The aim of this study was to develop a rapid microwave digestion procedure for the determination of the aqua regia content of lead, cadmium and chromium of.
Mikrochim.Acta 119, 233-241 (1995)
Mikrochimica Acta 9 Springer-Verlag1995 Printed in Austria
Comparison of Three Digestion Methods for the Determination of the Aqua Regia Soluble Content of Lead, Cadmium and Chromium in Sewage Sludges by ETAAS Nikolaos S. Thomaidis, Efrosini A. Piperaki, and Panayotis A. Siskos* Laboratory of AnalyticalChemistry,Department of Chemistry,Universityof Athens, Panepistimiopolis, 15771 Athens, Greece
Abstract. A procedure for the determination of the aqua regia soluble content of lead, cadmium and chromium in a sewage sludge reference material (CRM 145R) by electrothermal atomic absorption spectrometry (ETAAS) is described. A comparison of the dissolution procedure proposed in the certification report, to an oven-assisted digestion and a proposed microwave digestion procedure is performed. In the ETAAS method developed, 1 gg of Pt proved to be an appropriate modifier for each of the above heavy metals. Possible sources of error at each analytical step are addressed. The metal contents obtained with the proposed method are in a good agreement at 95% significance level with the certified values given for CRM 145R.
Key words: sewage sludge, trace metals, microwave digestion, electrothermal atomic absorption spectrometry, chemical modifiers. The determination of heavy metals in sewage sludges is of great importance because these materials can be used as fertilizers in agriculture, provided that the content of hazardous substances is low [1]. Hence, monitoring of the concentration of metals such as lead, cadmium and chromium in such samples is an everyday practice for many laboratories. The most commonly used techniques for these studies are atomic absorption spectrometry with flame (FASS) or electrothermal (ETAAS) atomization and inductively coupled plasma-atomic emission spectrometry (ICP-AES) or -mass spectrometry (ICP-MS). These techniques have recently been compared for the determination of metals in acid digests of solids [2]. Most analytical techniques require the sample in a liquid form. The conventional wet digestion procedures involve heating of the sample for some hours with several mixtures of acids and oxidizing agents such as H N O 3 / H 2 0 2 (EPA method 3050) [3], * To whom correspondenceshould be addressed
234
N.S. Thomaidis et al.
HNO3/H2SO 4 or HNO3/HC10 4 [4], HNO3/HF or I-INO3/HCI or just only HNO 3 [5]. A digestion using aqua regia usually provides enough information about the content of heavy metals l-l]. In the last years, microwave digestion techniques have become very popular and have been used for a variety of samples, such as soils [6,7], sediments [81 and biological samples [9]. The most attractive features of microwave digestion are its speed compared with conventional digestion procedures, the reduction in the reagents consumption since generally lower sample masses are used and the lower risk of contamination since the digestion is performed in closed vessels. The aim of this study was to develop a rapid microwave digestion procedure for the determination of the aqua regia content of lead, cadmium and chromium of a standard reference material (CRM 145R, sewage sludge). An alternative digestion method utilizing heating in an ordinary oven, existing in every laboratory, is developed. Platinum was used as chemical modifier for the above metals for the ETAAS measurements.
Experimental Instrumentation A Perkin-Elmer Model 5000 atomic absorption spectrometer equipped with a H G A 400 graphite furnace atomizer was used for the atomic absorption measurements. Pyrolytically coated graphite tubes with pyrolytic L'vov platforms for determinations of Pb and Cd, or without for the Cr determinations, were used throughout the study. A 20-1aL volume of the samples solution was dispensed in the graphite tubes with an AS-1 auto sampler and a 5-~tL volume of the modifier solution was injected with an Eppendorf micropipette with disposable polypropylene tips. The instrumental settings and the graphite furnace programmes are summarised in Tables 1 and 2. The time-resolved atomic absorption pulses were recorded with an IBM compatible PC Quest 286/16 interfaced with the spectrometer. This system was described elsewhere [10].
