Hierarchically porous composite scaffold composed of SBA-15

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Functional Inorganic Materials and Devices

Hierarchically porous composite scaffold composed of SBA-15 microrods and reduced graphene oxide functionalized with cyclodextrin for water purification Youngjin Choi, Arjyabaran Sinha, Jihye Im, Hooyeon Jung, and Jaeyun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01845 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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ACS Applied Materials & Interfaces

Hierarchically porous composite scaffold composed of SBA-15 microrods and reduced graphene oxide functionalized with cyclodextrin for water purification

Youngjin Choia,‡, Arjyabaran Sinhaa,‡,†, Jihye Ima, Hooyeon Junga and Jaeyun Kima,b,c.*

aSchool

of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Republic

of Korea bDepartment

of Health Sciences and Technology, Samsung Advanced Institute for Health

Science & Technology (SAIHST), Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea cBiomedical

Institute for Convergence at SKKU (BICS), Sungkyunkwan University (SKKU),

Suwon 16419, Republic of Korea ‡These authors contributed equally. † Present Addresses: Department of Microbiology, Immunology & Infectious Diseases, Cumming School of Medicine, University of Calgary, Canada

* To whom correspondence should be addressed: Jaeyun Kim, [email protected] Telephone: +82-31-290-7252. Fax: +82-31-290-7272

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ABSTRACT

Large-scale decontamination of bisphenol A (BPA) from wastewater under field conditions is an urgent need owing to the harmful toxic effects of BPA on living organisms. In this study, we report the fabrication of a three-dimensional (3D) hierarchically porous composite scaffold composed of mesoporous SBA-15 silica microrods and reduced graphene oxide (rGO-CD) functionalized with β-cyclodextrin (CD) and its application for BPA separation from contaminated water. The macroporous structure was achieved by sacrificial salt leaching and mesoporous structure was derived from the interparticle pores between compressed SBA-15 particles and intrinsic mesopores in SBA-15. The 3D hierarchical macroporous and mesoporous architecture of the scaffold enhances mass transport without any external forces, and the rGOCD component provides good capture sites for BPA in solution via inclusion complexation between CD and BPA. The inorganic SBA-15 component of the scaffold also allows long-term operation of filters by increasing the mechanical strength of the scaffold. The hierarchically porous SBA-15/rGO-CD composite scaffold could separate BPA from contaminated water significantly better than the scaffold without rGO-CD in both batch and filter systems. Our study indicates that the functional hierarchically porous composite scaffold can be a potential material in wastewater treatment technology.

KEYWORDS: reduced graphene oxide, cyclodextrin, bisphenol A, water purification, macroporous mesoporous silica scaffold


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INTRODUCTION Endocrine disrupting compounds (EDCs) are groups of emerging organic pollutants present in water and are a serious threat worldwide owing to their highly toxic effects on human and aquatic life. EDCs can imitate the biological activity of natural hormones, occupy the hormone receptors, or interfere with the transport and metabolic processes of natural hormones. As a result, serious conditions such as reproductive problems, hypospadias, miscarriages, infertility, birth defects, and cancer have emerged in humans and animals.1–6 Among the EDCs that are widespread in water, bisphenol A (BPA) is considered the most critical contaminant because estrogen-like effect of BPA is harmful to organisms. BPA is commonly used as a monomer to produce polycarbonate, epoxy resin, and other plastics, and could be released into the ecosystem during the production phase via wastewater. Thus, the separation of BPA from wastewater is an urgent need worldwide. Nowadays, various conventional methods are used for the separation of EDCs from water that include membrane filtration, biological treatment, photocatalytic degradation, and adsorption-based separation.7-22 Among them, adsorption-based separation is considered the most promising method owing to its high removal capacity, fast adsorption, simple operation, comparatively low cost, and few harmful secondary products.23–25 For this purpose, carbon-based materials, such as activated carbon,26,27 carbon nanotubes,28 porous carbon,29 graphene oxide (GO),30 and graphene,31,32 have received much attention as adsorbents due to their chemical stability, high surface area, extensive pore size distribution, and large-scale production. Among the carbon-based materials, GO has been considered as one of the promising adsorbent materials owing to its hydrophobic and hydrophilic nature of surface, high surface area, and twodimensional flat surface, which offers high loading of functional group on the surface and


