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Development of Novel Diol-Functionalized Silica Particles toward Fast and Efficient Boron Removal Yu Pan Tang,† Tai Shung Chung,*,† Martin Weber,‡ and Christian Maletzko§ †

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Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585, Singapore ‡ Advanced Materials & Systems Research, BASF SE, RAP/OUB - B001, 67056 Ludwigshafen, Germany § Performance Materials, BASF SE, G-PM/PU, 67056 Ludwigshafen, Germany ABSTRACT: Ion-exchange adsorption may be a promising way to tackle the boron contamination in various waterbodies on condition that an effective boron-specific adsorbent with fast sorption kinetics, high efficiency and capacity, easy regeneration, and low cost is accessible. In this work, a group of novel silica-based adsorbents were synthesized for boron removal, with the objectives of assessing their adsorption behaviors and improving their boron separation performance. The adsorption efficiency was systematically evaluated and optimized under various synthesis and operating conditions, i.e., reactant ratio, chelating temperature, particle loading, contact time, and ion strength. In addition, the adsorption kinetics and isotherm were adequately demonstrated. The adsorption kinetics followed the pseudo-second order kinetic model while the adsorption isotherm was described by Langmuir, Freundlich, and Sips models. The silica adsorbent exhibited a high adsorption rate; equilibrium was reached in few minutes, due to its high hydrophilicity and nontortuous structure. A high adsorption capacity was predicted, and a heterogeneous sorption behavior was validated by the isotherm models. Finally, regeneration performance of the adsorbent in both batch experiments and liquid chromatographic (LC) column-based experiments demonstrated that the adsorption capacity was marginally sacrificed (less than 10%) after three cycles of measurements, illustrating promising reusability. These findings may open up new ways to design high-performance boron-specific adsorbents.

1. INTRODUCTION Boron is not only an essential micronutrient for living beings but also an important raw material for numerous industries, e.g., the production of fiberglass, detergents, fuel cell, etc.1,2 However, it becomes toxic to plants and animals once exceeding a certain critical dose.3,4 The boron concentration in drinking water has been regulated in most countries and regions, for example, 2.4 mg L−1 recommended by WHO.5 In certain industries such as the semiconductor manufacturing sector, the control of boron in ultrapure water is much more stringent because boron is considered as a p-type impurity which may not only invert the n-type silicon but also cause an adverse effect on the concentration of carriers.6 Therefore, it is of paramount importance to remove boron from waterbodies in order to produce eligible water for various uses. A number of technologies such as reverse osmosis (RO),7−10 forward osmosis (FO),11−14 ion exchange with boron-specific resins (BSRs),15−18 hybrid adsorption-membrane filtration (AMF),19−21 and membrane distillation (MD)22−24 have been proposed to remove the boron from water. We have summarized the state-of-the-art deboronation technologies in a recent review article.25 We found that no simple and effective solution has been developed yet. RO seems to have great potential owing to its superior capability in desalination. © 2017 American Chemical Society

However, it is largely incapacitated due to the facts that (1) the boric acid molecule has no charge at the neutral condition and (2) its size is similar to the water molecule.26 In other words, boron can hardly be effectively separated by either size exclusion or Donnan exclusion. As a consequence, a low rejection of 40−80% to the nondissociated boric acid was attained in RO processes under normal operation conditions.27,28 On the other hand, multipass RO systems are proposed to reduce boron concentration effectively,7 which, nevertheless, involves additional energy and costs. The AMF process is attractive because it combines the sorption process with membrane separation. The technology has not been fully developed yet and the effective methods to regenerate adsorbents remain to be demonstrated.29,30 MD have been revealed to have very high rejections to nonvolatile solutes. However, it is still considered to be an energy-intensive process unless powered by waste heat or solar energy.31 Among various approaches, ion-exchange adsorption may be the most efficient way for boron removal.18,32 In order to form Received: Revised: Accepted: Published: 11618

July 27, 2017 September 20, 2017 September 20, 2017 September 20, 2017 DOI: 10.1021/acs.iecr.7b03115 Ind. Eng. Chem. Res. 2017, 56, 11618−11627

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Industrial & Engineering Chemistry Research

Figure 1. Synthesis routes of the diol-functionalized silica particle.

