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Adsorption mechanism for imprinted ion (Ni2+) of the surface molecular imprinting adsorbent (SMIA)
Biochemical Engineering Journal 39 (2008) 503–509
Adsorption mechanism for imprinted ion (Ni2+) of the surface molecular imprinting adsorbent (SMIA) Haijia Su ∗ , Jia Li, Tianwei Tan The Key Laboratory of Bioprocess of Beijing, Beijing University of Chemical Technology, P.O. Box 53, Beijing 100029, PR China Received 19 March 2007; received in revised form 23 August 2007; accepted 10 November 2007
Abstract The adsorption mechanism for the imprinted ion (Ni2+ ) of a novel surface molecular imprinting adsorbent (SMIA) prepared by the imprinting technique was studied. The interaction mechanism for the imprinted ion (Ni2+ ) with –OH and –NH2 groups on the chitosan molecules was testified by Fourier transform-infrared spectrometry (FT-IR) and X-ray photoelectron spectroscopy (XPS). By the means of FT-IR and XPS analysis, there exist two kinds of –NH2 groups on the chitosan molecule. Most –NH2 groups showed higher adsorption activity owing to using the imprinting technique in the adsorbent preparation, whereas a few –NH2 groups displayed a lower adsorption activity because of being cross-linked in preparation, which caused different adsorption mechanisms for Ni2+ . Compared with the surface molecular non-imprinting adsorbent (non-SMIA), both –OH and –NH2 groups on the imprinted adsorbent surface could provide higher adsorption activity to Ni2+ based on FT-IR and XPS analyses. SMIA had more pores and bigger specific surface area than non-SMIA according to the distribution of the pore diameter and specific surface area analyses. Based on the above research, a higher adsorption capacity and a better selectivity for the imprinted ions of SMIA could be interpreted. © 2007 Elsevier B.V. All rights reserved. Keywords: Adsorption mechanism; Metal ion; The surface molecular imprinting adsorbent (SMIA); Fourier transform-infrared spectrometry; X-ray photoelectron spectroscopy
1. Introduction Chitosan and chitin are well-known biosorbents for heavy metal ions owing to their high uptake and good selectivity. Using native chitin and chitosan or more sophisticated forms resulting from grafting of specific functional groups, biosorption behaviors for many heavy metal ions have been extensively studied [1–3]. And the adsorption mechanisms and reaction processes between metal ions and functional groups on the adsorbent surface are of interest. Muzzareui et al. [4] found that the noxious heavy metal ions, such as Hg2+ , Cd2+ , Ni2+ , and Cr3+ , could be effectively removed though the –NH2 and –OH groups on the chitosan molecule. Using X-ray photoelectron spectroscopy (XPS) technology, chemical interactions between three metal ions –Cu2+ , Mo6+ , and Cr6+ and chitosan were researched by Dambies et al. [5], who found that sorption mainly occurred on amine functional groups of the biopolymer for all the three
∗
Corresponding author. Tel.: +86 10 64451636; fax: +86 10 64416428. E-mail address: [email protected] (H. Su).
1369-703X/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.bej.2007.11.011
metals. Based on the changes of the binding energy, the N elements on the chitosan molecules were found to play an important role for Cu2+ sorption compared with the O and C elements by Ji [6]. More FT-IR and XPS techniques have been carried out for the interpretation of chemical interactions between chitosan adsorbent and several metal ions, such as Cu2+ , Ni2+ , Zn2+ , and Cr3+ . The results have shown that the main removal mechanisms for the heavy metal ions are adsorption, complexation, or ion-exchange processes [7–9]. The interactions between functional groups on the adsorbent surface and metal ions are of interest, however the studies were limited on the direct interaction of raw and untreated chitosan with metal ions [10]. But it is also well known that chitosan can be dissolved in the dilute acids solution and it as a biosorbent for the removal of heavy metal ions from wastewater is expensive. In addition, the direct use of waste biomass for wastewater treatment is also difficult due to limited reusability and lower adsorption capacity. For increasing uptake, enhancing stability in the acid conditions, and reducing cost, the advantages of the biomass adsorbent with a low cost and those of the imprinted chitosan adsorbent with a higher uptake and better
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H. Su et al. / Biochemical Engineering Journal 39 (2008) 503–509
rigidity were combined. Therefore, a new molecular imprinting adsorbent based on the surface molecular imprinting technique was prepared by Su et al. [11]. Biosorption performances for heavy metal ions of the surface imprinted adsorbent on Penicillium chrysogenum mycelium (SMIA) have been described [12]. The application in wastewater treatment and the stability of the adsorbent have been presented [13]. And a new surface active site (SAS) adsorption equilibrium model was brought forward, which explicitly accounted for the H+ competitive adsorption with Ni2+ in adsorption equilibrium [14]. But the adsorption mechanisms and reaction processes between the imprinted ions and functional groups on the adsorbent surface were not studied. So a more focused study of active sites was necessary to explain the main adsorption mechanisms between the functional groups and the imprinted ions. In this work, the adsorption mechanism of SMIA for the imprinted ions (Ni2+ ) was performed using several analytical techniques such as: FT-IR, XPS, the distribution of the pore diameter, and specific surface area. Simultaneously, a higher adsorption capacity and a better selectivity for the imprinted ion of SMIA could be proved. 2. Materials and methods 2.1. Adsorbents 2.1.1. Pretreatment of the biomass and chitosan adsorbent Biomass adsorbent obtained from waste P. chysogenum from fermentation industries was supplied by Dongchen Biochemical Engineering Company (Dongying, Shandong Province, China), Chitosan with 90% deacetylation was obtained from shrimp shells in our laboratory. 2.1.2. Preparation of the surface molecular non-imprinted adsorbent (non-SMIA) The preparation process was as follows: chitosan (0.1 g dry weight) was dissolved in a 10 cm3 2.5% (v/v) acetic acid solution. Then, 0.5 cm3 epichlorohydrin as cross-linking agent was added to the mixture and allowed to react for 12 h at room temperature. Then 2.0 g (dry weight) of mycelium was added into the cross-linking mixture and the suspension was stirred for 10 min. Finally, the non-SMIA was filtered using filter paper and dried at 60 ◦ C for 4–6 h. The dry adsorbent was sized by an 80-mesh sieve and stored in a sealed bottle for further use. 2.1.3. Preparation of the surface molecular imprinting adsorbent (SMIA) The preparation process of SMIA was as follows: NiSO4 ·6H2 O was dissolved in 2 mL dilute acetic acid solution (2.5%, v/v) to give a Ni2+ solution of 2.0 mg mL−1 . Then 0.1 g chitosan (dry weight) was dissolved in this solution. Two grams mycelium (dry weight) and enough deionized water were added into the above solution, and the mixture was stirred. Then, 0.5 mL epichlorohydrin as a cross-linking agent was added into the mixture and allowed to react for 4–6 h at room temperature.