Reagent~Materials All chemicals used in this study were of analytical grade. The acids were of Suprapur grade (Merck, Germany). Standards were prepared by diluting a 1000rag L-1 stock solution (Titrisol, Merck, Germany) with deionized distilled water and acidified to a final HC1 concentration of 0.5% v/v. Modifiers (Pd and Pt) stock solutions were prepared by dissolving appropriate amounts of their chloride salts in acid media and diluting to a final volume with water. Table 1. Instrumental operating conditions for the determination of lead, cadmium and chromium
Wavelength Bandpass EDL power or HCL current BG corrector Purge gas Sample volume
Lead
Cadmium
Chromium
228.8 nm 0.7 nm 10W D2 Ar 20 ~tL
283.3 nm 0.7 nm 5W D2 modifier volume measurement mode
357.9 nm 0.7 nm 28mA W 5 ~tL integrated absorbance (IA)
Comparison of Three Digestion Methods
235
Table 2. Temperature programmes for the determination oflead, cadmiumand chromium. The gas-stop facility and the READ command were used during the atomization step for all analytes
Step
Temperature/ ~C
Ramp time/s
Hold time/s
180 950 20 1800 2650
10 10 1 0 1
20 20 9 4 2
180 650 20 1400 2650
10 10 1 0 1
20 20 9 4 2
120 1300 20 2300 2650
10 10 1 0 1
20 20 9 4 2
Platform atomization
Lead Drying Pyrolysis Cool-down Atomization Cleaning
Cadmium Drying Pyrolysis Cool-down Atomization Cleaning Wall atomization
Chromium Drying Pyrolysis Cool-down Atomization Cleaning
The dry sewage sludge used throughout this study was a standard reference material, CRM 145R, (Community Bureau of Reference, CEC, Brussels). The sample was shaken manually for 5 min to ensure homogenisation before weighing. A conventional oven with maximum heating temperature of 240 ~ was used for the digestion of samples in the Procedure B. For this procedure we used screw-topped Teflon digestion vessels. A commercial microwave oven (maximum l150W) was used for the digestion of samples in the Procedure C. For the last procedure we used Parr 4749 digestion bombs. Procedures
Pre-digestion procedure. All glass and plastic ware were kept in 10% v/v H N O 3 for at least one night and then rinsed with 1% v/v HNO3 and subsequently with distilled water before use. The Teflon containers were cleaned thoroughly by adding 10 mL of conc. H N O 3 to the vessels and then were subjected to microwave heating at 550 W for 10 rain. The vessels were consequently rinsed with dilute HNO3 and finally with bidistilled water.
Digestion procedures. BCR digestion procedure (Procedure A). A modification of the method described in the certification report. (German standard D I N 38414-$7) [11] was applied. An approximately 0.5 g
236
N.S. Thomaidis et al.
subsample was weighted accurately in a round bottom flask. 0.5 mL bidistilled water was added to obtain a slurry, followed by 3.5 mL of conc. HC1 and 1.2 mL of conc. H N O 3. 10 mL of 0.5 M HNO 3 were added to the absorption vessel and connected with the reflux condenser. Both the apparatus were placed on the top of the reaction vessel. The samples were allowed to stand overnight in order to reduce the gases produced during the subsequent heating cycle. They were heated under reflux until boiling for 2h and allowed to cool slowly at room temperature. The contents and the following rinses of the absorption vessel were passed through the condenser into the reaction vessel. The content of the flask was transferred quantitatively to a 100 mL graduated flask and filled up to the mark with 0.5 M HNO 3. After the undissolved material has settled, the supernatant solution was subjected to atomic absorption analysis.
Oven-assisted digestion (procedure B). An approximately 0.2 g sample was weighed accurately in a 50-mL screw-topped Teflon vessel. 0.5 mL of bidistilled water was added, followed by 3.0 mL conc. HC1 and 1.0 mL cone. HNO 3. The samples were allowed to stand overnight, before capping. Then, they were heated at 120 ~ in a oven for 1 h. They were allowed to cool at room temperature before opening the vessels. The content of the vessel was transferred quantitatively into a 100 mL graduated flask, filled up to the mark with 0.5 M HNO3 and after the undissolved material was settled, the supernatant was subjected to atomic absorption analysis.
