Three-Dimensional Macroporous Alginate Scaffolds Embedded with

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Three-Dimensional Macroporous Alginate Scaffolds Embedded with Akaganeite Nanorods for the Filter-Based High-Speed Preparation of Arsenic-Free Drinking Water Arjyabaran Sinha,† Bong Geun Cha,† and Jaeyun Kim*,†,‡,§ †

School of Chemical Engineering, ‡Department of Health Sciences and Technology, Samsung Advanced Institute for Health Science & Technology, §Biomedical Institute for Convergence at Sungkyunkwan University, Sungkyunkwan University, Suwon 16419, Republic of Korea S Supporting Information *

ABSTRACT: Separation of arsenic from water is an urgent worldwide issue because of its serious toxic effect on human health and aquatic life. In this study, a filter device composed of three-dimensional (3D) macroporous alginate/akaganeite composite (MAAC) scaffolds is proposed for the convenient separation of arsenic from contaminated water. Akaganeite nanorods with superior arsenic adsorption capability are incorporated and distributed within the macroporous alginate scaffold, without significant aggregation. The micron-sized pores and oxygen functional groups in the MAAC scaffold offer enhanced mass transport of the contaminated water throughout the scaffold without the need for input of any force and allow easy contact of arsenic with the active adsorption sites of the scaffold. The high mechanical strength of the MAAC facilitates structural stability of the materials in aqueous solutions. Moreover, the scaffold is capable of excellent arsenic adsorption and can reduce the concentration of arsenic in contaminated water to the acceptable drinking-water level (10 μg L−1). We also demonstrate that a column filter device constructed by stacking several MAAC scaffolds enables a continuous supply of drinking water with a permissible limit of arsenic according to the World Health Organization in a highpurification speed (∼22−25 mL min−1 under gravity), which could potentially provide an appropriate technology to obtain arsenic-free drinking water in developing countries. KEYWORDS: arsenic removal, macroporous scaffolds, akaganeite nanoparticles, filters, water treatment

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INTRODUCTION Contamination of groundwater by toxic metals such as arsenic, lead, cadmium, and mercury is a serious global threat because of their high toxicity to human health and aquatic life.1−3 Exposure to toxic metals even in very trace amounts causes serious diseases like cancer, kidney damage, lung disease, skin pigmentation, birth defects, etc.4,5 Thus, the development of suitable materials and methods that efficiently separate toxic metals from contaminated water is becoming an urgent issue in environmental pollution and water purification. Currently, several traditional methods already exist for the separation of toxic metals from wastewater, such as precipitation and coagulation,6 ion exchange,7 membrane filtration, 8 bioremediation, 9 and adsorption-based approaches.10−12 However, most of these methods suffer from high costs, complex operating processes, sludge disposal problems, slow processing, and low removal efficiencies, particularly at low concentrations. Among these, adsorptionbased separation is considered to be a promising water treatment method because of its high removal capacity, fast adsorption rate, and simple operation process. For this approach, nanomaterials are very attractive candidates to be © XXXX American Chemical Society

used as good adsorbents because of their high surface areas and thermal stabilities.13−15 In the past few decades, various types of nanomaterials, including metal/metal oxide nanoparticles,16−18 porous materials,19−22 alumina,23,24 carbon-based materials,25,26 and metal/metal oxide composites,27−30 have been extensively studied. Of late, numerous groups have reported that ironbased materials, mainly including akaganeite (β-FeOOH), have a high affinity toward the adsorption of heavy metals.10−13 For example, Deliyanni et al. synthesized akaganeite microparticles and investigated the removal of arsenic from water by batch adsorption or a bed column.31 Zhang and Jia described the synthesis of akaganeite nanorods in solution phase and studied their arsenic separation efficiency from water by batch adsorption.32 Obregón et al. used ultrafine akaganeite nanoparticles for the adsorption-based separation of the toxic metal ion chromium from water.33 Patra and Kim reported the synthesis of mesoporous α-FeOOH by the self-assembly of akaganeite nanoparticles and studied mercury adsorption Received: March 10, 2018 Accepted: April 5, 2018 Published: April 5, 2018 A