Fig. 1. Schematic illustration of SMIA. 1, biomass core; 2, chitosan coat; 3, metal ion; 4, specific space of metal ion; a, coated; b, imprinted; c, desorbed; d, adsorbed.
Finally, the Ni2+ imprinted on the adsorbent was removed by treating with EDTA solution containing 0.2 g L−1 for 8–12 h. Regeneration was carried out by washing the adsorbent with 0.2 mol L−1 NaOH for 2 h by resuspension at shaking bath. SMIA was filtered using filter paper and dried at 60 ◦ C. The dry adsorbent was sized by an 80-mesh sieve and stored for further use. The schematic illustration of SMIA was shown in Fig. 1. 2.2. Experiment techniques 2.2.1. Experimental condition of the X-ray photoelectron spectroscopy The XPSAM800 surface analyzer produced by KRATOS Corp. (UK) was used. X-ray light source was AL K␣ (hυ = 1486.6 eV), power was 12 kV × 10 mA. The working conditions of electron input lens include middle times, fixed deceleration ratio, and high resolving power. Narrow seam width used was 5 mm. Vacuum was 1.3 × 10−6 Pa. The value of binding energy (BE) was checked by Cls (BE = 284.6 eV). 2.2.2. Experimental condition of Fourier transform-infrared spectrometry (FT-IR) FT-IR analyses were carried out on Nicolte Avatar 205FT-IR spectrometer. The infrared (KBr) spectra of the different adsorbents in the form of a KBr pellet (0.1%, w/w) at 4000–400 cm−1 were depicted. 2.2.3. Distribution of the pore diameter and specific surface area Adsorption isotherm of nitrogen in MCM-41 at temperature T = 77 K was measured by the ASAP2010 volumetric adsorption analyzer produced by Micromeritics Instrument Corp. The Brunauer–Emmett–Teller (BET) specific surface, Barrett–Joyner–Halenda (BJH) adsorption cumulative pore volume and average pore size were solved. In the experiment, high purity (99.99%) nitrogen was used, and before each measurement the sample was degassed at 473 K for 16 h.
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Fig. 2. The effect of the initial Ni2+ concentration in solution on the adsorption capacity.
2.2.4. Surface structure of adsorbent The morphology of SMIA was assessed using scanning electron micrographs (SEM) (S-250MK, Cambridge Company, Cambridge, UK). 2.3. Analysis of metal ion The concentration of Ni2+ was analyzed by an adsorption spectrophotometry according to reference [15]. 3. Results and discussion 3.1. Adsorption behavior of SMIA The adsorption capacity for Ni2+ of the different adsorbents was shown in Fig. 2. With the increase of the initial Ni2+ concentration, the adsorption capacities for the imprinted ion (Ni2+ ) of SMIA and non-SMIA increased, but SMIA presented a higher adsorption capacity than the latter. The adsorption capacity for Ni2+ of SMIA enhanced twice compared with that of the biomass adsorbent. However for the lowest Ni2+ concentration of 48 mg L−1 , no notable difference in adsorption capacity from Fig. 2 was noticed. The reason was that the uptake of adsorbent could not reach a highest value at the lower Ni2+ concentrations. But the removal ratio of SMIA reached 90.6% at the Ni2+ concentration of 48 mg L−1 , almost total Ni2+ could be removed from solution. Whereas the removal ratio of non-SMIA and biomass adsorbent only had 77.6% and 66.2%, respectively. So the difference in removal ratio was very remarkable. This also explained the predominance of SMIA. Simultaneously, SMIA had considerably higher adsorption stability and still possessed 90–95% of the adsorption ability after being reused for 10 cycles [11]. But non-SMIA only provided a lower adsorption capacity for Ni2+ , a decrease of 30–50% over that of SMIA. Two reasons are as follows: one reason might be that SMIA had an imprinted chitosan coat, so the functional groups on coat could provide a continually higher adsorption activity. Another reason might be that, after having been reused for many batches,
Fig. 3. SEM of the biomass, the novel and 10 batches reused of SMIA (a) biomass; (b) the novel SMIA; (c) 10 batches reused of SMIA.
the imprinted coat of adsorbent surface might appear with some cracks, or some cross-linked bands were broken. Those led to benefit from some interior functional groups (–NH2 or –OH) on the adsorbent coat or biomass core (see Fig. 3). However
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Table 1 The wavenumber for the main bands (FT-IR) of the different adsorbents Adsorbent
Chitosan (cm−1 )
NonSMIA (cm−1 )
The reused 10 batches of non-SMIA (cm−1 )
SMIA (cm−1 )
The reused 10 batches of SMIA (cm−1 )
νO–H and νN–H –NH2 C6 –OH C3 –OH
3394.09 1594.55 1028.10 1152.97
3415.66 1564.18 1037.20 1155.93
3389.88 1553.72 1038.64 1154.85
3424.34 1563.15 1037.60 1157.12
3412.19 1562.93 and 1550.68 1030.07 1152.47
after a 10-batch utilization, the surface chitosan imprinted coat was destroyed and some coat might shed, which also caused the degradation of adsorption ability. 3.2. FT-IR spectral analysis of the adsorbents The interaction of the chitosan molecule on the adsorbents surface with Ni2+ is of considerable interest. It is very important to study on the adsorption mechanisms. Fig. 4 showed the FT-IR spectra of chitosan, the novel, and 10 batches reused of SMIA and non-SMIA, respectively. The wavenumber for the main bands of the adsorbents were listed in Table 1.
Fig. 4. FT-IR spectroscopies of SMIA and non-SMIA. a, chitosan; b, non-SMIA; c, 10 batches reused of non-SMIA; d, SMIA; e, 10 batches reused of SMIA.
