Fabrication of a Magnetic Poly(aspartic acid)-Poly(acrylic acid

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Fabrication of a Magnetic Poly(aspartic acid)-Poly(acrylic acid) Hydrogel: Application for the Adsorptive Removal of Organic Dyes from Aqueous Solution Xiaojun Jv, Xiaowei Zhao,* Hucheng Ge, Jingyi Sun, Hai Li, Qiusheng Wang, and Hongguang Lu Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, China

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S Supporting Information *

ABSTRACT: At present, synthetic dyes have been widely used in several industries such as textiles, rubber, paper, plastic, and leather tanning. The dyes in effluents are among the most aggressive pollutions of all the industrial sectors and can lead to severe water contamination as well as resource waste without proper treatment and recovery. In the present work, magnetic poly(aspartic acid)-poly(acrylic acid) hydrogels (PAsp-PAA/Fe3O4) with semiinterpenetrating networks are successfully prepared. Their physicochemical properties are systematically characterized using scanning electron microscopy, Fourier transform infrared spectroscopy, the Brunauer−Emmett−Teller method, thermogravimetric analysis, and vibrating sample magnetometry. Organic dye methylene blue (MB) and neutral red (NR) adsorption onto the PAsp-PAA/Fe3O4 hydrogel are studied for the first time, and the maximum adsorption capacities of MB and NR are calculated by using the Langmuir model as 357.14 and 370.37 mg·g−1, respectively. The adsorption kinetics can be well described by a pseudo-second-order kinetic model. Furthermore, reusability tests indicate that the hydrogel has good reproducibility. In conclusion, the results demonstrate that the PAsp-PAA/Fe3O4 hydrogel can used as an excellent adsorbent material for removing dye pollutants from wastewater. variations in pH.26 PAA-based hydrogels such as N-maleyl chitosan/P(AA-co-VPA) hydrogel,20 katira gum-cl-poly(acrylic acid-co-N-vinyl imidazole) hydrogel,21 and poly(acrylic acid)grafted sodium alginate (SA-cl-PAA)-based hydrogel27 have been successfully synthesized and applied for treating organic dye wastewater. On the other hand, the poly(aspartic acid) (PAsp) hydrogel is an amino acid-based polymer that possesses various predominant properties, such as biocompatibility and biodegradability. The cross-linked PAsp hydrogel can absorb a large amount of water and is hardly removed even under pressure because of its excellent water-absorbing properties. These characteristics make it more suitable for application in the adsorption of dyes. However, it has the fatal drawback of lower mechanical properties resulting from the high charge density along the polymer chains.28 Considering the aforementioned semi-interpenetrating network (semi-IPN) technique that has been introduced, which is prepared by incorporating one polymer into the hydrogel network of the other polymer, this can attain satisfactory mechanical strength and various sensitivities.28 Therefore, when the PAsp hydrogel is combined with the PAA hydrogel by means of the semi-IPN

1. INTRODUCTION Currently, environmental contamination caused by synthetic organic dyes has become more and more serious as the dyeing industries develop. Dyes are important industrial materials which are widely used in printing, textile, varnishes, paper, dyestuffs, and other products.1,2 Unfortunately, approximately 2.37 billion tons of dyes are discharged in China alone each year for the textile industry.3 In addition, the migration of dyes and their degradation contaminants into the ecosystem is harmful to aquatic organisms and human health.4 Therefore, appropriate techniques are urgently required for organic dye removal from industrial wastewater. Among several physiochemical and biochemical methods for dye wastewater treatment, adsorption is considered to be a promising technology owing to numerous advantages such as high efficiency, simple operation, and various adsorbents. So far, a variety of materials have been investigated as adsorbents for dye removal including activated carbon,5 sorption resins,6 nanomaterials,7−10 and others.11−15 Hydrogels, as distinguished polymer materials with threedimensional networks structure, have attracted extensive attention. Until now, hydrogels have been used extensively in a variety of areas such as drug delivery systems,16 agriculture,17 tissue engineering,18 and wastewater treatment.19−25 As a widely utilized polymeric hydrogel, poly(acrylic acid) (PAA) exhibits swelling and deswelling behaviors in response to © XXXX American Chemical Society

