JP6778948B2 - Sericin-hydroxyapatite composite structure and its manufacturing method, adsorbent and water purifier - Google Patents

Description

The present invention relates to an organic-inorganic composite (hybrid) structure, and more particularly to a sericin-hydroxyapatite composite structure, a method for producing the same, and its application to a water purifier.

The structural complexity, ease of creation and applicability of the myriad natural patterns of functional hybrid structures that combine inorganic and organic components have always fascinated researchers and their assembly. It has continued to inspire efforts to understand and control the process (Non-Patent Documents 1 to 3). The approach of synthesizing a hierarchical composite structure, which has a micron-scale size as a whole and whose constituent parts are composed of complex nanoscale parts, on a laboratory basis is when manufacturing a well-defined structure. It cannot be controlled at the same level of precision (Non-Patent Documents 4 to 8). Organic-inorganic composite "flower-like structures" (also called bioinorganic hybrids) produced from biomolecules have a wide range of applications (non-biocatalysis) such as biocatalysis, drug delivery, and biosensing. Recently, it has been attracting more and more attention because it has patent documents 9 to 12). The interaction between biomolecules and inorganic materials plays a role in further increasing the functionality of the composite material itself. Compared to compact structures such as dense spheres or polyhedra, hierarchical floral structures have a much larger surface area, and their porosity further enhances their functionality. (Non-Patent Document 9). By immobilizing and incorporating biomolecules into the core of a nanoscale flower-like structure, they exhibit greater activity, durability and stability than molecules that freely move around in solution (Non-Patent Documents 9 to 13). ). Further, if the biomolecules contained in the composite structure can be removed without destroying the hierarchical morphology that constitutes the composite structure, the place where the biomolecules are contained becomes an empty area, and the porosity and surface area increase. The structure produced by such a method can be an excellent catalyst (Non-Patent Document 14).

Biomolecular complexes are not only due to the application of their combined functions, but also because of their chemical diversity, flexible composition and good biocompatibility, great efforts have been made to produce them. Has been poured. However, as previously reported, the majority of hierarchical bioinorganic complexes have been made using enzyme molecules. Therefore, in order to explore the possibilities in various aspects, it is strongly required to realize other functional biomolecules.

While silk, a natural polymer, is one of the most promising biopolymers, silk has a rich history of thousands of years in traditional applications in the textile and medical applications. (Non-Patent Documents 15 and 16). Silk is a promising candidate for multifunctional advanced materials because it is readily available, has good mechanical strength, optical properties and biocompatibility, and can control deterioration (non-patented). Documents 15 to 17).

Due to recent major developments, photonics (Non-Patent Document 18), resists for lithography (Non-Patent Document 19), microstructure patterns (Non-Patent Document 20), biomedical instruments (Non-Patent Document 21) and drug delivery (Non-Patent Document 21) A promising new possibility for the application of silk to Ref. 22) has been clarified, which further provides an innovative method for producing silk-based materials on multiple scales for use as a host of silk applications. It became necessary to develop. Silk obtained from silkworms mainly consists of two types of proteins, namely fibroin, which is a protein in the core part of the fiber, and glue protein, which is called sericin. Sericin coats fibroin fibers, which are the main component of silk threads supplied as a product, as a continuous adhesive layer, and forms cocoons by adhering the silk fibers to each other. Unlike fibroin, which is insoluble, sericin, which makes up 30% of the cocoon protein, has traditionally been treated as an industrial waste. However, sericin has unique properties such as good hydrophilicity, biodegradability, and resistance to oxidation, bacteria and ultraviolet light (Non-Patent Documents 23 to 26). Sericin also exhibits various biological and pharmacological functions, while promoting cell attachment, growth and proliferation by addition to culture media (Non-Patent Documents 28 and 29).

Despite these various advantageous properties, the use of sericin for the fabrication of functional structures has hitherto been due to the difficulty of processing sericin to obtain useful materials. Its range is still limited when compared to the well-studied fibroin. However, several recent studies have explored interesting applications of silk sericin, opening up new perspectives on the usefulness of sericin (Non-Patent Documents 29 and 30).

Apart from these possibilities, there are two main reasons that strongly motivate the choice of sericin to create a hierarchical complex floral structure. First, most of the amino acids contained in sericin have a molecular chain with strong polarity such as a hydroxyl group, a carboxyl group and an amino group. As a result, sericin has the characteristic of being water-soluble and strongly binding to metals (Non-Patent Documents 17 and 31). Second, sericin is a very inexpensive and abundant material that has usually been discarded as a by-product during the degumming process during raw silk production (Non-Patent Document 17). ..