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separated high amount of pollutant.31,32,34,35 Furthermore, reduced graphene oxide (rGO) obtained after reduction of GO could have more hydrophobic property than GO due to formation of more sp2-hybridized carbon atom, which is beneficial to separate organic pollutants with hydrophobic domain.34 However, their separation efficiencies vary depending on the chemical and physical nature of the sorbent and the chemical nature of the EDCs. In addition, the inherent aggregation properties of carbon-based materials significantly decrease the number of effective adsorption sites, which leads to a decrease of adsorption capacity and limits the potential application of these materials as good adsorbents. Hence, proper design of the carbon-based materials is still needed by increasing dispersibility in water and exposing effective active sites for the adsorption of pollutants. To overcome the aforementioned problems, various carbon-based nanomaterials modified with unique functionalities have been developed.19-22,33–35 For example, β (beta)cyclodextrin (CD), a cyclic oligosaccharide composed of seven glucose subunits, has shown promising results in separating organic pollutants as a functional molecule conjugated with carbon-based nanomaterials. As CD has a highly hydrophilic surface, is chemically inert, and is inexpensive, the conjugation of CD on the surface of carbon-based nanomaterials allows high colloidal dispersion and stability.36–38 Furthermore and most importantly, the presence of a hydrophobic cavity at the center of its molecular arrangement enables the formation of an inclusion complex with organic pollutants via a hydrophobic-hydrophobic interaction.[19-22] For instance, Liu et al.19 synthesized CD-functionalized magnetic graphene for the effective separation of organic dye molecules from water. Sinha et al.20 investigated the separation of microcystin-LR (Leucine and Arginine) using various CD-functionalized magnetic composites of graphene. Gupta et al.21 prepared CD-functionalized GO and used it for the removal of BPA.


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Hu et al.22 modified carbon nanotube (CNT) with CD and magnetic nanoparticles and used it for the removal of 1-naphthylamine from water. All these studies demonstrated that CDfunctionalized carbon-based nanomaterials have great potential for the removal of organic pollutants. However, most of the reported studies focus on batch adsorption-based removal of pollutants that leads to difficulties associated with the separation of the nano-sized absorbent materials from water after the adsorption process, leaching of nanomaterials into the purified water, and a complex operation process, which can limit the practical application of the materials.26–38 In addition, the low durability of the proposed materials for long-term operation and the great postprocessing challenges of these powder materials may inevitably increase the risk of release of nanoparticles into the treated water body and increase the water treatment cost. All these factors could restrict the practical application of nanomaterials for water treatment. Hence, the development of suitable materials and methods for the effective removal of organic pollutants from water in a large-scale and simple operation is desirable. For this purpose, the design of composite materials consisting of active absorbent nanomaterials and a three-dimensional (3D) scaffold with a hierarchical, multi-scale porous network could be a feasible strategy to solve the aforementioned issues. The presence of a hierarchical, multi-scale porous network composed of macropores and mesopores can allow fast mass transfer kinetics in wastewater, while the presence of abundant active adsorption sites associated with diverse pores in the matrix and the high surface area of active nanomaterials largely enhance the effective contact area with pollutants, consequently increasing water treatment performance.39,40 In addition, use of a 3D porous composite scaffold can avoid the separation of nanoadsorbents from the water with the help of magnetic field or centrifugation techniques.