DOI: 10.1021/acsanm.8b00389 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

Scheme 1. Schematic Representation of the Facile Synthesis of a 3D MAAC Scaffold for the Preparation of Arsenic-Free Drinking Water

properties from water.34 Although various sized iron-based nanomaterials have been used for the separation of heavy metal, their adsorption capacity significantly increased in smaller-sized particles compared to those of larger ones.35,36 However, smaller-sized particles have a high tendency to aggregate, resulting in a decrease in the active adsorption sites of akaganeite, and thus lead to a lower adsorption performance.37−39 In addition, difficulties associated with the separation of nanoparticles from water after the adsorption process, leaching of nanoparticles into purified water, and a complex operating process limit their practical application. To overcome these limitations, several materials have been used as supports to disperse the small-sized nanoparticles, which allow exposure of the adsorption sites to capture toxic metal ions and their facile separation after use.23,24,29,30 However, most studies have shown that large contents of nanoparticles encapsulated in supporting materials could aggregate into larger particles within the matrix, which affects mass diffusion, transportation, and the consequent performance of effective adsorption.40−42 Furthermore, the low durability of the proposed materials for longterm operation25−30 and great challenges of postprocessing of these powder-shaped particles31−34 may inevitably increase the release risk of nanoparticles into the treated water body and also increase the water treatment cost. All of these factors could restrict the practical application of nanomaterials for water remediation. Thus, it is highly desirable to develop suitable materials and methods that can efficiently separate toxic metals from water in large scales and enable the fast delivery of clean water with a simple operation procedure. For this purpose, the design of macroporous materials composed of active nanomaterials within their three-dimensional (3D) networks could be one of the most promising approaches to solving the aforementioned problems. In the 3D porous materials, the presence of macropores offers fast masstransfer kinetics and reduces mass-transfer resistance. Furthermore, the presence of abundant active adsorption sites associated with pores in the matrix and high surface area of active nanomaterials largely enhances the effective contact area with contaminants, consequently promoting the water treat-

ment performance.43,44 In this context, we present a preparative column consisting of a 3D macroporous alginate/akaganeite composite (MAAC) scaffold for the large-scale separation of toxic metals from water by a simple operation process. Alginates are polymers of (1−4)-linked β-D-mannuronic acid and α-L-guluronic acid monomers, and they form highly porous 3D networks via simple freeze-drying after covalent crosslinking.45 The presence of large amounts of carboxyl and hydroxyl groups on the alginate provides structural stability to the small-sized akaganeite nanorods against the aggregation and leaching of iron via hydrogen bonding, van der Waals interaction, or covalent binding between the akaganeite nanorods and alginate polymer chain during fabrication of the scaffold.29 This offers easy contact of water with the akaganeite nanorods, enabling better adsorption of the heavy metals and facilitating the separation of other toxic metal ions such as Pb2+ via coordination bonding of the carboxylate group and metal ion. Moreover, the high-mechanical-strength alginate aerogel with micron-sized interconnected macropores allows a high flow rate of the contaminated water throughout the scaffold, enhancing the separation efficiency compared to that of a nonmacroporous scaffold by facilitating the contact between arsenic ions and akaganeite within the scaffold matrix. We found that the MAAC scaffold efficiently separated the toxic arsenic ions from contaminated water in large scales at high flow rates and the amount of arsenic in the separated water is lower than the amount stipulated by the World Health Organization (WHO).

