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Determination of mercury in sewage sludge by direct slurry sampling graphite furnace atomic absorption spectrometry
Spectrochimica Acta Part B 60 (2005) 409 – 413 www.elsevier.com/locate/sab
Analytical note
Determination of mercury in sewage sludge by direct slurry sampling graphite furnace atomic absorption spectrometryB Danuta BaralkiewiczT, Hanka Gramowska, Maygorzata Ko´zka, Anetta Kanecka Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan´, Poland Received 25 July 2004; accepted 23 December 2004
Abstract Ultrasonic slurry sampling electrothermal atomic absorption spectrometry (ETAAS) method was elaborated to the determination of Hg in sewage sludge samples with the use of KMnO4+Pd modifier. The minimum sample amount required for slurry preparation with respect to sample homogeneity was evaluated by weighting masses between 3 and 30 mg directly into the autosampler cups. Validation of the proposed method was performed with the use of Certified Reference Materials of sewage sludge, CRM 007-040 and CRM 144R. Two sewage sludge samples from Poznan˜ (Poland) city were analysed using the present direct method and a method with sample digestion, resulting in no difference within statistical error. D 2005 Elsevier B.V. All rights reserved. Keywords: Ultrasonic slurry sampling; Electrothermal vaporization; Mercury; Sewage sludge
1. Introduction Mercury can be present as a trace contaminant in all environmental compartments as a result of both natural origin and anthropogenic activities, and the determination of this element is of considerable interest due to its toxicity and ability of bioaccumulation in many organisms [1]. The toxicological implications of the mercury content in the environment have encouraged the development of very sensitive methods for its determination. There is an increasing demand on the properties of sludge admissible for agricultural use in Europe [2] as well as in Poland [3]. The concentration of mercury in sewage sludge admitted for use in agriculture is 15–25 mg kg 1. In this context, an accurate, precise and quick determination of mercury in B
This paper was presented at the 6th European Furnace Symposium and 11th Solid Sampling Colloquium with Atomic Spectrometry, held in Balatonffldva´r, Hungary, 27–30 June 2004, and is published in the special issue of Spectrochimica Acta Part B, dedicated to that conference. T Corresponding author. E-mail address: [email protected] (D. Baralkiewicz). 0584-8547/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2005.01.010
sewage sludge has become an important analytical demand. A wide variety of decomposition methods for organic and inorganic matrices have been proposed for use in mercury determinations. These involve the usage of combinations of strong acids, oxidants, ultraviolet irradiation and elevated temperature and pressure. The main concerns with regard to possible error sources are volatilization and absorption losses during elevated temperature digestion procedures, sample contamination, and the problem with the use of a large amount of reagents during sample preparation, which gives rise to increased blank values and higher detection limits. The introduction of solid samples as slurries into ETAAS system can greatly reduce the time required for analysis by circumventing sample decomposition with wet or dry oxidation methods, and having the benefits of reduced risk of mercury losses and sample contamination. To date, the most number of applications dealing with slurry preparation for determination of mercury have used electrothermal atomic absorption spectrometry (ETAAS) to a wide range of materials [4–10]. In this work, the elaboration of a
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D. Baralkiewicz et al. / Spectrochimica Acta Part B 60 (2005) 409–413
procedure for rapid determination of mercury in sewage sludges using slurry sampling is discussed.
