Efficient Removal of Polycyclic Aromatic Hydrocarbons, Dyes, and

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Efficient Removal of PAHs, Dyes and Heavy Metal Ions by a Homopolymer Vesicle Hui Sun, Jinhui Jiang, Yufen Xiao, and Jianzhong Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15242 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 9, 2017

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ACS Applied Materials & Interfaces

Efficient Removal of PAHs, Dyes and Heavy Metal Ions by a Homopolymer Vesicle Hui Sun†, Jinhui Jiang†, Yufen Xiao† and Jianzhong Du*,†,‡ †

Department of Polymeric Materials, School of Materials Science and Engineering, Tongji

University, 4800 Caoan Road, Shanghai 201804, China. E-mail: [email protected]; Tel: +8621-6958 0239 ‡

Shanghai Tenth People's Hospital, Tongji University School of Medicine, Shanghai 200072,

China

ABSTRACT: It is an important challenge to effectively remove environmental pollutants such as polycyclic aromatic hydrocarbons (PAHs), dyes and heavy metal ions at a low cost. Herein, we present a multifunctional homopolymer vesicle self-assembled from a scalable homopolymer, poly(amic acid) (PAA) at room temperature. The vesicle can efficiently eliminate PAHs, cationic dyes and heavy metal ions from water based on π-π stacking, hydrophobic effect and electrostatic interactions with the pollutants. The residual concentrations of PAHs, cationic dyes and heavy metal ions (such as Ni2+) in water are lower than 0.6, 0.30 parts per billion (ppb) and 0.095 parts per million (ppm), respectively, representing a promising adsorbent for water remediation. Furthermore, precious metal ions such as Ag+ can be recovered into silver nanoparticles by in situ reduction on the membrane of PAA vesicles to form silver

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nanoparticle/vesicle composite (Ag@vesicle) that can effectively catalyze the reduction of toxic pollutants such as aromatic nitro-compounds and be recycled for more than ten times.

KEYWORDS: homopolymer vesicle, water purification, PAHs, cationic dyes, heavy metal ions 1. INTRODUCTION Water pollution is a worldwide problem that threatens millions of people.1-3 PAHs, phenols, dyes and heavy metal ions in industrial wastewater seriously influence the quality of fresh water, thus leading to a high risk of illness, disease and death of human beings, animals and plants.1,4 More seriously, some pollutants such as PAHs and heavy metal ions have a long-term threat to the water, soils and creatures due to the easy transfer and bioaccumulation.5,6 There are several approaches to decontaminate polluted water, including solvent extraction, precipitation, adsorption, and ion exchange, etc.7-9 However, one method usually cannot deal with different pollutants. For example, some lifelong organic pollutants such as PAHs and nonionic dyes are difficult to be removed from water by traditional water purification techniques.10-13 Biological and chemical techniques have been used to accelerate the degradation of PAHs. Nevertheless, bioremediation method lacks proper bacteria and environmental conditions for the growth of bacteria, such as nutrients, pH, gas atmosphere and temperature, etc. Besides, different strains are needed because of different physiochemical characteristics of PAHs.14,15 Chemical method is widely used but accompanied with heat production, toxic byproducts and the introduction of oxidants.16-18 Heavy metal ions contaminants such as Pb2+, Zn2+, Cu2+, and Ni2+ have been widely produced by mining, steel, and electroplating industries. Both chemical and physical methods have been applied to remove these contaminants.5,13,19-23 Chemical precipitation is widely used with an


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efficiency as high as 99%.7,24 However, this method only works when their concentrations are pretty high and accompanies with a high cost and secondary pollutions such as toxic foams. Electrochemical methods for reducing metal ions into zero-valent metals are quite efficient.5,25,26 Unfortunately, the cost is very high and only limited kinds of heavy metal ions can be effectively recovered. Ion-exchange resins and adsorbents with strong negative charges are good candidates for the removal of heavy metal ions from polluted water but may introduce hazardous substance and have similar disadvantages to the above mentioned methods.13,23,27 Functional nano-objects like micelles,9,28 vesicles,29 magnetic nanoparticles,27,30-33 silica nanoparticles,13,23,34 are widely used to capture both organic and inorganic pollutants from wastewater. For example, we coated silica nanoparticles with poly(ethylene oxide), azobenzene derivative and branched polyethylenimine to adsorb PAHs and anionic dyes from water.34 Gibson et al. used ion-supported silica nanoparticles as an adsorption platform for the removal of arsenate with a capacity of 69 mg/g. Binnemans and coworkers separated heavy rare-earth ions from aqueous solutions with a capacity as high as 100 - 400 mg/g by EDTA-functionalized magnetic nanoparticles.23,27 Tortora et al. used sodium-dodecyl-sulfate-based micelle to enhance the ultrafiltration of heavy metal ions from water with an efficiency more than 88%.35 Very recently, polymer vesicles were applied to adsorb PAHs from water with a residual concentration below ppb level by our group.29,36 However, most of adsorbents cannot meet the demand for simultaneous removal of various pollutants coexisted in the polluted water. Herein, we report a cheap and scalable multifunctional vesicle based on PAA homopolymer for effective water remediation. The synthetic process of the homopolymer is simple, energysaving and fast, which can be completed within two hours at room temperature, and the vesicle could be obtained by simply adding water to the polymer solution without purification. As


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shown in Scheme 1, the PAA vesicle shows great potential for the efficient removal of both PAHs and heavy metal ions due to strong π-π stacking, hydrophobic effect and electrostatic interactions. Besides, cationic dyes can be decontaminated from water with an extremely low residual concentration due to the synergistic effect between π-π stacking and electrostatic interactions. Furthermore, the pollutants-saturated PAA vesicles can be easily separated from water by precipitation induced by the partial neutralization of the negative charges on the vesicle coronas and the cationic metal ions and dyes. Moreover, the PAA vesicles are excellent supporters for immobilizing silver nanoparticles which can be used as highly efficient recyclable catalyst for the reduction of highly toxic nitro-compounds to less toxic amino compound.