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RESULTS AND DISCUSSION Fabrication of the MAAC Scaffold. The fabrication process of the MAAC scaffold and the strategy for the separation of toxic metals are illustrated in Scheme 1. Akaganeite nanorods synthesized by the hydrolysis of FeCl3 at 80 °C were mixed with an alginate solution to prepare a 1.5 wt % alginate solution.46 At this stage, akaganeite nanorods were homogeneously distributed in the alginate solution via hydrogen bonding or van der Waals interaction. Subsequently, alginate hydrogels loaded with akaganeite nanorods were B


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Figure 1. (a and b) Low- and high-resolution TEM images of akaganeite nanorods. (c) Digital image of the MAAC-50 scaffold. (d) TEM image of the MAAC-50 hydrogel before freeze-drying. (e and f) SEM images of the cross sections and surfaces of the MAAC-50 scaffold. (g) X-ray elemental mapping of the MAAC-50 scaffold exhibiting the presence of carbon, nitrogen, oxygen, and iron components. (h) FTIR spectra of the akaganeite nanorods before and after embedding in the scaffolds. Scaffolds embedded with akaganeite nanorods display fingerprint spectra of the alginate and akaganeite nanorods. (i) Compressive stress−strain curves for the MAAC-0 and MAAC-50 scaffolds in the wet state.

prepared by covalently cross-linking the alginate polymer via carbodiimide chemistry using adipic acid as a cross-linker, which allowed the scaffold to maintain its macroporous structure after lyophilization and subsequent rehydration of the alginate scaffold.47 Finally, the alginate hydrogel embedded with the akaganeite nanorods was freeze-dried to obtain a 3D MAAC scaffold. The amount of akaganeite nanorods was varied from 0 to 66 wt % with respect to alginate. The resulting scaffolds are referred to as MAAC-x, where x indicates the percentage of akaganeite nanorods. Characterization of the MAAC Scaffold. Low- and highresolution transmission electron microscopy (TEM) images clearly show the highly crystalline rod-shaped structure of the akaganeite nanorods with high crystallinity (Figure 1a,b). Photographs of the MAAC scaffold embedded with different amounts of akaganeite nanorods show the porous structure of the resulting MAAC scaffolds (Figures 1c and S1a). Before freeze-drying, the akaganeite nanorods embedded in the hydrogels were observed by TEM (Figures 1d and S1d). The akaganeite nanorods were homogeneously distributed in the hydrogel without significant aggregation, and their density in the hydrogel increased with an increase in the percentage of akaganeite. The macroporous structure of the MAAC scaffold was confirmed by scanning electron microscopy (SEM). The SEM images (Figures 1e,f and S1c) show the well-defined and

highly interconnected macroporous network of the scaffold with micron-sized macropores on the surface as well as the interior of the matrix, without any deformation of the 3D macroporous network with increased amounts of akaganeite nanorods. Elemental mapping of the MAAC scaffold shows that iron is well-distributed in the entire scaffold, indicating the uniform distribution of akaganeite nanorods in the 3D macroporous composite (Figure 1g). A powder X-ray diffraction (XRD) pattern of the MAAC scaffold displays characteristic peaks of the akaganeite nanorods, demonstrating their successful incorporation into the scaffold (Figure S2). The Fourier transform infrared (FTIR) spectrum of the MAAC scaffold shows characteristic peaks at 1030, 1405, 1617, and 2930 cm−1 corresponding to the C−O−C stretching of saccharides, amide N−H bending vibration, COO− stretching vibration, and CH2 stretching vibration, respectively (Figure 1h).48 The peak at 690 cm−1 is due to the Fe−O vibration of the akaganeite nanorods, revealing the existence of embedded akaganeite nanorods in the 3D macroporous alginate network. High-resolution X-ray photoelectron spectroscopy (XPS) of the MAAC scaffold exhibits the characteristic peaks of Fe 2p3/2 and Fe 2p1/2 and the satellite peak of Fe3+ at 710.8, 724.9, and 716.2 eV, respectively, also demonstrating the presence of akaganeite nanorods in the MAAC scaffold (Figure 4a).49 The narrow-scan C


