Uptake of Fe(III), Ag(I), Ni(II) and Cu(II) by salicylic acid-type chelating resin prepared via surface-initiated atom transfer radical polymerization

Abstract

With chloromethylated polystyrene resin being the base material, a new type of salicylic acid-type chelating resin with high capacity was prepared by grafting glycidyl methacrylate (GMA) on the surface of the resin via surface-initiated atom transfer radical polymerization and followed by the derivatization with 5-aminosalicylic acid. Fourier-transform infrared spectrometry and X-ray photoelectron spectroscopy were used to characterize the surface composition of the resin. It was found that the grafting degree of GMA increased linearly with the extension of ATRP time and GMA concentration. The maximum adsorption capacities of the resin were up to 4.23 mmol g⁻¹ for Fe(III), 4.02 mmol g⁻¹ for Ag(I), 2.36 mmol g⁻¹ for Ni(II) and 1.75 mmol g⁻¹ for Cu(II) at pH 4.0, respectively. The adsorption kinetics of the four metal ions conform to the model of pseudo second-order kinetics, and adsorption equilibria were better characterized by the Langmuir equation. Thermodynamic parameters were calculated to understand the nature of the adsorption process. The resin possesses excellent desorption rate and reusability. All these results indicate that the resin could be potentially applied to the efficient removal of heavy metal ions from waste water.

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1 Introduction

Adsorption is the common method for heavy metal wastewater treatment. The used chelating resin is the one that has the function of chelating ability for metal ions, which has been broadly used in aspects of analysis and detection, concentration and enrichment, segregation and desorption for the metal ions.^1–5 The surface function of the adsorbent not only impacts on the adsorption behavior, but also decides the adsorption mechanism. The adsorption selectivity and the adsorption capacity are two important characteristics to evaluate the performance of the chelating resin. In general, the increase of functional groups on the surface of the resin will improve the adsorption capacity. At present, small molecular ligands are often used for functionalizing the surface of materials, however, the low density of introduced functional group results in unsatisfactory adsorption capacity due to the limited number of reaction sites on the surface of materials.^6,7 To increase the density of surface functional group, polymers can be introduced into the materials via some physical methods; however, because no chemical bond has been shaped among them, the polymers will be washed away from the surface of materials easily during the adsorption process. The drawback of physical methods can be overcome by the formation of covalent bond between the polymer chain and the surface of materials via polymerization reaction, so the polymerization reaction has attracted much attention in the surface modification.^8,9

Atom transfer radical polymerization (ATRP) is a controlled radical polymerization technique, enabling the synthesis of well-defined, complex polymers.¹⁰ As shown on organic as well as inorganic substrates, ATRP is a useful tool in surface modifications as ATRP initiating functionalities are easily introduced on surfaces.^11–13 Our group had used SI-ATRP to make the tetrazole chelating resin¹⁴ and iminodiacetic acid resin,¹⁵ which proves that SI-ATRP is one of the new methods to produce the chelating resin with high capacity.

Glycidyl methacrylate (GMA) is a common functional monomer, in which epoxy is of reactive group, and it has been extensively applied in the preparation of functional materials.^16–22 5-Aminosalicylic acid, an organic compound with chelating function, contains one hydroxide radical, one carboxyl and one amine group, which can share double pairs or three pairs of electrons with metal ions, it will be easily introduced to the polymer chain via the reaction between the amine group and epoxy group.^23–25

In this work, considering that crosslinked polystyrene resin is a versatile platform to prepare various adsorbents, we attempted to take advantage of SI-ATRP to create a novel salicylic acid-type chelating resin with high capacity that is different from the reported adsorption resins in the structure. The adsorption property on Fe(III), Ag(I), Ni(II) and Cu(II) were investigated to indicate the application of the adsorbent in the treatment of heavy metal wastewater.

2 Experiment section

2.1 Materials and instruments

Chloromethylation polystyrene (CMPS (18% Cl) Sunresin New Materials Co. Ltd. Xi'an, China); glycidyl methacrylate (GMA) (Aladdin, Shanghai, China); 2,2-dipyridyl (Sinopharm Chemical Reagent Co., Ltd., China); cuprous bromide (Tianjin No. 1 Chemical Reagent Factory, China); 5-aminosalicylic acid (5-ASA) (Aladdin, Shanghai, China). All other chemicals were of analytical grade.

