- Email: [email protected]
The quality of our drinking water: Aluminium determination with an acoustic wave sensor
a n a l y t i c a c h i m i c a a c t a 6 1 7 ( 2 0 0 8 ) 162–166
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/aca
The quality of our drinking water: Aluminium determination with an acoustic wave sensor Marta I.S. Ver´ıssimo, M. Teresa S.R. Gomes ∗ CESAM & Department of Chemistry, Campus de Santiago, University of Aveiro, 3810-193 Aveiro, Portugal
a r t i c l e
i n f o
a b s t r a c t
Article history:
A new methodology based on an inexpensive aluminium acoustic wave sensor is presented.
Received 2 November 2007
Although the aluminium sensor has already been reported, and the composition of the
Accepted 23 December 2007
selective membrane is known, the low detection limits required for the analysis of drinking
Published on line 8 January 2008
water, demanded the inclusion of a preconcentration stage, as well as an optimization of the sensor. The necessary coating amount was established, as well as the best preconcentration
Keywords:
protocol, in terms of oxidation of organic matter and aluminium elution from the Chelex-
Bulk acoustic wave sensor
100.
Aluminium Drinking water
The methodology developed with the acoustic wave sensor allowed aluminium quantitation above 0.07 mg L−1 . Several water samples from Portugal were analysed using the acoustic wave sensor, as well as by UV–vis spectrophotometry. Results obtained with both methodologies were not statistically different (˛ = 0.05), both in terms of accuracy and precision. This new methodology proved to be adequate for aluminium quantitation in drinking water and showed to be faster and less reagent consuming than the UV spectrophotometric methodology. © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
Interest and concern about aluminium has considerably increased in recent years, due to the knowledge about the potential toxic effects of this element. Although there are no reported cases of acute aluminium poisoning of healthy individuals exposed to normal levels of aluminium, several studies have been published relating aluminium exposure and agerelated neurological disorders, such as Alzheimer’s disease [1]. Some authors even regard aluminium as the root cause of Alzheimer’s, while others believe that the cause lies elsewhere, and that aluminium is a simple spectator. Putting the controversy aside, aluminium has for long been known as a neurotoxic agent [2]. Encephalopathy, anemia, osteomalacic osteodistrophy and cardiotoxicity are disorders related to aluminium intoxication in patients who dialyzed at home [3].
∗
Corresponding author. Tel.: +351 234370722; fax: +351 234370084. E-mail address: [email protected] (M.T.S.R. Gomes). 0003-2670/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2007.12.034
Most experts agree that high levels of aluminium in dialysis fluids and in medications are responsible for dementia, and that controlling these levels of aluminium can significantly reduce the incidence of this disease. Aluminium has also been responsible for causing oxidative stress within brain tissue [4], for having a direct effect on hematopoiesis (the development of blood cells), and for inducing microcytic anemia (resulting of hemoglobin synthesis failure or insufficiency), among other diseases. All these findings cause alarming concern in public health, demanding accurate determination of aluminium. Although most of our daily aluminium intake comes from food, aluminium in food appears to be bound to other substances and it is thus present in a form that cannot be absorbed into the bloodstream. In contrast, research has shown that aluminium from drinking water can be absorbed
a n a l y t i c a c h i m i c a a c t a 6 1 7 ( 2 0 0 8 ) 162–166
to some extent both by animals and by humans. In fact, aluminium is one of the trace inorganic metals present in drinking water, in the low g L−1 range. In addition to the naturally occurring aluminium in raw waters, the use of aluminium-based coagulants, especially Al2 (SO4 )3 , often leads to an increase in the aluminium concentrations of the treated waters. Aluminium sulphate is frequently employed in water treatment processes for removing dissolved and suspended matter from raw waters, and a small faction of this hydrolyzed salt remains in solution. The maximum permissible concentration of aluminium in drinking water in the European Economic Community is fixed at 0.2 mg L−1 [5]. Also the World Health Organization (WHO) has proposed a guideline value of 0.2 mg L−1 , not based on any assessment of risks to health, but as a compromise between the need to limit the use of Al salts in water treatment, and the effective discoloration of distributed water. As very low detection limits are needed, the analysis of aluminium in water is difficult. Atomic absorption spectrometry with electrothermal atomization is the technique of choice to determine the low levels of aluminium in hemodialysis solutions and diluting water, although flame atomic absorption spectrometry (AAS), plasma inductively coupled spectrometry (ICP) and UV–vis spectrophotometry can be used, if combined with a preconcentration step. Among the preconcentration methods found in literature, the ion-exchange resins are the most suited for trace analysis [6]. Chelex 100 is often used for metal preconcentration [7–9], and possesses a high selectivity for polyvalent metal ions, without any retention of alkali and alkaline earth elements [10]. Besides, Chelex 100 is such a strong sorbing resin, that is able to sorb the metal ion combined in strong complexes. Another advantage for preconcentration of aluminium using Chelex 100 is the small volume of HNO3 used for elution, which allows a high concentration factor and low contaminations. With respect to aluminium speciation in drinking water, Srinivasan et al. [5] reported that total aluminium is the sum of suspended, colloidal and monomeric forms of aluminium. Therefore, UV-irradiation appears to be an essential step in the determination of total aluminium, as it removes the metal from colloids and complexes, by oxidizing organic matter [11], improving the retention efficiency of the Chelex 100 [12]. According to Golimowski and Golimowska [13] the addition of a strong oxidant, persulfate, to water samples, will accelerate the photooxidation process, allowing to oxidize nearly all organics to CO2 . All the instruments used in the analysis, especially for ICP and AAS, are expensive, and new methodologies combining preconcentration methodologies and inexpensive but reliable aluminium sensors, based on acoustic wave devices, can be attractive alternatives. An acoustic wave sensor for aluminium was already reported [14]. However, the analytical range for the sensor, from 2.7 to 7.2 mg L−1 , is not adequate for this analysis. The limit of detection is going to be improved by increasing the coating amount and by the implementation of a preconcentration methodology. Water from different sources will be analysed and the results compared with the ones obtained by UV–vis spec-
163
trophotometry. The new methodology will be evaluated in terms of its ability to check whether the aluminium level was below the maximum permitted value in the European Economic Community, fixed at 0.2 mg L−1 , the time of analysis, accuracy of the quantitation results, and cost, which will include not only the instruments, but also the reagents consumed.
