ORIGINAL PAPER Total reflection X-ray fluorescence ...

Chemical Papers 69 (5) 650–654 (2015) DOI: 10.1515/chempap-2015-0071

ORIGINAL PAPER

Total reflection X-ray fluorescence analysis of fly ash from Bulgarian coal-fired power plants‡ Albena K. Detcheva*, a Svilen E. Mitsiev, a Paunka S. Vassileva, a Juri H. Jordanov, b Metody G. Karadjov, a Elisaveta Ivanova

a Institute

rc op y

a

of General and Inorganic Chemistry, b Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Str. Bl. 11, 1113 Sofia, Bulgaria Received 10 April 2014; Revised 15 September 2014; Accepted 23 September 2014

ut

ho

The contents of Cl, Ca, K, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Rb, Sr, Ba and Pb in raw coal fly ash from five Bulgarian power plants were determined by total reflection X-ray fluorescence (TXRF), using gallium as the internal standard. The samples were analysed as in slurry form in TritonTM X-114. The experimental parameters, such as grain size, concentrations of fly ash slurry and excitation time were optimised. For validation of the method, the certified reference material BCR-176R fly ash was used. The precision of the results obtained is characterised by a relative standard deviation of approximately 10 %. The resulting data confirm the suitability of TXRF for the simultaneous determination of major, minor and trace elements in coal fly ash samples. Further advantages provided by TXRF are easy sample preparation (no sample dissolution) and the small sample amount required for analysis. c 2014 Institute of Chemistry, Slovak Academy of Sciences Keywords: coal fly ash, TXRF analysis, characterisation, major, minor and trace elements

A

Introduction

Coal-fired power plants in Bulgaria generate about 6 million tonnes of fly ash annually, of which less than 20 % is used, the rest being disposed of in landfills leading to serious risk of air, soil, ground- and surfacewater pollution (Shoumkova, 2006). Fly ash recycling continues to be insufficient, hence the need for novel applications. The chemical and physical properties of fly ash vary widely, depending on the kind of coalfired, boiler type and burning regime. Only a detailed study of the physical, chemical and morphological properties of a particular fly ash could afford an understanding of the potential environmental and health impacts associated with its disposal and use; however, these have not been adequately investigated (Száková et al., 2013). In a previous study, fly ash from five Bulgarian

power plants was characterised in terms of mineralogical and chemical composition, grain size distribution, specific surface area and pore size, conductivity, moisture, pH and loss on ignition by a combination of analytical techniques (Detcheva & Vassileva, 2014). Conventional analytical methods usually require wet digestion in sample preparation. Chemical sample pretreatment and sample decomposition are among the most crucial steps in the analytical procedures of conventional methods (Detcheva & Grobecker, 2008). Direct and slurry analyses of solids circumvent the welldocumented disadvantages of wet chemical analysis. A good alternative to the simultaneous determination of major, minor and trace elements in powdered samples proved to be total reflection X-ray fluorescence (TXRF) (Cariati et al., 2003; Wobrauschek, 2007). The calibration is simple – only one internal standard is required, unlike conventional X-ray fluores-

*Corresponding author, e-mail: [email protected] ‡ Presented at European Symposium on Atomic Spectrometry ESAS 2014 & 15th Czech – Slovak Spectroscopic Conference, Prague, Czech Republic, March 16–21, 2014

A. K. Detcheva et al./Chemical Papers 69 (5) 650–654 (2015)

cence (XRF) (Misra, 2011) and recently portable instrumentation has become available (Marguí et al., 2010). As an exemplary “green” analytical method, TXRF uses sample amounts at the milligram level and involves minimum sample pretreatment for the analysis of suspensions; the analysis is rapid and automated, with low energy, water and no gas consumption (Ivanova & Detcheva, 2012). In the present work, TXRF analysis was applied for the first time to characterise raw coal fly ash from five Bulgarian power plants.

