Agar Aerogel Containing Small-Sized Zeolitic Imidazolate Framework

Aug 21, 2017 - To this end, in this work, highly dispersed and small sized (14.95 nm) ... sustainable solution for the management of water contaminati...
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Research Article pubs.acs.org/journal/ascecg

Agar Aerogel Containing Small-Sized Zeolitic Imidazolate Framework Loaded Carbon Nitride: A Solar-Triggered Regenerable Decontaminant for Convenient and Enhanced Water Purification Wentao Zhang,† Shuo Shi,‡ Wenxin Zhu,† Lunjie Huang,† Chengyuan Yang,† Sihang Li,† Xinnan Liu,† Rong Wang,† Na Hu,§ Yourui Suo,§ Zhonghong Li,† and Jianlong Wang*,† †

College of Food Science and Engineering, Northwest A&F University, Yangling, 712100, Shaanxi, P. R. China School of Medicine, Xi’an Jiaotong University, Xi’an, 710061, Shaanxi, P. R. China § Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining, 810008, Qinghai, P. R. China ‡

S Supporting Information *

ABSTRACT: Although ZIF-based adsorbents are promising for environmental remediation, their performance is limited by their large crystal size, intrinsic fragility, and poor processability and reusability. To this end, in this work, highly dispersed and small sized (14.95 nm) metal−organic frameworks (MOFs), via the restriction of overgrowth and agglomeration on carbon nitride (C3N4) nanosheets, are integrated into agar aerogel, forming a flexible and efficient aerogel absorbent with photocatalytic activity. This separationfree hybrid aerogel mantaining the crystal structure of zeolitic imidazolate framework (ZIF-8) with smaller size exhibits a 1.44 times greater adsorption capacity for dye than that of pristine bulk ZIF-8. Most importantly, the introduction of C3N4 nanosheets not only regulates the growth of ZIF-8 crystals but also enables the hybdrid aerogel with stable reusability throughout five consecutive adsorption−degradation cycles, obtained via solar irradiation. This ecofriendly, regeneration technology provides a convenient and sustainable approach to reduce the cost and eliminate the use of hazardous substances for environmental remediation. Dramatically, for practical use under sunlight, attributed to the photocatalysis effect of the uniquely designed adsorbent, the active adsorption sites of the aerogel are continuously provided, resulting in enhanced dye removal ability. Therefore, this work establishes an innovative, low-cost, convenient, solar-triggered, sustainable solution for the management of water contamination by a MOF-based aerogel decontaminant. KEYWORDS: Aerogel, MOFs, Carbon nitride, Photocatalysis, Regeneration, Sustainable

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INTRODUCTION

format fabrications will facilitate the convenience of separation and simplify purification devices. To address these problems, the fabrication of MOFs into membranes, films, or localized growth on a variety of substrates is required for these important industrial processes.8−11 However, from a practical perspective, MOFs obtained using the aforementioned methods suffer from poor stability and fragility. Moreover, the fabrication often requires expensive, tedious time- and energy-consuming processes. As an alternative facile approach, loading nanoparticles into hydrogels/aerogels, has received much attention.12−18 On the other hand, although reusability of the adsorbents is one of the preconditions for practical use, because of the leaching of active/guest species during adsorption and/or regeneration of the used MOFs, they have not yet been

Annually, over 50 000 tons of dyes are discharged, making water contamination by dyes a major environmental concern.1 Most toxic dyes are often nonbiodegradable and even carcinogenic and teratogenetic, affecting aquatic products, as well as human health. To eliminate dye contamination, adsorption is one of the most attractive options owing to its process simplicity and ecofriendliness.2 For this prospective application, metal−organic frameworks (MOFs) have generated intense interest for their structural and functional tunability and received a fair share of attention for dye separation.3−6 When used as adsorbents, MOFs and their derivatives, which can efficiently adsorb organic pollutants, have been extensively investigated for environmental remediation.5,6 However, pristine MOFs with high crystallinity show big crystal size, intrinsic fragility, and poor processability, significant hurdles in achieving their effective performance.7 As MOF materials are currently available in powder forms which are not very convenient to retrieve from a sample matrix, suitable © 2017 American Chemical Society