respectively, for at least 48 h and rinsed with abundant deionized water before use. 2.3. Samples and reference materials
2. Experimental 2.1. Apparatus and conditions Varian SpectrAA plus atomic absorption spectrometer (Varian, USA) with deuterium lamp for background correction and an electrothermal atomiser GTA-96 with autosampler were applied. Slurries were sonicated by using an ultrasonic processor (Sonopuls, Germany) and were automatically agitated. Sample aliquots (20 Al) were delivered onto the graphite platform by injections with an autosampler. Mercury hollow cathode lamp and the Hg 253.7 nm line (spectral band width of 0.7 nm) were used in measuring time integrated absorbance signals. The heating program is given in Table 1. Argon 99.9% was used as the protective gas. For sample decomposition a closed vessel microwave oven, Mars 5 (CEM, USA), was used. 2.2. Reagents High purity deionized water (Milli-Q system Millipore) was used exclusively. Analytical reagent grade HNO3 and HF acids (Merck, Darmstadt, Germany) were used for preparation of slurries. Standard solutions were prepared from the stock solution containing 1000 mg l 1 Hg (Merck, Darmstadt, Germany). Palladium modifier for graphite furnace AAS (containing nitric acid, Merck Germany) and potassium permanganate (Sigma) (10% m/v) were also used. A 60% m/v PTFE emulsion (Sigma) was commercially available. All glassware and polyethylene autosampler cups were kept in 30% v/v and 10% v/v nitric acid,
Table 1 Instrumental parameters and thermal programs for mercury determination Optical parameters Wavelength, nm Spectral bandpass, nm Hg-hollow cathode lamp, current, mA Deuterium lamp background correction
253.7 0.5 6
Thermal programme Step
Temperature (8C)
Ramp time (s)
Hold time (s)
Drying Pyrolysis Atomization Cleaning
100 350 2000 2400
20 10 1 2
20 20 3 2
Atomizer: graphite tube furnace equipped with L’vov platform. The tube and platform were coated with pyrolytic graphite layer. The internal gas flow rate was 300 ml min 1 Ar in all steps except the atomization step, when it was interrupted (stop-flow). Reading was actuated during the atomization step.
Two sewage sludge samples from wastewater treatment plant in Poland (Poznan˜) were analysed. Certified Reference Materials (sewage sludge) of CRM 007-040 (RTC, USA) and CRM 144R (USA) were used for the evaluation of accuracy. 2.4. Preparation of slurry samples and standards The suspensions were prepared by weighing 20 mg samples directly in the autosampler cups and then adding the following: (1) 100 Al concentrated hydrofluoric acid, (2) 100 Al PTFE slurry (60%), (3) 500 Al palladium nitrate solution (1.5% Pd m/v) and (4) 60 Al potassium permanganate (10% m/v). The mixture was sonicated for a few minutes and aliquots of 20 Al were immediately taken and injected into the furnace. Calibration was performed against aqueous standard solutions of mercury to which concentrated hydrofluoric acid, PTFE slurry, solutions of palladium nitrate and potassium permanganate were added in the same amounts and concentrations as to the slurry samples. 2.5. Wet digestion For comparison, two sewage sludge samples and two Certified Reference Materials, as defined above, were also digested using a microwave-assisted method in closed vessels (Mars 5, CEM, USA). Sewage sludge was decomposed in triplicate according to the following procedure: 200 mg dried and ground material was accurately weighed in the vessel of microwave oven and then 2.0 ml of 60% v/v HNO3 was added. After decomposition, the microwave vessel was cooled, the digest was transferred to 10 ml volumetric flask and the volume adjusted with Milli-Q water.
3. Results and discussion 3.1. Pyrolysis curves Pyrolysis curves for Hg in aqueous solution and sewage sludge slurries with and without modifiers are shown in Fig. 1A and B, respectively. Without modifiers the mercury loss appears above 200 8C from both solution and slurry residues. Losses of mercury have been observed [11] even during the drying stage, when using Ultrasonic Slurry Sampling ETAAS technique. Such losses could be decreased by the addition of KMnO4 oxidizing agent. From the present experiments it can be seen that the stabilising effect of this modifier alone is higher for the solution residue at lower temperatures (A part) and the effect becomes stronger at higher temperatures in case of the slurry (B part).
D. Baralkiewicz et al. / Spectrochimica Acta Part B 60 (2005) 409–413
411
A Relative absorbance (%)
100 without modifier
90
with 0.6 mg KMnO4
80
with 0.6 mg KMnO4 +Pd
70 60 50 40 30 20 10 0 200
250
300
350
400
450
500
550
600
Temperature, (°C)
Relative absorbance (%)
B
100
without modifier
90
with 0.6 mg KMnO4
80
with 0.6 mg KMnO4 +Pd
70 60 50 40 30 20 10 0 200
250
300
350
400
450
500
550
600
Temperature, (°C) Fig. 1. (A). Pyrolysis curves of mercury (100 ng ml 1 Hg) in aqueous solution without modifier, with 0.6 mg KMnO4 and 0.6 mg KMnO4+7.5 Ag Pd modifier. (B). Pyrolysis curves of mercury (60.4 ng ml 1 Hg) in the slurry of CRM 007-040 sewage sludge without modifier, with 0.6 mg KMnO4 and 0.6 mg KMnO4+7.5 Ag Pd modifier.
