Al2O3 Sandwich

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Anti-oil Ag3PO4 nanoparticle/polydopamine/Al2O3 Sandwich Structure for Complex Wastewater Treatment: Dynamic Catalysis under Natural Light Ruixiang Qu, Weifeng Zhang, Na Liu, Qingdong Zhang, Yanan Liu, Xiangyu Li, Yen Wei, and Lin Feng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01469 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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ACS Sustainable Chemistry & Engineering

Anti-oil Ag3PO4 nanoparticle/polydopamine/Al2O3 Sandwich Structure for Complex Wastewater Treatment: Dynamic Catalysis under Natural Light Ruixiang Qu,† Weifeng Zhang,† Na Liu, ‡ Qingdong Zhang,† Yanan Liu, † Xiangyu Li, † Yen Wei †

and Lin Feng*,†

†

Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China

‡

Institute of Materials for Energy and Environment, School of Materials Science and

Engineering, Qingdao University, Qingdao 266071, P. R. China

KEYWORDS: anti-oil, heterojunction, natural light catalysis, low-cost

Affiliation: Tsinghua University, Beijing, China Postal address: Department of Chemistry, Tsinghua University, Haidian District, Beijing 100084, China Phone/Fax: +86 010 62792698 Email address: [email protected]

ACS Paragon Plus Environment

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ABSTRACT We have successfully fabricated sandwich structural Ag3PO4 nanoparticle/polydopamine/Al2O3 porous small balls (APPAOs) by a facile homogeneous precipitation method, which exhibit natural light catalysis capacity to degrade different kinds of water pollutants including industrial dyes and agricultural pesticides. The porous Al2O3 provides the substrate to form Ag3PO4/Al2O3 heterojunction, as well as increases the specific surface area (SSA) of Ag3PO4 nanoparticle, thus greatly enhance the photocatalytic capacity. Polydopamine (PDA) plays the role of adhesive between Al2O3 substrate and Ag3PO4 nanoparticle, aiming to stabilize the synthesized APPAO catalyst. A part of Ag3PO4 is reduced by PDA and transformed into Ag nanosphere, which further increases SSA and enhance the catalytic ability of the material by plasmonic effect. Further study shows there is a dynamic process between catalysis and adsorption/desorption equilibrium, i.e., with the catalysis going ahead, the adsorption/desorption equilibrium accordingly shifts thus thoroughly treat the pollutants. Besides, the superhydrophilic surface provides the APPAO with excellent anti-oil property, which greatly reduces second pollution. And the small ball structure makes the material easy to use and recycle. Due to its excellent reusability, mild catalytic conditions and easy-using, the APPAO has great potential to be used in the field of low-cost practical wastewater treatment.


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INTRODUCTION In recent years, pesticides are widely applied to boost agricultural productivity around the world, while it in turn threatens aquatic ecosystem as well as harms human body.

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In present

phase, a large quantity of pesticides are handled unreasonably, thus enrich in the food chain and bring incalculable consequences. Besides, common wastewater also contains various industry dyes, and makes freshwater scarcity problems more nonnegligible.

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These dyes and pesticides

are difficult to be degraded simultaneously for their diverse molecular structures and high chemical stability. As reported, developing materials with excellent catalytic capacity has been deemed as a promising way to reduce water pollution.

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However, most of the catalytic

materials fabricated recent years mainly focus on electrocatalysis, ultraviolet photocatalysis and visible light catalysis, which may bring additionally energy-consume. 15-18 Natural light catalysis material, in comparison, provides a greener and more convenient approach for water remediation, while it also calls for higher catalytic efficiency because of the weak natural light intensity, so there is still little research on it. Besides, the traget wastewater in practical water remediation always contains various insoluble oil, especially in the city watercourse, which presents a high demand on the anti-oil property of the catalyst. 19,20 Silver phosphate (Ag3PO4) is a particularly semiconductor material featured a typical band gap of ~ 2.42 eV, making it promising as a photocatalyst under natural light.

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By virtue of their

highly dispersive band structure at the conduction band minimum, Ag3PO4 shows higher electron transfer efficiency than many other semiconductor catalysts. most efficient photocatalysts currently.

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Thus, Ag3PO4 is one of the

Nevertheless, there are still some problems need to

overcome on the current situation. For example, the stability of Ag+ has troubled people for a long time, as Ag+ tends to transform into elemental silver (Ag0), which severely affects the


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catalytic performance of Ag3PO4.

