Graphene Oxide Aerogel Microspheres with Radially

Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11228−11240

pubs.acs.org/journal/ascecg

Silver Phosphate/Graphene Oxide Aerogel Microspheres with Radially Oriented Microchannels for Highly Efficient and Continuous Removal of Pollutants from Wastewaters Yaxin Liu,†,‡ Dongzhi Yang,*,†,‡ Yongzheng Shi,† Linna Song,† Ruomeng Yu,† Jin Qu,‡ and Zhong-Zhen Yu*,†,‡,§

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†

State Key Laboratory of Organic−Inorganic Composites, College of Materials Science and Engineering, ‡Beijing Key Laboratory of Advanced Functional Polymer Composites, §Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, 15 Beisanhuan East Road, Chaoyang District, Beijing 100029, China S Supporting Information *

ABSTRACT: Silver phosphate/graphene oxide aerogel microspheres (SGAMs) with radially oriented microchannels are constructed by electrostatically spraying the as-prepared silver phosphate/graphene oxide/ chitosan (Ag3PO4/GO/CS) suspension into numerous microdroplets followed by freezing, lyophilization and cross-linking for highly efficient and continuous removal of pollutants from wastewaters. The resultant aerogel microspheres exhibit both high adsorption rate and large adsorption capacity because of their short internal diffusion pathway and high specific surface area. The synergy between adsorption and visible light photocatalysis endows the porous microspheres an outstanding efficiency of removing pollutants under both static and continuous water treatment conditions. Furthermore, the presence of graphene oxide (GO) efficiently improves the recyclability of Ag3PO4 due to the fast electronic transfer at the interface. More than 95% Rhodamine B (50 mg L−1) is adsorbed by SGAMs in 5 min, and bisphenol A (10 mg L−1) is almost completely photodegraded in 20 min under visible light irradiation. The removal efficiency of bisphenol A in a continuous flow system (0.1 mL min−1) is maintained at 95% over 50 h. In addition, the SGAMs could photodegrade pharmaceuticals and pesticides effectively. This work provides a promising visible light photocatalysis approach for enhancing efficiency and durability of photocatalysts in a continuous water treatment system. KEYWORDS: Aerogel microspheres, Visible light photocatalyst, Silver phosphate, Graphene oxide, Photodegradation

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reusability. Ye et al.18 prepared compressible and lightweight graphene aerogels cross-linked with glutaraldehyde for water treatment. To achieve a thorough removal of pollutants and enhance the reusability of adsorbents, it would be an efficient way to combine adsorption and photodegradation together. Although silver phosphate (Ag3PO4) has a highly efficient visible light catalytic activity, its small specific surface area and easy photocorrosion in the absence of a sacrificial agent limit its wide application in the environmental field.19−21 Because GO is an effective acceptor of photoexcited electrons, the combination of Ag3PO4 with GO aerogels would efficiently prevent the photocorrosion of Ag3PO4 by accelerating charge transfer. Thus, in the present work, by utilizing ice templates, the radially diverged internal microchannels are formed in the GO-based aerogel microspheres, which not only possess high specific surface area to enhance the adsorption of pollutants but also favor the fast adsorption equilibrium by shortening the

INTRODUCTION In view of the increasing number of global water pollution accidents, it is always imperative to develop efficient and costeffective adsorption and photocatalysis technologies for water treatment and purification.1−3 Carbon-based adsorbents including graphene and its derivatives are promising in removing pollutants from wastewater due to their large specific surface area, high adsorption capacity, broad spectrum of adsorption, and high adsorption−desorption rates.4−8 For example, graphene oxide (GO) based nanomaterials are able to remove organic dyes, heavy metal ions, and gases.9−13 Due to its abundant active sites, GO could anchor semiconductor nanoparticles9 exhibiting enhanced adsorption capacity and photocatalytic efficiency. As two-dimensional (2D) sheets are particularly prone to agglomeration due to their strong intersheet attractions,14 it is highly preferable to construct three-dimensional (3D) porous architectures with rich pores and ultrahigh specific surface area.15,16 Recently, Liang et al.17 fabricated a GO based aerogel by hydrothermal self-assembly and freeze-drying, which exhibited a high adsorption capacity of Cr(VI) with a good © 2019 American Chemical Society

Received: January 28, 2019 Revised: May 11, 2019 Published: May 27, 2019 11228

DOI: 10.1021/acssuschemeng.9b00561 ACS Sustainable Chem. Eng. 2019, 7, 11228−11240

