Solid phase extraction–spectrophotometric determination of dissolved aluminum in soil extracts and ground waters

Journal of Inorganic Biochemistry 97 (2003) 173–178 www.elsevier.com / locate / jinorgbio

Solid phase extraction–spectrophotometric determination of dissolved aluminum in soil extracts and ground waters a,b a, Mingbiao Luo , Shuping Bi * a

Department of Chemistry, State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, PR China b Department of Applied Chemistry, East China Institute of Technology, Jiangxi 340000, PR China Received 1 April 2003; received in revised form 10 May 2003; accepted 3 June 2003

Abstract An on-line solid-phase extraction (SPE) technique, linked to spectrophotometry, has been developed to overcome the problem of high matrix concentration, which is thought to interfere with the determination of low levels of aluminum (Al) in environmental samples. Tiron modified resin was prepared and used as a SPE absorbent, which can quantitatively adsorb Al(III) at pH 4–6 with an adsorption capacity of 5.6 mg g 21 resin. The main advantages of this novel method are: (1) a much higher sensitivity has been obtained by SPE technology; and (2) a large amount of Na 1 , K 1 , Ca 21 and Mg 21 can be removed and the interference of Fe(III), Mn(II) and F 2 can be efficiently eliminated by eluting with 0.25 mol l 21 NaOH. It is a highly selective and sensitive method for simple and quick determination of dissolved Al in soil extracts and ground waters, particularly suitable for the analysis of complex environmental samples.  2003 Elsevier Inc. All rights reserved. Keywords: Solid-phase extraction; Aluminum; Spectrophotometry; Soil extracts; Ground waters; Eriochrome cyanine R; Environmental sample

1. Introduction Aluminum (Al) is the third most abundant element in the lithosphere. Normally it is very insoluble, and in most neutral natural waters its concentration is very low. Thus, its biological effect has not received much attention in the past. In recent years, however, a large amount of Al has been released into the environment and its solubility is significantly increasing because human activities have resulted in a serious problem of acidification. The maximum permissible content of Al in drinking water is only 0.2 mg l 21 [1]. However, determining total Al is generally not useful because the bioavailability of Al depends on the forms that are present in water [2]. Dissolved Al is significant to environmental science because of its bioavailability and toxicity [3–6]. It is very important to develop analytical methods for the determination of dissolved Al in soil extracts and natural waters. Recently, the determination of trace levels of Al has been reported by various methods, such as spectrophotometry [7,8], spectrophotofluorimetry [9], graphite furnace atomic absorption spectrometry (GF-AAS) [10], inductively coupled plasma *Corresponding author. Tel.: 186-25-6205840; fax: 186-25-3317761. E-mail address: [email protected] (S. Bi). 0162-0134 / 03 / $ – see front matter  2003 Elsevier Inc. All rights reserved. doi:10.1016 / S0162-0134(03)00243-5

atomic emission spectrometry (ICP-AES) [11], ICP mass spectrometry (ICP-MS) [12], and 27 Al-NMR [13–15]. The GF-AAS and ICP-AES are accepted reference methods for determining total Al in natural waters [16]. However, both methods suffer from serious matrix interferences. Moreover, the equipment for GF-AAS or ICP-AES is much more expensive and the accurate determination of Al requires considerable expertise [17]. Compared to ICP-MS, ICP-AES and 27 Al-NMR methods, spectrophotometry has advantages such as low cost, simple operation, easy spread and wide applications. The most effective spectrophotometry to monitor Al(III) are Eriochrome cyanine R (ECR) [16,18], pyrocathecol violet (PCV) [6], chromeazurol S (CAS) [19] and aluminon spectrophotometries [20], in which the ECR method recommended by the American Public Health Association has the highest sensitivity. However, some of the problems of the ECR method are the serious interferences by Fe(III), Mn(II) and F 2 , which widely exist in environment samples, and its sensitivity is not high enough to determine low concentrations of Al. It is therefore necessary to find a valid way to preconcentrate and separate Al(III) to eliminate the above problem. Only a few approaches have been proposed to enrich and separate Al because of the characteristics of Al(III), such as high electropositivity, amphoteric, slow reaction

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rates with many ligands. The most common methods recently developed in this area are liquid–liquid extraction and ion-exchange. In the extraction of oxine, the interference of Fe(III) can be eliminated partly by using a masking agent, but it is inconvenient in operation and not very sensitive because of the use of organic reagent. In the cation-exchange resin, Al(III) can be adsorbed under certain conditions, but this method has the problem of interference from Fe(III) and Mn(II). Recently, the solidphase extraction (SPE) technique has become increasingly popular. It has advantages over traditional liquid–liquid extraction such as a higher preconcentration factor, better efficiency and greater reproducibility [21–25]. The aim of this paper was to develop a simple and fast SPE spectrophotometry, which is free from the interference of Fe(III), Mn(II), and F 2 for the determination of Al(III) in environmental samples.

