Application of modified multiwalled carbon nanotubes as a sorbent for simultaneous separation and preconcentration trace amounts of Au(III) and Mn(II)

Journal of Hazardous Materials 168 (2009) 1548–1553

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Application of modified multiwalled carbon nanotubes as a sorbent for simultaneous separation and preconcentration trace amounts of Au(III) and Mn(II) Tayebeh Shamspur, Ali Mostafavi ∗ Chemistry Department, Shahid Bahonar University of Kerman, Kerman, Iran

a r t i c l e

i n f o

Article history: Received 11 January 2009 Received in revised form 5 March 2009 Accepted 6 March 2009 Available online 18 March 2009 Keywords: Multiwalled carbon nanotubes Preconcentration Gold determination Manganese determination

a b s t r a c t A solid phase extraction procedure is proposed for simultaneous separation and preconcentration trace amounts of Au(III) and Mn(II) in an aqueous medium by using a column of multiwalled carbon nanotubes modified with the analytical reagent N,N -bis(2-hydroxybenzylidene)-2,2 (aminophenylthio)ethane. An implementation, it was found that the sorption is quantitative in the pH range 5.0–7.5, whereas quantitative desorption occurs instantaneously with 4.0 mL of 0.1 mol L−1 Na2 S2 O3. Selected elements were also determined by flame atomic absorption spectrometry. Linearity was maintained between 0.2 ng mL−1 to 25 ␮g mL−1 for gold and 0.08 ng mL−1 to 5 ␮g mL−1 for manganese in the original solution. Various parameters such as the effect of pH, flow rate, type and amount of eluent, breakthrough volume and interference of a large number of anions and cations on the recovery of the selected ions was studied. Under optimum conditions, the detection limits (3 s, n = 10) for analytes were 0.03 ng mL−1 (gold) and 0.01 ng mL−1 (manganese). The method was successfully applied for separation and determination of gold and manganese ions in water and standard samples. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The determination of trace metals by flame atomic absorption spectrometry (FAAS) has a number of advantages which include high selectivity, speed and fairly low operational cost. Despite its significant analytical chemical capacities for metal determination at low concentration levels, FAAS often requires a suitable pretreatment step (preconcentration/separation) of the sample in order to facilitate the desired sensitivity and selectivity of measurement [1,2]. Solid phase extraction (SPE) is a popular technique for separation and preconcentration of metal ions in environmental samples [3–5] due to its simplicity, rapidity, minimal cost, low consumption of reagents and the ability of combination with different detection techniques in the form of on-line or off-line mode [6]. In the on-line procedures there is no sample manipulation between preconcentration and analysis, so loss and concentration risk are avoided and reproducibility values are better. Likewise, all the species are analyzed, so the volume of the sample can be smaller than in off-line procedures, the consumption of organic solvents is lower and the potential for automation is higher [7]. But the off-line SPE approach remains useful for analyzing complex samples due to its greater

∗ Corresponding author. Tel.: +98 341 3222033; fax: +98 341 3222033. E-mail address: [email protected] (A. Mostafavi). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.03.028

flexibility and the fact that it can analyze the same extract using various techniques. In addition, the detection limits and enrichment factor of the off-line methods are better than those utilizing on-line procedures [8–10]. In recent years, great attention has been paid to the application of nano-structure materials, especially carbon nanotubes (CNTs). In particular, there have been an increasing number of applications of CNTs in several fields of chemical analysis [11,12] CNTs can be visualized as a sheet of graphite that has been rolled into a tube. These tubes are classified as multiwalled carbon nanotubes (MWCNTs) [13] and single-walled carbon nanotubes (SWCNTs) [14] according to the carbon atom layers in the wall of the nanotubes. Because of the their special electronic, metallic and structural characteristics as well as the unique tubular structures of nano diameter and large length/diameter ratio [15], CNTs have been exploited in analytical chemistry and other fields [16,17]. The highly developed hydrophobic surface of CNTs exhibits strong sorption properties toward various compounds and therefore CNTs may be used for the separation and preconcentration of trace analytes [18–20]. MWCNTs have also been used for the preconcentration of trace amounts of organic materials [21–23] and the extraction of some ions from environmental samples [24,25]. However, according to our literature survey, until now, there were no references to the application of MWCNTs for simultaneous separation and preconcentration trace amounts of Au(III) and Mn(II).