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Schematic illustrating the preparation processes of (a) Ag3PO4/GO/CS suspension and (b) porous SGAMs with radially oriented microchannels. for 12 h to obtain Ag3PO4/GO/CS aerogel microspheres, which were denoted as SGAMx (x = 1, 2 and 3), where x is the mass ratio of Ag3PO4 to GO components. In the absence of Ag3PO4, GO/CS aerogel microspheres (GAMs) were also prepared using the same procedure. For comparison, Ag3PO4/GO/CS powders (SGPs) were obtained by precipitation; while randomly distributed Ag3PO4/GO/ CS aerogel bulks (SGABs) were fabricated by freezing of the Ag3PO4/ GO/CS suspension followed by lyophilizing with the freeze-drier. As a control, neat Ag3PO4 was synthesized by precipitation.25 In detail, AgNO3 was dissolved in deionized water under stirring, and after a solution of Na2HPO4 in equal stoichiometry was added into the AgNO3 solution, a yellow precipitate was generated immediately, which was dried at 70 °C in an oven and stored for use later. For comparison, two common adsorbents, activated carbon (AC) and Al2O3, were chosen as substrates to fabricate Ag3PO4/AC and Ag3PO4/Al2O3 photocatalysts by precipitation approaches (Figure S1). Characterization. The morphology and microstructure of the fabricated microspheres were observed with a Hitachi S-4700 scanning electron microscope (SEM), Hitachi HT7700 transmission electron microscope (TEM), and JEOL JEM-3010 high-resolution transmission electron microscope (HRTEM). Chemical states were analyzed using a Physical Electronics PHI5600 X-ray photoelectron spectroscopy (XPS). The crystal structures of the microspheres and Ag3PO4 powders were characterized by a Rigaku D/Max 2500 X-ray diffractometer (XRD) with Cu Kα irradiation, and a Nicolet Nexus 670 Fourier-transform infrared (FTIR) spectrophotometer. Photoluminescence (PL) and UV−vis absorption spectra of the SGAMs were recorded using a F-5400 fluorescence spectrophotometer with an excitation wavelength of 375 nm, and a Shimadzu UV-3600 UV− vis spectrometer (200−800 nm), respectively. Raman spectra were recorded on a Renishaw inVia with laser wavelength of 514 nm (Britain). The measurements of electrochemical impedance spectroscopy (EIS) and photocurrent transient responses were conducted on a Chenhua Instruments CHI760D electrochemical station (China). The BET surface area and pore size distribution were obtained by a mercury porosimeter on AutoPore IV 9500 V1.09 (America). The intermediates in the photocatalytic oxidation were identified with a Waters Xevo TQ-S mass spectrometer (USA) equipped with an ESI ion source. Static Adsorption. SGAMs samples (20 mg) were added into 20 mL of an aqueous solution of RhB, or MO (50 ppm) and agitated at a speed of 500 rpm. Aliquots (1 mL) of the suspension were taken at defined intervals, diluted, and filtered using a membrane filter (0.22 μm). The concentrations of RhB and MO were determined using a spectrophotometric analysis. The photocatalyst was recovered by a filtration using a 38 μm stainless steel membrane, and reused without further treatment.

diffusion pathway for pollutants entering into the porous aerogel microspheres. Ag3PO4 nanoparticles (∼15 nm) as visible light photocatalysts are in situ anchored onto the microchannel walls to construct Ag3PO4/GO aerogel microspheres (SGAMs), thereby cooperatively enhancing the photocatalytic efficiency. The cross-linking of the CScontaining aerogel microspheres with glutaraldehyde vapor ensures their structural stability during the adsorption and photocatalysis processes. The roles of Ag3PO4 and GO components, the advantages of the aerogel microspheres, and the factors influencing the adsorption and visible light photodegradation are also well investigated.

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EXPERIMENTAL SECTION

Materials. Graphite flakes (300 mesh) were provided by Huadong Graphite Factory (China). Acetic acid (99.5%), ethyl acetate, nhexane, chitosan (CS, Mw = 50000 g mol−1), silver nitrate (AgNO3), disodium hydrogen phosphate (Na2HPO4), disodium ethylene diamine tetraacetate EDTA, p-benzoquinone (BQ), n-butanol (nBuOH), Rhodamine B (RhB), methyl orange (MO), and bisphenol A (BPA) were supplied by Aladdin Chemical Reagents (China). All chemicals were used as received without further purification. Preparation of Ag3PO4/GO/Chitosan Suspensions. Graphite oxide was prepared by oxidizing graphite flakes with a modified Hummers method.22,23 An aqueous GO suspension (10 mg mL−1) was prepared by ultrasonic treatment of graphite oxide in deionized water. After aqueous AgNO3 solutions with different concentrations (0.12, 0.24, and 0.36 g mL−1) were added into the GO suspensions (10 mL), the mixtures were stirred for 12 h to achieve electrostatic self-assembly of positively charged silver ions with negatively charged groups of GO sheets. Then, an aqueous solution of Na2HPO4 of equal stoichiometry was added dropwise to the mixture within 30 min under stirring. Then, 0.8 wt % of CS solution and 1% of acetic acid24 were finally added, and the resultant mixture was stirred for 30 min to form Ag3PO4/GO/CS suspensions. Fabrication of Honeycomb-like Ag3PO4/GO/CS Aerogel Microspheres. The Ag3PO4/GO/CS suspension was loaded into the syringe of an Ucalery electrospinning apparatus (China). Numerous microdroplets were electro-sprayed into a beaker with nhexane solvent at −84 °C to be frozen to ice microspheres. Such a low temperature was achieved by putting the beaker in a Dewar bottle filled with ethyl acetate slush and solidified by liquid nitrogen. The distance between the fluid level and the syringe needle, the optimal flow rate, and the electrospray voltage were set to 10 cm, 8 mL h−1, and +11 kV, respectively. After the ice microspheres were filtered and lyophilized with a FD-1C-50 freeze-drier (China) at −80 °C under 10 Pa for 36 h, they were cross-linked by glutaraldehyde vapor at 60 °C 11229


