Preparation of Superhydrophilic Adsorbents with 3DOM Structure by

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Preparation of Super-hydrophilic Adsorbents with 3DOM Structure by Watersoluble Colloidal Crystal Templates for Boron Removal from Natural Seawater Xueri Nan, Jing Liu, Xiuli Wang, Xianhui Pan, Xiaomei Wang, and Xu Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11763 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 7, 2018

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ACS Applied Materials & Interfaces

Preparation of Super-hydrophilic Adsorbents with 3DOM Structure by Water-soluble Colloidal Crystal Templates for Boron Removal from Natural Seawater Xueri Nan,a Jing Liu,b Xiuli Wang,a Xianhui Pan,b Xiaomei Wang,a* Xu Zhang,a* a

School of Chemical Engineering, Hebei University of Technology, Tianjin 300130, P.R. China

b

The Institute of Seawater Desalination and Multipurpose Utilization, SOA, Tianjin, China

KEYWORDS: Three-dimensionally ordered macroporous, Water-soluble colloidal crystal template, Adsorption, Boron, Seawater

ABSTRACT:

Three-dimensionally

ordered

macroporous

cross-linked

poly(glycidyl

methacrylate) (3DOM CLPGMA) was constructed by water-soluble colloidal crystal templates (WS-CCTs) and further functionalized with N-methyl-D-glucamine (NMDG) to prepare superhydrophilic adsorbents for boron removal from natural seawater. 3DOM adsorbents possess features of interconnected macropore structure, ultrathin pore wall and super-hydrophilicity, makes efficient adsorption possible. The effect of cross-linking degree (CLD) on the adsorption

ACS Paragon Plus Environment

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capacity toward boron was investigated. The NMDG-modified 3DOM adsorbent with rich vicinal diols functional groups showed super-hydrophilicity and outstanding performance of adsorption. Significantly, its adsorption effect in boron removal from natural seawater indicated that the concentration of boron in natural seawater could decline to 0.16 mg·L-1 from 4.24 mg·L1

when the adsorbent dosage was 1 g·L-1, while the boron rejection reached 96.2%. After ten

regeneration-adsorption cycles, the adsorption capacity of 3DOM adsorbent remained over 85% of the initial value and the ordered structure was hardly changed. Additionally, 3DOM adsorbent could be directly and quickly separated from the seawater by filter mesh of 16 mesh number. Research shows that the 3DOM adsorbent exhibits an adsorption performance for practical application in boron removal from natural seawater.

INTRODUCTION As a widespread nonmetallic element, boron is a requisite trace nutrition for human and animal,1 and it also affects the growth of plan.2 However, it becomes toxic if the intake of boron is slightly excessive.3 At present, most countries in the world prescribes the limit that the highest boron concentration of drinking water is 0.5 ppm.4 Since the shortage of freshwater resources is more prevalent,5 seawater desalination has a high probability of solving this serious problem.6 Reports show that the average concentration of boron in natural seawater is about 4.5 mg·L-1,7 which is far higher than the drinking-water standard. However, it is difficult to effectively remove from untreated seawater by common desalination strategies due to its small size and uncharged state.8 Hence, it is urgent to exploit high-efficient, facile, and low-cost methods to remove boron from natural seawater.


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Currently, various separation methods have been developed rapidly to remove boron from waterbodies, including reverse osmosis (RO, all abbreviations are shown in Table S1), adsorption, electrocoagulation, donnan dialysis, chemical coagulation, and so on.9 Adsorption is considered as a low-cost and effective method among these technologies.10 Various adsorbents have been exploited in adsorption processes, such as selective resins,11 natural minerals,12 mesoporous silica,13 activated carbon,14 and complexing membranes.15 The study of polymeric adsorbent has received an extensive attention due to their special properties,16 which can be designed at the molecular level,17 and it also has other properties of high toughness, low density, and facile functionalization.18 In addition, boron usually exists as B(OH)3 and B(OH)4− in aqueous solutions according to the pH of aqueous solutions.19 However, in aqueous media, it can be formed a complex between boron and cis-diol of polyols,13 which provides a wide way to the design and synthesis of many types of boron-specific adsorbents. Recently, porous organic polymers (POPs) have attracted much attention, since their special properties achieved by combining the advanced properties of both porous structures and functionalities of polymers.20 Three-dimensionally ordered macroporous (3DOM) materials have interconnected macroporous structure, high porosity (approximately 75%), and ultrathin pore wall.21,22 which provide high-speed diffusion channels for diffusion of molecules inside the materials,23,24 and enable the entire surface to be modified uniformly by various surface modification techniques.25 Therefore, in this research, 3DOM structure and functional polymeric carrier were selected to design and prepare POPs of boron-specific adsorbents. To more facilely introduce the rich vicinal diols groups on POPs, cross-linked poly(glycidyl methacrylate) (CLPGMA) is selected as a polymer supporter and the N-methyl-D-glucamine (NMDG) is chosen as the functional group. The epoxy groups of CLPGMA are easily opened


