Adsorption behavior of heavy metal ions from aqueous solution onto composite dextran-chitosan macromolecule resin adsorbent

Adsorption behavior of heavy metal ions from aqueous solution onto composite dextran-chitosan macromolecule resin adsorbent

International Journal of Biological Macromolecules 141 (2019) 738–746 Contents lists available at ScienceDirect International Journal of Biological ...

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International Journal of Biological Macromolecules 141 (2019) 738–746

Contents lists available at ScienceDirect

International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac

Adsorption behavior of heavy metal ions from aqueous solution onto composite dextran-chitosan macromolecule resin adsorbent Yang Liu a, Lishuang Hu a,b,⁎, Bo Tan b, Jingru Li a, Xiaohui Gao a, Yaning He a, Xifeng Du a, Wei Zhang a, Weili Wang b a b

School of Environment and Safety Engineering, North University of China, Taiyuan 030051, China College of Weaponry Engineering, Naval University of Engineering, Wuhan 430033, China

a r t i c l e

i n f o

Article history: Received 1 August 2019 Received in revised form 1 September 2019 Accepted 5 September 2019 Available online 07 September 2019 Keywords: Resin Chitosan Dextran Absorption Heavy metal

a b s t r a c t Dextran-chitosan (DC) macromolecule resin was synthesized by ultrasonic heating and applied to adsorb various heavy metal ions (Cu2+, Co2+, Ni2+, Pb2+, Cd2+). The morphology and structure of the samples were characterized by various testing methods. The effects of five factors on the adsorption properties were studied. The adsorption kinetics, thermodynamics and isotherm models were discussed theoretically. The results show that the adsorption of heavy metal ions by DC resin is a spontaneous single molecule chemical adsorption, and the adsorption capacities of DC resin for Cu2+, Co2+, Ni2+, Pb2+ and Cd2+ were 342 mg g−1, 232 mg g−1, 184 mg g−1, 395 mg g−1, and 269 mg g−1, respectively at 20 °C, pH = 7 and adsorbent dose is 0.01 g. In addition, DC resin adsorbent has good reusability. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Unreasonable use of water resources leads to a lack of water resources and serious water pollution. Getting clean water is the common aspiration of people all over the world. The treatment of water pollution has also become a research hotspot of many scholars [1–3]. Water pollution includes inorganic water pollution and organic water pollution, and methods for different pollution treatments are also different. The methods for different pollution treatments are also different [4–8]. Redox, biomass, electrolysis, and precipitation methods are all applied to water treatment. However, these methods have some defects, which restrict the effective promotion and use. In many studies, the adsorption method stands out, and the characteristics of green environmental protection have become the research hotspot of water treatment [9,10]. There are two ways to adsorb metal ions. One is to electrostatically attract the metal ions through the surface of the material to achieve the purpose of adsorbing metal ions [11–22]. The other is that the material itself combines with metal ions to form a stable structure to form an adsorption. Compared with the former, this combination method is more applicable and less affected by the outside world. For example, the Cu and O elements form a regular tetrahedral structure. The polymer resin is a polymer polymer material and the synthesis method is ⁎ Corresponding author at: School of Environment and Safety Engineering, North University of China, Taiyuan 030051, China. E-mail address: imfl[email protected] (L. Hu).

https://doi.org/10.1016/j.ijbiomac.2019.09.044 0141-8130/© 2019 Elsevier B.V. All rights reserved.

simple. Due to the ease of compounding other materials, it is widely used in adsorbent materials. The combination of polymer composite chitosan, chitosan reinforcing material and metal ions. At the theoretical level, the adsorption capacity of adsorbed metal ions is increased [23–29]. In this study, hydrothermal synthesis of polymer resin was carried out, and acrylic acid was used as a carrier to synthesize chitosan and chitosan into acrylic resin through a crosslinking agent and an initiator. This study provides a more probable possibility for the study of polymer resins in water treatment. 2. Experiment 2.1. Material Acrylic acid (AA, Shanghai Maclean Biochemical Co., Ltd.), chitosan (Aladdin Industrial Co., Ltd.) and dextran (National Pharmaceutical Group Chemical Reagent Co., Ltd., Shanghai, China) were used as polymer monomers. Potassium persulfate (Shanghai Suran Chemical Reagent Co., Ltd.) and N′,N-methylenebisacrylamide (NMBA, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were used as initiators and crosslinkers, respectively. Other reagents were purchased from Nanjing Chemical Reagent Co., Ltd., such as sodium hydroxide, ethanol, cadmium nitrate tetrahydrate, copper nitrate pentahydrate, lead nitrate, nickel nitrate hexahydrate and hexahydrate nitric acid. All aqueous solutions used for polymerization, swelling and adsorption studies were prepared using deionized water.

