Selective Adsorption of Co(II) by Mesoporous Silica SBA-15-Supported Surface Ion Imprinted Polymer: Kinetics, Isotherms, and Thermodynamics Studies

FULL PAPER

* E-mail: [email protected]; Tel.: 13775539306; Fax: 0086-0511-88790683 Received September 25, 2011; revised November 26, 2011; accepted December 13, 2011. Project supported by the National Natural Science Foundation of China (No. 21077046), Ph. D. Programs Foundation of Ministry of Education of

China (No. 20093227110015), Ph.D. Innovation Programs Foundation of Jiangsu University (No. CX09B 12XZ).

Chin. J. Chem. 2011, 29, 387—398 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 387

Selective Adsorption of Co(II) by Mesoporous Silica SBA-15-Supported Surface Ion Imprinted Polymer: Kinetics,

Isotherms, and Thermodynamics Studies

Liu, Yana(刘燕) Liu, Zhanchaob(刘占超) Dai, Jiangdonga(戴江栋) Gao, Jiea(高捷) Xie, Jimina(谢吉民) Yan, Yongsheng*,a(闫永胜)

a School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China b School of Materials Science and Engineering, Jiangsu University of Science and Technology,

Zhenjiang, Jiangsu 212013, China

A novel surface ion imprinted adsorbent [Co(II)-IIP] using polyethyleneimine (PEI) as function monomer and ordered mesoporous silica SBA-15 as support matrix was prepared for Co(II) analysis with high selectivity. The prepared polymer was characterized by Fourier transmission infrared spectrometry, scanning electron microscopy, X-ray diffraction and nitrogen adsorption-desorption isotherm. Bath experiments of Co(II) adsorption onto Co(II)-IIP were performed under the optimum conditions. The experimental data were analyzed by pseudo-first-order and pseudo-second-order kinetic models. It was found that the pseudo-second-order model best correlated the kinetic data. The intraparticle diffusion and liquid film diffusion were applied to discuss the adsorp-tion mechanism. The results showed that Co(II) adsorption onto IIP was controlled by the intraparticle diffusion mechanism, along with a considerable film diffusion contribution. Langmuir, Freundlich and Dubinin-Radushke- vich adsorption models were applied to determine the isotherm parameters. Langmuir model fitted the experiment data well and the maximum calculated capacity of Co(II) reached 39.26 mg/g under room temperature. The ther-modynamic data were indicative of the spontaneousness of the endothermic sorption process of Co(II) onto Co(II)-IIP. Co(II)-IIP showed high affinity and selectivity for template ion compared with non imprinted polymer (NIP).

Keywords surface ion imprinted, Co(II), polyethyleneimine (PEI), SBA-15, adsorption, selective

Introduction

Metal ions are among the important pollutants in wastewaters as they are being discharged from several industries. Cobalt containing compounds are widely used in many industrial applications such as mining, metallurgical, electroplating, paints, pigments and elec-tronics.1 Cobalt, one of the common toxic and low-level radioactive metals affecting the environment, appears in wastewaters of nuclear power plants and many other industries and it can produce variety of undesirable ef-fects like nausea, vomiting and asthma, causing heart failure, damage to thyroid and liver on human beings.2 With a better awareness of the problems associated with cobalt, research studies related to the methods of re-moving cobalt from wastewater have drawn attention increasingly.

A number of processes exist for the removal of metal pollution, viz. precipitation,3 reverse osmosis,4 ion-exchange,5 coagulation6 and adsorption.7 Among them, adsorption of metals plays a significant role in

both natural and technological processes in terms of simplicity of design and operation. One of the adsorbent which is widely used for the removal of cobalt and other heavy metals is activated carbon.8 However, the high regeneration cost and poor selectivity of the activated carbon limit its large-scale applications.

Molecular imprinting is a convenient and powerful technique to synthesize molecularly imprinted polymer (MIP) which provides artificial receptor-like binding sites for template molecules. Therefore, the removal of template molecules leaves behind imprinted cavities inside the polymer which are complementary in shape, size and functional groups to the template molecules.9,10 Similar to MIP, ion-imprinted polymer (IIP) is a versa-tile material for producing imprints of target ions in cross-linked polymers, which displays the behavior of selective ions recognition.11,12 Three steps are involved in ion-imprinted process: (i) complexation of metal ion (template) to ligand; (ii) polymerization of this complex; (iii) removal of metal ion after co-polymerization.13 IIP

Liu et al.FULL PAPER

388 www.cjc.wiley-vch.de © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chin. J. Chem. 2011, 29, 387—398

has outstanding advantages of predetermined selectivity, simplicity and convenience to prepare. Removal of metal ions from environmental sources by IIP can lead to selective removal of analytes from matrix that can not be achieved by conventional methods. However, IIP synthesized by conventional methods has some disad-vantages, such as the number of recognition sites per unit volume of the polymer is relatively low and the template molecules are embedded in the matrices too deeply. In order to overcome these drawbacks effec-tively, the surface molecularly imprinting technique has been developed.14,15

The surface imprinting technique based on the sur-face modification of matrix materials has been studied widely.16 In our previous studies, the adsorption charac-teristics of Co17 and Sr18 on surface imprinted polymer supported by potassium titanate whisker were investi-gated. In recent years, ordered mesoporous materials have been universally reported as good solid support due to their stable mesoporous structure, tunable pore size, good chemical and mechanical stability, and well modified surface properties with abundant Si—OH ac-tive bonds on the pore walls.16 As mesoporous silica is generally considered as an ordered inert matrix, surface functionalization of ordered mesoporous silica thus sub-stantially expands its application in catalysis, separation and adsorption processes. Among the mesoporous sili-cas, SBA-15 characterized with highly ordered two-dimensional symmetry possesses hexagonal arrays of uniform pores with ultra large pore diameters (up to 30 nm), large surface area (up to 1000 m2/g), high pore volume (up to 1.3 cm3/g) and thick pore walls (3.1 to 6.4 nm).19 It also shows excellent homogeneity and chemical/mechanical stability.20 These characteristics of SBA-15 enable itself a potential candidate for inclusion of guest species on the surface. In the present work, we use SBA-15 as a new matrix in order to improve the adsorption capacity.