Received: November 23, 2018 Accepted: February 8, 2019

A

DOI: 10.1021/acs.jced.8b01117 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Figure 1. Characterization of the samples: (A) FTIR spectra of naked Fe3O4 and the PAsp-PAA/Fe3O4 hydrogel; (B) SEM images of the PAspPAA/Fe3O4 hydrogel; (C) TGA curves of naked Fe3O4 and the PAsp-PAA/Fe3O4 hydrogel; and (D) hysteresis loop of naked Fe3O4 and the PAspPAA/Fe3O4 hydrogel.

in response to variations in different pH values. The poly(aspartic acid) (PAsp) hydrogel is an amino acid-based polymer that possesses the properties of biocompatibility and biodegradability. When the PAsp hydrogel is combined with the PAA hydrogel by means of the semi-IPN technique, a type of biodegradable hydrogel will be obtained which possesses the merits of both. For easy solid−liquid separation, Fe3O4 nanoparticles are introduced into the semi-IPN hydrogel in this study. The Fe3O4 nanoparticles were synthesized according to the chemical coprecipitation method. Briefly, 3.2 g of FeCl3·6H2O and 1.6 g of FeSO4·7H2O were completely dissolved in 150 mL of deionized water, and the solution was transferred to a three-necked flask using nitrogen. The pH of the solution was adjusted to 10.0 by adding NH3· H2O, and mechanical stirring was continued for 1 h at 70 °C. After that, the resulting products were separated by means of an external magnetic field and then washed with deionized water and ethanol several times. Finally, the obtained products were dried in the vacuum for 24 h and stored for further use. PAsp-PAA/Fe3O4 hydrogels were synthesized as follows: 2 g of PAsp was uniformly dispersed in 30 mL of distilled water, and the solution was transferred to a 250 mL three-necked flask equipped with constant magnetic stirring and a nitrogen atmosphere. The reaction lasted for 1 h at 70 °C. Next, 5.4 g of neutralized acrylic acid, 0.057 g of N,N′-methylene bis(acrylamide), and 0.5 g of Fe3O4 nanoparticles were added to the above solution. Then, the mixture was cooled to 50 °C, 0.10 g of potassium persulfate as the initiator was introduced, and the reactants were slowly heated to 70 °C and kept for 1 h

technique, a type of biodegradable hydrogel will be obtained which possesses the merits of both. One more consideration after the adsorption of dyes from wastewater is how to separate the adsorbed hydrogel from the treated water. For easy solid−liquid separation, Fe 3 O 4 nanoparticles are introduced into the semi-IPN hydrogel in this study. Furthermore, the obtained magnetic hydrogel shows better mechanical strength than does the hydrogel without Fe3O4 nanoparticles. Herein, magnetic poly(aspartic acid)poly(acrylic acid) hydrogels (PAsp-PAA/Fe3O4) are prepared to be employed as an adsorbent for the adsorption of MB and NR from water. The effects of contact time, pH, and initial concentration of dyes are studied. The adsorption mechanism is discussed according to the adsorption behavior, kinetics studies, isothermal equilibrium, and thermodynamics. In addition, desorption and regeneration of the hydrogel are also investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. NH3·H2O (25%), FeSO4·7H2O, FeCl3· 6H2O, acrylic acid, and potassium persulfate were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). N,N′-Methylene bis(acrylamide), methylene blue, and neutral red were provided by Aladdin Reagent Inc. (Shanghai, China). Poly(aspartic acid) was obtained from Xiya Reagent Co., Ltd. (Shandong, China). All of the chemicals were used as received without further purification. 2.2. Synthesis of a Magnetic Poly(aspartic acid)Poly(acrylic acid) Hydrogel (PAsp-PAA/Fe3O4). Poly(acrylic acid) (PAA) shows swelling and deswelling behaviors B