Y. Liu et al. / International Journal of Biological Macromolecules 141 (2019) 738–746

2.2. Synthetic polymer resin The DC resin was synthesized by the following procedure, a certain amount of NaOH was weighed and dissolved in water, and 5 mL of acrylic acid (AA) was added dropwise with a dropper in an ice water bath, and continuously stirred with a glass rod. Subsequently, K2S2O8, N′,N-dimethyldiacrylamide (NMBA), chitosan and dextran were added, and the system was placed in an ultrasonic system, and the synthesis of the resin was completed by ultrasonication at 65 °C for 180 min. Finally, the synthesized resin was washed away with anhydrous ethanol to remove unreacted monomers, and dried under vacuum at 60 °C for 2 days to obtain a sample. The amounts of the above reagents are listed in the orthogonal list below. 2.3. Measuring the performance of resin A quantity of dried sample was added to a prepared 50 mL 100 mg L−1 M2+ (M = Cu, Co, Ni, Pb, Cd) solution. It is then placed in a constant temperature shaker until the adsorption equilibrium is absolutely reached. The residual M2+ concentration in the solution was measured by an atomic absorption spectrophotometer. The adsorption capacity of the hydrogel is calculated by the following equation: qe ¼

ðC 0 −C e ÞV m

ð1Þ

where Co (mg L−1) and Ce (mg L−1) represent the initial concentration and equilibrium concentration of M2+ in the solution, qe (mg g−1) is the amount of M2+ adsorbed on the adsorbent, which is used for V (L). The volume of the solution and the weight of the adsorbent used for m (g). When studying the regeneration of the adsorbent, using NaOH for desorption. The calculation formula for adsorption cycle regeneration is as follows: ∅n ¼

qn  100% q1

ð2Þ

where, q1 (mg g−1) is the first adsorption capacity, qn (mg g−1) is the nth adsorption capacity. 2.4. Characterization The Fourier transform infrared (FT-IR) spectrum of the sample was measured by the KBr disk method using a Nicolet Nexus 470 spectrometer (Nicolet, USA) at a wavenumber range of 4000–400 cm−1. Micromorphological analysis of the samples was investigated using a field emission scanning electron microscope (SEM) JSM-7021F (Jeol Ltd., Japan). The dried samples were covered with layered gold and imaged at an accelerating voltage of 15.0 kV. The samples were subjected to thermogravimetric analysis (TGA) using a STA 449C integrated thermal analyzer (Netzsch, Germany) in a nitrogen atmosphere at a heating rate of 20 °C to 800 °C at 5 °C min−1. 3. Result and discussion 3.1. Results of orthogonal experiments Each result in the orthogonal optimization table is the average of three experiments. The results showed that the amount of dextran had the greatest effect on the adsorption capacity, and the amount of chitosan had the second effect. This indicates that the adsorption of heavy metal ions by DC resin adsorbent mainly depends on two kinds of organic compounds, dextran and chitosan, which have a large number of organic functional groups, affecting the chelation of heavy metal ions. The neutralization degree of acrylic acid, the content of initiator and crosslinker have different effects on the adsorption properties of DC resin. These factors affect the overall structure of DC resin. The

