Design and controllable synthesis of ethylenediamine-grafted ion imprinted magnetic polymers for highly selective adsorption to perchlorate

Abstract

A series of ethylenediamine-grafted ion imprinted magnetic polymers (Fe₃O₄@IIPs) were synthesized via ultrasonic assisted suspension polymerization with perchlorate (ClO₄⁻) as an ion imprinting template. They were characterized by XRD, EA, VSM, FTIR and XPS and applied as adsorbents for ClO₄⁻ removal from aqueous solutions. The effects of the usage amount of crosslinking agent divinylbenzene (DVB) used for preparation on the structure and the adsorptive performance of Fe₃O₄@IIPs were investigated. The results show that the Fe₃O₄@IIPs have an average size of 200–800 nm, which increases with the increase of the amount of DVB from 0 to 2 g during the preparation process. The saturation magnetization intensities are at 35.6–42.8 emu g⁻¹, which decrease with the increase of the usage amount of DVB. The addition of DVB is beneficial to the formation and stability of the ion imprinted cavity of Fe₃O₄@IIPs. The effects of the solution pH value, initial concentration of ClO₄⁻, and adsorption time on the adsorption properties of ClO₄⁻ in aqueous solutions were investigated. The results show that the adsorption capability is affected significantly by solution pH value and reaches the maximum adsorption capacity at pH 3.0. The best adsorption capacity and selectivity of Fe₃O₄@IIPs to ClO₄⁻ can be obtained when the usage amount of DVB is at 0.5 g for synthesis. The adsorption mechanisms might include both ion exchange and electrostatic interaction. The isothermal adsorption curves mainly obey the Langmuir model with the theoretical maximum adsorption capacities (q_m,c) at 76.92–111.1 mg g⁻¹ and the experimental maximum adsorption capacities (q_m,e) at 75.7–108.9 mg g⁻¹, respectively, which are much higher than those of the non-ion imprinted material (Fe₃O₄@NIP, q_m,NIP: q_m,c at 60.61 mg g⁻¹ and q_m,e at 59.0 mg g⁻¹). The adsorption kinetic studies show that the adsorption processes reach equilibrium within 10 min and the kinetic data are well fitted to the pseudo-second-order model. There is almost no interference by the coexisting anions for the selective adsorption of ClO₄⁻, with a imprinting factor (α) at 1.8, and selectivity factor (β) larger than 5.9 for several kinds of common co-existing anions, respectively. The Fe₃O₄@IIPs are ideal candidates for removal of ClO₄⁻ from aqueous solution.

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1. Introduction

Perchlorate (ClO₄⁻) is a newly emerging, persistent environmental pollutant with high water solubility, mobility and stability, which poses a potential health threat to humans.¹ Because of its strong oxidative property, ClO₄⁻ is often used as a safe oxidant, and has been widely used in military manufacturing and industrial production, such as in solid rocket fuels, missiles, explosives, pyrotechnics, fireworks, etc.^2,3 ClO₄⁻ has also been widely used in clinical treatment of hyperthyroidism caused by immune system defects and clinical examination of thyroxine secretion, etc.⁴ Since the charge and ionic radius of ClO₄⁻ are very close those of the iodine ion (I⁻), ClO₄⁻ has been found to inhibit the uptake of iodine and disturb the synthesis of the thyroid hormone, which would affect the development of the central nervous system in newborns and induce thyroid cancer.⁵

Because of its high water solubility and stability, the degradation of ClO₄⁻ in the natural environment needs several decades or even longer.⁵ Conventional treatment processes, such as coagulation, precipitation, filtration and disinfection, cannot remove ClO₄⁻ efficiently.^6,7 Current technologies for ClO₄⁻ removal from drinking water and groundwater include adsorption,^8,9 ion exchange,^10,11 chemical reduction^12–14 bioremediation,^14,15 etc.

An ion imprinted polymer (IIP)^16–18 is able to recognize ions whilst keeping the unique virtues of molecular imprinting polymers (MIP), i.e., structure predictability, recognition specificity and application universality. Owing to special coordination or electrostatic interactions, IIPs are generally compatible with aqueous media and have advantages over most MIPs.¹⁶ IIPs can achieve effective identification of water-soluble ions. However, centrifugation or long-time subside is still needed for the separation of the IIPs from the post-adsorption solution, which is time-consuming and inefficient. Recently, functional group grafted nano-magnetic particles (FNMPs) have attracted greats attention because of their unique magnetic and structural properties.^19–24 It would be desirable if the high selectivity, good adsorptive efficiency and fast magnetic separation were associated by grafting the IIPs on the surface of the FNMPs via such a three-in-one strategy. The obtained material would be expected to not only retain the high selective recognition of IIPs to the target ion, but also to achieve the goal of high adsorption capacity, as well as fast removal of the contaminant.