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Figure 2. SEM images of (a) GAMs, (b) SGAM1 (50% Ag3PO4), (c) SGAM3 (75% Ag3PO4), and (d) cross-section of SGAM1. The insets are high-magnification SEM images of the corresponding microspheres. Static Photodegradation. The SGAMs catalyst (20 mg) was added into 20 mL of an aqueous solution of RhB (50 ppm) or BPA (20 ppm), and the suspension was magnetically stirred in dark for 10 min to ensure that the adsorption and desorption of RhB or BPA on the surface of the catalyst reach their equilibrium. Photodegradation was conducted in a glass reactor under illumination of a 210 W Xe arc lamp (Aulight, CEL-HXUV300) equipped with a cutoff filter (UVIRCUT400, λ > 400 nm) placed 15 cm above the glass reactor. Aliquots of 1 mL of the suspension were taken out at regular intervals, diluted and the absorbance values at 554 nm were determined to monitor the RhB concentration. The BPA concentration was determined by a Waters Acquity UPLC high performance liquid chromatography. Furthermore, the photocatalytic behaviors of SGAMs for three types of emerging pollutants of ibuprofen (IBU), sulfamethoxine (SMZ), and atrazine were investigated under the similar conditions. The concentrations of IBU and atrazine in the solutions were quantified using the liquid chromatography with the UV detector at 225 nm and SMZ at 254 nm. An Acquity UPLC BEH C18 column (1.7 μm × 2.1 mm × 50 mm) was employed for the solution separation. The elution phase was a mixture of 40% MiliQ water and 60% acetonitrile. The injection volume of samples was 1 μL, and the flow rate was 0.25 mL min−1. Dynamic Adsorption and Photocatalysis. In contrast to the static photocatalysis, dynamic adsorption and photocatalysis were studied using a custom-designed glass reactor (Figure S2). SGAMs (100 mg) were packed into a tubular glass photocatalytic reactor equipped with X400−316−0.035 stainless steel membranes (Anping Baidun Metal Screen manufacturing Co. Ltd.) on two sides, which served as inlet and outlet, through which RhB or BPA solution was fed at a constant flow rate of v = 0.1 mL min−1 with the help of a Kamoer FX-STP peristaltic pump. Other conditions were the same as those of the static adsorption and photocatalysis.

charged groups of GO sheets and subsequent in situ formation of Ag3PO4 nanoparticles on GO sheets in the presence of Na2HPO4, and CS molecules are used to enhance the interactions between GO sheets by electrostatic interaction and hydrogen bonding. Second, the resultant Ag3PO4/GO/CS suspension at the tip of needle is split into the gelid n-hexane coagulation bath to form frozen microspheres. During the freezing process, numerous ice crystals grow from the spherical surface to the center of a microsphere, and the Ag3PO4/GO components are expelled by the ice crystals and squeezed between adjacent ice crystals to form radially oriented structures, diverging from the center.26,27 After the frozen microspheres are freeze-dried to remove the ice crystals by subliming, the structural stability of the aerogel microspheres is enhanced by cross-linking of their CS component with glutaraldehyde vapor, finally generating porous SGAMs with radially oriented microchannels as visible light photocatalysts for efficiently removing pollutants from wastewaters. Figure 2 shows the SEM images of GAMs and SGAMs. It is seen that the GAMs has a perfectly spherical honeycomb shape with a diameter of ∼250 μm (Figure 2a), and the CS molecular chains are in the form of a cobweb anchored on the channels of the aerogel, enhancing the structural stability of the aerogel microsphere. With the in situ generated Ag3PO4 nanoparticles, the SGAMs reveals a rough surface (Figure S3) with open channel dimension of ∼4 μm (Figure 2b). However, with increasing the Ag3PO4 content, the interaction between the positive silver ions and the negative charges of the GO sheets is enhanced, and the open channel dimension increases to 9 μm (Figure 2c). The internal microstructure of the SGAMs is observed by cleaving the frozen microsphere prior to its lyophilization (Figure 2d), exhibiting radially oriented microchannels, where the thin walls consist of multiple layers of GO sheets, while the in situ grown Ag3PO4 nanoparticles are evenly

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RESULTS AND DISCUSSION Figure 1 illustrates the synthetic process of SGAMs. First, an Ag3PO4/GO suspension is prepared by electrostatic selfassembly of the positively charged silver ions with negatively 11230


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Figure 3. (a) XRD patterns, (b) FT-IR spectra, (c) Raman spectra, and (d) UV−vis diffuse reflectance spectra of GO, GAMs, Ag3PO4, and SGAM1.

