Cyanobacterial Polysaccharide Gels with Efficient Rare-Earth-Metal

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Biomacromolecules 2010, 11, 1773–1778

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Cyanobacterial Polysaccharide Gels with Efficient Rare-Earth-Metal Sorption Maiko K. Okajima,† Masatoshi Nakamura,† Tetsu Mitsumata,‡ and Tatsuo Kaneko*,† School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan, and Department of Polymer Science and Engineering, Graduate School of Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, 992-8510, Japan Received March 2, 2010; Revised Manuscript Received May 26, 2010

The cyanobacterial polysaccharide sacran, which contains carboxylate and sulfate groups, was extracted from Aphanothece sacrum, and the metal sorption behavior of sacran was investigated. Heterogels, where the sacran chains were trapped by polyvinyl alcohol networks, were prepared and immersed in NdCl3 solutions to shrink and cloud due to Nd binding. These heterogels had the ability to sorb excessive amounts of Nd ions, more than the stoichiometric ratio of 1:3 (sacran anion/Nd). Furthermore, the sacran-containing gels sorbed Nd ions under highly acidic conditions below pH 2 more efficiently than alginate-containing gels. We speculated that the strong Nd condensation effect of the sulfate groups in sacran under the acidic conditions may enhance the Nd sorption to the carboxylate groups.

Introduction Polysaccharides are among some of the most abundant renewable biomaterials on earth, and many are important as water-soluble giant polyols with various functions such as metal binding. Metal binding studies on polysaccharides have been performed for a long time in the fields of toxic element removal from natural water1 and the preparation of biomedical hydrogels2 and have shown that their metal binding properties are due to the coordinated function of their hydroxyl and other groups with negative charges. In particular, critical metal recovery from specific applications such as industrial waste management has gained recognition.3,4 Because most metal ions are more stable under acidic conditions, metal binding in high acidity is important for a wide variety of applications with various metals. Although sulfated molecules are strong electrolytes applicable over a wide pH range, sulfated polysaccharides are difficult to use due to their self-degradation under acidic conditions. Therefore, sulfated polysaccharides resistant against acidic conditions are required for the development of environmentally benign metal binding materials. In this study we focused on the cyanobacterial polysaccharide sacran, which contains sulfated groups, derived from Aphanothece sacrum biomaterials.5 We have already shown that sacran has an extremely high molecular weight of 1.6 × 107 g/mol and has carboxylate groups at 17 mol % and sulfate groups at about 12 mol % relative to the sugar residues.6 Furthermore, sacran formed rigid rods with an extremely high aspect ratio of X ∼ 1600 (from Flory’s lattice theory),7 and sacran is also difficult to hydrolyze.8 Because sacran chains are super giant, the negative charges are very condensed on these chains, and it has been reported that sacran efficiently bound to lanthanide (Ln) ions to form jelly aggregates.3 The binding showed a stoichiometric value 3:1 (anionic groups/Ln3+), typical of ionic adsorption.9 However, the amount of metal ions adsorbed onto the sacran chains was actually limited, because a dense skin * To whom correspondence should be addressed. Tel.: +81-761-51-1631. Fax: +81-761-51-1635. E-mail: [email protected]. † Japan Advanced Institute of Science and Technology. ‡ Yamagata University.

layer was formed on the surface of jelly aggregates in such a freely binding system.3 If the skin layer formation is restricted, then the metal ions may smoothly permeate into the aggregates. To restrict sacran chain mobility, we cross-linked sacran using diamine cross-linkers such as L-lysine and adipoyl dihydrazide and prepared hydrogels to sorb metal ions.4,10 However, hydrogels with very high swelling degrees of more than 700 were formed, presumably, by the sacran7 interfering with the cross-linking reaction, and thus, the skin layer formation was barely affected. Here we prepared hydrogels where the sacran chains were trapped by polyvinyl alcohol (PVA) networks under the concept of chain motility restriction and aggregation interruption. We observed that the hydrogels tended to sorb a larger amount of Nd ions than the stoichiometric amount, even under strongly acidic conditions.

