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Determination of thallium in soils by flow-injection-differential pulse anodic stripping voltammetry
Tahlo, Vol. 39, No. 3, pp. 221-227, 1992 Printed in Great Britain
0039-9140/92 s5.00 + 0.00 Pergamon FVessplc
DETERMINATION OF THALLIUM IN SOILS BY FLOW-INJECTION-DIFFERENTIAL PULSE ANODIC STRIPPING VOLTAMMETRY ZENONLUKASZEWSKI and WLODZIMIERZ ZEMe~zuwu Institute of Chemistry, Technical University of Poxnan, PL-60-965 Poxnan, Poland (Received 17 November 1990. Revised 8 July 1991. Accepted 26 July 1991) Summary-A relatively simple and quick method for the determination of thallium in soils is described. The method does not require any separation prior to determination. Total decomposition of the sample was performed in a teflon bomb. The interferences of iron, aluminum and manganese were removed by media exchange performed in a flow-injection measuring system, and the other interferenceswere removed by the use of the base electrolyte consisting of 0.15M EDTA and O.lM ascorbic acid. The contents of thallium in the examined samples of soil were between 100 and 350 ppb.
Thallium is a highly toxic element, however, it has not commanded a lot of attention from investigators because of its low content in the environment, owing to its low crystal abundance (0.7 pg/g). ‘.* On the other hand, the antropogenic sources of pollution with this metal exist and occasional ecological catastrophies caused by thallium arise (e.g., Chemovcy, 1988). It is reported that thallium which is preconcentrated in the soil can dampen the vitality of soil microorganisms.’ That is why the content of thallium in the environment, its speciation, circulation etc., has a certain importance and should be systematically controlled. A more extensive investigation of thallium in the environment cannot be performed because of the lack of quick and cheap methods for its determination which are resistant to interferences. This situation occurs because of the very low abundance of the element and the numerous interferences which are very difficult to overcome. That is why the majority of methods combines the final measurement with a preconcentration procedure, e.g., extraction with spectrophotometry3A or volatilization of thallium in the form of hydride with AAS.‘s6 Detection limits for different methods for determination of thallium are compared in a report by IUPAC.’ Liem et a1.5critically compared the detection limits of methods for determination of thallium also. The potential for stripping voltammetric methods is promising although both reportG’ differ very much in evaluation of detection limits of different methods. The lowest detec-
tion limit was achieved for the differential pulse mode. Also, the use of a mercury film electrode (MFE) gives a lower detection limit than HMDE. Extremely low concentration of thallium (10 ng/l.) was achieved when the mercury film was used together with a rotating disc.* Such a low detection limit is sufficient for the determination of thallium in environmental samples but interferences are the main problem. The main interferences with thallium in voltammetry are caused by lead, tin and cadmiurn9 Also indium interferes with thallium under the conditions of a MFE,‘O but this interference can be removed by the use of a proper voltage scan-rate. The signals of thallium, lead and cadmium can be separated by the use of certain base electrolytes having alkaline PH.” However, in the matrices having practical importance the separate determination of thallium seems to be a more practical solution because of the great difference in the level of thallium on the one hand and the interferents (e.g., lead) on the other hand. Dieker and van der Linden” proposed a method for the determination of thallium in the presence of a 105excess of lead with an MFE produced in situ. The use of complexons as the base electrolytes is a very effective measure against the interferences. EDTA solution together with acetates, tartrates or citrates is used.8*13*‘4 If surfactants are used together with the complexons the excess of lead against the thallium can be substantially increased (to 105-107).‘5~‘6 The interference of copper can also be removed in this way.” 221
222
ZENON
LUKASZEWSKI and
The interferences caused by the presence of iron and titanium turned out to be very difficult to overcome, although these metals do not accumulate in the mercury. These elements are present in many matrices at very high concentrations and their presence causes such problems that the determination is frequently impossible. ‘8*‘gThe interferences discussed can be limited by the use of properly selected surfactants. However, the simultaneous presence of several interferents is difficult to overcome, e.g., the use of EDTA which limits the interference of lead involves the appearance of the peak of titanium which does not appear in the noncomplexing electrolytes.‘g Removing the interferents which do not undergo deposition in the form of an amalgam (iron, titanium, etc.) seems to be possible by means of media exchange performed after the preconcentration has been completed. Simple removal of analyte containing interferents, after deposition of thallium is completed and its replacement by the pure electrolyte, is possible only in a flow-injection measuring system in which such an operation is possible without breaking the electric contact in the circuit. If these interferents were not present during the stripping stage their interfering influence would be eliminated. A similar solution has recently been used for removing the surfactant from the measuring system after the deposition of cadmium, lead and copper is complete but before the stripping stepm or for improving determination of copper.2’ The measuring system in these works was very similar to the flow-injection system.22 The aim of this paper is to develop a highly selective method for the determination of trace thallium, using media exchange in a flowinjection measuring system and proper selection of base electrolyte for removing interferences. Such a method should be sufficient for the determination of thallium in complicated environmental matrices. Soil seems to be a very good example of such a complicated environmental matrix which has large amounts of possible interferences. Limited deposition time, connected with the volume of analyte is a certain drawback in using a flow-injection system, but can be simply overcome by using circulation of the analyte during the time necessary for effective preconcentration of thallium. Information on thallium content in soil is comparatively scanty. Chattopadhyay and
WLODZMERZ zF!MsRzusKl
Jervis have determined 0.17-0.22 ppm in garden soi123(the detection limit of this method is only 0.15 ppm) and Ure and Bacon 0.27 ppm.24 Smith and Carson have reported 0.02-2.8 ppm in surface soils of the USA.” The largest antropogenic source of thallium is related to coal combustion.’ The normal level of lead in soil should not exceed 70 ppm,’ although in polluted soils can be much higher. Iron, aluminum, titanium and manganese have the most common range from 0.5 to 5%, from 0.45 to lo%, from 0.1 to 0.9% and from 200 to 800 ppm, respectively.’ The influence of antimony, bismuth and copper should also be taken into account. The content of antimony and bismuth does not exceed l-2 ppm but copper can be several hundred ppm.’ Such a ratio of these interferents should be expected in the solution obtained after the decomposition of the soil sample. The presence of iron(II1) in the determined solution obtained after the decomposition of soil is extremely troublesome. Iron(II1) causes the dissolution of the mercury film making the determination impossible. The use of supporting electrolyte consisting of EDTA and ascorbic acid can overcome this problem. The results of the determination of thallium in the soil with the use of such electrolyte and media exchange were preliminarily reported.26*27 EXPERIMENTAL