Fourier transform spectrometer (TENSOR 27, Bruker, Germany) was used to determine the structure of the chelating resin. X-ray photoelectron spectroscopy (PHI-5400X, Perkin Elmer, USA) was used to achieve the qualitative analysis of the surface elements. The concentrations of metal ions were measured by an atomic absorption spectrophotometry (AAS, novAA400, Analytik Jena AG, Germany).

2.2 Grafting of GMA onto CMPS beads

GMA was grafted onto the surface of the initiator-functionalized resins via SI-ATRP. Briefly, 2,2′-bipyridyl (0.5 g, 0.72 mmol), copper(I) bromide (0.05 g, 0.30 mmol) and CMPS beads (5.0 g) were placed in a reaction vessel, and the mixture was de-oxygenated by repeated vacuumizing. High-purity nitrogen was introduced into the tube after each evacuation stage. Then, GMA (10 mL) and 30 mL THF, de-oxygenated by three cycles of freezing–vacuum–thaw, was added into the tube using a syringe under a nitrogen atmosphere. The reaction was stirred at 40 °C under a nitrogen atmosphere. The beads were filtrated out from the reaction solution after the desired reaction time, and then immersed in 60 mL of a methanol/EDTANa₂ (1 : 1, v/v) solution for 24 h to remove copper. Finally, the resulting glycidyl methacrylate modified PS (PGMA-g-PS) was washed by water and methanol and dried at 35 °C under vacuum for the next reaction.

Polymerization time and monomer concentration were used as an independent variable to control the grafting degree (DG) of GMA on the CMPS resin. DG was measured by the increase in the mass percentage of PGMA-g-PS resins as described by eqn (1).

where W_a and W_b are the weight of CMPS before and after grafting, respectively.

2.3 Preparation of salicylic acid-type chelating resin (ASA/PGMA/PS)

5 g of the modified PGMA-g-PS beads, 6.0 g of 5-ASA, and 100 mL of 0.01 mol L⁻¹ NaOH were added to a three-necked flask under stirring for 30 min. pH of the mixture was adjusted to 12 and stirred at 80 °C for 12 h. The resulting ASA/PGMA/PS beads were filtered and washed thoroughly with 0.01 mol L⁻¹ HCl, 0.01 mol L⁻¹ NH₃·H₂O and deionized water in sequence, and finally dried at 35 °C in a vacuum oven.

2.4 Characterization of the chemical composition of resin

Chemical composition information about the surface of CMPS, PGMA-g-PS and ASA/PGMA/PS resins was analyzed using a Tensor 27 FT-IR spectrophotometer (Bruker Company, Germany) and X-ray photoelectron spectroscopy (XPS, PE, PHI-5400, USA).

2.5 Batch of adsorption experiment

The adsorption properties of the ASA/PGMA/PS resin for Fe(III), Ag(I), Ni(II), and Cu(II) ions were tested under non-competitive conditions by immersing the resin into a solution containing one type of metal ion.

2.5.1 Effect of pH. 0.100 g of ASA/PGMA/PS resin was added into a series of 100 mL of 6 mmol L⁻¹ metal ion solutions with different pH values, and the mixtures were shaken at 200 rpm for 12 h at 25 °C. The resins were removed by filtration, and the filtrates were collected to measure the final concentration of metal ion by atomic adsorption spectrometry. The amount of metal ion adsorbed was calculated according to eqn (2)where Q_e is the equilibrium adsorption capacity (mmol g⁻¹), C₀ and C_e are the initial and equilibrium metal ion concentrations (mmol L⁻¹), respectively; V is the solution volume (L), and W is the mass of the dried resins (g).

2.5.2 Adsorption isotherm. Adsorption isotherms were conducted at three different temperatures (25 °C, 35 °C, and 45 °C). 0.100 g of ASA/PGMA/PS resin was placed in a series of flasks containing different initial concentrations of each metal ion. The initial pH of the solution was adjusted to 4.0. Each flask was agitated at 200 rpm at a given temperature for 12 h. The filtrate was collected for the determination of the metal ion concentrations. The adsorption isotherm of each metal ion was obtained by plotting Q_e against C_e.

