chitosan for heavy metal ions

chitosan for heavy metal ions

Composites Part B 169 (2019) 45–54 Contents lists available at ScienceDirect Composites Part B journal homepage:

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Composites Part B 169 (2019) 45–54

Contents lists available at ScienceDirect

Composites Part B journal homepage:

Adsorption performance of a polysaccharide composite hydrogel based on crosslinked glucan/chitosan for heavy metal ions


Chenglong Jiang, Xiaohong Wang∗, Ganghu Wang, Chen Hao∗∗, Xin Li, Tihai Li School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, Jiangsu, 212013, China



Keywords: Hydrogel Adsorption Heavy metal ions Chitosan Glucan

Glucan/chitosan (GL/CS) hydrogels as adsorbents for heavy metal ions in wastewater treatment were synthesized by ultrasound-assisted free radical polymerization. The samples were characterized by Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). In the study of adsorption process, the effects of temperature, acidity and alkalinity, dosage of adsorbent, adsorption time and concentration of heavy metal ions on adsorption behavior of GL/CS adsorbent in simulated wastewater were discussed. On the basis of the above research, the adsorption thermodynamics, adsorption kinetics and adsorption mechanism were also studied. When the temperature was 20 °C, the pH value was 7.0 and the amount of adsorbent was 0.01 g, the results showed that the adsorption capacities of GL/CS hydrogels for Cu2+, Co2+, Ni2+, Pb2+ and Cd2+ were 342 mg g−1, 232 mg g−1, 184 mg g−1, 395 mg g−1, 269 mg g−1, respectively. The fitting results of the isothermal models, the kinetic models and thermodynamics of the experimental data of the ion adsorption process show that the adsorption of heavy metal ions by GL/CS is a spontaneous process of single-layer chemisorption.

1. Introduction The development of industry has brought convenience to people, but it has also caused serious harm to the environment. Among them, the pollution of water is more serious. The pollution of heavy metal ions in the wastewater has brought great damage to the biosphere. In essence, heavy metals mean that the density of the substance can be greater than 4.5 kg dm−3, and the metal in the periodic table is 21–83. It mainly includes about 40 species of lead (Pb), nickel (Ni), cobalt (Co), cadmium (Cd), copper (Cu) and so on [1–6]. There is no uniform definition of heavy metals. Generally, metals that cause pollution to the environment and cause harm to human body are heavy metals [7–10]. Heavy metal ions in wastewater cover the whole biological world, and they can do great damage to the natural water and the whole ecosystem through the cycle of biological action. Due to the enrichment in the biological chain, the final heavy metal ions will threaten the safety of human life. The removal of heavy metal ions is also a hot topic in the scientific community today. There are many measures to treat heavy metal ions in wastewater. The most commonly used ones are chemical flocculation, oxidationreduction, adsorption, electrolysis and so on [11–18]. Chemical flocculation is one of the most widely used methods to treat heavy metal ∗

ions. The application principle of the method is to add a chemical reagents capable of forming a water-insoluble precipitate with heavy metal ions in the wastewater, and then filter out the precipitates so as to achieve the purpose of removing heavy metal ions. The chemical flocculation method has the advantages of large removal, simple implementation, good removal efficiency, mature technology, low investment, low cost, and the like, and is widely applied to the removal of heavy metal ions. However, the deficiency of this method is that the addition of precipitant will produce a large amount of flocculent sludge, which will cause a secondary contamination. The heavy metal ions involved in oxidation-reduction methods usually have multiple valence states, and the ionic properties of different valence states are different. The method is to add a reducing agent or an oxidizing agent to the wastewater, convert the refractory valence state into a manageable valence state and form a precipitate, and the process generates a large amount of waste residue, resulting in a limited treatment range. Electrolysis is the process of chemical changes in a substance caused by electrical current. The positive ions in the solution move toward the cathode, and the negative ions move to the anode and undergo reduction and oxidation reactions, respectively. This process is well developed, but the sewage treatment capacity is not strong enough, the cost of ion removal is relatively high and causes secondary pollution, so

Corresponding author. Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (C. Hao).

∗∗ Received 30 October 2018; Received in revised form 16 March 2019; Accepted 31 March 2019 Available online 06 April 2019 1359-8368/ © 2019 Elsevier Ltd. All rights reserved.

