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Synthesis and application of surface-imprinted activated carbon sorbent for solid-phase extraction and determination of copper (II)
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 422–427
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis and application of surface-imprinted activated carbon sorbent for solid-phase extraction and determination of copper (II) Zhenhua Li a,⇑, Jingwen Li b, Yanbin Wang a, Yajun Wei a a b
College of Chemical Engineering, Northwest University for Nationalities, Lanzhou 730030, PR China Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Science, Suzhou 215163, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Solid phase extraction combined with
Cu(II)-imprinted sorbent was used for selective extraction of Cu(II). The Cu(II)-imprinted sorbent had higher selectivity and adsorption capacity for Cu(II). Cu(II)-imprinted sorbent showed high selectivity coefficients for Cu(II) relative Zn(II), Ni(II) and Pb(II). The precision and accuracy of the method are satisfactory. The adsorbent has great potential for preconcentration of Pb(II).
a r t i c l e
i n f o
Article history: Received 26 April 2013 Received in revised form 25 July 2013 Accepted 8 August 2013 Available online 21 August 2013 Keywords: Cu(II)-imprinted Activated carbon Preconcentration Solid-phase extraction (SPE) Inductively coupled plasma atomic emission spectrometry (ICP-AES).
a b s t r a c t A new Cu(II)-imprinted amino-functionalized activated carbon sorbent was prepared by a surface imprinting technique for selective solid-phase extraction (SPE) of Cu(II) prior to its determination by inductively coupled plasma atomic emission spectrometry (ICP-AES). Experimental conditions for effective adsorption of Cu(II) were optimized with respect to different experimental parameters using static and dynamic procedures in detail. Compared with non-imprinted sorbent, the ion-imprinted sorbent had higher selectivity and adsorption capacity for Cu(II). The maximum static adsorption capacity of the ion-imprinted and non-imprinted sorbent for Cu(II) was 26.71 and 6.86 mg g1, respectively. The relatively selectivity factor values (ar) of Cu(II)/Zn(II), Cu(II)/Ni(II), Cu(II)/Co(II) and Cu(II)/Pb(II) were 166.16, 50.77, 72.26 and 175.77, respectively, which were greater than 1. Complete elution of the adsorbed Cu(II) from Cu(II)-imprinted sorbent was carried out using 2 mL of 0.1 mol L1 EDTA solution. The relative standard deviation of the method was 2.4% for eleven replicate determinations. The method was validated for the analysis by two certified reference materials (GBW 08301, GBW 08303), the results obtained is in good agreement with standard values. The developed method was also successfully applied to the determination of trace copper in natural water samples with satisfactory results. Ó 2013 Elsevier B.V. All rights reserved.
Introduction In recent years, the toxicity and the effect of trace elements on human health and the environment are receiving increasing attention in pollution and nutritional studies. Copper is one of the most ⇑ Corresponding author. Tel./fax: +86 931 4512932. E-mail address: [email protected] (Z. Li). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.08.045
widespread heavy metal contaminants in environment. It is an important element for most life forms as a micronutrient, but is also toxic at high concentrations [1]. Therefore, sensitive, reproducible and accurate analytical methods are required for the determination of trace Cu in environmental and biological samples. Monitoring environmental pollutants at ultra-trace level needs an effective sample preconcentration step. Solid phase extraction (SPE) is the most common technique used for preconcentration
Z. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 422–427
of analytes in environmental waters because of its advantages of high enrichment factor, high recovery, rapid phase separation, low cost, low consumption of organic solvents and the ability of combination with different detection techniques in the form of on-line or off-line mode [2,3]. The choice of sorbent is therefore a key point in SPE because it can control the analytical parameters such as selectivity, affinity and capacity [4,5]. Activated carbon, among a large variety of sorbents, is still by far the most important one in current use in the environmental pollution control due to its large surface area, high adsorption capacity, porous structure, selective adsorption and high purity standards [6]. However, without any surface treatment, activated carbon presents an adsorption capacity for metal ions from fair to as low to none, due to the fact that metal ions often exist in solution either as ions or as hydrous ionic complex [7,8]. For this reason, modification and impregnation techniques have long been used to increase the surface adsorption to activated carbon. However, the selectivity of these functionalized sorbents is usually unremarkable because many metals have the ability to bind with the ligands without consideration of the stereo chemical interactions between the ligand and metal ion. Recently, highly selective molecularly imprinted materials have been extensively studied [9–11]. Molecular imprinting has become a powerful method for the preparation of robust materials that have the ability to recognize a specific chemical species [12]. In molecular imprinting, a molecular ‘‘memory’’ is imprinted on the polymer. Molecular imprinting polymers (MIPs) are capable of recognizing and binding the desired molecular target with a high affinity and selectivity [13]. Metal ion imprinted polymers (IIPs) are similar to MIPs, but they can recognize metal ions after imprinting and retain all the virtues of MIPs [14]. IIPs have been investigated as highly selective sorbents for SPE in order to concentrate and clean up samples prior to analysis. One potential application that has recently attracted widespread interest is their use for clean up and enrichment of analytes present at low concentrations in complex matrices [15,16]. Numerous studies on IIPs and their use for selective preconcentration and separation of metal ions have been reported [17,18]. However, to the best of our knowledge, there has been no report on using surface-imprinted activated carbon sorbent for Cu(II) enrichment. The main aim of the present work is to synthesize a new ionfunctionalized activated carbon sorbent by combining a surface molecular imprinting technique for selective separation and preconcentration of Cu(II) in natural water samples. The proposed method presented high selectivity and adsorption capacity for Cu(II), and possessed simple, convenient and accurate characteristics.
