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Trace Cd, Co, and Pb elements distribution during Sulcis coal pyrolysis: GFAAS determination with slurry sampling technique
Microchemical Journal 100 (2012) 48–54
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Trace Cd, Co, and Pb elements distribution during Sulcis coal pyrolysis: GFAAS determination with slurry sampling technique Silvera Scaccia ⁎, Roberta Mecozzi Sustainable Combustion Laboratory (UTTEI-COMSO), C.R. ENEA, Casaccia, Italy
a r t i c l e
i n f o
Article history: Received 8 July 2011 Received in revised form 2 September 2011 Accepted 2 September 2011 Available online 10 September 2011 Keywords: Trace elements Coal Sulcis GF AAS Slurry sampling Metal distribution
a b s t r a c t A simple and fast method based on graphite furnace atomic absorption spectrometry (GF AAS) and slurry sampling technique (SlS) was developed to determine trace Cd, Co and Pb in high-sulphur coal (Sulcis, Italy) and coal chars derived at 600, 750 and 950 °C under N2 atmosphere for developing a clean coal for electricity production. The proposed method was then coupled to a four-step sequential chemical extraction method for assessment of metals distribution in coaled samples. The slurries were prepared by varying sample mass (1–50 mg), volume (1–3 mL) and kind of dispersing medium (1% v/v Triton X-100 and 2 N HNO3), and sonication time (5–30 min). Pyrolysis/atomization temperatures as well carrier gas flow rate were optimised. Pd(NO3)2 and NH4H2PO4 were employed to stabilize Cd and Pb, respectively, in the pyrolysis stage of furnace program. The use of HNO3 as dispersing agent was found to be effective in lowering the high level of background absorption on the Cd analytical signal produced by raw coal matrix. Conversely, coal charred samples did not show significantly level of background interferences, so that Triton X-100 dispersing agent could be used for all analytes. Calibration curve against acid-matched standards was allowed for Cd, whereas the standard addition calibration was used for Co and Pb owing to chemical matrix interferences. The precision, expressed as relative standard deviation (% RSD, n = 5), was better than 5% for Cd, Co, and Pb at 1, 10, and 15 μg L − 1 levels, respectively. The accuracy of the analytical method was checked by analyzing a BCR No. 182 steam coal certificated reference material and the results were in good agreement with certificated and informed values. The solid detection limits (3σblank) were as low as 0.001 Cd, 0.01 Co, and 0.01 Pb mg kg− 1, using 30 mg sample mass and slurry concentration of 30 m v− 1 for Cd, and 50 mg sample mass and 50 m v− 1 slurry concentration for Co and Pb. The content of elements in Sulcis coal was found to be 0.33 Cd, 4.0 Co, and 3.8 Pb mg kg− 1 and mostly associated to sulphates and di-sulphides as indicated by the leaching test. Under pyrolysis conditions Cd significantly volatilised (about 64%) at temperature higher than 600 °C, whereas residue chars at 950 °C are enriched in Co and Pb up to 20%. The proposed method is suitable for routine metals monitoring in coaled samples. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Potentially hazardous and toxic trace metal elements present in coal are readily released into the air in great amount as millions of tons of coal per year are processed in coal-fired thermal power plants causing environmental and human health concerns. In order to moderate high emissions of metallic pollutants into atmosphere clean coal technologies are nowadays under development for getting a sustainable coal-based power generation [1]. Pyrolysis, the first thermo-chemical step common to all coal conversion processes, can be regarded as a coal cleaning procedure for obtaining clean feedstock of combustion/ gasification for power generation because it is operated at moderate temperature under inert atmosphere. Under certain pyrolysis conditions some elements, strongly associated with the organic fraction ⁎ Corresponding author at: V. Anguillarese 301, I-00123, Rome, Italy. Tel.: + 39 06 3048 4721; fax: + 39 06 3048 4811. E-mail address: [email protected] (S. Scaccia). 0026-265X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2011.09.001
and/or mineral sulphur of coal, may undergo to chemical reactions with sulphur forming unstable compounds; others elements occurring with the mineral matter such as silicates, phosphates, carbonates and sulphates may remain in stable form in the charred residue [2]. Therefore, well-defined analytical methods for the determination of trace metals as well their distribution in parent coal and derivatives are essential for properly design an environmentally friendly coal-processing system. There are many published papers dealing with trace element determinations in raw coal by the most commonly analytical instrumentations such as atomic spectroscopic techniques [3, 4]. The assessment of trace elements distribution among different phase of raw coal and chars is often carried out by analytical procedures based on sequential chemical extraction methods in combination with analytical techniques [5–7]. However, most of the above mentioned analytical methods comprise tedious and time-consuming sample dissolution steps prior to analysis. In addition, owing to the hardness of coal to acids attacks, high temperature and pressure
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of Cd, Co, and Pb for different phases of raw coal and char samples was assessed by a four-step sequential chemical extraction method followed by the analysis of the eluents from extraction procedure by conventional GF AAS, whereas the remaining solid residues were analyzed by the proposed GF AAS-SlS method. With no solid residue sample dissolution the risk of contamination and analyte loss is minimized thus precise and accurate results were obtained for mass balance.
conditions, using for example acid-assisted microwave irradiation and bomb acid digestion techniques, are often required for completely dissolution of solid sample [8, 9]. Thus, the risk of contamination and loss of volatile analyte can be increased, which will unavoidably invalidate the precision and accuracy of data. Finally, the use of large amounts of hazardous and toxic reagents to bring the coal complex matrix in liquid phase generates unsafe wastes as well an excessive dilution of the analyte that can worsen the detection limit of the method. The direct solid sampling based on slurry sampling (SlS) technique has been proposed to overcome the drawbacks associated with the intensive coal sample dissolution procedures because a finely powdered sample dispersed in a liquid can be successfully introduced into the atomizer of the AAS/AES [10]. The slurry sampling technique satisfactorily conjugates both the advantages of solid sampling (reduction of sample preparation time, minimizing contamination risks and volatile analyte losses) and liquid sampling (easily sample dispensing into atomizer, sample dilution). Moreover, calibration using aqueous standard solutions is compatible with SlS technique. Among the various spectroscopic instrumentations, such as flame and graphite furnace atomic absorption spectrometry (F and GF AAS) [11, 12], inductively coupled plasma-optical emission spectrometry (ICP-OES) [13], and inductively coupled plasma mass spectrometry (ICP-MS) [14], GF AAS still remains the most suitable instrumentation to be coupled to SlS technique because a conventional liquid sample handling auto-sampler can be used for slurry sampling injection into atomizer. In addition, the high detection power of GF AAS for certain elements is further increased using SlS technique as higher amounts of coal sample may be accommodated into the graphite tube. Trace elements in raw coal samples have been successfully determined by GF AAS using SlS [15, 16]. However, chemical and physical interferences yielded by coal matrix on the analytical signal of the elements may vary depending on coal rank (sum of carbon content and organic matter). Sulcis coal, from Sardinia region of Italy, posses a high content of sulphur accompanied with a high concentration of inorganic matters such as various sulphates, carbonates, pyrite and SiO2[17], which can severely contribute to matrix interferences, thus worsening the sensitivity. To the best of our knowledge, there are no information data available in literature about the content of trace elements in raw Sulcis coal and overall how these elements will be distributed into the different phase of coal during pyrolysis. In the present paper is described a fast and simple method for the determination of trace Cd, Co and Pb in high-sulphur Sulcis coal and coal chars derived at 600, 750 and 950 °C under N2 atmosphere by GF AAS using slurry sampling technique. Besides optimization of thermal program, such as pyrolysis and atomization temperatures, and internal gas flow rate of the graphite furnace, some parameters of slurry sampling such as the kind of dispersing media and concentration were optimised in order to obtain high sensitivities. Finally, the affinity
2. Experimental 2.1. Instrumentation A Varian Spectra 220FS (Margrave, Victoria, Australia) atomic absorption spectrometer equipped with a GTA110 graphite furnace, an auto sampler and a deuterium lamp as background correction system was used. Hollow cathode lamps (Varian) of Cd, Co and Pb were used as sources. The most sensitive wavelengths were used: 228.8, 240.7, and 283.3 nm with spectral bandwidth of 0.2 nm respectively. Pyrolytic graphite-coated graphite tubes without platform were used. Argon (99.99% Air Liquid) gas was fluxed in the graphite tube at the flow rate reported in Table 1. In some experiments the gas flow rate was varied in the atomization stage. A volume of 20 μL was automatically injected into the graphite tube and atomic absorption signals were measured as peak height (PH). Three replicates for each sample were performed. The temperature program of the graphite tube was optimised and reported also in Table 1. Mean particle sizes of samples was computed by scanning electron microscopy and all were lower than 100 μm. A Sonic 1200 ultrasonic bath (power 50 W, frequency 50/40 Hz) was used for the preparation of the slurries and for extraction procedure. A micro balance (AND HM-202) with 0.01 mg uncertainty was used for sample weighing. Plastic containers and other glassware were soaked in 10% (v/v) HNO3 for 24 h and then rinsed several times with ultrapure de-ionized water until no metal contamination from containers and glassware was detected. 2.2. Reagents and standards All reagents were of analytical reagent grade and supplied by Aldrich. Ultrapure water (18.2 MΩ cm− 1) produced through a Rios 5TM reverse osmosis and a Milli-Q Gradient water purification system (Millipore S.A. S, Molshem, France) and filtered through a 0.22 μm membrane filter was used to prepare all solutions. Single standard solutions of 2 Cd, 20 Co and 30 Pb μg L− 1 were daily prepared by making a serial dilutions of 1 g L− 1 atomic absorption stock solutions and acidified to 2 N HNO3. Calibration curves were constructed using five working standard solutions prepared directly into graphite tube through auto-sampler device
Table 1 Heating program of the graphite furnace. Element
Cd
Step Dry I II III Pyrolysis I II III Atomization I II III
T (°C)
hold time (s)
Ar (L min− 1)
T (°C)
Co hold time (s)
Ar (L min− 1)
T (°C)
hold time (s)
Ar (L min− 1)
85 95 120
5.0 40.0 10.0
1.1 2.0 3.0
85 95 120
5.0 40.0 10.0
1.1 2.0 3.0
85 95 120
5.0 40.0 10.0
1.1 2.0 3.0
300 300 300
5.0 1.0 2.0
3.0 3.0 3.0
800 800 800
5.0 1.0 2.0
3.0 3.0 3.0
450 450 450
5.0 1.0 2.0
3.0 3.0 3.0
1800 1800 1800
3.0 3.0 0.0
0.0 0.0 3.0
2300 2300 2300
3.0 3.0 0.0
0.0 0.0 3.0
2100 2100 2100
3.0 3.0 0.0
0.0 1.0 3.0
The underlined values correspond to optimized parameters.
