Preparation and application of Poly(AMPS-co-DVB) to remove Rhodamine B from aqueous solutions

Reactive and Functional Polymers 104 (2016) 53–61

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Preparation and application of Poly(AMPS-co-DVB) to remove Rhodamine B from aqueous solutions Luanluan Zhang a, Hejun Gao a,b,⁎, Yunwen Liao a,b,c,⁎ a b c

Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, China West Normal University, Nanchong 637000, China Institute of Applied Chemistry, China West Normal University, Nanchong 637000, China College of Environmental Sciences and Engineering, China West Normal University, Nanchong 637009, China

a r t i c l e

i n f o

Article history: Received 20 July 2015 Received in revised form 27 April 2016 Accepted 3 May 2016 Available online 7 May 2016 Keywords: Adsorption Application Removal Rhodamine B Polymer

a b s t r a c t A series of functional cross-linked polymer (Poly(AMPS-co-DVB)) were synthesized by 2-acrylamido-2methylpropane sulfonic acid (AMPS) and divinylbenzene (DVB). The physicochemical properties of the Poly(AMPS-co-DVB) were characterized by FT-IR, TGA, SEM, XRD, Zeta potential and UV–Vis. Those results showed that the Poly(AMPS-co-DVB) could provide mounts of adsorption sites from its special structure. The effects of the initial pH, dosage, contact time, and temperature on the adsorption of Rhodamine B (RhB) onto the Poly(AMPS-co-DVB) were investigated. It was found that the initial pH was an important factor for the molecules form of RhB and the surface formation of Poly(AMPS-co-DVB). With increasing of molar ratio of AMPS/DVB, the adsorption efficiency increased gradually. In the adsorption process, both physical and chemical mechanism is presence to adsorb RhB. The maximum adsorption capacity could get 407.9 mg/g within 2 h at room temperature. © 2016 Elsevier B.V. All rights reserved.

1. Introduction With the development of dye industry, a number of different dyes are discharged to the nature, which oversteps the self-purification capacity of environment. In the used dyes, about 10–15% of them is wasted and come directly into nature [1]. Water pollution with persistent chemicals, such as dyes, is a significant health hazard to environment and human even at extremely low concentrations. It is reported that dyes can cause different problems, such as skin and eye irritation, skin sensitization, which may increase the carcinogenicity [2–4]. Therefore, the removal and recovery of the used dyes from wastewater are attracting more and more attention than it was before [5,6]. The adsorption technology is one of the most popular methods to control dye pollutants. Numerous adsorbents have been surging in recent years, such as some porous materials functionalized with various organic or inorganic groups, tea factory waste [7], phenolated wood resin [8], kaolinite [9], zeolites [10], and modified jute [11] etc. They have been used to remove the heavy metal and organic pollutants from aqueous solution as adsorbents. There are still some shortcomings, such as low adsorption efficiency and secondary pollution in the adsorption process. So the most important work is to find a kind of new materials to overcome these shortcomings. Polymer has been widely applied in adsorption technology due to its excellent mechanical ⁎ Corresponding authors at: Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, China West Normal University, Nanchong 637000, China. E-mail addresses: [email protected] (H. Gao), [email protected] (Y. Liao).

http://dx.doi.org/10.1016/j.reactfunctpolym.2016.05.001 1381-5148/© 2016 Elsevier B.V. All rights reserved.

rigidity, large surface area, feasible regeneration and functional surface. Especially, polymer can effectively adsorb many nondegradable dyes [12–13]. Mahmoodi et al. [14] reported that the acid dyes can be effectively removed from aqueous solutions using poly(quaternary ammonium salt) and the maximum dye adsorption capacities were 2000 and 1667 mg/g for Acid Blue 25 and Acid Red 18, respectively. For different classes of dyes, a given polymer hardly reaches an excellent adsorptive capacity. In other words, the given polymer needs be modified to treat a type of dyes. The modification is expected to obtain some specific interaction between the dyes and polymer [15,16]. In this work, a novel cross-linked polymer (Poly(AMPS-co-DVB)) was prepared by free radical polymerization. Then, the structure and physicochemical properties of the Poly(AMPS-co-DVB) were characterized. Comprehensive studies of RhB removal from aqueous solutions using Poly(AMPS-co-DVB) were performed and the specific interaction between RhB and Poly(AMPS-co-DVB) were investigated.

