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Preparation and evaluation of Pb(II)-imprinted fucoidan-based sorbents
Reactive and Functional Polymers 115 (2017) 53–62
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Preparation and evaluation of Pb(II)-imprinted fucoidan-based sorbents ⁎
Vanessa R.A. Ferreira, Manuel A. Azenha , Carlos M. Pereira, A. Fernando Silva
MARK
CIQ-UP, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
A R T I C L E I N F O
A B S T R A C T
Keywords: Cation-imprinting Fucoidan Sol-gel Lead (II) Composite
Fucoidan, a sulfated polysaccharide extracted from brown seaweed, was, in the form of a silica composite, studied as a prospective cation imprinting matrix. The preparation of such composites in the presence of cations with a strong interaction with the biopolymer chains was expected to direct them towards arrangements, optimized for the sorption of those cations. As expected, the presence of Cu(II), a weakly fucoidan-binding cation, in the synthesis of the composites did not result in the production of significantly stronger Cu(II)-oriented binding arrangements, and therefore the imprinting was not successful. However, with Pb(II), with much stronger affinity for fucoidan, the materials obtained exhibited stronger (22%) binding as compared to the nonimprinted counterparts, and increased selectivity (1.4–1.6 fold) against Cd(II). Although these imprinting features were close to those observed previously with other sulfated polysaccharides, the fucoidan-based Pb(II) imprints developed here presented superior sorption properties, namely a higher capacity and higher binding strength for Pb(II). These features, demonstrated by the material developed here, may easily be put to work in different areas where Pb(II) sensing, determination, separation or remediation is of the utmost importance.
1. Introduction The utilisation of biodegradable, biocompatible polysaccharides extractable from abundant low-cost raw materials became appealing in different fields of research. In the case of the preparation of selective materials with higher loadings by imprinting techniques, for purposes such as separation, drug delivery or sensing, the employment of natural polysaccharides as cyclodextrin, chitosan, cellulose, alginic acid, agarose and starch has been documented [1]. The generic process comprises: (i) a template structure (a small molecule, a macromolecule, an ion, etc.) exhibiting a strong association affinity with the polysaccharide that results to some extent in the modulation of the conformational structure of the biopolymer; (ii) the fixation of the template-polysaccharide induced structure into a stable 2D or 3D network; (iii) the removal of the template from the network, which leaves the imprinted sites with the “memory” for an optimized rebinding of the template or closely related structures. A wide range of templates have been used in polysaccharide-based imprinting. The dextrins, for example, have been used along with steroidals, amino acids, polysaccharides, drugs, plant hormones, proteins, pesticides, and plastic additives (as reviewed in [2]). In what cationic templates are concerned, namely metallic cations (cf [3] for a comprehensive review on ion-imprinted polymers), chitosan and modified chitosan have been the polysaccharides of choice due to the presence of the amino groups, which, combined with hydroxyls or other added groups, confers metal-cation complexing ⁎
Corresponding author. E-mail address: [email protected] (M.A. Azenha).
http://dx.doi.org/10.1016/j.reactfunctpolym.2017.04.001 Received 2 February 2017; Received in revised form 20 March 2017; Accepted 2 April 2017 Available online 03 April 2017 1381-5148/ © 2017 Elsevier B.V. All rights reserved.
ability (cf. [4–7] for a few recent examples). However, the amino group in chitosan has a pKa value of ~6.5, which leads to a protonation in acidic to neutral solutions [8,9], thus impairing the cation-complexation ability in the lower half of the pH range. Nature, through specific biosynthesis routes, offers us a remarkable diversity of polysaccharides, many of which are virtually unexplored for their potentialities to interact with metal cations. As we recently have pointed out [10] the exploitation of negatively-charged sulfated polysaccharides for metal cation-related applications is still insipient. The presence of the eOeSO3− groups in the structure of these polysaccharides provides them with permanent negatively-charged points even at pH values as low as 2. The eventual cooperativity between sulfate and other nearby functionalities attached to the backbone of the biopolymers foresees strong binding modes towards cationic species with different selectivity than the one associated to the most commonly studied polysaccharides. We developed already a first example of sulfated-polysaccharide-based imprinted materials by preparing chondroitin sulfate/silica Pb(II) imprinted composites [11]. Chondroitin sulfate, a disaccharide comprising an amino sugar (Nacetylgalactosamine) and glucuronic acid linked by β-(1 → 3) glycosidic bonds, contains in average one carboxylate and one sulfate group per monomer. The materials exhibited a “memory” effect for the Pb(II) ions, expressed in the observation of stronger (up to 44%) binding as compared to the non-imprinted counterparts, and increased selectivity (1.5–2 fold) against Cd(II).
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Fig. 1. Chemical structures of Fd (I), GPTMS (II), TMOS (III) and schematics of the Fd cation-imprinting process.
2. Materials and methods
In the present work, we studied fucoidan, another sulfated polysaccharide, extracted from brown seaweed, generally consisting of a backbone of L-fucopyranose residues linked by β(1 → 3) bonds, which may enclose side branches containing fucopyranoses or other glycosyl units, e.g. glucuronic acid [12] (I in Fig. 1). In this case, the typical unit contains three sulfate groups per disaccharide, therefore constituting a more densely-charged polyanionic polysaccharide than chondroitin sulfate. The preparation scheme (Fig. 1) and the features of the Pb (II)-imprinted and Cu (II)-imprinted fucoidan-based materials are presented and discussed, inclusively from the comparative perspective with the chondroitin-based imprints. The Ag(I) and Au(III) cations were studied at an initial exploratory stage because of the high commercial value of these cations. Cd (II) and Zn (II) cations were also evaluated, mainly for ascertaining selectivity features.
2.1. Materials Fucoidan (Fd, > 90% purity, containing 5% uronic acids, average MW 73.7 kDa) was provided by Marinova, Australia, extracted from Fucus vesiculosus. The metal cation salts used in the synthesis of imprinted materials and for the sorption tests, such as Pb(NO3)2, Cu(NO3)2·3H2O, Zn (NO3)2·6H2O and Cd(NO3)2·6H2O, were provided by Merck (Darmstadt, Germany, > 99% purity). Glycidyloxypropyl-trimethoxysilane (GPTMS, II in Fig. 1) and tetramethoxysilane (TMOS, III in Fig. 1) were purchased from Sigma-Aldrich (Deisenhofen, Germany) and were used without further purification (> 98%). Water was of Milli-Q 54
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0.7 mL min− 1, in a Visiprep (Supelco) SPE station manifold. After sample loading, 5 mL of acetate buffer plus 5 mL HNO3 0.001 mol L− 1 were used as washing solutions. The cartridges were thereafter subjected to a three elution step of 3 × 5 mL of HNO3 0.01 mol L− 1. The fractions (loading, washing and elution) eluting from the SPE column were directly monitored by atomic absorption spectrophotometry as described in Section 2.2. The resulting absorbances were used to estimate the fraction of sorbent-bound analyte. The SPE retention % was calculated according to Eq. (2).
(Millipore, Italy) purity. All other reagents were analytical grade. 2.2. Synthesis of cation-imprinted materials (MIM) The cation-imprinting within reticulated Fd-silica composites was performed according to a procedure adapted from our previous work [11]. A 1.5% m:v Fd solution was prepared in deionized water (pH was adjusted from 6.7 to 7.5 with 1 mol L− 1 of ammonia solution) and mixed with GPTMS (Fd monomer: GPTMS molar ratio of 1:1). This mixture was stirred for 17 h at room temperature. Then, the template cation (in the form of Pb(NO3)2 or Cu(NO3)2 salt) was added in the molar proportion of 1 cation to 1 Fd monomer). Thereafter, TMOS (Fd monomer:TMOS molar ratio 1:1) was added into the mixture to form a gel. The precipitate was centrifuged at 6000 rpm (PrO-Analitycal, Centurion Scientific Lid CR4000R5 Model) for 10 min at 25 °C. The pellet was dried overnight in the oven at 50 °C. The metal cations were removed by washing six times with HNO3 0.01 mol·L− 1. Every washing step was intercalated with centrifugation at 6000 rpm for 10 min. The washings were monitored by analyzing the supernatant of centrifugation by atomic absorption spectrophotometry (Perkin Elmer, AAnalyst 200 Model, operated at the instrumental parameters recommended by the manufacturer for every element analyzed). For reference purposes, non-imprinted materials (NIM) were obtained similarly, but without adding the cation salt. Also, purely sol-gel reference materials, with and without cation imprinting, were prepared and their synthesis was the same as described previously, but without addition of Fd.