The main spectra of chitosan were characterized by a series of narrow peaks in Fig. 4(a). The 3400 cm−1 was assigned to the shake-up adsorption peak of –OH and –NH2 groups on the adsorbent surfaces (ν(–OH) and ν(–NH2 )). It could be seen that, compared with chitosan, the peaks of SMIA and nonSMIA had some changes. It displayed that the –OH and –NH2 groups on the chitosan molecules of the adsorbent surface underwent a derivative reaction with the cross-linking agent in the preparation. In the mean time, the peak width at 3400 cm−1 increased; and the peak width of SMIA increased more apparently. However, having been reused for 10 batches, both the peak wavenumbers of two adsorbents at 3400 cm−1 decreased. The wavenumber of SMIA reduced 12.15 cm−1 , whereas that of non-SMIA decreased double about 25.78 cm−1 (See Table 1). That could explain why the adsorption ability of the latter after being reused 10 batches declined more. The presence of the –NH2 groups on the chitosan molecules with one characteristic band at 1594.55 cm−1 was indicated by the distinctive peak in Fig. 4(a). But this peak was found at about 1563 cm−1 for the novel SMIA and non-SMIA (Fig. 4(b and d)). There was only one peak at 1553.72 cm−1 after nonSMIA being reused 10 batches, and this peak shifted from 1564.18 cm−1 to 1553.72 cm−1 , a decrease of 10.46 cm−1 compared with the novel adsorbent (see Fig. 4(c)). Whereas we could find the two adsorption peaks of –NH2 at 1562.93 cm−1 and 1550.68 cm−1 in the FT-IR spectra after SMIA being reused 10 batches (see Fig. 4(e)). Compared with the characteristic peak at 1563.15 cm−1 of the novel SMIA, no obvious change of the former peak (at 1562.93 cm−1 ) was observed, but the latter peak made a considerable shift from 1563.15 cm−1 to 1550.68 cm−1 . The reason could be that there existed two kinds of –NH2 groups on the chitosan molecules to the Ni2+ adsorption. One kind of the –NH2 groups was imprinted and protected in the adsorbent preparation, so the peaks of which had no change after 10 batches reused. Another one was cross-linked by the cross-linking agent in preparation. And the cross-linked bands might break, which led to some cross-linked –NH2 groups of the adsorbent coat be exposed. The characteristic peak of the cross-linked –NH2 groups of SMIA was similar to that of non-SMIA. Therefore the changes of their adsorption peaks in FT-IR spectra were also similar. The characteristic peak of first hydroxyl (C6 –OH) on the chitosan molecules was indicated at 1028.10 cm−1 , the second hydroxyl (C3 –OH) had a characteristic peak at 1152.97 cm−1 . The first hydroxyl (C6 –OH) on SMIA and non-SMIA had more changes than the second hydroxyl (C3 –OH). However, when the adsorbent was reused 10 batches, the first and second hydroxyls
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Table 2 Binding energy of N and O on the adsorbent surface Element
Binding energy Non-SMIA
C O(–OH) N(–NH2 ) Ni2+
Fig. 5. O element––XPS spectrogram of the adsorbent. a, non-SMIA; b, SMIA; c, 10 batches reused of SMIA.
on SMIA had a bigger shift than those of non-SMIA. The results showed that the first and second hydroxyls on SMIA were higher active. Both –OH and –NH2 groups on the adsorbent surfaces could adsorb Ni2+ based on FT-IR spectra. But if the adsorbent was imprinted in preparation, the –OH and –NH2 groups could provide higher activity to the adsorption of Ni2+ than those of non-SMIA. 3.3. X-ray photoelectron spectroscopy of adsorbents Recently, XPS techniques have been widely used for the interpretation of chemical interactions between native chitosanor chitin and several metal ions such as Ag+ , Ni2+ , Cu2+ , Hg2+ , and Zn2+ . They have shown that the sorption of these metal ions mainly occurs on the –NH2 groups of the chitosan molecules. So XPS analysis was also carried out to elucidate the surface adsorption mechanism in this work. Figs. 5 and 6 showed the O and N elements spectrogram of the adsorbent, respectively. The comparison of XPS spectrogram between SMIA and non-SMIA showed that the binding energies of O and N elements had obvi-
Fig. 6. N element––XPS spectrogram of the adsorbent. a, non-SMIA; b, SMIA; c, 10 batches reused of SMIA.
284.6 532.55 399.35 –
SMIA 284.6 532.65 399.85 –
The reused 10 batches of SMIA 284.6 531.65 399.5 855.7, 856.8
ously changed. The BE of N element on SMIA increased from 0.5 eV to 399.85 eV. This showed the adsorption ability for Ni2+ of –NH2 increased because the N element had a much stronger ability of losing electron than that of non-SMIA. However, after being reused 10 batches of the adsorbent, the BE of N element decreased to 399.5 eV (BE = 0.15 eV). That led to the adsorption ability went down. The BE of N and O elements on the adsorbent surface in XPS were listed in Table 2. Compared with non-SMIA, the BE of O element in the novel SMIA had no obvious change (see Table 2). However after SMIA being reused 10 batches, the BE of O element decreased to 531.65 eV from 532.65 eV. This showed that the ability of the gaining electron of O element increased, so the –OH groups could also make contributions to the Ni2+ adsorption. The reason was that, after being reused 10 batches, the cross-linking bands on the adsorbent could be broken. Based on FT-IR and XPS analyses, the same adsorption mechanism between the –OH, –NH2 groups on the adsorbent surface and the imprinted ions (Ni2+ ) could be obtained. The existence of nickel on adsorbent surface was indicated by the Ni2+ -partial XPS spectrogram with two peaks of binding energy at 855.70 eV and weaker 856.80 eV in Fig. 7, while the BE of nickel in the NiSO4 solution is 856.80 eV. It could also be seen that there existed two kinds of reaction mechanism for Ni2+ . The BE at 855.7 eV of Ni2+ on the adsorbent surface decreased 1.1 eV compared with the free Ni2+ in the NiSO4 solution. This indicated that the Ni2+ with the BE at 855.7 eV could easily gain electrons. By using the imprinting technology in the preparation, most –NH2 groups on the chitosan molecules
Fig. 7. Partial XPS spectrogram of Ni2+ on the reused 10 batches of SMIA.