Figure 4. (a) XPS survey spectrum and (d) N 1s spectrum of SGAM1. (b) P 2p and (e) Ag 3d XPS spectra of Ag3PO4 and SGAM1. C 1s XPS spectra of (c) GO and (f) SGAM1. (g) Photoluminescence spectra, (h) EIS curves, and (i) photocurrent transient responses of Ag3PO4 and SGAMs.

distributed on the GO sheets. The radially oriented microchannel structure is attributed to the rapid inward growth of

ice crystals during the freezing process and the subliming of these ice crystals during subsequent lyophilization process. 11231


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Figure 5. Adsorptions of (a) RhB and (b) MO with Ag3PO4, SGAM1, SGABs, and GAMs with insets showing the color variations of the solutions as a function of adsorption time (min). Residual amounts of (c) RhB and (d) MO over SGAMs with different contents of Ag3PO4 after adsorbing for 60 min (C0 = 50 mg L−1). (e) ζ potential curves of SGAMs and Ag3PO4 suspension at different pH values. (f) Fitting results of RhB on SGAMs for kinetic models.

C skeleton, deformation stretching of C−O (alkoxy/alkoxide), stretching vibration of C−O (epoxy/ether), and stretching vibration of CO (carbonyl), respectively.6 The strong absorption peak at 3432 cm−1 is assigned to the stretching vibration of −OH due to adsorbed water. The peaks at 549 and 1390 cm−1 are attributed to the OP−O bending vibration and the PO stretching vibration, respectively. The absorption peaks at 863 and 1058 cm−1 are assigned to symmetric and asymmetric stretching vibrations of P−O−P, respectively.30 When inserted into the GO network, the P−O− P stretching vibration peak initially at 1058 cm−1 red-shifts to 1085 cm−1, suggesting a strong interaction between Ag3PO4 and GO in the hybrid. The combination of Ag3PO4 and GO in SGAMs is also reflected in their Raman spectra (Figure 3c). Clearly, GO exhibits two distinct peaks at 1350 and 1598 cm−1, corresponding to the well-documented D and G bands of graphite, respectively; while neat Ag3PO4 has many peaks in the range of 500−1200 cm−1, consistent with reports.30 Reasonably, SGAM1 exhibits the peaks of both GO and Ag3PO4, confirming the presence of Ag3PO4 in the SGAMs. Figure 3d shows the UV−vis diffuse reflectance spectra of GO, GAMs, Ag3PO4 and SGAM1. Neat Ag3PO4 presents a sharp absorption edge at ∼530 nm with a band gap of 2.37 eV,21

TEM image of neat GO reveals a layered structure with chiffon-like ripples and wrinkles (Figure S4a), while SGAMs present uniform Ag3PO4 nanoparticles, and their sizes increase from 10 to 20 nm with increasing the loading of Ag3PO4 (Figure S4b−d). As shown in a HRTEM image (Figure S4e), the lattice spacing of 0.270 nm for the crystalline Ag3PO4 is consistent with the interplanar spacing of the (210) plane in a cubic crystal of Ag3PO4. Compared to neat Ag3PO4 nanocrystals with an average size of ∼485 nm (Figure S5), the presence of the GO sheets prevents the agglomeration of Ag3PO4 efficiently, leading to smaller Ag3PO4 nanoparticles with enhanced dispersion. Figure 3a shows the XRD patterns of GO, Ag3PO4, GAMs, and SGAMs. GO has a typical diffraction peak at 10.8°, and the intercalation of CS molecules enlarges the interlayer distance between GO sheets.28,29 After hybridization with Ag3PO4, however, the main diffraction peak of GO is not observed, indicating the disordered distribution of the GO sheets. The typical diffraction peaks of cubic Ag3PO4 are well reserved in the XRD patterns of the SGAMs, implying the unchanged crystal form of Ag3PO4 (Figure S6). The FT-IR spectrum of GO (Figure 3b) shows four main peaks at 1630, 1055, 1403, and 1735 cm−1, corresponding to the vibration of aromatic C− 11232


