Hui Jiang, Zijian Zhao, Ning Yu, Yi Qin, Zhengwei Luo, Wenhua Geng,*, Jianliang Zhu

1 College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing 211816, China

2 School of Environmental Science and Engineering, Nanjing Tech University, Nanjing 211816, China

Keywords:

ABSTRACT

1. Introduction

Mineral resource is an important support to economic development. With the increasing demand for mineral resources, the exploration and development of liquid mineral resources such as salt lake brine has become the focus of research [1]. Boron (B) is a trace element which is widely distributed in the earth’s hydrosphere and lithosphere.Due to the special valence bond structure,there are abundant types of boron compounds,which usually exist in nature in the form of boric acid and borate[2].The physical and chemical properties of boron lied between those of metals and non-metals. Because of its special properties, it has become an important raw material in the industrial field. Boron is used in many fields such as glass,ceramics,semiconductors,pharmaceuticals, cosmetics, catalysts, and cleaning products [3,4]. Meanwhile,boron is also an essential element for plants,animals and humans,and plays an important role in the growth,development and metabolism of various organisms [5,6]. However, excess boron can be harmful to living organisms.On one hand,there is a huge demand for boron [7]. On the other hand, there are abundant boron resources in natural water bodies. Meanwhile, wastewater will be generated during the production and use of boron. Therefore, it is necessary to study the method of extracting boron from water bodies [8].

In the aqueous solution, the form of boron mainly depends on the pH and the concentration of the solution. Boric acid is a weak Lewis acid found in natural waters as:

Boron exists in water as boric acid molecules and a variety of boron oxyanions. At lower pH values, boron mainly exists in the form of H3BO3, whereas it is mainly found in the form of anions under alkaline conditions. B(OH)3and B(OH)–4mostly exist in low-concentration solutions. The degree of polymerization and the existing form of borate anions are different at different pH values, temperatures and concentrations.

The treatment of water resources, including salt lake brine and industrial wastewater, is of significance for environmental protection and resource recovery. There are many methods for extracting boron from water, including chemical precipitation [9,10], membrane separation [11,12], acidification, solvent extraction [13],ion exchange [14–16], and adsorption. The adsorption method achieves the purpose of enriching ions by adding adsorbents into the solution that have special properties. The method has the advantages of high adsorption efficiency, recyclability and good enrichment effect [17]. Adsorbents are mainly divided into inorganic and organic adsorbents. Common inorganic adsorbents include metal hydroxides [18], natural ores [19], cellulose derivatives [20], and metal oxides. Organic adsorbents are mostly those containing active dihydroxy functional groups [21,22]. Among them, N-methyl-D-glucosamine (NMDG) is the most widely used adsorbent [4]. Boron can be adsorbed through the chelation effect of adsorbents and boron, which has the underlying principle of forming a complex with boric acid or borate ion through cisdihydroxyl group.

The use of polyhydroxyl groups as chelating agents for boron adsorption is a promising approach [23]. The existing adsorbents may be insufficient in terms of adsorption capacity, adsorption rate and adsorption selectivity, and therefore, it is necessary to develop novel adsorbents having better performance. Imprinted adsorption is undoubtedly better than other adsorption techniques. In imprinted adsorption, a large number of specific active sites are located on the surface of the adsorbent, which not only improves the adsorption rate, but also improves the selectivity of the adsorbent, thus having a good impact on the overall adsorption performance [24]. Molecular imprinting technology (MIT) is a process of complexing monomers and template molecules or ions to produce polymers with specific recognition sites [25]. Ion imprinting technology (IIT) originates from MIT to selectively enrich target ions [26]. The molecularly imprinted polymers (MIPs) or ionimprinted polymers (IIPs) show high affinity for template molecules or ions. MIPs and IIPs have been used as effective solid adsorbents for the separation and removal of harmful substances in aqueous solutions [27]. Compared with the traditional imprinting method, the surface imprinting method is carried out on the surface of the substrate, so that almost all the adsorption sites are distributed on the outer surface of the substrate, which is convenient for the binding of target ions, and results in a good adsorption rate,stability and high selectivity [28,29]. Meanwhile, the matrix of the surface ion-imprinted polymer is also very important, which affects the adsorption performance of the prepared IIPs [30–32].

To prepare imprinted material, many reports are based on the boron affinity principle [33]. Many boron affinity materials are derived for biomedical applications such as separation, sensing analysis and disease diagnosis [34–36]. However, there are only a handful of reports focusing on the preparation of adsorption materials based on boron affinity mechanism and ion imprinting technology for boron adsorption, whereas studies focusing on anion imprinted polymers are also relatively rare[37].Based on the idea of reverse synthesis of imprinted boron affinity materials, monomers with cis-dihydroxyl functional groups were modified and grafted on the surface of a suitable substrate to prepare boron adsorption materials [38].

In this work, based on boron affinity mechanism and ion imprinting technology, we synthesized a surface ion imprinted polymer BMIIP (boron magnetic ion-imprinted polymer) based on magnetic Fe3O4as a support for selective boron adsorption[39–41].For comparison, bulk polymeric boron adsorbent glycidyl methacrylate-NMDG (GMA-NMDG) and magnetic GMA-NMDG polymer (MGN) were synthesized simultaneously. The structure and composition of the synthesized adsorbents were analyzed by using FT-IR, TGA, SEM, EDS, XRD, VSM and BET. At the same time,combining with the adsorption isotherm, adsorption kinetics,adsorption thermodynamics, and reusability analysis, BMIIP was compared with GMA-NMDG, MGN, and other reported boron adsorbents and showed advantages referring to adsorption ability,adsorption equilibrium time, adsorption selectivity, and reuse performance.

