A novel mixed matrix membrane allowing for flow-through removal of boron

Chemical Engineering Journal 308 (2017) 557–567

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

A novel mixed matrix membrane allowing for flow-through removal of boron Zhe Wang, Pan Wang, Jingjing Cao, Yufeng Zhang, Bowen Cheng, Jianqiang Meng ⇑ State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, China

h i g h l i g h t s
A MMM was prepared by phase inversion of polysulfone and a boron specific resin.
The resin blending leads to increased surface area and roughness.
The resin blending leads to increased surface hydrophilicity and membrane flux.
The adsorption shows S-type isotherm and obeys to pseudo first-order kinetic model.
The MMM has similar boron uptake but much higher adsorption rate than the resin.

a r t i c l e

i n f o

Article history: Received 18 July 2016 Received in revised form 17 September 2016 Accepted 17 September 2016 Available online 19 September 2016 Keywords: Non-solvent induced phase inversion Polysulfone (PSF) membrane Mixed matrix membrane Complexing membrane Boron removal

a b s t r a c t In this work, a novel mixed matrix membrane (MMM) was fabricated by Non-solvent induced phase inversion of the blend of polysulfone (PSF) with a boron selective resin (BSR) and used for boron removal. The membrane surface chemical composition was characterized by ATR-FTIR and XPS. The membrane morphology was observed by FESEM, AFM and BET measurements. The effects of BSR content on boron adsorption properties were studied in detail. Addition of BSR leads to rougher and more hydrophilic membrane surface and higher permeation flux. The boron uptake increases obviously with the BSR content and has the maximum over 2.0 mmol/g. The membrane shows S-type isotherm and the adsorption kinetics can be well described by the pseudo-first-order model. Compared with the polymeric resin, the PSF/BSR membrane shows similar boron uptake but much higher adsorption rate. With a membrane of 1.76 cm2, a boron removal efficiency as high as 97.6% can be obtained for 10 mL of 5 mg/L boron solution in the flow-through experiment at flux rate of 25 L/m2h. It is concluded that the MMM can combine the advantage of the resin on high uptake and that of the membrane on convective transport. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction Boron exists in the hydrosphere and lithosphere of the earth. It is widely found in the environment in the form of boric acid and borate salts [1]. As a micronutrient, boron is essential for plants and animals. It plays a crucial role in the growth of plants in many ways. However, boron becomes harmful or even lethal to both plants and animals when its amount is greater than required [2]. In 2011, the World Health Organization (WHO) regulated a boron content standard of less than 2.4 mg/L for drinking water [3], while the boron content of irrigation water is limited to 1 mg/L or less [4]. On the other hand, many water resources in the world and many waste water effluents from the industry have the boron content well above the WHO regulations. For example, the average ⇑ Corresponding author. E-mail (J. Meng).

addresses:

[email protected],

http://dx.doi.org/10.1016/j.cej.2016.09.094 1385-8947/Ó 2016 Elsevier B.V. All rights reserved.

[email protected]

boron content in seawater is 5 mg/L or so [5] and that in some industrial waste water reaches 100 mg/L or more. For example, the boron concentration is about 100 mg/L in geothermal water and can be as high as 1000 mg/L for effluent from boron releasing manufacturing industries [6,7]. Therefore, it is indispensable to develop efficient and cheap technologies for boron removal. Currently, the reported boron removal methods include boron extraction [8], adsorption [9], ion exchange [10], reverse osmosis (RO) [11], electrodialysis (ED) [12] etc. However, boron removal still remains a big challenge for most commercial desalination technologies due to its small molecular size, high diffusion rate and nonionic nature at normal pH range [11]. For example, RO technologies are getting the leading position for boron removal [11] and there is a tendency of the RO membrane units replacing the traditional operational units in the water treatment process. However, the boron rejection by RO is highly dependent on the pH of solution which is related to the dissociation ratio of boric acid. A single pass RO for sea water desalination is usually only able to

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produce permeate with boron content of 0.9–1.8 mg/L [13]. The reason is that boron is predominantly in the form of non-ionic boric acid at the natural pH and has the ability to diffuse through the RO membrane via hydrogen bonding [14]. The complexation of boric acid with compounds having adjacent hydroxy groups (Fig. 1) has been demonstrated to be a more effective solution for boron removal than other methods based on charge and size exclusions [15]. Various adsorbents have been developed using multihydric compounds as the complexing ligands. As a novel polymeric adsorbent [16,17], the complexing membranes offer the opportunity of flow-through removal of the target solute in a dynamic filtration process. Compared with traditional polymeric resins, the complexing membranes can provide better adsorption kinetics thanks to the large mesopores, as is characterized by fast equilibration of solutes between a mobile liquid phase and a stationary solid phase on the mesopore walls. In this context, we have synthesized a series of complexing membranes via surface grafting of glycopolymers onto the surface of microporous membranes [5,16,17]. The grafted membrane could adsorb 2.0 mmol/g boron within 2 h in a 300 mg/L boron solution. In addition, it is hard to simultaneously achieve high adsorption rate and high uptake for a specific membrane material. Because the enhancement of the uptake by increasing the ligand density usually leads to increased grafted layer thickness with hindered diffusion and lowered adsorption rate as a result. Mixed matrix membrane (MMM) is heterogeneous matrix containing ‘‘inserts” in a polymer matrix. The MMM concept combines the advantage of each phase. It has been successfully used to combine the high selectivity of organic fillers with desirable mechanical property and economical processability of polymers for gas separation [18]. Inspired by this, we envisaged that a MMM membrane comprising dispersed microporous resins in a mesoporous structure will combine the high uptake of microporous polymer resins with the convective transport advantage of the mesoporous membrane [19]. Hence, in this work, we decided to explore the boron adsorption property of a MMM membrane via phase inver-