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to finish the polymerization. Finally, the hydrogel was cut into small blocks and dried for further use. 2.3. Characterization. The structure and interaction of the samples were analyzed on a Vertex 70 (Bruker, Germany) Fourier transform infrared (FTIR) spectrometer. The surface morphology was observed using a scanning electron microscope (Zeiss Merlin Compact, Germany). The specific surface area was measured by the BET method (Micromeritics TriStar II 3020, USA). The magnetic property of the hydrogel was evaluated using an MPMS-XL-7 Quantum Design SQUID magnetometer. The thermal characterizations of the hydrogel were obtained by means of thermogravimetric analysis (TG209, Germany). Dye concentrations were analyzed with a UV/vis spectrophotometer (U-3310, Shimadzu, Japan). 2.4. Swelling Characteristics. The swelling behavior of the PAsp-PAA/Fe3O4 hydrogel in deionized water under different pH conditions and in different salt solutions was investigated. The swelling ratio (SR) of the hydrogel can be evaluated by using eq 1 SR(g g −1) =

Wt − W0 W0

3. RESULTS AND DISCUSSION 3.1. Characterization. FTIR spectra of naked Fe3O4 and the PAsp-PAA/Fe3O4 hydrogel are illustrated in Figure 1A. It is obvious that the broad bands of both samples at 580 cm ̵ 1 are ascribed to Fe−O vibrations, while the peaks appearing at 3421, 1581, and 1398 cm ̵ 1 in the spectrum of the PAsp-PAA/ Fe3O4 hydrogel are due to the −NH stretching vibration, amide group, and carboxylic acid group. The above information indicates that PAsp-PAA/Fe3O4 hydrogels are successfully prepared. Figure 1B shows an SEM image of the PAsp-PAA/Fe3O4 hydrogel. It is found that Fe3O4 nanoparticles are dispersed in the hydrogel, and a macroporous structure is also observed. This structure is advantageous for improving the adsorption capacity and adsorption rate. The specific surface area, average pore diameter, and pore volume of the PAsp-PAA/Fe3O4 hydrogel are listed in Table 1. As can Table 1. Physicochemical Properties of PAsp-g-PAA/Fe3O4

(1)

(C0 − Ce)V m

average pore diameter (nm)

pore volume (cm3 g−1)

62.7312

28.3041

0.3347

be observed, the Brunauer−Emmett−Teller (BET) specific surface area of the hydrogel is calculated to be 62.7312 m2 g−1. During the adsorption process, more adsorption sites can be provided through the larger surface area, which can contribute to the adsorption applications. Thermogravimetric analysis (TGA) of naked Fe3O4 and the PAsp-PAA/Fe3O4 hydrogel was performed in the temperature range from 35 to 600 °C (Figure 1C). For naked Fe3O4, only about a 3.7% weight loss is observed over the temperature range, which is caused by the loss of residual water in the sample. The TGA curve of the PAsp-PAA/Fe3O4 hydrogel shows four mass-loss stages in the temperature ranges of 35 to 180, 180 to 330, 330 to 430, and 430 to 600 °C. The first weight loss is due to the removal of absorbed physical and chemical water, while the second step of weight loss corresponds to the dehydration and the breaking of CC− OH bonds in the chain of PAsp. The third step of weight loss is related to the elimination of the CO2 molecule from the polymeric backbone. With increasing temperature (430 to 600 °C), there is no distinct weight loss, suggesting that only iron oxide is present in the last step. Furthermore, by comparing two TGA curves, it can be found that the amount of Fe3O4 in PAsp-PAA/Fe3O4 is about 22%. FTIR analysis is also studied in order to analyze the residue of the hydrogel, and the results are shown in Figure S1 (Supporting Information). Magnetization properties of naked Fe3O4 and the PAspPAA/Fe3O4 hydrogel are investigated with the MPMS-XL-7 Quantum Design SQUID magnetometer and the magnetic hysteresis loop of the sample, as illustrated in Figure 1D. It is found that the magnetization curve shows S-curve behavior over the applied magnetic field for both samples, exhibiting typical superparamagnetic behavior with negligible coercivity (Ce) and remanence (Mr) values. The saturation magnetization (Ms) of the PAsp-PAA/Fe3O4 hydrogel obtained at 298 K is 29.59 emu g−1, while that of naked Fe3O4 is 61.12 emu g−1. These results indicate that the prepared adsorbent has high magnetism, which can be beneficial to reuse by magnetic separation. 3.2. Swelling Characteristics of the PAsp-PAA/Fe3O4 Hydrogel. 3.2.1. Swelling in Deionized Water. As observed

