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Pyrocatechol-modified resins for boron recovery from water: Synthesis, adsorption and isotopic separation studies
Reactive and Functional Polymers 112 (2017) 1–8
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Pyrocatechol-modified resins for boron recovery from water: Synthesis, adsorption and isotopic separation studies Jiafei Lyu, Zhouliangzi Zeng, Nan Zhang, Hongxu Liu, Peng Bai, Xianghai Guo ⁎ Department of Pharmaceutical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China Key Laboratory of Systems Bioengineering, Ministry of Education, Tianjin University, Tianjin 300350, China
a r t i c l e
i n f o
Article history: Received 22 November 2016 Received in revised form 27 December 2016 Accepted 28 December 2016 Available online 02 January 2017 Keywords: Boron Pyrocatechol Resin Adsorption Isotopic separation
a b s t r a c t Two kinds of pyrocatechols, pyrocatechol (CL) and nitropyrocatechol (NCL), were chosen by the conductive value change (Δ) they cause in the boric acid-polyol solutions, and corresponding modified resins (CL-RESIN and NCLRESIN) were synthesized by new methodology for effective boron removal and isotopic separation. The optimized boron adsorption occurs at pH = 9.06 for CL-RESIN, and pH = 6.70 for NCL-RESIN, with the maximum adsorption capacity 0.7886 mmol·g−1 and 0.7931 mmol·g−1, which were comparable to commercial IRA 743. Boron adsorption on prepared resins was saturated within 12 h and can be well described by pseudo-secondorder kinetic model. Freundlich isotherm model fits well at low boron concentration while Langmuir isotherm model fits better at high concentration. Furthermore, the boron isotopic separation factors S on two prepared resins are 1.080 for CL-RESIN and 1.140 for NCL-RESIN, which are far higher than all previous results. Boron removal and isotopic separation capacities make it possible that three problems, boron removal, isotopic separation and boron reusability, can be addressed in single adsorption process. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Boron is an essential element as a micronutrient for plants, animals and human beings, but the range between deficiency and excess is very narrow. Östurk et al. [1] regards the ingestion of large amounts of boron can affect the central nervous system and the reproductive system in humans. Besides, boron is detrimental to some plants at high concentration [2]. Excess boron can not only reduce fruit yield, but also induce premature ripening and massive leaf damage. The recommended boron concentration in irrigation water is between 0.3 and 1 ppm according to different countries. For the health of human beings, the boron concentration in drinking water was limited below 2.4 ppm in 2011 by WHO, and the limits in some countries are set lower [3]. However, boron is widely distributed in seawater at around 5 ppm and in some groundwater of active volcanic and geothermal activities, where boron concentration may reach up to 119 ppm. Furthermore, wide applications of boron compounds in the industry such as glass production and detergent production continuously lead to an enrichment of boron in the wastewater, which has become one of the most widespread environmental problems in this century [4,5]. In the future, the increasing demand for fresh water is a pressing concern, and although Earth is composed mainly of water, only 1% of this is consumable fresh water ⁎ Corresponding author at: Department of Pharmaceutical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China. E-mail address: [email protected] (X. Guo).
http://dx.doi.org/10.1016/j.reactfunctpolym.2016.12.016 1381-5148/© 2016 Elsevier B.V. All rights reserved.
[6]. Therefore effective boron removal techniques for seawater desalination and treatment of wastewater and geothermal water are highly demanded to address the lack of fresh water [7]. Among many proposed methods of boron removal, adsorption [7–12] is the most important one because of its low cost and effective boron uptake capacity. The most popular boron adsorbents in the industry, boron-selective chelating resins, are designed by modifying resins with polyol groups which are capable to capture boron acid in water along with releasing a proton and increasing the conductive value (Δ) of the boric acid-polyol solution. Various polyols and the Δ they cause are summarized in Table S1. N-methyl-D-glucamine (Δ = 794 μs·cm−1) group has been employed to modify resin for commercial IRA 743 successfully with a boron removal capacity of 1.010 mmol·g−1. In this study, we chose two pyrocatechols, pyrocatechol (named as CL) and nitropyrocatechol (named as NCL) as functional groups, which have similar Δ (516 μs·cm−1 and 1163 μs·cm−1) with N-methyl-Dglucamine, for novel boron chelating resins and verifying whether the increase of conductive value (Δ) can be selective criteria for functional groups of boron adsorbents. Our previous research [13] has demonstrated that chelating resins bearing pyrocatechol can form complexes with boron at wider pH range. The work also proved that the regeneration process could be achieved by 10% AcOH solution at mild conditions, avoiding using corrosive strong hydrochloride acids. It is of great importance in industrial application except for low adsorption capacity (0.4200 mmol·L−1) which could be explained by that part of pyrocatechol groups might have been oxidized to yield quinines during
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Scheme 1. Synthetic method of the pyrocatechol functional groups 3 and 6.
Scheme 2. Synthesis of pyrocatechol-modified resins.
preparation process. In this wok, the hydroxyls of two functional groups were protected by benzyl groups which can bond the hydroxyls more tightly than the methoxymethyl group used in previous research [13]. On the other hand, boron has two stable isotopes 10B and 11B, of which 10B with a large attenuation cross section for thermal neutrons is irreplaceably used in nuclear power plant as neutron shielding and in radioactive waste disposal equipment, such as components for compact fuel storage racks and transportation baskets [14,15]. 10B also plays a critical role in treating cancers that cannot be controlled effectively by conventional means, as called boron neutron capture therapy (BNCT) [16]. Although 10B is highly demanded for the nuclear industry and medical treatment, the separation process is practically very difficult because of tiny differences between 10B and 11B. Some methods have been observed including exchange distillation [17–19], adsorption-based separation [20–22], thermal ionization mass spectrometry [23,24] and laser assisted retardation of condensation (SILARC) method [25], among which only isotopic exchange distillation has been successfully
applied in practical 10B production. Except for small separation factor around 1.03, disadvantages including instability of anisole-boron trifluoride complex, severe causticity to the equipment and insecurity as a result of the toxicity of boron trifluoride greatly impede the process. Furthermore, technologies like mass spectrometry and laser are too costly to be adopted in the industrial scale. A promising alternative for boron isotopes separation is adsorption-based process using a boron adsorbent, which is more efficient, easier to operate and less costly. And some adsorbents [21,26] have been packed in a chromatography column for boron isotopic separation (Table S2). The N-methyl-Dglucamine type resin has the up-to-date highest isotopic separation factor of 1.027, which is still far from satisfaction from the angle of industrial application. Moreover, research about the boron isotopic separation capacity on pyrocatechol-modified resin has yet been reported. In this contribution, three important novelties were presented. First, the increase of conductive value (Δ) [27] was chosen as criteria for selecting boron-specific chelating group. Two pyrocatechol groups (pyrocatechol and nitropyrocatechol) with similar Δ with N-methyl-Dglucamine are chosen to test it in this study. Second, based on our success of pyrocatechol functional group [13] to some extent, a new route was used to address the problem of lower boron adsorption capacity due to partial oxidation of chelating groups during synthetic process. Third, the boron isotopic separation capacities of two pyrocatecholmodified resins were further explored for the first time, which provided important insights on reusability of boron resource and 10B production techniques. 2. Materials and methods
Fig. 1. FT-IR spectra of the chloromethyl resin and the two pyrocatechol-modified resins.
Two protected pyrocatechol functional groups, pyrocatechol (CL) and nitropyrocatechol (NCL), were synthesized from 3,4dihydroxybenzaldehyde and 3,4-dihydroxy-5-nitrobenzaldehyde respectively by new methods shown in Scheme 1, and the detailed procedures were provided in the Supplementary material. The hydroxyls of
J. Lyu et al. / Reactive and Functional Polymers 112 (2017) 1–8
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Fig. 2. SEM surface morphology of resins. The chloromethyl resin (a–b); CL-RESIN (c–d); NCL-RESIN (e–f).
