Kinetic and Isotherm Studies of Ni2+Adsorption on Poly(methacrylic

Article pubs.acs.org/IECR

Kinetic and Isotherm Studies of Ni2+Adsorption on Poly(methacrylic acid) Synthesized through a Hierarchical Double-Imprinting Method Using a Ni2+ Ion and Cationic Surfactant as Templates Fernanda Midori de Oliveira,† Bruna Fabrin Somera,† Emerson Schwingel Ribeiro,‡ Mariana Gava Segatelli,† Maria Josefa Santos Yabe,† Evgeny Galunin,† and César Ricardo Teixeira Tarley†,§,* †

Departamento de Química, Universidade Estadual de Londrina (UEL), Centro de Ciências Exatas, Rodovia Celso Garcia Cid, PR 445, Km 380, Londrina, PR, 86050-482, Brazil ‡ Instituto de Química, Universidade Federal do Rio de Janeiro (UFRJ), CEP 21941-909, Rio de Janeiro- RJ, Brazil § Departamento de Química Analítica, Instituto de Química, Universidade Estadual de Campinas (UNICAMP), Instituto Nacional de Ciência e Tecnologia (INCT) de Bioanalítica, Cidade Universitária Zeferino Vaz, s/n, Campinas, SP, 13083-970, Brazil S Supporting Information *

ABSTRACT: A novel poly(methacrylic acid) material (IIP/CTAB) was prepared by a hierarchical double-imprinting process with Ni2+ ion and cationic surfactantcetyltrimethylammonium bromide (CTAB) as templates, and it was employed to adsorb Ni2+ ions from aqueous medium. Other poly(methacrylic acid) materials single-imprinted (IIP/no CTAB) and nonimprinted (NIP/no CTAB) were investigated in adsorption studies. All the synthesized polymers were characterized by FTIR, SEM, and nitrogen adsorption−desorption isotherm. The maximum Ni2+ adsorption capacities of IIP/CTAB and NIP/no CTAB were found to be 33.31 and 18.64 mg g−1, respectively, at pH 7.25. The relative selectivity coefficient (k′) values for Ni2+/Cu2+, Ni2+/ Mn+, Ni2+/Co2+ and Ni2+/Pb2+ systems were higher than 1, thus confirming the significant improvement in the selectivity of the polymer. The kinetic data were described very well by the pseudo-second-order model, thereby confirming the chemical nature of the Ni2+ adsorption (chemisorption), whereas the dual-site Langmuir−Freundlich equation provided the best fit to the isotherm data, suggesting the existence of two kinds of adsorption sites (with low and high binding energies) on the polymer surface. The high chemical stability of IIP/CTAB was verified with 300 Ni2+ adsorption−desorption cycles using 1.0 mol L−1 HNO3 as stripping agent.

1. INTRODUCTION Nickel is one of numerous trace metals found in air, water and soil. The main sources of its emissions to air include volcanic eruptions, rock weathering by wind and fire, waste incineration, fossil fuel combustion and even burning tobacco, whereas kitchen utensils, inexpensive jewelry, metal alloys, and agricultural activity are the common causes of water and soil pollution.1 Human exposure to this metal usually takes place during inhalation of gases and ingestion of food and drinks. Although nickel is considered moderately toxic compared to other transition metals, it is among those causing some skin disorders such as dermatitis and allergic reactions in highly sensitive people. Chronic exposure may lead to pulmonary fibrosis, cardiovascular diseases, and exert carcinogenic activity.1−3 In aquatic systems, nickel forms insoluble hydroxide, oxide, and sulfide compounds, and thus, its bioavailability depends on its concentration, salinity, pH, and the amount of organic matter present.4,5 Therefore, considering nickel toxic levels, there has been a need to treat industrial wastewater employing common techniques such as filtration, chemical precipitation, membrane separation, ion exchange, and adsorption in order to reduce the environmental impact caused by indiscriminate disposal of effluents in water bodies. Adsorption is considered © XXXX American Chemical Society