where W0 (g) and Wt (g) are the masses of the dried and swollen hydrogels, respectively. Additionally, the equilibrium swelling ratio (ESR) was also calculated by replacing Wt with We (the mass of saturated swollen hydrogel) in eq 1. 2.5. Adsorption Study in a Single System. Batch experiments were performed to study the adsorption behavior of the hydrogel. Organic dyes MB and NR were chosen in this study. Typically, 20 mg of the PAsp-PAA/Fe3O4 hydrogel was added to 20 mL of dye solution (50 mg·L−1) for 1 h except for the contact time study. After adsorption equilibrium was achieved, the adsorbed hydrogel was removed magnetically from the solution using a permanent hand-held magnet (0.25 T). The amount of each dye in the solution before and after adsorption was measured with a UV/vis spectrophotometer at λmax = 665 nm for MB and 530 nm for NR. The equilibrium adsorption amount (qe, mg·g−1) was evaluated by using eq 2 qe =

specific surface area (m2 g−1)

(2)

where C0 and Ce are the initial and equilibrium concentrations (mg·L−1) of dye in solution, respectively. V is the volume (L) of the dye solution, and m is the mass of the hydrogel (g). 2.6. Adsorption Study in Binary Systems. A binary component system composed of MB and NR was employed to explore the effect of coexisting dyes. PAsp-PAA/Fe3O 4 hydrogel (20 mg) was added to a 20 mL mixed dye solution at pH 7.0, and then the solution was shaken on a shaker for 1 h. Finally, the same method as above was carried out to determine the adsorption behavior of the hydrogel in binary systems. 2.7. Regeneration. The recyclability of the PAsp-PAA/ Fe3O4 hydrogel was investigated by conducting six consecutive sorption cycles. For these experiments, 20 mg of PAsp-PAA/ Fe3O4 hydrogel was added to 20 mL of a 50 mg·L−1 dye solution at pH 7.0 and shaken for 1 h. Afterward, the adsorbed hydrogel was separated with a magnet and dispersed into 20 mL of 0.01 moL L−1 HCl and shaken for 1 h. Then, the collected hydrogel was dried and reused for adsorption. C