Received: July 15, 2017 Revised: August 13, 2017 Published: August 21, 2017 9347

DOI: 10.1021/acssuschemeng.7b02376 ACS Sustainable Chem. Eng. 2017, 5, 9347−9354

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ACS Sustainable Chemistry & Engineering

enhanced water purification. As a proof of concept, we demonstrate that the synthesized MOF-based hybrid aerogel is a solar-regenerable adsorbent with excellent adsorption performance for continues water purification under the irradiation of visible light.

successfully reused. So far, most of MOFs-based adsorbents are often disposable or regenerated by various unsustainable and harsh methods, such as elution with organic solvent, NaOH, or HCl solution, which debase the performance of adsorbents and are adverse to sustainable development.19−22 As such, considerable effort has been dedicated to broadening the species and improving adsorption ability of MOFs, but minimal studies of their reutilization have been reported.10,23−25 At this point, an attractive strategy for the regeneration of MOFs-based adsorbents is to combine MOFs with photocatalytic compounds for photodegradation of absorbed organic molecules. Photocatalysis-driven degradation of organic dyes might provide a convenient and green approach to reduce the cost and eliminate the use of hazardous substances. As such, the development of adsorbent system that can effectively absorb organic dye and simultaneously harvest visible light for organic dye degradation is an essential task in order to utilize this technology. As an analogue of graphene, graphitic C3N4 has recently been reported to possess the performance of decomposing organic pollutants for water purification under visible light irradiation.26,27 With large surface area, C3N4 nanosheets have been an attractive choice as the matrix for uniformly growing and anchoring well-dispersed nanoparticles, showing a fair share of potential as a platform to integrate with MOFs.28 The performance of MOFs also significantly depends on its morphology, size and dispersity as well. 29 This C 3 N 4 combination procedure increases the surface area and the number of surface active sites of guest nanocrystals, thus greatly improving the activity of nanocomposites.30,31 For example, decorated with transparent MOFs, the obtained C3N4/ZIF-8 (zeolitic imidazolate framework) nanocomposite showed enhanced efficiency to photocatalytically convert CO2 into solar fuels.31 Although many C3N4/MOF nanocomposites have been reported,28,31−33 growth regulation of MOF nanocrystals with different morphologies and sizes by C3N4 sheets has not been explored. One can envisage C3N4 as a potential platform and as a structure-directing agent for the growth and stabilization of MOF nanoparticles with enhanced dye adsorption ability. Most importantly, combined with photocatalytic C3N4 nanosheets, a MOF-based decontaminant can be regenerated in a facile and environmentally friendly manner by visible light for the management of organic pollutants. Inspired by the advantages of aerogel in retaining nanoparticles and photocatalytic 2D C3N4 nanosheets in restricting the overgrowth and improving the dispersity of guest nanoparticles, herein, we report an advanced hybrid nanoformulation that integrates photocatalytic MOF-based heterojunction within aerogel to greenly regenerate adsorbent for high efficient water purification. Agar labeled as cheap biomass, commonly used as solid substrate for microbiological culture, was selected as precursor to form gel for nanomaterials loading. Such a nano-hetero-junction−hydrogel formulation judiciously integrates three materials into one hybrid system with unique physicochemical properties that either one of the three building blocks cannot achieve independently. Via the restriction of overgrowth and agglomeration by C3N4 nanosheets, highly dispersed ZIF-8 with smaller size is obtained. Compared with free ZIF-8, C3N4, or ZIF-8/C3N4, entrapping them within aerogel can overcome the challenge of processing these powders into a convenient, separation free, and tailorable form. Therefore, this hybrid aerogel mantaining the crystal structure of ZIF-8 and C3N4 shows enormous potential for

