Branched Polyethyleneimine-Functionalized Polystyrene Resin

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

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Branched Polyethyleneimine-Functionalized Polystyrene Resin: Preparation and Adsorption of Cu2+ Qi Meng, Jiang Liu, Yan Jiang, and Qiaoqiao Teng* Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China J. Chem. Eng. Data Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/28/19. For personal use only.

S Supporting Information *

ABSTRACT: A polyamine-functionalized polystyrene adsorption resin (PSAPA) containing CN functional groups was prepared via condensation reaction of acetylated polystyrene resin (PSA) with branched polyethyleneimine (PEI). Multiple reaction factors for the PSAPA preparation were screened to afford an optimal total exchange capacity of 3.953 mmol/g. This PSAPA showed decent adsorptive activity toward Cu2+ in aqueous with an equilibrium adsorption capacity of 1.766 mmol/g. Kinetic studies indicated that the adsorption of Cu2+ by resin PSAPA conforms to the pseudosecond-order kinetics, and the adsorption rate was controlled by the intraparticle diffusion. Sorption isotherms were well fitted by the Freundlich model. Thermodynamic parameters suggested an exothermic process, which mainly relies on physical adsorption.

1. INTRODUCTION As industrialization progresses, heavy metal pollution in wastewater has aroused significant public concern because of their high toxicity and nonbiodegradability, and they will thus accumulate in the environment. A number of technologies and methods including chemical precipitation,1 ion-exchange,2 electrocoagulation,3 membrane separation,4 and adsorption5 have been developed to tackle this problem. However, most of them suffered from drawbacks of secondary pollution, low efficiency, and high operational costs.6 Among the heavy metal ions, Cu2+ is one of the most common contaminants, which has caused significant issues to the ecosystem.7 To date, adsorption is proved to be one of the most effective methods to remove Cu2+ from aqueous solutions attributing to its low cost, environmental friendliness, and satisfactory efficiency in general. Current research endeavors are mainly focused on the development of adsorbents with high stability, large adsorption capacity, and great selectivity for the sake of Cu recovery and reuse. It is well known that nitrogen-containing ligands are highly favorable ligands for Cu2+ coordination8,9 based on hard and soft (Lewis) acids and bases theory.10 Because of the so-called chelate effect, polyamines could form even more stable complexes. Therefore, polyamine-functionalized chelating resin has attracted broad attention in the past few decades, and indeed, adsorbents with polyamine functional groups have evidently shown advantageous effects in selectively removing Cu2+. Bai et al. have functionalized poly(glycidyl methacrylate) (PGMA) with ethylenendiamine, diethylenetriamine (DETA), triethylenetetramine, and tetraethylenepentamine (TEPA) and found that DETA-functionalized PGMA gave the best Cu2+ adsorption capacity of 1.18 mmol/g.11,12 Purolite S984 resin is © XXXX American Chemical Society

a commercially available chelating resin characterized with polyamine functional groups. Liu and Bai found this resin to be highly efficient in selectively adsorbing Cu2+ (equilibrium adsorption capacity: 0.6 mmol/g) to give ultrapure nickel solutions.13 Denizli et al. have immobilized poly(methyl methacrylate) (PMMA) with PEI, and the Cu2+ adsorption capacity is significantly increased from 3.6 μmol/g (Cu2+ adsorption capacity of PMMA) to 0.224 mmol/g.14 Gao and co-workers have successfully grafted PEI onto the surface of silica gel, and the experimental results showed that the adsorption capacity of Cu2+ could reach 0.64 mmol/g.15 Han’s group has modified the wheat straw with PEI and used it to adsorb Cu2+ in aqueous solution, and the saturated adsorption capacity was up to 0.77 mmol/g.16 Our group has also synthesized various polyamine-functionalized polystyrene resins to obtain highly efficient Cu2+ adsorbents. We have installed TEPA and 1,4,7,10-tetraazacyclododecane to the resin beads via substitution of chloroacetylated and chloromethylated polystyrene, respectively. Their adsorption capacity toward Cu2+ was enhanced to 46.15 mg/mL and 1.50 mmol/g, respectively.17,18 The aforementioned polyamine groups were mostly grafted to the resin via C−N single bonds realized by nucleophilic substitution19 or addition reactions.11,12 As a continuous contribution to this field, we will report an unprecedented polyamine-type adsorption resin PSAPA, which was prepared via condensation of acetylated polystyrene resin with branched polyethyleneimine (PEI). Cu2+ adsorption capability of this Received: January 28, 2019 Accepted: April 12, 2019