Apparatus
A Telpod (Poland) pulse polarograph model PP-04 was used. Voltamperograms were displayed on an Endim (GDR) 620.02 XYrecorder. The differential pulse amplitude was 50 mV and the scan-rate was 11.l mV/sec. The flow-injection voltammetric system shown in Fig. 1 was used. A wall-jet type three-electrode flow-through cell was used. This cell consisted of: a mercury film electrode based on the epoxy resin impregnated graphite (manufactured in Maria Sklodowska-Curie-University), saturated calomel electrode (SCE) and platinum wire auxiliary electrode. The geometrical surface of MFE was 3.14 mm2. The mercury film was deposited over 10 min from the solution consisting of 50pM mercury(I1) nitrate and O.lM potassium nitrate, and was washed with potassium nitrate solution. Only one mercury film was required for a whole day of measurements. The peristaltic pump 372.C (Elpan, Poland) was used with a flow-rate of 15 ml/min. During the
Determination of thallium in soils
(a)
223
peroxide (POCh) were used. The samples of soil were received from the Institute of Soil Science and Cultivation of Plants, Pulawy, Poland together with the determined contents of lead, iron and manganese. Decomposition of soil
Fig. I. The voltammetric flow-injection measuring system (a) with flow-through cell of wall-jet type (b). MFE-mercury film electrode based on epoxy resin impregnated pyrolitic graphite, Ref.-reference electrode (SCE), ptauxiliary electrode (Pt wire).
measurement, circulation of measured solution was used by junction of the outlet from the measuring system with an analysed sample in the beaker. Another flow-injection voltammetric system was used for part of the experiments. This system was the first version of the final system and was characterized by the lower hydrodynamic effectiveness of the cell and, as a result, weaker analytical signal. Reagents
The supporting electrolyte was 0.15M EDTA containing O.lM ascorbic acid (pH 4.5), which was prepared from the analytical grade reagents (POCh). The pH of this solution was adjusted with sodium hydroxide solution (Merck). The carrier solution was 0.05M EDTA solution which was continuously deaerated with the purified nitrogen. The standard solution of Tl(1) was prepared from thallium(I) nitrate. All solutions were prepared in triply distilled water in a fused-silica apparatus. Suprapure nitric and hydrochloric acids (Merck) and the analytical grade hydrofluoric acid and hydrogen
The soil sample (approx. 0.5 g) was transferred to the beaker of a teflon bomb and moistened with water. Hydrofluoric acid (2.5 ml) and 3.5 ml of a mixture of hydrochloric and nitric acids (3: 1) were added. The teflon bomb was closed and put in the drying-oven for 3 hr (135”). The obtained solution was transferred into a teflon beaker and heated on a graphite heater until evaporated. In the final stage of the evaporation, 2 ml of hydrogen peroxide was added in a few portions, for decomposition of residual organic substances originating from soil. The residue was dissolved in 1 ml of hot hydrochloric acid. The solution of ascorbic acid in the amount corresponding to the final concentration of O.lM was added and after a few minutes the corresponding amount of EDTA solution was added. The pH was adjusted to 4.5 and the volume was supplemented to 25 ml with water in the measuring flask. Extraction of soil (slightly modljied procedure of Wolf et al.‘“)
The sample of soil (ca. 5 g) was placed in the conical flask equipped with a reflux condenser and treated with 20 ml of a mixture of hydrochloric and nitric acids (3 : 1) and put aside for 48 hr. Then the sample was heated for 2 hr and filtered. The filter was washed with 2M nitric acid and the filtrates were mixed. The obtained solution was evaporated. In the final stage of evaporation 2.5 ml of hydrogen peroxide were added in a few portions to completely destroy any organic matter. The residue was dissolved in 1 ml of hydrochloric acid and the procedure was continued as for decomposition of soil. Measuring procedure
A portion of the solution (8 ml) was transferred to the beaker and was introduced into the flow-injection system. The solution was directed to the measuring cell and then back to the beaker with the sample. In this way the solution was in circulation. Now the preconcentration was started by application of a preconcentration potential. In all experiments preconcentration was carried out at a potential of -0.850 V with the exception of the experiments in which the
224
ZENONLUWZEWSK~ and
influence of preconcentration potential on the peak height was examined. The preconcentration time was l-3 min depending on the thallium concentration. After the deposition stage was complete the valve was switched off and the analysed solution was replaced by the deaerated carrier solution (0.05M EDTA). Then the flow was stopped and the voltamperogram recorded after 15 set of quiescent time. The evaluation of the concentration of thallium was performed by the method of two additions of standard. The pH remained constant throughout the measuring cycle and so did not require correction. RESULTS AND DISCUSSION
The presence of ascorbic acid in the base electrolyte transforms iron(II1) into iron( which highly improves possibilities for the measurement. However, it is still insufficient for the sensitive determination of thallium because of too high a base-line current. This is visible in Fig. 2(a), which shows the voltamperogram of a solution containing EDTA, ascorbic acid, aluminum, iron(I1) [introduced in the form of iron(III)] and 1OnM thallium. Aluminum and iron are present here in the ratio typical of their
~LODZIMEZRZ
ihLtRZUSK1
contents in the soil. Only a small bulge resulting from thallium is visible on this voltamperogram. Media exchange for pure 0.05M EDTA removes iron(I1) as well as other interferents which do not form amalgams, and in this way radically improves the conditions for the determination of thallium [see Fig. 2(b)]. The correct selection of a deposition potential is very important. On the one hand, a sufficiently negative deposition potential guarantees a stable peak height of thallium (if it is located on the plateau of dependence of the peak height on the deposition potential). On the other hand, with the shifting of potential towards negative direction, the lead signal increases. Thus the selection of deposition potential is the compromise between these two tendencies. This is shown in Figs 3(a) and 3(b). Figure 3(a) shows the dependence of height of the peak of thallium on the deposition potential both in the pure EDTA solution (circles) and in the solution containing additionally iron(I1) and aluminum(II1) in the ratio typical of the solution obtained after decomposition of soiP (crosslets). Iron was introduced in the form of iron(II1) for the control of effectiveness of its reduction. These two dependences are identical and their plateau is located starting from a potential of -0.850 V us. SCE. The discussed
-cl7
-08
-G9
-10
-If
E,IVI
Fig. 2. The voltamperograms with signals of thallium in the presence of iron@) (before medium exchange) (a) and in the absence of iron (after the medium exchange) (6). The base electrolyte for the case (a) whole cycle and for the deposition stage in the case (b) 0.15M EDTA + 0. IM ascorbic acid + 70mM of aluminum(II1) + 20mM of iron(D) (PH 4.5). The base electrolyte for stripping stage in the case (b) 0.05M EDTA @H 4.5). Concentration of thallium: 1OnM. Preconcentration potential: -0900 V vs. SCE; preconcentration time: 180 sec.
