Research Note pubs.acs.org/IECR
Spongy Hydrogels of Cyanobacterial Polyanions Mediate EnergySaving Electrolytic Metal-Refinement Maiko K. Okajima,† Masatoshi Nakamura,† Tetsuya Ogawa,‡ Hiroki Kurata,‡ Testu Mitsumata,§ and Tatsuo Kaneko†,* †
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa, 923-1292, Japan Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan § Department of Polymer Science and Engineering, Graduate School of Engineering, Yamagata University, Yonezawa 992-8510, Japan ‡
S Supporting Information *
ABSTRACT: The freeze-drying of anionic megamolecules extracted from gelatinous cyanobacteria, Aphanothece sacrum, formed spongy materials capable of metal adsorption to form hydrogels. Cryogenic-transmission electron microscopy demonstrated that thicker nanofibers were formed in sacran/In3+ complexes than in sacran/Sn4+. The preferential sorption of In3+ into the hydrogels occurred in the mixed solution of In3+ and Sn4+ with concentrations below 40 mM. Since In3+ was condensed in the sponges, electrolytic refinement of indium was made using the ion-complex hydrogels at room temperature to obtain indium metal foil at a pure grade over 99.9%. Furthermore, the sponges were recovered and used again, to successfully obtain highly pure indium metal, in even the second and third trials. Thus epochal energy-saving methods for indium refinement were established using spongy hydrogels of cyanobacterial polyanions.
2.2. Metal Ion Sorption. Sacran was dissolved in hot water to a 1 wt % solution, and the solution was freeze-dried in a lyophilizer and annealed at 160 °C for 3 h in vacuo, to create spongy materials. The sacran sponge was cut into cubes (Figure 1a) and weighed. Aqueous solutions of indium (In3+) chloride,
1. INTRODUCTION The development of a metal recovery agent taking advantage of biofunctionality is proceeding, but the metals salvaged by the recovery agent are in an ionic state, or those obtained by treatment at a very high temperature are in a metal oxide state. Molten salt methods for electronic refining also require very large thermal energy (800−900 °C for indium).1 On the other hand, biosorptions using microorganisms2−9 such as cyanobacteria or anionic polysaccharides10−16 were widely reported, but these studies did not extend to the metal recovery. As a consequence, there have been no reports on a facile approach of metal refinement in mild conditions at room temperature. Here we propose the usage of hydrogels containing the metal ions to salvage metal ions in a metallic state. For this purpose, the development of an ideal adsorbent which can selectively adsorb heavy metal ions and subsequently enable the retrieval of the metal ions as elemental metal is desired. Here we report the absorption of In3+(III) and Sn4+(IV) which are used in transparent electrodes to cyanobacterial polysaccharide hydrogels, and the selective recovery of indium metal.
Figure 1. (a) Freeze-dried sponges of Aphanothece sacrum polysaccharides, sacran. (b) The gelatinous cyanobacterial, Aphanothece sacrum, biomaterial. (c) Scanning electron microscopy (SEM) image of freeze-dried sacran sponge surface. (d) Hydrogels of freezedried sacran sponge used for metal refinement at three times the original size.
2. EXPERIMENTAL SECTION 2.1. Sacran Extraction. The extraction method of polysaccharides, sacran, from A. sacrum is given briefly as follows. After the A. sacrum biomaterials were washed in water and ethanol, the samples were dissolved in hot alkaline water. The obtained solution was dialyzed with pure water until the pH value decreased to 8.0−9.0, and then filtrated. The filtrate was concentrated and reprecipitated over ethanol to get the white fibrous material of sacran. The sacran was purified by repeating precipitation and size exclusion chromatography. The detail extraction procedure is shown in the Supporting Information. © 2012 American Chemical Society
Received: Revised: Accepted: Published: 8704
May 1, 2012 June 2, 2012 June 8, 2012 June 8, 2012 dx.doi.org/10.1021/ie301117p | Ind. Eng. Chem. Res. 2012, 51, 8704−8707
Industrial & Engineering Chemistry Research
Research Note
polysaccharides with molecular weights over 1.0 × 107 g/mol and with deep negative-charge potential wells.17,18 The polysaccharide, sacran, derived from Aphanothece sacrum (Figure 1b) has better absorption properties to trivalent metal ions than to divalent ones, and could adsorb metal ions under rather low pH conditions due to their sulfate groups.19−21 Then we focus sacran in the present study. However, if the metal ions were absorbed to the hydrated sacran, then a skin layer was instantaneously formed in the region of the sacran/metal ions complex. In addition, since the water affinity of sacran was extremely high,22,23 the sacran content became very low in the equilibrated-swollen state of the chemically cross-linked gel.19 Therefore, it seemed that the absorption efficiency of metal ions to the sacran chains in either solution or chemically cross-linked gel was insufficient. Then we used the sacran materials in the dried solid state, in order to improve the metal-absorption efficiency and handling ability. We first tried to use fibrous and powdery samples for metal absorption, to create the metal ion-complex hydrogels by immersing them into the solution of trivalent metal ions such as In3+ with a concentration of 10−2 M. However the resulting gels were not self-supported, and then it was difficult to take them out of the metal ion solution. Next we tried to use freeze-dried sacran (Figure 1a) with microporous structures prepared using a sacran solution of 1% concentration, as illustrated by SEM image (Figure 1c), just like as sponges. When the sacran sponges were immersed into the In3+ solution with a concentration range over 10−2 M, the self-supporting gels were formed (Figure 1d). From this phenomenon, one can conclude that the sponge materials were useful for metalrecovery. However, the sponges were dissolved or too-highly swollen in the metal ion solution with a low concentration range below 10−2 M. Then the sacran sponge with a moisture percentage of 17 wt % (Supporting Information, Figure S2) was further dried at 160 °C for 3 h in vacuo (degradation temperature, ∼250 °C (Figure S2)), possibly to form physical cross-linking such as interchain hydrogen bonding. Expectedly, the thermally treated sponge became insoluble even in a metalion solution with a low concentration of 10−4 M, and was used for all metal ion absorption experiments in the present study. 3.2. Metal Absorption. The absorption behavior of sacran sponge in a mixed system of In3+ and Sn4+ (1:1 mol/mol) was investigated. As a result, the absorption ratio of In3+ was higher than Sn4+ below a concentration of 0.05 M, and especially the absorption ratio of Sn4+ approached asymptotically to zero (Figure 2a). The morphology difference of the sacran−metal complexes between these two metal ions was then observed by cryo-TEM. The cryo-TEM image revealed the formation of thin strings (Figure 2c). In particular, the image of the sacran/ In3+ complexes showed thicker strings (more than 5 nm diameter) than the sacran/Sn4+ complexes (less than 2 nm diameter). These results suggest that complexation with In3+ caused the sacran chains to bundle most effectively. A bundle with a diameter of 5 nm should be composed of ca. 100 sacran chains with diameters of ∼0.5 nm each. The larger diameter in the sacran/In3+ complexes corresponds to the stronger affinity with sacran chains. Sacran sponge showed a swelling degree ranging 25−47 g/g close to that of the individual absorption test of In3+ at low concentrations (