2. Experimental

2.1. Materials

NMDG was obtained from MacLean Biochemical Technology Co., Ltd., China. Glycidyl methacrylate (GMA), γ-methylacryloxy propyl trimethoxy silane (MPS), N,N’-methylene bisacrylamide(Bis),and azodiisobutyronitrile(AIBN)were obtained from Aladdin Industrial Corp., China. Ferrous chloride tetrahydrate (FeCl2?4H2O)and ferric chloride hexahydrate(FeCl3?6H2O)were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Acetonitrile (ACN),1,4-dioxane, tetraethyl orthosilicate (TEOS) and boric acid(H3BO3) were purchased from Lingfeng Chemical Reagents Co.,Ltd., China. All chemical reagents employed in this work were of reagent grade and used without further purification.

2.2. Preparation of GMA-NMDG polymer

0.4 ml of GMA[42]and 0.6 g of NMDG were dissolved in 80 ml of ACN.Then,under mechanical stirring,80 mg of AIBN was added to the solution. The mixture was heated to 363.15 K, and refluxed for 4 h to obtain white GMA-NMDG polymer. The polymers were washed with deionized water and ethanol, and then, dried in vacuum at 323.15 K for 12 h.

2.3. Preparation and silanization of Fe3O4 NPs

The preparation of Fe3O4NPs was based on a previous work[43].First,2.12 g FeCl2?4H2O and 4.86 g FeCl3?6H2O were dissolved in 110 ml of deionized water, and stirred under N2atmosphere at 353.15 K.Meanwhile,10 ml of ammonia was added dropwise until the color of the solution changed from brown to black.After 60 min of reaction, black Fe3O4NPs were obtained. The prepared Fe3O4NPs (1.0 g) were dispersed in 120 ml of absolute ethanol and 20 ml of deionized water,followed by the addition of 4 ml ammonia. Under the condition of mechanical stirring, 2 ml of TEOS was added dropwise, and the stirring was continued for 12 h at room temperature to obtain silanized Fe3O4NPs (Fe3O4@SiO2) [44]. The products were separated with an external magnetic field and washed with deionized water, followed by drying in vacuum at 323.15 K for 12 h.

2.4. Functionalization of Fe3O4@SiO2

0.5 g of prepared Fe3O4@SiO2was dissolved in 80 ml aqueous ethanol solution(15%(vol)),and 2 ml of MPS was added to it dropwise. The mixture was stirred continuously at 313.15 K for 12 h.The functionalized Fe3O4@SiO2(Fe3O4@SiO2@MPS) was separated using external magnets,washed with ethanol and deionized water,and dried in vacuum at 323.15 K for 12 h.

2.5. Graft polymerization of GMA

0.15 g of Fe3O4@SiO2@MPS was dispersed in 80 ml ACN,whereas 0.5 ml of GMA was added under stirring. Meanwhile,0.3 g of crosslinking agent Bis and 30 mg of AIBN were added at the same time. The reaction lasted for 12 h at 353.15 K under the protection of N2.The obtained Fe3O4@SiO2@MPS-GMA was washed with deionized water for several times and dried in vacuum at 323.15 K for 12 h.

2.6. Ring opening reaction for NMDG

0.1 g of Fe3O4@SiO2@MPS-GMA was dispersed in 80 ml of aqueous 1,4-dioxane solution(volume ratio=9:1),and 0.4 g NMDG was added to it[45].Under mechanical stirring,the mixture was heated to 353.15 K and the reaction lasted for 12 h.The obtained MGN was washed with deionized water and dried in vacuum at 323.15 K for 12 h.

2.7. Preparation of BMIIP

First,0.6 g NMDG and 0.5 ml GMA were dissolved and stirred in 80 ml aqueous 1,4-dioxane solution (volume ratio = 9:1) at 353.15 K for 12 h. After the ring opening reaction was completed,0.2 g of H3BO3was added and the system was stirred for 120 min to homogenize it. Then, 0.4 g Fe3O4@SiO2@MPS was evenly dispersed in the solution, and 20 ml of aqueous 1,4-dioxane solution containing 6 mmol Bis and 70 mg AIBN was added to it. Subsequently,N2was introduced for 10 min to eliminate air in the bottle.After 24 h of polymerization at 338.15 K under N2protection,BMIIP containing borate ions was obtained.The BMIIP was washed with ethanol and deionized water for several times to remove the residual reagent, and the template ions were eluted with 0.05 mol?L–1NaOH for several times. Each elution time was 120 min until boron could not be detected in the eluent.After elution, BMIIP was rinsed with deionized water to neutral, and dried in vacuum at 323.15 K for 12 h.

Fig. 1 shows the synthesis process of boron adsorbents.