sion of a blend of a boron selective resin (BSR) with polysulfone (PSF). Because polymeric resins used to suffer from hindered transport because of the diffusion in tutorised pores [20], the polymer resins were milled into finer particles so that more boron selective ligands can be present on the mesopore walls of the membrane matrix where the resin particulates are dispersed [21,22]. The mechanism of the MMM removing boron via complexation was shown in Fig. 1. In addition to the size sieving effect from a UF membrane, an additional separation mechanism was added for boron removal via the complexation of boric acid with the vicinal diol group in the incorporated BSR [16,17]. The obtained membranes were characterized in detail with the focus on their surface chemical structure, morphology and mechanical properties. The boron adsorption properties of the MMM from different blend ratios were systematically investigated and effects of initial boron concentration, adsorption time, pH of the solution, ion strength were discussed. The recyclability of the membrane as an adsorbent was also addressed.

2. Materials and methods 2.1. Materials ZXC-700 BSR was purchased from Zhengzhou Xidian Power Resin Co. LTD, China. The resin has been claimed to have a crosslinked polystyrene scaffold anchored with a large amount of Nmethyl-D-glucamine (NMG) and the particle diameter ranged from 0.315 to 1.25 mm. It was milled by wet milling for 4 h with ethanol as the wetting agent and dried in vacuum oven at 50 °C for 24 h. The obtained BSR particulate was screened out of the 200 mesh screen (the particle diameter was lower than 75 lm) and used as the membrane additive for blending. PSF (P-3500 LCD, Mw = 75,000–81,000 g/mol) was purchased from Solvay, USA. Curcumine was purchased from Tianjin Kemiou Chemical Reagent Company, China. Phosphate buffered saline solution (PBS,

Fig. 1. Mechanism of the MMM removing boron by complexation.

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pH = 7.4, KH2PO4 0.24 g, Na2HPO4 1.44 g, NaCl 8.0 g, KCl 0.2 g, 1 L DI water) was prepared prior to use. Bovine serum albumin (BSA) was purchased from Sigma-Aldrich. Deionized (DI) water (18.2 MX cm at 25 °C, 1.2 lg/L TOC) was obtained from a Millipore Milli-Q Advantage A10 water purification system (Billerica, MA, USA). All the other chemical reagents were used as received and were analytical grade. 2.2. MMM preparation MMM was prepared by the phase inversion technique [17]. The casting solution was prepared by dispersing the PSF/BSR blend in N,N-dimethyl acetamide with the total solid content of PSF and BSR fixed at 17 wt% [23]. The weight ratios of PSF:BSR were 10:0, 9:1, 8:2, 7:3 and 6:4. The obtained membranes are respectively represented as PSF, PSF/BSR1, PSF/BSR2, PSF/BSR3, PSF/BSR4. Poly (vinylpyrrolidone) (PVP K30, Mw = 30,000 g/mol) was added at a concentration of 8 wt% as the porogen [17]. The casting solutions were kept stirred at 60 °C for 24 h. After cooling down and degassed standing, the casting solution was casted with a casting knife on a cleaned glass plate. After that, the plate was immediately immersed in DI water at room temperature. When the membrane was stripped off from the plate, it was extracted with DI water and stored in a 5% sodium bisulfite solution. 2.3. Membrane surface characterization The membrane surface chemical composition was characterized using FT-IR spectrophotometer (Bruker, Vector-22) with Zinc Selenide (ZnSe) as an internal reflection element at an incident angle of 45°. The XPS measurements were performed on a Quanta200 spectrometer (FEI Co., Ltd. USA) with a monochromatic Al Ka X-ray source (1486.6 eV photons) at a pass energy of 93.9 eV. The measurements were conducted at a take-off angle of 45 °C. Survey spectra were run in the binding energy range of 0–1000 eV, followed by high-resolution scan of the N1s. Binding energies were calibrated using the containment carbon (C1s = 284.6 eV). Field emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan) was used under high vacuum condition to investigate membrane surface morphology. The membranes were cut into 0.5 cm2 samples, attached with double-side tape to steel stabs, and shadowed with gold prior to the observation. Membrane surface morphologies were also measured by AFM (Agilent 5500 scanning probe microscope, Agilent technologies, USA) imaging and analysis, equipped with a standard silicon nitride cantilever. Tensile properties of the membrane were measured in reference to ISO 1184: 1983 standard at test speed of 2 mm/min on a SNAS electronic universal testing machine (MTS, China) in order to estimate the mechanical properties of the PSF and PSF/BSR membranes. The water contact angle (WCA) of the membrane surface was determined on a Krüss Instrument (CM3250-DS3210, Germany) at ambient temperature. One drop of water (1 lL) was put on the membrane surface with an automatic piston syringe and photographed. The obtained value was an average of three measurements. The specific surface areas were determined by nitrogen adsorption/desorption isotherms with standard Brunauer-Emmett-Teller (BET) (Autosorb-1, Quantachrome Instrument Corporation, USA). Data were analyzed by a computer controlled sorption analyzer (Autosorb-1 Software) operated in the continuous mode. 2.4. Membrane permeation and rejection properties The pure water flux of membrane samples was measured using a stirred filtration cell (Amicon 8010, Millipore) under the pressure of 0.1 MPa. The effective area of the testing cell is 15.9 cm2. The membranes were prewetted in ethanol for 1 min prior to perme-