Table 1 Comparison between the pyrocatechol-modified resins prepared in this study and commercial IRA 743. Resin
Functional group
Particle size (μm)
Nitrogen content (%)
Cfunctional group (mmol·g−1)
Adsorption capacitya (mmol·g−1)
CL-RESIN NCL-RESIN IRA 743
Pyrocatechol Nitropyrocatechol N-Methyl-D-glucamine
722–855 710–845 550–700
2.51 4.86 1.60
1.793 1.736 1.143
0.7886 0.7931 1.010
a
Maximum adsorption capacity obtained from adsorption isotherms.
CL and NCL functional chains were protected by benzyl groups which can bond the aromatic hydroxyls more tightly during reaction than the methoxymethyl groups used in previous study. The pyrocatechols protected by benzyl were attached to the chloromethyl resins by nucleophilic substitution. After removal of protective groups by TFA, the modified resins were then characterized by elemental analysis, FT-IR and SEM. 2.1. Materials The starting resin is Merrifield resin (Tianjin Nankai Hecheng, crosslinking: 6.5%DVB, bead size: 20–50 mesh, substitution: 5.1 mmol·g−1). All the reagents used in this research are analytic grade products. 3,4dihydroxybenzaldehyde (Damas-beta, 99%), dry potassium carbonate (Yuanli, AR), potassium iodide (Yuanli, AR), benzyl chloride (Yuanli, AR), ethanol (Yuanli, AR), 3 Å molecular sieve powder (Kemiou), methylamine methanol solution (HWRK, 30 wt%), Sodium borohydride (HEOWNS), diatomite (Yuanli), 3,4-dihydroxy-5-nitrobenzaldehyde (TCI, 98%), tetrabutylammonium iodide (Yuanli, AR), titanium isopropoxide (TCI, AR) and trifluoroacetic acid (HEOWNS, AR) were used as pursed. N,N-Dimethylformamide (Macklin, AR) was dried with 3 Å molecular sieve overnight before using as solvent. Ultra-pure water was used throughout the study. All aqueous solutions prepared were stored in polyethylene/polypropylene containers in case of pollution of boron from glassware.
filtered out and washed with methanol. Then trifluoroacetic acid was used for removal of the benzyl group [28].
2.3. Characterization NMR spectra of the compounds in CDCl3 were obtained with a BRUKER spectrometer. Spectra of the resins were recorded using FT-IR (Thermo NICOLET 6700). Elemental analyzer (Elementar Vario MICRO cube) was used to determine concentration of the functional group. The surface morphology of the resins was determined by SEM (JEM2100F), and the samples were coated with a gold layer. ICP-OES (Perkin-elmer Optima 8000) was employed to determine the concentration of boron in the aqueous solution. ICP-MS (Thermo Electron Corporation, USA) was used to determine the boron isotope abundance. 11B NMR (AVANCE III, Bruker, 400 MHz) was employed to observe the chemical environment of boron in the boron aqueous solutions which was prepared by dissolving boric acid into D2O and adjusting pH with sodium hydroxide or hydrochloric acid. 11B MAS NMR (Infinityplus 300, Varian, 300 MHz) was performed to evaluate the chemical environment of boron in the exhausted materials.
2.2. Preparation of modified resins 2.2.1. Synthesis of two functional groups Benzyl chloride was used for protecting the aromatic hydroxyl. Then methylamine was induced into the structure by reductive amination (Scheme 1). The detailed synthetic procedure and 1H NMR spectra of two functional groups are described in Section 3 of Supplementary material. 2.2.2. Modification of chloromethyl resins The procedure was shown in Scheme 2 and the optimization of grafting method was summarized in Section 2 of Supplementary material. The chloromethyl resins (2.00 g), functional groups (compound 3 or compound 6) (20.4 mmol) and potassium carbonate (3.52 g, 25.5 mmol) were added to DMF (80 mL) in a round-bottom flask. The mixture was stirred for 15 h at 50 °C. The as-synthesized resins were
Fig. 3. Adsorption capacity of two modified resins at different pHs (initial boron concentration: 10 mmol·L−1, dosage: 10 g·L−1, temperature: 25 °C, adsorption time: 30 h).
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Scheme 3. Formation of boron complexes proposed by Van Duin.
Fig. 5. Pseudo-second-order kinetics of boron adsorption on two modified resins.
Scheme 4. Formation of the boron complex proposed by Bishop [20].
from 2.5 to 11.5. The pH was adjusted by adding 1 M sodium hydroxide aqueous solution or 1 M hydrochloric acid. Two pyrocatechol-modified resins were added to these boric acid solutions for boron removal respectively. Thirty hours later, the resins were filtered off and the filtrate was tested for the boron concentration. 2.6. Adsorption kinetics
2.4. Boron adsorption In boron adsorption studies, the as-synthesized pyrocatecholmodified resins were pretreated, soaped in deionized water, and then washed with 1 M hydrochloric acid and 1 M sodium hydroxide aqueous solution, respectively. In a typical adsorption experiment, a 100 mL plastic flask was added by the as-synthesized chelating resin at the dosage of 10 g·L−1, followed by boric acid solution with a known boron concentration. The mixture was placed in the water bath shaker at 140 rpm for 30 h. The chelating resins were filtered off with a syringe filter (aquo system, 0.22 μm) and the filtrate was tested for the boron concentration.
2.5. Effect of pH on boron uptake Boric acid appears in various structures with the change of pH in the aqueous solution. So pH has a huge impact on the adsorption capacity of adsorbents. To observe the effect, we conducted the boron adsorption experiments with a series of boric acid solutions of 0.01 M at pH range
A 100 mL plastic flask was added by 400 ± 0.1 mg pyrocatecholmodified resins and 40 ± 0.1 mL boric acid solution of 10 ± 0.1 mmol·L−1. The mixture was shaken up for 30 h at 25 °C. The supernatant was collected at different shaking time intervals. The collected samples were filtered with a syringe filter and the content of unabsorbed boric acid was analyzed with ICP-OES for a kinetic study. 2.7. Adsorption isotherms A series of aqueous solutions with a known concentration between 2 mmol·L−1 and 75 mmol·L−1 were prepared for the study of the adsorption isotherms. The as-synthesized chelating resins were dispersed to these boron solutions respectively with same dosage 10 g·L−1. The mixtures were placed in plastic flasks and shaken at 25 °C. Thirty hours later, the resins were filtered off and the boron concentrations of the remaining solutions were tested. 2.8. Boron isotopic separation A series of 10 mmol·L−1 boric acid aqueous solutions were prepared with pH range from 2.5 to 11.5. The as-synthesized chelating resins were dispersed to these boron solutions respectively with same dosage 10 g·L−1. The mixtures were placed in plastic flasks and shaken at 25 °C. Thirty hours later, the resins were filtered off and the boron concentration and boron isotopic abundance of the remaining solutions were tested with ICP-OES and ICP-MS. 2.9. Regeneration The regeneration capacity of prepared resins with four eluents (5% hydrochloric acid, 5% acetic acid, 10% acetic acid, 15% acetic acid) are Table 2 Kinetic parameters for boron adsorption on two modified resins.
Fig. 4. The effect of contact time on boron adsorption on two modified resins (initial boron concentration: 10 mmol·L−1, dosage: 10 g·L−1, temperature: 25 °C, adsorption time: 30 h, pH: 9.07).