Experimental Section Materials. Frozen and discolored samples of A. sacrum were donated by Kisendou Inc. (Asakura, Japan) and were used as received. Sodium hydroxide, ethanol, and isopropanol, which were used for sacran extraction, were used as received (KANTO Chemical Co. Ltd.). Sodium alginate (molecular weight: 100000-200000), used for comparisons with sacran, was used as received (KANTO Chemical Co. Ltd.). PVA (molecular weight: ca. 20000), used for trapping the polysaccharide chains, was used as received (KANTO Chemical Co. Ltd.). Sacran Extraction. We extracted the sacran by a procedure previously reported5,6,8 as follows. Briefly, after the samples were washed by water and then ethanol, we used aqueous NaOH for the elution of the sacran. After the insoluble material was filtered, the solution was thoroughly dialyzed with pure water while repeatedly replacing the solution outside of the dialysis membrane (MWCO: 14000). After the solution inside the dialysis membrane was concentrated, it was slowly poured into isopropanol to create fibrous precipitates that were reprecipitated twice more to give colorless fibers. Preparation of Heterogels. Heterogels composed of PVA networks with polysaccharide chains (sacran or alginate) were prepared as follows. A 1 wt % PVA solution of 100 mL and a 0.8 wt % polysaccharide solution of 100 mL were mixed with agitation to a homogeneous solution at pH 6. After agitation, glutaraldehyde (2 mol

10.1021/bm100231q  2010 American Chemical Society Published on Web 06/18/2010

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Figure 1. (a) Schematic illustration of heterogels composed of polysaccharide chains such as sacran and PVA networks: a partial structure of sacran is shown on the right; (b) Pictures of the heterogels: left, before Nd3+ sorption; right, after Nd3+ sorption.

% to the PVA) was added to cross-link the PVA chains, and then the pH of the solution was adjusted to 2.4-2.5 by the careful addition of 0.1 M HCl to accelerate the acetyl reaction of the glutaraldehyde with the PVA via hydroxyls. The mixed solution was centrifuged to gather the air bubbles on the surface, and the solution was rested until the gels formed. Normal PVA gels cross-linked by glutaraldehyde were also prepared for comparison. The gels were then swelled in distilled water for 1 week while replacing the water for purification. The swelling degree of the gels was measured by the method shown below. Four block gels (size: 1 cm3) in an equilibrated-swollen state were weighed. The degree of swelling was the mean of the weight ratio of four block gels before and after drying. Nd Ion Sorption. Aqueous solutions of Nd ions at concentrations of 10-1, 10-2, 10-3, 10-4, and 10-5 M were prepared just before the sorption tests. The gels cut into about 1 × 1 × 1 cm3 cubes and were immersed into individual metal ion solutions of 10 mL in closed containers and shaken (100 strokes/min) for a week. After the Nd ions were fully sorbed into each gel, the gels were removed, and the aqueous solutions of the Nd ions at each concentration were treated by HNO3 to decontaminate any organic substances. The residual Nd ion concentration was finally measured by ICP (inductively coupled plasma) analysis using a Shimadzu ICPS-8100. Based on the Nd ion concentration, the sorption ratio per gel weight and the sorption ratio per negative charge were then calculated. When pH effects on the sorption were investigated, the hydrogels were presoaked for more than 5 days in each aqueous solution of the given pH and then transferred to the NdCl3 solution of the same pH. The evaluation of the amount of Nd3+ sorbed was the same as per the normal NdCl3 solution. Desorption of Nd Ions from Gels. Each gel was cut into 1 × 1 × 1 cm3 cubes and were stored in NdCl3 solutions (10 mL) at various concentrations of 7 × 10-1, 7 × 10-2, 7 × 10-3, 7 × 10-4, and 7 × 10-5 M, respectively. The Nd ions were then sorbed into the gels for a week. The gels were taken out of the solution, and the solution was assayed for the Nd ion concentration (M) inside the gel after sorption. Next, gels withdrawn from the NdCl3 solutions (10 mL) were adequately wiped and immersed in 10 mL of pure water. After 7 days, the gels were removed from the pure water, and the Nd ion concentration in the pure water was measured to calculate the concentration (M) of Nd ions remaining in the gel. From these results, the residual ratio of the Nd ions in the gel was calculated.