ation test, and then were pressured for 10 min to obtain a stable flux value. The flux was calculated by measuring the permeation volume in 5 min for 3 times to obtain the average value. The flux was calculated using Eq. (1)

F ¼ V=ðAtÞ

ð1Þ 2

where F is the water permeation flux (L/m h), V the permeation volume (L), A the effective membrane area (m2), and t is the permeate collection time (h). The membrane retention properties were evaluated by deadend filtration of 1 g/L BSA solution in PBS at room temperature in an Amicon 8010 cell with a stirring speed of 600 rpm. The BSA solution was filtered through 0.22 lm cellulose esters membrane in vacuum filter holder immediately prior to use. The BSA rejection R was calculated by the following equation

  Cp  100% R ¼ 1 Cf

ð2Þ

where Cp and Cf are the concentration of BSA in permeate and feed solution, respectively. The concentration of BSA was measured via a UV–vis spectrophotometer at wavelength of 280 nm. 2.5. Boron adsorption experiments Before the static adsorption experiments, the complexing membrane sample was dried and weighed. Then, it was wetted with methanol and placed into a high-density polyethylene (HDPE) conical flask containing 20 mL of boric acid solutions. Then, the flask was sealed and shaken with a continuous shaker at 25 °C for 6 h. At the end of the experiment, a sample of 0.5 mL of solution was withdrawn. The boron concentration was determined spectrophotometrically (k = 540 nm) by the Curcumine method [24]. For the kinetic adsorption study, aliquots of 0.5 mL were withdrawn from a 50 mL boron solution at predetermined time intervals to measure its concentration. As a control experiment, the boron adsorption kinetic of the milled BSR particulate was also investigated. Aliquots of boron solution with BSR particulate suspended in it were withdrawn at predetermined time intervals and centrifuged for 10 min at 10,000 rpm. The boron concentration of the supernatant fraction was measured as the equilibrium concentration of boron in the solution. The boron uptake (q) can be calculated by the following equation:

q¼

ðc0  ce ÞV mM

ð3Þ

where c0 and ce (mg/L) are the initial and equilibrium concentrations of boron in the solution volume V (L), m the mass of dry membrane (g) and M is the mole weight of boron acid (g/mol). The dynamic filtration of boron solution with the membrane was conducted using laboratory-made filtration equipment having a dead-end cell connected with a nitrogen gas cylinder. A pressure regulator was used to adjust the flow rate. The filtration cell had an effective area of 1.76 cm2. The boron acid solution content was 5 mg/L and the pH was adjusted to 9.0 before loading into the cell. At first, 10 mL of boron acid solution was filtrated at different flow rates (0.075 mL/min, 0.5 mL/min, 1 mL/min and 3 mL/min). Then, a series of boron solutions of different volume (5 mL, 10 mL, 15 mL, 20 mL) was filtrated at 0.075 mL/min. The boron acid removal efficiency (%) by the adsorptive membrane was determined by the following equation [25]:

Removal efficiency ¼

c0  ct  100% c0

ð4Þ

where c0 and ct (mg/L) are the initial and final concentration of boron acid solution before and after filtration, respectively.

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2.6. Regeneration of the complexing membrane The membrane can be regenerated by the acid leaching method [26]. At first, the membrane samples were placed into a HDPE conical flask containing 20 mL of 100 mg/L boric acid solution and kept shaking for 4 h. The boron adsorption capacity q1 was measured. Then the saturated membranes were soaked into 0.1 mol/L HCl solution and kept stirring for 1 h. After that, the membrane was rinsed with 0.1 mol/L NaOH solution for 1 h. Then, the treated membrane was withdrawn and washed with excess of DI water. The boron adsorption capacity q2 was measured. For the membrane regeneration of the dynamic filtration experiment, the membrane sample was placed into the dead-end cell and the cell was loaded with 10 mL of 5 mg/L boric acid solution at pH = 9. The flow rate was controlled at 0.0075 mL/min by adjust pressure of nitrogen gas. The boron adsorption capacity q1 was measured in the permeation. In order to the desorption of saturated membranes, the experiment was put 0.1 mol/L HCl solution into the dead-end filtration equipment and repeatedly permeate for 1 h. After that, the solution was changed into 0.1 mol/L NaOH solution for neutralize HCl solution. Then, the solution was changed into DI water again for washed the membrane. Repeat the above step to measure the boron adsorption capacity q2. The regeneration efficiency (RE) of the membrane was defined as:

q RE ¼ 2  100% q1

ð5Þ

where q1 and q2 are the initial boron uptake and that for the cycled use. 3. Results and discussion 3.1. Membrane characterization 3.1.1. Membrane surface compositions Fig. 2 shows the ATR-FTIR spectra of the PSF and PSF/BSR membranes surface with different blending ratios. There is an adsorption at 1680 cm1 corresponding to the carbonyl group. A broad adsorption at about 3300 cm1 belongs to the stretching vibration of the –OH groups. We see the intensity of these two adsorptions increases significantly. This result is attributed to the NMG groups in BSR and indicates successful incorporation of BSR in the mem-

brane matrix. It should be noted that the PSF spectrum also shows trivial adsorption at 1680 cm1, which should be attributed to the acrylamide group in the residue PVP. The composition of the membrane surfaces was further analyzed by XPS. Fig. 3 shows typical XPS wide scans for PSF and PSF/BSR4 membrane surfaces. There are two main emission peaks at 531 eV and 285 eV assigned to O1s and C1s. The other two emission peaks at 399 eV and 168 eV can be assigned to N1s and S2p. The presence of N1s emission peak for the PSF membrane confirmed that there is residue PVP in the membrane surface. The peak components for C1s in the XPS spectra are further analyzed, and the core level C1s spectra are also shown in Fig. 3. The C1s peak components at the binding energy of 284.5 eV and 286.2 eV are attributable to CAH/CAC, CAN species. It can be seen that the peak percentage of PSF/BSR4 membrane at 286.2 eV assigned to CAN is obviously greater than that of PSF membrane. Besides, the corresponding composition of the membrane surfaces is listed in Table 1. The increase of the N content with the BSR content is obvious so that the N/S ratio increases when the BSR content increases. All the above results are because of the incorporation of NMG groups into membrane matrix and indicate successful blending of BSR particulates into the membrane. 3.1.2. Membrane surface morphology Fig. 4 shows FESEM images of the PSF and PSF/BSR membranes surface and cross section. We can clearly see that blending of BSR has altered the membrane morphology. With the increase of BSR content, the membrane surface appears to be rougher. This result is confirmed by the AFM results as shown in Fig. 5. We see the membrane surface roughness increases dramatically with the BSR content. The average surface roughness (Ra) of the PSF, PSF/ BSR1, PSF/BSR2, PSF/BSR3 and PSF/BSR4 membranes are 0.32 lm, 0.45 lm, 0.64 lm, 0.96 lm and 1.03 lm, respectively. We note here that the scale of the Z-axis of the AFM images varied for different membranes. In addition, the membrane with a high BSR content appears to have a higher porosity, which is especially true for the PSF/BSR4 membrane whose surface pores become visible at the magnification of 20,000. For the cross-section images, all the membranes show a typical morphology from phase inversion. There is a thin skin layer on the membrane surface with fingerlike morphology developed underneath. With the increase of the BSR content, the finger-like morphology becomes more irregular.

Fig. 2. ATR-FTIR spectra of the PSF (a), PSF/BSR1 (b), PSF/BSR2 (c), PSF/BSR3 (d) and PSF/BSR4 (e) membranes.

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Fig. 3. XPS wide spectra (a, c) and C1s core level spectra (b, d) of PSF (a, b) and PSF/BSR4 (c, d) membranes.

Table 1 Elemental surface compositions of the PSF and PSF/BSR membranes analyzed by XPS. Membrane

PSF PSF/BSR1 PSF/BSR2 PSF/BSR3 PSF/BSR4

PSF/BSR weight ratio

NA 9:1 8:2 7:3 6:4

Element (at.%)

N/S

C

O

S

N

79.93 79.21 78.59 77.63 78.81

13.08 13.76 14.25 13.43 13.66

3.04 2.78 2.73 2.37 2.15

1.72 1.92 2.16 2.67 2.98

0.57 0.69 0.79 1.13 1.39

Fig. 4. FESEM images of top views (20,000) and the cross section (600) of the membranes: PSF (a, a1), PSF/BSR1 (b, b1), PSF/BSR2 (c, c1), PSF/BSR3 (d, d1), PSF/BSR4 (e, e1).

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Fig. 5. AFM images of PSF (a), PSF/BSR1 (b), PSF/BSR2 (c), PSF/BSR3 (d) PSF/BSR4 and (e) membranes.