2. Materials and methods 2.1. Materials 2-Acrylamido-2-methylpropane sulfonic acid (AR, 98%) and divinylbenzene (DVB) (AR, 80%) were purchased from Aladdin Chemical Co., Ltd. Methanol (AR, 99.7%) was provided by J&K Scientific Ltd. 2, 2′-Azobis(2-methylpropionitrile) (AR, 99%) was obtained from Chengdu Kelong Chemical Reagent Co. All chemicals are without

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L. Zhang et al. / Reactive and Functional Polymers 104 (2016) 53–61

Fig. 1. Synthesis of Poly(AMPS-co-DVB).

Fig. 2. SEM of Poly(AMPS-co-DVB).

treatment in advance. The pH values of dye solutions were adjusted with 37 wt% HCl or 6 M NaOH aqueous solutions. 2.2. Methods 2.2.1. Preparation of the Poly(AMPS-co-DVB) A certain proportion of AMPS, DVB, and 2, 2′-azobis(2methylpropionitrile) (2 wt.% of the amounts of AMPS)were added into 20 mL of methanol. And the mixture was stirred and refluxed at 70 °C under a nitrogen atmosphere for 6 h. After filtering, the solid was washed several times with water and ethanol. The product was dried under vacuum at 40 °C for 5 h. Then, the products (different mole ratios of AMPS/DVB) were obtained and the polymer yields were about 85%, 70% and 65% for mole ratio of 1:1, 2:1, and 4:1(AMPS/DVB), respectively. The preparation process of Poly(AMPSco-DVB) is shown in Fig. 1. The Poly(divinylbenzene) (PDVB) was prepared by the same method in the absence of AMPS. 2.2.2. Characterization of the Poly(AMPS-co-DVB) The morphology of the Poly(AMPS-co-DVB) was investigated by scanning electron microscope (SEM, JEOL JEM-6510LV). Powder X-ray diffraction(XRD) spectra were recorded on a Rigaku Dmax/Ultima IV diffractometer with monochromatized Cu Kα radiation (data interval of 0.02°), operating at a current of 30 mA with a voltage of 40 kV at a scanning velocity of 5°/min in the range of 5–80°. The functional groups of the Poly(AMPS-co-DVB) were characterized using Fourier transform infrared (FTIR, NICOLET 6700, Thermo scientific instrument) in the range of 4000 to 400 cm−1 with KBr powder in a resolution of 4 cm−1. The thermogravimetric Analysis (TGA) of the Poly(AMPS-coDVB) were investigated using standard equipment (TGA-1500

Rheometric Scientific Inc. Piscataway, NJ) at a heating rate of 5 °C/min under N2 atmosphere from room temperture to 600 °C. The BrunauerEmmett-Teller (BET) surface area, pore size distribution, and average pore volume were measured using an Autosorb-iQ-MP (Quantachrome, U.S.A.) under standard measurement conditions. The zeta potential of adsorbent was investigated uing a Brookhanven Zeta PALS (Brookhaven Crop., USA). All dye concentrations in aqueous solutions and were measured by UV–Vis spectroscopy (UV-2550, Shimadzu Japan) at λmax = 554 nm.

Fig. 3. XRD analysis of the Poly(AMPS-co-DVB).

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55

Fig. 6. Effect of initial pH (adsorbate conc. = 21 mg/L; adsorbent conc. = 200 mg/L). Fig. 4. FT-IR spectra of Poly(AMPS-co-DVB), Poly(AMPS-co-DVB)-RhB (after adsorption), and PDVB.