The stabilising effect of organic matrices has been reported [12]. Finally the most important observation in curves A and B is that with KMnO4+Pd modifier the mercury stabilisation can be ensured up to 400 8C, for both solution residue and sewage sludge slurry. The effect Pd modifier alone was studied and discussed in [13] and it was shown that Pd could diminish Hg vaporization up to about 250 8C. In conclusion, KMnO4+Pd was found to be a suitable modifier for resulting in a similar thermal behaviour of mercury, present in aqueous solutions and in sewage sludge slurries. In further experiments, atomization curves for Hg were determined for the aqueous solutions and slurries using KMnO4+Pd modifier applying 350 8C pyrolysis temperature. The mercury signal increased up to about 2000 8C atomization temperature and then slightly decreased stepwise. For this reason atomization temperature of 2000 8C was chosen for analytical purposes. 3.2. Optimization of slurry concentration As known, the success of slurry sampling technique depends on the sample particle diameter, subsample homogeneity, suspension medium, stirring method and
sampling depth. In this work all of these parameters were taken into account in order to achieve the best available accuracy and precision. It has been estimated in [14] that the influence of the sample inhomogeneity on the precision and accuracy becomes usually detrimental at and below 0.2% m/v slurry concentration. Therefore in the present studies the role of slurry concentration at and above 1% m/v were selected. From other relevant studies it was also concluded [15,16] that sewage sludge samples must be ground to produce particle size smaller than 10 Am for attaining the required representativeness. For the two CRM standards used in this study the required properties had been ensured by the producer. The suspensions were prepared directly in the autosampler cups with 1 ml mixture of 4% v/v HF and 0.5% v/v HNO3 in which 10, 20 30, 40, 50 and 60 mg portions of CRM standards (corresponding to 1–6% m/v concentrations) were suspended after addition of KMnO 4+Pd modifier (500 Al of palladium nitrate 1.5% and 60 Al potassium permanganate 10%). As seen in Fig. 2, the concentration normalized mercury signals do not show systematic change up to 3% m/v slurry
D. Baralkiewicz et al. / Spectrochimica Acta Part B 60 (2005) 409–413
1,0
Integrated absorbance (s)
0,15
a 0,10
0,5
b 0,05
d
c 0,00
Integrated absorbance (s)
412
0,0 1
2
3
4
5
6
Suspension concentration (% m/v) Fig. 2. Integrated absorbance of mercury normalized to the loaded sample mass, as a function of slurry concentration. Curve a: CRM 007-040, spiked with 150 ng ml 1 Hg. Curve b: CRM 144R spiked with 70 ng ml 1 Hg. Curves c and d: represent the corresponding background signals, respectively.
concentration for both of the two CRM standards (curves a and b). The background signals for these two materials (curves c and d) are near constant up to about 5% m/v slurry concentration, and it increases strongly above this concentration. Taking into account the mean mercury content in the samples and the best precision of measurements the use of 2.0% m/v slurry concentration was selected. In order to minimize the build-up of inorganic residue on the graphite platform a cleaning temperature of 2400 8C was required. 3.3. Calibration curves and analytical performance data Three types of calibration curves were determined: (1) using matrix-free calibration solutions, (2) using the slurry of the CRM 007-040 sewage slag standard to which matrixfree solution standards were added, (3) the same as (2) with the use of the other CRM 144R sewage slag standard. Obviously, in the three sets of standards the same amount of KMnO4+Pd modifier was applied. Each standard series consisted of four concentrations and triplicate measurements were made to each concentrations. The calibration curves covered the concentration range from 10.00 ng ml 1 to 100 ng ml 1 Hg content in the slurries, corresponding to mercury content from 37.25 ng ml 1 to 85.45 ng ml 1, in the original samples. The calibration Table 2 Slopes of calibration graphs for aqueous standard and for two sewage sludge standards to which aqueous standards were added (standard addition calibration curves) 4
Sample
Slope10 /A ng ml
Aqueous standard solution CRM 007-040 sewage sludge (2% m/v) CRM 144R sewage sludge (2% m/v)
6.65F0.28 (0.999) 6.40F0.52 (0.997) 6.21F0.44 (0.998)
The correlation coefficient, r, is given in parentheses. a MeanFstandard deviation (n=3).