Polyethyleneimine (PEI) is a kind of water-soluble polyamine, and there is a large quantity of nitrogen at-oms of amino groups on the line-type macromolecular chains of PEI, which makes it a well-recognized or-ganic-based polymer with high metal complexation ca-pability.21 As a result, PEI is a kind of new trapping agent for heavy-metal ions. PEI impregnation on acti-vated carbon for adsorption of metal ions has been studied by Yin et al.22 and PEI modified SiO2 for ad-sorption of Pb2＋,23 Cu2＋, Cd2＋, Zn2＋ 24 and 2

4CrO － 25 have been studied by Gao et al. The research results show that PEI modified inorganic materials possessed excellent adsorption property for heavy metal ions. However, the basic disadvantage of the adsorbents is lack of metal selectivity.

In present work, we attempted to synthesize a novel surface imprinted polymer using PEI as functional monomer based on support matrix of SBA-15. Ion im-printing was performed using Co(II) as template, and

epichlorohydrin as crosslinking agent. The structural characteristics, adsorption properties of the IIP-PEI/ SBA-15 adsorbents including kinetic, isotherm, ther-modynamic and selectivity towards Co(II) in aqueous solution, were described and discussed in detail.

Experimental

Chemicals

PEI, molecular weight of 10000—20000, was pur-chased from Qianglong Chemical Limited Company (Wuhan, China). γ-Chloropropyl trimethoxysilane (CP, Aladdin, Shanghai, China), Pluronic P123, triblock poly- (ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), (EO20PO70EO20, molecular weight 5800) (Sigma, USA) were used in our work. Standard stock solution of Co(II) (1.0 g/L) were prepared by dissolving CoCl2• 6H2O. Standard working solutions were further diluted prior to use, also the other metal ions. All the chemicals used were of analytical grade. Doubly distilled water (DDW) was used for all dilutions.

Apparatus and measurements

Spectrometric measurements were carried out with a TAS-986 flame atomic adsorption spectrometer (FAAS) (Beijing, China); the main operating conditions were as follows: lamp current: 2.0 mA; narrow aperture: 0.4 nm; burning equipment height: 5.0 mm; acetylene flow rate: 1.2 L/min; burner position: 3.0 mm.

A pHS-3C digital pH meter and DDS-11A conduc-tivity meter (LIDA Instrument Factory, Shanghai, China) were used for the pH value adjustments and conductiv-ity determination, respectively. Fourier transmission infrared spectra (FT-IR, 4000—400 cm－1) in KBr were recorded on a NICOLET NEXUS 470 FT-IR spec-trometer (Nicolet, USA). Scanning electron micro-graphs (SEM) was obtained with a JSM-6480 Field-Emission Scanning Electron Microscope (Jeol, Japan). Low-angle X-ray powder diffraction (XRD) measurements were carried out with a Bruker D8 dif-fractometer (Bruker, Germany) using a wavelength of 1.5418 Å (Cu Kα) and an angle of 0.5°—10° with 0.002° steps. A NOVA2000 surface area and pore size analyzer (Quntachrome, USA) was used to measure surface area and pore size distribution. Brunauer- Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) method were employed to calculate the surface area and pore diameter.

Conductometric titration

Conductometric titration was operated according to reference.26 Briefly, PEI solution with a molarity (monomeric unit molarity) of 20 mmol/L was obtained to titrate 5.0 mL of 20 mmol/L Co(II) ion solution in buffer solution (pH value 5.0). The conductivity changes of ion solution during titration were recorded with a digit conductivity meter, and at the same time, the volumes of consumed PEI solution were noted.

Selective Adsorption of Co(II) by Surface Ion Imprinted Polymer

Chin. J. Chem. 2011, 29, 387—398 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 389

Preparation of Co(II)-IIP

Preparation and activation of SBA-15 SBA-15 was synthesized as described by Zhao et al. using P123 as the structure-directing agent and TEOS as the silica source.27 It was then activated with methane sulfonic acid by refluxing for 24 h, followed by washing with doubly distilled water (DDW) to neutral and heated up at 110 ℃ for 2 h for drying. The activated SBA-15 was obtained.

Preparation and characterization of PEI/SBA-15 5 g of activated SBA-15 was reacted with 7 mL of CP at 80 ℃ by using 40 mL of xylene as solvent, into which 1.4 mL of water was added, and chloropropylation of SBA-15 was realized and CP-SBA-15 was prepared. Finally, 5 g of CP-SBA-15 was added into 50 mL of 20% PEI aqueous solution. The reaction was carried out at 90 ℃ for 6 h, PEI was grafted onto SBA-15 surface in coupling manner, and PEI/SBA-15 was prepared. The amount of amino groups on PEI/SBA-15 was deter-mined with conductivity titration method by using hy-drochloric acid as titrant, and the grafting amount of PEI was calculated to be 4.29 g/ 100 g.