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optimum conditions for the final factors are A1B3C2D4E1, and the adsorption capacity of DC resin adsorbent synthesized under the optimum conditions is 342 mg g−1. 3.2. Characterization of DC resin In this paper, the monomers and synthesized samples were tested by Fourier transform infrared spectroscopy. In Fig. 1(A-a), 3467 cm−1 is the stretching vibration absorption peak of O\\H; 2869 cm−1 and 1659 cm−1 are the stretching vibration absorption peaks of N\\H; the absorption peaks of the ether bonds in chitosan are 1598 cm−1 and 1067 cm−1. Appears; C\\H vibration absorption peak is 661 cm−1. In Fig. 1(A-b), 3443 cm−1 is a stretching vibration absorption peak of O\\H; 847 cm−1 is a glycosidic bond absorption peak. From Fig. 1(Ac), it can be seen that a glucosidic bond unique to dextran appears at 856 cm−1 in DC resin, a stretching vibration peak of N\\H occurs at 1560 cm−1, and dextran is found in DC resin. Specific peaks with chitosan indicate that the two monomers were successfully grafted into DC resin. (See Scheme 1.) (See Tables 1–3.) It can be clearly seen from Fig. 1(B) that chitosan has a maximum heat loss rate temperature at 317 °C, dextran exhibits a maximum heat loss rate temperature at 295 °C, and DC resin exhibits a heat maximum loss weight temperature at 443 °C. The final retention rates for chitosan, dextran and DC resin are 10%, 29% and 49%, respectively. It can be seen from Fig. 1(B) and (C) that an absorption peak appears around 70 °C due to the loss of moisture in the heat of chitosan and dextran. Chitosan loses 10% of its weight at this stage, and dextran loses 12% of its weight at this stage. The second stage is the thermal decomposition of chitosan and dextran molecules, chitosan begins thermal decomposition at 282 °C until the weight drops to 10%; GL begins thermal decomposition at 258 °C until it drops to 29% by weight. Compared with chitosan and dextran, the thermal stability of DC resin is obviously improved. The first stage DC resin loses its moisture at 87 °C, and the second stage DC resin starts thermal decomposition at 400 °C until it drops to 49% of the weight. The hydrogel resin formed by the grafting of chitosan, dextran and acrylic acid by the action of a crosslinking agent has better thermal stability than the thermal stability of each monomer. The resin crosslinked by chitosan, dextran and acrylic acid has better stability than the thermal stability of each monomer. From a theoretical point of view, it can be analyzed that after the resin is crosslinked, it will exhibit a three-dimensional network structure. It can be seen from the SEM (E) and (F) that the resin has a three-dimensional structure in which a plurality of pore structures can be seen and a large number of pleats are observed. It can be clearly seen from (G) and (H) that there are a plurality of pore structures on the surface of the resin, and gully cracks appear on the surface. The three-dimensional structure of DC resin is obtained by crosslinking three monomers (acrylic acid, chitosan and dextran) with crosslinking agent (NMBA). The porous structure on the surface of DC resin is formed by the loss of water after vacuum drying. In the process of resin synthesis, water will remain in the structure of resin. Under the condition of vacuum drying, water in the system will accelerate to volatilize into the environment, which will lead to the appearance of holes on the surface of resin, thus it can be inferred that honeycomb-like hollow structure will appear in the interior of resin. It is also speculated that a honeycomb hollow structure will appear inside the resin. The special structure of the resin makes it have excellent performance in adsorption. The three-dimensional structure, pore structure and wrinkles increase the surface area, which provides more adsorption sites for the adsorbed substances and improves the adsorption performance. 3.3. Effect factors of adsorption performance The pH in industrial wastewater is often not 7, so the effect of pH on adsorption performance is also particularly important. It is clear from Fig. 2(A) that qe increases with increasing pH, and the value of qe is

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c

A

B

100

0.0 80 1312

-0.4

1404

847

1651

a

60 -0.8

DTG

2937

856

TG(%)

b

1560

40 -1.2

1150 3443 664

1659 1598

4000

-1.6

1065

2869

3467

20

3000

2000 Wavenumber/cm

0

1000

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-1

400

600

800

T

0.3

C

100

D

0.3 100

0.0

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0.0

40

-0.3 60

DTG

-0.3

TG(%)

60

DTG

TG(%)

80

-0.6

-0.6 20 40

-0.9

-0.9 0

0

200

400

600

800

T

0

200

400

600

800

T

Fig. 1. Infrared spectra of chitosan (A-a), dextran (A-b) and DC resin (A-c); TG and DTG maps for chitosan (B), dextran (C) and DC resin (D); SEM of DC resin in different sizes (E–H).