1a
curves were apparently linear up to 150 ng ml 1 Hg concentration in the slurry. The linearity is characterized numerically by the correlation coefficient (r), in Table 2, where the slope values for the three sets of standards are also listed. The accuracy of the slurry sampling method was investigated by comparing the analysis results to that found with wet digestion–dissolution method (Section 2.4) and with the known concentrations of CRM standards. The significance of the numerical deviations seen in Table 3 was estimated by the statistical bpaired t-test b method, and a value of t= 210 ( P=0.857) was found. This indicates that there were no significant differences between the results obtained by the slurry method and the wet digestion method. Similar statements are valid for the comparison of the results obtained by proposed method for the CRMs and their certified concentrations. The precision of the slurry sampling method could be characterized by an average relative standard deviation Table 3 Comparison of mercury content (Ag g 1) of two sewage sludge samples and reference materials determined by dissolution (digestion) based and direct slurry sampling methods Samples and CRMs
Digesta (n=3)
Slurryb (n=3)
Sludge Koziegyowy Sludge Poznan´ CRM 144R Recovery, % CRM 007- 040 Recovery, %
4.25F0.34 1.86F0.15 1.52F0.18 83.5 2.86F0.26 90.8
4.62F0.38 2.08F0.19 1.75F0.08 96,2 3.02F0.25 95,8
Reference value
1.82F0.24 3.15F0.42
Recoveries for the CRM standards were also calculated. F standard deviation (S.D.). a Triplicate digests were prepared and triplicate measurements were made from each digest. b Triplicate slurries were prepared in three autosampler cups and triplicate measurements were made from each cup.
D. Baralkiewicz et al. / Spectrochimica Acta Part B 60 (2005) 409–413
RSDaverage=8.3% in the concentration range 1.5–4.6 Ag g 1 Hg. The detection limit was determined by making 10 replicate measurements with previously preheated sludge samples as slurries. Applying the 3r concept of calculation, detection limit of 17 ng g 1 (ppb) Hg was found.
4. Conclusion The evaluation of the direct slurry sampling atomic absorption spectrometry for the trace determination of mercury in sewage sludge samples was made. The determination was possible by the addition of KMnO4+Pd to the aqueous standards and slurries, in order to stabilize the analyte. The modifier proved to be very advantageous, since it is an easily available reagent, and has shown to be very efficient. The results obtained in this work show that slurry sampling is a rapid and economical alternative to the conventional acid digestion procedures for sewage sludge analysis. Sample preparation and the analysis can be performed for about 15 min and only small amounts of reagents are required. Low risk of contamination, simple handling, and the possibility to use aqueous calibration are very convenient for routine application.
References [1] A. Tessier, D. Turner, Metal Speciation and Bioavailability in Aquatic Systems, John Willey & Sons, Chichester, UK, 1995, ch. 3. [2] Directions of Council Concerning the Protection of Environment, in Particular Soils, When Using Waste Deposits in Agriculture, 86/278/ EWG, 12 June 1986. [3] Decrete of the Minister for Protection of Environment, Natural Resources and Forestry Concerning the Conditions on Which Waste Deposits Can be Used for Non-Industrial Purposes, 11 August 1999. [4] E. Lqcker, Direct solid sampling ETAAS determination of mercury Fresenius, J. Anal. Chem. 358 (1997) 848 – 853.