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Figure 6. Photodegradation of RhB by (a) Ag3PO4, SGAMs, SGPs, Ag3PO4/AC, and Ag3PO4/Al2O3 and (b) SGAMs with different contents of Ag3PO4. (c) RhB photodegradation by synergistic effect of absorption-photocatalysis over different photocatalysts. (d) Photocatalytic activities of SGAM2 for RhB degradation in the presence of different scavengers.

and cationic RhB. Because of the oxygen-containing groups of the GO component, the SGAMs adsorb more than 95% of RhB and 49% of MO in 5 min (Figure 5a,b), more efficient than those of the randomly distributed Ag3PO4/GO/CS aerogel bulks (SGABs) (Figure S9) and Ag3PO4 powders. However, the adsorption capacity is significantly reduced with increasing the Ag3PO4 contents (Figure 5c). Similar results are obtained for adsorptions of MO on SGAMs (Figure 5d). Notably, compared to neat Ag3PO4, the SGAMs composite has a greatly improved adsorption capacity because of its increased specific surface area (Figure S10) and the presence of the active sites. The ζ potentials of both Ag3PO4/GO/CS suspension and Ag3PO4 suspension remain negative under the experimental conditions used (Figure 5e). The curve fitting results reveal that the adsorption kinetics of RhB on SGAMs has a better fit to the pseudo-second-order kinetic model than to the pseudo-first-order kinetic model (Figure 5f), indicating that the chemisorption of RhB on the active sites, rather than the internal diffusion, controls the adsorption process.24 Clearly, the positively charged RhB is preferentially adsorbed on SGAMs by electrostatic self-assembly. The cyclic adsorption of RhB on SGAMs is conducted by recycling the catalyst with a filtration membrane. Only 48% of RhB is removed by SGAM1 after five cycles (Figure S11), indicating that the adsorption sites are saturated with adsorbed RhB and could not be regenerated. For the SGAM3 with a high content of Ag3PO4, its removal efficiency is much smaller than that of SGAM1. Interestingly, by combining the adsorption with photocatalysis, the SGAMs exhibits greatly enhanced efficiency of RhB removal, which is still more than 90% after five cycles (Figure S12). To compare the effect of structure and different substrates of photocatalysts, Figure 6a shows the photodegradation of RhB

while SGAM1 shows clearly enhanced visible light absorption and a small band gap of 1.73 eV benefited from the light absorption of GO. With increasing the GO content, SGAMs exhibit intensive visible light absorptions (Figure S7), consistent with the color change of Ag 3PO 4 /GO/CS suspensions from tawny to bistre (Figure S8). The interaction between GO and Ag3PO4 is corroborated by XPS spectra (Figure 4a,b,e). The high-resolution Ag 3d spectrum is split into two peaks at 373.9 eV (3d5/2) and 367.9 eV (3d3/2), attributed to silver ions. However, these two peaks shift, respectively, to 373.8 and 367.8 eV in SGAM1, along with the shift of P 2p peak from 132.9 to 132.7 eV. These shifts are attributed to the interaction between Ag3PO4 and GO sheets.25 The N 1s spectrum at 399.1 eV (Figure 4d) is observed due to the presence of CS.31 In addition, the C 1s spectrum of GO presents its characteristic peaks at 286.6 eV (C−O) and 288.5 eV (OC−OH), and their intensities are significantly reduced in the SGAMs (Figure 4c,f). In fact, the chemical bonding between GO and Ag3PO4 benefits the charge transfer and prevents the recombination of electron− hole pairs, which is supported by the PL spectra, EIS curves, and photocurrent transient responses of Ag3PO4 and SGAMs (Figure 4g−i). The greatly weakened PL peak intensity of SGAMs is due to the slow recombination of electron−hole pairs. The rapid charge transfer is confirmed by the small semicircle diameters of SGAMs, corresponding to small charge-transfer resistance and greatly enhanced current density. Compared to conventional aerogels with random pores, the radially oriented microchannels of SGAMs are expected to shorten the diffusion pathway of adsorbates, benefiting the rapid adsorption of pollutants. Figure 5 shows the adsorption behaviors of SGAMs with commonly used dyes of anionic MO 11233


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ACS Sustainable Chemistry & Engineering Scheme 1. Proposed Photocatalytic Degradation Pathway of RhB in the Presence of SGAMs

under visible light irradiation over Ag3PO4 nanoparticles, SGPs, SGAMs, Ag3PO4/AC, and Ag3PO4/Al2O3 powders. To eliminate the effect of photosensitivity, the photodegradation in the absence of photocatalysts is set as a blank control. Thanks to the 3D porous structure with a large specific surface area and radially oriented microchannels, the SGAM1 has the highest adsorption capacity and the fastest adsorption of RhB among all the samples tested. SGAM1 presents the highest photodegradation rate for RhB degradation among all the samples due to its low PL intensity and fast electron transfer (Figure S13). RhB is almost completely degraded in 8 min by SGAM1 (Figure 6a). Among the SGAMs with different Ag3PO4 contents, SGAM1 exhibits the highest adsorption capacity. The fastest photocatalytic rate of SGAM3 is mainly attributed to its high Ag3PO4 content (Figure 6b). In addition, the photodegradation of RhB by GAMs (Figure S14) indicates the photocatalytic effect of GO. Figure 6c shows the RhB photodegradation under conditions of synergistic adsorptionphotocatalysis. With respect to the photocatalysis of SGAMs without the preadsorption, SGAMs with different contents of