Results and Discussion 1. Heterogel Preparation. Sacran was extracted from Aphanothece sacrum biomaterials by a previously reported method,3 and the molar ratio of anionic groups to the sugar residues was 29.5 mol % (carboxylate: 17.0 mol %, sulfate: 12.5 mol %), which was almost the same value as the previous report. The sacran chains extracted here can be used for metal sorption; in fact, gel beads with skin layers were formed very quickly by dropping the sacran solution (0.5 wt %) into the NdCl3 solution (10-2 M). To restrict the mobility of the sacran chains, they were trapped within networks of nonionic chains with little Coulombic interaction. We selected PVA, which was easily cross-linked by glutaraldehyde, to form tough hydrogels. Tough heterogels composed of PVA networks and alginate chains (sacran/PVA gels) were prepared by adding glutaraldehyde to a mixture of PVA and sacran at pH 2-3. Heterogels composed of PVA networks and alginate chains (alginate/PVA gel) and normal PVA gels without polysaccharides were also prepared for comparison. We did not confirm the formation of hydrogels following the reaction of the sacran or alginate chains with glutaraldehyde, but a slight increase in viscosity was observed. This finding indicates that the reaction of glutaraldehyde with the PVA chains occurs preferentially to the reaction with the polysaccharides, and polysaccharide chains may be trapped by the PVA networks, as illustrated in Figure 1. As a result of the swelling degree measurement of these gels, the sacran/PVA gel swelled to 22-fold of the dry weight, the alginate/PVAs swelled by 18-fold, and the PVA gel swelled by 5-fold. These data showed that the sacran chains had stronger effects on the increase in the water sorption of the gels than alginate chains. 2. Sorption to Hydrogels. Ln-based materials are becoming increasingly important both in terms of research activity11 and their use in commercial products such as optical materials,12 ion conductors,13 microelectronics,14 and ferromagnetic materials composed of neodymium (Nd).15 We selected Nd from the Ln elements because Nd is gaining attention in various applications, such as high-performance magnets in electric vehicle motors.16 Cubic gels of 1 cm3 were immersed in NdCl3 solutions at initial concentrations of 10-1, 10-2, 10-3, 10-4, and

Sacran Metal Sorption

Figure 2. Changes in the degree of swelling of heterogels composed of PVA networks and polysaccharide chains as a function of the concentration of NdCl3. The swelling degree of PVA gels containing no polysaccharides is also shown for comparison.

10-5 M and then incubated with shaking for a week. We observed that the size of the heterogels decreased and the opacity increased, as seen in the photo from Figure 1b. The change in the swelling degree of the various gels depending on the concentration of the NdCl3 solutions was then estimated (Figure 2). As a result, the swelling degree of the heterogels decreased to 13 g/g with an increase in concentration from 10-5 to 10-3 M but showed almost constant values below 10-5 M. At a metal ion concentration of 10-3 to 100 M, the swelling degree of the gels showed a slight increase from 13 to 17 g/g. On the other hand, the PVA gels with no anionic groups barely showed any change in their swelling degree. Therefore, we simply speculate that the complex change in the swelling degree of the heterogels may be related to the Nd3+ sorption to the polysaccharide anions and that the slight change in the PVA gels without polysaccharides may be due to the negligibly low interaction of PVA with Nd3+. To quantify the sorption ratio of Nd3+ into the gels, the concentration decrease of the metal ion in the supernatant was measured by an ICP emission method. The Nd3+ sorption ratio to 1 g of polymer components, such as polysaccharide chains or PVA networks, was then calculated (Figure 3a). In both