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[5] S. Rio-Segade, C. Bendicho, Determination of mercury in soil by electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 14 (1999) 1907 – 1912. [6] G.E.M. Hall, P. Pelchat, Evaluation of direct solid sampling atomic absorption spectrometry for the trace determination of mercury in geological samples, Analyst 122 (1997) 921 – 924. [7] I. Lopez Garcia, M. Sanchez Merlos, M. Hernandez Cordoba, Determination of mercury in soils and sediments by graphite furnace atomic absorption spectrometry with slurry sampling, Spectrochim. Acta, Part B 52 (1997) 2085 – 2092. [8] D. Baralkiewicz, Fast determination of lead in lake sediment samples using electrothermal atomic absorption spectrometry with slurry samples introduction, Talanta 56 (2002) 105 – 112. [9] D. Baralkiewicz, H. Gramowska, Study of parameters associated with the determination of cadmium in lake sediments by slurry sampling electrothermal atomic absorption spectrometry, Anal. Chim. Acta 510 (2004) 249 – 256. [10] D. Baralkiewicz, Usama El-Sayed, M. Filipiak, H. Gramowska, M. Mleczek, Determination of selenium in infant foods using electrothermal atomic absorption spectrometry with slurry sample introduction, Cent. Eur. J. Chem. 2 (2004) 334 – 346. [11] A.F. Silva, B. Welz, A.J. Curtius, Noble metals as permanent chemical modifiers for determination of mercury in environmental reference materials using solid sampling graphite furnace atomic absorption spectrometry and calibration against aqueous standards, Spectrochim. Acta, Part B 57 (2002) 2031 – 2045. [12] N.J. Miller-Ihli, Advances in ultrasonic slurry graphite furnace atomic absorption spectrometry, Fresenius’ J. Anal. Chem. 339 (1993) 482 – 489. [13] Shu Feng Chen, Shiuh Jen Jiang, Determination of cadmium, mercury and lead in soil samples by slurry sampling electrothermal vaporization inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 13 (1998) 1113 – 1117. [14] Z.A. Benzo, M. Velosa, C. Ceccarelli, M. de la Guardia, A. Salvador, Application of the slurry technique to biological materials: a survey of literature, Fresenius’ J. Anal. Chem. 339 (1991) 235 – 239. [15] T. Nakamura, T. Kusata, H. Matsumoto, J. Sato, Atomic absorption spectrometric determination of cadmium and lead in human and artificial teeth by direct atomization technique, Anal. Biochem. 226 (1995) 256 – 269. [16] D. Baralkiewicz, Slurry sampling for electrothermal atomic absorption spectrometric determination of chromium, nickel, lead and cadmium in sewage sludge, Anal. Chim. Acta 437 (2001) 11 – 16.
Analytical note
Determination of mercury in sewage sludge by direct slurry sampling graphite furnace atomic absorption spectrometryB Danuta BaralkiewiczT, Hanka Gramowska, Maygorzata Ko´zka, Anetta Kanecka Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan´, Poland Received 25 July 2004; accepted 23 December 2004
Abstract Ultrasonic slurry sampling electrothermal atomic absorption spectrometry (ETAAS) method was elaborated to the determination of Hg in sewage sludge samples with the use of KMnO4+Pd modifier. The minimum sample amount required for slurry preparation with respect to sample homogeneity was evaluated by weighting masses between 3 and 30 mg directly into the autosampler cups. Validation of the proposed method was performed with the use of Certified Reference Materials of sewage sludge, CRM 007-040 and CRM 144R. Two sewage sludge samples from Poznan˜ (Poland) city were analysed using the present direct method and a method with sample digestion, resulting in no difference within statistical error. D 2005 Elsevier B.V. All rights reserved. Keywords: Ultrasonic slurry sampling; Electrothermal vaporization; Mercury; Sewage sludge
1. Introduction Mercury can be present as a trace contaminant in all environmental compartments as a result of both natural origin and anthropogenic activities, and the determination of this element is of considerable interest due to its toxicity and ability of bioaccumulation in many organisms [1]. The toxicological implications of the mercury content in the environment have encouraged the development of very sensitive methods for its determination. There is an increasing demand on the properties of sludge admissible for agricultural use in Europe [2] as well as in Poland [3]. The concentration of mercury in sewage sludge admitted for use in agriculture is 15–25 mg kg 1. In this context, an accurate, precise and quick determination of mercury in B
This paper was presented at the 6th European Furnace Symposium and 11th Solid Sampling Colloquium with Atomic Spectrometry, held in Balatonffldva´r, Hungary, 27–30 June 2004, and is published in the special issue of Spectrochimica Acta Part B, dedicated to that conference. T Corresponding author. E-mail address: [email protected] (D. Baralkiewicz). 0584-8547/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2005.01.010
sewage sludge has become an important analytical demand. A wide variety of decomposition methods for organic and inorganic matrices have been proposed for use in mercury determinations. These involve the usage of combinations of strong acids, oxidants, ultraviolet irradiation and elevated temperature and pressure. The main concerns with regard to possible error sources are volatilization and absorption losses during elevated temperature digestion procedures, sample contamination, and the problem with the use of a large amount of reagents during sample preparation, which gives rise to increased blank values and higher detection limits. The introduction of solid samples as slurries into ETAAS system can greatly reduce the time required for analysis by circumventing sample decomposition with wet or dry oxidation methods, and having the benefits of reduced risk of mercury losses and sample contamination. To date, the most number of applications dealing with slurry preparation for determination of mercury have used electrothermal atomic absorption spectrometry (ETAAS) to a wide range of materials [4–10]. In this work, the elaboration of a
410
D. Baralkiewicz et al. / Spectrochimica Acta Part B 60 (2005) 409–413
procedure for rapid determination of mercury in sewage sludges using slurry sampling is discussed.