PSF/BSR1 and PSF/BSR2 membranes show parallel pore channels, while for PSF/BSR3 and PSF/BSR4 membranes, these finger-like pores are disrupted by other macropores and communicate with each other. This observation may be caused by the interfacial stresses between polymer and BSR particulates during the demixing process [27]. To investigate the mechanism of the alteration of membrane morphology upon blending of BSR, we measured the viscosity of casting solutions. As shown in Fig. 6, the casting solution viscosity decreases gradually with the BSR content. In addition to the decrease of the polymer content, the introduction of BSR particulates also deteriorates the entanglement of PSF polymer chains, so that the PSF/BSR solution viscosity decreases. As a result, the solvent and nonsolvent exchange in the demixing process is sped up and further promoted by the interaction of hydrophilic BSR particulates with water as a coagulant. Therefore, a greater turbulence can occur on the interface of separated phase in the demixing process leading to high roughness and porosity. It is

known that high porosity and low casting solution viscosity usually have negative impacts on the mechanical strength of obtained membranes [28]. To investigate effects of the BSR blending on membrane strength, the tensile strength of membranes were measured. As shown in Fig. 6, the tensile strength of membranes decreases with the BSR content, as is similar to the trend of casting viscosity as a function of the BSR content. In spite of that, the PSF/ BSR4 membrane has a tensile strength of 2.0 MPa, which is still fair for an UF membrane [29]. 3.1.3. BET measurements To further characterize the membrane as an adsorbent, the specific surface area (SSA) of membranes was measured and shown in Fig. 7. We see the membrane SSA drops upon adding of BSR, presumably due to the significant alteration of the phase inversion process by the addition of BSR leading to large pore size. However, when the BSR content is high, such as PSF/BSR3 and PSF/BSR4

Fig. 6. Mechanical properties of PSF/BSR membranes and viscosity of the casting solution.

Z. Wang et al. / Chemical Engineering Journal 308 (2017) 557–567

563

Fig. 9. Pure water flux and BSA rejection of the PSF (a), PSF/BSR1 (b), PSF/BSR2 (c), PSF/BSR3 (d) and PSF/BSR4 (e) membranes. Fig. 7. Specific surface area of the PSF and PSF/BSR membranes.

3.2. Boron adsorption properties membranes, the SSA again increases. The SSA of PSF/BSR4 membrane is almost double that of PSF/BSR1 membrane, which is believed to be caused by the incorporation of a large amount of BSR particulates with high SSA. A high SSA value is beneficial to the adsorption capacity of the membrane. 3.1.4. Surface wettability and permeation properties The hydrophilicity of the membrane surface was evaluated by water contact angle (WCA) measurements. As shown in Fig. 8, the WCA of PSF/BSR4 membrane decrease significantly, compared with other membranes, presumably due to the incorporation of hydrophilic BSR particulates onto the membrane surface. According to the SEM and AFM results, the increased surface porosity and roughness should also contribute. Permeation flux is the most important parameter defining membrane productivity and efficiency. Fig. 9 shows the pure water flux and BSA rejection of different membranes. As the BSR content increases, the membrane water flux increases gradually. This observation should be due to the increased porosity and surface hydrophilicity with the BSR content. The flux of the PSF/BSR4 membrane is 565 L/m2h at 0.1 MPa, which is typical for high flux UF membranes. We also see that the BSA rejection slightly decreases with increase of the BSR content, indicating minimal variation of mean pore size upon BSR blending. It should be cautioned that the membrane pore size can be underestimated due to the quick adhesion and deposition of BSA aggregate onto membrane surface [30].

Fig. 8. WCA values of the PSF membrane and PSF/BSR membranes.

3.2.1. Effect of the initial boron concentration The effects of initial boron concentration on boron uptake were investigated. Fig. 10 shows the boron uptake as a function of initial boron concentration. We see that the boron uptake increases with the BSR content. It should be noted that the boron uptake of the PSF membrane is minimal, indicating that the boron adsorption is resulted from the complexation reaction of NMG moieties in BSR with boron. It can also be seen that the boron uptake q increases with the boron initial concentration from 5 mg/L up to 900 mg/L. The increase of the boron uptake with the initial boron concentration was believed to be due to the increase of the probability of NGM moieties in BSR reacting with boron species and the increased concentration gradient leading to increased diffusion rate. When the concentration of initial boron solution is higher than 900 mg/L, the boron concentration change upon adding of the membrane cannot be accurately determined, thereby boron uptake cannot be calculated. 3.2.2. Adsorption isotherm In order to elucidate the boron adsorption mechanism, the adsorption isotherm was obtained by plotting boron uptake q as a function of the equilibrium boron concentration ce. Fig. 11 shows the boron adsorption isotherm of the membranes with different BSR content. We see the isotherms have marked inflection points. They start convex and then become concave. Therefore, they can-

Fig. 10. Effects of the initial boron concentration on the boron uptake.