A

DOI: 10.1021/acs.jced.9b00090 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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polyamine-grafted polystyrene functionalized with additional CN will be evaluated and compared with those of the existing counterparts.

Table 1. Detailed Information on the Experimental Materials Used in This Work

2. EXPERIMENTAL SECTION 2.1. Material and Instruments. The polystyrene (PS) resin was used after pretreatment. PEI, 1,2,4-trimethylbenzene, toluene-4-sulfonic acid, CuSO4·5H2O, and all other reagents were used as received. The detailed information was presented in Table 1. Deionized water was used to prepare all solutions throughout the experiments. Instruments used in this study are as follows: 752XP UV− visible spectrophotometer (Shanghai Lengguang Technology Co., Ltd, China), IS50 Fourier transform infrared spectrometer (Thermo Fisher Scientific Company, America), Elementar Vario EL cube (Elementar Co., Germany), ASAP2460 Automatic specific surface area and pore size analyzer (Quantachrome Company, China), DTA6300 Thermal gravimetric analyzer (PerkinElmer Instruments Co., Ltd. China), PHS-3C pH meter (Shanghai INESA Scientific Instrument Co., Ltd. China). 2.2. Synthesis of PSAPA Resin. The acetylated PS resin was prepared according to the literature:20 in a three-neck flask, the PS resins (10 g) were swollen in 100 mL of CH2Cl2 at 298 K for 12 h before AlCl3 (20 g) and acetyl chloride (10 g) were added. The reaction mixture was then stirred for 8 h at room temperature. The mixture was filtered, and the resulting resins were rinsed successively with CH2Cl2, tetrahydrofuran, 3% hydrochloric acid, and deionized water to give PSA resins, which were dried at 333 K in vacuum, and the acetyl content was determined as 3.161 mmol/g. In a three-neck flask, the prepared PSA resins (0.5 g) were swollen in 8 mL of 1,2,4-trimethylbenzene at 298 K for 12 h before PEI (0.5 g) and TsOH·H2O (0.5 g) were added. The reaction mixture was then stirred for 24 h at 433 K. The mixture was filtered, and the resulting resins were rinsed successively with absolute ethanol, 1 M hydrochloric acid, 1 M sodium hydroxide, and deionized water to give PSAPA resins, which were dried at 333 K in vacuum. 2.3. Characterization of PSAPA Resin. The functional groups in PS, PSA, and PSAPA were identified by a Fourier transform infrared spectrometer using the conventional KBr pellet technique within the wavenumber range of 4000−500 cm−1. The nitrogen content was estimated by CHN elemental analysis. The specific surface area and pore volume of the three resins were determined by nitrogen adsorption/desorption isotherms at 77 K. The specific surface area, the pore volume, and average pore diameter were measured and calculated by the Brunauer−Emmett−Teller (BET) method and the Barrett−Joyner−Halenda model. The thermal stability of resins was investigated by thermogravimetric analysis (TGA) in the nitrogen atmosphere at a heating rate of 15 °C/min. Total exchange capability (E, mmol/g) was measured with the acid−base neutralization titration method: PSAPA (a, ∼0.1 g) was dispersed in 50 mL of HCl solution (C1, ∼0.04 mol/L) before oscillating for 12 h at 298 K in water bath. The supernatant liquid (10 mL) was transferred into a 100 mL conical flask with a pipette. It was titrated with NaOH solution (C2, ∼0.04 mol/L, pretitrated by potassium acid phthalate) using phenolphthalein as an indicator. The volume of NaOH solution was recorded as V (mL). The E (mmol/g) value of the PSAPA resin was calculated according to the following eq 121