Fig. 3. The dependence of the peak height of thallium (a) and lead (b) on the deposition potential and the peak height of lead on its concentration (c) under conditions of the medium exchange. The base electrolyte for the deposition stage: 0.15M EDTA @H 4.5) [Fig. 3(a)-circles] or 0.15M EDTA + 0.1&f ascorbic acid + 70mM of aluminum(II1) + 20mM of iron(I1) t’pH 4.5) (other cases). The base electrolyte for stripping stage (all cases): 0.05M EDTA @H 4.5). Concentration of thallium: (a) lOnM, (b, c) 0; concentration of lead: (a) 0, (b) lo@, (c) variable. Preconcentration potential: (a, b) variable, (c) -0.850 V vs. SCE. Preconcentration time: (a, b and c) 300 sec.
225
Determination of thallium in soils
dependence is shifted toward more negative values in the comparison with similar dependence measured with the use of HMDE.16 The potential peak of thallium is also located at about - 150 mV more negative than when using HMDE (- 0.600 V 11s.SCE). The dependence of the peak height of lead on the deposition potential under these conditions is shown in Fig. 3(b). It is obvious from this picture that the lead peak appears at a potential of - 0.850 V us. SCE which is necessary for the determination of thallium. The dependence of the peak height of lead on the concentration of this metal is shown in Fig. 3(c). This dependence makes possible the evaluation of the positive error of determination of thallium caused by the presence of a certain concentration of lead. This error is negligible for < 10jN lead which corresponds to 100 ppm lead in a 0.5-g soil sample. The masking of the lead signal is caused by the presence of EDTA and the question arises as to whether the excess of free EDTA, i.e., nonbounded with iron, aluminum, etc. is necessary for the effective masking of lead. The dependence of the peak height of lead on the preconcentration potential was examined under the following conditions: pure EDTA solution, a solution with an EDTA to iron ratio of 1: 1 and a solution with an excess of iron vs. EDTA. The results are shown in Fig. 4 and the representative voltamperograms are shown in Fig. 5. The results shown in Fig. 4 were performed with the first version of a measuring system which
a/
/
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-07
-08 E. IV)
-09
- IO
Fig. 4. The dependence of the peak height of lead on the preconcentration potential under conditions of absence (a) or in the presence of iron in the sample (b, c). The ratio of iron US.EDTA: (a) 0, (b) 1, (c) 1.1. The base electrolyte for the deposition stage: 0.1 M EDTA + 0.1 M ascorbic acid + O.lmM of lead(D) @H 4.5). The base electrolyte of stripping stage: 0.05M EDTA. Preconcentration time: 180 sec.
l76n-M
I
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,-:
Fig. 5. The effect of excess or deficit of nonbounded EDTA on the tolerance of the system on the presence of lead. The ratio of EDTA us. iron: (u) 7.5 : 1, (b) 1: 1,(c) 1:2. The base electrolyte of deposition stage: 0.15M EDTA + 0.30M ascorbic acid @H 4.5). Concentration of lead (deposition stage): O.OlmM. Concentration of iron (deposition stage): (a) 2OmM, (b) 150mM and (c) 300mM. The base electrolyte of stripping stage: 0.05M EDTA (PH 4.5). Preconcentration potential: -0.850 V; preconcentration time: 900 sec.
was less sensitive and the results were further checked with the system finally used (Fig. 5). The higher concentration of ascorbic acid was used in the last case for more effective reduction of introduced amounts of iron(II1) to iron(I1). It is clearly visible in Fig. 4 and supported in Fig. 5, that if the concentration of iron and aluminum exceeds EDTA concentration lead is no more effectively masked and a huge peak arising from this metal appears. Thus the ratio of the weight of soil sample to the EDTA concentration must be synchronized and an excess of EDTA in the measured solution is necessary. The possible interferences of antimony, bismuth and copper with the determination of thallium were predicted.s For this reason the tolerated excess of these metals was checked. Antimony or bismuth (0.1@4) or 1pM of copper was added to the 1OnM thallium solution. Such a ratio is required because of the reported highest level of these elements in the soil.’ The results of these experiments are shown in Fig. 6. The peaks of considered metals are located at (- 0.240) (- 0.240) and (-0.200) V us. SCE respectively, under the
226
ZENONLUKASZ~WSKI and WUIDZIMERZZJZ~RZUSKI
Fig. 6. The voltamperograms of solution containing thallium in the presence of an excess of antimony (a), copper (b) and bismuth (c). Concentration of thallium: 1OnM. Concentration of antimony or bismuth: lOOnM, concentration of copper: 1pAf. The base electrolyte of deposition stage: O.lSM EDTA + O.lM ascorbic acid @H 4.5). The base electrolyte of stripping stage: O.OSMEDTA. Preconcentration potential: -0.850 V us. SCE, preconcentration time: 6OOsec.