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Figure 2. (a) Effect of the amount of akaganeite embedded in the scaffolds on the separation of arsenic. (b) Effect of the contact time on the adsorption of arsenic by MAAC-50. A total of 100 mg of the material was used for 100 mL of a 2 mg L−1 arsenic solution. (c) Arsenic removal efficiency of the MMAC-50 scaffold at various initial concentrations of arsenic. (d) Adsorption isotherms of arsenic(III) and arsenic(V) on the MAAC-50 scaffold, where qe is the amount of arsenic adsorbed on MAAC-50 and Ce is the equilibrium concentration of arsenic. (e) Effect of the coexisting anion on the separation of arsenic using the MAAC-50 scaffold.

C 1s XPS spectrum of the MAAC scaffold shows that the peaks located at binding energies of 284.7, 286.2, and 287.9 eV can be assigned to C−C, C−O, and CO bonds, indicating the presence of hydroxyl and carboxyl groups in the scaffold (Figure S3a).50 The relative contents of C−O (43%) and C O (20%) present in the scaffold were calculated from the deconvoluted XPS spectrum of C 1s, which were mainly coming from hydroxyl and carboxyl groups, respectively. To check the porous structure of the scaffolds, N2 adsorption/ desorption isotherms of only alginate and akaganeite nanorods embedded alginate scaffolds were employed (Figure S3). N2 adsorption/desorption curves show that both the MMAC-0 and MAAC-50 scaffolds exhibited type IV isotherms, indicating the presence of mesopores in the materials. The corresponding Brunauer−Emmett−Teller surface areas of the MAAC-0 and MAAC-50 scaffolds were 27 and 20 m2 g−1, respectively. The Barrett−Joyner−Halenda pore-size distribution curves obtained from the desorption of both samples also showed the presence of 37- and 72-nm-sized pores at the peaks. The pore volumes for MAAC-0 and MAAC-50 were 0.58 and 0.38 cm3 g−1, respectively. The presence of mesopores and macropores can predominately increase the effective contact area with the toxic metal ion, and a macropore can also allow easy transport of contaminated water through scaffolds without an external force. It is important to consider the mechanical properties of the MAAC scaffold for developing a structurally stable waterpurifying filter device for durable use. To evaluate the mechanical properties of the MAAC scaffold, compressive stress−strain curves of the scaffolds were measured in the wet state (Figure 1i). The maximum compressive strengths of MAAC-0 and MAAC-50 are 8 and 5.5 kPa, respectively, and

the maximum strain reached over 75%. Such high strength and strain values suggest that the high structural stability of MAAC could prevent leaching of the embedded akaganeite nanorods from the matrix during possible mechanical deformations, which is essential for materials used in filter-based water purification systems. Adsorption Behaviors of Arsenic/Lead/Chromium on the MAAC Scaffold. The application potential of the MAAC scaffold for water purification was investigated by investigating the removal efficiency of arsenic from contaminated water at a high initial arsenic concentration (2 mg L−1; Figure 2a). The experimental procedure used for heavy-metal separation is depicted in Figure S4. First, MAAC scaffolds were added to contaminate water, followed by gentle stirring for a few hours to achieve maximum adsorption of arsenic. Subsequently, MAAC scaffolds were removed from the water, and the remnant arsenic in the water was measured using inductively coupled plasma mass spectroscopy (ICP-MS). The results show that the removal of arsenic by the MAAC-0 scaffold was negligibly small, which might be due to the electrostatic repulsion between the negatively charged carboxyl groups in alginate and the negative HAsO4− ions in water. In contrast, the removal efficiency of arsenic was increased with increasing amounts of akaganeite nanorods embedded in the MAAC scaffold (Figure 2a). This is because the active sites for arsenic adsorption were increased with increasing amounts of akaganeite nanorods. However, the removal efficiency of arsenic did not increase significantly after a certain amount of akaganeite nanorods (50%) embedded in the scaffolds, which might be due to partial blockage of the active adsorption sites of arsenic in scaffolds with akaganeite nanorod contents greater D