ously hydrophobised quartz plate. The same procedure was applied to analysis of the fly ash samples from BD, MI, R, RI and S. Aliquots of 5 L of the slurries mixed with the internal standard were pipetted on the quartz reflector and were dried under an IR lamp. The area of the dry residues, obtained as above, was less than 10 mm2 (Georgieva et al., 2013). Three replicates of each sample were prepared. The dry residues were measured for 1000 s and each dry residue was measured twice at two perpendicular positions of the quartz reflector. Several blank quartz reflectors were measured as described in order to determine the blank value. For the evaluation of the TXRF spectra, Spectra 5.1 software (Bruker AXS) was used. The quantification was performed in accordance with Hoefler et al. (2006). The lower limit of detection (LLDi ) for each analyte (i) was determined in each spectrum by the equation:

rc op y

Experimental

A

ut

ho

In the present study, coal fly ash from five Bulgarian power plants: “Bobov Dol”, “Maritza Iztok 3”, “Republika”, “Russe Iztok” and “Sliven”, denoted as BD, MI, R, RI and S, respectively, was studied. The materials were obtained directly from the electrostatic gas cleaning systems in the plants. A TXRF analyser S2 PICOFOX (Bruker AXS, Germany) was used for the measurements. The instrument was equipped with an air-cooled low-power X-ray tube (Mo target), a Ni/C monochromator with 80 % reflectivity and a liquid nitrogen-free Silicon Drift Detector (SDD) 10 mm2 in area with an energy resolution of < 159 eV (MnKα ). The sampleholders were quartz optical plates 30 mm in diameter and 3 mm in thickness (Perspex Distribution, UK). The power was 50 kV and the current was 1 mA. The surface of the quartz reflectors was hydrophobised by applying 10 L of silicone solution in propanol (Serva, Germany) and drying in air for several minutes. For slurry preparation, a 0.01 vol. % aqueous solution of the surfactant TritonTM X-114 (Sigma–Aldrich, Germany) was used to preclude agglomeration and sedimentation of the slurry particles. For internal calibration, a working standard solution with a concentration of 100 mg L−1 was prepared by diluting a stock single-element Ga solution with a concentration of 1 g L−1 (Merck, Germany). Deionised water (ColeParmer deioniser model 01503-20, Germany) and analytical grade reagents were used throughout the experiments. As the size of the particles in the fly ash samples was not uniform (Detcheva & Vassileva, 2014), additional grinding for 30 min using a Pulverisette type 02.102 N.3515 mill (Fritsch, Germany) was necessary. The certified reference material BCR-176R fly ash (Held et al., 2007) meets the requirements for quantitative TXRF analysis of particles as reported in Bruker AXS Microanalysis (2007), hence no further milling of this material was applied. Amounts of 5 mg, 10 mg and 20 mg of the certified reference material (CRM) BCR-176R fly ash were dispersed in 1 mL of 0.01 vol. % TritonTM X-114 containing Ga internal standard with a concentration of 2 mg L−1 . 5 L of the magnetically stirred slurry (PV-1 Vortex mixer, Grant Instruments, UK) was pipetted onto the previ-

651

LLDi = 3Ci (Nbg )1/2 Ni−1

(1)

where Ci is the concentration of the element (i), Ni is the area of the fluorescence peak in counts, and Nbg is the background area subjacent to the fluorescence peak (Bruker AXS Microanalysis, 2007).

Results and discussion

Effect of sample deposition and measurement time (exposition) As reported previously (Georgieva et al., 2013), the sample volume of a single pipetting should not exceed 5 L so that the dry residue area falls within the effective detector area (10 mm2 ). Sample volumes of less than 5 L were not used due to insufficient precision of the slurry-pipetting. Three replicates of each sample were measured at two perpendicular positions of the quartz reflector in order to obtain an averaged picture of the dry residue layer below the detector with a view to eliminate the effect of any possible unevenness of the layer on the precision. A similar homogeneity check is reported in Cataldo (2012). The 6 signals registered for each sample were statistically treated. Typical X-ray spectra obtained for CRM BCR176R fly ash and sample BD (as representative of all the fly ash samples investigated) are shown in Fig. 1. The exposition was varied between 300 s and 1500 s. As the optimal exposition, 1000 s was chosen, combining low LLDs with reasonable duration of the analysis. TXRF, like other techniques using a semiconductor detector (e.g. silicon-drift detector), exhibits several disadvantages associated with poor spectral resolution and overlapping peak; these give rise to over- or underestimation of results for elements at very low concentrations (De La Calle et al., 2013). Moreover, elements such as Mo cannot be detected because a Mo tube is