T. Shamspur, A. Mostafavi / Journal of Hazardous Materials 168 (2009) 1548–1553

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2.3. Sampling Water samples were collected from two regions in Kerman province, Iran (Kerman drinking water and waste water of Copper factory, Sarcheshmeh). Before the analysis, the organic content of the water samples was oxidized in the presence of 1% H2 O2 and then concentrated nitric acid was added. These water samples were filtered through a cellulose membrane filter (Millipore) of pore size 0.45 ␮m to remove particulate matter. The pH of the filtered water samples were adjusted to approximately 6 using sodium acetate/acetic acid buffer solution. 2.4. Preparation of modified MWCNTs

Fig. 1. N,N -bis(2-hydroxybenzylidene)-2,2 (aminophenylthio)ethane (NBHAE).

In the present investigation, the analytical potential of MWCNTs modified with N,N -bis(2-hydroxybenzylidene)-2,2 (aminophenylthio)ethane [NBHAE] (Fig. 1) was examined for simultaneous preconcentration of Au(III) and Mn(II) in aqueous samples prior to their flame atomic absorption spectrometric determinations. The analytical conditions for the preconcentration (quantitative retentions) of analyte elements were investigated.

2.0 g of MWCNTs was suspended in 20 mL 0.2% (m/v) solution of NBHAE and the mixture was refluxed with stirring for 4 h. The solid was filtered, washed with water and dried at 80 ◦ C. The amount of NBHAE deposited on the MWCNTs was estimated by spectrometric measurements from the residual amount of NBHAE in solution. 2.5. Column preparation 40.0 mg of modified MWCNTs was slurred in water, and then poured into a funnel-tipped glass tube (80 mm × 10 mm) plugged with a small portion of cotton at the end. The column was used repeatedly after washing with distilled water.

2. Experimental 2.6. Recommended procedure 2.1. Apparatus A Varian model Spectr AA 220 (Palo Alto, CA) atomic absorption spectrometer equipped with deuterium background correction and gold and manganese hollow-cathode lamps as the radiation source were used for absorbance measurements at wavelength of 242.8 and 279.5 nm, respectively. All measurements were carried out in an air/acetylene flame. The instrumental parameters were adjusted according to the manufacturer’s recommendations. A digital pH meter 827 (Metrohm-Peak LLC, Houston, TX) equipped with a combined glass calomel electrode was used for the pH adjustment. A funnel-tipped glass tube (80 mm × 10 mm) was used as column for preconcentration. All glassware and columns were washed with mixture of concentrated hydrochloric acid and concentrated nitric acid (1:1) before application. 2.2. Reagents and materials High purity reagents from Sigma (St. Louis, MO, USA) and Merck (Darmstadt, Germany) were used for all preparations of the standard and sample solution. Stock solutions of analyte ions, 1000 mg L−1 , were diluted daily for obtaining reference and working solutions. Stock solution of diverse elements was prepared from high purity compounds. Multiwalled carbon nanotubes of 95% purity and length 1–10 ␮m, number of walls 3–15 were purchased from Plasma Chem GmbH (Berlin, Germany). MWCNTs materials were oxidized with concentrated HNO3 according to literature with minor modification before being used. In order to create binding sites onto the surface, 3.0 g of MWCNTs were first soaked in 30 mL of concentration HNO3 for 12 h at room temperature while being stirred [25]. Then the solution was filtered through a 0.45 ␮m membrane filter and the MWCNTs were washed with distilled water until the pH was neutral. The Schiff base ligand NBHAE was synthesized and purified as reported in the literature [26]. A 0.2% (m/v) solution of NBHAE was prepared by dissolving 50 mg of NBHAE in 1.0 mL of DMF and diluting to 25.0 mL with ethanol.

