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Polysaccharide–fibrous clay bionanocomposites
CLAY-02936; No of Pages 7 Applied Clay Science xxx (2014) xxx–xxx
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Research paper
Polysaccharide–fibrous clay bionanocomposites☆ Ana C.S. Alcântara 1,⁎, Margarita Darder, Pilar Aranda ⁎, Eduardo Ruiz-Hitzky Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 17 June 2013 Received in revised form 18 February 2014 Accepted 20 February 2014 Available online xxxx Keywords: Sepiolite Palygorskite Bionanocomposites Polysaccharides Biosorbents Heavy metal adsorption
a b s t r a c t The present paper introduces results on the preparation, characterization and exploration of properties of bionanocomposites prepared by the assembly of polysaccharides to sepiolite and palygorskite. The effect of the incorporation of these fibrous clay minerals was analyzed in three types of polysaccharide matrices: neutral, such as starch, and provided with either negatively or positively charged groups, such as alginate or chitosan, respectively. The effect of the amount of clay mineral and of its interaction with each type of biopolymer was analyzed by diverse physicochemical techniques. The resulting bionanocomposites processed as films exhibit not only improved mechanical properties and water resistance, but also biocompatibility and biodegradability, as well as a significant reduction of water absorption, which could make them very attractive for uses in wide variety of applications from bioplastics to adsorbents. In this work it has been also tested the retention properties of these materials towards heavy metal ions in view to enlarge the scope of their applications in the field of environmental remediation. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The interaction of polymers with clay minerals through different mechanisms has been a topic extensively studied in Clay Science (RuizHitzky and Van Meerbeek, 2006; Theng, 1979). Nowadays, the association of clay minerals of different structures and habit (laminar or fibrous) with different polymers attains a great success, being not only a subject of interest for clay scientists but also a topic of huge incidence in the areas of science and technology of polymer-based advanced materials (Avérous and Pollet, 2012; Galimberti, 2011; Lagaly, 1999; Mittal, 2011; Pinnavaia and Beall, 2000). Thus, the resulting materials are more commonly known as polymer–clay nanocomposites (Pinnavaia and Beall, 2000), or clay–polymer nanocomposites usually abbreviated CPN (Bergaya et al., 2013; Carrado and Bergaya, 2007; Lagaly, 1999; RuizHitzky and Van Meerbeek, 2006), being this last term retained in this paper for coherence with the nomenclature recommended from the Handbook of Clay Science (Bergaya et al., 2006). Depending on the clay mineral and its interaction with the polymer, the resultant CPN can exhibit drastic modifications in their properties, as improved mechanical and barrier properties, thermal stability, high transparency, etc., actuating the inorganic counterpart as fillers in the polymer matrix. When the involved polymer is of natural origin, i.e. biopolymers, the resulting ☆ Manuscript recognized with the 2013 AIPEA Bradley Award at the 15th International Clay Conference, Rio de Janeiro 2013. ⁎ Corresponding authors. Tel.: +34 91334900; fax: +34 913720623. E-mail addresses: [email protected] (A.C.S. Alcântara), [email protected] (P. Aranda). 1 Present address: Federal University of Rio Grande do Norte, UFRN, Department of Chemistry, Lagoa Nova, 59072-970, Natal-RN, Brazil.
materials are classed as a new group of hybrids so-called bionanocomposites (Darder et al., 2007; Ruiz-Hitzky et al., 2008). In addition to improved structural features, clay mineral–biopolymer nanocomposites usually offer the biocompatible and biodegradable character associated with the biopolymer, and can be also provided with functional properties of interest in relevant areas as heterogeneous catalysts, chemical sensors and as active components in optical, magnetic and electrochemical devices, and bioplastics among others (Avérous and Pollet, 2012; Darder et al., 2007; Lagarón, 2011; Ruiz-Hitzky et al., 2013). Most of the studies on bionanocomposites are related to polylactic acid (PLA), polycaprolactone (PCL), proteins and polysaccharides, and incorporating layered silicates of the smectite group (Mittal, 2011). However, recent studies show that the use of microfibrous clay minerals such as sepiolite and palygorskite results in an interesting reinforcement of polymer as well as biopolymer matrices in the development of clay–polymer nanocomposites (Chivrac et al., 2010; Fernandes et al., 2009; Fukushima et al., 2010; Ruiz-Hitzky et al., 2011, 2013). Although they do not exhibit intercalation properties, these natural magnesium silicates offer interesting characteristics, such as microporosity and large specific surface area (around 320 and 150 m2/g for sepiolite and palygorskite, respectively), and the presence of OH groups at their external surface which can be functionalized to introduce new properties (Alvarez et al., 2011; Ruiz-Hitzky et al., 2011). In this work, three different polysaccharides (chitosan, starch and alginate) were associated with fibrous sepiolite and palygorskite for the preparation of functional reinforced bionanocomposites. The properties of the chitosan biopolymer assembled to sepiolite were previously explored, revealing their potential use as components of electrochemical sensors (Darder et al., 2006). Chivrac et al. (2010) observed that the
http://dx.doi.org/10.1016/j.clay.2014.02.018 0169-1317/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: Alcântara, A.C.S., et al., Polysaccharide–fibrous clay bionanocomposites, Appl. Clay Sci. (2014), http://dx.doi.org/ 10.1016/j.clay.2014.02.018
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incorporation of sepiolite in a starch matrix results in a higher increase of Young's modulus and stress at break of the reinforced starch films than in bionanocomposites incorporating montmorillonite. The assembly of alginate and palygorskite was previously reported for the preparation of ternary hydrogel systems involving carboxymethyl cellulose-g-poly(acrylic acid), which showed reduced swelling ability and controlled drug release (Wang et al., 2012). These polysaccharides show different charges in their structure (Fig. 1): chitosan derived from chitin is provided with positively charged amino groups; alginate extracted from sea algae, is negatively charged bearing carboxylate groups in its structure; and starch obtained from maize, does not have ionic sites in the polymer backbone (neutral charge). For this reason, it is expected that the assembly of each type of biopolymer to the sepiolite and palygorskite may lead to different interactions and, consequently, to interesting properties of the resulting clay–polysaccharide bionanocomposite materials that have not been explored in the previous works mentioned above. Thus, a detailed study about the possible interactions, the effect of the fibrous silicates content and the resulting properties of these polysaccharide– microfibrous clay materials were investigated here.
2.2. Preparation of fibrous clays–biopolymers nanocomposites A series of clay–biopolymer systems that incorporate different percentages of clay mineral/biopolymer ratio (3, 17, 33 or 50% mass/ mass) were prepared from 2% Sep or Pal aqueous dispersion mixed with the biopolymer solution. Alginate-based bionanocomposites were prepared by mixing the desired amount of clay mineral dispersion with a 2% (mass) aqueous polymeric solution heated at 60 °C, forming a single batch that is kept under constant stirring until complete homogenization. The resulting dispersion was placed onto a glass plate and dried at room temperature. Chitosan-based bionanocomposites were prepared using a similar procedure, except that in this case acetic acid (1% v/v) was used to prepare the polymer solution. Starch-based bionanocomposites were prepared using a 2% (mass) aqueous starch solution prepared at 80 °C. After complete homogenization, glycerol (Sigma-Aldrich) in a starch:glycerol 80:20 (mass/mass) ratio was added to the starch solution together with the clay mineral dispersion, and once homogenized the system films were casted onto a plastic plate and dried at room temperature. 2.3. Characterization
2. Experimental section 2.1. Starting materials and reagents Sepiolite from Vicálvaro (Spain) commercialized as Pangel® S9 (Sep) was provided by TOLSA, S.A., and Brazilian palygorskite (Pal) from Piauí state was kindly provided by Prof. L.S. Barreto (Universidade Federal de Sergipe). A detailed characterization of this palygorskite can be found in previous works reported by Oliveira et al. (2013) and by dos Santos Soares et al. (2013). The biopolymers sodium alginate from brown algae (ALG), chitosan of medium molecular mass and deacetylation degree of ca. 75% (CHT), and starch (STH) from maize were purchased from Sigma-Aldrich. Deionized water (resistivity of 18.2 MΩ cm) was obtained with a Maxima Ultrapure Water from Elga.