3. Results and discussions 2.2.3. Adsorption process The batch experiments of RhB adsorbed on Poly(AMPS-co-DVB) were conducted in order to evaluate the adsorption process. All the experiments were accomplished as follow: 20 mL of RhB aqueous solution (20 mg/L, optimum pH = 2.85) and 2 mg of the Poly(AMPS-co-DVB) were mixed into a 25 mL conical beaker about 2 h, which was shaken at 150 rpm by a oscillator (SHZ-82, Changzhou Shaipu Experimental Instrument Factory, China). After centrifugation (10,000 r/min, 5 min), the residual concentrations were analyzed using UV–vis spectroscopy. The specific adsorbed amounts of RhB on the Poly(AMPS-co-DVB) are calculated by the following Eq. (1) [17]. The adsorption percent for the dye removal efficiency is determined using the expression (2) [18]: qt ¼

ðC 0 −C t ÞV m

%R ¼

3.1. Characterization results of the Poly(AMPS-co-DVB) The macroscopic appearance of the Poly(AMPS-co-DVB) is powder. The morphology of the Poly(AMPS-co-DVB) is observed by SEM

ð1Þ

100ðC 0 −C t Þ C0

ð2Þ

where C0 and Ct (mg/L) are the initial concentrations of RhB in solution and concentrations of RhB in solution at any time t. qt is the adsorption capacity at time of t (mg/g). V is the volume of RhB aqueous solution (L) and m is the mass of the Poly(AMPS-co-DVB) (g). %R represents the removal percentage.

Fig. 5. TG curves of Poly(AMPS-co-DVB) and PDVB.

Fig. 7. Existence forms of Poly(AMPS-co-DVB) and RhB.

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L. Zhang et al. / Reactive and Functional Polymers 104 (2016) 53–61

the cationic RhB, other peaks are decreased. Those indicated that RhB was successfully adsorbed on Poly(AMPS-co-DVB). The thermal stability of Poly(AMPS-co-DVB) and PDVB are investigated by TGA. TGA plots of Poly(AMPS-co-DVB) and PDVB are showed in Fig. 5. PDVB shows apparent thermal decomposition at one stage, which is from ~ 400 to 500 oC. While the Poly(AMPS-co-DVB) has three weight losses, which take place at ~ 100–200 °C, ~ 200–300 °C, and ~ 400– 500 °C. The initial decompositon of the Poly(AMPS-co-DVB) is mainly due to the losses of water. The weight loss in the range of ~ 200– 300 °C could be due to the decomposition of AMPS and the weight loss in the range of ~ 400–500 °C is assigned to the decomposition of DVB. With the increase ratio of AMPS in the polymer, the weight loss increases gradually in the range of ~200–300 °C and the water content of the Poly(AMPS-co-DVB) also increases gradually, which may due to the hydrophilic groups of AMPS. 3.2. Effect of initial pH Fig. 8. Zeta potential of the Poly(AMPS-co-DVB) at different initial pH.

(Fig. 2), which is not uniform shape, and its structure is an amorphous nature (Fig. 3). The shape of polymer is in blocks as a result of crosslinking. The diameters of the Poly(AMPS-co-DVB) are in the range of 10–100 μm (Fig. 2a). Fig. 2b shows that the surface of the Poly(AMPS-co-DVB) is rough and irregular in shape, indicating that Poly(AMPS-co-DVB) could provide more adsorption sites for adsorption. Fig. 4 shows the FT-IR spectra of the PDVB, Poly(AMPS-co-DVB), and Poly(AMPS-co-DVB)-RhB (after adsorption). In the curve of the Poly(AMPS-co-DVB), the broad band at 3440 cm−1 is attributed to the N\\H stretching vibration. The bands at 2920 and 2850 cm−1 are due to the C\\H asymmetric and symmetric tensile vibration, respectively [19]. The band at 1630 cm− 1 is due to bending mode of C_O, N\\H groups and adsorbed water molecules [20]. The bands of 1450 and 1300 cm−1 are assigned to the bending vibrations of \\CH2\\ and \\CH\\. The bands at 1200 cm−1 and 1040 cm−1 are indicative of the presence of \\SO3H groups. The speak at 626 cm− 1 was due to the C\\H vibration of aromatic ring. Above results show that the Poly(AMPS-co-DVB) was successfully prepared by AMPS and DVB via free radical polymerization. In the spectrum of the Poly(AMPS-coDVB)-RhB, the peaks at 3440 and 1630 cm− 1 are also strong, which could be attributed to the functional groups N\\H, C_O in the AMPS. Due to the electrostatic interaction between the sulfonate ions with