2.8. Characterization of adsorbents

The functional groups of materials and intermediates were analyzed using Fourier transform infrared spectroscopy(FT-IR,Nicolet 6700, Thermo Fisher, USA) within the scanning range of 500–4000 cm-1. Scanning electron microscopy (SEM, Thermo Fisher)was used to analyze the surface morphology, whereas energy dispersive spectroscopy (EDS) was used to conduct the elemental analysis of boron adsorbents.X-ray diffraction(XRD)patterns were measured within the range of 10°–90°using the X-ray diffractometer (SmartLab, Rigaku, Japan). The magnetism was analyzed using the vibrating sample magnetometer (VSM, VersaLab, Quantum Design, USA). The thermal stabilities of the adsorbed materials were tested using thermogravimetric analysis (TGA, STA 449C,New Castle, USA), whereas the maximum temperature was 1073.15 K in N2atmosphere. The Brunauer–Emmett–Teller (BET)method was used to test the specific surface area and pore size(V-Sorb 2800 Series Analyzer, Gold APP Instrument, China).

2.9. Adsorption experiments

The simulated boron solution was prepared for the adsorption experiment.First,10 mg of adsorbents were added into each screw bottle,and 10 ml of boron solution was also pipetted out.The two solutions were shaken in the oscillator at 200 r?min-1for adsorption. Then, the adsorbed materials were separated using the magnet, and the supernatant was filtered using the filter head(0.45 μm). The concentrations of boron before and after the adsorption were determined using the inductively coupled plasma-optical emission spectrometer (ICP-OES, iCAP 6300,Thermo Fisher).The adsorption capacity qeis calculated by Eq.(1).

where qeis the adsorption quantity (calculated based upon boron element), mg?g-1; Ciand Ceare the initial concentration of boron solution and the concentration at adsorption equilibrium, mg?L–1,respectively; V is the volume of the adsorbed liquid, L; and m is mass of adsorbents, g.

3. Results and Discussion

3.1. Characterization

3.1.1. FT-IR analysis

FT-IR analysis was used to analyze the functional groups of the prepared materials. Fe3O4NPs, Fe3O4@SiO2, Fe3O4@SiO2@MPS,Fe3O4-GMA, MGN, BMIIP and GMA-NMDG were investigated within the range of 500–4000 cm-1, and the results are shown in Fig. 2. For Fe3O4NPs, the absorption peak at 575 cm-1was due to the asymmetric vibration of Fe—O, whereas the peak of —OH appeared at 3391 cm-1because of the presence of hydroxyl group on the surface of Fe3O4NPs and the residual water [46,47]. These peaks can prove the successful preparation of Fe3O4NPs [48]. For Fe3O4@SiO2, Si—O stretching vibration resulted in peaks at 709 and 949 cm-1,while the peak of Si—O—Si was at 1097 cm-1.These peaks proved the successful modification of TEOS [49]. Moreover,the peaks at 1399 and 3217 cm-1were of—CH2and—CH3,respectively.For Fe3O4@SiO2@MPS,the peak at 1650 cm-1was due to the asymmetric vibration of C=C, while the peak at 1719 cm-1was ascribed to C=O. These peaks indicated the functionalization of MPS [50]. For Fe3O4-GMA, the peaks at 756 and 909 cm-1were epoxy peaks,while those at 1526 and 3060 cm-1were N—H peaks,which originated from the crosslinking agent Bis. Therefore, GMA was grafted on the surface through polymerization [50]. By comparing the results with those of MGN, it was found that the epoxy peak disappeared, while other peaks did not change significantly,indicating that the ring-opening reaction was completed and NMDG was loaded successfully. BMIIP and GMA-NMDG had the same reaction, indicating that other characteristic peaks existed in curves (f) and (g), while the epoxy group was absent. These results indicated that GMA-NMDG, MGN and BMIIP were successfully prepared.

3.1.2. TGA analysis

Fig. 1. Schematic for the synthesis of boron adsorbents.

Fig. 2. FT-IR spectra of (a) Fe3O4 NPs, (b) Fe3O4@SiO2, (c) Fe3O4@SiO2@MPS, (d) Fe3O4-GMA, (e) MGN, (f) BMIIP, and (g) GMA-NMDG.

The thermal stabilities of Fe3O4NPs, Fe3O4@SiO2@MPS, MGN,BMIIP and GMA-NMDG were investigated using thermogravimetric analysis, as shown in Fig. 3. The curve of Fe3O4NPs was flat within the temperature range of 293.15–973.15 K, whereas the mass of Fe3O4NPs exhibited almost no obvious change. The slight decrease may be due to the evaporation of residual water on the surface. The TGA curves of Fe3O4@SiO2@MPS and Fe3O4NPs showed similar variation trends within the temperature range of 293.15–973.15 K. However, there was a slight decrease, which might have been caused by the decomposition of silane coupling agent on Fe3O4@SiO2@MPS at high temperature. The final mass fraction was still 92.8%.The curves of MGN and BMIIP were similar,because both of them were grafted on the surface of Fe3O4@SiO2@-MPS. Their mass loss mainly went through three stages. The first stage occurred within the temperature range of 293.15–523.15 K,while the small decrease of mass was caused by the evaporation of water and residual reagent in the adsorbent.In the second stage,the mass fractions of MGN and BMIIP decreased sharply from 523.15 to 773.15 K.At this moment,the imprinted polymer rapidly decomposed and volatilized at high temperature. Within the temperature range of 773.15–973.15 K, the mass fraction decreased slowly. At this time, the organic imprinted layer on the surface had almost completely decomposed.The remaining mass fractions of MGN and BMIIP were 21.2% and 24.7%, respectively. GMANMDG had no support; however, the TGA curve was consistent with those of the MGN and BMIIP. It also went through the same three stages; however, the final mass fraction was only 5.4%, indicating that the above analysis was reasonable. The TGA curves indicated that the prepared materials had superior thermal stabilities.