The present invention solves the above-mentioned problems of the prior art and uses sericin, which has not been sufficiently utilized as a by-product in the production of raw silk, and has a complex nanostructure, sericin-hydroxyapatite. It is an object of the present invention to provide a HAP) composite structure (hereinafter, also referred to as a sericin-HAP composite flower-like structure, or simply a composite flower-like structure). In addition, adsorption that removes harmful substances from the wastewater by satisfactorily adsorbing substances harmful to the human body such as heavy metal ions dissolved in water and dyes such as Congo Red (CR), a dye for fiber dyeing that is carcinogenic. The purpose is to utilize this congo red structure as a material and a water purifying agent.

According to one aspect of the present invention, a sericin-hydroxyapatite composite structure having a plurality of sericin-hydroxyapatite composite nanosheets and the plurality of nanosheets bonded at the central portion so as to extend from the central portion in different directions. Is given.
Here, the sericin-hydroxyapatite composite structure may have a substantially spherical shape or a porous shape.
Further, the average size may be in the range of 2 to 10 μm.
The average thickness of the sericin-hydroxyapatite composite nanosheet may be in the range of 10 to 25 nm.
According to another aspect of the present invention, there is provided a method for producing a sericin-hydroxyapatite composite structure as a precipitate from a phosphate buffered saline in which sericin and calcium chloride are dissolved.
Here, the sericin concentration may be 0.1 to 3 mg / mL, and the calcium chloride concentration may be 10 to 300 mM.
According to still another aspect of the present invention, an adsorbent for adsorbing at least one of the heavy metal ion and the dye consisting of any of the sericin-hydroxyapatite composite structures is provided.
Here, the heavy metal ions and dyes may be one or more selected from the group consisting of Pb (II), Cd (II), Hg (II) and Congo Red.
According to still another aspect of the present invention, a water purifying agent composed of any of the above adsorbents is provided.
According to still another aspect of the present invention, a water purifying agent comprising a film-like filter in which any of the above sericin-hydroxyapatite composite structures is laminated is provided.
According to still another aspect of the present invention, there is provided a method of regenerating any of the adsorbents, which separates the adsorbed heavy metal ions from the sericin-hydroxyapatite composite structure by a reagent that dissolves the heavy metal ions. ..
According to still another aspect of the present invention, there is provided a method of regenerating the water purifying agent, which separates the adsorbed heavy metal ions from the sericin-hydroxyapatite composite structure by a reagent that dissolves the heavy metal ions.

According to the present invention, a composite structure having a very large surface area can be easily produced. Furthermore, since this composite flower-like structure has a large surface area, it adsorbs heavy metal ions such as Pb (II), Cd (II), and Hg (II) and harmful pigments well, so that it can be used as a water purifier. It is useful.

SEM image and TEM image of the sericin-HAP composite flower-like structure of one embodiment of the present invention. (A) SEM image of a population of sericin-HAP composite structures. (B) SEM image of a single sericin-HAP composite structure. (C) An image obtained by further magnifying the above (b). (D) TEM image of a single sericin-HAP composite structure. (E) TEM image of petal-shaped nanosheets constituting the sericin-HAP composite structure. (F) High-resolution TEM image showing the crystal lattice of petal-shaped nanosheets (the inset is an enlarged image showing the lateral configuration of crystal lattice fringes). The upper left figure is a HAADF-TEM image of the sericin-HAP composite floral structure of one embodiment of the present invention, and the remaining figures are elemental mappings of C, N, O, P and Ca. It can be seen that all of these elements are uniformly distributed although their concentrations are different. (A) The figure which shows the XRD pattern of the crystalline composite structure and sericin which is an amorphous silk protein. The XRD pattern of standard hydroxyapatite (JCPDS 09-0432) is shown at the bottom of the figure. Since the XRD pattern of the composite structure and the peak position of the hydroxyapatite crystal are the same, it is suggested that the composite flower-like substance is mainly formed of hydroxyapatite. (B) It is a figure which shows the XPS spectrum of the crystalline composite structure and sericin which is a silk protein. (A) The figure which shows the nitrogen gas desorption curve of the hydroxyapatite powder synthesized with the sericin-hydroxyapatite composite structure. The surface area of the sericin-hydroxyapatite composite structure is more than doubled. (B) The figure which shows the pore size distribution curve of HAP composite flower-like thing and synthesized HAP. (A) The figure which shows the result of mixing the sericin-hydroxyapatite composite structure in the ratio of 5mg / 20mL with respect to the Pb ion aqueous solution of the concentration of 15mg / L, and plotting the ratio of removal by adsorption with respect to time. The inset is a diagram in which the Pb ion concentration in the aqueous solution was changed from 2 mg / L to 60 mg / L, and the adsorption amount per unit complex after 15 minutes was plotted. (B) The ratio of each heavy ion (Pb, Cd, Hg) (concentration: 15 mg / L) aqueous solution adsorbed and removed by the composite structure (concentration 5 mg / 20 mL) plotted over time. .. SEM image of Pb nanorods composed of Pb ions (concentration 15 mg / L) adsorbed on an aqueous solution (5 mg / L) of a sericin-hydroxyapatite composite structure. (A) TEM images of a large number of nanorods grown on the sericin-HAP composite structure. (B) STEM-EDX mapping. (C) and (d) HAADF-TEM images showing the formation of Pb nanowires (bright-looking parts indicate the presence of Pb, and slightly bright parts indicate the presence of Ca). (A) A solution in which congo red is mixed at about 0.05 mg / mL in an aqueous solution (2 mg / L) of a hierarchical sericin-hydroxyapatite composite structure, and the congo red molecules are adsorbed on the composite structure in the solution. An absorption spectrum showing how the concentration decreases and the absorption rate changes over time. Inset: Photographs of solution and complex powder before and after adsorption. (B) The figure which showed the time change of the absorption rate of a solution. (A) Conceptual diagram of a filter system for performing water purification using a sericin-hydroxyapatite composite structure. The inset in the upper right is a photo of the filter part. (B) Overall photograph of the filter system.