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under mild conditions, while NMDG possesses polyols and secondary amine ends which can induce nucleophilic reaction of epoxy groups. However, it is difficult to obtain 3DOM CLPGMA using conventional colloidal crystal templates (CCTs) technology. In general, hydrofluoric acid and organic solvents are chosen to remove the monodisperse silica and oil-soluble linear polymeric microspheres, respectively.26 Unfortunately, epoxy groups will be opened if silica CCTs are used, and if linear polymeric microspheres act as CCTs, it will cause the CCTs to be corroded by the monomers precursor. Meanwhile, these methods are dangerous for operators and the environment. Hence, it is great vital for the preparation of 3DOM materials that CCTs can be removed under mild conditions. Polyacrylamide (PAM) is a water-soluble (WS) polymer and it is hardly dissolved by most organic solvents.27 Additionally, it can be degradated by biodegradation and bioconversion technologies.28 Therefore, PAM microspheres were selected to assemble the WS-CCTs and further to prepare the 3DOM CLPGMA. In this work, the 3DOM CLPGMA with different cross-linking degrees (CLDs) were prepared by WS-CCTs and further functionalized with NMDG (3DOM CLPGMA-NMDG). The preparation process is depicted in Scheme 1. The effect of CLDs, hydrophilicity, and pH levels on the adsorption capacity as well as adsorption isotherms and adsorption kinetics were researched by batch adsorption experiments. Furthermore, the practical adsorption performances of the 3DOM adsorbent in natural seawater were investigated.


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Scheme 1. Process diagram for preparing 3DOM CLPGMA-NMDG by WS-CCTs.

EXPERIMENTAL SECTION Chemicals. Glycidyl methacrylate (GMA), ethylene glycol dimethacrylate (EGDMA), acrylamide (AM), polyvinylpyrrolidone (PVP-K30, Mw=30,000 g·mol−1), N-methyl-Dglucamine (NMDG), boric acid, methanol, ethanol and N, N-dimethylformamide (DMF) were obtained from Aladdin Industrial Corporation. Benzoyl peroxide (BPO) and 2, 2'-azobis (2methylpropionitrile) (AIBN) refined by methanol were purchased from Shanghai Dibai. The pH value was adjusted by sodium hydroxide (NaOH) and hydrochloric acid (HCl) solutions. Deionized (DI) water was supplied by an Ulupure water purification apparatus (18.25 MΩ·cm-1 at 25 oC, UPR-11-10T, Sichuan, China). The natural seawater was provided by the Institute of Seawater Desalination and Multipurpose Utilization, SOA (Tianjin, China). Fabrication of WS-CCTs. PAM microspheres with average diameter size of 600 nm were prepared by suspension polymerization. Typically, AM (40 g), PVP (16 g), 120 mL of ethanol and 120 mL of DI water were put into the 1 L reactor equipped with a mechanical stirrer under the protection of argon. After stirring for 30 min, the initiator solution (0.4 g BPO in 160 mL ethanol) was slowly added at 80 oC. The polymerization last for 8 h under mechanical stirring at