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Scheme 1. Synthesis of PAA Vesicles and Their Applications in Water Remediation #)

#)

(a) Synthesis of PAA homopolymer and its self-assembly into vesicles without purification of homopolymer; (b) Applications of PAA vesicles as a multifunctional adsorbent for the removal of both organic/inorganic pollutants, and a supporter to in situ reduce waste precious metal ions into valuable nanoparticle catalyst for catalyzing the reduction of p-nitrophenol. 2. MATERIALS AND METHODS 2.1 Materials. Pyromellitic dianhydride (PMDA, 99%), 4,4′-oxydianiline (ODA, 98%), silver nitrate (AgNO3, 99.8%), p-nitrophenol (p-NP, 98%), sodium citrate (99%), phosphotungstic acid (PTA, AR), magnesium sulfate anhydrous (MgSO4, 99.5%), naphthalene (98%), anthracene (96%), fluoranthene (98%), pyrene (97%), Victoria blue B (80%), methylene blue (70%), crystal


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violet (90%), nickel sulfate hexahydrate (NiSO4·6H2O, 99.9%), copper sulfate pentahydrate (CuSO4·5H2O, 99.99%), zinc chloride (ZnCl2, 99.95%) and lead nitrate (Pb(NO3)2, 99%) were purchased from Aladdin Chemistry, Co. Ltd. and used as received. DMSO-d6 was purchased from J&K Scientific Ltd. Sodium borohydride (NaBH4 96%) and other reagents and solvents were purchased from Sinopharm Chemical Reagent Co., Ltd and used without further purification. 2.2 Synthesis of Poly(Amic Acid). The synthesis and self-assembly of PAA homopolymer followed our previous work.37 Briefly, PDMA (0.023 mol) dispersed in DMF was added to the DMF solution of ODA (0.023 mol) batch-by-batch within 1 h, and the mixture was stirred for another hour to complete the reaction. 2.3 Self-Assembly of PAA Homopolymer into Vesicles. Two methods were used to prepare PAA vesicles by self-assembly. Method One: The purified PAA homopolymer was dissolved in DMF with a concentration of 2.0 mg/mL, and then deionized water was added dropwise to induce the formation of PAA vesicles. DMF was removed by dialysis against water. Method Two: Water was directly added into the unpurified polymer solution to simplify the selfassembly process. After the polymerization reaction, the PAA homopolymer solution was diluted to different concentrations of 1.0, 2.0, 3.0, 4.0, 6.0, 8.0 mg/mL without purification, followed by adding deionized (DI) water dropwise to self-assemble into vesicles. The volume of water added was twice of that of the polymer solution. Finally, the PAA vesicle solution was dialyzed in water to remove DMF. The size of the PAA vesicles can be controlled by the initial concentrations of PAA polymer in DMF.


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For Method One, the PAA homopolymer was purified by precipitating in acetone for three times, so some polymer chains with low molecular weight were removed. Subsequently, the sizes of vesicles self-assembled from purified and unpurified PAA were different (e.g., 200 nm and 300 nm for Methods One and Two at 2.0 mg/mL). But for Method Two, the self-assembly procedure is much simpler than Method One, and the diameters of the PAA vesicles can be controlled from 261 to 714 nm. Therefore, PAA vesicles were prepared by Method Two in this work. 2.4 The Stability Study of PAA Vesicle at Different pH by Dynamic Light Scattering. The diameters and polydispersities (PDs) of PAA vesicles in water at different pHs were measured by dynamic light scattering (DLS). 2.5 The Stability Study of PAA Vesicles at Different Ionic Strength by DLS. Aqueous MgSO4 solution was added into the PAA vesicle solution to investigate the ionic responsiveness. The final concentrations of MgSO4 in the PAA vesicle solutions were 0, 0.05, 0.1, 0.2, 0.3, 0.5, 1.0, and 1.5 M, respectively. After equilibrated for 30 min, the diameters and PDs of PAA vesicles were measured by DLS. 2.6 Adsorption of PAHs from Polluted Water by PAA Vesicles. Typically, 1.0 mL of stock solution of PAHs, various volumes of PAA vesicle solutions and DI water were mixed in a quartz cuvette to give certain concentrations of vesicles (The final volume was 2.0 mL). After different equilibrium time, the fluorescence quenching process was recorded via fluorescence spectroscopy and the UV absorbance of the solution was determined by UV-vis spectrophotometer after filtration. The residual concentrations of PAHs were determined based on the calibration curves of naphthalene, anthracene, fluoranthene and pyrene.