Pb
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programmed to aspirate sequentially known amount of metal single standard solution and blank. Matrix modifiers 5 μg Pd(NO3)2, 5 μg Pd(NO3)2+ 5 μg Mg(NO3)2, and 200 μg NH4H2PO4 were used for Cd and Pb and dispensed into atomizer through the auto-sampler. One % (v/v) Triton X-100 was used as dispersing medium of slurries. The extraction solutions were prepared by appropriate dissolution of a known amount of CH3COONa salt and dilution of hydrochloric acid (37%), H2O2 (30%), HNO3 (65%) in deionised water.
2.3. Samples Raw coal sample was collected from Sulcis field, Sardinia, Italy. The bulk coal sample was crushed by means of a jaw crusher and ground in a ball mill and dried in air at 110 °C for 2 h. Coal char samples were produced from raw coal submitted to heating in an horizontal tubular furnace. A known amount of raw coal sample (3.0 g) was loaded on an alumina boat hanging from a stainless-steel rod, which was then inserted in a quartz tube (20 mm in diameter and 600 mm in length) with a Teflon stopper with inlet and outlet tubing gases and inlet thermocouple. The quartz tube was tightly set into the tubular furnace, heated at a rate of 20 °C min − 1 under N2 atmosphere at a flow rate of 200 mL min − 1, kept at the final temperature of 600, 750 and 950 °C for 1 h and then cooled to ambient temperature switching-off the power furnace. The char yields were stored without any grinding in desiccators before analysis. BCR No. 182 steam coal certified reference material (Community Bureau of Reference, Brussels, Belgium) was used to checked the accuracy of the entire analytical method. The average particle size of CRM ranged from 63 to 212 μm and was analysed as received without any grinding and sieving. Although, CRM particle sizes were wider
than those of Sulcis coal sample no major problems were observed during the analysis. 2.4. Analytical procedures 2.4.1. Slurry preparation Slurries of coal samples were prepared by weighing accurately between 1.0 and 50.0 mg of sample mass in a 5 mL test-tube and adding between 0.025 and 0.100 mL concentrated HNO3, controlled previously for blank, and balanced with de-ionized water to a total volume between 1 and 3 mL. The final concentration in the slurries was 2 N HNO3. Slurries were also prepared in 1% (v/v) Triton X-100. Because the parameters of the sonication such as power and frequency cannot be varied, the slurries were sonicated at room temperature at different times (5–30 minutes) for testing the homogeneous dispersing degree of coal powder. The homogeneity of the slurries was maintained with manual shaking with an Eppendorf micropipette made just before the capillary of the auto-sampler withdraw the sample. 2.4.2. Sequential chemical extraction procedure The four-step sequential chemical extraction procedure is schematically depicted in Fig. 1. A known amount of sample (about 200 mg) was immersed in the suitable solvent and after a certain magnetic agitation time and heating the suspension was filtered through a Teflon filter with pore size of 1 μm in a Millipore filtration apparatus under vacuum and washed thoroughly with de-ionized water. The residue filtrate from each solvent extraction step was recovered and submitted to air treatment at 120 °C for 2 h, weighed and then stored in a dessicator prior to be submitted to subsequent step. The supernatant, referred to associated to carbonates (F1), sulphates and sulphides (F2), organically metal bonded (F3), and disulphides (F4), was analysed for the metal content by GF AAS
ELUATE ANALYSIS BY GF AAS
RESIDUE ANALYSED BY GF AAS- SlS
200 mg coal 1 M CH3COONa (500 mL)
Acid soluble and carbonate bonded
F1
Sulphates, sulphides
F2
Disulphides
F3
Organic matter
F4
room temperature, 24 h
4 M HCl (200 mL) 100 °C, 2h
2 N HNO3 (200 mL) 80°C, 3h
F5
Alumino-silicates
H2O2 (100 mL) 60°C, 3h
Fig. 1. Schematic drawing of sequential chemical extraction procedure.
S. Scaccia, R. Mecozzi / Microchemical Journal 100 (2012) 48–54
3.0.1. Optimization of the graphite furnace heating program Pyrolysis/atomization curves were built up for 0.01 ng Cd, 0.10 ng Co, and 0.15 ng Pb in aqueous standard and raw coal sample as slurries using both 2 N HNO3 and 1% (v/v) Triton X-100 dispersing medium with/without chemical modifiers (Fig. 2a–c). The use of conventional chemical modifiers such as Pd(NO3)2, Pd(NO3)2 plus Mg(NO3)2, and NH4H2PO4 was tested owing to the tendency of cadmium and lead to volatilize at high pyrolysis temperatures. The best effectiveness in stabilising analyte signals in coaled sample as slurries was exhibited by Pd(NO3)3 for cadmium up to pyrolysis temperature of 700 °C and NH4H2PO4 for lead up to pyrolysis temperature of 800 °C, thus only the data using these chemical modifiers were depicted. The effect of pyrolysis temperature on the analytical signal of the elements was similar for both the dispersing media chosen. From Fig. 2 it was also seen that the atomization temperatures had little influence on the absorption signal of the analytes in both aqueous standard and coal as slurries. Exception is made for the analytical signal of cadmium, which was very temperature-sensitive to the atomization temperature in absence of a thermally stabilising agent. 3.0.2. Effect of dispersing medium on analyte signals In Fig. 3 is depicted the absorbance-time profile of cadmium in raw coal as slurries in different dispersing media along with the background absorbance produced by coal matrix under optimised furnace heating program. At the most sensitive cadmium analytical wavelength a high background absorption level (about 2 A/a. u.) was exhibited by 30 m v − 1 coal as slurry in Triton X-100 medium. Otherwise, using 2 N HNO3 as dispersing medium the background absorption was significantly attenuated at a level (0.6 A/a. u.) easily handling by conventional deuterium background corrector. It is well known that nitric acid can act also as chemical modifier allowing to remove the effect of the coal matrix during the ash step. The shape of the lead atomic signal peak coming from 50 m v − 1 raw coal as slurry using both 1% (v/v) Triton X-100 and 2 N HNO3 dispersing media is showed in Fig. 4. As it can be seen the lead analytical signals in coal as slurries is narrower compared to the signals coming from aqueous standard solution. The release of lead atomic signal in coal as slurry dispersed in Triton X-100 was anticipated in appearance compared to the analyte signal in aqueous solution and was accompanied by a non-specific absorption background structured as a sharp peak. A second broad non-specific absorption peak was also detected, which was likely due to the radiation scattering as it was easily eliminated by enhancing the internal argon flow rate through the graphite furnace in the atomization step II. These findings are in accordance with other results reported in literature [15]. The atomic signal peak of lead in coal as slurry in 2 N HNO3 was almost coincident in appearance time with the corresponding background absorption and with the lead atomic signal coming from the aqueous standard solution. It is interesting to note that coal chars as slurry showed lower background interferences as the volatile organic fraction of raw coal was destroyed during coal pyrolysis processing. Excessive residual unburnt carbon into graphite tube were avoided by discharging periodically the accumulated residues and any deterioration of the graphite tubes, which could worsen the analytical signals, were observed.