SPE retention (%) = 100 −
× 100
(2)
2.5. Batch sorption studies: isotherms and kinetics The sorption assays were conducted in glass flasks containing 10 mg of biosorbent in 5 mL of acetate buffer 0.01 mol L− 1 at pH 5.5 and supplemented with different concentrations of each metal cation, within the range of 10 to 100 mg L– 1. All experiments were conducted under stirring (120 rpm) and at constant temperature (25 °C). For the study of sorption kinetics, samples were collected at approx. regular intervals until 6 h, while for the study of the isotherms all samples were collected at the end of 6 h. Afterwards, the samples were centrifuged at 6000 rpm for 10 min at 25 °C, the supernatant was collected, acidified to pH 4 with concentrated nitric acid and stored protected from light, until AAS analysis. Non-linear fitting of theoretical isotherms to experimental data was performed using IGOR PRO software Package. In this study, three models, Langmuir (Eq. (3)), Freundlich (Eq. (4)) and LangmuirFreundlich isotherms (L–F) (Eq. (5)) were tested to describe the biosorption of the metallic cations.
Attenuated Total Reflectance Fourier Transform Infrared (ATRFTIR) measurements were performed using a Bruker FT-IR System Tensor 27 spectrophotometer in the range of 600–4000 cm− 1. The degree of swelling (Sw) was determined by keeping 10 mg (W0) of the biosorbent in 2 mL of deionized water (pH 7). The increase in weight (Wt − W0) of the biosorbent, after 12 h of agitation, in comparison to initial weight (W0) of the biosorbent was used to calculate the degree of swelling (Sw) using Eq. (1)
Wt − W0 × 100 W0
Ai
where Af corresponds to the absorbance representative of the analyte concentration found in the eluate and Ai to the absorbance representative of the analyte concentration in the original sample. It was always assured that both Ai and Af were kept within the linear response range of the technique. Regeneration of the SPE cartridge was achieved by re-conditioning with HNO3 0.01 mol L− 1 and acetate buffer (0.01 mol L− 1) at pH 5.5. The re-conditioning process was monitored by AAS until the release of traces of the used cations ceased being detected.
2.3. Characterization of the imprinted materials
Sw (%) =
Af
(1)
where W0 and Wt are the initial and final weights of the biosorbent, respectively. The surface area and the pore parameters were determined by a nitrogen adsorption analyser (TriStar Plus, Micromeritics). The experiments were conducted with 200–500 mg of sample, which were previously dried overnight in the oven at 50 °C followed by at least 2 h under a flow of nitrogen. The specific surface areas (S) were evaluated using the BET method, the specific pore volumes (Vp) following the Gurvitch method and the average pore diameter (Dp) using the BJH theory applied to the desorption branch of the isotherm. The amount of Fd present in composites was determined indirectly by quantifying the amount of biopolymer remaining in the supernatant of the reaction mixture. The quantification was based on the precipitation reaction between Fd (due to the sulfate groups) and Ba2 +, followed by turbidimetric analysis (Portable Turbidimeter 2100Qis, HACH) according to a method described in the literature (Dodgson, 1961 [13]).
qe =
qmax KC 1 + KC
(3)
qe =
aC m
(4)
qe =
(KC )m
qmax 1 + (KC )m
(5)
where qe corresponds to the sorbed concentration in equilibrium (mg g− 1); C is the equilibrium concentration in solution (mg/L) and a and m are fitting constants. The constant a is related with the binding affinity and the parameter m refers to the heterogeneity index and its value ranges from 0 to 1, increasing as heterogeneity decreases. The Langmuir model assumes that one class of site is present on the surface, with saturation capacity qmax, to form a complete monolayer on the surface with dissociation constant K (L mg− 1). The Freundlich isotherm, on the other hand, assumes sites with a Gaussian distribution of binding strengths. Here the width of the Gaussian distribution describes the degree of heterogeneity, through the index m. The difference between the L–F model and the Freundlich one is evident at high sorbate concentrations, for which the L–F model is able to represent the saturation behaviour. At low sorbate concentrations, the L–F equation reduces to the classical Freundlich equation. On the other hand, if m approaches unity, it is indicative of a completely homogeneous sorbent surface (i.e., energetic equivalence of all binding sites), the L–F equation reduces to the classical Langmuir equation. Thus, the hybri-
2.4. Solid phase extraction (SPE) SPE cartridges were packed individually with 200 mg of each biosorbent. The cartridges were conditioned with 10 mL HNO3 0.01 mol L− 1 and 5 mL acetate buffer (0.01 mol L− 1) at pH 5.5. Samples (5 mL) containing one of the following mixed cation solutions in acetate buffer (0.01 mol L− 1, pH 5.5) were then introduced: 10 ppm Cu(II)/Zn(II), 50 ppm Pb (II)/Au(III), 50 ppm Pb(II)/Ag(I) and 50 ppm Pb (II)/Cd (II). The percolation proceeded at a constant flow rate of 55
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dized L–F isotherm is able to model adsorption of solutes at high and low concentrations on to both homogeneous and heterogeneous sorbents. Sorption kinetics fitting was tested by linear regression using pseudo-first order (Eq. (6)) and pseudo-second order (Eq. (7)) models.
qt = qe (1 − qt =
e kt )
Table 1 Microstructural properties and chemical composition of the composites. Sample
(6)
Controla NIM Controla MIM Pb(II) Controla MIM Cu(II) Fd NIM Fd MIM Pb(II) Fd MIM Cu(II)
qe2 k2 t 1 + qe k2 t
(7)
where qt is the amount of each compound sorbed at time t (mg g − 1), and k is the rate constant of each model. 2.6. Computation of imprinting features
a
The calculated binding constants and capacity values were the inputs for deriving the imprinting and selectivity factors (IF and α, respectively). The IF is here defined as the ratio between the template binding constants for the MIM and corresponding NIM, IF = KMIM / KNIM, and expresses the relative gain in binding strength due to the imprinting process. Selectivity was evaluated according to two different definitions. The first one relates the binding constants of the template and a test cation (the pairs Pb(II)/Cd(II) and Cu(II)/Zn(II) were employed, respectively, for the Pb(II)- and Cu(II)-imprinted materials): α(K) = K(template) / K(test cation). The second definition is similar but uses the capacity of the materials: α(qmax) = qmax(template) / qmax(test cation).
BET
Mass fraction of Fd in biosorbents (mg/mg)
Sw (%)
S (m2/g)
Vp (mL/g)
12 17
0.032 0.016
– –
3.5 3.8
13
0.011
–
3.9
6.4 7.2 6.9
0.004 0.006 0.004
0.33 0.38 0.27
2.4 2.2 2.7
Control — Material without Fd.
biopolymer, which appears to cooperate with the crosslinking process. The recorded ATR-FTIR measurements agree with the enrichment of Fd in the Pb(II)-imprinted composites. In Fig. 2, the typical spectrum of fucoidan reference (a) is presented alongside with those of the nonimprinted (b) and Pb(II)-imprinted composites (c). The spectrum of a purely sol-gel material (d) is also shown. Using the band at 1687 cm− 1 as a fucoidan marker, since it does not appear in the purely sol-gel material, we promptly identify the fucoidan component in the composites. It is also evident the increased importance of that band in the Pb (II)-imprinted material comparing with the non-imprinted one, what is compatible with a fucoidan-richer material, as suggested by turbidimetry analysis. On the other hand, the silica component of the composites can be identified by the intense band at ~ 1050 cm− 1, which nevertheless appears also in the fucoidan spectrum as a minor band.
3. Results and discussion Recently developed fucoidan/silica composites exhibited a remarkable feature discovered during the study of their acid-base properties: a much higher proton deficiency (~ 4 fold) was found for the surface of the composites (33% fucoidan in mass) as compared to a purely fucoidan suspension [10]. This behaviour implicates that the composites possess a surface much richer in anionic (proton-exchanging) or basic sites. As these differences could not be ascribed to the silica component, a likely explanation put forward was the unfolding of the fucoidan chains during the synthesis process, exposing otherwise inaccessible proton-accepting sites (cation-exchange processes should be dominating in this case). Due to this feature, the composites exhibited outstanding sorptive properties towards metallic cations, most noticeably Pb(II) and Cd(II) [10]. Hence, this composite seemed a very appealing matrix to attempt the imprinting of metallic cations. A strongly sorbed cation (Pb(II); affinity constant 370 L mg− 1) and a significantly weakly sorbed cation (Cu(II); affinity constant 120 L mg− 1) were tested as templates.