Resin
Qe(exp) (mg·g−1)
Qe( mg·g−1)
K2 (g·mg−1·h−1)
R2
CL-RESIN NCL-RESIN
3.212 3.016
3.451 3.229
0.1529 0.1834
0.9989 0.9876
J. Lyu et al. / Reactive and Functional Polymers 112 (2017) 1–8
Fig. 6. Adsorption isotherms on two modified resins (dosage: 10 g·L−1, temperature: 25 °C, adsorption time: 30 h, pH: 9.07).
investigated respectively. The regeneration condition was that exhausted resins (0.1 g) were stirring in the eluent (10 mL) for twelve hours and every three hours the eluent was removed and fresh eluent was added into the exhausted resin. After elution, the regenerated resins were washed with water (3 × 10 mL) and methanol (3 × 10 mL), and dried under air. The regenerated resins were characterized with SEM to observe if the morphology changed during regeneration procedure. Meanwhile, the regenerated resins (0.1 g) were added into boron solution (10 mmol·L−1, 10 mL) for boron adsorption. After adsorption, the elution and adsorption procedures were performed for another two times to see the stability of prepared resins. 3. Results and discussion 3.1. Characterization of synthesized chelating resins Based on the FT-IR spectra (Fig. 1), the peaks (C\\Cl) of chloromethyl resin at 670 cm−1 and 1262 cm−1 disappeared, which suggested the successful modification. Meanwhile, both assynthesized resins represented strong O\\H s2tretching vibrations at 3400 cm−1, 1198 cm−1 and 1125 cm−1, which means the protective group (benzyl) has been successfully removed, and C\\N stretching vibrations at 1268 cm−1 which is related to tertiary amine in both resins. All the changes in the FT-IR spectra indicated the functional groups were attached to the chloromethyl resins successfully. The surface morphology of the chloromethyl resin and the two pyrocatechol-modified resins were observed by SEM, and the images were show in Fig. 2. The particle size of chloromethyl resin is 645–
Fig. 7. Fitting of boron adsorption isotherm on two modified resins by Langmuir model.
5
Fig. 8. Fitting of boron adsorption isotherm on two modified resins by Freundlich model.
722 μm. There are irregular macro pores on the surface the bead. After modification the particle size of both two pyrocatechol-modified resins increased because of swelling during synthesis, CL-RESIN at around 722–855 μm and NCL-RESIN at about 710–845 μm. No obvious change of macro pores was observed after chemical treatment. Besides, the surface of the chloromethyl resin was rather smooth, while some cracks were found on the surface of as-synthesized resins, as shown in Fig. 2c and e. We thought the treatment with TFA may be the reason for the damage of the surface structure. In this study, hydrogenolysis with Pd/C catalyst was first tried to remove the benzyl protecting group. However, it turned out that supported palladium catalyst was hard to remove protecting group inside resins so that the as-synthesized chelating resins exhibited very low adsorption capacity of boron (0.8500 mg·g−1 for CL-RESIN and 0.9700 mg·g−1 for NCL-RESIN). So trifluoroacetic acid was applied to deprotect hydroxyl groups, and the modified resins in this study are summarized in Table 1. The concentration of functional groups can be calculated according to the N% of the prepared resin which is obtained from elemental analysis. The equation is given as: For CL-RESIN C functional group ¼ N% 1000=14
ð1Þ
For NCL-RESIN C functional group ¼ N% 1000=28
ð2Þ
Fig. 9. Langmuir isotherm of boron adsorption on two modified resins at high concentrations.
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Fig. 10. Freundlich isotherm of boron adsorption on two modified resins at low concentrations.
Fig. 11. 11B NMR of boric acid aqueous solutions and exhausted chelating resins (CL-RESIN and IRA 743).
3.3. Kinetics for boron adsorption on two as-synthesized resins where Cfunctional group means the concentration of functional groups in the as-synthesized resins, mmol·g−1, N% is the nitrogen content of the as-synthesized resin, %. The results are listed in Table 1. 3.2. Effect of pH on boron adsorption Being a weak acid, the distribution of boric acid (H3BO3) and borate ion (B(OH)− 4 ) essentially depends on the pH of the medium. The form of borate ion dominates at higher pH, while the non-ionized boric acid dominates at lower pH [29]. It is meaningful to observe the effect of pH on boron adsorption of pyrocatechol-modified resins. Fig. 3 shows the results of boron uptake at pH range 2.50–11.50. For CL-RESIN, the diagram shows boron uptake capacity increased from pH 2.50 to 9.06 and achieved the maximum value at pH = 9.06, then further increase in pH leaded to decrease of adsorption capacity. For NCL-RESIN, the trend was same as the CL-RESIN, and the highest adsorption capacity appeared at pH = 6.70. Van Duin [30] suggested that boron complex of glycol can only be expected in the region where pH N 9.07 because the complex between non-ionized boric acid and glycol could not be demonstrated. In his research, an increase of pH from 11 has no effect on the concentration of borate, and the concentration of the complex reached a maximum. Then, they suggested the classic mechanism as shown in Scheme 3. In this study, adsorption occurred at pH = 2.5, while little borate ion existed at this condition according to Van Duin's research. Moreover, further increasing pH leaded to decrease of adsorption capacity, which is also different from the phenomenon observed by Van Duin. Bishop et al. [20] proposed that the formation of the complex most likely occurred via the reaction of boric acid rather than borate. They found that the concentration of the complex increased with increasing pH and reached a maximum, then further increase of pH leaded to decrease of adsorption capacity. The trend observed by Bishop is consistent with our results. So we think the mechanism proposed by Bishop can explain our results better as shown in Scheme 4. We can see it from Fig. 3 that the NCL-RESIN reached the maximum adsorption value at lower pH than the CL-RESIN. This fact can be explained by comparing the pKa of the CL and NCL functional groups. The NCL functional groups, with lower pKa value, can deprotonate more easily. Table 3 Langmuir and Freundlich isotherm constants of boron adsorption on modified resins. Resins
Langmuir constants
Freundlich constants
KA (L·mmol−1) Qm (mmol·g−1) R2
1/n
CL-RESIN 0.1054 NCL-RESIN 0.05340
0.7886 0.7931
KF
R2
0.9756 2.186 0.00460 0.9933 0.9783 1.535 0.01230 0.9914
In order to examine the adsorption mechanism, the time-dependent adsorption capacity was tested for the kinetics on two pyrocatecholmodified resins synthesized. Fig. 4 shows the boron adsorption capacity of two resins with respect to the adsorption time. The boron adsorption capacity of CL-RESIN increased gradually until about twelve hours, and then the adsorption process reached equilibrium. The boron adsorption capacity of NCL-RESIN increased rapidly in the first 5 h and reached equilibrium in thirteen hours. The pseudo-second-order model was used for the study of adsorption kinetics on the resins, which is given as t 1 1 ¼ þ t Q t K 2 Q 2e Q e
ð3Þ
where K2 (g·mg−1·h−1) is the rate constant of second-order adsorption. Qe (mg·g−1) and Qt (mg·g−1) are the amount of boron adsorbed at equilibrium and time t. The plot of t/Qt against t was shown in Fig. 5. The calculated value of K2 along with relevant correlation coefficients (R2) are shown in Table 2. According to the results, boron adsorption on prepared resins was a slow process which might be related to the low diffusion rate of boric acid in the heterogeneous surface of particles. The adsorption behavior of boron on two pyrocatechol-modified resins was well described by pseudo-second-order model, which indicated the adsorption of boron on modified resin was a chemical adsorption [13]. 3.4. Adsorption isotherms for boron adsorption on two as-synthesized resins The adsorption isotherms of two as-synthesized chelating resins with pyrocatechol functional groups were obtained from boron adsorption experiments at different initial boron concentrations. The uptake of boron was studied in the concentration range from 2 mmol·L− 1 to 75 mmol·L−1 while the dosage of the sorbent in each solution was held constant at 10 g·L−1. The plot on the adsorption capacity of resins (Qe) against the residual boron concentration in solution phase (Ce) was shown in Fig. 6. The boron adsorption capacity (Qe) increased with the increasing initial concentration gradually. The adsorption capacity achieved the maximum at equilibrium concentration of 48 mmol·L−1 for CL-RESIN and 72 mmol·L−1 for NCL-RESIN. As we all know, the relationship between the adsorption capacity of sorbents (Qe) and the residual boron concentration in solution phase (Ce) can be described by various isotherm models. In this study the Langmuir and Freundlich adsorption isotherms were applied for
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Scheme 5. Structures of boron complexes.