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In addition, although Ag3PO4 has been used as a popular

catalyst, most of them are ineligible for natural light catalysis as the light intensity is lower than 500 lux indoor, making Ag3PO4 difficult to be applied to practical water remediation. Besides, oil pollution can severely reduce the catalytic efficency of Ag3PO4, as well as cause second pollution. Therefore, it is necessary to explore anti-oil Ag3PO4 materials with higher photostability and catalytic efficiency to versatility eliminate various pollutants under natural light in wastewater. Herein, we report a brand new anti-oil Ag3PO4 nanoparticle/polydopamine/ Al2O3 small ball (APPAO) with dynamic wastewater remediation property under natural light. The Ag3PO4/Al2O3 heterojunction structure provides the as prepared APPAO with super high photocatalytic performance, thus APPAO can absorb low-intensity natural light and generate electron hole pair, followed by the electron hole pair being separated and transferred. In this way, the APPAO can easily degrade various common water pollutants including cationic dyes, anionic dyes and phosphorous pesticide. Due to the small ball structure has an average diameter of 5 mm, this material can be easily used and recycled. Dynamic wastewater remediation means the APPAO firstly reaches an adsorption-desorption equilibrium of pollutants, then the adsorbed pollutants are degraded, accompanied by an equilibrium shifting. Furthermore, the PDA layer in the material has two important functions. One is acting as adhesive between Al2O3 substrate and Ag3PO4 nanoparticle and stabilizing the synthesized APPAO catalyst, the other one is reducing part of the Ag3PO4 and generating Ag nanosphere, which enhances the catalytic ability of the material by plasmonic effect, as well as increases the SSA of APPAO. Owning to these excellent features, the as prepared APPAO can degrade most of the water pollutants in several quarters under natural light. In the meanwhile, the high specific surface energy as well as micro/nano


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composite structure caused by the accumulating of Ag and Ag3PO4 provides the APPAO with superhydrophilic surface, which protect it from being polluted by the oil in wastewater. Compared to the previous excellent work, the as prepared APPAO shows higher catalytic rate as well as milder catalytic condition, and is much easier to be used and recycled. As illustrated in Scheme 1a, after a facile homogeneous precipitation process, Ag3PO4 nanoparticles were uniformly coated on the PDA pre-treated Al2O3 molecular sieve to form the stable APPAO catalyst.

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The hydroxyl groups on PDA reacted with Ag+ and generated Ag

nanosphere, which brought plasmonic effect and greatly enhanced the catalytic capability of the as prepared material.

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Due to the porous structure of APPAO, the as prepared APPAO could

adsorb various pollutants in wastewater and reached adsorption-desorption equilibrium. Subsequently, as the pores on the APPAO were full of nano-structured Ag3PO4, the adsorbed pollutants would be degraded through a green natural light photocatalytic process, accompanied

Scheme 1. The synthetic process and dynamic catalytic mechanism of the as prepared APPAOs. After being synthesized by a simple homogeneous precipitation method, the APPAOs can adsorb pollutants such as pesticide and dyes, and reach an adsorption desorption equilibrium. Heterojunction enhances the photocatalytic property of APPAOs, the pollutants will be degraded under natural light, with the adsorption desorption equilibrium shifting.


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by the shifting of adsorption-desorption equilibrium, finally the polluted water was fully cleaned. Ag3PO4/Al2O3 heterojunction played an important role in the catalytic process. As the conduction band and valance band of Ag3PO4 (2.87 eV, 0.45 eV) were lower than Al2O3 (8.70 eV, 2.35 eV) 37,38,when the heterojunction was exposed to natural light, electron hole pairs were generated and electrons were transferred to Al2O3 while holes were transferred to Ag3PO4, followed by further oxidation reaction.

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In this way, the rapid and high-efficiency water

remediation process was accomplished. Scheme 1b illuminated the possible combination way of Ag/Ag3PO4 and PDA. As reported in the previous works, the adjacent hydroxide groups on PDA showed strong reducing property and could reduce part of the Ag3PO4 to Ag, followed by the Ag chelated with adjacent hydroxide groups. 40 Besides, the remaining Ag3PO4 could form hydrogen bond with the amino of PDA, and was tightly immobilized on the substrate.

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The combination

of Ag and Ag3PO4/PDA Al2O3 sandwich structure generated the integrated natural light catalyst.