2. EXPERIMENTAL SECTION 2.1. Materials. The 3-aminopropyl-functionalized silica gel with a functional group loading of 1 mmol g−1 and an average particle size of 50 μm was purchased from Sigma-Aldrich. Glycidol (96%) from Sigma-Aldrich, sodium chloride from Merck, and sulfuric acid (96.3%) from J.T. Baker were used as received without further purification. Boric acid (99.9%) provided by Sigma-Aldrich was used to prepared the boron solution. 2.2. Synthesis of Diol-Functionalized Silica Particles. The diol-functionalized silica particles were synthesized by grafting glycidol on the 3-aminopropyl-functionalized silica (silica-NH2) via the epoxy-amine nucleophilic addition reaction.42,43 Figure 1 illustrates the synthesis routes. Briefly, the silica-NH2 particle was first dispersed in DI water to a solid content of 10 wt % via vigorous ultrasonication for 2 h. Argonpurged glycidol was subsequently introduced dropwise into the silica suspension at ambient temperature (23 °C). The reactant ratio was varied based on their epoxy-to-amine molar ratio, which was preset at 1:1, 2:1, 3:1, 5:1, and 10:1. The mixture was kept at 60 °C in a water bath with stirring to proceed the reaction for 3.5 h. Afterward, the products were extracted via centrifugation, washed with DI water to remove the excess glycidol, and finally dried at 100 °C. These diol-functionalized particles were referred as silica-N1, silica-N2, silica-N3, silicaN4, and silica-N5, respectively, according to the reactant ratios. 2.3. Batch Experiments for Boron Removal. Effects of various parameters in synthesis and chelation processes were studied in batch experiments. These variables of interest include reactant ratio, chelating temperature, particle loading, contact time, ion strength, etc. In one experiment, a predetermined amount of silica particles was introduced in a 10 mL boron solution. The solution pH value was kept at neutral (about 6.8), and the boron concentration was fixed at about 11.5 ppm unless otherwise stated. The adsorbent loading, denoted as the molar ratio of the tertiary amines on silica particles to the boron molecules (eq 1), was varied in the range of 0−100.

a stable complex, the adsorbent should bear diol or polyol groups which must orient properly to match the structural parameters of the tetrahedrally coordinated boron.3,33 For example, a typical commercially available BSR consists of a macro-porous polystyrene backbone and a boron-specific functional group based on N-methyl-D-glucamine (NMDG).34 However, such a kind of resin suffers from a number of disadvantages. First, the hydrophobic polystyrene polymer hinders the diffusion of hydrophilic solute molecules into the resin and leads to slow adsorption.18 Second, the frequent swelling and shrinking of the resin during the adsorption− desorption cycles could shorten its lifetime and lower its packing density.35 Dead volumes are also often found within the packed column.36 Third, the pressure drop across the packed ion exchange column can be significant which leads to high energy consumption.37,38 Exploration of new high-performance adsorbents is one of the most important imperatives for the large-scale implementation of this treatment. As backbone materials, inorganic silica particles may be more advantageous than the reported polymeric resins. First of all, due to their hydrophilic feature, silica particles may have high wettability in boron solutions such that the contact of functional groups with boron may be sufficient and fast. Second, a high density of diol functionalization may be achievable due to their versatility in chemical reactions. Third, other than the polymeric BSRs, the nontortuous structure of the silica particles may facilitate the boron diffusion and, hence, result in a fast adsorption process. Therefore, silica particles are promising adsorbents by virtue of their high hydrophilicity, high density of functional groups, fast sorption kinetics, and low cost. So far there are only limited studies to explore silica as an adsorbent for boron removal.35,39−41 However, all of these studies are restricted to the widely known NMDG functionalization. Thus, breakthroughs on silica functionalization are urgently needed. In this work, a group of novel diol-functionalized silica particles were synthesized for boron separation from water. The effects of synthesis and adsorption parameters were systematically studied and compared with regard to the adsorption performance. The adsorption behaviors were correlated with the pseudo-second order kinetic model and three isotherm models in order to assess the optimal contact time and adsorption capacity. Regeneration of the functionalized silica was also investigated. Finally, a LC column-based system was demonstrated for continuous operations of boron removal from boron-containing water. Results indicated that the silica particles exhibited an extremely high adsorption rate, outstanding adsorption capacity, and excellent reusability, which may be promisingly applied for commercial use.