3.2. Sorption of metallic cations in dynamic mode The composites and respective sol-gel controls were subjected to an initial expeditious evaluation in dynamic mode, as SPE stationary phases for the sorption of a few cations. Solutions of mixed cations in pH 5.5 buffer were percolated through the sorbents and the retained cations were thereafter recovered by a flow of HNO3 solution. The cations screened were Cu (II) and Zn (II) for the Cu(II)-imprinted materials, and Pb (II), Ag (I), Au (III) and Cd (II) for the Pb(II)imprinted materials. The behaviour of the Cu(II)-imprinted fucoidan composite is presented in Fig. 3 a) and b). It was not observed any apparent gain from the process of imprinting the fucoidan composite with Cu(II), since no improvement in retention nor changes in the washing profile could be observed for both Cu(II) and Zn(II). Interestingly, in their corresponding sol-gel materials (control MIM and NIM), templating with Cu(II) resulted in significantly increased retention (from 6 to 16% to 47–60%) and a different washing profile – most of the cation being washed by 0.001 mol L− 1 HNO3 instead of mostly being directly eluted during the sample loading. Nevertheless, these findings were not investigated deeper since our interest was focused on the fucoidan composites which presented always higher retention and washing profiles indicating stronger binding (most of the retained cation being eluted only with the stronger eluent 0.01 mol L− 1 HNO3). In Fig. 3 c), d), e) and f) the sorption behaviour of the Pb(II)imprinted MIM is displayed. In this case, the results were indicative of an effective gain in the retention features of the MIM against NIM, both containing fucoidan, namely an increased retention of Pb(II) (92% vs. 81%) and increased fraction eluted with the strongest eluant (0.01 mol L− 1 HNO3). In the corresponding control materials, only very small retention (11–18%) was obtained and elution occurred easily with the weaker washing eluants. Control materials presented also weak retention behaviour towards Ag(I), Au(III) and Cd(II). The Pb (II)-imprinted fucoidan composites also did not show improvements for the sorption of any of these cations in dynamic mode, thus providing a
3.1. Structural and chemical characterization Table 1 condensates the major textural features obtained for all the materials. All of them are essentially microporous, as based on the small surface area (6–17 m2/g) and pore volume (0.00–0.032 mL/g) ranges. Most noticeably, within the fucoidan composites the addition of the cation (either Pb(II) or Cu(II)) did not implicate any major changes in the measured surface area and pore volume, as determined by nitrogen adsorption analysis. Additionally, very small swelling ratios (2–4%) were observed. Everything considered, it may be concluded that a dense, highly crosslinked, structure for the composites was obtained, both using the cations or not. Table 1 contains also the data of the mass fraction of Fd within the composites. It was observed, in the case of the Pb(II)-imprinted composite only, that the presence of the cation template caused a small increase of Fd contents in the final product. This effect has been found before with Pb(II)-imprinted composites of chondroitin sulfate [11] and is likely related with an extra aggregational influence exerted by the divalent cations upon the sulfated 56
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Fig. 2. ATR-FTIR spectra of: a) Fd; b) Fd NIM; c) Fd MIM Pb(II); d) Sol-gel control NIM. All spectra represented with the same scale (0.4 Au).
further indication of a successful selective imprinting.
batches of non-imprinted and Pb(II)-imprinted composites showed that the variability of the SPE retention of Pb(II) was below 10%, what may be considered acceptable reproducibility.
3.2.1. Shelf life stability and synthesis reproducibility These features where also evaluated by SPE. It was found that the composites could be stored (room temperature, protected from light) at least 3 months before use without losing any retention capacity. In what regards reproducibility, the results obtained from 3 independent
3.3. Assessment of the imprinting features The evaluation of the imprinting features relied essentially on the 57
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a)
60%
16%
absorbance
85%
84%
W2
0.4
W1
S 0.2 E1
E2 E3
0 Control MIM Cu(II)
b)
47%
absorbance
0.4
Control NIM 6%
Fd MIM Cu(II) 62%
Fd NIM 70%
W2
S
W1
0.2
E1 E2 E3
0 Control MIM Cu(II)
c)
18%
Control NIM
11%
Fd MIM Cu(II)
92%
Fd NIM
81%
S
absorbance
0.4
0.2
W1 W2 E1E2
E3
0 Control MIM Pb(II)
Control NIM
Fd MIM Pb(II)
Fd NIM
Fig. 3. SPE results, expressed as the AAS absorbance measured in the consecutive eluates, found for the different sorbents with: a) Cu (II), b) Zn (II), c) Pb (II), d) Ag (I), e) Au (III), f) Cd (II) The retention (%) observed upon loading of the metal cation solution is also shown. SPE steps (from left to right in the graphs): sample introduction – 10 mL of metal cation solution in acetate buffer pH 5.5 (S-red); wash 1–6 mL acetate buffer pH 5.5 (W1-purple); wash 2–6 mL HNO3 0.001 mol L− 1(W2-black); elution 1–6 mL HNO3 0.01 mol L− 1(E1-grey); elution 2–6 mL HNO3 0.01 mol L− 1(E2-green); elution 3–6 mL HNO3 0.01 mol L− 1(E3-blue).
Pb(II), it was observed a higher constant rate (k = 1.6 g/(mg min− 1) than the one obtained for the corresponding NIM (k = 1.0 g/ (mg min− 1). However, the difference fell within the uncertainty range. For the sorption of Cd(II) a similar situation was found. It may thus be concluded that the imprinting process did not bring significant changes in the kinetics of cation sorption. The sorption isotherms (Fig. 5) could reveal the actual effect of the imprinting process upon the sorption properties of major interest, such as capacity (maximum sorbable amount) and binding strength. The isotherm (qe vs C) data points were fitted to different isotherm models, such as Langmuir (Eq. (3)) Freundlich (Eq. (4)) and hybrid L–F (Eq. (5)) by non-linear regression analysis. The best fitting was selected according to the lower χ2 (chi-squared) values obtained, and qmax proximity between the calculated values and the experimental values. In all cases the L–F model provided the best fit. The corresponding isotherm parameters were condensed in Table 3.
evaluation of the imprinting and selectivity factors (IF and α, respectively), as defined in 2.6. Cd(II) was chosen as selectivity test cation for Pb(II) due to similar properties relevant to sorption, namely crystal ionic radius above 100 pm [14], common biological effects (similar coordination properties), and from our previous experience on the composites, which made us realize that the sorption kinetics were similar for the two cations. Likewise, we chose Zn(II) as selectivity test cation for Cu(II) since they are adjacent in the Irving-Williams series (similar coordination properties), have similar crystal ionic radius (87–88 pm) [14] and similar sorption kinetics with our composites (significantly faster than Pb(II) and Cd(II)). For the computation of IF and α, input data were obtained from the mathematical treatment of the cation sorption isotherm data. The sorption profiles (Fig. 4) helped establishing the time given for the equilibrium in the isotherm experiments (6 h). The results, from the kinetic studies, were treated by two mathematical models, pseudo first order and pseudo second order, and the model that best fitted all the experimental data sets was pseudo second order model. The constant rates for the sorption by Cu(II)-imprinted fucoidan composites showed statistically equivalent constant rates for imprinted (k = 13 g/ (mg min– 1) and non-imprinted composites (k = 14 g/(mg min− 1) (cf. Table 2). In the case of the Pb(II)-imprinted material, for the sorption of
3.3.1. Assessment of the Cu(II)-imprint For the Cu(II)-imprinted material, it may be observed that the values of the binding constant, for both Cu(II) and Zn(II) (125 and 112 L mg− 1, respectively) were statistically undistinguishable from those of the corresponding non-imprinted material (120 and 58
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d) 0.4
15%
12%
20%
27%
absorbance
S
0.2
W1
E1 E3 W2 E2
0.0 Control MIM Pb(II)
e) absorbance
0.4
9%
Control NIM 5%
Fd MIM Pb(II) 21%
Fd NIM 33%
S W1 W2 E1
0.2
E2 E3 0.0 Control MIM Pb(II)
absorbance
f)
18% 0.4
0.2
Control NIM 15%
Fd MIM Pb(II) 61%
Fd NIM 65%
S
W1 W2 E1 E2 E3
0 Control MIM Pb(II)
Control NIM
Fd MIM Pb(II)
Fd NIM
Fig. 3. (continued)
110 L mg− 1, respectively). This means that the presence of Cu(II) in the synthesis of the composite did not result in the production of significantly stronger Cu(II)-oriented binding arrangements, and therefore the imprinting was not successful. However, larger capacities were observed in the Cu(II)-imprinted for Zn (II) (3.8 vs. 1.9 mg g− 1), which are, nevertheless, very low capacities when compared to those for Cu (II) and especially Cd (II) and Pb(II), as will be described next.
binding sites, this successful Pb(II)-imprinting scheme also promoted a selective capacity increase. 3.3.3. Fucoidan vs. chondroitin sulfate Pb(II)-imprinted composites The results obtained here for the imprinting of Pb(II) in a network based on the polysulfated polysaccharide fucoidan are, in terms of the imprinting figures of merit, in line with those obtained with other sulfated polysaccharides, chondroitin sulfates from various sources [11]. The IF of 1.22 for the fucoidan composite falls within the IF range observed with the chondroitin sulfate ones, IF 1.13–1.44. In the case of selectivity, α(K) and α(qmax), the gains were also found within the range obtained with chondroitin: fucoidan composites gains — 38% and 64%, respectively; chondroitin sulfate composite gains — 23–68% and 58–108%. We may, thus, consider that we could be similarly successful in the Pb(II) imprinting within both matrices. However, as referred above, we had here a presumably better starting point due to the intrinsic superior sorption properties of fucoidan composites. Conjugating that with similar improvements due to imprinting, it resulted that Pb(II) fucoidan-based imprints presented overall higher capacity (30 vs. 15–20 mg g− 1) and binding strength for Pb(II) (452 vs. 241–332 L mg− 1) than chondroitin-based imprints, with a comparable degree of selectivity. Finally, one more common feature between the two imprinting schemes was the unsuccessful attempts of Cu(II) imprinting, reinforcing the view that only for the most strongly biopolymer-interacting cations such as Pb(II), the templating of the composites seems feasible.