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The PAsp-PAA/Fe3O4 hydrogel contains carboxyl and hydroxy groups which are anionic-type adsorbents. Under an acidic medium (pH 10.0), the excess Na+ in the swelling medium can combine with −COO−, decreasing the osmotic pressure of the polymer network and resulting in hydrogel shrinkage.29,30 3.2.3. Swelling in Different Salt Solutions. The swelling behavior of the PAsp-PAA/Fe3O4 hydrogel is estimated in different concentrations of NaCl, CaCl2, and FeCl3 (0.1−0.4% w/w) salt solutions. As observed in Figure 2C, the ionic species have a great impact on the swelling ability of the hydrogel. For example, at a given concentration of the salt (0.2 wt %), the ESR values are 23.65, 11.29, and 6.05 g g−1 for NaCl, CaCl2 and FeCl3 salt solutions, respectively. This phenomenon may be attributed to the fact that multivalent cations (for example, Fe3+) can be integrated with the carboxylate groups in the PAsp-PAA/Fe3O4 hydrogel, limiting water incorporation, which leads to a small swelling ratio. Besides, the concentration of salt solution also has an effect on the ESR of the PAsp-PAA/Fe3O4 hydrogel. As illustrated in Figure 2C, the water absorbency decreases with increasing salt solution concentration. For instance, the ESR values are 28.32, 23.65, 19.63, and 13.58 g g−1 for 0.1, 0.2, 0.3, and 0.4 wt % NaCl, respectively. The shrinking behavior can be explained by the fact that the osmotic pressure between the hydrogel network and the salt solution is strengthened by increasing the concentration of the salt solution. Overall, these results indicate that both the ionic species and the concentration of salt solution influence the ESR of the hydrogel.30 3.3. Adsorption Study in a Single System. The properties of PAsp-PAA/Fe3O4 hydrogel adsorption to MB and NR have been studied in a single system. The effects of pH, contact time, and initial dye concentration as well as the adsorption kinetics, adsorption isotherm, and thermodynamics are systematically investigated. 3.3.1. Effect of Solution pH. In the present study, the influence of solution pH on the removal of MB and NR by the PAsp-PAA/Fe3O4 hydrogel is researched by altering the solution pH value from 3.0 to 9.0. As shown in Figure 3A, the absorption of both dyes shows a remarkable increase with the increase in pH from 2.0 to 6.0 and then slightly increases when the pH value increases from 6.0 to 9.0. The results indicate that at low pH values the adsorbents are protonated, which weakens the electrostatic attraction toward MB or NR molecules. Also, the surface of the PAsp-PAA/Fe3O4 hydrogel is more negatively charged with increasing pH values, which is more favorable to the adsorption of organic dyes such as MB and NR. 3.3.2. Effects of Contact Time and Adsorption Kinetics. Figure 3B depicts the influence of contact time on MB and NR adsorption onto the PAsp-PAA/Fe3O4 hydrogel with a contact time in the range from 10 to 150 min. It can be observed that the uptake of the PAsp-PAA/Fe3O4 hydrogel increases rapidly within the initial 10 min and then slows gradually over time.

from Figure 2A, the swelling ratios of the hydrogel are appreciably increased during the first 200 min (from 10.10 to

Figure 2. Swelling behavior of the PAsp-PAA/Fe3O4 hydrogel: (A) swelling ratio in deionized water; (B) ESR values for different pH values; and (C) ESR values for different salt solutions.

66.32 g g−1), and then they gradually become slower with time. The largest swelling ratio occurs at about 350 min (73.30 g g−1). Further increases in contact time do not enhance the adsorption percentage obviously. 3.2.2. Swelling at Various pH Values. The swelling properties of the PAsp-PAA/Fe3O4 hydrogel are investigated by changing the solution pH values from 2.0 to 12.0 with HCl and NaOH. As shown in Figure 2B, it is obvious that the equilibrium water absorbency of the PAsp-PAA/Fe3O4 hydrogel shows pH sensitivity. ESR shows a dramatic increase with increasing pH from 2.0 to 6.0, increases slightly when the pH value increases from 6.0 to 10.0, and then decreases slightly within pH 10.0 to 12.0. D



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log(qe − qt) = log qe −

k1 t 2.303

(3)

t 1 1 = + t qt qe k 2qe 2

(4)

