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Specific mercury(II) adsorption by thymine-based sorbent
Talanta 78 (2009) 253–258
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Specific mercury(II) adsorption by thymine-based sorbent Xiangjun Liu, Cui Qi, Tao Bing, Xiaohong Cheng, Dihua Shangguan ∗ Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
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Article history: Received 18 August 2008 Received in revised form 31 October 2008 Accepted 5 November 2008 Available online 13 November 2008 Keywords: Selective adsorption Thymine Polymer sorbent Mercury(II)
a b s t r a c t A new kind of polymer sorbent based on the specific interaction of Hg(II) with nucleic acid base, thymine, is described for the selective adsorption of Hg(II) from aqueous solution. Two types of sorbents immobilized with thymine were prepared by one-step swelling and polymerization and graft polymerization, respectively. The maximum static adsorption capacity of the new polymer sorbents for Hg(II) is proportional to the density of thymine on their surface, up to 200 mg/g. Moreover, the new kind polymer sorbent shows excellent selectivity for Hg(II) over other interfering ions, such as Cu(II), Cd(II), Zn(II), Co(II), Ca(II) and Mg(II), exhibits very fast kinetics for Hg(II) adsorption from aqueous solution, and can be easily regenerated by 1.0 M HCl. It also has been successfully used for the selective adsorption of spiked Hg(II) from real tap water samples. This new thymine polymer sorbent holds a great promise in laboratory and industrial applications such as separation, on-line enrichment, solid-phase extraction, and removal of Hg(II) from pharmaceutical, food and environmental samples. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Heavy metal ion pollution has become widespread throughout the world as a result of industrialization, which significantly threats the ecosystem, especially the people’s health due to their severe toxicity. Mercury, as one of the most toxic heavy metal, can cause human disease, such as kidney toxicity, neurological damage and chromosome breakage, even at low concentration [1]. Mercury and mercury compounds are included in all lists of priority pollutants. Different regulations and guidelines have been developed for limiting their levels in water and sediments [2,3]. Burning coal, inorganic mercury used in chloro-alkali cells, electrical apparatus and chemical engineering process are the main sources of mercury pollutants introduced into the environment. Hg(II) is one of the main species of mercury pollutants in the environment, thus monitoring or removal of Hg(II) is quite necessary. However, in real samples, there are many coexisting interferences which seriously influence the detection or the removal efficiency of Hg(II), so the specific adsorption material for separation, preconcentration and removal of Hg(II) from the complex matrix is essential. Many methods have been reported for the adsorption of Hg(II) from aqueous media, such as solvent extraction [4], solid-phase extraction by polymer sorbents [5,6], activated carbon [7], chitosan [8], clay [9] and other materials [10,11]. Particularly polymer sorbents have recently attracted more attention because of their larger adsorption capacity, higher efficiency and easier prepara-
∗ Corresponding author. Tel.: +86 10 62528509; fax: +86 10 62528509. E-mail address: [email protected] (D. Shangguan). 0039-9140/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2008.11.005
tion. However, these sorbents commonly lack selectivity for Hg(II). Some attempts have been made to solve this problem, for example, thiol [12,13] or amide [14,15] modified sorbents have improved the selectivity, but it is still not enough since the interaction of these functional groups with Hg(II) is not specific. Recently, molecularly imprinted polymers (MIPs) for Hg(II) have been developed [16–18]. Although the MIPs sorbent showed better selectivity for Hg(II), the preparation process of the MIPs is complex and the adsorption capacity for Hg(II) is low. Moreover, using Hg(II) as template during the preparation of MIPs introduces a potential new source of mercury contamination. In recent years, Hg(II) has been reported to selectively bind to thymine/thymine (T/T) mismatched base pairs in DNA sequences, by forming T–Hg(II)–T complexes (Fig. 1a) [19–21]. NMR studies have demonstrated that Hg(II) binds directly to N-3 of two thymidine residues in place of two imino protons and forms N–Hg(II)–N bond (Fig. 1a) [21,22]. Several Hg(II) probes or sensors based on the specific interaction of T and Hg(II) have also been developed and showed great selectivity and sensitivity [23–25]. To date, all these studies focused on the single nucleotide polymorphisms analysis and Hg(II) detection, no attempts have been carried out to adsorb Hg(II) using this specific interaction. In this study, we extend the application of this specific interaction to selective adsorption of Hg(II) from aqueous solution samples. Thymine-modified polymer sorbents were prepared by copolymerizing of allyl thymine (AT, Fig. 1b) with ethylene dimethacrylate (EDMA) or by grafting of AT onto the surface of a support. The characteristic of these sorbents was evaluated by SEM micrographs, infrared (IR) spectra and X-ray photoelectron spectrometer (XPS). The adsorption capability, adsorption kinetics, selectivity, and reusability of the new kind
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2.4. Preparation of thymine polymer sorbent 2.4.1. Preparation of polyglycidyl methacrylate (PGMA) and polychloromethylstyrene (PCMSt) PGMA and PCMSt were prepared using dispersion polymerization [27]. Five milliliters of GMA or CMSt, 0.15 g of AIBN and 0.5 g of PVP were dissolved in 50 mL of ethanol and purged with N2 for 10 min. And then the reaction was carried out at 70 ◦ C for 24 h. The resultant beads were washed with ethanol and dried under vacuum at room temperature.
Fig. 1. Chemical structure of (a) proposed structure of T–Hg(II)–T and (b) allyl thymine (AT).
thymine polymer sorbent were investigated. Finally, the thymine polymer sorbent was successfully applied for the selective adsorption of Hg(II) which spiked in real water samples. 2. Experimental 2.1. Materials Thymine (T) was purchased from Sigma (USA). Allyl bromide was purchased from Beijing Xingjin Chemical Plant (Beijing, China). Sodium hydride (NaH) was purchased from Tianjin Fuchen Chemical Reagent Plant (Tianjin, China). Chloromethylstyrene (CMSt) and EDMA were purchased from Acros Organics (USA). Glycidyl methacrylate (GMA) and N,N,N ,N, ,N, pentamethyldiethylenetriamine (PMDETA) were purchased from Aldrich (USA). 2,2-Azobis(2-isobutyronitrile) (AIBN), benzoyl peroxide (BPO), poly(vinyl pyrrolidone) (PVP), poly(vinyl alcohol) (PVA) and sodium dodecylsulphonate (SDS) were purchased from Beijing Chemical Plant (Beijing, China). Dithizone and cuprous bromide (CuBr) were purchased from Beijing Chemical Reagent Company (Beijing, China). All other reagents (AR) used were purchase from Beijing Chemical Plant (Beijing, China). Doubly deionised water was used throughout this work. AIBN, BPO, CuBr and dithizone were re-purification prior to use. 2.2. Instrumentation 1 H NMR spectra were recorded on Bruker AV400 instrument. UV–vis absorption spectra were recorded on Shimadzu UV-1601PC spectrophotometer. FT-IR spectra in KBr were recorded on Bruker Tensor 27 instrument. SEM micrographs of the sorbent were obtained on a JEOL JSM-6700F field emission scanning electron microscopy. The concentration of mercury and other ions in a mixed solution were measured using an Inductive Coupled Plasma Optical Emission Spectrometer (ICP-OES) (PE-Optima 2000DV). The nitrogen content on the surface of the sorbents was determined by ESCALab220I-XL X-ray photoelectron spectrometer (XPS).