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Fig. 8. Distribution of the pore diameter of SMIA and non-SMIA. Table 3 Specific surface area of SMIA and non-SMIA Adsorbent Specific surface area
(m2 g−1 )
Non-SMIA
SMIA
1.68
1.99
were imprinted without being cross-linked, so the active sites for the imprinted ions (Ni2+ ) were left on the adsorbent surface, that caused Ni2+ easily adsorbed by the active site. At the weaker peaks of 856.80 eV, the adsorption mechanism maybe occur ion-exchange process between the cross-linked –NH2 or –OH groups and Ni2+ . The results showed that there existed different adsorption mechanisms of the imprinted –NH2 group and the cross-linked –NH2 group for Ni2+ . This also explained why the adsorption ability of SMIA increased more than that of non-SMIA. Actually, this had been proved by the adsorption of Cu2+ by chitosan molecule [6]. 3.4. Distribution of the pore diameter and specific surface area The distribution of the pore diameter on the adsorbent sur˚ 60 A, ˚ and 108 A ˚ were indicated face with three peaks at 33 A, (see Fig. 8). At those positions, the pore diameter of non˚ the number SMIA could also be observed. But at 27–33 A, of the pores on SMIA was almost tenfold that of non-SMIA, ˚ the number of the former was twice that of the and at 60 A, latter. The specific surface area of SMIA of 1.99 m2 g−1 , an increase of 18% over that of non-SMIA, was listed in Table 3. This showed that the pores on the adsorbent surface were very flat, so the adsorption process for Ni2+ mainly occurred on the adsorbent surface. The higher adsorption capacity and better selectivity for the imprinted ions of SMIA were testified again. 4. Conclusions The adsorption mechanism for Ni2+ of the –OH and –NH2 groups on the adsorbent surface was testified by FT-IR and
XPS analyses. By using imprinting technique in the adsorbent preparation, there existed two kinds of –NH2 groups on the chitosan molecules, most –NH2 groups protected and only a few –NH2 groups cross-linked, which caused different adsorption mechanism with Ni2+ . After the adsorbent being reused 10 batches, the cross-linking bands could be opened, or the adsorbent coat appeared with some cracks. This made some interior –OH groups and a few cross-linked NH2 groups made some contribution to the removal of Ni2+ . In contrast to non-SMIA, the BE of N element on SMIA increased 0.5 eV, which enhanced the adsorption ability for Ni2+ of –NH2 groups. The distribution of ˚ The non-SMIA the pore diameter on SMIA was mainly at 33 A. had only a few amounts of the pores, but the specific surface area of which merely decreased 18% compared with SMIA. So the pore on the SMIA surface was very flat and the adsorption process for Ni2+ mainly occurred on the adsorbent surfaces. Based on the above study, if the adsorbent was imprinted in preparation, the –OH and –NH2 groups could obtain more activity and a better selectivity for the imprinted ions (Ni2+ ) than non-SMIA. On the whole, the study on the adsorption mechanism would be propitious to the development of the adsorption technique. Acknowledgements The authors express their thanks for the supports from National Natural Science Foundation of China (20406002, 20636010, 20576013) and National Basic Research Program (973 Program) of China (2007CB714305). References [1] T.W. Tan, P. Chen, Biosorption of heavy metal ion with penicillin biomass, Biosep. Eng. 6 (2000) 169–173. [2] S. Nagib, K. Inoue, T. Yamaguchi, T. Tamaru, Recovery of Ni from a large excess of Al generated from spent hydrodesulfurization catalyst using picolylamine type chelating resin and complexane types of chemically modified chitosan, Hydrometallurgy 51 (1999) 73–85. [3] T. Becker, M. Schlaak, H. Strasdeit, Adsorption of nickel(II), zinc(II) and cadmium(II) by new chitosan derivatives, React. Funct. Polym. 44 (2000) 289–298. [4] R.A.A. Muzzareui, X. Weckx, O. Filippinj, F. Sigon, Removal of trace metal irons from industrial waters, nuclear effluents and drinking water, with the aid of cross-linked N-carboxymethyl chitosan, Carbohydr. Polym. 11 (4) (1989) 293–301. [5] L. Dambies, C. Guimon, S. Yiacoumi, E. Guibal, Characterization of metal ion interactions with chitosan by X-ray photoelectron spectroscopy Laurent, Colloid Surf. A: Physicochem. Eng. Aspects 177 (2001) 203–214. [6] J.H. Ji, XPS study on Cu2+ chitosan chelate and adsorption mechanism of chitosan for Cu2+ , Chin. J. Appl. Chem. 17 (1) (2000) 115–118. [7] M. Tsezos, Z. Georgousis, E. Remoundaki, Mechanism of aluminum interference on uranium biosorption by Rhizopus arrhizus, Biotechnol. Bioeng. 55 (1) (1997) 16–27. [8] Y.M. Robert, R.P. Huang, G.Y. Moon, Crosslinked chitosan composite membrane for the pervaporation dehydration of alcohol mixtures and enhancement of structural stability of chitosan/polysulfone composite membranes, J. Membrane Sci. 160 (1999) 17–30. [9] O. Genc, C. Arpa, G. Bayramog, M.Y. Arıca, S. Bektas, Selective recovery of mercury by Procion Brown MX 5BR immobilized poly (hydroxyethylmethacrylate/chitosan) composite membranes, Hydrometallurgy 67 (2002) 53–62.
H. Su et al. / Biochemical Engineering Journal 39 (2008) 503–509 [10] R.S. Juang, H.J. Shao, A simplified equilibrium model for sorption of heavy metal ions from aqueous solutions on chitosan, Water Res. 36 (2002) 2999–3008. [11] H.J. Su, Z.X. Wang, T.W. Tan, Preparation of a surface molecular imprinted adsorbent for Ni2+ based on Penicillium chysogenum, J. Chem. Technol. Biot. 4 (80) (2005) 439–444. [12] H.J. Su, Z.X. Wang, T.W. Tan, Adsorption behavior of surface molecular imprinting adsorbent from Penicillium chysogenum mycelium, Biotechnol. Lett. 25 (12) (2003) 949–953.
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[13] H.J. Su, Y. Zhao, J. Li, T.W. Tan, Biosorption of Ni2+ by the surface molecular imprinting adsorbent, Process Biochem. 41 (2006) 1422– 1426. [14] H.J. Su, S. Chen, T.W. Tan, Surface active site model for Ni2+ adsorption of the surface imprinted adsorbent, Process Biochem. 42 (2007) 612– 619. [15] National Environment Protect Bureau, The examining and analyzing method of water and wastewater. Environment Press of China,1989, pp. 156–161.