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In this article, we present a hierarchically porous composite scaffold (HPCS) for efficient separation of the BPA, a representative EDC organic pollutant, from contaminated wastewater. The HPCS was fabricated by combining mesoporous silica microparticles and CD-modified reduced graphene oxide (rGO-CD) nanoparticles via a pressing and salt leaching method (Figure 1). The microrods of SBA-15, a well-known hexagonally ordered mesoporous silica, after being adsorbed with rGO-CD nanoparticles were blended with sodium chloride (NaCl) crystals, which are porogens for macropores, and with a sodium silicate solution, which is a crosslinker between silica particles. After pressing the mixture powder in a cylindrical mold to produce a pellet, the sacrificial NaCl crystals were removed from the resulting disc-shaped pellet to generate a hierarchically porous composite scaffold (HPCS). The resulting scaffold has hierarchical porosity composed of macropores and mesopores, which can contribute to high mass transfer and improved separation efficacy. We investigated the detailed adsorption behavior of BPA on the scaffold under batch conditions and in a trial column filter system for future water purification applications.

EXPERIMENTAL PROCEDURES Synthesis of β-cyclodextrin-functionalized rGO (rGO-CD) nanoparticles For the synthesis of rGO-CD, first a colloidal solution of GO was prepared from graphite powder via a modified Hummer’s method.41 Next, 750 mg of β-cyclodextrin (CD) was dissolved in 20 mL of water followed by the addition of 300 µL of NH3 solution (28 wt%), and this mixture was mixed with 10 mL of 3 mg/mL GO solution while stirring. After 1 h, 50 µL of hydrazine solution was added and the whole solution was heated at 70 °C for 1 h to reduce GO into rGO. A sodium chloride (NaCl) solution was added to the resultant stable black solution and centrifuged


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to remove the unreacted β-cyclodextrin. The washing procedure was repeated three times to remove the unreacted β-cyclodextrin completely, and to produce a neutral pH in the solution with the removal of excess ammonia. Finally, the resulting rGO-CD was dispersed in water and stored for further use. The rGO conjugated with dextran (rGO-dextran) was also prepared in a same synthetic procedure used in rGO-CD. Separation of Bisphenol A (BPA) by rGO-CD nanoparticles The adsorption experiment was performed by adding 1 mg of rGO-CD (or rGO-dextran) to 10 mL solutions of varying BPA concentrations; these solutions were stirred overnight. Next, rGOCD nanoparticles adsorbed with BPA were separated by centrifugation (8000 rpm for 3 min) and the supernatant was used for the determination of the remaining BPA. Synthesis of SBA-15 mesoporous silica microrods and SBA-15/rGO-CD To prepare SBA-15 microrods, 4 g of Pluronic P-123 and 46 mg of ammonium fluoride (NH4F) were dissolved in 150 mL of 1.6 M hydrochloric acid (HCl) solution at room temperature. After complete dissolution, 9.2 mL of tetraethyl orthosilicate (TEOS) were added while stirring at 500 rpm and the mixture was reacted for 20 h at 40oC. After the reaction, the solution was treated at 100oC under static conditions for 24 h. The white precipitates were collected and washed by ethanol and deionized water to remove the HCl residue. After the powder was dried, it was calcined at 550oC for 4 h. The final product (SBA-15) was kept at room temperature for further use. To prepare SBA-15 adsorbed with rGO-CD (SBA-15/rGO-CD), 8 mg of SBA-15 was mixed with rGO-CD solution (2.8 mg/L) to contain 200, 400, or 800 μg of rGO-CD. The mixture was dried at 100oC and the resulting powder (SBA-15/rGO-CD) was stored for further use. Preparation of hierarchically porous scaffold