Viscosity measurements were performed in a RVDVII+ Pro Brookfield viscometer at 100 rpm and 25 ± 0.5 °C. IR spectra were obtained in a FTIR spectrophotometer BRUKER IFS 66v/S or by means of attenuated total reflectance mode (ATR-IR), using a Shimadzu GladiATR 10 equipment. In both cases, pure samples prepared as self-supporting films were scanned from 4000 to 400 cm−1 with 2 cm−1 and 4 cm−1 of resolution for FTIR and ATR measurements, respectively. Crosssections of the bionanocomposite films were observed in FE-SEM equipment FEI-NOVA NanoSEM 230 in samples directly adhered on a carbon tap without conductive coating on the surface. 2.4. Mechanical properties Tensile modulus (E) of the film samples was evaluated with a Model 3345 Instron Universal Testing Machine (Instron Engineering Corporation). The samples were mounted between the grips with an initial separation of 50 mm, and the cross-head speed was set at 2 mm/min. Three replicates were run for each film. 2.5. Water absorption determination The bionanocomposite films were placed in a Petri dish, immersed in distilled water at room temperature. After 24 h, the films were withdrawn, the excess of water was removed, and then weighed on an analytical balance. The water uptake was calculated from Eq. (1): water absorption ðg=gÞ ¼ W t –W 0 =W t
ð1Þ
where Wt and W0 are the wet and initial mass of films, respectively. 2.6. Uptake of heavy metals A weighed part of each bionanocomposite film (aprox. 12.5 mg) was added as adsorbent to reaction flasks containing 6.25 mL of 4 × 10−3 M aqueous solution of copper and lead ions at pH values of 5.25 and 5.00, respectively. All these systems were maintained at 28 ± 2 °C in an incubator shaker at 100 rpm. After 24 h the supernatant was taken and analyzed by ICP technique (PerkinElmer, Optima 2100 DV model) to determine the amount of metal ions. The amount of metal ions adsorbed per gram of the material was calculated by the difference between the initial and final amounts of ions in each solution by Eq. (2): Fig. 1. Chemical structures of biopolymers used in this work.
adsorbed metal ions ðmg=gÞ ¼ ðC 0 −C e ÞV=W
ð2Þ
Please cite this article as: Alcântara, A.C.S., et al., Polysaccharide–fibrous clay bionanocomposites, Appl. Clay Sci. (2014), http://dx.doi.org/ 10.1016/j.clay.2014.02.018
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where C0 and Ce (mg/L) are the initial and final concentrations, respectively, V (L) is the volume of the heavy metal solution and W (g) is the weight of the film. 3. Results and discussion 3.1. Bionanocomposites characterization Viscosity of the prepared clay–biopolymer dispersions was studied as it depends not only on the chemical structure and molecular mass of the polymer, but also on the possible interactions of the inorganic solid assembled to the polysaccharide matrix. Thus, Fig. 2 collects the results on the rheological characterization of the dispersions of starch (STH), chitosan (CHT) and alginate (ALG) containing 3, 17, 33 and 50% of Sep and Pal. Among all the studied polysaccharides, starch and their bionanocomposites showed the highest viscosity values, around 500 cPs for the materials loaded with 50% of clay minerals (Fig. 2). Dispersions containing Pal (Fig. 2b) afforded lower viscosity values than those including Sep (Fig. 2a). A common behavior for the three biopolymers is that the viscosity values increased linearly with the content of added Pal (Fig. 2b), reaching values between 350 and 500 cPs for the bionanocomposites loaded with 50% of this clay mineral. In contrast, these same values were already reached with a lower amount of added Sep (17%), and then the viscosity slightly rises with increasing amounts of clay mineral up to 50%. As reported by Ristolainen et al. (2006) in clay-PVA viscosity studies, this substantial increase in the viscosity with the addition of clay mineral could indicate the existence
Fig. 2. Evolution of the viscosity of fibrous clay–polysaccharide water dispersions with sepiolite (a) and palygorskite (b).
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of hydrogen bonding interactions between silanol groups on the clay mineral surface and the biopolymer, possibly hydroxyl groups and also carboxyl or amino groups in alginate and chitosan, respectively. Furthermore, the significant increase in viscosity in the dispersions containing Sep could be related to the rheological properties of sepiolite Pangel® S9, since this product considered of rheological degree shows “disagglomerated” microfibers that favor interactions with the polymer chains (Bokobza et al., 2004). In contrast, Pal apparently has not suffered any previous industrial process, and the 50% dispersion has a viscosity of 17.2 cPs, while 50% Sep shows a value of 31.7 cPs. This may suggest that the Pal fibrils could be agglomerated, hindering a good dispersion of the same in the biopolymer matrix, leading to weaker interactions between the polymer and the clay mineral. Such effect is reflected in the final viscosity of their bionanocomposites and could have influence on the physical properties of the resulting films. The IR spectra of the pure biopolymers and the corresponding bionanocomposites loaded with 33% of Sep are shown in Fig. 3. The spectra of the polysaccharides (Fig. 3a) show a strong vibration band at around 1000–1023 cm−1 attributed to C\O stretching vibrations of the C\OH groups present in all the polysaccharides backbone. In starch-based bionanocomposites, the characteristic bands of the bending vibration mode (δHOH) of water molecules in different environments of Sep, typically in the 1655 to 1624 cm−1 region, appear as a unique band centered at 1652 cm−1 that may also involve the vibration band related to the water adsorbed in the amorphous regions of starch (Kizil et al., 2002). In the chitosan-based bionanocomposites, the band at 1574 cm− 1, assigned to the N–H deformation vibrations (δNH3) of the protonated amino groups in the pristine chitosan, is shifted towards 1544 cm−1 indicating electrostatic interactions between the protonated amino groups and the negatively charged sites of Sep fibrils, as previously reported (Darder et al., 2006). In the spectra of the alginate–
Fig. 3. IR spectra of starting polysaccharides in the 4000–500 cm−1 region (a), the bionanocomposites based on their assembly to sepiolite (33% mass/mass clay mineral content) in the 2100–1100 cm−1 region (ATR) (b) and in the 3750–3650 cm−1 region (FTIR) (c).
Please cite this article as: Alcântara, A.C.S., et al., Polysaccharide–fibrous clay bionanocomposites, Appl. Clay Sci. (2014), http://dx.doi.org/ 10.1016/j.clay.2014.02.018
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sepiolite bionanocomposites (Fig. 3b) the band assigned to the asymmetric stretching vibration of \COO− groups, appearing at 1593 cm−1 in the pristine alginate (Fig. 3a), is overlapped by the characteristic bands δHOH of water molecules. This fact complicates the ascription of changes in this spectral zone to the existence of interactions between the polymer and the clay mineral. However, the possible interaction between these carboxylate groups and the Sep cannot be completely discarded, considering that the frequency of the νas(CO) vibration of those groups seems to be shifted towards higher wavenumber (1605 cm− 1) in the bionanocomposites. A relevant characteristic detected in the spectra of all the bionanocomposites is the disappearance of the band at 3720 cm− 1, assigned to the OH stretching vibration of free silanol groups located on the external surface in the pristine Sep (Fig. 3c). An analogous behavior was observed in related hybrid systems (Ahlrichs et al., 1975; Alcântara et al., 2012; Darder et al., 2006; Wicklein et al., 2011) and ascribed to a shift in the band to lower frequencies as a consequence of the interactions of silanol groups typically by hydrogen bonding with the hydrophilic species. In contrast, the band attributed to the stretching OH vibration of Mg\OH groups located inside the block structure of Sep appearing at 3681 cm−1 remains practically unaltered in relative intensity and position in all the bionanocomposites, due to the inaccessibility of the biopolymer chains to the internal structure of the clay mineral. These results are especially relevant as they confirm the existence of interactions between the biopolymer and the clay mineral, being possible that several groups of the biopolymer structure (hydroxyl, carboxyl and amino groups) could be implicated in such interactions. FE-SEM images of the chitosan and starch biopolymers and their bionanocomposites resulting from the incorporation of 17% mass/ mass of Sep or Pal are shown in Fig. 4. In the images of the bionanocomposites, chitosan and starch appear to be fully associated with the silicates, being the clay mineral fibrils well integrated within the biopolymer matrix, presenting a more compact morphology in the starch-based material. The good integration between the silicate fibers and both biopolymers can result in interesting mechanical and barrier properties. In the chitosan-based bionanocomposites, the interaction
of the clay mineral with chitosan appears to induce the formation of a layered arrangement. This behavior can be related to the high tendency of chitosan to be conformed as layers, probably favored by the processing of the materials as films. 3.2. Properties of the bionanocomposite films The influence of the microfibrous clay mineral loading on the mechanical properties of the polysaccharides in bionanocomposite films is displayed in Fig. 5. A reinforcement effect can be clearly observed for both bionanocomposites incorporating Sep and Pal with respect to the starting biopolymers, being the Young's modulus increased with the increase of the filler content. The highest values were found for alginate bionanocomposite films, showing a tensile modulus around 5.2 and 4.8 GPa for the materials loaded with 50% of Sep (Fig. 5a) and Pal (Fig. 5b), respectively. The tensile modulus of chitosan, 1 GPa, was hugely improved with the addition of sepiolite or palygorskite, with an increment of 4.1 and 3.8 times more in the bionanocomposites, respectively. Interestingly, in contrast to the behavior reported for analogous materials based on layered silicates, where the modulus is reduced at high clay mineral loadings (Rhim, 2011), the Young's modulus of the bionanocomposites prepared here continues to increase as the load of fibrous clay mineral increases, even with high loading amounts of 50% (mass/mass ). This is a trend previously observed in other bionanocomposites based on fibrous clay minerals and gelatin (Fernandes et al., 2009), chitosan (Darder et al., 2006), starch (Chivrac et al., 2010), and PLA and PCL (Fukushima et al., 2010). This salient feature has been attributed to the existence of strong interactions at the nanometer range between both components of the bionanocomposite, i.e. the fibrous clay minerals and the biopolymer chains (Bilotti et al., 2009; Chivrac et al., 2010; Fernandes et al., 2009). Mechanical properties of bionanocomposites containing Sep are better than those containing Pal, independently of the clay mineral content or the involved polysaccharide. This fact can be narrowly related to the higher external surface area available in Sep (150 m2 g− 1) for
Fig. 4. FE-SEM images of the chitosan (a) and starch (d) films, and their respective bionanocomposites containing 17% of fibrous clay minerals: CHT-Sep (b), CHT-Pal (c), STH-Sep (e) and STH-Pal (f).
