CHEN Han WU Chun-Hua HUANG Yi WU Hua-Hua PANG Ji

a (Engineering Research Centre of Fujian-Taiwan Special Marine Food Processing and Nutrition, Ministry of Education, Fuzhou 350002, China) b (State Key Laboratory of Food Safety Technology for Meat Products, Xiamen 361100, China) c (College of Food Science, Fujian Agriculture and Forestry University, Fuzhou 350002, China) d (Key Laboratory of Marine Biotechnology of Fujian Province, Institute of Oceanology, Fujian Agriculture and Forestry University, Fuzhou 350002, China) e (Fujian University of Technology, Fuzhou 350118, China)

ABSTRACT It is emergent to develop a green waste water adsorbent with high efficiency. Therefore, a type of low-cost, green and environmentally friendly konjac glucomannan (KGM) -silk fibroin (SF) composite aerogels were compounded via simple chemical grafting and vacuum freeze drying, and a study on its adsorption capacity was also conducted. The characterizations of FT-IR, SEM, XRD and DSC indicate that the modified aerogels show a porous network space structure and there is a strong hydrogen bond effect between the KGM and SF molecules, which improves the density, compressive strength and thermal stability of aerogel materials. The adsorption experiments show that KGM-SF aerogels can effectively adsorb the water pollutants Cr(Ⅲ) with a maximal adsorption capacity of 82 mg·g-1. In addition, the adsorption isotherm and dynamic model analysis are used to elaborate the adsorption mechanism of KGM-SF aerogels and explain that the composite aerogels can be single molecule chemisorption. KGM-SF aerogels have potential adsorption capacity.

Keywords: konjac glucomannan, silk fibroin, aerogel, adsorption, Cr(Ⅲ)

1 INTRODUCTION

With the advancement of industrialization, heavy metal pollution in oceans and other water is becoming increasingly serious, and its teratogenicity and carcinogenicity are threatening on human health and have attracted great concern[1-4]. As a method for the separation and disposal of heavy metals, the adsorption method has arose great concern and research due to its advantages of simple operation, technical maturity and wide application[5,6].

Aerogels are a kind of advanced materials with promising applications in chemical engineering, biomedical, food and pharmacetical area[7,8]. Recent researches discover that aero- gel materials as the carrier for immobilization have good adsorption effects[9-13]. However, the recycling[14]and degradation[15]after adsorption are another difficult process and also cause recontamination, which to great extent restricts its application. Therefore, producing a natural, degradable and high-performance aerogel material has become one of the research hotspots in the polymer material field[16-22].

KGM, a natural polysaccharide polymer with excellent biocompatibility, biodegradability, and hydrophilicity has been used in materials, food, and biomedical fields[23-27]. Taking the water pollutant Cr(Ⅲ) as an example, this research used SF to modify KGM, prepared KGM-SF aerogels, characterized its structure by the use of fourier infrared spectrometer (FT-IR), scanning electron microscope (SEM), X-ray diffractometer (XRD) and differential scanning calo- rilmeter (DSC), and researched its strength and thermal stability. Meanwhile, the effects of the dosage of KGM-SF aerogels adsorbent, contact time, the pH of the solution and initial concentration on adsorbing Cr(Ⅲ) were investigated so as to provide a theoretical basis and guide for the development and application of aerogels made of natural polymer polysaccharide in the field of immobilization carrier materials.

2 EXPERIMENTAL

2. 1 Materials

Konjac glucomannan (KGM) (purity ≥ 95%, viscosity: 1.5% solution, ≥ 35,000 MPas) was purchased from Yizhi Konjac biological technology Co. Ltd. (Hubei, China). Silk fibroin (SF) (average molecule weight (Mw): 90) was provided by Huzhou Xintiansi Bio-tech Co., Ltd. (Zhejiang, China). The adsorbates (Cr(Ⅲ)) used in this study were supplied by Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). All other reagents (analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Purified water with a resistance greater than 18 MΩ· cm-1was used in all experiments.