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Figure 3. (a) Photograph of a column-type filter device prepared by stacking MAAC-50 scaffolds for the purification of arsenic-contaminated water (200 μg of arsenic L−1) mimicking natural contaminated water. The contaminated water was passed through the column in a continuous flow. The flow rate of water through the MAAC column was ∼22−25 mL min−1 under gravity. (b) Enlarged image of the filter consisting of several stacks of MAAC-50. (c) Concentration of arsenic in purified water filtered through a filter device over time.

mg−1 for arsenic(III) and 24.4 mg g−1 and 0.15 L mg−1 for arsenic(V), respectively. These arsenic separation values are higher compared to those of other reported methods (Table S2). We calculated the RL values for a better understanding of the feasibility of adsorption and the shape of the Langmuir isotherm. RL > 1 indicates that adsorption is unfavorable, 0 < RL < 1 indicates that adsorption is favorable, and RL = 1 and 0 indicate linear and irreversible adsorption isotherms, respectively.34,52 The calculated RL values are 0.30 and 0.21 for arsenic(III) and arsenic(V) adsorption on MAAC, respectively, indicating that arsenic adsorption is favorable. The values of the Freundlich constants n and KF are 1.96 and 5.7 for arsenic(III) and 2.56 and 4.9 for arsenic(V), respectively, representing the high adsorption affinity of arsenic on MAAC-50. The arsenic removal efficiency in the presence of coexisting anions that are generally present in natural water was also tested (Figure 2e). The presence of Cl−, NO3−, SO42−, and PO43− did not have any significant effect on the removal efficiency of arsenic, suggesting the practical application of MAAC-50 for real arsenic separation from contaminated water. Recycling after regeneration is another essential factor for the practical application of such materials. For regeneration, the arsenic-adsorbed MAAC scaffold was first treated with a 0.5 M NaOH solution for 2 h and repeatedly washed with water until the pH became neutral. In a highly alkaline medium, all of the carboxylate and hydroxyl groups of alginate and akaganeite become negatively charged, resulting in desorption of the anionic arsenic species.21,53 In the second cycle, the air-dried regenerated materials were used for the separation of arsenic (Figure S5e), demonstrating that the MAAC scaffold could be reused with little loss in the removal efficiency. This slight loss of the removal efficiency might be arising from the inefficient detachment of arsenic from akaganeite nanorods present inside scaffolds in the desorption step. Filter-Based Approach for the Large-Scale and HighSpeed Preparation of Arsenic-Free Drinking Water. On the basis of the positive results of the arsenic adsorption performance by MAAC-50 scaffold in the batch experiment, we further evaluated the performance of an arsenic-separating column composed of several stacked MAAC scaffolds for largescale, continuous purification of arsenic-contaminated water (Figure 3). A total of 15 MAAC-50 scaffolds with a total weight of ∼1 g were tightly packed in the column, and highly arseniccontaminated water (200 μg L−1) mimicking natural contaminated water was passed through the column (Figure 3a,b). The flow rate of water through the MAAC column was ∼22−