Preparing and characterization of ion-imprinted polymer Co(II)-IIP 5 g of PEI/SBA-15, 0.1552 g of CoCl2•6H2O and 10 mL of epichlorohydrin (ECH) were added into absolute ethanol and the reaction was al-lowed to carry out for 30 min at room temperature with continuous stirring. Afterwards, 10 mL of 0.01 mol/L sodium hydroxide was added. The reaction continued for 30 min at room temperature under continuous stir-ring. Then, the dry product was grinded and treated with 2.0 mol/L HCl to completely leach the coordinated and non-coordinated Co(II), thus, Co(II)-IIP was obtained. At last, the polymer was filtered with DDW to neutrali-zation, dried at 60 ℃ under vacuum, grinded and sifted with 100 meshes. By comparison, the non-imprinted polymer (NIP) was also prepared as a blank in parallel but without the addition of CoCl2•6H2O.

Adsorption experiments

Co(II) adsorption batch experiments Adsorption of Co(II) from aqueous solutions was investigated in batch experiments. 0.02 g of Co(II)-IIP was added into 25.0 mL colorimetric tube containing certain amount of Co(II) at a pH value of 5.0 (adjusted with 0.1 mol/L HCI and 0.1 mol/L NH3•H2O). Then the mixture was shaken vigorously for 10 min, and the adsorption time was maintained for certain moment. After centrifugation, the concentration of Co(II) in the residues solution was determined by FAAS. The adsorption ratio (E) and ab-sorption capacity qe (mg/g) were calculated by Eqs. 1, 2.

0 e

0

100%C C

EC

－

＝ × (1)

0 ee

( )C C Vq

W

－

＝ (2)

where C0 (mg/L) and Ce (mg/L) are initial and equilib-rium concentrations of Co(II), respectively. V (mL) and W (g) are the volume of solution and the mass of sor-bent, respectively.

Selectivity study In order to measure the selectiv-ity of the imprinted polymer, competitive adsorption of Co(II)/Pb(II), Co(II)/Cu(II), Co(II)/Zn(II), Co(II)/Sr(II) and Co(II)/Ni(II) from their mixtures were investigated by using Co(II)-IIP and NIP, respectively. The distribu-tion coefficient Kd (mL/g), selectivity coefficient k, and the relative selectivity coefficient k′ 28 were given as follows:

i fd

f

( )C C VK

C W

－

＝ (3)

d ( C o )

d ( M )

Kk

K＝ (4)

( I IP )

( N IP )

kk '

k＝ (5)

where Ci and Cf represent the initial and equilibrated concentrations of the given metal ions in solution, re-spectively. Kd(Co) and Kd(M) represent the distribution coefficient of Co(II) and M [M＝Pb(II), Cu(II), Zn(II), Sr(II), Ni(II)] respectively. k(IIP) and k(NIP) represent the selectivity coefficient of Co(II)-IIP and NIP, respec-tively.

Results and discussion

Preparation

According to the steps described in Section “Con-ductometric titration”, 5.0 mL of Co(II) ion solution was titrated by PEI solution with the same concentration. During the titration process the conductivity decreases with the consuming of PEI solution because of chelation of PEI towards Co(II), then a clear inflection point ap-pears at the volume of 30 mL, which reveals that the stoichiometric amounts of the coordination reaction between PEI and Co(II) is in the ratio of 6∶1, and the chelates with six ligands have been formed.

The preparing process of Co(II)-IIP is expressed in Figure 1. The first step of reaction process is the prepa-ration of PEI/SBA-15. γ-Chloropropyl trimethoxysilane was chosen as coupling agent to link silica gel and func-tional monomer macromolecular PEI. The activated SBA-15 with great deals of silanol groups producing on mesoprous surface was first functionalized by grafting procedure. γ-Chloropropyl trimethoxysilane reacted with silanol groups by self-condensation to form modi-fied SBA-15 (CP-SBA-15). The molecules of commer-cial PEI often contains primary, secondary and ternary amino groups in a ratio of approximately 1∶2∶1.29 The chloropropyl groups on CP-SBA-15 reacted further with amine groups on PEI chains, macromolecule PEI



was grafted onto silica surface in coupling mode, so that the composite particles of PEI/SBA-15 were formed finally.

The second step of reaction process is the prepara-tion of Co(II)-IIP. After PEI/SBA-15 was swelled fully in water medium, they would produce strong chelation towards Co(II) ion. Firstly, the ring opening reaction between ECH and amine groups of PEI chain took place when the crosslink agent ECH was added. Then the de-hydrochlorination reaction between ECH and another amine group of PEI chain occured when NaOH was added. Finally, the template ion was removed by hy-drochloric acid solution, and ionic imprinting polymer Co(II)-IIP was formed. A net work of condensed or-ganosilanes was formed in the pores of SBA-15, lower-ing the specific surface area.