Herein, in this work, we report the design and successful synthesis of a serial of core–shell microspheres ion imprinted magnetic polymers (Fe₃O₄@IIPs), by using ClO₄⁻ as the template. Their applications as adsorbents for ClO₄⁻ removal from aqueous solutions were studied. By varying the usage amount of DVB during the preparation procedure, the structure and adsorption properties on ClO₄⁻ were tuned. Based on the investigation of the effects of solution pH values and common co-existing anions on the adsorption properties, adsorption thermodynamic, kinetic parameters, as well as comparing with that of the non-ion imprinted magnetic polymer composite (Fe₃O₄@NIP), the adsorption mechanism was intensively studied. The results show that the Fe₃O₄@IIPs are ideal candidates for adsorption and removal ClO₄⁻ from aqueous solution.

2. Experimental

2.1. Materials

Ferric chloride (FeCl₃·6H₂O), sodium acetate (NaAc), ethylene glycol (EG), potassium perchlorate (KClO₄), were analytical grade, and purchased from Sinopharm Chemical Reagent Co., Ltd. ethylenediamine (EDA), styrene (St), divinylbenzene (DVB) and glycidylmethacrylate (GMA) were supplied by Aladdin Chemical Reagent Co., Ltd. (Shanghai, China) and purified by vacuum distillation. Benzoyl peroxide (BPO) was purchased from J&K Chemical (99%) and used as an initiator without further purification. Distilled water was used to prepare all the solutions. 0.1 mol L⁻¹ HCl and 0.1 mol L⁻¹ NaOH solutions were used for pH adjustment.

2.2. Synthesis of nFe₃O₄@IIPs

The overall preparation of ClO₄⁻-imprinted magnetic polymers (nFe₃O₄@IIPs) is depicted in Scheme 1. 4 kinds of nFe₃O₄@IIPs, as listed in Table 1, were prepared by the following steps via suspension polymerization and surface imprinting technique. The preparation procedure of nFe₃O₄@IIP-D0.5-G8 (no. 2) was taken as an example for discussion.

Scheme 1 Schematic procedure of preparation of nFe₃O₄@IIP.

Table 1 Content of Fe₃O₄, N, saturation moments and average diameter of nFe₃O₄@IIP

No.	Adsorbents	Content of Fe3O4 (%)	Content of N (mmol g^−1)	Saturation moments (emu g^−1)	Average diameter (nm)
1	nFe3O4@IIP-D0-G8	41.5	7.82	42.8	200
2	nFe3O4@IIP-D0.5-G8	39.5	7.45	40.5	400
3	nFe3O4@IIP-D1-G8	37.6	7.22	38.7	500
4	nFe3O4@IIP-D2-G8	35.4	7.08	35.6	400–800
5	nFe3O4@NIP-D0.5-G8	39.8	7.38	40.8	200–600
6	nFe3O4	96.8	—	72.8	200

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Firstly, the nano-Fe₃O₄ (nFe₃O₄, no. 6) was produced using a polyol-media one-pot solvothermal method (Scheme 1(a)). 4.0 g of FeCl₃·6H₂O, and 12.0 g of NaAc were dissolved in 120 mL ethylene glycol (EG). This solution was stirred vigorously at room temperature for 10 min to form a stable orange solution and then transferred to a Teflon-lined autoclave and heated at 180 °C for 8 h. After the autoclave cooled to room temperature, the resulting nFe₃O₄ was isolated under magnetic field and washed with water and ethanol to remove redundant reagents and impurities. The as-prepared nFe₃O₄ was dried in a vacuum oven at 60 °C for 12 h and stored in a sealed bottle for further use.

Then, the ClO₄⁻-template solution was prepared (Scheme 1(b)). 5 mL of 0.2 mol L⁻¹ EDA ethanol solution and 8 mL of 0.2 mol L⁻¹ GMA ethanol solution were mixed and stirred at room temperature for 30 min, followed by adding 10 mL of 0.2 mol L⁻¹ KClO₄ ethanol solution dropwisely, and stirred at room temperature for another 2 h, to obtain the ClO₄⁻-templated functional monomer solution at the concentration of 0.1 mol L⁻¹ (calculated based on the ClO₄⁻).

Afterward, nFe₃O₄@IIPs was prepared by dispersive polymerization (Scheme 1(c)). 1.0 g of nFe₃O₄ was dispersed 25 mL ethanol under ultrasonication, then the above obtained 20 mL of ClO₄⁻-template solution, and 10 mL 0.2 mol L⁻¹ of St ethanol solution, and 0.5 g DVB were added accordingly. Finally, 0.1 g of benzoyl peroxide (BPO) dissolved in 25 mL ethanol was added dropwisely under vigorously stirring. The mixture was continuously reacted at 80 °C for 3 h. The template ion, ClO₄⁻, was cleaned with 0.5 mol L⁻¹ NaOH ethanol solution for several times under ultrasound until ClO₄⁻ could not be detected by photometric method.²⁵ Then protonated by 0.5 mol L⁻¹ HAc ethanol solution to obtain the final nFe₃O₄@IIP, named as nFe₃O₄@IIP-D0.5-G8. The as-prepared nFe₃O₄@IIP-D0.5-G8 were washed with water three times, dried in a vacuum oven at 60 °C and stored in a sealed bottle for further use.