2.3. Synthesis of allyl thymine (AT) The synthesis of AT was performed as describing previously [26]. To a suspension of 2 g of thymine in 80 mL of dry DMF was slowly added 0.72 g of NaH. The suspension was stirred at room temperature for 1 h until no more gas evolved. Then 1.8 mL of allyl bromide was added dropwise. After 24 h, the excess NaH was quenched with saturated NH4 Cl aqueous solution. The mixture was concentrated under a vacuum and subjected to column chromatography (eluent: 40% ethyl acetate/petroleum ether) to yield some white solid. 1 H NMR (CDCl3 ): ı 8.61 (s, 1H, pyrimidine NH), 6.96 (d, 1H, pyrimidine H-6), 5.81–5.92 (m, 1H, –CH ), 5.24–5.33 (m, 2H, CH2 ), 4.33 (d, 2H, –CH2 –), 1.92 (d, 3H, pyrimidine–CH3 ).
2.4.2. Preparation of copolymerized thymine polymer sorbent The copolymerized thymine polymer sorbent (P1, P2 and P3) was prepared using a one-step swelling and polymerization method [27]. The typical process is as follows: A suspension of 0.1 g PGMA seeds in 10 mL of homogeneous solution containing PVA, SDS and distilled water was stirred at room temperature. Then an emulsion made of PVA, SDS, distilled water, 40 mg of BPO, 2 mL of chloroform and 1 mL of acetonitrile and monomer mixture (10 mmol) of AT and EDMA (molar amount of AT/EDMA: P1, 0/10; P2, 1/9 and P3, 5/5) was added dropwise to the suspension of PGMA seeds. After the monomer mixture was absorbed by the PGMA seeds completely, the polymerization was carried out at 60 ◦ C for 24 h. The polymer beads were washed with water and ethanol, and dried under vacuum at room temperature. 2.4.3. Preparation of grafted thymine polymer sorbent The grafted thymine polymer sorbent (P4) was prepared using the atom transfer radical polymerization (ATRP) method [28]. 0.5 g PCMSt was suspended in a solution of 5 mL of distilled water and methanol (1:1, v/v) containing 0.5 g of AT, and then 32 mg of CuBr and 140 L of PMDETA were added. The polymerization was carried out at room temperature for 24 h. The resultant beads (P4) were washed with water and ethanol, and dried under vacuum at room temperature. 2.5. Equilibration adsorption studies Adsorption of Hg(II) from aqueous solutions was investigated in batch experiments. Fifty milligrams of sorbent was equilibrated with 30 mL of different solutions at room temperature. Then, an aliquot of the supernatant was separated, and the quantity of the ion was determined by the dithizone spectrophotometric method (GB 7469-87, China). The adsorption capacity of the sorbent and the removal efficiency of the ion could be calculated using the following equations: Q =
(C0 − Ce )V W
E (%) =
C0 − Ce × 100 C0
(1) (2)
where Q represents the adsorption capacity (mg/g), C0 and Ce are the initial and equilibrium concentration (mg/L), and E is the removal efficiency (%). 2.6. Recycling test 0.1 g of thymine polymer sorbent was equilibrated with 30 mL of 2.0 mg/L Hg(II) solution (pH 8.0, 20 mM Na2 HPO3 /H3 PO4 ). After 2 h, the sorbent was regenerated for the next cycle by washing with 20 mL of 1.0 M HCl solution once, 10 mL of water twice and 20 mL of pH 8.0 Na2 HPO3 /H3 PO4 buffer once. The equilibrium concentration was determined using the dithizone spectrophotometric method (GB 7469-87, China) and the removal efficiency was calculated according to Eq. (2).
X. Liu et al. / Talanta 78 (2009) 253–258
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Fig. 2. SEM image of (A) copolymerized thymine polymer sorbent P3 and (B) grafted thymine polymer sorbent P4.
2.7. Selectivity studies The selectivity of the thymine polymer sorbent was investigated by ICP-OES. Fifty milligrams of thymine polymer or blank sorbent was suspended in 30 mL of aqueous solution of containing Hg(II) and other ions. After adsorption equilibrium, the concentration of each ion in the remaining solution was measured by ICP-OES. 2.8. Adsorption test in real tap water sample Tap water samples were taken from our research laboratory and filtered through a cellulose membrane filter (pore size 0.22 m) prior to use. Different concentration of Hg(II) were spiked to tap water samples, and then disposed with the sorbents. After adsorption equilibrium, the concentration of Hg(II), Ca(II) and Mg(II) in the solution was measured by ICP-OES. 3. Results and discussion 3.1. Preparation and characterization of thymine polymer sorbent Thymine has been previously reported to form T–Hg–T complexes (Fig. 1a) with Hg(II) between DNA sequences [19–22], polymer chains [24] and fluorophore-modified thymine [23,25]. Therefore by immobilizing thymine on solid support, we expected to obtain a Hg(II) specific sorbent. In order to prove the feasibility
of this approach, we prepared two kinds of sorbents with different densities of thymine by radical polymerization using two different methods. Since the N-3 of thymine has been reported to be the specific binding site to Hg(II), the thymine was allylated at N-1 position by allyl bromide and used as the polymerization monomer (Fig. 1b). Using the first method, sorbent P1 (blank sorbent), P2 and P3 were synthesized by copolymerization of three different ratios of AT and EDMA (0/10, 1/9 and 5/5, respectively) using a one-step swelling and polymerization method, a simple and convenient technique to prepare beads several microns in diameter. The other kind of sorbent (P4) was synthesized by grafting AT onto the surface of PCMSt beads using the ATRP method. SEM micrographs of the two kinds of thymine polymer sorbents showed that the diameter of the beads was about 7 m and 3 m for copolymerizing and grafting beads, respectively (Fig. 2). The sorbent with the small beads will have high adsorption capacity because of its high surface area. Moreover, it may have the potential to be used for on-line enrichment of Hg(II) or as a specific ion chromatography stationary phase directly. The IR spectra (Fig. 3) demonstrated that thymine was copolymerized (P2 and P3) or grafted (P4) on the polymer beads successfully. IR spectra of different copolymerizing thymine polymer sobent (P1, P2 and P3) are shown in Fig. 3A. In the three spectra, the adsorption peaks of C O bond at 1732 cm−1 , C–O bond at 1155 cm−1 and aliphatic C–H at 2958 and 2991 cm−1 showed the presence of polymethacrylate. Furthermore, when compared the
Fig. 3. FT-IR spectra of polymer sorbent (a) P1, (b) P2, (c) P3, (d) P4 and (e) PCMSt.