experimental conditions (deposition from 0.1 SM EDTA + O.lM ascorbic acid solution at -0.850 V and stripping from 0.05M EDTA solution). No serious interferences of the investigated metals at such a ratio are observed. Removal of interferences makes the determination of thallium in the real soil samples possible. The recovery and precision of such a determination has been checked. The representative soil sample (No. 126D8437-see Table 1) contained 1.07% iron, 270 ppm manganese and 8.5 ppm lead. It was analysed
according to the described procedure, both alone as well as spiked with 250 ppb thallium. A 213-ppb concentration of thallium was found in the analysed soil (7 independent measurements, S = 30.5 ppb, S, = 14.3%). The spiked thallium was recovered in 94% (6 independent measurements, S = 31.5 ppb, S, = 7.2%). Such results seem to be satisfactory for such a complicated matrix and difficult decomposition procedure. Precision of measurement is satisfactory because it is typical of measurement on such a concentration level. A concentration of 1nM thallium can be comparatively easily determined with 10-l 5 min preconcentration. Hence, the detection limit of 10 ppb (with 0.5 g of soil) can be achieved, and, if necessary, improved by prolonging deposition time. Lead is tolerated up to 100 ppm but higher contents cause a small positive error. The developed method was used for the determination of thallium in several samples of soil. Parallel samples were totally decomposed or extracted with a mixture of nitric and hydrochloric acids according to the procedure described above. The results are given in Table 1, together with the content of iron, manganese and lead in the soil samples. It is visible that the content of thallium in the totally decomposed samples is roughly located between 100 and 350 ppb. The extraction of thallium leads to much lower results, which indicates only partial extraction (21-74%). Thus the extraction is the unproper way for the evaluation of total thallium content in the soil in contrast to the determination of zinc, lead, cadmium and copper in the soil,” where extraction is complete. The contents of thallium seem to be correlated with the content of iron and manganese. Of course, such a conclusion should be supported with much extensive examination.
Table 1. The results of the determination of thallium in different soil samples performed after the total decomposition of the samples and after the extraction of samnles with the mixture of nitric and hydrochloric acids Content of iron, manganese and lead Sample number
Fe, %
Mn,
Pb,
mm
pm
99D8448A 127D8446 26338321B 126D8437 29D8325B 36D8309F
0.23 0.44 0.58 1.07 1.36 2.91
120 210 750 270 400 710
9.0 32 240 8.5 31 29
Thallium content (average) Total decomposition, ppb 97 175 203 213 353 355
Extraction, % ppb 27 28 37 21 150 74 74 35 148 42 176 50
Determination of thallium in soils
The promising results for the determination of thallium in the soil, i.e., sample having silicium, aluminum, iron, titanium, manganese and lead indicate that this element can also be determined in geological samples of similar composition, by means of a very similar procedure. Acknowledgements-This work was supported by Research Program CPBP 01.17. We thank Professor A. KabataPendias of the Institute of Soil Science and Cultivation of Plants, Pulawy, Poland for providing the soil samples with determined contents of iron, manganese and lead and for valuable discussion. We are grateful to Miss Anna Piela for her technical assistance in the measurements. REFERENCES 1. A. Kabata-Pendias and H. Pendias, Trace Elements in Soils and Plants, CRC Press, Boca Raton, Florida, 1984. 2. R. D. Reeves and R. R. Brooks, Trace Element Analysis of Geological Materials, Wiley, New York, 1978. 3. M. Sager and G. Tolg, Mkrochim. Acta, 1982 II, 231. 4. Z. Marczenko, W. Kalowska and M. Mojski, Talanta, 1974, 21, 93. 5. I. Liem, G. Kaiser, M. Sager and G. Tolg, Anal. Chim. Acta, 1984, 158, 179. 6. J. Aznarez, J. Vidal, L. Marco and J. Galban, Euroanalysis VI, Paris 1987. 7. IUPAC Report, Pure Appl. Chem., 1982, 54, 1565. 8. J. E. Bonelli, H. E. Taylor and R. K. Skogerboe, Anal. Chim. Acta, 1980, 118, 243. 9. G. Henze and R. Neeb, Elektrochemische Analyrik, Springer-Verlag, 1986.
227
10. K. Wikiel and Z. Kublik, J. Elcctrwnal. Chem., 1984,
165, 71. 11. R. Neeb and I. Kiehnast, Z. An& C/rem., 1968, 241,
142. 12. J. Dieker and van der Linden, ibid., 1975, 274, 97. 13. V. Gemmer-Colos, I. Kiehnast, J. Trenner and R. Neeb, ibid., 1981, 306, 149. 14. R. G. Dhaneshwar and L. R. Zarapkar, Analyst, 1980, 105, 386. 15. N. You and R. Neeb, Z. AMI. Chem., 1983, 314, 394. 16. A. Ciszewski and Z. Lukaszewski, Talanfa, 1983, 30, 873. 17. I&m, ibid., 1985, 32, 1101. 18. Z. Lukaszewski, A. Ciszewski and A. Saymanski, Chem. Analit. (Warsaw), 1987, 32, 903. 19. A. Ciszewski and Z. Lukaszewski, Tafanta, 1988, 35, 191. 20. F. Wahdat, Z. Lukasaewski and R. Neeb, Anal. Chem., 1990, 33ll, 163. 21. Z. Lukaszewski, F. Wahdat and R. Neeb, ibid., 1990, 337, 885. 22. T. Frank and R. Neeb, ibid., 1987, 327, 670. 23. A. Chattopadhyay and R. E. Jervis, Anal. Chem., 1974, 45, 1630. 24. A. M. Ure and J. R. Bacon, Analysr, 1978, 103, 807. 25. I. C. Smith and B. L. Carson, Trace Metals in rhe Environment, Vol. 1, Ann Arbor, Michigan, 1977. 26. Z. Lukasaewski and W. Zembrzuski, ElectroFinnAnalysis, Turku, 1988. 27. Z. Lukaszewski and W. Zembrzuski, I lth International Symposium on Microchemical Techniques, Wiesbaden, 1989; Z. Anal. Chem., 1989,334, 624. 28. A. Wolf, P. Schrammel, G. Lill and H. Hohn, ibid., 1984, 317, 512.