a part in the adsorption process. The larger adsorption capacity than Wang's result can be attributed to the better protective group, benzyl, which keeps complexing phenolic hydroxyl groups by avoiding the oxidation of phenols during preparation. 3.5. Adsorption mechanism
Fig. 12. Effect of pH on boron isotopic separation factor.
explaining the results. Detailed equations and simulation methods were attached in the Supplementary material. For Langmuir model, Ce/Qe was plotted against Ce as shown in Fig. 7, and for Freundlich model the plot of logQe against logCe was shown in Fig. 8. Apparently the Langmuir model cannot explain the adsorption of boron at low concentrations (0–5.4 mmol·L−1), while it did well at higher concentrations (5.4–75 mmol·L−1) as shown in Fig. 9. For Freundlich model, it fit well at low concentrations as shown in Fig. 10, but failed to work at higher concentrations. The parameters of the Langmuir model and Freundlich model were determined from the intercepts and slopes of the straight lines as presented in Table 3. The parameter KA reflects the adsorption energy. We can see it from Table 3 that the KA value of CL-RESIN was higher than that of NCL-RESIN, which indicated that the adsorption isotherm of CL-RESIN increased more rapidly than that of NCL-RESIN at concentration range 5.4–30 mmol·L− 1 as shown in Fig. 6. According to the Langmuir model, the maximum adsorption capacity was determined to 0.7886 mmol·g− 1 for CL-RESIN and 0.7931 mmol·g− 1 for NCLRESIN, which were much larger than results of Wang [13] (0.4200 mmol·g−1) and came close to the adsorption capacity of commercial IRA 743 (1.010 mmol·g−1). About 45.9% functional groups play
To explore the boron adsorption mechanism of as-synthesized chelating resins, 11B NMR was performed to observe the chemical environment of boron in the aqueous solutions and in the exhausted resins (Fig. 11). In agreement of the results of previous researches [20], boric acid appears in various structures with the change of pH in the aqueous solution, B(OH)− 4 of 1.64 ppm at alkaline condition and B(OH)3 of 19.4 ppm at acid condition. Two peaks show in the 11B MAS NMR of exhausted IRA 743, of which, the chemical shift at 5.4 ppm can be assigned to the monochelate B(OH)2L− (Scheme 5A) and the chemical shift at 9.8 ppm can be assigned to the bichelate BL− 2 (Scheme 5B). Comparing the patterns of exhausted CL-RESIN and IRA 743, the chemical shift at 7.2 ppm and 13.8 ppm can be assigned to monochelate B(OH)2L− and bichelate BL− 2 , respectively. This result also can be proved by previous research [20]. The chemical shift at 10.0 ppm suggests a third complex in the exhausted chelating resins, which can be attributed to the complex B(OH)L (Scheme 5C). 3.6. Boron isotopic separation factor The isotopic separation factor can be calculated according to the boron concentration and isotopic abundance of initial boron solution and residual solution. The formula is shown as following: S
10
B=11 B ¼ 10 B=11 B resin = 10 B=11 B solution c0 α 0 ð1 þ α 1 Þ−c1 α 1 ð1 þ α 0 Þ ¼ c0 α 1 ð1 þ α 1 Þ−c1 α 1 ð1 þ α 0 Þ
ð4Þ
where c0 represents initial boron concentration, mol·L−1; c1 represents residual boron concentration, mol·L−1; α0 represents initial 10B/11B abundance, 0.24779; α1 represents 10B/11B abundance of the residual solution. The effect of pH on boron isotopic separation was studied which is shown in Fig. 12. Different from conventional boron isotopic separation
Fig. 13. Recycling of prepared resins for boron adsorption.
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techniques [31], pyrocatechol-modified resins exhibit stronger selectivity for 11B than 10B. The separation factor increases gradually with increasing pH, reaches to a maximum value S = 1.080 at pH = 6.70 for CL-RESIN and S = 1.140 at pH = 7.37 for NCL-RESIN, then decreases with further increasing pH. We also observed the boron isotopic separation factor of commercial IRA 743 under the same condition, which had a performance of 1.0046. The separation factors of two pyrocatecholmodified resins are much higher than commercial IRA 743 [31] which is, to the best of our knowledge, the best reported separation factors. In practical applications, high separation factor can efficiently decrease the length of the column and reduce the cost apparently, which makes them promising chromatographic solid phase for boron isotopic separation and provides important insights on reusability of boron resource. 3.7. Regeneration To explore the regeneration capacity and stability of prepared resins, 5% hydrochloride acid and acetic acid of different concentrations (5%, 10%, 15%) were used for the elution of exhausted resins. The regeneration results were summarized in the Fig. 13. It can be found that all of four eluents could regenerate most parts of prepared resins, of which 5% hydrochloride acid and 15% acetic acid were slightly better than 5% acetic acid and 10% acetic acid. Regenerated for four times, boron adsorption capacity on two prepared resins showed a tiny decrease, which demonstrated the stability of prepared resins. 4. Conclusions In this study, two aromatic hydroxyls (pyrocatechol and nitropyrocatechol) were selected based on the conductive value change (Δ) they caused for the first time to modify chloromethyl resin. New synthetic methodology with benzyl involved in replacement of methoxymethyl group avoided oxidation of hydroxyls. Boron removal properties on two as-synthesized resins were further evaluated including the maximum capacity, optimum pH, adsorption kinetics, isotherms and mechanism. Boron adsorption capacity and easy regeneration under mild acidic condition [13] make the two pyrocatechol-modified resins promising boron specific adsorbents. Meanwhile, they exhibit the highest reported boron separation factors so far. Great boron removal and isotopic separation capacities make it possible that three problems, boron removal, isotopic separation and boron reusability, can be addressed in single adsorption process. Acknowledgments Research supported by the National Natural Science Foundation of China (No. 21202116), Independent Innovation Foundation of Tianjin University (2016XZC-0071) and Natural Science Foundation of Tianjin (16JCYBJC20300). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.reactfunctpolym.2016.12.016. References [1] D.K.N. Öztürk, T. Ennil Köse, Boron removal from aqueous solution by reverse osmosis, Desalination 223 (1–3) (2008) 1–9. [2] N. Geffen, R. Semiat, M.S. Eisen, Y. Balazs, I. Katz, C.G. Dosoretz, Boron removal from water by complexation to polyol compounds, J. Membr. Sci. 286 (1–2) (2006) 45–51.