the smallest at pH = 1. The effect of pH on the five ions is basically the same. At pH = 7, the difference in the adsorption amount of the five ions is larger, and the difference becomes smaller as the pH decreases. The process of resin adsorption of heavy metal ions, the chelation of heavy metal ions, ion exchange, and the decrease of pH makes the H+ in water increase, and the process of resin adsorption of heavy metal ions is a reversible process, H+ is added. Will compete with heavy

metal ions [30,31]. As H+ increases, H+ binds to some hydrogencontaining functional groups (C_O, N\\H, O\\H), and the sites associated with heavy metal ions decrease, resulting in heavy metals. The amount of ions adsorption is reduced. As shown in Fig. 2(B), when the other conditions remain unchanged, when the amount of the adsorbent is in the range of 0 to 0.02 g, the increase in the amount of the adsorbent in the solution is favorable for the

Scheme 1. Schematic illustration for synthesis process of DC resin.

Y. Liu et al. / International Journal of Biological Macromolecules 141 (2019) 738–746 Table 1 5-factor 4-level orthogonal experiment. Factors

Chitosan/g

Dextran/g

NMBA/g

AA%

K2S2O8/g

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

0.01 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

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

0.07 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

Table 2 L16 (54) experimental results. Factors Chitosan/g Dextran/g NMBA/g AA %

K2S2O8/g Result (Cu2+qe/mg g−1)

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

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

0.01 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

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

249.69 253.81 286.84 295.84 232.71 239.19 248.86 231.53 268.27 260.39 239.54 257.06 287.20 180.63 247.31 244.59

3.4. Adsorption kinetics The effect of time on the adsorption of five ions by DC resin was studied. The kinetic model was fitted and the kinetic mechanism was discussed. Fig. 3 shows the effect of adsorption time on adsorption kinetics of five ions and DC resin. It can be seen from Fig. 3(f) that with the prolongation of adsorption time, the adsorption capacity increases from 0 min to 60 min, and slowly increases after 60 min until the adsorption equilibrium reaches 180 min. In order to study dynamics, we use the following formulas to study pseudo-first-order and pseudosecond-order dynamic models [7,12,33–35]. The equation for the pseudo-first-order kinetic model is as follows: ln ðqe −qt Þ ¼ lnqe −k1t

ð3Þ

The equation for the pseudo-second-order kinetic model is as follows: t 1 1 ¼ þ qt k2 qe 2 qe

Table 3 Analysis of L16 (54) experimental results. Analysis

A

B

C

D

E

K1 K2 K3 K4 Range

271.54 238.07 256.32 239.93 33.47

247.23 253.88 267.57 237.18 30.39

243.26 247.72 241.82 241.82 5.90

234.06 253.02 256.13 262.65 28.59

259.47 233.51 255.64 257.25 25.96

adsorption of the resin, and in this range. As the amount of adsorbent added increases, the adsorbent will provide more adsorption sites, resulting in an increase in the amount of adsorption of the resin. 400

However, when the amount of the adsorbent exceeds 0.02 g, the adsorption amount of the resin decreases with the increase of the amount of the adsorbent, which is probably because although the amount of the adsorbent is high, the metal ion concentration remains unchanged, resulting in water condensation. Many adsorption sites in the gel are in an unsaturated state because there are not enough metal ions to combine with them. At the same time, the concentration of the adsorbent in the solution is too high, so that agglomeration occurs between them, resulting in a large amount of empty adsorption sites disappearing, resulting in resin. The amount of adsorption is reduced [32]. It can also be observed from the Fig. 2(B) that the adsorption performance of the resin is optimal when the amount of the adsorbent reaches 0.015 g, regardless of Cu2+ or Co2+, wherein the adsorption amount of Co2+ is 385 mg L−1 and the Cu2+ is 290 mg L−1.

ð4Þ

where qe (mg g−1) is the equilibrium adsorption capacity, t (mg g−1) is the adsorption time, t is the adsorption time, k1 (min−1), and k2 (g) are the rate constants of pseudo-first-order kinetic models and pseudosecond-order kinetic models, respectively. The fitting parameters of adsorption kinetics are listed in Table 4. The pseudo-first-order kinetic model is based on the membrane diffusion theory, and it is considered that the adsorption reaction rate of the adsorbent is proportional to the square of the difference between the equilibrium adsorption amounts in the system. A pseudo-secondorder kinetic model was established on the adsorption rate limiting step, including the adsorption mechanism, including electron sharing or electron transfer between the adsorbent 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. The correlation coefficient (R2) of the pseudo-second-order kinetic model (R2) is higher than the pseudo-first-order kinetic model (R2) in Table 4, indicating that the

B

400

A

300 qe/mg L-1

300

200

qe/ mg L-1

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

741

200

Cu Co Ni Pb Cd

100 0 1

2

3

4 pH

5

6

7

Cu Co Ni Pb Cd

100

0

0.01

0.02

0.03

0.04

Adsorption Dose

Fig. 2. Effect of pH (A) and adsorbent dose (B) on adsorption capacity.