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EXPERIMENTAL SECTION

Chemicals and Materials. Dicyandiamide (99%) and zinc acetate dehydrate (ACS reagent, ≥98%) were purchased from Sigma-Aldrich. 2-Methylimidazole (MIM) was obtained from Sinopharm Chemical Reagent Co., Ltd. Organic Dye, including Methylene Blue and Congo Red, were purchased from Aladdin Chemical Co., Ltd. Agar powder was purchased from Solarbio Science & Technology Co., Ltd. All chemicals were used as received without any further purification. Deionized water was used in all experiments. Characterizations. Fourier transform infrared (FT-IR) spectroscopy was recorded on a Vetex70 (BRUKER Corp., Germany). The powder X-ray diffraction (XRD) patterns were obtained by Bruker D8 Advanced Diffractometer System, equipped with Cu Kα radiation (40 kV, 40 mA). The UV−vis spectra were measured with a UV-2550 spectrophotometer (Shimadzu, Japan) at room temperature. Field emission scanning electron microscope (SEM) image was taken by an S-4800 (Hitachi, Japan). Transmission electron microscopy (TEM) measurements were performed with a JEM-2100 (JEOL, Japan). Synthesis of Bulk and Nanosheets of C3N4. Bulk and nanosheets of C3N4 were synthesized according to an earlier study.34 Typically, the bulk C3N4 was prepared by polymerization of dicyandiamide under high temperature. In detail, 10 g dicyandiamide was heated at 550 °C for 3 h under argon condition with a ramp rate of 3 °C min−1. This heating rate allows the slow condensation of dicyandiamide so that the polymerization process for C3N4 formation could occur more completely, and the porous structures of resultant C3N4 could be retained (Figure S1).35 To prepare ultrathin C3N4 nanosheets, the obtained yellow powder, bulk C3N4, was liquid exfoliated. In detail, 200 mg of bulk C3N4 powder was dispersed in 100 mL water and then ultrasound was performed for about 20 h, followed by centrifuging at 3000 rpm for 10 min to remove the residual unexfoliated C3N4 nanoparticles. Growth of ZIF-8 on C3N4 Nanosheets. To grow ZIF-8 on C3N4 nanosheets, 10 mL of a C3N4 nanosheet (∼0.8 mg mL−1) dispersion was mixed with 40 mL of water under ultrasound, followed by addition of freshly prepared MIM solution (60 mL) and Zn(OAC)2 (50 mL) in sequence with mild stirring. After 2 h reaction at room temperature, the resulting solution was centrifuged at 3000 rpm for 10 min and washed three times with water. Preparation of Hybrid Aerogels. A 0.4% agar powder was added slowly to a beaker at room temperature with vigorous stirring to create a cloudy suspension. The suspension was then heated until forming a clear solution. Then solution containing ZIF-8/C3N4 heteronanosheets at different concentration was added. The resulting suspensions were than sonicated for 2 min. All suspensions were in ultrapure water without pH adjustment. The final suspension of cross-linked clusters was transferred into the well of a 6-wells plate (9.6 cm2) or 24-wells plate (2 cm2) and left undisturbed for 2 h until hydrogel formed, followed by freezing for another 2 h in −80 °C freezer. The final hybrid aerogels were obtained by freeze-drying the ice-gel. Experiments for the Removal of Water Contaminants. To determine the adsorption efficacy of the as-synthesized hybrid aerogels for organic pollutants, two dyes (Congo Red and Methylene Blue) were investigated. In a typical experiment, 5 mg of aerogel was soaked into 20 mL of the contaminant solution and shaken at 120 rpm in a shaker for a predetermined time. The solution pH was not controlled. For kinetic studies, a typical adsorption experiment was performed with 200 mg L−1 Congo Red. And the organic pollutants concentration at different time intervals was determined by UV/vis spectrometer and calculated by the standard spectrophotometric method at the maximum absorbance. Adsorption isotherm experiments were conducted by adding 5 mg adsorbent into 20−1000 mg L−1 Congo Red. 9348

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Figure 1. (a) Schematic illustration of the fabrication of ZIF-8/C3N4 heterostructure containing hybrid aerogels by sol−gel process. The diagram is not drawn to scale. (b) SEM image of the ZIF-8/C3N4 heterostructure. (c) HRTEM image of ZIF-8/C3N4 heterostructure. (d) Photographs of aerogel loading with 0% (top, Hgel-0) and 50% (bottom, Hgel-50) ZIF-8/C3N4. (e) XRD patterns of Hgel-0 and Hgel-50. (f, g) SEM images of hybrid aerogel with 50.0% ZIF-8/C3N4 (Hgel-50) at different magnifications. (h) SEM-EDX elemental mappings of C, N, and Zn for hybrid aerogels. For the synergistic removal of organic pollutants, 10 mg hybrid aerogel was added into prepared 20 mL 100 mg L−1 organic pollutant solution. The suspensions were then mildly magnetically stirred with visible light irradiation. The visible-light source was a solar simulator 300 W Xe lamp. During photocatalytic processes, the sample was periodically withdrawn. The concentration of organic pollutants during the synergistic removal process was monitored using a Shimadzu UV2500 spectrometer. The amounts of CR adsorbed at specific time were determined by eq 1:

Q=

(C0 − Ct )V m −1

10.4°, 16.5°, and 18.1° are consistent with the simulated XRD pattern of ZIF-8, while other three peaks at 29.3°, 31.7°, and 34.4° are changing in different degrees, indicating a change in crystal structure of ZIF-8 due to the immobilization.36 The existence of ZIF-8 on C3N4 is also evidenced by FT-IR spectra on the basis of characteristic absorbance peaks of C3N4 and ZIF-8 (Figure S3). The morphology of heterojunctions is then investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. Compared to the smooth surface of the as-prepared C3N4 nanosheets (Figure S4), the surface of ZIF-8/C3N4 heterostructure becomes rough (Figure 1b), indicating a substantial loading of small ZIF-8 with a size of about 14.95 nm and high dispersivity rather than large-sized bulks (Figure S5 and S6).37 In the inset of Figure 1c, about 0.287 nm size of the lattice fringe is clearly observed, which can be indexed as the (334) plane of ZIF-8.36 When compared with pristine ZIF-8 synthesized by conventional methods, ZIF-8 involved in heterojunctions shows much better dispersity and smaller sizes (14.95 nm), corroborating that C3N4 nanosheets can effectively restrict the overgrowth and agglomeration of ZIF8.10,15,38 With the certitude of successful interaction between ZIF-8 and C3N4, finally, the hybrid aerogels were prepared by mixing dispersed ZIF-8/C3N4 heterostructure with boiled agar solution, cooling, and freeze-drying. The characterizations of hybrid aerogel were then conducted. XRD of the aerogels, in Figure 1e, showed that the crystallinity of ZIF-8/C3N4 was retained during the processing, indicating that the hybrid aerogel maintains the function ZIF-8/C3N4 heterostructure. The presence of ZIF-8/C3N4 in hybrid aerogel was also verified by FT-IR. Figure S7 shows the FT-IR spectra

(1) −1

Herein, Q (mg g ) and Ct (mg L ) are the amounts of adsorbed CR and the concentration of CR at time specific time, respectively, and m is the dosage of adsorbent.

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RESULTS AND DISCUSSION The fabrication of the hybrid aerogel is illustrated in Figure 1a. First, C3N4 nanosheets were prepared by dicyandiamide condensation followed by ultrasonic exfoliation in water.34 Then they were dispersed in the 2-methylimidazole precursor solution, where the ZIF-8 was in situ formed on C3N4 nanosheets by adding zinc acetate. All of synthesis procedures were conducted in water without the use of organic solvent. The formation of ZIF-8/C3N4 heterostructure is first confirmed by X-ray diffraction (XRD). The XRD patterns of ZIF-8/C3N4 are shown in Figure S2. The ZIF-8/C3N4 samples present a two-phase composition: ZIF-8 and C3N4. A pronounced peak is found at 27.4°, corresponding to the interplanar staking peak of aromatic systems of C3N4.26 The peak located at 12.7°, in our result, might be the overlay of the characteristic interlayer structural packing peak of C3N4 and (112) planes of ZIF-8 and cannot be distinguished in the XRD patterns.26,36 Peaks at 9349

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Figure 2. (a) Photographs of the contaminated CR (50 ppm) aqueous solution (left) and Hgel-50 before (middle) and after (right) adsorption. (b) Adsorption capacities of CR from an aqueous solution (200 ppm) over different adsorbents including ZIF-8, C3N4, ZIF-8/C3N4, Hgel-CN, Hgel-0, Hgel-11.1, Hgel-33.3, and Hgel-50.