3.3.2. Assessment of the Pb(II) imprints In the case of Pb(II)-imprinted material a stronger binding of Pb(II) was found in the imprinted composite (452 vs. 370 L mg− 1). This corresponded to an IF value of 1.22, what translates to 22% stronger binding sites, in average, existing in the imprinted composite. Moreover, the binding constant found for Cd(II) was lowered (140 to 126 L mg− 1) due to the Pb(II) imprinting, however, not in a statistically significant (p = 0.05) way. Therefore, it appears that the stronger Pb (II) binding sites induced by imprinting are not more fit to divalent cations in general, but rather selective to Pb(II), the template. Such selectivity improvement towards Pb(II) is depicted by the α(K) values presented in Table 3. The non-imprinted material presented an original selectivity to Pb(II) over Cd(II) of 2.6, which increased to 3.6 in the imprinted counterpart. Concerning the selectivity described by α(qmax), which expresses the gains of capacity for the template cation against the test cation, it is observed that the non-imprinted composite presented very similar capacities for Pb(II) and Cd(II), α(qmax) 1.1, while in the Pb (II)-imprint the α(qmax) increased to 1.8. So, besides generating stronger 59
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a)
6
qe(mg g-1)
4
2
0
b)
0
1
2
3 time(h)
4
5
6
3 time(h)
4
5
6
10
qe(mg g-1)
8 6 4 2 0
0
1
2
Fig. 4. Kinetic profiles of cation sorption by a) eFd NIM and eFd MIM-Cu(II); b) eFd NIM and eFd MIM-Pb(II), for Cu (II) (■), Zn (II) (▲), Pb (II) (●) and Cd (II) (x), (Ci = 20 mg L– 1). Error bars represent the standard deviation of the mean result (n = 3).
with other sulfated polysaccharides (chondroitin sulfates of bovine and fish sources) composites. However, the fucoidan-based Pb(II) imprints developed here presented superior sorption properties, namely a higher capacity (30 vs. 15–20 mg g− 1) and higher binding strength for Pb(II) (452 vs. 241–332 L mg− 1) over the chondroitin-based imprints. The material developed here may easily be put to work in different areas where Pb(II) sensing, determination, separation or remediation is of importance. Currently, we are working on the development of solidphase extraction protocols aiming at the complete isolation of Pb(II) from a mixture of several common divalent cations. Extending this work to other cations capable of a strong interaction with fucoidan, such as Ba2 + [15], can be a promising way of designing sorbents bearing properties tuned towards a preferential uptake of targeted cations.
Table 2 Pseudo second ordera constant rates calculated for the sorption of divalent cations onto fucoidan imprinted and non-imprinted composites. Sorbent
Fd NIM
Fd MIM Pb(II) Fd MIM Cu (II) a
Sorbate
Pb (II) Cd (II) Cu (II) Zn (II) Pb (II) Cd (II) Cu (II) Zn (II)
Sorption kinetics (Ci = 20 mg L– 1) Model pseudo second ordera k (g/ (mg min− 1))
qe (mg g− 1)
1.0 ± 0.5 2.1 ± 0.6 14 ± 2 14 ± 3 1.6 ± 0.6 1.2 ± 0.8 13 ± 2 13 ± 3
9 ± 3 8 ± 2 6 ± 2 1.9 ± 0.8 9 ± 2 7 ± 1 6 ± 2 4 ± 1
Model fitting by linear regression exhibiting r2 ≥ 0.998 in every instance.
Acknowledgments 4. Conclusion This work was financed by Programa de Cooperação Transfronteiriça Espanha-Portugal through the Fundo Europeu de Desenvolvimento Regional (FEDER) with support from the European Union under the project Novomar, Grant 0687-Novomar-1-P. Additional financial support was granted from the Pest-C/QUI/ UI0081/2013 project (FEDER/COMPETE and FCT). The Programa Operacional do Norte (ON2) and FCT/MEC(PIDDAC) through the project NORTE-07-0124-FEDER-000065, awarded to CIQUP, is acknowledged for the gas adsorption facilities.
It was shown that fucoidan, a polysulfated polysaccharide extracted from the brown seaweed Fucus vesiculosus, may be used for the preparation of Pb(II)-imprinted materials. The materials here described consist of fucoidan/silica composites, whose preparation in the presence of Pb(II) cations originated a “memory” of those cations (cationimprinting). The imprinting effect was expressed in the observation of stronger (22%) binding and increased selectivity (1.4–1.6 fold) against Cd(II). Such imprinting features were close to those observed previously 60
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a) 7 6
qe (mg g-1)
5 4 3 2 1 0 0
5
10
15 Ci (mg L-1)
20
25
30
b) 30
qe (mg g-1)
25 20 15 10 5 0 0
20
40
60
80
100
Ci (mg L-1) Fig. 5. Equilibrium binding isotherms for the sorption of Cu (II) (■), Pb (II) (●), Zn (II) (▲) and Cd (II) (x) onto a) eFd NIM, eFd MIM Cu(II); b) eFd NIM, eFd MIM Pb(II). Error bars represent the standard deviation of the mean result (n = 3). Table 3 Isotherm models fitting parameters and imprinting features derived for all the composite materials studied. Sorbent
Fd NIM
Fd MIM Pb(II) Fd MIM Cu (II)
a
Langmuir-Freundlich isotherm modela Sorbate
K (L mg
Pb (II) Cd (II) Cu (II) Zn (II) Pb (II) Cd (II) Cu (II) Zn (II)
370 140 120 110 452 126 125 112
± ± ± ± ± ± ± ±
−1
33 21 22 16 59 26 22 19
)
Imprinting features qmax (mg g
−1
)
m
24 ± 3 21 ± 2 5.2 ± 0.9 1.9 ± 0.7 30 ± 3 17 ± 1 5.8 ± 0.9 3.8 ± 0.5
0.98 0.95 0.97 0.96 0.98 0.98 0.98 0.97
± ± ± ± ± ± ± ±
0.01 0.04 0.03 0.02 0.04 0.07 0.08 0.09
IF(KMIM/KNIM)
α(Κ)
α(qmax)
– – – – 1.22 0.90 1.04 1.02
2.6 – 1.1 – 3.6 – 1.1 –
1.1 – 2.7 – 1.8 – 1.5 –
Non-linear fitting exhibiting χ2 < 10− 7. 008. [5] L. Bingjie, C. Wei, P. Xiaofeng, G. Yu, Biosorption of lead from aqueous solutions by ion-imprinted tetraethylenepentamine modified chitosan beads, Int. J. Biol. Macromol. 86 (2016) 562–569, http://dx.doi.org/10.1016/j.ijbiomac.2016.01.100. [6] F. Lulu, L. Chuannan, L.V. Zhen, H. Qiu, Removal of Ag+ from water environment using a novel magnetic thiourea-chitosan imprinted Ag+, J. Hazard. Mater. 194 (2011) 193–201, http://dx.doi.org/10.1016/j.jhazmat.2011.07.080. [7] L. Bingjie, W. Dongfeng, X. Ying, G. Huang, Adsorption properties of Cd(II)imprinted chitosan resin, J. Mater. Sci. 46 (5) (2011) 1535–1541, http://dx.doi. org/10.1007/s10853-010-4958-6. [8] D.W. Lee, C. Lim, N.I. Jacob, S.H. Dong, Strong adhesion and cohesion for chitosan in aqueous solutions, Langmuir 29 (46) (2013) 14222–14229, http://dx.doi.org/10. 1021/la403124u. [9] C. Lim, D.W. Lee, N.I. Jacob, S.H. Dong, Contact time and pH-dependent adhesion and cohesion of low molecular weight chitosan coated surfaces, Carbohydr. Polym.
References [1] H. Wang, L. Zhang, Molecular imprinted functional materials base on polysccharides, Prog. Chem. 22 (11) (2010) 2165–2172. [2] L. Sovichea, N. Xiaofeng, Y. Haining, et al., State-of-the-art applications of cyclodextrins as functional monomers in molecular imprinting techniques: a review, J. Sep. Sci. 39 (12) (2016) 2321–2331, http://dx.doi.org/10.1002/jssc. 201600003. [3] C. Branger, W. Meouche, A. Margaillan, Recent advances on ion-imprinted polymers Review Article, React. Funct. Polym. 73 (6) (2013) 859–875, http://dx. doi.org/10.1016/j.reactfunctpolym.2013.03.021. [4] P.R. Dhakal, T. Oshima, Y.Y. Baba, Planarity-recognition enhancement of N-(2pyridylmethyl) chitosan by imprinting planar metal ions, React. Funct. Polym. 68 (11) (2008) 1549–1556, http://dx.doi.org/10.1016/j.reactfunctpolym.2008.08.