adsorbent loading =

nA nB

(1)

where nA and nB are the amount of substance of the tertiary amines on the silica particles and that of boron molecules in the solution, respectively. The ion strength was adjusted by adding NaCl to the boron solution. The mixture was vigorously stirred for a preset time duration. Afterward, the silica particles were isolated via centrifugation, and the supernatant was collected for boron analyses by an inductively coupled plasma optical emission spectrometer (ICP-OES). The boron rejection, R, was described by the following equation: 11619

DOI: 10.1021/acs.iecr.7b03115 Ind. Eng. Chem. Res. 2017, 56, 11618−11627

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Figure 2. Schematic illustration of the liquid chromatographic column-based testing system.

⎛ Cp ⎞ R(%) = ⎜1 − ⎟ × 100% Cf ⎠ ⎝

concentration and pH were kept at 50 ppm and neutral, respectively. A circulation pump was employed to regulate the feed flow rate. Boron concentrations in the feed and effluent were analyzed using ICP-OES. Regeneration of the column was carried out using 0.125 M sulfuric acid. 2.6. Characterizations. The successful surface grafting of diols was evidenced by X-ray photoelectron spectroscopy (XPS, Kratos AXIS UltraDLD, Kratos Analytical Ltd., England). The characterization procedures were introduced elsewhere.44,45 Generally, wide scans in the binding energy range of 0−1100 eV and narrow scans of core-level C 1s were performed on the pristine and diol-functionalized silica particles. All binding energies (BEs) were referenced to that of the neutral C 1s hydrocarbon peak at 284.7 eV. The core-level C 1s spectra were deconvoluted with the aid of a XPSSPEAK41 software by applying the respective binding energy of each element-related structure involved in different chemistry environments. Surface elemental stoichiometries were determined from peak-area ratios, after correcting with the experimentally determined sensitivity factors. The morphology of the silica particles was observed by a field emission scanning electron microscope (FESEM JEOL JSM-6700F). To prevent charging effects, the samples were coated with platinum using a JEOL JFC-1300 platinum coater.

(2)

where Cp and Cf are the boron concentrations of supernatant and feed, respectively. The boron uptake (Q, mg g−1) was calculated based on the following equation: Q=

mass of boron adsorbed(mg) mass of adsorbent(g)

(3)

The adsorption isotherm of silica-N4 was carried out under the following conditions: a contact time of 10 min, the chelating temperature at 23 °C, and a particle loading of 50 mol mol−1 (= molar ratio of the tertiary amines on silica particles to the boron molecules). The initial boron concentration was varied from 1 to 1000 ppm. 2.4. Multicycle Studies. Reusability is one of the critical indexes of an eligible adsorbent. Regeneration of the BSRs were generally accomplished via acidity adjustment.3 The recycling capability of the silica adsorbents was evaluated as follows. Sulfuric acid was employed as the regenerant and its concentration was varied from 0.125 to 2.0 M. In general, 10 mL of acid was introduced to the silica after it being saturated and isolated. The mixture was centrifuged after stirring on a Stuart roller mixer for 2 h. The supernatant was analyzed by ICP-OES, and the sedimentary particles were washed thoroughly with DI water to remove the acid and bring pH back to neutral. A fresh boron feed solution of 10 mL was then introduced for the second and third sorption−desorption cycles in the same manner. 2.5. LC Column-Based Continuous Adsorption. Figure 2 shows the LC column-based bench-scale system customized for continuous adsorption measurements. The column was packed with 4.0 g of diol-functionalized silica particles. Its internal diameter is 1.0 cm and effective length is 12.0 cm. The experiments were performed at a constant flow rate of 0.5 mL min−1 and an ambient temperature of 23 °C. The boron feed