0039-9140/92 s5.00 + 0.00 Pergamon FVessplc
DETERMINATION OF THALLIUM IN SOILS BY FLOW-INJECTION-DIFFERENTIAL PULSE ANODIC STRIPPING VOLTAMMETRY ZENONLUKASZEWSKI and WLODZIMIERZ ZEMe~zuwu Institute of Chemistry, Technical University of Poxnan, PL-60-965 Poxnan, Poland (Received 17 November 1990. Revised 8 July 1991. Accepted 26 July 1991) Summary-A relatively simple and quick method for the determination of thallium in soils is described. The method does not require any separation prior to determination. Total decomposition of the sample was performed in a teflon bomb. The interferences of iron, aluminum and manganese were removed by media exchange performed in a flow-injection measuring system, and the other interferenceswere removed by the use of the base electrolyte consisting of 0.15M EDTA and O.lM ascorbic acid. The contents of thallium in the examined samples of soil were between 100 and 350 ppb.
Thallium is a highly toxic element, however, it has not commanded a lot of attention from investigators because of its low content in the environment, owing to its low crystal abundance (0.7 pg/g). ‘.* On the other hand, the antropogenic sources of pollution with this metal exist and occasional ecological catastrophies caused by thallium arise (e.g., Chemovcy, 1988). It is reported that thallium which is preconcentrated in the soil can dampen the vitality of soil microorganisms.’ That is why the content of thallium in the environment, its speciation, circulation etc., has a certain importance and should be systematically controlled. A more extensive investigation of thallium in the environment cannot be performed because of the lack of quick and cheap methods for its determination which are resistant to interferences. This situation occurs because of the very low abundance of the element and the numerous interferences which are very difficult to overcome. That is why the majority of methods combines the final measurement with a preconcentration procedure, e.g., extraction with spectrophotometry3A or volatilization of thallium in the form of hydride with AAS.‘s6 Detection limits for different methods for determination of thallium are compared in a report by IUPAC.’ Liem et a1.5critically compared the detection limits of methods for determination of thallium also. The potential for stripping voltammetric methods is promising although both reportG’ differ very much in evaluation of detection limits of different methods. The lowest detec-
tion limit was achieved for the differential pulse mode. Also, the use of a mercury film electrode (MFE) gives a lower detection limit than HMDE. Extremely low concentration of thallium (10 ng/l.) was achieved when the mercury film was used together with a rotating disc.* Such a low detection limit is sufficient for the determination of thallium in environmental samples but interferences are the main problem. The main interferences with thallium in voltammetry are caused by lead, tin and cadmiurn9 Also indium interferes with thallium under the conditions of a MFE,‘O but this interference can be removed by the use of a proper voltage scan-rate. The signals of thallium, lead and cadmium can be separated by the use of certain base electrolytes having alkaline PH.” However, in the matrices having practical importance the separate determination of thallium seems to be a more practical solution because of the great difference in the level of thallium on the one hand and the interferents (e.g., lead) on the other hand. Dieker and van der Linden” proposed a method for the determination of thallium in the presence of a 105excess of lead with an MFE produced in situ. The use of complexons as the base electrolytes is a very effective measure against the interferences. EDTA solution together with acetates, tartrates or citrates is used.8*13*‘4 If surfactants are used together with the complexons the excess of lead against the thallium can be substantially increased (to 105-107).‘5~‘6 The interference of copper can also be removed in this way.” 221
222
ZENON
LUKASZEWSKI and
The interferences caused by the presence of iron and titanium turned out to be very difficult to overcome, although these metals do not accumulate in the mercury. These elements are present in many matrices at very high concentrations and their presence causes such problems that the determination is frequently impossible. ‘8*‘gThe interferences discussed can be limited by the use of properly selected surfactants. However, the simultaneous presence of several interferents is difficult to overcome, e.g., the use of EDTA which limits the interference of lead involves the appearance of the peak of titanium which does not appear in the noncomplexing electrolytes.‘g Removing the interferents which do not undergo deposition in the form of an amalgam (iron, titanium, etc.) seems to be possible by means of media exchange performed after the preconcentration has been completed. Simple removal of analyte containing interferents, after deposition of thallium is completed and its replacement by the pure electrolyte, is possible only in a flow-injection measuring system in which such an operation is possible without breaking the electric contact in the circuit. If these interferents were not present during the stripping stage their interfering influence would be eliminated. A similar solution has recently been used for removing the surfactant from the measuring system after the deposition of cadmium, lead and copper is complete but before the stripping stepm or for improving determination of copper.2’ The measuring system in these works was very similar to the flow-injection system.22 The aim of this paper is to develop a highly selective method for the determination of trace thallium, using media exchange in a flowinjection measuring system and proper selection of base electrolyte for removing interferences. Such a method should be sufficient for the determination of thallium in complicated environmental matrices. Soil seems to be a very good example of such a complicated environmental matrix which has large amounts of possible interferences. Limited deposition time, connected with the volume of analyte is a certain drawback in using a flow-injection system, but can be simply overcome by using circulation of the analyte during the time necessary for effective preconcentration of thallium. Information on thallium content in soil is comparatively scanty. Chattopadhyay and
WLODZMERZ zF!MsRzusKl
Jervis have determined 0.17-0.22 ppm in garden soi123(the detection limit of this method is only 0.15 ppm) and Ure and Bacon 0.27 ppm.24 Smith and Carson have reported 0.02-2.8 ppm in surface soils of the USA.” The largest antropogenic source of thallium is related to coal combustion.’ The normal level of lead in soil should not exceed 70 ppm,’ although in polluted soils can be much higher. Iron, aluminum, titanium and manganese have the most common range from 0.5 to 5%, from 0.45 to lo%, from 0.1 to 0.9% and from 200 to 800 ppm, respectively.’ The influence of antimony, bismuth and copper should also be taken into account. The content of antimony and bismuth does not exceed l-2 ppm but copper can be several hundred ppm.’ Such a ratio of these interferents should be expected in the solution obtained after the decomposition of the soil sample. The presence of iron(II1) in the determined solution obtained after the decomposition of soil is extremely troublesome. Iron(II1) causes the dissolution of the mercury film making the determination impossible. The use of supporting electrolyte consisting of EDTA and ascorbic acid can overcome this problem. The results of the determination of thallium in the soil with the use of such electrolyte and media exchange were preliminarily reported.26*27 EXPERIMENTAL