Please cite this article as: Alcântara, A.C.S., et al., Polysaccharide–fibrous clay bionanocomposites, Appl. Clay Sci. (2014), http://dx.doi.org/ 10.1016/j.clay.2014.02.018
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Fig. 6. Water resistance and macroscopic appearance of swollen chitosan film and their bionanocomposite films loaded with 3% and 50% of sepiolite after the water absorption assay.
Fig. 5. Evolution of the tensile modulus (E) with the increase in sepiolite (a) and palygorskite (b) content in alginate, chitosan and starch based bionanocomposites.
interactions with the polymer matrix compared with Pal (120 m2 g−1) (Ruiz-Hitzky et al., 2011), and also to the degree of dispersion of the individual clay mineral fiber in the polymer matrix. As discussed above, the clay–polysaccharide dispersions containing Sep showed higher values of viscosity compared to those containing Pal, attributed to a more effective clay–biopolymer interaction, which is later reflected in the better mechanical properties of the sepiolite-based bionanocomposites. Actually, needle-like shape and high specific surface area are factors often used to explain the reinforcement efficiency in materials involving fibrous clay minerals as filler in comparison to those based on the most classical smectites (e.g. montmorillonite) (Bilotti et al., 2009; Xue et al., 2006). In fact, smectites present low external surface area (N 10 m2 g−1) and they need to undergo exfoliation to reach the maximum surface for interaction, which sometimes is not possible and originates weakness of the mechanical properties at high level of loading. The control of the water absorption in biopolymer-based materials is a key challenge, since these polymers are normally formed by hydrophilic chains very sensitive to water. It was not possible to determine the water absorption of unmodified biopolymer films, because they underwent a fast swelling and disintegrated after a short contact time with the solvent (Fig. 6). However, the incorporation of Sep and Pal in the biopolymer matrix significantly improved the water resistance, maintaining its integrity during and after the measurement. Water absorption of the bionanocomposites as a function of the clay mineral content is displayed in Fig. 7. In all cases, the water absorption properties were clearly influenced by the clay mineral content, showing a decrease
Fig. 7. Water uptake values of bionanocomposite films containing different amounts of sepiolite (a) and palygorskite (b) fibrous clay minerals exposed to deionized water during 24 h.
Please cite this article as: Alcântara, A.C.S., et al., Polysaccharide–fibrous clay bionanocomposites, Appl. Clay Sci. (2014), http://dx.doi.org/ 10.1016/j.clay.2014.02.018
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of water uptake as the filler loading increased. Alginate and starch based films exhibit good stability in water and the effect of clay mineral content is not as significant as in chitosan-based bionanocomposites. For these last materials, a reduction in water uptake of approximately 32.7% and 7.7% is observed in films with 17% of Sep and Pal, respectively, in comparison to films containing only 3% of clay mineral. Moreover, these values can be further reduced to more than 68% and 57% for Sep and Pal, respectively, for clay mineral content of 50% in the chitosan matrix. Again the great stability to water and reduced water uptake in the bionanocomposites can be related to the interactions between the filler and the biopolymer matrix, reducing the number of available OH groups in the polysaccharide chains, and consequently decreasing the sites for incorporation of water molecules. The improved mechanical properties and the resistance to water achieved with the assembly of these fibrous clay minerals to polysaccharide matrices make them interesting fillers with a good low cost–benefit ratio. 3.3. Uptake of heavy metals in aqueous media Adsorption processes are generally known to be one of the most effective techniques for the removal of environmentally hazardous metals. Diverse bionanocomposite materials have been recently evaluated for the uptake of different pollutants (Liu et al., 2010; Zou et al., 2011). Bionanocomposites with 33% mass/mass in Sep and Pal were selected for evaluation in the retention of heavy metals in aqueous solution due to their great water stability and good mechanical properties. Thereby, films of these bionanocomposites were tested in the
adsorption of Pb(II) and Cu(II) ions following batch experiments. Although starch–sepiolite films present also good water stability, they were not stable in the heavy metals solutions, undergoing complete disintegration and making the measurements impossible. The other bionanocomposite systems are more stable and the uptake of heavy metal ions by neat alginate and chitosan as well as their based bionanocomposites are displayed in Fig. 8. A general observation is that the adsorption of the metal ions is affected by the presence of the clay mineral, occurring a clear diminution in the adsorption capacity of the bionanocomposites compared to the biopolymers alone, although bionanocomposites show higher stability in aqueous media. The carboxyl groups of alginate chains can be involved in the adsorption of the metal ions. As part of the carboxyl groups are in interaction with the silicate, as previously discussed, the availability of carboxyl groups in the bionanocomposites is lower and consequently their removal capacity decreases compared to alginate. In spite of this, the effectiveness of fibrous clay–alginate bionanocomposites is higher than that of bionanocomposites based on layered silicates, as reported for the uptake of lead (Shawky, 2011) and copper (Ely et al., 2009). Chitosan and its derived bio-hybrids present very similar amounts of adsorbed copper ions, with values around 110 mg/g. The mechanism of copper adsorption by chitosan is well known, being related to the ability of the amino groups in the polymeric structure to form coordination complexes with these ions (Varma et al., 2004). When Sep or Pal is added to the biopolymer matrix only a fraction of the amino groups interact electrostatically with the fibers, while an excess of amino groups remains available for other interactions (Darder et al., 2006). As a result, these amino groups can form chelate compounds with the copper ions, presenting its bionanocomposites a similar behavior to that of pure chitosan. In lead adsorption experiments (Fig. 8a), the importance of the clay–chitosan interaction in the bio-hybrid films is remarkable, as unmodified chitosan films were totally disintegrated in the Pb(II) solutions while the bionanocomposites showed a great stability. This behavior could be associated with the unfavorable acid media of the lead solution for the pure chitosan films, together with the different interaction of this polysaccharide with Pb(II) in comparison to Cu(II) ions. Thus, the bionanocomposite films present very similar adsorption capacities, independent of the filler employed, being able to remove lead ions around 100 mg/g. The results of heavy metal adsorption presented in this work are satisfactory and comparable with other bionanocomposite systems, allowing their application as biosorbents for environmental remediation. 4. Concluding remarks The systematic study carried out shows the possibility to prepare bionanocomposites based on the assembly of fibrous clay minerals with polysaccharides of neutral character as well as positively or negatively charged. The interaction mechanism between the polysaccharides and the clay minerals occurs mainly through the interaction of the OH groups in the biopolymers backbone and the silanol groups on the silicate surface. In the case of alginate and chitosan the presence of other functionalities, carboxylate and amino groups, respectively, may be also implicated. The existence of strong interactions between both components of the bionanocomposites results in materials provided with good mechanical properties, improved water resistance and reduction of water absorption in comparison to biopolymer alone, which make them very attractive for a wide range of applications. These films also offer interesting results for the retention of heavy metal ions such as copper and lead, which together with the biocompatibility and biodegradability afforded by the biopolymer may enlarge the scope of applications of these bionanocomposite materials. Acknowledgments
Fig. 8. Uptake of Cu(II) and Pb(II) ions in polysaccharide films and their bionanocomposites with 33% of clay mineral. (The adsorption of Pb2+ on the CHT films could not be determined due to the instability of this film in this solution).
This work was supported by the CICYT (Spain) project MAT201231759 and the UE COST Programme (project MP1202). A.C.S.A.