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its application is limited. The adsorption method is simple in operation, excellent in treatment effect, low in adsorbent price, and no secondary pollution in the treatment process. Therefore, the method is widely applied to the removal of heavy metal ions in wastewater, and is considered to be one of the greenest and most promising methods for treating heavy metal ion wastewater [19–26]. Hydrogel is a kind of polymer with three-dimensional network structure [27–30], which was used in medical treatment when it was first discovered. Due to its high water absorption, it is widely used in agriculture, health care and so on. In recent years, it has been found that the hydrogel has a polyfunctional structure so that it also exhibits excellent performance as an adsorbent for heavy metal ion treatment. Chitosan is produced by deacetylation of chitin, so chitosan is also called deacetylated chitin [31–33]. Since the French Rouget first got chitosan in 1895, the biocompatibility, blood compatibility, safety, microbial degradation and other advantages of this natural polymer have been widely concerned by all walks of life. Significant progress has been made in the fields of medicine, food, chemical, cosmetic water treatment, metal extraction and recovery, biochemistry and biomedical engineering [34–39]. Chethan et al. [40] prepared ethylenediaminemodified chitosan microspheres by chemical crosslinking method to adsorb Cu2+,Zn2+,Pb2+,Cr4+. Glucan is a kind of homologous polysaccharide composed of glucose monosaccharide and glucan units are linked by glycoside bonds. According to the type of glycosidic bond, alpha-glucan and beta-glucan can be further divided into two groups. Alpha-glucan, also known as glucan, is a polysaccharide found in the mucus secreted by certain microorganisms during their growth. Fabrication and application of composite materials is the development direction of modern functional materials research,.and the composite materials can make up for the shortcomings of different components, and give full play to the advantages of different components, so as to achieve performance efficiency [41,42]. Adding glucan into the hydrogel not only improves the adsorption performance, but also greatly reduces its biological toxicity. In recent years, many researchers have used the excellent adsorption properties of hydrogels to treat heavy metal ions. Wang et al. [43] used the unique adsorption properties of hydrogels to treat Co2+ in wastewater. Kankeu et al. [44] prepared hydrogels from graft copolymerization of ammonium acrylate and gelatin and used them to adsorb Cd2+ in mine wastewater. Ultrasound-assisted synthesis has been proved to be a simple and effective technology for polymer synthesis. Due to its high acoustic frequency and large energy, ultrasonic assisted synthesis can increase the chemical reaction rate, reduce the reaction conditions, shorten the reaction induction time and improve the reaction selectivity. In ultrasonic polymerization, ultrasonic cavitation effects can increase the chance of collision between molecules and generate free radicals in the liquid, thereby initiating and accelerating the polymerization between the monomers [45,46]. In this paper, GL/CS hydrogel was synthesized by ultrasonic assisted method using glucan and chitosan as the main raw materials. The hydrogel belongs to acrylic hydrogel and is applied to the treatment of Cu2+, Co2+, Ni2+, Pb2+ and Cd2+ in wastewater. The hydrogel exhibits excellent adsorption properties, outstanding use value, and is easy to promote.

Table 1 The results and analysis of L16 (54) orthogonal experiment design. Factors






Result(Cu2+ qe/mg g−1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

0.1 0.1 0.1 0.1 0.2 0.2 0.2 0.2 0.3 0.3 0.3 0.3 0.4 0.4 0.4 0.4

0.07 0.09 0.11 0.13 0.07 0.09 0.11 0.13 0.07 0.09 0.11 0.13 0.07 0.09 0.11 0.13

0.0085 0.0115 0.0145 0.0175 0.0115 0.0085 0.0175 0.0145 0.0145 0.0175 0.0085 0.0115 0.0175 0.0145 0.0115 0.0085