Apparatus
A Telpod (Poland) pulse polarograph model PP-04 was used. Voltamperograms were displayed on an Endim (GDR) 620.02 XYrecorder. The differential pulse amplitude was 50 mV and the scan-rate was 11.l mV/sec. The flow-injection voltammetric system shown in Fig. 1 was used. A wall-jet type three-electrode flow-through cell was used. This cell consisted of: a mercury film electrode based on the epoxy resin impregnated graphite (manufactured in Maria Sklodowska-Curie-University), saturated calomel electrode (SCE) and platinum wire auxiliary electrode. The geometrical surface of MFE was 3.14 mm2. The mercury film was deposited over 10 min from the solution consisting of 50pM mercury(I1) nitrate and O.lM potassium nitrate, and was washed with potassium nitrate solution. Only one mercury film was required for a whole day of measurements. The peristaltic pump 372.C (Elpan, Poland) was used with a flow-rate of 15 ml/min. During the
Determination of thallium in soils
(a)
223
peroxide (POCh) were used. The samples of soil were received from the Institute of Soil Science and Cultivation of Plants, Pulawy, Poland together with the determined contents of lead, iron and manganese. Decomposition of soil
Fig. I. The voltammetric flow-injection measuring system (a) with flow-through cell of wall-jet type (b). MFE-mercury film electrode based on epoxy resin impregnated pyrolitic graphite, Ref.-reference electrode (SCE), ptauxiliary electrode (Pt wire).
measurement, circulation of measured solution was used by junction of the outlet from the measuring system with an analysed sample in the beaker. Another flow-injection voltammetric system was used for part of the experiments. This system was the first version of the final system and was characterized by the lower hydrodynamic effectiveness of the cell and, as a result, weaker analytical signal. Reagents
The supporting electrolyte was 0.15M EDTA containing O.lM ascorbic acid (pH 4.5), which was prepared from the analytical grade reagents (POCh). The pH of this solution was adjusted with sodium hydroxide solution (Merck). The carrier solution was 0.05M EDTA solution which was continuously deaerated with the purified nitrogen. The standard solution of Tl(1) was prepared from thallium(I) nitrate. All solutions were prepared in triply distilled water in a fused-silica apparatus. Suprapure nitric and hydrochloric acids (Merck) and the analytical grade hydrofluoric acid and hydrogen
The soil sample (approx. 0.5 g) was transferred to the beaker of a teflon bomb and moistened with water. Hydrofluoric acid (2.5 ml) and 3.5 ml of a mixture of hydrochloric and nitric acids (3: 1) were added. The teflon bomb was closed and put in the drying-oven for 3 hr (135”). The obtained solution was transferred into a teflon beaker and heated on a graphite heater until evaporated. In the final stage of the evaporation, 2 ml of hydrogen peroxide was added in a few portions, for decomposition of residual organic substances originating from soil. The residue was dissolved in 1 ml of hot hydrochloric acid. The solution of ascorbic acid in the amount corresponding to the final concentration of O.lM was added and after a few minutes the corresponding amount of EDTA solution was added. The pH was adjusted to 4.5 and the volume was supplemented to 25 ml with water in the measuring flask. Extraction of soil (slightly modljied procedure of Wolf et al.‘“)
The sample of soil (ca. 5 g) was placed in the conical flask equipped with a reflux condenser and treated with 20 ml of a mixture of hydrochloric and nitric acids (3 : 1) and put aside for 48 hr. Then the sample was heated for 2 hr and filtered. The filter was washed with 2M nitric acid and the filtrates were mixed. The obtained solution was evaporated. In the final stage of evaporation 2.5 ml of hydrogen peroxide were added in a few portions to completely destroy any organic matter. The residue was dissolved in 1 ml of hydrochloric acid and the procedure was continued as for decomposition of soil. Measuring procedure
A portion of the solution (8 ml) was transferred to the beaker and was introduced into the flow-injection system. The solution was directed to the measuring cell and then back to the beaker with the sample. In this way the solution was in circulation. Now the preconcentration was started by application of a preconcentration potential. In all experiments preconcentration was carried out at a potential of -0.850 V with the exception of the experiments in which the
224
ZENONLUWZEWSK~ and
influence of preconcentration potential on the peak height was examined. The preconcentration time was l-3 min depending on the thallium concentration. After the deposition stage was complete the valve was switched off and the analysed solution was replaced by the deaerated carrier solution (0.05M EDTA). Then the flow was stopped and the voltamperogram recorded after 15 set of quiescent time. The evaluation of the concentration of thallium was performed by the method of two additions of standard. The pH remained constant throughout the measuring cycle and so did not require correction. RESULTS AND DISCUSSION
The presence of ascorbic acid in the base electrolyte transforms iron(II1) into iron( which highly improves possibilities for the measurement. However, it is still insufficient for the sensitive determination of thallium because of too high a base-line current. This is visible in Fig. 2(a), which shows the voltamperogram of a solution containing EDTA, ascorbic acid, aluminum, iron(I1) [introduced in the form of iron(III)] and 1OnM thallium. Aluminum and iron are present here in the ratio typical of their
~LODZIMEZRZ
ihLtRZUSK1
contents in the soil. Only a small bulge resulting from thallium is visible on this voltamperogram. Media exchange for pure 0.05M EDTA removes iron(I1) as well as other interferents which do not form amalgams, and in this way radically improves the conditions for the determination of thallium [see Fig. 2(b)]. The correct selection of a deposition potential is very important. On the one hand, a sufficiently negative deposition potential guarantees a stable peak height of thallium (if it is located on the plateau of dependence of the peak height on the deposition potential). On the other hand, with the shifting of potential towards negative direction, the lead signal increases. Thus the selection of deposition potential is the compromise between these two tendencies. This is shown in Figs 3(a) and 3(b). Figure 3(a) shows the dependence of height of the peak of thallium on the deposition potential both in the pure EDTA solution (circles) and in the solution containing additionally iron(I1) and aluminum(II1) in the ratio typical of the solution obtained after decomposition of soiP (crosslets). Iron was introduced in the form of iron(II1) for the control of effectiveness of its reduction. These two dependences are identical and their plateau is located starting from a potential of -0.850 V us. SCE. The discussed
-cl7
-08
-G9
-10
-If
E,IVI
Fig. 2. The voltamperograms with signals of thallium in the presence of iron@) (before medium exchange) (a) and in the absence of iron (after the medium exchange) (6). The base electrolyte for the case (a) whole cycle and for the deposition stage in the case (b) 0.15M EDTA + 0. IM ascorbic acid + 70mM of aluminum(II1) + 20mM of iron(D) (PH 4.5). The base electrolyte for stripping stage in the case (b) 0.05M EDTA @H 4.5). Concentration of thallium: 1OnM. Preconcentration potential: -0900 V vs. SCE; preconcentration time: 180 sec.