Please cite this article as: Alcântara, A.C.S., et al., Polysaccharide–fibrous clay bionanocomposites, Appl. Clay Sci. (2014), http://dx.doi.org/ 10.1016/j.clay.2014.02.018
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acknowledges CSIC for a JAE-Predoc fellowship. The authors also thank Mr. A. Valera and Mr. C. Sebastián for their technical assistance in the FESEM study. References Ahlrichs, J.L., Serna, C., Serratosa, J.M., 1975. Structural hydroxyls in sepiolite. Clay Clay Miner. 23, 119–124. Alcântara, A.C.S., Darder, M., Aranda, P., Ruiz-Hitzky, E., 2012. Zein-fibrous clays biohybrid materials. Eur. J. Inorg. Chem. 5216–5522. Alvarez, A., Santarén, J., Esteban-Cubillo, A., Aparicio, P., 2011. Current industrial applications of palygorskite and sepiolite. In: Galán, E., Singer, A. (Eds.), Developments in Palygorskite-Sepiolite Research. A New Outlook of These Nanomaterials. Elsevier B.V., Oxford, pp. 281–298. Avérous, L., Pollet, E., 2012. Environmental Silicate Nano-Biocomposites. Springer-Verlag, London. Bergaya, F., Theng, B.K.G., Lagaly, G., 2006. Handbook of Clay Science. Elsevier, Oxford. Bergaya, F., Detellier, C., Lambert, J.-F., Lagaly, G., 2013. Introduction to clay–polymer nanocomposites (CPN), In: Bergaya, F., Lagaly, G. (Eds.), Handbook of Clay Science, Fundamentals, second edition. Developments in Clay Science, Vol5A. Elsevier, Oxford. Bilotti, E., Zhang, R., Deng, H., Quero, F., Fischer, H.R., Peijs, T., 2009. Sepiolite needle-like Clay for PA6 nanocomposites: an alternative to layered silicates? Compos. Sci. Technol. 69, 2587–2595. Bokobza, L., Burr, A., Garnaud, G., Perrin, M.Y., Pagnotta, S., 2004. Fibre reinforcement of elastomers: nanocomposites based on sepiolite and poly(hydroxyethyl acrylate). Polym. Int. 53, 1060–1065. Carrado, K.A., Bergaya, F. (Eds.), 2007. Clay based Polymer Nanocomposites (CPN), in the CMS Workshop Lecture, vol. 15. The Clay Minerals Society, Chantilly (VA). Chivrac, F., Pollet, E., Schmutz, M., Averous, L., 2010. Starch nano-biocomposites based on needle-like sepiolite clays. Carbohydr. Polym. 80, 145–153. Darder, M., López-Blanco, M., Aranda, P., Aznar, A.J., Bravo, J., Ruiz-Hitzky, E., 2006. Microfibrous chitosan-sepiolite nanocomposites. Chem. Mater. 18, 1602–1610. Darder, M., Aranda, P., Ruiz-Hitzky, E., 2007. Bionanocomposites: a new concept of ecological, bioinspired, and functional hybrid materials. Adv. Mater. 19, 1309–1319. dos Santos Soares, D., Santana Fernandes, C., da Costa, A.C.S., Nervo Raffin, F., Acchar, W., de Lima e Moura, T.F.A., 2013. Characterization of palygorskite clay from Piauí, Brazil and its potential use as excipient for solid dosage forms containing anti-tuberculosis drugs. J. Therm. Anal. Calorim. 113, 551–558. Ely, A., Baudu, M., Basly, J.-P., Kankou, M.O.S.A.O., 2009. Copper and nitrophenol pollutants removal by Na-montmorillonite/alginate microcapsules. J. Hazard. Mater. 171, 405–409. Fernandes, F.M., Ruiz, A.I., Darder, M., Aranda, P., Ruiz-Hitzky, E., 2009. Gelatin-clay bionanocomposites: structural and functional properties as advanced materials. J. Nanosci. Nanotechnol. 9, 221–229. Fukushima, K., Tabuani, D., Abbate, C., Arena, M., Ferreri, L., 2010. Effect of sepiolite on the biodegradation of poly(lactic acid) and polycaprolactone. Polym. Degrad. Stab. 95, 2049–2056. Galimberti, M. (Ed.), 2011. Rubber–Clay Nanocomposites: Science, Technology and Applications. John Wiley & Sons. Kizil, R., Irudayaraj, J., Seetharaman, K., 2002. Characterization of irradiated starches by using FT-Raman and FTIR spectroscopy. J. Agric. Food Chem. 50, 3912–3918.
7
Lagaly, G., 1999. Introduction: from clay mineral–polymer interactions to clay mineral– polymer nanocomposites. Appl. Clay Sci. 15, 1–9. Lagarón, J.M., 2011. Multifunctional and Nanoreinforced Polymers for Food Packaging. Woodhead Publising Ltd., Cambridge. Liu, Y., Wang, W., Wang, A., 2010. Adsorption of lead ions from aqueous solution by using carboxymethylcellulose-g-poly(acrylic acid)/attapulgite hydrogel composites. Desalination 259, 258–264. Mittal, V., 2011. Nanocomposites with biodegradable polymers. Synthesis, properties and future perspectives, Oxford University Press, New York. Oliveira, R.N., Acchar, W., Soares, G.D.A., Barreto, L.S., 2013. The increase of surface area of a brazilian palygorskite clay activated with sulfuric acid solutions using a factorial design. Mater. Res. 16, 924–928. Pinnavaia, T.J., Beall, G.W. (Eds.), 2000. Polymer–clay nanocomposites. John Wiley & Sons, West Sussex. Rhim, J.-R., 2011. Effect of clay contents on mechanical and water vapor barrier properties of agar-based nanocomposite films. Carbohydr. Polym. 86, 91–699. Ristolainen, N., Heikkilä, P., Harlin, A., Seppälä, J., 2006. Poly(vinyl alcohol) and polyamide-66 nanocomposites prepared by electrospinning. Macromol. Mater. Eng. 291, 114–122. Ruiz-Hitzky, E., Van Meerbeek, A., 2006. Clay mineral- and organoclay–polymer nanocomposite. In: Bergaya, F., Theng, B.K.G., Lagaly, G. (Eds.), Handbook of Clay Science. Elsevier, Oxford, pp. 583–621. Ruiz-Hitzky, E., Aranda, P., Darder, M., 2008. Bionanocomposites. Kirk Othmer Encyclopedia of Chemical Technology. John Wiley Sons, Inc., pp. 1–28. http://dx.doi.org/ 10.1002/0471238961.bionruiz.a01/abstract. Ruiz-Hitzky, E., Aranda, P., Alvarez, A., Santarén, J., Esteban-Cubillo, A., 2011. Advanced materials and new applications of sepiolite and palygorskite. In: Galán, E., Singer, A. (Eds.), Developments in Palygorskite-Sepiolite Research. A New Outlook of These Nanomaterials. Elsevier B.V., Oxford, pp. 393–452. Ruiz-Hitzky, E., Darder, M., Fernandes, F.M., Wicklein, B., Alcântara, A.C.S., Aranda, P., 2013. Fibrous clays based bionanocomposites. Prog. Polym. Sci. 38, 1392–1414. Shawky, H.A., 2011. Improvement of water quality using alginate/montmorillonite composite beads. J. Appl. Polym. Sci. 119, 2371–2378. Theng, B.K.G., 1979. Formation and properties of clay–polymer complexes. Elsevier, New York. Varma, A.J., Deshpande, S.V., Kennedy, J.F., 2004. Metal complexation by chitosan and its derivatives: a review. Carbohyd. Polym. 55, 77–93. Wang, Q., Wang, W., Wu, J., Wang, A., 2012. Effect of attapulgite contents on release behaviors of a pH sensitive carboxymethylcellulose-g-poly(acrylic acid)/attapulgite/ sodium alginate composite hydrogel bead containing diclofenac. J. Appl. Polym. Sci. 124 (6), 4424–4432. Wicklein, B., Darder, M., Aranda, P., Ruiz-Hitzky, E., 2011. Phospholipid-sepiolite biomimetic interfaces for the immobilization of enzymes. ACS Appl. Mater. Interfaces 3, 4339–4348. Xue, S., Reinholdt, M., Pinnavaia, T.J., 2006. Palygorskite as an epoxy polymer reinforcement agent. Polymer 47, 3344–3350. Zou, X., Pan, J., Ou, H., Wang, X., Guan, W., Li, C., Yan, Y., Duan, Y., 2011. Adsorptive removal of Cr(III) and Fe(III) from aqueous solution by chitosan/attapulgite composites: equilibrium, thermodynamics and kinetics. Chem. Eng. J. 167, 112–121.