Experimental Apparatus An Iris Advantage ER/S inductively coupled plasma emission spectrometer, Thermo Jarrel Ash (Franklin, MA, USA) was used for all metal ions determination. The instrumental parameters were those recommended by the manufacturer. The wavelength selected for Cu was 216.999 nm. The pH value was controlled with a pHs-3C digital pH meter (Shanghai Lei Ci Device Works, China). Infrared spectra (4000–400 cm1) in KBr were recorded on a Nicolet NEXUS 670 FT-IR apparatus (USA). A JSM-6701F (JEOL, Japan) was used to examine scanning electron microscope (SEM) images. A reciprocating shaker bath SHZ-88-1, (Taicang Laboratorial Equipment Factory, Jiangsu, China) was used for controlling the shaking speed. An YL-110 peristaltic pump (General Research Institute for Non-ferrous Metals, Beijing, China) was used in the
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separation/preconcentration process. A PTFE (polytetrafluoroethylene) column (50 mm 9.0 mm i.d.) (Tianjin Jinteng Instrument Factory, Tianjin, China) was used. Chemicals and reagents Reagents of analytical and spectral purity were used for all experiments and doubly distilled deionized water (DDW) was used throughout. Standard labware and glassware used were repeatedly cleaned with HNO3 and rinsed with double distilled water, according to a published procedure [19]. Standard stock solutions of Cu(II), Pb(II), Ni(II) and Zn(II) (1 mg mL1) were prepared by dissolving appropriate amounts of analytical grade salts in DDW with addition of in 1.0% HNO3 and further diluted daily prior to use. Activated carbon (AC), gas chromatographic grade, 40–60 mesh, (Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China) and diethylenetriamine (DETA) (The First Reagent Factory, Shanghai, China) were used in this work. N,N0 -dicyclohexylcarbodiimide (DCC) was purchased from Sinopharm Chemcial Reagent Co. Ltds. (Shanghai, China). The certified reference materials (GBW 08301, river sediment; GBW 08303, polluted farming soil) were provided by the National Research Center for Certified Reference Materials (Beijing, China). Sample preparation River water was collected from the Yellow River, Lanzhou, China. The water samples were filtered through a 0.45 lm PTFE Millipore filter, and acidified to a pH of about 2 with concentrated HCl prior to storage for use. Tap water samples taken from our research laboratory were analyzed without pretreatment. The pH value was adjusted to 2 with 0.1 mol L1 HCl prior to use. The certified reference materials (GBW 08301, river sediment; GBW 08303, polluted farming soil) were digested according to literature [20]. A portion (50–100 mg) of the certified sediment sample was accurately weighed into a 50 mL container (or beaker) and aquaregia (12 mL concentrated hydrochloric acid and 4.0 mL of concentrated nitric acid) was added to the sample. The container was covered with a watch glass and the mixture was evaporated on a hot plate at 95 °C almost to dryness. Then 8.0 mL of aquaregia was added to the residue and the mixture was again evaporated to dryness. After cooling, resulting mixture was filtered through a 0.45 lm polytetrafluoroethylene (PTFE) Millipore filter. The sample was diluted to 10 mL with double distilled water and was analyzed by the preconcentration procedure. Synthesis of SPE sorbent Preparation of carboxylic derivative of activated carbon (ACACOOH) Activated carbon powder was first purified with 10% (v/v) hydrochloric acid solution for 24 h so as to remove the metal ions and other impurities sorbed on it. Then 10 g of purified activated carbon was suspended in 300 mL of 32.5% (v/v) nitric acid solution under stirring and heating for 5 h at 60 °C. The mixture was filtered and washed with deionized water to neutral and dried under vacuum at 80 °C for 8 h. The product was carboxylic derivative of activated carbon (ACACOOH). Preparation of the Cu(II) imprinted and non-imprinted sorbent 2.125 g of CuCl26H2O was dissolved in 80 mL of ethanol and 3 mL of diethylenetriamine (DETA) was slowly added to this solution with continuous stirring at room temperature. Then 5.0 g amount of ACACOOH and 1.0 g of N,N0 -dicyclohexylcarbodiimide (DCC) were also added into the suspension and refluxed for 48 h, the product was recovered by filtration, washed with ethanol and stirred in 100 mL of 0.1 mol L1 EDTA to remove Cu(II) ions for
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10 h. The final product (Cu(II)-imprinted sorbent) was cleaned with doubly distilled water and then dried under vacuum at 70 °C for 12 h. The non-imprinted sorbent was also prepared using an identical procedure without adding CuCl26H2O. The scheme for preparation of Cu(II)-imprinted sorbent described in Fig. 1. Procedures Static procedure Cu(II)-imprinted or non-imprinted sorbent (30 mg) were added to 10 mL metal ions solution after adjusting to the desired pH value. Then the mixture was shaken vigorously for 10 min to facilitate adsorption of the metal ions onto the sorbent. After the solution was centrifuged, the concentrations of the metal ions in the solution were determined by ICP-AES. The adsorption capacity, the distribution ratio, the selectivity coefficient and the relative selectivity coefficient were calculated as the following equation:
Q¼
ðC o C e ÞV ; W
E¼
ðC o C e Þ ; Co
D¼
Q ; Ce
a¼
DCu ai ; ar ¼ DM an
where Q represents the adsorption capacity (mg g1), Co and Ce represent the initial and equilibrium concentration of Cu(II) (lg mL1), W is the mass of Cu(II)-imprinted sorbent (g) and V is the volume of metal ion solution (L), E is the extraction percentage, D is the distribution ratio (mL g1), DCu and DM represent the distribution ratios of Cu(II) and Ni(II), Co(II), Zn(II), or Pd(II), a is the selectivity coefficient, ar is the relative selectivity coefficient, ai and an represent the selectivity factor of Cu(II)-imprinted sorbent and non-imprinted sorbent, respectively. Dynamic procedure A total of 50 mg Cu(II)-imprinted sorbent was filled into a PTFE column. The ends of the column were plugged with a small portion of glass wool to retain the sorbent in the column. After cleaning by passing through ethanol, 0.1 mol L1 HCl solution and doubly distilled water once more, the column was conditioned to the desired pH with 0.1 mol L1 NH4Cl buffer solutions. Each solution was passed through the column at a flow rate of 3.0 mL min1. Afterwards, the metal ions retained on the column were eluted with 0.1 mol L1 EDTA solution at a fiow rate of 3.0 mL min1. The analytes in the elution were determined by ICP-AES. Results and discussion Characterization of Cu(II)-imprinted sorbent SEM was conducted to characterize the activated carbon and Cu(II)-imprinted sorbent. The representative SEM images were shown in Fig. 2. The activated carbon was formed a dense and robust structure (see Fig. 2a). In contrast, the surface structure and physical characteristics of Cu(II)-imprinted sorbent were changed.