Adsorption mechanism for imprinted ion (Ni2+) of the surface molecular imprinting adsorbent (SMIA) Haijia Su ∗ , Jia Li, Tianwei Tan The Key Laboratory of Bioprocess of Beijing, Beijing University of Chemical Technology, P.O. Box 53, Beijing 100029, PR China Received 19 March 2007; received in revised form 23 August 2007; accepted 10 November 2007
Abstract The adsorption mechanism for the imprinted ion (Ni2+ ) of a novel surface molecular imprinting adsorbent (SMIA) prepared by the imprinting technique was studied. The interaction mechanism for the imprinted ion (Ni2+ ) with –OH and –NH2 groups on the chitosan molecules was testified by Fourier transform-infrared spectrometry (FT-IR) and X-ray photoelectron spectroscopy (XPS). By the means of FT-IR and XPS analysis, there exist two kinds of –NH2 groups on the chitosan molecule. Most –NH2 groups showed higher adsorption activity owing to using the imprinting technique in the adsorbent preparation, whereas a few –NH2 groups displayed a lower adsorption activity because of being cross-linked in preparation, which caused different adsorption mechanisms for Ni2+ . Compared with the surface molecular non-imprinting adsorbent (non-SMIA), both –OH and –NH2 groups on the imprinted adsorbent surface could provide higher adsorption activity to Ni2+ based on FT-IR and XPS analyses. SMIA had more pores and bigger specific surface area than non-SMIA according to the distribution of the pore diameter and specific surface area analyses. Based on the above research, a higher adsorption capacity and a better selectivity for the imprinted ions of SMIA could be interpreted. © 2007 Elsevier B.V. All rights reserved. Keywords: Adsorption mechanism; Metal ion; The surface molecular imprinting adsorbent (SMIA); Fourier transform-infrared spectrometry; X-ray photoelectron spectroscopy
1. Introduction Chitosan and chitin are well-known biosorbents for heavy metal ions owing to their high uptake and good selectivity. Using native chitin and chitosan or more sophisticated forms resulting from grafting of specific functional groups, biosorption behaviors for many heavy metal ions have been extensively studied [1–3]. And the adsorption mechanisms and reaction processes between metal ions and functional groups on the adsorbent surface are of interest. Muzzareui et al. [4] found that the noxious heavy metal ions, such as Hg2+ , Cd2+ , Ni2+ , and Cr3+ , could be effectively removed though the –NH2 and –OH groups on the chitosan molecule. Using X-ray photoelectron spectroscopy (XPS) technology, chemical interactions between three metal ions –Cu2+ , Mo6+ , and Cr6+ and chitosan were researched by Dambies et al. [5], who found that sorption mainly occurred on amine functional groups of the biopolymer for all the three
∗
Corresponding author. Tel.: +86 10 64451636; fax: +86 10 64416428. E-mail address: [email protected] (H. Su).
1369-703X/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.bej.2007.11.011
metals. Based on the changes of the binding energy, the N elements on the chitosan molecules were found to play an important role for Cu2+ sorption compared with the O and C elements by Ji [6]. More FT-IR and XPS techniques have been carried out for the interpretation of chemical interactions between chitosan adsorbent and several metal ions, such as Cu2+ , Ni2+ , Zn2+ , and Cr3+ . The results have shown that the main removal mechanisms for the heavy metal ions are adsorption, complexation, or ion-exchange processes [7–9]. The interactions between functional groups on the adsorbent surface and metal ions are of interest, however the studies were limited on the direct interaction of raw and untreated chitosan with metal ions [10]. But it is also well known that chitosan can be dissolved in the dilute acids solution and it as a biosorbent for the removal of heavy metal ions from wastewater is expensive. In addition, the direct use of waste biomass for wastewater treatment is also difficult due to limited reusability and lower adsorption capacity. For increasing uptake, enhancing stability in the acid conditions, and reducing cost, the advantages of the biomass adsorbent with a low cost and those of the imprinted chitosan adsorbent with a higher uptake and better
504
H. Su et al. / Biochemical Engineering Journal 39 (2008) 503–509
rigidity were combined. Therefore, a new molecular imprinting adsorbent based on the surface molecular imprinting technique was prepared by Su et al. [11]. Biosorption performances for heavy metal ions of the surface imprinted adsorbent on Penicillium chrysogenum mycelium (SMIA) have been described [12]. The application in wastewater treatment and the stability of the adsorbent have been presented [13]. And a new surface active site (SAS) adsorption equilibrium model was brought forward, which explicitly accounted for the H+ competitive adsorption with Ni2+ in adsorption equilibrium [14]. But the adsorption mechanisms and reaction processes between the imprinted ions and functional groups on the adsorbent surface were not studied. So a more focused study of active sites was necessary to explain the main adsorption mechanisms between the functional groups and the imprinted ions. In this work, the adsorption mechanism of SMIA for the imprinted ions (Ni2+ ) was performed using several analytical techniques such as: FT-IR, XPS, the distribution of the pore diameter, and specific surface area. Simultaneously, a higher adsorption capacity and a better selectivity for the imprinted ion of SMIA could be proved. 2. Materials and methods 2.1. Adsorbents 2.1.1. Pretreatment of the biomass and chitosan adsorbent Biomass adsorbent obtained from waste P. chysogenum from fermentation industries was supplied by Dongchen Biochemical Engineering Company (Dongying, Shandong Province, China), Chitosan with 90% deacetylation was obtained from shrimp shells in our laboratory. 2.1.2. Preparation of the surface molecular non-imprinted adsorbent (non-SMIA) The preparation process was as follows: chitosan (0.1 g dry weight) was dissolved in a 10 cm3 2.5% (v/v) acetic acid solution. Then, 0.5 cm3 epichlorohydrin as cross-linking agent was added to the mixture and allowed to react for 12 h at room temperature. Then 2.0 g (dry weight) of mycelium was added into the cross-linking mixture and the suspension was stirred for 10 min. Finally, the non-SMIA was filtered using filter paper and dried at 60 ◦ C for 4–6 h. The dry adsorbent was sized by an 80-mesh sieve and stored in a sealed bottle for further use. 2.1.3. Preparation of the surface molecular imprinting adsorbent (SMIA) The preparation process of SMIA was as follows: NiSO4 ·6H2 O was dissolved in 2 mL dilute acetic acid solution (2.5%, v/v) to give a Ni2+ solution of 2.0 mg mL−1 . Then 0.1 g chitosan (dry weight) was dissolved in this solution. Two grams mycelium (dry weight) and enough deionized water were added into the above solution, and the mixture was stirred. Then, 0.5 mL epichlorohydrin as a cross-linking agent was added into the mixture and allowed to react for 4–6 h at room temperature.