Fig. 3. Mass loss curves of (a) Fe3O4 NPs, (b) Fe3O4@SiO2@MPS, (c) MGN, (d) BMIIP and (e) GMA-NMDG.

3.1.3. SEM analysis

The surface morphologies and structures of Fe3O4@SiO2@MPS,GMA-NMDG, MGN and BMIIP were investigated using SEM, as shown in Fig. 4. According to the SEM of Fe3O4@SiO2@MPS shown in Fig.4(a),the prepared support consisted of spherical nanoparticles with uniform particle size,whereas the independent spherical contour could still be observed when they were stacked together.By observing the SEM of GMA-NMDG in Fig. 4(b), it can be seen that the polymer synthesized directly had a compact structure and a massive agglomeration, with uneven and rough surface. By comparing the SEM images of MGN and BMIIP (Fig. 4(c) and (d)),it can be found that the surface organic polymers led to distinct structural layers and roughness. In contrast, the surface layer of MGN was more uneven. The beads of BMIIP were connected through agglomeration, with uniform load and the same coarse and porous structure. These results showed that the prepared materials had good appearance, structure and morphology.

3.1.4. EDS analysis

Fig. 5 shows the EDS of Fe3O4@SiO2@MPS, GMA-NMDG, MGN,and BMIIP. It can be seen from Fig. 5(a) that C, O, Fe and Si were the main elements of Fe3O4@SiO2@MPS. The existence of Fe and Si indicated the successful preparation and functionalization of Fe3O4NPs. From Fig. 5(b), it can be seen that the polymer directly formed by GMA-NMDG reaction was mainly composed of C and O.The comparison of MGN,BMIIP and Fe3O4@SiO2@MPS(as shown in Fig. 5(c), (d) and (a), respectively) shows that the proportion of C was significantly increased, because the organic layer was loaded on the surface of Fe3O4NPs, indicating that the subsequent graft polymerization was successful. Fig. 5(e) shows the BMIIP after the adsorption process.The presence of B indicated that the adsorbent had the ability to adsorb boron.Meanwhile,the template ions could be removed using elution.

3.1.5. XRD analysis

The results from the XRD analyses of prepared Fe3O4NPs, Fe3-O4@SiO2@MPS, MGN, BMIIP and GMA-NMDG are shown in Fig. 6.The characteristic peaks of Fe3O4NPs at 2θ of 30.14°, 35.38°,43.08°, 53.42°, 57.04° and 62.66° were observed, which were attributed to(2 2 0),(3 1 1),(4 0 0),(4 2 2),(5 1 1)and(4 4 0)crystal planes, respectively. This was consistent with the diffraction peaks of Fe3O4(JCPDS card number 19-0629), indicating the successful preparation of Fe3O4NPs [51]. After modification with silane coupling agent, the diffraction peak of Fe3O4@SiO2@MPS remained almost unchanged. It can also be observed that the XRD patterns of MGN and BMIIP had their diffraction peaks in the original position.Although their intensity was attenuated,they were still clearly visible.However,in GMA-NMDG,the above characteristic peaks disappeared,and only one peak appeared at 18.48°.This indicated that the organic imprinting layer did not change the crystal phase of Fe3O4NPs, and the crystal structure remained intact even after the polymerization process[52].This proved that the magnetic materials had stable crystal structure.

3.1.6. VSM analysis

The magnetic saturation intensities of Fe3O4@SiO2@MPS, MGN and BMIIP were measured using VSM and the specific results are illustrated in Fig.7.The prepared magnetic materials had the classical ‘‘S” shaped curves, and no coercivity or hysteresis phenomenon was observed, indicating that Fe3O4@SiO2@MPS, MGN and BMIIP all had super paramagnetism [53]. Meanwhile, it can be clearly observed that the magnetic saturation intensities of MGN and BMIIP were lower than that of Fe3O4@SiO2@MPS. Moreover, the imprinted polymer layer was loaded onto the surface,which led to the decline of magnetism of the composite materials.The magnetic saturation intensity of Fe3O4@SiO2@MPS could be as high as 60.03 A?m2?kg-1,while those of MGN and BMIIP were 8.89 and 6.01 A?m2?kg-1, respectively. Even after loading the organic imprinted layer,the magnetic saturation intensity of the materials decreased. However, MGN and BMIIP could still be quickly separated from the solution using external magnetic field. The results show that the dark particles were attracted to the bottle wall.From the figure (right), in the same time, the adsorbent loaded with magnets separated faster from the solution, while the GMANMDG without the loaded magnet had no magnetism. It can be seen from the physical comparison figure that the GMA-NMDG was still dispersed in the solution.

3.1.7. BET analysis

Fig. 4. SEM micrographs of (a) Fe3O4@SiO2@MPS, (b) GMA-NMDG, (c) MGN and (d) BMIIP.