one of the simplest, most effective, and inexpensive methods for removing metal ions from aqueous solutions.6 Among the wide range of adsorbents (e.g., polymers, activated carbon, inorganic oxides, and biosorbents), ionimprinted polymers (IIPs), prepared by using molecular imprinting technology, have received significant attention.7 These materials can be synthesized in the presence of a metal ion template by means of analyte−monomer interactions. After polymerization, the template is leached out from the matrix with acids or complexing agents, thereby resulting in the formation of analyte-selective binding sites.8 The bulk polymerization reaction conducted in a homogeneous medium is best known today. However, when employing this method, synthesized organic IIPs contain structures that disfavor rapid mass transfer of analytes toward the polymer surface, thereby exhibiting slow retention kinetics and low adsorption capacity.8 A synthesis approach based on surfactantassisted sol−gel processing has been proposed to overcome these problems.9 Materials synthesized through this technique Received: January 30, 2013 Revised: May 5, 2013 Accepted: June 2, 2013

A

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Reagenti, Rodano, Italy) by making appropriate dilutions with ultrapure water produced by a Milli-Q system (Millipore, Bedford, MA, USA). 2.3. Polymer Syntheses. To synthesize the doubleimprinted poly(methacrylic acid) material (IIP/CTAB), CTAB (0.50 g) was placed into a round-bottom flask and dissolved in C2H5OH (15.0 mL), and then, NiCl2·6H2O (0.21 g) and MAA (3.73 g) were added. The obtained solution was stirred for 30 min. In the next step, TMSPMA (1.00 g), AIBN (0.30 g), C2H5OH (10.0 mL), and EGDMA (10.3 g) were added to the solution. The mixture was purged with argon gas for 5 min, and after the flask was sealed, it was polymerized at 60 °C for 24 h. Thereafter, the polymer was ground and sieved to obtain particles with diameters of 0.998) for the four materials. This model assumes that the adsorption site occupancy rate is proportional to the square of the number of unoccupied sites, thus considering that the analyte can be bound to different binding sites. Besides, this model recognizes a chemisorption process that involves electron sharing or exchange between the adsorbent and adsorbate.18,19 The binding sites able to interact with the Ni2+ ions can be attributed to selective cavities formed by the carboxyl groups of the MAA monomer, carbonyl groups of the EGDMA cross-linker and nonselective sites generated during the polymer synthesis. The Elovich model was also used to evaluate the existence of chemical sorption on highly heterogeneous adsorbents.20 A good linear relationship between Qt and ln t especially for the first portion (before reaching the sorption equilibrium) makes possible to affirm, in a similar way to pseudo-second-order equation, that the adsorption process is of a chemical nature. The intraparticle diffusion model is employed to assess if the sorption process is only controlled by intraparticle diffusion, that is, when the plot of Qt and t1/2 gives a straight line. However, the existence of two linear regressions characterizes a two-stage adsorption process, where the first stage represents instantaneous adsorption on the outer surface through film diffusion and the second stage is attributed to gradual adsorption with intraparticle diffusion. As noted from Table 2, in general, a better adjust was observed for the first straight portion in comparison to the second portion of intraparticle diffusion model, thus indicating that Ni2+ was transported to the external surface of polymers through film diffusion. However, a value of intercept (C) different from zero, which provides an estimation of the boundary layer thickness, indicates the existence of a complex mechanism consisting of both surface adsorption and intraparticle transport within the pores of polymers. The larger is the intercept value, the greater is the boundary layer effect, thus the higher values obtained for IIP/CTAB when compared with those from IIP/no CTAB, shows that for this material, the diffusion of Ni2+ through the solution to the external surface of the adsorbent contributes strongly to the adsorption process as well as enhancing the rate diffusion within pores. 3.5. Ni2+ Adsorption Isotherms. The Ni2+ adsorption isotherms obtained for the four polymers are presented in Figure 6. It can be observed that double imprinting with the Ni2+ ions and surfactant significantly increased the adsorption capacity of poly(methacrylic acid). Moreover, it was also possible to elucidate interactions between the analyte and selective binding sites on the adsorbent surface by applying the following isotherm models: Dubinin−Radushkevich, Scatchard, nonlinear Langmuir, and nonlinear Freundlich as well as singlesite and dual-site Langmuir−Freundlich.21−23 The parameters evaluated for the IIP/CTAB material are presented in Table 3. As can be seen, the best fit to the experimental data was achieved with the dual-site Langmuir−Freundlich model for all the materials. This result indicates that the Ni2+ adsorption probably took place on the energetically heterogeneous solid surface with different binding sites. To confirm the existence of two kinds of the binding sites on the polymer surfaces, the Scatchard model was employed.22 For IIP/CTAB, it failed to distinguish between the energies, giving only one linear segment with the poorest fit. Therefore, the failure in