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to examine whether the limiting step has a diffusional or reactive nature, three models, each assuming a different rate-limiting step in the adsorption process, were used to model the kinetic data [32],

Fig. 11. Boron adsorption isotherms of the PSF/BSR membranes and BSR.

not be modeled either with the Langmuir or the Freundlich equations. According to the Giles classification, the isotherm for PSF/BSR membranes can be classified as S-type isotherm. This result suggests that cooperative adsorption occurs in the high equilibrium boron content range when the adsorbate-adsorbate interaction is stronger than the adsorbate-adsorbent interaction, i.e., the adsorbed boric acid molecules may associates forming polyboric acids at high concentration. Similar isotherms were reported by others [31]. It is obvious that the boron adsorption capacity increases with the BSR content. However, it is also interesting that the boron uptake is not proportional to the BSR content. The increase of boron uptake with the BSR content becomes significant for PSF/BSR3 and PSF/BSR4 membranes, which may be ascribed to the increased accessibility of complexing ligands besides their density. 3.2.3. Adsorption kinetics The adsorption kinetics was systematically investigated for PSF/ BSR4 membrane by varying the initial boron concentration from 5 mg/L to 300 mg/L. Fig. 12 shows the results. All the curves have identical profiles. We see that the adsorption equilibrium was reached after 120 min for the PSF/BSR4 membrane, while for the BSR, it takes more than 600 min to reach the equilibrium. In spite that the adsorption capacities of the PSF/BSR4 membrane and the BSR are very similar, as is speculated from the plateau value, the adsorption rate of the PSF/BSR4 membrane is much higher. In order

Film diffusion model : lnð1  FÞ ¼ kf t

ð6Þ

Intraparticle diffusion model : qt ¼ kp t1=2

ð7Þ

Chemical reaction model : 1  ð1  FÞ1=3 ¼ kr t

ð8Þ

where kf, kp and kr are the rate constants for film diffusion, intraparticle diffusion and chemical reaction, respectively; F (qt/qe) is the fractional attainment of the equilibrium. The fitted results are shown in Table 2. Obviously the chemical reaction model shows the highest R2 value and leads to the best regression coefficients, indicating that the adsorption process is limited by complexation reaction between boric acid and vicinal diols. In order to illuminate the mechanism involved in the sorption process, two classical kinetic models, Lagergren pseudo-firstorder Eq. (9) and pseudo-second-order kinetic models Eq. (10) were used to describe the experimental data. The equations are shown as following,Pseudo-first-order model:

  1 qt ¼ qe 1  k t e1

ð9Þ

Pseudo-second-order model:

qt ¼

k2 q2e t 1 þ k 2 qe t

ð10Þ

where qt and qe are boron uptake at time t and the equilibrium respectively, k1 and k2 (min1) are the adsorption rate constants of the pseudo first-order and the pseudo second-order kinetic models respectively. The model parameters and fitted results are shown in Table 3. As shown in Table 3, the pseudo first-order kinetic model has higher R2 values indicating that the pseudo-first order model fits better with the experimental results for both PSF/BSR membrane and BSR. In addition, the theoretical qe value from the pseudo first-order kinetic model and the experimental qe value are close. By comparing the adsorption kinetics of the PSF/BSR4 and the BSR at the 5 mg/L boric acid solution, it can be seen that the PSF/ BSR4 membrane has a similar boron uptake to the BSR but has much higher adsorption rates (k1) than the BSR, which is especially true at low boron concentrations. The higher boron adsorption rate of the PSF/BSR4 membrane than the BSR should be attributed to the segregation of the NMG group along the membrane pore lumen due to the interaction of hydrophilic BSR particulates with water as the coagulant. Interestingly, the membrane has a BSR content less

Fig. 12. Boron adsorption kinetics of the PSF/BSR4 membrane (a) and BSR (b) at different initial boron concentrations.

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Z. Wang et al. / Chemical Engineering Journal 308 (2017) 557–567 Table 2 The parameters of rate-limiting step for boron adsorption of the PSF/BSR4 membrane. c0 (mg/L)

Film diffusion model

5 10 50 100 300

Intraparticle diffusion model 2

Kf

R

0.025 0.021 0.028 0.014 0.027

0.970 0.939 0.923 0.981 0.940

Chemical reaction model

2

Kp

R

0.010 0.025 0.032 0.037 0.116

0.862 0.861 0.929 0.949 0.873

Kr

R2

0.006 0.005 0.007 0.004 0.006

0.989 0.984 0.985 0.970 0.988

Table 3 Kinetic parameters for boron adsorption of the PSF/BSR4 membrane and the BSR. Membrane

PSF/BSR4 PSF/BSR4 PSF/BSR4 PSF/BSR4 BSR

c0 (mg/L)

100 50 10 5 5

First-order fitting constants a

Second-order fitting constants

qe (mmol/g) (experimental)

qe (mmol/g)

k1 (min1)

R2

qe (mmol/g) (calculated)

k2 (min1)

R2

0.547 0.387 0.324 0.120 0.160

0.014 0.019 0.017 0.020 0.002

0.967 0.990 0.977 0.972 0.957

0.695 0.469 0.405 0.147 0.169

0.020 0.043 0.040 0.140 0.138

0.952 0.969 0.948 0.936 0.944

than 40%, considering potential loss of the BSR particulates in the phase inversion process, but still has competitive boron uptake compared with the pristine BSR, which should benefit from the increased accessibility of the NMG group upon the milling process. It is known that the pore blockage due to the porosity in polymer resins can impede access of the complexing ligands with low uptake as a result [33]. The buried ligands can be ‘‘recovered” by milling the resin into finer particulates.