3. RESULTS AND DISCUSSION 3.1. Characterizations of the Diol-Functionalized Silica Particles. Figure 3 summarizes the XPS wide scanned spectra of the pristine and diol-functionalized silica particles. The peaks of Si 2p, O 1s, C 1s, and N 1s can be identified from all of the spectra. Core-level spectra of C 1s and their deconvolution results were plotted in Figure 4. The C 1s spectrum of the pristine silica was deconvoluted into three peaks at the binding energies of approximately 284.0, 284.7, and 286.0 eV, corresponding to the C−Si, C−C, and C−N bonds, respectively.46 Upon reaction with glycidol, one more peak appears at 287.0 eV, attributed to the C−O groups.20 11620


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Figure 5. Oxygen-to-silicon atomic ratios of the pristine and diolfunctionalized silica particles. Figure 3. XPS wide scanned spectra of the pristine and diolfunctionalized silica particles.

change much in comparison to that of the pristine silica. However, the particle size significantly decreases after diol functionalization possibly due to the trituration by the vigorous magnetic stirring during the reaction with glycidol. 3.2. Parameter Studies on the Adsorption Behaviors. 3.2.1. Effects of the Reactant Ratios. Figure 7a shows the boron adsorption performance of the five diol-functionalized silica and their counterpart silica-NH2 at adsorbent loadings of 10 and 50. The dosage of the reactant glycidol defines the grafting density of the diol groups on the silica particles. The silica-NH2 has very low boron rejections at both adsorbent loadings. The boron rejection gradually increases to a plateau as a function of epoxy-to-amine ratio due to the increasing diol content. Figure 7b shows good linear correlations between the boron rejection and the oxygen-to-silicon ratio. A high grafting density is always preferred for a high adsorption capacity. SilicaN4 was chosen for further studies in what follows since the

Figure 5 presents the oxygen-to-silicon atomic ratios of the silica particles derived from XPS analyses. The three dotted lines indicate the ratios for three ideal scenarios: the pristine silica, the functionalized silica with all primary amine groups completely and solely converted to secondary amine groups (Figure 1b), and the one with those completely converted to tertiary amine groups (Figure 1a). It is interesting to observe that the experimental values are all higher than the ideal values. In addition to the instrumental error, the physical adsorption of excess glycidol on the particles via hydrogen bonding may be another reason. Figure 6 shows the FESEM morphology. The particle size of the pristine silica is in the range of 40−80 μm which is quite consistent with the reported value from the supplier. Figure 6b presents the image of the pristine particles after 2 h ultrasonication. It appears that the particle size does not

Figure 4. C 1s core-level spectra of the pristine and diol-functionalized silica particles. 11621


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Figure 6. FESEM morphology of (a) silica-NH2, (b) silica-NH2 after 2 h ultrasonication, and (c−g) silica-N1, silica-N2, silica-N3, silica-N4, and silica-N5.

Figure 7. (a) Boron rejection performance of silica particles under different reaction conditions (chelating temperature at 60 °C) and (b) linear correlation between the boron rejection and the O/Si atomic ratio.

Figure 8. Effects of adsorbent loading on boron rejections of (a) silica-N5 at 60 °C and (b) silca-N4 at 60 and 23 °C.