Ag3PO4 (50%, 66.7%, 75%) show nearly equal photocatalytic efficiency (98.5%, 98.7%, 98.6%), much higher than that for other photocatalysts after 14 min irradiation, highlighting the synergistic effect between the adsorption of RhB and its photodegradation within the aerogels. To explore the catalytic degradation mechanism of pollutants during the photocatalysis of SGAMs, three control experiments are conducted by adding 10 mM of EDTA, nBuOH, or BQ as radical scavengers. As seen in Figure 6d, the concentration evolution of RhB over the 14 min of photocatalysis reveals that the presences of EDTA and BQ hinder the degradation of RhB. In contrast, n-BuOH does not show any negative impact. These results suggest that h+ and ·O2− are the predominant active species that are responsible for the degradation of RhB. Furthermore, the degradation products due to the deethylation of RhB are detected by the mass spectrum analysis (Figure S15), and a photocatalytic degradation pathway of RhB is illustrated in Scheme 1. The reactive radicals attack the N-diethyl group of RhB molecules, making the N-de-ethylation occur by the formation of N11234


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Ionic strength (IS) is also an important chemical parameter for describing the state of a solution. Figure 7d indicates that the increase in IS has a distinctly adverse effect on the photodegradation of BPA. With the increase of IS from 0 to 0.5 M, the rate constant decreases gradually from 0.206 to 0.074 min−1, which is probably related to the occupation of Na+ on the limited photoreactivity sites of the SGAMs, hindering the move of BPA molecules onto SGAMs.35 To investigate the photocatalysis mechanism for the degradation of BPA, radical trapping experiments are conducted by adding different scavengers. Similar to that of RhB, n-BuOH, EDTA, and BQ (10 mM) are injected into the BPA solution (20 ppm, 20 mL) as ·OH, h + , and ·O 2 − scavengers, respectively. Under the visible light irradiation, the photocatalytic efficiency decline markedly, and the BPA with n-BuOH exhibits the lowest rate constant of 0.017 min−1 (Figure 7e), suggesting that ·OH is the dominant reactive specie for the BPA degradation. The introduction of EDTA and BQ also decreases the rate constant from 0.237 min−1 to 0.043 min−1 and 0.058 min−1, indicating that h+ and ·O2− are significant reactive species. The degradation intermediate products of BPA in the presence of the SGAM1 catalyst are investigated by mass spectrum analysis (Figure S16) and a degradation pathway for BPA is illustrated in Scheme 2. Generally, the degradation is initiated by the ·OH attack to the electron-rich C3 in the phenyl group of BPA, generating 4isopropanolphenol and phenol via the photocleavage of phenyl groups. The oxidation of 4-isopropanolphenol leads to the formation of isopropenylphenol and 4-hydroxybenzenaldehye, which is in agreement with previous studies on the removal of BPA.36 Subsequently, the obtained single aromatic intermediates are oxidized by ring-opening reactions to short chain intermediates. All these intermediates are eventually mineralized into CO2 and H2O by a sequential photocatalytic process.37 The structural stability of the photocatalysts is evaluated by cyclic photodegradation 6 times. For each cycle, the sample is illuminated for 30 min. The degradation efficiency of neat Ag3PO4 initially reaches 99% but decreases to 39% at the sixth cycle. Whereas, the SGAM1 exhibits a significantly higher cycling stability, and its degradation efficiency is still 96% at the sixth cycle (Figure 8a). Additionally, postphotocatalysis characterizations of the SGAM1 with XRD, SEM, and TEM also confirm that its structural stability is clearly better than that of neat Ag3PO4. Only a faint diffraction peak is seen due to the silver crystal, and very little aggregation or shedding is detected (Figure 8b−d). Given that the photocatalysis of SGAM1 and Ag3PO4 follow a similar mechanism, the remarkable difference in stability between the two catalysts clearly points to the positive influence of GO with fast charge transfer capability and the fast electron−hole separation on the stability of SGAMs. To simulate the photocatalytic processes in actual application, a continuous flow system (Figure 9a) is set up to evaluate the adsorption and photocatalysis performance of SGAMs. Generally, reaching a concentration of 3−5% of pollutants in the outflow liquid is considered as the breakthrough point.38 In this study, the breakthrough point is defined to be 5% of the pollutant. Figure 9b shows that, in the dark, BPA reaches the breakthrough point at the end of 20 h due to the presence of a large number of adsorption sites on SGAM1. Afterward, the removal of BPA starts to drop because of the gradual saturation of the adsorption sites. After 110 h,