2. Experimental

2.1. Apparatus and chemicals A spectrophotometer (UV-260, Japan) with a light path of 2 cm or longer was utilized to carry out ECR spectrometric measurement of Al. A pH meter (pHS-3C, Shanghai Rex Instrumentation Factory, China) was used for checking the pH of solutions. A homemade microcolumn (70 mm32.5 mm i.d.) was made from polypropylene. A peristaltic pump (HL-1, Shanghai Huxi Instrumentation Factory, Shanghai, China) was used to control the flow rate. An ICP-atomic emission spectrometer (Atomscan16, TJA, USA) was used for the determination of metal elements. An Al stock solution (500 mg ml 21 ) was prepared by dissolving 0.1250 g high purity Al foil in 10 ml of concentrated HCl and heated until the Al was completely dissolved. Then the solution was cooled and transferred to a 250-ml flask, and diluted to the mark with deionizeddistilled water. A 5 mg ml 21 Al working standard solution was prepared by diluting 10 ml stock solution to 1000 ml with deionized-distilled water. A 0.1% (w / v) ECR solution was prepared by dissolving 0.25 g of ECR with a little deionized-distilled water, and the solution was diluted to 250 ml in a calibrated flask, which can be used after standing for 4 h. A series of buffer solutions were prepared by dissolving 150 g hexamethylenetetramine to 500 ml of doubly distilled water, and the solution was adjusted to the desired pH (from 4.5 to 6.2) with 6 mol l 21 HCl and 6 mol l 21 NH 3 ?H 2 O, checked by glass electrode; 20138 strong base anion-exchange resin (100–120 mesh, styrene–divinylbenzene copolymers containing quaternary ammonium) was purchased from Shanghai Yaolong Chemical Factory (Shanghai, China). All reagents used were of analytical-reagent grade, and the solutions were prepared with deionized-distilled water.

2.2. Modified resins and micro-column preparation 2.2.1. Modified resins preparation The appropriate amount of 20138 strong base anionic exchange resin was immersed in 0.1 mol l 21 NaOH for 10 h and then in 1 mol l 21 HCl for another 10 h. The treated resin was washed to be neutral with water and dried at 60 8C, then stored in a desiccator. Three grams of the treated 20138 resin was weighed into the solution containing 100 ml of 1% 1,2-dihydroxy-3,5-benzenedisulfonic acid (Tiron), and settled for 12 h. Then the mixture was filtered and washed with 3 mol l 21 HCl and 1 mol l 21 NaOH, respectively, then conditioned to neutrality with deionized-distilled water, and finally dried at 60 8C. Then it was stored in a desiccator before use. 2.2.2. Resin regeneration The micro-column can be reused by regeneration with 3.0 ml 3.0 mol l 21 of HCl, 0.5 ml 0.5 mol l 21 of HCl, and 5 ml distilled water. The desired pH value was adjusted with the buffer solution at the same pH as that of the samples. 2.2.3. Resin stability According to the procedure, the capacity of the resin was found to be constant before its repeated use at least 35 times for the Tiron modified resin. While here the resin will usually be adopted for 25 times before it is renewed. 2.2.4. Micro-column preparation Packed 0.20 g Tiron modified resins into a homemade micro-column 6–7 cm high using the slurry technique. 2.3. Solid-phase extraction The resin modified by Tiron was filled into a microcolumn, which was connected to a peristaltic pump. The resin column was equilibrated with the buffer solution at pH 5 before analysis. If the pH of the sample solution is in the pH range 4.0–6.0, it can be directly pumped through the micro-column, otherwise it should be adjusted to pH 5 with 10% (v / v) HCl and then passed through the microcolumn packed with modified resin at 1 ml min 21 , and rinsed with 3 ml of water, and then desorbed with 2.5 ml 0.25 mol l 21 of NaOH. The Al in the eluent was estimated by ECR spectrophotometry.