Please cite this article as: Alcântara, A.C.S., et al., Polysaccharide–fibrous clay bionanocomposites, Appl. Clay Sci. (2014), http://dx.doi.org/ 10.1016/j.clay.2014.02.018
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Research paper
Polysaccharide–fibrous clay bionanocomposites☆ Ana C.S. Alcântara 1,⁎, Margarita Darder, Pilar Aranda ⁎, Eduardo Ruiz-Hitzky Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 17 June 2013 Received in revised form 18 February 2014 Accepted 20 February 2014 Available online xxxx Keywords: Sepiolite Palygorskite Bionanocomposites Polysaccharides Biosorbents Heavy metal adsorption
a b s t r a c t The present paper introduces results on the preparation, characterization and exploration of properties of bionanocomposites prepared by the assembly of polysaccharides to sepiolite and palygorskite. The effect of the incorporation of these fibrous clay minerals was analyzed in three types of polysaccharide matrices: neutral, such as starch, and provided with either negatively or positively charged groups, such as alginate or chitosan, respectively. The effect of the amount of clay mineral and of its interaction with each type of biopolymer was analyzed by diverse physicochemical techniques. The resulting bionanocomposites processed as films exhibit not only improved mechanical properties and water resistance, but also biocompatibility and biodegradability, as well as a significant reduction of water absorption, which could make them very attractive for uses in wide variety of applications from bioplastics to adsorbents. In this work it has been also tested the retention properties of these materials towards heavy metal ions in view to enlarge the scope of their applications in the field of environmental remediation. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The interaction of polymers with clay minerals through different mechanisms has been a topic extensively studied in Clay Science (RuizHitzky and Van Meerbeek, 2006; Theng, 1979). Nowadays, the association of clay minerals of different structures and habit (laminar or fibrous) with different polymers attains a great success, being not only a subject of interest for clay scientists but also a topic of huge incidence in the areas of science and technology of polymer-based advanced materials (Avérous and Pollet, 2012; Galimberti, 2011; Lagaly, 1999; Mittal, 2011; Pinnavaia and Beall, 2000). Thus, the resulting materials are more commonly known as polymer–clay nanocomposites (Pinnavaia and Beall, 2000), or clay–polymer nanocomposites usually abbreviated CPN (Bergaya et al., 2013; Carrado and Bergaya, 2007; Lagaly, 1999; RuizHitzky and Van Meerbeek, 2006), being this last term retained in this paper for coherence with the nomenclature recommended from the Handbook of Clay Science (Bergaya et al., 2006). Depending on the clay mineral and its interaction with the polymer, the resultant CPN can exhibit drastic modifications in their properties, as improved mechanical and barrier properties, thermal stability, high transparency, etc., actuating the inorganic counterpart as fillers in the polymer matrix. When the involved polymer is of natural origin, i.e. biopolymers, the resulting ☆ Manuscript recognized with the 2013 AIPEA Bradley Award at the 15th International Clay Conference, Rio de Janeiro 2013. ⁎ Corresponding authors. Tel.: +34 91334900; fax: +34 913720623. E-mail addresses: [email protected] (A.C.S. Alcântara), [email protected] (P. Aranda). 1 Present address: Federal University of Rio Grande do Norte, UFRN, Department of Chemistry, Lagoa Nova, 59072-970, Natal-RN, Brazil.
materials are classed as a new group of hybrids so-called bionanocomposites (Darder et al., 2007; Ruiz-Hitzky et al., 2008). In addition to improved structural features, clay mineral–biopolymer nanocomposites usually offer the biocompatible and biodegradable character associated with the biopolymer, and can be also provided with functional properties of interest in relevant areas as heterogeneous catalysts, chemical sensors and as active components in optical, magnetic and electrochemical devices, and bioplastics among others (Avérous and Pollet, 2012; Darder et al., 2007; Lagarón, 2011; Ruiz-Hitzky et al., 2013). Most of the studies on bionanocomposites are related to polylactic acid (PLA), polycaprolactone (PCL), proteins and polysaccharides, and incorporating layered silicates of the smectite group (Mittal, 2011). However, recent studies show that the use of microfibrous clay minerals such as sepiolite and palygorskite results in an interesting reinforcement of polymer as well as biopolymer matrices in the development of clay–polymer nanocomposites (Chivrac et al., 2010; Fernandes et al., 2009; Fukushima et al., 2010; Ruiz-Hitzky et al., 2011, 2013). Although they do not exhibit intercalation properties, these natural magnesium silicates offer interesting characteristics, such as microporosity and large specific surface area (around 320 and 150 m2/g for sepiolite and palygorskite, respectively), and the presence of OH groups at their external surface which can be functionalized to introduce new properties (Alvarez et al., 2011; Ruiz-Hitzky et al., 2011). In this work, three different polysaccharides (chitosan, starch and alginate) were associated with fibrous sepiolite and palygorskite for the preparation of functional reinforced bionanocomposites. The properties of the chitosan biopolymer assembled to sepiolite were previously explored, revealing their potential use as components of electrochemical sensors (Darder et al., 2006). Chivrac et al. (2010) observed that the
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Please cite this article as: Alcântara, A.C.S., et al., Polysaccharide–fibrous clay bionanocomposites, Appl. Clay Sci. (2014), http://dx.doi.org/ 10.1016/j.clay.2014.02.018
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incorporation of sepiolite in a starch matrix results in a higher increase of Young's modulus and stress at break of the reinforced starch films than in bionanocomposites incorporating montmorillonite. The assembly of alginate and palygorskite was previously reported for the preparation of ternary hydrogel systems involving carboxymethyl cellulose-g-poly(acrylic acid), which showed reduced swelling ability and controlled drug release (Wang et al., 2012). These polysaccharides show different charges in their structure (Fig. 1): chitosan derived from chitin is provided with positively charged amino groups; alginate extracted from sea algae, is negatively charged bearing carboxylate groups in its structure; and starch obtained from maize, does not have ionic sites in the polymer backbone (neutral charge). For this reason, it is expected that the assembly of each type of biopolymer to the sepiolite and palygorskite may lead to different interactions and, consequently, to interesting properties of the resulting clay–polysaccharide bionanocomposite materials that have not been explored in the previous works mentioned above. Thus, a detailed study about the possible interactions, the effect of the fibrous silicates content and the resulting properties of these polysaccharide– microfibrous clay materials were investigated here.
2.2. Preparation of fibrous clays–biopolymers nanocomposites A series of clay–biopolymer systems that incorporate different percentages of clay mineral/biopolymer ratio (3, 17, 33 or 50% mass/ mass) were prepared from 2% Sep or Pal aqueous dispersion mixed with the biopolymer solution. Alginate-based bionanocomposites were prepared by mixing the desired amount of clay mineral dispersion with a 2% (mass) aqueous polymeric solution heated at 60 °C, forming a single batch that is kept under constant stirring until complete homogenization. The resulting dispersion was placed onto a glass plate and dried at room temperature. Chitosan-based bionanocomposites were prepared using a similar procedure, except that in this case acetic acid (1% v/v) was used to prepare the polymer solution. Starch-based bionanocomposites were prepared using a 2% (mass) aqueous starch solution prepared at 80 °C. After complete homogenization, glycerol (Sigma-Aldrich) in a starch:glycerol 80:20 (mass/mass) ratio was added to the starch solution together with the clay mineral dispersion, and once homogenized the system films were casted onto a plastic plate and dried at room temperature. 2.3. Characterization
2. Experimental section 2.1. Starting materials and reagents Sepiolite from Vicálvaro (Spain) commercialized as Pangel® S9 (Sep) was provided by TOLSA, S.A., and Brazilian palygorskite (Pal) from Piauí state was kindly provided by Prof. L.S. Barreto (Universidade Federal de Sergipe). A detailed characterization of this palygorskite can be found in previous works reported by Oliveira et al. (2013) and by dos Santos Soares et al. (2013). The biopolymers sodium alginate from brown algae (ALG), chitosan of medium molecular mass and deacetylation degree of ca. 75% (CHT), and starch (STH) from maize were purchased from Sigma-Aldrich. Deionized water (resistivity of 18.2 MΩ cm) was obtained with a Maxima Ultrapure Water from Elga.