50.0 mL of an aqueous solution containing 5.0 ␮g of analyte ions was taken and the pH was adjusted to about 6. The resulting solution was passed through the column at flow rate of about 1 mL min−1 . After the solution passed completely, the column was rinsed with 5 mL of water and the adsorbed ions were eluted with 4.0 mL of 0.1 mol L−1 Na2 S2 O3 with a flow rate of 1 mL min−1 . The analyte ions in the eluent were determined by FAAS. 3. Results and discussion Preliminary experiments showed that MWCNTs have a tendency for the retention of metal ions adsorption (62% for Mn(II) and 78% for Au(III)) but were not selective. Recent works [19,21–23] indicates that the MWCNTs can adsorb organic material, so we decided to add NBHAE to MWCNTs. These modified multiwalled carbon nanotubes have a greater capacity for adsorption and selectivity of ions. In other words, the sorbent was selective and sensitive for separation and preconcentration of trace amounts of gold and manganese in the sample solution. In order to achieve the best performance, the separation/preconcentration procedure was optimized for various analytical parameters, such as the nature and concentration of eluent, pH of the sample solution, volume and type of the eluent solution, the flow rate of eluent and sample solutions and volume of the sample solution. Various ions interference effects were also investigated. 3.1. Effect of the sample pH Since the pH of the aqueous solutions is an important analytical factor in the solid phase extraction studies of metal ions [27–29], the influence of pH on the recovery of analyte ions were examined in the pH range of 1–10. The results are shown in Fig. 2. Au(III) ions were quantitatively recovered in the pH range of 2.0–7.5, while Mn(II) ions were recovered at pH range of 5.0–10.0. In subsequent studies, the pH was kept at approximately 6 using sodium acetate/acetic acid buffer solution.

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Fig. 2. Effect of the pH of a sample solution on the recovery of analyte ions.

3.2. Choice of eluent Another important factor which affects the preconcentration procedure is the type, volume and concentration of the eluent used for the removal of metal ions from the sorbent [28]. Optimization of the elution conditions were performed in order to obtain the maximum recovery with the minimal concentration and volume of the elution solution. The ions were stripped with 4.0 mL of different concentrations of various eluting solutions. From the results shown in Table 1, it is obvious that 0.1 mol L−1 Na2 S2 O3 is the best eluent. 3.3. Effect of flow rates of sample and eluent solution The retention of an element on an adsorbent also depends on the flow rate of the sample solution [28]. Thus, the effect of flow rates of the sample and elution solutions on the retention and recovery of ions was investigated under optimum conditions. It was found that retention and recovery of the ions was independent of flow rate in a range of 0.5–1.5 mL min−1 . Therefore a flow rate of 1 mL min−1 was applied for sample and elution solutions in all experiments. 3.4. Breakthrough volume The measurement of breakthrough volume is important in solid phase extraction because breakthrough volume represents the sample volume that can be preconcentrated without loss of analyte during elution of the sample [28]. The breakthrough volume of the sample solution was tested by dissolving 5.0 ␮g of ions (Mn(II) and Au(III)) in different volumes (50–1600 mL) and the recommended procedure was followed. The recovery of Au(III) ions throughout the working range was significant and acceptable. Manganese was recovered quantitatively in the range of 50–1000 mL. The preconcentration factor (the ratio of the highest sample volume for both the analytes (1000 mL) and the lowest eluent volume (4.0 mL)) was 250.