Fig. 1. Schematic illustration of SMIA. 1, biomass core; 2, chitosan coat; 3, metal ion; 4, specific space of metal ion; a, coated; b, imprinted; c, desorbed; d, adsorbed.
Finally, the Ni2+ imprinted on the adsorbent was removed by treating with EDTA solution containing 0.2 g L−1 for 8–12 h. Regeneration was carried out by washing the adsorbent with 0.2 mol L−1 NaOH for 2 h by resuspension at shaking bath. SMIA was filtered using filter paper and dried at 60 ◦ C. The dry adsorbent was sized by an 80-mesh sieve and stored for further use. The schematic illustration of SMIA was shown in Fig. 1. 2.2. Experiment techniques 2.2.1. Experimental condition of the X-ray photoelectron spectroscopy The XPSAM800 surface analyzer produced by KRATOS Corp. (UK) was used. X-ray light source was AL K␣ (hυ = 1486.6 eV), power was 12 kV × 10 mA. The working conditions of electron input lens include middle times, fixed deceleration ratio, and high resolving power. Narrow seam width used was 5 mm. Vacuum was 1.3 × 10−6 Pa. The value of binding energy (BE) was checked by Cls (BE = 284.6 eV). 2.2.2. Experimental condition of Fourier transform-infrared spectrometry (FT-IR) FT-IR analyses were carried out on Nicolte Avatar 205FT-IR spectrometer. The infrared (KBr) spectra of the different adsorbents in the form of a KBr pellet (0.1%, w/w) at 4000–400 cm−1 were depicted. 2.2.3. Distribution of the pore diameter and specific surface area Adsorption isotherm of nitrogen in MCM-41 at temperature T = 77 K was measured by the ASAP2010 volumetric adsorption analyzer produced by Micromeritics Instrument Corp. The Brunauer–Emmett–Teller (BET) specific surface, Barrett–Joyner–Halenda (BJH) adsorption cumulative pore volume and average pore size were solved. In the experiment, high purity (99.99%) nitrogen was used, and before each measurement the sample was degassed at 473 K for 16 h.
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Fig. 2. The effect of the initial Ni2+ concentration in solution on the adsorption capacity.
2.2.4. Surface structure of adsorbent The morphology of SMIA was assessed using scanning electron micrographs (SEM) (S-250MK, Cambridge Company, Cambridge, UK). 2.3. Analysis of metal ion The concentration of Ni2+ was analyzed by an adsorption spectrophotometry according to reference [15]. 3. Results and discussion 3.1. Adsorption behavior of SMIA The adsorption capacity for Ni2+ of the different adsorbents was shown in Fig. 2. With the increase of the initial Ni2+ concentration, the adsorption capacities for the imprinted ion (Ni2+ ) of SMIA and non-SMIA increased, but SMIA presented a higher adsorption capacity than the latter. The adsorption capacity for Ni2+ of SMIA enhanced twice compared with that of the biomass adsorbent. However for the lowest Ni2+ concentration of 48 mg L−1 , no notable difference in adsorption capacity from Fig. 2 was noticed. The reason was that the uptake of adsorbent could not reach a highest value at the lower Ni2+ concentrations. But the removal ratio of SMIA reached 90.6% at the Ni2+ concentration of 48 mg L−1 , almost total Ni2+ could be removed from solution. Whereas the removal ratio of non-SMIA and biomass adsorbent only had 77.6% and 66.2%, respectively. So the difference in removal ratio was very remarkable. This also explained the predominance of SMIA. Simultaneously, SMIA had considerably higher adsorption stability and still possessed 90–95% of the adsorption ability after being reused for 10 cycles [11]. But non-SMIA only provided a lower adsorption capacity for Ni2+ , a decrease of 30–50% over that of SMIA. Two reasons are as follows: one reason might be that SMIA had an imprinted chitosan coat, so the functional groups on coat could provide a continually higher adsorption activity. Another reason might be that, after having been reused for many batches,
Fig. 3. SEM of the biomass, the novel and 10 batches reused of SMIA (a) biomass; (b) the novel SMIA; (c) 10 batches reused of SMIA.
the imprinted coat of adsorbent surface might appear with some cracks, or some cross-linked bands were broken. Those led to benefit from some interior functional groups (–NH2 or –OH) on the adsorbent coat or biomass core (see Fig. 3). However
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Table 1 The wavenumber for the main bands (FT-IR) of the different adsorbents Adsorbent
Chitosan (cm−1 )
NonSMIA (cm−1 )
The reused 10 batches of non-SMIA (cm−1 )
SMIA (cm−1 )
The reused 10 batches of SMIA (cm−1 )
νO–H and νN–H –NH2 C6 –OH C3 –OH
3394.09 1594.55 1028.10 1152.97
3415.66 1564.18 1037.20 1155.93
3389.88 1553.72 1038.64 1154.85
3424.34 1563.15 1037.60 1157.12
3412.19 1562.93 and 1550.68 1030.07 1152.47
after a 10-batch utilization, the surface chitosan imprinted coat was destroyed and some coat might shed, which also caused the degradation of adsorption ability. 3.2. FT-IR spectral analysis of the adsorbents The interaction of the chitosan molecule on the adsorbents surface with Ni2+ is of considerable interest. It is very important to study on the adsorption mechanisms. Fig. 4 showed the FT-IR spectra of chitosan, the novel, and 10 batches reused of SMIA and non-SMIA, respectively. The wavenumber for the main bands of the adsorbents were listed in Table 1.
Fig. 4. FT-IR spectroscopies of SMIA and non-SMIA. a, chitosan; b, non-SMIA; c, 10 batches reused of non-SMIA; d, SMIA; e, 10 batches reused of SMIA.