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2.7 Adsorption of Cationic Dyes from Polluted Water by PAA Vesicles. Different cationic dyes (e.g., Victoria blue B, methylene blue and crystal violet) were added into DI water to give a concentration of 50 µg/mL. Then PAA vesicle solution was mixed with the dye solutions for 10 min. The concentrations of PAA vesicles and cationic dyes are 228 and 10 µg/mL, respectively, and the pH of the solution was 6.9. After filtration, the absorbance of the mixtures was determined by UV-vis spectrophotometer. Subsequently, the concentrations of the residual dyes were calculated according to the calibration curves. 2.8 Adsorption of Heavy Metal Ions from Polluted Water by PAA Vesicles. Different heavy metal salts (NiSO4·6H2O, CuSO4·5H2O, ZnCl2, Pb(NO3)2) were added into water to prepare stock solutions with a concentration of 10 µmol/mL. Then the solution was diluted tenfold and the PAA vesicle was added to give a concentration of 1.0 mg/mL. After 10 min, the solution was filtered by nanofiltration membrane and then detected by Agilent 7700 series inductively coupled plasma mass spectrometer (ICP-MS). 2.9 Removal of Mixed Pollutants from Water by PAA Vesicles. The stock solutions of fluoranthene and Victoria Blue B were mixed together, and then PAA vesicles were added. The final concentrations of fluoranthene, Victoria Blue B and PAA vesicles were 5.0, 10.0, and 500 µg/mL, respectively. After 30 min, the mixture was filtered by nanofiltration membrane and tested by fluorescence spectrophotometer and UV-vis spectrometer. For the removal of the combination of PAH and metal ions, the stock solutions of fluoranthene and ZnCl2 were mixed, and then PAA vesicles were added. The final concentrations of fluoranthene, Zn2+ and PAA vesicles were 5.0, 207 µg/mL and 1.0 mg/mL, respectively. After 30


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min, the residual concentrations of fluoranthene and Zn2+ were determined by fluorescence spectrophotometer and ICP. 2.10 In situ Reduction of Waste Silver Ions into Valuable Silver Nanoparticles (Ag@vesicle). The aqueous PAA vesicle solution (0.4 mg/mL; 6.0 mL) was mixed with AgNO3 solution (1.0 mg/mL) at a molar ratio of 1:2 [AgNO3/(–COOH)]. After gently stirring for 30 min in the dark at room temperature, the aqueous NaBH4 solution (3.0 mg/mL) was then added to the vesicle solution immediately. The Ag@vesicle was obtained either by dialysis or centrifugation. 2.11 Traditional Silver Nanoparticles without a Vesicle Supporter. The traditional silver nanoparticles were prepared using a chemical reduction method.38 The spherical silver hydrosols were prepared by reducing aqueous AgNO3 with sodium citrate at near boiling temperature. In a typical procedure, an aqueous solution of AgNO3 (1.0 mM) was heated to boiling temperature, then an aqueous solution of sodium citrate (1.0 mM) was added. The solution was heated until the color was greenish yellow and then cooled to room temperature. 2.12 Reduction of p-NP Catalyzed by Ag@Vesicle. The catalytic reduction of p-NP was carried out in a quartz cuvette in the presence of Ag@vesicle and NaBH4. As control, the catalyst was replaced by PAA vesicles and Ag gel, respectively. To study the catalytic efficiency, the catalyst doses were 5.0 and 15.0 µg/mL, keeping the final concentrations of p-NP at 5.0 × 10-5 M and NaBH4 at 6.7 × 10-3 M. The stock solutions of p-NP (10 µL, 0.01 M) and NaBH4 (133 µL, 0.1 M) were added to a quartz cuvette one after the other. At this stage, the n-nitrophenol was converted to n-nitrophenolate anion. After that, the pre-calculated volume of catalyst and DI water were added to keep the volume of the mixture to 2.0 mL. The reaction was monitored at 400 nm by the UV-vis spectrometer at different time intervals. The Ag@vesicle could be


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recycled by high-speed centrifugation after the reduction reaction. In order to reduce the influence of the loss of Ag@vesicle during recycling process, the volume of the reaction solution increased to 20 mL but with the same concentrations of different additives. 2.13 Characterization. DLS. DLS analysis was conducted to determine the hydrodynamic diameter (Dh) and PD of PAA vesicles using a ZETASIZER Nano series instrument (Malvern Instruments). Zeta Potential. Zeta potential studies of PAA homopolymer vesicles and Ag@vesicles were conducted at 25 oC using a ZETASIZER Nano series instrument (Malvern Instruments). UV−vis Spectrometer. The UV−vis absorbance was recorded using a UV759S UV–vis spectrophotometer (Shanghai Precision & Scientific Instrument Co., Ltd.). Fluorescence Spectroscopy. Fluorescence experiments were carried out to monitor the π-π stacking effect between PAA vesicles and different PAHs via a Lumina Fluorescence Spectrometer (ThermoFisher). Transmission Electron Microscopy (TEM). All the vesicle solutions were diluted at ambient temperature. Copper grids were surface-coated to form a thin layer of amorphous carbon. Each sample (10 µL) was then dropped onto the carbon-coated grid and dried at ambient conditions. To stain vesicles, a drop of PTA (2 wt.%) solution was dropped onto a hydrophobic film (Parafilm), then those sample-loaded grids were laid upside down on the top of the PTA solution droplet and soaked for 1 min. After that a filter paper was used to carefully blot the excess PTA solution. The grids were dried under ambient environment overnight. Imaging was performed on a JEOL JEM-2100F instrument at 200 kV equipped with a Gatan 894 Ultrascan 1k CCD camera. The Ag@vesicle was viewed by TEM without staining. Scanning Electron Microscopy (SEM). SEM was utilized to observe silver nanoparticles and the surface morphologies of Ag@vesicle. To obtain SEM images, a drop of solution was spread on a silicon wafer and left until dryness. It was coated with platinum and viewed by a FEI Quanta 200 FEG