652

rc op y


Fig. 1. Typical X-ray spectra for CRM BCR-176R fly ash (a) and sample BD (b).

ut

ho

recovery = Cexp /Ccert × 100 %

A

Fig. 2. Recovery data – mean recoveries (8 replicates) ± recovery standard deviation for TXRF analysis of CRM BCR-176R fly ash: – 5 mg, – 10 mg, – 20 mg. The selected area describes the recovery tolerance ((100 ± 10) % recovery).

used to generate X-rays. Some interferences have been observed (Fig. 1) due to X-ray line-overlapping, for example in the case of AsKα (10.53 keV) and PbLα (10.55 keV) (Karjou, 2007). Other issues are encountered with elements, such as Cd and Sb, when analysed by TXRF using a Mo tube, since they are detected using their low-intensity L-lines, these lines being overlapped by the strong K-lines of K and Ca (matrix elements). Effect of slurry concentration For method validation, the CRM BCR-176R fly ash was used. Fig. 2 presents the recoveries of selected elements, where Cexp is the analyte concentration obtained by the developed TXRF-method and Ccert is the analyte concentration reported in the certificate:

(2)

(interference-free) in this material. In addition, Fig. 2 illustrates the effect of slurry concentration on the accuracy and precision of minor element determination in the CRM BCR-176R fly ash. The best accuracy and precision were obtained with a slurry concentration of 10 g L−1 and recoveries of between 90 % and 110 % were registered, indicating that errors due to sedimentation during pipetting are negligible. The lower precision at slurry concentrations of around 5 g L−1 may be related to the poor representation of dilute slurries containing a relatively small number of particles (Detcheva et al., 2002; Gentscheva et al., 2004). Accordingly, a slurry concentration of 10 g L−1 was further applied to fly ash sample analysis. Under the optimal conditions, a good agreement between experimental and certified values was obtained (Table 1). Element content in fly ash investigated The following elements Ca, K, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Rb, Sr, Ba and Pb were determined in the fly ash samples under the experimental conditions adopted and are presented in Table 2. The light elements (Li, Be and B) were not measured due to strong absorption of the characteristic lines of these elements by the detector window since the Be window thickness in the SDD detector is 8 m. Ag, Cd and Mo were not detected because of the Mo-excitation. Weak L-lines of Ag and Cd could be analysed, however, they are overlapped with the strong Ca and K signals in the fly ash samples. The precision of the TXRF element content measurements in the fly ash samples investigated is characterised by a relative standard deviation (RSD) of approximately 10 %. The TXRF lower limits of detection ( g g−1 ) in the fly ash samples analysed were determined as given in Table 2. The limits of

653


Table 1. Experimental TXRF results (in %) for CRM BCR-176R fly ash (mean values of 8 replicates) compared with the certified values (Held et al., 2007) Element Parameter

Cexp /% SD RSD/% Ccert /% uncertainty

Cr

Mn

Fe

Ni

Cu

Zn

Rb

Ba

Pb

0.077 0.004 5 0.081 0.007

0.079 0.007 9 0.073a 0.005

1.34 0.05 4 1.31 0.05

0.012 0.002 17 0.012 0.001

0.107 0.007 7 0.105 0.007

1.75 0.06 3 1.68 0.04

0.0094 0.0004 4 0.0102b –

0.47 0.03 5 0.47b –

0.48 0.02 5 0.50 0.05

a) Indicative value (Held et al., 2007); b) non-certified, determined by NAA (Held et al., 2007); SD – standard deviation.

rc op y

Table 2. TXRF results for fly ash samples (mean values of 6 replicates ± SD); concentrations in % except for minor components and LLD (in g g−1 ) indicated by asterisk Sample/%

Element BD 0.014 0.9 5.7 0.30 (110 0.094 0.094 6.2 0.051 (79 (48 (16 (2.3 (80 0.084 0.122 (33

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.004 0.1 0.3 0.02 9)a 0.009 0.007 0.5 0.005 6)a 4)a 1)a 0.2)a 6)a 0.006 0.004 2)a