0.20
Cd
Absorbance (a.u.)
0.15
0.10
0.05
0.00
Aqueous standard without modifier Aqueous standard with modifier Raw coal as slurry in 1% (v/v) Triton X-100 with modifier Raw coal as slurry in 2N HNO3 with modifier
-0.05
-0.10 200 400 600 800 1000
1600
1800
2000
2200
Temperature (°C)
b
0.15
Co 0.10
Absorbance (a.u.)
3. Results and discussion
a
0.05
0.00
Aqueous standard Raw coal as slurry in 1% (v/v) Triton X-100 Raw coal as slurry in 2N HNO3
-0.05 400
600
800
1000
2000
2200
2400
Temperature (°C)
c 0.15
Pb
0.10
Absorbance (a.u.)
according to the thermal program of Table 1. An aliquot of solid residue coming from each extraction step was analysed by the proposed GF AAS–SlS method. The results of the analysis of F1–F5 residues were utilised for mass balance, being the final residue F5 containing metals that have the highest affinity to the crystalline structures of minerals (alumino-silicates). The % leached amount was defined as the ratio between the metal content in leached solution/metal content in solid sample x100. Blank extraction tests (with no solid sample) were carried out through the whole procedure.
51
0.05
0.00
-0.05
Aqueous standard without modifier Aqueous standard with modifier Raw coal as slurry in 1% (v/v) Triton-X 100 with modifier Raw coal as slurry in 2N HNO3with modifier
-0.10 200 400 600 800 1000 1800
2000
2200
Temperature (°C) Fig. 2. Pyrolysis and atomization temperature curves for: (a) Cd (0.01 ng), (b) Co (0.10 ng), and (c) Pb (0.15 ng) in aqueous solutions and Sulcis coal as slurry in different dispersing media with and without chemical modifier. Modifiers: Pd(NO3)2 for Cd; and NH4H2PO4 for Pb. Pyrolysis (300, 800, and 450 °C) and atomization (1800, 2300, and 2100 °C) temperatures for Cd, Co, and Pb, respectively. Sample mass: 0.6 mg for Cd; 1 mg for Co and Pb.
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S. Scaccia, R. Mecozzi / Microchemical Journal 100 (2012) 48–54 Table 2 The characteristic parameters of the calibration curves. Element Correlation coefficient, Slope of regression Working concentration (n = 5)/r2 line equation/ L μg− 1 range/ μg L− 1 Matrix Cd Co Pb
Aqueous Slurry 0.9999 0.9999 0.9999 0.9999 0.9999 0.9999
Aqueous Slurry 0.15610 0.15725 0.00969 0.00632 0.00989 0.00832
Aqueous Slurry 0.1–2.5 0.1–2.5 5.0–30 5.0–30 5.0–30 5.0–30
value. Thus, high concentrated slurries gave rise to higher reproducibility and no occlusion of the auto-sampler capillary tip was seen. The best analytical results were obtained using extraction time of 30 minutes. Prolonged sonication times were not further beneficial to extraction of metals from coal samples.
Fig. 3. Atomic and background absorption peak profiles of: (a) 0.01 ng Cd and (b) 0.15 ng Pb in Sulcis coal as slurry in different dispersing media. Sample mass: 0.6 mg for Cd; and 1 mg for Pb.
3.0.3. Optimization of slurry sample The particle size of the powdered sample is a very important issue in slurry-sampling technique as it can affect the reproducibility of measurements as large size particles can give rise to low reproducibility [18]. Thus, the ratio between sample mass (1–50 mg) and volume of dispersing medium (1–3 mL) was investigated in the range 0.2–50 m v− 1. For cadmium 30 mg of coal sample mass was the maximum value used in the slurry preparation as larger amounts produced higher background signal. Because dilute slurries gave to poorly reproducible measurements it was chosen to operate with the volume minimum
3.1. Total content of trace Cd, Co and Pb in Sulcis coal during pyrolysis
6.0
0.40 0.35
5.5
0.25
Cd Co Pb
0.20
5.0
0.15
4.5
0.10 0.05
200
400
600
800
1000
Temperature (°C) Fig. 4. Total content of Cd, Co, and Pb in Sulcis coal and chars.
In Fig. 4 is showed the variation of Cd, Co, and Pb content in Sulcis raw coal as a function of pyrolysis temperature. It seems that under pyrolysis conditions cadmium is stable up to 600 °C, whereas vaporization of total cadmium occurs within the temperature range 600– 950 °C. The final cadmium concentration was 0.12 mg kg− 1 at 950 °C, thus a noticeable losses of about 64% is reached at high pyrolysis Table 3 Results of analysis of BCR No. 182 steam coal and Sulcis coal by GF AAS-SlS. Concentration in mg kg− 1.
4.0
Sample
3.5
BCR No. 182 steam coal Certified Found Sulcis coal Found
0.00 0
Co, Pb (mg kg-1)
Cd (mg kg-1)
0.30
3.0.4. Analytical features Calibration curves were established for each element employing either aqueous standard solutions or the standard addition technique to slurries. The characteristic parameters of the calibration curves are reported in Table 2. The correlation coefficients, r 2, of the calibration curves were better than 0.9999 in the entire range of concentrations studied and the intercepts did not significantly deviate from zero. The slopes of the calibration curves for cobalt and lead under the optimized operating conditions were dissimilar indicating a chemical interference of the coal matrix, therefore calibration with the standard additions method was used. For cadmium the slopes of the calibration curves were similar in both slurry and aqueous matrices so that calibration against aqueous standard solutions could be suitable. The limits of detection (LOD), based on three times the standard deviation (sblank) of seven measurements of blank sample solution to the slope of calibration curve, were calculated to be 0.003, 0.5, and 0.5 μg L − 1 for Cd, Co, and Pb, respectively. The limits of detection of solid were 0.001, 0.01, and 0.01 mg for Cd, Co, and Pb, respectively, per kilogram of sample, which were calculated taking into account the maximum amount of sample that gives linearity, i.e. 30 mg sample mass and slurry concentration of 30 m v − 1 for cadmium, and 50 mg sample mass and 50 m v − 1 slurry concentration for Co and Pb. The precision, expressed as relative standard deviation (% RSD, n = 5), for Cd, Co, and Pb at 1, 10, and 15 μg L − 1 levels, respectively, was better than 5%. The accuracy of the method was checked by analyzing the BCR N°182 steam coal certificated reference material for all the analytes under study and the results were in good accordance with both the certificated and informed values (recovery better than 98%) as reported in Table 3. The concentration of Cd, Co, and Pb in Sulcis raw coal was also reported in Table 3.
*Informed value.
Cd
Co
Pb
0.057 ± 0.004 0.054 ± 0.003
8.87* 8.8 ± 0.5
15.3* 15.1 ± 0.6
0.33 ± 0.02
4.0 ± 0.2
3.8 ± 0.2
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Table 4 Distribution of Cd, Co, and Pb in raw Sulcis coal and chars pyrolysed at 600 and 950 °C under N2 atmosphere. Concentration in mg kg− 1. Cd Step Sample Raw coal Char600°C Char950°C Co Step Sample Raw coal Char600°C Char950°C Pb Step Sample Raw coal Char600°C Char950°C
F1 (Carbonates)
F2 (Sulphates, sulphides)
F3 (Organic)
F4 (Disulphides)
F5 (Alumino-silicates)
b LOD§ b LOD§ b LOD§
0.20 ± 0.01 0.21 ± 0.01 0.024 ± 0.01
0.077 ± 0.006 0.073 ± 0.007 0.040 ± 0.005
0.043 ± 0.005 0.033 ± 0.006 0.042 ± 0.005
0.010 ± 0.001 0.010 ± 0.001 0.0042 ± 0.0006
F1 (Carbonates)
F2 (Sulphates, sulphides)
F3 (Organic)
F4 (Disulphides)
F5 (Alumino-silicates)
0.70 ± 0.01 b LOD§ b LOD§
2.50± 3.80± 3.90±
0.50 ± 0.01 0.80 ± 0.01 0.90 ± 0.02
0.20 ± 0.01 0.25 ± 0.01 0.31 ± 0.01
0.100 ± 0.005 0.151 ± 0.008 0.21 ± 0.01
F1 (Carbonates)
F2 (Sulphates, sulphides)
F3 (Organic)
F4 (Disulphides)
F5 (Alumino-silicates)
b LOD§ b LOD§ b LOD§
2.90 ± 0.04 2.80 ± 0.04 2.40 ± 0.05
0.32 ± 0.01 0.45 ± 0.01 0.53 ± 0.01
0.45 ± 0.01 0.52 ± 0.01 1.00 ± 0.02
0.110 ± 0.005 0.23 ± 0.01 0.51 ± 0.02
§ LOD = Limit of detection was defined as the concentration corresponding to three times the standard deviation of seven measurements of blank multiplied by the dilution factor.