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electron microscopy operated at 15 kV. The images were recorded by a digital camera. Elemental Analysis. Agilent 7700 series inductively coupled plasma mass spectrometer (ICP-MS) was used to determine the concentration of metal ions adsorbed by PAA vesicles. 3. RESULTS AND DISCUSSION 3.1 Synthesis and Self-assembly of PAA Homopolymer. The detailed synthetic and selfassembly procedures as well as characterizations of PAA homopolymer were discussed in our previous work.37 PAA homopolymer can be massively synthesized at mild conditions and selfassembled by simply adding water into the polymer solution (Scheme 1). The critical vesiculation concentration (CVC) is as low as 0.1 µg/mL. Besides, PAA vesicles show good stability at a broad range of pHs (from 3 to 12, Figure S1 in the Supporting Information), which makes it possible to apply the PAA vesicles at different circumstances. 3.2 TEM Analysis of PAA Vesicles. To investigate their vesicular structure, PAA vesicles were frozen by liquid nitrogen and freeze-dried under vacuum, which maintained the morphology of vesicles in solution. As shown in Figure 1A and B, the stack-up vesicle (Highlighted by the red arrow) clearly demonstrates the classical vesicular structure after stained by PTA solution. The diameter of PAA vesicles is 185 ± 29 nm by TEM analysis. The membrane thickness of the PAA vesicle is calculated to be 5 nm by analyzing the electron transmittance diagram combined with the mathematical modeling as reported previously by our group.37,39,40


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Figure 1. (A) and (B) TEM image of PAA vesicles with different magnifications. The red arrow in (B) highlights the stack-up vesicle on the other five vesicles. The black circles are membranes stained by PTA and the bright areas are hollow cavities of PAA vesicles. 3.3 Control of the Size of PAA Vesicles by Initial PAA Concentrations in DMF and the Aggregation Behavior at Different Ion Strength. The size of PAA vesicles can be easily controlled by tuning the initial concentration (Ci) of PAA homopolymer in DMF (Figure 2A). When the Ci’s increase from 1.0 to 8.0 mg/mL, the Dh’s of the PAA vesicles increase from 261 to 714 nm gradually while the corresponding PDs of vesicles remain low. For all the samples, the preparation and characterization conditions are the same except that the initial concentration of PAA homopolymer in DMF is different. During self-assembly process, the volume ratio of DMF to water is 1:2, and all the DMF is removed by dialysis in DI water before conducting DLS analysis. The vesicles with various sizes by can meet different demands in real world applications.


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Figure 2. Control of the size of PAA vesicles by the initial concentration of PAA in DMF and separation of PAA vesicles from water by Mg2+-induced precipitation. (A) DLS studies of PAA vesicle at different initial concentrations by directly adding water into polymer solution; (B) Size and size distribution of PAA vesicles at different concentration of Mg2+ in the solution after 30 min; (C) PAA vesicles (a) before and (b) after treated with 1.0 M MgSO4 for 12 h; (D) Schematic illustration of the aggregation of PAA vesicles induced by Mg2+. Furthermore, the aggregation of PAA vesicles can be easily tuned by the concentration of divalent cation in solution. For example, when the initial concentration of the PAA homopolymer is 6.0 mg/mL in DMF, the Dh of PAA vesicle is 530 nm with good size


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distribution, as confirmed by DLS studies (Figure 2A). Without the addition of MgSO4, the size and size distribution are quite stable, as shown in Figure 2B. By contrast, the diameter of PAA vesicle dramatically increases to 969 nm in the presence of 0.05 M MgSO4. The Dh further increases with the addition of MgSO4, leading to the precipitation of the PAA vesicles from water after 12 h when the concentration of MgSO4 is 1.0 M (Figure 2C). The reason for the saltinduced precipitation is that the added Mg2+ ions interact with carboxyl groups on the vesicle surface and bridge more vesicles together as more Mg2+ ions are added in the vesicle solution (Figure 2D). It is noteworthy that positively charged ionic pollutants in the PAA vesicles can be separated from water by the Mg2+-induced precipitation. 3.4 Ultrafast Removal of PAHs from Water by PAA Vesicles. Although the PAHs have low solubility in water, they are highly bioaccumulative and may cause mutation or cancer.41,42 The removal of trace of PAHs from aqueous solution remains an important technical challenge. Since the backbone of PAA homopolymer consists of aromatic rings, PAA vesicles can effectively adsorb PAHs from water based on π–π stacking.29 As shown in Table 1 and Figure 3, typical PAHs including naphthalene, anthracene, fluoranthene, and pyrene are used to investigate the adsorption capability of PAA vesicles. As shown in Figure 3A, the adsorption capacity of naphthalene reaches 35 mg/g, but its residual concentration is also as high as 3.33 ppm after filtered by nanofiltration membrane even after 24 h (The calibration curves of PAHs are presented in Figure S2). We ascribe the poor adsorption efficiency to the weak π–π stacking between naphthalene and benzene rings. In principle, the adsorption rate and efficiency can be significantly improved if the π–π interaction between the PAHs and vesicles is enhanced.