Table 1 Percent recovery of Au(III) and Mn(II) from the modified columna . b

Stripping agent −1

EDTA 0.1 (mol L ) HNO3 0.1 (mol L−1 ) HCL 0.1 (mol L−1 ) NH2 CSNH2 0.1 (mol L−1 ) KSCN 0.1 (mol L−1 ) Na2 SO3 0.1 (mol L−1 ) Na2 S2 O3 0.05 (mol L−1 Na2 S2 O3 0.1 (mol L−1 ) Na2 S2 O3 0.5 (mol L−1 ) a b

Recovery Au (%)

16.0 18.0 22.0 27.2 32.3 93.4 97.3 99.1 99.0

± ± ± ± ± ± ± ± ±

0.8 0.6 1.0 0.5 0.5 1.0 1.1 0.9 0.7

b

Recovery Mn (%) 80.2 92.0 99.8.0 35.0 99.0 93.0 98.1 98.6 98.8

± ± ± ± ± ± ± ± ±

Initial samples contained 5.0 ␮g Au(III) and Mn(II) ions in 50 mL water. Average of five determination, ±standard deviation.

0.5 0.8 0.2 1.0 0.6 0.5 .1.1 0.8 1.0

Fig. 3. Calibration curves for gold and manganese.

3.5. Adsorption capacity In order to evaluate the adsorptive capacity of the modified MWCNTs, a batch method was used. 50 mL of solution containing 5.0 mg of gold or 1.0 mg of manganese (individually) at pH 6 was added to 40.0 mg sorbent. The mixture was filtered, after shaking for 30 min. 10.0 mL of the supernatant solution was diluted to 100.0 mL and determined by FAAS. The capacity of sorbent for Au(III) and Mn(II) was found 75 and 7.5 mg g−1 , respectively. 3.6. Analytical performance The reproducibility of the procedure was evaluated by a model solution containing 5.0 ␮g of analyte ions (n = 10). The relative standard deviations (RSD) were found to be ±1.61% for gold and ±0.83% for manganese. The calibration curves for the determination of gold and manganese according to the general procedure under the optimized conditions are shown in Fig. 3. Linearity in the final solution was maintained between 0.05 and 25.0 ␮g mL−1 for gold and 0.02–5 ␮g mL−1 for manganese with correlation factors of 0.9997 and 0.9999, respectively. The limit of detection (LOD) for the analyte ions based on 3 bl /m (n = 10) were 0.03 ng mL−1 for gold and 0.01 ng mL−1 for manganese in initial solution. The sensitivities of 1% absorbance for Au(III) and Mn(II) were 115 and 23.9 ng mL−1 , respectively. 3.7. Matrix effects Matrix effects are important problems in the determination of metals in real samples [30,31]. In order to assess the possible analytical applications of the recommended procedure, the interference of several cations and anions were examined under optimized conditions. For these studies an aliquot of aqueous solution (50 mL) containing 5.0 ␮g of analyte ions was taken with different amounts of foreign ions and the procedure was implemented. The tolerance limit was defined as the highest amount of foreign ions that produced an error no greater than ±3% in the determination of investigated analyte ions. The results, summarized in Table 2, show that the proposed method is selective and can be used in various samples for determination of these metals without interference. 3.8. Analysis of ions in standard alloy This method was applied for determination of analyte ions in Nippon Keikinzoku Kogyo (NKK) No. 920, aluminum alloy. A 100.0 mg sample of the standard alloy was dissolved completely

T. Shamspur, A. Mostafavi / Journal of Hazardous Materials 168 (2009) 1548–1553 Table 2 Effect of foreign ions on the recovery of metal ionsa . Diverse Ion −

−

CH3COO , NO3 CO3 2 ,SO4 2− EDTA Tartrate Oxalate Pb2+ Pd2+ Rh3+ Al3+ Ni2+ K+ ,Na+ , NH4 + Cd2+ Fe3+ Zn2+ , Co2+ Ca2+ , Ba2+ , Mg2+ Cu2+

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results, shown in Table 3, are in good agreement with the certified values.