temperature. Otherwise, cobalt and lead were stable up to 950 °C as their concentrations in the final residue chars were 5.2 and 4.5 mg kg − 1, respectively. 3.2. The distribution of trace elements in raw Sulcis coal and chars The solubility of metals among sequential solvent extraction steps, namely CH3COONa (F1), HCl (F2), HNO3 (F3) and H2O2 (F4), of raw coal and chars along with the metal content in the final residue (F5) are showed in Table 4. 3.2.1. Cadmium The main forms of cadmium in raw Sulcis coal are associated with sulphates–sulphides (F2) and pyrite (F3), and to organic matter (F4), whereas cadmium bonded to alumino-silicates fraction (F5) is of minor extent. In the pyrolyzed char at 600 °C the mass fraction of cadmium distributed in the various phases remains nearly unchanged with respect to raw coal. In low-rank high sulphur coal pyrite decomposes to sulphide and sulphur at temperature above 600 °C under pyrolysis conditions, thus that cadmium presents in FeS2 and in organic matter could be swept out during coal pyrolysis step [19]. In char 950 °C the dominant forms of cadmium still remains sulphate–disulphide, being the amount of cadmium associated to organic matrix noticeably decreased. 3.2.2. Cobalt The mass fraction of cobalt extracted in F2 phase from raw coal was more than 50% of the total suggesting a strong association with sulphates and mono-sulphides. The cobalt associated with carbonates (F1) and organic matter (F3) was of the same order of magnitude. The affinity of cobalt to the pyrite form (F4) as well the content of cobalt remaining in the alumino-silicate minerals was very low. The mode of occurrence of cobalt to carbonates (F1) completely disappeared at the high pyrolysis temperatures, whereas cobalt noticeably enriched in phase F2 in char samples at 600 and 950 °C as well in organic matter, whereas the mass fraction of cobalt associated to pyrite and that remaining into residual phase F5 did not change. 3.2.3. Lead Lead in Sulcis coal was chiefly associated with sulphates and mono-sulphides (F2) and pyrite (F4), whereas a low content of lead was associated to organic matter (F3) and the final residue (F5).
Lead usually occurs in association with sulphides because PbS (galena) is a very stable species under pyrolysis conditions. In the char obtained at 600 °C the sulphate/sulphide related-lead compounds decreased, while the amount of lead in the others forms increased. The behaviour of lead in char 950 °C followed the same patterns distribution of char 600 °C even though lead associated with the pyritic form was enhanced as well higher enrichment in the stable residue F5. This indicates that some lead was transformed into thermally stable forms during coal pyrolysis [20]. 4. Conclusions The proposed GF AAS-SlS method allowed to determinate trace levels of Cd, Co and Pb in high-sulphur Sulcis raw coal and in coal chars derived at 600, 750 and 950 °C under N2 atmosphere with good accuracy and precision. Triton-X was chosen as dispersing medium for Co and Pb determination in raw coal slurry, whereas the use of HNO3 as dispersing agent was able in lowering the background interferences of matrix on the Cd signal. The charred samples did showed background interferences on the analytical signals. Finally, the proposed method was used in combination with a four-step sequential extraction procedure for the assessment of the analyte affinity to different phase of coal samples. The method is simple, sensitive, speed and easy handling for coaled sample. References [1] S. Derenne, P. Sartorelli, J. Bustard, R. Stewart, S. Sjostrom, P. Johnson, M. McMillian, F. Sudhoff, R. Chang, TOXECON clean coal demonstration for mercury and multipollutant control at the Presque Isle Power Plant, Fuel Pro. Technol. 90 (2009) 1400. [2] J. Wang, A. Tomita, A Chemistry on the Volatility of Some Trace Elements during Coal Combustion and Pyrolysis, Energy & Fuels 17 (2003) 954. [3] B. Fairman, M.W. Hinds, S.M. Nelms, D.M. Penny, P. Goodall, Atomic Spectrometry Update. Industrial analysis: metals, chemicals and advanced materials, J. Anal. At. Spectrom. 16 (2001) 1446. [4] P. Monkhouse, On-line spectroscopic and spectrometric methods for the determination of metal species in industrial process, Prog. Energy Combust. Sci. 37 (2011) 125. [5] F.E. Huggins, Overview of analytical methods for inorganic constituents in coal, Int. J Coal Geol. 50 (2002) 169. [6] C.G. Yuan, Leaching characteristics of metals in fly ash from coal-fired power plant by sequential extraction procedure, Microchim. Acta 165 (2009) 91. [7] A. Smeda, W. Zyrnicki, Application of sequential extraction and the ICP-AES method for study of the partitioning of metals in fly ashes, Microchem. J. 72 (2002) 9.
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S. Scaccia, R. Mecozzi / Microchemical Journal 100 (2012) 48–54
[8] K. Srogi, Microwave-assisted sample preparation of coal and coal fly ash for subsequent metal determination, Anal. Lett. 40 (2007) 199. [9] É.M.M. Flores, J.S. Barin, M.F. Mesko, G. Knapp, Sample preparation techniques based on combustion reactions in closed vessels — A brief overview and recent applications, Spectrochim. Acta Part B 62 (2007) 1051. [10] B. Welz, M. Sperling, Atomic Absorption Spectrometry, 2nd, Completely Revised Edition, Viley-VCH, 1991. [11] S.L.C. Ferreira, M. Mirò, E.G.P. Da Silva, G.D. Matos, P.S. Dos Reis, G.C. Brandao, W.N.L. Dos Santos, A.T. Duarte, M.G.R. Vale, R.G.O. Araujo, Slurry Sampling—An Analytical Strategy for the Determination of Metals and Metalloids by Spectroanalytical Techniques, Appl. Spectrosc. Rev. 45 (2010) 44. [12] M. Felipe-Sotelo, A. Carlosena, J.M. Andrade, M.J. Cal-Prieto, E. Fernàndez, D. Prada, Slurry-based procedures to determine chromium, nickel and vanadium in complex matrices by ETAAS, Microchem. J. 81 (2005) 217. [13] M. Mujuru, R.I. McCrindle, N. Panichev, Characterisation of coal slurries for introduction into ICP OES for multi-element determinations, J. Anal. At. Spectrom. 24 (2009) 494. [14] M.A. Vieira, A.S. Ribeiro, A.J. Curtius, Determination of As, Ge, Hg, Pb, Sb, Se and Sn in coal slurries by CVG-ETV-ICP-MS using external or isotopic dilution calibration, Microchem. J. 82 (2006) 127.
[15] M.M. Silva, M.G.R. Vale, E.B. Caramão, Slurry sampling graphite furnace atomic absorption spectrometry: determination of trace metals in mineral coal, Talanta 50 (1999) 1035. [16] M.J. Cal-Pietro, M. Felipe-Sotelo, A. Carlosena, J.M. Andrade, P. Lopez-Mahia, S. Muniategui, D. Prada, Slurry sampling for direct analysis of solid materials by electrothermal atomic absorption spectrometry (ETAAS). A literature review from 1990 to 2000, Talanta 56 (2002) 51. [17] A. D'Alessio, P. Vergamini, E. Benedetti, FT-IR investigation of the structural changes of Sulcis and South Africa coals under progressive heating in vacuum, Fuel 29 (2000) 1215. [18] N.J. Miller-ihli, Influence of slurry preparation on the accuracy of ultrasonic slurry electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 9 (1994) 1129. [19] S. Furfari, R. Cyprès, Hydropyrolysis of a high-sulphur, high-calcite Italian Sulcis coal.3. Pyrite behaviour, Fuel 62 (1983) 615. [20] P. Shah, V. Strezov, P.F. Nelson, X-Ray absorption near edge structure spectrometry study of nickel and lead speciation in coals and coal combustion products, Energy & Fuels 23 (2009) 1518.