0.518 0.367 0.307 0.116 0.124

tion in an acid solution. It should be an advantage of the adsorption process over RO. In the latter process a much higher pH needs to be reached to pursue a sufficient boron rejection, with high scaling risk added as a result [16].

3.2.4. Effect of pH Boric acid is a weak acid having pKa of 9.25. B(OH) 4 is the dominant species at pH > 9 while H3BO3 is the dominant species at pH < 9, which will render the complexation-based process sensitive to solution pH. Fig. 13 shows the effect of the initial solution pH on the boron adsorption for a 100 mg/L boron solution. The solution pH shows the same tendency to our previous work using NMG as the complexing ligand [17]. The maximum adsorption was observed at pH = 9, which is very close to the pKa of boric acid. The OH- ion can compete with B(OH) 4 to interact with the hydroxy group in an alkaline solution, while the presence of a large amount of H+ can shift the complexing equilibrium to the desorption direc-

3.2.5. Effect of ion strength There are various inorganic salt ions in sea water. The presence of inorganic ions should have impact on the boron removal performance of the membrane since the boron complexing reaction can be regarded as an ion exchange reaction. By adding NaCl from 10 to 500 mg/L into the boron solution, the effect of ion strength on boron adsorption was studied. As shown in Fig. 14, the boron adsorption gradually increases with ionic strength. This phenomenon is attributed to compression of the electrical double layer in high ionic strength conditions, so that the formation of the innersphere surface complexation between boron and vicinal diols is promoted [34]. The evolution of boron uptake with solution ionic strength holds another advantage of the complexing reaction over RO. In the latter case, RO loses its boron rejection when the ionic strength of the feed solution increases because of the suppression of Donnan rejection in high ionic strength conditions [2].

Fig. 13. Effect of pH on boron adsorption (PSF/BSR4, c0 = 100 mg/L).

Fig. 14. Effect of ion strength on boron adsorption (PSF/BSR4, c0 = 100 mg/L, pH = 9.0).

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3.2.6. Flow-through experiments To evaluate the potential of the PSF/BSR membrane removing boron in the field, the flow-through experiments were conducted via filtrating the boron solution through the PSF/BSR4 membrane with a dead-end filtration equipment. The solution pH was adjusted to be 9.0 and the boron solution content was chosen to be 5 mg/L, which is typical for seawater boron content. The flow rate and permeate volume dependence of the boron removal efficiency was investigated and the results are shown in Fig. 15. We see the removal efficiency decreases with the increase of the flow rate. Other researchers have also reported that higher flow rate led to lower dynamic binding capacity for adsorptive fiber materials, which indicated that the boron diffusion from the liquid to the particle surface across the boundary layer is the rate-determining step defining the dynamic adsorption capacity [35]. Quantitative boron removal with boron removal efficiency as high as 97.6% is obtained for the lowest flow rate (0.075 mL/min), which corresponds to a membrane flux of 25 L/m2h and actually falls at the higher end of the typical UF flux range designed for a MBR or UF pretreatment unit [11,36]. The flow rate of 0.075 mL/min is the lowest one we can obtain by adjusting the pressure regulator. But it is reasonable to postulate that for the flow rate used for the UF membrane in the field, quantitative boron removal can be obtained with the PSF/BSR4 membrane. The evolution of the boron removal efficiency with the permeate volume was probed at the flow rate of 0.075 mL/min and the results are shown in Fig. 16(b). We see the removal efficiency is over 95% within the first 10 mL and then drops afterwards, indicating breakthrough of the adsorptive membrane. In this work, we define a drop of boron removal efficiency across 90% as the breakthrough of the membrane, which corresponds to a 11 mL elution volume. By calculating with a typical flow rate of 10 L/m2h for UF in the field, a boron rejection of over 90% can be maintained over 6 h [36]. Based on the above results,

we conclude that the PSF/BSR membrane show good potential replacing current UF membranes to quantitatively remove boron in the pretreatment stage of a RO seawater desalination process. 3.2.7. Cycled use of the membrane The cycled use of the membrane is very important for the application as an adsorbent from the economic perspective. Fig. 16(a) shows the evolution of the boron uptake with the regeneration cycle. We see the boron adsorption capacity of the membrane maintains, although shows slight decline, even after 10 cycled use, the RE value of PSF/BSR4 membrane is still 95.9%, indicating robust recyclability of the membrane. To investigate the cycled use of membranes in flow-through experiments, the membrane was also regenerated by flushing with acid solution. A 5 mg/L solution was used as the feed to correspond with the experiments in Section 3.2.6. Fig. 16(b) shows the boron uptake with cycled times in flow-through experiments. We can see that the RE value is 90.5% after 10 cycled use. The RE value is slightly lower compared with static regeneration experiments, presumably due to the reduced contact time. In spite of that, the PSF/BSR4 membrane still show good potential for application. 4. Conclusions A mixed matrix membrane has been prepared by blending milled boron-specific polymer resin with polysulfone membrane materials via phase inversion. The blending of polymer resin particulates into the membrane matrix leads to increased SSA, improved surface hydrophilicity and enhanced membrane flux. More importantly, the obtained membrane as an adsorbent combines the superiority of the polymer resins on adsorption capacity and superiority of mesoporous membranes on convective transport. As a result, the MMM shows competitive uptake but much