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100-120 r/min. The synthesized PAM microspheres were rinsed with fresh absolute ethanol for more than three times, and then collected by centrifugation and finally dried in the dryer for further use. Preparation of 3DOM CLPGMA. The precursors with different ratios of GMA, EGDMA, and AIBN were made up, and the recipe of the precursor was listed in Table S2. After vacuum of the WS-CCT to remove air, the precursor mixture was injected into the WS-CCT and further immersed for 1 h. After removal of the excess precursor, polymerization was carried out under argon protection at 70 oC for 24 h to prepare PGMA@WS-CCT composite. Subsequently, the WS-CCT in the composite was removed with DI water. The 3DOM CLPGMA with different CLDs were obtained according to the same way and by drying at 45 oC under vacuum. The products labeled as 3DOM CLPGMA-0.5, 3DOM CLPGMA-6, 3DOM CLPGMA-10, 3DOM CLPGMA-20, 3DOM CLPGMA-40, 3DOM CLPGMA-50, 3DOM CLPGMA-70, and 3DOM CLPGMA-90, respectively. The number in labeler symbolized the CLD, and it was determined by the equation as shown in Table S3. Preparation of 3DOM CLPGMA-NMDG adsorbents. The 3DOM CLPGMA-NMDG adsorbents were prepared as follows: 3DOM CLPGMA (0.5 g), NMDG (1.61 g), and DMF (50 mL) were added in the single-neck flask equipped with a magnetic stirrer. After stirring for 12 h at 25 oC, the reaction last for 24 h at 80 oC. Subsequently, the products were filtrated and rinsed with anhydrous methanol for more than three times. Finally, the 3DOM CLPGMA-NMDG adsorbents were freeze-dried within 1, 4-dioxane and labeled as 3DOM CLPGMA-NMDG-0.5, 3DOM CLPGMA-NMDG-6, 3DOM CLPGMA-NMDG-10, 3DOM CLPGMA-NMDG-20, 3DOM CLPGMA-NMDG-40, 3DOM CLPGMA-NMDG-50, 3DOM CLPGMA-NMDG-70, and 3DOM CLPGMA-NMDG-90, respectively.


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Characterization. Diameter of PAM microsphere was measured in ethanol using a Malvern NanoZS90 Zetasizer. The Fourier transform infrared (FT-IR) spectra were recorded on Bruker VECTOR-22 with KBr pellets. The morphology of material was characterized using fieldemission scanning electron microscopy (SEM, FEI Nova NanoSEM450, USA). The 3DOM adsorbents before and after adsorption of boron were analyzed through energy-dispersive X-ray spectrometer (EDX) analysis of SEM with an EDAX TEAM Octane Pro analyzer. X-ray photoelectron spectroscopy patterns were collected on an X-Ray photoelectron spectrometer (XPS, Thermo ESCALAB-250Xi, USA). The pore structure of the material was determined by ASAP 2020M+C surface area and porosity analyzer. The number of NMDG groups was measured according to the nitrogen content analyzing by a Flash EA 1112 elemental analyzer. Water contact angle (WCA) was measured by contact angle analyzer (KRÜSS DSA 30) using 10 µL deionized water droplet and fitted by Young-Laplace equation (20-180o) and Circle method (0-20o), respectively. The boron element contents and the main components of the natural seawater were measured on an inductively coupled plasma optical emission spectrometer (ICPOES, iCAP 6000 SERIES ICP Spectrometer, Thermo Fisher Scientific), inductively coupled plasma mass spectrometry (ICP-MS, ICAP Q ICP-MS, Thermo Fisher Scientific), and total organic carbon (TOC) analyzer (Shimadzu TOC Analyzer, Japan). Boron batch adsorption experiments. For each batch, the accurately weighted adsorbents (0.1 g) were put into a conical flask of high-density polyethylene and mixed with 20 mL of boric acid solutions with different initial concentrations (5~500 mg·L-1) at 25 oC. After shaking for 24 h, the boron concentration was measured by ICP-MS and ICP-OES. For the research of kinetic adsorption, boron solution of 100 µL was taken at the constant time from 20 mL solution to


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determine the boron content. The adsorption capacity could be computed from the equation (Table S3). Boron adsorption in natural seawater and regeneration. The practical efficiency of the adsorbent for boron removal from the nature seawater was evaluated. The sorbents dosages of 1, 3 and 5 g·L-1 were tested, respectively. Other operations were described in the boron adsorption experiments. The main components of natural seawater were analyzed by ICP-OES, ICP-MS and TOC analyzer. The saturated adsorbents were desorbed with 30 mL HCl (0.1 M) solution immersion for 12 h. Afterwards, the adsorbents were immersed in 30 mL NaOH solution (0.1 M) and washed by DI water to neutral. The regeneration efficiency (RE) of adsorbents was evaluated by the equation (Table S3). RESULT AND DISCUSSION Preparation and characterization of materials Figure 1 demonstrates the FT-IR spectra of the PAM WS-CCTs, 3DOM CLPGMA-6, and 3DOM CLPGMA-NMDG-6. In the PAM WS-CCTs spectrum (Figure 1a), around 3208 and 1633 cm-1 were attributed to the stretching vibration of -NH2 and C=O for PAM, respectively.29 In the 3DOM CLPGMA-6 spectrum (Figure 1b), the band presenting in 1735 cm-1 is a distinct peak, attributed to the stretching vibration of carbonyl in the CLPGMA. Compared with Figure 1a, the peaks at 1633 and 3208 cm-1 are disappeared, which prove that the PAM WS-CCTs were completely removed. The peaks at 907 and 844 cm-1 result from symmetric stretching of the epoxy ring. Moreover, the peaks existing in 1261 and 1151 cm-1 symbolize the C-O symmetric