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[12] M.T. Ale, A.S. Meyer, Fucoidans from brown seaweeds: an update constructors, extraction techniques and use of enzymes as tools for structural elucidation, RSC Adv. 3 (2013) 8131–8141, http://dx.doi.org/10.1039/c3ra23373a. [13] K.S. Dodgson, Determination of inorganic sulphate in studies on the enzymic and non-enzymic hydrolysis of carbohydrate and other sulphate esters, Biochem. J. 78 (2) (1961) 312–319. [14] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. A 32 (1976) 751–767. [15] A. Paskins-Hurlburt, S. Skoryna, Y. Tanaka, W. Moore, F. Stara, Fucoidan its binding of lead and other metals, Bot. Mar. 11 (1978) 13–22.
117 (6) (2015) 887–894, http://dx.doi.org/10.1016/j.carbpol.2014.10.033. [10] V.R.A. Ferreira, M.A. Azenha, A.G. Bustamante, M.T. Mêna, C. Moura, C.M. Pereira, A.F. Silva, Metal cation sorption ability of immobilized and reticulated chondroitin sulfate or fucoidan through a sol-gel crosslinking scheme, Mater. Today Communication 8 (2016) 172–182, http://dx.doi.org/10.1016/j.mtcomm.2016.08. 003. [11] V.R.A. Ferreira, M.A. Azenha, M.T. Mêna, C. Moura, C.M. Pereira, R.I. Pérez-Martin, J.A. Vásquez, A.F. Silva, Cationic imprinting of Pb (II) within composite networks based on bovine or fish chondroitin sulfate, J. Mol. Recognit. (2017) e2614, http:// dx.doi.org/10.1002/jmr.2614.
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Reactive and Functional Polymers journal homepage: www.elsevier.com/locate/react
Preparation and evaluation of Pb(II)-imprinted fucoidan-based sorbents ⁎
Vanessa R.A. Ferreira, Manuel A. Azenha , Carlos M. Pereira, A. Fernando Silva
MARK
CIQ-UP, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
A R T I C L E I N F O
A B S T R A C T
Keywords: Cation-imprinting Fucoidan Sol-gel Lead (II) Composite
Fucoidan, a sulfated polysaccharide extracted from brown seaweed, was, in the form of a silica composite, studied as a prospective cation imprinting matrix. The preparation of such composites in the presence of cations with a strong interaction with the biopolymer chains was expected to direct them towards arrangements, optimized for the sorption of those cations. As expected, the presence of Cu(II), a weakly fucoidan-binding cation, in the synthesis of the composites did not result in the production of significantly stronger Cu(II)-oriented binding arrangements, and therefore the imprinting was not successful. However, with Pb(II), with much stronger affinity for fucoidan, the materials obtained exhibited stronger (22%) binding as compared to the nonimprinted counterparts, and increased selectivity (1.4–1.6 fold) against Cd(II). Although these imprinting features were close to those observed previously with other sulfated polysaccharides, the fucoidan-based Pb(II) imprints developed here presented superior sorption properties, namely a higher capacity and higher binding strength for Pb(II). These features, demonstrated by the material developed here, may easily be put to work in different areas where Pb(II) sensing, determination, separation or remediation is of the utmost importance.
1. Introduction The utilisation of biodegradable, biocompatible polysaccharides extractable from abundant low-cost raw materials became appealing in different fields of research. In the case of the preparation of selective materials with higher loadings by imprinting techniques, for purposes such as separation, drug delivery or sensing, the employment of natural polysaccharides as cyclodextrin, chitosan, cellulose, alginic acid, agarose and starch has been documented [1]. The generic process comprises: (i) a template structure (a small molecule, a macromolecule, an ion, etc.) exhibiting a strong association affinity with the polysaccharide that results to some extent in the modulation of the conformational structure of the biopolymer; (ii) the fixation of the template-polysaccharide induced structure into a stable 2D or 3D network; (iii) the removal of the template from the network, which leaves the imprinted sites with the “memory” for an optimized rebinding of the template or closely related structures. A wide range of templates have been used in polysaccharide-based imprinting. The dextrins, for example, have been used along with steroidals, amino acids, polysaccharides, drugs, plant hormones, proteins, pesticides, and plastic additives (as reviewed in [2]). In what cationic templates are concerned, namely metallic cations (cf [3] for a comprehensive review on ion-imprinted polymers), chitosan and modified chitosan have been the polysaccharides of choice due to the presence of the amino groups, which, combined with hydroxyls or other added groups, confers metal-cation complexing ⁎
Corresponding author. E-mail address: [email protected] (M.A. Azenha).
http://dx.doi.org/10.1016/j.reactfunctpolym.2017.04.001 Received 2 February 2017; Received in revised form 20 March 2017; Accepted 2 April 2017 Available online 03 April 2017 1381-5148/ © 2017 Elsevier B.V. All rights reserved.
ability (cf. [4–7] for a few recent examples). However, the amino group in chitosan has a pKa value of ~6.5, which leads to a protonation in acidic to neutral solutions [8,9], thus impairing the cation-complexation ability in the lower half of the pH range. Nature, through specific biosynthesis routes, offers us a remarkable diversity of polysaccharides, many of which are virtually unexplored for their potentialities to interact with metal cations. As we recently have pointed out [10] the exploitation of negatively-charged sulfated polysaccharides for metal cation-related applications is still insipient. The presence of the eOeSO3− groups in the structure of these polysaccharides provides them with permanent negatively-charged points even at pH values as low as 2. The eventual cooperativity between sulfate and other nearby functionalities attached to the backbone of the biopolymers foresees strong binding modes towards cationic species with different selectivity than the one associated to the most commonly studied polysaccharides. We developed already a first example of sulfated-polysaccharide-based imprinted materials by preparing chondroitin sulfate/silica Pb(II) imprinted composites [11]. Chondroitin sulfate, a disaccharide comprising an amino sugar (Nacetylgalactosamine) and glucuronic acid linked by β-(1 → 3) glycosidic bonds, contains in average one carboxylate and one sulfate group per monomer. The materials exhibited a “memory” effect for the Pb(II) ions, expressed in the observation of stronger (up to 44%) binding as compared to the non-imprinted counterparts, and increased selectivity (1.5–2 fold) against Cd(II).
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Fig. 1. Chemical structures of Fd (I), GPTMS (II), TMOS (III) and schematics of the Fd cation-imprinting process.
2. Materials and methods
In the present work, we studied fucoidan, another sulfated polysaccharide, extracted from brown seaweed, generally consisting of a backbone of L-fucopyranose residues linked by β(1 → 3) bonds, which may enclose side branches containing fucopyranoses or other glycosyl units, e.g. glucuronic acid [12] (I in Fig. 1). In this case, the typical unit contains three sulfate groups per disaccharide, therefore constituting a more densely-charged polyanionic polysaccharide than chondroitin sulfate. The preparation scheme (Fig. 1) and the features of the Pb (II)-imprinted and Cu (II)-imprinted fucoidan-based materials are presented and discussed, inclusively from the comparative perspective with the chondroitin-based imprints. The Ag(I) and Au(III) cations were studied at an initial exploratory stage because of the high commercial value of these cations. Cd (II) and Zn (II) cations were also evaluated, mainly for ascertaining selectivity features.
2.1. Materials Fucoidan (Fd, > 90% purity, containing 5% uronic acids, average MW 73.7 kDa) was provided by Marinova, Australia, extracted from Fucus vesiculosus. The metal cation salts used in the synthesis of imprinted materials and for the sorption tests, such as Pb(NO3)2, Cu(NO3)2·3H2O, Zn (NO3)2·6H2O and Cd(NO3)2·6H2O, were provided by Merck (Darmstadt, Germany, > 99% purity). Glycidyloxypropyl-trimethoxysilane (GPTMS, II in Fig. 1) and tetramethoxysilane (TMOS, III in Fig. 1) were purchased from Sigma-Aldrich (Deisenhofen, Germany) and were used without further purification (> 98%). Water was of Milli-Q 54
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0.7 mL min− 1, in a Visiprep (Supelco) SPE station manifold. After sample loading, 5 mL of acetate buffer plus 5 mL HNO3 0.001 mol L− 1 were used as washing solutions. The cartridges were thereafter subjected to a three elution step of 3 × 5 mL of HNO3 0.01 mol L− 1. The fractions (loading, washing and elution) eluting from the SPE column were directly monitored by atomic absorption spectrophotometry as described in Section 2.2. The resulting absorbances were used to estimate the fraction of sorbent-bound analyte. The SPE retention % was calculated according to Eq. (2).