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Fig. 4. XPS of N1s of (a) P2, (b) P3 and (c) P4 thymine polymer sorbent.
IR spectra of P2 (b) with P1 (a), it could be clearly found that a new peak of C O band of acylamide group appeared at 1672 cm−1 which indicates that the AT was copolymerized with EDMA in the sorbent successfully. And this peak was weak adsorption because of the lower content of AT in the sorbent P2. But this peak disappeared when the AT content rising (P3, c), and compared the spectra c with b and a, it could be found that the peak at 1732 cm−1 was become broader obviously. The phenomenon might be caused from the overlap of peaks of C O band of acylamide and ester. IR spectra of grafting sobent (P4) and PCMSt are shown in Fig. 3B. The peaks of C C bond at 1610, 1511, 1445, and 1421 cm−1 and aliphatic C–H at 3021 cm−1 indicated that the presence of benzene. Compared the IR spectra of d with e, it could be clearly found that some new peaks appeared. The new broad peak at 3419 cm−1 showed the presence of O–H or N–H in P4, and new peaks at 1696 and 1664 cm−1 indicated the presence C O band of acylamide group. All these new peaks showed that AT had grated on the surface of the PCMSt beads successfully. Additionally, since no nitrogen is contained in blank sorbent beads (P1 and PCMSt), any nitrogen content represents the amount of thymine immobilized on polymer beads. As the adsorption capability of the sorbent was determined by the amount of thymine located on the surface of sorbent, the nitrogen content on the surface of P2, P3 and P4 were measured by XPS (Fig. 4). The nitrogen content on P2 surface was below the detection limit and the P3 was 1.75%. Whereas, the nitrogen content on the surface of P4, which was obtained by graft polymerization method, was 4.76% (about 2.7 times of P3). This might led to the larger adsorption capacity for Hg(II) of the grafting polymer sorbent P4. XPS results also clearly showed that the thymine polymer sorbent was prepared successfully.
Fig. 5. Effect of pH on adsorption of Hg(II) on the thymine polymer sorbent. Hg(II) concentration: 2.0 mg/L; pH 2.0 (20 mM NaH2 PO4 /H3 PO4 ); pH 4.0 (20 mM CH3 COONa/CH3 COOH); pH 6.0 (20 mM CH3 COONa/CH3 COOH); pH 7.0 (20 mM NaH2 PO4 /NaOH); pH 8.0 (20 mM Na2 HPO4 /H3 PO4 ); pH 10.0 (20 mM Na2 HPO4 /NaOH); sorbent P3: 50 mg; sample volume: 30 mL.
pH was 4.0, more than 80% of Hg(II) was removed; the removal efficiency reached 100% in the pH range of 7.0–10.0. Moreover, the loss of the thymine polymer’s adsorption ability at low pH indicates that this kind of Hg(II) sorbent may be regenerated conveniently by an acidic solution. 3.3. Adsorption capacity of thymine polymer sorbent for Hg(II) Adsorption capacity of the new thymine polymer sorbents for Hg(II) from aqueous solution was investigated in batch experiments. Different initial concentrations of Hg(II) aqueous solutions were equilibrated with polymer sorbents P1, P2, P3, P4 and PCMSt, respectively. Fig. 6 shows the amount of the Hg(II) adsorbed on the different sorbent depends on the initial concentration of Hg(II) in solution. Hg(II) was hardly adsorbed on the blank sorbent P1 (0.78 mg/g dry weight) and PCMSt (0.98 mg/g dry weight). For P2, P3 and P4, however, the amount of Hg(II) adsorbed increased with increasing concentrations of Hg(II) in the initial solution. Finally at a high enough concentration the curve reached a plateau, which
3.2. Effect of pH on adsorption Hg(II) probes based on thymine were reported to be pH sensitive [23]. This indicated that the adsorption capability of our thymine polymer sorbent may also depend on the pH of the solution. The effect of pH on the Hg(II) adsorption of sorbent beads P3 is shown in Fig. 5. The thymine sorbent shows weak sorption ability for Hg(II) at lower pHs. Only 22% of Hg(II) was adsorbed onto the P3 sorbent when the pH was 2.0. This might be due to that low pH is unfavorable for dissociation of the imino protons of thymine residues, which reduce the coordination ability of thymine with the Hg(II) in aqueous solution. As the pH of the solution is increased, the extent of dissociation of the imino protons becomes easier causing the Hg(II) adsorption to greatly increase. As shown in Fig. 5, when the
Fig. 6. The adsorption curve of Hg(II) on different sorbent: (a) P4, (b) P3, (c) P2, (d and e) P1 and PCMSt. Fifty milligrams of each sorbent in 30 mL of buffer (20 mM NaH2 PO4 /NaOH, pH 7.0) with different concentration of Hg(II). Q denote the adsorption capacity (mg/g), C0 denote the initial concentration of Hg(II) (mg/L).