Material characterization

FT-IR results To identify synthesis process, SBA-15, PEI/SBA-15 and Co(II)-IIP were characterized by IR spectra as shown in Figure 2. In the spectra of SBA-15, the siloxane (Si—O—Si) bands appeared as a broad strong one centered at 1086 and 800 cm－1, while the peaks near 3442 and 1632 cm－1 were attributed to the stretching and bending vibrations of the surface si-lanol groups.30 In the spectrum of PEI/SBA-15, the characteristics absorptions of flex vibration and bend vibration of N—H bond appeared at 3641 and 1701 cm－1, respectively, and the characteristics absorptions of bend vibration of C—N bond appeared at 1464 and

1560 cm－1.23 The appearance of these absorption bands showed that PEI macromolecules have been grafted onto SBA-15 surface. It was found that after ion im-printing the absorption bands at 3641 and 1701 cm－1, which were vibration absorption of N—H bond, disap-peared, indicating that the H atoms of the primary and secondary amine groups in PEI chains have been sub-stituted completely by methylene of ECH (via ring- opening reaction and dehydrochlorination reaction), namely, all of primary and secondary amine groups in PEI chains have changed into tertiary groups. At the same time, the vibration absorption of O—H groups reappeared at 3442 cm－1, and it further indicated that the ring-opening reaction of ECH has been occurred. Besides, after ion imprinting the vibration absorption bands of C—H bond at 2958 and 1464 cm－1 have shifted to 2972 and 1454 cm－1, respectively, owing to the change of the chemical surroundings around C—H bond after cross-linking reaction.26 These phenomena suggested that macromolecule PEI was successfully grafted onto the surface of the mesoporous silica of SBA-15 and subsequently a layer of ion-imprinted polymer has been formed on the surface of SBA-15.

SEM Figure 3 displays the particle morphology of SBA-15 and Co(II)-IIP. As seen in Figure 3, the mor-phology of the standard SBA-15 sample displays a spherical shape with a size of 1—2 µm and smooth sur-faces. After polymerization, the solid particles of Co(II)-IIP agglomerated and then formed cylindrical

Figure 1 Schematic illustration of preparation process of Co(II)-IIP supported by SBA-15.



Figure 2 FT-IR spectra of SBA-15, PEI/SBA-15 and Co(II)-IIP.

Figure 3 SEM micrographs of (a): SBA-15; (b): Co(II)-IIP.

shape polymer. Moreover, polymer film existing on the external surface of the mesoporous material could be observed. As a result, regular shaped and more stable ion imprinted polymer was obtained.

Low angle XRD pattern results are shown in Figure 4. Three XRD peaks at 0.77°, 1.44° and 1.64° were ob-served for Co(II)-IIP, which corresponded to the (100), (110) and (200) reflections of SBA-15,28 respectively, suggesting that the structure of SBA-15 was well pre-served. The decrease in the (100) XRD diffraction peak in Co(II)-IIP compared with SBA-15 provided evidence that grafting mainly occurs inside the mesopore chan-nels, since the attachment of functional groups in the mesopore channels tends to reduce the scattering power of the mesoporous silicate wall.31

The nitrogen adsorption-desorption isotherms for SBA-15, PEI/SBA-15 and Co(II)-IIP are shown in

Figure 4 Low angle XRD patterns of SBA-15 and Co(II)-IIP.

Figure 5. It can be seen that three isotherms were all of type IV and exhibited a hysteresis loop of H1 type ac-cording to the IUPAC classification, typical for materi-als with pores of constant cross section, which indicated that the parent mesoporous structure was well main-tained during the synthesis process.32 The narrower pore size distribution of Co(II)-IIP shown in insert curve of Figure 5 further provided evidence for their uniform framework mesoporosity. The BET surface area, pore volume and average pore diameter of SBA-15 were 546.03 m2/g, 0.82 cm3/g and 6.6 nm, respectively, while the three parameters of Co(II)-IIP decreased to 136.93 m2/g, 0.22 cm3/g and 2.58 nm, respectively, which sug-gested the incorporation of macromolecular functional groups into the pore channels of SBA-15.

Figure 5 Nitrogen adsorption-desorption isotherm at 77 K of SBA-15, PEI/SBA-15 and Co(II)-IIP. Inset: Pore size distribution for samples.

Adsorption experiments

Effect of pH and mass of sorbent From the known hydrolysis constants up to pH 8, Co ions exists mainly as Co2 ＋ ions and at pH 10 [Co(OH)] ＋

predominates.33 Present adsorption studies were carried



out at pH lower than 7 and, therefore, Co2＋ ion was the main species in the solution that was adsorbed on Co(II)-IIP. The adsorption ratios of Co(II) on Co(II)-IIP at different pH values are shown in Table 1. The ad-sorption ratio increased with the enhancing of pH values, and at pH 7, the adsorption ratio reached maximum. In acidic solution, most of N atoms of the amino groups in the macromolecular chains of PEI are in protonation state,21 the protonation degree of N atoms of amino groups decreases with the decrease of acidity, and the coordination ability of N atoms of amino groups to-wards metal ions strengthens, so that the adsorption ra-tio of Co(II) increased with the rising of pH value. However, for most of heavy metal ions, precipitation occurred at pH of 6 to 7. In order to investigate selectiv-ity of Co(II)-IIP for other metal ions, thereafter, initial pH value of 5 was selected as the experimental pH. As shown in Table 1, the adsorption ratio of Co(II)-IIP to-wards Co(II) showed no obvious change between 0.02 and 0.08 g. So the minimum amount of adsorbent of 0.02 g was chosen as the optimum mass of sorbent.

Table 1 Effect of initial pH and mass of sorbent on adsorption of Co(II) onto Co(II)-IIPa

pH Adsorption ratio/% Mass of sorbent/g Adsorption ratio/%

1 18.88 0.02 84.74

2 19.16 0.03 84.76

3 20.10 0.05 84.78

4 46.12 0.08 84.74

5 84.74

6 85.16

7 95.16 a Experimental conditions: Co(II) 5.0 mg/L; shaking time 5 min; adsorption time 200 min; temperature 25 ℃.