Viscosity measurements were performed in a RVDVII+ Pro Brookfield viscometer at 100 rpm and 25 ± 0.5 °C. IR spectra were obtained in a FTIR spectrophotometer BRUKER IFS 66v/S or by means of attenuated total reflectance mode (ATR-IR), using a Shimadzu GladiATR 10 equipment. In both cases, pure samples prepared as self-supporting films were scanned from 4000 to 400 cm−1 with 2 cm−1 and 4 cm−1 of resolution for FTIR and ATR measurements, respectively. Crosssections of the bionanocomposite films were observed in FE-SEM equipment FEI-NOVA NanoSEM 230 in samples directly adhered on a carbon tap without conductive coating on the surface. 2.4. Mechanical properties Tensile modulus (E) of the film samples was evaluated with a Model 3345 Instron Universal Testing Machine (Instron Engineering Corporation). The samples were mounted between the grips with an initial separation of 50 mm, and the cross-head speed was set at 2 mm/min. Three replicates were run for each film. 2.5. Water absorption determination The bionanocomposite films were placed in a Petri dish, immersed in distilled water at room temperature. After 24 h, the films were withdrawn, the excess of water was removed, and then weighed on an analytical balance. The water uptake was calculated from Eq. (1): water absorption ðg=gÞ ¼ W t –W 0 =W t
ð1Þ
where Wt and W0 are the wet and initial mass of films, respectively. 2.6. Uptake of heavy metals A weighed part of each bionanocomposite film (aprox. 12.5 mg) was added as adsorbent to reaction flasks containing 6.25 mL of 4 × 10−3 M aqueous solution of copper and lead ions at pH values of 5.25 and 5.00, respectively. All these systems were maintained at 28 ± 2 °C in an incubator shaker at 100 rpm. After 24 h the supernatant was taken and analyzed by ICP technique (PerkinElmer, Optima 2100 DV model) to determine the amount of metal ions. The amount of metal ions adsorbed per gram of the material was calculated by the difference between the initial and final amounts of ions in each solution by Eq. (2): Fig. 1. Chemical structures of biopolymers used in this work.
adsorbed metal ions ðmg=gÞ ¼ ðC 0 −C e ÞV=W
ð2Þ
Please cite this article as: Alcântara, A.C.S., et al., Polysaccharide–fibrous clay bionanocomposites, Appl. Clay Sci. (2014), http://dx.doi.org/ 10.1016/j.clay.2014.02.018
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where C0 and Ce (mg/L) are the initial and final concentrations, respectively, V (L) is the volume of the heavy metal solution and W (g) is the weight of the film. 3. Results and discussion 3.1. Bionanocomposites characterization Viscosity of the prepared clay–biopolymer dispersions was studied as it depends not only on the chemical structure and molecular mass of the polymer, but also on the possible interactions of the inorganic solid assembled to the polysaccharide matrix. Thus, Fig. 2 collects the results on the rheological characterization of the dispersions of starch (STH), chitosan (CHT) and alginate (ALG) containing 3, 17, 33 and 50% of Sep and Pal. Among all the studied polysaccharides, starch and their bionanocomposites showed the highest viscosity values, around 500 cPs for the materials loaded with 50% of clay minerals (Fig. 2). Dispersions containing Pal (Fig. 2b) afforded lower viscosity values than those including Sep (Fig. 2a). A common behavior for the three biopolymers is that the viscosity values increased linearly with the content of added Pal (Fig. 2b), reaching values between 350 and 500 cPs for the bionanocomposites loaded with 50% of this clay mineral. In contrast, these same values were already reached with a lower amount of added Sep (17%), and then the viscosity slightly rises with increasing amounts of clay mineral up to 50%. As reported by Ristolainen et al. (2006) in clay-PVA viscosity studies, this substantial increase in the viscosity with the addition of clay mineral could indicate the existence
Fig. 2. Evolution of the viscosity of fibrous clay–polysaccharide water dispersions with sepiolite (a) and palygorskite (b).
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of hydrogen bonding interactions between silanol groups on the clay mineral surface and the biopolymer, possibly hydroxyl groups and also carboxyl or amino groups in alginate and chitosan, respectively. Furthermore, the significant increase in viscosity in the dispersions containing Sep could be related to the rheological properties of sepiolite Pangel® S9, since this product considered of rheological degree shows “disagglomerated” microfibers that favor interactions with the polymer chains (Bokobza et al., 2004). In contrast, Pal apparently has not suffered any previous industrial process, and the 50% dispersion has a viscosity of 17.2 cPs, while 50% Sep shows a value of 31.7 cPs. This may suggest that the Pal fibrils could be agglomerated, hindering a good dispersion of the same in the biopolymer matrix, leading to weaker interactions between the polymer and the clay mineral. Such effect is reflected in the final viscosity of their bionanocomposites and could have influence on the physical properties of the resulting films. The IR spectra of the pure biopolymers and the corresponding bionanocomposites loaded with 33% of Sep are shown in Fig. 3. The spectra of the polysaccharides (Fig. 3a) show a strong vibration band at around 1000–1023 cm−1 attributed to C\O stretching vibrations of the C\OH groups present in all the polysaccharides backbone. In starch-based bionanocomposites, the characteristic bands of the bending vibration mode (δHOH) of water molecules in different environments of Sep, typically in the 1655 to 1624 cm−1 region, appear as a unique band centered at 1652 cm−1 that may also involve the vibration band related to the water adsorbed in the amorphous regions of starch (Kizil et al., 2002). In the chitosan-based bionanocomposites, the band at 1574 cm− 1, assigned to the N–H deformation vibrations (δNH3) of the protonated amino groups in the pristine chitosan, is shifted towards 1544 cm−1 indicating electrostatic interactions between the protonated amino groups and the negatively charged sites of Sep fibrils, as previously reported (Darder et al., 2006). In the spectra of the alginate–
Fig. 3. IR spectra of starting polysaccharides in the 4000–500 cm−1 region (a), the bionanocomposites based on their assembly to sepiolite (33% mass/mass clay mineral content) in the 2100–1100 cm−1 region (ATR) (b) and in the 3750–3650 cm−1 region (FTIR) (c).
Please cite this article as: Alcântara, A.C.S., et al., Polysaccharide–fibrous clay bionanocomposites, Appl. Clay Sci. (2014), http://dx.doi.org/ 10.1016/j.clay.2014.02.018
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sepiolite bionanocomposites (Fig. 3b) the band assigned to the asymmetric stretching vibration of \COO− groups, appearing at 1593 cm−1 in the pristine alginate (Fig. 3a), is overlapped by the characteristic bands δHOH of water molecules. This fact complicates the ascription of changes in this spectral zone to the existence of interactions between the polymer and the clay mineral. However, the possible interaction between these carboxylate groups and the Sep cannot be completely discarded, considering that the frequency of the νas(CO) vibration of those groups seems to be shifted towards higher wavenumber (1605 cm− 1) in the bionanocomposites. A relevant characteristic detected in the spectra of all the bionanocomposites is the disappearance of the band at 3720 cm− 1, assigned to the OH stretching vibration of free silanol groups located on the external surface in the pristine Sep (Fig. 3c). An analogous behavior was observed in related hybrid systems (Ahlrichs et al., 1975; Alcântara et al., 2012; Darder et al., 2006; Wicklein et al., 2011) and ascribed to a shift in the band to lower frequencies as a consequence of the interactions of silanol groups typically by hydrogen bonding with the hydrophilic species. In contrast, the band attributed to the stretching OH vibration of Mg\OH groups located inside the block structure of Sep appearing at 3681 cm−1 remains practically unaltered in relative intensity and position in all the bionanocomposites, due to the inaccessibility of the biopolymer chains to the internal structure of the clay mineral. These results are especially relevant as they confirm the existence of interactions between the biopolymer and the clay mineral, being possible that several groups of the biopolymer structure (hydroxyl, carboxyl and amino groups) could be implicated in such interactions. FE-SEM images of the chitosan and starch biopolymers and their bionanocomposites resulting from the incorporation of 17% mass/ mass of Sep or Pal are shown in Fig. 4. In the images of the bionanocomposites, chitosan and starch appear to be fully associated with the silicates, being the clay mineral fibrils well integrated within the biopolymer matrix, presenting a more compact morphology in the starch-based material. The good integration between the silicate fibers and both biopolymers can result in interesting mechanical and barrier properties. In the chitosan-based bionanocomposites, the interaction
of the clay mineral with chitosan appears to induce the formation of a layered arrangement. This behavior can be related to the high tendency of chitosan to be conformed as layers, probably favored by the processing of the materials as films. 3.2. Properties of the bionanocomposite films The influence of the microfibrous clay mineral loading on the mechanical properties of the polysaccharides in bionanocomposite films is displayed in Fig. 5. A reinforcement effect can be clearly observed for both bionanocomposites incorporating Sep and Pal with respect to the starting biopolymers, being the Young's modulus increased with the increase of the filler content. The highest values were found for alginate bionanocomposite films, showing a tensile modulus around 5.2 and 4.8 GPa for the materials loaded with 50% of Sep (Fig. 5a) and Pal (Fig. 5b), respectively. The tensile modulus of chitosan, 1 GPa, was hugely improved with the addition of sepiolite or palygorskite, with an increment of 4.1 and 3.8 times more in the bionanocomposites, respectively. Interestingly, in contrast to the behavior reported for analogous materials based on layered silicates, where the modulus is reduced at high clay mineral loadings (Rhim, 2011), the Young's modulus of the bionanocomposites prepared here continues to increase as the load of fibrous clay mineral increases, even with high loading amounts of 50% (mass/mass ). This is a trend previously observed in other bionanocomposites based on fibrous clay minerals and gelatin (Fernandes et al., 2009), chitosan (Darder et al., 2006), starch (Chivrac et al., 2010), and PLA and PCL (Fukushima et al., 2010). This salient feature has been attributed to the existence of strong interactions at the nanometer range between both components of the bionanocomposite, i.e. the fibrous clay minerals and the biopolymer chains (Bilotti et al., 2009; Chivrac et al., 2010; Fernandes et al., 2009). Mechanical properties of bionanocomposites containing Sep are better than those containing Pal, independently of the clay mineral content or the involved polysaccharide. This fact can be narrowly related to the higher external surface area available in Sep (150 m2 g− 1) for
Fig. 4. FE-SEM images of the chitosan (a) and starch (d) films, and their respective bionanocomposites containing 17% of fibrous clay minerals: CHT-Sep (b), CHT-Pal (c), STH-Sep (e) and STH-Pal (f).