(Millipore, Italy) purity. All other reagents were analytical grade. 2.2. Synthesis of cation-imprinted materials (MIM) The cation-imprinting within reticulated Fd-silica composites was performed according to a procedure adapted from our previous work [11]. A 1.5% m:v Fd solution was prepared in deionized water (pH was adjusted from 6.7 to 7.5 with 1 mol L− 1 of ammonia solution) and mixed with GPTMS (Fd monomer: GPTMS molar ratio of 1:1). This mixture was stirred for 17 h at room temperature. Then, the template cation (in the form of Pb(NO3)2 or Cu(NO3)2 salt) was added in the molar proportion of 1 cation to 1 Fd monomer). Thereafter, TMOS (Fd monomer:TMOS molar ratio 1:1) was added into the mixture to form a gel. The precipitate was centrifuged at 6000 rpm (PrO-Analitycal, Centurion Scientific Lid CR4000R5 Model) for 10 min at 25 °C. The pellet was dried overnight in the oven at 50 °C. The metal cations were removed by washing six times with HNO3 0.01 mol·L− 1. Every washing step was intercalated with centrifugation at 6000 rpm for 10 min. The washings were monitored by analyzing the supernatant of centrifugation by atomic absorption spectrophotometry (Perkin Elmer, AAnalyst 200 Model, operated at the instrumental parameters recommended by the manufacturer for every element analyzed). For reference purposes, non-imprinted materials (NIM) were obtained similarly, but without adding the cation salt. Also, purely sol-gel reference materials, with and without cation imprinting, were prepared and their synthesis was the same as described previously, but without addition of Fd.
SPE retention (%) = 100 −
× 100
(2)
2.5. Batch sorption studies: isotherms and kinetics The sorption assays were conducted in glass flasks containing 10 mg of biosorbent in 5 mL of acetate buffer 0.01 mol L− 1 at pH 5.5 and supplemented with different concentrations of each metal cation, within the range of 10 to 100 mg L– 1. All experiments were conducted under stirring (120 rpm) and at constant temperature (25 °C). For the study of sorption kinetics, samples were collected at approx. regular intervals until 6 h, while for the study of the isotherms all samples were collected at the end of 6 h. Afterwards, the samples were centrifuged at 6000 rpm for 10 min at 25 °C, the supernatant was collected, acidified to pH 4 with concentrated nitric acid and stored protected from light, until AAS analysis. Non-linear fitting of theoretical isotherms to experimental data was performed using IGOR PRO software Package. In this study, three models, Langmuir (Eq. (3)), Freundlich (Eq. (4)) and LangmuirFreundlich isotherms (L–F) (Eq. (5)) were tested to describe the biosorption of the metallic cations.
Attenuated Total Reflectance Fourier Transform Infrared (ATRFTIR) measurements were performed using a Bruker FT-IR System Tensor 27 spectrophotometer in the range of 600–4000 cm− 1. The degree of swelling (Sw) was determined by keeping 10 mg (W0) of the biosorbent in 2 mL of deionized water (pH 7). The increase in weight (Wt − W0) of the biosorbent, after 12 h of agitation, in comparison to initial weight (W0) of the biosorbent was used to calculate the degree of swelling (Sw) using Eq. (1)
Wt − W0 × 100 W0
Ai
where Af corresponds to the absorbance representative of the analyte concentration found in the eluate and Ai to the absorbance representative of the analyte concentration in the original sample. It was always assured that both Ai and Af were kept within the linear response range of the technique. Regeneration of the SPE cartridge was achieved by re-conditioning with HNO3 0.01 mol L− 1 and acetate buffer (0.01 mol L− 1) at pH 5.5. The re-conditioning process was monitored by AAS until the release of traces of the used cations ceased being detected.
2.3. Characterization of the imprinted materials
Sw (%) =
Af
(1)
where W0 and Wt are the initial and final weights of the biosorbent, respectively. The surface area and the pore parameters were determined by a nitrogen adsorption analyser (TriStar Plus, Micromeritics). The experiments were conducted with 200–500 mg of sample, which were previously dried overnight in the oven at 50 °C followed by at least 2 h under a flow of nitrogen. The specific surface areas (S) were evaluated using the BET method, the specific pore volumes (Vp) following the Gurvitch method and the average pore diameter (Dp) using the BJH theory applied to the desorption branch of the isotherm. The amount of Fd present in composites was determined indirectly by quantifying the amount of biopolymer remaining in the supernatant of the reaction mixture. The quantification was based on the precipitation reaction between Fd (due to the sulfate groups) and Ba2 +, followed by turbidimetric analysis (Portable Turbidimeter 2100Qis, HACH) according to a method described in the literature (Dodgson, 1961 [13]).
qe =
qmax KC 1 + KC
(3)
qe =
aC m
(4)
qe =
(KC )m
qmax 1 + (KC )m
(5)
where qe corresponds to the sorbed concentration in equilibrium (mg g− 1); C is the equilibrium concentration in solution (mg/L) and a and m are fitting constants. The constant a is related with the binding affinity and the parameter m refers to the heterogeneity index and its value ranges from 0 to 1, increasing as heterogeneity decreases. The Langmuir model assumes that one class of site is present on the surface, with saturation capacity qmax, to form a complete monolayer on the surface with dissociation constant K (L mg− 1). The Freundlich isotherm, on the other hand, assumes sites with a Gaussian distribution of binding strengths. Here the width of the Gaussian distribution describes the degree of heterogeneity, through the index m. The difference between the L–F model and the Freundlich one is evident at high sorbate concentrations, for which the L–F model is able to represent the saturation behaviour. At low sorbate concentrations, the L–F equation reduces to the classical Freundlich equation. On the other hand, if m approaches unity, it is indicative of a completely homogeneous sorbent surface (i.e., energetic equivalence of all binding sites), the L–F equation reduces to the classical Langmuir equation. Thus, the hybri-
2.4. Solid phase extraction (SPE) SPE cartridges were packed individually with 200 mg of each biosorbent. The cartridges were conditioned with 10 mL HNO3 0.01 mol L− 1 and 5 mL acetate buffer (0.01 mol L− 1) at pH 5.5. Samples (5 mL) containing one of the following mixed cation solutions in acetate buffer (0.01 mol L− 1, pH 5.5) were then introduced: 10 ppm Cu(II)/Zn(II), 50 ppm Pb (II)/Au(III), 50 ppm Pb(II)/Ag(I) and 50 ppm Pb (II)/Cd (II). The percolation proceeded at a constant flow rate of 55
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dized L–F isotherm is able to model adsorption of solutes at high and low concentrations on to both homogeneous and heterogeneous sorbents. Sorption kinetics fitting was tested by linear regression using pseudo-first order (Eq. (6)) and pseudo-second order (Eq. (7)) models.
qt = qe (1 − qt =
e kt )
Table 1 Microstructural properties and chemical composition of the composites. Sample
(6)
Controla NIM Controla MIM Pb(II) Controla MIM Cu(II) Fd NIM Fd MIM Pb(II) Fd MIM Cu(II)
qe2 k2 t 1 + qe k2 t
(7)
where qt is the amount of each compound sorbed at time t (mg g − 1), and k is the rate constant of each model. 2.6. Computation of imprinting features
a
The calculated binding constants and capacity values were the inputs for deriving the imprinting and selectivity factors (IF and α, respectively). The IF is here defined as the ratio between the template binding constants for the MIM and corresponding NIM, IF = KMIM / KNIM, and expresses the relative gain in binding strength due to the imprinting process. Selectivity was evaluated according to two different definitions. The first one relates the binding constants of the template and a test cation (the pairs Pb(II)/Cd(II) and Cu(II)/Zn(II) were employed, respectively, for the Pb(II)- and Cu(II)-imprinted materials): α(K) = K(template) / K(test cation). The second definition is similar but uses the capacity of the materials: α(qmax) = qmax(template) / qmax(test cation).
BET
Mass fraction of Fd in biosorbents (mg/mg)
Sw (%)
S (m2/g)
Vp (mL/g)
12 17
0.032 0.016
– –
3.5 3.8
13
0.011
–
3.9
6.4 7.2 6.9
0.004 0.006 0.004
0.33 0.38 0.27
2.4 2.2 2.7
Control — Material without Fd.
biopolymer, which appears to cooperate with the crosslinking process. The recorded ATR-FTIR measurements agree with the enrichment of Fd in the Pb(II)-imprinted composites. In Fig. 2, the typical spectrum of fucoidan reference (a) is presented alongside with those of the nonimprinted (b) and Pb(II)-imprinted composites (c). The spectrum of a purely sol-gel material (d) is also shown. Using the band at 1687 cm− 1 as a fucoidan marker, since it does not appear in the purely sol-gel material, we promptly identify the fucoidan component in the composites. It is also evident the increased importance of that band in the Pb (II)-imprinted material comparing with the non-imprinted one, what is compatible with a fucoidan-richer material, as suggested by turbidimetry analysis. On the other hand, the silica component of the composites can be identified by the intense band at ~ 1050 cm− 1, which nevertheless appears also in the fucoidan spectrum as a minor band.
3. Results and discussion Recently developed fucoidan/silica composites exhibited a remarkable feature discovered during the study of their acid-base properties: a much higher proton deficiency (~ 4 fold) was found for the surface of the composites (33% fucoidan in mass) as compared to a purely fucoidan suspension [10]. This behaviour implicates that the composites possess a surface much richer in anionic (proton-exchanging) or basic sites. As these differences could not be ascribed to the silica component, a likely explanation put forward was the unfolding of the fucoidan chains during the synthesis process, exposing otherwise inaccessible proton-accepting sites (cation-exchange processes should be dominating in this case). Due to this feature, the composites exhibited outstanding sorptive properties towards metallic cations, most noticeably Pb(II) and Cd(II) [10]. Hence, this composite seemed a very appealing matrix to attempt the imprinting of metallic cations. A strongly sorbed cation (Pb(II); affinity constant 370 L mg− 1) and a significantly weakly sorbed cation (Cu(II); affinity constant 120 L mg− 1) were tested as templates.