E=

50 × C1 − 5 × C2 × V a

(1)

2.4. Batch Adsorption. Batch adsorption studies were performed by dispersing 0.05 g of PSAPA resins in 50 mL of Cu2+ solution at various concentrations (0.1−0.5 mg/mL with 0.1 mg/mL intervals), temperature, and pH (adjusted by B



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Table 2. Optimization of Synthetic Conditions of PSAPA Resina

entry

solvent

temperature (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

dioxane DMF DMSO toluene mixed xylene o-xylene p-xylene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene 1,2,4-trimethylbenzene

373 373 373 373 373 373 373 373 393 413 433 443 433 433 433 433 433 433 433 433 433 433 433 433 433 433 433 433

catalyst

catalyst loading

time (h)

Eb (mmol/g)

0.2 0.2 0.2 0.05 0.1 0.3 0.5 0.8 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 1 3 6 10 15 20 24 30

0.570 1.009 1.171 1.415 1.745 1.781 1.807 1.898 2.358 2.651 2.805 2.571 1.683 2.985 3.116 2.853 2.929 3.465 3.753 3.681 2.853 2.929 3.465 3.753 3.821 3.940 3.953 3.957

H2SO4 HOAc TsOH·H2O TsOH·H2O TsOH·H2O TsOH·H2O TsOH·H2O TsOH·H2O TsOH·H2O TsOH·H2O TsOH·H2O TsOH·H2O TsOH·H2O TsOH·H2O TsOH·H2O TsOH·H2O

a

Reaction Conditions: PSA (0.5 g), PEI (0.5 g), solvent (8 mL). bTotal exchange capacity.

adding diluted HCl and NaOH). The remaining Cu2+ in aqueous solution was analyzed via a 752XP UV−visible spectrophotometer. The absorbance of the Cu−EDTA (ethylenediaminetetraacetate) complex was measured at 735 nm.22 The amount of Cu2+ adsorbed by PSAPA resins was calculated by the following eq 2 qe =

(C0 − Ce) ·V m·M

(2)

where qe is the adsorption capacity (mmol/g) at equilibrium, C0 and Ce are the initial and equilibrium concentrations (mg/ mL) of Cu2+, respectively, V is the solution volume (mL), m is the weight of PSAPA resin (g), and M is the molar mass (g/ mol) of Cu2+. Kinetic studies were conducted by dispersing 0.20 g PSAPA in 200 mL of Cu2+ aqueous solutions (Cu2+ concentration: 0.5 mg/mL) at different temperatures (298, 303, 313, and 323 K). Samples were analyzed at times. All the experiments were triplicated, and the error is less than 3%.

Figure 1. FTIR spectra of PS, PSA, and PSAPA.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of PSAPA. 3.1.1. Optimization of Synthetic Conditions. The effects of solvent, catalyst, temperature, and time on the PSAPA formation were screened evaluated by the total exchange capacity (E) of the resulting PSAPA resins (Table 2). For C