65 70 75 80 75 80 65 70 80 75 70 65 70 65 80 75

0.01 0.06 0.11 0.16 0.16 0.11 0.06 0.01 0.06 0.01 0.16 0.11 0.11 0.16 0.01 0.06

249.69170 253.80796 286.83799 295.83709 232.71007 239.19275 248.85845 231.52685 268.27318 260.39063 239.54272 257.05763 287.20462 180.63194 247.30861 244.59221







k1 k2 k3 k4 Range

271.544 238.072 256.316 239.934 33.472

259.470 233.506 255.637 257.253 25.964

243.255 247.721 241.817 273.818 31.256

234.06 253.021 256.133 262.653 28.593

247.229 253.883 267.573 237.18 30.393

nitrate, nickel nitrate hexahydrate, and hexahydrate nitric acid. All aqueous solutions used for reactant polymerization, water swelling and adsorption studies of hydrogels were prepared using deionized water. 2.2. Synthetic GL/CS hydrogel The GL/CS hydrogel was synthesized according to the following steps, a certain amount of NaOH was weighed and dissolved in water, and 5 mL of acrylic acid was added dropwise with a dropper to neutralize the above NaOH solution in an ice water bath, and a continuous stirring was accompanied. Then potassium persulfate, NMBA, CS and GL were added, and then the system was placed in a KQ-100 ultrasonic cleaner (bath, 250 W, 40 kHz) and the reaction was maintained at 80 °C for 3 h. Finally, the synthesized hydrogel was washed with anhydrous ethanol to remove unreacted monomer, and the sample was dried under vacuum at 70 °C for 24 h. The amount of the above reagents is listed in the orthogonal list below (Table 1).(Detailed experiments see support information (SI) Fig. S1). 2.3. Experiment and reaction mechanism Both GL and CS are polysaccharides with multiple hydroxyl groups, and the O-H bond is very easy to break under the action of heat and initiator. Firstly, persulfate radical dissociates the hydroxyl groups on GL and CS, and transforms them from stable molecules into active radical groups. The active sites will receive the attack of acrylonitrile and produce a large number of polymer branched chains. Then, all the grafted chains are cross-linked together to form reticulated macromolecules under the action of crosslinking agents. The intact monomer and branched chain were removed by anhydrous ethanol, and finally the hydrogel samples were obtained by vacuum drying. The specific process of the reaction is shown in Fig. 1.

2. Experiment 2.1. Experimental materials Acrylic acid (AA) came from Shanghai McLean Biochemical Co., Ltd. Chitosan (CS), glucan (GL) and N'N-methylenebisacrylamide (NMBA) were from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Potassium persulfate (K2S2O8) was purchased from Shanghai Suran Chemical Reagent Co., Ltd. Other experimental reagents were from Nanjing Chemical Reagents Co., Ltd., such as sodium hydroxide, ethanol, cadmium tetrahydrate, copper nitrate pentahydrate, lead

2.4. Hydrogel swelling capacity At 25 °C, the dry GL/CS hydrogel was weighed as m1 g and put into deionized water until the hydrogel reached adsorption equilibrium. Then the mass of the adsorbed hydrogel was weighed as m2 g. The swelling rate of GL/CS was calculated by the following formula: 46

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Fig. 1. Ultrasound-assisted synthesis reaction mechanism of GL/CS hydrogel.

Qe =

m2 − m1 m1

3. Experimental results and discussion (1) 3.1. Orthogonal test results and analysis

Where,Qe (g g−1) is the quality of adsorbed water per unit mass of hydrogel. The data obtained in this paper are averaged by multiple measurements.

In the experiment, ultrasonic synthesis was used to accelerate the collisions between liquid molecules and free radicals, and the synthesis conditions were optimized by orthogonal experimental design. The results of orthogonal experiment of hydrogel are shown in Table 1, and each qe in the table is the average of multiple experimental data. The results of orthogonal experiments of GL/CS hydrogels are shown in Table 1. The magnitude of the influence of these several influencing factors is the amount of GL > the amount of NMBA > the amount of CS > the degree of neutralization of AA > the amount of K2S2O8. The best level of each factor is A1B1C4D4E3. It can be seen that among the several factors investigated, the amount of GL is the biggest factor affecting the adsorption of heavy metal ions by hydrogels. This may be due to the fact that the dextran contains more hydroxyl groups capable of chelating with heavy metal ions, so it contributes greatly to the adsorption of heavy metal ions on the hydrogel. The effect of crosslinking agent was the second, which also confirmed that the crosslinking degree of hydrogels had great influence on the overall structure of hydrogels. The amount of CS also has a certain effect, indicating that the functional groups of CS have great influence on the adsorption of heavy metal ions.