The present invention relates to a method for easily creating a composite structure of environment-friendly sericin (organic material) and HAP (inorganic material) known as a main component of bones and teeth, and a water purification function of the structure. is there.

Sericin is a water-soluble protein, which is mixed with basic calcium phosphate showing high biohydrophilicity in a phosphate buffer, shaken, and then left for about 1 day to grow self-organized and has a very large surface area. Produces a large hierarchical microstructure. Since this sericin-HAP composite structure is an aggregate of flakes and has a flower-like appearance as a whole, this shape is referred to as "flower-like" in the present application. In addition, this composite flower-like structure has a large surface area of 200 cm ² / g that can be adsorbed, and has a high water purification ability for an aqueous solution mixed with heavy ions and dyes. The sericin-hydroxyapatite composite structure on which heavy ions are adsorbed can be easily removed with a filter or the like, and only heavy ions can be removed and reused with an acetic acid aqueous solution having a concentration of about 1%. This composite flower-like structure has hydrophilicity and functions as a remover for toxic heavy ions and dyes dissolved in water, a highly efficient catalyst material, a cell culture skeleton, a sensor, and a water retention / moisturizer.

In the present invention, a hierarchical HAP composite structure is produced by using an environmentally friendly and simple coprecipitation method and taking advantage of the preferable material properties of silk protein, which is a water-soluble biopolymer, that is, sericin. Sericin plays an important role in the formation of flower-shaped HAP composite structures, accounting for only a few percent of the total mass. This synthesized hierarchical structure has a large BET surface area and exhibits excellent adsorption properties for toxic heavy metal ions such as Pb (II), Cd (II) and Hg (II) from wastewater. As a result, the sericin-HAP composite structure of the present invention becomes an excellent adsorbent for water purification. One of the most important properties of this adsorbent is to adsorb 99% or more of Pb (II) ions from the aqueous solution in a short time (less than 15 minutes). This is due to the phenomenon found by the inventor of the present application that Pb nanorods are uniformly distributed and grown on the hierarchical surface of the sericin-HAP composite structure after adsorption, so that Pb (II) is compared with other heavy metals. ) Is efficiently adsorbed. In addition, the HAP composite structure exhibited promising absorption properties for removing the carcinogenic dye CR from contaminated water.

As described above, the present invention has various advantages such as easy production, low cost, large surface area, very high speed and a large amount of adsorption capacity, and easy separation. It turns out that the sericin-HAP composite flower-like structure is an attractive adsorbent for water purification. Furthermore, biocompatible silk-derived sericin is well known for its application to culture media, while HAP-based materials have been widely applied in the biomedical field. Therefore, the combination of the two forming a hierarchical porous structure is a very effective scaffold in a manner of exerting a synergistic effect as a composite material having a possibility of being applied to biomedicine. Overall, the HAP composite structure of the present invention is a fruitful industrial waste sericin in that it develops high value-added materials with great potential for environmental and economic benefits. It will be useful.

Hereinafter, the present invention will be described in more detail based on Examples. It should be noted that, of course, the following examples are presented to aid in understanding the invention and are not intended to limit the invention to any particular form.