[3] E. Güler, C. Kaya, N. Kabay, M. Arda, Boron removal from seawater: state-of-the-art review, Desalination 356 (2015) 85–93. [4] G. Zelmanov, R. Semiat, Boron removal from water and its recovery using iron (Fe + 3) oxide/hydroxide-based nanoparticles (NanoFe) and NanoFeimpregnated granular activated carbon as adsorbent, Desalination 333 (1) (2014) 107–117. [5] H. Demey, T. Vincent, M. Ruiz, A.M. Sastre, E. Guibal, Development of a new chitosan/Ni(OH)2-based sorbent for boron removal, Chem. Eng. J. 244 (2014) 576–586. [6] H. Demey, T. Vincent, M. Ruiz, M. Nogueras, A.M. Sastre, E. Guibal, Boron recovery from seawater with a new low-cost adsorbent material, Chem. Eng. J. 254 (2014) 463–471. [7] Z. Guan, J. Lv, P. Bai, X. Guo, Boron removal from aqueous solutions by adsorption — a review, Desalination 383 (2016) 29–37. [8] M. Gazi, N. Bicak, Selective boron extraction by polymer supported 2hydroxylethylamino propylene glycol functions, React. Funct. Polym. 67 (10) (2007) 936–942. 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Vujic, Development of a sealed-accelerator-tube neutron generator, Appl. Radiat. Isot. 53 (2000) 801–809. [15] M. Zawisky, M. Bastürk, R. Derntl, F. Dubus, E. Lehmann, P. Vontobel, Nondestructive 10B analysis in neutron transmission experiments, Appl. Radiat. Isot. 61 (4) (2004) 517–523. [16] W.S. Kiger, M.R. Palmer, K.J. Riley, R.G. Zamenhof, P.M. Busse, A pharmacokinetic model for the concentration of 10B in blood after boronophenylalanine-fructose administration in humans, Radiat. Res. 155 (4) (2001) 611–618. [17] F. Wei, W. Zhang, M. Han, L. Han, X. Zhang, S. Zhang, Operational policy of the boron isotopes separation by chemical exchange reaction and distillation, Chem. Eng. Process. 47 (1) (2008) 17–21. [18] T. Lin, W. Zhang, L. Wang, Theoretical calculation of separation factors for boron isotopic exchange between BF3 and BF3 center dot C6H5OCH3, J. Phys. Chem. A 113 (26) (2009) 7267–7274. [19] M. Abdollahi, S.J. Ahmadi, Application of ideal temperature gradient technology to optimize the chemical exchange and distillation process of boron isotopes separation by (CH3)(2)O-BF3 complex, Chem. Eng. Process. 76 (2014) 26–32. [20] M. Bishop, N. Shahid, J. Yang, A.R. Barron, Determination of the mode and efficacy of the cross-linking of guar by borate using MAS 11B NMR of borate cross-linked guar in combination with solution 11B NMR of model systems, Dalton Trans. 17 (2004) 2621–2634. [21] A. Sonoda, Y. Makita, T. Hirotsu, Boron isotope fractionation in column chromatography with glucamine type resins, J. Nucl. Sci. Technol. 43 (4) (2006) 437–440. [22] T. Oi, H. Shimazaki, R. Ishii, M. Hosoe, Boron isotope fractionation in liquid chromatography with boron-specific resins as column packing material, Sep. Sci. Technol. 32 (11) (1997) 1821–1834. [23] Q. Xu, Y. Dong, H. Zhu, A. Sun, Separation and analysis of boron isotope in high plant by thermal ionization mass spectrometry, Int. J. Anal. Chem. 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Reactive and Functional Polymers journal homepage: www.elsevier.com/locate/react
Pyrocatechol-modified resins for boron recovery from water: Synthesis, adsorption and isotopic separation studies Jiafei Lyu, Zhouliangzi Zeng, Nan Zhang, Hongxu Liu, Peng Bai, Xianghai Guo ⁎ Department of Pharmaceutical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China Key Laboratory of Systems Bioengineering, Ministry of Education, Tianjin University, Tianjin 300350, China
a r t i c l e
i n f o
Article history: Received 22 November 2016 Received in revised form 27 December 2016 Accepted 28 December 2016 Available online 02 January 2017 Keywords: Boron Pyrocatechol Resin Adsorption Isotopic separation
a b s t r a c t Two kinds of pyrocatechols, pyrocatechol (CL) and nitropyrocatechol (NCL), were chosen by the conductive value change (Δ) they cause in the boric acid-polyol solutions, and corresponding modified resins (CL-RESIN and NCLRESIN) were synthesized by new methodology for effective boron removal and isotopic separation. The optimized boron adsorption occurs at pH = 9.06 for CL-RESIN, and pH = 6.70 for NCL-RESIN, with the maximum adsorption capacity 0.7886 mmol·g−1 and 0.7931 mmol·g−1, which were comparable to commercial IRA 743. Boron adsorption on prepared resins was saturated within 12 h and can be well described by pseudo-secondorder kinetic model. Freundlich isotherm model fits well at low boron concentration while Langmuir isotherm model fits better at high concentration. Furthermore, the boron isotopic separation factors S on two prepared resins are 1.080 for CL-RESIN and 1.140 for NCL-RESIN, which are far higher than all previous results. Boron removal and isotopic separation capacities make it possible that three problems, boron removal, isotopic separation and boron reusability, can be addressed in single adsorption process. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Boron is an essential element as a micronutrient for plants, animals and human beings, but the range between deficiency and excess is very narrow. Östurk et al. [1] regards the ingestion of large amounts of boron can affect the central nervous system and the reproductive system in humans. Besides, boron is detrimental to some plants at high concentration [2]. Excess boron can not only reduce fruit yield, but also induce premature ripening and massive leaf damage. The recommended boron concentration in irrigation water is between 0.3 and 1 ppm according to different countries. For the health of human beings, the boron concentration in drinking water was limited below 2.4 ppm in 2011 by WHO, and the limits in some countries are set lower [3]. However, boron is widely distributed in seawater at around 5 ppm and in some groundwater of active volcanic and geothermal activities, where boron concentration may reach up to 119 ppm. Furthermore, wide applications of boron compounds in the industry such as glass production and detergent production continuously lead to an enrichment of boron in the wastewater, which has become one of the most widespread environmental problems in this century [4,5]. In the future, the increasing demand for fresh water is a pressing concern, and although Earth is composed mainly of water, only 1% of this is consumable fresh water ⁎ Corresponding author at: Department of Pharmaceutical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China. E-mail address: [email protected] (X. Guo).
http://dx.doi.org/10.1016/j.reactfunctpolym.2016.12.016 1381-5148/© 2016 Elsevier B.V. All rights reserved.
[6]. Therefore effective boron removal techniques for seawater desalination and treatment of wastewater and geothermal water are highly demanded to address the lack of fresh water [7]. Among many proposed methods of boron removal, adsorption [7–12] is the most important one because of its low cost and effective boron uptake capacity. The most popular boron adsorbents in the industry, boron-selective chelating resins, are designed by modifying resins with polyol groups which are capable to capture boron acid in water along with releasing a proton and increasing the conductive value (Δ) of the boric acid-polyol solution. Various polyols and the Δ they cause are summarized in Table S1. N-methyl-D-glucamine (Δ = 794 μs·cm−1) group has been employed to modify resin for commercial IRA 743 successfully with a boron removal capacity of 1.010 mmol·g−1. In this study, we chose two pyrocatechols, pyrocatechol (named as CL) and nitropyrocatechol (named as NCL) as functional groups, which have similar Δ (516 μs·cm−1 and 1163 μs·cm−1) with N-methyl-Dglucamine, for novel boron chelating resins and verifying whether the increase of conductive value (Δ) can be selective criteria for functional groups of boron adsorbents. Our previous research [13] has demonstrated that chelating resins bearing pyrocatechol can form complexes with boron at wider pH range. The work also proved that the regeneration process could be achieved by 10% AcOH solution at mild conditions, avoiding using corrosive strong hydrochloride acids. It is of great importance in industrial application except for low adsorption capacity (0.4200 mmol·L−1) which could be explained by that part of pyrocatechol groups might have been oxidized to yield quinines during
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Scheme 1. Synthetic method of the pyrocatechol functional groups 3 and 6.
Scheme 2. Synthesis of pyrocatechol-modified resins.
preparation process. In this wok, the hydroxyls of two functional groups were protected by benzyl groups which can bond the hydroxyls more tightly than the methoxymethyl group used in previous research [13]. On the other hand, boron has two stable isotopes 10B and 11B, of which 10B with a large attenuation cross section for thermal neutrons is irreplaceably used in nuclear power plant as neutron shielding and in radioactive waste disposal equipment, such as components for compact fuel storage racks and transportation baskets [14,15]. 10B also plays a critical role in treating cancers that cannot be controlled effectively by conventional means, as called boron neutron capture therapy (BNCT) [16]. Although 10B is highly demanded for the nuclear industry and medical treatment, the separation process is practically very difficult because of tiny differences between 10B and 11B. Some methods have been observed including exchange distillation [17–19], adsorption-based separation [20–22], thermal ionization mass spectrometry [23,24] and laser assisted retardation of condensation (SILARC) method [25], among which only isotopic exchange distillation has been successfully
applied in practical 10B production. Except for small separation factor around 1.03, disadvantages including instability of anisole-boron trifluoride complex, severe causticity to the equipment and insecurity as a result of the toxicity of boron trifluoride greatly impede the process. Furthermore, technologies like mass spectrometry and laser are too costly to be adopted in the industrial scale. A promising alternative for boron isotopes separation is adsorption-based process using a boron adsorbent, which is more efficient, easier to operate and less costly. And some adsorbents [21,26] have been packed in a chromatography column for boron isotopic separation (Table S2). The N-methyl-Dglucamine type resin has the up-to-date highest isotopic separation factor of 1.027, which is still far from satisfaction from the angle of industrial application. Moreover, research about the boron isotopic separation capacity on pyrocatechol-modified resin has yet been reported. In this contribution, three important novelties were presented. First, the increase of conductive value (Δ) [27] was chosen as criteria for selecting boron-specific chelating group. Two pyrocatechol groups (pyrocatechol and nitropyrocatechol) with similar Δ with N-methyl-Dglucamine are chosen to test it in this study. Second, based on our success of pyrocatechol functional group [13] to some extent, a new route was used to address the problem of lower boron adsorption capacity due to partial oxidation of chelating groups during synthetic process. Third, the boron isotopic separation capacities of two pyrocatecholmodified resins were further explored for the first time, which provided important insights on reusability of boron resource and 10B production techniques. 2. Materials and methods
Fig. 1. FT-IR spectra of the chloromethyl resin and the two pyrocatechol-modified resins.