One of the most important factors of the adsorption process is the initial pH value of dye solution, which can change the surface charge of the Poly(AMPS-co-DVB) and the degree of ionization of dye molecule [21]. Different dyes have diversified suitable pH depending on nature of the adsorbent being used [22]. The effect of pH on adsorption was studied in the pH range of 2.0–11.0. Fig. 6 shows the effect of pH on the adsorption capacities of RhB on the Poly(AMPS-co-DVB). It is obvious that the adsorption capacities of the Poly(AMPS-co-DVB) are highly dependent on the initial pH of solution. With the increase of pH from 2.0 to 3.0, the adsorption capacity increase. While the adsorption capacity decreased gradually at the pH from 3.0 to 11.0. The changes in adsorption behavior of RhB on the Poly(AMPS-co-DVB) are different from some other adsorbents [23–25], which have been reported previously. This result could be due to the surface charge of adsorbent and the structure of RhB molecule at different pH values (Fig. 7). On one hand, the surface of the adsorbent could be more protonated by H+ ions at pH = 2.0. With the increase of pH value, the extent of protonation is decreasing and the number of \\SO− 3 groups increase gradually. In other words, the availability of\\SO− 3 groups increases with increasing value of pH. Therefore, the adsorption capacity increased with increasing of pH form 2.0 to 3.0. On the other hand, the pKa value is about 3.0 [26]. At pH b 3, RhB molecules are monomeric form, indicating that RhB molecule can easily come into the cross-linked structure of Poly(AMPS-coDVB) [27]. The structure of RhB is electropositive and the surface negative charge density of the adsorbent is high (Fig. 8), which can activate

Fig. 9. Effect of adsorbent dosage. (adsorbate conc. = 21 mg/L; pH = 2.85; temp. = 25 °C; contact time = 2 h).

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that all the active sites on the Poly(AMPS-co-DVB) surface are gradually exploited. The chemisorption, such as ion-exchange, and physisorption are present in the second adsorption process [31]. During the final stage of the adsorption process, the adsorption equilibriums are approached. The adsorption capacities of RhB on Poly(AMPS-co-DVB) at equilibrium time are 119.57, 192.60 and 205.35 mg/g for mole ratio of 1:1, 2:1, and 4:1(AMPS/DVB), respectively. The result indicates that the adsorption capacity is increase with increasing the molar ratio of AMPS/DVB. In order to further investigate the adsorption kinetics of dye adsorption onto the Poly(AMPS-co-DVB), two well-known models of pseudofirst-order and pseudo-second-order were used, which could be expressed as Eqs. (3) and (4), respectively.

Fig. 10. Effect of contact time. (adsorbent conc. = 200 mg/L; adsorbate conc. = 21 mg/L; pH = 2.85; temp. = 25 ± 1 °C).

the electrostatic interaction between RhB molecule and \\SO− 3 groups of adsorbent. At pH N 3, RhB molecules are zwitterionic form in the solution, which could aggregate to bigger molecules. It decreases the density of positive charge of RhB and the RhB molecule is hard to enter to the cross-linked structure. Then, the electrostatic interaction disappeared gradually. Other reason can be owing to the release of H+ from carboxylic groups of RhB and increase of repulsion force between adsorbate and sorbent. Therefore, the adsorption capacity decreased with increasing of pH form 3.0 to 11.0. The optimum initial pH value is 2.85 at this work and the next experiments were accomplished at this pH. 3.3. Effect of adsorbent dosage The effect of adsorbent dosage on the RhB removal from aqueous solution is illustrated in Fig. 9. The percentage of RhB removal is a significant increase with the adsorbent dosage up to a limit value and then it keep a constant value (Fig. 9A). This may be due to the more available adsorption functional sites and the increasing adsorbent surface area [28]. However, the adsorption capacities decrease gradually with increase of dosage, indicating that the adsorption saturation capacities of RhB on the Poly(AMPS-co-DVB) could not be reached under the dye concentration of 21 mg/L (Fig. 9B). The more surface area and adsorption functional sites can be available in high dye wastewater concentrations [29]. The adsorbent dosage of 2 mg is chosen for the next studies. 3.4. Adsorption kinetics In the practical process of dye treatment, the adsorption rate is an important parameter. With the increase of contact time, the change of the adsorption capacity is shown in Fig. 10. The variations of adsorption capacity go through three distinct stages. The instantaneous adsorption is in the initial 10 min, which is a general characteristic of physisorption in the first stage. It could be attributed to the rough and irregular surface [30]. In the second stage, the curves show a gradually attainment of the equilibrium with increase of the contact time. This result could be due to