2.
Experimental
2.1.
Reagents
The complexing resin Chelex 100 (Na-form, 50-100 mesh) was obtained from Sigma–Aldrich (C-7901). ¨ 30702) and ammonium Nitric acid was 69% (Riedel deHaen hydroxide was 25% (Panreac 131129). Besides the ionophore, the coating membrane was prepared with polyvinyl chloride—PVC (Fluka 81388), onitrophenyloctyl ether—NPOE (Fluka 73732), and potassium tetrakis (p-chlorophenyl) borate—KTpClPB (Fluka 60591). The ionophore for aluminium, bis(5-phenylazo salicylaldehyde) 2,3-naphatene diimine, was synthesized according to the procedure described by Abbaspour et al. [15] and Khandar and Rezvani [16]. For the conventional UV–vis methodology, sulphuric acid 95–97% (Fluka 84720), ascorbic acid p.a. (Panreac 131013), sodium acetate anhydrous (Riedel-deHaen 32319) and acetic acid (Panreac 131008.1212) were used. The indicator was Eriochrome cyanine R (Sigma E2505). For organic matter oxidation potassium persulfate p.a. (Merck 5091) was used. Aluminium standards were prepared with aluminium nitrate standard solution for atomic spectroscopy (BDH Chemicals 14031). Nitrogen was Alphagaz from “Arliquido”.
2.2.
Apparatus
The batch photo-oxidation apparatus consisted of a circular array of quartz sample tubes held around a 1000 W mercury lamp. Samples were fan-cooled to maintain operating temperature constant. The analysis of aluminium in preconcentrated water samples was performed by flow injection analysis (FIA), using a coated 9 MHz piezoelectric quartz crystal. ¨ Delta 10 BM) was used to coat the quartz A spin coater (Suss crystal. Fig. 1 shows the FIA methodology used, where Milli-Q water carries the sample into the crystal cell. Polyethylene tubes of 0.8 mm internal diameter were used in the flow system. Experimental setup has been described elsewhere [17]. The oscillator was home-made and the frequency of oscillation of the crystal was monitored with a Counter/Timer Device PXI 6608 from National Instruments, and recorded on a PC, with data acquisition software written in Lab View. Aliquots of the same samples were also analysed by the conventional method, using UV–vis spectroscopy, in a scanning spectrophotometer Shimadzu, UV-2101 PC, using a 1 cm path quartz cell.
164
a n a l y t i c a c h i m i c a a c t a 6 1 7 ( 2 0 0 8 ) 162–166
Fig. 1 – Experimental layout for quartz crystal methodology: (A) pressure regulator, (B) Milli-Q water, (C) injection port, (D) crystal cell, (E) oscillator, (F) counter/timer device PXI 6608, (G) personal computer, (H) waste and (I) nitrogen.
3.
Procedure
3.1.
UV-irradiations
The quartz tubes for UV-irradiations were cleaned by washing with concentrated HNO3 for several hours, and then rinsed with Milli-Q water. 10 mL of a potassium persulfate saturate solution was added to each 1.0 L of water sample. Initially, several irradiation times were tested for the same water sample.
3.2.
Aluminium preconcentration
In order to preserve the original pH of the solution during sorption, the sorbing resin was in the ammonium form [18]. The conversion of the resin to the ammonium form was achieved by equilibration with NH4 OH 1 M, after which the resin was rinsed with Milli-Q water [19]. All the determinations were performed by batch equilibration of 1.0 L of sample with 1 g of the chelating resin [19,20]. The equilibration was made at room temperature and under stirring, for 24 h. The solid was separated by filtration and washed with 50 mL Milli-Q water. The metal sorbed on the resin was in contact with 10 + 10 mL of 1 M HNO3 , for 24 h. The solution was diluted to 25.0 or 50.0 mL with Milli-Q water and buffered to pH 6.0 before the analysis.
3.3.