[Preparation of sericin-HAP composite flower-like structure]
High-purity sericin (Wako Pure Chemical Industries, Ltd.), calcium chloride dihydrate (Sigma Aldrich Japan) and phosphate buffered saline (Lonza, 1X PBS 17-516F) as precursor reagents for the coprecipitation reaction. It was used. Milli-Q pure deionized water was used to prepare the reservoir.

200 μL of CaCl ₂ aqueous solution (100 mM) was added to 4 mL of 1X PBS (pH 7.4) containing sericin at a concentration of 0.5 mg / mL, and the mixture was gently shaken for several minutes. Then, it was kept at 25 degreeC for 24 hours. The precipitate of the white flower-like composite structure thus produced was collected, washed repeatedly with deionized water, and then dried at room temperature. All work was performed at room temperature (25 ° C.). Therefore, the aqueous solution has the same temperature.

[Measurement of characteristics of the prepared sericin-HAP composite flower-like structure]
<Analysis by SEM>
After thoroughly washing the prepared sericin-HAP composite flower-like structure with water, a high-concentration solution was deposited on a glass substrate and dried, and then platinum-coated. The SEM image was obtained using a Hitachi S4800 and SU8000 field radiation microscope.

<Analysis by TEM>
2 μL of a diluted dispersion of a sericin-HAP composite floral structure was applied to a TEM grid made of molybdenum and copper and dried at room temperature. Then, a TEM image was obtained at an operating voltage of 200 kV using a JEOL JEM-2100F high-resolution electron microscope. In addition, HAADF (high angle annular dark field) images and EDX mapping were obtained in scanning transmission electron microscope (STEM) mode.

<XRD analysis>
XRD analysis was performed using Cu Kα radiation (λ = 1.5406 Å, 40 kV / 40 mA) with a Rigaku Rint 2000 Ultima III X-ray diffractometer. The 2θ scanning range was 1 ° to 80 ° in 0.02 ° steps, and the scanning speed was 1 ° / min.

<XPS analysis>
XPS analysis was performed using Thermo Fisher Scientific's Theta Probe system.

<Thermogravimetric (TG) analysis>
Thermogravimetric analysis was performed with a SII Exstar TG / DTA 6200 thermal analyzer in a dynamic atmosphere of dinitrogen (flow velocity = 30 cm ³ / min). The sample was heated from 25 ° C. to 500 ° C. at a rate of 5 ° C./min in an alumina crucible.

<Surface area and hole size measurement>
The specific surface area was determined by the Brunauer-Emmett-Teller (TET) method from the nitrogen adsorption deisolated temperature line obtained by vacuum drying the sample at 100 ° C. for 20 hours and then using the Auantachrome Autosorb iQ2 automated gas sorption analyzer. I asked. The pore size distribution curve of the sericin-HAP composite floral structure was calculated using the Barrett-Joyner-Halenda (BHJ) method based on the desorption branch of nitrogen isotherm.

<Measurement of heavy metal ion adsorption>
_{By Pb (NO 3) 2, Cd} (NO 3) 2 · 4H 2 O and _{Hg (NO 3) 2 · H} 2 O as the heavy metal ion source is dissolved in water, the concentration of each various Pb (II), Aqueous solutions of Cd (II) and Hg (II) ions were prepared. 5 mg of the sericin-HAP composite flower-like structure was mixed with each of the above heavy metal aqueous solutions (initial concentration 15 mg / L), and the mixture was stirred at room temperature for 2 hours. After the end of a particular period, the solution was removed and immediately separated into a solid and a supernatant liquid by centrifugation. The concentration of metal ions remaining in the supernatant solution was analyzed by inductively coupled plasma atomic emission spectrometry (ICP-OES). This indicates the concentration of metal ions remaining in the solution without being adsorbed on the composite structure. The adsorption isotherm of Pb (II) was determined by changing the initial concentration (2 to 60 mg / L) and stirring at room temperature for a certain period of time.

On the other hand, a sericin-HAP complex was used as the filter material. About 10 mg of the complex powder was deposited on a commercially available paper filter. The filtration capacity was evaluated by permeating an aqueous lead ion solution (concentration 15 mg / L) through this simple filter and analyzing the ion concentration in the permeated solution by inductively coupled plasma emission spectrometry (ICP-OES).

An experiment was conducted in which heavy ions were redissolved and recovered from the complex adsorbed with heavy ions using a low-concentration acetic acid solution.