The N2adsorption–desorption isotherms of Fe3O4@SiO2@MPS,MGN,BMIIP and GMA-NMDG are shown in Fig.8.All the prepared materials exhibited Type IV isotherms.When the relative pressure was low, the adsorption capacity of N2increased slowly. When 0.8 < P/P0<1.0, the adsorption capacity increased sharply, indicating that they were mesoporous structures [54]. The N2adsorption capacity of Fe3O4@SiO2@MPS was much higher than MGN, BMIIP and GMA-NMDG. The pore size, pore volume and specific surface area were determined using Barrett–Joyner–Halenda method[55]. The results are presented in Table 1. After the polymer was loaded onto the surface, the pore size and specific surface area decreased[56].However,the prepared materials still had good surface morphologies. In addition, the pore sizes of BMIIP, MGN and GMA-NMDG were 10.33, 8.20 and 7.66 nm, all of which were within the range of mesopores (2 nm < d < 50 nm), proving that the prepared adsorbents were suitable mesoporous materials[57].

3.2. Adsorption experiments

The solution used in the adsorption experiments was the simulated boron solution, which used H3BO3as the boron source. The concentration was calculated as the boron element (B).

3.2.1. Influence of pH

When the adsorbent was used to adsorb boron, the pH value had an important effect on the adsorption. Boron can exist in various ionic forms in aqueous solutions. B(OH)3and B(OH)4–were predominant at low concentrations. When the pH was low, the boron in the solution was mainly B(OH)3.When the pH was higher,B(OH)4–was dominant, and they could get transformed into each other in the solution. The initial solution prepared was basically neutral,and its pH was about 6.5.Therefore,the adsorption capacity of adsorbents within the pH range of 3.5–10.5 was explored at the concentration of 100 mg?L–1,and the corresponding results are shown in Fig. 9. When the pH was 3.5, slightly better adsorption effect was achieved,and boric acid was present mainly in the form of B(OH)3. Stable boric acid molecules and functional groups had good complexation, and the negatively charged adsorbents improved the electrostatic effect.However,B(OH)3had weak complexation ability as compared to that of B(OH)4–. Generally, under acidic conditions, the degree of protonation at the adsorption site was high.Due to this reason,the electrostatic effect was weakened,and the complexation effect was inhibited by H+. When the pH value was above 4.0, the adsorption capacity of boron adsorbents was very low.With the conversion of boron to B(OH)4–in the solution,the adsorption capacity increased gradually with the increase of pH within the range of 4.5 < pH < 6.5. The presence of B(OH)4–gradually increased. The functional groups of adsorbents and B(OH)4–formed a more stable complex,and the degree of protonation of surface functional groups was weakened. When the pH value was 6.5, the adsorption capacity reached the maximum value, and the complexation was stable, whereas the electrostatic effect was strengthened. With the further increase of pH, the negatively charged functional groups and boric acid anions generated electrostatic repulsion, which resulted in the decrease in adsorption capacity. Because boron mainly existed in the form of B(OH)4–, and the alkaline environment could neutralize the hydrogen ions produced in the complexation process (which was conducive to the complexation), the overall adsorption effect improved. These results indicated that the adsorbents had good adsorption capacity within a wide range of pH,and the adsorption effect was better in the neutral and alkaline environments.In order to consider energy-savings, environmental protection and low consumption of adsorbents, we chose not to add acid and alkali solution, and used the initial solution pH of 6.5 as the optimum pH condition for adsorption.

Fig. 5. EDS spectra of (a) Fe3O4@SiO2@MPS, (b) GMA-NMDG, (c) MGN and BMIIP (d) before and (e) after the adsorption process.

3.2.2. Influence of the adsorbent amount

The amount of adsorbents added had a great influence on the adsorption capacity. The amount of the adsorbent was also one of the important considerations for cost control. The removal rate E is calculated by Eq. (2).

where Ciand Ceare the initial concentration and the concentration at adsorption equilibrium, mg?L–1, respectively.

The influence of the adsorbent’s amount within the range of 0.2–3.0 g?L–1on the adsorption effect was explored,and the results are shown in Fig. 10. When the concentration is 100 mg?L–1, after 120 min, with the increase in adsorbent’s quantity, the unit adsorption capacity decreased, while the removal efficiency increased. Adding more adsorbent will provide more adsorption sites, thus improving the overall adsorption capacity. Meanwhile,the total amount of boron in a fixed volume of solution was certain.The adsorption site reached saturation, due to which, the unit adsorption capacity decreased. When the dosage lied within the range of 0.2–1.0 g?L–1, the unit adsorption capacity decreased significantly, while the rate of drop was slow when the dosage exceeded 1.0 g?L–1. However, the removal rate always presented an obvious upward trend with the increase of the amount of adsorbent. Based upon a comprehensive comparison, 1.0 g?L–1was selected as the optimal amount of adsorbent.

Fig. 6. XRD patterns of Fe3O4 NPs, Fe3O4@SiO2@MPS, MGN, BMIIP and GMA-NMDG.

Fig.7. Magnetization curves of Fe3O4@SiO2@MPS,MGN and BMIIP(the pictures in the insets show the rapid separation of MGN,and the comparison between BMIIP(left)and GMA-NMDG (right) under the action of magnet).