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solvents were studied here to disclose their effects on the total exchange capacity of the resulting adsorbent (entry 1−8). It was found that aromatic hydrocarbons afford the highest total exchange capacities, which might be due to the structural similarity with that of the PSA skeleton. Among the aromatic hydrocarbon reactions, the resultant total exchange capacities rank in a decreasing order of 1,2,4-trimethylbenzene > p-xylene > o-xylene > mixed xylenes > toluene. The total exchange capacity of PSAPA varies significantly when synthesized at different temperatures. It reached a peak value of 2.805 mmol/ g at 433 K (entry 11), at which the mixture was refluxing. It is well known that such condensation reactions could be promoted by acid catalysis via acetyl protonation and thus the enhancement of electrophilicity.23 However, it should also be borne in mind that the amino groups in PEI are competing in the protonation process as well. The latter seems to be more favorable in this case as they are both in the liquid phase. After careful selection, we found that TsOH·H2O gives the best performance. Again, its aryl ring might lead to easier approach and diffusion into PSA, de promotes the acetyl protonation and condensation. With 0.5 equiv TsOH·H2O added, the PSAPA obtained after 12 h has an E value of 3.753 mmol/g (entry 19), which could be further enhanced by prolonging the reaction time. Overall, the optimum conditions are as follows: 1,2,4trimethylbenzene as the reaction medium, TsOH·H2O as the catalyst, reaction temperature of 433 K for 24 h, and a molar ratio of n(PSA)/n(PEI)/n(TsOH·H2O) being 1:2.5:0.5 (Table 2, entry 27). The total exchange capacity of the resulting PSAPA synthesized under optimal conditions is 3.953 mmol/g. 3.1.2. Characterization of the Resins. 3.1.2.1. Fourier Transform Infrared Spectroscopy and Elemental Analysis. IR spectroscopy was employed to track the variation of functional groups (Figure 1). After acetylation, a sharp and intense peak at 1685 cm−1 (a) was observed for PSA, which is clearly due to the stretching of the CO group.24 After condensation, the CO characteristic peak obviously weakened and migration to 1676 cm−1 (c) was observed, which probably stems from hydrogen bond association. More importantly, a characteristic peak appeared at 1637 cm−1 (d), corroborating the CN formation.25 The CHN contents of PSAPA were determined as 73.91, 8.37, and 7.65%, respectively. The appearance of the significant amount of nitrogen indicates a successful graft of PEI onto the PSA beads. The grafting ratio of PEI on PSA is about 43% (see calculation details in the Supporting Information). Notably, the nitrogen amount calculated from the total exchange capacity is 5.54%, indicating that 72% (see calculation details in the Supporting Information) of the nitrogen is accessible for the adsorption of H+ and metal species. 3.1.2.2. Brunauer−Emmett−Teller. The specific surface areas and pore volumes of the PSA and PSAPA resins are significantly reduced in comparison with those of PS (the N2 adsorption/desorption curves and the pore size distribution graph are shown in Figure S1). In contrast, the average pore diameters increased. We thus believed that the acetylation and the subsequent condensation mainly occurred inside the PS resin pores (Table 3). As the PEI has a high average molecular weight and thus high volumes, it may induce pore blockings thus leading to the specific surface areas and pore volume decrease, while the average pore diameters increase.26

Table 3. Pore Structure Parameters of Resins resin

BET surface area (m2/g)

pore volume (cm3/g)

average pore diameter (nm)

PS PSA PSAPA

644.3 321.4 122.6

1.872 1.367 0.8796

5.831 8.542 12.83

Figure 2. TGA analysis curves of PS (a), PSA (b), and PSAPA (c).

Figure 3. pH influence on the adsorption capacity (pH = 1.32−4.72, C0 = 0.5 mg/mL, T = 298 K, t = 24 h, m(PSAPA) = 0.05 g, V = 50 mL).

Figure 4. Equilibrium adsorption isotherms of Cu2+ on PSAPA from aqueous solutions (C0 = 0.1−0.5 mg/mL, T = 303−323 K, t = 24 h, m(PSAPA) = 0.05 g, pH = 4.72, V = 50 mL).

heterogeneous reactions, the reaction medium often plays a significant role on the adsorption capability. Therefore, eight D



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Figure 5. Langmuir (left) and Freundlich (right) isothermal fitting curves.