2.5. Adsorption properties of GL/CS hydrogels A certain amount of dried hydrogel was added to the prepared 50 mL 100 mg L−1 M2+ (M = Cu, Co, Ni, Pb, Cd) solution. It was then placed in a thermostatic shaker until the adsorption equilibrium is absolutely reached. The residual M2+ concentration in the solution was determined using a TAS-986 atomic absorption spectrophotometer. The hydrogel adsorption capacity is calculated by the following equation:

qe =

(C0 − Ce ) V m −1

(2) −1

When Co (mg L ) and Ce (mg L ) represent the initial concentration and the equilibrium concentration of M2+ in the solution, respectively. qe (mg g−1) is the amount of M2+ adsorbed on the adsorbent, V (L) is the volume of M2+ solution, and m (g) is the weight of adsorbent. (See Fig. S2 in SI before and after adsorption of ions). 2.6. Characterization of GL/CS hydrogels

3.2. GL/CS hydrogel properties The synthesis of GL/CS hydrogel was completed in KQ-100 ultrasonic cleaner. In order to characterize the functional groups on the surface of GL/CS hydrogels before and after adsorption of heavy metal ions, Nicolet-Nexus 470 (Nicolet, USA) Fourier transform infrared spectrometer was used. To observe the morphology of GL/CS hydrogel, JSM-7001F scanning electron microscopy was utilized. The thermal analysis of GL/CS hydrogel was carried out by STA 449 C integrated thermal analyzer.

3.2.1. Photographs and SEM From a theoretical point of view, it can be analyzed that the crosslinking of the monomers will produce a hydrogel with a three-dimensional network structure. From the photographs of the hydrogels (Fig. 2(A) and (B)), it is shown that the GL/CS hydrogel has a highly developed three-dimensional structure of pale brown-yellow honeycomb morphology accompanied by a large number of folds, and the 47

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Fig. 2. Photographs and SEM images of GL/CS (A–D); Macrostructure of GL/CS (E–F); FT-IR spectra of CS (a), GL (b), GL/CS (c) (G), after adsorption heavy metal ions (d = Cu, e = Co, f = Ni, g = Pb, h = Cd) (H); TG and DTG curves of GL, CS and GL/CS (I).

1560 cm−1. The specific peaks of glucan and chitosan are found in GL/ CS, indicating that two monomers were successfully grafted into GL/CS. Fig. 2(H) d∼h are the FT-IR spectra of GL/CS adsorbed heavy metal ions. It is clear that some of the absorption peaks after the adsorption of heavy metal ions are offset. For example, -OH shifts from 3423 cm−1 to about 3500 cm−1. Some similar absorption peaks produce a degeneracy of the Fermi resonance effect, for example, the -NH2 characteristic absorption peak and C-O are degenerate at a position of about 1300 cm−1. The phenomenon of the deviation and degeneracy of these absorption peaks is due to the fact that during the adsorption process, heavy metal ions M2+ will chelate with O, N and form coordination bonds, which makes the electron cloud density change and causes the vibration band shift of the bond.

hydrogel is translucent and has a thin wall. As shown in Fig. 2(C) and (D), the hydrogel is rich in pores of various sizes, and the surface is relatively smooth, but some gully-like cracks are distributed on the surface. The tri-dimensional structure of the hydrogel was obtained by cross-linking three monomers (acrylic acid, chitosan, glucan) with a crosslinking agent (NMBA). The pore structure on the surface is caused by the loss of moisture of the hydrogel during vacuum drying. Because water will remain in the structure of the hydrogel during the process of synthesis. Under the condition of vacuum drying, the water in the system will accelerate to the environment, which will lead to pores on the surface of the hydrogel. And it is also speculated that the honeycomb-like hollow structure will appear inside the hydrogel, which is confirmed by Fig. 2(A) and (B). The water content of each part of the hydrogel is different. Because of the water loss, the collapse of each part is different, resulting in a wrinkled structure on the surface. Fig. 2(E) and (F) is a partial enlarged view of the inner wall of the tunnel, the surface of the inner wall is rugged, and many short rods and short pieces are randomly stacked to form a plurality of smaller microspores. This porous special structure of the hydrogel makes it exhibit excellent performance in terms of adsorption. Its three-dimensional structure, pore structure and surface folds are beneficial to increase the specific surface area and provide more adsorption sites for the adsorbed substances, thereby effectively improving its adsorption performance.