2. 2 Preparation of KGM aerogels

The quantitative KGM was placed at normal temperature and dispersed in 100 mL deionized water under a 350 rpm stirring for 60 mins. Then a KGM sol solution of 3% (w/v, w in gram and v in milliliter) was obtained and stored in a 4 ℃ refrigerator for 24 h. The KGM aerogels were then obtained after 18 h of liquid nitrogen quick freezing and vacuum freeze drying.

2. 3 Preparation of KGM-SF aerogels

KGM-SF aerogels synthesized by adopting a sol-gel method under freeze-drying are shown in Fig. 1. The SF solution was obtained through dissolving 300 mg SF in 10 mL deionized water[28]. The SF solution and KGM sol solution at a ratio of 1:1 were stirred with a rotation speed of 350 rpm in a constant temperature water bath for 60 mins and stored in a 4 ℃ refrigerator for 24 h. Finally, KGM-SF aerogels were obtained after 18 h of liquid nitrogen quick freezing and vacuum freeze drying.

Fig. 1. Conceptual graph of KGM-SF aerogels preparation and adsorption

2. 4 Characterization of aerogels

The Fourier transform infrared spectra of the aerogels were performed by using a Nicolet 6700 FTIR spectrometer (FT-IR, Nicolet 6700, Thermo Fisher Scientific Co., Ltd., USA) in the range of 4000～500 cm-1. The structural morphology of the aerogels was determined using scanning electron microscopy (SEM, Hitachi S-4800, Hitachi, Japan). The phase structures of KGM/SF aerogels were investigated through X-ray diffraction on a D8 Advance XRD diffractometer (XRD, Bruker AXS D8 Advance, Bruker Inc., Germany) with Cu-NF filter at a scan rate of 0.1/s. DSC was measured with a DSC200F3 instrument (Zetzsch, Germany) from 25 to 150 ℃ at a rate of 5 ℃·min-1under nitrogen flow at a rate of 50 mL·min-1.

2. 5 Adsorption test of Cr(Ⅲ) by KGM-SF aerogels

Approximately, 15 mg KGM-SF aerogels was added to the conical flask containing 25 mL Cr(Ⅲ) solution, and its pH was adjusted by adding HClO4and NaOH solutions. Then it was placed in a constant temperature water bath shaker and shaken for a certain time. The concentration of Cr(Ⅲ) before and after adsorption in the solution was measured by using ICP-OES in the Ultraviolet and Visible Spectrophotometry (n = 652 nm)[29,30]. All the experiments were performed twice to get the average results.

The equilibrium adsorption capacity (qe/mg·g-1), partition coefficient (Kd/ml·g-1), and the adsorption rate (S) of KGM-SF aerogels for Cr(Ⅲ) were calculated respectively via the equations (1～3).

In the above equations, C0represents the initial con- centration of the metal ions, Ceis the equilibrium con- centration of the metal ions, V means the volume of the solu- tion measured, and m stands for the mass of the adsorbent.

3 RESULTS AND DISCUSSION

3. 1 FT-IR, SEM, XRD, and DSC analyses

The FT-IR spectra of KGM and KGM-SF aerogels in the range from 4000 to 500 cm-1are shown in Fig. 2. The peaks between 3000 and 2800 cm-1are the C-H stretching vibration of CH2group and C-H of CH3in the saturated structure, and the absorption peak intensity of KGM-SF aerogels within the range of 1500～1350 cm-1becomes stronger, which explains that Maillard reaction can lead to the increase of C-OH vibration and C-H deformation vibration[31-34], and these two groups of peaks also verify the existence of carbohydrate in KGM aerogels. Amide Ⅱ band can vividly display the hydrogen-bond interaction between or in macromolecules[35,36]. Compared with KGM aerogels, the peak intensities of KGM-SF aerogels in the amide I band (1700～1600 cm-1) and amide II band (1600～1500 cm-1) show an increasing trend, and the corresponding absorption peaks also shift toward high wave numbers. In Fig. 2, the changes of the peak shape of the amide I band in KGM-SF aerogels further explain that the SF intramolecular structure is destroyed; in the KGM-SF aerogels, an absorption peak disappearing in the KGM aerogels appears in the amide II band, which indicates the presence of strong hydrogen bonding between protein and polysaccharide molecules after the covalent bonding of KGM and SF (Fig. 3).