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and asymmetric vibrations in CLPGMA.30 After the ring cleavage reaction of epoxy groups with NMDG (Figure 1c), it is worth noting that the distinctive epoxy bands are disappeared. Moreover, a stronger absorption peak of 3420 cm-1, results from the stretching vibration of hydroxyl from 3DOM CLPGMA-NMDG-6.31 Meanwhile, the FT-IR spectrum of the resultant NMDG-containing resin shows many significant changes. The signals in the region of 1161 and 1087 cm-1 are characteristic of the NMDG group, which attribute to the CH-OH and C-O vibrational bands and confirm the introduction of NMDG groups in the polymer.32

Figure 1. FT-IR spectra of (a) PAM WS-CCTs, (b) 3DOM CLPGMA-6, and (c) 3DOM CLPGMA-NMDG-6. The chemical composition of adsorbents before and after modification were ascertained by XPS. The wide spectra of 3DOM CLPGMA-6, 3DOM CLPGMA-NMDG-6, and 3DOM CLPGMA-NMDG-6 after adsorption are shown in the Figure S1 (a, c, and e). The emission peaks at 191.3 eV, 285 eV, 398.8 eV, and 531.8 eV are attributed to B 1s, C 1s, N 1s, and O 1s, respectively. Compare Figure S1 b with Figure S1 d the new peak at 285.3 eV of C 1s is attributed to C-N, which indicates successful anchoring of NMDG into the 3DOM CLPGMA-6.


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In addition, other components of C 1s peaks at 288.5 eV, 286.0 eV, and 284.6 eV are attributed to COO, C-O, and C-C/C-H species, respectively. The wide spectra of 3DOM CLPGMANMDG-6 after adsorption of boron are shown in Figure S1 e. The new peak of B 1s appears in the spectrum, which testifies boric acid is adsorbed into the adsorbents. The O 1s peak (Figure S1 f) of adsorbent after adsorption at 531.3 eV and 532.5 eV, which assign to COO and O-B/OH, respectively. The corresponding compositions of materials (Table S4). The increased O/N ratio for 3DOM CLPGMA-NMDG-6 after adsorption are attributed to boric acid adsorbed into the adsorbents. The SEM images of PAM CCTs, 3DOM CLPGMA-6, and 3DOM CLPGMA-NMDG-6 are presented in Figure 2. The PAM microspheres with the average diameter of 600 nm are regular arrangement into WS-CCTs (Figure 2a). As shown in Figure 2b~c, 3DOM CLPGMA-6 and 3DOM CLPGMA-NMDG-6 exhibit 3DOM structure and the macroporous are interconnected with small windows. This highly interconnected pore structure greatly increases mass transfer efficiency and the ultrathin pore walls increase the utilization of functional groups within the materials.24,25 Meanwhile, the EDX was performed on the 3DOM CLPGMA-NMDG-6 before and after adsorption of boron (Figure S2). It can be seen that the percentage of oxygen after adsorption is significantly increased, but due to the small atomic mass of boron, the peak of boron may be covered by the adjacent peak of carbon. However, the increased of oxygen indirectly indicates that boric acid was adsorbed into the adsorbents, and the peak of nitrogen demonstrates that the material has been successfully modified.