<Congo red dye adsorption measurement>
The dye adsorption properties of the sericin-HAP composite floral structure were investigated by examining the time-dependent UV adsorption of dye molecules. In a typical experiment, 6 mg of sericin-HAP complex floral structure was added to 3 mL of Congo red aqueous solution (0.05 mg / mL). UV measurements were performed using UV-VIS-NIR Spectrophotometer (V-570, JASCO) for samples with various retention times.

[Experimental results and analysis / examination]
The hybrid composite structure of one embodiment of the present invention was first made by a simple mixture of two elements: an aqueous solution of CaCl ₂ was first added to PBS containing a clear solution of the silk protein sericin. The resulting mixture was then gently shaken for a few minutes and kept at room temperature (about 25 ° C.). After 24 hours, a whitish, porous, uniformly grown flower-like precipitate appeared. FIGS. 1A and 1B show images of the overall morphology of this composite structure observed by SEM. The high-resolution SEM image of FIG. 1 (c) clearly shows the hierarchical composition in the flower-like morphology in the range of 10 to 15 nm in thickness. In this flower-like form, multiple petals extend from the center in various directions. Sericin itself has the function of self-assembling to form various nanoparticles. On the other hand, when calcium chloride itself was mixed with 1X PBS, it precipitated and formed large lumpy crystals, but no structure having a flower-like morphology was formed.

TEM images of the composite flower-like material in the synthesized state are shown in FIGS. 1 (d) to 1 (f). The petals made of nanosheets shown in FIG. 1 (e) have a curved and wavy shape, and have many voids between adjacent petals. The plaids in the high-resolution TEM image shown in FIG. 1 (f) correspond to the periodicity of the structure.

Energy dispersive X-ray (EDX) spectroscopic analysis of the complex flower-like material revealed the presence of the elements C, N, O, P and Ca. As shown in the EDX element mapping, as shown in FIG. 2, the elements Ca, P and O, which are the main elements of the HAP material, are compared with other elements in the petals made of nanosheets constituting the composite flower-like substance. It was predominantly distributed. Furthermore, it was also shown from FIG. 2 that the elements C and N derived from the sericin protein are uniformly distributed in the petals.

X-ray diffraction (XRD) was used to examine the characteristics of the crystal structure of the composite flower. As shown in FIG. 3 (a), the high crystallinity of the composite flower-like material was confirmed from the comparison between the sharp diffraction peak of the composite flower-like material and the broad diffraction peak of the amorphous silk sericin. Further, as also shown in FIG. 3A, the diffraction peak of the composite flower-like material was in good agreement with the diffraction peak of the natural HAP material (JCPSD No. 09-0432).

Furthermore, the surface characteristics of the composite flower-like material were investigated by X-ray photoelectron spectroscopy (XPS). The XPS profile of the sample scanned in the range 0 to 1400 eV as shown in FIG. 3 (b) showed the presence of the elements C, N, O, P and Ca. The high resolution XPS spectra of C1s and N1s clearly showed the presence of characteristic functional groups of the silk protein sericin. On the other hand, the binding energy data of Ca shows the presence of calcium phosphate (Ca2p: 347eV, 350.5eV), and the two main elements, O1s peak (531.5eV) and P2p peak (133.3eV), are PO. _It is thought that there are _{4 to} ³ units. Therefore, the feature binding energy peak clearly indicates the presence of protein molecules and calcium phosphate in the complex structure.

Thermogravimetric analysis (TGA) was performed to determine the proportions of the various components in the complex flower by taking advantage of the different weight loss steps of these elements. The initial weight loss in the temperature range of 25 ° C to 200 ° C was about 5.6% due to the removal of physically adsorbed and bound water molecules. The second weight loss step occurred at 200 ° C. to 450 ° C., which is considered to be due to the complete decomposition of the sericin molecule in the complex flower-like substance (Non-Patent Document 32). The magnitude of weight loss in this temperature range was approximately 5%, which should be considered to correspond to the total weight percent of sericin molecules in this composite. On the other hand, the mass of the residue after the heat treatment corresponds to the inorganic component in the composite structure, and the amount was calculated to be about 89.4%. This phenomenon was further confirmed by TG analysis of the precursor material sericin.