RI

0.013 ± 0.004 1.8 ± 0.1 2.8 ± 0.1 0.38 ± 0.04 0.023 ± 0.003 0.031 ± 0.002 0.032 ± 0.003 5.6 ± 0.4 0.012 ± 0.001 (109 ± 9)a 0.012 ± 0.001 (31 ± 3)a n.d. 0.013 ± 0.001 0.043 ± 0.004 0.084 ± 0.006 (80 ± 8)a

ho

0.048 ± 0.006 1.9 ± 0.1 7.8 ± 0.4 0.36 ± 0.02 0.014 ± 0.001 0.027 ± 0.003 0.051 ± 0.003 7.4 ± 0.4 0.012 ± 0.001 (88 ± 6)a 0.025 ± 0.002 (67 ± 2)a n.d. 0.015 ± 0.002 0.081 ± 0.008 0.130 ± 0.010 0.012 ± 0.001

R

ut

Cl K Ca Ti V Cr Mn Fe Ni Cu Zn As Se Rb Sr Ba Pb

MI

(130 1.4 1.9 0.41 (72 0.070 0.160 5.7 0.025 (78 (84 (12 (2.5 0.012 0.101 0.220 (87

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

LLD/( g g−1 )

S

8)a 0.1 0.2 0.04 5)a 0.006 0.010 0.5 0.002 6)a 5)a 3)a 0.2)a 0.001 0.009 0.020 9)a

0.019 1.9 3.3 0.51 (86 0.160 0.103 7.5 0.056 (78 0.016 (13 (3.8 0.018 0.180 0.134 (69

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.002 0.1 0.1 0.04 7)a 0.010 0.008 0.7 0.004 6)a 0.001 1)a 0.5)a 0.001 0.010 0.009 4)a

50–100 20–50 10–200 8–100 7–40 5–80 4–0 3–40 1–20 0.1–2 0.1–6 0.04–4 0.05–1 0.06–10 0.1–2 20–300 0.1–20

a) In g g−1 ; n.d. – not detectable.

A

detection could be lowered using longer measurement times. The TXRF results for Ca, Fe, K, As and Cl in the fly ash samples are in good agreement with those obtained by an independent method – wet chemical analysis after sample digestion (Detcheva & Vassileva, 2014). The ash samples examined have similar chemical composition. They all consist of silica and contain significant amounts of alumina and small amounts of oxides of iron, alkaline earth metals, alkali metals and other minor constituents. The high content of SiO2 in all samples affords them credibility as good adsorbents (Detcheva & Vassileva, 2014).

Conclusions In the present work, raw coal fly ash from five Bulgarian power plants was analysed by means of TXRF using gallium as the internal standard. Milling of the samples for 30 min was sufficient to obtain the particle size required for analysis of the slurries. For validation of the method, the certified reference material

BCR-176R fly ash was used. The best accuracy and precision were obtained with a slurry concentration of 10 g L−1 and recoveries of between 90 % and 110 % were registered. Under the optimal conditions, the elements Cl, Ca, K, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Rb, Sr, Ba and Pb could be determined across a wide concentration range. Hence, TXRF proved to be a fast and simple method for the simultaneous determination of a large number of elements in fly ash samples at concentrations ranging from per cents down to trace level. Acknowledgements. The authors wish to thank the National Science Fund of Bulgaria (contract no. DTK-02/5, 19 January 2010) and the National Centre for New Materials UNION (contract no. DCVP-02/2/2009) for the financial support received.

References Bruker AXS Microanalysis (2007). Bruker S2 PICOFOXTM – user manual. Berlin, Germany: Bruker AXS Microanalysis. Cariati, F., Fermo, P., Gilardoni, S., Galli, A., & Milazzo, M. (2003). A new approach for archaeological ceramics analysis