Chemical form of the compound

Tolerance limit (mg)

CH3 COONa·3H2 O, KNO3 , Na2 SO4 , Na2 CO3 C10 H14 N2 Na2 O8 ·2H2 O Sodium potassium tartrate Sodium oxalate Pb(CH3 COO)2 PdCl2 RhCl3 ·3H2 O AlCl3 NiSO4 NaCl, NH4 Cl, KNO3 Cd(NO3 )2 Fe(NO3 )3 ZnCl2 , Co(NO3 )2 CaCl2 , BaCl2 , MgCl2 Cu(NO3 )2

1000 400 20 400 400 300 300 200 30 200 700 500 300 300 350 500

3.9. Analysis of ions in gold Certified Reference Materials The accuracy and applicability of the proposed method was applied to the determination of ions in Canadian Certified Reference Materials Project (CCRMP) (CCU-1b and MA-1b). A 200.0 mg sample was taken and dissolved completely by heating in a mixture of nitric acid (∼2 mL), hydrochloric acid (∼6 mL) and HF (∼1 mL) with heating. The solution was cooled, diluted and filtered. The volume of the filtrate was raised to 50.0 mL with distilled water in a calibrated flask. An aliquot of the sample solution was taken individually and ions were determined by the recommended procedure. The results, given in Table 4, are in good agreement with the certified value. 3.10. Analysis of ions in real samples

a Initial samples contained 5.0 ␮g Au(III), Mn(II) and different amounts of diverse ions in 50 mL water.

In order to assess the applicability of the method to real samples, it was applied to the separation and recovery of ions in different sample waters including tap water, (Kerman drinking water) well water (University of Kerman) and waste water (Copper factory Sarcheshmeh). For the preconcentration and determination of ions, 250.0 mL of the prepared water sample was passed through a column and analyzed by a recommended procedure. The reliability was checked by spiking experiments. The results are shown in

in a mixture of nitric acid (∼5 mL), hydrochloric acid (∼5 mL) and H2 O2 (∼3 mL) by heating. The solution was cooled, diluted and filtered. The volume of the filtrate was increased to 100.0 mL with distilled water in a calibrated flask and then 5.0 ␮g of Au(III) ions was added to it. An aliquot of the sample solution was taken and these metal ions were determined by the usual procedure. The

Table 3 Determination of Mn(II) ion in standard alloy and recovery of Au(III). Sample

Composition (%)

a

NKK, No.920 aluminum alloy

Mn; 0.20, Sn; 0.20, Si; 0.78, Fe; 0.72, Zn; 0.80, Cu; 0.71, Ni; 0.29, Ti; 0.15, Mg; 0.46, Pb; 0.10, Cr; 0.27, V; 0.15, Co; 0.10, Bi; 0.06, Sb; 0.01, Ga; 0.05, Ca; 0.03

0.19 ± 0.02

Found Mn(II) (%)

Au(III) added (␮g)

a

5.0

4.9 ± 0.1

Found (␮g)

a

Recovery Au(III) (%)

98 ± 2

NKK: Nippon Keikinzoku Kogyo. a Average of five determination, ±standard deviation.

Table 4 Determination of Mn(II) and Au(III) ions in the Canadian Certified Reference Materials Project. Composition (% or ␮g g−1 )

Sample

MA-1b reference gold ore

a

Found −1

S; 34.8 ± 0.2, Fe; 29.95 ± 0.13, Zn; 5.70 ± 0.05, Cu; 24.5 ± 0.08, SiO2 , 1.4 ± 0.05 MgO; 0.568 ± 0.015, Pb; 1.32 ± 0.02, Al2O3; 0.188 ± 0.012, CaO; 0.184 ± 0.014%, Cd; 181.0 ± 5.0, Ag; 178. ± 2.0, As; 58.0 ± 7.0, Au; 5.89 ± 0.1 ␮g g−1 . Si; 24.5, Al; 6.11, Fe; 4.62, Ca; 4.60, K; 4.45, Mg; 2.56, C; 2.44, Na; 1.49, S; 1.17, Ti; 0.38, Ba; 0.18, P; 0.16, Mn; 0.09%, Cr; 200.0, Pb; 200.0, Rb; 160.0, Zr; 140.0, Cu; 100.0, Zn; 100.0, Bi; 100.0, Ni; 90.0, Mo;80.0, Te; 40.0, Co; 30.0, Y; 20.0, Au; 17.0, W; 15.0, Sc; 13.0, As; 8.0, Ag, 3.9, Sb; 3.0 ␮g g−1

CCU-1b copper flotation concentrate

a

a

Recovery (%)

Au: 5.87 ± 0.08 ␮g g

99.7 ± 0.8

Au: 16.9 ± 0.2 ␮g g−1 Mn: 0.09 ± 0.18%

99.4 ± 1.1 100.0 ± 1.0

Average of five determination, ±standard deviation.