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Fig. 7. Kinetics of Hg(II) adsorption on thymine polymer sorbent. Hg(II) concentration: 100.0 mg/L; pH 7.0 (20 mM NaH2 PO4 /NaOH buffer); sorbent P3: 100 mg; sample volume: 20 mL. E denote removal efficiency.
represents saturation of the active binding sites. The maximum adsorption capacity of P2, P3 and P4 is about 14.4, 78.5 and 200 mg/g dry sorbent, respectively, which agree well with the amount of thymine immobilized on the surface of different polymer beads (Fig. 4). Although the nitrogen content on P2 sorbent is undetectable, the molar amount of thymine used for preparation of sorbent P2 is one fifth of that for sorbent P3, this ratio is similar to the ratio of maximum adsorption capacity of P2 to P3. And the maximum adsorption capacity of P4 is about 2.55 times that of P3, which also agrees well with the ratio of the surface nitrogen content of P4 to P3. These results indicate that the adsorption capacity for Hg(II) depends on the density of thymine on the surface of the polymer sorbent, which illuminate that the specific coordination interaction of thymine and Hg(II) is the key factor for the adsorption performance of the new thymine polymer sorbent. The adsorption capacity of P4 is 200 mg/g, which is lower than the reported thiol (696 mg/g) [13] or amide (496 mg/g) [14] modified polymer sorbents, but higher than the MIP sorbent (41 mg/g [16] and 78.5 mg/g [17]). Because the adsorption capacity of the new sorbent is determined by the surface density of thymine, improvement of the surface density of thymine on the polymer sorbent would further increase the adsorption capacity. 3.4. Adsorption kinetics The speed of adsorption of Hg(II) on P3 sorbent is shown in Fig. 7. Nearly 90% of Hg(II) was adsorbed within 2.5 min, and more than 95% of Hg(II) was removed from water after 5 min. This is much faster than some MIPs and other commonly used Hg(II) sorbents, which require 20 or more minutes for 95% adsorption of Hg(II) [13–16]. This unique pace could make the thymine polymer sorbent suitable for chemical sensing, chromatographic separation, solid-
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Fig. 8. Reusability of the thymine polymer sorbent. Hg(II) concentration: 2.0 mg/L; pH 8.0 (20 mM Na2 HPO4 /H3 PO4 buffer); sorbent P3: 100 mg; sample volume: 30 mL.
phase extraction or other applications that require fast adsorption of Hg(II). 3.5. Recycling test 1.0 M HCl was found to elute the adsorbed Hg(II) from the P3 sorbent completely. Thus it was used to regenerate the thymine polymer. To test the reusability of the thymine polymer sorbent, we subjected it to a series of loading and elution batch experiments. As shown in Fig. 8, the Hg(II) adsorption ability of the thymine sorbent did not decrease even after 15 cycles of loading and elution, which shows good reusability and stability. 3.6. Selectivity studies To investigate the selectivity of the thymine polymer sorbent, competitive adsorption of Hg(II) with other ions, such as Cu(II), Cd(II), Zn(II), Co(II), Ca(II) and Mg(II) was performed. From Fig. 9, it can be seen that only Hg(II) was completely removed by thymine polymer sorbent, while less than 10% of the Co(II), Ca(II) and Mg(II) ions were removed by both thymine and blank polymer sorbent. About 60% of Cu(II), 30% of Cd(II) and Zn(II) were adsorbed by the thymine polymer sorbent as well as by the blank polymer sorbent, indicating that the adsorption of Cu(II), Cd(II), and Zn(II) was caused by the non-specific adsorption of the matrix, and not by thymine. Since Cu(II), Cd(II) and Zn(II) were found to be adsorbed by the sorbent matrix, the adsorption capacities for these ions were further investigated. The maximum adsorption capacities of thymine polymer sorbent P3 for Cu(II), Cd(II) and Zn(II) were found to be 0.4, 0.2 and 0.1 mg/g, respectively, which are much lower than that for Hg(II) (78.5 mg/g), indicating the good selectivity of the thymine polymer sorbent for Hg(II).
Table 1 Adsorption of Hg(II) by thymine polymer sorbent from real tap water samples (n = 3)a . Hg(II) added (mg/L)
1.0 5.00 20.0 a b
After adsorption by thymine polymer sorbent (P3) (mg/L)
After adsorption by blank polymer sorbent (P1) (mg/L)
Hg(II)
Ca(II)
Mg(II)
Hg(II)
Ca(II)
Mg(II)
–b 0.37 ± 0.07 0.59 ± 0.14
50.99 ± 0.39 49.48 ± 0.21 49.83 ± 0.74
24.45 ± 0.32 24.19 ± 0.14 24.87 ± 0.31
1.03 ± 0.16 4.61 ± 0.04 19.42 ± 0.87
50.25 ± 0.97 49.65 ± 1.07 51.08 ± 0.11
24.40 ± 0.45 24.27 ± 0.11 24.71 ± 0.76
The concentration of Ca(II) and Mg(II) in real tap water samples were measured to be 50.71 ± 1.54 and 25.02 ± 0.50 mg/L, respectively. (−) means not detected.
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amount of thymine located on the sorbent surface. This kind of sorbent can be easily regenerated by acidic solution, showing great potential for selective separation and removal of Hg(II) from real water samples. Furthermore, the fast adsorption kinetics of Hg(II) onto the thymine polymer sorbent make it an attractive option for chemical sensing, solid-phase extraction, or selective on-line preconcentration and separation of Hg(II) from complex samples. Acknowledgements We gratefully acknowledge financial support from NSF of China (20775082 and 90717119), grant 973 Program (2007CB935601), 863 Program (2007AA02Z221 and the Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences. We would also like to thank Meghan B. O’Donoghue for carefully reading and correcting this manuscript. References Fig. 9. Selectivity of thymine polymer sorbent. Mixture of different ions in 20 mM Na2 HPO4 /NaOH (pH 7.0); sorbent P1 or P3: 50 mg; sample volume: 30 mL.