3.2.3. Adsorption kinetics

Fig. 11 shows changes in the adsorption capacities of GMANMDG, MGN and BMIIP over time. At the concentration of 100 mg?L–1, the adsorption capacity kept on increasing with time until the adsorption saturation was finally reached. As shown in Fig. 11(a), the adsorption capacity of GMA-NMDG showed little change in the initial time range, but increased significantly after 30 min,reaching saturation after 120 min.The process of complexation took some time. Compared with Fig.11(b)and (c),MGN and BMIIP maintained a good increasing trend of adsorption capacity over time, because they had uniform magnetic particles on their surfaces. The adsorption sites were evenly distributed, due to which, the mass transfer efficiency was better. The adsorption capacities of MGN and BMIIP increased rapidly at 30 min. BMIIP reached saturation at 60 min, while MGN approached saturation at 60 min and reached complete saturation at 120 min. Because BMIIP had abundant imprinted sites, the adsorption was faster.The adsorption sites on the surface of adsorbents gradually got exhausted over time, until the final adsorption equilibrium was reached. All three adsorbents showed good adsorption capacity for boron.

The kinetic equations, pseudo-first-order (Eq. (3)) and pseudosecond-order (Eq. (4)) were used to fit the experimental data of adsorption for the three sorbents [58,59].

where t is the adsorption time,min;qtis the adsorption capacity at time t,mg?g-1;qeis the adsorption capacity at equilibrium,mg?g-1;k1is the adsorption rate constant of pseudo-first-order equation,min-1; and k2is the adsorption rate constant of pseudo-secondorder equation, g?mg-1?min-1.

Fig.11 shows the kinetic fitting curve of boron adsorption using GMA-NMDG, MGN and BMIIP, whereas Table 2 presents the relevant fitting parameters. After fitting, the correlation coefficient(R2) of pseudo-second-order kinetic equation was found to be closer to unity than that of pseudo-first-order kinetic equation. The pseudo-second-order kinetic model could better describe the process of boron adsorption using the synthesized sorbents[60],indicating that the process of boron adsorption using GMA-NMDG,MGD and BMIIP mainly belonged to chemisorption. However, the R2of two models of GMA-NMDG and MGD were similar, and the gap between the theoretical and experimental values of pseudofirst-order was smaller, indicating that physical adsorption might be happening at the same time.

Fig. 8. N2 adsorption–desorption isotherms of (a) Fe3O4@SiO2@MPS, (b) MGN, (c) BMIIP and (d) GMA-NMDG.

Table 1 BET surface areas and pore parameters of Fe3O4@SiO2@MPS, MGN, BMIIP and GMANMDG

3.2.4. Adsorption isotherms

The adsorption isotherm experiments were conducted with the initial concentration of 50–600 mg?L–1, and the results are shown in Fig. 12. From 50 to 300 mg?L–1, the adsorption capacities of GMA-NMDG, MGN and BMIIP increased significantly with the increase of the initial concentration.Then,after 300 mg?L–1,it gradually leveled off,and got saturated for the value of 400 mg?L–1.The maximum saturated adsorption capacity of GMA-NMDG was 43.4 mg?g-1, whereas those of MGN and BMIIP were 32.5 and 28.3 mg?g-1, respectively. The high concentration of borate ions pushed them closer to the adsorption sites,so that most of the sites could be filled,thus increasing the adsorption capacity [61]. However, the imprinted polymer BMIIP was different from the traditional ion-imprinted polymers. The adsorption capacity of BMIIP was lower than those of the other two non-imprinted polymers.The selected NMDG monomer was complexed with boric acid through the structure of its hydroxyl functional group, which is how it achieved the adsorption effect.It already had the adsorption sites for boron on its own structure,and had a good matching ability for boron adsorption. Therefore, the polymer GMA-NMDG that was prepared directly retained most of the adsorption sites, while the proportion of functional monomers was relatively large per unit mass. By loading the functionalized Fe3O4NPs, the magnetic properties could be obtained for easy recovery.However,the grafting process would lose some of the sites,and the support occupies some of the mass, resulting in the decrease in unit adsorption capacity. On this basis, ion-imprinted technology was combined to strengthen the binding ability of monomer and target ion, so as to make the adsorption sites more matched. However, it would further deplete the adsorption sites, resulting in a decrease in adsorption capacity. The prepared GMA-NMDG, MGN and BMIIP still had satisfactory adsorption effect.Table 3 compares the effects of three kinds of adsorbents for boron adsorption with other reported adsorbents.

Fig. 9. Effects of pH on boron adsorption of adsorbents.

Fig. 10. Effects of the addition of adsorbent on the adsorption of boron.