respectively, for at least 48 h and rinsed with abundant deionized water before use. 2.3. Samples and reference materials
2. Experimental 2.1. Apparatus and conditions Varian SpectrAA plus atomic absorption spectrometer (Varian, USA) with deuterium lamp for background correction and an electrothermal atomiser GTA-96 with autosampler were applied. Slurries were sonicated by using an ultrasonic processor (Sonopuls, Germany) and were automatically agitated. Sample aliquots (20 Al) were delivered onto the graphite platform by injections with an autosampler. Mercury hollow cathode lamp and the Hg 253.7 nm line (spectral band width of 0.7 nm) were used in measuring time integrated absorbance signals. The heating program is given in Table 1. Argon 99.9% was used as the protective gas. For sample decomposition a closed vessel microwave oven, Mars 5 (CEM, USA), was used. 2.2. Reagents High purity deionized water (Milli-Q system Millipore) was used exclusively. Analytical reagent grade HNO3 and HF acids (Merck, Darmstadt, Germany) were used for preparation of slurries. Standard solutions were prepared from the stock solution containing 1000 mg l 1 Hg (Merck, Darmstadt, Germany). Palladium modifier for graphite furnace AAS (containing nitric acid, Merck Germany) and potassium permanganate (Sigma) (10% m/v) were also used. A 60% m/v PTFE emulsion (Sigma) was commercially available. All glassware and polyethylene autosampler cups were kept in 30% v/v and 10% v/v nitric acid,
Table 1 Instrumental parameters and thermal programs for mercury determination Optical parameters Wavelength, nm Spectral bandpass, nm Hg-hollow cathode lamp, current, mA Deuterium lamp background correction
253.7 0.5 6
Thermal programme Step
Temperature (8C)
Ramp time (s)
Hold time (s)
Drying Pyrolysis Atomization Cleaning
100 350 2000 2400
20 10 1 2
20 20 3 2
Atomizer: graphite tube furnace equipped with L’vov platform. The tube and platform were coated with pyrolytic graphite layer. The internal gas flow rate was 300 ml min 1 Ar in all steps except the atomization step, when it was interrupted (stop-flow). Reading was actuated during the atomization step.
Two sewage sludge samples from wastewater treatment plant in Poland (Poznan˜) were analysed. Certified Reference Materials (sewage sludge) of CRM 007-040 (RTC, USA) and CRM 144R (USA) were used for the evaluation of accuracy. 2.4. Preparation of slurry samples and standards The suspensions were prepared by weighing 20 mg samples directly in the autosampler cups and then adding the following: (1) 100 Al concentrated hydrofluoric acid, (2) 100 Al PTFE slurry (60%), (3) 500 Al palladium nitrate solution (1.5% Pd m/v) and (4) 60 Al potassium permanganate (10% m/v). The mixture was sonicated for a few minutes and aliquots of 20 Al were immediately taken and injected into the furnace. Calibration was performed against aqueous standard solutions of mercury to which concentrated hydrofluoric acid, PTFE slurry, solutions of palladium nitrate and potassium permanganate were added in the same amounts and concentrations as to the slurry samples. 2.5. Wet digestion For comparison, two sewage sludge samples and two Certified Reference Materials, as defined above, were also digested using a microwave-assisted method in closed vessels (Mars 5, CEM, USA). Sewage sludge was decomposed in triplicate according to the following procedure: 200 mg dried and ground material was accurately weighed in the vessel of microwave oven and then 2.0 ml of 60% v/v HNO3 was added. After decomposition, the microwave vessel was cooled, the digest was transferred to 10 ml volumetric flask and the volume adjusted with Milli-Q water.