3.7. Application to real tap water sample Water is probably the most studied environment sample and Hg(II) is the main species of mercury pollutant existed in natural water. The thymine polymer was also investigated for the selective adsorption and removal of spiked Hg(II) in tap water samples. Twenty milliliters of tap water sample spiked with Hg(II) at different concentrations were disposed with 50 mg of thymine sorbent P3 and blank sorbent P1, respectively. The concentrations of Hg(II), Ca(II) and Mg(II) before and after treatment are shown in Table 1. Ca(II) and Mg(II), the main interference ions existed in tap water, were hardly adsorbed by the thymine polymer sorbent and blank sorbent. The spiked Hg(II) was almost adsorbed by the thymine polymer sorbent completely, but was hardly adsorbed by the blank polymer sorbent. These results indicate that the thymine polymer sorbent can be used for the selective separation, preconcentration, or removal of Hg(II) from real water samples. 4. Conclusions A new kind of sorbent for specific Hg(II) adsorption based on thymine was developed, which was stable, reusable, and easy to be prepared and environment friendly. The good specificity of this sorbent for Hg(II) relies on the specific interaction of Hg(II) with thymine. The Hg(II) adsorption capacity is highly depended on the
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Talanta journal homepage: www.elsevier.com/locate/talanta
Specific mercury(II) adsorption by thymine-based sorbent Xiangjun Liu, Cui Qi, Tao Bing, Xiaohong Cheng, Dihua Shangguan ∗ Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
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Article history: Received 18 August 2008 Received in revised form 31 October 2008 Accepted 5 November 2008 Available online 13 November 2008 Keywords: Selective adsorption Thymine Polymer sorbent Mercury(II)
a b s t r a c t A new kind of polymer sorbent based on the specific interaction of Hg(II) with nucleic acid base, thymine, is described for the selective adsorption of Hg(II) from aqueous solution. Two types of sorbents immobilized with thymine were prepared by one-step swelling and polymerization and graft polymerization, respectively. The maximum static adsorption capacity of the new polymer sorbents for Hg(II) is proportional to the density of thymine on their surface, up to 200 mg/g. Moreover, the new kind polymer sorbent shows excellent selectivity for Hg(II) over other interfering ions, such as Cu(II), Cd(II), Zn(II), Co(II), Ca(II) and Mg(II), exhibits very fast kinetics for Hg(II) adsorption from aqueous solution, and can be easily regenerated by 1.0 M HCl. It also has been successfully used for the selective adsorption of spiked Hg(II) from real tap water samples. This new thymine polymer sorbent holds a great promise in laboratory and industrial applications such as separation, on-line enrichment, solid-phase extraction, and removal of Hg(II) from pharmaceutical, food and environmental samples. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Heavy metal ion pollution has become widespread throughout the world as a result of industrialization, which significantly threats the ecosystem, especially the people’s health due to their severe toxicity. Mercury, as one of the most toxic heavy metal, can cause human disease, such as kidney toxicity, neurological damage and chromosome breakage, even at low concentration [1]. Mercury and mercury compounds are included in all lists of priority pollutants. Different regulations and guidelines have been developed for limiting their levels in water and sediments [2,3]. Burning coal, inorganic mercury used in chloro-alkali cells, electrical apparatus and chemical engineering process are the main sources of mercury pollutants introduced into the environment. Hg(II) is one of the main species of mercury pollutants in the environment, thus monitoring or removal of Hg(II) is quite necessary. However, in real samples, there are many coexisting interferences which seriously influence the detection or the removal efficiency of Hg(II), so the specific adsorption material for separation, preconcentration and removal of Hg(II) from the complex matrix is essential. Many methods have been reported for the adsorption of Hg(II) from aqueous media, such as solvent extraction [4], solid-phase extraction by polymer sorbents [5,6], activated carbon [7], chitosan [8], clay [9] and other materials [10,11]. Particularly polymer sorbents have recently attracted more attention because of their larger adsorption capacity, higher efficiency and easier prepara-
∗ Corresponding author. Tel.: +86 10 62528509; fax: +86 10 62528509. E-mail address: [email protected] (D. Shangguan). 0039-9140/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2008.11.005
tion. However, these sorbents commonly lack selectivity for Hg(II). Some attempts have been made to solve this problem, for example, thiol [12,13] or amide [14,15] modified sorbents have improved the selectivity, but it is still not enough since the interaction of these functional groups with Hg(II) is not specific. Recently, molecularly imprinted polymers (MIPs) for Hg(II) have been developed [16–18]. Although the MIPs sorbent showed better selectivity for Hg(II), the preparation process of the MIPs is complex and the adsorption capacity for Hg(II) is low. Moreover, using Hg(II) as template during the preparation of MIPs introduces a potential new source of mercury contamination. In recent years, Hg(II) has been reported to selectively bind to thymine/thymine (T/T) mismatched base pairs in DNA sequences, by forming T–Hg(II)–T complexes (Fig. 1a) [19–21]. NMR studies have demonstrated that Hg(II) binds directly to N-3 of two thymidine residues in place of two imino protons and forms N–Hg(II)–N bond (Fig. 1a) [21,22]. Several Hg(II) probes or sensors based on the specific interaction of T and Hg(II) have also been developed and showed great selectivity and sensitivity [23–25]. To date, all these studies focused on the single nucleotide polymorphisms analysis and Hg(II) detection, no attempts have been carried out to adsorb Hg(II) using this specific interaction. In this study, we extend the application of this specific interaction to selective adsorption of Hg(II) from aqueous solution samples. Thymine-modified polymer sorbents were prepared by copolymerizing of allyl thymine (AT, Fig. 1b) with ethylene dimethacrylate (EDMA) or by grafting of AT onto the surface of a support. The characteristic of these sorbents was evaluated by SEM micrographs, infrared (IR) spectra and X-ray photoelectron spectrometer (XPS). The adsorption capability, adsorption kinetics, selectivity, and reusability of the new kind
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2.4. Preparation of thymine polymer sorbent 2.4.1. Preparation of polyglycidyl methacrylate (PGMA) and polychloromethylstyrene (PCMSt) PGMA and PCMSt were prepared using dispersion polymerization [27]. Five milliliters of GMA or CMSt, 0.15 g of AIBN and 0.5 g of PVP were dissolved in 50 mL of ethanol and purged with N2 for 10 min. And then the reaction was carried out at 70 ◦ C for 24 h. The resultant beads were washed with ethanol and dried under vacuum at room temperature.
Fig. 1. Chemical structure of (a) proposed structure of T–Hg(II)–T and (b) allyl thymine (AT).
thymine polymer sorbent were investigated. Finally, the thymine polymer sorbent was successfully applied for the selective adsorption of Hg(II) which spiked in real water samples. 2. Experimental 2.1. Materials Thymine (T) was purchased from Sigma (USA). Allyl bromide was purchased from Beijing Xingjin Chemical Plant (Beijing, China). Sodium hydride (NaH) was purchased from Tianjin Fuchen Chemical Reagent Plant (Tianjin, China). Chloromethylstyrene (CMSt) and EDMA were purchased from Acros Organics (USA). Glycidyl methacrylate (GMA) and N,N,N ,N, ,N, pentamethyldiethylenetriamine (PMDETA) were purchased from Aldrich (USA). 2,2-Azobis(2-isobutyronitrile) (AIBN), benzoyl peroxide (BPO), poly(vinyl pyrrolidone) (PVP), poly(vinyl alcohol) (PVA) and sodium dodecylsulphonate (SDS) were purchased from Beijing Chemical Plant (Beijing, China). Dithizone and cuprous bromide (CuBr) were purchased from Beijing Chemical Reagent Company (Beijing, China). All other reagents (AR) used were purchase from Beijing Chemical Plant (Beijing, China). Doubly deionised water was used throughout this work. AIBN, BPO, CuBr and dithizone were re-purification prior to use. 2.2. Instrumentation 1 H NMR spectra were recorded on Bruker AV400 instrument. UV–vis absorption spectra were recorded on Shimadzu UV-1601PC spectrophotometer. FT-IR spectra in KBr were recorded on Bruker Tensor 27 instrument. SEM micrographs of the sorbent were obtained on a JEOL JSM-6700F field emission scanning electron microscopy. The concentration of mercury and other ions in a mixed solution were measured using an Inductive Coupled Plasma Optical Emission Spectrometer (ICP-OES) (PE-Optima 2000DV). The nitrogen content on the surface of the sorbents was determined by ESCALab220I-XL X-ray photoelectron spectrometer (XPS).
