urea aqueous solution

urea aqueous solution

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Carbohydrate Polymers xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol

Preparation of cationic konjac glucomannan in NaOH/urea aqueous solution ⁎

Kai Wang, Shanjun Gao , Chunhui Shen, Junwei Liu, Siyu Li, Jiqin Chen, Xuechao Ren, Yuan Yuan Department of Polymer Materials and Engineering, School of Material Sciences and Engineering, Wuhan University of Technology, Luoshi Road 122#, Wuhan 430070, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Konjac glucomannan Cationization Flocculation

Cationic derivatives of konjac glucomannan (KGM) were homogeneously synthesized by reacting KGM with 3chloro-2-hydroxypropyltrimethylammonium chloride (CHPTAC) in NaOH/urea aqueous solutions. The derivatives were characterized by FT-IR, 1H NMR, elemental analysis, thermogravimetric analysis and zeta potential analysis. The results showed that the degree of substitution (DS) of the CKGM with the value of 0.15–0.46 could be obtained by changing the molar ratio between KGM and CHPTAC. The TG/DTG revealed that the thermal stability of KGM after cationization was lower than that of raw KGM. Among a wide range of pH value, the zeta potential in CKGM solution was shown with positive charges. The flocculation capacity of the CKGM was assessed through Kaolin suspension using the light transmission test. The results exhibited that the CKGM had excellent flocculation performance and could be used as an emerging flocculant agent in the wastewater treatment.

1. Introduction

Chen, & Yang, 2014). Besides, the impact of the acetyl on the KGM has been researched deeply (Gao & Nishinari, 2004). KGM is water-soluble, and when sodium hydroxide solution is added, the whole system turns into an irreversible gel (Nishinari, 2000). In accordance with the spectrum of FT-IR, the peak at 1735 cm−1 which belongs to the absorption peak of acetyl disappeared, confirming acetyl actually endows KGM with a certain water solubility. The utilization of KGM is inseparable from the chemical modification. Thus, finding a proper solvent and appropriate method is vital to broaden the scope of KGM usage. Owing to its strong inter- and intra- molecular hydrogen bond, KGM could be barely dissolved in most organic solvents. Traditional study claims that KGM could be dissolved in the solution of distilled water (Lei, Yang, Jia, & Zhang, 2010), NaOH/thiourea (Yang, Xiong, & Zhang, 2002) and isoamyl acetate (Torigata, Inagaki, & Kitano, 1951), but at room temperature, it became unstable or formed a heterogeneous system which would led a low efficiency. According to a recent study, some ionic liquid such as 1-allyl-3-methyl imidazolium chloride (AMIMCl) and 1-butyl-3-methylimidazolium chloride (BMIMCl) were used as efficient solvent for KGM because of the hydrogen bond formed between KGM and ionic liquid (Shen, Li, Zhang, Wan, & Gao, 2011). But the ionic liquid was easy to react with other reagents, which limits its application. Many breakthroughs have been made in the research of the modification of KGM, such as cross-linking modification (Gao, Guo, Wu, &

Nowadays, the treatment of wastewater has become an effective measure to solve the serious issue of water resources shortage (Luo et al., 2014). Therefore, the development of flocculants becomes the focal point of solving this problem. Among all sorts of flocculants, inorganic coagulants are being commonly used due to its low cost and ease of use, but their application is constrained with low flocculating efficiency and the presence of residue metal ion in the treated water. As for the organic polymeric flocculants, owing to its remarkable ability to flocculate efficiently with low dosage, they are widely used in the water purification industry. However, its lack of biodegradability and dispersion of monomers residue in water that may cause a health issues. Therefore, natural polymers which own lower toxicity, low expense and biodegradability, have been identified as a kind of very promising flocculants and have drawn the attention of numerous researchers (Lu, Zhao, Qi, & Chen, 2015; Yang, Yan et al., 2013; Yang, Yang et al., 2013). Konjac glucomannan (KGM), one of the polysaccharides which is extracted from the tuber of Amorphophallus konjac K, Koch (Chua et al., 2012), is a very significant bio-renewable resource. It consists of β-1, 4 linked D-mannose and D-glucose in a ratio of 1.6:1 (Kato & Matsuda, 1969). Although about 1 in 19 units on the side chains of the molecules are acetylated (Maekaji, 1978; Maeda, Shimahara, & Sugiyama, 1980), the exact branched position is still in debate (Zhang, ⁎