The hole structure was more looser, and a high specific surface area was also obtained (see Fig. 2b). The modified activated carbon was also confirmed by IR analysis (see Fig. 3). Comparison of the IR spectrum of ACACOOH with activated carbon, a new bang (1710 cm1) appeared in ACACOOH due to C@O stretching vibration of the carboxylic acid group, which indicated the carboxylic derivative of activated carbon was prepared successfully. Several new peaks also appeared in the spectrum of Cu(II)-imprinted sorbent. According to the literatures [21,22], the new peaks can be assigned as follows: the peak at 1710 cm1 is due to C@O stretching vibration. The peak at 1594 cm1 is caused by CAN stretching vibration and NAH bending vibration. The bands around 3424 cm1 can be assigned to NAH stretching vibration. Imprinted and non-imprinted sorbent showed a very similar location and appearance of the major bands. It indicated that NAH was recovered after removal of Cu(II) in the imprinted sorbent. These results suggested that ANH2 has been grafted onto the surface of activated carbon after modification. Effect of pH The effect of varying pH values on Cu(II) uptake was investigated using the recommended procedure (static method). Aliquots of 10 mL of the buffer solutions containing 1.0 lg mL1 of Cu(II) was tested by equilibrating 30 mg of Cu(II)-imprinted sorbent under different pH conditions. It can be seen from Fig. 4, the adsorption quantity of Cu(II) increases with the increase in the studied pH ranges. Below pH 2.0, the adsorption quantity for Cu(II) was very low which is attributed to the protonation of the sorbent, but the adsorption rate is increasing rapidly above pH 2.0 and the adsorption quantity is near the maximum quantity above pH 4.0. To avoid hydrolyzing at higher pH value, pH 4.0 was selected as the enrichment acidity for further study. The same procedure was used for preconcentration and separation of Cu(II) by non-imprinted sorbent. Compared with Cu(II)-imprinted sorbent, the adsorption of Cu(II) was low at the same pH value. Effect of elution condition on recovery The elution of the analyte from the column containing the Cu(II)-imprinted sorbent was studied by using 2 mL of various concentrations of HCl or EDTA as eluent following the dynamic procedure. We found that Cu(II) ions could be quantitatively eluted with 2 mL of 0.1 mol L1 EDTA solution. When EDTA is used as a desorption agent, the coordination spheres of chelated Cu(II) ions is disrupted and subsequently Cu(II) ions are released from the iron templates into the desorption medium. The quantitative recoveries (>95%) of Cu(II) can be obtained using 2 mL of 0.1 mol L1 EDTA as eluent. Therefore, 2 mL of 0.1 mol L1 EDTA was used as eluent in subsequent experiments.
Fig. 1. Scheme for preparation of Cu(II)-imprinted sorbent.
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Fig. 2. SEM images of activated carbon (a) and Cu(II)-imprinted sorbent (b).
Fig. 3. FT-IR spectra of activated carbon (a), ACACOOH (b) and Cu(II)-imprinted sorbent (c).
Adsorption rate Uptake kinetics of Cu(II) by the imprinted sorbent was also examined by static procedure. The results in Fig. 5 showed that the 95% uptake of Cu(II) was achieved within 10 min. It is clear that the solid-phase extraction process of the present sorbent is faster than most of the traditional ion-imprinted sorbent [23–25]. This indicated that the surface imprinting greatly facilitates diffusion of Cu(II) to the binding site and the adsorption is a rapid kinetic process.
Therefore, the effect of the flow rate of sample solution was examined under the optimum conditions (pH, eluent, etc.) by passing 50 mL of sample solution through the microcolumn with a peristaltic pump. The flow rate was adjusted in a range of 0.5–5.0 mL min1. It was found from Fig. 6 that the retention of Cu(II) was practically not changed up to 3.0 mL min1 flow rate. The recovery of Cu(II) decreased slightly when the flow rate is over 3.0 mL min1. Thus, a flow rate of 3.0 mL min1 is employed in this work.
Maximum sample volume and enrichment factor Effect of flow rate In a SPE system, the flow rate of sample solution not only affects the recoveries of analytes, but also controls the analysis time.
The enrichment factor was studied by recommended dynamic procedure by increasing volume of Cu(II) solution and keeping the total amount of loaded Cu(II) constant to 1.0 lg. For this
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Effect of coexisting ions
Adsorption (%)
100 80 60 40 Cu(II)-imprinted sorbent Non-imprinted sorbent
20 0
1
2
3
4
5
6
7
pH Fig. 4. Effect of pH on adsorption of 1.0 lg mL1 Cu(II) on Cu(II)-imprinted sorbent and non-imprinted sorbent.
100
Different foreign ions were added to equal quantities of the diluted mixed standard solutions and enriched and determined according to the general procedure. The results showed that up 2 to 2000-fold K+, Na+, Ca2+, Mg2+, Cl, NHþ 4 ; 500-fold NO3 , SO4 ; 2+ 2+ 2+ 2+ 3+ 2+ 2+ 100-fold Co , Mn , Pb , Cd , Cr , Hg , Ni and 50-fold PO3 4 ions do not affect the separation process. There are three possible factors for this reason [26]. One is the amino-functionalized group inherent selectivity. The amino group is a soft base and it would not interact with alkali metal and alkali earth metal ions that are classified as hard acids. The second is the hole-size selectivity. The size of Cu(II) exactly fits the cavity of the Cu(II)-imprinted sorbent. The third is the coordination-geometry selectivity because the Cu(II)-imprinted sorbent can provide the ligand groups arranged in a suitable way required for coordination of Cu(II). Although some ions have similar size with Cu(II), and some ions have high affinity with the amino ligand, the Cu(II)-imprinted sorbent still exhibits high selectivity for extraction of Cu(II) in the presence of other metal ions. These results suggest that the coordination-geometry selectivity may dominate in the selectivity enhancement.
Adsorption (%)
90
Adsorption capacity of Cu(II)-imprinted sorbent for Cu(II) 80 70 60 50 0
5
10
15
20
25
30
Shaking time (min) Fig. 5. Adsorption rate of Cu(II) on Cu(II)-imprinted sorbent. Other conditions: pH 4.0, temperature 20 °C.
100
Recovery (%)
95 90 85 80 75 0
1
2
3
4
5
The flow rate (mL/min) Fig. 6. Effect of solution fiow rate on recovery of Cu(II). Other conditions: volume 50 mL, pH 4.0, temperature 20 °C.
purpose, 10, 50, 100, 150, 200, 250 and 300 mL of sample solutions containing 1.0 lg of Cu(II) were passed through the column at the optimum flow rate. The maximum sample volume can be up to 250 mL with the recovery >95%. Therefore, 250 mL of sample solution was adopted for the preconcentration of analytes from sample solutions. And a high enrichment factor of 125 was obtained because 2 mL of 0.1 mol L1 EDTA was used as eluent in these experiments.