Aluminium quantification
All labwares, made of high-density polyethylene, was kept in 1:1 nitric acid for at least 48 h, and rinsed with Milli-Q water just before use. The aluminium-sensitive membrane used to coat the piezoelectric quartz crystal was prepared as previously reported elsewhere [14]. The piezoelectric quartz crystal was spin-coated on one of the faces with the aluminium-sensitive membrane and dried. Frequency decrease due to coating was computed. The procedure was repeated until the desired frequency decrease was obtained. In order to perform the analysis, the piezoelectric-coated quartz crystal, was inserted into the cell and the coated face was under a constant flow of Milli-Q water. Fixed quantities of 0.50 mL of aluminium standard solutions and samples were
injected. The differences in the frequency of the crystal before injection, while Milli-Q water was flowing through the cell, and the minimum frequency observed after each sample injection, were recorded. Baseline stability and complete recover from previous experiment were assured, and no standard or sample was injected before a constant reading, equal to the baseline frequency, was attained. The conventional UV–vis methodology, also used to quantify the aluminium, was based on the principle that aluminium solutions, buffered to a pH of 6.0, produced, with Eriochrome cyanine R dye, a red to pink complex with a maximum absorption at 535 nm. Absorption at 535 nm was proportional to the amount of aluminium present in the sample, although the maximum absorption occurs after 15 min and starts decreasing [21]. Maximum absorption was registered for both standards and samples. Whenever necessary, the samples were diluted to a concentration as close as possible to the centroid of the linear calibration curve.
4.
Results and discussion
4.1.
Optimization of the analytical methodologies
The aluminium present in 1.0 L of the water to be analysed was concentrated in a total volume of 25.0 or 50.0 mL. As already said, the sample needed to be oxidized before the metal was sorbed on Chelex 100 and eluted with nitric acid. The efficiency of the oxidation depends upon the time under UV light and the efficiency of the elution with a total quantity of 20 mL of nitric acid, depends on the procedure used. Standard solutions of 0.20 mg L−1 of Al were sorbed in the Chelex resin, following the described procedure, eluted with nitric acid and analysed by UV–vis spectrophotometry. The elution with nitric acid was done in one, two or four steps. The recovery factor [22] of the preconcentration of aluminium in the sorbing resin, Chelex 100, was calculated and is shown in Table 1. The highest recovery factor obtained (99.9%) was achieved with an elution in two steps: a first elution with 10 mL of nitric acid followed by a second elution with another 10 mL portion of nitric acid. Degradation of organic matter present in the water samples was attempted by UV light, addition of persulfate [13], or both. Table 2 shows the results for aluminium found in Aveiro tap water, after oxidation and preconcentration with Chelex 100. Time under UV light was also varied. As can be seen, aluminium detected increased after persulfate addition as well as after UV irradiation. 30 min were found to be enough for UV light irradiation.
Table 1 – Recovery factor for preconcentration of 0.20 mg L−1 Al solutions Volume of HNO3 (mL) 20 10 + 10 5+5+5+5
Recovery factor (%) 97.6 ± 0.9 99.9 ± 0.9 99.2 ± 0.9
a n a l y t i c a c h i m i c a a c t a 6 1 7 ( 2 0 0 8 ) 162–166
Table 2 – Aluminium content (mg L−1 ) in Aveiro water after different pretreatment and preconcentration on Chelex 100 resin UV-irradiation time (min) 0 30 60 90 a
Without persulfate [Al] mg L−1
With persulfatea [Al] mg L−1
<0.04 <0.04 <0.04 <0.04
<0.04 0.04 ± 0.01 0.04 ± 0.01 0.04 ± 0.01
Mean values ± the 95% confidence limit (n = 5).
Limit of detection and sensitivity depend on the coating amount, which must not exceed the limit at which vibration is impaired, but must be high enough to assure the quantitation of the quantities present in the concentrated samples. A 9 MHz piezoelectric quartz crystal was spin-coated with a solution of the specific membrane for aluminium previously reported [14]. Solution was applied until an homogeneous membrane, which caused a frequency decrease at least of 14.5 kHz after drying, was obtained. Fig. 2 shows a typical frequency decrease of the piezoelectric quartz crystal, when 0.50 mL of a preconcentrated water sample was injected into the FIA system. When the aluminium ions start to interact with the crystal membrane, the frequency decrease, until a minimum is reached, and then start to increase, while the Milli-Q water flow carries the aluminium ions to waste. Complete recovery of the sensor was accomplished 2 min after the sample injection. A linear calibration line f = 3.2[Al3+ ] + 19.2, where f is expressed in Hz and Al3+ in mg L−1 , was obtained in the range between 1 and 14 mg L−1 of aluminium. The quantification limit for the quartz crystal methodology with the Chelex preconcentration was calculated to be 0.07 mg L−1 , below the guideline limit for aluminium in drinking water. This new methodology is therefore suitable to check if the water is according to the legislation or not. For the UV–vis methodology, a linear calibration line of Abs = 2.8194 [Al3+ ] − 0.0247 was obtained, and the method including the Chelex preconcentration presents a quantification limit of 0.04 mg L−1 .
Fig. 2 – Frequency of the coated crystal observed during the injection of 0.50 mL of a water sample into the FIA system.
4.2.