Fig. 2. FT-IR spectra of KGM and KGM-SF

Fig. 3. Hydrogen bonds between KGM and SF

As shown in Fig. 4(a) and 4(b), both KGM and KGM-SF aerogels show white sponge-like shape when observed with the naked eyes. Using SEM to observe its microstructure, it can be seen from the figure that compared with KGM aerogels, the hydrogen bonding between KGM and SF in KGM-SF aerogels forms a dense and ordered porous network cross-linked structure, indicating that SF can improve the stability of KGM aerogels; the interaction between KGM and SF reduces the aggregation of KGM molecules, further reduces the pore sizes of aerogels, increases the surface area, and enhances the interaction between molecules. Therefore, these regular porous network structure pores can provide more metal adsorption sites, which is conducive to obtaining nano porous network adsorption materials with ideal adsorption capacity[37,38]. This may be due to the strong hydrogen bonding interaction between SF and the functional groups.

Fig. 4. Photographs of (a) KGM and (b) KGM-SF; SEM images of (c) KGM and (d) KGM-SF

It can be seen from Fig. 5(a) that KGM aerogels have no significant crystallization peak within the whole scanned area and shows an undefined structure, and there is only a wide diffuse peak at 2θ = 20.14º. From the diffraction spectrum of KGM-SF aerogels, it can be discovered that the diffuse peak of KGM aerogels is strengthened due to the addition of SF. This shows a strong interaction between KGM and SF. Through cross-linking, intramolecular and intermolecular hydrogen bonding, KGM and SF make the molecular chains intertwined, thus forming a stable porous network structure, and can serve as a physical cross-linking point to increase the cross-linking density of aerogel materials, so that the density and compressive strength of aerogels can be improved[39]. It also proves a good compatibility between KGM and SF.

Fig. 5. (a) XRD patterns of KGM and KGM-SF; (b) DSC patterns of KGM and KGM-SF; (c) Effect of sample volumes on the adsorption capacity (qe) of Cr(Ⅲ); (d) Effect of pH on the adsorption capacity of Cr(Ⅲ)

It can be seen that both KGM and KGM-SF aerogels exhibit two main thermal transition peaks from Fig. 5(b). The Td of the KGM-SF aerogel sample is 99.0 ℃ and the Ti is 31.6 ℃, while those of the KGM aerogels sample are 117.2 and 29.1 ℃, respectively. Compared with KGM aerogels, the endothermic peaks of KGM-SF aerogels are sharper and narrower, which indicates that the latter are more stable. In terms of enthalpy (ΔH), 0.2226 J/g of KGM-SF aerogels is higher than that of KGM aerogels, which explains the SF can improve the thermal stability of KGM aerogels[40].

3. 2 Effect of the amount of adsorbent on adsorption

As shown in Fig. 5(c), with the increase of sample amount, the adsorption capacity of KGM-SF aerogels to Cr(Ⅲ) decreases, but the adsorption rate increases. When the 15 mg sample was added, the adsorption capacity of KGM-SF aerogels to Cr(Ⅲ) exceeded 50 mg·g-1, and the adsorption rate of KGM-SF aerogels to Cr(Ⅲ) could reach about 62%. Since the sample amount needed under this condition was small and the better adsorption effect could be achieved, all subsequent experiments chose 15 mg KGM-SF aerogels as the amount of adsorbent at the optimal point.

3. 3 Effect of pH on the adsorption

Fig. 5(d) represents the effect of pH on the adsorption capacity of KGM-SF aerogels to Cr(Ⅲ). In water solution, the pH value significantly affects the binding sites, surface charge and existence form of Cr(Ⅲ), thus becoming an important parameter of affecting the adsorption capacity of Cr(Ⅲ). From Fig. 5(d), it can be seen that when the pH of the solution is 1.0～4.0, the growth rate of the adsorption capacity of the sample is the fastest; it gradually slows down when the pH value is 4.0～8.0; and it finally stabilizes at the pH of 8.0～10.0. The reason for this phenomenon is that when pH < 4.0, H+in the solution competes with the adsorption sites on the KGM-SF aerogels; when 4.0 8.0, the adsorption capacity of KGM-SF aerogels increases because of the precipitation in the system.