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Figure 9. (a) Boron rejection as a function of contact time (silica-N4, chelating temperature at 23 °C) and (b) linear fitting by the pseudo-second order kinetic model.

rejections reached plateaus at a reactant ratio of 5 for both loadings. 3.2.2. Effects of the Adsorbent Loadings and Chelating Temperatures. Adsorbent loading is another important factor in determining the effectiveness of boron removal. Due to the kinetic and thermodynamic limitations, adsorption efficiency can also be improved by providing more active sites.20 However, a high adsorbent loading would cause a high volume of regenerant and high cost. To search for the optimal adsorbent loading, the chelating temperature was kept at 60 °C for silica-N5 while two temperatures, 23 and 60 °C, were employed for silica-N4 for comparison. Figure 8 shows the performance as a function of adsorbent loading. The boron rejection dramatically increases before reaching a plateau at a loading of 50. Since the chelation is a reversible process, chemical equilibrium at the solid−liquid interface is responsible for such behavior. Another interesting observation is that the adsorption capacity is not affected by the chelation temperature as shown in Figure 8b. This is somewhat at odds with our previous work.20 This is probably due to the inherently high chelation affinity between the boron molecules and the diol groups as well as the low barrier for the boron diffusion. 3.2.3. Adsorption Kinetics. Adsorption of boron was measured as a function of contact time to evaluate the adsorption kinetics. The silica-N4 was used for the measurements at two loadings. The boron rejection in Figure 9a exhibits a substantial increase at first and then turns to a plateau. Both loadings display a similar trend and have the inflection points at 5 min, suggesting fast adsorption processes for both cases. A pseudo-second order kinetic model as expressed in eq 4 was employed to fit the experimental data.47 ⎛ ⎞ t 1 ⎜ 1 ⎟t = + ⎜Q ⎟ Qt k 2Q eq 2 ⎝ eq ⎠

t1/2 =

1 k 2Q eq

(5)

The half adsorption time is often used as a measure of the adsorption rate. It is defined as the time consumed for the adsorbent to take up one-half of its equilibrium value. Table 1 shows the values of k2, Qeq, and t1/2. It appears that a higher adsorption rate but a lower equilibrium uptake are Table 1. Fitting Parameters from the Pseudo-Second Order Kinetic Models case

conditions

k2 (g mg−1 min−1)

Qeq (mg g−1)

t1/2 (min)

1 2

silica-N4, loading 100 silica-N4, loading 50

1.949 0.713

1.007 1.219

0.509 1.151

achieved for the case with a higher adsorbent loading, which agrees well with those reported in the literature.39 The half adsorption time attained in this work is much shorter than those reported in the literature,40 indicating fast adsorption kinetics. This may be due to the high hydrophilicity and the nonporous structure of the silica particles which induce rapid wetting and provide excellent contact with boron molecules. In general, the adsorption time is the sum of boron diffusion time and adsorption time, while the diffusion consists of external diffusion in the bulk solution and internal diffusion in the adsorbent matrix.40 In addition, diffusion is the dominant step in the adsorption process.34 The slow adsorption observed for the BSRs may be caused by the slow internal diffusion rate of boron molecules due to their tortuous hydrophobic matrix. Different from BSRs, the silica adsorbents have a nontortuous structure. The absence of internal diffusion results in their extremely fast adsorption of boron molecules. 3.2.4. Effects of Ion Strength. In seawater and produced wastewater, large amounts of cations and anions coexist with boron. Therefore, the effects of ion strength are worthy of study. As shown in Figure 10a, the boron rejections have very similar values with and without the presence of 0.6 M NaCl at different adsorbent loadings. Figure 10b shows that the NaCl concentration has negligibly negative effects on boron rejection. 3.3. Adsorption Isotherm. The boron adsorption isotherm of silica-N4 was studied at 23 °C and a adsorbent loading of 50. Figure 11a shows the boron rejection as a function of boron concentration in the feed, whereas Figure 11b shows the isotherm.

(4)

where k2 is the rate constant (g mg−1 min−1) and Qeq is the amount of boron adsorbed at equilibrium (mg g−1). The pseudo-first order model was not suitable for this work as it is for high concentration range.40,48 Since the boron concentration is only 11.5 ppm (section 2.3), the pseudo-second order model was chosen. Figure 9b presents excellent linear fitting for both cases with the R2 > 0.9997. Based on the second order model, the half adsorption time (t1/2) was estimated by the following equation: 11623


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Figure 10. Effects of ion strength on boron rejection of silica-N4 (chelating temperature at 23 °C and adsorbent loading at 50).