The adsorption capacity is an important factor because it determines how much sorbent is required to quantitatively concentrate the analytes from a given solution. The adsorption capacity was tested following the general procedure. To measure the adsorption capacity, 30 mg of Cu(II)-imprinted or non-imprinted sorbent was equilibrated with 50 mL of various concentrations (10– 300 lg mL1) of Cu(II) solutions adjusted with 0.1 mol L1 of HNO3 or NH3H2O at pH 4. A breakthrough curve was gained by plotting the concentration (lg mL1) vs. the micrograms of Cu(II) adsorbed per gram of sorbent. From the breakthrough curve, the maximum adsorption capacity of the ion-imprinted and non-imprinted sorbent for Cu(II) were found to be 26.71 and 6.86 mg g1, respectively. The adsorption capacity of the ion-imprinted was about four times of non-imprinted one. The results showed that the ion-imprinted sorbent had a high adsorption capacity for Cu(II). This difference indicates the imprinting play an important role in the adsorption behavior. During the preparation of the imprinted sorbent, the presence of Cu(II) made the ligands arrange orderly. After the removal of Cu(II), the imprinted cavity and specific binding sites of functional groups in a predetermined orientation were formed, whereas no such specificity was found in non-imprinted sorbent. In addition, compared with other traditional ion-imprinted sorbent for the separation and preconcentration for Cu(II) [27–29], the new surface-imprinted activated carbon sorbent had a relatively high capacity. Effect of column reuse In order to examine the long-term stability of the adsorbent, several extraction and elution operations cycles were carried out in this work. The adsorbent can be reused after desorption with 0.1 mol L1 EDTA solution. The results provided that the column was relatively stable without obvious decrease of recoveries up to at least 9 extraction–elution cycles. Therefore, repeated use of the adsorbent is possible. Selectivity of the imprinted sorbent In order to evaluate the selectivity of the imprinted sorbent, competitive enrichment of Cu(II)/Zn(II), Cu(II)/Ni(II), Cu(II)/Co(II) and Cu(II)/Pb(II) from their mixture was investigated in static pro-
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Z. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 422–427 Table 1 Competitive loading of Cu(II) and Zn(II), Ni(II), Co(II), Pd(II) ions by the Cu(II)-imprinted and non-imprinted sorbent. Metal ions
D (mL g1)
E (%)
Cu(II) Zn(II) Ni(II) Co(II) Pb(II)
Non-imprinted
Imprinted
Non-imprinted
ai
an
ar
99.45 32.51 55.38 57.61 29.31
65.31 45.27 39.44 50.37 42.69
60732.60 160.59 413.80 453.07 138.21
627.56 275.72 217.07 338.30 251.05
378.18 146.77 134.05 439.42
2.276 2.891 1.855 2.500
166.16 50.77 72.26 175.77
Table 2 Analytical results for the determination of Cu(II) in Yellow River and tap water samples. Ion
Added
Founda
Recovery (%)
0 5 10
4.35 ± 0.24 9.28 ± 0.15 14.55 ± 0.21
99.3 101.3
0 5 10
3.28 ± 0.11 8.13 ± 0.20 13.07 ± 0.13
98.2 98.4
1
Cu(II) (lg mL ) Yellow River water
Tap water
a
a
Imprinted
The value following ‘‘±’’ is the standard deviation (n = 5).
cedure because these five ions have the same charge and similar ionic radius. In their binary mixture, the two metal ions had the same concentration of 5 lg mL1 and the sorbent was 30 mg. As can be seen in Table 1, the distribution ratio (D) values of Cu(II)imprinted sorbent for Cu(II) were highly greater than that of other metals. The relatively selectivity factor values (ar) of Cu(II)/Zn(II), Cu(II)/Ni(II), Cu(II)/Co(II) and Cu(II)/Pb(II) were 166.16, 50.77, 72.26 and 175.77, respectively, which were greater than 1. The results indicate that the Cu(II)-imprinted sorbent had higher selectivity for Cu(II). This means that Cu(II) can be determined even in the presence of Zn(II), Ni(II), Co(II) and Pd(II). Analytical precision and accuracy of the proposed method Under the selected conditions, eleven portions of standard solutions were analyzed simultaneously following the general procedure. The result showed that the relative standard deviation (RSD) was 2.4%, which indicated that the method had good precision for the analysis of trace Cu(II) in solution samples. The detection limit (blank + 3r) defined as by IUPAC were found to be 0.19 lg L1. In order to ascertain the accuracy of the proposed procedure, the method has been applied to analysis of Cu(II) in two certified reference materials (GBW 08301, river sediment; GBW 08303, polluted farming soil). The results (53 ± 2.5 and 122 ± 3.7 lg g1) obtained were good agreement compare to the certified values (53 ± 6.0 and 120 ± 6.0 lg g1) using a t-test at 95% confidence limits [30]. Moreover, the suitability of the method for the analysis of natural water samples was checked by spiking samples of the tap water and Yellow River water with different concentrations of Cu(II). The results are given in Table 2. It was found that the recoveries of analyte were in the range of 98–102%. Evidently, the method is reliable, feasible and provides satisfactory results. Conclusions Ion-imprinted sorbent have attracted widespread attention as highly selective sorbent to remove metal ions selectively in the presence of other metal ions. In this study, a surface Cu(II)imprinted sorbent was prepared. The imprinted sorbent showed
high affinity, selectivity and good accessibility for Cu(II) than non-imprinted sorbent. In addition, the adsorption capacity of Cu(II)-imprinting for Cu(II) was larger than that of non-imprinted sorbent and other methods reported. A selective and sensitive method was developed and successfully applied to the analysis of trace copper in certified and natural samples with satisfactory results. Acknowledgments This work was supported by the research start-up funds for introduced talents, Northwest University for Nationalities (NO. xbmuyjrc201109) and the Fundamental Research Funds for the Central Universities, Northwest University for Nationalities (NO. zyz2011065). References [1] J.X. Li, J. Hu, G.D. Sheng, G.X. Zhao, Q. Huang, Colloids Surf. A: Physicochem. Eng. Aspects 349 (2009) 195–201. [2] K. Pyrzynska, Crit. Rev. Anal. Chem. 29 (1999) 313–321. [3] S.A. Ahmed, J. Hazard. Mater. 156 (2008) 521–529. [4] C.F. Poole, Anal. Chem. 22 (2003) 362–373. [5] J.R. Dean, Extraction Methods for Environmental Analysis, Wiley, New York, 1998. [6] V.A. Lemos, L.S.G. Teixeira, M.A. Bezerra, A.C.S. Costa, J.T. Castro, L.A.M. Cardoso, DS. Jesus, E.S. Santos, P.X. Baliza, L.N. Santos, Appl. Spectrosc. Rev. 43 (2008) 303–334. [7] D. Mohan, S. Chander, Colloids Surf. A 177 (2000) 183–196. [8] A. Ozer, M. Tanyildizi, F. Tumen, Environ. Technol. 19 (1998) 1119–1121. [9] A.C.B. Dias, E.C. Figueiredo, V. Grassi, E.A.G. Zagatto, M.A.Z. Arruda, Talanta 76 (2008) 988–996. [10] C. He, Y. Long, J. Pan, K. Li, F. Liu, J. Biochem. Biophys. Meth. 70 (2007) 133–150. [11] J. Tan, Z. Jiang, R. Li, X. Yan, TrAC Trends Anal. Chem. 39 (2012) 207–217. [12] H.H. Yang, S.Q. Zhang, W. Yang, X.L. Chen, Z.X. Zhang, J.G. Xu, X.R. Wang, J. Am. Chem. Soc. 126 (2004) 4054–4055. [13] K. Haupt, Analyst 126 (2001) 747–756. [14] S.Y. Bae, G.L. Southard, G.M. Murray, Anal. Chim. Acta 397 (1999) 173–181. [15] K. Ensing, T. Boer, Anal. Chem. 18 (1999) 138–145. [16] Y. Liu, X. Chang, S. Wang, Anal. Chim. Acta 519 (2004) 173–179. [17] B. Gao, F. An, Y. Zhu, Polymer 48 (2007) 2288–2297. [18] T. Prasada Rao, R. Kala, S. Daniel, Anal. Chim. Acta 578 (2006) 105–116. [19] D.P.H. Laxen, R.M. Harrison, Anal. Chem. 53 (1981) 345–350. [20] Y.W. Liu, X.J. Chang, Y. Guo, S.M. Meng, J. Hazard. Mater. B 135 (2006) 389– 394. [21] H.T. Tang, Organic Compound Spectra Determination, Publishing house of Beijing University, Beijing, 1992, pp. 124–159. [22] Q.N. Dong, IR Spectrum Method, Publishing house of the Chemical Industry, Beijing, 1979, pp. 104–168. [23] Y. Liu, Y. Zai, X. Chang, Y. Guo, S. Meng, F. Feng, Anal. Chim. Acta 575 (2006) 159–165. [24] S. Büyüktiryaki, R. Say, A. Ersöz, E. Birlik, A. Denizli, Talanta 67 (2005) 640– 645. [25] H. Ebrahimzadeh, M. Behbahani, Y. Yamini, L. Adlnasab, A. Asgharinezhad, React. Funct. Polym. 