2.5.3 Adsorption kinetics. 0.100 g of ASA/PGMA/PS resin was placed in a series of flasks containing 100 mL of each metal ion solution (6.0 mmol L⁻¹). The initial pH of the solution was adjusted to 4.0. Each flask was agitated at 200 rpm for a given time at 25 °C. The filtrate was collected for the determination of the metal ion concentrations. The kinetic curve was obtained by plotting the adsorption capacities of the metal ions vs. the adsorption time.

2.6 Multicomponent adsorption

0.100 g of ASA/PGMA/PS resin was placed in a series of flasks containing 100 mL of the mixture solution including four metal ions (the concentration of each metal ion was 6.0 mmol L⁻¹). The initial pH of the solution was adjusted to 4.0. The method of adsorption measure is the same as the method mentioned above.

2.7 Regeneration

The Fe(III), Ag(I), Ni(II), and Cu(II)-loaded resins were exposed to 0.1 mol L⁻¹ HNO₃ solution to be regenerated for 24 h and then were collected from the solutions by filtration, washed several times with distilled water and methanol, and then dried under vacuum at 35 °C. The regenerated resin was reused in the next cycle of adsorption experiments. The adsorption–desorption experiments were conducted at room temperature for ten cycles.

2.8 The stability experiments

The stability of ASA/PGMA/PS resin was tested by a dissolution experiment. Two portions of 2.000 g of the resins were immersed into 1000 mL of 0.2 mol L⁻¹ HCl and 0.2 mol L⁻¹ NaOH solutions, separately, for one month. Then, the mixture was filtered. The filtrates were extracted using 50.0 mL of ethyl acetate three times, and the combined extract was concentrated to 5.0 mL and analyzed by gas chromatography-mass spectrometry (GC/MS) (GC7980-MS5975, Agilent Technologies, Santa Clara, CA). Samples were injected in splitless mode onto a DB-5 MS column (30 m × 250 mm × 0.25 mm, Agilent Technologies, Santa Clara, CA). Data were processed using Agilent Technologies MSD Chemstation ver. E.02. The treated ASA/PGMA/PS beads were washed with distilled water and dried for mass and elemental analysis.

3 Results and discussion

3.1 Synthesis of salicylic acid-type chelating resin (ASA/PGMA/PS)

A schematic diagram of the synthetic route for the preparation of the salicylic acid-type chelating resins is outlined in Scheme 1. In the first step, PGMA was grafted onto the surface of CMPS beads by SI-ATRP. In the second step, the 5-aminosalicylic acid (5-ASA) is introduced into the polymer chain by the ring opening reaction between epoxy group and amine of 5-ASA.

Scheme 1 Synthetic route for the preparation of ASA–PGMA–PS resins.

Fig. 1 and 2 show the effect of ATRP time and GMA dosage on DG and adsorption capacity of PGMA-g-PS, respectively. As seen from Fig. 1, DG was linearly increased with the grafting time, indicating that the thickness of PGMA increases linearly on the surface of resin. With the process of reaction, the concentration of GMA is reduced remarkably, and the initial high speed of SI-ATRP can be controlled because the double coupling and double disproportionation termination occupy the dominant. As seen in Fig. 2, DG increases linearly with the increasing of GMA dosage. All these indicate the living/controlled character of SI-ATRP.

Fig. 1 Effects of ATRP time on the grafting density and adsorption capacity.

Fig. 2 Effects of GMA amounts on the grafting density and adsorption capacity.

It was also found the Fig. 1 that the adsorption capacity increases linearly with increasing ATRP time within 6 h. But after 6 h, the increase in the adsorption capacity trends to slow down. With the extension of polymerization time, longer PGMA polymer brush is covered on the surface and inside of resin pore. When the polymer jams the pore, the reaction of ATRP will stop, so the ATRP time was maintained at 6 h in the following experiments. The same phenomenon was also found for the effect of GMA usage on the adsorption capacity of metal ions, as shown in Fig. 2. The adsorption capacity increased linearly with increasing GMA dosage within 10 mL, which is in proportional to the number of GMA grafted on the surface of PS resin. After 10 mL, the increase of adsorption capacity slowed down, which is possibly related to the pore blockage by the excess grafted polymer. According to the adsorption capacity obtained in Fig. 2, the GMA dosage was maintained at 10 mL.