3. Results and discussion 3.1. Pyrolysis curves Pyrolysis curves for Hg in aqueous solution and sewage sludge slurries with and without modifiers are shown in Fig. 1A and B, respectively. Without modifiers the mercury loss appears above 200 8C from both solution and slurry residues. Losses of mercury have been observed [11] even during the drying stage, when using Ultrasonic Slurry Sampling ETAAS technique. Such losses could be decreased by the addition of KMnO4 oxidizing agent. From the present experiments it can be seen that the stabilising effect of this modifier alone is higher for the solution residue at lower temperatures (A part) and the effect becomes stronger at higher temperatures in case of the slurry (B part).
D. Baralkiewicz et al. / Spectrochimica Acta Part B 60 (2005) 409–413
411
A Relative absorbance (%)
100 without modifier
90
with 0.6 mg KMnO4
80
with 0.6 mg KMnO4 +Pd
70 60 50 40 30 20 10 0 200
250
300
350
400
450
500
550
600
Temperature, (°C)
Relative absorbance (%)
B
100
without modifier
90
with 0.6 mg KMnO4
80
with 0.6 mg KMnO4 +Pd
70 60 50 40 30 20 10 0 200
250
300
350
400
450
500
550
600
Temperature, (°C) Fig. 1. (A). Pyrolysis curves of mercury (100 ng ml 1 Hg) in aqueous solution without modifier, with 0.6 mg KMnO4 and 0.6 mg KMnO4+7.5 Ag Pd modifier. (B). Pyrolysis curves of mercury (60.4 ng ml 1 Hg) in the slurry of CRM 007-040 sewage sludge without modifier, with 0.6 mg KMnO4 and 0.6 mg KMnO4+7.5 Ag Pd modifier.
The stabilising effect of organic matrices has been reported [12]. Finally the most important observation in curves A and B is that with KMnO4+Pd modifier the mercury stabilisation can be ensured up to 400 8C, for both solution residue and sewage sludge slurry. The effect Pd modifier alone was studied and discussed in [13] and it was shown that Pd could diminish Hg vaporization up to about 250 8C. In conclusion, KMnO4+Pd was found to be a suitable modifier for resulting in a similar thermal behaviour of mercury, present in aqueous solutions and in sewage sludge slurries. In further experiments, atomization curves for Hg were determined for the aqueous solutions and slurries using KMnO4+Pd modifier applying 350 8C pyrolysis temperature. The mercury signal increased up to about 2000 8C atomization temperature and then slightly decreased stepwise. For this reason atomization temperature of 2000 8C was chosen for analytical purposes. 3.2. Optimization of slurry concentration As known, the success of slurry sampling technique depends on the sample particle diameter, subsample homogeneity, suspension medium, stirring method and
sampling depth. In this work all of these parameters were taken into account in order to achieve the best available accuracy and precision. It has been estimated in [14] that the influence of the sample inhomogeneity on the precision and accuracy becomes usually detrimental at and below 0.2% m/v slurry concentration. Therefore in the present studies the role of slurry concentration at and above 1% m/v were selected. From other relevant studies it was also concluded [15,16] that sewage sludge samples must be ground to produce particle size smaller than 10 Am for attaining the required representativeness. For the two CRM standards used in this study the required properties had been ensured by the producer. The suspensions were prepared directly in the autosampler cups with 1 ml mixture of 4% v/v HF and 0.5% v/v HNO3 in which 10, 20 30, 40, 50 and 60 mg portions of CRM standards (corresponding to 1–6% m/v concentrations) were suspended after addition of KMnO 4+Pd modifier (500 Al of palladium nitrate 1.5% and 60 Al potassium permanganate 10%). As seen in Fig. 2, the concentration normalized mercury signals do not show systematic change up to 3% m/v slurry
D. Baralkiewicz et al. / Spectrochimica Acta Part B 60 (2005) 409–413
1,0
Integrated absorbance (s)
0,15
a 0,10
0,5
b 0,05
d
c 0,00
Integrated absorbance (s)
412
0,0 1
2
3
4
5
6
Suspension concentration (% m/v) Fig. 2. Integrated absorbance of mercury normalized to the loaded sample mass, as a function of slurry concentration. Curve a: CRM 007-040, spiked with 150 ng ml 1 Hg. Curve b: CRM 144R spiked with 70 ng ml 1 Hg. Curves c and d: represent the corresponding background signals, respectively.