Two protected pyrocatechol functional groups, pyrocatechol (CL) and nitropyrocatechol (NCL), were synthesized from 3,4dihydroxybenzaldehyde and 3,4-dihydroxy-5-nitrobenzaldehyde respectively by new methods shown in Scheme 1, and the detailed procedures were provided in the Supplementary material. The hydroxyls of
J. Lyu et al. / Reactive and Functional Polymers 112 (2017) 1–8
3
Fig. 2. SEM surface morphology of resins. The chloromethyl resin (a–b); CL-RESIN (c–d); NCL-RESIN (e–f).
Table 1 Comparison between the pyrocatechol-modified resins prepared in this study and commercial IRA 743. Resin
Functional group
Particle size (μm)
Nitrogen content (%)
Cfunctional group (mmol·g−1)
Adsorption capacitya (mmol·g−1)
CL-RESIN NCL-RESIN IRA 743
Pyrocatechol Nitropyrocatechol N-Methyl-D-glucamine
722–855 710–845 550–700
2.51 4.86 1.60
1.793 1.736 1.143
0.7886 0.7931 1.010
a
Maximum adsorption capacity obtained from adsorption isotherms.
CL and NCL functional chains were protected by benzyl groups which can bond the aromatic hydroxyls more tightly during reaction than the methoxymethyl groups used in previous study. The pyrocatechols protected by benzyl were attached to the chloromethyl resins by nucleophilic substitution. After removal of protective groups by TFA, the modified resins were then characterized by elemental analysis, FT-IR and SEM. 2.1. Materials The starting resin is Merrifield resin (Tianjin Nankai Hecheng, crosslinking: 6.5%DVB, bead size: 20–50 mesh, substitution: 5.1 mmol·g−1). All the reagents used in this research are analytic grade products. 3,4dihydroxybenzaldehyde (Damas-beta, 99%), dry potassium carbonate (Yuanli, AR), potassium iodide (Yuanli, AR), benzyl chloride (Yuanli, AR), ethanol (Yuanli, AR), 3 Å molecular sieve powder (Kemiou), methylamine methanol solution (HWRK, 30 wt%), Sodium borohydride (HEOWNS), diatomite (Yuanli), 3,4-dihydroxy-5-nitrobenzaldehyde (TCI, 98%), tetrabutylammonium iodide (Yuanli, AR), titanium isopropoxide (TCI, AR) and trifluoroacetic acid (HEOWNS, AR) were used as pursed. N,N-Dimethylformamide (Macklin, AR) was dried with 3 Å molecular sieve overnight before using as solvent. Ultra-pure water was used throughout the study. All aqueous solutions prepared were stored in polyethylene/polypropylene containers in case of pollution of boron from glassware.
filtered out and washed with methanol. Then trifluoroacetic acid was used for removal of the benzyl group [28].
2.3. Characterization NMR spectra of the compounds in CDCl3 were obtained with a BRUKER spectrometer. Spectra of the resins were recorded using FT-IR (Thermo NICOLET 6700). Elemental analyzer (Elementar Vario MICRO cube) was used to determine concentration of the functional group. The surface morphology of the resins was determined by SEM (JEM2100F), and the samples were coated with a gold layer. ICP-OES (Perkin-elmer Optima 8000) was employed to determine the concentration of boron in the aqueous solution. ICP-MS (Thermo Electron Corporation, USA) was used to determine the boron isotope abundance. 11B NMR (AVANCE III, Bruker, 400 MHz) was employed to observe the chemical environment of boron in the boron aqueous solutions which was prepared by dissolving boric acid into D2O and adjusting pH with sodium hydroxide or hydrochloric acid. 11B MAS NMR (Infinityplus 300, Varian, 300 MHz) was performed to evaluate the chemical environment of boron in the exhausted materials.
2.2. Preparation of modified resins 2.2.1. Synthesis of two functional groups Benzyl chloride was used for protecting the aromatic hydroxyl. Then methylamine was induced into the structure by reductive amination (Scheme 1). The detailed synthetic procedure and 1H NMR spectra of two functional groups are described in Section 3 of Supplementary material. 2.2.2. Modification of chloromethyl resins The procedure was shown in Scheme 2 and the optimization of grafting method was summarized in Section 2 of Supplementary material. The chloromethyl resins (2.00 g), functional groups (compound 3 or compound 6) (20.4 mmol) and potassium carbonate (3.52 g, 25.5 mmol) were added to DMF (80 mL) in a round-bottom flask. The mixture was stirred for 15 h at 50 °C. The as-synthesized resins were
Fig. 3. Adsorption capacity of two modified resins at different pHs (initial boron concentration: 10 mmol·L−1, dosage: 10 g·L−1, temperature: 25 °C, adsorption time: 30 h).
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Scheme 3. Formation of boron complexes proposed by Van Duin.
Fig. 5. Pseudo-second-order kinetics of boron adsorption on two modified resins.
Scheme 4. Formation of the boron complex proposed by Bishop [20].
from 2.5 to 11.5. The pH was adjusted by adding 1 M sodium hydroxide aqueous solution or 1 M hydrochloric acid. Two pyrocatechol-modified resins were added to these boric acid solutions for boron removal respectively. Thirty hours later, the resins were filtered off and the filtrate was tested for the boron concentration. 2.6. Adsorption kinetics
2.4. Boron adsorption In boron adsorption studies, the as-synthesized pyrocatecholmodified resins were pretreated, soaped in deionized water, and then washed with 1 M hydrochloric acid and 1 M sodium hydroxide aqueous solution, respectively. In a typical adsorption experiment, a 100 mL plastic flask was added by the as-synthesized chelating resin at the dosage of 10 g·L−1, followed by boric acid solution with a known boron concentration. The mixture was placed in the water bath shaker at 140 rpm for 30 h. The chelating resins were filtered off with a syringe filter (aquo system, 0.22 μm) and the filtrate was tested for the boron concentration.
2.5. Effect of pH on boron uptake Boric acid appears in various structures with the change of pH in the aqueous solution. So pH has a huge impact on the adsorption capacity of adsorbents. To observe the effect, we conducted the boron adsorption experiments with a series of boric acid solutions of 0.01 M at pH range
A 100 mL plastic flask was added by 400 ± 0.1 mg pyrocatecholmodified resins and 40 ± 0.1 mL boric acid solution of 10 ± 0.1 mmol·L−1. The mixture was shaken up for 30 h at 25 °C. The supernatant was collected at different shaking time intervals. The collected samples were filtered with a syringe filter and the content of unabsorbed boric acid was analyzed with ICP-OES for a kinetic study. 2.7. Adsorption isotherms A series of aqueous solutions with a known concentration between 2 mmol·L−1 and 75 mmol·L−1 were prepared for the study of the adsorption isotherms. The as-synthesized chelating resins were dispersed to these boron solutions respectively with same dosage 10 g·L−1. The mixtures were placed in plastic flasks and shaken at 25 °C. Thirty hours later, the resins were filtered off and the boron concentrations of the remaining solutions were tested. 2.8. Boron isotopic separation A series of 10 mmol·L−1 boric acid aqueous solutions were prepared with pH range from 2.5 to 11.5. The as-synthesized chelating resins were dispersed to these boron solutions respectively with same dosage 10 g·L−1. The mixtures were placed in plastic flasks and shaken at 25 °C. Thirty hours later, the resins were filtered off and the boron concentration and boron isotopic abundance of the remaining solutions were tested with ICP-OES and ICP-MS. 2.9. Regeneration The regeneration capacity of prepared resins with four eluents (5% hydrochloric acid, 5% acetic acid, 10% acetic acid, 15% acetic acid) are Table 2 Kinetic parameters for boron adsorption on two modified resins.