As seen in Fig. 3, the adsorption capacity increased quickly within initial 6 h, and the equilibrium was achieved at around 10 h. Since the introduced carboxyl and hydroxide groups of 5-ASA were responsible for metal ions adsorption, the increasing adsorption capacity indicated that more 5-ASA molecules reacted with the epoxy groups in the grafted PGMA. The ring-opening reaction was fixed at 10 h to prepare the ASA/PGMA/PS resins.

Fig. 3 Effects of ring-opening time on the grafting density and adsorption capacity.

3.2 Characterization of resin

The morphologies of the resin before and after ATRP modification and ring-opening reaction were observed using scanning electron microscopic (SEM). As shown in Fig. 4a and b, the surface of the PS resin was relatively smooth. After the modification at ATRP time of 8 h, some netlike polymers were clearly observed on the surface of PGMA-grafted resin (Fig. 4c), indicating that the PGMA brushes were successfully grafted on the resin. Due to no steric hindrance, long PGMA brushes might be formed on the surface, whereas short PGMA chains may occur within the resin. Fig. 4d illustrates the morphology of the ASA/PGMA/PS resin after the ring-opening reaction, which is completely different from the resins after ATRP (Fig. 4c). The surface of ASA/PGMA/PS resin became more hydrophilic after carboxyl and hydroxide groups were introduced on the surface, possibly resulting in the change of surface morphology.

Fig. 4 SEM micrograph of (a) chloromethylated PS, (b) chloromethylated PS, (c) PS-g-PGMA, and (d) ASA/PGMA/PS.

The infrared spectrograms of PS, PGMA-g-PS and ASA/PGMA/PS were used to identify the reactions (Fig. 5). In Fig. 5a, the absorption peaks at 759 cm⁻¹ and 698 cm⁻¹ can be assigned to C–Cl stretching vibration of –CH₂Cl. After the resin surface modification, the typical peaks in the spectra in Fig. 5b and c changed significantly compared with those in Fig. 5a. In Fig. 5b, the absorption peak at 1723 cm⁻¹ is the C=O, and the peaks at 907 cm⁻¹ and 841 cm⁻¹ show the characteristic absorption peaks of epoxy, indicating that PGMA has been grafted on the surface of PS resin. In Fig. 5c, the characteristic adsorption peaks of the epoxy bond at 907 cm⁻¹ and 841 cm⁻¹ almost disappear, and three new peaks at 1678 cm⁻¹, 1385 cm⁻¹, 1408 cm⁻¹ appears corresponding to the characteristic adsorption peak of C=O in carboxy, phenolic hydroxyl and C–N bond respectively, which certify 5-ASA has been reacted with epoxy group, and accordingly the chelating resin ASA/PGMA/PS is produced.

Fig. 5 FTIR spectrum of (a) chloromethylated PS, (b) PS-g-PGMA, (c) ASA/PGMA/PS and (d) ASA/PGMA/PS–Fe(III).

In order to further prove that ASA/PGMA/PS has adsorption function for Fe(III), the infrared spectrum of ASA/PGMA/PS after adsorption was determined (Fig. 5d). The characteristic absorption peak of C=O in carboxy was shifted from 1678 cm⁻¹ to 1519 cm⁻¹, this is because the carboxyl group participates in coordination, electronic cloud of O will be transferred to the Fe(III), the bond energy of C=O reduce, so the characteristic absorption peak is shifted to the low wave number, at the same time, the characteristic absorption peak at 1385 cm⁻¹ of phenolic hydroxyl groups disappeared. It was proved that the adsorption for Fe(III) was mainly depended on the chelating action of carboxyl and hydroxyl groups of salicylic acid molecule.