Figure 3. (a) Dual-site adsorption model for the adsorption isotherms of CR by Hgel-50. (b) Adsorption kinetics of CR on Hgel-50: the full line represents the pseudo-second-order until 2 h, and the dashed line represents simulated data at higher times.

which retains the favorable properties of ZIF-8 and C3N4 and integrates the feature of aerogel, is ready as an excellent and reusable adsorbent for water purification. Specifically, we looked at using the hybrid aerogels to remove the Congo Red (CR), which is widely used in the textile industry and has resistance to oxidation and decoloration under natural conditions. The maximum contamination level of CR is defined to be 30 ppm by the European Union. In view of actual application, we selected CR concentrations higher than 30 ppm for further adsorption and degradation experiments. The uptake of CR by hybrid aerogel from solution was studied by the batch method at room temperature. Figure 2a shows the color change of aerogels and CR solution. The light-yellow hybrid aerogel turned to red, while conversely the CR contaminated water became colorless. In contrast, the ZIF-8/ C3N4-free aerogel cannot entirely scavenge the color of CR solution, indicating the better adsorption ability of hybrid aerogel than pure aerogel. Detailly, the adsorption capacities of ZIF-8/C3N4 and hybrid aerogels loaded with C3N4 or different amount of ZIF-8/C3N4 for CR were demonstrated (Figure 2b). All of C3N4, Hgel-0, and C3N4 containing aerogel (Hgel-CN) show limited adsorption capacity for CR. The adsorption capacity of hybrid aerogels is positively related to the content of ZIF-8/C3N4 in aerogels. In all cases, comparable adsorption capacities to calculated values were observed for all aerogels (Table S1), indicating that the agar molecules have not suppressed the function of ZIF-8/C3N4. Moreover, compared with the calculated value derived from the big-sized bulk ZIF-8, ZIF-8/C3N4 with small-sized dispersed ZIF-8 shows obviously higher adsorption ability. It is reasonable that the restriction of C3N4 can partly improve the activity of ZIF-8. These results indicate Hgel-50 possesses good adsorption ability for CR and also highlight that the ZIF-8 unit is the primary contributor to

of agar, ZIF-8/C3N4 heterostructure, and hybrid aerogel. The pure aerogel shows a rough surface (Figure 1d). SEM was then carried out to investigate the morphology and hierarchical structure of hybrid aerogel. As shown in Figure 1f, at low magnification, the hybrid aerogel exhibited the randomly oriented, interconnected microporous three-dimensional structures. The microporous are templated from the ice crystals that grow when the hybrid hydrogel is frozen. In contrast, loading with ZIF-8/C3N4, the hybrid aerogels show good shape. From the SEM images shown in Figures 1g, S8, and S9, the surfaces of aerogels are observed to be rough with the increase of loading ZIF-8/C3N4 amount. Additionally, Figure 1g shows that ZIF-8/C3N4 heterostructure were physically embedded or encapsulated within the aerogels. This structure can minimize the loss and leaching of ZIF-8/C3N4 during use. Moreover, ZIF-8/C3N4 heterostructures are well dispersed within the aerogel and no agglomerates are observed. The energy dispersive X-ray (EDX) spectrum (Figure 1h) shows the presence of carbon (C), nitrogen (N), and zinc (Zn) elements in the sample, thereby confirming the presence of ZIF-8/C3N4 heterostructure and the formation of a hybrid aerogel. The hybrid aerogels have ultralow densities, increasing with ZIF-8/ C3N4 loading from 7.52 ± 0.7 mg cm−3 for 11.1 wt % to 11.02 ± 0.3 and 14.13 ± 0.6 mg cm−3 for 33.3 wt % and 50 wt % hybrid aerogel (abbreviated as Hgel-11.1, Hgel-33.3, and Hgel50), respectively, lighter than that of a previously reported hybrid aerogel.15 Hybrid aerogels with higher loading amount of ZIF-8/C3N4 are unstable in aqueous solution and visibly broken under mild shake, inapplicable for experimental or practical water remediation. Therefore, we can facilely control over the ZIF-8/C3N4 loading and density of hybrid aerogel. With a high loading amount, and without crystallinity change of ZIF-8/C3N4 heterostructure, the obtained hybrid aerogel, 9350