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Table 1. The Adsorption Capabilities and Residual Concentrations of Several PAHs in Water after Treatment with PAA Vesicles

Naphthalene

Anthracene

Fluoranthene

Pyrene

Adsorption capacity (mg/g)

35

29

5.8

0.41

Residual concentrations (ppb)

3328

41

0.85

0.60

To verify the above hypothesis, anthracene, fluoranthene and pyrene were tested, and we found that the fluorescence could be quenched within several minutes due to π–π stacking and excellent water dispersity of PAA vesicles. The residual concentration of anthracene reduces to 41 ppb with an adsorption efficiency of 99.2% (Figure 3B). In order to investigate the influence of the solid content on the adsorption efficiency, the adsorption experiments with different water volumes were carried out, as shown in Figure S3 in the Supporting Information. The results manifest that the water volume hardly affects the adsorption efficiency. As shown in Figure 3C, the adsorption capacity of fluoranthene is 5.8 mg/g with the residual concentration reducing for more than three orders of magnitude (> 99.91%) to 0.85 ppb. For pyrene, the adsorption efficiency is also as high as 99.1% and the residual concentration is only 0.6 ppb (Figure 3D).


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0.4 0h 0.5 h 2h 4h 8h 16 h 24 h

Cvesicle = 278 µg/mL

Absorbance

0.3

0.2

20000

(B)

Ref: 5 µg/mL Cvesicle (µg/mL)

5.36

15000

Intensity (a.u.)

(A) Cnaphthalene = 10 µg/mL

6

×2

10000

0.1

240

260

280

0 360

300

Wavelength (nm)

40000

40000

(C)

342.4

5000

0.0

380

400

420

5.36

(D) Cvesicle (µg/mL) 5.20

30000

6

×2

460

Ref: 68.5 ppb

Intensity (a.u.)

30000

440

Wavelength (nm)

Cvesicle (µg/mL)

Ref: 1.0 µg/mL Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20000

20000

342.4

5

×2

10000

10000

166.5 0 400

420

440

460

480

500

520

540

0 360

380

Wavelength (nm)

400

420

440

460

Wavelength (nm)

Figure 3. (A) UV-vis absorption of naphthalene solution at different time with the addition of PAA vesicles; (B), (C) and (D) fluorescence intensities of aqueous solutions of residual anthracene, fluoranthene and pyrene, respectively. The adsorption time is 1 min. 3.5 Effective Adsorption of Cationic Dyes from Water by PAA Vesicles. Many dyes with different chemical structures are used in the textile industry during fiber bleaching and dyeing processes.43 Some industrial dyes are harmful to humans, especially for the lifelong pollutants with aromatic moieties due to the difficulty in the degradation.44 Therefore, different absorbents have been developed for the removal of dyes.44,45 The adsorption efficiency of PAA vesicles for cationic dyes was investigated based on several typical dyes (Victoria blue B, methylene blue, and crystal violet). The ionic strength strongly influence the adsorption efficiency of cationic dyes because of the competitive effects, especially divalent ions, so the adsorption experiments were conducted in the absence of other cationic ions. The calibration curves are shown in Figure S4 in the Supporting Information. Figure 4 illustrates


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the detailed adsorption efficiency of PAA vesicles for (A) Victoria blue B, (B) methylene blue, (C) crystal violet and (D) crystal violet. The concentration of vesicles in (A-C), (a) and (b) in (E) and (F) is 228 µg/mL, while it is 720 µg/mL in (D), (c) in (E), and (F). The concentration of dyes is 10 µg/mL and the pH is 6.9. For Victoria blue B, after adsorption for 10 min, the UV-vis spectrum of the mixed solution was measured by UV-vis spectrometer after filtration by a nanofiltration membrane. Notably, there are no signals between 500 and 700 nm, indicating the ultrahigh adsorption efficiency of Victoria blue B, which was also confirmed by the digital photo inserted in Figure 4A. Then the filtrate was concentrated for 100 times and measured by UV-vis spectrometer again, but there were still no signals. Considering that the resolution of UV-vis spectrometer is 0.005, the residual concentration of Victoria blue B should be less than 0.54 ppb, which is much lower than previously reported values.43,44,46-49 For methylene blue, PAA vesicles also show excellent adsorption efficiency (Figure 4B). The residual concentration of methylene blue is less than 0.31 ppb as determined by UV-vis spectrometer using the same method as for Victoria blue B. For crystal violet, the adsorption efficiency is not as good as that for the above mentioned dyes when small amount of PAA vesicle is used (Figure 4C), which is owing to that the electropositivity of ammonium ion is less than that of Victoria blue B and methylene blue due to the electro-donating property of the triphenylamine moiety in the crystal violet molecule. However, when the concentration of PAA vesicle increases to 720 µg/mL, nearly all the crystal violet can also be removed from water as detected by UV-vis spectroscopy (Figure 4D). The residual concentration of crystal violet is < 0.28 ppb.