165
Results of the analysis
Both methodologies were applied to water samples from domestic supply networks, public sources or private wells, from several different places in Portugal, marked in Fig. 3. Table 3 shows aluminium determined by the new methodology with the coated piezoelectric quartz crystal, and with the conventional methodology of UV–vis spectrophotometry. Five replicates of each sample have been analysed. As we can see from Table 3, all water samples from domestic network supplies () showed very low aluminium values, under 0.2 mg L−1 . A large number of those samples presented aluminium values below the quantification limit. These results show that, although aluminium sulphate is used in water treatment to flocculate suspended particles, the treated water respects legislation. In untreated water samples from natural sources, e.g. public fountains () or private wells (䊉), it is possible to find some samples with aluminium content higher than the guideline value, respectively one sample from “Caminha”, two from “Barcelos” and one from “Santa Maria da Feira”. Curiously, shortly after the analysis, a warning reportage on the high aluminium contamination of the public fountain, in “Caminha”, was printed in a daily national newspaper. Observing the location of the fountains and wells with high levels of aluminium on Fig. 3, it can be concluded that all of them are located in the northest region of the country. Soils in the north of Portugal are mainly granitic, while calcareous soils are encountered in the central region. It can be postulated the aluminium source to be linked to the granitic soil
Fig. 3 – Portugal map with the sampling spots marked (—water from the domestic supply system network, —water from public fountains, 䊉—water from private wells).
166
a n a l y t i c a c h i m i c a a c t a 6 1 7 ( 2 0 0 8 ) 162–166
Table 3 – Al content (mg L−1 ) of water samples from different places in Portugal: results obtained with both methodologies Water samples
Aveiro Aveiro 䊉 Aveiro 䊉 Aveiro Barcelos 䊉 Barcelos Barcelos 䊉 Caminha Coimbra Esponsende Gouveia Leiria 䊉 Leiria Ovar 䊉 Sta. Maria da Feira 䊉 Tomar 䊉 Tomar Milli-Q water 1 Milli-Q water 2
Quartz crystal methodologya [Al] mg L−1 <0.07 <0.07 <0.07 <0.07 0.17 ± 0.01 0.21 ± 0.03 0.24 ± 0.04 0.26 ± 0.03 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 0.74 ± 0.02 <0.07 0.09 ± 0.04 <0.07 <0.07
UV–vis spectrometrya [Al] mg L−1 <0.04 <0.04 <0.04 <0.04 0.17 ± 0.01 0.21 ± 0.01 0.24 ± 0.01 0.27 ± 0.01 <0.04 <0.04 <0.04 <0.04 <0.04 <0.04 0.75 ± 0.01 <0.04 0.09 ± 0.01 <0.04 <0.04
(): water from the domestic supply system network; (): water from public fountains; (䊉): water from private wells. a
Mean values ± the 95% confidence limit (n = 5).
[23]. However, it is not the purpose of this work to explain why high aluminium concentration are found in some places, but to demonstrate that this new methodology, based on piezoelectric quartz sensors, is reliable and can be used to detect water sources not in accordance with the aluminium guideline value of 0.2 mg L−1 . The values in Table 3, obtained with the two methodologies, were statistically compared, and the difference in the mean values between the two groups was not great enough to exclude the possibility that the difference was due to random sampling variability. A test F was running for the comparison of standard deviations, and no significant differences in the variances of the two methods were found (˛ = 0.05). Besides being analytically adequate for the purpose, the new methodology is faster: 2 min of analysis versus the 15 min needed for colour development in UV spectrophotometry, to which colour development sample preparation time must be added. The equipment needed in the piezoelectric quartz crystal methodology is inexpensive, as data acquisition is surplus. An home-made oscillator was used, and an inexpensive frequency meter with 1 Hz resolution is all that is required. The quartz crystal is not an expensive item, and the aluminium ionophore was synthesized starting from reagents easily available.
5.
Conclusion
As a conclusion, it can be said that this new methodology is less reagent, sample and time consuming than UV–vis spectrophotometric methodology, and that it needs
inexpensive instruments. However, the new methodology is capable of quantifying aluminium in concentrations as low as 0.07 mg L−1 , and is therefore adequate to test if waters conform to legislation, and possess aluminium concentrations below the 0.2 mg L−1 . With respect to aluminium contamination found, results are consistent with the drinking water from domestic network supply being safe, and point to the need to test water from the well in the backyard, or from the fountain next to our house. Comparing the results obtained by both methodologies, no statistical significant differences (˛ = 0.05) were found, both in the average of the replicate analysis, or in the precision of the results.
Acknowledgments This project was financed by the Portuguese Foundation for Science and Technology (FCT), POCTI, FEDER and CESAM.