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heterogels, the Nd3+ sorption ratio showed a two-step increase; the first increase occurred in a NdCl3 concentration range from 10-5 to 10-3 M, and the second increase occurred at over 10-3 M. Moreover, the Nd3+ sorption ratio of the heterogels was much higher than the PVA gels, indicating that the Nd3+ was taken up more efficiently into gels containing anionic polysaccharides. From these results, we speculated that the swelling degree decrease of both heterogels over the concentration range from 10-5 to 10-3 M was related to the first increase in Nd3+ sorption because Nd3+ binding to the anions may reduce the hydration force of the polysaccharide chains. On the other hand, the Nd3+ sorption ratio of normal PVA gels increased with an increase in the Nd3+ concentration. The inclination of the dotted line gradually increased, and then the Nd3+ sorption ratio of the normal PVA gels became comparable with those of heterogels over 10-2 M, which suggested that the Nd3+ binding to the PVA hydroxyls may be more effective at a higher range of Nd3+ concentrations.17 The slight increase in the swelling degree at a concentration range over 10-3 M may be due to the second increase in the Nd3+ sorption ratio, which increased the osmotic pressure of the heterogels such that a large amount of ions penetrated through PVA coordination. To compare the polysaccharide capability to bind with Nd3+, the data from Figure 3a were converted into Nd3+ sorption ratios to the molar amount of polysaccharide anions: 29.5 mol % to the sugar residues for sacran anions and 100 mol % for alginate (Figure 3b). The sorption ratio of the sacran chains was higher than that of the alginate chains over the whole concentration range, indicating that sacran anions have a higher efficiency to sorb Nd3+. These results are in good agreement with the reported phenomenon that sacran formed jellylike complexes with Ln3+ more efficiently than alginate.3 On the other hand, the difference between the Nd3+ sorption ratios of heterogels composed of sacran and alginate became smaller at higher concentrations of Nd3+, presumably due to the stronger coordination force of the PVA hydroxyls. The sorption ratio of Nd3+ in both heterogels above Nd3+ concentrations of 10-5 M was much higher than the stoichiometric ratio 0.333 (3 anions/1 Nd), even if the Nd3+ sorption ratio to the PVA gels was subtracted from the value of the heterogels (Figure S1). This suggests that the condensation of the trivalent metal ion Nd3+ occurred in the heterogels. This condensation may be due to Nd3+ drawing the anionic polysaccharides through Coulombic interactions, as shown in the illustration in Figure 4a. We suggest that the excess Nd3+

Figure 3. Changes in the Nd3+ adsorption ratios in heterogels composed of PVA networks and polysaccharide chains as a function of the concentration of NdCl3: (circles) heterogels composed of PVA networks and sacran chains; (triangles) heterogels composed of PVA networks and alginate chains. (a) Sorption ratios to the dry weight of the gels. PVA gels containing no polysaccharides are also shown for comparison (squares and dotted line). (b) Sorption ratios to moles of anionic groups in the polysaccharides.

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Figure 4. Schematic illustration of the Nd3+ sorption behavior into heterogels composed of PVA networks and polysaccharide chains. (a) Nd3+ was condensed in heterogels. (b) Many Nd3+ ions sorbed onto the polysaccharide anions were accompanied by Cl- as one or two counterions when the polysaccharide chains were restricted by the PVA networks from getting close to the Nd3+ ion and cross-linking with it.

Figure 5. Residual ratios of Nd3+ in the heterogels after the Nd3+ ions were desorbed by immersion in pure water for one week: (circles) heterogels composed of PVA networks and sacran chains; (triangles) heterogels composed of PVA networks and alginate chains. The results from PVA gels containing no polysaccharides are also shown for comparison (squares and dotted line).

sorption might be caused by the entrapment of some anionic polysaccharides in the nonionic PVA networks. Because the polysaccharides are constructed from chains of continuous heterorings, they are difficult to deform. If the polysaccharides are trapped in the polymer networks, then deformation and translational movement may become even more difficult. Even if multivalent metal ions sorb to the polysaccharide chains, crosschain binding may be difficult in such a restricted state. At the molecular level, the Cl- of NdCl3 is replaced by the anion of the polysaccharides during sorption, and all three Cl- ions should be replaced in the free state. On the other hand, only one or two Cl- ions may be replaced in the restricted state, that is, [NdCl2]+ or [NdCl]2+ ions sorbed onto the polysaccharide chains, as shown in Figure 4b. Thus, the anionic polysaccharides fixed in the polymer networks may induce 1:2 or 1:1 adsorption, as well as 1:3 adsorption, which leads to an adsorption ratio excess versus that in normal ionic adsorption. In fact, the normalized Nd3+ adsorption ratio in the heterogels was lower than 1.0 at Nd3+ concentrations below 10-3 M (Figure S1). If Nd3+ did not bind to the polysaccharide anions very strongly, then Nd3+ should be desorbed from the gels easily. We performed a desorption test by immersing the Nd3+-sorbing gels in pure water (Figure 5). One can confirm that drastic desorption occurred from both heterogels sorbing Nd3+ from the NdCl3 solution at concentrations between 10-2-10-3 M, whereas normal PVA gels did not show a large dependence on the Nd3+ concentration during sorption.