Fig. 3. The dependence of the peak height of thallium (a) and lead (b) on the deposition potential and the peak height of lead on its concentration (c) under conditions of the medium exchange. The base electrolyte for the deposition stage: 0.15M EDTA @H 4.5) [Fig. 3(a)-circles] or 0.15M EDTA + 0.1&f ascorbic acid + 70mM of aluminum(II1) + 20mM of iron(I1) t’pH 4.5) (other cases). The base electrolyte for stripping stage (all cases): 0.05M EDTA @H 4.5). Concentration of thallium: (a) lOnM, (b, c) 0; concentration of lead: (a) 0, (b) lo@, (c) variable. Preconcentration potential: (a, b) variable, (c) -0.850 V vs. SCE. Preconcentration time: (a, b and c) 300 sec.
225
Determination of thallium in soils
dependence is shifted toward more negative values in the comparison with similar dependence measured with the use of HMDE.16 The potential peak of thallium is also located at about - 150 mV more negative than when using HMDE (- 0.600 V 11s.SCE). The dependence of the peak height of lead on the deposition potential under these conditions is shown in Fig. 3(b). It is obvious from this picture that the lead peak appears at a potential of - 0.850 V us. SCE which is necessary for the determination of thallium. The dependence of the peak height of lead on the concentration of this metal is shown in Fig. 3(c). This dependence makes possible the evaluation of the positive error of determination of thallium caused by the presence of a certain concentration of lead. This error is negligible for < 10jN lead which corresponds to 100 ppm lead in a 0.5-g soil sample. The masking of the lead signal is caused by the presence of EDTA and the question arises as to whether the excess of free EDTA, i.e., nonbounded with iron, aluminum, etc. is necessary for the effective masking of lead. The dependence of the peak height of lead on the preconcentration potential was examined under the following conditions: pure EDTA solution, a solution with an EDTA to iron ratio of 1: 1 and a solution with an excess of iron vs. EDTA. The results are shown in Fig. 4 and the representative voltamperograms are shown in Fig. 5. The results shown in Fig. 4 were performed with the first version of a measuring system which
a/
/
-06
-07
-08 E. IV)
-09
- IO
Fig. 4. The dependence of the peak height of lead on the preconcentration potential under conditions of absence (a) or in the presence of iron in the sample (b, c). The ratio of iron US.EDTA: (a) 0, (b) 1, (c) 1.1. The base electrolyte for the deposition stage: 0.1 M EDTA + 0.1 M ascorbic acid + O.lmM of lead(D) @H 4.5). The base electrolyte of stripping stage: 0.05M EDTA. Preconcentration time: 180 sec.
l76n-M
I
05pA
,-:
Fig. 5. The effect of excess or deficit of nonbounded EDTA on the tolerance of the system on the presence of lead. The ratio of EDTA us. iron: (u) 7.5 : 1, (b) 1: 1,(c) 1:2. The base electrolyte of deposition stage: 0.15M EDTA + 0.30M ascorbic acid @H 4.5). Concentration of lead (deposition stage): O.OlmM. Concentration of iron (deposition stage): (a) 2OmM, (b) 150mM and (c) 300mM. The base electrolyte of stripping stage: 0.05M EDTA (PH 4.5). Preconcentration potential: -0.850 V; preconcentration time: 900 sec.
was less sensitive and the results were further checked with the system finally used (Fig. 5). The higher concentration of ascorbic acid was used in the last case for more effective reduction of introduced amounts of iron(II1) to iron(I1). It is clearly visible in Fig. 4 and supported in Fig. 5, that if the concentration of iron and aluminum exceeds EDTA concentration lead is no more effectively masked and a huge peak arising from this metal appears. Thus the ratio of the weight of soil sample to the EDTA concentration must be synchronized and an excess of EDTA in the measured solution is necessary. The possible interferences of antimony, bismuth and copper with the determination of thallium were predicted.s For this reason the tolerated excess of these metals was checked. Antimony or bismuth (0.1@4) or 1pM of copper was added to the 1OnM thallium solution. Such a ratio is required because of the reported highest level of these elements in the soil.’ The results of these experiments are shown in Fig. 6. The peaks of considered metals are located at (- 0.240) (- 0.240) and (-0.200) V us. SCE respectively, under the
226
ZENONLUKASZ~WSKI and WUIDZIMERZZJZ~RZUSKI
Fig. 6. The voltamperograms of solution containing thallium in the presence of an excess of antimony (a), copper (b) and bismuth (c). Concentration of thallium: 1OnM. Concentration of antimony or bismuth: lOOnM, concentration of copper: 1pAf. The base electrolyte of deposition stage: O.lSM EDTA + O.lM ascorbic acid @H 4.5). The base electrolyte of stripping stage: O.OSMEDTA. Preconcentration potential: -0.850 V us. SCE, preconcentration time: 6OOsec.