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The adsorption capacities are 332, 68.8 and 47.7 mg/g for Victoria blue B, methylene blue and crystal violet, respectively. The synergistic effect between electrostatic interaction and π–π interaction of PAA vesicles and cationic dyes contributes to the excellent adsorption efficiency. Besides, after adsorption for 12 h, PAA vesicles and dyes can be precipitated from the mixed solution together, as shown in Figure 4E and F. The removal process of adsorbents in our case is much easier compared to traditional nano-adsorbents.23,27,29,30,35 The desorption experiments were also conducted at pH 4.5, as shown in Figure S5 in the Supporting Information. After filtration of the adsorbed dye solution, the color was still very deep, which was much different from that in Figure 4, indicating the successful desorption of cationic dyes at acidic condition.

Figure 4. The adsorption efficiencies of (A) Victoria blue B, (B) methylene blue, (C) and (D) crystal violet with different vesicle concentrations at 10 min by PAA vesicles; (E) and (F) corresponding photos of different dye solutions before/after adsorbed for 24 h. The concentration of dyes is 10 µg/mL. The concentration of PAA vesicle is 228 µg/mL in (A), (B), (C) and (a) and (b) in (E) and (F), while it is 720 µg/mL in (D) and (c) in (E) and (F).


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3.6 Efficient Elimination of Heavy Metal Ions from Water by PAA Vesicles. To investigate the adsorption efficiency of heavy metal ions, several typical industrial discharged ions are selected (Ni2+, Cu2+, Zn2+, Pb2+). After adding PAA vesicles, the metal ions and carboxyl groups on the vesicle will bind together by electrostatic adsorption and complexation. And for this adsorption mechanism, the adsorption efficiency of metal ions is significantly affected by the ionic strength. The loose structure of the vesicle membrane37 makes it an metal ion sponge, which dramatically increases its adsorption capacity and efficiency for metal ions. As shown in Figure 5, the PAA vesicle was mixed with different metal ions with a concentration of 1.0 mg/mL. After 10 min, the mixed solutions were filtered and measured by inductively coupled plasma mass spectrometer to determine the residual concentration of metal ions. For Ni2+, PAA vesicles show excellent adsorption efficiency (Figure 5A). After filtration, the concentration of Ni2+ is reduced from 58.7 to 0.095 ppm with an efficiency of 99.84%, which is far below the national discharged standard of sewage of China (GB 20426-2006, below 0.5 ppm). For Cu2+, the residual concentration is 2.47 ppm, which is not as good as Ni2+, but the adsorption efficiency is still as high as 96.11% (Figure 5B). For Zn2+, the concentration is reduced from 65.4 to 0.58 ppm after filtration, corresponding to an adsorption efficiency of 99.11% (Figure 5C), which also meets the national discharge standard of China (GB 20426-2006, below 2.0 ppm). Pb2+ is also tested to verify the generality of PAA vesicle as an excellent adsorbent for heavy metal pollutants. After filtration, nearly 99.58% of Pb2+ is adsorbed by PAA vesicle with a residual concentration of 0.87 ppm (Figure 5D), which is lower than the permitted concentration (1.0 ppm) of national discharge standard of China (GB 20426-2006). As expected, the adsorption efficiencies of different heavy metal ions strongly depend on the pH of the solution, which are hard to reach 50% when the pH of the solution is 5.0 (Figure S6 in the Supporting


19


Information).50,51 Most importantly, the desorption efficiencies of different heavy metal ions are also as high as above 80%, which guarantees the cyclic utilization of PAA vesicles (Figure S7 in the Supporting Information). It is noteworthy that the maximal adsorption capacities of PAA vesicle to different heavy metal ions are as high as 58.6, 61.1, 64.8 and 206.3 mg/g for Ni2+, Cu2+, Zn2+ and Pb2+, which are better than most of current adsorbents.19,20,22,23 70

80

(A)

58.7 ppm

70

30 20

Copper sulfate (ppm)

40

100

60

Copper sulfate (ppm)

50

10

1

0.1

Before

10

After

50 40 30 20

Before

1

Before

After

2.47 ppm

0

After

Before

80

After

250

(C)

65.4 ppm 100

50 40 30

10

150

1

100

20 0.1

Before

10

After

50

Before

After

100

10

1

0.1

Before

0.58 ppm

0

(D)

207.2 ppm 200

Lead nitrate (ppm)

Zinc chloride (ppm)

60

Lead nitrate (ppm)

70

10

0.1

10

0.095 ppm 0

(B)

63.5 ppm

100

Nickel sulfate (ppm)

Nickel sulfate (ppm)

60

Zinc chloride (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 37

After

0.87 ppm 0

Before

After

Figure 5. Concentrations of heavy metal pollutants before and after adsorbed by PAA vesicles. (A) nickel sulfate, (B) copper sulfate, (C) zinc chloride and (D) lead nitrate. Insets: plots on a logarithmic scale. The concentration of PAA vesicle is 1.0 mg/mL and the pH is 6.5. 3.7 Removal of Mixed Pollutants from Water by PAA Vesicles. It is very important for an adsorbent to possess the capability of adsorbing different pollutants simultaneously since there are several contaminants coexisting in the polluted water. In this study, different combinations of water pollutants (e.g., PAHs/dyes, PAHs/metal ions) were used to investigate the potential of PAA vesicles to eliminate various contaminants concurrently. The removal of PAHs and dyes at the meantime by PAA vesicles was firstly studied. Fluoranthene and Victoria blue B aqueous