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3. pH Effects. Sacran has strong electrolyte groups, such as sulfates together with carboxylates, that may possess sorbability regardless of pH. We investigated the influence of pH changes on the sorption of Nd ions into the gels. Heterogel cubes of 1 cm3 were immersed into HCl solutions whose pH values were prepared at 0, 0.5, 1, 1.5, 2, 2.5, 3, 3.5 4, 4.5, and 5, respectively, for two days. The gels were then taken out of the solutions, their surfaces wiped, and then weighed. The swelling degree change as a function of the pH is shown in Figure 6. Both heterogels showed a decrease in the swelling degree with a pH decrease, whereas the swelling degrees of the PVA gels did not depend on the pH. The pH dependence of the swelling degree may be attributed to the protonation of carboxylate anions under acidic conditions. The sacran/PVA gels showed a simple decrease in swelling degree from pH 5.5 to pH 0, whereas the alginate/PVA gels showed a decrease from pH 2 to pH 0. This difference in dependency may be attributed to the structural complexity difference between sacran and alginate; sacran chains have 11 sugar residues that create a variety of local environments around the carboxylates and sulfates, whereas alginate has only two sugar residues. Thus, the sacran/PVA gels showed pH dependency over a wider range than alginate/PVA gels. The heterogels taken out of the HCl solutions were stored in the NdCl3 solutions at a concentration of 0.3 mM (10 mL), at pH values corresponding to the preimmersion HCl solutions, for four days. The swelling degrees of all gels decreased, and in particular, the sacran/PVA gels showed dramatic decreases over a pH range of 3.0-5.5, but slight decreases over a pH range of 2.5-0. One can speculate that the sacran carboxylate anions are easily protonated under mildly acidic conditions over pH 2.5 and, thus, may be preferentially bound with Nd3+ to reduce the swelling degree. A quantitative sorption ratio was calculated in a similar fashion to the former section (Figure 7). As a result, both heterogels also showed higher sorption ratio than the PVA gels (Figure 7a). In the alginate/PVA gel immersed in a NdCl3 solution of 0.3 mM at pH 5, the sorption ratio per negative charge was about 0.5 mol/mol, and this decreased with a decrease in pH to attain a minimum value of about 0.25 mol/ mol around pH 2. The pH value of the sorption ratio decreased, corresponding to the swelling degree decrease, which may be due to the protonation of the alginate carboxylates. On the other hand, the sorption ratio of the sacran/PVA gel was significantly higher in NdCl3 prepared at various pH values as compared to the alginate/PVA gels, and the sorption ratio per negative charge was 2.6 mol/mol at pH 5, and decreased with lower pH values at pH 0 (Figure 7b). The high sorption ratio at over pH 3 corresponded to the dramatic decrease in the swelling degree observed in Figure 6a. Here it was demonstrated that the sacran/ PVA gel could sorb Nd ions even below pH 2, although the alginate/PVA gel barely sorbed anything. Alginate has more negative charges than sacran, but has only carboxylic acid, and although sacran has one-third of the total alginate negative charge, it has both carboxylate and sulfate groups. Therefore, we hypothesized that the reason for the high sorption ratio of the sacran/PVA gel at a low pH was the presence of sulfate in the sacran chains. We speculated that the Nd cations may bind to the carboxylate anions more strongly than the sulfate anions, because Nd2(SO4)3 is readily soluble in water but Nd2(CO3)3 is poorly soluble. The Coulombic force from the sulfates may be effective in collecting cations, even in the Nd-binding state, in contrast to carboxylates. This effect condensed the Nd3+ around the sacran chains and, of course, the carboxylate groups. In this state, the conversion from the carboxylic acids of the sacran

Sacran Metal Sorption

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Figure 6. Changes in the swelling degree of gels in the presence (open marks) and absence (closed marks) of Nd3+ as a function of the pH: (a) heterogels composed of sacran and PVA networks; (b) heterogels composed of alginate and PVA networks; and (c) PVA gels.