references
[1] A. Campbell, A. Becaria, D.K. Lahiri, K. Sharman, S.C. Bondy, J. Neurosci. Res. 75 (2004) 565. [2] P. Zatta, R. Lucchini, S.J. Van Rensburg, A. Taylor, Brain Res. Bull. 62 (2003) 15. [3] H.L. Elliott, F. Dryburgh, G.S. Fell, S. Sabet, A.I. MacDougall, Br. Med. J. 29 (1978) 1101. [4] A. Becaria, D.K. Lahiri, S.C. Bondy, D.M. Chen, A. Hamadeh, H. Li, R. Taylor, A. Campbell, J. Neuroimmunol. 176 (2006) 16. [5] P.T. Srinivasan, T. Viraraghavan, K.S. Subramanian, Water SA. 25 (1999) 47. [6] B.S. Garg, R.K. Sharma, N. Bhojak, S. Mittal, Microchem. J. 61 (1999) 94. [7] M.G. Pereira, M.A.Z. Arruda, Microchim. Acta 141 (2003) 115. ´ [8] M.S. Jimenez, R. Velarte, J.R. Castillo, Spectrochim. Acta B 57 (2002) 391. [9] K.C. Bowles, S.C. Apte, G.E. Batley, L.T. Hales, N.J. Rogers, Anal. Chim. Acta 558 (2006) 237. [10] BioRad Laboratories Guide to ion exchange, no. 140-9997, 1996, pp. 56. [11] D.E.W. Hofbauer, S.A. Andrews, Water Sci. Technol.: Water Supply 4 (2004) 41. ´ [12] C. Gueguen, C. Belin, B.A. Thomas, F. Monna, P.-Y. Favarger, J. Dominik, Anal. Chim. Acta 386 (1999) 155. [13] J. Golimowski, K. Golimowska, Anal. Chim. Acta 325 (1996) 111. [14] M.I.S. Ver´ıssimo, J.A.B.P. Oliveira, M.T.S.R. Gomes, Sens. Actuators B 118 (2006) 192. [15] A. Abbaspour, A.R. Esmaeilbeing, A.A. Jarrahpour, B. Khajeh, R. Kia, Talanta 58 (2002) 397. [16] A.A. Khandar, Z. Rezvani, Polyhedron 18 (1999) 129. [17] M.T.S.R. Gomes, K.S. Tavares, J.A.B.P. Oliveira, Analyst 125 (2000) 1983. [18] M. Pesavento, R. Biesuz, G. Alberti, M. Sturini, J. Sep. Sci. 26 (2003) 381. [19] G. Alberti, G. D’Agostino, G. Palazzo, R. Biesuz, M. Pesavento, J. Inorg. Biochem. 99 (2005) 1779. [20] M. Pesavento, R. Biesuz, React. Funct. Polym. 36 (1998) 135. [21] M.S.S. Pereira, B.F. Reis, Quim. Nova. 25 (2002) 931. [22] D.T. Burns, K. Danzer, A. Townshend, Pure Appl. Chem. 74 (2002) 2201. ´ [23] A. Ferreira, M.M. Inacio, P. Morgado, M.J. Batista, L. Ferreira, V. Pereira, M.S. Pinto, Appl. Geochem. 16 (2001) 1323.
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/aca
The quality of our drinking water: Aluminium determination with an acoustic wave sensor Marta I.S. Ver´ıssimo, M. Teresa S.R. Gomes ∗ CESAM & Department of Chemistry, Campus de Santiago, University of Aveiro, 3810-193 Aveiro, Portugal
a r t i c l e
i n f o
a b s t r a c t
Article history:
A new methodology based on an inexpensive aluminium acoustic wave sensor is presented.
Received 2 November 2007
Although the aluminium sensor has already been reported, and the composition of the
Accepted 23 December 2007
selective membrane is known, the low detection limits required for the analysis of drinking
Published on line 8 January 2008
water, demanded the inclusion of a preconcentration stage, as well as an optimization of the sensor. The necessary coating amount was established, as well as the best preconcentration
Keywords:
protocol, in terms of oxidation of organic matter and aluminium elution from the Chelex-
Bulk acoustic wave sensor
100.
Aluminium Drinking water
The methodology developed with the acoustic wave sensor allowed aluminium quantitation above 0.07 mg L−1 . Several water samples from Portugal were analysed using the acoustic wave sensor, as well as by UV–vis spectrophotometry. Results obtained with both methodologies were not statistically different (˛ = 0.05), both in terms of accuracy and precision. This new methodology proved to be adequate for aluminium quantitation in drinking water and showed to be faster and less reagent consuming than the UV spectrophotometric methodology. © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
Interest and concern about aluminium has considerably increased in recent years, due to the knowledge about the potential toxic effects of this element. Although there are no reported cases of acute aluminium poisoning of healthy individuals exposed to normal levels of aluminium, several studies have been published relating aluminium exposure and agerelated neurological disorders, such as Alzheimer’s disease [1]. Some authors even regard aluminium as the root cause of Alzheimer’s, while others believe that the cause lies elsewhere, and that aluminium is a simple spectator. Putting the controversy aside, aluminium has for long been known as a neurotoxic agent [2]. Encephalopathy, anemia, osteomalacic osteodistrophy and cardiotoxicity are disorders related to aluminium intoxication in patients who dialyzed at home [3].