Other 3 kinds of nFe₃O₄@IIPs (no. 1, 3, and 4) were synthesized with different usage amount of DVB at 0 g, 1 g, and 2 g, respectively, listed in Table 1. In parallel, the non-imprinted magnetic polymer (nFe₃O₄@NIP-D0.5-G8, no. 5) was synthesized almost the same procedures described above without the addition of the templates.

2.3. Characterization

The morphology and dimensions of as-prepared nFe₃O₄@IIPs were obtained on a Hitachi H-7650 transmission electron microscopy (TEM) (Hitachi, Japan) at an accelerating voltage of 75 kV. The magnetic properties of magnetic particles were measured using a vibrating sample magnetometer (VSM, Lake Shore 7410). Powder X-ray diffraction (XRD) patterns were collected on an X-ray diffractometer (Bruker D8 Advance) with CuKα radiation at λ = 0.154 nm operating at 40 kV and 40 mA. The content of Fe₃O₄ in each of nFe₃O₄@IIPs was calculated from the amount of leached Fe, which was measured by using a spectrophotometer (722, Shanghai, China) according to the standard colorimetric method²⁶ after digesting nFe₃O₄@IIPs in 12 mol L⁻¹ HCl solution. The elementary analysis results of the nitrogen contents in nFe₃O₄@IIP were measured using an elementary analysis (EA, Thermo Fisher Flash-1112). Fourier Transform Infrared spectrometer (FTIR, Thermo Nicolet, USA) were applied for characterization.

2.4. Adsorption experiments

Batch adsorption experiments were carried out in 150 mL stoppered flasks, each of which contained 25 mL of KClO₄ solution. A 20 mg amount of adsorbents was added into each flask and shaken at 180 rpm in a thermostatic shaker. The solution pH was adjusted by 0.1 mol L⁻¹ HCl or 0.1 mol L⁻¹ NaOH solution. The KClO₄ concentration in the supernatant was measured by colorimetric method.²⁵ According to the ClO₄⁻ concentrations before and after adsorption, the equilibrium adsorption capacity (q, mg g⁻¹) of ClO₄⁻ bound to the nFe₃O₄@IIP is calculated using eqn (1):²⁷where C₀ and C_e represent the initial solution concentration and the equilibrium concentration of ClO₄⁻ (mg L⁻¹), V is the volume of the ClO₄⁻ solution (mL), m is the adsorbent dosage (mg), the same hereinafter.

To investigate the effect of pH, 25 mL of 100 mg L⁻¹ ClO₄⁻ with pH ranging from 2.0 to 10.0 were mixed with 20 mg of magnetic adsorbents for 1 h at 308 K, respectively. In the kinetic experiments, the nFe₃O₄@IIPs was also investigated with contacting time ranging from 1 to 180 min. The pseudo-first-order model (eqn (2)),²⁷ pseudo-second-order model (eqn (3)),^27,28 and intraparticle diffusion model (eqn (4)),²⁷ were used to fit the experimental data.

q_t = k_idt^1/2 + C (4)

where, q_e and q_t are the adsorption capacities at equilibrium and at time t (mg g⁻¹), respectively. k₁ (min⁻¹), k₂ (g (mg⁻¹ min⁻¹)) are the adsorption rate constants, k_id is the intraparticle diffusion rate constant (mg (g⁻¹ min^−1/2)), C is the intercept (mg g⁻¹).

The adsorption isotherm studies were investigated with ClO₄⁻ initial concentration ranging from 0 to 500 mg L⁻¹ at 308 K for 3 h. Two adsorption isotherms, Langmuir model (eqn (5))^27,29 and Freundlich model (eqn (6)) were applied to analyze the adsorption data^27,29

log q_e = log K_F + (1/n)log C_e (6)

where q_m and K_L are the Langmuir constants related to the maximum adsorption capacity and apparent heat change, respectively, while K_F is a Freundlich constant related to adsorption capacity and 1/n is a Freundlich constant related to the adsorption intensity.