3.2.3. TG/DT It can be clearly seen from Fig. 2(I) that the decomposition temperatures of CS, GL and GL/CS are 317 °C, 295 °C and 443 °C, respectively. After thermal decomposition from 25 °C to 800 °C, the final residual mass fractions of CS, GL and GL/CS were 10%, 29% and 49%, respectively. From Fig. 2(I), it can be found that CS and GL lose weight by 10% and 12%, respectively, at around 70 °C, which is caused by the loss of moisture when heated. The second stage is the thermal decomposition of CS and GL molecules, and CS begins thermal decomposition at 282 °C until the weight drops to 10%, while GL thermally decomposes from 258 °C until it falls to 29% by weight. Compared to CS and GL, the thermal stability of GL/CS is significantly improved. First, GL/ CS loses its moisture at 87 °C. In the second stage, GL/CS begins to thermally decompose at around 400 °C until it reduce to 49% of the weight. GL/CS hydrogel formed by crosslinking CS, GL and acrylic acid is more stable than that of other monomers.

3.2.2. FT-IR In this paper, the monomers and synthesized samples were tested by FT-IR spectroscopy. In Fig. 2(G) a, 3467 cm−1 is a stretching vibration absorption peak of -OH; 2869 cm−1 and 1659 cm−1 are the stretching vibration absorption peaks of -NH2, respectively; the absorption peak of the ether bond in chitosan appears at 1598 cm−1; the peak at about 661 cm−1 is assigned to the C-H vibration absorption. In Fig. 2(G) b, 3443 cm−1 is related to the stretching vibration absorption peak of -OH and 847 cm−1 is the glycoside bond absorption peak. It can be seen from Fig. 2(G) c that a glucosidic bond unique to glucan appears at 856 cm−1 in GL/CS, and a stretching vibration peak of -NH2 occurs at

3.3. Adsorption properties of GL/CS hydrogels 3.3.1. Swelling behaviors of the GL/CS In the fields of industry, agriculture and medical treatment, the 48

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Fig. 3. (A) Effect of time on water absorption performance of GL/CS; (B) Effect of pH on ions adsorption behavior of GL/CS; (C) Effect of adsorbent dosage on ions adsorption behavior of GL/CS.

of hydrogels. In this range, with the increase of the amount of adsorbents, the adsorbents will provide more adsorption sites and increase the adsorption capacity of the hydrogel. However, when the amount of GL/CS hydrogel exceeds 0.02 g, the adsorption capacity decreases as the amount of GL/CS hydrogel increases. This is because the concentration of the metal ions remains unchanged, the amount of the adsorbent increases, and the number of free adsorption sites increases, resulting in a decrease in the overall adsorption capacity of the hydrogel. At the same time, the content of the adsorbent in the solution is too high, so that agglomeration occurs between them, causing some adsorption sites to disappear, thereby reducing the adsorption amount of the hydrogel. It is also observed from the diagram that the adsorption properties of the hydrogels are in the best state when the amount of adsorbents is up to 0.02 g, no matter which kind of ions. And the adsorption capacity of Cu2+ is 365 mg g−1, Co2+ is 251 mg g−1, Ni2+is 207 mg g−1, Pb2+ is 424 mg g−1, and Cd2+ is 356 mg g−1.

swelling properties of the hydrogels is an important indicator of hydrogels. Liquid-solid contact time is one of the parameters for hydrogel application in water absorption and swelling. Fig. 3 (A) shows that the water absorption rate of GL/CS hydrogel increases rapidly in 40 min and reaches saturation slowly after 60 min. The swelling process of GL/ CS hydrogels can be divided into two stages: initial fast stage and final slow stage. Initially, the hydrogel contains a large number of binding sites, which can bind to water rapidly, and the water absorption rate is very fast. In the slow stage, fewer binding sites lead to lower adsorption rate and finally saturation. 3.3.2. Effect of pH and adsorbent dosage on adsorption properties The pH value in industrial wastewater is often not 7, so the impact of pH on adsorption performance is also particularly important. As is clear from Fig. 3(B), qe increase with an increase in pH value and the minimum value of qe is at pH = 1. The effect of pH on the five ions is basically the same. At pH = 7, the difference of the adsorption capacity of the five ions is larger, but with the gradual decrease of the pH value, the difference will become smaller. In the adsorption process of heavy metal ions, there are both chelation of the hydrogel and heavy metal ions, as well as ion exchange. The adsorption of heavy metal ions by hydrogels is a reversible process. H+ is easily bonded to some polar functional groups (-COO-, -NH-, -O-), and therefore, H+ competes with heavy metal ions. The decrease in pH causes an increase in H+ concentration in water. As H+ increases, the position of the hydrogel capable of binding to heavy metal ions is reduced, resulting in a decrease in the amount of adsorption of heavy metal ions. As shown in Fig. 3(C), when other conditions remain unchanged, and the amount of adsorbents is in the range of 0–0.02 g, the increase in the amount of adsorbents in the solution is beneficial to the adsorption