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The hierarchically porous scaffold was fabricated using the pressing and salt-leaching method. Briefly, 8 mg of SBA-15 or SBA-15/rGO-CD with different loading amount of rGO-CD was blended with 150 mg of sodium chloride (NaCl) with a size ranged from 355 to 500 μm and 2 μL of sodium silicate solution. The powder mixture was placed in a designed cylindrical mold with an inner diameter of 7 mm and was pressed with a pellet press at 127 MPa for 5 min to produce a disc-shaped pellet. The pellet was incubated in an oven at 100oC overnight to induce the siloxane bonding between the adjacent SBA-15 particles. The resulting pellet was immersed in water to remove the sacrificial NaCl templates for generating the macropores. The resulting hierarchical macroporous and mesoporous scaffold (HPSS or HPCS) was finally retrieved, dried and stored at room temperature for further use. The dimension of the scaffold was increased by increasing the total amounts of the components added in the mold or by changing the dimension of mold. Separation of BPA by scaffolds In a batch-type separation, the designated number of HPSS or HPCS loaded with 0, 200, 400, or 800 μg of rGO-CD was immersed into 500 μL of BPA solution (20 mg/L) under static conditions. After 1, 2, 4, or 20 h of immersion, the supernatant was collected and its BPA concentration was measured using UV-Vis spectrometry. In column filter-type separation, six or eight HPSSs or HPCSs loaded with 800 μg rGO-CD were stacked in a pipe with an inner diameter of 7 mm similar to the diameter of the scaffold disc. After stacking of scaffolds in the pipe, 3 mL of 5 mg/L BPA solution was flowed through the pipe and the filtered solution was collected to measure BPA concentration using UV-Vis spectrometry. The separation of Rhodamine B and 1-Naphthol was also conducted in a same separation procedure.

RESULTS AND DISCUSSION


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Synthesis of β-cyclodextrin functionalized rGO First, to provide rGO with a strong affinity for the BPA present in wastewater, rGO nanoparticles conjugated with β-cyclodextrin (CD) was prepared (Figure 1A). In the conjugation procedure, CD was dissolved in water in presence of ammonia and mixed with GO under stirring condition. At this stage, CD were attached on the surface of GO via hydrogen bonding or chemical reaction between hydroxyl groups of CD and epoxide groups of GO. Next, GO was reduced with hydrazine to form CD-functionalized reduced graphene oxide (rGO-CD).19,20,22 Fourier transform infrared (FT-IR) spectra of CD, rGO, and rGO-CD (Figure 2A) show that the rGO-CD exhibits the characteristic bands of CD at 1025, 1160 and 1640 cm-1 derived from the coupled C-O/C-C stretching/O-H bending vibration, the coupled C-O-C stretching/O-H bending vibration, and the HOH vibration of hydroxyl groups, respectively. This clearly indicates CD functionalization on the rGO surface. Thermogravimetric analysis (TGA) was used to estimate the amount of CD attached on the surface of the rGO (Figure 2B). The rGO-CD displayed 18% weight loss in the 200 to 400°C temperature region, whereas bare rGO did not show any weight loss in that temperature region. Such weight loss observed in rGO-CD could be attributed to the decomposition of CD attached to the surface of the rGO. Furthermore, the CD functionalization on rGO did not significantly affect rGO properties such as particle size and surface charge. The rGO and rGO-CD had similar particle size distributions as proven via dynamic light scattering (DLS) (Figure 2C). The surfaces of the rGO and rGO-CD were negatively charged in water, at 40 mV and -30 mV, respectively (Figure 2D). The slight increase of zeta potential in the rGOCD was due to a weak positive surface charge of CD in water.42

rGO-CD as an adsorbent of BPA


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To verify the separation functionality of rGO-CD nanoparticles, 10 mL of BPA solutions in concertation of 20, 40, 60 mg/L were treated with 1 mg of rGO-CD and a dextran-modified rGO (rGO-dextran) as control group. Dextran was used as counterpart material for CD because both are polysaccharide materials composed of same repeating unit glucose, but they have different molecular geometry; dextran is a linear form of repeating glucose units and CD is a cyclic form of 7 glucose units. The cyclic structure of CD makes hydrophobic cavity at the center which capture hydrophobic organic pollutant via hydrophobic-hydrophobic interaction. Therefore, a use of rGO-dextran as a comparing material could reveal the BPA-binding characteristic of rGO-CD due to the existence of the hydrophobic pockets in CDs. After an overnight incubation, the concentration of remaining BPA in solution were measured using UV-Vis spectrometry. In all concentration conditions, BPA solution treated with rGO-CD (blue line) exhibited a significant peak drop at 278 nm wavelength compared to rGO-dextran (red line) and original BPA solution (black line) (Figure 3A-C). Maximal BPA removal efficiency (71%) and removal capacity (7.34 mg/g) was achieved with rGO-CD in 60 mg/L BPA solution, whereas rGO-dextran showed less removal with every concentration with values more than two times lower (Figure 3D, 3E). Such high separation efficiency with rGO-CD compared to rGO-dextran is attributed to formation of a stable inclusion complex between the hydrophobic center at CD functional group and the BPA present in solution.22 This result indicates that rGO-CD can be used for highly efficient separation of BPA.