2.5. Selectivity

Common ions with similar ionic radii of ClO₄⁻, such as CrO₄²⁻, H₂PO₄⁻, NO₃⁻, HSO₄⁻, MnO₄⁻ and I⁻ were chosen to evaluate the selectivity of the nFe₃O₄@IIP and nFe₃O₄@NIP and individually dispersed into 25 mL of 100 mg L⁻¹ ClO₄⁻. The mixtures were shaken for 3 h at 308 K, and the concentrations of ClO₄⁻ and other ions in the supernatants were analyzed by ion chromatography (IC). The IL analysis was performed on a Thermo Scientific Dionex DX-500 with a gradient pump (Dionex GP50) at a flow of 0.25 mL min⁻¹, an autosampler (Dionex AS50) with 1000 μL-injection loop, a column thermostat set at 30 °C (Dionex Ultimate 3000 TCC-3000), an eluent generator (Dionex RFC-30) equipped with an eluent generator cartridge (Dionex EGC-III KOH), continuously regenerated trap column (Dionex CR-ATC), and a conductivity detector (Dionex CD-25). Separation was performed on a Dionex IonPac AS20 column and guard column-set. Columns and suppressor were in the 2 mm format, and data acquisition and evaluation were done using the Dionex Chromeleon 6.70 chromatography software. The eluent (35 mmol L⁻¹ KOH) was produced electrolytically in situ. For eluent suppression prior to conductivity detection, a Dionex AERS 500 was used at a current setting of 22 mA. Samples were injected with volumes of 200 μL. The adsorptive amounts of ClO₄⁻ and other ions to nFe₃O₄@IIPs and nFe₃O₄@NIP were then compared.

The specific recognition properties of nFe₃O₄@IIPs are evaluated by imprinting factor (α), defined as eqn (7), and selectivity factor (β) defined as eqn (8):³⁰

where q_IIP and q_NIP are the adsorption capacity of the template or the analogue on IIP and NIP, respectively.where, α_tem and α_ana are the imprinting factor toward the template ion and analogue, respectively.

3. Results and discussion

3.1. Characterization of nFe₃O₄@IIPs

The TEM spectra of the nFe₃O₄@IIPs were shown in Fig. 1. It revealed that the diameter of nFe₃O₄@IIPs increased from 200 nm to 800 nm with the increasing of the usage amount of DVB (Fig. 1(a)–(d)) during the co-polymerization procedure. It might be due to the fact that the more DVB used, the more cross-linking of polymer layer grafted onto the nFe₃O₄ magnetic core, which leading an increasing of the average diameter. When the usage amount of the DVB reached 1 g (Fig. 1(d)), the particle size distribution became not uniform and varied from 400 nm to 800 nm. The TEM spectra of the nFe₃O₄@NIP-D0.5-G8 (Fig. 1(e)) and nFe₃O₄ (Fig. 1(f)) were also investigated, and showed that the particle size distribution of the non-imprinted magnetic polymer was not as uniform as the imprinted analogues, ranging from 200–600 nm, while the bare nFe₃O₄ microspheres had a multidispersed spherical shape with a rough surface, with an average diameter of around 200 nm. The average diameter of the 4 kinds of nFe₃O₄@IIPs and the nFe₃O₄@NIP, as well as the bare nFe₃O₄ was summarized in Table 1.

Fig. 1 TEM images of (a) no. 1 nFe₃O₄@IIP-D0-G8; (b) no. 2 nFe₃O₄@IIP-D0.5-G8; (c) no. 3 nFe₃O₄@IIP-D1-G8; (d) no. 4 nFe₃O₄@IIP-D2-G8; (e) no. 5 nFe₃O₄@NIP-D0.5-G8; (f) no. 6 nFe₃O₄.

The XRD of nFe₃O₄@IIPs was shown in Fig. 2. It indicated that nFe₃O₄@IIPs had retained the spinel structure of Fe₃O₄, in which the identical peaks for Fe₃O₄ located at 30.1°, 35.5°, 43.1°, 53.4°, 57.0° and 62.6°, corresponding to their indices (220), (311), (400), (422), (511) and (400) appeared.³¹ The diffraction peaks of the nFe₃O₄@IIPs were slightly more broaden than that of pure nFe₃O₄. The FWHM of the peak at 35.5° (311) was taken as an example, increasing from 0.59933 (a, nFe₃O₄) to 0.64522 (b, nFe₃O₄@IIP-D0-G8); 0.65178 (c, nFe₃O₄@IIP-D0.5-G8); 0.69442 (d, nFe₃O₄@IIP-D1-G8); and 0.72884 (e, nFe₃O₄@IIP-D2-G8), which indicated that the nFe₃O₄@IIPs have smaller scale, which were consistent to the TEM results and might be favorable for the rapid adsorption and the increase of adsorption capacity.³¹

Fig. 2 The XRD of (a) nFe₃O₄; (b) nFe₃O₄@IIP-D0-G8; (c) nFe₃O₄@IIP-D0.5-G8; (d) nFe₃O₄@IIP-D1-G8; (e) nFe₃O₄@IIP-D2-G8.