3.4. Adsorption thermodynamic It can be seen from Fig. 4 that the adsorption capacity of GL/CS hydrogels increases with the increase of temperature, because the adsorption process is an endothermic process, and the increase of temperature can move the adsorption equilibrium to the direction of strong adsorption. In order to study thermodynamics, the obtained data are fitted by the following formulas, the results are shown in Fig. 4 and the corresponding thermodynamic parameters ΔG0, ΔS0 and ΔH0 are shown in Table 2 (A).

ΔG 0 = ΔH 0 − T ΔS 0 49


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Fig. 4. Thermodynamic fitting of the five kinds of ions adsorbed by GL/CS and the influence of temperature on adsorption of the ions.

Table 2 (A) Adsorption thermodynamic parameters. Thermodynamic



△G0 (KJ mol−1)

Parameters Cu2+ Co2+ Ni2+ Pb2+ Cd2+

(KJ mol−1) 6.923 17.02 21.08 4.953 10.41

(J mol−1 K−1) 43.72 70.55 81.78 41.40 49.86

293.15K −5.89 −3.66 −2.89 −7.18 −4.21

303.15K −6.33 −4.36 −3.70 −7.60 −4.71

313.15K −6.77 −5.07 −4.53 −8.01 −5.21

323.15K −7.21 −5.78 −5.34 −8.43 −5.71

333.15K −7.64 −6.48 −6.16 −8.84 −6.21

Kinetic models and parameters






qe,exp (mg g−1) Pseudo-first-order kinetics qe (mg g−1) k1 (min−1) R2 Pseudo-second-order kinetics qe (mg g−1) k2 (g mg−1min−1) × 105 R2






1625 0.060 0.717

495.4 0.035 0.787

241.1 0.026 0.979

703.2 0.038 0.705

461.7 0.030 0.908

381.7 121.6 0.999

285.7 8.137 0.998

234.7 8.623 0.991

436.7 10.55 0.999

343.6 5.602 0.995


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Fig. 5. Effect of adsorption time on adsorption ion behavior of GL/CS (F); and the pseudo-first-order kinetic fitting (a∼e), pseudo-second-order kinetic fitting (A∼E).

In (qe / Ce ) =

ΔS 0 ΔH 0 − R RT

Pseudo-first-order kinetic model: (4)

In (qe − qt ) = Inqe − k1 t

Where, qe (mg g−1) is the adsorption capacity of adsorbent, and Ce (mg L−1) is the concentration of adsorbate in the equilibrium of adsorption. R (J K−1 mol−1) is the universal constant of gas and T (K) is the temperature of reaction. From Table 2(A), it can be seen that the ΔG0 is negative, the adsorption process is a spontaneous process, and the temperature rise is beneficial to the adsorption, which is consistent with the trend of the qeT diagram. ΔH0 is a positive value, indicating that the adsorption process is accompanied by endothermic process. The fitting of thermodynamic curve shows that the adsorption process by which GL/CS adsorbs ions is accompanied by a spontaneous endothermic reaction.