where qe (mg·g−1) and qt (mg·g−1) are the adsorption capacity at equilibrium and at time t, respectively. k1 (min−1) and k2 (g mg−1 min−1) are the pseudo-first-order and pseudo-secondorder rate constants, respectively. The relevant kinetic parameters are summarized in Table 2. Compared to the pseudo-first-order model, the pseudosecond-order kinetic model gives a relatively higher R2. In addition, the adsorption capacities (qe, cal) measured from the pseudo-second-order kinetic model are closer to the experimental values (qe, exp) than that measured from the pseudo-first-order model. In this case, it suggests that the pseudo-second-order model is more reasonable for explaining the kinetics of the uptake process of dyes onto the PAsp-PAA/ Fe3O4 hydrogel. Therefore, the adsorption mechanism depended on the adsorbate and adsorbent, and the adsorption process is primarily governed by chemisorption. This behavior could be attributed to the electrostatic interactions that arose as a result of an exchange or sharing of electrons between the cationic groups of dyes and anionic groups of the PAsp-PAA/ Fe3O4 hydrogel. Similar results are reported in the literature.31 3.3.3. Adsorption Isotherms. The effects of the initial concentration of MB and NR adsorption onto the PAsp-PAA/ Fe3O4 hydrogel vary from 10 to 500 mg·L−1 at pH 7.0, and the results are shown in Figure 3C. Obviously, the dye adsorption capacities on the PAsp-PAA/Fe3O4 hydrogel increase with increasing dye concentration until adsorption is saturated at a concentration of 350 mg·L−1. This is due to the fact that the driving force for the adsorption of dye is increased with increasing dye concentration until the adsorption sites are saturated. Then, Langmuir and Freundlich isotherm models are applied to correlate the adsorption isotherms, described as follows Ce C 1 = + e qe qmax b qmax Figure 3. Effects of (A) initial pH, (B) contact time, and (C) initial concentration upon the adsorption of MB and NR on the hydrogel.

log(qe) =

(5)

1 log(Ce) + log K f n

(6) −1

where qe and qmax are the amounts of dyes (mg·g ) adsorbed at equilibrium and the maximum adsorption capacity, respectively, and Ce (mg·L−1) represents the equilibrium concentration of dyes. b represents the Langmuir constant (L mg−1) that is related to the affinity of the adsorbate for the adsorbent surface. Kf and n are the constants of a Freundlich isotherm characterizing the adsorption capacity and intensity of adsorption, respectively.

To ensure the attendance of adsorption equilibrium, a 60 min contact time is used in the following equilibrium adsorption experiments. To evaluate the dynamic adsorption behavior, the pseudofirst-order and pseudo-second-order models are utilized to correlate the adsorption kinetic data

Table 2. Kinetic Parameters for the Adsorption of MB and NR onto PAsp-g-PAA/Fe3O4 pseudo-first-order model qe,exp −1

qe,cal −1

pseudo-second-order model

k1

qe,cal

−1

2

dye

(mg·g )

(mg·g )

(min )

R

MB NR

49.56 51.60

4.49 4.32

0.015 0.021

0.8151 0.9010 E

−1

(mg·g ) 49.26 51.55

K2 −1

(g mg

min−1)

0.015 0.016

R2 0.9997 0.9999



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Table 3. Isotherm Parameters for the Adsorption of MB and NR onto PAsp-g-PAA/Fe3O4 Langmuir model

Freundlich model

b

qm

dye

(L mg−1)

(mg·g−1)

RL

R2

(L mg−1)

1/n

R2

MB NR

0.15 0.10

357.14 370.37

0.01 0.02

0.9984 0.9960

46.45 33.87

0.49 0.57

0.9293 0.9181

kf

Considering the Freundlich isotherm, the values of Kf and 1/n are calculated from the intercept and slope of the plot of ln qe vs ln Ce. In the present study, the values of Kf are 46.65 and 33.87 L mg−1 for MB and NR, respectively. The 1/n values of 0.49 and 0.57 are observed for MB and NR, respectively. The slope 1/ n, ranging between 0 and 1, is a measure of the adsorption intensity or surface heterogeneity, becoming more heterogeneous as its value gets closer to zero. A value for 1/n of 1 is indicative of cooperative adsorption. Additionally, the magnitudes of Kf and n values indicate the facile removal of MB and NR and imply favorable adsorption.35 3.3.4. Thermodynamic Studies. In this work, four temperatures, 308, 318, 328, and 338 K, are considered to understand the effect of temperature because it provides an in-depth understanding of energy changes during adsorption. Thermodynamic parameters such as the free-energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) for MB and NR adsorption onto the PAsp-PAA/Fe3O4 hydrogel are calculated with the following equations