73 (2013) 23–29. [26] G.Z. Fang, J. Tan, X.P. Yan, Anal. Chem. 77 (2005) 1734–1739. [27] M. Shamsipur, J. Fasihi, A. Khanchi, R. Hassani, K. Alizadeh, H. Shamsipur, Anal. Chim. Acta 599 (2007) 294–301. [28] I. Dakova, I. Karadjova, I. Ivanov, V. Georgieva, B. Evtimova, G. Georgiev, Anal. Chim. Acta 584 (2007) 196–203. [29] X. Luo, S. Luo, Y. Zhan, H. Shu, Y. Huang, X. Tu, J. Hazard. Mater. 192 (2011) 949–955. [30] J.C. Miller, J.N. Miller, Statistics for Analytical Chemistry, Ellis Horwood, New York, 1988.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis and application of surface-imprinted activated carbon sorbent for solid-phase extraction and determination of copper (II) Zhenhua Li a,⇑, Jingwen Li b, Yanbin Wang a, Yajun Wei a a b
College of Chemical Engineering, Northwest University for Nationalities, Lanzhou 730030, PR China Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Science, Suzhou 215163, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Solid phase extraction combined with
Cu(II)-imprinted sorbent was used for selective extraction of Cu(II). The Cu(II)-imprinted sorbent had higher selectivity and adsorption capacity for Cu(II). Cu(II)-imprinted sorbent showed high selectivity coefficients for Cu(II) relative Zn(II), Ni(II) and Pb(II). The precision and accuracy of the method are satisfactory. The adsorbent has great potential for preconcentration of Pb(II).
a r t i c l e
i n f o
Article history: Received 26 April 2013 Received in revised form 25 July 2013 Accepted 8 August 2013 Available online 21 August 2013 Keywords: Cu(II)-imprinted Activated carbon Preconcentration Solid-phase extraction (SPE) Inductively coupled plasma atomic emission spectrometry (ICP-AES).
a b s t r a c t A new Cu(II)-imprinted amino-functionalized activated carbon sorbent was prepared by a surface imprinting technique for selective solid-phase extraction (SPE) of Cu(II) prior to its determination by inductively coupled plasma atomic emission spectrometry (ICP-AES). Experimental conditions for effective adsorption of Cu(II) were optimized with respect to different experimental parameters using static and dynamic procedures in detail. Compared with non-imprinted sorbent, the ion-imprinted sorbent had higher selectivity and adsorption capacity for Cu(II). The maximum static adsorption capacity of the ion-imprinted and non-imprinted sorbent for Cu(II) was 26.71 and 6.86 mg g1, respectively. The relatively selectivity factor values (ar) of Cu(II)/Zn(II), Cu(II)/Ni(II), Cu(II)/Co(II) and Cu(II)/Pb(II) were 166.16, 50.77, 72.26 and 175.77, respectively, which were greater than 1. Complete elution of the adsorbed Cu(II) from Cu(II)-imprinted sorbent was carried out using 2 mL of 0.1 mol L1 EDTA solution. The relative standard deviation of the method was 2.4% for eleven replicate determinations. The method was validated for the analysis by two certified reference materials (GBW 08301, GBW 08303), the results obtained is in good agreement with standard values. The developed method was also successfully applied to the determination of trace copper in natural water samples with satisfactory results. Ó 2013 Elsevier B.V. All rights reserved.
Introduction In recent years, the toxicity and the effect of trace elements on human health and the environment are receiving increasing attention in pollution and nutritional studies. Copper is one of the most ⇑ Corresponding author. Tel./fax: +86 931 4512932. E-mail address: [email protected] (Z. Li). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.08.045
widespread heavy metal contaminants in environment. It is an important element for most life forms as a micronutrient, but is also toxic at high concentrations [1]. Therefore, sensitive, reproducible and accurate analytical methods are required for the determination of trace Cu in environmental and biological samples. Monitoring environmental pollutants at ultra-trace level needs an effective sample preconcentration step. Solid phase extraction (SPE) is the most common technique used for preconcentration
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of analytes in environmental waters because of its advantages of high enrichment factor, high recovery, rapid phase separation, low cost, low consumption of organic solvents and the ability of combination with different detection techniques in the form of on-line or off-line mode [2,3]. The choice of sorbent is therefore a key point in SPE because it can control the analytical parameters such as selectivity, affinity and capacity [4,5]. Activated carbon, among a large variety of sorbents, is still by far the most important one in current use in the environmental pollution control due to its large surface area, high adsorption capacity, porous structure, selective adsorption and high purity standards [6]. However, without any surface treatment, activated carbon presents an adsorption capacity for metal ions from fair to as low to none, due to the fact that metal ions often exist in solution either as ions or as hydrous ionic complex [7,8]. For this reason, modification and impregnation techniques have long been used to increase the surface adsorption to activated carbon. However, the selectivity of these functionalized sorbents is usually unremarkable because many metals have the ability to bind with the ligands without consideration of the stereo chemical interactions between the ligand and metal ion. Recently, highly selective molecularly imprinted materials have been extensively studied [9–11]. Molecular imprinting has become a powerful method for the preparation of robust materials that have the ability to recognize a specific chemical species [12]. In molecular imprinting, a molecular ‘‘memory’’ is imprinted on the polymer. Molecular imprinting polymers (MIPs) are capable of recognizing and binding the desired molecular target with a high affinity and selectivity [13]. Metal ion imprinted polymers (IIPs) are similar to MIPs, but they can recognize metal ions after imprinting and retain all the virtues of MIPs [14]. IIPs have been investigated as highly selective sorbents for SPE in order to concentrate and clean up samples prior to analysis. One potential application that has recently attracted widespread interest is their use for clean up and enrichment of analytes present at low concentrations in complex matrices [15,16]. Numerous studies on IIPs and their use for selective preconcentration and separation of metal ions have been reported [17,18]. However, to the best of our knowledge, there has been no report on using surface-imprinted activated carbon sorbent for Cu(II) enrichment. The main aim of the present work is to synthesize a new ionfunctionalized activated carbon sorbent by combining a surface molecular imprinting technique for selective separation and preconcentration of Cu(II) in natural water samples. The proposed method presented high selectivity and adsorption capacity for Cu(II), and possessed simple, convenient and accurate characteristics.