The main spectra of chitosan were characterized by a series of narrow peaks in Fig. 4(a). The 3400 cm−1 was assigned to the shake-up adsorption peak of –OH and –NH2 groups on the adsorbent surfaces (ν(–OH) and ν(–NH2 )). It could be seen that, compared with chitosan, the peaks of SMIA and nonSMIA had some changes. It displayed that the –OH and –NH2 groups on the chitosan molecules of the adsorbent surface underwent a derivative reaction with the cross-linking agent in the preparation. In the mean time, the peak width at 3400 cm−1 increased; and the peak width of SMIA increased more apparently. However, having been reused for 10 batches, both the peak wavenumbers of two adsorbents at 3400 cm−1 decreased. The wavenumber of SMIA reduced 12.15 cm−1 , whereas that of non-SMIA decreased double about 25.78 cm−1 (See Table 1). That could explain why the adsorption ability of the latter after being reused 10 batches declined more. The presence of the –NH2 groups on the chitosan molecules with one characteristic band at 1594.55 cm−1 was indicated by the distinctive peak in Fig. 4(a). But this peak was found at about 1563 cm−1 for the novel SMIA and non-SMIA (Fig. 4(b and d)). There was only one peak at 1553.72 cm−1 after nonSMIA being reused 10 batches, and this peak shifted from 1564.18 cm−1 to 1553.72 cm−1 , a decrease of 10.46 cm−1 compared with the novel adsorbent (see Fig. 4(c)). Whereas we could find the two adsorption peaks of –NH2 at 1562.93 cm−1 and 1550.68 cm−1 in the FT-IR spectra after SMIA being reused 10 batches (see Fig. 4(e)). Compared with the characteristic peak at 1563.15 cm−1 of the novel SMIA, no obvious change of the former peak (at 1562.93 cm−1 ) was observed, but the latter peak made a considerable shift from 1563.15 cm−1 to 1550.68 cm−1 . The reason could be that there existed two kinds of –NH2 groups on the chitosan molecules to the Ni2+ adsorption. One kind of the –NH2 groups was imprinted and protected in the adsorbent preparation, so the peaks of which had no change after 10 batches reused. Another one was cross-linked by the cross-linking agent in preparation. And the cross-linked bands might break, which led to some cross-linked –NH2 groups of the adsorbent coat be exposed. The characteristic peak of the cross-linked –NH2 groups of SMIA was similar to that of non-SMIA. Therefore the changes of their adsorption peaks in FT-IR spectra were also similar. The characteristic peak of first hydroxyl (C6 –OH) on the chitosan molecules was indicated at 1028.10 cm−1 , the second hydroxyl (C3 –OH) had a characteristic peak at 1152.97 cm−1 . The first hydroxyl (C6 –OH) on SMIA and non-SMIA had more changes than the second hydroxyl (C3 –OH). However, when the adsorbent was reused 10 batches, the first and second hydroxyls
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Table 2 Binding energy of N and O on the adsorbent surface Element
Binding energy Non-SMIA
C O(–OH) N(–NH2 ) Ni2+
Fig. 5. O element––XPS spectrogram of the adsorbent. a, non-SMIA; b, SMIA; c, 10 batches reused of SMIA.
on SMIA had a bigger shift than those of non-SMIA. The results showed that the first and second hydroxyls on SMIA were higher active. Both –OH and –NH2 groups on the adsorbent surfaces could adsorb Ni2+ based on FT-IR spectra. But if the adsorbent was imprinted in preparation, the –OH and –NH2 groups could provide higher activity to the adsorption of Ni2+ than those of non-SMIA. 3.3. X-ray photoelectron spectroscopy of adsorbents Recently, XPS techniques have been widely used for the interpretation of chemical interactions between native chitosanor chitin and several metal ions such as Ag+ , Ni2+ , Cu2+ , Hg2+ , and Zn2+ . They have shown that the sorption of these metal ions mainly occurs on the –NH2 groups of the chitosan molecules. So XPS analysis was also carried out to elucidate the surface adsorption mechanism in this work. Figs. 5 and 6 showed the O and N elements spectrogram of the adsorbent, respectively. The comparison of XPS spectrogram between SMIA and non-SMIA showed that the binding energies of O and N elements had obvi-
Fig. 6. N element––XPS spectrogram of the adsorbent. a, non-SMIA; b, SMIA; c, 10 batches reused of SMIA.
284.6 532.55 399.35 –
SMIA 284.6 532.65 399.85 –
The reused 10 batches of SMIA 284.6 531.65 399.5 855.7, 856.8
ously changed. The BE of N element on SMIA increased from 0.5 eV to 399.85 eV. This showed the adsorption ability for Ni2+ of –NH2 increased because the N element had a much stronger ability of losing electron than that of non-SMIA. However, after being reused 10 batches of the adsorbent, the BE of N element decreased to 399.5 eV (BE = 0.15 eV). That led to the adsorption ability went down. The BE of N and O elements on the adsorbent surface in XPS were listed in Table 2. Compared with non-SMIA, the BE of O element in the novel SMIA had no obvious change (see Table 2). However after SMIA being reused 10 batches, the BE of O element decreased to 531.65 eV from 532.65 eV. This showed that the ability of the gaining electron of O element increased, so the –OH groups could also make contributions to the Ni2+ adsorption. The reason was that, after being reused 10 batches, the cross-linking bands on the adsorbent could be broken. Based on FT-IR and XPS analyses, the same adsorption mechanism between the –OH, –NH2 groups on the adsorbent surface and the imprinted ions (Ni2+ ) could be obtained. The existence of nickel on adsorbent surface was indicated by the Ni2+ -partial XPS spectrogram with two peaks of binding energy at 855.70 eV and weaker 856.80 eV in Fig. 7, while the BE of nickel in the NiSO4 solution is 856.80 eV. It could also be seen that there existed two kinds of reaction mechanism for Ni2+ . The BE at 855.7 eV of Ni2+ on the adsorbent surface decreased 1.1 eV compared with the free Ni2+ in the NiSO4 solution. This indicated that the Ni2+ with the BE at 855.7 eV could easily gain electrons. By using the imprinting technology in the preparation, most –NH2 groups on the chitosan molecules
Fig. 7. Partial XPS spectrogram of Ni2+ on the reused 10 batches of SMIA.