Besides, on the basis of the Langmuir model, a dimensionless factor called the separation factor, RL, can be calculated using the value of b (eqs 5), where Ci is the maximum initial concentration of dye: RL =

1 1 + Cib

(7)

According to eq 7, the RL value suggests whether the adsorption is favorable (0 < RL < 1), unfavorable (RL > 1), linear (RL = 1), or irreversible (RL = 0). The theoretical parameters and correlation coefficients (R2) of adsorption isotherms for the adsorption of MB and NR are shown in Table 3. By comparing the correlation coefficient (R2), it is found that the Langmuir isotherm model fits this adsorption data better than does the Freundlich isotherm model. The results suggest that the binding sites of the PAspPAA/Fe3O4 hydrogel are uniformly distributed on its surface, and the adsorption of dyes is regarded as monolayer adsorption. On the basis of the Langmuir model, the maximum adsorption capacity of the PAsp-PAA/Fe3O4 hydrogel is calculated to be 357.14 mg·g−1 for MB and 370.37 mg·g−1 for NR, which are higher than for most of the recent advanced adsorbents (Table 4).5,7−15,20−23,32−34 The values of RL are in the range from 0 to 1, suggesting that adsorption is favorable.

adsorption capacity mg·g−1 adsorbent

MB 71.42 80.4 178.57 43.82

NR 171.20 56.0 76.3

61.50 141.0 165.0 833.0 45.10 64.46 331.50 137.2 254.0

ref 5 7 8 9 10 11

23 32

26.92 35.96

33 34

357.14

370.37

ΔG° = ΔH ° − T ΔS°

(9)

Table 5. Thermodynamic Parameters for the Adsorption of Dyes onto PAsp-g-PAA/Fe3O4 ΔG°

12 13 14 15 20 21 22

20.73

(8)

where T(K) is the absolute temperature and R is the universal gas constant (8.314 J·mol−1·K−1). The values of enthalpy (ΔH°) and entropy (ΔS°) can be obtained from the slope and the intercept, respectively, according to eq 8, and the magnitude of ΔG° is given by eq 9. The results of thermodynamic parameters (ΔH°, ΔS°, and ΔG°) are observed in Table 5. The negative values of ΔG°

Table 4. Comparison of Adsorption Capacity for MB and NR by Different Adsorbents

magnetic biomass activated carbon PANI−ZrPB nanocomposite bioglass nanospheres (BGN) magnetite/silica/pectin NPs graphene nanosheet (GNS)/magnetite monodisperse hollow porous magnetic particles (MHPMs) GO−Fe3O4 hybrid Chi/nGO 3:1 NH2-MIL-125(Ti) magnetic chitosan composite (CCM) NMC cross-linked P(AA-co-VPA) hydrogel KG-cl-poly(AA-co-NVI) hydrogel poly(vinyl alcohol)-sodium alginatechitosan-montmorillonite hydrogel beads chitosan/TEOS hydrogel poly-CD/PAA-azo hydrogel PVA/PAA/GO−COOH@PDA composite membrane Fe3O4@(PAH/GO−COOH)n nanocomposites PAsp-PAA/Fe3O4

ij q yz −ΔH ° ΔS° lnjjj e zzz = + j Ce z RT R k {

dyes

T(K)

MB

308 318 328 338 308 318 328 338

NR

kJ·mol

−1

−10.602 −9.971 −9.340 −8.709 −12.481 −11.415 −10.350 −9.284

ΔH° kJ·mol

−1

ΔS° kJ·mol−1·K−1

−30.031

−0.063

−45.302

−0.106

suggest that the adsorption of dyes is a feasible and spontaneous process. The magnitude of ΔG° decreases with increasing temperature, meaning that the affinity will be strengthened at lower temperature, which is beneficial to practical applications. The negative values of ΔH° show that the adsorption of MB and NR onto the PAsp-PAA/Fe3O4

this study F



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MB and 91.64% for NR during the sixth cycle, suggesting that the adsorption of the MB and NR onto PAsp-PAA/Fe3O4 hydrogels are reversible. The experimental results demonstrate that PAsp-PAA/Fe3O4 hydrogel hold potential for practical applications as a highly efficient and economic adsorbent.