The formation mechanism of these hierarchical composite structures is similar to the recently published paper on the same calculated complex flower-like product (Non-Patent Documents 33 to 37). As shown in FIG. 3 (a), the peak position in the XRD pattern of the complex structure is the same as the peak position for the HAP crystal, and as shown in FIG. 1 (f), the HR-TEM of the petals which are nanosheets. From the widespread and uniformly arranged lattice structure observed in the image, it can be concluded that the petals of the composite flower-like material are formed by the regular arrangement of the HAP material. The uniform distribution of the elements C and N of the sericin protein indicates that the HAP frame of the petals is covered with the sericin layer. Sericin, which is an inherently hydrophilic protein, forms a complex with the surface of HAP mainly by the coordination mechanism of the amino acid backbone, and acts as an adhesive for forming petals, which are nanosheets. These nanosheets are further self-assembled to form a hierarchical porous structure with a flower-like morphology. The protein sericin plays an important role in self-organizing growth, as it does not form a hierarchical structure without HAP sericin.

The specific surface area, average pore size and pore volume of the hierarchical structure play a prominent role in enhancing the adsorption capacity of the material. These synthetic flower-like hierarchical composites were expected to have a large surface area. This surface area was measured by the standard Brunauer-Emmett-Teller (BET) method. FIG. 4A shows a typical nitrogen adsorption deisothermal line of HAP in the composite structure and synthesized state of the present invention. According to the results of the investigation, the three-dimensional HAP composite flower-like material in the prepared state clearly has a very large BET surface area of 373.2 m ² / g, which is in the synthesized state. Not only is it much larger than the BET surface area of the precursor HAP material, 153.9 m ² / g, but it is also significantly increased from the previously reported hierarchical HAP structures (Non-Patent Documents 38-41). doing. The adsorption results show type IV isotherms, and the characteristic hysteresis loops they have indicate the presence of mesopores in the microstructure (Non-Patent Documents 41 and 42), these mesopores. Is relatively narrow and has a mesopore size distribution mainly centered in the range of 3 to 20 nm, and as shown in FIG. 4 (b), from the Barrett-Joyner-Halenda (BJH) desorption method, 1. It has a cumulative pore volume of 239 cm ³ / g.

[Adsorption of heavy metals and organic dyes]
The composite flower-like substance according to the present invention can adsorb heavy metals and organic dyes as it is, and therefore can be used as a water purifying agent for purifying water contaminated with these. Typical heavy metal ions Pb (II), Cd (II) and Hg (II) are selected as adsorption targets, and the contact time with these aqueous solutions and the concentration of these ions as adsorbents in the aqueous solution are changed. The experiment was conducted. These heavy metal ions selected are one of the major environmentally harmful and highly toxic major pollutants in water resources, and their efficient removal is very important for ensuring the safety of drinking water. It is important (Non-Patent Documents 43-49).

FIG. 5 (a) shows the adsorption rate of Pb (II) ions having an initial concentration C ₀ of 15 mg / L on the hierarchical flower-like product prepared in the above embodiment of the present invention, that is, Pb (II) at each time point. The time change of C / C ₀ when the ion concentration is C is shown. From this, it was found that the adsorption process proceeded rapidly and reached an equilibrium state within 15 minutes from the start of adsorption. This rapid adsorption rate could be observed even when the concentration of heavy metal ions was changed. In order to further investigate the adsorption characteristics, the initial concentration of Pb (II) was changed in the range of 2 mg / L to 60 mg / L, and the adsorption isotherm experiment of the HAP composite flower-like substance was carried out at various time intervals (5 mg per 20 mL of the sample). Input) was performed. A part of the result is shown in the inset diagram of FIG. 5 (a). The maximum adsorption capacity of the HAP complex flower-like substance for Pb (II) was calculated to be 248 mg / g, which is the maximum adsorption capacity of the HAP structure under the same experimental conditions reported so far (Non-Patent Documents). It was much higher than 39-41) and was faster. Furthermore, as a comparative standard, the adsorption capacity of the bulk HAP as it was produced under the same experimental conditions was investigated, but the adsorption rate of the bulk HAP was as high as that of the HAP composite flower even after 2 hours of adsorption. It was much lower than the state. This confirms the need for a sericin-mediated composite flower structure having a large surface area for excellent adsorption efficiency from other aspects as well.

In addition, as shown in FIG. 5 (b), this hierarchical composite flower is an efficient adsorption for removing other dangerous heavy metals such as Cd (II), Hg (II) from contaminated water. It turned out to be a drug. Again, a rapid adsorption process was observed within the first 15 minutes, with subsequent adsorption progressing relatively slowly to achieve equilibrium. However, when the adsorption rates were compared between these ions, there was a considerable difference, and when arranged in order of decreasing rate, Hg (II) << Cd (II) <Pb (II). Further, as shown in FIG. 5 (b), the removal rates of these heavy metal ions Hg (II), Cd (II) and Pb (II) 15 minutes after the start of adsorption were 42.5%, 89% and respectively. It was 99.9%. This result clearly shows that the adsorption efficiency for Pb (II) is considerably higher than that for Cd (II), and when viewed at the same adsorption time, the adsorption efficiency for Hg (II) is that of the other two heavy metals. It was shown to be very low compared to ions.