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Microchemical Journal journal homepage: www.elsevier.com/locate/microc
Trace Cd, Co, and Pb elements distribution during Sulcis coal pyrolysis: GFAAS determination with slurry sampling technique Silvera Scaccia ⁎, Roberta Mecozzi Sustainable Combustion Laboratory (UTTEI-COMSO), C.R. ENEA, Casaccia, Italy
a r t i c l e
i n f o
Article history: Received 8 July 2011 Received in revised form 2 September 2011 Accepted 2 September 2011 Available online 10 September 2011 Keywords: Trace elements Coal Sulcis GF AAS Slurry sampling Metal distribution
a b s t r a c t A simple and fast method based on graphite furnace atomic absorption spectrometry (GF AAS) and slurry sampling technique (SlS) was developed to determine trace Cd, Co and Pb in high-sulphur coal (Sulcis, Italy) and coal chars derived at 600, 750 and 950 °C under N2 atmosphere for developing a clean coal for electricity production. The proposed method was then coupled to a four-step sequential chemical extraction method for assessment of metals distribution in coaled samples. The slurries were prepared by varying sample mass (1–50 mg), volume (1–3 mL) and kind of dispersing medium (1% v/v Triton X-100 and 2 N HNO3), and sonication time (5–30 min). Pyrolysis/atomization temperatures as well carrier gas flow rate were optimised. Pd(NO3)2 and NH4H2PO4 were employed to stabilize Cd and Pb, respectively, in the pyrolysis stage of furnace program. The use of HNO3 as dispersing agent was found to be effective in lowering the high level of background absorption on the Cd analytical signal produced by raw coal matrix. Conversely, coal charred samples did not show significantly level of background interferences, so that Triton X-100 dispersing agent could be used for all analytes. Calibration curve against acid-matched standards was allowed for Cd, whereas the standard addition calibration was used for Co and Pb owing to chemical matrix interferences. The precision, expressed as relative standard deviation (% RSD, n = 5), was better than 5% for Cd, Co, and Pb at 1, 10, and 15 μg L − 1 levels, respectively. The accuracy of the analytical method was checked by analyzing a BCR No. 182 steam coal certificated reference material and the results were in good agreement with certificated and informed values. The solid detection limits (3σblank) were as low as 0.001 Cd, 0.01 Co, and 0.01 Pb mg kg− 1, using 30 mg sample mass and slurry concentration of 30 m v− 1 for Cd, and 50 mg sample mass and 50 m v− 1 slurry concentration for Co and Pb. The content of elements in Sulcis coal was found to be 0.33 Cd, 4.0 Co, and 3.8 Pb mg kg− 1 and mostly associated to sulphates and di-sulphides as indicated by the leaching test. Under pyrolysis conditions Cd significantly volatilised (about 64%) at temperature higher than 600 °C, whereas residue chars at 950 °C are enriched in Co and Pb up to 20%. The proposed method is suitable for routine metals monitoring in coaled samples. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Potentially hazardous and toxic trace metal elements present in coal are readily released into the air in great amount as millions of tons of coal per year are processed in coal-fired thermal power plants causing environmental and human health concerns. In order to moderate high emissions of metallic pollutants into atmosphere clean coal technologies are nowadays under development for getting a sustainable coal-based power generation [1]. Pyrolysis, the first thermo-chemical step common to all coal conversion processes, can be regarded as a coal cleaning procedure for obtaining clean feedstock of combustion/ gasification for power generation because it is operated at moderate temperature under inert atmosphere. Under certain pyrolysis conditions some elements, strongly associated with the organic fraction ⁎ Corresponding author at: V. Anguillarese 301, I-00123, Rome, Italy. Tel.: + 39 06 3048 4721; fax: + 39 06 3048 4811. E-mail address: [email protected] (S. Scaccia). 0026-265X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2011.09.001
and/or mineral sulphur of coal, may undergo to chemical reactions with sulphur forming unstable compounds; others elements occurring with the mineral matter such as silicates, phosphates, carbonates and sulphates may remain in stable form in the charred residue [2]. Therefore, well-defined analytical methods for the determination of trace metals as well their distribution in parent coal and derivatives are essential for properly design an environmentally friendly coal-processing system. There are many published papers dealing with trace element determinations in raw coal by the most commonly analytical instrumentations such as atomic spectroscopic techniques [3, 4]. The assessment of trace elements distribution among different phase of raw coal and chars is often carried out by analytical procedures based on sequential chemical extraction methods in combination with analytical techniques [5–7]. However, most of the above mentioned analytical methods comprise tedious and time-consuming sample dissolution steps prior to analysis. In addition, owing to the hardness of coal to acids attacks, high temperature and pressure
S. Scaccia, R. Mecozzi / Microchemical Journal 100 (2012) 48–54
49
of Cd, Co, and Pb for different phases of raw coal and char samples was assessed by a four-step sequential chemical extraction method followed by the analysis of the eluents from extraction procedure by conventional GF AAS, whereas the remaining solid residues were analyzed by the proposed GF AAS-SlS method. With no solid residue sample dissolution the risk of contamination and analyte loss is minimized thus precise and accurate results were obtained for mass balance.
conditions, using for example acid-assisted microwave irradiation and bomb acid digestion techniques, are often required for completely dissolution of solid sample [8, 9]. Thus, the risk of contamination and loss of volatile analyte can be increased, which will unavoidably invalidate the precision and accuracy of data. Finally, the use of large amounts of hazardous and toxic reagents to bring the coal complex matrix in liquid phase generates unsafe wastes as well an excessive dilution of the analyte that can worsen the detection limit of the method. The direct solid sampling based on slurry sampling (SlS) technique has been proposed to overcome the drawbacks associated with the intensive coal sample dissolution procedures because a finely powdered sample dispersed in a liquid can be successfully introduced into the atomizer of the AAS/AES [10]. The slurry sampling technique satisfactorily conjugates both the advantages of solid sampling (reduction of sample preparation time, minimizing contamination risks and volatile analyte losses) and liquid sampling (easily sample dispensing into atomizer, sample dilution). Moreover, calibration using aqueous standard solutions is compatible with SlS technique. Among the various spectroscopic instrumentations, such as flame and graphite furnace atomic absorption spectrometry (F and GF AAS) [11, 12], inductively coupled plasma-optical emission spectrometry (ICP-OES) [13], and inductively coupled plasma mass spectrometry (ICP-MS) [14], GF AAS still remains the most suitable instrumentation to be coupled to SlS technique because a conventional liquid sample handling auto-sampler can be used for slurry sampling injection into atomizer. In addition, the high detection power of GF AAS for certain elements is further increased using SlS technique as higher amounts of coal sample may be accommodated into the graphite tube. Trace elements in raw coal samples have been successfully determined by GF AAS using SlS [15, 16]. However, chemical and physical interferences yielded by coal matrix on the analytical signal of the elements may vary depending on coal rank (sum of carbon content and organic matter). Sulcis coal, from Sardinia region of Italy, posses a high content of sulphur accompanied with a high concentration of inorganic matters such as various sulphates, carbonates, pyrite and SiO2[17], which can severely contribute to matrix interferences, thus worsening the sensitivity. To the best of our knowledge, there are no information data available in literature about the content of trace elements in raw Sulcis coal and overall how these elements will be distributed into the different phase of coal during pyrolysis. In the present paper is described a fast and simple method for the determination of trace Cd, Co and Pb in high-sulphur Sulcis coal and coal chars derived at 600, 750 and 950 °C under N2 atmosphere by GF AAS using slurry sampling technique. Besides optimization of thermal program, such as pyrolysis and atomization temperatures, and internal gas flow rate of the graphite furnace, some parameters of slurry sampling such as the kind of dispersing media and concentration were optimised in order to obtain high sensitivities. Finally, the affinity
2. Experimental 2.1. Instrumentation A Varian Spectra 220FS (Margrave, Victoria, Australia) atomic absorption spectrometer equipped with a GTA110 graphite furnace, an auto sampler and a deuterium lamp as background correction system was used. Hollow cathode lamps (Varian) of Cd, Co and Pb were used as sources. The most sensitive wavelengths were used: 228.8, 240.7, and 283.3 nm with spectral bandwidth of 0.2 nm respectively. Pyrolytic graphite-coated graphite tubes without platform were used. Argon (99.99% Air Liquid) gas was fluxed in the graphite tube at the flow rate reported in Table 1. In some experiments the gas flow rate was varied in the atomization stage. A volume of 20 μL was automatically injected into the graphite tube and atomic absorption signals were measured as peak height (PH). Three replicates for each sample were performed. The temperature program of the graphite tube was optimised and reported also in Table 1. Mean particle sizes of samples was computed by scanning electron microscopy and all were lower than 100 μm. A Sonic 1200 ultrasonic bath (power 50 W, frequency 50/40 Hz) was used for the preparation of the slurries and for extraction procedure. A micro balance (AND HM-202) with 0.01 mg uncertainty was used for sample weighing. Plastic containers and other glassware were soaked in 10% (v/v) HNO3 for 24 h and then rinsed several times with ultrapure de-ionized water until no metal contamination from containers and glassware was detected. 2.2. Reagents and standards All reagents were of analytical reagent grade and supplied by Aldrich. Ultrapure water (18.2 MΩ cm− 1) produced through a Rios 5TM reverse osmosis and a Milli-Q Gradient water purification system (Millipore S.A. S, Molshem, France) and filtered through a 0.22 μm membrane filter was used to prepare all solutions. Single standard solutions of 2 Cd, 20 Co and 30 Pb μg L− 1 were daily prepared by making a serial dilutions of 1 g L− 1 atomic absorption stock solutions and acidified to 2 N HNO3. Calibration curves were constructed using five working standard solutions prepared directly into graphite tube through auto-sampler device
Table 1 Heating program of the graphite furnace. Element
Cd
Step Dry I II III Pyrolysis I II III Atomization I II III
T (°C)
hold time (s)
Ar (L min− 1)
T (°C)
Co hold time (s)
Ar (L min− 1)
T (°C)
hold time (s)
Ar (L min− 1)
85 95 120
5.0 40.0 10.0
1.1 2.0 3.0
85 95 120
5.0 40.0 10.0
1.1 2.0 3.0
85 95 120
5.0 40.0 10.0
1.1 2.0 3.0
300 300 300
5.0 1.0 2.0
3.0 3.0 3.0
800 800 800
5.0 1.0 2.0
3.0 3.0 3.0
450 450 450
5.0 1.0 2.0
3.0 3.0 3.0
1800 1800 1800
3.0 3.0 0.0
0.0 0.0 3.0
2300 2300 2300
3.0 3.0 0.0
0.0 0.0 3.0
2100 2100 2100
3.0 3.0 0.0
0.0 1.0 3.0
The underlined values correspond to optimized parameters.