ln ðqe −qt Þ ¼ lnqe −k1 t

ð3Þ

t 1 t ¼ þ qt k2 qe 2 qe

ð4Þ

where qe (mg/g) is the adsorption capacity of RhB on adsorbent at equilibrium time. The parameter values of the kinetic models are listed in Table 1. The determination coefficient (R2) values are above 0.89 for pseudo-first-order kinetic model. However, the calculated qe do not agree with experimental values, implying that the pseudo-first-order model is not fitting for describing the adsorption process. While, all the R2 values are above 0.99 for pseudo-second-order kinetic model, indicating that the pseudo-second-order kinetic model could better describe the adsorption process of RhB on the Poly(AMPS-co-DVB) than that of pseudo-first-order kinetic model. Moreover, the experimental qe shows a good agreement with the calculated values of pseudosecond-order kinetic model. It shows that the rate-limiting step may be chemisorption involving valency forces through sharing or exchange of electrons between adsorbent and adsorbate [32]. As we all know, physical adsorption is a fast process, while chemical adsorption is a slow process. The rate constant of the pseudo-second-order kinetic model (K2) of 4:1 is the least in the three samples. This result could be caused by chemical adsorption, which increases with increasing the molar ratio of AMPS/DVB and decreases the values of K2 (Table 1). 3.5. Adsorption isotherms The equilibrium relationship between adsorbent and adsorbate can be well explained by adsorption isotherms, which usually adopt the Langmuir, Freundlich, and Tempkin models. Adsorption isothermal process is carried at 298, 313 and 328 K using optimum experimental conditions (contact time = 2 h, pH = 2.85) (Fig. 11). The Langmuir isotherm is often applicable to a homogeneous adsorption surface with all the adsorption sites having equal adsorbate affinity [33]. The Langmuir isotherm is expressed as follows: Ce 1 Ce ¼ þ qe qm K L qm

ð5Þ

Where qm represents the maximum theoretical adsorption capacity (mg/g). KL is the Langmuir constant related to adsorption mechanism of adsorption (L/mg). The Freundlich isotherm model assumes that different sites are involved with several adsorption energies, so it can be applied to nonideal

Table 1 Pseudo-first-order and pseudo-second-order kinetic parameters for adsorption rate of RhB onto Poly(AMPS-co-DVB). Adsorbents (nAMPS/nDVB)

Pseudo-first-order model K1 × 10

1:1 2:1 4:1

0.25 0.29 0.27

−1

(min

−1

)

Pseudo-second-order model 2

qe (mg/g)

R

K2 × 10

26.95 66.85 67.22

0.8951 0.9897 0.9574

6.72 1.88 1.73

−3

(mg/g min)

qreal (mg/g) 2

qe (mg/g)

R

119.05 192.31 208.33

0.9998 0.9999 0.9999

119.57 192.60 205.35

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L. Zhang et al. / Reactive and Functional Polymers 104 (2016) 53–61

adsorption on heterogeneous surfaces as well as multilayer adsorption [34,35]. The Freundlich model is described as follows:

lnqe ¼ ln K F þ

  1 ln C e n

ð6Þ

where KF (mg g−1(L/mg)1/n) is the Freundlich constant and 1/n is the heterogeneity factor. The Tempkin isotherm model usually describes the chemical adsorption of that adsorption heat linear decline along with the change of surface coverage and the heat of the adsorption and the adsorbent– adsorbate interaction [36]. The follow is the Tempkin model: qe ¼ B1 ð ln K T þ lnC e Þ