20

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solutions were mixed, and PAA vesicle solution was added to the mixture. The final concentrations of fluoranthene, Victoria blue B and vesicle were 5, 10 and 500 µg/mL, respectively. After 30 min, the mixture was filtered by nanofiltration membrane and tested by fluorescence spectrophotometer and UV-vis spectrometer. As shown in Figure 6A, the elimination efficiencies of fluoranthene and Victoria blue B are 99.83% and 99.97%, respectively, which are similar to the adsorption efficiencies of single compounds. Similarly, the fluoranthene and lead nitrate solutions were also mixed to study the adsorption efficiency of PAA vesicles against the mixture solution of PAHs/metal ions. The final concentrations of fluoranthene, Zn2+ and vesicle were 5, 207 µg/mL and 1.0 mg/mL, respectively. The residual concentrations of fluoranthene and Zn2+ were measured by fluorescence spectrophotometer and inductively coupled plasma mass spectrometer respectively, after nanofiltration (30 min). Figure 6B indicates that the adsorption efficiencies of fluoranthene and Zn2+ are 99.78% and 99.45%, respectively, which is consistent with the experimental results of single pollutant. It can be concluded from the above experimental results that several pollutants can be adsorbed by PAA vesicles simultaneously but still with very high elimination efficiency. The adsorption capacity and efficiency of PAA vesicles are compared with other adsorbents (Table S1 in the Supporting Information).52-58 The adsorption capacities are comparable with reported adsorbents, but the adsorption efficiencies of PAA vesicles for PAHs, cationic dyes and heavy metal ions are much better than other adsorbents.


21


99.83%

99.97%

(A)

100

Adsorption efficiency (%)

100

Adsorption efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

80 60 40 20 0

Page 22 of 37

99.78%

99.45%

Fluoranthene

Lead nitrate

(B)

80 60 40 20 0

Fluoranthene

Victoria blue B

Figure 6. The adsorption efficiency of (A) fluoranthene and Victoria blue B in the mixture of PAH/dye solution and (B) fluoranthene and lead nitrate in the mixture of PAH/metal ion solution by PAA vesicles. The adsorption kinetics of different pollutants by PAA vesicles was also studied. However, it is difficult to monitor the adsorption process of pyrene and methylene blue since the adsorption reaches equilibrium very quickly (e.g., 30 s), as shown in Figure S8 A-B in the Supporting Information. Therefore, efforts have been made to measure the adsorption amounts of pyrene and methylene blue at the very beginning of the adsorption process. In 0.5 min, ca. 90% of pyrene and 94% of methylene blue were adsorbed by PAA vesicles. In 10 min, 99% of Zn2+ was adsorbed. These results confirmed the ultrafast adsorption rate of PAA vesicles for different pollutants, which was ascribed to the hollow structure of PAA vesicles with a thin membrane (5 nm) and excellent water dispersity.37 The adsorption kinetics of Zn2+ by PAA vesicles was studied using pseudo-second order model,59 as shown in Equation 1. ௧ ௤೟

=௞

ଵ మ మ ௤೐

௧

+ ௤ ···························1 ೐

ℎ = ݇ଶ ‫ݍ‬௘ଶ ································2


22

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Here, t is the adsorption time; qt is the amount of adsorbate adsorbed at time t; qe is the amount adsorbed at equilibrium; k2 is the adsorption rate constant, h is the adsorption rate at the initial stage. As shown in Figure S9 in the Supporting Information, the adsorption kinetics of Zn2+ was fitted by pseudo-second order model with an R2 of 0.998. The adsorption reaches equilibrium within one hour with an adsorption rate constant (k2) of 0.737 g/(mg·h), corresponding to an adsorption rate (h) of 3120 mg/(g·h) at the initial stage, as calculated using Equation 2. These results demonstrated the ultrafast adsorption rate of Zn2+ by PAA vesicles. The equilibrium adsorption capability (qe) is 65.1 mg/g, which matches the experimental result (64.8 mg/g) very well. 3.9 In situ Reduction of Waste Precious Metal Ions to Valuable Nanoparticles to Eliminate Toxic Nitro-Compounds in Water. Precious metal ions such as silver, gold and palladium as pollutants in water are difficult to recover.60-62 So we propose an in situ reduction strategy to reduce precious metal ions into nanoparticles on the membrane of PAA vesicles to prepare metal nanoparticle/vesicle composite.