Fig. 4. The effect of contact time on boron adsorption on two modified resins (initial boron concentration: 10 mmol·L−1, dosage: 10 g·L−1, temperature: 25 °C, adsorption time: 30 h, pH: 9.07).
Resin
Qe(exp) (mg·g−1)
Qe( mg·g−1)
K2 (g·mg−1·h−1)
R2
CL-RESIN NCL-RESIN
3.212 3.016
3.451 3.229
0.1529 0.1834
0.9989 0.9876
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Fig. 6. Adsorption isotherms on two modified resins (dosage: 10 g·L−1, temperature: 25 °C, adsorption time: 30 h, pH: 9.07).
investigated respectively. The regeneration condition was that exhausted resins (0.1 g) were stirring in the eluent (10 mL) for twelve hours and every three hours the eluent was removed and fresh eluent was added into the exhausted resin. After elution, the regenerated resins were washed with water (3 × 10 mL) and methanol (3 × 10 mL), and dried under air. The regenerated resins were characterized with SEM to observe if the morphology changed during regeneration procedure. Meanwhile, the regenerated resins (0.1 g) were added into boron solution (10 mmol·L−1, 10 mL) for boron adsorption. After adsorption, the elution and adsorption procedures were performed for another two times to see the stability of prepared resins. 3. Results and discussion 3.1. Characterization of synthesized chelating resins Based on the FT-IR spectra (Fig. 1), the peaks (C\\Cl) of chloromethyl resin at 670 cm−1 and 1262 cm−1 disappeared, which suggested the successful modification. Meanwhile, both assynthesized resins represented strong O\\H s2tretching vibrations at 3400 cm−1, 1198 cm−1 and 1125 cm−1, which means the protective group (benzyl) has been successfully removed, and C\\N stretching vibrations at 1268 cm−1 which is related to tertiary amine in both resins. All the changes in the FT-IR spectra indicated the functional groups were attached to the chloromethyl resins successfully. The surface morphology of the chloromethyl resin and the two pyrocatechol-modified resins were observed by SEM, and the images were show in Fig. 2. The particle size of chloromethyl resin is 645–
Fig. 7. Fitting of boron adsorption isotherm on two modified resins by Langmuir model.
5
Fig. 8. Fitting of boron adsorption isotherm on two modified resins by Freundlich model.
722 μm. There are irregular macro pores on the surface the bead. After modification the particle size of both two pyrocatechol-modified resins increased because of swelling during synthesis, CL-RESIN at around 722–855 μm and NCL-RESIN at about 710–845 μm. No obvious change of macro pores was observed after chemical treatment. Besides, the surface of the chloromethyl resin was rather smooth, while some cracks were found on the surface of as-synthesized resins, as shown in Fig. 2c and e. We thought the treatment with TFA may be the reason for the damage of the surface structure. In this study, hydrogenolysis with Pd/C catalyst was first tried to remove the benzyl protecting group. However, it turned out that supported palladium catalyst was hard to remove protecting group inside resins so that the as-synthesized chelating resins exhibited very low adsorption capacity of boron (0.8500 mg·g−1 for CL-RESIN and 0.9700 mg·g−1 for NCL-RESIN). So trifluoroacetic acid was applied to deprotect hydroxyl groups, and the modified resins in this study are summarized in Table 1. The concentration of functional groups can be calculated according to the N% of the prepared resin which is obtained from elemental analysis. The equation is given as: For CL-RESIN C functional group ¼ N% 1000=14
ð1Þ
For NCL-RESIN C functional group ¼ N% 1000=28
ð2Þ
Fig. 9. Langmuir isotherm of boron adsorption on two modified resins at high concentrations.
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Fig. 10. Freundlich isotherm of boron adsorption on two modified resins at low concentrations.
Fig. 11. 11B NMR of boric acid aqueous solutions and exhausted chelating resins (CL-RESIN and IRA 743).
3.3. Kinetics for boron adsorption on two as-synthesized resins where Cfunctional group means the concentration of functional groups in the as-synthesized resins, mmol·g−1, N% is the nitrogen content of the as-synthesized resin, %. The results are listed in Table 1. 3.2. Effect of pH on boron adsorption Being a weak acid, the distribution of boric acid (H3BO3) and borate ion (B(OH)− 4 ) essentially depends on the pH of the medium. The form of borate ion dominates at higher pH, while the non-ionized boric acid dominates at lower pH [29]. It is meaningful to observe the effect of pH on boron adsorption of pyrocatechol-modified resins. Fig. 3 shows the results of boron uptake at pH range 2.50–11.50. For CL-RESIN, the diagram shows boron uptake capacity increased from pH 2.50 to 9.06 and achieved the maximum value at pH = 9.06, then further increase in pH leaded to decrease of adsorption capacity. For NCL-RESIN, the trend was same as the CL-RESIN, and the highest adsorption capacity appeared at pH = 6.70. Van Duin [30] suggested that boron complex of glycol can only be expected in the region where pH N 9.07 because the complex between non-ionized boric acid and glycol could not be demonstrated. In his research, an increase of pH from 11 has no effect on the concentration of borate, and the concentration of the complex reached a maximum. Then, they suggested the classic mechanism as shown in Scheme 3. In this study, adsorption occurred at pH = 2.5, while little borate ion existed at this condition according to Van Duin's research. Moreover, further increasing pH leaded to decrease of adsorption capacity, which is also different from the phenomenon observed by Van Duin. Bishop et al. [20] proposed that the formation of the complex most likely occurred via the reaction of boric acid rather than borate. They found that the concentration of the complex increased with increasing pH and reached a maximum, then further increase of pH leaded to decrease of adsorption capacity. The trend observed by Bishop is consistent with our results. So we think the mechanism proposed by Bishop can explain our results better as shown in Scheme 4. We can see it from Fig. 3 that the NCL-RESIN reached the maximum adsorption value at lower pH than the CL-RESIN. This fact can be explained by comparing the pKa of the CL and NCL functional groups. The NCL functional groups, with lower pKa value, can deprotonate more easily. Table 3 Langmuir and Freundlich isotherm constants of boron adsorption on modified resins. Resins
Langmuir constants
Freundlich constants
KA (L·mmol−1) Qm (mmol·g−1) R2
1/n
CL-RESIN 0.1054 NCL-RESIN 0.05340
0.7886 0.7931
KF
R2
0.9756 2.186 0.00460 0.9933 0.9783 1.535 0.01230 0.9914
In order to examine the adsorption mechanism, the time-dependent adsorption capacity was tested for the kinetics on two pyrocatecholmodified resins synthesized. Fig. 4 shows the boron adsorption capacity of two resins with respect to the adsorption time. The boron adsorption capacity of CL-RESIN increased gradually until about twelve hours, and then the adsorption process reached equilibrium. The boron adsorption capacity of NCL-RESIN increased rapidly in the first 5 h and reached equilibrium in thirteen hours. The pseudo-second-order model was used for the study of adsorption kinetics on the resins, which is given as t 1 1 ¼ þ t Q t K 2 Q 2e Q e
ð3Þ
where K2 (g·mg−1·h−1) is the rate constant of second-order adsorption. Qe (mg·g−1) and Qt (mg·g−1) are the amount of boron adsorbed at equilibrium and time t. The plot of t/Qt against t was shown in Fig. 5. The calculated value of K2 along with relevant correlation coefficients (R2) are shown in Table 2. According to the results, boron adsorption on prepared resins was a slow process which might be related to the low diffusion rate of boric acid in the heterogeneous surface of particles. The adsorption behavior of boron on two pyrocatechol-modified resins was well described by pseudo-second-order model, which indicated the adsorption of boron on modified resin was a chemical adsorption [13]. 3.4. Adsorption isotherms for boron adsorption on two as-synthesized resins The adsorption isotherms of two as-synthesized chelating resins with pyrocatechol functional groups were obtained from boron adsorption experiments at different initial boron concentrations. The uptake of boron was studied in the concentration range from 2 mmol·L− 1 to 75 mmol·L−1 while the dosage of the sorbent in each solution was held constant at 10 g·L−1. The plot on the adsorption capacity of resins (Qe) against the residual boron concentration in solution phase (Ce) was shown in Fig. 6. The boron adsorption capacity (Qe) increased with the increasing initial concentration gradually. The adsorption capacity achieved the maximum at equilibrium concentration of 48 mmol·L−1 for CL-RESIN and 72 mmol·L−1 for NCL-RESIN. As we all know, the relationship between the adsorption capacity of sorbents (Qe) and the residual boron concentration in solution phase (Ce) can be described by various isotherm models. In this study the Langmuir and Freundlich adsorption isotherms were applied for
J. Lyu et al. / Reactive and Functional Polymers 112 (2017) 1–8
7
Scheme 5. Structures of boron complexes.