ð7Þ

where B1 is the Tempkin constant related to heat of the adsorption (J/mol). KT is the equilibrium binding constant (L/mg). Table 2 shows the results of fitting datas to the Langmuir, Freundlich, and Tempkin models. Comparison of the R2 values of the Langmuir, Freundlich, Tempkin models, the results show that the Langmuir is best fitting for the experimental data than that of the Freundlich, Tempkin models using the linear method. Those results indicate that the surface of the Poly(AMPS-co-DVB) is covered with monolayer of RhB in the experimental temperature. Ranking of these adsorbents in terms of adsorption capacity is the following order: AMPS : DVB ¼ 4 : 1 N AMPS : DVB ¼ 2 : 1 N AMPS : DVB ¼ 1 : 1 It is obvious that the adsorption capacity increases with the increase of the part AMPS in the Poly(AMPS-co-DVB). The reason could be due to that the introduction of the AMPS moieties into the cross-linked polymer brings a significant improvement in the adsorption performance. The qm values are 140.94, 266.87, and 407.90 mg/g for the 1:1, 2:1, and 4:1, respectively. It is clear that the adsorption performance of the Poly(AMPS-co-DVB) (4:1) is better than most of the other adsorbents reported earlier (Table 3). The reason could be attributed to the strong adsorption force of the Poly(AMPS-co-DVB), which comes from the unusual surface morphologies (4:1, Specific surface area = 1.233 m2/g, Pore Volume = 0.012 ml/g, Pore Diameter = 3.409 nm) and the functional groups. As a result it can be concluded that RhB adsorption not only is owing to electrostatic interaction but hydrophobic interaction, hydrogen bonding, π-π interaction as well as pore structure can also participate in the adsorption process [42]. Moreover, the essential feature of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor (RL), which is an important indicator of the adsorption process, given by the following equation [43]: RL ¼

1 : 1 þ K L C0

ð8Þ

The RL indicates that the adsorption process is irreversible (RL = 0), favorable (0 b RL b 1), linear (RL = 1), or unfavorable (RL N 1). Table 2 shows that the RL values are all below 1. Thereby, the RhB adsorption onto Poly(AMPS-co-DVB) is favorable process. 3.6. Thermodynamics The temperature is another important factor to influence the process of adsorption, which can be used to calculate the thermodynamic parameters. The standard free energy change (ΔG0), enthalpy (ΔH0), and entropy (ΔS0) for adsorption can be obtained from the following equations [44]: ΔG0 ¼ −RT lnK e Ke ¼

Fig. 11. Adsorption isotherm of RhB on Poly(AMPS-co-DVB) (adsorbent conc. = 200 mg/L; contact time = 2 h; pH = 2.85).

C ad Ce

ln K e ¼ −

ð9Þ ð10Þ

ΔH0 ΔS0 þ RT R

ð11Þ

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59

Table 2 Parameters of adsorption isotherm of RhB onto Poly(AMPS-co-DVB). (C0 = 10–80 mg/L, pH = 2.85, contact time = 2 h). AMPS:DVB = 1:1

Langmuir qm (mg/g) KL (L/mg) R2 RL

AMPS:DVB = 4:1

313 K

328 K

298 K

313 K

328 K

298 K

313 K

328 K

149.25 0.26 0.9989 0.0459

147.06 0.42 0.9991 0.0289

156.25 0.37 0.9979 0.3270

277.78 0.60 0.9980 0.0204

303.03 0.77 0.9990 0.0160

370.37 0.79 0.9991 0.0156

434.78 0.52 0.9982 0.0235

454.55 0.73 0.9973 0.0168

476.19 0.88 0.9981 0.0141

53.26 0.2519 0.7886

74.22 0.1667 0.8933

77.74 0.1690 0.9382

139.45 0.2033 0.8397

136.20 0.2299 0.8468

156.96 0.2554 0.8968

158.18 0.2918 0.8953

195.14 0.2472 0.8339

218.31 0.2334 0.8193

67.28 24.09 0.8540 140.94

61.39 17.56 0.9311 142.93

55.43 17.79 0.9634 151.63

52.90 34.91 0.9146 266.87

30.45 42.83 0.9153 293.62

24.74 54.15 0.9632 359.99

12.88 68.14 0.9562 407.90

38.89 60.78 0.9666 435.63

62.92 60.73 0.9109 457.83

Freundlich KF (mg/g)(L/mg)1/n 1/n R2 Tempkin KT (L/mg) B1 (J/mol) R2 qe (Experiment) (mg/g)