XPS is applied to analyze the resin composition and change of functional group before and after modification, as shown in Fig. 6. The peak at 531.2 eV is due to O1s (Fig. 6c), suggesting that the GMA is introduced into surface of the resin via SI-ATRP; after ring-opening reaction, the appearance of the peak of N1s at 395.7 eV shows that the 5-ASA has been successfully introduced into the PS-g-PGMA resin (Fig. 6c). Overall, all the results indicated the GMA has been successfully grafted to the PS resin via SI-ATRP, and the salicylic acid is introduced into the molecule chain by means of the ring-opening reaction between amine group and epoxy group.

Fig. 6 XPS wide scan of (a) chloromethylated PS, (b) PS-g-PGMA, (c) ASA/PGMA/PS and (d) ASA/PGMA/PS–Fe(III).

XPS analysis was performed to investigate the molecular level information of iron adsorption on the ASA/PGMA/PS resin. Fig. 6d shows the XPS spectra of the Fe(III) loaded ASA/PGMA/PS. Before iron adsorption, the O1s binding energy located at 532.6 eV is composed of a signal, which belongs to the oxygen in C=O and OH in the resin. After iron adsorption, the O1s band was shifted to 533.3 eV, due to the formation of bonds between the oxygen and the iron, indicating that a complex between iron and oxygen occurred and the adsorption of iron in the resin.

3.3 Adsorption isotherm

Fig. 7 shows the adsorption isotherms of Fe(III), Ag(I), Ni(II), and Cu(II) at 25 °C. The Langmuir and Freundlich adsorption isotherm models were used to explain the adsorption experimental data. The Langmuir model holds that the adsorption of metal ions is the monolayer adsorption on the homogeneous surface, while Freundlich model considers that the adsorption of metal ions is on the heterogeneous surface. The adsorption experimental data were analyzed with the Langmuir model (3) and Freundlich model (4), respectively, and the results are shown in Table 1.where Q_e is the adsorption capacity, mmol g⁻¹; C_e is the equilibrium concentration of metal ions, mmol L⁻¹; Q₀ is the saturated adsorption capacity, mmol g⁻¹; K_F is an empirical parameter; n is the Freundlich constant; and K_F is the binding energy constant reflecting the affinity of the adsorbents to metal ions; b is the Langmuir adsorption constant, and it is related to the adsorption equilibrium constant between the solid and liquid (aqueous solution), as expressed by the following equation:

b = (K_C − 1) × M/ρ (5)

where M and ρ are the molar mass (g mol⁻¹) and density (g L⁻¹) of the solvent, respectively.

Fig. 7 Adsorption isotherms of ASA/PGMA/PS resin for Fe(III), Ag(I), Ni(II), Cu(II) at 25 °C (pH 4.0; contact time: 12 h; adsorbent dose: 0.1 g).

Table 1 Langmuir and Freundlich constants for the adsorption of Fe(III), Ag(I), Ni(II), Cu(II) on ASA/PGMA/PS resins at 25 °C

Metal ions	Langmuir constants				Freundlich constants
	Q0 (mmol g^−1)	b	KC	RL^2	KF		RF^2
Fe(III)	4.904	1.89	105.72	0.98514	0.959	0.7373	0.97423
Ag(I)	4.26	1.70	95.44	0.99251	2.266	0.77	0.96542
Ni(II)	2.886	1.13	63.78	0.99491	1.320	0.73	0.9593
Cu(III)	2.119	1.00	57.11	0.9808	0.973	0.756	0.96869

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As seen in Table 1, the value of R_L² verifies Langmuir model fits the experimental data better than Freundlich, therefore, the adsorption process is of Langmuir adsorption. According to the Langmuir equation, the maximum adsorption capacity (Q_m) was calculated to be 4.23 mmol g⁻¹ for Fe(III), 4.02 mmol g⁻¹ for Ag(I), 2.36 mmol g⁻¹ for Ni(II) and 1.75 mmol g⁻¹ for Cu(II). The adsorption capacity of the ASA/PGMA/PS resin was compared with the reported resins (Table 2). Obviously, the adsorption capacity of the ASA/PGMA/PS resin is higher than those of the reported resins. Higher adsorption capacities can produce higher productivity for water treatment. Therefore, the resin should be of value in water treatment on the basis of sustainability principles.