3. 4 Analysis of adsorption isotherm

The effect of different initial concentrations on the KGM-SF aerogels’ adsorption capacity of Cr(Ⅲ) is shown in Fig. 6(a), in which the KGM-SF aerogels’ adsorption capacity of Cr(Ⅲ) displays a growing trend with increasing the initial concentration of Cr(Ⅲ). When the initial concentration of Cr(Ⅲ) reaches 200 mg·L-1, the adsorption capacity of the sample is about 82 mg·g-1. Researches on the adsorption isotherm are also made to further understand the functional mechanism of the interaction between the adsorbent substances and the adsorbent solvents[41,42]. This process describes the relationship between the concentration of adsorbent and the adsorption capacity through two isothermal adsorption model equations: Freundlich isotherm and Langmuir isotherm[43]. The equation linear representations of Langmuir and Freundlich are respectively shown in equations (4) and (5). From the results of Fig. 6(b) and 6(c), the assumed Langrnuir model based on the single molecule adsorption has a higher fitting correlation coefficient compared with the correlation coefficient of the Freundlich model. Therefore, the KGM-SF aerogels’ adsorption of Cr(Ⅲ) may be a process of single molecular adsorption.

In the above equations, b is the equilibrium constant, qmis the maximal adsorption capacity of Langmuir monolayer adsorption, and KFand n represent the empirical coefficients in the process of adsorption within a certain range.

Fig. 6. (a) Effect of the initial concentration on adsorption capacity of Cr(Ⅲ); (b) Langmuir isotherm model; (c) Freundlich isotherm model; (d) Effect of contact time on the adsorption capacity of Cr(Ⅲ); (e) Pseudo-first order kinetic model; (f) Pseudo-second order kinetic model

3. 5 Analysis of the adsorption kinetics

Fig. 6(d) represents the effect of time on the adsorption capacity of KGM-SF aerogels. It can be seen that the growth rate of adsorption capacity is rapid in a few hours of the initial contact, reaching about 65% of the equilibrium adsorption capacity, and the adsorption reached equilibrium at about 10 h. The pseudo-first and pseudo-second order kinetic models can be applied to the researching process of adsorption[44]. The two models are respectively represented by equations (6) and (7).

In the above equations, qestands for the adsorption capacity at the equilibrium state, qtrepresents the adsorption capacity at any time (t), and k1and k2are the adsorption rate constants of the pseudo-first and pseudo-second order kinetics, respectively. Fig. 6(e) shows the pseudo-first order kinetic model, and Fig. 6(f) represents the pseudo-second kinetic model. Though making an analysis and comparison, the pseudo-second order kinetic model based on chemical adsorption is found to be more suitable to describe the process of KGM-SF aerogels adsorption of Cr(Ⅲ). Therefore, KGM-SF aerogels adsorption of Cr(Ⅲ) is mainly a chemical adsorption process. The KGM-SF aerogels prepared in this study are a perfect support that can hold the micro particles to perform the adsorption ability. The obtained KGM-SF aerogels constitute a feasible option as a Cr(Ⅲ) sorbent due to its potentially low cost and environmentally benign nature.

4 CONCLUSION

In conclusion, this research has developed KGM-SF aerogels with a porous network structure, which endows it with high order, high strength, stable thermal properties and more unique mechanical performance. The results of FT-IR, SEM, XRD and DSC spectra indicate that there is good compatibility between KGM and SF, and there is a strong hydrogen bond between KGM and SF molecules in KGM-SF aerogels, which may be the main reasons for the improvement of its performance; the adsorption results indicate that KGM-SF aerogels can stably exist in the water solution and have certain adsorption effects on the water pollutant Cr(Ⅲ). Its maximal adsorption capacity is about 82 mg·g-1, and the equilibrium time is about 10 h, which also verifies that the pH of the adsorbent has a significant effect on the adsorption capacity; the experimental results show that the Langmuir isothermal model and pseudo-second order kinetic model are more suitable to describe KGM-SF aerogels’ adsorption process of the water pollutant Cr(Ⅲ), and that the process may be single molecule chemisorption.

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