Microwavedigestion(procedureC.) An approximately 0.1 g sample was weighted accurately into the Teflon vessel of a Parr bomb 4749.0.2 mL ofbidistilled water was added, followed by 1.5 mL conc. HC1 and 0.5 mL conc. H N O 3. The samples were allowed to stand overnight, then the bombs were sealed and heated in a domestic commercial microwave oven at 550 W for 10 rain and at 880 W for 10 rain. They were allowed to cool at room temperature before opening the bombs. The content of the vessel was transferred quantitatively into a 100 mL graduated flask, filled up to the mark with 0.5 M HNO 3 and after the undissolved material was settled, the supernatant was subjected to atomic absorption analysis. Measurementprocedure. For the optimization of the ETAAS procedures, 5 pL of the modifier solution was dispensed into the graphite tube, followed by 20 ~L of the sample solution. The calibration curves were constructed by injecting appropriate aqueous standards of Pb, Cd and Cr and applying the temperature programmes shown in Table 2. Quantification was performed measuring the peak area of the AA signals. The characteristic mass, m o (pg), was calculated from the slope (b) of the standard curve, using the equation m o = 0.0044 x 20/b for a sample volume of 20 pL (see results in Table 5).
Results and Discussion
Preparation of Digestion Apparatus It w a s o b s e r v e d d u r i n g this s t u d y t h a t t h e T e f l o n vessels a p p e a r e d to be c o n t a m i n a t e d w i t h C r a n d s o m e t i m e s w i t h P b after s o m e successive d i g e s t i o n s . T h i s p r o b l e m h a s b e e n a v o i d e d b y a d d i n g 10 m L o f c o n c . H N O 3 t o t h e T e f l o n c o n t a i n e r s a n d t h e n s u b j e c t e d t o m i c r o w a v e h e a t i n g at 5 5 0 W f o r 1 0 m i n . T h e vessels w e r e c o n s e q u e n t l y r i n s e d w i t h d i l u t e H N O 3 a n d finally w i t h bidistilled w a t e r (see a l s o ref. [12]).
Comparison of Digestion Procedures T h e results f r o m t h e p r o p o s e d m e t h o d s f o r t h e d e t e r m i n a t i o n o f lead, c a d m i u m a n d c h r o m i u m a r e p r e s e n t e d in T a b l e 3. F r o m this t a b l e it m a y be c o n c l u d e d t h a t all t h e
Comparison of Three Digestion Methods
237
Table 3. Determination of the aqua regia content of Pb, Cd and Cr in the reference material C R M 145R. Results expressed as mean ,+ one standard deviation Experimental value (ggg-1) Proc. A (n = 3) Pb Cd Cr
276.3 ,+ 8.0 3.18 + 0.06 288.8,+11.2
Certified value (ggg-1) Proc. B (n = 5)
Proc. C (n = 5)
289.3 ,+ 10.1 3.33 __ 0.26 322.5_+3.7
282.1 _ 12.5 3.19 ,+ 0.06 315.2_-t-7.9
282 _+ 9" 3.43 +_ 0.17 b 307___ 13a
Proc. A = BCR digestion procedure. Proc. B = Oven assisted digestion procedure. Proc. C = Microwave digestion procedure. a Mean value _+ uncertainty (95 % significance level). b Indicative value: mean value -+_one standard deviation.