Corresponding author. E-mail address: [email protected] (S. Gao).

https://doi.org/10.1016/j.carbpol.2017.11.084 Received 8 September 2017; Received in revised form 22 November 2017; Accepted 22 November 2017 0144-8617/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: Wang, K., Carbohydrate Polymers (2017), https://doi.org/10.1016/j.carbpol.2017.11.084

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In the above equation, N% represents for the nitrogen content, 162.15 is the molecular weight of anhydroglucose units, and 151.5 is the molecular weight of glycidyl trimethylammonium chloride. FT-IR spectra of raw and cationic KGM were recorded using a Nicolet6700 by the KBr disc method, and the samples were mixed with dry KBr evenly. Then, the sample in an appropriate amount was poured into a clean tableting mold. The spectra of 400–4000 cm−1 was obtained. The 1H NMR spectrum of cationic KGM was recorded on Bruker AVANCE III™ operating at 500 MHz. 20 mg cationic KGM was dissolved in D2O at room temperature, and the chemical shifts were represented in δ(ppm) relative to the resonance.

Wang, 2008; Gao, Guo, & Nishinari, 2008; Gao & Zhang, 2001), carboxymethyl modification (Wang et al., 2014), ethylation (Lan et al., 2009), cyanoethylation (Shen, Li, Zhang, Wan, & Gao, 2012), oxidation (Yu, Lu, & Xiao, 2010) and so on. However, only a few studies focus on the subject of cationized KGM, which has been widely used in water treatment (Ren, Zhang, Qin, & Li, 2016), cosmetic (Pang, Lin, Zhang, Tian, & Sun, 2003), papermaking (Wang, He, Yan, & Song, 2017) and bio-technical (Yu, Huang, Ying, & Xiao, 2007). As for the preparation methods, cationic KGM was prepared by dissolving KGM into isopropyl alcohol (Niu, Wu, Wang, & Shi, 2006) and water (Tian, Wu, Liu, & Xie, 2010). However, it could transform into a large viscosity system even if the concentration of KGM was very low. In this study, KGM was dissolved in NaOH/urea aqueous solution to get a homogeneous system, after that, the etherification agent 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHPTAC) was added to the solution. The influence of molar ratio between KGM and CHPTAC on the degree of substituent (DS) has been taken into consideration. Furthermore, the derivation of KGM was characterized by elemental analyzer, FT-IR, thermogravimetric analysis and zeta potential test. The flocculation performance of CKGM was also studied in our experiment. Colloids in water are usually negatively charged, therefore, the more cation ions are ionized by flocculants, and the more superior wastewater treatment might be. In our test, light transmittance test worked as the tool to study the flocculation performance of cationic KGM, and the effect of different DS on flocculation performance was also evaluated.

2.4. Thermal analysis The thermogravimetric analysis (TGA) of the raw KGM and cationic KGM (with DS = 0.46) were conducted by using TGA Q50 V20.13 Build 39 in the Nitrogen atmosphere. The temperature of the experiment heated from room temperature to 600 °C in a rate of 10 °C/min. All samples were placed in a dry environment prior to testing. 2.5. Zeta potential analysis Zeta potential of KGM and cationic KGM with different degree of substitution were detected by using an electrophoretic light scattering with a zetasizer (nano ZS90, Malvern, UK). The solution of the test was prepared by dissolving 5 mg sample in 100 ml deionized water. The pH of the system was titrated with 0.25 mol/l hydrochloric acid and 0.25 mol/l sodium hydroxide.

2. Experiment 2.1. Material

2.6. Flocculation experiments 6

KGM (Mv = 1.35 × 10 ) was purchased from Hubei Konson Konjac Co., Ltd. (further purified by mixing KGM with its four times weight of 50 wt% and 80 wt% ethanol for 2 h, respectively, and with anhydrous ethanol for 4 h, and then vacuum-dried at 60 °C for 4 h). CHPTAC, chemically pure, was provided by Shanghai Macklin Biochemical Co. Ltd. sodium hydroxide and urea were supplied by Sinopharm Chemical Reagent Co., Ltd. Kaolin was purchased from Suzhou Kaolin Production Co. (Suzhou, China). Other chemicals were of analytical reagent.