Hierarchically Porous Silica Scaffold (HPSS) made of SBA-15 While the BPA separation capability of rGO-CD nanoparticles is good, retrieval of those nanoparticles after water purification remains a problem. We hypothesized that a hierarchically


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porous scaffold made of mesoporous silica microparticles combined with rGO-CD could provide easy retrieval of the rGO-CD nanoparticles while maintaining their efficiency in organic pollutant separation. Hierarchical pores including macropores, interparticle pores generated between mesoporous silica microparticles, and intrinsic mesopores of the mesoporous silica component would be helpful for enhancing the contact between functional surface and pollutants (Figure 4A). In this context, we first fabricated a hierarchically porous silica scaffold (HPSS) with simultaneous macropores and mesopores based on our previous report,43 and analyzed the pore structure of the scaffold. We used SBA-15, hexagonally ordered mesoporous silica microrods that have high surface area (642 m2/g), high pore volume (1.2 cm3/g), and 4.3 nm mesopores, to prepare the HPSS. The SBA-15 microrods were blended with sodium chloride (NaCl) crystals ranging in size from 355 to 500 μm, which are porogens for macropores, and with sodium silicate solution, which is a crosslinker between silica particles. After pressing the mixture powder in a cylindrical mold to produce a disc-shaped pellet, the sacrificial NaCl crystals were removed from the pellet to generate the HPSS. The pore structures of pristine SBA15 and HPSS prepared using 2 μL of sodium silicate as an interparticle crosslinker were analyzed by N2 sorption analysis (Figures 4B, 4C and Table 1). The pristine SBA-15 showed a typical hysteresis between 0.6 and 0.8 P/P0 derived from the cylindrical mesopores (Figure 4B). The HPSS showed a similar hysteresis in the isotherm with slightly lower pore volume at approximately 4 nm compared to the pristine SBA-15 (Figure 4C). This is presumably because the high pressure applied to the mixture of SBA-15 and NaCl crystals to prepare the HPSS may shrink the mesopores and because the sodium silicate could block the pore entrance of the SBA15. Almost no hysteresis between 0.6 and 0.8 P/P0 was found and the intrinsic mesopore peak disappeared for the scaffold prepared using more sodium silicate (4 μL) (Figures S1A and S1B),


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indicating that the excess sodium silicate blocked most mesopores or pore entrances during fabrication. As a result, lower surface area (19.99 m2/g) and pore volume (0.75 cm3/g) occurred compared with SBA-15 and HPSS. Interestingly, the HPSS exhibited a significantly high pore volume with a distinct second hysteresis between 0.9 and 1.0 P/P0 while pristine SBA-15 showed only a small adsorption in the same region. We speculated that the second hysteresis and the resulting significantly high pore volume are derived from interparticle pores generated between the compressed SBA-15 microparticles in HPSS. To prove the existence of interparticle pores in HPSS, we crushed the HPSS into powder to destroy interparticle pores and analyzed the pores in the crushed HPSS. The second hysteresis in the isotherm was significantly reduced in the crushed HPSS (Figures 4B and 4C). Furthermore, the pore volumes corresponding to the mesopores between 30 and 100 nm were significantly reduced compared to those of the intact HPSS. The pore volumes of the crushed HPSS and intact HPSS measured at approximately 4.3 nm pore size (intrinsic mesopore of SBA-15) were similar (Figure 4C), while the pore volume measured at approximately 70 nm (interparticle mesopore generated between pressed SBA-15 particles) was decreased by half for the crushed scaffold. Crushing of the HPSS did not destroy the intrinsic mesopores of the SBA15 component but destroyed approximately half of the interparticle mesopores in the HPSS. This observation implies that the HPSS possesses interparticle mesopores between compressed SBA15 particles that are larger than the intrinsic mesopores in SBA-15. Scanning electron microscope (SEM) images of the HPSS show the macropores generated by removal of NaCl crystals, and the absence of macropores in the HPSS before NaCl leaching (Figure 4D and Figures S1C, S1D). Taken together, HPSS prepared with SBA-15 mesoporous silica via NaCl templating has multi-scale pores including macropores (~400 μm), interparticle mesopores (30-