Table 5 Determination of Au(III) and Mn(II) ions in water samples. Analyte

Added (␮g L−1 )

Tap water (Kerman) a

Found (␮g L−1 )

Well water (Kerman) a

Recovery (%)

a

Found (␮g L−1 )

a

Wastewater (1) Sarcheshmeh, (Kerman) Recovery (%)

a

Found (␮g L−1 )

a

Recovery (%)

Wastewater (2) Sarcheshmeh, (Kerman) a

Found (␮g L−1 )

a

Recovery (%)

Au(III)

0.0 5.0


– 102 ± 1


– 98 ± 2

6.3 ± 1.5 11.1 ± 1.8

– 96 ± 3

4.5 ± 1.3 9.2 ± 2.1

– 94 ± 4

Mn(II)

0.0 5.0

4.7 ± 0.6 9.6 ± 0.8

– 98 ± 1

6.5 ± 1.6 11.6 ± 1.2

– 102 ± 3

12.8 ± 1.6 17.5 ± 1.7

– 94 ± 4

8.3 ± 1.4 13.1 ± 1.9

– 96 ± 3

LOD: limit of detection. Wastewater (1) = befor precipitation. Wastewater (2) = after precipitation. a Average of five determination, ±standard deviation.

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Table 6 Comparative data from some recent studies on preconcentration of analyte ions. Technique

Analytes

System

SPE SPE

Au Au

MWCNT Amberlite XAD-2000/DDTX, column

SPE Coprecipitation

Au Fe, Pb, Co, Cr, Mn, Ni, Cd, Au, Bi, U, Th Au

Silica gel/nanometer TiO2 Cu(II)-9-phenyl-3-fluorone

SPE SPE SPE SPE SPE

Mn Mn Cu, Cd, Pb, Mn, Fe, Ni, Co Mn, Au

Eluent

1.0 (mol L−1) HNO3 in acetone 0.1 (mol L−1) HNO3 HNO3

Octadecyl silica membrane disks modified with pentathia-15-crown-5 Kaolinite/5-Br-PADAP, column Amberlite XAD-2/P.Ox column Penicillium italicum/Sepabeads SP 70 MWCNT/NBHAE

PF

Detection limit (␮g L−1 )

RSD (%)

Ref.

75 200

0.15 16.6

3.1 <6

[11] [12]

0.21 1.9a , 0.3b

1.8 –

[32] [33]

1

2.1

[34]

0.71 – – 0.83a , 6.1b

[35] [36] [37] This work

50 30

0.5 (mol L−1) Na2 S2 O3

200

−1)

160 60 25 250

4.0 (mol L 1.0 (mol L−1) 1.0 (mol L−1) 0.1 (mol L−1)

H2 SO4 HCl HCl Na2 S2 O3

4.3 12 0.55 0.03a , 0.01b

PF: preconcentration factor. a Parameter for Mn. b Parameter for Au.

Table 5. The recovery of spiked samples was satisfactory, and hence the presented procedure may be applied for the preconcentration of metal ions in different samples.

[13] [14]

4. Conclusions

[15] [16]

The present study demonstrates preparation and use of a sorbent based on the modification of MWCNTs with a Schiff base ligand. The modification of MWCNTs is simple, and the reagent remains in the column, which allows it to be used several times. The procedure offers a useful, rapid and reliable enrichment technique for preconcentration of Au(III) and Mn(II) in various samples with acceptable accuracy and precision. Analyte ions were quantitatively recovered by the investigated matrix ions. The detection limit and enrichment factor were better than for some previously reported methods for the determination of Au(III) or Mn(II) (Table 6).

[17]

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