5 mg cationic KGM with different degree of substitution was dissolved in 20 ml deionized water to configure aqueous solution with the concentration of 0.25 mg/ml. Kaolin suspension, which is one of the main components of river sediment or mud, was often used to assess the effect of flocculants in water purification owing to its relatively small particle size, favorable dispersion in water, stable composition and low cost. In this experiment, we obtained a stable 0.25 wt% kaolin suspension by mechanical agitation at different pH value to simulate the wastewater environment. 0.1 ml to 1.0 ml cationic aqueous solution was added to the Kaolin suspension. The whole system was intensely stirred for 10 min, and followed by slow stirred for 5 mins. After that, the suspension system was settled in a static state for 5 mins. Finally, the upper liquid of the system was extracted by a centrifuge tube for light transmittance experiments at a wavenumber of 420 nm by a 722N UV–vis spectrophotometer.

2.2. Preparation of cationic KGM Homogeneous KGM solution was prepared according to the previous method (Gao, Wu, & Nishinari, 2008). In a 100 ml round-bottomed flask equipped with mechanical agitation, 1.98 g sodium hydroxide, 1.65 g urea and 29.37 g distilled water (6:5:89 in weight) were stirred till a transparent solvent was generated. Afterwards, 0.5 g dry KGM powder was dispersed into it and a transparent and low viscosity solution was obtained after intensely stirred for 8 h at room temperature. Furthermore, the requisite amount of cationization reagent 3chloro-2-hydroxypropyltrimethylammonium chloride, was added through pressure-equalizing dropping funnel. After stirring for the required time, the reaction mixture was neutralized with 10 wt% hydrochloric acid aqueous solution, then the excess chloride ions were removed by dialysis through an 8000–14,000 molecular weight cut-off dialysis tubing. At last, the crude product was vacuum freeze-dried to obtain the purified white cationic KGM flocculation.

3. Results and discussion 3.1. Synthesis of the cationic konjac glucomannan The mechanism of dissolving KGM in NaOH/urea was speculated according to some related research of cellulose (Luo & Zhang, 2013). On the one hand, because of the deacetylation reaction in the alkaline environments, the KGM became water-insoluble. On the other hand, new hydrogen bond networks was formed through the interaction between NaOH and KGM, and the urea could be self-assembled at the surface of the NaOH hydrogen-bonded KGM to form an inclusion complex (IC). With regard to the cationic KGM, it was synthesized by using CHPTAC as etherifying agent under the alkaline environment. Firstly, the epoxide was produced in situ from CHPTAC by utilizing appropriate base. Moreover, due to the nucleophilic effect of sodium hydroxide, the hydroxyl groups of the KGM can react with epoxy functional groups easily. The reaction was presented in Scheme 1.

2.3. Characterization of the products The degree of substitution (DS) was determined by nitrogen contents which were measured by elemental analyzer (Vario EL cube) and calculated through the following equation: DS = 162.15 × N% /(1400 − 151.5 × N%) 2

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Scheme 1. The reaction of cationic KGM with CHPTAC.