The paramagnetic property of the nFe₃O₄@IIPs was verified by the magnetization curve measured by VSM (Fig. 3). The saturation moments obtained from the hysteresis loops of were summarized in Table 1, and found to be at 35.6–42.8 emu g⁻¹, varying with the usage amount of DVB. The nFe₃O₄@IIPs was expected to respond well to magnetic field without any permanent magnetization, therefore making the solid and liquid phases separate easily. For comparing, the saturation moments, Fe₃O₄ contents and nitrogen contents of all the prepared nFe₃O₄@IIPs and nFe₃O₄@NIP were also investigated. Results were shown in Table 1. The Fe₃O₄ content of the nFe₃O₄@IIP decreased from 41.5% to 35.4% (no. 1 to 4 in Table 1) with the usage amount of the DVB increased during the preparation procedure. The results were consistent to the VSM results of the nFe₃O₄@IIPs. With the increasing of the usage amount of the DVB, the nonmagnetic sensitive polymer shell grafted onto the surface of the nFe₃O₄ increased, leading the Fe₃O₄ content of the nFe₃O₄@IIPs decreased as well as the saturation moments decreased. The nitrogen content of the nFe₃O₄@IIPs decreased from 7.82 to 7.08 mmol g⁻¹, with the usage amount of the DVB. With fixed usage amount of the GMA, the increasing the usage amount the DVB (no. 1 to 4 in Table 1) would cause a decrease of the percentage of the GMA groups in polymer shell, thus leading a slight decrease of amino-groups via ring-opening reaction, and eventually leading a decrease of the nitrogen percentage (Table 1).

Fig. 3 The VSM of (a) nFe₃O₄@IIP-D0-G8; (b) nFe₃O₄@IIP-D0.5-G8; (c) nFe₃O₄@IIP-D1-G8; (d) nFe₃O₄@IIP-D2-G8.

3.2. Effect of solution pH and the presumed adsorption mechanism

The adsorption capacity of the nFe₃O₄@NIP-D0.5-G8 and the nFe₃O₄@IIPs were investigated for the pH values ranging from 1.5 to 7.0. The results were shown in Fig. 4. The adsorption capacity of nFe₃O₄@NIP-D0-G8 to ClO₄⁻ was dependent on solution pH (Fig. 4(a)). With the solution pH increasing, the adsorption capacities firstly increased with the solution pH ranging from 1.5 to 3.0, and reached to the maximum at pH 3.0, then decreased. Those of the nFe₃O₄@IIPs were of similar trends. For the 4 kinds of nFe₃O₄@IIPs obtained with the usage of DVB varied (Fig. 4(b)–(e)), the highest adsorption capacities was obtained for the nFe₃O₄@IIP-D0.5-G8, while the lowest one was the nFe₃O₄@IIP-D2-G8, with almost 2 times of the adsorption capacities as that of nFe₃O₄@NIP-D0-G8. Except nFe₃O₄@IIP-D0-G8, this result was consistent with the nitrogen content of the nFe₃O₄@IIPs. The higher nitrogen content implied the more amino groups anchored on the surface of the nFe₃O₄@IIPs, leading a higher adsorption capacities, which was consistent to the results of the nitrogen percentage of the nFe₃O₄@IIPs, as discussed above and shown in Table 1. Interestingly, the solution pH affect the adsorption of the nFe₃O₄@IIP-D0-G8 much more significantly than the other nFe₃O₄@IIPs obtained via adding a certain amount of DVB. As shown in Fig. 4(b), with the solution pH increasing, the adsorption capacities of the nFe₃O₄@IIP-D0-G8 increased sharply and reached to the maximum at pH 3.0, then decreased sharply, as well. However, the adsorption capacities varying of the other nFe₃O₄@IIPs (Fig. 4(c)–(e)) was relatively much milder in the range of pH value 2–4. This implied that the presence of DVB might be contributed to the stabilization of ion imprinting holes, which would be favorable to the improvement of adsorption capacity and to the stabilization of a wider pH range.

Fig. 4 Effect of solution pH value on the adsorption of ClO₄⁻ by nFe₃O₄@NIP-D-0.5-G8 (a), nFe₃O₄@IIPs (with DVB varied (b)–(e)).

The FTIR spectra of nFe₃O₄@IIP-D-0.5-G8 before and after adsorption of ClO₄⁻ were showed in Fig. 5 and could be used for the confirmation of the adsorption mechanism. As shown in Fig. 5(a), the characteristic band of Fe₃O₄ occurred at ∼589 cm⁻¹. The broad peak appeared at ∼3400 cm⁻¹ and 1635, ∼1568 cm⁻¹ can be assigned to be the stretching and bending vibrations of the –OH, –NH₂⁺ and –NH₃⁺ groups. The stretching vibration absorptions of –CH₂ and –CH₃ appeared at ∼2924 cm⁻¹, ∼2853 cm⁻¹, while that of C=O appeared at ∼1630 cm⁻¹. After adsorption (Fig. 5(b)), the characteristic bands at ∼1568 cm⁻¹ shifted to ∼1530 cm⁻¹ along with the appearance of two new bands at ∼1090 cm⁻¹ and ∼1145 cm⁻¹. After desorption (Fig. 5(c)), the latter two peaks disappeared, which might be attributed to the stretching vibration of the ClO₄⁻ (ref. 32), indicating that the interaction between –NH₂⁺ and –NH₃⁺ groups and the ClO₄⁻ were realized via the electrostatic interaction.

Fig. 5 FTIR of the nFe₃O₄@IIP-D-0.5-G8 (a) before, (b) after adsorption of ClO₄⁻ and (c) reused.