than 50 wt %. Thus, we used the MAAC-50 scaffold for further the heavy-metal removal application because the arsenic removal efficiency was not significantly increased with additional amounts of akaganeite. Next, we checked the ability of MAAC scaffolds to separate chromium from the water (Figure S5a). The MAAC-50 scaffold effectively separated chromium from the water, but MAAC-0 failed to separate chromium (data not shown), which was expected because chromium also present in negative ions forms HCrO−4 in water. Although alginate played no role in the separation of arsenic and chromium, it effectively separated lead, another toxic metal from water, owing to the electrostatic or coordination binding of lead(2+) with the oxygen-containing group of alginate (Figure S5b,c). We also observed that the removal efficiency of lead was not affected by the amount of akaganeite nanorods in the MAAC scaffold, suggesting that lead adsorption to akaganeite is negligible. Next, to optimize the adsorbent dose, we measured the removal efficiency of arsenic using different weights of MAAC50 for a fixed arsenic concentration of 2 mg L−1 (Figure S5d). We found that the removal efficiency of arsenic increased with an increase in the adsorbent dose because of the availability of more active sites for arsenic adsorption. The effect of the contact time on the adsorption of arsenic on MAAC-50 was investigated (Figure 2b). The adsorption kinetic graph displays that MAAC-50 removed approximately 75% of arsenic within 120 min, followed by a relatively slow process; adsorption equilibrium is reached at ∼200 min. This fast adsorption of arsenic is attributed to the 3D macroporous structure of the MAAC scaffold, which offers excellent permeability of water into the scaffold and easy contact of heavy metals with the active adsorption sites of the akaganeite nanorods in the scaffold. We further evaluated the application potential of the MAAC scaffold toward the purification of arsenic-contaminated water by comparing the removal efficiency of both arsenic(III) and arsenic(V) from water with various arsenic ion concentrations ranging from 0.2 to 10 mg L−1 to mimic real arsenic-contaminated water (Figure 2c). High removal efficiencies (>85%) of both arsenic(III) and arsenic(V) are observed at all of the tested concentrations, suggesting the potential of these materials for water purification. The adsorption isotherm shows that arsenic(III) and arsenic(V) adsorption could be analyzed by the Langmuir and Freundlich isotherm models (Figure 2d).51,52 The corresponding isotherm parameters are presented in Table S1. The calculated values of the Langmuir constants qm and KL are 70.5 mg g−1 and 0.03 L E


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Figure 4. High-resolution XPS spectra of the MAAC-50 scaffold corresponding to Fe 2p (a), O 1s (b), and As 3d expanded regions (c) before and after arsenic(III)/arsenic(V) adsorption.

25 mL min−1 under gravity, and such high flow rates could be achieved thanks to the highly macroporous structure of the MAAC scaffold. The output water from the column was collected over time, and the remnant arsenic in the output water was analyzed by ICP-MS (Figure 3c). The arsenic concentration in the output water was immediately reduced to a concentration lower than 10 μg L−1, the permissible limit of arsenic in drinking water according to WHO. The purification capability of the MAAC scaffold (1 g) was maintained for at least 3 h, producing a total of 5 L of arsenic-safe drinking water. We also analyzed the amount of iron in the output water to verify the release of iron from akaganeite embedded in the MACC scaffold by ICP-MS and found that no iron was present in the output water, suggesting that akaganeite nanorods were stably embedded in the alginate matrix without leaching out during the purification process. Overall, all of these results strongly suggest the promising potential of the MAAC scaffold for drinking-water treatment. Stability of Materials and Adsorption Mechanism. The stability of the MAAC scaffold after arsenic adsorption was further investigated using elemental mapping coupled with SEM, XRD, and XPS studies. Elemental mapping of the MAAC scaffold before and after the adsorption of arsenic shows the presence of arsenic throughout the scaffold after adsorption, revealing the homogeneous adsorption of arsenic (Figure S6). Powder XRD pattern of the MAAC scaffold after arsenic adsorption shows no significant difference from intact MAAC50, indicating the high stability of akaganeite within the scaffold even after arsenic adsorption (Figure S7). XPS analysis of MAAC-50 was carried out before and after adsorption of both arsenic(III) and arsenic(V) to confirm the adsorption properties of the adsorbent, valence states of the elements, and surface composition of the adsorbent (Figure 4). The high-resolution iron spectra of the MAAC scaffold show two broad peaks of Fe 2p3/2 and Fe 2p1/2 with binding energies of 710.8 and 724.9 eV, respectively, with a satellite peak at 716.2 eV corresponding to iron(3+).49 Similar peaks of iron were observed after the adsorption of both arsenic(III) and arsenic(V), with a slight change in the peak positions owing to the surface interaction of arsenic with iron(III) (Figure 4a). The deconvoluted highresolution O 1s spectrum of the MAAC scaffold shows three major peaks located at 530.2, 531.38, and 532.6 eV, attributed to the lattice oxygen atom bound to iron (Fe−O), surface