AMPS:DVB = 2:1

298 K

ΔG0 ¼ ΔH0 −TΔS0

ð12Þ

where R (8.314 J/mol K) is the universal gas constant and T (K) represents the absolute temperature. Cad (mg/L) is the concentration of solute adsorbed at equilibrium. The all results are shown in Table 4. The negative values of ΔG0 suggest that the adsorption process is feasible and spontaneous [45]. In general, the values of energy ΔG0 in between 0 and ~ 20 kJ/mol indicate that the adsorption process is physisorption, while the values between ~ 80 and ~ 400 kJ/mol correspond to chemisorptions [46,47]. The values of ΔG0 suggest that both physical and chemical mechanism is presence to adsorb RhB [48,49], which is caused by the unusual surface morphologies and electrostatic interactions. Those positive ΔH0 confirms that the adsorption process is an endothermic nature. The value of ΔH0 of 1:1(AMPS:DVB) is the biggest one in the three samples, which indicates that it need more heat to obtain the good adsorption capacity. In other words, the increase of the adsorption capacity is the least in those samples at the same temperature range (Table 2). Those positive ΔS0 indicate that the randomness increases gradually at the solid/solution interface [50]. This may be caused by the water molecules, which escaped from the solid/solution interface into the bulk solution.

the first step. In the next step, the electrostatic attraction could be caused by the protonated RhB and negatively adsorbents at pH b 3.0, which could be the rate-determining step [22]. The values of ΔG0 (− 1.70 to − 10.84) are in good agreement with the existing physisorption (electrostatic attraction) between the RhB and the adsorbents. At pH N 3.0, the zwitterionic form of RhB in the solution will aggregate to bigger molecules, which could decrease the density of positive charge on RhB and the adsorption capacity.

4. Conclusions The novel crosse-linked adsorbent of the Poly(AMPS-co-DVB) was successfully prepared by free radical polymerization. Our experiments show the preparation and application of the novel adsorbent to adsorb RhB, which can be used to theoretical calculations such as kinetics and thermodynamics. The good adsorption of the Poly(AMPS-co-DVB) could due to two aspects: the molecules form of RhB and the surface formation of Poly(AMPS-co-DVB) which result in the electrostatic interaction. The increasing molar ratio of AMPS/DVB that introduces chemical adsorption.

Acknowledgments 3.7. Adsorption mechanism The mechanism for the adsorption of RhB onto the Poly(AMPS-coDVB) is showed in Fig. 12. RhB was dissolved in aqueous solution with pH b pHpzc (pH 3.0) to protonate the functional groups \\N\\ [51] in

Table 3 Comparison adsorption of RhB onto Poly(AMPS-co-DVB) with other adsorbents reported. Adsorbents

Sodium montmorillonite Fe3O4/HA Surface modified tannery waste AIMCM-41 Poly(methacrylic acid)modified biomass Activated carbon Poly(AMPS-DVB)

Condition Dosage, pH, temperature

Reference

42.19 161.80 212.77 41.86 234.50

0.3 g/L, 7.0, 30 °C 0.5 g/L, 6.0, 20 °C 1.0 g/L, 3.5, 30 °C 0.025 g/L, 7.0, 25 °C 1.0 g/L, 6.5, 25 °C

[37] [38] [27] [39] [40]

263.85 457.80

1.0 g/L, 5.7, 20 °C 0.1 g/L, 2.9, 25 °C

[41] Present work

qm (mg/g)

We acknowledged the financial support of the Applied Basic Research Programs of Science and Technology Department of Sichuan Province, China (2015JY0042), the Key Fund Project of Education Department of Sichuan Province, China (15ZA0147), and the Fundamental Research Funds of China West Normal University, China (14E015).

Table 4 Thermodynamic parameters for adsorption of RhB on Poly(AMPS-co-DVB) (C0 = 10 mg/L). Adsorbent

T (K)

ΔG0 (kJ/mol)

AMPS:DVB = 1:1

298 313 328 298 313 328 298 313 328

−1.70 −3.38 −5.05 −6.69 −8.18 −9.68 −7.96 −9.40 −10.84

AMPS:DVB = 2:1

AMPS:DVB = 4:1

ΔH0 (kJ/mol)

ΔS0 (J/mol K)

31.55

111.59

23.04

99.75

20.69

96.13

60

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Fig. 12. Adsorption process of RhB from aqueous solutions by Poly(AMPS-co-DVB).

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