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Figure 2. The SEM images of (a) WS-CCTs; (b) 3DOM CLPGMA-6; (c) 3DOM CLPGMANMDG-6. The scale bars are 3 µm. The pore structure was analyzed by surface area and porosity analyzer (Table S5). The pore size and BET surface area of 3DOM CLPGMA-NMDG-6 are 31.4 nm and 6.06 m2·g-1, respectively. Figure S3 demonstrates N2 adsorption-desorption isotherms and pore diameter distribution of materials before and after functionalization. Both 3DOM CLPGMA-6 and 3DOM CLPGMA-NMDG-6 show Type III isotherm,33 which indicates material possess the macroporous structure.34 The result of pore diameter distribution (Figure S3b) is attributed to the cross-linking agent that it formed a bridge among the polymer molecular chains. Furthermore, the specific surface area and the average pore diameter of 3DOM adsorbent slightly decreased after modification, may be the reason that plenty of polar groups were uploaded into the pore wall of the 3DOM CLPGMA-NMDG-6.35 Generally, the wettability of porous adsorbent can affect the diffusion of adsorbate solution into the adsorbent interior and accordingly affect adsorption property. Figure 3a shows the evolution of WCA before and after modified materials with different CLDs. Before modification, the 3DOM CLGMA are strong hydrophobicity and WCA are over 130o. After immersing in aqueous solution even for 24 h, the 3DOM CLGMA-6 still floated on the solution (Figure 3b,


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left). The 3DOM CLGMA has a hydrophobic surface, since ester and epoxy groups are weakly polar. After modification, when the CLD is greater than 10%, the WCA increases with increasing of CLDs. When the CLD is less than 10%, the 3DOM CLPGMA-NMDG exhibits super-hydrophilicity (WCA is approximately 0°) due to the rough surface and abundant hydroxyl groups.36,37 Because of stronger hydrogen bonding, the hydrophilic of the hydroxy group is high than ester or epoxy groups. In the aqueous solution, the 3DOM CLPGMA-NMDG-6 sunk in the solution rapidly (Figure 3b, right). As shown in video S1, water completely infiltrated into the 3DOM CLPGMA-NMDG-6 after 15 seconds due to abundant hydroxyl groups and the interconnected macroporous structure. Super-hydrophilicity and rapid infiltration should promote the diffusion of boric acid aqueous solution inside the material and rapidly reach the adsorption equilibrium. Furthermore, the 3DOM adsorbent is the millimeter-scale size (Figure 3b), which is convenient to separate the adsorbent from solution.

Figure 3. (a) The changes of WCA before and after modification of materials with different CLDs; (b) the photograph of 3DOM CLPGMA-6 (left) and 3DOM CLPGMA-NMDG-6 (right).


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Boron bath adsorption The 3DOM CLPGMA-NMDG adsorbents with multi-hydroxyl groups of NMDG can chelate boron via single or double cis-diol structure (Figure 4a). The mechanism of boron chelation with multi-hydroxyl groups was presented in the 1997 and 1998.38,39 Therefore, it is likely that the more multi-hydroxyl groups will increase the adsorption capacity toward boron. Furthermore, stabilized interconnected macroporous structure and the ultrathin pore wall of the former CLPGMA matrix are also significant, which facilitates boron to active sites.

Figure 4. (a) Proposed mechanism of boron chelation for NMDG modified adsorbents and (b) the effect of CLD on adsorption capacity and N/C mole ratio. The adsorption capacities of the 3DOM CLPGMA-NMDG and the 3DOM CLPGMA samples with different CLDs were compared. It can be seen that the adsorption capacities of 3DOM CLPGMA towards boron are low and the maximum is only 0.13 mmol·g-1 at 6% CLD (Figure S4). After modification, the adsorption capacity of the 3DOM CLPGMA-NMDG is significantly improved. The 3DOM CLPGMA-NMDG-6 exhibits the largest adsorption capacity than others. As shown in Figure 4b, the adsorption capacity and N/C mole ratios of 3DOM CLPGMA-


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NMDG are extremely influenced by CLDs (Figure 4b, the N/C mole ratio is calculated according to the data in Table S6). The quantity of multi-hydroxyl groups decreased with the increased of the CLDs, leading to the adsorption capacity decreasing. However, the adsorption capacity of 3DOM CLPGMA-NMDG-0.5 is lower than 3DOM CLPGMA-NMDG-6/ -10. The CLD not only affects the amounts of functional groups but also impacts the morphology of materials. As shown in Figure S5a, the morphology of 3DOM CLPGMA-NMDG-0.5 shows that the structure is severely collapsed and the interconnected windows are disappeared after functionalization due to the low CLD. The structures of the other 3DOM CLPGMA-NMDG with the high CLD still maintain their original morphology (Figure S5b-h). Therefore, the well-interconnected macroporous structure is dominant in the adsorption process, which increases the utilization of reactive sites in the pore walls and reduces the material diffusion resistance. Although the specific surface area of 3DOM adsorbent is not high (Table S5), abundant functional groups, interconnected macropore structure and ultrathin pore wall are the main reasons to obtain the outstanding adsorption performance. In the subsequent investigation, 3DOM CLPGMA-NMDG6 is selected as the optimized experimental adsorbents since it has the highest adsorption capacity among all adsorbents. pH value is a significant parameter for boron adsorption. As shown in Figure 5, the adsorption capacity toward boron increases with pH from 3 to 8 and reaches the maximum (1.13 mmol·g-1) at around pH 8. The adsorption capacity decreases with the increase of pH when the pH is over 8 (Figure 5a). Meanwhile, 3DOM CLPGMA-6 also has the similar boron adsorption behavior toward boron, but its adsorption capacities at different pH value are few (Figure 5b). The effect of pH on adsorption can be interpreted according to the property of boric acid as a quite weak Lewis acid in water (the pKa is 9.2 at 298 K).16 Distribution diagrams of boron in the different