EXPERIMENTAL SECTION Materials. The commercial aluminium oxide molecular sieve (Shanghai Zeolite Molecular Sieve Co. Ltd., Shanghai, China) were used as purchased. Silver nitrate (AgNO3, Xiya Chemical Co., Ltd, Shangdong, China) were of analytical grade from Xiya Reagents. Ammonium hydroxide (NH3·H2O, Beijing Chemical Co. Ltd, Beijing, China) and disodium hydrogen phosphate (Na2HPO4, Beijing Chemical Co. Ltd, Beijing, China) were of analytical grade from Sinopharm Chemical Reagents. Methyl blue, orange II sodium salt, Congo red, acid black and methylene blue were purchased from J&K Scientific Ltd. And ammonium glyphosate and acephate was purchased from Nanjing Tai Chemical Co., Ltd. Vitamin C (C6H8O6), EDTA (C10H16N2O8) and antimony potassium tartrate (C8H4K2O12Sb2) and ammonium molybdate


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((NH4)6Mo7O24) were purchased from Sinopharm Chemical Reagents. There is no further purification process taken for all other reagents. Fabrication of APPAO. The polydopamine pre-treated Al2O3 molecular sieves (PAO) were washed by deionized water and acetone alternately followed by dried in an oven to elute impurities in the porous. Appropriate amount of ammonium hydroxide was added to silver phosphate solution until the solution became clear. Then a certain concentration of disodium hydrogen phosphate solution was added dropwise. Subsequently, the cleaned molecular sieves were stand in room temperature for 30 minutes. The resultant molecular sieves were taken out and washed with deionized water and dried under 60℃. Water Remediation. In the water remediation test, APPAOs were put into a mixed solution and stood for 10 minutes in the dark to get saturation adsorption. Then the whole system was exposed to natural light for several minutes to conduct natural light catalysis. And sample solution was extracted at regular intervals. The natural light photocatalysis performance of the as prepared APPAOs was measured by recording the absorbance peaks of the collected solution gotten from UV spectrometer. All of the photocatalysis experiments were conducted on sunny days to ensure the light intensity is almost the same. The degradation rate was evaluated by the equation D = (1 - Ct/Ci) × 100 %, in which Ct means contaminants concentration in the solution taken out at different times, and Ci means the original contaminants concentration. Mechanism study. Synthetic process of APPAO was investigated by testing the ion absorption capacity of its precursor PAO. And the concentrations of PO43- and Ag+ before and after adsorption were quantitated by ammonium molybdate spectrophotometric method and inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES),


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respectively. In the ammonium molybdate spectrophotometric process, 1 g vitamin C and 0.02 g EDTA were dissolved in 20 ml deionized water, then 0.8 ml methanoic acid was added to the solution, and the solution was diluted to 50ml (The as prepared solution was called Vc solution). Besides, 1.3 g ammonium molybdate and 0.05 g antimony potassium tartrate were dissolved in 20 ml deionized water, then 23ml 50% H2SO4 was added to the solution, the solution was also diluted to 50 ml (The as prepared solution was called AM solution). 20 ml PO43- solution was mixed with 2.0ml AM solution and 3.0 ml Vc solution, the whole solution was diluted to 50ml and stood for 10 minutes. And UV spectrum of the solution was measured to calculate the PO43-concentration. Characterization. The SEM images of the APPAO were obtained using a field-emission scanning electron microscope (SU-8010, Hitachi Limited, Japan). X-ray diffraction patterns were obtained on a polycrystalline X-ray diffractometer with a Cu Kα radiation source (Bruker D8 Advance, Bruker-AXS, Germany). Infrared spectrometer (Bruker, Horiba Scientific, Germany) and Specific Surface Area and Porosity Analyzer (Tristar II 3020 Micromeritics, America) were used to obtain infrared spectroscopy and porosity. Optical images were taken on a digital camera (Canon EOS 60D, Japan). Dye concentration in different solution was tested with Perkin Elmer Lambda-750 UV spectrometer (United Kingdom). Light intensity was measured by illuminometer (MS6612, Peakmeter, China). Oil content in the filtrate was measured with the infrared spectrometer oil content analyzer (Oil480, Beijing Chinainvent Instrument Tech. Co. Ltd., China).And energy dispersive X-ray images were measured using energy-dispersive X-ray analysis (HORIBA, Ltd., Japan). RESULTS AND DISCUSSION Surface Morphology and Chemical Composition.