hydroxyl group (Fe−OH), and oxygen atoms present in the alginate template, respectively (Figure 4b).21,22,34 However, after the adsorption of arsenic(III) and arsenic(V), the position of O 1s at 530.2 eV is shifted to higher binding energies, 530.4 and 530.7 eV, respectively, indicating the strong interaction of oxygen(2−) with iron(III) and arsenic(III)/arsenic(V) after adsorption. Similarly, the O 1s peak at 531.38 eV is slightly shifted to a higher binding energy after the adsorption of arsenic(III) and arsenic(V), owing to the formation of Fe−O− As. The high-resolution spectrum of the As 3d region also shows characteristic peaks of arsenic at 44.5 and 44.6 eV after the adsorption of arsenic(III) and arsenic(V) on MAAC-50, respectively (Figure 4c). All of these results strongly support the successful adsorption of both arsenic(III) and arsenic(V) on akaganeite encapsulated in the 3D network of alginate. MAAC Scaffold as a Unique Material for the Decontamination of Arsenic. Compared to the previously reported materials and methods, arsenic separation by the 3D macroporous alginate akaganeite composite presents several distinct advantages. First, the biocompatible alginate polymer composed with large amounts of carboxylate (20% of the total content of the scaffold) and hydroxyl groups (43% of the total content of the scaffold) provides stability and uniform loading of large amounts of akaganeite nanorods in the scaffold (0.5 g g−1 scaffold). Second, the large quantities of the carboxylate and hydroxyl groups present on the alginate chain also allow easy contact of the contaminated water with the active sites of the akaganeite nanorods for superior adsorption of arsenic (71 and 25 mg of arsenic(III) and arsenic(V) per g of MAAC scaffold, respectively). Third, the presence of macropores in the scaffolds facilitates easy passage of water (22−25 mL min−1) without the aid of any external force. Fourth, the high mechanical strength (5.5 kPa compressive strength and a maximum strain of more than 75%) of the materials provides good structural stability for a durable water purification system. Finally, assembly of the MAAC scaffold into a controllable column enables the purification of contaminated water to supply large amounts of drinking water (∼1.5 L h−1) by a simple continuous operation process, which may provide an appropriate technology for the preparation of arsenic-free drinking water in developing countries. F


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each scaffold. Next, 5 L of arsenic-contaminated water was placed in the upper part of the column and the passed through the column-type filter device. Finally, output water from the column was collected at different times, and the remaining arsenic in the output water was measured using ICP-MS.

CONCLUSION In summary, we have successfully synthesized a 3D macroporous scaffold composed of an alginate matrix loaded with large amounts of uniformly dispersed akaganeite nanorods for efficient arsenic adsorption. The scaffold shows a high arsenic removal efficiency because of the presence of micron-sized pores and oxygen-containing functional groups, which allow a facile contact of water with the active adsorption sites of the scaffold for superior adsorption. Moreover, the materials demonstrated excellent performance in a column-based, largescale separation of arsenic without the aid of any external force, and the residual arsenic concentration was reduced to lower than the permissible limit of arsenic in drinking water. This 3D macroporous alginate/akaganeite nanorod composite material can be used in a practical water treatment process.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00389. Detailed characterization of MAAC-0, MAAC-5, MAAC25, and MAAC-66, an adsorption isotherm study, removal of chromium by scaffolds, effect of akaganeite nanorods in the composite on the separation of lead, removal of lead by scaffolds, effect of the amount of scaffolds on the separation of arsenic, and recycling of materials (PDF)