Table 4. Langmuir and Freundlich Isothermal Parameters Langmuir isotherm model −1

Freundlich isotherm model 2

temperature (K)

qm (mmol·g )

KL

R

303 313 323

2.0783 2.0540 2.8208

9.3795 7.7821 2.2379

0.9864 0.9864 0.9701

n

KF

R2

2.4768 2.2832 1.3687

2.4716 2.4101 2.7218

0.9989 0.9974 0.9836

N bonds. The grafting ratio of PEI on PSA is calculated to be ∼33% (see calculation details in the Supporting Information), which is close to that calculated from EA analysis. Above 380 °C, both the cross-linked network and the polystyrene skeleton broke in all three resins leading to a commonly sharp weight decline.27 3.2. Adsorption Properties of PSAPA Resin toward Cu2+. 3.2.1. pH Effect on Adsorption. The aqueous pH is one of the determinant factors for effective adsorption. It is observed that for PSAPA, the adsorption capacity at equilibrium maximized at pH 4.72 (Figure 3), which is the original value measured for a 0.5 mg/mL Cu2+ aqueous solution. At more acidic environment, competitive H + adsorption is more favored, and the resulting ammonium group will thus be electrostatically repulsive to the Cu2+ ions, leading to a low Cu2+ uptake. When NaOH was added to increase the pH, Cu(OH)2 precipitation occurred, which affected the accurate evaluation. 3.2.2. Adsorption Isotherms. Temperature is another essential factor influencing adsorptions. From Figure 4, it could be concluded that Cu2+ adsorption with PSAPA is favored at lower temperatures. This suggests an exothermic

Figure 6. Adsorption kinetic curves of Cu2+ (t = 0−1500 min, T = 298−323 K, C0 = 0.5 mg/mL, m(PSAPA) = 0.20 g, pH = 4.72, V = 200 mL).

3.1.2.3. Thermogravimetric Analysis. The thermogravimetric curves suggest that the PSAPA resin is thermally stable till 300 °C, while PS did not show any significant weight loss up to 380 °C (Figure 2). The large weight loss for PSAPA between 300 and 380 °C is thus ascribed to the decomposition of C

Figure 7. Pseudo-first-(left) and pseudo-second-order (right) rate equation fitting curves. E



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Table 5. Pseudo-First- and Pseudo-Second-Order Kinetics Parameters pseudo-first-order rate equation

pseudo-second-order rate equation

temperature (K)

k1 (min−1)

qe (mmol/g)

R2

k2 (g·mmol−1·min−1)

qe (mmol/g)

R2

298 303 313 323

0.006440 0.007740 0.01047 0.01719

1.9620 1.5705 1.1914 0.6412

0.9212 0.9707 0.9920 0.8271

0.004777 0.009149 0.01996 0.07002

1.9562 1.7647 1.6254 1.3649

0.9953 0.9979 0.9991 0.9996

Table 6. Liquid Film and Intraparticle Diffusion Parameters liquid film diffusion equation

intraparticle diffusion equation

temperature (K)

kf/min−1

C

R2

kp/(mmol·g−1·h−0.5)

C

R2

298

0.006440

0.1050

0.9212

303

0.007740

−0.05514

0.9707

313

0.01047

−0.2788

0.9920

323

0.01719

−0.7445

0.8271

0.08549 0.05697 0.09323 0.04272 0.09840 0.01784 0.1193 0.005590

−0.01725 0.3968 0.03247 0.7030 0.1358 1.2025 0.07050 1.2501

0.9886 0.9884 0.9947 0.9777 0.9744 0.9525 0.9086 0.9498

Figure 8. Liquid film (left) and intraparticle (right) diffusion fitting curves.