Table 1. Textural Parameters Obtained for the Polymers polymer IIP/CTAB IIP/no CTAB NIP/CTAB NIP/no CTAB

surface area (m2 g−1)

pore volume (cm3 g−1)

average pore diameter (Å)

107.832 148.026

0.208 0.277

1.942 1.629

138.800 156.900

0.269 0.332

4.239 2.124

NIP/no CTAB were higher than for NIP/CTAB; however, the average pore diameter for NIP/no CTAB was lower than for NIP/CTAB. These data show that the addition of the surfactant to the polymerization reactional medium caused the decrease in the surface area and pore volume for poly(methacrylic acid), but on the other hand, the average pore diameter increased twice due to the formation of spherical micelles. When comparing IIP/no CTAB and NIP/no CTAB, the negative Ni2+ imprinting effect on the textural properties of poly (methacrylic acid) was observed. It should be emphasized that the presence of CTAB in the polymer synthesis always promotes an increase in the average pore diameter and decrease in the pore volume and surface area, as can be seen when relating NIP/CTAB to NIP/no CTAB and IIP/CTAB to IIP/ no CTAB. Therefore, although adsorption is a surface-based process, materials with larger surface areas may have greater adsorption capacity, but the pore volume and diameter may also significantly contribute to the process. Herein, IIP/CTAB demonstrated the highest adsorption capacity among the other studied materials, as will be further shown. This finding enables confirmation that the binding sites created by carboxyl groups were located at the outer surface and not only inside the micropores, which is very desirable for rapid adsorption kinetics due to the enhanced mass transfer of the adsorbate toward the adsorbent sites. Moreover, it should be mentioned that the highest adsorption capacity of IIP/CTAB can also be associated with the Ni2+ and CTAB synergistic effects on the formation of selective binding sites. 3.4. Ni2+ Adsorption Kinetics. Figure 5 shows the Ni2+ adsorption profiles for the polymers. It can be seen that the

Figure 5. Shaking time effect on the Ni2+ adsorption.

time required for the adsorbate to reach equilibrium between the liquid and solid phases was obtained quickly, approximately in 15 min, for all the adsorbents. The pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion kinetic models applied to the obtained E

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Table 2. Kinetic Model Parameters Estimated for the Studied Polymers. Qexp = 26.912 (IIP/CTAB), 14.444 (IIP/no CTAB), 17.698 (NIP/CTAB), and 16.371(NIP/no CTAB) mg g−1a

a K1 is the pseudo-first-order adsorption rate constant (min−1), K2 is the pseudo-second-order adsorption rate constant (g mg−1 min−1), β is the extent of surface coverage and activation energy for chemisorption (g mg−1), α is the initial adsorption rate constant (min−1 mg g−1), Kid is the internal diffusion coefficient (mg g−1 min−1/2), and C is the boundary layer thickness.

same functional groups (carboxyl) responsible for the Ni2+ adsorption. For IIP/no CTAB, NIP/CTAB, and NIP/no CTAB, the Scatchard model presented two segments (Supporting Information, Tables S1, S2, S3), indicating the existence of nonselective binding sites with high and low affinities characterized by the carboxyl and carbonyl groups of EGDMA, respectively. The maximum adsorption capacity of Ni2+ by polymers determined with the dual-site Langmuir−Freundlich model decreased in the following sequence: IIP/CTAB > NIP/CTAB > IIP/no CTAB > NIP/no CTAB, thus demonstrating the synergistic effect of the double-imprinting procedure. The chemical nature of the Ni2+ adsorption on the materials was verified by estimating the average free energy (E, kJ mol−1) according to eq 5.