∗
Corresponding author. Tel.: +351 234370722; fax: +351 234370084. E-mail address: [email protected] (M.T.S.R. Gomes). 0003-2670/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2007.12.034
Most experts agree that high levels of aluminium in dialysis fluids and in medications are responsible for dementia, and that controlling these levels of aluminium can significantly reduce the incidence of this disease. Aluminium has also been responsible for causing oxidative stress within brain tissue [4], for having a direct effect on hematopoiesis (the development of blood cells), and for inducing microcytic anemia (resulting of hemoglobin synthesis failure or insufficiency), among other diseases. All these findings cause alarming concern in public health, demanding accurate determination of aluminium. Although most of our daily aluminium intake comes from food, aluminium in food appears to be bound to other substances and it is thus present in a form that cannot be absorbed into the bloodstream. In contrast, research has shown that aluminium from drinking water can be absorbed
a n a l y t i c a c h i m i c a a c t a 6 1 7 ( 2 0 0 8 ) 162–166
to some extent both by animals and by humans. In fact, aluminium is one of the trace inorganic metals present in drinking water, in the low g L−1 range. In addition to the naturally occurring aluminium in raw waters, the use of aluminium-based coagulants, especially Al2 (SO4 )3 , often leads to an increase in the aluminium concentrations of the treated waters. Aluminium sulphate is frequently employed in water treatment processes for removing dissolved and suspended matter from raw waters, and a small faction of this hydrolyzed salt remains in solution. The maximum permissible concentration of aluminium in drinking water in the European Economic Community is fixed at 0.2 mg L−1 [5]. Also the World Health Organization (WHO) has proposed a guideline value of 0.2 mg L−1 , not based on any assessment of risks to health, but as a compromise between the need to limit the use of Al salts in water treatment, and the effective discoloration of distributed water. As very low detection limits are needed, the analysis of aluminium in water is difficult. Atomic absorption spectrometry with electrothermal atomization is the technique of choice to determine the low levels of aluminium in hemodialysis solutions and diluting water, although flame atomic absorption spectrometry (AAS), plasma inductively coupled spectrometry (ICP) and UV–vis spectrophotometry can be used, if combined with a preconcentration step. Among the preconcentration methods found in literature, the ion-exchange resins are the most suited for trace analysis [6]. Chelex 100 is often used for metal preconcentration [7–9], and possesses a high selectivity for polyvalent metal ions, without any retention of alkali and alkaline earth elements [10]. Besides, Chelex 100 is such a strong sorbing resin, that is able to sorb the metal ion combined in strong complexes. Another advantage for preconcentration of aluminium using Chelex 100 is the small volume of HNO3 used for elution, which allows a high concentration factor and low contaminations. With respect to aluminium speciation in drinking water, Srinivasan et al. [5] reported that total aluminium is the sum of suspended, colloidal and monomeric forms of aluminium. Therefore, UV-irradiation appears to be an essential step in the determination of total aluminium, as it removes the metal from colloids and complexes, by oxidizing organic matter [11], improving the retention efficiency of the Chelex 100 [12]. According to Golimowski and Golimowska [13] the addition of a strong oxidant, persulfate, to water samples, will accelerate the photooxidation process, allowing to oxidize nearly all organics to CO2 . All the instruments used in the analysis, especially for ICP and AAS, are expensive, and new methodologies combining preconcentration methodologies and inexpensive but reliable aluminium sensors, based on acoustic wave devices, can be attractive alternatives. An acoustic wave sensor for aluminium was already reported [14]. However, the analytical range for the sensor, from 2.7 to 7.2 mg L−1 , is not adequate for this analysis. The limit of detection is going to be improved by increasing the coating amount and by the implementation of a preconcentration methodology. Water from different sources will be analysed and the results compared with the ones obtained by UV–vis spec-
163
trophotometry. The new methodology will be evaluated in terms of its ability to check whether the aluminium level was below the maximum permitted value in the European Economic Community, fixed at 0.2 mg L−1 , the time of analysis, accuracy of the quantitation results, and cost, which will include not only the instruments, but also the reagents consumed.
2.
Experimental
2.1.
Reagents
The complexing resin Chelex 100 (Na-form, 50-100 mesh) was obtained from Sigma–Aldrich (C-7901). ¨ 30702) and ammonium Nitric acid was 69% (Riedel deHaen hydroxide was 25% (Panreac 131129). Besides the ionophore, the coating membrane was prepared with polyvinyl chloride—PVC (Fluka 81388), onitrophenyloctyl ether—NPOE (Fluka 73732), and potassium tetrakis (p-chlorophenyl) borate—KTpClPB (Fluka 60591). The ionophore for aluminium, bis(5-phenylazo salicylaldehyde) 2,3-naphatene diimine, was synthesized according to the procedure described by Abbaspour et al. [15] and Khandar and Rezvani [16]. For the conventional UV–vis methodology, sulphuric acid 95–97% (Fluka 84720), ascorbic acid p.a. (Panreac 131013), sodium acetate anhydrous (Riedel-deHaen 32319) and acetic acid (Panreac 131008.1212) were used. The indicator was Eriochrome cyanine R (Sigma E2505). For organic matter oxidation potassium persulfate p.a. (Merck 5091) was used. Aluminium standards were prepared with aluminium nitrate standard solution for atomic spectroscopy (BDH Chemicals 14031). Nitrogen was Alphagaz from “Arliquido”.
2.2.
Apparatus
The batch photo-oxidation apparatus consisted of a circular array of quartz sample tubes held around a 1000 W mercury lamp. Samples were fan-cooled to maintain operating temperature constant. The analysis of aluminium in preconcentrated water samples was performed by flow injection analysis (FIA), using a coated 9 MHz piezoelectric quartz crystal. ¨ Delta 10 BM) was used to coat the quartz A spin coater (Suss crystal. Fig. 1 shows the FIA methodology used, where Milli-Q water carries the sample into the crystal cell. Polyethylene tubes of 0.8 mm internal diameter were used in the flow system. Experimental setup has been described elsewhere [17]. The oscillator was home-made and the frequency of oscillation of the crystal was monitored with a Counter/Timer Device PXI 6608 from National Instruments, and recorded on a PC, with data acquisition software written in Lab View. Aliquots of the same samples were also analysed by the conventional method, using UV–vis spectroscopy, in a scanning spectrophotometer Shimadzu, UV-2101 PC, using a 1 cm path quartz cell.
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Fig. 1 – Experimental layout for quartz crystal methodology: (A) pressure regulator, (B) Milli-Q water, (C) injection port, (D) crystal cell, (E) oscillator, (F) counter/timer device PXI 6608, (G) personal computer, (H) waste and (I) nitrogen.
3.
Procedure
3.1.