The high adsorption efficiency of Pb (II) may be interpreted by using HSAB law (hard and soft acid-base theory) (Non-Patent Documents 50 and 51). In aqueous solution, Pb (II) ions are harder than Cd (II) and Hg (II) ions and behave as borderline Lewis acids. As a result, the Pb (II) ion preferentially interacts with the orthophosphate ion, which is a borderline Lewis base in the complex flower. Furthermore, it should be noted that electrostatic interactions, surface complex formation, flexibility and / or ion exchange may be involved in the realization of this hierarchical structure adsorption mechanism for heavy metal ions. Must be (Non-Patent Documents 52-54). The superior performance of the HAP complex as it is made is a layer with abundant surface functional groups derived from silk sericin, which provides a large surface area and a large number of active sites for heavy metal removal. It can be said that it is due to the structure.

In general, the binding of a metal to the HAP surface can be explained by an ion exchange mechanism that exchanges Ca (II) ions with M ^{+ 2} ions, that is, Pb (II), Cd (II), Hg (II), etc. (Non-Patent Documents) 55-57). Alkaline earth metals such as Ca (II) are easily replaced by divalent metal ions due to their tendency to change due to the ion exchange phenomenon (Non-Patent Documents 55 to 57). However, the observed differences in the efficiency of the adsorption process for various heavy metals depend on the effect of the combination of electronegativity and ionic radius to assist the ion exchange mechanism.

Furthermore, the very fast adsorption process (less than 15 minutes) and the destruction of the flower-like structure of the HAP complex after adsorption also show the involvement of the ion exchange mechanism (Non-Patent Documents 55 and 58). As mentioned above, Pb (II) has a strong affinity for the HAP complex, which is in good agreement with previous reports (Non-Patent Document 59). The relatively large adsorption of Pb (II) ions to the composite flower-like material when compared to other heavy metals is due to the appropriate electrical negativeness and ionic radius of the Pb (II) ions. It may promote better cation exchange between Pb (II) ions in the aqueous solution and Ca (II) ions in HAP in the sericin-mediated hierarchical porous structure. Thus, large amounts of Pb (II) ions accumulated very rapidly on the complex flower-like material, and then a series of longitudinally elongated nanorods with hexagonal faces began to grow. It is interesting to see that the densely packed nanorod structures with diameters of 20 nm to 200 nm cause distinctive growth on the hierarchical surface of HAP composite flowers (FIGS. 6 and 7).

By EDX analysis and sequential element mapping with a scanning transmission electron microscope (STEM), it was confirmed that Pb was present in the newly grown nanorod morphology as shown in FIGS. 7 (b) to 7 (d). No detectable nanorod morphology was observed at low initial concentrations of Pb (II) ions, but at higher concentrations a large number of nanorods grew on the composite flower. However, crystalline lead nitrate formed hexagonal microstructures in the aqueous solution. Differences in the degree and morphology of nanorod growth were due to differences in the concentration of Pb (II) ions adsorbed on and near the surface for crystal growth (Non-Patent Document 60). This result proved to be direct experimental evidence of very fast and strong fixation of heavy metal ions on the adsorbent surface and the resulting growth of nanorod morphology. On the contrary, for Cd (II) ions or Hg (II) ions, the electrical negativeness and ionic radius are not so preferable, so that the cation exchange phenomenon between the adsorbent and these heavy metal ions becomes relatively inactive. Under the same experimental conditions, very small amounts of these heavy metals were accumulated on the surface of the composite flower-like material, and the peculiar growth of the corresponding heavy metal nanorods as described above did not occur. This is very important for the complete and rapid removal of dangerous cations from water and for safe disposal to avoid leaching from adsorbents. In addition, the densely and uniformly distributed structure of nanorods may provide a means for recovering and reusing incorporated heavy metals.