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The morphologies of Al2O3 molecular sieves in different reaction stages were characterized by field emission scanning electron microscope (FESEM). Figure 1a and the inset photos illustrated typical image of original Al2O3 molecular sieves (AOs). The original AOs were white and showed irregular surface with varying nanoscale folds on it, as well no obvious crystal or particle was observed on the surface. In comparison, the AOs distinctly turned yellow when pretreated by PDA and composed PAOs, accompanied by the surface became relative smooth (Figure 1b). Besides, after directly treating AOs with homogeneous precipitation without pretreating process, white AOs were coated by scattered Ag3PO4 nanoparticles and turned in pale yellow, thus became Ag3PO4 coated Al2O3 molecular sieves (APAOs). The SEM image showed the Ag3PO4

Figure 1. SEM images of molecular sieves in different reaction periods, inset is the optical photograph of the corresponding molecular sieves: a) Al2O3 molecular sieves (AO) before reaction; b) Polydopamine coated Al2O3 molecular sieves (PAO); c) Ag3PO4 coated Al2O3 molecular sieves (APAO); d) Ag3PO4 nanosphere/polydopamine coated Al2O3 molecular sieves (APPAO).


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on the APAOs surface was sparse and had an average diameter of 300 nm (Figure 1c). When AOs went through both PDA pretreating process and homogeneous precipitation process, its color would turn black and a great amount of Ag3PO4 nanoparticles (with an average diameter of 300 nm) as well as Ag nanospheres (with an average diameter of 25 nm) would generate on the surface (Figure 1d). These black small balls were called Ag3PO4 nanoparticle/PDA coated Al2O3 molecular sieves (APPAOs). The Ag nanospheres on the surface were generated by reducing Ag3PO4 with PDA and could assist the catalysis. 42-44 The influence of precipitation time was also studied on the final morphology of APPAOs and observed by FESEM to conclude 30 minutes was confirmed as the optimal reaction time (Figure S1). Energy Dispersive X-Ray Spectroscopy (EDX) image of the APPAO surface further illustrated the elemental composition and material distribution of the as prepared APPAO. As shown in Figure S2, the mass ratio of O element, Al element, Ag element, C element and P element were 39.0%, 26.0%, 20.4%, 12.9% and 1.5%, respectively. The value of Ag : P on the APPAO surface (13.6) was a little bit higher than in Ag3PO4 (10.4), which could be due to the existence of Ag metal. Thus, we could deduce that Ag mainly located on the surface of APPAO, and were uniformly distributed. Besides, the element mapping images of the edge of APPAO cross-section (cut into slices manually) were collected to confirm the formation of heterojunctions. As shown in Figure S3, Al element and Ag element were uniformly distributed on the edge, and no boundary was observed, indicating that the Ag3PO4 directly touch the Al2O3 in spite of the existence of PDA, which was the requirement for formatting heterojunctions. Chemical composition of the as prepared APPAO were studied to determine the growth pattern of Ag3PO4 on the balls. As shown in Figure 2a, in order to verify the successful modify of PDA on AO surface, infrared spectroscopy (IR) was employed. Obviously, a wide peak at


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Figure 2. Chemical composition and microstructure of APPAO: a) Infrared spectroscopy of AO and PAO; b) XRD pattern of the final production; c) The pore size distribution of PAO and APPAO; d) Comparison of specific surface area (SSA), pore volume and pore diameter between PAO and APPAO; e) Possible growth model of APPAO, being speculated on the base of SSA, pore volume and pore diameter. 3300 cm-1 appeared in the infrared spectrum of PAO, which signify the –OH in PDA. Subsequently, after the PAO was treated with homogeneous precipitation, X-Ray Diffraction (XRD) was proceeded to confirm the existence of Ag3PO4. It was worth noting that the APPAO small balls had an average diameter of 5 mm and was inconvenient to be characterized by XRD directly. So we grinded the small balls and collected the powder sample for further characterization. The lattice of Ag3PO4 was partly broken during grind, making the signal peaks in XRD pattern weak. As shown in Figure 2b, typical signal peaks of the small ball in different stages were detected. The AO and PAO showed nearly no signal peak, and the peak at 2θ = 67.6°