3.3. Kinetic studies

Fig. 6 presented the adsorption kinetics of ClO₄⁻, onto nFe₃O₄@IIPs and nFe₃O₄@NIP. As shown in Fig. 6 (left, (a) and (b)), for nFe₃O₄@NIP-D-0.5-G8 and nFe₃O₄@IIP-D-0-G8, the adsorption capacity increased gradually and reached equilibrium in nearly 60 min, while for nFe₃O₄@IIP-D-0.5-G8, nFe₃O₄@IIP-D-1-G8 and nFe₃O₄@IIP-D-2-G8 (Fig. 6, left, (c)–(e), respectively), the adsorption capacity increased rapidly, and only 10 min were needed to reach adsorption equilibrium. This revealed that the existing of the ClO₄⁻-template and the presence of DVB for the crosslinking during the preparation process would stabilize the surface imprinting cavities in polymer matrix of the nFe₃O₄@IIPs and eventually shorten the equilibrium time. The kinetic curves of nFe₃O₄@NIP-D-0.5-G8 and nFe₃O₄@IIP-D-0-G8, could be divided into three portions, which could be described by intraparticle diffusion model (shown in Fig. 6, right, (f) and (g)) and indicated that the intra-particle process³³ might be one of the rate-limiting steps for ClO₄⁻ removal. The kinetic curves of the other nFe₃O₄@IIPs could only be divided into two portions (Fig. 6, right, (h)–(j)), thus, the intra-particle process might not be involved in the rate-limiting steps. Therefore, in the present case, ClO₄⁻ reached the surface imprinting cavities of nFe₃O₄@IIPs easily and took less time to reach adsorption equilibrium, implying that the surface imprinting and uniform structures of nFe₃O₄@IIPs allowed efficient mass transport, thus overcoming some drawbacks of traditionally non-imprinting materials. The presence of DVB for the crosslinking took an important role for the stabilization of the imprinting cavities of the nFe₃O₄@IIPs and favorable for the adsorption kinetic processes.

Fig. 6 Effect of adsorption time on the adsorption of ClO₄⁻ by nFe₃O₄@NIPs-D-0.5-G8 (a), (f), nFe₃O₄@IIPs (with DVB varied (b)–(e), (g)–(j)).

Pseudo-first-order and pseudo-second-order models were used to describe the adsorption kinetic data. The results were shown in ESI, Fig. S1 and S2.‡ The correlation coefficient values indicated a better fit of the pseudo-second-order model with the experimental data compared to the pseudo-first-order for all the nFe₃O₄@IIPs and nFe₃O₄@NIP (Table 2). The calculated q_e values were in agreement with the theoretical ones, and the plots showed good linearity with R² above 0.999. Therefore, the adsorption behaviors followed the pseudo-second-order model, suggesting a chemisorption process,³⁴ including both ion exchange and electrostatic interaction.

Table 2 Pseudo-first-order and pseudo-second-order models and constants

Adsorbents	qe,exp (mg g^−1)	Pseudo-first-order model			Pseudo-second-order model
		k1 (min^−1)	qe,cal (mg g^−1)	R^2	k2 (g (mg^−1 min^−1))	qe,cal (mg g^−1)	R^2
nFe3O4@IIP-D0-G8	75.8	0.0689	24.7	0.6301	0.0032	77.5	0.9992
nFe3O4@IIP-D0.5-G8	102	0.052	7.58	0.3553	0.0122	102	0.9997
nFe3O4@IIP-D1-G8	92.7	0.735	13.7	0.532	0.0072	93.5	0.9999
nFe3O4@IIP-D2-G8	81.9	0.0619	16.14	0.6124	0.0067	82.6	0.9999
nFe3O4@NIP-D0.5-G8	59.5	0.081	28.77	0.8166	0.00243	62.5	0.9998

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3.4. Adsorption capacities

The adsorption capacities of ClO₄⁻ were investigated for all the nFe₃O₄@IIPs and nFe₃O₄@NIP (Fig. 7). Both Langmuir and Freundlich adsorption models were applied for data fitting analysis. The results were shown in ESI, Fig. S3 and S4.‡ The represented parameters using Langmuir and Freundlich adsorption models indicated that the Langmuir model could effectively describe the adsorption data with R² > 0.995 (Table 3), suggesting a better fit of the Langmuir isotherm rather than Freundlich isotherm, which suggested a monolayer adsorption.

Fig. 7 Adsorption isotherm of ClO₄⁻ onto nFe₃O₄@NIPs-D-0.5-G8 (a) and nFe₃O₄@IIPs (with DVB varied (b)–(e)).