2.4. ECR spectrophotometry A simplified ECR procedure was used. An appropriate volume of sample solution containing Al was pipetted into a 50-ml calibrated flask, and 20 ml of water was added, then one drop of 0.1% p-nitrophenol was added and mixed. The treated solution turned a yellow color with 0.1 mol l 21 NaOH. Then the yellow of the solution was faded with 0.5 mol l 21 HCl. Next, 5 ml of hexamethylenetet-

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ramine buffer solution at pH 6 was added and mixed as well. The absorbance was measured at 535 nm with a 2-cm cell against a blank reagent prepared in a similar way without Al.

2.5. Sample preparation 2.5.1. Synthetic waters Standard solutions of Al, Fe, Mn, Ca and Mg were used to make up the synthetic water samples. Synthetic water sample 1 was prepared with a composition of 5.0 mg ml 21 Al, 8.0 mg ml 21 Fe, 0.5 mg ml 21 Mn, 20 mg ml 21 Ca and 3.0 mg ml 21 Mg. Synthetic water sample 2 was prepared with a composition of 0.01 mg ml 21 Al, 5.0 mg ml 21 Fe, 0.5 mg ml 21 Mn, 10 mg ml 21 Ca and 3.0 mg ml 21 Mg. Their pH values were adjusted to 5.5. 2.5.2. Ground waters Four ground waters were collected from Jiangxi province in east China. They were stored in closed polyethylene bottles and analyzed within 10 h of sampling. Before analysis samples were filtered through a 0.45-mm membrane filter. 2.5.3. Soil extracts Soil samples (0.50 g; collected from China) were shaken for 35 min with 100 ml of 0.5 mol l 21 NaOH in an oscillator, centrifuged (10,000 rpm, 20 min) and decanted [26]. The soil extract was filtered using a 0.45-mm filter. The filtrate was stored in a 100-ml polypropylene bottle for the investigation.

3. Results and discussion

Fig. 1. Effect of pH on the sorption of aluminum. Concentration of Al is 10 mg ml 21 ; resin 0.2 g, flow rate 1 ml min 21 .

concentration of Al(III) in the eluent was determined by ECR spectrophotometry, and Fe(III) and Mn(II) can be eluted by using 3 ml 1 mol l 21 of HCl. The principal interferent for analysis in environmental samples is Fe(III). Tiron can form stable complexes with Al(III) and Fe(III) with log K of 19 and 20, respectively. Fig. 2 shows that 50 mg of Fe(III) retained on the resin was not eluted with 2.5 ml of 0.25 mol l 21 NaOH and thus did not interfere in down-line Al chemistry, but Al(III) was removed from the resin. This result was consistent with the much lower stability of the ferrate complex [Fe(OH) 42 ] compared with aluminate [Al(OH) 42 ] [27]. An experimental retention of Fe(III) and Mn(II) by the column was confirmed. The 50 mg Fe(III) and 10 mg Mn(II) adsorbed on resin were eluted completely with 3 mol l 21 HCl and detected by ICP-AES in eluent. The present method was used to determine the Al(III) in a mine lot water containing 8.2 mg ml 21 Fe. Results were in good agreement with those obtained by ICP-AES.

3.1. Effect of pH on sorption The influence of pH on the metal recovery was determined. The standard solution was adjusted to the desired pH with the buffer solution. These solutions were then passed through the micro-column at 1.0 ml min 21 according to the fore-mentioned procedure. It was found that maximum retention of Al was achieved at pH 4–6.5. As is shown in Fig. 1, the recovery of Al (defined as Al in eluent / total Al) begins to decrease when the solution is over pH 6.5. Therefore, the buffer solution at pH 5.0 must be pumped simultaneously through the micro-column with the sample solution.

3.2. Influence of elution agent concentration and the separation of Fe( III), Mn( II) and Al( III) The elution of Al(III) from the column was studied by using NaOH solution at varying concentrations (0.1–0.25 mol l 21 ) as a stripping agent. The optimum eluant concentration found for Al(III) was 0.25 mol l 21 NaOH. The

Fig. 2. Eluting curves for aluminum and iron. Concentration of Al is 10 mg ml 21 ; concentration of Fe is 50 mg ml 21 ; resin 0.2 g; flow rate 1 ml min 21 ; solid, Al; dash, Fe.

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3.3. Effect of flow rate on sorption The retention of Al(III) on the Tiron modified resin was studied at different flow rates. The recovery was greater than 98% at a flow rate between 0.5 and 4 ml min 21 . However, at a flow rate above 4 ml min 21 , there was a decrease in the sorption percentage. Al(III) reacts slowly with many ligands. Some Al(III) cannot be adsorbed on the Tiron modified resin if the flow rate is too high. The experimental results showed that the recovery would decrease if the flow rate was over 4 ml min 21 . In this work, a 1.0 ml min 21 flow rate was selected as optimum flow rate for the further study.