Table 2 Comparison of heavy metal ions adsorption capacity of various adsorbents

Adsorbents	Adsorption capacities (mmol g^−1)				Reference
	Fe(III)	Ag(I)	Ni(II)	Cu(II)
Thiourea-modified magnetic chitosan microspheres	—	—	0.26	1.04	26
Expanded perlite	—	0.08	—	0.03	27
Of water bamboo husk biosorbents	0.084	—	0.143	—	28
Poly(acrylamide) (PACM)	—	—	0.12	0.11	29
Chelating terpolymer resin	0.18	—	0.29	0.28	30
Polystyrene-coated magnetic particles functionalized with 2-(3-(2-aminoethylthio)propylthio)ethanamine	—	0.44	—	—	31
Silica gel encapsulated by 5-sulfosalicyclic acid functionalized polystyrene (SG-PS-azo-SSA)	—	0.822	—	0.472	32
ASA/PGMA/PS resin	4.23	4.02	2.36	1.75	This work

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3.4 Effect of pH

The effect of solution pH on the adsorption of metal ions is shown in Fig. 8. The adsorption of ASA/PGMA/PS resin to metal ions displays the strong pH dependency. When the pH is 1.0, there are no adsorption basically; the adsorption capacity will dramatically increase with the increase of pH value. When the solution pH is 4.0, the adsorption capacity for Fe(III), Ag(I), Ni(II), and Cu(II) tends to be stable and arrives to the maximum. Under the acidic condition, amino group is protonated by H⁺, the chelating interaction between ASA and metal ions weakens, as a result, the adsorption capacity on Fe(III), Ag(I), Ni(II), and Cu(II) is reduced. With the increase of pH value, the competitive effect of H⁺ is weakened, meanwhile, the ionization degree of the functional group (carboxyl and hydroxyl group) on the adsorption material increases accordingly, so the materials not only has the coordination chelating interaction with metal cation, but also has the electrostatic interaction. Clearly, with the increase of pH value, the cooperativity between the electrostatic interaction and the coordination chelating interaction is increased, thus the adsorption capacity increases with the increase of pH value.

Fig. 8 Effect of pH on the adsorption of ASA/PGMA/PS resin for Fe(III), Ag(I), Ni(II), Cu(II) (initial concentration: 6 mmol L⁻¹; 25 °C; adsorbent dose: 0.1 g).

On the other hand, because the resin basically doesn't have adsorption for Fe(III), Ag(I), Ni(II), and Cu(II) under the acidic condition, we may use 0.1 mol L⁻¹ HNO₃ to desorb the saturated resin after adsorption and regenerate the adsorbent resin.

3.5 Adsorption thermodynamics

Fig. 9 shows the adsorption isotherms of Fe(III), Ag(I), Ni(II), and Cu(II) at 25 °C, 35 °C and 45 °C. From Fig. 9, the adsorption capacity of resin is increased for the same metal ion, but the increased increment is not so obvious, confirming that the adsorption was an endothermic process, but the impact of temperature on the adsorption process is not significant. Generally, the physical absorption is an exothermic process, clearly, the adsorption ASA/PGMA/PS resin for Fe(III), Ag(I), Ni(II), and Cu(II) is deduced to be the chemical adsorption process.

Fig. 9 Adsorption isotherms of ASA/PGMA/PS resin for (a) Fe(III), (b) Ag(I), (c) Ni(II) and (d) Cu(II) at different temperatures (pH 4.0; contact time: 12 h; adsorbent dose: 0.1 g).

The thermodynamic parameters provide in-depth information on inherent energetic changes that are associated with adsorption, including free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS), which can be estimated respectively using the following equations:

ΔG = −RT ln K_C (6)

ΔG = ΔH − TΔS (7)

ln K_C = −ΔH/RT + ΔS/R (8)

where R is the universal gas constant (8.314 J mol⁻¹ K⁻¹), T the absolute temperature (K), and K_C the adsorption equilibrium constant, which was obtained from the Langmuir isotherm at different temperatures.

Using a linear plot between ln K_C versus 1/T, the adsorption enthalpy and entropy for the adsorption process of the ions could be obtained according to the slope and intercept of the plot, as shown in Table 3. In the process of adsorption, the negative values of ΔG indicated the spontaneous nature of the adsorption process, and the positive values of ΔH and ΔS demonstrated the endothermic nature of this process and the spontaneous process of entropy drive. Before adsorption, each ion exist in the form of hydration ion, after the ion is coordinated and chelated by the resin, the result of competitive role makes the water molecule replaced, and released via the form of free molecule.