concentration for both of the two CRM standards (curves a and b). The background signals for these two materials (curves c and d) are near constant up to about 5% m/v slurry concentration, and it increases strongly above this concentration. Taking into account the mean mercury content in the samples and the best precision of measurements the use of 2.0% m/v slurry concentration was selected. In order to minimize the build-up of inorganic residue on the graphite platform a cleaning temperature of 2400 8C was required. 3.3. Calibration curves and analytical performance data Three types of calibration curves were determined: (1) using matrix-free calibration solutions, (2) using the slurry of the CRM 007-040 sewage slag standard to which matrixfree solution standards were added, (3) the same as (2) with the use of the other CRM 144R sewage slag standard. Obviously, in the three sets of standards the same amount of KMnO4+Pd modifier was applied. Each standard series consisted of four concentrations and triplicate measurements were made to each concentrations. The calibration curves covered the concentration range from 10.00 ng ml 1 to 100 ng ml 1 Hg content in the slurries, corresponding to mercury content from 37.25 ng ml 1 to 85.45 ng ml 1, in the original samples. The calibration Table 2 Slopes of calibration graphs for aqueous standard and for two sewage sludge standards to which aqueous standards were added (standard addition calibration curves) 4
Sample
Slope10 /A ng ml
Aqueous standard solution CRM 007-040 sewage sludge (2% m/v) CRM 144R sewage sludge (2% m/v)
6.65F0.28 (0.999) 6.40F0.52 (0.997) 6.21F0.44 (0.998)
The correlation coefficient, r, is given in parentheses. a MeanFstandard deviation (n=3).
1a
curves were apparently linear up to 150 ng ml 1 Hg concentration in the slurry. The linearity is characterized numerically by the correlation coefficient (r), in Table 2, where the slope values for the three sets of standards are also listed. The accuracy of the slurry sampling method was investigated by comparing the analysis results to that found with wet digestion–dissolution method (Section 2.4) and with the known concentrations of CRM standards. The significance of the numerical deviations seen in Table 3 was estimated by the statistical bpaired t-test b method, and a value of t= 210 ( P=0.857) was found. This indicates that there were no significant differences between the results obtained by the slurry method and the wet digestion method. Similar statements are valid for the comparison of the results obtained by proposed method for the CRMs and their certified concentrations. The precision of the slurry sampling method could be characterized by an average relative standard deviation Table 3 Comparison of mercury content (Ag g 1) of two sewage sludge samples and reference materials determined by dissolution (digestion) based and direct slurry sampling methods Samples and CRMs
Digesta (n=3)
Slurryb (n=3)
Sludge Koziegyowy Sludge Poznan´ CRM 144R Recovery, % CRM 007- 040 Recovery, %
4.25F0.34 1.86F0.15 1.52F0.18 83.5 2.86F0.26 90.8
4.62F0.38 2.08F0.19 1.75F0.08 96,2 3.02F0.25 95,8
Reference value
1.82F0.24 3.15F0.42
Recoveries for the CRM standards were also calculated. F standard deviation (S.D.). a Triplicate digests were prepared and triplicate measurements were made from each digest. b Triplicate slurries were prepared in three autosampler cups and triplicate measurements were made from each cup.
D. Baralkiewicz et al. / Spectrochimica Acta Part B 60 (2005) 409–413
RSDaverage=8.3% in the concentration range 1.5–4.6 Ag g 1 Hg. The detection limit was determined by making 10 replicate measurements with previously preheated sludge samples as slurries. Applying the 3r concept of calculation, detection limit of 17 ng g 1 (ppb) Hg was found.
4. Conclusion The evaluation of the direct slurry sampling atomic absorption spectrometry for the trace determination of mercury in sewage sludge samples was made. The determination was possible by the addition of KMnO4+Pd to the aqueous standards and slurries, in order to stabilize the analyte. The modifier proved to be very advantageous, since it is an easily available reagent, and has shown to be very efficient. The results obtained in this work show that slurry sampling is a rapid and economical alternative to the conventional acid digestion procedures for sewage sludge analysis. Sample preparation and the analysis can be performed for about 15 min and only small amounts of reagents are required. Low risk of contamination, simple handling, and the possibility to use aqueous calibration are very convenient for routine application.
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