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100 nm), and intrinsic mesopores of SBA-15 (4.3 nm), which would be beneficial for access by the pollutant small molecules throughout the composite scaffold combined with rGO-CD.

Hierarchically porous composite scaffold (HPCS) functionalized with rGO-CD for BPA separation To investigate BPA separation by the hierarchical scaffold functionalized with rGO-CD, four types of scaffold were prepared with rGO-CD amounts varying from 0 to 800 μg. The SBA-15 microrods were first adsorbed with rGO-CD nanoparticles, and the resulting SBA-15/rGO-CD composite materials were used to fabricate the HPCS following the same procedure as for HPSS preparation. The color of the scaffold turned darker with increasing rGO-CD amount adsorbed on the SBA-15 microrods (Figure 5A). The incorporation of rGO-CD in the scaffold showed no changes in surface morphology and macropores at the top and in the cross-section (Figure S2A), or in intrinsic mesopores (Figure S2B), compared to the HPSS. The adsorbed rGO-CD on SBA15 microrod was not detached from the scaffold during NaCl leaching step (Figure S2C, S2D). The dimension of the scaffold could be easily controlled by simply increasing the amounts of basic component materials (SBA-15 and NaCl) or by using larger-sized templating mold (Figure 5B). For example, using double or triple weights of the basic component materials in 7 mm diameter templating mold, HPSS and HPCS with a height of 4.2 mm and 6.6 mm were fabricated, respectively. In addition, HPSS with a larger diameter (13 mm) was also successfully fabricated using quadruple weight of SBA-15 and NaCl in 13 mm diameter template mold. The comparison between HPCS and non-macroporous composite pellet prepared by pressing SBA15/rGO-CD powder without adding NaCl clearly demonstrate the existence of macropores in HPCS (Figure 5C). The resulting non-macroporous composite pellet was very thin (0.28 mm)


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compared to macroporous 1x HPCS (2 mm in height). When even triple amounts of SBA15/rGO-CD was used, the pellet was still thinner (0.87 mm) than 1x HCPS. These data verified that hierarchically porous composite scaffold can be easily prepared in diverse scale by controlling amount of base material or size of template mold, which might be applicable for the scale-up fabrication demanded in industrial level. To prove the BPA separation capability of the HPCS, BPA separation from contaminated water was performed under static batch conditions (Figure 6A). First, HPCS with varying rGOCD amounts was incubated with BPA solution (Figure 6B). One of each scaffold (0, 200, 400, and 800 μg) was immersed into a 20 mg/L BPA solution for 20 h. The HPSS without incorporation of rGO-CD showed no BPA separation ability, while all HPCSs showed higher BPA separation. Increased rGO-CD amounts in the HPCS led to higher BPA separation efficiencies. After 20 h of immersion, the HPCS loaded with 800 μg rGO-CD reduced the BPA concentration to 44.8%, whereas the HPCSs loaded with smaller amounts of rGO-CD showed lower BPA separation. This data indicates that rGO-CD has the main role in adsorbing BPA and that rGO-CD incorporated in the HPCS is still active probably due to the high porosity of the scaffold. Based on these data, HPCSs with 800 μg rGO-CD were chosen optimal HPCS for further experiment. We further tested the BPA separation by simply increasing the quantity of HPCSs (Figure 6C). One, two, and four HPCSs, denoted as 1x, 2x, 4x HPCS, were immersed in 20 mg/L BPA solution for 20 h, showing that a greater quantity of scaffolds leads to a faster and greater BPA separation ability. Four scaffolds reduced the BPA concentration to 30% within 1 h and to 13% within 20 h. Furthermore, contaminated water by Rhodamine B (20 mg/L) and 1Naphthol (20 mg/L) was treated with HPCSs for 24h to verify the separation function of HPCS for other organic pollutants. The results showed that HPCS can effectively separate Rhodamine