Table 7. Adsorption Thermodynamic Data of PSAPA toward Cu2+ ΔG (kJ/mol) C0 (mg/mL)

ΔH (kJ/mol)

0.1 0.2 0.3 0.4 0.5

−8.4199 −6.6341 −5.4029 −3.1795 −2.5641

303 K

−6.2394

313 K

−5.9415

nature of the adsorption. Moreover, the metal uptake increases as its initial concentration increases. The adsorption mechanism could often be described by two isotherm models. One is the Langmuir model, which is indicative of a monolayer adsorption process by an entirely homogeneous adsorption surface. The other is the Freundlich isotherm model, and it is an empirical isotherm equation which represents a multilayer, heterogenous adsorption process. The Langmuir (3) and Freundlich (4) isotherm equation28 can be expressed as Ce C 1 = e + qe qm KLqm

ΔS (J/(mol·K)) 323 K

303 K

313 K

323 K

−3.6755

−7.1964 −1.3025 2.7606 10.0986 12.1297

−7.9182 −2.2126 1.7207 8.8243 10.7904

−14.6885 −9.1596 −5.3480 1.5356 3.4409

log qe = log KF +

1 log Ce n

(4)

where Ce is the equilibrium concentration (mg/mL), qe is the equilibrium adsorption capacity (mmol/g), qm is the maximum adsorption capacity (mmol/g), KL and KF are the Langmuir and Freundlich constants, respectively, and n is the Freundlich exponent related to adsorption intensity. The adsorption of Cu2+ by PSAPA resin is better described by the Freundlich equation as indicated by the better R2 values at all temperatures (Figure 5 and Table 4). This suggests that the Cu2+ adsorption by PSAPA is heterogenous adsorption and follows a multilayer adsorption mechanism. The adsorption is favorable at all the studied temperatures with n values greater than 1.29

(3) F



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Table 8. Comparison of Adsorption toward Cu2+ Cu2+ adsorption capacity

author

adsorbents

Pavasant et al. Liu et al.

activated carbon prepared from eucalyptus bark Polyamine-modified Purolite S984 resin PEI-immobilized silica gel PEI-modified wheat straw DETA-functionalized PGMA TEPA-functionalized chloracetylated polystyrene resin 1,4,7,10-tetraazacyclododecanemodified chloromethylated polystyrene microsphere PSA

Gao et al. Han et al. Bai et al.

our group present study

PSAPA

3.2.4. Adsorption Thermodynamics. The thermodynamic parameters were calculated from the adsorption isotherms of Cu2+ by PSAPA at different initial concentrations. When the adsorption fits the Freundlich equation, the changes in the Gibbs free energy change (ΔG) could be directly related to the Freundlich constant n by eq 9.33 The enthalpy change (ΔH) was determined by measuring the slope of the ln Ce/T−1 graph and the entropy change (ΔS) was calculated by eq 11. The values are listed in Table 7.

refs

0.45 mmol/g

37

0.6 mmol/g

13

0.64 mmol/g 0.77 mmol/g 1.18 mmol/g 46.15 mg/mL

15 16 17

1.50 mmol/g

18

11,12

ln Ce = ln K 0 +

0 mmol/g

ln(qe − qt ) = ln qe − k1t

(5)

t 1 t = + 2 qt qe k 2·qe

(6)

(7) (8) −1

(11) −1

where R is the gas constant (8.314 J·mol ·K ), and T is the absolute temperature (K). The adsorption is confirmed to be exothermic as indicated by the negative ΔH values. This explains the aforementioned temperature dependence during adsorption. ΔG is negative, confirming a spontaneous adsorption of Cu2+ by PSAPA. Both ΔH and ΔG are low during adsorption, indicating a dominant physisorption, while those of chemisorption is higher than 10 kJ/mol for ΔH34 and from −80 to −400 kJ/mol for ΔG.35 The PEI was introduced to form a stable chelate with Cu2+, which was designed to be a chemical process. The high specific surface area nature of the resin clearly favors the physical adsorption to a much greater extent, making the overall process appear as physical adsorption. The entropy change showed interesting trends. At low concentrations, the Cu2+ ions were adsorbed by PSAPA resulting in a more ordered arrangement and thus negative ΔS.33 At higher concentrations, more mobile molecules such as H2O will be swapped out leading to an overall entropy increase.36 The ΔS decreases at higher temperatures, which is in line with the variation trends for adsorption capacity. This could also be explained by the decrease of H2O molecules in the system. 3.2.5. Comparison of Cu2+ Removal with the Literature. Table 8 includes the Cu2+ adsorption capacity of PSAPA and related materials. The data show the outperformance of PSAPA, indicating the beneficial effect of grafting PEI to the polystyrene adsorbent via CN bonds. Currently, we are investigating the adsorption potentials of this adsorbent on other heavy metals and the possibility of selective removal. Moreover, dynamic adsorption and desorption applications in industrial copper-containing wastewater treatment are underway.