stretching vibration absorption peak of CeOeC. As for the product of cationic modified KGM, the broad band at 3419 cm−1 is due to the stretching vibration peak of OeH groups. Besides, the peak at 1735 cm−1 disappears due to removal of the acetyl side groups from KGM under deacetylation. The peak at 1479 cm−1 is demonstrated to belong to the methyl groups of ammonium (Loubaki, Ourevitch, & Sicsic, 1991), which is the biggest striking difference between KGM and QKGM spectra. The FT-IR spectrum showed the introduction of the quaternary ammonium salt group on KGM backbone successfully. Fig. 2 illustrate the 1H NMR spectrum of cationic KGM in D2O at ambient temperature. The chemical shift at 4.71 is attributed to the peak of residual solvent. The three signals in the 1H NMR spectrum of area A at δ = 3.2, 4.4 and 3.47 ppm are assigned to the hydrogen proton located at position 1, 2, 3 in CKGM chain. The peak at δ = 3.2 ppm is turned out to be the methyl groups of quaternary ammonium salt group (Lim & Hudson, 2004), indicating that cationic group is successfully grafted to the KGM chains through etherification. The spectrum result revealed that the cationic KGM is synthesized in NaOH/urea solution through the homogeneous way. The influence of the molar ratio of CHPTAC and KGM on the DS value was depicted in Fig. 3. The molar ratio between CHPTAC and KGM had a significant influence on the DS value. Based on Fig. 3, we could know that the overall trend is divided into two parts. In the first stage, the DS increases with the increasing proportion of CHPTAC to KGM, and it can reach 0.46 when the ratio between CHPTAC and KGM is 10:1. However, further increase of CHPTAC was ineffective for the enhancement of the DS value. The reasons are as follows. When the molar ratio between CHPTAC and KGM is 4, the DS of the product is 0.2. This is owing to a low amount of CHPTAC, which led to a low efficiency, and the presence of water will makes the effect of side reaction more severe. However, further increase in the amount of CHPTAC lead to the decline in the value of DS, which is owing to the steric effect between the cationic charges, there are no excess position is provided for substitution reaction, besides, the amount of water in the system also increase with the increase of CHPTAC, resulting in the more intense side reaction. So the DS value decrease with the increase of proportion between CHPTAC and KGM.

From the reaction we could know that the role of sodium hydroxide in this system is more complicate. First of all, NaOH/urea is the right solvent which stimulates KGM to show its best solubility. Secondly, in the cationic reaction, NaOH is a nucleophile which could let KGM a certain reactivity. Besides, sodium hydroxide is also an important material for the formation of epoxy groups. However, the side effect of NaOH could not be ignored. When NaOH is at high amount, it could hydrolyze CHPTAC and produce the diol. In addition, KGM would also be degraded in such condition. Therefore appropriate dosage of NaOH is very important for the experimental results. 3.2. Characterization of the cationic konjac glucomannan The FT-IR spectra of raw KGM and cationic derivative are depicted in Fig. 1. In the IR spectrum of raw KGM, the extremely broad band at 3415 cm−1 is attributed to the stretching vibration modes of OeH groups, the peaks at 2929 cm−1 and 2887 cm−1 are corresponded to the stretching vibration of CeH bond. The absorption peak of the carbonyl group appears at 1735 cm−1. Besides, the band at 1638 cm−1 is caused by the bending vibration absorption peak of the hydroxyl group. Meanwhile, the peak of 1050 m−1 and 1028 cm−1 are ascribed to

Fig. 1. FT-IR spectra of raw KGM (RKGM) and cationic KGM (CKGM).

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Fig. 2. 1H NMR spectrum of cationic KGM in D2O at room temperature.

water. Then the weight of the sample is not changed until the temperature rise to 230 °C, which is the beginning of the second stage. At this stage the weight loss rate drop rapidly, and loses 53% of its weight up to 350 °C. From the DTG plot of Fig. 4(a), the maximum value in rate occurs at 300 °C. Afterwards, as the temperature increased, the changing ratio of the sample is not significant and leaves about 24% residue at 600 °C. However, in the case of cationic KGM, the situation is slightly different. Same as what happens in the experiment of raw KGM, at the first stage, the phenomenon is the loss of moisture content, which occurs at 38 °C. After that, the mass of the sample changes little as the temperature rise. The degradation in the second stage take place at 220 °C, followed by a sharp declines in weight till 330 °C. The temperature at which the weight drop fastest in DTG plot is at 275 °C. The overall decomposition leaves about 11% residue. Compared with raw KGM, the cationic KGM after dehydration reaction lost much mass than that of raw KGM, which is the result of substitution of the hydroxyl groups in cationic modification. Differing from raw KGM, the start of decomposition temperature of cationic KGM in the second stage is 10 °C lower. Furthermore, 63% of the total weight of cationic KGM is lost in this stage while only about 53% lost happened in the case of raw KGM. It means that the cationic groups in KGM would leads to a weak thermal stability. Finally at the 600 °C, the amount of residue raw KGM and cationic KGM are 24% and 11%, respectively. In general, after the cationic modification treatment on KGM, the thermal performance of cationic KGM is less stable than raw KGM itself.