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B and 1-Naphthol from the contaminated water (Figure S3). The concentration of Rhodamine B was reduced to 33.7% and 8.5% with single HPCS (1x HPCS) and triple HPCS (3x HPCS) after 24h in batch-type treatment, respectively. The concentration of 1-naphthol was also reduced to 45.1% and 23.2% with 1x HPCS and 3x HPCS after 24 h, respectively. These data suggest that HPCS can be used to remove diverse organic pollutants from contaminated water. Lastly, as a proof of concept, the application of the scaffold to organic pollutant separation in a column filter system was further investigated (Figure 6D). The filter-type separation system was built by stacking the scaffolds in a pipe, and the gap between the scaffold outer surfaces and the pipe inner surface was sealed to enhance water flow through the stacked scaffold filter. To first verify the importance of macroposity in HPCS in filter-type separation system, we set up two filter systems using six macroporous HPCS or six non-macroporous SBA15/rGO-CD composite pellets and Rhodamine B solution (1 mL, 5 mg/L) was flowed through the pipes (Figure 6E). The whole Rhodamine B solution easily passed HPCS layers within 60 seconds without additional pressure, while no penetration was observed in case of nonmacroporous composite filters during the same time. This result clearly presents the importance of macropores for the efficient mass transfer in the filter-type separation system. Next, to check BPA separation by HPCS over time, a solution with 20 mg/L BPA was supplied through the pipe filled with six HCPS without external pressure. The BPA concentration of the filtered solution was quickly dropped to 80% within 70 seconds (Figure 6F). The filter composed of eight HPCSs showed a significantly higher separation of up to 27.5% of BPA from the contaminated solution compared to a filter prepared with the same quantity of HPSSs that showed only 7.5% removal (Figure 6G). The efficiency of filter-type system is relatively lower than that of the static batch condition, which would be attributed to the shorter contact time between the scaffold


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and the contaminated water in filter system (around 2 min) compared to that in the batch system (20 h). For industrial application, separation efficiency of flow condition should be enhanced. As the advantage of filter system is more rapid and continuous processing than the batch system, achieving both facile processing and high separation efficiency in the filter system needs to be further investigated. Increasing the number of scaffolds in the filter or controlling the macropore size of the scaffolds could be a possible way to enhance the contact time between the active part of the scaffold and the contaminated water and thus to increase the separation efficiency while maintaining the rapid processing compared to the batch system. Taken together, in both batch and column filter systems, the HPCS could be a promising platform to separate BPA from contaminated water.

CONCLUSIONS For organic pollutant separation, carbon-based nanoparticles and cyclodextrin are promising for chemical capture. Use of these nanoparticles for organic pollutant separation has limitations such as difficulty of retrieval during usage under batch conditions. To solve the problems, a threedimensional macroporous nanocomposite is a desirable form. We presented a three-dimensional hierarchically porous scaffold that enables efficient BPA separation from water by incorporation of cyclodextrin-modified reduced graphene oxide nanoparticles into SBA-15 silica scaffold. The resulting composite scaffold could efficiently separate BPA from the polluted system. The purification capability could be controlled by the amount of rGO-CD. The potential use of the composite porous scaffold was demonstrated in a batch-type system as well as in a filtration system. The hierarchically porous composite scaffold as an organic pollutants adsorbent is a promising material in filtration systems for wastewater treatment.