3.4. Breakthrough studies The breakthrough volume and the sorption capacity by column method were investigated. A series of 20-ml standard solutions of 10 mg ml 21 Al were passed through the micro-column packed with 0.20 g of modified resin, and the Al in the effluent was estimated by ECR spectrophotometry. The breakthrough curve was obtained and is shown in Fig. 3. The maximum sorption capacity (C /Co 5 0.5) by the column method was found to be 5.6 mg Al g 21 resin.

3.5. Tolerance of foreign ions Five micrograms of Al and a certain amount of concomitant metal were added to 50 ml of the chromomeric system and determined by ECR spectrophotometry. The tolerance levels of the main concomitant ions with and without the separation of SPE are given in Table 1 when the recovery of Al was defined as over 95%. As is shown in Table 1, the tolerance levels of Fe(III), Mn(II), F 2 had increased 80, 10, and 100 times, respec-

Fig. 3. Breakthrough curve for aluminum. Co , concentration of Al is 10 mg ml 21 ; C, concentration of Al in effluent; resin 0.2 g; flow rate 1 ml min 21 .

Table 1 The tolerance levels of the concomitant ions with and without SPE (5 mg Al) The concomitant metal

The tolerance levels without the SPE / mg

The tolerance levels with the SPE / mg

K1 Na 1 Ca 21 Mg 21 Fe(III) Mn(II) Zn 21 F2 PO 32 4

500 500 50 100 5 10 150 1 15

100 100 50 50 0.4 1.0 1.0 0.1 0.5

tively. The reason is that Tiron can form stable complexes with Al(III) (log K1 519.1) [5], while that of F 2 is weak (log K1 57.0) [28]. This made it possible to eliminate the main interfering ions such as F 2 , alkali metal and alkali earth metal ions after passing through the SPE. Although Fe(III) is still adsorbed, it can be separated if 0.5 mol l 21 of NaOH is selected as the eluant, which results in the first elution of Al(III) and the detention of the majority of Fe(III) on the micro-column. Therefore, the tolerance levels of Fe(III) were increased 80 times.

3.6. Preconcentration limit, preconcentration factor, detection limit and precision The limits of preconcentration and preconcentration factors were investigated by using the column procedure. For this purpose, a series of 100, 200, 300, 500 and 800 ml of standard solutions containing 2 mg of Al, were passed through a micro-column by using a continuous column procedure. The concentration range of Al was between 2.5 and 20 mg l 21 . The results showed that the limit of preconcentration was 4 mg l 21 when the quantitative recovery was considered up to 97%. Recoveries from more dilute solutions were not quantitative. The preconcentration factor was up to 200 times because 2 mg of Al from a 500-ml standard solution on the micro-column could be eluted with 2.5 ml 0.25 mol l 21 of NaOH. However, in practice, 25–50-ml samples were studied in the analysis and procedure and the preconcentration factor was found to be 20 times. According to the definition of the IUPAC, the detection limit (3s ) is determined by performing 10 repeated measurements of the blank value. The detectable minimum Al concentration was found to be 6 mg l 21 by ECR spectrophotometry in the absence of fluorides and complex phosphates [16]. In the present paper, it was found to be 0.3 mg l 21 Al if the preconcentration factor of 20 times was adopted. The precision of the determination was measured by five successive retention and elution cycles for 10 mg of Al in 50 ml of solution. It was found that the recovery was 99.765.7% at the 95% confidence level.

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Table 2 Results of synthetic and ground waters by the proposed method and ICP-AES [n53, Al(III) / mg ml 21 ] Samples

1 Synthetic water (1) 2 Synthetic water (2) 3 Ground water (1) 4 Ground water (2) 5 Ground water (3) 6 Ground water (4)

pH

Proposed method

5.5 5.5 6.0 6.5 6.3 6.8

ICP-AES

Al

R.S.D. (%)

Al

R.S.D. (%)

4.92 0.009 0.297 0.119 0.096 0.002

3.4 5.3 4.2 3.5 4.5 5.0

5.10 0.007 0.301 0.121 0.092 ,LOD

4.4 8.3 7.3 5.9 4.4

NaOH is higher than with 1 mol l 21 HCl in Chinese soils. The results which contain mainly dissolved Al (Al–organic acids, Al 31 , Al(OH) 3 , Al(OH)21 , Al(OH) 21 ) are better to reflect most Al forms existing in soil. Previous studies have shown that it is an appropriate method to extract Al(III) with 0.5 mol l 21 NaOH in Chinese soils.