Effect of contact time at different initial ion con-centrations and temperatures A study of the effect of contact time on the adsorption of Co(II) by Co(II)-IIP at different initial concentrations was performed as shown in Figure 6a. It was observed that the adsorption rate was extremely high from 0 min to about 20 min and became slow during the latter stages. The adsorption processes were near equilibrium after 200 min for all the initial concentrations studied. The value of present work was lower than that of our previous studies (300 min).18 With changing the concentrations of the solution from 3.0 to 8.0 mg/L, the absolute amount of Co(II) ions per unit of adsorbent increased from 2.55 to 8.50 mg/g at 25 ℃.

As can be seen from Figure 6b, the adsorption of Co(II) onto the surface of Co(II)-IIP took place quickly for three temperatures during first 20 min, then became constant till 200 min. The absorbed amount of Co(II) ions slightly increased from 5.30 to 5.90 mg/g when increasing temperature from 25 to 55 ℃. A little pre-doninance was observed of high temperature effect on

adsorption of Co(II). To ensure the easy control of ex-perimental conditions, in addition, the adsorption capac-ity was satisfied under room temperature, further ad-sorption experiments were carried out at 25 ℃.

Figure 6 Effect of contact time and comparison of three kinetic models for adsorption of Co(II) onto Co(II)-IIP at (a): different initial concentrations; (b): different temperatures.

Kinetics studies To evaluate the adsorption kinet-ics of Co(II) ions, three different kinetic models were applied for the experimental data: (1) the pseudo-first- order model; (2) the pseudo-second-order model; (3) the Elovich model.

The pseudo-first-order equation is:34

e t e 1ln( ) lnq q q k t－＝－ (6)

The pseudo-second-order equation is:34

2e2 e

1 1

t

tt

q qk q

⎛ ⎞⎜ ⎟⎝ ⎠

＝＋ (7)

where qe (mg/g) is the adsorption capacity at equilib-rium, qt (mg/g) is the adsorption amount at time t (min), k1 (min－1) and k2 (mg•g－1•min－1) are pseudo-first-order and pseudo-second-order rate constants of adsorption, respectively.

The Elovich equation is one of the most useful mod-



els for describing chemisorption,35

1 1( ) ln( ) lntq ab tb b

＝＋ (8)

where a (mg•g－1•min－1), is the initial sorption rate, and b (g/mg), is related to the extent of surface coverage and activation energy for chemisorption.

To compare the validity of each model, a normalized standard deviation ∆qe (%) was calculated using

2exp cal exp

e

( ) /100%

1

q q qq

N

∑∆

－＝ ×

－

(9)

where qexp and qcal (mg/g) are the experimental and cal-culated amount of Co(II), respectively, that was ad-sorbed on Co(II)-IIP, and N is the number of measure-ments made. If data from a model are similar to the ex-perimental data, the value of ∆qe (%) will be low; oth-erwise, if they differ, the value of ∆qe will be high. To confirm the best fit kinetic and isotherms models for the adsorption system, it is necessary to analyze the data set using ∆qe combined with the values of the determined coefficient R2.

Pseudo-first-order model is rendered the rate of occupation of the adsorption sites to be proportional to the number of unoccupied sites; pseudo-second-order kinetic model is assumed the chemical reaction mecha-nisms, and that the adsorption rate is controlled by chemical adsorption through sharing or exchange of electrons between the adsorbrate and adsorbent. All ki-netic parameters, correlation coefficients, and ∆qe are listed in Table 2. It can be seen from Figure 6 that the pseudo-first-order kinetic curves do not give a good fit to the experimental kinetic data. This disagreement can also be corroborated by low R2 and high ∆qe. As a result, it was suggested that the adsorption of Co(II) onto Co(II)-IIP did not follow the pseudo-first-order model. On the contrary, the results presented an ideal fit to the pseudo-second-order model in Figure 6 together with the extremely high R2. A good agreement can further be supported by the low values of ∆qe. Otherwise, the

Elovich model could also adapt to the experimental data according to the relatively low ∆qe (%) value, which indicated a chemical sorption mechanism.35 The con-stant b is very small indicating the rate of chemisorption is fast in the whole adsorption process.

Adsorption mechanism The pseudo-first-order and pseudo-second-order models can not identify the diffusion mechanism, thus, to gain insight into the mechanism and rate-controlling steps affecting the kinetics of adsorption, intraparticle diffusion and external mass transfer have been applied to investigate the ad-sorption process.

Intraparticle diffusion equation was given as cited in Ref. 36:

1/2dtq k t y＝＋ (10)

where kd (mg•L－1•min－0.5) is the initial rate of intrapar-ticular diffusion and y is the intercept. Value of y gives information on to the thickness of the boundary layer, that is, the larger the intercept, the greater the bound-ary-layer effect.

The liquid film diffusion model namely external mass transfer37 may be applied to determine the trans-port of the solute molecules from the liquid phase up to the solid phase boundary:

fln(1 )F k t－＝－ (11)

where F is the fractional attainment of equilibrium (F＝qt/qe) and kf (cm/s) is the film diffusion rate constant.