Table 3 Thermodynamic parameters estimated for adsorption of four metal ions on ASA/PGMA/PS resins

Metal ion	ΔH (kJ mol^−1)	ΔS (J mol^−1 K^−1)	ΔG (kJ mol^−1)
			25 °C	35 °C	45 °C
Fe(III)	36.771	159.5	−10.76	−12.12	−14.42
Ag(I)	27.54	130.3	−11.29	−12.59	−13.90
Ni(II)	22.33	109.5	−10.30	−11.39	−12.50
Cu(III)	20.79	103.4	−10.02	−11.06	−12.075

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3.6 Adsorption kinetics

Fig. 10 shows the adsorption kinetics on ASA/PGMA/PS resin for Fe(III), Ag(I), Ni(II), and Cu(II).

Fig. 10 Adsorption kinetics of ASA/PGMA/PS resin for Fe(III), Ag(I), Ni(II), Cu(II) (initial concentration: 6 mmol L⁻¹; pH 4.0; 25 °C; adsorbent dose: 0.1 g).

The adsorption of Fe(III) is the fastest among the four metal ions. The adsorption time at equilibrium was 4 h for Fe(III), 4.5 h for Ag(I), 5 h for Ni(II) and 7 h for Cu(II). Although the initial concentration of the four metal ions were all 7 mmol L⁻¹, the adsorption capacity was different at the equilibrium time. The adsorption process of porous adsorbent generally includes the adsorption of external diffusion, internal diffusion and activity, and the internal diffusion plays an important role. The pseudo second-order kinetic equation was employed to analyze the desorption kinetics of three ions. The pseudo second-order equation is as follows:

where t is the adsorption time (min); k the adsorption rate constant (min⁻¹); and Q_t and Q_e are the adsorption amount at given time t and equilibrium time, respectively.

The data in Fig. 10 could be converted into plots of t/Q_t vs. t, and good linearity was found for the three metal ions with correlation coefficients (R²) greater than 0.998, indicating that the adsorption kinetics of the metal ions on ASA/PGMA/PS resins followed the pseudo second-order kinetic model. The adsorption rate constants obtained from the slopes and intercept of the plots were 0.217, 0.196, 0.162 and 0.102 g mmol⁻¹ h⁻¹ for Fe(III), Ag(I), Ni(II) and Cu(II), respectively. The kinetic data would be very useful for the fabrication and design of wastewater treatment and noble metal recovery systems.

3.7 Multicomponent adsorption

The competitive adsorption among the four metal ions was investigated, with the results shown in Table 4.

Table 4 Metal uptake on ASA/PGMA/PS of competitive and non-competitive adsorption

Metal ion	Q (mmol g^−1)
	Non-competitive adsorption	Competitive adsorption
Fe(III)	4.23	3.89
Ag(I)	4.02	2.78
Ni(II)	2.36	1.03
Cu(II)	1.75	0.38

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As seen in Table 4, compared with the adsorption of single metal ion, the adsorption capacity of four metal ions is reduced accordingly under the condition of competition, the adsorption capacity reduces by 8.04%, 30.85%, 56.36% and 78.29% for Fe(III), Ag(I), Ni(II), and Cu(II) respectively. Besides, the competitive adsorption experiment indicates the adsorption of Fe(III) almost has not impacted by other co-existing ions. For four metal ions, the ASA/PGMA/PS resins had a preferential adsorption of Fe(III) > Ag(I) > Ni(II) > Cu(II). The different adsorption behaviors might be relevant to the nature of metal ionic and the interaction between adsorbent and adsorbate.

3.8 Reusability and stability

To demonstrate the reusability of the adsorbent, an adsorption–desorption cycle was repeated ten times using the preparation methods. The results are shown in Fig. 11. After ten adsorption–desorption cycles, the adsorption capacities of the resins did not significantly change during the repeated adsorption–desorption operations. These results show that ASA/PGMA/PS resins could be repeatedly used in heavy-metal adsorption studies without any detectable change in adsorption capacity.