Figure 11. (a) Boron rejection of silica-N4 as a function of the initial feed concentration, (b) boron adsorption isotherm and the fitting curve by the Sips model, (c) Langmuir isotherm plot, and (d) Freundlich isotherm plot.

Table 2. Fitting Parameters from the Three Isotherm Models Sips isotherm model −1

−1

Langmuir isotherm model −1

Freundilich isotherm model −1

Qm,2 (mg g )

kS (L mg )

n

Qm,1 (mg g )

kL (L mg )

kF

n

57.09

0.0006

0.5407

13.26

0.0514

0.6295

0.5919

concentration at equilibrium in the aqueous phase (mg L−1), and kL is the Langmuir adsorption constant (L mmol−1). Figure 11c shows the plot of 1/Qeq versus 1/ceq, and Table 2 summarizes the values of Qm,1 and kL. Although it seems a good fitting with R2 > 0.9999, the data points at the high concentration range diverge from the fitting line as zoomed in the inset of Figure 11c. It should be noted that the speciation of boron highly relies on its concentration. The mononuclear B(OH)3/B(OH)4− dominates at low boron concentrations while polyborate species are dominant at concentrations higher than 220 ppm.1,3,4 The divergence is probably attributed to the formation of polyborates. The adsorption at high boron

The adsorption isotherms are generally described by Langmuir and Freundlich isotherm models.35,49 The Langmuir model assumes a monolayer coverage of adsorbate over a homogeneous adsorbent surface. The binding sites are also assumed to be energetically equivalent and distant from each other. The Langmuir model can be represented by the following equation: 1 1 1 = + Q eq Q m,1 Q m,1kLceq

(6)

where Qm,1 is the adsorption capacity when the adsorbent surface is completely covered with adsorbate. ceq is the boron 11624


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Figure 12. Chelation mechanisms of boric acid with diols at (a) a low boron concentration (220 ppm) where B4O4(OH)4 takes place.

concentrations may not be in a monolayer manner as proposed in Figure 12. Thus, the Langmuir model may not be applicable to describe the whole concentration range. On the other hand, the Freundlich adsorption isotherm model describes the reversible adsorption and is not restricted to the formation of the monolayer. A linear Freundlich equation was therefore applied to the data:50 log Q eq = log kF + n log ceq

(7)

where kF is the Freundlich constant and n is the index of heterogeneity, which varies from 0 to 1; n = 1 for homogeneous materials. Figure 11d shows the plot of log Qeq versus log ceq, and Table 2 summarizes the values of kF and n values. It is interesting to observe that n is equal to 0.8864 at the low boron concentration range of 1−50 ppm and 0.3798 at the high range of 50−1000 ppm. This phenomenon implies that the adsorption gradually becomes heterogeneous at elevated concentrations. It is consistent with the observation from the Langmuir fitting. In order to attain a close fitting, the Sips isotherm model, which combines the Langmuir and Freundlich sorption, was employed to describe the adsorption behaviors:51,52 1 1 1 = + Q eq Q m,2 Q m,2(k Sceq)n

Figure 13. Multicycle experiments for silica-N4.

further decreasing in the third cycle. The decrease is probably due to two reasons: the loss of silica particles during the centrifugation and the incomplete decomposition of the diolboron complex. Furthermore, the effectiveness of regeneration seems not to be affected by the acid concentration in the range of 0.125−2.0 M. 3.5. Applications in LC Column Experiments. Figure 14 shows the breakthrough curve for three adsorption−desorption