Figure 7. pH dependence on the sorption ratio of Nd3+ onto heterogels. The concentration of Nd3+ was 3 × 10-4 M. (a) The sorption ratio to gel weight; data from the PVA gels were plotted for comparison. (b) The sorption molar ratio to the polysaccharide anions.

chains into carboxylate anions was enhanced, even under strongly acidic conditions of pH 0-2, and thus the Nd3+ sorbed onto the carboxylate groups. This is one possible mechanism for the excess Nd3+ binding to the sacran chains with both carboxylates and sulfates.

Conclusion The cyanobacterial polysaccharide sacran, which possesses both carboxylate and sulfate groups, was extracted from Aphanothece sacrum, and heterogels composed of PVA networks and sacran chains were prepared. These heterogels containing sacran chains shrank and clouded in aqueous solutions of NdCl3 and sorbed and condensed Nd3+ excessively to a ratio of 3 (sacran anions)/1 (Nd3+). This phenomenon suggests that [NdCl2]+ and [NdCl]2+ as well as Nd3+ cations

sorbed strongly to the rigid sacran chains restricted by the PVA networks. An investigation of the pH effects on Nd3+ adsorption showed an advantage of sacran under acidic conditions below pH 2 as compared with alginate. Nd3+ condensation due to the sulfate groups of sacran may enhance the Nd-binding to carboxylate anions, even in such highly acidic conditions. An analogy of the heterosystem is in the Aphanothece sacrum biomaterials, where the sacran chains are fixed by various crosslinking anchors such as cells and cationic proteins. Multivalent metal ions such as Fe3+, Al3+, and Ca2+ were condensed in these biomaterials from the river by a mechanism involving Ndsorption. Furthermore, one can propose that another possible sacran application would be Ln collection from acidic streams.18,19 Acknowledgment. This research was financially supported by a Grant-in-Aid for NEDO (08C46218d). We acknowledge

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Kisendou Inc. (Asakura, Japan) for dedicating A. sacrum biomaterials. Supporting Information Available. The difference between Nd-sorption ratios of heterogels and PVA gels. This material is available free of charge via the Internet at http://pubs.acs.org.

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Okajima et al. (8) Okajima, M. K.; Kaneko, D.; Mitsumata, T.; Kaneko, T.; Watanabe, J. Macromolecules 2009, 42, 2881–3218. (9) Okajima, M. K.; Nakamura, M.; Mitsumata, T.; Kaneko, T. Proceedings of the 10th International Symposium on Biomimetic Materials Processing (BMMP-10), Nagoya University, Nagoya, Japan, Jan 2629, 2010, BMMP, Nagoya, Japan, 2010; Vol. 10, p 15. (10) Okajima, M. K.; Miyazato, S.; Kaneko, T. Trans. Mater. Res. Soc. Jpn. 2008, 33, 497–498. (11) Molander, G. A. Chem. ReV. 1992, 92, 29–68. (12) Binnemansm, K. Chem. ReV. 2009, 109, 4283–4374. (13) Adachi, G.-Y.; Imanaka, N.; Tamura, S. Chem. ReV. 2002, 102, 2405– 2430. (14) Jones, A. C.; Aspinall, H. C.; Chalker, P. R. Surf. Coat. Technol. 2007, 201, 9046–9054. (15) Benelli, C.; Gatteschi, D. Chem. ReV. 2002, 102, 2369–2388. (16) Qadeer, R. J. Radioanal. Nucl. Chem. 2005, 265, 377–381. (17) Roy, S. J. Appl. Polym. Sci. 2008, 110, 2693–2697. (18) Raju, C. S. K.; Subramanian, M. S. Sep. Purif. Technol. 2007, 55, 16–22. (19) Yantasee, W.; Fryxell, G. E.; Addleman, R. S.; Wiacek, R. J.; Koonsiripaiboon, V.; Pattamakomsan, K.; Sukwarotwat, V.; Xu, J.; Raymond, K. N. J. Hazard. Mater. 2009, 168, 1233–1238.

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