DOI: 10.1021/acssuschemeng.7b02376 ACS Sustainable Chem. Eng. 2017, 5, 9347−9354

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ACS Sustainable Chemistry & Engineering the hybrid aerogel as a highly effective filter for decontamination of CR. To determine the adsorption process and the Hgel-50 uptake capacity for CR, adsorption experiments were performed with initial CR concentrations ranging from 20 to 1000 mg L−1. Figure 3a shows the adsorption isotherms of CR on 50%aerogel at 25 °C. In the initial section, this shows that the growth of Ce gives rise to Qe, while the amount of CR adsorbed is not significantly increased with the further increase of Ce, indicating the saturated adsorption. It is also further verified that the adsorption process includes two phases, namely, surface diffusion and intraparticle diffusion. The experimental data were then analyzed by single-site adsorption, dual-site adsorption, and Freundlich isotherm models, displayed as eqs 2,39 3,39 and 4,40 respectively. Qe =

constant adsorption capacities (Qe). The rapid adsorption rate of CR at the early state could be attributed to the abundant adsorption sites on the adsorbents surface. Based on those data, three kinetics models, including the pseudo-first-order model (eq 5), pseudo-second-order kinetics model (eq 6), and intraparticle diffusion model (eq 7),40 are used to simulate adsorption kinetics of CR by hybrid aerogel, according to the following equations: Q t = Q e(1 − e−k1t )

Qt =

KLQ maxCe 1 + KLCe

(3)

Qe =

Q e 2K 2t 1 + Q eK 2t

(6)

Q t = k pt 1/2 + C

(7)

Where K1 and K2 are the equilibrium coefficients of the pseudofirst-order and pseudo-second-order equation, respectively. Qe and Qt represent the adsorption capacity of CR at equilibrium and time t, respectively. kp and C represent the diffusion rate constant of intraparticle and the thickness of boundary layer, respectively. Importantly, the fundamentals behind pseudosecond-order equation ensure its validity just under intermediate reaction time and in the case that desorption process is insignificant.39 Therefore, pseudo-second-order kinetics analysis at lower time was properly conducted (Figures S15 and 3b).39 For comparison, pseudo-second-order kinetics of full time was also performed (Figure S16). The parameters of adsorption kinetics are summarized in Table S3. From Figure 3b, S16, and S17 and Table S3, the adsorption process is better fitted to the pseudo-second-order model of lower time than that of full time and pseudo-first-order model, suggesting that the chemisorption in the CR adsorption process may be the rate limiting step. In the region of simulated data at the higher time of Figure 3b, the curve deviates from the experimental data, indicating the invalidity of pseudo-second-order kinetics model at higher time. Besides, the experimental data are smaller than the calculated ones, which might be due to the slight desorption of CR during the adsorption process. For the intraparticle diffusion model of hybrid aerogel, a plot of Qt versus t1/2 in Figure S18 shows that there are two stages during the adsorption process of CR onto a hybrid aerogel. The first stage is attributed to external diffusion with high slope and intercept. In this stage, the adsorption rate of CR is higher due to the instantaneous availability of large active adsorption sites on the surface of aerogel. The second stage is a gradual adsorption stage, where intraparticle diffusion is rate-controlled. From Table S3, the Kp1 (256.24 mg g−1 h1/2) is obviously higher than Kp2 (20.84 mg g−1 h1/2), suggesting the external diffusion plays a main role in the adsorption kinetics. Besides, a similar KP1 value (247.03 mg g−1 h1/2) was calculated for half of ZIF-8/C3N4 conducted at the same conditions for a hybrid aerogel, further indicating the coated agar molecule has not suppressed the function of ZIF-8/C3N4. One of the main questions that newly developed sorbents face is the suitability for regeneration as this represents a significant factor in determining the final cost of the technology. Herein, with the integration of photocatalytic C3N4, our MOFs-based hybrid aerogel shows apparent potential to be regenerated by photocatalytically degrading the adsorbed organic pollutant molecules under the irradiation of visible light, a green and low-cost way for sorbent regeneration. As shown in Figure 4a, being short of

(2)

2 ⎛ ⎛ 1 ⎞ 1 ⎞ Q e = Q max ⎜1 + ⎟ − Q max ⎜1 + ⎟ −1 2KdCe ⎠ 2KdCe ⎠ ⎝ ⎝

K f Ce1/ n

(5)

(4)