Table 3 Langmuir and Freundlich isotherms and constants of ClO₄⁻ adsorption

Adsorbents	qm,e (mg g^−1)	Langmuir isotherm			Freundlich isotherm
		KL (L mg^−1)	qm,c (mg g^−1)	R^2	KF	1/n	R^2
nFe3O4@IIP-D0-G8	80.9	0.1105	83.33	0.9989	2.976	0.3827	0.7725
nFe3O4@IIP-D0.5-G8	108.9	0.1445	111.1	0.9987	3.269	0.4045	0.7854
nFe3O4@IIP-D1-G8	94.0	0.1537	95.24	0.9986	3.158	0.3852	0.694
nFe3O4@IIP-D2-G8	75.7	0.1593	76.92	0.9994	2.825	0.3915	0.7986
nFe3O4@NIP-D0.5-G8	59.0	0.0978	60.61	0.9977	2.349	0.4146	0.8310

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As shown in Fig. 7(a) and Table 3, the maximum adsorption capacities of nFe₃O₄@IIPs (q_m,c = 76.92–111.1 mg g⁻¹; q_m,e = 75.7–108.9 mg g⁻¹) were much higher than that of nFe₃O₄@NIPs-D-0.5-G8 (q_m,c = 60.61 mg g⁻¹; q_m,e = 59.0 mg g⁻¹). It can be seen that by using the ion imprinting technology, the adsorptive capacities of the obtained materials (nFe₃O₄@IIPs) to ClO₄⁻ can be improved. Although with the usage amount of the crosslinking agent DVB increased, the contents of the protonated amino groups in the nFe₃O₄@IIPs reduced 1.1%∼3.4% from nFe₃O₄@IIPs-D-0-G8 to nFe₃O₄@IIPs-D-2-G8 (Table 1), the adsorption capacities of the nFe₃O₄@IIPs did not decrease as expected, but an increase of around 34.6% for nFe₃O₄@IIP-D0.5-G8 and 16.2% for nFe₃O₄@IIP-D1-G8, were observed (Table 3). And till to the amount of DVB increased to 2.0 g, the decrease of the adsorption capacity at about 4.9% appeared for nFe₃O₄@IIP-D2-G8 (Table 3). It implied that the usage amount of DVB not only affected the structure of the material in the process of material synthesis, but also played a positive correlation with the adsorption properties of the material at a suitable usage amount. Too much amount of crosslinking agent DVB, 2.0 g in the present work, might cause the active site of the protonated amino group wrapped inside the polymer chain, thus reducing the adsorption capacity.

3.5. Adsorption selectivity

It is of great importance to assess the selective recognition towards the template for a novel imprinted material. Adsorption selectivity is known to be related to the size, shape, and functionality of the template of the imprinted cavities in imprinted materials.¹⁶ Herein, the selectivity of nFe₃O₄@IIP-D0.5-G8 and nFe₃O₄@NIP-D0.5-G8 towards ClO₄⁻ and some other ions with similar ionic radii,³⁵ i.e., CrO₄²⁻, H₂PO₄⁻, NO₃⁻, HSO₄⁻, MnO₄⁻ and I⁻ at individual concentration of 100 mg L⁻¹ was studied. The imprinting factor (α) of nFe₃O₄@IIP-D0.5-G8 for ClO₄²⁻ was 1.8, which was much larger than those of the other ions with similar ionic radii (Table 4). The high selectivity factors (β) of the analogs (5.9–49.4) also exhibited high selectivity of nFe₃O₄@IIP-D0.5-G8 toward ClO₄⁻. This could be attributed to the fact that in adsorbing process, specific recognition sites respect to template ion (here, i.e., ClO₄⁻) were generated on the surface of nFe₃O₄@IIP-D0.5-G8, thus ClO₄⁻ was strongly bound to the specific binding sites. As the competitive ions, the recognition sites of the imprinting cavities were not complementary to them, so it had less chance to be adsorbed on the nFe₃O₄@IIP-D0.5-G8. In contrast, the nFe₃O₄@NIP-D0.5-G8 adsorbed template much less than that of nFe₃O₄@IIP-D0.5-G8 since nFe₃O₄@NIP-D0.5-G8 had not generated specific recognition sites due to the absence of template ion.

Table 4 Imprinting factors (α) and selectivity factors (β) of Fe₃O₄@IIP(St–HPMA–DVB)-0.5 ^a

Ion	Ionic radius (aq.)/nm	qIIP (mg g^−1)	qNIP (mg g^−1)	α	β
a qIIP: Adsorption capacity of Fe3O4@IIP(St–HPMA–DVB)-0.5; qNIP: adsorption capacity of Fe3O4@NIP(St–HPMA–DVB)-0.5; α: imprinting factor; β: the selectivity factor.
ClO4^−	0.240	111.1	60.6	1.8
CrO4^2−	0.240	28.5	122.5	0.2	7.7
H2PO4^−	0.238	22.1	72.5	0.3	5.9
NO3^−	0.200	2.5	68.6	0.0	49.4
HSO4^−	0.230	6.5	62.8	0.1	17.4
MnO4^−	0.240	7.6	60.8	0.1	14.4
I^−	0.285	2.8	42.3	0.1	27.2

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3.6. Reusability investigation