(8)

where Qm,2 is the adsorption capacity (mg g−1) and kS is the affinity constant for adsorption (L mg−1). The equation becomes the Langmuir adsorption equation when n = 1, which means that the sorption is homogeneous. Figure 11b shows the fitting curve, and Table 2 summarizes the values of Qm,2, kS, and n. The Qm,2 calculated from the Sips model is much higher than that predicted by the Langmuir model. This is probably attributed to the formation of polyborate species at high boron concentrations as aforementioned, which significantly increases the adsorption capacity. In addition, n is equal to 0.5407, indicating that the adsorption is heterogeneous. 3.4. Multicycle Studies. The ability to regenerate adsorbent materials is an important measure of their reusability. An ideal adsorbent material must be easily regenerated in multiple cycles without sacrificing its adsorption efficiency. High stability of the adsorbent material under acidic conditions as well as minimum weight loss during regeneration are key factors for its large scale applications in industries. To evaluate the reusability of the diol-functionalized silica, multicycle tests were carried out. Figure 13 shows the results at different acid concentrations. The boron uptake at equilibrium was normalized based on that of the first cycle. The boron uptake decreases to 95% in the second cycle and shows almost no

Figure 14. Breakthrough curve for three cycles.

cycles in the LC column-based system as shown in Figure 2. The adsorption capacity is 19.03 mg g−1 calculated from the first cycle, which is lower than the theoretical value derived from the Sips isotherm model. This is because of the low boron concentration in the feed such that the formation of polyborate species is limited. In other words, the adsorption behaviors may 11625


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HSE/WSH/09.01/2; World Health Organization press: Switzerland, 2009; Vol. WHO/HSE/WSH/09.01/2. (6) Wen, R.; Deng, S.; Zhang, Y. The removal of silicon and boron from ultra-pure water by electrodeionization. Desalination 2005, 181, 153−159. (7) Xu, J.; Gao, X.; Chen, G.; Zou, L.; Gao, C. High performance boron removal from seawater by two-pass SWRO system with different membranes. Water Sci. Technol.: Water Supply 2010, 10, 327− 336. (8) Dydo, P.; Nemś, I.; Turek, M. Boron removal and its concentration by reverse osmosis in the presence of polyol compounds. Sep. Purif. Technol. 2012, 89, 171−180. (9) Rahmawati, K.; Ghaffour, N.; Aubry, C.; Amy, G. L. Boron removal efficiency from Red Sea water using different SWRO/BWRO membranes. J. Membr. Sci. 2012, 423−424, 522−529. (10) Hu, J.; Pu, Y.; Ueda, M.; Zhang, X.; Wang, L. Charge-aggregate induced (CAI) reverse osmosis membrane for seawater desalination and boron removal. J. Membr. Sci. 2016, 520, 1−7. (11) Jin, X.; Tang, C. Y.; Gu, Y.; She, Q.; Qi, S. Boric Acid Permeation in Forward Osmosis Membrane Processes: Modeling, Experiments, and Implications. Environ. Sci. Technol. 2011, 45, 2323− 2330. (12) Kim, C.; Lee, S.; Shon, H. K.; Elimelech, M.; Hong, S. Boron transport in forward osmosis: Measurements, mechanisms, and comparison with reverse osmosis. J. Membr. Sci. 2012, 419−420, 42−48. (13) Valladares Linares, R.; Li, Z. Y.; Sarp, S.; Park, Y. G.; Amy, G.; Vrouwenvelder, J. S. Higher boron rejection with a new TFC forward osmosis membrane. Desalin. Water Treat. 2015, 55, 2734−2740. (14) Luo, L.; Zhou, Z.; Chung, T.-S.; Weber, M.; Staudt, C.; Maletzko, C. Experiments and Modeling of Boric Acid Permeation through Double-Skinned Forward Osmosis Membranes. Environ. Sci. Technol. 2016, 50, 7696−7705. (15) Kabay, N.; Yilmaz, I.̇ ; Bryjak, M.; Yüksel, M. Removal of boron from aqueous solutions by a hybrid ion exchange−membrane process. Desalination 2006, 198, 158−165. (16) Kabay, N.; Sarp, S.; Yuksel, M.; Kitis, M.; Koseoğlu, H.; Arar, Ö .; Bryjak, M.; Semiat, R. Removal of boron from SWRO permeate by boron selective ion exchange resins containing N-methyl glucamine groups. Desalination 2008, 223, 49−56. (17) Yan, C.; Yi, W.; Ma, P.; Deng, X.; Li, F. Removal of boron from refined brine by using selective ion exchange resins. J. Hazard. Mater. 2008, 154, 564−71. (18) Nasef, M. M.; Nallappan, M.; Ujang, Z. Polymer-based chelating adsorbents for the selective removal of boron from water and wastewater: A review. React. Funct. Polym. 2014, 85, 54−68. (19) Geffen, N.; Semiat, R.; Eisen, M. S.; Balazs, Y.; Katz, I.; Dosoretz, C. G. Boron removal from water by complexation to polyol compounds. J. Membr. Sci. 2006, 286, 45−51. (20) Tang, Y. P.; Yuwen, S.; Chung, T. S.; Weber, M.; Staudt, C.; Maletzko, C. Synthesis of hyperbranched polymers towards efficient boron reclamation via a hybrid ultrafiltration process. J. Membr. Sci. 2016, 510, 112−121. (21) Wei, Y.-T.; Zheng, Y.-M.; Chen, J. P. Design and fabrication of an innovative and environmental friendly adsorbent for boron removal. Water Res. 2011, 45, 2297−2305. (22) Boubakri, A.; Bouguecha, S. A.-T.; Dhaouadi, I.; Hafiane, A. Effect of operating parameters on boron removal from seawater using membrane distillation process. Desalination 2015, 373, 86−93. (23) Francis, L.; Ghaffour, N.; Alsaadi, A. S.; Nunes, S. P.; Amy, G. L. Performance evaluation of the DCMD desalination process under bench scale and large scale module operating conditions. J. Membr. Sci. 2014, 455, 103−112. (24) Hou, D.; Wang, J.; Sun, X.; Luan, Z.; Zhao, C.; Ren, X. Boron removal from aqueous solution by direct contact membrane distillation. J. Hazard. Mater. 2010, 177, 613−9. (25) Tang, Y. P.; Luo, L.; Thong, Z.; Chung, T. S. Recent advances in membrane materials and technologies for boron removal. J. Membr. Sci. 2017, 541, 434−446.