Pb
50
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programmed to aspirate sequentially known amount of metal single standard solution and blank. Matrix modifiers 5 μg Pd(NO3)2, 5 μg Pd(NO3)2+ 5 μg Mg(NO3)2, and 200 μg NH4H2PO4 were used for Cd and Pb and dispensed into atomizer through the auto-sampler. One % (v/v) Triton X-100 was used as dispersing medium of slurries. The extraction solutions were prepared by appropriate dissolution of a known amount of CH3COONa salt and dilution of hydrochloric acid (37%), H2O2 (30%), HNO3 (65%) in deionised water.
2.3. Samples Raw coal sample was collected from Sulcis field, Sardinia, Italy. The bulk coal sample was crushed by means of a jaw crusher and ground in a ball mill and dried in air at 110 °C for 2 h. Coal char samples were produced from raw coal submitted to heating in an horizontal tubular furnace. A known amount of raw coal sample (3.0 g) was loaded on an alumina boat hanging from a stainless-steel rod, which was then inserted in a quartz tube (20 mm in diameter and 600 mm in length) with a Teflon stopper with inlet and outlet tubing gases and inlet thermocouple. The quartz tube was tightly set into the tubular furnace, heated at a rate of 20 °C min − 1 under N2 atmosphere at a flow rate of 200 mL min − 1, kept at the final temperature of 600, 750 and 950 °C for 1 h and then cooled to ambient temperature switching-off the power furnace. The char yields were stored without any grinding in desiccators before analysis. BCR No. 182 steam coal certified reference material (Community Bureau of Reference, Brussels, Belgium) was used to checked the accuracy of the entire analytical method. The average particle size of CRM ranged from 63 to 212 μm and was analysed as received without any grinding and sieving. Although, CRM particle sizes were wider
than those of Sulcis coal sample no major problems were observed during the analysis. 2.4. Analytical procedures 2.4.1. Slurry preparation Slurries of coal samples were prepared by weighing accurately between 1.0 and 50.0 mg of sample mass in a 5 mL test-tube and adding between 0.025 and 0.100 mL concentrated HNO3, controlled previously for blank, and balanced with de-ionized water to a total volume between 1 and 3 mL. The final concentration in the slurries was 2 N HNO3. Slurries were also prepared in 1% (v/v) Triton X-100. Because the parameters of the sonication such as power and frequency cannot be varied, the slurries were sonicated at room temperature at different times (5–30 minutes) for testing the homogeneous dispersing degree of coal powder. The homogeneity of the slurries was maintained with manual shaking with an Eppendorf micropipette made just before the capillary of the auto-sampler withdraw the sample. 2.4.2. Sequential chemical extraction procedure The four-step sequential chemical extraction procedure is schematically depicted in Fig. 1. A known amount of sample (about 200 mg) was immersed in the suitable solvent and after a certain magnetic agitation time and heating the suspension was filtered through a Teflon filter with pore size of 1 μm in a Millipore filtration apparatus under vacuum and washed thoroughly with de-ionized water. The residue filtrate from each solvent extraction step was recovered and submitted to air treatment at 120 °C for 2 h, weighed and then stored in a dessicator prior to be submitted to subsequent step. The supernatant, referred to associated to carbonates (F1), sulphates and sulphides (F2), organically metal bonded (F3), and disulphides (F4), was analysed for the metal content by GF AAS
ELUATE ANALYSIS BY GF AAS
RESIDUE ANALYSED BY GF AAS- SlS
200 mg coal 1 M CH3COONa (500 mL)
Acid soluble and carbonate bonded
F1
Sulphates, sulphides
F2
Disulphides
F3
Organic matter
F4
room temperature, 24 h
4 M HCl (200 mL) 100 °C, 2h
2 N HNO3 (200 mL) 80°C, 3h
F5
Alumino-silicates
H2O2 (100 mL) 60°C, 3h
Fig. 1. Schematic drawing of sequential chemical extraction procedure.
S. Scaccia, R. Mecozzi / Microchemical Journal 100 (2012) 48–54
3.0.1. Optimization of the graphite furnace heating program Pyrolysis/atomization curves were built up for 0.01 ng Cd, 0.10 ng Co, and 0.15 ng Pb in aqueous standard and raw coal sample as slurries using both 2 N HNO3 and 1% (v/v) Triton X-100 dispersing medium with/without chemical modifiers (Fig. 2a–c). The use of conventional chemical modifiers such as Pd(NO3)2, Pd(NO3)2 plus Mg(NO3)2, and NH4H2PO4 was tested owing to the tendency of cadmium and lead to volatilize at high pyrolysis temperatures. The best effectiveness in stabilising analyte signals in coaled sample as slurries was exhibited by Pd(NO3)3 for cadmium up to pyrolysis temperature of 700 °C and NH4H2PO4 for lead up to pyrolysis temperature of 800 °C, thus only the data using these chemical modifiers were depicted. The effect of pyrolysis temperature on the analytical signal of the elements was similar for both the dispersing media chosen. From Fig. 2 it was also seen that the atomization temperatures had little influence on the absorption signal of the analytes in both aqueous standard and coal as slurries. Exception is made for the analytical signal of cadmium, which was very temperature-sensitive to the atomization temperature in absence of a thermally stabilising agent. 3.0.2. Effect of dispersing medium on analyte signals In Fig. 3 is depicted the absorbance-time profile of cadmium in raw coal as slurries in different dispersing media along with the background absorbance produced by coal matrix under optimised furnace heating program. At the most sensitive cadmium analytical wavelength a high background absorption level (about 2 A/a. u.) was exhibited by 30 m v − 1 coal as slurry in Triton X-100 medium. Otherwise, using 2 N HNO3 as dispersing medium the background absorption was significantly attenuated at a level (0.6 A/a. u.) easily handling by conventional deuterium background corrector. It is well known that nitric acid can act also as chemical modifier allowing to remove the effect of the coal matrix during the ash step. The shape of the lead atomic signal peak coming from 50 m v − 1 raw coal as slurry using both 1% (v/v) Triton X-100 and 2 N HNO3 dispersing media is showed in Fig. 4. As it can be seen the lead analytical signals in coal as slurries is narrower compared to the signals coming from aqueous standard solution. The release of lead atomic signal in coal as slurry dispersed in Triton X-100 was anticipated in appearance compared to the analyte signal in aqueous solution and was accompanied by a non-specific absorption background structured as a sharp peak. A second broad non-specific absorption peak was also detected, which was likely due to the radiation scattering as it was easily eliminated by enhancing the internal argon flow rate through the graphite furnace in the atomization step II. These findings are in accordance with other results reported in literature [15]. The atomic signal peak of lead in coal as slurry in 2 N HNO3 was almost coincident in appearance time with the corresponding background absorption and with the lead atomic signal coming from the aqueous standard solution. It is interesting to note that coal chars as slurry showed lower background interferences as the volatile organic fraction of raw coal was destroyed during coal pyrolysis processing. Excessive residual unburnt carbon into graphite tube were avoided by discharging periodically the accumulated residues and any deterioration of the graphite tubes, which could worsen the analytical signals, were observed.