For a solid-liquid adsorption process, the solute transfer is usually characterized by either external mass transfer (boundary layer diffusion) for non porous me-dia or intraparticle diffusion for porous matrices, or both combined. If the intercept of plot qt vs. t1/2 passed through the origin, then intraparticle diffusion is the sole rate-limiting step.37

While the plot of ln(1－F) vs. t with zero intercept would suggest that the kinetics of the adsorption process is controlled by diffusion through the liquid film sur-

Table 2 Parameters of three kinetic models for adsorption of Co(II) onto Co(II)-IIP

Pseudo-first-order kinetics Pseudo-second-order kinetics Elovich Parameter

qe(exp)/ (mg•g－1) qe(cal)/

(mg•g－1) k1/min－1 R2 ∆qe/%

qe(cal)/

(mg•g－1) k2/

(g•mg－1•min－1) R2 ∆qe/%

a/ (mg•g－1•min－1)

b/(g•mg－1) R2 ∆qe/%

Temp./℃

25 5.30 4.161 1.20×10－2 0.940 26.32 5.50 6.90×10－3 0.999 4.62 0.90 1.16 0.992 6.03

35 5.58 3.546 1.02×10－2 0.942 44.64 5.73 7.7×10－3 0.999 3.29 1.29 1.16 0.978 6.40

55 5.90 2.209 1.03×10－2 0.949 76.62 6.02 1.37×10－2 0.999 2.49 4.93 1.29 0.932 9.96

C0/(mg•L－1)

3.0 2.55 0.706 1.14×10－2 0.954 88.57 2.56 4.90×10－2 0.999 0.48 29.14 4.14 0.890 7.35

5.0 5.30 4.161 1.20×10－2 0.940 26.32 5.50 6.90×10－3 0.999 4.62 0.90 1.16 0.992 6.03

8.0 8.50 3.050 8.99×10－3 0.945 78.53 8.62 9.69×10－3 0.999 1.73 19.82 1.06 0.932 6.34



Table 3 Intraparticle mass transfer, external mass transfer parameters for adsorption of Co(II) onto Co(II)-IIP

Intraparticle mass transfer External mass transfer

Parameter kd1/(mg•

g－1•min1/2)

kd2/(mg•

g－1•min1/2)

kd3/(mg•

g－1•min1/2) y1 y2 y3 (R1)

2 (R2)2 (R3)

2 kf/(cm•s－1) Intercepts R2

Temp/℃

25 0.611 0.189 0.041 0.034 2.098 4.330 0.9955 0.9752 0.9002 0.012 0.241 0.9399

35 0.517 0.170 0.057 0.494 2.694 4.244 0.9934 0.9241 0.9159 0.010 0.453 0.9423

55 0.573 0.132 0.007 1.081 3.833 5.735 0.9430 0.8541 0.8849 0.010 0.982 0.9487

C0/(mg•L－1)

3.0 0.048 0.053 0.003 0.159 1.759 2.486 0.9647 0.9843 0.7368 0.011 1.284 0.9538

5.0 0.611 0.189 0.041 0.034 2.098 4.330 0.9955 0.9752 0.9002 0.012 0.241 0.9399

8.0 1.297 0.172 0.035 0.626 5.562 7.677 0.9776 0.9851 0.6440 0.009 1.026 0.9452

rounding the solid adsorbents. As shown in Table 3, intercept of plot of qt vs. t1/2 (y1) passed through closely from the origin, which indicated the boundary layer ef-fect. However the intercept of plot of －ln(1－F) vs. t was not zero, which suggested that Co(II) adsorption onto Co(II)-IIP was controlled by the intraparticle diffu-sion mechanism, along with a considerable film diffu-sion contribution of the mechanism.

As seen in Figure 7, it can be concluded that three adsorption stages occurred in the whole adsorption process. Firstly, in the initial 20 min, it is known that the Co(II) species migrate from the bulk liquid phase to the outer surface of adsorbent particles through film diffu-sion (external mass transfer) and this adsorption process was very fast. Secondly, the portion from 20 to 200 min was the gradual adsorption stage, which has been vali-dated by the above analysis that both film diffusion and intra-particle diffusion were simultaneously operating. Thirdly, the portion (after 200 min) was the final equi-librium stage where the two diffusion mechanisms start to slowdown due to the extremely low solute concentra-tion in solution. It can be considered that a first straight line portion represents macro-pore diffusion, the second and the third one represent meso-pore diffusion, respec-tively. Moreover, the three straight lines represented the relative values of intercept C, which give an idea about the boundary layer thickness. It was observed that the intercept C increased with increasing contact time for three linear portions, which implies that film diffusion (boundary layer diffusion) mechanism should become more significant when the adsorption time becomes longer.

Adsorption isotherms Adsorption isotherms de-scribe qualitative information on the nature of the sol-ute-surface interaction as well as the specific relation between the concentration of adsorbate and its degree of accumulation onto adsorbent surface at constant tem-perature. Adsorption isotherm experiments for Co(II)- IIP and NIP were carried out as a function of different initial concentrations of Co(II) solution at temperature

Figure 7 Intraparticle diffusion model for the adsorption of Co(II) onto Co(II)-IIP at (a): different initial concentrations; (b): different temperatures.

settings of 25, 35 and 55 ℃ (Figure 8). With increas-ing temperature, the adsorption capacity for Co(II)-IIP increased obviously. The information thus obtained specifies an endothermic nature of the existing process. Three isotherm models—Langmuir, Freundlich and Dubbin-Radushkevich (D-R) were used to analyze the equilibrium experimental data as shown in Figure 8.

Langmuir isotherm assumes monolayer adsorption



onto a surface with a finite number of identical sites. It was expressed as follows:38

e e

e m m

1C C

q q b q＝＋ (12)

b (L/mg) and qm (mg/g) are the Langmuir coefficients, representing the adsorption equilibrium constant and the monolayer capacity, respectively.

Figure 8 Adsorption isotherms and comparison of differenrt isotherm models for adsorption of Co(II) onto (a) Co(II)-IIP; (b) NIP at different temperatures.