654


Held, A., Kramer, G. N., Robouch, P., & Wätjen, U. (2007). The certification of the mass fractions of As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Sb, Se, Tl, V and Zn in fly ash BCR-176R. Luxembourg, Luxembourg: Office for Official Publications of the European Communities. (EUR Report 22667 EN) Hoefler, H., Streli, C., Wobrauschek, P., Óvári, M., & Záray, G. (2006). Analysis of low Z elements in various environmental samples with total reflection X-ray fluorescence (TXRF) spectrometry. Spectrochimica Acta Part B: Atomic Spectroscopy, 61, 1135–1140. DOI: 10.1016/j.sab.2006.07.005. Ivanova, E. H., & Detcheva, A. K. (2012). Green analytical chemistry and its perspectives in Bulgaria. Bulgarian Chemical Communications, 44(1), 5–10. Karjou, J. (2007). Matrix effect on the detection limit and accuracy in total reflection X-ray fluorescence analysis of trace elements in environmental and biological samples. Spectrochimica. Acta Part B: Atomic Spectroscopy, 62, 177–181. DOI: 10.1016/j.sab.2007.02.003. Marguí, E., Floor, G. H., Hidalgo, M., Kregsamer, P., RománRoss, G., Streli, C., & Queralt, I. (2010). Analytical possibilities of total reflection X-ray spectrometry (TXRF) for trace selenium determination in soils. Analytical Chemistry, 82, 7744–7751. DOI: 10.1021/ac101615w. Misra, N. L. (2011). Total reflection X-ray fluorescence and energy-dispersive X-ray fluorescence characterization of nuclear materials. Pramana-Journal of Physics, 76, 201–212. DOI: 10.1007/s12043-011-0046-y. Shoumkova, A. (2006). Physico–chemical characterization and magnetic separation of coal fly ashes from “Varna”, “Bobov dol” and ”Maritza-Istok I” power plants, Bulgaria. I – physico–chemical characteristics. Journal of the University of Chemical Technology and Metallurgy, 41, 175–180. Száková, J., Ochecová, P., Hanzlíček, T., Perná, I., & Tlustoš, P. (2013). Variability of total and mobile element contents in ash derived from biomass combustion. Chemical Papers, 67, 1376–1385. DOI: 10.2478/s11696-013-0399-4. Wobrauschek, P. (2007). Total reflection X-ray fluorescence analysis—a review. X-Ray Spectrometry, 36, 289–300. DOI: 10.1002/xrs.985.

A

ut

ho

rc op y

using total reflection X-ray fluorescence spectrometry. Spectrochimica Acta Part B: Atomic Spectroscopy, 58, 177–184. DOI: 10.1016/s0584-8547(02)00253-7. Cataldo, F. (2012). Multielement analysis of a municipal landfill leachate with total reflection X-ray fluorescence (TXRF). A comparison with ICP-OES analytical results. Journal of Radioanalytical and Nuclear Chemistry, 293, 119–126. DOI: 10.1007/s10967-012-1777-z. De La Calle, I., Cabaleiro, N., Romero, V., Lavilla, I., & Bendicho, C. (2013). Sample pretreatment strategies for total reflection X-ray fluorescence analysis: A tutorial review. Spectrochimica Acta Part B: Atomic Spectroscopy, 90, 23–54. DOI: 10.1016/j.sab.2013.10.001. Detcheva, A., Gentscheva, G., Havezov, I., & Ivanova, E. (2002). Slurry sampling ETAAS determination of sodium impurities in optical crystals of potassium titanyl phosphate and potassium gadolinium tungstate. Talanta, 58, 489–495. DOI: 10.1016/s0039-9140(02)00286-2. Detcheva, A., & Grobecker, K. H. (2008). Determination of trace elements in aquatic plants by solid sampling Zeeman atomic absorption spectrometry (SS-ZAAS). Environmental Chemistry Letters, 6, 183–187. DOI: 10.1007/s10311-0070124-z. Detcheva, A., & Vassileva, P. (2014). Characterization of fly ashes from Bulgarian coal-fired power plants with respect to their adsorption properties by the combined use of analytical techniques. Comptes Rendus de l’Académie Bulgare des Sciences, 67(3), 331–338. Gentscheva, G., Detcheva, A., Havezov, I., & Ivanova, E. (2004). Slurry sampling electrothermal atomic absorption spectrometric determination of sodium and iron impurities, in optical crystals of rubidium titanyl phosphate. Microchimica Acta, 144, 115–118. DOI: 10.1007/s00604-003-0081-6. Georgieva, R., Detcheva, A., Karadjov, M., Jordanov, J., & Ivanova, E. (2013). Total reflection X-ray fluorescence analysis of trace elements in Bulgarian bottled mineral waters of low and high mineral content. International Journal of Environmental Analytical Chemistry, 93, 1043–1051. DOI: 10.1080/03067319.2013.775277.