0.20
Cd
Absorbance (a.u.)
0.15
0.10
0.05
0.00
Aqueous standard without modifier Aqueous standard with modifier Raw coal as slurry in 1% (v/v) Triton X-100 with modifier Raw coal as slurry in 2N HNO3 with modifier
-0.05
-0.10 200 400 600 800 1000
1600
1800
2000
2200
Temperature (°C)
b
0.15
Co 0.10
Absorbance (a.u.)
3. Results and discussion
a
0.05
0.00
Aqueous standard Raw coal as slurry in 1% (v/v) Triton X-100 Raw coal as slurry in 2N HNO3
-0.05 400
600
800
1000
2000
2200
2400
Temperature (°C)
c 0.15
Pb
0.10
Absorbance (a.u.)
according to the thermal program of Table 1. An aliquot of solid residue coming from each extraction step was analysed by the proposed GF AAS–SlS method. The results of the analysis of F1–F5 residues were utilised for mass balance, being the final residue F5 containing metals that have the highest affinity to the crystalline structures of minerals (alumino-silicates). The % leached amount was defined as the ratio between the metal content in leached solution/metal content in solid sample x100. Blank extraction tests (with no solid sample) were carried out through the whole procedure.
51
0.05
0.00
-0.05
Aqueous standard without modifier Aqueous standard with modifier Raw coal as slurry in 1% (v/v) Triton-X 100 with modifier Raw coal as slurry in 2N HNO3with modifier
-0.10 200 400 600 800 1000 1800
2000
2200
Temperature (°C) Fig. 2. Pyrolysis and atomization temperature curves for: (a) Cd (0.01 ng), (b) Co (0.10 ng), and (c) Pb (0.15 ng) in aqueous solutions and Sulcis coal as slurry in different dispersing media with and without chemical modifier. Modifiers: Pd(NO3)2 for Cd; and NH4H2PO4 for Pb. Pyrolysis (300, 800, and 450 °C) and atomization (1800, 2300, and 2100 °C) temperatures for Cd, Co, and Pb, respectively. Sample mass: 0.6 mg for Cd; 1 mg for Co and Pb.
52
S. Scaccia, R. Mecozzi / Microchemical Journal 100 (2012) 48–54 Table 2 The characteristic parameters of the calibration curves. Element Correlation coefficient, Slope of regression Working concentration (n = 5)/r2 line equation/ L μg− 1 range/ μg L− 1 Matrix Cd Co Pb
Aqueous Slurry 0.9999 0.9999 0.9999 0.9999 0.9999 0.9999
Aqueous Slurry 0.15610 0.15725 0.00969 0.00632 0.00989 0.00832
Aqueous Slurry 0.1–2.5 0.1–2.5 5.0–30 5.0–30 5.0–30 5.0–30
value. Thus, high concentrated slurries gave rise to higher reproducibility and no occlusion of the auto-sampler capillary tip was seen. The best analytical results were obtained using extraction time of 30 minutes. Prolonged sonication times were not further beneficial to extraction of metals from coal samples.
Fig. 3. Atomic and background absorption peak profiles of: (a) 0.01 ng Cd and (b) 0.15 ng Pb in Sulcis coal as slurry in different dispersing media. Sample mass: 0.6 mg for Cd; and 1 mg for Pb.
3.0.3. Optimization of slurry sample The particle size of the powdered sample is a very important issue in slurry-sampling technique as it can affect the reproducibility of measurements as large size particles can give rise to low reproducibility [18]. Thus, the ratio between sample mass (1–50 mg) and volume of dispersing medium (1–3 mL) was investigated in the range 0.2–50 m v− 1. For cadmium 30 mg of coal sample mass was the maximum value used in the slurry preparation as larger amounts produced higher background signal. Because dilute slurries gave to poorly reproducible measurements it was chosen to operate with the volume minimum
3.1. Total content of trace Cd, Co and Pb in Sulcis coal during pyrolysis
6.0
0.40 0.35
5.5
0.25
Cd Co Pb
0.20
5.0
0.15
4.5
0.10 0.05
200
400
600
800
1000
Temperature (°C) Fig. 4. Total content of Cd, Co, and Pb in Sulcis coal and chars.
In Fig. 4 is showed the variation of Cd, Co, and Pb content in Sulcis raw coal as a function of pyrolysis temperature. It seems that under pyrolysis conditions cadmium is stable up to 600 °C, whereas vaporization of total cadmium occurs within the temperature range 600– 950 °C. The final cadmium concentration was 0.12 mg kg− 1 at 950 °C, thus a noticeable losses of about 64% is reached at high pyrolysis Table 3 Results of analysis of BCR No. 182 steam coal and Sulcis coal by GF AAS-SlS. Concentration in mg kg− 1.
4.0
Sample
3.5
BCR No. 182 steam coal Certified Found Sulcis coal Found
0.00 0
Co, Pb (mg kg-1)
Cd (mg kg-1)
0.30
3.0.4. Analytical features Calibration curves were established for each element employing either aqueous standard solutions or the standard addition technique to slurries. The characteristic parameters of the calibration curves are reported in Table 2. The correlation coefficients, r 2, of the calibration curves were better than 0.9999 in the entire range of concentrations studied and the intercepts did not significantly deviate from zero. The slopes of the calibration curves for cobalt and lead under the optimized operating conditions were dissimilar indicating a chemical interference of the coal matrix, therefore calibration with the standard additions method was used. For cadmium the slopes of the calibration curves were similar in both slurry and aqueous matrices so that calibration against aqueous standard solutions could be suitable. The limits of detection (LOD), based on three times the standard deviation (sblank) of seven measurements of blank sample solution to the slope of calibration curve, were calculated to be 0.003, 0.5, and 0.5 μg L − 1 for Cd, Co, and Pb, respectively. The limits of detection of solid were 0.001, 0.01, and 0.01 mg for Cd, Co, and Pb, respectively, per kilogram of sample, which were calculated taking into account the maximum amount of sample that gives linearity, i.e. 30 mg sample mass and slurry concentration of 30 m v − 1 for cadmium, and 50 mg sample mass and 50 m v − 1 slurry concentration for Co and Pb. The precision, expressed as relative standard deviation (% RSD, n = 5), for Cd, Co, and Pb at 1, 10, and 15 μg L − 1 levels, respectively, was better than 5%. The accuracy of the method was checked by analyzing the BCR N°182 steam coal certificated reference material for all the analytes under study and the results were in good accordance with both the certificated and informed values (recovery better than 98%) as reported in Table 3. The concentration of Cd, Co, and Pb in Sulcis raw coal was also reported in Table 3.
*Informed value.
Cd
Co
Pb
0.057 ± 0.004 0.054 ± 0.003
8.87* 8.8 ± 0.5
15.3* 15.1 ± 0.6
0.33 ± 0.02
4.0 ± 0.2
3.8 ± 0.2
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Table 4 Distribution of Cd, Co, and Pb in raw Sulcis coal and chars pyrolysed at 600 and 950 °C under N2 atmosphere. Concentration in mg kg− 1. Cd Step Sample Raw coal Char600°C Char950°C Co Step Sample Raw coal Char600°C Char950°C Pb Step Sample Raw coal Char600°C Char950°C
F1 (Carbonates)
F2 (Sulphates, sulphides)
F3 (Organic)
F4 (Disulphides)
F5 (Alumino-silicates)
b LOD§ b LOD§ b LOD§
0.20 ± 0.01 0.21 ± 0.01 0.024 ± 0.01
0.077 ± 0.006 0.073 ± 0.007 0.040 ± 0.005
0.043 ± 0.005 0.033 ± 0.006 0.042 ± 0.005
0.010 ± 0.001 0.010 ± 0.001 0.0042 ± 0.0006
F1 (Carbonates)
F2 (Sulphates, sulphides)
F3 (Organic)
F4 (Disulphides)
F5 (Alumino-silicates)
0.70 ± 0.01 b LOD§ b LOD§
2.50± 3.80± 3.90±
0.50 ± 0.01 0.80 ± 0.01 0.90 ± 0.02
0.20 ± 0.01 0.25 ± 0.01 0.31 ± 0.01
0.100 ± 0.005 0.151 ± 0.008 0.21 ± 0.01
F1 (Carbonates)
F2 (Sulphates, sulphides)
F3 (Organic)
F4 (Disulphides)
F5 (Alumino-silicates)
b LOD§ b LOD§ b LOD§
2.90 ± 0.04 2.80 ± 0.04 2.40 ± 0.05
0.32 ± 0.01 0.45 ± 0.01 0.53 ± 0.01
0.45 ± 0.01 0.52 ± 0.01 1.00 ± 0.02
0.110 ± 0.005 0.23 ± 0.01 0.51 ± 0.02
§ LOD = Limit of detection was defined as the concentration corresponding to three times the standard deviation of seven measurements of blank multiplied by the dilution factor.