0.05

0.06

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a

A 0.6

4

Cu

t/qt

In(qe-qt)

0

0.4

-4

Cu 0.2

-8 -12

0.0 0

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100 t/min

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50

100 t/min

150

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1.0

6

b

B 0.8

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Co

0.6 t/qt

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In(qe-qt)

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Co

0.4 -2 0.2

-4 0

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100

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t/min

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6

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t/qt

In(qe-qt)

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Ni

0.4 0 0

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250

0.0

0

50

t/min

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150

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t/min

8 0.6

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D

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Pb t/qt

In(qe-qt)

4

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Pb

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-4

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0

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t/min

t/min

8 0.8

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Cd

2

t/qt

In(qe-qt)

E

e

6

0.4

Cd

0 0.2

-2 0

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250

t/min

0

50

100

150

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250

t/min

f

400

qt/mg g

-1

300 200

Cu Co Ni Pb Cd

100 0

0

50

100

150

200

250

t/min

Fig. 3. Effects of adsorption time on adsorption capacity (f), adsorption kinetics, pseudo-first-order kinetics (a–e) and pseudo-second-order kinetics (A–E).

Y. Liu et al. / International Journal of Biological Macromolecules 141 (2019) 738–746 Table 4 Dynamic model parameters. Kinetic models and parameters

Cu2+

Co2+

Ni2+

Pb2+

Cd2+

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−1 min−1) × 105 R2

342.6

232.8

185.5

395.2

269.8

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

adsorption process of heavy metal ions is more consistent with the pseudo-second-order kinetic model. 3.5. Adsorption isotherm and cycles Isotherm is an important factor in understanding the adsorption efficiency and exploring the adsorption mechanism. Adsorption isotherm is to explore the relationship between adsorption capacity and adsorption concentration in adsorption equilibrium, that is to say, adsorption capacity is only related to equilibrium concentration at constant temperature. Langmuir adsorption isotherm model, Freundlich adsorption isotherm model, Temkin adsorption isotherm model and D-R adsorption isotherm model are commonly used in isotherm research [15,16,35,36]. The Langmuir adsorption isotherm model assumes that there is an atomic force field on the surface of the adsorbent that is not saturated. When the gas is in contact with it, it is adsorbed on the surface of the adsorbent. Once the surface is covered with a layer of gas molecules, the force field is saturated. Adsorption does not occur anymore, so the adsorption is monolayer. The Langmuir adsorption isotherm equation is as follows: Ce Ce 1 ¼ þ qe qm;L K Lqm;L

ð5Þ

where Ce (mg L−1) is the equilibrium concentration, qe (mg g−1) is the equilibrium adsorption amount, and qm,L (mg g−1) is the maximum saturated adsorption capacity. KL (L mg−1) is the Langmuir adsorption constant related to the adsorption energy. The Freundlich adsorption isotherm takes into account the diversity of the surface of the adsorbent. The irregularity of the adsorption heat on the surface is suitable for multi-molecular layer adsorption. There is an interaction between the adsorbed substances. The linear Freundlich adsorption isotherm equation is as follows: ln qe ¼ ln K F þ

1 ln C e n

ð6Þ

where KF (L g−1) is the Freundlich adsorption constant associated with the adsorption capacity. For the same adsorbent, the larger the KF value, the stronger the adsorbent has the adsorption capacity. n is the Freundlich constant which reflects the difficulty of reaction. It represents the degree of surface heterogeneity and the energy distribution of adsorption sites. If n N 1, the adsorption process is easy to occur, and n b 1/2, the adsorption process is not easy to occur. The Temkin adsorption isotherm model is based on the interaction between the adsorbent and the adsorbed species. The model assumes that the heat of adsorption decreases linearly with the degree of adsorption process. The linear expression is as follows: qe ¼ B ln A þ B ln C e