follow the Langmuir calculation. In addition, the adsorption capacity is only sacrificed less than 10% of its original value after three cycles of measurements, implying a promising reusability of the diol-functionalized silica particles.

4. CONCLUSIONS In this work, a group of novel boron-specific silica particles were developed as adsorbents for boron separation from water. The variables of interest, i.e., reactant ratio, chelating temperature, particle loading, contact time, and ion strength, were carefully tuned and optimized in batch experiments. Adsorption behaviors were correlated with the pseudo-second order kinetic model and three isotherm models. Regeneration of the adsorbent was also investigated in both batch experiments and LC column-based experiments. The following conclusions can be drawn from this work: 1. The diol-functionalized silica particles showed promising boron adsorption performance under ambient conditions and neutral pH. 2. The half adsorption time of silica-N4 is much shorter than the commercially available BSRs and those adsorbents reported in the literature. The extremely fast adsorption kinetics is probably due to the high hydrophilicity and nontortuous structure of the silica particles. 3. Adsorption isotherm was described and compared by three sorption models. The sorption behavior follows the Sips model due to the heterogeneity of the adsorbent. A high adsorption capacity was predicted by the Sips model. 4. The adsorbent was easily regenerated by using 0.125 M sulfuric acid. Both the batch experiments and the continuous column adsorption experiments demonstrated that the adsorption capacity is marginally sacrificed (less than 10%) after three cycles of measurements, illustrating a promising reusability of the diol-functionalized silica.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (65)67791936. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.P.T. and T.S.C. would like to thank BASF SE, Germany for funding this work with Grant No. R-279-000-411-597. Thanks are also given to Dr. S. Japip, Dr. N. Widjojo, and Dr. M. Jung for their kind help and suggestions.

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Development of Novel Diol-Functionalized Silica ... - ACS Publications

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