temperature. Otherwise, cobalt and lead were stable up to 950 °C as their concentrations in the final residue chars were 5.2 and 4.5 mg kg − 1, respectively. 3.2. The distribution of trace elements in raw Sulcis coal and chars The solubility of metals among sequential solvent extraction steps, namely CH3COONa (F1), HCl (F2), HNO3 (F3) and H2O2 (F4), of raw coal and chars along with the metal content in the final residue (F5) are showed in Table 4. 3.2.1. Cadmium The main forms of cadmium in raw Sulcis coal are associated with sulphates–sulphides (F2) and pyrite (F3), and to organic matter (F4), whereas cadmium bonded to alumino-silicates fraction (F5) is of minor extent. In the pyrolyzed char at 600 °C the mass fraction of cadmium distributed in the various phases remains nearly unchanged with respect to raw coal. In low-rank high sulphur coal pyrite decomposes to sulphide and sulphur at temperature above 600 °C under pyrolysis conditions, thus that cadmium presents in FeS2 and in organic matter could be swept out during coal pyrolysis step [19]. In char 950 °C the dominant forms of cadmium still remains sulphate–disulphide, being the amount of cadmium associated to organic matrix noticeably decreased. 3.2.2. Cobalt The mass fraction of cobalt extracted in F2 phase from raw coal was more than 50% of the total suggesting a strong association with sulphates and mono-sulphides. The cobalt associated with carbonates (F1) and organic matter (F3) was of the same order of magnitude. The affinity of cobalt to the pyrite form (F4) as well the content of cobalt remaining in the alumino-silicate minerals was very low. The mode of occurrence of cobalt to carbonates (F1) completely disappeared at the high pyrolysis temperatures, whereas cobalt noticeably enriched in phase F2 in char samples at 600 and 950 °C as well in organic matter, whereas the mass fraction of cobalt associated to pyrite and that remaining into residual phase F5 did not change. 3.2.3. Lead Lead in Sulcis coal was chiefly associated with sulphates and mono-sulphides (F2) and pyrite (F4), whereas a low content of lead was associated to organic matter (F3) and the final residue (F5).
Lead usually occurs in association with sulphides because PbS (galena) is a very stable species under pyrolysis conditions. In the char obtained at 600 °C the sulphate/sulphide related-lead compounds decreased, while the amount of lead in the others forms increased. The behaviour of lead in char 950 °C followed the same patterns distribution of char 600 °C even though lead associated with the pyritic form was enhanced as well higher enrichment in the stable residue F5. This indicates that some lead was transformed into thermally stable forms during coal pyrolysis [20]. 4. Conclusions The proposed GF AAS-SlS method allowed to determinate trace levels of Cd, Co and Pb in high-sulphur Sulcis raw coal and in coal chars derived at 600, 750 and 950 °C under N2 atmosphere with good accuracy and precision. Triton-X was chosen as dispersing medium for Co and Pb determination in raw coal slurry, whereas the use of HNO3 as dispersing agent was able in lowering the background interferences of matrix on the Cd signal. The charred samples did showed background interferences on the analytical signals. Finally, the proposed method was used in combination with a four-step sequential extraction procedure for the assessment of the analyte affinity to different phase of coal samples. The method is simple, sensitive, speed and easy handling for coaled sample. References [1] S. Derenne, P. Sartorelli, J. Bustard, R. Stewart, S. Sjostrom, P. Johnson, M. McMillian, F. Sudhoff, R. Chang, TOXECON clean coal demonstration for mercury and multipollutant control at the Presque Isle Power Plant, Fuel Pro. Technol. 90 (2009) 1400. [2] J. Wang, A. Tomita, A Chemistry on the Volatility of Some Trace Elements during Coal Combustion and Pyrolysis, Energy & Fuels 17 (2003) 954. [3] B. Fairman, M.W. Hinds, S.M. Nelms, D.M. Penny, P. Goodall, Atomic Spectrometry Update. Industrial analysis: metals, chemicals and advanced materials, J. Anal. At. Spectrom. 16 (2001) 1446. [4] P. Monkhouse, On-line spectroscopic and spectrometric methods for the determination of metal species in industrial process, Prog. Energy Combust. Sci. 37 (2011) 125. [5] F.E. Huggins, Overview of analytical methods for inorganic constituents in coal, Int. J Coal Geol. 50 (2002) 169. [6] C.G. Yuan, Leaching characteristics of metals in fly ash from coal-fired power plant by sequential extraction procedure, Microchim. Acta 165 (2009) 91. [7] A. Smeda, W. Zyrnicki, Application of sequential extraction and the ICP-AES method for study of the partitioning of metals in fly ashes, Microchem. J. 72 (2002) 9.
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S. Scaccia, R. Mecozzi / Microchemical Journal 100 (2012) 48–54
[8] K. Srogi, Microwave-assisted sample preparation of coal and coal fly ash for subsequent metal determination, Anal. Lett. 40 (2007) 199. [9] É.M.M. Flores, J.S. Barin, M.F. Mesko, G. Knapp, Sample preparation techniques based on combustion reactions in closed vessels — A brief overview and recent applications, Spectrochim. Acta Part B 62 (2007) 1051. [10] B. Welz, M. Sperling, Atomic Absorption Spectrometry, 2nd, Completely Revised Edition, Viley-VCH, 1991. [11] S.L.C. Ferreira, M. Mirò, E.G.P. Da Silva, G.D. Matos, P.S. Dos Reis, G.C. Brandao, W.N.L. Dos Santos, A.T. Duarte, M.G.R. Vale, R.G.O. Araujo, Slurry Sampling—An Analytical Strategy for the Determination of Metals and Metalloids by Spectroanalytical Techniques, Appl. Spectrosc. Rev. 45 (2010) 44. [12] M. Felipe-Sotelo, A. Carlosena, J.M. Andrade, M.J. Cal-Prieto, E. Fernàndez, D. Prada, Slurry-based procedures to determine chromium, nickel and vanadium in complex matrices by ETAAS, Microchem. J. 81 (2005) 217. [13] M. Mujuru, R.I. McCrindle, N. Panichev, Characterisation of coal slurries for introduction into ICP OES for multi-element determinations, J. Anal. At. Spectrom. 24 (2009) 494. [14] M.A. Vieira, A.S. Ribeiro, A.J. Curtius, Determination of As, Ge, Hg, Pb, Sb, Se and Sn in coal slurries by CVG-ETV-ICP-MS using external or isotopic dilution calibration, Microchem. J. 82 (2006) 127.
[15] M.M. Silva, M.G.R. Vale, E.B. Caramão, Slurry sampling graphite furnace atomic absorption spectrometry: determination of trace metals in mineral coal, Talanta 50 (1999) 1035. [16] M.J. Cal-Pietro, M. Felipe-Sotelo, A. Carlosena, J.M. Andrade, P. Lopez-Mahia, S. Muniategui, D. Prada, Slurry sampling for direct analysis of solid materials by electrothermal atomic absorption spectrometry (ETAAS). A literature review from 1990 to 2000, Talanta 56 (2002) 51. [17] A. D'Alessio, P. Vergamini, E. Benedetti, FT-IR investigation of the structural changes of Sulcis and South Africa coals under progressive heating in vacuum, Fuel 29 (2000) 1215. [18] N.J. Miller-ihli, Influence of slurry preparation on the accuracy of ultrasonic slurry electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 9 (1994) 1129. [19] S. Furfari, R. Cyprès, Hydropyrolysis of a high-sulphur, high-calcite Italian Sulcis coal.3. Pyrite behaviour, Fuel 62 (1983) 615. [20] P. Shah, V. Strezov, P.F. Nelson, X-Ray absorption near edge structure spectrometry study of nickel and lead speciation in coals and coal combustion products, Energy & Fuels 23 (2009) 1518.