Experimental Apparatus An Iris Advantage ER/S inductively coupled plasma emission spectrometer, Thermo Jarrel Ash (Franklin, MA, USA) was used for all metal ions determination. The instrumental parameters were those recommended by the manufacturer. The wavelength selected for Cu was 216.999 nm. The pH value was controlled with a pHs-3C digital pH meter (Shanghai Lei Ci Device Works, China). Infrared spectra (4000–400 cm1) in KBr were recorded on a Nicolet NEXUS 670 FT-IR apparatus (USA). A JSM-6701F (JEOL, Japan) was used to examine scanning electron microscope (SEM) images. A reciprocating shaker bath SHZ-88-1, (Taicang Laboratorial Equipment Factory, Jiangsu, China) was used for controlling the shaking speed. An YL-110 peristaltic pump (General Research Institute for Non-ferrous Metals, Beijing, China) was used in the
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separation/preconcentration process. A PTFE (polytetrafluoroethylene) column (50 mm 9.0 mm i.d.) (Tianjin Jinteng Instrument Factory, Tianjin, China) was used. Chemicals and reagents Reagents of analytical and spectral purity were used for all experiments and doubly distilled deionized water (DDW) was used throughout. Standard labware and glassware used were repeatedly cleaned with HNO3 and rinsed with double distilled water, according to a published procedure [19]. Standard stock solutions of Cu(II), Pb(II), Ni(II) and Zn(II) (1 mg mL1) were prepared by dissolving appropriate amounts of analytical grade salts in DDW with addition of in 1.0% HNO3 and further diluted daily prior to use. Activated carbon (AC), gas chromatographic grade, 40–60 mesh, (Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China) and diethylenetriamine (DETA) (The First Reagent Factory, Shanghai, China) were used in this work. N,N0 -dicyclohexylcarbodiimide (DCC) was purchased from Sinopharm Chemcial Reagent Co. Ltds. (Shanghai, China). The certified reference materials (GBW 08301, river sediment; GBW 08303, polluted farming soil) were provided by the National Research Center for Certified Reference Materials (Beijing, China). Sample preparation River water was collected from the Yellow River, Lanzhou, China. The water samples were filtered through a 0.45 lm PTFE Millipore filter, and acidified to a pH of about 2 with concentrated HCl prior to storage for use. Tap water samples taken from our research laboratory were analyzed without pretreatment. The pH value was adjusted to 2 with 0.1 mol L1 HCl prior to use. The certified reference materials (GBW 08301, river sediment; GBW 08303, polluted farming soil) were digested according to literature [20]. A portion (50–100 mg) of the certified sediment sample was accurately weighed into a 50 mL container (or beaker) and aquaregia (12 mL concentrated hydrochloric acid and 4.0 mL of concentrated nitric acid) was added to the sample. The container was covered with a watch glass and the mixture was evaporated on a hot plate at 95 °C almost to dryness. Then 8.0 mL of aquaregia was added to the residue and the mixture was again evaporated to dryness. After cooling, resulting mixture was filtered through a 0.45 lm polytetrafluoroethylene (PTFE) Millipore filter. The sample was diluted to 10 mL with double distilled water and was analyzed by the preconcentration procedure. Synthesis of SPE sorbent Preparation of carboxylic derivative of activated carbon (ACACOOH) Activated carbon powder was first purified with 10% (v/v) hydrochloric acid solution for 24 h so as to remove the metal ions and other impurities sorbed on it. Then 10 g of purified activated carbon was suspended in 300 mL of 32.5% (v/v) nitric acid solution under stirring and heating for 5 h at 60 °C. The mixture was filtered and washed with deionized water to neutral and dried under vacuum at 80 °C for 8 h. The product was carboxylic derivative of activated carbon (ACACOOH). Preparation of the Cu(II) imprinted and non-imprinted sorbent 2.125 g of CuCl26H2O was dissolved in 80 mL of ethanol and 3 mL of diethylenetriamine (DETA) was slowly added to this solution with continuous stirring at room temperature. Then 5.0 g amount of ACACOOH and 1.0 g of N,N0 -dicyclohexylcarbodiimide (DCC) were also added into the suspension and refluxed for 48 h, the product was recovered by filtration, washed with ethanol and stirred in 100 mL of 0.1 mol L1 EDTA to remove Cu(II) ions for
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10 h. The final product (Cu(II)-imprinted sorbent) was cleaned with doubly distilled water and then dried under vacuum at 70 °C for 12 h. The non-imprinted sorbent was also prepared using an identical procedure without adding CuCl26H2O. The scheme for preparation of Cu(II)-imprinted sorbent described in Fig. 1. Procedures Static procedure Cu(II)-imprinted or non-imprinted sorbent (30 mg) were added to 10 mL metal ions solution after adjusting to the desired pH value. Then the mixture was shaken vigorously for 10 min to facilitate adsorption of the metal ions onto the sorbent. After the solution was centrifuged, the concentrations of the metal ions in the solution were determined by ICP-AES. The adsorption capacity, the distribution ratio, the selectivity coefficient and the relative selectivity coefficient were calculated as the following equation:
Q¼
ðC o C e ÞV ; W
E¼
ðC o C e Þ ; Co
D¼
Q ; Ce
a¼
DCu ai ; ar ¼ DM an
where Q represents the adsorption capacity (mg g1), Co and Ce represent the initial and equilibrium concentration of Cu(II) (lg mL1), W is the mass of Cu(II)-imprinted sorbent (g) and V is the volume of metal ion solution (L), E is the extraction percentage, D is the distribution ratio (mL g1), DCu and DM represent the distribution ratios of Cu(II) and Ni(II), Co(II), Zn(II), or Pd(II), a is the selectivity coefficient, ar is the relative selectivity coefficient, ai and an represent the selectivity factor of Cu(II)-imprinted sorbent and non-imprinted sorbent, respectively. Dynamic procedure A total of 50 mg Cu(II)-imprinted sorbent was filled into a PTFE column. The ends of the column were plugged with a small portion of glass wool to retain the sorbent in the column. After cleaning by passing through ethanol, 0.1 mol L1 HCl solution and doubly distilled water once more, the column was conditioned to the desired pH with 0.1 mol L1 NH4Cl buffer solutions. Each solution was passed through the column at a flow rate of 3.0 mL min1. Afterwards, the metal ions retained on the column were eluted with 0.1 mol L1 EDTA solution at a fiow rate of 3.0 mL min1. The analytes in the elution were determined by ICP-AES. Results and discussion Characterization of Cu(II)-imprinted sorbent SEM was conducted to characterize the activated carbon and Cu(II)-imprinted sorbent. The representative SEM images were shown in Fig. 2. The activated carbon was formed a dense and robust structure (see Fig. 2a). In contrast, the surface structure and physical characteristics of Cu(II)-imprinted sorbent were changed.