2.3. Synthesis of allyl thymine (AT) The synthesis of AT was performed as describing previously [26]. To a suspension of 2 g of thymine in 80 mL of dry DMF was slowly added 0.72 g of NaH. The suspension was stirred at room temperature for 1 h until no more gas evolved. Then 1.8 mL of allyl bromide was added dropwise. After 24 h, the excess NaH was quenched with saturated NH4 Cl aqueous solution. The mixture was concentrated under a vacuum and subjected to column chromatography (eluent: 40% ethyl acetate/petroleum ether) to yield some white solid. 1 H NMR (CDCl3 ): ı 8.61 (s, 1H, pyrimidine NH), 6.96 (d, 1H, pyrimidine H-6), 5.81–5.92 (m, 1H, –CH ), 5.24–5.33 (m, 2H, CH2 ), 4.33 (d, 2H, –CH2 –), 1.92 (d, 3H, pyrimidine–CH3 ).
2.4.2. Preparation of copolymerized thymine polymer sorbent The copolymerized thymine polymer sorbent (P1, P2 and P3) was prepared using a one-step swelling and polymerization method [27]. The typical process is as follows: A suspension of 0.1 g PGMA seeds in 10 mL of homogeneous solution containing PVA, SDS and distilled water was stirred at room temperature. Then an emulsion made of PVA, SDS, distilled water, 40 mg of BPO, 2 mL of chloroform and 1 mL of acetonitrile and monomer mixture (10 mmol) of AT and EDMA (molar amount of AT/EDMA: P1, 0/10; P2, 1/9 and P3, 5/5) was added dropwise to the suspension of PGMA seeds. After the monomer mixture was absorbed by the PGMA seeds completely, the polymerization was carried out at 60 ◦ C for 24 h. The polymer beads were washed with water and ethanol, and dried under vacuum at room temperature. 2.4.3. Preparation of grafted thymine polymer sorbent The grafted thymine polymer sorbent (P4) was prepared using the atom transfer radical polymerization (ATRP) method [28]. 0.5 g PCMSt was suspended in a solution of 5 mL of distilled water and methanol (1:1, v/v) containing 0.5 g of AT, and then 32 mg of CuBr and 140 L of PMDETA were added. The polymerization was carried out at room temperature for 24 h. The resultant beads (P4) were washed with water and ethanol, and dried under vacuum at room temperature. 2.5. Equilibration adsorption studies Adsorption of Hg(II) from aqueous solutions was investigated in batch experiments. Fifty milligrams of sorbent was equilibrated with 30 mL of different solutions at room temperature. Then, an aliquot of the supernatant was separated, and the quantity of the ion was determined by the dithizone spectrophotometric method (GB 7469-87, China). The adsorption capacity of the sorbent and the removal efficiency of the ion could be calculated using the following equations: Q =
(C0 − Ce )V W
E (%) =
C0 − Ce × 100 C0
(1) (2)
where Q represents the adsorption capacity (mg/g), C0 and Ce are the initial and equilibrium concentration (mg/L), and E is the removal efficiency (%). 2.6. Recycling test 0.1 g of thymine polymer sorbent was equilibrated with 30 mL of 2.0 mg/L Hg(II) solution (pH 8.0, 20 mM Na2 HPO3 /H3 PO4 ). After 2 h, the sorbent was regenerated for the next cycle by washing with 20 mL of 1.0 M HCl solution once, 10 mL of water twice and 20 mL of pH 8.0 Na2 HPO3 /H3 PO4 buffer once. The equilibrium concentration was determined using the dithizone spectrophotometric method (GB 7469-87, China) and the removal efficiency was calculated according to Eq. (2).
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Fig. 2. SEM image of (A) copolymerized thymine polymer sorbent P3 and (B) grafted thymine polymer sorbent P4.
2.7. Selectivity studies The selectivity of the thymine polymer sorbent was investigated by ICP-OES. Fifty milligrams of thymine polymer or blank sorbent was suspended in 30 mL of aqueous solution of containing Hg(II) and other ions. After adsorption equilibrium, the concentration of each ion in the remaining solution was measured by ICP-OES. 2.8. Adsorption test in real tap water sample Tap water samples were taken from our research laboratory and filtered through a cellulose membrane filter (pore size 0.22 m) prior to use. Different concentration of Hg(II) were spiked to tap water samples, and then disposed with the sorbents. After adsorption equilibrium, the concentration of Hg(II), Ca(II) and Mg(II) in the solution was measured by ICP-OES. 3. Results and discussion 3.1. Preparation and characterization of thymine polymer sorbent Thymine has been previously reported to form T–Hg–T complexes (Fig. 1a) with Hg(II) between DNA sequences [19–22], polymer chains [24] and fluorophore-modified thymine [23,25]. Therefore by immobilizing thymine on solid support, we expected to obtain a Hg(II) specific sorbent. In order to prove the feasibility
of this approach, we prepared two kinds of sorbents with different densities of thymine by radical polymerization using two different methods. Since the N-3 of thymine has been reported to be the specific binding site to Hg(II), the thymine was allylated at N-1 position by allyl bromide and used as the polymerization monomer (Fig. 1b). Using the first method, sorbent P1 (blank sorbent), P2 and P3 were synthesized by copolymerization of three different ratios of AT and EDMA (0/10, 1/9 and 5/5, respectively) using a one-step swelling and polymerization method, a simple and convenient technique to prepare beads several microns in diameter. The other kind of sorbent (P4) was synthesized by grafting AT onto the surface of PCMSt beads using the ATRP method. SEM micrographs of the two kinds of thymine polymer sorbents showed that the diameter of the beads was about 7 m and 3 m for copolymerizing and grafting beads, respectively (Fig. 2). The sorbent with the small beads will have high adsorption capacity because of its high surface area. Moreover, it may have the potential to be used for on-line enrichment of Hg(II) or as a specific ion chromatography stationary phase directly. The IR spectra (Fig. 3) demonstrated that thymine was copolymerized (P2 and P3) or grafted (P4) on the polymer beads successfully. IR spectra of different copolymerizing thymine polymer sobent (P1, P2 and P3) are shown in Fig. 3A. In the three spectra, the adsorption peaks of C O bond at 1732 cm−1 , C–O bond at 1155 cm−1 and aliphatic C–H at 2958 and 2991 cm−1 showed the presence of polymethacrylate. Furthermore, when compared the
Fig. 3. FT-IR spectra of polymer sorbent (a) P1, (b) P2, (c) P3, (d) P4 and (e) PCMSt.