Figure 6. Adsorption isotherms for IIP/CTAB, IIP/no CTAB, NIP/ CTAB, and NIP/no CTAB.

distinguishing between the energies shown by the Scatchard model can be interpreted regarding the similar Ni2+ amount adsorbed on these sites (15.23 and 18.08 mg g−1) as well as the

E=

1 2K

(5)

Table 3. Parameters Estimated from the Isotherm Models for IIP/CTAB. Qexp= 33.51 mg g−1a model

K1

equation

K2

b1

b2

n1

n2

R2

sum of error squared (SSE)

Dubinin− Radushkevich

ln Q eq = ln Q max − KE2

2.41 × 10−9

Scatchard

Q eq /Ceq = Q eqKb − QKb

3.62

37.39

0.6769

nonlinear Langmuir nonlinear Freundlich single-site Langmuir− Freundlich

Q eq = KbCeq /(1 + KCeq)

5.34

34.20

0.9565

41.51

0.15

0.7574

231.40

1.54

0.9832

16.02

0.9994

0.57

dual-site Langmuir− Freundlich

Q eq = KCeq1/ n

0.8700

21.60

Q eq = b(KCeq)n1 /1 + (KCeq)n Q eq =

2.57 × 10−3

b1(K1Ceq)n1 n1

1 + (K1Ceq)

+

b2(K 2Ceq)n2 1 + (K 2Ceq)n2

5.80

33.34

11.72

2.92

15.23

18.08

11.02

3.85

In the Langmuir and Freundlich equations, K1,2 (Langmuir) (L g−1), K1,2 (Freundlich) (mg g−1) (L g−1) are the adsorbate−adsorbent affinities, b1,2 are the maximum adsorption capacities (mg g−1), and n1 are the intensities or degrees of favorability for adsorption. In the Scatchard equation, Kb is the equilibrium dissociation constant. Ceq = equilibrium concentration (mg L−1), Qeq = concentration in solid phase (mg g−1). In the Dubinin− Radushkevich equation: Qmax = b1 (mol g−1), and E = mean free energy of adsorption (mol2 J2). a

F

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where K is the coefficient obtained from the slope of the linear Dubinin−Radushkevich model. When the average free energy is less than 8 kJ mol−1 a physisorption process is indicated and when it is within the range 8−16 kJ mol−1, this indicates a chemisorption process.23 The values obtained [11.39, 13.78, 12.55, and 11.75 kJ mol−1 for IIP/CTAB, IIP/no CTAB, NIP/ CTAB and NIP/no CTAB, respectively] confirm the chemical nature of adsorption process. The maximum adsorption capacity of Ni2+ on doubleimprinted poly(methacrylic acid) material was compared to other adsorbents24−31 (Supporting Information). A superior maximum adsorption capacity was obtained by using the proposed polymer in comparison to the other adsorbents including ion imprinted polymers for nickel ions. Besides this advantage, the synthesis based on bulk polymerization in the presence of cationic surfactant does not require auxiliary chelating agents to create the selective binding sites. 3.6. Selectivity Parameters. The selectivity study was performed through the Ni2+, Cu2+, Co2+, Mn2+, and Pb2+ competitive adsorption. Table 4 shows the selectivity

(Supporting Information, Table S5), even with the conditions optimized for removing Ni2+, thus indicating the ability of IIP/ CTAB for removing other metallic ions from polluted water samples. 3.7. Reusability Assessment for IIP/CTAB. Under the procedure described in section 2.8, it was observed that the Ni2+ adsorption capacity of IIP/CTAB was unchanged after 300 preconcentration cycles and no memory effect was observed after each preconcentration/elution step. The data showed that IIP/CTAB had excellent generation ability.