hydroxymethylated intermediates as degradation products (m/ z = 415 and m/z = 387), which are eventually mineralized into CO2 and H2O by de-ethylation, oxidation, and ring-rupturing reactions. These results are in agreement with those of previous studies where CeO2 was used as the photocatalyst.32 To further investigate the photocatalytic activity under visible light, colorless pollutant of BPA is chosen as a degradation target, which has been widely used for producing polycarbonate plastics and epoxy resins but is a highly toxic compound that may seriously affect the physiological metabolism of humans. At a low concentration of 5 ppm, BPA can be completely photodegraded in 10 min, and a high concentration of BPA (20 ppm) can be photodegraded to 98% after 20 min (Figure 7a). Parts b−d of Figures 7 show the influences of pH value, temperature, and ionic strength on the photocatalytic degradation of BPA by SGAM1. The insets are kinetic linear fitting curves, and the corresponding pseudo-firstorder rate constants of each scenario are listed in Table 1. As Table 1. Pseudo-First-Order Rate Constants for Photocatalytic Degradation of BPA by SGAM1 Water Parameters temperature (°C)

pH value

ionic strength (M)

scavenger

30 40 50 60 2 4 6 8 0 0.1 0.3 0.5 blank n-BuOH EDTA BQ

rate constant (min−1)

correlation coefficient

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.997 0.994 0.990 0.995 0.995 0.990 0.992 0.992 0.967 0.980 0.975 0.96 0.998 0.978 0.997 0.979

0.208 0.247 0.242 0.162 0.202 0.284 0.227 0.050 0.206 0.150 0.076 0.074 0.237 0.017 0.043 0.058

0.004 0.007 0.004 0.004 0.011 0.012 0.008 0.002 0.012 0.008 0.006 0.007 0.004 0.001 0.001 0.003

shown in Figure 7b, the pH value has a volcanic effect on the photodegradation of BPA over SGAMs. Increasing of the pH value from 4 to 8 reduces the rate constant from 0.284 to 0.05 min−1, while the decrease of the pH value to 2 reduces the rate constant by 28.9%, indicating that both highly acidic and alkali adversely affect the photodegradation of BPA. This is because Ag3PO4 is partially dissolved under strong acid conditions, deteriorating the degradation performances of BPA. In addition, the ionization of BPA under alkaline conditions generates BPA− or BPA2−, and the adverse electrostatic repulsion between BPA anions and SGAMs weakens the degradation of BPA. Proper increase in temperature is in favor of the photocatalytic degradation of BPA (Figure 7c). When the temperature increases from 30 to 50 °C, the rate constant increases from 0.208 to 0.242 min−1, because the increase in temperature enhances the molecular collision frequencies, and the change in interatomic interactions weakens the chemical bonds, resulting in a fast degradation rate of BPA.33,34 Nevertheless, a rather high temperature reduces the stability of Ag3PO4, which is unfavorable for the BPA removal, leading to the reduced rate constant of 0.162 min−1 at the high temperature of 60 °C. 11235


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Figure 7. (a) Photodegradation of BPA with different initial concentrations by SGAM1. Effects of (b) pH value, (c) temperature, and (d) ionic strength (IS). (e) Effects of scavengers on photodegradation of BPA over SGAM1. Insets are their kinetic linear fitting curves.

Scheme 2. Proposed Photocatalytic Degradation Pathway of BPA in the Presence of SGAMs

the SGAM1 is saturated, and there is little removal ability for BPA. In contrast, when both adsorption and photocatalysis take place simultaneously, the removal efficiency of BPA maintains at 95% over 50 h under visible light irradiation, demonstrating the advantageous photocatalytic activity and excellent photocatalytic stability of SGAMs. In addition, SEM, FTIR spectra, and XRD patterns of the SGAM1 before and after the photocatalysis prove its satisfactory stability (Figure S17). Interestingly, the SGAMs are also efficient in photodegrading three kinds of emerging pollutants: ibuprofen (IBU), sulfamethoxine (SMZ), and atrazine under visible

light irradiation (Figure 10). The pseudo-first-order rate constants are much higher than those reported,39,40 confirming that the SGAMs is also an efficient photocatalyst for removing pharmaceuticals and pesticides. To the best of our knowledge, there are rare reports on such a high removal efficiency and long-term stability of aerogelbased photocatalysts in a continuous photodegradation system (Figure 11a, Table 2).30,41−45 The better photocatalytic performance of the SGAMs is attributed to the advantages of Ag3PO4 and its association with honeycomb-shaped GO aerogel microspheres. First, numerous ice crystals induce the formation of a honeycomb-like structure with channels radially 11236


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Figure 8. (a) Cyclic photodegradation of BPA by photocatalysts of Ag3PO4 and SGAM1. (b) XRD patterns, (c) SEM image, and (d) TEM image of SGAM1 after six degradation cycles.