3.7. Practical applications to determine dissolved Al concentration in ground waters and soil extracts In order to establish the validity of the method, synthetic and natural waters were selected for analysis of Al(III). The proposed method and ICP-AES [29] method have been applied to the analysis of Al(III) in synthetic waters and ground waters, respectively. The results are shown in Table 2. Both methods obtained consistant results indicating the proposed SPE approach is accurate and reliable. Five soil extracts were analyzed in the recovery study (Table 3). The experimental results indicated that the interferences of Fe(III) and Mn(II) were removed and the selectivity as well as sensitivity were improved with this method. It is worth mentioning here about the leaching of dissolved Al(III). There are many methods to extract Al(III) in soils; 0.5 mol l 21 NaOH, 1 mol l 21 HCl, 1 mol l 21 KCl, 1 mol l 21 NH 4 Ac and 0.1 mol l 21 BaCl 2 are usually used as extractants. However, 0.5 mol l 21 NaOH is usually used for leaching Al(III) in Chinese soils [30–33], which is quite different from European soils. The majority of Chinese soil is variable charge soil and consists of red soil [30]. The content of Fe 2 O 3 is about 8% in the red soil. Fe(III) will be released when red soil solution is extracted with 1 mol l 21 HCl and the concentration of Fe(III) is up to 30–50 mg ml 21 , and Fe(OH) 3 (s) will be formed when the pH is adjusted to 5.0. This adsorbs some Al(III) and affects the successful operations. When 0.5 mol l 21 NaOH is used as an extractant to extract red soil solution, the dissolution of Fe(III) is less than Al(III) and it can be eliminated completely by coupling with the SPE technique. The concentration of Al(III) extracted with 0.5 mol l 21

4. Conclusions From the above experiments, we can make the following conclusions:

1. ECR is a very sensitive chromogenic reagent for Al. However, the concentration of sodium in leaching solution often exceeds 10 mg ml 21 . Furthermore, the 2 concentrations of Fe(III), Mn(II) and F are sometimes also much higher. This seriously interferes with the determination of Al(III) by the above method. Therefore, it is necessary to find a suitable new way to solve these problems. ECR spectrometry coupled with SPE is one such promising method. 2. The tolerance levels of Fe(III), Mn(II), F 2 , by using SPE technology–Tiron modified resin, can be increased 80, 10 and 100 times, respectively. This resolves the long-term critical problem of the interferences of Fe(III), Mn(II), F 2 for determining Al(III) by spectrophotometry. 3. A large concentration factor (20 times for a 50-ml sample volume) can be reached using SPE technology and a much higher sensitivity (20 times) has been obtained by ECR spectrophotometry coupled to SPE

Table 3 Determination of dissolved Al in real soil samples (n53) Samples

Determined 21

1 Weihai Shandong soil extracts 2 Linchuan soil extracts 3 Jiangxi soil extracts 4 Nanjing soil extracts 5 Guangdong soil extracts

21

(g kg )

(mg ml )

1.65 3.26 6.80 1.13 4.84

8.25 16.3 34.0 6.65 24.2

Added (mg ml 21 )

Found (mg ml 21 )

Recovery (%)

10.0 10.0 10.0 10.0 10.0

17.8 27.3 44.0 17.5 34.6

95.5 110 100 108 104

DL, limit of determination (0.6 mg l 21 ); R.S.D., relative standard deviation; ground waters 1–4 were collected from Lingchuan river, Ganjian river, Fuzhou river, Yihuang river, and Shicheng river, respectively.

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technology. This enables ECR spectrophotometry to reach the sensitivity requirement for determining dissolved Al in soil extracts and ground waters. In summary, the proposed method possesses distinct advantages such as the simplicity in handling and transferring, the rapidness, and the economic advantages. Its sensitivity and selectivity are satisfactory for the determination of total reactive Al in soil extracts and other complex environmental samples.

Acknowledgements This project was supported by the National Natural Science Foundation of China (Nos. 20075011 and 49831005) and research funding for young teachers in key universities by the State Education Administration of China. The authors would like to thank Profs. Hongyuan Chen, Xiaoru Wang, Guoliang Ji and Tianren Yu for their help.

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