A dimensionless constant separation factor is de-noted as RL and defined as:39

L0

1

1R

bC＝

＋ (13)

The RL value indicates whether the type of the iso-therm is favorable (0＜RL＜1), unfavorable (RL＞1), linear (RL＝1), or irreversible (RL＝0).

Freundlich isotherm40 is an empirical equation based on adsorption on a heterogeneous surface. The equation is commonly represented as follows:

e f e1

ln ln lnq K Cn

＝＋ (14)

where Kf (mg/g(L/mg)1/n) and n are the Freundlich con-

stants characteristics of the system, indicating the ad-sorption capacity and the adsorption intensity, respec-tively.

The D-R isotherm is also widely used in adsorption studies because it does not assume a homogeneous sur-face or constant adsorption potential.41 The D-R equa-tion is given by the following relationship:

2e m DRln lnq q K ε＝－ (15)

where KDR is the constant related to the mean free en-ergy of sorption. Value of KDR＜1.0 represents the rough surface with many cavities.41 qm is the theoretical saturation capacity, and ε can be correlated as:

e

1ln(1 )RT

Cε＝＋ (16)

where R is the gas constant (8.314 J/mol•k-1) and T is the absolute temperature.

The D-R constant can give the valuable information regarding the mean energy42 (E, kJ/mol) by the follow-ing equation:

1/2DR=(2 )E K − (17)

The magnitude of E can be used for estimating the type of adsorption. The adsorption behavior can be de-scribed as the physical adsorption when E is between 1.0 and 8.0 kJ/mol. However, the chemical adsorption is more than 8.0 kJ/mol of the mean adsorption energy.42

The three model adsorption constants calculated from the corresponding isotherms with the correlation coefficients are presented in Table 4. Comparisons of R2 and ∆qe values showed that the adsorption process could be well described by the Langmuir equation. The con-formity of the adsorption data to the Langmuir isotherm (correlation coefficient＞0.995) could be interpreted as indicating a homogeny adsorption process, leading to monolayer binding. Based on the effect of separation factor RL values are in the range of 0＜RL＜1, which indicated that Co(II)-IIP was a favourable adsorbent for adsorption of Co(II) ions from aqueous solution. The mean adsorption energies (E) of the adsorption process at 25, 35 and 55 ℃ were 31.54, 32.82 and 38.82 kJ/mol, respectively, reflecting that the adsorption was predominantly chemical adsorption in the process. As illustrated in Figure 1, chelation of amino groups on PEI with Co(II) is considered the dominating mechanism. A comparison based on Langmuir calculated maximum capacity for Co(II) at 25 ℃, indicated that Co(II)-IIP (39.26 mg/g) was more efficient than NIP (12.21 mg/g). Moreover, the saturation adsorption capacity in present work is higher than the values reported in our prerious studies for Co(II) adsorption on IIP supported by potas-sium titanate whisker (22.5 mg/g).18



Table 4 Parameters of three isotherm models for adsorption of Co(II) onto Co(II)-IIP and NIP at different temperatures

Langmuir isotherm Freundlich isotherm Dubinin-Radushkevich

Temp./℃ qe(exp)/

(mg•g－1) qm/

(mg•g－1) K/(1/mg) R2 RL

∆qe/

%

Kf/

(mg/g)•(L/mg)1/n n R2 ∆qe/%

qm/

(mg•g－1)

KDR/

(mol2•kJ－2)

E/

(kJ•mol－1) R2

∆qe/

%

25 37.96 39.26 4.94×10－2 0.995 0.8709—0.0430 4.19 5.10 2.80 0.919 20.39 29.47 5.028×10－4 31.54 0.809 27.39

35 41.80 43.33 5.23×10－2 0.995 0.8644—0.0408 4.48 5.83 2.84 0.934 20.51 33.25 4.642×10－4 32.82 0.851 25.05 IIP

55 44.97 46.19 6.18×10－2 0.996 0.8436—0.0347 3.32 7.01 3.00 0.941 19.45 34.74 3.317×10－4 38.82 0.801 27.86

25 11.63 12.21 2.48×10－2 0.974 0.9308—0.0822 6.11 1.33 2.72 0.948 8.95 8.66 9.840×10－4 22.54 0.810 31.28

35 12.87 13.24 2.81×10－2 0.976 0.9223—0.0733 3.52 1.70 2.92 0.945 7.04 9.70 8.834×10－4 24.49 0.843 30.17 NIP

55 13.82 14.03 3.03×10－2 0.976 0.9167—0.0683 1.86 1.94 3.03 0.944 5.94 10.47 8.282×10－4 24.57 0.860 29.69

The thermodynamic parameters In environ-

mental engineering practice, both energy and entropy factors must be considered in order to determine which process will occur spontaneously. The thermodynamic parameters of Gibbs free energy, enthalpy and entropy for the adsorption of Co(II) can be obtained using the following equation:43

DlnH S

KRT R

∆ ∆＝－＋

� �

(kJ/mol) (18)

G H T S∆ ∆ ∆＝－

� � �

(kJ/mol) (19)

where KD is the equilibrium constant (KD＝qe/Ce). At different temperatures, the corresponding Ce values for different qe were calculated by the Langmuir equation. Table 5 summarizes the values of these thermodynamic parameters at different ion concentrations.