Fig. 3. Effect of the molar between CHPTAC and KGM on DS value.

3.3. Thermal analysis results The results of the TGA were depicted in Fig. 4. Each sample has two lines which represented TG and DTG, respectively. From the two pictures we could see that both the raw KGM and cationic KGM involve two steps of degradation. As for the picture of raw KGM, the first step of degradation take place at 42.6 °C which is attributed to the loss of

Fig. 4. Thermal analysis of the raw KGM (a) and cationic KGM (b).

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Fig. 5. Zeta potential of KGM, CKGM (a) and Kaolin suspension (b).

Fig. 6. Flocculation performance of cationic KGM with different DS in kaolin suspension at (a) pH = 4.0, (b) pH = 7.0, and (c) pH = 10.0, respectively.

for the change in zeta potential of CKGM, it’s remain stable at pH less than 7, however, when the pH value is greater than 7, due to the presence of hydroxide, the zeta potential of CKGM will decrease owing to the deprotonation of quaternary ammonium salt (Yang, Yan et al., 2013; Yang, Yang et al., 2013). Besides, all the isoelectric point of the three samples are near the pH = 12, that is to say, KGM after cationization could ionize cations at a wide pH value range.

3.4. Zeta potential analysis results The relation between zeta potential and pH value about the KGM, cationic KGM and Kaolin are depicted in Fig. 5. We could know from the picture that the isoelectric point of raw KGM is around pH = 2, but as for the cationic KGM, the value is pH = 12, which is the evidence of the successful occurrence of cationic modification of KGM. From the data of three different DS products in the graph, the higher of DS, the greater the quantity of positive charges are in the solution. Moreover, as 5

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Fig. 7. Zeta potential of the supernatants as a function of dosage of CKGM with different DS.

the relationship between zeta potential of the supernatants and the dosage of CKGM with different DS. We could know from the picture that the zeta potential always decreases with increased dosage of CKGM solution, which indicate that the flocculation processes are controlled by the charge neutralization effect. Besides, owing to the long molecule chain of cationic polysaccharides, the CKGM molecules attached to the solid particles still had free active sites that could be adsorbed on other particles. Therefore, the bridging effect was happened and the consequent formation of a three-dimensional network structure. Naturally, when the amount of flocculant was too low, it could not form a bridge between the suspended kaolin particles. With the increasing in the addition of cationic KGM, the polymer segments can fully absorb the anion particles, hence, the sedimentation efficiency is enhanced. The maximum flocculation performance was obtained at a polymer amount corresponding to a zeta potential zero/close to zero (Figure a: the dosage is 0.6 ml when pH = 10. Figure b: the dosage is 0.2 ml when pH = 7; the dosage is 0.6 ml when pH = 10) (Yang, Yan et al., 2013; Yang, Yang et al., 2013). However, continue adding cationic polymer solution, the particle charge is neutralized, and this neutralization results in an isoelectric point and rapid aggregation of the colloidal particles (Kleimann, GehinDelval, Auweter, & Borkovec, 2005). Besides, the surface of the suspended particles are positively charged, resulting in electrostatic repulsion, which causes the suspension to stabilize again. Besides, excessive addition of flocculants will occlude and encapsulate the polymer, which will also weaken the bridging effect. In addition, from the results depicted in Fig. 5. The zeta potential of Kaolin suspension is −10 mV, so the potential of DS = 0.20 is relatively too low to be efficient. Besides, when the DS = 0.46, the surface