hydrogel is an exothermic process, which is also supported by the decrease in the amount of adsorbed hydrogel with increased temperature. The activation entropy change ΔS° may be divided into the dissociative mechanism and associative mechanism, depending on whether the value is positive or negative.36 In this experiment, negative values are observed for both MB and NR, and thus the process contained an associative mechanism. Meanwhile, the negative values of ΔS° give evidence for the decreased randomness at the solid− solution interface. These results are in accordance with the previous results.37−40 3.4. Adsorption of a Binary Mixture of MB and NR. A binary mixture of MB and NR is used to evaluate the adsorption behavior of the PAsp-PAA/Fe3O4 hydrogel in the presence of various cationic dyes. The absorbance is measured at wavelengths of 665 and 530 nm. In this study, the initial concentration of the dye is selected to be 350 mg L−1. The reason is that the amount of adsorption increases as the initial concentration increases until adsorption is saturated at a concentration of 350 mg·L−1 (Figure 3C). As shown in Table 6, the adsorption capacities of both NR and MB on the

4. CONCLUSIONS In this work, PAsp-PAA/Fe3O4 hydrogels are successfully synthesized, and their adsorption behavior toward MB and NR dyes in single and binary systems is studied. In our findings, the adsorption of organic dyes on the hydrogel is affected by the contact time, solution pH, and initial dye concentration. An adsorption isotherm study indicates that the adsorption of dyes on the hydrogel is a monolayer adsorption, and the maximum adsorption capacities for MB and NR are estimated to be 357.14 and 370.37 mg·g−1, respectively. The adsorption kinetics can be well described by a pseudo-second-order kinetic model. In summary, the prepared PAsp-PAA/Fe3O4 hydrogel with a high swelling ratio, pH sensitivity, convenient magnetic separation, excellent adsorption behavior to dyes, and remarkable regeneration performance possessed greater competitiveness compared to that of other adsorbents.

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Table 6. Comparison of the Adsorption Capacity for MB and NR onto PAsp-PAA/Fe3O4 Hydrogels in Single and Binary Systems initial concentration −1

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b01117. FTIR analysis of the residue of the PAsp-PAA/Fe3O4 hydrogel (PDF)

adsorption capacity −1

dye

mg·L

system

mg·g

MB NR MB NR

350 350 350 350

single single binary binary

308.09 317.48 196.84 212.58

ASSOCIATED CONTENT

S Supporting Information *

■

AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86-22-60214259. E-mail: [email protected].

hydrogel decrease in the binary dye system compared to in the single dye system. The possible reason may be explained by the existence of competitive adsorption for limited adsorption sites on the surface of the PAsp-PAA/Fe3O4 hydrogel. 3.5. Reusability of PAsp-PAA/Fe3O4. Herein, six cycles of desorption and regeneration experiments are conducted to evaluate the regeneration ability of the PAsp-PAA/Fe3O4 hydrogel. In this study, 0.01 moL L−1 HCl is used as the eluent. Figure 4 shows the variation of adsorption capacities and desorption ratios for MB and NR onto the PAsp-PAA/ Fe3O4 hydrogel during the six cycles. Only a tiny loss of adsorption capacity is observed for both MB and NR after six cycles. Additionally, the desorption ratio exceeds 94.11% for

ORCID

Xiaowei Zhao: 0000-0002-3408-3209 Hongguang Lu: 0000-0002-2911-762X Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge the National Natural Science Foundation of China (nos. 51503149 and 21777113) and the Tianjin Natural Science Foundation (15JCQNJC43100). The authors also gratefully acknowledge the Training Project

Figure 4. Effect of cycle times on the adsorption capacity and desorption ratio of the hydrogel toward (A) MB and (B) NR. G



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of Innovation Team of Colleges and Universities in Tianjin (TD13-5020).

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