ð7Þ

743

where A (mg L−1) is an equilibrium constant related to binding energy, and B is a constant related to heat of adsorption. The D-R adsorption isotherm model does not need to make the above ideal assumptions, and is based on the Polanyi potential energy theory. According to the theory, the adsorption space on the surface of the adsorbent is constant, and there is a temperature-independent adsorption potential at each point of the adsorption space. The D-R model application is more extensive than the above two models, and its linear equation is as follows: ln qe ¼ βε2 þ lnqm;D−R

ð8Þ

where qm,D-R (mg g−1) is the theoretical maximum adsorption amount of the D-R adsorption isotherm model, β (mol2 kJ−2) is the constant related to the adsorption energy, and ε is the Polanyi adsorption potential energy. Among them,

ε ¼ RT ln

  Ce þ 1 Ce

ð9Þ

where R is a universal gas constant (8.314 J K−1 mol−1), and T (K) is a thermodynamic temperature. The mean free energy E (kJ mol−1) of the adsorption process can also be calculated by the β constant of the D-R model. The calculation formula is as follows: 1 E ¼ pffiffiffiffiffiffi 2β

ð10Þ

According to E, it can be judged whether the adsorption type of the process is physical or chemical adsorption, when E b 8 kJ mol−1 is physical adsorption, when 8 b E b 16 kJ mol−1, it is ion exchange type chemical adsorption, when E N 16 kJ mol −1 is chemically adsorbed. Fig. 2 shows the effect of heavy metal ion concentration on the adsorption of five ions by DC resin and the isotherm model. As shown in Fig. 4, the adsorption capacity (qe) increases with the increase of ion concentration. The results show that the adsorption capacity of the DC resin is concentration dependent. When the concentration of metal ions increases, the mass transfer dynamics such as ion exchange and electrostatic attraction between solid and liquid phases also increase, which makes it easier for metal ions to overcome the resistance of adsorption. Table 5 shows the adsorption process of five ions by DC resin, and the corresponding isotherm model parameters are obtained by fitting. It is clear from the table that the adsorption process of DC resin for heavy metal ions is in good agreement with Langmuir linear fitting. The correlation coefficients R2 are all above 0.99, which are better than those of other isothermal models. It can be concluded that the adsorption process of heavy metal ions by DC resin is mainly in accordance with the chemical adsorption of Langmuir monolayer, and combined with the parameter 1/n of Freundlich model, it shows that the adsorption process is easy to occur. Combining thermodynamic and kinetic models, it is shown that the whole adsorption process is mainly monolayer, endothermic chemical adsorption of spontaneous reaction [32,37–39]. Reuse efficiency is an important indicator of adsorbent. DC resin adsorbent can achieve five adsorption-desorption cycles in a single ion system. As can be seen from Fig. 4, DC resin adsorbent still has high adsorption efficiency after five cycles, with the lowest efficiency being 75%. This indicates that DC resin is a green and environmentally friendly adsorbent for heavy metal ions.

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Y. Liu et al. / International Journal of Biological Macromolecules 141 (2019) 738–746

2.5

2.5

2.0

2.0 1.5 Ce/qe

Ce/qe

1.5 1.0

1.0

Co

Cu

0.5

Langmuir

0.5

Langmuir

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800

Ce/ mg L-1

Ce /mg L-1

2.0 4 1.6 3 Ce/qe

Ce/qe

1.2 2

Ni

1

0

0.8

Pb

0.4

Langmuir

Langmuir

0.0 0

200

400

600

800

1000

1200

0

200

400

600

800

1000

Ce/mg L-1

-1 Ce/mg L

600 3

500 400

Ce/qe

qe/mg L-1

2

1

Cd Langmuir

300

Cu Co Ni Pb Cd

200 100

0 0

200

400

600

800

1000

0

1200

0

200

400

Cu Co Ni Pb Cd

100 90 ∅/%

600

800

1000

1200

Ions Concentration

Ce/mg L-1

75%

80 70 60 50

1

2

3

4

5

Cycle times Fig. 4. Effect of heavy metal ion concentration on adsorption capacity, isotherm model and effect of cycles times on the adsorption capacity of DC resin.