Please cite this article as: Alcântara, A.C.S., et al., Polysaccharide–fibrous clay bionanocomposites, Appl. Clay Sci. (2014), http://dx.doi.org/ 10.1016/j.clay.2014.02.018
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Fig. 6. Water resistance and macroscopic appearance of swollen chitosan film and their bionanocomposite films loaded with 3% and 50% of sepiolite after the water absorption assay.
Fig. 5. Evolution of the tensile modulus (E) with the increase in sepiolite (a) and palygorskite (b) content in alginate, chitosan and starch based bionanocomposites.
interactions with the polymer matrix compared with Pal (120 m2 g−1) (Ruiz-Hitzky et al., 2011), and also to the degree of dispersion of the individual clay mineral fiber in the polymer matrix. As discussed above, the clay–polysaccharide dispersions containing Sep showed higher values of viscosity compared to those containing Pal, attributed to a more effective clay–biopolymer interaction, which is later reflected in the better mechanical properties of the sepiolite-based bionanocomposites. Actually, needle-like shape and high specific surface area are factors often used to explain the reinforcement efficiency in materials involving fibrous clay minerals as filler in comparison to those based on the most classical smectites (e.g. montmorillonite) (Bilotti et al., 2009; Xue et al., 2006). In fact, smectites present low external surface area (N 10 m2 g−1) and they need to undergo exfoliation to reach the maximum surface for interaction, which sometimes is not possible and originates weakness of the mechanical properties at high level of loading. The control of the water absorption in biopolymer-based materials is a key challenge, since these polymers are normally formed by hydrophilic chains very sensitive to water. It was not possible to determine the water absorption of unmodified biopolymer films, because they underwent a fast swelling and disintegrated after a short contact time with the solvent (Fig. 6). However, the incorporation of Sep and Pal in the biopolymer matrix significantly improved the water resistance, maintaining its integrity during and after the measurement. Water absorption of the bionanocomposites as a function of the clay mineral content is displayed in Fig. 7. In all cases, the water absorption properties were clearly influenced by the clay mineral content, showing a decrease
Fig. 7. Water uptake values of bionanocomposite films containing different amounts of sepiolite (a) and palygorskite (b) fibrous clay minerals exposed to deionized water during 24 h.
Please cite this article as: Alcântara, A.C.S., et al., Polysaccharide–fibrous clay bionanocomposites, Appl. Clay Sci. (2014), http://dx.doi.org/ 10.1016/j.clay.2014.02.018
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of water uptake as the filler loading increased. Alginate and starch based films exhibit good stability in water and the effect of clay mineral content is not as significant as in chitosan-based bionanocomposites. For these last materials, a reduction in water uptake of approximately 32.7% and 7.7% is observed in films with 17% of Sep and Pal, respectively, in comparison to films containing only 3% of clay mineral. Moreover, these values can be further reduced to more than 68% and 57% for Sep and Pal, respectively, for clay mineral content of 50% in the chitosan matrix. Again the great stability to water and reduced water uptake in the bionanocomposites can be related to the interactions between the filler and the biopolymer matrix, reducing the number of available OH groups in the polysaccharide chains, and consequently decreasing the sites for incorporation of water molecules. The improved mechanical properties and the resistance to water achieved with the assembly of these fibrous clay minerals to polysaccharide matrices make them interesting fillers with a good low cost–benefit ratio. 3.3. Uptake of heavy metals in aqueous media Adsorption processes are generally known to be one of the most effective techniques for the removal of environmentally hazardous metals. Diverse bionanocomposite materials have been recently evaluated for the uptake of different pollutants (Liu et al., 2010; Zou et al., 2011). Bionanocomposites with 33% mass/mass in Sep and Pal were selected for evaluation in the retention of heavy metals in aqueous solution due to their great water stability and good mechanical properties. Thereby, films of these bionanocomposites were tested in the
adsorption of Pb(II) and Cu(II) ions following batch experiments. Although starch–sepiolite films present also good water stability, they were not stable in the heavy metals solutions, undergoing complete disintegration and making the measurements impossible. The other bionanocomposite systems are more stable and the uptake of heavy metal ions by neat alginate and chitosan as well as their based bionanocomposites are displayed in Fig. 8. A general observation is that the adsorption of the metal ions is affected by the presence of the clay mineral, occurring a clear diminution in the adsorption capacity of the bionanocomposites compared to the biopolymers alone, although bionanocomposites show higher stability in aqueous media. The carboxyl groups of alginate chains can be involved in the adsorption of the metal ions. As part of the carboxyl groups are in interaction with the silicate, as previously discussed, the availability of carboxyl groups in the bionanocomposites is lower and consequently their removal capacity decreases compared to alginate. In spite of this, the effectiveness of fibrous clay–alginate bionanocomposites is higher than that of bionanocomposites based on layered silicates, as reported for the uptake of lead (Shawky, 2011) and copper (Ely et al., 2009). Chitosan and its derived bio-hybrids present very similar amounts of adsorbed copper ions, with values around 110 mg/g. The mechanism of copper adsorption by chitosan is well known, being related to the ability of the amino groups in the polymeric structure to form coordination complexes with these ions (Varma et al., 2004). When Sep or Pal is added to the biopolymer matrix only a fraction of the amino groups interact electrostatically with the fibers, while an excess of amino groups remains available for other interactions (Darder et al., 2006). As a result, these amino groups can form chelate compounds with the copper ions, presenting its bionanocomposites a similar behavior to that of pure chitosan. In lead adsorption experiments (Fig. 8a), the importance of the clay–chitosan interaction in the bio-hybrid films is remarkable, as unmodified chitosan films were totally disintegrated in the Pb(II) solutions while the bionanocomposites showed a great stability. This behavior could be associated with the unfavorable acid media of the lead solution for the pure chitosan films, together with the different interaction of this polysaccharide with Pb(II) in comparison to Cu(II) ions. Thus, the bionanocomposite films present very similar adsorption capacities, independent of the filler employed, being able to remove lead ions around 100 mg/g. The results of heavy metal adsorption presented in this work are satisfactory and comparable with other bionanocomposite systems, allowing their application as biosorbents for environmental remediation. 4. Concluding remarks The systematic study carried out shows the possibility to prepare bionanocomposites based on the assembly of fibrous clay minerals with polysaccharides of neutral character as well as positively or negatively charged. The interaction mechanism between the polysaccharides and the clay minerals occurs mainly through the interaction of the OH groups in the biopolymers backbone and the silanol groups on the silicate surface. In the case of alginate and chitosan the presence of other functionalities, carboxylate and amino groups, respectively, may be also implicated. The existence of strong interactions between both components of the bionanocomposites results in materials provided with good mechanical properties, improved water resistance and reduction of water absorption in comparison to biopolymer alone, which make them very attractive for a wide range of applications. These films also offer interesting results for the retention of heavy metal ions such as copper and lead, which together with the biocompatibility and biodegradability afforded by the biopolymer may enlarge the scope of applications of these bionanocomposite materials. Acknowledgments
Fig. 8. Uptake of Cu(II) and Pb(II) ions in polysaccharide films and their bionanocomposites with 33% of clay mineral. (The adsorption of Pb2+ on the CHT films could not be determined due to the instability of this film in this solution).
This work was supported by the CICYT (Spain) project MAT201231759 and the UE COST Programme (project MP1202). A.C.S.A.