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form at various pH in solution are also shown in Figure 5. When the pH is high, it can form －

B(OH)4− by coupling OH .9 A large number of OH− ions are free in aqueous solution. There is a competition between the OH− ions and polyol groups interaction with boric acid. When the pH is low, the complex reaction is gradually restrained because of the protonation of boric acid, leading to boron uptake declining.

Figure 5. The relationship between adsorption capacity and pH value: (a) 3DOM CLPGMANMDG-6 and (b) 3DOM CLPGMA-6 towards boron in aqueous solution, C0=100 mg·L-1 at 298 K (species distribution of boron with the different forms at different pH in solution are shown together). Figure 6 displays the boron adsorption isotherm. It can be observed that the adsorption capacity improves with the increasing of initial concentration of boron. The adsorption capacity of 3DOM CLPGMA-NMDG-6 reaches 2.34 mmol·g-1 at the original concentration is 500 mg·L1

. The experimental data were analyzed by Freundlich and Langmuir adsorption equations (Table

S3). Table 1 and Figure 6 present the fitting results. It can be found that the adsorption behavior of the 3DOM CLPGMA-NMDG-6 can be well described by the Freundlich isotherm since R2 >


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0.98. Furthermore, it can conclude that the adsorption for boron is a favorable process because the n values are much more than 1.40 Table 1. Characteristic parameters for the boron adsorption on the 3DOM CLPGMA-NMDG-6 from aqueous solution at 298 K. (pH=8.0±0.1)

Material 3DOM CLPGMA-NMDG-6

Freundlich constants kf 1/n R2 (mmol/g)(L/mmol)1/n 0.311 0.321 0.982

Langmuir constants qm b R2 (mmol/g) (L/mmol) 2.22 0.022 0.884

Figure 6. Equilibrium adsorption isotherm of boron on 3DOM CLPGMA-NMDG-6 from aqueous solution at 298 K (pH=8.0±0.1). Figure 7 shows kinetic curves for adsorption of boron on 3DOM CLPGMA-NMDG-6 at diverse initial boron concentrations. It illustrates that all the rates of boron adsorptions are fast at the beginning. Pseudo-first-order and pseudo-second-order kinetic models (Table S3) as classical kinetic models were applied to fit the kinetic data. The corresponding constants are summarized in Table S7. The kinetic data at different initial concentrations can be well fitted by the pseudosecond-order model since the calculated values (qe) are similar to experimental values as well as


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R

2

> 0.99. Therefore, the combination of above adsorption mechanism analysis, it can be

concluded that the chemisorption may be the main controlling factor in the adsorption process. This result is consistent with the reported adsorption mechanism.38,39 In addition, as the values of constant k2 increasing with the initial concentration suggests that the higher initial concentration is in favor of improving the rate of boron adsorption.

Figure 7. Boron adsorption kinetics of the 3DOM CLPGMA-NMDG-6 at the different initial concentrations of boron: (a) C0=108.8 mg·L-1, (b) C0=54.3 mg·L-1, and (c) C0=5.4 mg·L-1. (pH=8.0±0.1, 298 K) Adsorption in natural seawater Currently, seawater reverse osmosis (SARO) is a widely used technology to desalinate seawater.41,42 However, many operating parameters for SARO extremely affect boron rejection, such as pH value, pressure, temperature, and so on.7,43 For example, when the pH is around 7, the boron rejection is 30~83% by using of low-pressure SARO membranes; but it remarkably increases to 90~99% when the pH is 10.5.44,45 However, in practice, it is complicated and tedious to adjust the pH of seawater. In contrast, the operating parameters have the small effect for