Fig. 11 Adsorption capacity of the resin after ten adsorption–desorption cycles.

As is well known, crosslinked polystyrene resins are usually stable and widely used in water treatment. To demonstrate the stability of ASA/PGMA/PS resins, the ASA/PGMA/PS resins were immersed into strong acid and base solutions for one month, respectively. No weight loss was found after treatment. The filtration solution was analyzed by GC-MS, and the ingredients of the resins, divinylbenzene, vinylbenzene and other aromatic compounds were not checked out in compound libraries, indicating no leakage of the harmful compounds from the materials. These results indicated that ASA/PGMA/PS resins do not dissolve in aqueous solution, and the salicylic acid group in resins could keep the stability in solution.

3.9 Adsorption mechanism

There are bidentate coordinating groups such as carboxyl and hydroxide group in salicylic acid molecule, it was proved that the salicylic acid molecule on the surface of ASA/PGMA/PS resin played the strong coordination chelation interaction for metal ions by FTIR spectrum and XPS. When the density of functional groups is lower, the distance between the functional groups on the same microspheres or adjacent microspheres are larger, the adsorption of heavy metal ions is mainly depended on the chelating action of carboxyl and hydroxyl groups on the metal ions (Fig. 12), so the salicylic acid molecule on the surface of ASA/PGMA/PS resin plays the strong coordination chelation interaction for metal ions, and forms the stable six-membered chelate ring, the adsorption capacity is lower because of the limited chelating sites. When the density of functional groups is higher, the distance between the functional groups is smaller, the close salicylic acid groups at the same microspheres or salicylic acid groups at adjacent microspheres may take part in the chelation on the metal ions to form multi-ligand chelate. So adsorption of metal ions on ASA/PGMA/PS resins is depended on the chelating effect of functional groups inside of molecular chain (Scheme 2a) and also in the interchain (Scheme 2b). As a result of using ATRP technology, the number and density of functional groups on the surface of the resin were increased, the chelating sites and the stability of the chelate were improved, so the adsorption capacity was significantly increased.

Fig. 12 Adsorption mechanism of single molecule salicylic acid for metal ions.

Scheme 2 Adsorption mechanism of ASA/PGMA/PS resins for metal ions.

4 Conclusion

In this work, a novel salicylic acid-type chelating resin (ASA/PGMA/PS) with high capacity was prepared by SI-ATRP of polyglycidyl methacrylate (PGMA) and subsequent ring-opening reaction. The graft amount of PGMA on the chloromethylated PS resin was found to increase linearly with ATRP time and dosage of GMA, indicating the living/controlled character of SI-ATRP. FTIR and XPS confirmed the successful grafting of PGMA and the introduction of ASA onto the surface of the resin.

The ASA/PGMA/PS resin was used to remove Fe(III), Ag(I), Ni(II), and Cu(II) ions from aqueous solution. The adsorption capacity increased with increasing ATRP time and GMA dosage. Compared with the results in the literature, the adsorption capacities with Langmuir fitting were higher and could be up to 4.23 mmol g⁻¹ for Fe(III), 4.02 mmol g⁻¹ for Ag(I), 2.36 mmol g⁻¹ Ni(II) for and 1.75 mmol g⁻¹ for Cu(II) at pH 4.0 on the ASA/PGMA/PS resin. The adsorption process conforms to the model of pseudo second-order kinetic, which can be described with Langmuir adsorption model, belonging to the monolayer adsorption; the adsorption process is the concurrent results of the ion exchange and chelation. Competitive adsorption among the four metal ions displayed a preferential adsorption of Fe(III) > Ag(I) > Ni(II) > Cu(II). The regeneration experiment showed that ASA/PGMA/PS could be repeatedly used in heavy-metal adsorption studies without any detectable change in adsorption capacity. Consequently, the ASA/PGMA/PS resin has potential application prospect in the field of wastewater treatment of heavy metal ions.

Acknowledgements

This work was supported by the Natural Science Foundation of Shaanxi Province, China (no. 2013JM2016), Special Research Projects of Education Department of Shaanxi provincial Government (no. 15JK1782), and the Key Project of Specialized Research Foundation, Xianyang Normal College (no. 15XSYK037).

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