where qt (mmol/g) is the adsorption capacity at time t (min), and k1 (min−1) and k2 (g·mmol−1·min−1) are the adsorption rate constants. These results in Figure 7 and Table 5 indicate that the adsorption of Cu2+ follows a pseudo-second-order kinetic. Moreover, the calculated equilibrium adsorption capacity qe is in good agreement with the experimental data. The liquid film diffusion model (7)31 and intraparticle diffusion model (8)30 are used to investigate the diffusion mechanism of metal ions into the adsorbent, and their diffusion equations are given as

−1

(10)

−1

3.2.3. Kinetic Experiments. The adsorption rate was monitored at different temperatures (Figure 6). The results demonstrated a rapid adsorption in the beginning at all temperatures. Although the overall adsorption capacity is the best at room temperature, elevating the temperature has clear beneficial effects on the adsorption rate. At 323 K, the adsorption could be almost completed within 2 h. The kinetic data were further analyzed by a pseudo-firstorder rate eq 5 and pseudo-second-order rate eq 6 expressed as30

qt = k pt 0.5 + C

ΔH RT

ΔS = (ΔH − ΔG)/T

1.766 mmol/g

ij q yz lnjjjj1 − t zzzz = −k f t + C j qe z k {

(9)

ΔG = −nRT

4. CONCLUSIONS In conclusion, we have prepared an unprecedented polyamine functionalized polystyrene resin via condensation reactions of acetylated PS with branched PEI. Compared with polyaminegrafted PS via C−N single bonds, the introduction of CN bonds increases the total exchange capacity in general. More importantly, the PSAPA has a stronger adsorption capability toward Cu2+ than most of the existing adsorbents of the similar type. Kinetic and thermodynamic studies indicate that (1) the Cu2+ adsorption conforms to the pseudo-second-order rate equation, and the adsorption rate is controlled by intraparticle diffusion; (2) the sorption isotherms fit the Freundlich model,

−0.5

where kf (min ) and kp (mmol·g ·h ) are the liquid and film diffusion rate coefficients, and C is a constant related to the thickness of the boundary layer. R2 values suggest a better fit with the intraparticle diffusion way (Table 6 and Figure 8). Looking in depth, the fitting curves comprise two linear portions. The first linear portion was better assigned as the external surface adsorption (liquid film diffusion), and it is fast. The second linear portion was ascribed to the intraparticle diffusion. Overall, these results revealed that both film diffusion and intraparticle diffusion were involved in Cu2+ adsorption, but the latter is the ratelimiting step.32 G



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disclosing a multilayer, heterogenous adsorption; (3) it is physical adsorption and a lower temperature is favored.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.9b00090. Additional calculation details and figures, calculation of grafting ratio of PEI on PSA, calculation of the percent of nitrogen for absorbing H+, and N2 adsorption/ desorption curves and pore size distribution of resins (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Qiaoqiao Teng: 0000-0002-7343-8092 Funding

The authors thank the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (18KJB150002) and the Start-up Foundation of Changzhou University (ZMF 17020129) for financial supports. Notes

The authors declare no competing financial interest.

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REFERENCES

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Branched Polyethyleneimine-Functionalized Polystyrene Resin

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