Please cite this article as: Alcântara, A.C.S., et al., Polysaccharide–fibrous clay bionanocomposites, Appl. Clay Sci. (2014), http://dx.doi.org/ 10.1016/j.clay.2014.02.018
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acknowledges CSIC for a JAE-Predoc fellowship. The authors also thank Mr. A. Valera and Mr. C. Sebastián for their technical assistance in the FESEM study. References Ahlrichs, J.L., Serna, C., Serratosa, J.M., 1975. Structural hydroxyls in sepiolite. Clay Clay Miner. 23, 119–124. Alcântara, A.C.S., Darder, M., Aranda, P., Ruiz-Hitzky, E., 2012. Zein-fibrous clays biohybrid materials. Eur. J. Inorg. Chem. 5216–5522. Alvarez, A., Santarén, J., Esteban-Cubillo, A., Aparicio, P., 2011. Current industrial applications of palygorskite and sepiolite. In: Galán, E., Singer, A. (Eds.), Developments in Palygorskite-Sepiolite Research. A New Outlook of These Nanomaterials. Elsevier B.V., Oxford, pp. 281–298. Avérous, L., Pollet, E., 2012. Environmental Silicate Nano-Biocomposites. Springer-Verlag, London. Bergaya, F., Theng, B.K.G., Lagaly, G., 2006. Handbook of Clay Science. Elsevier, Oxford. Bergaya, F., Detellier, C., Lambert, J.-F., Lagaly, G., 2013. Introduction to clay–polymer nanocomposites (CPN), In: Bergaya, F., Lagaly, G. (Eds.), Handbook of Clay Science, Fundamentals, second edition. Developments in Clay Science, Vol5A. Elsevier, Oxford. Bilotti, E., Zhang, R., Deng, H., Quero, F., Fischer, H.R., Peijs, T., 2009. Sepiolite needle-like Clay for PA6 nanocomposites: an alternative to layered silicates? Compos. Sci. Technol. 69, 2587–2595. Bokobza, L., Burr, A., Garnaud, G., Perrin, M.Y., Pagnotta, S., 2004. Fibre reinforcement of elastomers: nanocomposites based on sepiolite and poly(hydroxyethyl acrylate). Polym. Int. 53, 1060–1065. Carrado, K.A., Bergaya, F. (Eds.), 2007. Clay based Polymer Nanocomposites (CPN), in the CMS Workshop Lecture, vol. 15. The Clay Minerals Society, Chantilly (VA). Chivrac, F., Pollet, E., Schmutz, M., Averous, L., 2010. Starch nano-biocomposites based on needle-like sepiolite clays. Carbohydr. Polym. 80, 145–153. Darder, M., López-Blanco, M., Aranda, P., Aznar, A.J., Bravo, J., Ruiz-Hitzky, E., 2006. Microfibrous chitosan-sepiolite nanocomposites. Chem. Mater. 18, 1602–1610. Darder, M., Aranda, P., Ruiz-Hitzky, E., 2007. Bionanocomposites: a new concept of ecological, bioinspired, and functional hybrid materials. Adv. Mater. 19, 1309–1319. dos Santos Soares, D., Santana Fernandes, C., da Costa, A.C.S., Nervo Raffin, F., Acchar, W., de Lima e Moura, T.F.A., 2013. Characterization of palygorskite clay from Piauí, Brazil and its potential use as excipient for solid dosage forms containing anti-tuberculosis drugs. J. Therm. Anal. Calorim. 113, 551–558. Ely, A., Baudu, M., Basly, J.-P., Kankou, M.O.S.A.O., 2009. Copper and nitrophenol pollutants removal by Na-montmorillonite/alginate microcapsules. J. Hazard. Mater. 171, 405–409. Fernandes, F.M., Ruiz, A.I., Darder, M., Aranda, P., Ruiz-Hitzky, E., 2009. Gelatin-clay bionanocomposites: structural and functional properties as advanced materials. J. Nanosci. Nanotechnol. 9, 221–229. Fukushima, K., Tabuani, D., Abbate, C., Arena, M., Ferreri, L., 2010. Effect of sepiolite on the biodegradation of poly(lactic acid) and polycaprolactone. Polym. Degrad. Stab. 95, 2049–2056. Galimberti, M. (Ed.), 2011. Rubber–Clay Nanocomposites: Science, Technology and Applications. John Wiley & Sons. Kizil, R., Irudayaraj, J., Seetharaman, K., 2002. Characterization of irradiated starches by using FT-Raman and FTIR spectroscopy. J. Agric. Food Chem. 50, 3912–3918.
7
Lagaly, G., 1999. Introduction: from clay mineral–polymer interactions to clay mineral– polymer nanocomposites. Appl. Clay Sci. 15, 1–9. Lagarón, J.M., 2011. Multifunctional and Nanoreinforced Polymers for Food Packaging. Woodhead Publising Ltd., Cambridge. Liu, Y., Wang, W., Wang, A., 2010. Adsorption of lead ions from aqueous solution by using carboxymethylcellulose-g-poly(acrylic acid)/attapulgite hydrogel composites. Desalination 259, 258–264. Mittal, V., 2011. Nanocomposites with biodegradable polymers. Synthesis, properties and future perspectives, Oxford University Press, New York. Oliveira, R.N., Acchar, W., Soares, G.D.A., Barreto, L.S., 2013. The increase of surface area of a brazilian palygorskite clay activated with sulfuric acid solutions using a factorial design. Mater. Res. 16, 924–928. Pinnavaia, T.J., Beall, G.W. (Eds.), 2000. Polymer–clay nanocomposites. John Wiley & Sons, West Sussex. Rhim, J.-R., 2011. Effect of clay contents on mechanical and water vapor barrier properties of agar-based nanocomposite films. Carbohydr. Polym. 86, 91–699. Ristolainen, N., Heikkilä, P., Harlin, A., Seppälä, J., 2006. Poly(vinyl alcohol) and polyamide-66 nanocomposites prepared by electrospinning. Macromol. Mater. Eng. 291, 114–122. Ruiz-Hitzky, E., Van Meerbeek, A., 2006. Clay mineral- and organoclay–polymer nanocomposite. In: Bergaya, F., Theng, B.K.G., Lagaly, G. (Eds.), Handbook of Clay Science. Elsevier, Oxford, pp. 583–621. Ruiz-Hitzky, E., Aranda, P., Darder, M., 2008. Bionanocomposites. Kirk Othmer Encyclopedia of Chemical Technology. John Wiley Sons, Inc., pp. 1–28. http://dx.doi.org/ 10.1002/0471238961.bionruiz.a01/abstract. Ruiz-Hitzky, E., Aranda, P., Alvarez, A., Santarén, J., Esteban-Cubillo, A., 2011. Advanced materials and new applications of sepiolite and palygorskite. In: Galán, E., Singer, A. (Eds.), Developments in Palygorskite-Sepiolite Research. A New Outlook of These Nanomaterials. Elsevier B.V., Oxford, pp. 393–452. Ruiz-Hitzky, E., Darder, M., Fernandes, F.M., Wicklein, B., Alcântara, A.C.S., Aranda, P., 2013. Fibrous clays based bionanocomposites. Prog. Polym. Sci. 38, 1392–1414. Shawky, H.A., 2011. Improvement of water quality using alginate/montmorillonite composite beads. J. Appl. Polym. Sci. 119, 2371–2378. Theng, B.K.G., 1979. Formation and properties of clay–polymer complexes. Elsevier, New York. Varma, A.J., Deshpande, S.V., Kennedy, J.F., 2004. Metal complexation by chitosan and its derivatives: a review. Carbohyd. Polym. 55, 77–93. Wang, Q., Wang, W., Wu, J., Wang, A., 2012. Effect of attapulgite contents on release behaviors of a pH sensitive carboxymethylcellulose-g-poly(acrylic acid)/attapulgite/ sodium alginate composite hydrogel bead containing diclofenac. J. Appl. Polym. Sci. 124 (6), 4424–4432. Wicklein, B., Darder, M., Aranda, P., Ruiz-Hitzky, E., 2011. Phospholipid-sepiolite biomimetic interfaces for the immobilization of enzymes. ACS Appl. Mater. Interfaces 3, 4339–4348. Xue, S., Reinholdt, M., Pinnavaia, T.J., 2006. Palygorskite as an epoxy polymer reinforcement agent. Polymer 47, 3344–3350. Zou, X., Pan, J., Ou, H., Wang, X., Guan, W., Li, C., Yan, Y., Duan, Y., 2011. Adsorptive removal of Cr(III) and Fe(III) from aqueous solution by chitosan/attapulgite composites: equilibrium, thermodynamics and kinetics. Chem. Eng. J. 167, 112–121.
Please cite this article as: Alcântara, A.C.S., et al., Polysaccharide–fibrous clay bionanocomposites, Appl. Clay Sci. (2014), http://dx.doi.org/ 10.1016/j.clay.2014.02.018