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could be attributed to Al2O3 (JCPDS: 04-0880). After directly modifying Ag3PO4 on the Al2O3 surface, the APAO showed signal peaks at 2θ = 29.7°, 33.4°, 36.6°, which signified the (200), (210) and (211) of Ag3PO4, respectively (JCPDS: 06-0505). Besides, the finally production APPAO showed an extra peak at 2θ = 38.1°, which was the typical signal peak of Ag (111) (JCPDS: 04-0783). It was worth noting that APAO also showed a weak peak at 2θ = 38.1°. This could be attributed to the partially reduced of Ag3PO4 on APAO under natural light. In this way we could confirm that the PDA redued part of Ag3PO4 and generated Ag nanospheres. Figure 2c gave the pore size distribution of PAO and APPAO, which was characterized by nitrogen adsorption desorption method. The average pore diameter of APPAO was much smaller than PAO, indicating that a large amount of Ag3PO4 and Ag generated in the pore of molecular sieve and narrowed the pore. Figure 2d gave the concrete data on specific surface area (SSA), pore volume and pore diameter of PAO and APPAO, as a lot of Ag3PO4 nanoparticles distributed in the pore, the SSA became higher while pore volume and pore diameter became lower. As the pore diameter changed from 5.65nm to 5.26nm, we could deduce the average particle size of Ag3PO4 and Ag in the pore was 0.20 nm. A growth pattern of Ag3PO4 was came up and displayed in Figure 2e, in which the small gray balls signified Ag3PO4 nanoparticles, the nanoparticles were inlaid in the PDA layer and got in touch with Al2O3, so that they could form Ag3PO4/Al2O3 heterojunction. Every data of the pattern was in consistence with Figure 2d. In this situation, we could learn the specific morphology of the as prepared APPAO thoroughly. Sunlight Photocatalysis The photocatalysis property of Ag3PO4 had been investigated for a long time, while two major problems still remained unsolved. One was the stability of Ag3PO4 was poor that Ag+ tended to turn into Ag0, the other one was the catalytic efficiency of Ag3PO4 was not high enough to


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Figure 3. Degradation process of different pollutants under natural light with the participation of APPAO: a) The remediation of complex wastewater and recycle of APPAO; b) Degradation curve of 5 ppm methylene blue, 5 ppm rhodamine B, 5 ppm orange ΙΙ, 100 ppm ammonium glyphosate (AG) and 100 ppm acephate (Ace); c) UV-Vis adsorption spectra of mixture solution of 5ppm methylene blue, 5 ppm orange Ⅱ and 100 ppm acephate extracted at same time intervals; d) Digital Photos of 5 ppm methylene blue, 5 ppm rhodamine B, 5 ppm orange ΙΙ extracted at same time intervals. conduct a fast natural light photocatalysis. In this work, these two problems were well resolved by introducing PDA and building Ag3PO4/Al2O3 heterojunction. Besides, due to the small ball structure of APPAO had an average diameter of 5 mm, the as prepared material was easy to be used and recycled before and after catalysis. To study the natural light photocatalytic activity of APPAOs, degradation tests of several different pollutants were conducted under sunny day with the participation of APPAOs. Before the catalytic process started, the APPAOs were immersed in the wastewater and stood for 10 minutes in the dark to get saturation adsorption. Then the whole system was exposed to natural light for several minutes until the water turned colorless. And the used APPAO was easily recycled by decantation (Figure 3a). Cationic dyes (methylene blue, rhodamine B), anionic dyes (orange ΙΙ) and phosphorous pesticides (ammonium glyphosate,


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acephate) were chosen as the representative pollutants. In consideration of data credibility and scientificity, the total mass of APPAOs was kept close to 15 g, and the natural light intensity was measured and recorded. As shown in Figure S4, the natural light intensity in an ordinary sunny day was 220 lux, only one fifth of that of 90 mw/cm2 Xe lamp and one ninth of that of 180 mw/cm2 Xe lamp. Figure 3b illustrated the natural light photocatalytic results of different kinds of pollutants with the participation of APPAOs. Obviously, APPAOs showed excellent catalytic ability towards these pollutants even though the light intensity was weak. 5 ppm methylene blue, 5 ppm rhodamine B and 5 ppm orange ΙΙ were completely degraded in 40 minutes, 60 minutes and 30 minutes, respectively (The corresponding UV-Vis adsorption spectra were exhibited in Figure S3). As for pesticides, a degradation rate of more than 75 % occurred in 20 minutes to ammonium glyphosate and nearly all of the ammonium glyphosate was degraded in 50 minutes. Besides, a similar phenomenon appears to acephate that more than 75 % acephate was degraded in 5 minutes and all of the acephate was degraded in 20 minutes, the corresponding UV-Vis adsorption spectra were also exhibited in Figure S5. Figure S6 exhibited the rate constants of the above photocatalytic tests (first order reaction). The rate constant of degradation of MeB, orange ΙΙ, RB, AG and Ace using APPAO were 0.0759 s-1, 0.0858 s-1, 0.0380 s-1, 0.0462 s-1 and 0.0955 s-1, respectively. To further illustrate the superiority of APPAOs over previous Ag3PO4 materials, a detailed table of comparisons was given in Table S1, which indicated that APPAOs has the best catalytic ability among all of the outstanding work. The degradation process of the mixture solution with different pollutants in it was listed in Figure 3c, in which acephate, orange ΙΙ and rhodamine B were chosen as the representative pollutants. The spectrum indicated there was no negative synergism among the pollutants and APPAOs had potential to energy-saving remediation of complex wastewater in one step. Figure 3d were the digital photos of three dyes