4. CONCLUSIONS The hierarchical double-imprinting process was used for the first time to prepare an ion-imprinted organic polymer [poly(methacrylic acid) for the Ni2+ ions adsorption. The synergistic effect of the Ni2+ ions and CTAB micelles, employed as templates, caused increases in the Ni2+ adsorption capacity and selectivity for poly(methacrylic acid), as demonstrated by the distribution and relative selectivity coefficient values, respectively. The double-imprinting procedure provided lower surface area and pore volume for the polymer (IIP/CTAP) compared to the single-imprinted (IIP/no CTAB) and nonimprinted (NIP/CTAB and NIP/no CTAB) materials; however, it was observed that the use of the surfactant template in the polymer synthesis led to an increase in the average pore diameter. Therefore, the great adsorption capacity (33.31 mg g−1), high selectivity, and short adsorption time (15 min) obtained for IIP/CTAB is resulted from its textural properties (low pore volume and high average pore diameter) as well as from the formation of imprinted sites onto its surface. Besides, the kinetics data allowed disclosure of the chemical nature of the Ni2+ adsorption (i.e., chemisorption). Moreover, IIP/CTAB can be reused up to 300 times when employing 1.0 mol L−1 HNO3 as stripping agent. Considering the aforementioned results, the synthesis technique implemented can be a promising tool for the development of separation and preconcentration methods to remove toxic metal ions from aqueous media

Table 4. Selectivity Parameters Obtained for the Ni2+, Cu2+, Co2+, Mn2+, and Pb2+. Competitive Adsorption on IIP/ CTAB and NIP/no CTAB polymer IIP/CTAB NIP/no CTAB IIP/CTAB NIP/no CTAB IIP/CTAB NIP/no CTAB IIP/CTAB NIP/no CTAB

metal ion

Kd

k

k′

Ni2+ Cu2+ Ni2+ Cu2+ Ni2+ Co2+ Ni2+ Co2+ Ni2+ Mn2+ Ni2+ Mn2+ Ni2+ Pb2+ Ni2+ Pb2+

83545 8488 23100 5231 11240 4253 2376 1218 4642 3008 2054 1444 21851 72000 4192 109000

9.84

2.22

4.42 2.64

1.35

1.95 1.54

1.08

1.42 0.30

■

7.50

0.04

ASSOCIATED CONTENT

* Supporting Information S

Tables of parameters estimated from the isotherm models for IIP/no CTAB, NIP/CTAB, and NIP/no CTAB and comparison of maximum adsorption capacity by IIP/CTAB with other adsorbents. This material is available free of charge via the Internet at http://pubs.acs.org.

parameters calculated for the double-imprinted (IIP/CTAB) and nonimprinted (NIP/no CTAB) materials. Higher selectivity coefficients (k) for IIP/CTAB in relation to NIP/ no CTAB were obtained considering the Ni2+ binding in the presence of Cu2+, Co2+, Mn2+ or Pb2+. These results confirm the self-assembly complex between Ni2+ and methacrylic acid during the polymer synthesis and the imprinting effect. The imprinting effect may still be evidenced based on the formation constant for the metallic complexes with poly(methacrylic acid), where the stability constant for nickel [K = 2.34 × 109] is lower than that for copper [K = 1.58 × 1011] and cobalt [K = 6.39 × 109].32 To evaluate the effective behavior of IIP/CTAB for removing of Ni2+ in the presence of other metallic ions, a simulated electroplating effluent33 was submitted to the batch adsorption process under optimized conditions. An amount of 50.0 mL of simulated electroplating effluent were stirred with 50.0 mg of polymer in polyethylene flask. After the adsorption process, as expected, higher amounts of Ni2+ were retained by IIP/CTAB, but excellent results were also achieved for other metals

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +55 43 3371 4366. Fax +55 43 3371 4286. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Conselho Nacional de ́ Desenvolvimento Cientifico e Tecnológico (CNPq) [Process No. 471655/2011-2, 308580/2010-9, and 471912/2011-5], ́ Superior Coordenaçaõ de Aperfeiçoamento de Pessoal de Nivel (CAPES), Instituto Nacional de Ciência e Tecnologia de ́ Bioanalitica (INCT) [Process No. 573672/2008-3], Fundaçaõ G

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de Amparo a Pesquisa do Estado do Rio de Janeiro (FAPERJ) for their financial support and fellowships.

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