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polymeric sorbent of 3DOM CLPGMA-NMDG-6, and the pH of the natural seawater is around 7.5~8.5,45 which is favorable for the 3DOM adsorbent adsorption (Figure 5). In this research, the 3DOM adsorbent was applied to remove boron from the natural seawater. Table S8 lists the primary chemical components of the natural seawater and the concentration of boron is 4.24 mg·L-1. After adsorption using 3DOM CLPGMA-NMDG-6 with the different dosage in the seawater, as shown in Table 2, all the final concentration at equilibrium are below 0.2 mg·L-1, which is far less than 0.5 mg·L-1. The boron rejection is higher than 95% at the different adsorbent dosage, which is the outstanding performance for most of the other current technologies. Most of boron can be removed from seawater by only 1 g·L-1 adsorbent, and the adsorption capacity is 4.08 mg·g-1. Figure 8a shows the changes of the boron concentration in the seawater with the contact time. It takes around 240 min to achieve the adsorption equilibrium, longer than that of boron batch adsorption experiments, which may be caused by the effect of the other ions with the high concentration in the natural seawater. Moreover, elemental compositions of the 3DOM CLPGMA-NMDG-6 before and after desorption are analyzed by XPS (Figure 8b and c, respectively), and the results are summarized in Table S9. After adsorption, small amount ions of seawater remain in the adsorbent. However, these ions are almost completely removed after desorption by the acid soaking method, which provide a great chance to regenerate the 3DOM adsorbent.


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Table 2. Boron rejection at different adsorbent dosage. (T=298 K) Adsorbent Dosage (g·L-1) 1 3 5 a

C 0a (mg·L-1) 4.24

Initial concentration of boron in the natural seawater.

C eb (mg·L-1) 0.163 0.013 0.004 b

Boron rejection (%) 96.16% 99.69% 99.91%

Equilibrium concentration of boron in

the natural seawater after adsorption.

Figure 8. (a) The equilibrium concentration of different time at 1 g·L-1 adsorbent dosage. (T=298 K); the XPS wide spectra of 3DOM CLPGMA-NMDG-6 (b) after adsorption and (c) after desorption. The reusability of the adsorbent is very important in the practical applications. Hence, the regeneration performance of the 3DOM CLPGMA-NMDG-6 was investigated in this work. As


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shown in Figure 9a, the regeneration efficiency (RE) is nearly 100% in the first of successive three cycles and the RE is above 85% in the following sequential seven cycles, demonstrating high reliable reusability. The morphology of the adsorbent after ten regeneration-adsorption cycles is perfectly preserved (Figure 9b). Meanwhile, the N/C mole ratio of 3DOM CLPGMANMDG-6 does hardly decrease after ten recycles (Table S5). Especially, it is easy to separate the 3DOM adsorbent from the seawater quickly by filter mesh of 16 mesh number due to the millimeter-scale size (Figure 3b). The results indicate that the 3DOM CLPGMA-NMDG-6 excellent reusable performance to be used in desalination.

Figure 9. (a) The evolution of the RE value of the 3DOM CLPGMA-NMDG-6 with the reuse cycles and (b) SEM image of adsorbent after 10 cycles. Comparison with other adsorbents on seawater In addition, the boron adsorption performance of 3DOM CLPGMA-NMDG-6 was compared with literatures reported on seawater (Table S10, Supporting information). In fact, there are a few reports about the practical application in removal of boron from seawater. It can be seen that the 3DOM CLPGMA-NMDG-6 has the highest boron rejection but the lowest adsorbent dosages as


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well as the excellent reusability. Hence, it should be more efficient and economic for the 3DOM polymeric adsorbent is combined with SARO, and the combination can motivate the application of SARO technology. CONCLUSION In summary, the WS-CCTs were used to prepare boron-specific 3DOM adsorbents, and NMDG was further anchored in the ultrathin pore wall as adsorption group, which result in the adsorption capacity increases with the decreasing of the CLD. For the 3DOM adsorbent, although it has the low specific surface area, adsorbents obtain the excellent adsorption performance due to high functional group content, interconnected macropore structure, and ultrathin pore wall. It exhibited the rapid adsorption equilibrium of boron within 45 min. The Freundlich model and the pseudo-second-order model could well describe the adsorption isotherm and the kinetic data, respectively. The optimal adsorption occurs near a neutral pH region. The adsorbents of 3DOM CLPGMA-NMDG-6 could efficiently remove (over 95%) boron from natural seawater without any treatment. In addition, the RE is above 85% after ten regeneration-adsorption cycles and it can facilely separate from the seawater by the filter mesh. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The abbreviations of manuscript; The recipe of the precursor; the equations of manuscripts; the XPS spectra of material before and after modification; EDX spectra of before and after adsorption; N2 adsorption isotherms and pore size distribution; the adsorption capacity of 3DOM