In addition, an attempt was made to extend the sericin-embedded HAP complex to the removal of contaminated and dangerous dyes. Congo red (CR), which is a benzidine-based anionic dye, is mainly discharged from factories in the textile industry, and this dye was taken up as a specific example of a removal target (Non-Patent Documents 62 and 63). CR is acutely toxic to aquatic organisms and has carcinogenicity to humans (Non-Patent Document 61). Therefore, it is very important to remove the CR remaining in the water before it is released into the environment. FIG. 8 shows the results of an experiment in which a CR dye having an initial concentration of 50 mg / L was adsorbed using a hierarchical HAP composite structure. The upper side of the inset of FIG. 8A is the CR aqueous solution (the left is just before the start of adsorption, the right is 12 hours after the start of adsorption), and the lower side is the hierarchical HAP composite structure powder (the left is the injection into the CR aqueous solution). As can be seen from the front and right, the powder that was adsorbed for 12 hours was taken out), most of the dye molecules moved from water to the adsorbent, and after 12 hours, the color of the aqueous solution became considerably lighter and pale red (Fig. Since 8 (a) is monochrome, it looks gray). As shown in FIG. 8B, almost 50% of the dye molecules were adsorbed in the first 4 hours after the start of adsorption, and the subsequent adsorption rate decreased. The large adsorption rate in the early stages is probably due to the rapid contact of CR molecules with the active sites on the outer surface of the adsorbent. The hierarchical HAP complex is generally covered with various surface active groups such as hydroxyl groups, carboxyl groups and amino groups derived from the amino acid skeleton of silk sericin. Thus, CR dyes functionalized with amino and sulfone groups have a strong interaction with the sericin-mediated HAP complex by electrostatic interaction or hydrogen bonding, which makes this dangerous dye prominent. (Non-Patent Document 62).

A sericin-HAP complex was used as the material for the filtration filter that could be used for water purification. About 10 mg of the complex powder was deposited on a commercially available paper filter (see FIG. 9 (a)). When a lead ion aqueous solution (concentration 15 mg / L) is permeated through this simple filter system (FIG. 9), lead ions are adsorbed on the complex and purified. Filtration capacity was evaluated by analyzing the ion concentration in the permeated solution by inductively coupled plasma atomic emission spectrometry (ICP-OES). The lead ion concentration in the water before filtration was 9.02 mg / L to 0.01 mg / L after filtration, which was almost 100% efficient.

An experiment was conducted in which heavy ions were redissolved and recovered from the complex on which heavy ions were adsorbed using a low-concentration acetic acid solution. First, 5.7 mg of the hybrid complex is mixed with 20 mL of an aqueous solution having a lead ion concentration (15 mg / L) and stirred for 30 minutes to adsorb lead ions on the complex. The complex on which lead ions are adsorbed is separated by centrifugation, 20 mL of a 1% aqueous acetic acid solution is added thereto, and the mixture is left for half a day. Then, the solution and the complex are separated by centrifugation to determine the lead ion concentration in the supernatant. When examined by ICP-OES, it was 6.70 mg / L. We succeeded in separating lead ions. This indicates the possibility of separation and reuse of lead ions.

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Claims (12)

Has multiple sericin-hydroxyapatite composite nanosheets,
A sericin-hydroxyapatite composite structure in which the plurality of nanosheets are bonded at the central portion so as to extend from the central portion in different directions. The sericin-hydroxyapatite composite structure according to claim 1, which has a substantially spherical shape or a porous shape. The sericin-hydroxyapatite composite structure according to claim 1 or 2, wherein the average size is in the range of 2 to 10 μm. The sericin-hydroxyapatite composite structure according to any one of claims 1 to 3, wherein the average thickness of the sericin-hydroxyapatite composite nanosheet is in the range of 10 to 25 nm. The method for producing a sericin-hydroxyapatite composite structure according to any one of claims 1 to 4, as a precipitate from a phosphate buffered saline in which sericin and calcium chloride are dissolved. The method for producing a sericin-hydroxyapatite composite structure according to claim 5, wherein the sericin concentration is 0.1 to 3 mg / mL and the calcium chloride concentration is 10 to 300 mM. An adsorbent for adsorbing at least one of a heavy metal ion and a dye composed of the sericin-hydroxyapatite composite structure according to any one of claims 1 to 4. The adsorbent according to claim 7, wherein the heavy metal ions and dyes are one or more selected from the group consisting of Pb (II), Cd (II), Hg (II) and Congo Red. A water purifying agent comprising the adsorbent according to claim 7 or 8. A water purifying agent comprising a film-like filter in which the sericin-hydroxyapatite composite structure according to any one of claims 1 to 4 is laminated. A method for regenerating the adsorbent according to any one of claims 7 to 9, wherein the adsorbed heavy metal ion is separated from the sericin-hydroxyapatite composite structure by a reagent that dissolves the heavy metal ion. The method for regenerating the water purifying agent according to claim 9 or 10, wherein the adsorbed heavy metal ions are separated from the sericin-hydroxyapatite composite structure by a reagent that dissolves the heavy metal ions.