3.6. Adsorption thermodynamics

change ΔH° are shown in Table 4.

As can be seen from Fig. 5, the adsorption capacity of ions increases as the adsorption temperature increases. In order to study the thermodynamics, the obtained data was fitted by the following formula. The results are shown in Fig. 5. The corresponding thermodynamic parameters, Gibbs free energy ΔG°, entropy change ΔS° and enthalpy

ΔG ° ¼ ΔH °−TΔS ° ln ðqe =C e ¼

ΔS ° ΔH ° − R RT

ð11Þ ð12Þ

where Ce (mg L−1) is the equilibrium ion concentration, qe (mg g−1) is

Y. Liu et al. / International Journal of Biological Macromolecules 141 (2019) 738–746

4. Conclusion

Table 5 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 E (kJ mol−1) × 103 R2

745

Cu2+

Co2+

Ni2+

Pb2+

Cd2+

469.5 0.202 0.993

332.2 0.028 0.998

287.4 0.013 0.0992

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

94,792 17.03 0.723

417.1 3.330 0.087 0.383

296.3 12.01 0.045 0.670

246.1 19.72 0.036 0.527

507.0 1.943 0.113 0.640

301.4 4.179 0.077 0.223

the equilibrium adsorption capacity, R is the universal gas constant, and T (K) is the thermodynamic temperature. It can be seen from Table 6 that ΔG° is negative and decreases with the increase of temperature, which indicates that the adsorption process is a spontaneous process and the increase of temperature is conducive to adsorption, which is consistent with the trend of qe-T. The positive value of ΔH° indicates that the adsorption process is accompanied by endothermic process. The fitting of thermodynamic curves shows that the adsorption process of five ions adsorbed by DC resin is accompanied by a confused spontaneous endothermic reaction [5,32–34].

Acknowledgments This work was supported by Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (STIP, 2019L0517).

Table 6 Adsorption thermodynamic parameters. Thermodynamic parameters

Cu2+ Co2+ Ni2+ Pb2+ Cd2+

Cd

2.0

ΔH°

ΔG° (kJ mol−1)

ΔS°

(kJ (J mol−1) mol−1 K−1)

293.15 K

303.15 K

313.15 K

323.15 K

333.15 K

6.923 17.02 21.08 4.953 10.41

−5.89 −3.66 −2.89 −7.18 −4.21

−6.33 −4.36 −3.70 −7.60 −4.71

−6.77 −5.07 −4.53 −8.01 −5.21

−7.21 −5.78 −5.34 −8.43 −5.71

−7.64 −6.48 −6.16 −8.84 −6.21

43.72 70.55 81.78 41.40 49.86

Ni

2.0

In(qe/Ce)

2.2 In(qe /Ce)

DC resin adsorbent with excellent adsorption performance was synthesized by ultrasonic heating. Five heavy metal ions, Cu2+, Co2+, Ni2+, Pb2+ and Cd2+, were adsorbed. The adsorption experiments show that the adsorption performance is the best under neutral conditions. The adsorption experimental data are close to the pseudo-second-order kinetic model and Langmuir isotherm, which proves that the adsorption process is a spontaneous endothermic chemical monolayer adsorption. In addition, the adsorption efficiency of DC resin adsorbent for heavy metal ions is at least 75% of the first adsorption capacity after five adsorption-desorption cycles.

1.6

1.2 1.8 0.0030

0.0031

0.0032

0.0033

0.0034

0.0030

0.0031

1/T

0.0032

0.0033

3.2

2.4

Co

Pb 3.1 In(qe/Ce)

In(qe/Ce)

0.0034

1/T

2.0

3.0 1.6

0.0030

0.0031

0.0032

0.0033

2.9

0.0034

0.0030

0.0031

1/T

0.0032

0.0033

0.0034

1/T

2.8 400 350 qe/mg g-1

In(qe/Ce)

Cu 2.6

300

Cd Ni Co Pb Cu

250 2.4

200 0.0030

0.0031

0.0032 1/T

0.0033

0.0034

290

300

310

320

T/K

Fig. 5. Adsorption thermodynamics and effect of temperature on adsorption capacity.

330

340

746

Y. Liu et al. / International Journal of Biological Macromolecules 141 (2019) 738–746

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