3.2. Sorption of metallic cations in dynamic mode The composites and respective sol-gel controls were subjected to an initial expeditious evaluation in dynamic mode, as SPE stationary phases for the sorption of a few cations. Solutions of mixed cations in pH 5.5 buffer were percolated through the sorbents and the retained cations were thereafter recovered by a flow of HNO3 solution. The cations screened were Cu (II) and Zn (II) for the Cu(II)-imprinted materials, and Pb (II), Ag (I), Au (III) and Cd (II) for the Pb(II)imprinted materials. The behaviour of the Cu(II)-imprinted fucoidan composite is presented in Fig. 3 a) and b). It was not observed any apparent gain from the process of imprinting the fucoidan composite with Cu(II), since no improvement in retention nor changes in the washing profile could be observed for both Cu(II) and Zn(II). Interestingly, in their corresponding sol-gel materials (control MIM and NIM), templating with Cu(II) resulted in significantly increased retention (from 6 to 16% to 47–60%) and a different washing profile – most of the cation being washed by 0.001 mol L− 1 HNO3 instead of mostly being directly eluted during the sample loading. Nevertheless, these findings were not investigated deeper since our interest was focused on the fucoidan composites which presented always higher retention and washing profiles indicating stronger binding (most of the retained cation being eluted only with the stronger eluent 0.01 mol L− 1 HNO3). In Fig. 3 c), d), e) and f) the sorption behaviour of the Pb(II)imprinted MIM is displayed. In this case, the results were indicative of an effective gain in the retention features of the MIM against NIM, both containing fucoidan, namely an increased retention of Pb(II) (92% vs. 81%) and increased fraction eluted with the strongest eluant (0.01 mol L− 1 HNO3). In the corresponding control materials, only very small retention (11–18%) was obtained and elution occurred easily with the weaker washing eluants. Control materials presented also weak retention behaviour towards Ag(I), Au(III) and Cd(II). The Pb (II)-imprinted fucoidan composites also did not show improvements for the sorption of any of these cations in dynamic mode, thus providing a
3.1. Structural and chemical characterization Table 1 condensates the major textural features obtained for all the materials. All of them are essentially microporous, as based on the small surface area (6–17 m2/g) and pore volume (0.00–0.032 mL/g) ranges. Most noticeably, within the fucoidan composites the addition of the cation (either Pb(II) or Cu(II)) did not implicate any major changes in the measured surface area and pore volume, as determined by nitrogen adsorption analysis. Additionally, very small swelling ratios (2–4%) were observed. Everything considered, it may be concluded that a dense, highly crosslinked, structure for the composites was obtained, both using the cations or not. Table 1 contains also the data of the mass fraction of Fd within the composites. It was observed, in the case of the Pb(II)-imprinted composite only, that the presence of the cation template caused a small increase of Fd contents in the final product. This effect has been found before with Pb(II)-imprinted composites of chondroitin sulfate [11] and is likely related with an extra aggregational influence exerted by the divalent cations upon the sulfated 56
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Fig. 2. ATR-FTIR spectra of: a) Fd; b) Fd NIM; c) Fd MIM Pb(II); d) Sol-gel control NIM. All spectra represented with the same scale (0.4 Au).
further indication of a successful selective imprinting.
batches of non-imprinted and Pb(II)-imprinted composites showed that the variability of the SPE retention of Pb(II) was below 10%, what may be considered acceptable reproducibility.
3.2.1. Shelf life stability and synthesis reproducibility These features where also evaluated by SPE. It was found that the composites could be stored (room temperature, protected from light) at least 3 months before use without losing any retention capacity. In what regards reproducibility, the results obtained from 3 independent
3.3. Assessment of the imprinting features The evaluation of the imprinting features relied essentially on the 57
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a)
60%
16%
absorbance
85%
84%
W2
0.4
W1
S 0.2 E1
E2 E3
0 Control MIM Cu(II)
b)
47%
absorbance
0.4
Control NIM 6%
Fd MIM Cu(II) 62%
Fd NIM 70%
W2
S
W1
0.2
E1 E2 E3
0 Control MIM Cu(II)
c)
18%
Control NIM
11%
Fd MIM Cu(II)
92%
Fd NIM
81%
S
absorbance
0.4
0.2
W1 W2 E1E2
E3
0 Control MIM Pb(II)
Control NIM
Fd MIM Pb(II)
Fd NIM
Fig. 3. SPE results, expressed as the AAS absorbance measured in the consecutive eluates, found for the different sorbents with: a) Cu (II), b) Zn (II), c) Pb (II), d) Ag (I), e) Au (III), f) Cd (II) The retention (%) observed upon loading of the metal cation solution is also shown. SPE steps (from left to right in the graphs): sample introduction – 10 mL of metal cation solution in acetate buffer pH 5.5 (S-red); wash 1–6 mL acetate buffer pH 5.5 (W1-purple); wash 2–6 mL HNO3 0.001 mol L− 1(W2-black); elution 1–6 mL HNO3 0.01 mol L− 1(E1-grey); elution 2–6 mL HNO3 0.01 mol L− 1(E2-green); elution 3–6 mL HNO3 0.01 mol L− 1(E3-blue).
Pb(II), it was observed a higher constant rate (k = 1.6 g/(mg min− 1) than the one obtained for the corresponding NIM (k = 1.0 g/ (mg min− 1). However, the difference fell within the uncertainty range. For the sorption of Cd(II) a similar situation was found. It may thus be concluded that the imprinting process did not bring significant changes in the kinetics of cation sorption. The sorption isotherms (Fig. 5) could reveal the actual effect of the imprinting process upon the sorption properties of major interest, such as capacity (maximum sorbable amount) and binding strength. The isotherm (qe vs C) data points were fitted to different isotherm models, such as Langmuir (Eq. (3)) Freundlich (Eq. (4)) and hybrid L–F (Eq. (5)) by non-linear regression analysis. The best fitting was selected according to the lower χ2 (chi-squared) values obtained, and qmax proximity between the calculated values and the experimental values. In all cases the L–F model provided the best fit. The corresponding isotherm parameters were condensed in Table 3.
evaluation of the imprinting and selectivity factors (IF and α, respectively), as defined in 2.6. Cd(II) was chosen as selectivity test cation for Pb(II) due to similar properties relevant to sorption, namely crystal ionic radius above 100 pm [14], common biological effects (similar coordination properties), and from our previous experience on the composites, which made us realize that the sorption kinetics were similar for the two cations. Likewise, we chose Zn(II) as selectivity test cation for Cu(II) since they are adjacent in the Irving-Williams series (similar coordination properties), have similar crystal ionic radius (87–88 pm) [14] and similar sorption kinetics with our composites (significantly faster than Pb(II) and Cd(II)). For the computation of IF and α, input data were obtained from the mathematical treatment of the cation sorption isotherm data. The sorption profiles (Fig. 4) helped establishing the time given for the equilibrium in the isotherm experiments (6 h). The results, from the kinetic studies, were treated by two mathematical models, pseudo first order and pseudo second order, and the model that best fitted all the experimental data sets was pseudo second order model. The constant rates for the sorption by Cu(II)-imprinted fucoidan composites showed statistically equivalent constant rates for imprinted (k = 13 g/ (mg min– 1) and non-imprinted composites (k = 14 g/(mg min− 1) (cf. Table 2). In the case of the Pb(II)-imprinted material, for the sorption of
3.3.1. Assessment of the Cu(II)-imprint For the Cu(II)-imprinted material, it may be observed that the values of the binding constant, for both Cu(II) and Zn(II) (125 and 112 L mg− 1, respectively) were statistically undistinguishable from those of the corresponding non-imprinted material (120 and 58
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d) 0.4
15%
12%
20%
27%
absorbance
S
0.2
W1
E1 E3 W2 E2
0.0 Control MIM Pb(II)
e) absorbance
0.4
9%
Control NIM 5%
Fd MIM Pb(II) 21%
Fd NIM 33%
S W1 W2 E1
0.2
E2 E3 0.0 Control MIM Pb(II)
absorbance
f)
18% 0.4
0.2
Control NIM 15%
Fd MIM Pb(II) 61%
Fd NIM 65%
S
W1 W2 E1 E2 E3
0 Control MIM Pb(II)
Control NIM
Fd MIM Pb(II)
Fd NIM
Fig. 3. (continued)
110 L mg− 1, respectively). This means that the presence of Cu(II) in the synthesis of the composite did not result in the production of significantly stronger Cu(II)-oriented binding arrangements, and therefore the imprinting was not successful. However, larger capacities were observed in the Cu(II)-imprinted for Zn (II) (3.8 vs. 1.9 mg g− 1), which are, nevertheless, very low capacities when compared to those for Cu (II) and especially Cd (II) and Pb(II), as will be described next.