The hole structure was more looser, and a high specific surface area was also obtained (see Fig. 2b). The modified activated carbon was also confirmed by IR analysis (see Fig. 3). Comparison of the IR spectrum of ACACOOH with activated carbon, a new bang (1710 cm1) appeared in ACACOOH due to C@O stretching vibration of the carboxylic acid group, which indicated the carboxylic derivative of activated carbon was prepared successfully. Several new peaks also appeared in the spectrum of Cu(II)-imprinted sorbent. According to the literatures [21,22], the new peaks can be assigned as follows: the peak at 1710 cm1 is due to C@O stretching vibration. The peak at 1594 cm1 is caused by CAN stretching vibration and NAH bending vibration. The bands around 3424 cm1 can be assigned to NAH stretching vibration. Imprinted and non-imprinted sorbent showed a very similar location and appearance of the major bands. It indicated that NAH was recovered after removal of Cu(II) in the imprinted sorbent. These results suggested that ANH2 has been grafted onto the surface of activated carbon after modification. Effect of pH The effect of varying pH values on Cu(II) uptake was investigated using the recommended procedure (static method). Aliquots of 10 mL of the buffer solutions containing 1.0 lg mL1 of Cu(II) was tested by equilibrating 30 mg of Cu(II)-imprinted sorbent under different pH conditions. It can be seen from Fig. 4, the adsorption quantity of Cu(II) increases with the increase in the studied pH ranges. Below pH 2.0, the adsorption quantity for Cu(II) was very low which is attributed to the protonation of the sorbent, but the adsorption rate is increasing rapidly above pH 2.0 and the adsorption quantity is near the maximum quantity above pH 4.0. To avoid hydrolyzing at higher pH value, pH 4.0 was selected as the enrichment acidity for further study. The same procedure was used for preconcentration and separation of Cu(II) by non-imprinted sorbent. Compared with Cu(II)-imprinted sorbent, the adsorption of Cu(II) was low at the same pH value. Effect of elution condition on recovery The elution of the analyte from the column containing the Cu(II)-imprinted sorbent was studied by using 2 mL of various concentrations of HCl or EDTA as eluent following the dynamic procedure. We found that Cu(II) ions could be quantitatively eluted with 2 mL of 0.1 mol L1 EDTA solution. When EDTA is used as a desorption agent, the coordination spheres of chelated Cu(II) ions is disrupted and subsequently Cu(II) ions are released from the iron templates into the desorption medium. The quantitative recoveries (>95%) of Cu(II) can be obtained using 2 mL of 0.1 mol L1 EDTA as eluent. Therefore, 2 mL of 0.1 mol L1 EDTA was used as eluent in subsequent experiments.
Fig. 1. Scheme for preparation of Cu(II)-imprinted sorbent.
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Fig. 2. SEM images of activated carbon (a) and Cu(II)-imprinted sorbent (b).
Fig. 3. FT-IR spectra of activated carbon (a), ACACOOH (b) and Cu(II)-imprinted sorbent (c).
Adsorption rate Uptake kinetics of Cu(II) by the imprinted sorbent was also examined by static procedure. The results in Fig. 5 showed that the 95% uptake of Cu(II) was achieved within 10 min. It is clear that the solid-phase extraction process of the present sorbent is faster than most of the traditional ion-imprinted sorbent [23–25]. This indicated that the surface imprinting greatly facilitates diffusion of Cu(II) to the binding site and the adsorption is a rapid kinetic process.
Therefore, the effect of the flow rate of sample solution was examined under the optimum conditions (pH, eluent, etc.) by passing 50 mL of sample solution through the microcolumn with a peristaltic pump. The flow rate was adjusted in a range of 0.5–5.0 mL min1. It was found from Fig. 6 that the retention of Cu(II) was practically not changed up to 3.0 mL min1 flow rate. The recovery of Cu(II) decreased slightly when the flow rate is over 3.0 mL min1. Thus, a flow rate of 3.0 mL min1 is employed in this work.
Maximum sample volume and enrichment factor Effect of flow rate In a SPE system, the flow rate of sample solution not only affects the recoveries of analytes, but also controls the analysis time.
The enrichment factor was studied by recommended dynamic procedure by increasing volume of Cu(II) solution and keeping the total amount of loaded Cu(II) constant to 1.0 lg. For this
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Effect of coexisting ions
Adsorption (%)
100 80 60 40 Cu(II)-imprinted sorbent Non-imprinted sorbent
20 0
1
2
3
4
5
6
7
pH Fig. 4. Effect of pH on adsorption of 1.0 lg mL1 Cu(II) on Cu(II)-imprinted sorbent and non-imprinted sorbent.
100
Different foreign ions were added to equal quantities of the diluted mixed standard solutions and enriched and determined according to the general procedure. The results showed that up 2 to 2000-fold K+, Na+, Ca2+, Mg2+, Cl, NHþ 4 ; 500-fold NO3 , SO4 ; 2+ 2+ 2+ 2+ 3+ 2+ 2+ 100-fold Co , Mn , Pb , Cd , Cr , Hg , Ni and 50-fold PO3 4 ions do not affect the separation process. There are three possible factors for this reason [26]. One is the amino-functionalized group inherent selectivity. The amino group is a soft base and it would not interact with alkali metal and alkali earth metal ions that are classified as hard acids. The second is the hole-size selectivity. The size of Cu(II) exactly fits the cavity of the Cu(II)-imprinted sorbent. The third is the coordination-geometry selectivity because the Cu(II)-imprinted sorbent can provide the ligand groups arranged in a suitable way required for coordination of Cu(II). Although some ions have similar size with Cu(II), and some ions have high affinity with the amino ligand, the Cu(II)-imprinted sorbent still exhibits high selectivity for extraction of Cu(II) in the presence of other metal ions. These results suggest that the coordination-geometry selectivity may dominate in the selectivity enhancement.
