric Power Plant of Yangzi Petrochemical. The authors also would like to acknowledge Professor Liu and his staff at Institute of Soil Science of Academy of Science in Nanjing for all AAS analytical work and Professor Yang and his staff at Center for Materials Analysis in Nanjing University for all XRF analytical work.

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