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extracted at same time intervals, which clarified the bright color of original solution gradually disappeared along with increasing the degradation and was almost bleached after dozens of minutes. The degradation test in the presence of humic acids was also conducted as humic acids was a common interfering compounds in water.

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It was found that the degradation rate of

orange ΙΙ remained nearly unchanged after 5 ppm humic acids was added (Figure S7a), and the orange ΙΙ was completely degraded while the humic acids was changed into intermediate products after 30 minutes irradiation (Figure S7b). As a result, we could preliminary verify the feasibility of natural light photocatalysis of APPAO. In order to further confirm the excellent catalytic property of APPAO, a series of controlled experiments were conducted, and the corresponding results showed the APPAO had satisfactory performance either on catalytic efficiency or on reusability. Figure S8 gave the catalytic results of PAO and APAO. The UV-Vis adsorption spectra of 5 ppm orange ΙΙ showed little reduction in 60 minutes, which could be attributed to the absence of Ag3PO4 in PAO and APAO (Ag3PO4 tended to fall of rapidly from APAO without the existence of PDA). Besides, the adsorption test of APPAO to three kinds of dyes in the dark was proceeded and the results were listed in Figure S9. Obviously, the dynamic catalytic process was killed without illumination, and APPAO was not able to degrade dyes in the dark. Figure S10 exhibited the catalytic results of free Ag3PO4 nanopowder synthesized by the same approach. Continuous stir was applied to the system as the powder precipitated easily in aqueous phase. The dosage of Ag3PO4 was determined by the mass difference between PAO and APPAO. As displayed in Figure S10, there was little reduction happened to the spectra of orange ΙΙ and rhodamine B, and the spectrum reduction of methylene blue was about 60% in 60 minutes, much lower than APPAO. The rate constant of degradation of MeB, orange ΙΙ and RB using Ag3PO4 nanoparticles were 0.0121 s-1, 0.00381 s-1 and 0.00432


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s-1, respectively (Figure S11). Thus, we could demonstrate the significance of heterojunction structure to the catalytic ability of APPAO. Besides, the influence of Ag nanoparticle to the photocatalysis was studied subsequently by comparing the catalytic property of 0.4g Ag3PO4 and 0.4g Ag/Ag3PO4/PDA (Figure S12), and it was found that Ag/Ag3PO4/PDA had better catalytic property than Ag3PO4, indicating Ag nanospheres could assist the catalysis. As the APPAO was easy to be recycled, the reusability and stability of APPAO were also tested by reusing APPAO for several as well as treating APPAO with physical and chemical force. In the previous researches, photostability of Ag+ has troubled people for a long time, as Ag+ tends to transform into elemental silver (Ag0) under light. However, in this work we utilized this disadvantage and combined Ag3PO4 and Ag together, and greatly enhanced the catalytic ability by plasmonic effect. With the existence of PDA adhesive, the Ag nanospheres were tightly chelated and the Ag3PO4 was immobilized by hydrogen bond. So the as prepared APPAO showed excellent durability and was promising to be used in practical application. Figure S13a and b exhibited the SEM images of APPAO used for 10 times, in which the surface morphology remained almost unchanged, showing the high reusability of APPAO. And Figure S13c illustrate that the degradation rate of orange ΙΙ kept higher than 90% in ten times of use. Besides, Figure S14 was the results of stability test. As shown in Figure S14a, after treating the as prepared APPAO with ultrasonic for 60 minutes, no reduction on catalytic ability was observed that APPAO could still degrade 5 ppm methylene blue in 40 minutes. Furthermore, APPAOs suffered from 60 minutes of ultrasonic treatment in solutions with different pH values stayed unchanged, and the solutions remained clear (Figure S14b). This was because the unstable Ag3PO4 on the ball surface had been removed by ultrasonic treatment in fabrication process, and the remained


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Ag3PO4 were tightly immobilized by PDA. In this situation, we could learn the excellent reusability and stability of the as prepared APPAO thoroughly. Mechanism discussion In order to further study the synthetic and catalytic mechanism of APPAO, we conducted a series of further experiments, and the corresponding results were listed in Figure 4. Synthetic process of APPAO was investigated by testing the ion absorption capacity of its precursor PAO (Figure 4a).