experimental conditions (deposition from 0.1 SM EDTA + O.lM ascorbic acid solution at -0.850 V and stripping from 0.05M EDTA solution). No serious interferences of the investigated metals at such a ratio are observed. Removal of interferences makes the determination of thallium in the real soil samples possible. The recovery and precision of such a determination has been checked. The representative soil sample (No. 126D8437-see Table 1) contained 1.07% iron, 270 ppm manganese and 8.5 ppm lead. It was analysed
according to the described procedure, both alone as well as spiked with 250 ppb thallium. A 213-ppb concentration of thallium was found in the analysed soil (7 independent measurements, S = 30.5 ppb, S, = 14.3%). The spiked thallium was recovered in 94% (6 independent measurements, S = 31.5 ppb, S, = 7.2%). Such results seem to be satisfactory for such a complicated matrix and difficult decomposition procedure. Precision of measurement is satisfactory because it is typical of measurement on such a concentration level. A concentration of 1nM thallium can be comparatively easily determined with 10-l 5 min preconcentration. Hence, the detection limit of 10 ppb (with 0.5 g of soil) can be achieved, and, if necessary, improved by prolonging deposition time. Lead is tolerated up to 100 ppm but higher contents cause a small positive error. The developed method was used for the determination of thallium in several samples of soil. Parallel samples were totally decomposed or extracted with a mixture of nitric and hydrochloric acids according to the procedure described above. The results are given in Table 1, together with the content of iron, manganese and lead in the soil samples. It is visible that the content of thallium in the totally decomposed samples is roughly located between 100 and 350 ppb. The extraction of thallium leads to much lower results, which indicates only partial extraction (21-74%). Thus the extraction is the unproper way for the evaluation of total thallium content in the soil in contrast to the determination of zinc, lead, cadmium and copper in the soil,” where extraction is complete. The contents of thallium seem to be correlated with the content of iron and manganese. Of course, such a conclusion should be supported with much extensive examination.
Table 1. The results of the determination of thallium in different soil samples performed after the total decomposition of the samples and after the extraction of samnles with the mixture of nitric and hydrochloric acids Content of iron, manganese and lead Sample number
Fe, %
Mn,
Pb,
mm
pm
99D8448A 127D8446 26338321B 126D8437 29D8325B 36D8309F
0.23 0.44 0.58 1.07 1.36 2.91
120 210 750 270 400 710
9.0 32 240 8.5 31 29
Thallium content (average) Total decomposition, ppb 97 175 203 213 353 355
Extraction, % ppb 27 28 37 21 150 74 74 35 148 42 176 50
Determination of thallium in soils
The promising results for the determination of thallium in the soil, i.e., sample having silicium, aluminum, iron, titanium, manganese and lead indicate that this element can also be determined in geological samples of similar composition, by means of a very similar procedure. Acknowledgements-This work was supported by Research Program CPBP 01.17. We thank Professor A. KabataPendias of the Institute of Soil Science and Cultivation of Plants, Pulawy, Poland for providing the soil samples with determined contents of iron, manganese and lead and for valuable discussion. We are grateful to Miss Anna Piela for her technical assistance in the measurements. REFERENCES 1. A. Kabata-Pendias and H. Pendias, Trace Elements in Soils and Plants, CRC Press, Boca Raton, Florida, 1984. 2. R. D. Reeves and R. R. Brooks, Trace Element Analysis of Geological Materials, Wiley, New York, 1978. 3. M. Sager and G. Tolg, Mkrochim. Acta, 1982 II, 231. 4. Z. Marczenko, W. Kalowska and M. Mojski, Talanta, 1974, 21, 93. 5. I. Liem, G. Kaiser, M. Sager and G. Tolg, Anal. Chim. Acta, 1984, 158, 179. 6. J. Aznarez, J. Vidal, L. Marco and J. Galban, Euroanalysis VI, Paris 1987. 7. IUPAC Report, Pure Appl. Chem., 1982, 54, 1565. 8. J. E. Bonelli, H. E. Taylor and R. K. Skogerboe, Anal. Chim. Acta, 1980, 118, 243. 9. G. Henze and R. Neeb, Elektrochemische Analyrik, Springer-Verlag, 1986.
227
10. K. Wikiel and Z. Kublik, J. Elcctrwnal. Chem., 1984,
165, 71. 11. R. Neeb and I. Kiehnast, Z. An& C/rem., 1968, 241,
142. 12. J. Dieker and van der Linden, ibid., 1975, 274, 97. 13. V. Gemmer-Colos, I. Kiehnast, J. Trenner and R. Neeb, ibid., 1981, 306, 149. 14. R. G. Dhaneshwar and L. R. Zarapkar, Analyst, 1980, 105, 386. 15. N. You and R. Neeb, Z. AMI. Chem., 1983, 314, 394. 16. A. Ciszewski and Z. Lukaszewski, Talanfa, 1983, 30, 873. 17. I&m, ibid., 1985, 32, 1101. 18. Z. Lukaszewski, A. Ciszewski and A. Saymanski, Chem. Analit. (Warsaw), 1987, 32, 903. 19. A. Ciszewski and Z. Lukaszewski, Tafanta, 1988, 35, 191. 20. F. Wahdat, Z. Lukasaewski and R. Neeb, Anal. Chem., 1990, 33ll, 163. 21. Z. Lukaszewski, F. Wahdat and R. Neeb, ibid., 1990, 337, 885. 22. T. Frank and R. Neeb, ibid., 1987, 327, 670. 23. A. Chattopadhyay and R. E. Jervis, Anal. Chem., 1974, 45, 1630. 24. A. M. Ure and J. R. Bacon, Analysr, 1978, 103, 807. 25. I. C. Smith and B. L. Carson, Trace Metals in rhe Environment, Vol. 1, Ann Arbor, Michigan, 1977. 26. Z. Lukasaewski and W. Zembrzuski, ElectroFinnAnalysis, Turku, 1988. 27. Z. Lukaszewski and W. Zembrzuski, I lth International Symposium on Microchemical Techniques, Wiesbaden, 1989; Z. Anal. Chem., 1989,334, 624. 28. A. Wolf, P. Schrammel, G. Lill and H. Hohn, ibid., 1984, 317, 512.