EXPERIMENTAL SECTION

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Materials and Reagents. Alginic acid, adipic acid dihydrazide (AAD), N-hydroxysuccinimide (NHS), sodium (meta)arsenite (NaAsO2), sodium arsenate dibasic heptahydrate (Na2HAsO4), lead(II) chloride (PbCl2), and FeCl3 were purchased from SigmaAldrich and used as received. 1-Ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC) was received from TCI Chemical. Sodium chloride (NaCl), sodium nitrate (NaNO3), sodium hydrogen phosphate (Na2HPO4), sodium sulfate (Na2SO4), and chromium oxide (CrO3) were purchased from JUNSEI. Synthesis of Akaganeite Nanorods. Akaganeite nanorods were synthesized by hydrolyzing FeCl3. In brief, 400 mL of a 0.2 M FeCl3 aqueous solution in a three-neck, round-bottomed flask equipped with a condenser and a temperature controller was heated to 80 °C. After 12 h, the solution was cooled to room temperature, and the assynthesized akaganeite nanorods were purified by centrifugation. Fabrication of the 3D MAAC Scaffold. For the synthesis of MAAC, 6 mL of a 3 wt % alginate solution in a 2-(Nmorpholino)ethanesulfonic acid buffer (pH 6.5) was homogeneously mixed with 4 mL of various amounts of akaganeite nanorods (0− 66%). Then, 0.8 mL of 22.5 mg mL−1 AAD, 0.2 mL of 90 mg mL−1 NHS, and 1 mL of 180 mg mL−1 EDC were added sequentially to the resultant mixture. The final weight percent ratio of alginate/AAD/ NHS/EDC was 1:0.05:0.05:1. The mixture was immediately poured into a well plate and placed in a refrigerator (2−8 °C) for complete solidification of the gel. After 6 h, the gels were placed in a large volume of water to remove the excess reagents and to swell the gels. The gels were then frozen at −20 °C and lyophilized to obtain the 3D macroporous alginate akaganeite composites. Metal Adsorption Study. In this experiment, first, arsenic(III), arsenic(V), chromium(VI), and lead(II) solutions were prepared by dissolving NaAsO2, Na2HAsO4, CrO3, and PbCl2 salts in water, respectively. Next, the MAAC scaffold (50 mg) was added to 50 mL of an aqueous solution of metal ions at various concentrations. When necessary, the amounts of the materials were changed and the pH of the solution was adjusted by adding NaOH or HCl. Subsequently, the solution was magnetically stirred for 3 h, and the MAAC scaffold was separated from the solution using tweezers. Finally, the aqueous solution was used for the estimation of the remaining metal ions by ICP-MS. Effect of a Coexisting Anion. To examine the effect of a coexisting anion on the adsorption of arsenic, various salts (NaCl, NaNO3, Na2HPO4, and Na2SO4) of a final concentration in solution of 5 mg L−1 were added to 50 mL of a 2 mg L−1 arsenic solution. Next, the MAAC-50 scaffold (50 mg) was added followed by stirring for 3 h. Finally, the MAAC-50 scaffold was removed from the solution, and the aqueous solution was used for the estimation of the remaining arsenic by ICP-MS. Preparation of a Column and Purification of Water. To make the column, 15 freeze-dried MAAC-50 scaffolds of total weight of ∼1 g were stacked one by one inside the column. There was no gap between

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +82-31-290-7252. Fax: +82-31-290-7272. ORCID

Jaeyun Kim: 0000-0002-4687-6732 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by grants funded by the National Research Foundation under the Ministry of Science and ICT, Republic of Korea (Grants 2015R1A2A2A01005548 and 20100027955), and a grant from the Korea Health Technology R&D Project through the Korea Health Industry Development Institute, funded by the Ministry of Health & Welfare, Republic of Korea (Grant HI17C0076).

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