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Fig. 4. XPS of N1s of (a) P2, (b) P3 and (c) P4 thymine polymer sorbent.
IR spectra of P2 (b) with P1 (a), it could be clearly found that a new peak of C O band of acylamide group appeared at 1672 cm−1 which indicates that the AT was copolymerized with EDMA in the sorbent successfully. And this peak was weak adsorption because of the lower content of AT in the sorbent P2. But this peak disappeared when the AT content rising (P3, c), and compared the spectra c with b and a, it could be found that the peak at 1732 cm−1 was become broader obviously. The phenomenon might be caused from the overlap of peaks of C O band of acylamide and ester. IR spectra of grafting sobent (P4) and PCMSt are shown in Fig. 3B. The peaks of C C bond at 1610, 1511, 1445, and 1421 cm−1 and aliphatic C–H at 3021 cm−1 indicated that the presence of benzene. Compared the IR spectra of d with e, it could be clearly found that some new peaks appeared. The new broad peak at 3419 cm−1 showed the presence of O–H or N–H in P4, and new peaks at 1696 and 1664 cm−1 indicated the presence C O band of acylamide group. All these new peaks showed that AT had grated on the surface of the PCMSt beads successfully. Additionally, since no nitrogen is contained in blank sorbent beads (P1 and PCMSt), any nitrogen content represents the amount of thymine immobilized on polymer beads. As the adsorption capability of the sorbent was determined by the amount of thymine located on the surface of sorbent, the nitrogen content on the surface of P2, P3 and P4 were measured by XPS (Fig. 4). The nitrogen content on P2 surface was below the detection limit and the P3 was 1.75%. Whereas, the nitrogen content on the surface of P4, which was obtained by graft polymerization method, was 4.76% (about 2.7 times of P3). This might led to the larger adsorption capacity for Hg(II) of the grafting polymer sorbent P4. XPS results also clearly showed that the thymine polymer sorbent was prepared successfully.
Fig. 5. Effect of pH on adsorption of Hg(II) on the thymine polymer sorbent. Hg(II) concentration: 2.0 mg/L; pH 2.0 (20 mM NaH2 PO4 /H3 PO4 ); pH 4.0 (20 mM CH3 COONa/CH3 COOH); pH 6.0 (20 mM CH3 COONa/CH3 COOH); pH 7.0 (20 mM NaH2 PO4 /NaOH); pH 8.0 (20 mM Na2 HPO4 /H3 PO4 ); pH 10.0 (20 mM Na2 HPO4 /NaOH); sorbent P3: 50 mg; sample volume: 30 mL.
pH was 4.0, more than 80% of Hg(II) was removed; the removal efficiency reached 100% in the pH range of 7.0–10.0. Moreover, the loss of the thymine polymer’s adsorption ability at low pH indicates that this kind of Hg(II) sorbent may be regenerated conveniently by an acidic solution. 3.3. Adsorption capacity of thymine polymer sorbent for Hg(II) Adsorption capacity of the new thymine polymer sorbents for Hg(II) from aqueous solution was investigated in batch experiments. Different initial concentrations of Hg(II) aqueous solutions were equilibrated with polymer sorbents P1, P2, P3, P4 and PCMSt, respectively. Fig. 6 shows the amount of the Hg(II) adsorbed on the different sorbent depends on the initial concentration of Hg(II) in solution. Hg(II) was hardly adsorbed on the blank sorbent P1 (0.78 mg/g dry weight) and PCMSt (0.98 mg/g dry weight). For P2, P3 and P4, however, the amount of Hg(II) adsorbed increased with increasing concentrations of Hg(II) in the initial solution. Finally at a high enough concentration the curve reached a plateau, which
3.2. Effect of pH on adsorption Hg(II) probes based on thymine were reported to be pH sensitive [23]. This indicated that the adsorption capability of our thymine polymer sorbent may also depend on the pH of the solution. The effect of pH on the Hg(II) adsorption of sorbent beads P3 is shown in Fig. 5. The thymine sorbent shows weak sorption ability for Hg(II) at lower pHs. Only 22% of Hg(II) was adsorbed onto the P3 sorbent when the pH was 2.0. This might be due to that low pH is unfavorable for dissociation of the imino protons of thymine residues, which reduce the coordination ability of thymine with the Hg(II) in aqueous solution. As the pH of the solution is increased, the extent of dissociation of the imino protons becomes easier causing the Hg(II) adsorption to greatly increase. As shown in Fig. 5, when the
Fig. 6. The adsorption curve of Hg(II) on different sorbent: (a) P4, (b) P3, (c) P2, (d and e) P1 and PCMSt. Fifty milligrams of each sorbent in 30 mL of buffer (20 mM NaH2 PO4 /NaOH, pH 7.0) with different concentration of Hg(II). Q denote the adsorption capacity (mg/g), C0 denote the initial concentration of Hg(II) (mg/L).