Here, the Qe and Ce are the adsorption capacity of CR and the concentration of CR at equilibrium, respectively. The parameter Qmax is the maximum adsorption capacity of adsorption isotherm. KL, Kd, and Kf are adsorption coefficients of the single-site adsorption, dual-site adsorption, and Freundlich isotherm models. All of the parameters of singlesite adsorption (Figure S10), dual-site adsorption (Figure 3a), and Freundlich (Figure S11) models are summarized in Table S2. The adsorption data of CR on 50%-aerogel is well fitted with dual-site adsorption model with higher R2 values than those of single-site adsorption and Freundlich model, proving that the combination of CR molecules with aerogels is dual-site adsorption.39 According to the calculated data from two-side adsorption model, the maximum adsorption capacity of Hgel50 is 287.35 mg g−1. Meanwhile, aerogel containing 21.8% ZIF8 (Hgel-Z, Figure S12), which was equal to the content of ZIF8 in Hgel-50, was prepared. For CR removal, even after 10 h, this Hgel-Z shows a weaker maximum adsorption capacity than Hgel-50, calculated to be 208.75 mg g−1 based on the well fitted dual-side adsorption model (Figure S13, Table S2). As reported, the diameter of the CR molecule is estimated to be approximately 21 Å,41 which is much greater than the aperture of ZIF-8 (3.4 Å).42 As ZIF-8 is the primary contributor of hybrid aerogel, the CR molecule is mainly adsorbed on the surface of ZIF-8. Therefore, this maximum adsorption capacity difference between ZIF-8 aerogel and ZIF-8/C3N4 aerogel contributed to the C3N4 nanosheets performance by efficiently regulating the growth of nanocrystals on its surface, resulting in small and high dispersed ZIF-8, which in turn shows higher performance for water purification. Furthermore, this ZIF-8/ C3N4 hybrid aerogel shows well capacity for the removal of methylene blue, and the maximum adsorption capacity is calculated to be 154.87 mg g−1 (Figure S14). Based on the data shown in Figure 2b, the adsorption equilibrium time and the kinetic interaction between CR and Hgel-50 were then calculated. As shown in Figure 3b, the adsorption capacities (Qt) increase rapidly in the first 1 h and achieve the adsorption process equilibrium after 1.5 h with 9351

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dependent dye removal rate was recorded and shown in Figure 4d. Impressively, the introduction of visible light greatly enhances the removal rate of CR, for which hybrid aerogel with light illumination can almost completely remove CR within 50 min. This is reasonable that, under the irradiation of visible light, the hybrid aerogel can degrade the adsorbed CR molecules, continuously maintaining the availability of active adsorption sites on the surface of aerogel. The color of resulting hybrid aerogel was light red, owing to the faster adsorption rate than the degradation progress. Subsequently, the light red hybrid aerogel was resoaked into another fresh 20 mL 100 mg L−1 CR solution under visible light and can bleach the red color within 70 min. Even at the fourth run, the used hybrid aerogel showed good removal efficiency. Inversely, a hybrid aerogel without light irradiation adsorbs 74.6% CR within 50 min and 91.6% for 1.5 h for the first time and shows unsatisfied performance for the second run. Besides, with poor adsorption ability, Hgel-CN also exhibits weaker dye removal performance. The condition of Hgel-50 after sunlight irradiation was then checked, exhibiting good stability (Figure S19). Moreover, no leaching or breakdown of ZIF-8 was detected by using atomic absorption spectrometer (Table S4). These results indicate that our hybrid aerogel, combining adsorption and photocatalytic degradation, shows continuously excellent performance for water purification with the presence of sunlight.

Figure 4. Visible light triggered the regeneration and continuous enhanced removal activity of Hgel-50 for CR removal. (a) Photographs of CR engulfed Hgel-Z (i) and Hgel-50 (ii−vi) with visible light irradiation for 2 h (photographs ii−vi show five regenerations of Hgel-50). (b) Photograph of the contaminated CR aqueous solution and the fifth run Hgel-50 before (i) and after adsorption (ii). (c) Reusability of Hgel-50 for CR removal after multiple cycles. (d) Continuous synergistic CR removal by Hgel-CN and Hgel-50 with (on) or without (off) sunlight irradiation.