Pseudo-second-order kinetic model:

t 1 1 = + qt qe k2 qe2

(6) −1

Where, qe (mg g ) is the theoretical equilibrium adsorption capacity, t is the adsorption time, qt (mg g−1) is the adsorption capacity at time t, and k1 (min−1) is the pseudo first-order kinetic model rate constant, k2 (g) is the pseudo-second-order kinetic rate constant, and the adsorption kinetic parameters obtained by the model are listed in Table 2(B). The pseudo-first-order kinetic model is based on the membrane diffusion theory, and it is considered that the adsorption reaction rate of the adsorbate is proportional to the square of the difference between the equilibrium adsorption amounts in the system. The pseudo-secondorder kinetic model is established on the adsorption rate limiting step, including the adsorption mechanism, electron sharing or electron transfer between the adsorbate and the adsorbent. The degree to which the correlation coefficient (R2) is close to 1 reflects the closeness of the actual situation to the model. From the results of the fitted parameters, the theoretical adsorption capacity obtained by pseudo-second-order kinetics is closer to the actual measurement results than that of the firstorder kinetics, and the correlation coefficient (R2) of the pseudosecond-order kinetic model is higher. These results indicate that the pseudo second-kinetic model is more in line with the process of describing heavy metal ions adsorbed by GL/CS hydrogels.

3.5. Adsorption kinetics By studying the influence of time on the adsorption of the five ions by GL/CS, the kinetic model was fitted and the dynamic mechanism was explored. Fig. 5 shows the effect of adsorption time on the adsorption of the ions and adsorption kinetics of GL/CS. It can be seen from Fig. 5(F) that the adsorption capacity increases rapidly with increasing adsorption time from 0 to 60 min, and the adsorption capacity increases slowly after 60 min until the adsorption equilibrium reaches 180 min later. In order to further explore the kinetics of adsorption, the pseudo-first-order kinetic model and the pseudo-second-kinetic model were used to study this process. 51

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Fig. 6. Effect of heavy metal ions concentration on the adsorption of GL/CS and the isotherm model.

Ce C 1 = e + qe qm, L KL qm, L

3.6. Adsorption isotherm As can be seen from Fig. 6, the adsorption capacity of GL/CS hydrogel increases gradually with the increase of initial heavy metal ion concentration until the final adsorption capacity of the hydrogel tends to be stable. This is due to the fixed amount of hydrogel, the number of adsorption sites is certain, and the increase of metal ion concentration leads to fewer and fewer adsorption sites. Finally, the adsorption capacity of the adsorbent reaches saturation. In order to further study the adsorption mechanism, the relationship between adsorption capacity and metal ion concentration was discussed by using adsorption isotherms. The Langmuir adsorption isotherm model assumes that there is no saturated atomic force field on the surface of the adsorbent. Once the surface is covered by a layer of adsorbate molecules, the force field will be saturated and the adsorption will not occur, so the adsorption is monolayer adsorption. The linear Langmuir adsorption isotherm equation is as follows:



Where, Ce (mg L ) is the concentration of adsorbate at equilibrium, qe (mg g−1) is the adsorption capacity of adsorbent at equilibrium, qm,L (mg g−1) is the theoretical maximum adsorption capacity, and KL (L mg−1) is the Langmuir adsorption constant. Freundlich adsorption isotherm assumes that the adsorbent surface is heterogeneous, the adsorbate adsorption is multi-layered, and there is interaction between adsorbents.

Inqe = InKF +

1 InCe n −1


Where, Ce (mg L ) is the concentration of adsorbate at equilibrium, qe (mg g−1) is the adsorption capacity at equilibrium, KF (L g−1) is the adsorption constant related to adsorption capacity, and n is the adsorption constant related to surface inhomogeneity. Temkin adsorption isotherm model discussed the interaction between adsorbent and adsorbate, assuming that the heat of adsorption decreases with the degree of adsorption process. 52

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The correlation coefficient R2 is above 0.99, which is better than the correlation coefficient of other isotherm models. This shows that the process of adsorption of heavy metal ions by GL/CS is mainly in accordance with the chemical adsorption of the single molecular layer of Langmuir. Combined with the parameter 1/n in the Freundlich model, it can be inferred that the adsorption process is more likely to occur. Based on the fitting results of the thermodynamic and kinetic models, it is concluded that the whole adsorption process is mainly endothermic chemical adsorption with single molecular layer and spontaneous reaction. (The other three isotherm fitting curves can be found in Figs. S3, S4, and S5 of SI).