UV-irradiations
The quartz tubes for UV-irradiations were cleaned by washing with concentrated HNO3 for several hours, and then rinsed with Milli-Q water. 10 mL of a potassium persulfate saturate solution was added to each 1.0 L of water sample. Initially, several irradiation times were tested for the same water sample.
3.2.
Aluminium preconcentration
In order to preserve the original pH of the solution during sorption, the sorbing resin was in the ammonium form [18]. The conversion of the resin to the ammonium form was achieved by equilibration with NH4 OH 1 M, after which the resin was rinsed with Milli-Q water [19]. All the determinations were performed by batch equilibration of 1.0 L of sample with 1 g of the chelating resin [19,20]. The equilibration was made at room temperature and under stirring, for 24 h. The solid was separated by filtration and washed with 50 mL Milli-Q water. The metal sorbed on the resin was in contact with 10 + 10 mL of 1 M HNO3 , for 24 h. The solution was diluted to 25.0 or 50.0 mL with Milli-Q water and buffered to pH 6.0 before the analysis.
3.3.
Aluminium quantification
All labwares, made of high-density polyethylene, was kept in 1:1 nitric acid for at least 48 h, and rinsed with Milli-Q water just before use. The aluminium-sensitive membrane used to coat the piezoelectric quartz crystal was prepared as previously reported elsewhere [14]. The piezoelectric quartz crystal was spin-coated on one of the faces with the aluminium-sensitive membrane and dried. Frequency decrease due to coating was computed. The procedure was repeated until the desired frequency decrease was obtained. In order to perform the analysis, the piezoelectric-coated quartz crystal, was inserted into the cell and the coated face was under a constant flow of Milli-Q water. Fixed quantities of 0.50 mL of aluminium standard solutions and samples were
injected. The differences in the frequency of the crystal before injection, while Milli-Q water was flowing through the cell, and the minimum frequency observed after each sample injection, were recorded. Baseline stability and complete recover from previous experiment were assured, and no standard or sample was injected before a constant reading, equal to the baseline frequency, was attained. The conventional UV–vis methodology, also used to quantify the aluminium, was based on the principle that aluminium solutions, buffered to a pH of 6.0, produced, with Eriochrome cyanine R dye, a red to pink complex with a maximum absorption at 535 nm. Absorption at 535 nm was proportional to the amount of aluminium present in the sample, although the maximum absorption occurs after 15 min and starts decreasing [21]. Maximum absorption was registered for both standards and samples. Whenever necessary, the samples were diluted to a concentration as close as possible to the centroid of the linear calibration curve.
4.
Results and discussion
4.1.
Optimization of the analytical methodologies
The aluminium present in 1.0 L of the water to be analysed was concentrated in a total volume of 25.0 or 50.0 mL. As already said, the sample needed to be oxidized before the metal was sorbed on Chelex 100 and eluted with nitric acid. The efficiency of the oxidation depends upon the time under UV light and the efficiency of the elution with a total quantity of 20 mL of nitric acid, depends on the procedure used. Standard solutions of 0.20 mg L−1 of Al were sorbed in the Chelex resin, following the described procedure, eluted with nitric acid and analysed by UV–vis spectrophotometry. The elution with nitric acid was done in one, two or four steps. The recovery factor [22] of the preconcentration of aluminium in the sorbing resin, Chelex 100, was calculated and is shown in Table 1. The highest recovery factor obtained (99.9%) was achieved with an elution in two steps: a first elution with 10 mL of nitric acid followed by a second elution with another 10 mL portion of nitric acid. Degradation of organic matter present in the water samples was attempted by UV light, addition of persulfate [13], or both. Table 2 shows the results for aluminium found in Aveiro tap water, after oxidation and preconcentration with Chelex 100. Time under UV light was also varied. As can be seen, aluminium detected increased after persulfate addition as well as after UV irradiation. 30 min were found to be enough for UV light irradiation.
Table 1 – Recovery factor for preconcentration of 0.20 mg L−1 Al solutions Volume of HNO3 (mL) 20 10 + 10 5+5+5+5
Recovery factor (%) 97.6 ± 0.9 99.9 ± 0.9 99.2 ± 0.9
a n a l y t i c a c h i m i c a a c t a 6 1 7 ( 2 0 0 8 ) 162–166
Table 2 – Aluminium content (mg L−1 ) in Aveiro water after different pretreatment and preconcentration on Chelex 100 resin UV-irradiation time (min) 0 30 60 90 a
Without persulfate [Al] mg L−1
With persulfatea [Al] mg L−1
<0.04 <0.04 <0.04 <0.04
<0.04 0.04 ± 0.01 0.04 ± 0.01 0.04 ± 0.01
Mean values ± the 95% confidence limit (n = 5).
Limit of detection and sensitivity depend on the coating amount, which must not exceed the limit at which vibration is impaired, but must be high enough to assure the quantitation of the quantities present in the concentrated samples. A 9 MHz piezoelectric quartz crystal was spin-coated with a solution of the specific membrane for aluminium previously reported [14]. Solution was applied until an homogeneous membrane, which caused a frequency decrease at least of 14.5 kHz after drying, was obtained. Fig. 2 shows a typical frequency decrease of the piezoelectric quartz crystal, when 0.50 mL of a preconcentrated water sample was injected into the FIA system. When the aluminium ions start to interact with the crystal membrane, the frequency decrease, until a minimum is reached, and then start to increase, while the Milli-Q water flow carries the aluminium ions to waste. Complete recovery of the sensor was accomplished 2 min after the sample injection. A linear calibration line f = 3.2[Al3+ ] + 19.2, where f is expressed in Hz and Al3+ in mg L−1 , was obtained in the range between 1 and 14 mg L−1 of aluminium. The quantification limit for the quartz crystal methodology with the Chelex preconcentration was calculated to be 0.07 mg L−1 , below the guideline limit for aluminium in drinking water. This new methodology is therefore suitable to check if the water is according to the legislation or not. For the UV–vis methodology, a linear calibration line of Abs = 2.8194 [Al3+ ] − 0.0247 was obtained, and the method including the Chelex preconcentration presents a quantification limit of 0.04 mg L−1 .