3.5. Flocculation analysis Kaolin is composed of aluminosilicate with layered structure, and the structural units are formed by silicon oxide tetrahedron and aluminum oxide octahedral in a ratio of 1:1 (Zhao, Zhang, Luo, Wang, & Yu, 1994). From Fig. 5(b), we could know that the Kaolin suspension was negatively charged in the pH range 4–12. Besides, with the increase of pH, more negative charges would be carry on the surface of the suspended particles owing to the deprotonation reaction of Si-OH, which would lead to the increase of active center. Therefore, we selected three different pH values to study the flocculation performance of cationic KGM, and the flocculation results of the cationic KGM samples with different DS values are illustrated in Fig. 6. When pH value equals to 4. We could know from Fig. 6(a). The transmittance of the three suspensions increase with the increase of cationic polymer solution. However, after reaching its maximum value, the transmittance begin to decline. Furthermore, at pH = 4, the dosage of flocculant is relatively high due to a small amount of active centers. The highest flocculation performance is the product with a DS of 0.30, and the maximum transmittance could reach 81.3% when the volume of cationic polymer solution is 0.8 ml. Besides, the second high flocculation performance is the DS of 0.46 when the content of cationic polymer solution is 0.5 ml, and the light transmittance reach up to 56.3%. The product with the lowest performance is DS = 0.20, the maximum transmittance is 25% at the dosage of 0.3 ml. Flocculation via adsorption of polymers can be explained according to the following mechanisms: Bridging and charge neutralization. In the case of CKGM, its mechanism is a combination of the two. Fig. 7 depicts 6

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of Kaolin particles are positively charged due to the adsorption of too much cation, this is the reason why the flocculation performance of DS = 0.30 was greater than DS = 0.46, which is same as pH = 7. At pH = 7, cationic KGM with different DS values shows similar trends with pH = 4 in flocculation performance. However, when a small amount of cationic agent is added, the transmittance could be enormously improved. The three samples reach their maximum flocculation performance at the dosage of 0.2 ml, which indicate that samples with different DS at pH = 7 could be highly effective in the case of small additions. Moreover, the flocculation performance is the best when the DS is 0.3. At pH = 10, Si-OH structure in Kaolin is fully deprotonated and the surface of the particle is filled with negative charge, moreover, the active center is completely exposed in this environment, so the more cations were ionized, the more efficient flocculation would become. In the zeta potential experiment, cations at DS = 0.3 and DS = 0.2 were higher than that at DS = 0.46, so the light transmittance at DS = 0.46 is the lowest. It was surprising that the transmittance would reach 99.7% at the dosage of 0.5 ml in DS = 0.30 and 99.5% at the volume of 0.6 ml in DS = 0.20. By studying the flocculation performance of other polysaccharides, we could know that the highest flocculation performance of cationic chitosan is 87% (Wang, Chen, Ge, & Yu, 2007), cationic cellulose is 83% (Yan, Tao, & Bangal, 2009), and 95% of cationic starch (Du, Wei, Li, & Yang, 2017). As for the flocculation performance of CKGM, the maximum value of transmittance was 99.7% which took place at pH = 10 in the DS = 0.30, so CKGM is the most efficiency cationic agent compared with the other samples. Then, in the pH = 7, no matter how much DS was equal to, a good performance of flocculation could be achieved when a few dosage of cationic agent was added. Under neutral and alkaline conditions, in general, the sample of different DS showed a good flocculation performance.

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4. Conclusion Cationic KGM was synthesized by the reaction between KGM and the etherification agent 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHPTAC) in NaOH/urea aqueous solution at room temperature in a homogeneous way. The optimal condition for the cationization reaction such as the molar ratio between KGM and CHPTAC was obtained. The cationic product was evaluated by FT-IR, 1H NMR, elemental analysis, thermal analysis, zeta potential and flocculation test. Results revealed that the thermal performance of cationic KGM was lower than raw KGM, the beginning temperature of the decomposition became lower after modification, and the residue of cationic KGM was only about half of the raw KGM. As for the zeta potential, the cationic KGM show positively charged at pH = 1–12, combined with flocculation experiments, cationic KGM had high flocculation capacity over a wide pH range, especially under neutral and alkaline conditions, the maximum value of transmittance was 99.7%, which is the highest among other cationic polysaccharides. Although the DS value of CKGM in our experiment is the highest at this stage of the relevant experiments, but there still need some improvement because of its relatively low yield and high cost. In the future research of cationic KGM, we hope to find a solvent that is more efficient in dissolving KGM. In this way, we could improve the efficiency of the reaction and reduce costs at the same time. Acknowledgement The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (50703031). References Chua, M., Chan, K., Hocking, T. J., Williams, P. A., Perry, C. J., & Baldwin, T. C. (2012).

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