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Fig. 8. Distribution of the pore diameter of SMIA and non-SMIA. Table 3 Specific surface area of SMIA and non-SMIA Adsorbent Specific surface area
(m2 g−1 )
Non-SMIA
SMIA
1.68
1.99
were imprinted without being cross-linked, so the active sites for the imprinted ions (Ni2+ ) were left on the adsorbent surface, that caused Ni2+ easily adsorbed by the active site. At the weaker peaks of 856.80 eV, the adsorption mechanism maybe occur ion-exchange process between the cross-linked –NH2 or –OH groups and Ni2+ . The results showed that there existed different adsorption mechanisms of the imprinted –NH2 group and the cross-linked –NH2 group for Ni2+ . This also explained why the adsorption ability of SMIA increased more than that of non-SMIA. Actually, this had been proved by the adsorption of Cu2+ by chitosan molecule [6]. 3.4. Distribution of the pore diameter and specific surface area The distribution of the pore diameter on the adsorbent sur˚ 60 A, ˚ and 108 A ˚ were indicated face with three peaks at 33 A, (see Fig. 8). At those positions, the pore diameter of non˚ the number SMIA could also be observed. But at 27–33 A, of the pores on SMIA was almost tenfold that of non-SMIA, ˚ the number of the former was twice that of the and at 60 A, latter. The specific surface area of SMIA of 1.99 m2 g−1 , an increase of 18% over that of non-SMIA, was listed in Table 3. This showed that the pores on the adsorbent surface were very flat, so the adsorption process for Ni2+ mainly occurred on the adsorbent surface. The higher adsorption capacity and better selectivity for the imprinted ions of SMIA were testified again. 4. Conclusions The adsorption mechanism for Ni2+ of the –OH and –NH2 groups on the adsorbent surface was testified by FT-IR and
XPS analyses. By using imprinting technique in the adsorbent preparation, there existed two kinds of –NH2 groups on the chitosan molecules, most –NH2 groups protected and only a few –NH2 groups cross-linked, which caused different adsorption mechanism with Ni2+ . After the adsorbent being reused 10 batches, the cross-linking bands could be opened, or the adsorbent coat appeared with some cracks. This made some interior –OH groups and a few cross-linked NH2 groups made some contribution to the removal of Ni2+ . In contrast to non-SMIA, the BE of N element on SMIA increased 0.5 eV, which enhanced the adsorption ability for Ni2+ of –NH2 groups. The distribution of ˚ The non-SMIA the pore diameter on SMIA was mainly at 33 A. had only a few amounts of the pores, but the specific surface area of which merely decreased 18% compared with SMIA. So the pore on the SMIA surface was very flat and the adsorption process for Ni2+ mainly occurred on the adsorbent surfaces. Based on the above study, if the adsorbent was imprinted in preparation, the –OH and –NH2 groups could obtain more activity and a better selectivity for the imprinted ions (Ni2+ ) than non-SMIA. On the whole, the study on the adsorption mechanism would be propitious to the development of the adsorption technique. Acknowledgements The authors express their thanks for the supports from National Natural Science Foundation of China (20406002, 20636010, 20576013) and National Basic Research Program (973 Program) of China (2007CB714305). References [1] T.W. Tan, P. Chen, Biosorption of heavy metal ion with penicillin biomass, Biosep. Eng. 6 (2000) 169–173. [2] S. Nagib, K. Inoue, T. Yamaguchi, T. Tamaru, Recovery of Ni from a large excess of Al generated from spent hydrodesulfurization catalyst using picolylamine type chelating resin and complexane types of chemically modified chitosan, Hydrometallurgy 51 (1999) 73–85. [3] T. Becker, M. Schlaak, H. Strasdeit, Adsorption of nickel(II), zinc(II) and cadmium(II) by new chitosan derivatives, React. Funct. Polym. 44 (2000) 289–298. [4] R.A.A. Muzzareui, X. Weckx, O. Filippinj, F. Sigon, Removal of trace metal irons from industrial waters, nuclear effluents and drinking water, with the aid of cross-linked N-carboxymethyl chitosan, Carbohydr. Polym. 11 (4) (1989) 293–301. [5] L. Dambies, C. Guimon, S. Yiacoumi, E. Guibal, Characterization of metal ion interactions with chitosan by X-ray photoelectron spectroscopy Laurent, Colloid Surf. A: Physicochem. Eng. Aspects 177 (2001) 203–214. [6] J.H. Ji, XPS study on Cu2+ chitosan chelate and adsorption mechanism of chitosan for Cu2+ , Chin. J. Appl. Chem. 17 (1) (2000) 115–118. [7] M. Tsezos, Z. Georgousis, E. Remoundaki, Mechanism of aluminum interference on uranium biosorption by Rhizopus arrhizus, Biotechnol. Bioeng. 55 (1) (1997) 16–27. [8] Y.M. Robert, R.P. Huang, G.Y. Moon, Crosslinked chitosan composite membrane for the pervaporation dehydration of alcohol mixtures and enhancement of structural stability of chitosan/polysulfone composite membranes, J. Membrane Sci. 160 (1999) 17–30. [9] O. Genc, C. Arpa, G. Bayramog, M.Y. Arıca, S. Bektas, Selective recovery of mercury by Procion Brown MX 5BR immobilized poly (hydroxyethylmethacrylate/chitosan) composite membranes, Hydrometallurgy 67 (2002) 53–62.
H. Su et al. / Biochemical Engineering Journal 39 (2008) 503–509 [10] R.S. Juang, H.J. Shao, A simplified equilibrium model for sorption of heavy metal ions from aqueous solutions on chitosan, Water Res. 36 (2002) 2999–3008. [11] H.J. Su, Z.X. Wang, T.W. Tan, Preparation of a surface molecular imprinted adsorbent for Ni2+ based on Penicillium chysogenum, J. Chem. Technol. Biot. 4 (80) (2005) 439–444. [12] H.J. Su, Z.X. Wang, T.W. Tan, Adsorption behavior of surface molecular imprinting adsorbent from Penicillium chysogenum mycelium, Biotechnol. Lett. 25 (12) (2003) 949–953.
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[13] H.J. Su, Y. Zhao, J. Li, T.W. Tan, Biosorption of Ni2+ by the surface molecular imprinting adsorbent, Process Biochem. 41 (2006) 1422– 1426. [14] H.J. Su, S. Chen, T.W. Tan, Surface active site model for Ni2+ adsorption of the surface imprinted adsorbent, Process Biochem. 42 (2007) 612– 619. [15] National Environment Protect Bureau, The examining and analyzing method of water and wastewater. Environment Press of China,1989, pp. 156–161.