Table 3 Adsorption isotherm parameters. Parameters Langmuir qm,L (mg g−1) KL (L mg−1) R2 Freundlich KF (L g−1) 1/n R2 Temkin A (mg L−1) B R2 D-R qm,D-R (mg g−1) β(mol2 KJ−2) × 10−5 R2






469.5 0.202 0.993

332.2 0.028 0.998

287.4 0.013 0.992

552.5 0.031 0.998

329.0 0.027 0.997

251.1 0.081 0.831

144.1 0.121 0.997

100.5 0.130 0.943

301.6 0.083 0.956

209.2 0.058 0.734

947.5 31.97 0.798

19.32 33.44 0.992

4.442 31.43 0.913

1078 38.46 0.938

94792 17.03 0.723

417.1 3.330 0.383

296.3 12.01 0.670

246.1 19.72 0.527

507.0 1.943 0.640

301.4 4.179 0.223

4. Conclusion The polysaccharide-based hydrogel GL/CS with excellent adsorption performance was successfully prepared by a high-efficiency, environmentally friendly, inexpensive, and easy-to-use ultrasonic-assisted polymerization method. It was used to adsorb five heavy metal ions, such as Cu2+, Co2+, Ni2+, Pb2+ and Cd2+. The adsorption experiments showed that GL/CS had the best adsorption performance under neutral conditions. The kinetics, thermodynamics and adsorption isotherms of the hydrogel adsorption process indicate that the adsorption of the five ions conforms to the pseudo-second-order kinetic model and Langmuir isotherm model, which proves that the adsorption process is a spontaneous endothermic chemical monolayer adsorption. In addition, GL/CS hydrogel also has good water absorption performance, and the water adsorption capacity is 1566 g g−1, indicating that GL/CS also has a good development prospect in water absorption.

When the amount of adsorbent is 0.01 g, the GL/CS hydrogel exhibits excellent adsorption performance in a neutral solution at 20 °C, and its adsorption capacity for Cu2+, Co2+, Ni2+, Pb2+ and Cd2+ is 342 mg g−1, 232 mg g−1, 184 mg g−1, 395 mg g−1 and 269 mg g−1, respectively. As shown in Table 4, GL/CS hydrogels have outstanding adsorption properties for heavy metal ions compared with other adsorbents, taking Cu2+ adsorption as an example. Table 4 Comparison of adsorption capacities of several adsorbents for Cu2+. Adsorbent

Adsorption capacity(mg g−1)


PE-PD/GO BAS M-CS-AnGS Cellulosic biopolymer Claymineral NIPAM-co-AA HMO-P(HMAm/HEA) SBM CCS Fe3O4-SC nZVMn GL/CS

87 18 83 76 43 108 54 64 299 73 181 342

[4] [6] [7] [10] [14] [17] [18] [23] [24] [26] [30] This study

qe = BInA + BInCe

Acknowledgments We gratefully acknowledge the Natural Science Foundation of Jiangsu Province (BK20131249), the Senior Personnel Scientific Research Foundation of Jiangsu University (15JDG084), Natural Science Fund Project of Colleges in Jiangsu Province(16KJB430008) and the College Students Innovative Practice Fund of Jiangsu University Industrial Center (ZXJG2018114) for financial support of this research. Appendix A. Supplementary data Supplementary data to this article can be found online at https://


where, Ce (mg L−1) is the concentration of adsorbate at equilibrium, qe (mg g−1) is the capacity of adsorbent at equilibrium, A (mg L−1) is the equilibrium constant related to binding energy, and B is the equilibrium constant related to adsorption heat. Dubinin-Radushkevich (D-R) isotherm model is based on Polanyi potential energy theory. It is pointed out that the adsorption space on the adsorbent surface is constant, and the adsorption potential is independent of temperature.

Inqe = −βε 2 + Inqm, D − R

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where, qe (mg g ) is the adsorption capacity of the adsorbent at equilibrium, qm,D-R (mg g−1) is the theoretical maximum adsorption capacity, β (mol2 KJ−2) is the adsorption constant related to the adsorption energy, and Ɛ is the adsorption potential.

Among them, ε = RTIn ⎛⎜ ⎝

Ce + 1 ⎞ ⎟ Ce ⎠



Where, Ce (mg L ) is the concentration of adsorbate at equilibrium, R (J K−1 mol−1) is the universal constant of gas, and T (K) is the temperature of reaction. Table 3 is the process of GL/CS adsorption of the five kinds of ions, and the corresponding isotherm model parameters are obtained by fitting. It is clear from the table that the process of GL/CS adsorption of heavy metal ions is more consistent with the linear fitting of Langmuir. 53

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C. Jiang, et al.

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