binding sites, this successful Pb(II)-imprinting scheme also promoted a selective capacity increase. 3.3.3. Fucoidan vs. chondroitin sulfate Pb(II)-imprinted composites The results obtained here for the imprinting of Pb(II) in a network based on the polysulfated polysaccharide fucoidan are, in terms of the imprinting figures of merit, in line with those obtained with other sulfated polysaccharides, chondroitin sulfates from various sources [11]. The IF of 1.22 for the fucoidan composite falls within the IF range observed with the chondroitin sulfate ones, IF 1.13–1.44. In the case of selectivity, α(K) and α(qmax), the gains were also found within the range obtained with chondroitin: fucoidan composites gains — 38% and 64%, respectively; chondroitin sulfate composite gains — 23–68% and 58–108%. We may, thus, consider that we could be similarly successful in the Pb(II) imprinting within both matrices. However, as referred above, we had here a presumably better starting point due to the intrinsic superior sorption properties of fucoidan composites. Conjugating that with similar improvements due to imprinting, it resulted that Pb(II) fucoidan-based imprints presented overall higher capacity (30 vs. 15–20 mg g− 1) and binding strength for Pb(II) (452 vs. 241–332 L mg− 1) than chondroitin-based imprints, with a comparable degree of selectivity. Finally, one more common feature between the two imprinting schemes was the unsuccessful attempts of Cu(II) imprinting, reinforcing the view that only for the most strongly biopolymer-interacting cations such as Pb(II), the templating of the composites seems feasible.
3.3.2. Assessment of the Pb(II) imprints In the case of Pb(II)-imprinted material a stronger binding of Pb(II) was found in the imprinted composite (452 vs. 370 L mg− 1). This corresponded to an IF value of 1.22, what translates to 22% stronger binding sites, in average, existing in the imprinted composite. Moreover, the binding constant found for Cd(II) was lowered (140 to 126 L mg− 1) due to the Pb(II) imprinting, however, not in a statistically significant (p = 0.05) way. Therefore, it appears that the stronger Pb (II) binding sites induced by imprinting are not more fit to divalent cations in general, but rather selective to Pb(II), the template. Such selectivity improvement towards Pb(II) is depicted by the α(K) values presented in Table 3. The non-imprinted material presented an original selectivity to Pb(II) over Cd(II) of 2.6, which increased to 3.6 in the imprinted counterpart. Concerning the selectivity described by α(qmax), which expresses the gains of capacity for the template cation against the test cation, it is observed that the non-imprinted composite presented very similar capacities for Pb(II) and Cd(II), α(qmax) 1.1, while in the Pb (II)-imprint the α(qmax) increased to 1.8. So, besides generating stronger 59
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a)
6
qe(mg g-1)
4
2
0
b)
0
1
2
3 time(h)
4
5
6
3 time(h)
4
5
6
10
qe(mg g-1)
8 6 4 2 0
0
1
2
Fig. 4. Kinetic profiles of cation sorption by a) eFd NIM and eFd MIM-Cu(II); b) eFd NIM and eFd MIM-Pb(II), for Cu (II) (■), Zn (II) (▲), Pb (II) (●) and Cd (II) (x), (Ci = 20 mg L– 1). Error bars represent the standard deviation of the mean result (n = 3).
with other sulfated polysaccharides (chondroitin sulfates of bovine and fish sources) composites. However, the fucoidan-based Pb(II) imprints developed here presented superior sorption properties, namely a higher capacity (30 vs. 15–20 mg g− 1) and higher binding strength for Pb(II) (452 vs. 241–332 L mg− 1) over the chondroitin-based imprints. The material developed here may easily be put to work in different areas where Pb(II) sensing, determination, separation or remediation is of importance. Currently, we are working on the development of solidphase extraction protocols aiming at the complete isolation of Pb(II) from a mixture of several common divalent cations. Extending this work to other cations capable of a strong interaction with fucoidan, such as Ba2 + [15], can be a promising way of designing sorbents bearing properties tuned towards a preferential uptake of targeted cations.
Table 2 Pseudo second ordera constant rates calculated for the sorption of divalent cations onto fucoidan imprinted and non-imprinted composites. Sorbent
Fd NIM
Fd MIM Pb(II) Fd MIM Cu (II) a
Sorbate
Pb (II) Cd (II) Cu (II) Zn (II) Pb (II) Cd (II) Cu (II) Zn (II)
Sorption kinetics (Ci = 20 mg L– 1) Model pseudo second ordera k (g/ (mg min− 1))
qe (mg g− 1)
1.0 ± 0.5 2.1 ± 0.6 14 ± 2 14 ± 3 1.6 ± 0.6 1.2 ± 0.8 13 ± 2 13 ± 3
9 ± 3 8 ± 2 6 ± 2 1.9 ± 0.8 9 ± 2 7 ± 1 6 ± 2 4 ± 1
Model fitting by linear regression exhibiting r2 ≥ 0.998 in every instance.
Acknowledgments 4. Conclusion This work was financed by Programa de Cooperação Transfronteiriça Espanha-Portugal through the Fundo Europeu de Desenvolvimento Regional (FEDER) with support from the European Union under the project Novomar, Grant 0687-Novomar-1-P. Additional financial support was granted from the Pest-C/QUI/ UI0081/2013 project (FEDER/COMPETE and FCT). The Programa Operacional do Norte (ON2) and FCT/MEC(PIDDAC) through the project NORTE-07-0124-FEDER-000065, awarded to CIQUP, is acknowledged for the gas adsorption facilities.
It was shown that fucoidan, a polysulfated polysaccharide extracted from the brown seaweed Fucus vesiculosus, may be used for the preparation of Pb(II)-imprinted materials. The materials here described consist of fucoidan/silica composites, whose preparation in the presence of Pb(II) cations originated a “memory” of those cations (cationimprinting). The imprinting effect was expressed in the observation of stronger (22%) binding and increased selectivity (1.4–1.6 fold) against Cd(II). Such imprinting features were close to those observed previously 60
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a) 7 6
qe (mg g-1)
5 4 3 2 1 0 0
5
10
15 Ci (mg L-1)
20
25
30
b) 30
qe (mg g-1)
25 20 15 10 5 0 0
20
40
60
80
100
Ci (mg L-1) Fig. 5. Equilibrium binding isotherms for the sorption of Cu (II) (■), Pb (II) (●), Zn (II) (▲) and Cd (II) (x) onto a) eFd NIM, eFd MIM Cu(II); b) eFd NIM, eFd MIM Pb(II). Error bars represent the standard deviation of the mean result (n = 3). Table 3 Isotherm models fitting parameters and imprinting features derived for all the composite materials studied. Sorbent
Fd NIM
Fd MIM Pb(II) Fd MIM Cu (II)
a
Langmuir-Freundlich isotherm modela Sorbate
K (L mg
Pb (II) Cd (II) Cu (II) Zn (II) Pb (II) Cd (II) Cu (II) Zn (II)
370 140 120 110 452 126 125 112
± ± ± ± ± ± ± ±
−1
33 21 22 16 59 26 22 19
)
Imprinting features qmax (mg g
−1
)
m
24 ± 3 21 ± 2 5.2 ± 0.9 1.9 ± 0.7 30 ± 3 17 ± 1 5.8 ± 0.9 3.8 ± 0.5
0.98 0.95 0.97 0.96 0.98 0.98 0.98 0.97
± ± ± ± ± ± ± ±
0.01 0.04 0.03 0.02 0.04 0.07 0.08 0.09
IF(KMIM/KNIM)
α(Κ)
α(qmax)
– – – – 1.22 0.90 1.04 1.02
2.6 – 1.1 – 3.6 – 1.1 –
1.1 – 2.7 – 1.8 – 1.5 –
Non-linear fitting exhibiting χ2 < 10− 7. 008. [5] L. Bingjie, C. Wei, P. Xiaofeng, G. Yu, Biosorption of lead from aqueous solutions by ion-imprinted tetraethylenepentamine modified chitosan beads, Int. J. Biol. Macromol. 86 (2016) 562–569, http://dx.doi.org/10.1016/j.ijbiomac.2016.01.100. [6] F. Lulu, L. Chuannan, L.V. Zhen, H. Qiu, Removal of Ag+ from water environment using a novel magnetic thiourea-chitosan imprinted Ag+, J. Hazard. Mater. 194 (2011) 193–201, http://dx.doi.org/10.1016/j.jhazmat.2011.07.080. [7] L. Bingjie, W. Dongfeng, X. Ying, G. Huang, Adsorption properties of Cd(II)imprinted chitosan resin, J. Mater. Sci. 46 (5) (2011) 1535–1541, http://dx.doi. org/10.1007/s10853-010-4958-6. [8] D.W. Lee, C. Lim, N.I. Jacob, S.H. Dong, Strong adhesion and cohesion for chitosan in aqueous solutions, Langmuir 29 (46) (2013) 14222–14229, http://dx.doi.org/10. 1021/la403124u. [9] C. Lim, D.W. Lee, N.I. Jacob, S.H. Dong, Contact time and pH-dependent adhesion and cohesion of low molecular weight chitosan coated surfaces, Carbohydr. Polym.
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