The reusable of the Fe₃O₄@IIPs was evaluated by comparing the adsorption efficiency. The ClO₄⁻ loaded Fe₃O₄@IIPs was extracted with 0.5 mol L⁻¹ methanol/NaOH solution for 10 min twice, and then for adsorption to get the adsorption efficiency. Table 5 listed the reusability of Fe₃O₄@IIPs for ClO₄⁻. The results indicated that nFe₃O₄@IIP-D0.5-G8 and nFe₃O₄@IIP-D1-G8 could be used for at least five cycles with a loss of less than 5% upon recovery on average, while desorption efficiencies decreased to 78.8–80.6%, and the re-adsorption capacities decreased to 43.4–45.5%, for nFe₃O₄@NIP-D0.5-G8, nFe₃O₄@IIP-D0-G8 and nFe₃O₄@IIP-D2-G8, which further implied that the usage amount of DVB played an important role for the stabilization of the active site of the protonated amino group and the imprinting cavities of the nFe₃O₄@IIPs when the usage amount at 0.5–1 g during the preparation procedure.

Table 5 Reusability of Fe₃O₄@IIP(St–HPMA–DVB) for ClO₄⁻

Adsorbent	Reusability^a (%)	Run 1	Run 2	Run 3	Run 4	Run 5
a Des: desorption efficiency (%); Ads: adsorption efficiency (%).
nFe3O4@IIP-D0-G8	Des	95.1	90.6	89.0	82.4	80.6
	Ads	70.3	68.8	57.9	48.8	45.5
nFe3O4@IIP-D0.5-G8	Des	95.3	90.1	88.0	82.0	80.4
	Ads	72.3	69.7	56.4	48.6	44.5
nFe3O4@IIP-D1-G8	Des	99.1	99.6	99.0	98.4	98.6
	Ads	98.3	98.8	97.9	98.8	98.5
nFe3O4@IIP-D2-G8	Des	99.0	98.9	98.6	98.2	98.2
	Ads	97.5	98.3	97.9	97.4	97.9
nFe3O4@NIP-D0.5-G8	Des	93.9	90.3	86.8	80.8	78.8
	Ads	72.0	69.0	58.1	48.0	43.4

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3.7. Adsorption comparison

The adsorption capacities of nFe₃O₄@IIPs and nFe₃O₄@NIP compared with other adsorbents examined for the removal of ClO₄⁻ under similar conditions are summarized in Table 6. The results indicate that the as-prepared nFe₃O₄@IIPs in this work has 3.7–14 times of adsorption capacity as that of other adsorbents reported in the literature, and twice as that of the non-printed one, i.e., nFe₃O₄@NIP-D0.5-G8. Hence, the newly developed nFe₃O₄@IIPs has promising potential applications in the removal of ClO₄⁻ from environmental water.

Table 6 Comparison of adsorption properties of different adsorbents ClO₄⁻

Adsorbent	Functional group	Model	qm/(mg g^−1)	Ref.
a NA: not available.b L: Langmuir.c F: Freundlich.d T: Tempkin.
Activated carbon	NA^a	L^b, F^c,T^d	8.04–13.00	36
Cationic surfactant loaded activated carbon	Cationic surfactant	NA^a	21.14–29.59	8
Calcined Zn/Al layered double hydroxides	–NR4^+	F^c	NA^a	37
nFe3O4@NIP-D0.5-G8	–NH2^+, –NH3^+	L^b	60.6	This work
nFe3O4@IIP-D0-G8	–NH2^+, –NH3^+	L^b	83.3	This work
nFe3O4@IIP-D0.5-G8	–NH2^+, –NH3^+	L^b	111.1	This work
nFe3O4@IIP-D1-G8	–NH2^+, –NH3^+	L^b	95.2	This work
nFe3O4@IIP-D2-G8	–NH2^+, –NH3^+	L^b	76.9	This work

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4. Conclusion

A series of novel protonated amino functionalized core–shell ionic imprinted magnetic polymers (nFe₃O₄@IIPs) was controlled prepared by ultrasonic assisted suspension polymerization and surface imprinting technique. The as-prepared nFe₃O₄@IIPs exhibited a homogeneous morphology, highly selective recognition, strong affinity ability, and high magnetic responsiveness for the adsorption of ClO₄⁻ in aqueous. The adsorption mechanism of the ClO₄⁻ onto the nFe₃O₄@IIPs is mainly related to ion exchange and electrostatic attraction, in which the protonated amino groups and imprinting cavities play a cooperative role in the adsorption of ClO₄⁻.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We would like to thank the National Natural Science Foundation of China (51608479), the National Natural Science Foundation of Zhejiang Province (LY14B04003), the National Natural Science Foundation of Ningbo (2014A610092), the National College Students’ innovation and entrepreneurship training program (201713022009), the Xinmiao Students' innovation training program of Zhejiang Province (2017R401181) for the financial support.

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Footnotes

† Dedicated to Prof. Dai-Zheng Liao on the occasion of his 80th birthday.

‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra06085a

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