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CONCLUSION In summary, a MOF-containing aerogel with photocatalytic activity was fabricated by regulating the growth of ZIF-8 on C3N4 nanosheets and the straightforward sol−gel process of agar. Such a nano-hetero-junction−aerogel formulation judiciously integrates three materials into one hybrid system with unique physicochemical properties that either one of the three building blocks cannot achieve independently. Compared with free ZIF-8, C3N4, or ZIF-8/C3N4, entrapping them within aerogel can overcome the challenge of processing these powders into a convenient, separation free, and tailorable form. In this way, Hgel-50 exhibits fast adsorption for CR with a high saturation CR uptake capacity of 287.35 mg g−1. This high performance can be economically and sustainably recovered by the irradiation of visible light. Most importantly, for real-world wastewater treatment exposed under sunlight, continuous active sites can be provided on hybrid aerogel, achieving enduring high performance for pollutant removal. Therefore, our approach of creating aerogel adsorbent integrated with ZIF-8 loaded C3N4 can be practically applied for continuously effective removal of organic pollutant under sunlight.

photocatalytic activity, ZIF-8 containing aerogel cannot be discolored by the irradiation of visible light for 2 h after adsorption experiment, while the color of 50%-aerogel engulfed CR molecules turns from red to yellow, the color of Hgel-50. Following the irradiation of visible light, the hybrid aerogel was compressed to squeeze out the water for reuse. The effect of five consecutive adsorption−regeneration cycles was studied, and the results are shown in Figure 4c. It shows that the adsorption capacity of CR on hybrid aerogel decreases slowly with increasing cycle number. Even at least the fifth run, the adsorption ability of hybrid aerogel was excellent (Figure 4b) and the color of aerogel can be recovered by light irradiation (Figure 4avi). As shown in Figure 4bii, owing to good water absorption and retention ability from the agar macromolecule, at the fifth round the Hgel-50 sank down to the bottom of the bottle, which however had no effect on the adsorption efficiency of Hgel-50. These results show that, with the ability of photocatalytic regeneration, our hybrid aerogel can be recycled for effective CR removal. For industrial wastewater treatment in real-world environments, adsorbents are exposed to the sunlight, allowing simultaneous dye adsorption and photocatalytic degradation by our hybrid aerogel. Most importantly, fitted well with twosite adsorption model, progress of CR adsorbed by our hybrid aerogel is mainly affected by the active sites of exposed surface area. Predictably, continuously providing active sites can facilitate the removal of adsorbates. To demonstrate the synergistic removal of organic pollutants by adsorption and photocatalytic degradation, ∼10 mg hybrid aerogel was added into prepared 20 mL 100 mg L−1 CR solution. The suspensions were then mildly magnetically stirred with visible light irradiation. In contrast, parallel suspensions were magnetically stirred without the presence of visible light. The time

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02376. XRD patterns and FT-IR spectra of ZIF-8, C3N4, and ZIF-8/C3N4; SEM image of C3N4; size distribution of ZIF-8 nanoparticles on C3N4; SEM image and size distribution of big-sized bulk ZIF-8; FT-IR spectra of agar, ZIF-8, C3N4, and Hgel-50; SEM of Hgel-11.1 and Hgel-33.3; adsorption capacities of various absorbents for CR; the single-site adsorption and Freundlich isotherm model of CR adsorption by Hgel-50; thermodynamic 9352

DOI: 10.1021/acssuschemeng.7b02376 ACS Sustainable Chem. Eng. 2017, 5, 9347−9354

Research Article

ACS Sustainable Chemistry & Engineering

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parameters of CR adsorption by Hgel-50; photograph, SEM images, and XRD pattern of Hgel-Z; thermodynamic isotherm model for the adsorption isotherms of CR by Hgel-Z; thermodynamic isotherm model for the adsorption isotherms of Methylene Blue by Hgel-50; invalidity of the pseudo-second-order conditions for the full time range; pseudo-second-order, pseudo-first-order, and intraparticle diffusion model of CR adsorption by Hgel-50 of full time; adsorption kinetic parameters of CR adsorption by Hgel-50; XRD pattern of the regenerated Hgel-50 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jianlong Wang: 0000-0003-3529-542X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (21675127, 31371813) and the Development Project of Qinghai Key Laboratory (2017-ZJ-Y10) for financial support.

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DOI: 10.1021/acssuschemeng.7b02376 ACS Sustainable Chem. Eng. 2017, 5, 9347−9354