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Two sets of PAO with the same mass were respectively put into the solution of

Na2HPO4 and AgCl (two reagents used for preparing Ag3PO4 in this work)and the systems were stood for 30 minutes to get adsorption saturation. The concentration of PO43- and Ag+ was kept the same with it in the synthetic process so that the whole test could be more reliable. The concentration of PO43- was tested by ammonium molybdate spectrophotometric method and the corresponding colors of solution were inserted in Figure 4a. Apparently, PAO had better absorption efficiency to PO43- than Ag+, embodied in 55% adsorption rate in 30 minutes to PO43while 20% adsorption rate in the same time interval to Ag+. Thus, PAO tended to adsorb more PO43- than Ag+ in the mixed solution of PO43- than Ag+. It was worth noting that the pH value of reaction solution was higher than 4 (Figure S15), leading to the PAO carrying negative charges,

Figure 4. a) The PAO could adsorb both PO43- and Ag+, and more than 50% PO43- was adsorbed in 30 minutes; b) The catalytic performance of APPAOs obviously reduced when APPAOs adsorbed indigo and reached saturation of adsorption capacity. And the catalytic performance returned to normal after desorption of indigo.


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so the electrostatic attraction between PAO and PO43- was excluded, and the hydrogen bond between PO43- and –NH2 was thought as a leading role in the adsorption of phosphate ions. 48 As a result, we could speculate that in the synthetic process PAO firstly absorbed PO43- by its porous structure, followed by the Ag+ combined with the absorbed PO43- and turned into Ag3PO4. In this way the Ag3PO4 nanoparticles were able to generate in the pores of molecular sieve. Catalytic mechanism of APPAO was also thoroughly studied by tampering with adsorption process in the dynamic equilibrium. As shown in Figure 4b, the APPAOs were firstly put into 100 ppm indigo solution, which was stable and hard to be degraded by APPAOs. Nearly 42% indigo was adsorbed by APPAOs in 120 minutes, the adsorption time was long enough to get saturation adsorption. Then the treated APPAOs were put into 5 ppm orange ΙΙ solution under natural light for 120 minutes. There was a sharp decrease on the degradation rate of orange ΙΙ that only 37% dye was degraded in 120 minutes. In comparison, almost all of the orange ΙΙ was degraded in an hour in the above catalytic test. This was due to the indigo molecules occupied the adsorption sites on the APPAO and hindered the combination between APPAO orange ΙΙ, thus cut off the integrated dynamic catalytic process. In contrast, after putting the treated APPAOs into deionized water for 120 minutes to desorb the indigo, the catalytic capacity of APPAO restored and all of the remained orange ΙΙ was degraded in 30 minutes under natural light. Thus we could verify that APPAO treated wastewater by an integrated dynamic catalytic process, in which the pollutants molecules were adsorbed and the system reached an adsorptiondesorption equilibrium, then the adsorbed pollutants were degraded, accompanied by an equilibrium shifting. And it was feasible to control the remediation by tamper with adsorption process. Anti-oil property of APPAO


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In the practical wastewater treatment process, the target wastewater always contained insoluble oil such as gasoline, soybean oil and toluene, especially in the city watercourse. As a result, traditional catalyst might face the problem that the catalyst itself would polluted by oil phase. Although this pollution phenomenon could be relieved by cleaning catalyst after use, second pollution was still difficult to solve. In this work, the second pollution was well avoided by endowing the catalyst with superhydrophilic/under water superoleophobic suface. The high specific surface energy of Ag3PO4 provided the APPAO with hydrophily, while the rough structure caused by Ag nanoparticle enhanced the wetting behaviour and make the APPAO superhydrophilic. As shown in Figure 5a, when the APPAO was immersed in wastewater, the water could instantly infiltrate APPAO and be trapped into its rough structures. In consequence,

Figure 5. a) The APPAO was covered by water layer due to its superhydrophilic property, and the oil drops would be isolated by water layer; b) The turbidity of the water phase with adsorbed oil in it; c) The anti-oil property of APPAO, compared with original AO substrate.