Fig. 2 – Frequency of the coated crystal observed during the injection of 0.50 mL of a water sample into the FIA system.
4.2.
165
Results of the analysis
Both methodologies were applied to water samples from domestic supply networks, public sources or private wells, from several different places in Portugal, marked in Fig. 3. Table 3 shows aluminium determined by the new methodology with the coated piezoelectric quartz crystal, and with the conventional methodology of UV–vis spectrophotometry. Five replicates of each sample have been analysed. As we can see from Table 3, all water samples from domestic network supplies () showed very low aluminium values, under 0.2 mg L−1 . A large number of those samples presented aluminium values below the quantification limit. These results show that, although aluminium sulphate is used in water treatment to flocculate suspended particles, the treated water respects legislation. In untreated water samples from natural sources, e.g. public fountains () or private wells (䊉), it is possible to find some samples with aluminium content higher than the guideline value, respectively one sample from “Caminha”, two from “Barcelos” and one from “Santa Maria da Feira”. Curiously, shortly after the analysis, a warning reportage on the high aluminium contamination of the public fountain, in “Caminha”, was printed in a daily national newspaper. Observing the location of the fountains and wells with high levels of aluminium on Fig. 3, it can be concluded that all of them are located in the northest region of the country. Soils in the north of Portugal are mainly granitic, while calcareous soils are encountered in the central region. It can be postulated the aluminium source to be linked to the granitic soil
Fig. 3 – Portugal map with the sampling spots marked (—water from the domestic supply system network, —water from public fountains, 䊉—water from private wells).
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Table 3 – Al content (mg L−1 ) of water samples from different places in Portugal: results obtained with both methodologies Water samples
Aveiro Aveiro 䊉 Aveiro 䊉 Aveiro Barcelos 䊉 Barcelos Barcelos 䊉 Caminha Coimbra Esponsende Gouveia Leiria 䊉 Leiria Ovar 䊉 Sta. Maria da Feira 䊉 Tomar 䊉 Tomar Milli-Q water 1 Milli-Q water 2
Quartz crystal methodologya [Al] mg L−1 <0.07 <0.07 <0.07 <0.07 0.17 ± 0.01 0.21 ± 0.03 0.24 ± 0.04 0.26 ± 0.03 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 0.74 ± 0.02 <0.07 0.09 ± 0.04 <0.07 <0.07
UV–vis spectrometrya [Al] mg L−1 <0.04 <0.04 <0.04 <0.04 0.17 ± 0.01 0.21 ± 0.01 0.24 ± 0.01 0.27 ± 0.01 <0.04 <0.04 <0.04 <0.04 <0.04 <0.04 0.75 ± 0.01 <0.04 0.09 ± 0.01 <0.04 <0.04
(): water from the domestic supply system network; (): water from public fountains; (䊉): water from private wells. a
Mean values ± the 95% confidence limit (n = 5).
[23]. However, it is not the purpose of this work to explain why high aluminium concentration are found in some places, but to demonstrate that this new methodology, based on piezoelectric quartz sensors, is reliable and can be used to detect water sources not in accordance with the aluminium guideline value of 0.2 mg L−1 . The values in Table 3, obtained with the two methodologies, were statistically compared, and the difference in the mean values between the two groups was not great enough to exclude the possibility that the difference was due to random sampling variability. A test F was running for the comparison of standard deviations, and no significant differences in the variances of the two methods were found (˛ = 0.05). Besides being analytically adequate for the purpose, the new methodology is faster: 2 min of analysis versus the 15 min needed for colour development in UV spectrophotometry, to which colour development sample preparation time must be added. The equipment needed in the piezoelectric quartz crystal methodology is inexpensive, as data acquisition is surplus. An home-made oscillator was used, and an inexpensive frequency meter with 1 Hz resolution is all that is required. The quartz crystal is not an expensive item, and the aluminium ionophore was synthesized starting from reagents easily available.
5.
Conclusion
As a conclusion, it can be said that this new methodology is less reagent, sample and time consuming than UV–vis spectrophotometric methodology, and that it needs
inexpensive instruments. However, the new methodology is capable of quantifying aluminium in concentrations as low as 0.07 mg L−1 , and is therefore adequate to test if waters conform to legislation, and possess aluminium concentrations below the 0.2 mg L−1 . With respect to aluminium contamination found, results are consistent with the drinking water from domestic network supply being safe, and point to the need to test water from the well in the backyard, or from the fountain next to our house. Comparing the results obtained by both methodologies, no statistical significant differences (˛ = 0.05) were found, both in the average of the replicate analysis, or in the precision of the results.
Acknowledgments This project was financed by the Portuguese Foundation for Science and Technology (FCT), POCTI, FEDER and CESAM.
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