Fig. 15. Effects of the flow rate (a) and permeate volume (b) on the boron removal efficiency of 5 mg/L boron solution with PSF/BSR4 membrane.

Fig. 16. The evolution of the PSF/BSR4 membrane boron uptake with static reuse cycles (a: c0 = 100 mg/L) and with flow-through regeneration cycles (b: c0 = 5 mg/L, V = 10 mL,) at pH = 9.0.

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higher adsorption rate than the polymer resin, which is especially true when the initial concentration is low. By filtrating 10 mL of 5 mg/L boron solution with PSF/BSR4 membrane at a flux of 25 L/m2h, the boron removal efficiency of PSF/BSR4 membrane is 97.6%, indicating quantitative boron removal in a flow-through process. The preparation of hollow fiber membrane elements using the same technique and the exploration of their field performance are underway in the lab. We believe the MMM reported in this work not only is good complexing membrane for boron removal, but also provides a new way to fabricate adsorbents for rapid removal or recycle of gradients of trace amount in aqueous solution. Acknowledgements We gratefully acknowledge support from the National Basic Research Program of China (2014CB660813), the National Natural Science Foundation of China (Grant Nos. 21574100, 21274108) and the Program for Chang-jiang Scholars and Innovative Research Team in University of Ministry of Education of China (Grant No. IRT13084). References [1] S.P. Yu, H. Xue, Y.G. Fan, R.F. Shi, Synthesis, characterization of salicylic-HCHO polymeric resin and its evaluation as a boron adsorbent, Chem. Eng. J. 219 (2013) 327–334. [2] H.N. Liu, B.J. Qing, X.S. Ye, Q. Li, K. Lee, Z.J. Wu, Boron adsorption by composite magnetic particles, Chem. Eng. J. 151 (2009) 235–240. [3] WHO, Guidelines for Drinking-Water Quality, fourth ed., World Health Organization, Geneva, Switzerland, 2011. _ Kıpçak, M. Özdemir, Removal of boron from aqueous solution using calcined [4] I. magnesite tailing, Chem. Eng. J. 189 (2012) 68–74. [5] X.L. Du, J.Q. Meng, R.S. Xu, Q. Shi, Y.F. Zhang, Polyol-grafted polysulfone membranes for boron removal: effects of the ligand structure, J. Membr. Sci. 476 (2015) 205–215. [6] N. Hilal, G.J. Kim, C. Somerfield, Boron removal from saline water: a comprehensive review, Desalination 273 (2011) 23–35. [7] N. Nadav, Boron removal from seawater reverse osmosis permeate utilizing selective ion exchange resin, Desalination 124 (1999) 131–135. [8] A. Fortuny, M.T. Coll, A.M.D. Sastre, Use of methyltrioctyl/decylammonium bis 2,4,4-(trimethylpentyl) phosphinate ionic liquid (ALiCY IL) on the boron extraction in chloride media, Sep. Purif. Technol. 97 (2012) 137–141. [9] D. Kavak, Removal of boron from aqueous solutions by batch sorption on calcined alunite using experimental design, J. Hazard. Mater. 163 (2009) 308– 314. [10] A.E. Yilmaz, R. Boncukoglu, M.T. Yilmaz, M.M. Kocakerim, Sorption of boron from boron containing wastewaters by ion exchange in continuous reactor, J. Hazard. Mater. 117 (2005) 221–226. [11] W. Bouguerra, A. Mnif, B. Hamrouni, M. Dhahbi, Boron removal by adsorption onto activated alumina and by reverse osmosis, Desalination 223 (2008) 31– 37. [12] N. Kabay, O. Arar, F. Acar, A. Hazal, U. Yuksel, M. Yuksel, Removal of boron from water by electrodialysis: effect of feed characteristics and interfering ions, Desalination 223 (2008) 63–72. [13] N. Kabaya, S. Sarpa, M. Yuksela, M. Kitisb, H. Koseog˘lub, Ö. Ararc, M. Bryjakd, R. Semiate, Removal of boron from SWRO permeate by boron selective ion exchange resins containing N-methyl glucamine groups, Desalination 223 (2008) 49–56. [14] M.R. Pastor, A.F. Ruiz, M.F. Chillon, D.P. Rico, Influence of pH in the elimination of boron by means of reverse osmosis, Desalination 140 (2001) 145–152.

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