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the spherical APPAO was tightly covered by a water layer, which prevented the APPAO from contacting with oil droplets. Thus, APPAO could still keep unpolluted after practical application. To further investigate the anti-oil property of APPAO, a series of controlled experiments were conducted. 15 g APPAO as well as 15 g original AO were both wetted by deionized water and immersed in toluene for 30 minutes to get adsorption saturation. Then they were took out and immersed in 40 ml deionized water, followed by ultrasonic treatment for 30 minutes. The resultant water phase was collected for further study. As shown in Figure 5b, the deionized water in APPAO group kept clear after 30 minutes ultrasonic treatment, and the letters behind the water was legible. In contrast, the deionized water in AO group became turbid that the letters behind the water was illegible. This was because AO could adsorb a large amount of oil, and the adsorbed oil entered water phase during ultrasonic process. N-hexane, gasoline and diesel were also used to measure the anti-oil property, and infrared oil content analyser was utilized to measure the oil content of the treated water above. The corresponding result was exhibited in Figure 5c. The calculation formula of oil concentration was listed as below:

C oil =

(1)

in which Moil meant the quality of oil in the treated water and Vwater meant the volume of water phase. As shown in Figure 5c, the oil contents in AO group were higher than 1×105 ppm to four kinds of different oils, while in APPAO group they were lower than 3 ppm. Figure S16 exhibited the oil droplets distribution in the treated water under optical microscope. And the oil droplets density in the AO group was much higher than it in the APPAO group. The photocatalytic tests in the presence of oils were also carried out to investigate how the presence of oil and the claimed anti-oil property will affect the wastewater remediation results.


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As shown in Figure S17, a layer of oil was added and float on the orange ΙΙ solution to simulate the real water environment When the APPAO was put into and take out from the solution, it will unavoidably pass through oil layer. The experimental result showed the existence of oil layer had no influence on the degradation of orange ΙΙ. More than 90% orange ΙΙ was degraded in 30 minutes under the catalysis of APPAO and nearly no oil was adsorbed. In contrast, when original Al2O3 molecular sieves (AO) were put into the system, no orange ΙΙ was degraded while oil concentrations in DI water was higher than 2700 ppm. In this way, we could confirm the excellent anti-oil property of the as prepared APPAO. CONCLUSIONS In summary, we report a brand new Ag3PO4/ nanoparticle/polydopamine coated Al2O3 porous small ball (APPAO) with dynamic natural light catalytic ability. The as prepared APPAO shows typical Ag3PO4/Al2O3 heterojunction structure, which provides the APPAO with super high photocatalytic ability to make use of low-intensity natural light. Catalytic can be easily controlled by tampering with adsorption-desorption equilibrium of pollutants. The PDA layer in the material has two important functions. On is acting as adhesive between Al2O3 substrate and Ag3PO4 nanoparticle and making the production more stable than previous catalyst, the other on is reducing part of the Ag3PO4 and generating Ag nanosphere, which increases the SSA and enhances the catalytic ability of the material by plasmonic effect. Accordingly, it is capable of recyclable remediation of complex wastewater under natural light with high catalytic efficiency as well as outstanding reusability. Besides, the superhydrophilic surface provides it with excellent anti-oil property, and great reduces second pollution. We believe that the as prepared material can provide a promising pathway for energy-saving, controllable treatment of wastewater and the potential to be utilized in practical application.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. SEM images of APPAOs synthesized with different reaction times, The EDX image of APPAO surface, The element mapping images of the edge of APPAO cross-section, Light intensity measured by illuminometer, UV-Vis adsorption spectra of different pollutants in the natural light catalytic process, Comparison of this approach with existing technologies, The rate constant of degradation tests using APPAO, The natural light catalytic test with the existence of humic acids, The catalytic property of PAO and APAO, APPAO showed low degradation rate to pollutants in the dark, The catalytic property of free Ag3PO4 powder, The rate constant of degradation tests using Ag3PO4 nanoparticles, The catalytic performace of Ag/Ag3PO4/PDA, reusability of APPAO, Mechanical stability and acid-base resistance property of APPAO, The pH value of the reaction solution, The droplets density of four oils in Al2O3 group and APPAO group, The photocatalytic property of APPAO and AO in the presence of four oils. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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Table of Contents: An anti-oil sandwich structural Ag3PO4 nanosphere/polydopamine/Al2O3 porous small ball with capable of ultrafast natural light catalysis is fabricated, which can be used for low cost complex wastewater treatment. It is believed there is a dynamic process between catalysis and adsorption/desorption equilibrium.


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Al2O3 Sandwich

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