In order to further analyze the process of boron adsorption,the experimental data were fitted using three common isotherm models[64],namely the Langmuir isotherm equation(Eq.(5)),the Freundlich isotherm equation (Eq. (6)) and the Temkin isotherm equation (Eq. (7)) [28]. The adsorption process that was suitable for Langmuir model usually belongs to chemical adsorption, and indicates that the adsorption process is taking place on the homogeneous surface. The Freundlich model is more suitable for physical adsorption, and its multilayer adsorption takes place at different types of adsorption sites on a non-homogeneous surface[65].Temkin isotherm model assumes that the enthalpy of adsorption decreases linearly with the increase of adsorption capacity[66].

where Ceis the concentration of adsorption equilibrium, mg?L–1; qeand qmare the equilibrium adsorption capacity and the theoretical adsorption capacity,respectively,mg?g-1;KLis the Langmuir model constant, L?mg-1; KFis the Freundlich model constant, mg?g-1; n is the binding constant of Freundlich model, bTeis the Temkin constant related to adsorption heat, aTeis the Temkin constant, L?g-1;R is the gas constant, 8.314 J?mol-1?K-1; and T is the thermodynamic temperature, K.

Fig. 11. Variations of the adsorption capacity of boron using (a) GMA-NMDG, (b) MGN and (c) BMIIP with time and their fitting curves.

Table 2 Fitting parameters of kinetic models for boron’s adsorption

Fig. 12. Effect of initial concentration on adsorption capacity of adsorbents.

Table 3 Comparison of boron adsorption using different adsorbents

Fig. 13 shows the isotherm model fitting diagram for GMANMDG, MGN and BMIIP, whereas the specific fitting parameters are presented in Table 4. By comparing the fitting results of the three isotherm models and the obtained correlation coefficients,the R2values of the Langmuir isotherm models of GMA-NMDG,MGN and BMIIP were all higher than those of the other two models, indicating that the Langmuir isotherm model was more suitable for describing the process of boron adsorption. Meanwhile,the excellent fitting of Langmuir isotherm model also indicated that the adsorption of boron consisted of chemical adsorption,which was consistent with the results of the kinetic model. These results proved that the adsorbents had superior adsorption surface and all adsorption sites had the same adsorption affinity [67,68].

3.2.5. Influence of temperature on adsorption properties

The adsorption performance at different temperatures was investigated to explore the applicability of adsorbents within different temperature ranges, so as to facilitate the application in the actual environment and meet the requirements of low energy consumption. In the initial concentration of 50–600 mg?L–1, the adsorption experiments were carried out at 288.15, 298.15 and 308.15 K. Calculate the entropy change (ΔS, kJ?mol-1?K-1),enthalpy change (ΔH, kJ?mol-1) and Gibbs free energy(ΔG, kJ?mol-1) using equations (Eqs. (8) and (9)):

where K is the distribution coefficient, L?g-1; R is the gas constant,J?mol-1?K-1; T is the absolute temperature, K.

The results are illustrated in Figs.14 and 15.The influence of the change in temperature on GMA-NMDG, MGN and BMIIP was slightly low. The complexation of NMDG and boron was an exothermic reaction. In terms of thermodynamics, the adsorption capacity increased with the increase of concentration and the decrease of temperature, whereas the increase of temperature would lead to the dissociation of B(OH)4–in the solution. Meanwhile, temperature affected the chemical equilibrium and ion exchange rates. In general, for the kinetics, increasing the temperature facilitated the mass transfer of reactions. In addition, with the increase in temperature, the viscosity of solution decreased and the diffusion rate of borate ions increased, which was more conducive to adsorption.

The adsorption thermodynamic parameters are obtained by fitting the slope and intercept of the straight line. As can be seen in Table 5,ΔG is greater than zero in the studied temperature range,indicating that the boron adsorption processes of GMA-NMDG,MGN and BMIIP are non-spontaneous. The calculated ΔH values are all negative, indicating that the boron adsorption process of the three materials is exothermic, a conclusion that is consistent with that obtained from the experimental measurements. In addition, the ΔS value is lower than zero because the combination of boron and adsorption sites on the materials forms a stable structure that makes the system less chaotic. Therefore, the adsorption of boron using the synthesized adsorbents was affected by many aspects. The degree of the influence of temperature indicated the possible existence of various types of adsorption.

Salt lakes are abundant in Qinghai and Tibet, China, which are generally located in areas of low temperature. Therefore, the performances of adsorbents at lower temperature (≤288.15 K), room temperature (298.15 K) and higher temperature (308.15 K) were considered.The results showed that GMA-NMDG,MGN and BMIIP showed satisfactory adsorption performance for boron within the normal temperature range. Moreover, the low temperature was conducive to the complex of boron, which was suitable for the development and utilization of salt lakes.The appropriate changes in temperature could not affect the performance of the adsorbents.For the consideration of energy consumption, or meeting the requirements of actual environment, room temperature can be used for adsorption using the synthesized adsorbents.

Fig. 13. Langmuir, Freundlich and Temkin isotherm model fitting diagrams of boron adsorption onto (a) GMA-NMDG, (b) MGN and (c) BMIIP.