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Fig. 7. Kinetics of Hg(II) adsorption on thymine polymer sorbent. Hg(II) concentration: 100.0 mg/L; pH 7.0 (20 mM NaH2 PO4 /NaOH buffer); sorbent P3: 100 mg; sample volume: 20 mL. E denote removal efficiency.
represents saturation of the active binding sites. The maximum adsorption capacity of P2, P3 and P4 is about 14.4, 78.5 and 200 mg/g dry sorbent, respectively, which agree well with the amount of thymine immobilized on the surface of different polymer beads (Fig. 4). Although the nitrogen content on P2 sorbent is undetectable, the molar amount of thymine used for preparation of sorbent P2 is one fifth of that for sorbent P3, this ratio is similar to the ratio of maximum adsorption capacity of P2 to P3. And the maximum adsorption capacity of P4 is about 2.55 times that of P3, which also agrees well with the ratio of the surface nitrogen content of P4 to P3. These results indicate that the adsorption capacity for Hg(II) depends on the density of thymine on the surface of the polymer sorbent, which illuminate that the specific coordination interaction of thymine and Hg(II) is the key factor for the adsorption performance of the new thymine polymer sorbent. The adsorption capacity of P4 is 200 mg/g, which is lower than the reported thiol (696 mg/g) [13] or amide (496 mg/g) [14] modified polymer sorbents, but higher than the MIP sorbent (41 mg/g [16] and 78.5 mg/g [17]). Because the adsorption capacity of the new sorbent is determined by the surface density of thymine, improvement of the surface density of thymine on the polymer sorbent would further increase the adsorption capacity. 3.4. Adsorption kinetics The speed of adsorption of Hg(II) on P3 sorbent is shown in Fig. 7. Nearly 90% of Hg(II) was adsorbed within 2.5 min, and more than 95% of Hg(II) was removed from water after 5 min. This is much faster than some MIPs and other commonly used Hg(II) sorbents, which require 20 or more minutes for 95% adsorption of Hg(II) [13–16]. This unique pace could make the thymine polymer sorbent suitable for chemical sensing, chromatographic separation, solid-
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Fig. 8. Reusability of the thymine polymer sorbent. Hg(II) concentration: 2.0 mg/L; pH 8.0 (20 mM Na2 HPO4 /H3 PO4 buffer); sorbent P3: 100 mg; sample volume: 30 mL.
phase extraction or other applications that require fast adsorption of Hg(II). 3.5. Recycling test 1.0 M HCl was found to elute the adsorbed Hg(II) from the P3 sorbent completely. Thus it was used to regenerate the thymine polymer. To test the reusability of the thymine polymer sorbent, we subjected it to a series of loading and elution batch experiments. As shown in Fig. 8, the Hg(II) adsorption ability of the thymine sorbent did not decrease even after 15 cycles of loading and elution, which shows good reusability and stability. 3.6. Selectivity studies To investigate the selectivity of the thymine polymer sorbent, competitive adsorption of Hg(II) with other ions, such as Cu(II), Cd(II), Zn(II), Co(II), Ca(II) and Mg(II) was performed. From Fig. 9, it can be seen that only Hg(II) was completely removed by thymine polymer sorbent, while less than 10% of the Co(II), Ca(II) and Mg(II) ions were removed by both thymine and blank polymer sorbent. About 60% of Cu(II), 30% of Cd(II) and Zn(II) were adsorbed by the thymine polymer sorbent as well as by the blank polymer sorbent, indicating that the adsorption of Cu(II), Cd(II), and Zn(II) was caused by the non-specific adsorption of the matrix, and not by thymine. Since Cu(II), Cd(II) and Zn(II) were found to be adsorbed by the sorbent matrix, the adsorption capacities for these ions were further investigated. The maximum adsorption capacities of thymine polymer sorbent P3 for Cu(II), Cd(II) and Zn(II) were found to be 0.4, 0.2 and 0.1 mg/g, respectively, which are much lower than that for Hg(II) (78.5 mg/g), indicating the good selectivity of the thymine polymer sorbent for Hg(II).
Table 1 Adsorption of Hg(II) by thymine polymer sorbent from real tap water samples (n = 3)a . Hg(II) added (mg/L)
1.0 5.00 20.0 a b
After adsorption by thymine polymer sorbent (P3) (mg/L)
After adsorption by blank polymer sorbent (P1) (mg/L)
Hg(II)
Ca(II)
Mg(II)
Hg(II)
Ca(II)
Mg(II)
–b 0.37 ± 0.07 0.59 ± 0.14
50.99 ± 0.39 49.48 ± 0.21 49.83 ± 0.74
24.45 ± 0.32 24.19 ± 0.14 24.87 ± 0.31
1.03 ± 0.16 4.61 ± 0.04 19.42 ± 0.87
50.25 ± 0.97 49.65 ± 1.07 51.08 ± 0.11
24.40 ± 0.45 24.27 ± 0.11 24.71 ± 0.76
The concentration of Ca(II) and Mg(II) in real tap water samples were measured to be 50.71 ± 1.54 and 25.02 ± 0.50 mg/L, respectively. (−) means not detected.
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amount of thymine located on the sorbent surface. This kind of sorbent can be easily regenerated by acidic solution, showing great potential for selective separation and removal of Hg(II) from real water samples. Furthermore, the fast adsorption kinetics of Hg(II) onto the thymine polymer sorbent make it an attractive option for chemical sensing, solid-phase extraction, or selective on-line preconcentration and separation of Hg(II) from complex samples. Acknowledgements We gratefully acknowledge financial support from NSF of China (20775082 and 90717119), grant 973 Program (2007CB935601), 863 Program (2007AA02Z221 and the Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences. We would also like to thank Meghan B. O’Donoghue for carefully reading and correcting this manuscript. References Fig. 9. Selectivity of thymine polymer sorbent. Mixture of different ions in 20 mM Na2 HPO4 /NaOH (pH 7.0); sorbent P1 or P3: 50 mg; sample volume: 30 mL.
3.7. Application to real tap water sample Water is probably the most studied environment sample and Hg(II) is the main species of mercury pollutant existed in natural water. The thymine polymer was also investigated for the selective adsorption and removal of spiked Hg(II) in tap water samples. Twenty milliliters of tap water sample spiked with Hg(II) at different concentrations were disposed with 50 mg of thymine sorbent P3 and blank sorbent P1, respectively. The concentrations of Hg(II), Ca(II) and Mg(II) before and after treatment are shown in Table 1. Ca(II) and Mg(II), the main interference ions existed in tap water, were hardly adsorbed by the thymine polymer sorbent and blank sorbent. The spiked Hg(II) was almost adsorbed by the thymine polymer sorbent completely, but was hardly adsorbed by the blank polymer sorbent. These results indicate that the thymine polymer sorbent can be used for the selective separation, preconcentration, or removal of Hg(II) from real water samples. 4. Conclusions A new kind of sorbent for specific Hg(II) adsorption based on thymine was developed, which was stable, reusable, and easy to be prepared and environment friendly. The good specificity of this sorbent for Hg(II) relies on the specific interaction of Hg(II) with thymine. The Hg(II) adsorption capacity is highly depended on the
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