procedures performed yielded accurate and precise results. No significant difference exists at p = 0.05 confidence level between the results obtained by the three digestion procedures. The only exception was the results for Cr obtained with the proposed procedures which give significantly higher results than the BCR procedure. This could be attributed to the contamination problems observed in the procedures involved Teflon vessels, as already mentioned previously. Nevertheless, the results obtained with the latter two procedures are not significant different from the certified value. The relative standard deviation of the digestion procedures were always lower than 4.0%, except for Cd with the second digestion procedure, where the RSD was 7.9%. A careful optimization of the power programme for the microwave digestion was performed in order to obtain complete recovery of the metals and to avoid leaching of the sample through the top of the Teflon vessels. Comparison of the above procedures (Table 4) indicates that the microwave digestion procedure offers the following advantages: a) it is faster than the other two, b) the reagents consumption is kept to a minimum of 2 mL of suprapur-grade acids because a small mass of the sewage sludge sample is used and c) closed Teflon vessels reduce the risk of contamination from the laboratory environment, therefore the blank values are lower. The only disadvantage is the compulsory use of Parr bombs in order to control the pressure in the reaction vessel. The alternative oven-assisted digestion procedure could be used as well; it is also faster than the BCR procedure. When the digestion time was reduced to 30 rain the metals were not recovered completely. Only 17% of Pb, 41% of Cd and 10% of Cr were recovered. The procedure proposed in the certification report requires higher sample mass, thus higher acid consumption and rather complicated apparatus. The sample mass was reduced to 0.5g rather than the recommended 1-3 g in order to reduce the undissolved solids. The undissolved material from a 0.5 g subsample was settled very quickly and there was no need to filtrate or centrifuge it. The number of samples that could be digested at one time is also lower because each sample requires separate
238
N.S. Thomaidis et al.
Table 4. Comparison of practicability of the digestion procedures Step
Proc. A
Proc. B
Proc. C
0.5 a
0.2 b
0.1 b
Acid volume, mL
5.0
4.0
2.0
Apparatus
d
c
c
Time
2h
1h
20 min
Preparation Sample mass, g Clean up
Digestion
a Simple cleaning. b Rigorous cleaning. c Simple apparatus. Proc. B. Screw-topped Teflon vessels and conventional oven Proc. C: Parr bombs and microwave oven. d Complex apparatus: Round-bottomed flasks, condensers, absorption vessels, heating devices.
instrumentation (round-bottomed reaction flasks, condensers, absorption vessels, heating device, etc). On the other hand, with the other two digestion procedures a large number of samples could be digested simultaneously. It is very important to notice that the samples in all digestion methods must be allowed to stand some time to digest at room temperature in order to avoid over-pressurization of the vessels during the initial stages of heating. It is also important to notice that the vessels must be opened cool otherwise significant losses of Pb and Cd may occur.
Optimization of the ETAAS Determinations We first compared the maximum pyrolysis temperature (Tpyr) and sensitivity obtained when Pt and Pd used as chemical modifiers.
Lead. Without chemical modifier, the maximum Tpy r w a s 700 ~ In the presence of 1 gg of modifier (Pd or Pt) the maximum Tpyr was 1000 ~ However, the peak height sensitivity was better only in the presence of Pt. It was reported that Pb forms refractory species when Pd is present, thus analytical errors are to be expected when Pd is used as chemical modifier for Pb [13]. This was not the case when Pt was used instead of Pd. The optimum atomization temperature (Tatom) was 1800 ~ The peak profiles of Pb with or without Pt is shown in Fig. la. Sometimes, double peaks were observed in the absence of a chemical modifier, especially with old graphite tubes. These peaks disappeared in the presence of Pt (Fig. lb). Generally, use of modifier leads to a constant behaviour for a large number of determinations, thus repetitive calibrations are avoided. Comparison of the slopes of matrix-matched standard curves and standard curves in aqueous solution revealed that when Pt was used as modifier with peak area measurement, the quantification may be done with aqueous standards. These conditions were used for the final determination of Pb.
Comparison of Three Digestion Methods Table 5. Maximum pyrolysis temperatures
239
(Tpyr, ~
and characteristics masses (m0, pg) for Pb, Cd
and Cr in the presence of different chemical modifiers
Element
Modifier
Tpyr (~
PH
mo (Pg) IA
Pb
none 1 gg Pd 1 gg Pt
700 1000 1000
7.5 6.4 4.8
11.7 11.0 11.0
Cd
none 1 gg Pd 1 gg Pt
300 650 650
0.37 0,35 0,23
0.51 0.36 0.38
Cr
none 1 gg Pt
1000 1300
1.9 1.4
3.7 3.2
PH = peak height absorbance measurement, IA = integrated absorbance measurement.
0.5
I
I
I'
2
3
(a) 0.4 O E 0
0.3
0 I/I ,m
0.2
B
A