Figure 9. (a) Schematic diagram for a continuous photodegradation system. (b) Removal efficiency of BPA over SGAMs in the continuous photodegradation system.

Figure 10. (a) Photodegradation curves of ibuprofen (IBU), sulfamethoxine (SMZ), and atrazine over SGAM1 under visible light irradiation and (b) their photodegradation kinetics.

diverging from the center, which shortens the diffusion pathway of pollutants and allows to achieve rapid adsorption equilibrium; second, the interaction between Ag3PO4 nanoparticles and the GO matrix facilitates fast electron hopping

from Ag3PO4 to the GO sheets, which significantly reduces the photocorrosion of Ag3PO4 and enhances the photocatalytic activity of Ag3PO4 (Figure 11b). Third, the cross-linked micrometer-sized aerogel with a stable structure ensures the 11237


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Figure 11. (a) Comparison of the continuous photocatalytic performance of SGAM1 with those of previously reported aerogel photocatalysts. (b) Schematic illustrating the charge transfer and the pollutant degradation on SGAM1.

Table 2. Comparison of the Photocatalytic Activity of SGAMs with Those Reported in the Literaturea pollutant

Cpollutant (ppm)

mca. (g)

f (10−3 mg min−1 g−1)

t (h)

DP (%)

C3N4-agar TiO2-rGH

MB Cr(VI)

AgCl@RGO-(5 wt %) rGH-2 Ag3PO4/rGH rGH-AgBr@RGO g-C3N4/rGH SGAM1

BPA BPA BPA Cr(VI) RhB

9.6 10 5 10 10 10 2 50 30 10

0.2 0.25 0.25 0.1 0.15 0.5 0.1 0.1 0.1 0.1

48 6.4 3.2 20 10.7 3.2 3.2 50 30 10

100 20 20 9 30 45 25 46 26 100

22 55 73 75 67 40 100 68 98 81

Catalyst

BPA

ref 41 42 43 30 44 45 this work

a

Cpollutant: the concentration of a pollutant. mca: the mass of a photocatalyst. t: the photodegradation time. DP: the degradation percentage of a pollutant. f: the photocatalytic degradation efficiency of a pollutant by 1 g of a catalyst within 1 min.

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long-term treatment of pollutants, and allows the photocatalyst to be separated by filtering with a stainless steel membrane. In view of these advantages, SGAMs have the best photocatalytic performances among photocatalysts with similar structures reported (Table 2).

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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.9b00561. SEM images of Ag3PO4/AC, Ag3PO4/Al2O3, and AGAM1; schematic diagram for continuous adsorption and photocatalysis; TEM images of GO and SGAMs; SEM image and size distribution of Ag3PO4; XRD patterns; UV−vis spectra; digital images of Ag3PO4/ GO/CS suspensions; SEM images and time-dependent concentration of RhB and MO; BET surface area and pore size distribution; cycle runs of RhB removal; photoluminescence spectra; photodegradation of RhB by GAMs; mass spectra of RhB; and SEM images, FT-IR spectra, and XRD patterns of SGAM1 (PDF)

CONCLUSION

To fully taking advantage of the high visible-light photocatalytic efficiency of Ag3PO4 nanoparticles and the high adsorption and recyclability of GO aerogels, Ag3PO4/GO/CS aerogel microspheres with numerous microchannels diverging radially from the center to outward are fabricated by electrostatically spraying. The radially oriented microchannels not only shorten the internal diffusion pathway and thus promote the rapid adsorption equilibrium of pollutants but also improve the light utilization efficiency. Furthermore, the interaction between the Ag3PO4 nanoparticles and the porous matrix of GO effectively prevents the photocorrosion of Ag3PO4. Interestingly, SGAMs can be used for continuous water treatment, and its removal efficiency to BPA maintains at 95% over 50 h. These outstanding photodegradation efficiency and cyclic stability make the porous aerogel microspheres highly promising for continuous removal of pollutants from wastewaters.

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AUTHOR INFORMATION

Corresponding Authors

*(D.Y.) Fax: +86-10-64428582. E-mail: [email protected]. edu.cn. *(Z.-Z.Y.) Fax: +86-10-64428582. E-mail: [email protected]. edu.cn. ORCID

Dongzhi Yang: 0000-0003-2592-7833 Jin Qu: 0000-0001-8962-3260 11238


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Zhong-Zhen Yu: 0000-0001-8357-3362 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51273015, 51533001) and the Fundamental Research Funds for the Central Universities (YS201402) is gratefully acknowledged.

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Graphene Oxide Aerogel Microspheres with Radially

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