The negative values of G∆ � indicate the feasibility of the process and the spontaneous nature of adsorption process. When the initial concentration increased from 100.0 to 400.0 mg/L, the magnitude of free energy change shifts to low negative value, suggesting that the adsorption was more spontaneous at low concentration. The value of H∆ � was positive, indicating that the binding of Co(II) ions onto Co(II)-IIP was endothermic in nature. In addition, the positive value of S∆ � suggested an increase in the randomness at the

solid/solution interface during the adsorption process. In adsorption of Co(II), the adsorbed solvent molecules, which were displaced by the adsorbate species, gain more translational entropy than that lost by the adsor-bate ions, thus allowing for the prevalence of random-ness in the system.44 Compared with Gibbs free energy for the adsorption of Co(II) on NIP, the spontaneous adsorptive forces were stronger on Co(II)-IIP due to the specific recognition sites on Co(II)-IIP.

Selectivity study Pb(II), Cu(II), Zn(II), Sr(II) and Ni(II) were chosen as competitive metal ions because they are belong to heavy metal ions with the same charge with Co(II), additionally Sr(II) and Co(II) are belong to mid-low radionuclide. As shown in Table 6, the Kd(IIP) value of Co(II)-IIP for Co(II) was the largest, while Kd(IIP) decreased significantly for Pb(II), Cu(II), Zn(II), Sr(II) and Ni(II), respectively. NIP has low k value due to having random distribution of ligand func-tionalities in the polymeric network. High k value of Co(II)-IIP synthesized for Co(II) had a higher selectiv-ity specialism for this ion. The reason for this is that the cavities created after removal of the template were complementary to the imprint ion in size, shape and coordination geometries. The chelate of Co(II) with PEI is six ligands, whereas the chelate of Pb(II) or Cu(II) with PEI is four ligands. Pb(II) or Cu(II) is difficult to combine with Co(II)-IIP because of non-matched com-bining sites. The radii of Zn(II), Sr(II) and Ni(II)

Table 5 Thermodynamic paramaters for the adsorption of Co(II) onto Co(II)-IIP and NIP

G∆ � /(kJ•mol－1) Polymer C0/(mg•L－1) H∆ � /(kJ•mol－1) S∆ � /(J•mol－1•K－1)

25 ℃ 35 ℃ 55 ℃

100.0 8.154 75.978 －14.487 －15.247 －16.767

200.0 5.060 60.951 －13.103 －13.713 －14.932

300.0 4.967 57.656 －12.214 －12.791 －13.944 IIP

400.0 4.860 54.922 －11.507 －12.056 －13.154

100.0 6.330 57.169 －10.706 －11.278 －12.421

200.0 3.611 43.767 －9.432 －9.870 －10.745

300.0 3.230 41.002 －8.989 －9.399 －10.219 NIP

400.0 4.332 42.856 －8.439 －8.868 －9.725



Table 6 Competitive sorption of different ions by Co(II)-IIP and NIP sorbenta

Co(II)-IIP NIP Metal type

kd/(mL•g－1) k

kd/(mL•g－1) k k′

Co(II) 1016.72 94.72

Pb(II) 138.76 7.33 190.40 0.50 14.73

Cu(II) 117.41 8.66 125.85 0.75 11.51

Zn(II) 117.82 8.63 119.19 0.79 10.86

Sr(II) 27.43 37.06 39.77 2.38 15.56

Ni(II) 123.65 8.22 125.30 0.76 10.88 a Experimental conditions: Initial concentrations of metal ions 10.0 mg/L; Co(II)-IIP 0.08 g; shaking time 5 min; adsorption time 200 min; temperature 25 ℃.

ions were 74, 118 and 72 pm, respectively, which are bigger than that of Co(II) (65 pm), so they could not enter into the cavity imprinted by Co(II).

Desorption experiments To make the adsorption process more economical and to obtain practical infor-mation about the recovery of Co(II)-IIP, desorption from spent adsorbent material was studied. Attempts were made to desorb Co(II) ions using 2.0 mol/L H2SO4, HCl, HNO3 which have good potential to dissolve metal ions. The desorption experiments were conducted for 6.0 h at 298 K. The optimum desorption was happened in 2 mol/L HCl. It showed that 3.0 mL of 2.0 mol/L HCl could desorb 95.8% of the adsorbed Co(II). As a result, the quantitative recoveries of Co(II) can be obtained using 3.0 mL of 2.0 mol/L HCl as eluent.

Subsequently, the effect of six consecutive adsorp-tion-desorption cycles on the adsorption of Co(II) by Co(II)-IIP was studied. Adsorption/desorption cycle of Co(II)-IIP decreased slightly as the number of cycle increased. The adsorption capacity of the recycled Co(II)-IIP retained more than 95% after 6 cycles. It can be concluded that the Co(II)-IIP can be used for several times without significantly decreasing its adsorption capacities.

Conclusion

A novel surface ion imprinted adsorbent using poly-ethyleneimine (PEI) as function monomer and ordered mesoporous silica SBA-15 as support matrix was pre-pared for Co(II) analysis with high selectivity. The ad-sorption behavior of Co(II) onto Co(II)-IIP belonged to the pseudo-second-order kinetic model and the adsorp-tion process was controlled by the intra-particle diffu-sion mechanism, along with a considerable film diffu-sion contribution. Langmuir was well fitted the data and indicated the formation of monolayer coverage of Co(II) ions on the surface of the sorbent. Thermodynamic pa-rameters indicated the adsorption process was sponta-neous, endothermic and of good affinity nature. Com-petitive adsorption studies showed that Co(II)-IIP of-fered the advantages of high selectivity toward targeted Co(II) even in the presence of other metal ions.

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Documents

Selective Adsorption of Co(II) by Mesoporous Silica SBA-15-Supported Surface Ion Imprinted Polymer: Kinetics, Isotherms, and Thermodynamics Studies