Table 4 Fitting parameters of Langmuir, Freundlich and Temkin isotherm model for boron absorption using the synthesized adsorbents

3.2.6. Selectivity

The adsorption capacity and selectivity of adsorbents for boron in multi-ion solution were explored,and the influence of other ions on the adsorption capacity was investigated, as shown in Figs. 16 and 17. Various ions such as Na+, K+, Mg2+, S and Cl–not only widely exist in industrial wastewaters,butso have a high content in salt lakes. In the prepared multi-ion competitive solution, the adsorbents had almost no adsorption capacity for Cl–, whereaswhich had a similar structure to B(OH)was also affected only slightly, indicating that anions had almost no influence on the adsorption of boron. However, Na+, K+and Mg2+were also adsorbed. On one hand, some adsorption sites may have been occupied by cations, and formed complexes. However, the overall boron adsorption was still satisfactory, because the extra salt ions balanced the electrostatic and non-electrostatic forces,and had little influence on boron adsorption.Meanwhile,magnesium hydroxide was produced in the solution and would have certain adsorption capacity for boron [13]. On the other hand, because of the mesoporous structure of the adsorbents,the metal cations with smaller particle size were attached. Concurrently, there was electrostatic interaction between the negatively charged functional groups and the cations. Although the adsorption of boron was based on the hydroxyl structure of NMDG,the adsorption capacity of BMIIP was less affected and its selectivity for boron was higher.Because BMIIP had adsorption sites with memory effects that better matched with boron due to the preparation process of imprinting.

Based upon the results shown in Fig.17,the influence of cations on boron adsorption was investigated.Na+had a great influence on boron adsorption, while the adsorption capacity of boron was low under same conditions,which may be due to the influence of transformed NaOH in the aqueous solution. However, K+and Mg2+had little effect on the performance of boron adsorption. The adsorption capacity of boron was even slightly higher than that of pure boron solution. Therefore, the influence of metal ions can be ignored. According to the comprehensive comparison of GMANMDG, MGN and BMIIP, the GMA-NMDG had the highest adsorption capacity and could simultaneously act on boron and metal ions, due to which, joint adsorption and staged desorption could be considered to maximize the effect. BMIIP could be used to recover and purify boron due to its strong adsorption selectivity.

Fig. 14. Effect of temperature on boron absorption performance of adsorbents.

3.2.7. Reusability

Reusability is important in practical application and has great significance for cost control and resource saving. As shown in Fig. 18, multiple sorption-elution cycles were conducted for GMA-NMDG,MGN and BMIIP.Moreover,HCl and NaOH could generally be used as the eluents. In the present work, 0.05 mol?L–1NaOH was selected as the eluent.After the first cycle of adsorption and desorption, the adsorption capacity of GMA-NMDG decreased by nearly 50%,indicating that a large number of its adsorption sites were destroyed.For the second time,it decreased slightly,through still retained some adsorption sites.The decrease of MGN was significantly smaller than that of GMA-NMDG, because some functional groups had been lost in the loading process of polymerization. Moreover, MGN had good rigidity when grafted on the surface of the support. The adsorption capacity of BMIIP decreased after five cycles of adsorption,while the imprinted sites were damaged due to repeated elution.However,they still showed good stability and reusability [69]. Therefore, although imprinted polymerization lost part of the adsorption capacity, BMIIP had the capacity of function continuously. When considering the cost,the overall adsorption capacities of GMA-NMDG, MGN and BMIIP should be compared in many aspects before reaching a decision.

4. Conclusions

In this work, three adsorbents, GMA-NMDG, MGN and BMIIP,were synthesized for the efficient treatment of boron in water bodies.The adsorbents maintained satisfactory adsorption performance within a certain temperature range and adapted to various environments for practical application.Other ions had no negative effect on the adsorption of boron using the synthesized adsorbents.The optimum performances of GMA-NMDG,MGN and BMIIP were obtained in the initial neutral solution(pH of 6.5).Moreover,GMA-NMDG and MGN reached the maximum adsorption capacity at 120 min,whereas BMIIP reached adsorption saturation at 60 min.The synthesis of GMA-NMDG was simple,and the adsorption capacity was the highest. The maximum adsorption capacity of GMA-NMDG was found to be 43.4 mg?g-1,while those of MGN and BMIIP were 32.5 and 28.3 mg?g-1, respectively. It was suitable for co-adsorption,and graded desorption could be considered.MGN had high adsorption capacity,good comprehensive performance,and excellent magnetic properties,which were convenient for separation and recovery form water.BMIIP had better adsorption rate,selectivity and recyclabilitythan the other adsorbents.Throughthe optimization of synthesis conditions, the adsorption capacity of the traditional monomer NMDG polymer was increased, and the magnetism was given to facilitate rapid recovery.Combined with the ion imprinting technology, it showed higher boron adsorption selectivity in the presence of competitive ions.

Fig. 15. Plots of lnK vs 1/T for (a) GMA-NMDG, (b) MGN and (c) BMIIP.

Table 5 Thermodynamic parameters for boron adsorption

Fig. 16. Adsorption capacities of adsorbents for various ions.

Fig. 17. Effects of cations on boron adsorption capacity.

Fig. 18. Reusability of adsorbents.

By fitting the adsorption experimental data,the pseudo-secondorder kinetic model and the Langmuir isotherm model were more suitable for describing the adsorption process. They proved that boron adsorbed by adsorbents belonged to chemical adsorption.Benefiting from ion imprinting technology, BMIIP can achieve saturated adsorption faster than the other two materials and has good adsorption stability. And the thermodynamic data indicated that the three materials were exothermic. The experimental results showed that the adsorbents in this work had the good adsorption process for boron.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (22078157), the Natural Science Foundation of the Jiangsu Higher Education institutions of China(21KJB610011)and Postgraduate Research and Practice Innovation Program of Jiangsu Province (SJCX21_0468).