a part in the adsorption process. The larger adsorption capacity than Wang's result can be attributed to the better protective group, benzyl, which keeps complexing phenolic hydroxyl groups by avoiding the oxidation of phenols during preparation. 3.5. Adsorption mechanism
Fig. 12. Effect of pH on boron isotopic separation factor.
explaining the results. Detailed equations and simulation methods were attached in the Supplementary material. For Langmuir model, Ce/Qe was plotted against Ce as shown in Fig. 7, and for Freundlich model the plot of logQe against logCe was shown in Fig. 8. Apparently the Langmuir model cannot explain the adsorption of boron at low concentrations (0–5.4 mmol·L−1), while it did well at higher concentrations (5.4–75 mmol·L−1) as shown in Fig. 9. For Freundlich model, it fit well at low concentrations as shown in Fig. 10, but failed to work at higher concentrations. The parameters of the Langmuir model and Freundlich model were determined from the intercepts and slopes of the straight lines as presented in Table 3. The parameter KA reflects the adsorption energy. We can see it from Table 3 that the KA value of CL-RESIN was higher than that of NCL-RESIN, which indicated that the adsorption isotherm of CL-RESIN increased more rapidly than that of NCL-RESIN at concentration range 5.4–30 mmol·L− 1 as shown in Fig. 6. According to the Langmuir model, the maximum adsorption capacity was determined to 0.7886 mmol·g− 1 for CL-RESIN and 0.7931 mmol·g− 1 for NCLRESIN, which were much larger than results of Wang [13] (0.4200 mmol·g−1) and came close to the adsorption capacity of commercial IRA 743 (1.010 mmol·g−1). About 45.9% functional groups play
To explore the boron adsorption mechanism of as-synthesized chelating resins, 11B NMR was performed to observe the chemical environment of boron in the aqueous solutions and in the exhausted resins (Fig. 11). In agreement of the results of previous researches [20], boric acid appears in various structures with the change of pH in the aqueous solution, B(OH)− 4 of 1.64 ppm at alkaline condition and B(OH)3 of 19.4 ppm at acid condition. Two peaks show in the 11B MAS NMR of exhausted IRA 743, of which, the chemical shift at 5.4 ppm can be assigned to the monochelate B(OH)2L− (Scheme 5A) and the chemical shift at 9.8 ppm can be assigned to the bichelate BL− 2 (Scheme 5B). Comparing the patterns of exhausted CL-RESIN and IRA 743, the chemical shift at 7.2 ppm and 13.8 ppm can be assigned to monochelate B(OH)2L− and bichelate BL− 2 , respectively. This result also can be proved by previous research [20]. The chemical shift at 10.0 ppm suggests a third complex in the exhausted chelating resins, which can be attributed to the complex B(OH)L (Scheme 5C). 3.6. Boron isotopic separation factor The isotopic separation factor can be calculated according to the boron concentration and isotopic abundance of initial boron solution and residual solution. The formula is shown as following: S
10
B=11 B ¼ 10 B=11 B resin = 10 B=11 B solution c0 α 0 ð1 þ α 1 Þ−c1 α 1 ð1 þ α 0 Þ ¼ c0 α 1 ð1 þ α 1 Þ−c1 α 1 ð1 þ α 0 Þ
ð4Þ
where c0 represents initial boron concentration, mol·L−1; c1 represents residual boron concentration, mol·L−1; α0 represents initial 10B/11B abundance, 0.24779; α1 represents 10B/11B abundance of the residual solution. The effect of pH on boron isotopic separation was studied which is shown in Fig. 12. Different from conventional boron isotopic separation
Fig. 13. Recycling of prepared resins for boron adsorption.
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techniques [31], pyrocatechol-modified resins exhibit stronger selectivity for 11B than 10B. The separation factor increases gradually with increasing pH, reaches to a maximum value S = 1.080 at pH = 6.70 for CL-RESIN and S = 1.140 at pH = 7.37 for NCL-RESIN, then decreases with further increasing pH. We also observed the boron isotopic separation factor of commercial IRA 743 under the same condition, which had a performance of 1.0046. The separation factors of two pyrocatecholmodified resins are much higher than commercial IRA 743 [31] which is, to the best of our knowledge, the best reported separation factors. In practical applications, high separation factor can efficiently decrease the length of the column and reduce the cost apparently, which makes them promising chromatographic solid phase for boron isotopic separation and provides important insights on reusability of boron resource. 3.7. Regeneration To explore the regeneration capacity and stability of prepared resins, 5% hydrochloride acid and acetic acid of different concentrations (5%, 10%, 15%) were used for the elution of exhausted resins. The regeneration results were summarized in the Fig. 13. It can be found that all of four eluents could regenerate most parts of prepared resins, of which 5% hydrochloride acid and 15% acetic acid were slightly better than 5% acetic acid and 10% acetic acid. Regenerated for four times, boron adsorption capacity on two prepared resins showed a tiny decrease, which demonstrated the stability of prepared resins. 4. Conclusions In this study, two aromatic hydroxyls (pyrocatechol and nitropyrocatechol) were selected based on the conductive value change (Δ) they caused for the first time to modify chloromethyl resin. New synthetic methodology with benzyl involved in replacement of methoxymethyl group avoided oxidation of hydroxyls. Boron removal properties on two as-synthesized resins were further evaluated including the maximum capacity, optimum pH, adsorption kinetics, isotherms and mechanism. Boron adsorption capacity and easy regeneration under mild acidic condition [13] make the two pyrocatechol-modified resins promising boron specific adsorbents. Meanwhile, they exhibit the highest reported boron separation factors so far. Great boron removal and isotopic separation capacities make it possible that three problems, boron removal, isotopic separation and boron reusability, can be addressed in single adsorption process. Acknowledgments Research supported by the National Natural Science Foundation of China (No. 21202116), Independent Innovation Foundation of Tianjin University (2016XZC-0071) and Natural Science Foundation of Tianjin (16JCYBJC20300). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.reactfunctpolym.2016.12.016. References [1] D.K.N. Öztürk, T. Ennil Köse, Boron removal from aqueous solution by reverse osmosis, Desalination 223 (1–3) (2008) 1–9. [2] N. Geffen, R. Semiat, M.S. Eisen, Y. Balazs, I. Katz, C.G. Dosoretz, Boron removal from water by complexation to polyol compounds, J. Membr. Sci. 286 (1–2) (2006) 45–51.
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