Adsorption (%)
90
Adsorption capacity of Cu(II)-imprinted sorbent for Cu(II) 80 70 60 50 0
5
10
15
20
25
30
Shaking time (min) Fig. 5. Adsorption rate of Cu(II) on Cu(II)-imprinted sorbent. Other conditions: pH 4.0, temperature 20 °C.
100
Recovery (%)
95 90 85 80 75 0
1
2
3
4
5
The flow rate (mL/min) Fig. 6. Effect of solution fiow rate on recovery of Cu(II). Other conditions: volume 50 mL, pH 4.0, temperature 20 °C.
purpose, 10, 50, 100, 150, 200, 250 and 300 mL of sample solutions containing 1.0 lg of Cu(II) were passed through the column at the optimum flow rate. The maximum sample volume can be up to 250 mL with the recovery >95%. Therefore, 250 mL of sample solution was adopted for the preconcentration of analytes from sample solutions. And a high enrichment factor of 125 was obtained because 2 mL of 0.1 mol L1 EDTA was used as eluent in these experiments.
The adsorption capacity is an important factor because it determines how much sorbent is required to quantitatively concentrate the analytes from a given solution. The adsorption capacity was tested following the general procedure. To measure the adsorption capacity, 30 mg of Cu(II)-imprinted or non-imprinted sorbent was equilibrated with 50 mL of various concentrations (10– 300 lg mL1) of Cu(II) solutions adjusted with 0.1 mol L1 of HNO3 or NH3H2O at pH 4. A breakthrough curve was gained by plotting the concentration (lg mL1) vs. the micrograms of Cu(II) adsorbed per gram of sorbent. From the breakthrough curve, the maximum adsorption capacity of the ion-imprinted and non-imprinted sorbent for Cu(II) were found to be 26.71 and 6.86 mg g1, respectively. The adsorption capacity of the ion-imprinted was about four times of non-imprinted one. The results showed that the ion-imprinted sorbent had a high adsorption capacity for Cu(II). This difference indicates the imprinting play an important role in the adsorption behavior. During the preparation of the imprinted sorbent, the presence of Cu(II) made the ligands arrange orderly. After the removal of Cu(II), the imprinted cavity and specific binding sites of functional groups in a predetermined orientation were formed, whereas no such specificity was found in non-imprinted sorbent. In addition, compared with other traditional ion-imprinted sorbent for the separation and preconcentration for Cu(II) [27–29], the new surface-imprinted activated carbon sorbent had a relatively high capacity. Effect of column reuse In order to examine the long-term stability of the adsorbent, several extraction and elution operations cycles were carried out in this work. The adsorbent can be reused after desorption with 0.1 mol L1 EDTA solution. The results provided that the column was relatively stable without obvious decrease of recoveries up to at least 9 extraction–elution cycles. Therefore, repeated use of the adsorbent is possible. Selectivity of the imprinted sorbent In order to evaluate the selectivity of the imprinted sorbent, competitive enrichment of Cu(II)/Zn(II), Cu(II)/Ni(II), Cu(II)/Co(II) and Cu(II)/Pb(II) from their mixture was investigated in static pro-
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Z. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 422–427 Table 1 Competitive loading of Cu(II) and Zn(II), Ni(II), Co(II), Pd(II) ions by the Cu(II)-imprinted and non-imprinted sorbent. Metal ions
D (mL g1)
E (%)
Cu(II) Zn(II) Ni(II) Co(II) Pb(II)
Non-imprinted
Imprinted
Non-imprinted
ai
an
ar
99.45 32.51 55.38 57.61 29.31
65.31 45.27 39.44 50.37 42.69
60732.60 160.59 413.80 453.07 138.21
627.56 275.72 217.07 338.30 251.05
378.18 146.77 134.05 439.42
2.276 2.891 1.855 2.500
166.16 50.77 72.26 175.77
Table 2 Analytical results for the determination of Cu(II) in Yellow River and tap water samples. Ion
Added
Founda
Recovery (%)
0 5 10
4.35 ± 0.24 9.28 ± 0.15 14.55 ± 0.21
99.3 101.3
0 5 10
3.28 ± 0.11 8.13 ± 0.20 13.07 ± 0.13
98.2 98.4
1
Cu(II) (lg mL ) Yellow River water
Tap water
a
a
Imprinted
The value following ‘‘±’’ is the standard deviation (n = 5).
cedure because these five ions have the same charge and similar ionic radius. In their binary mixture, the two metal ions had the same concentration of 5 lg mL1 and the sorbent was 30 mg. As can be seen in Table 1, the distribution ratio (D) values of Cu(II)imprinted sorbent for Cu(II) were highly greater than that of other metals. The relatively selectivity factor values (ar) of Cu(II)/Zn(II), Cu(II)/Ni(II), Cu(II)/Co(II) and Cu(II)/Pb(II) were 166.16, 50.77, 72.26 and 175.77, respectively, which were greater than 1. The results indicate that the Cu(II)-imprinted sorbent had higher selectivity for Cu(II). This means that Cu(II) can be determined even in the presence of Zn(II), Ni(II), Co(II) and Pd(II). Analytical precision and accuracy of the proposed method Under the selected conditions, eleven portions of standard solutions were analyzed simultaneously following the general procedure. The result showed that the relative standard deviation (RSD) was 2.4%, which indicated that the method had good precision for the analysis of trace Cu(II) in solution samples. The detection limit (blank + 3r) defined as by IUPAC were found to be 0.19 lg L1. In order to ascertain the accuracy of the proposed procedure, the method has been applied to analysis of Cu(II) in two certified reference materials (GBW 08301, river sediment; GBW 08303, polluted farming soil). The results (53 ± 2.5 and 122 ± 3.7 lg g1) obtained were good agreement compare to the certified values (53 ± 6.0 and 120 ± 6.0 lg g1) using a t-test at 95% confidence limits [30]. Moreover, the suitability of the method for the analysis of natural water samples was checked by spiking samples of the tap water and Yellow River water with different concentrations of Cu(II). The results are given in Table 2. It was found that the recoveries of analyte were in the range of 98–102%. Evidently, the method is reliable, feasible and provides satisfactory results. Conclusions Ion-imprinted sorbent have attracted widespread attention as highly selective sorbent to remove metal ions selectively in the presence of other metal ions. In this study, a surface Cu(II)imprinted sorbent was prepared. The imprinted sorbent showed
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