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AUTHOR INFORMATION Corresponding Author * Corresponding author. E-mail: [email protected] Telephone: +82-31-290-7252. Fax: +82-31-290-7272 Present Addresses † Department of Microbiology, Immunology & Infectious Diseases, Cumming School of Medicine, University of Calgary, Canada Author Contributions All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Conflicts of interest There are no conflicts to declare.

ACKNOWLEDGMENT This work was supported by the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Science and ICT, Republic of Korea (20100027955, 2014M3A9B8023471, 2019R1A2C2004765).


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Figure 1. Schematic of fabrication of by hierarchically porous composite scaffold (HPCS) using reduced graphene oxide conjugated to cyclodextrin (rGO-CD) and SBA-15 microrods and their application to organic pollutant separation. (A) Preparation of rGO-CD through the reduction of graphene oxide (GO) in the presence of cyclodextrin (CD). (B) Adsorption of rGO-CD with SBA-15 microrods to prepare SBA-15/rGO-CD. (C) Fabrication of HPCS using a mixture of SBA-15/rGO-CD and NaCl crystals via pressing and salt-leaching process. (D) Separation of BPA with HPCS in batch- or column filter-type systems.


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Figure 2. Characterization of rGO and rGO-CD nanoparticles using (A) Fourier transform infrared (FT-IR), (B) thermogravimetric analysis (TGA), (C) dynamic light scattering (DLS) for hydrodynamic size measurement, and (D) zeta potential.


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Figure 3. Ultraviolet-visible spectra of BPA solutions at concentrations of (A) 20 mg/L, (B) 40 mg/L, and (C) 60 mg/L, separated by cyclodextrin-functionalized rGO nanoparticle (rGO-CD) and the control dextran-functionalized rGO nanoparticle (rGO-dextran). (D) BPA removal efficiency and (E) removal capacity of rGO-CD and rGO-dextran depending on concentration of BPA solution.


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Figure 4. (A) Schematic of pristine SBA-15, hierarchically porous silica scaffold (HPSS), and pressed SBA-15 particles in HPSS. Interparticle pores of pressed SBA-15 are indicated by red arrows. (B) Isotherms and (C) pore size distributions for nitrogen gas sorption with pristine SBA-15, HPSS, and crushed HPSS. (D) Scanning electron microscopy (SEM) images of HPSS before and after NaCl leaching.


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Pristine SBA-15

HPSS

Crushed HPSS

HPSS w/ 4μL S.S.

641

244

410

20.0

1.20

1.12

1.22

0.75

Surface area (m2/g) Pore volume (cm3/g)

Table 1. Surface area and pore volume of pristine SBA-15, HPSS, crushed HPSS and HPSS prepared with an addition of 4 μL sodium silicate.


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Figure 5. Photographs of (A) HPCSs with varying amounts of rGO-CD loaded in the scaffold (0, 200, 400, and 800 μg), (B) HPSSs and HPCSs with different heights (upper) and diameters (lower). (C) Photograph of HPCS with macroporosity (left), non-macroporous composite pellets prepared with triple (middle) and same (right) amounts of SBA-15/rGO-CD.


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Figure 6. (A) Schematic image of batch-type separation system. The BPA concentration after HPCS treatment under static batch condition depending on (B) rGO-CD amount in scaffold and (C) the number of HPCSs with loaded with 800 μg rGO-CD. (D) Schematic image of filter-type separation system. (E) Photographs of Rhodamine B separation by filter-type separation system built with six HPCSs (left tower) and six non-macroporous composite pellets (right tower) over time. (F) The BPA concentration in filtered solution based on filter-type separation with six HPCSs over time. (G) The BPA concentration in filtered solution by filter-type separation with eight HPSSs and eight HPCSs.


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TOC Hierarchically porous composite scaffold composed of mesoporous SBA-15 silica microrods and reduced graphene oxide (rGO-CD) functionalized with β-cyclodextrin (CD) were designed and fabricated for BPA separation.


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Hierarchically porous composite scaffold composed of SBA-15

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