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CLPGMA; elemental analysis of different CLD adsorbents; SEM images of adsorbents with different CLDs; chemical composition of seawater. (PDF) The video of WCA measures for 3DOM adsorbent. (MPG) AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] and [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 51573038, 51403049, 50903027) and the Natural Science Foundation of Hebei Province (No. E2016202261 and E2017202036). REFERENCES (1) Hunt, C. D. Dietary Boron: An Overview of the Evidence for its Role in Immune Function. J. Trace Elem. Exp. Med. 2003, 16 (4), 291-306. (2) Xu, F.; Goldbach, H. E.; Brown, P. H.; Bell, R. W.; Fujiwara, T.; Hunt, C. D.; Goldberg, S.; Shi, L. Advances in Plant and Animal Boron Nutrition, Springer, Dordrecht, the Netherlands, 2007, pp 83-90. (3) Weir, R. J.; Fisher, R. S. Toxicologic Studies on Borax and Boric Acid. Toxicol. Appl. Pharmacol. 1972, 23, 351-364.


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Table of Contents (TOC)


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Scheme 1. Process diagram for preparing 3DOM CLPGMA-NMDG by WS-CCTs. 40x9mm (600 x 600 DPI)



Figure 1. FT-IR spectra of (a) PAM WS-CCTs, (b) 3DOM CLPGMA-6 and (c) 3DOM CLPGMA-NMDG-6. 66x51mm (300 x 300 DPI)


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Figure 2. The SEM images of (a) WS-CCTs; (b) 3DOM CLPGMA-6; (c) 3DOM CLPGMA-NMDG-6. All the scale bars are 3 μm. 43x10mm (300 x 300 DPI)



Figure 3. (a) The change of WCA before and after modification of materials with different CLDs; (b) the photograph of 3DOM CLPGMA-6 (left) and 3DOM CLPGMA-NMDG-6 (right). 64x23mm (600 x 600 DPI)


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Figure 4. (a) Proposed mechanism for the boron adsorption by NMDG functionalized adsorbents and (b) the effect of CLD on adsorption capacity and N/C mole ratio. 65x24mm (300 x 300 DPI)



Figure 5. Effect of the solution pH on the adsorption capacity of (a) 3DOM CLPGMA-NMDG-6 and (b) 3DOM CLPGMA-6 towards boron in aqueous solution, C₀=100 mg·L⁻¹ at 298 K (distribution diagram of boric acid and borate ions in solution and adsorption capacity at various pH are shown together). 56x38mm (300 x 300 DPI)


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Figure 6. Equilibrium adsorption isotherms of boron on 3DOM CLPGMA-NMDG-6 from aqueous solution at 298 K (fitted to Langmuir and Freundlich models are shown together with the experimental data points, pH=8.0±0.1). 63x48mm (300 x 300 DPI)



Figure 7. Boron adsorption kinetics of the 3DOM CLPGMA-NMDG-6 at the different initial boron concentrations: (a) C₀=108.8 mg·L⁻¹, (b) C₀=54.3 mg·L⁻¹ and (c) C₀=5.4 mg·L⁻¹. (pH=8.0±0.1, T=298 K) 60x43mm (600 x 600 DPI)


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Figure 8. (a) The equilibrium concentration of different time at 1 g·L⁻¹ adsorbent dosage. (T=298 K); the XPS wide spectra of 3DOM CLPGMA-NMDG-6 (b) after adsorption and (c) after desorption. 89x45mm (300 x 300 DPI)



Figure 9. (a) The evolution of the RE value of the 3DOM CLPGMA-NMDG-6 with the reuse cycles and (b) SEM image of adsorbent after 10 cycles. 63x47mm (300 x 300 DPI)


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Table of Contents (TOC) 35x14mm (300 x 300 DPI)


Preparation of Superhydrophilic Adsorbents with 3DOM Structure by

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