23


Page 24 of 37

Figure 7. The recovery of silver ions into nanoparticles by using PAA vesicles as supporter. (A) DLS studies and digital photos (inlet image) before and after the reduction of silver ions; (B) SEM; (C) TEM images and (D) magnified TEM image of Ag@vesicles. The silver ions are chosen as the typical expensive metal ions, which can be adsorbed and concentrated on the membrane of PAA vesicles and then reduced to nontoxic silver nanoparticles to form Ag@vesicle with excellent catalytic capacity. As shown in Figure 7, the recovery of waste silver ions into nanoparticles preserve the structure of PAA vesicles, but the negative charges on the surface of vesicles are partially neutralized (Figure S10 in the Supporting Information). The color of PAA vesicle solution turns into brown, but the diameter of Ag@vesicles barely changes after the reduction of silver ions (Figure 7A). SEM and TEM images in Figure 7B-D illustrate the morphology and size distribution of silver nanoparticles on the membrane of PAA vesicles. The relative content of silver nanoparticles is 112.0 mg/g vesicle, as confirmed by inductive coupled plasma spectroscopy, which demonstrates that the PAA vesicle can dramatically concentrate silver ions from solution as the concentration of silver ions is calculated to be 0.15 mg/mL. The reduction reaction of p-nitrophenol (p-NP) to p-aminophenol (p-AP) was chosen as a typical reaction to demonstrate the catalytic capability of Ag@vesicle. UV-vis spectroscopy was used to monitor the reaction at 400 nm. In the absence of Ag@vesicles, p-NP was not reduced throughout the experiment period (see curves a & b in Figure 8A), whereas the UV-vis absorption decreased sharply (from 0.90 to 0.02 a.u.) within 10 min after the addition of Ag@vesicles (Figure 8A and Figure S11 in the Supporting Information), confirming excellent catalytic activity of Ag@vesicles. The lag time (ca. 2 min) results from the dissolved oxygen in the solution.63 By contrast, even at the same concentration and with the similar diameter (Figure


24

Page 25 of 37

S12 in the Supporting Information), the Ag nanoparticles without polymer vesicle as a supporter barely catalyze the reduction reaction. Unless the concentration of Ag increases to 15.0 µg/mL (Three times higher than that of Ag@vesicle solution), only part of p-NP is reduced in 30 min (see curves c and e in Figure 8A). The enhancement of catalytic capability of Ag@vesicle is originated from the π–π stacking between p-NP and the aromatic membrane of PAA vesicles. Owing to the strong affinity between aromatic compounds, p-NP is adsorbed by the aromatic vesicle membrane, creating a local domain with amplified concentration of p-NP, where also allocates the Ag nanoparticles (Scheme 1). Such synergetic mechanism ensures a better catalytic activity of the Ag@vesicles than Ag nanoparticles without a vesicle supporter, which also tend to aggregate over time. More importantly, the Ag@vesicle could be recovered after the reduction reaction. The amount of pNP reduced in 20 min was used to evaluate the catalytic ability of Ag@vesicle. As shown in Figure 8B, for the first five cycles, the catalytic capability of Ag@vesicle barely attenuated due to the excellent stability of silver nanoparticles immobilized on the membrane of PAA vesicles. After that the catalytic activity of Ag@vesicle gradually decreased, which might be attributed to the loss of catalyst during recycling process. However, the p-NP could also be completely reduced within 30 min, confirming excellent catalytic activity and recyclability of Ag@vesicle. 0

5

10

15

20

25

30

(a)

100

35

105

(A)

80

60

40

(B)

100

(b) (c) (a) blank control (b) PAA vesicle (c) Ag without vesicles: 5.0 µg/mL (d) Ag@vesicles: 5.0 µg/mL (e) Ag without vesicles: 15.0 µg/mL

Catalytic capability (%)

Relative 4-NP content (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(e) 20

(d)

95 90 85 80 75

0 0

5

10

15

20

Time (min)

25

30

35

70 0

2

4

6

8

10

Cycle number


25


Page 26 of 37

Figure 8. (A) with different additives: (a) blank control; (b) pure PAA vesicle solution; (c) traditional silver nanoparticles without vesicles (control, 5.0 µg/mL); (d) Ag@vesicle solution (5.0 µg/mL); (e) traditional silver nanoparticles without vesicles (control, 15.0 µg/mL); (B) the catalytic capability of Ag@vesicle at different recycle number. 4. CONCLUSIONS In summary, the results of our study imply that PAA vesicles can be used for effectively eliminating different water pollutions including PAHs, cationic dyes and heavy metal ions. The PAA homopolymer is synthesized by one-step condensation polymerization of commercially available monomers at ambient conditions within 2 h, which is further self-assembled into vesicles by directly adding water into polymer solution without purification. Considering the simplicity of the synthesis and self-assembly, the PAA vesicles can be massively producible. PAA vesicle showed high adsorption efficiencies of 99.91% and 99.84% for PAHs and heavy metal ions and very quick adsorption rates. Notably, the cationic dyes can be removed from water with extremely low residual concentrations (0.5 ppb) as a result of the synergistic effect between electrostatic interaction and π-π stacking. More importantly, mixed water pollutants (PAHs/dyes and PAHs/metal ions) can be removed by PAA vesicles simultaneously with high adsorption efficiency as that for single pollutant. Thus, these PAA vesicles could be considered as an ideal candidate to purify polluted water since most of industrial wastewater contains several kinds of pollutants. In addition, PAA vesicles allow the recovery and recycle of valuable precious metal ions contaminants into nanoparticles and endow them with new functions such as excellent catalytic activity, providing a new strategy to recover waste metal ions into valuable materials.


26

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ASSOCIATED CONTENT Supporting Information. DLS studies of PAA vesicles at different pH, calibration curves of PAHs and dyes, adsorption kinetics of PAHs, dyes and metal ions, desorption results of dye and metal ion at acidic condition, zeta potential results, UV-vis spectrum of catalytic process, TEM image of silver nanoparticles, and comparison of adsorption capacity and efficiency with other adsorbents. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by Shanghai International Scientific Collaboration Fund (15230724500), Shanghai 1000 Talents Plan, NSFC (21374080, 21674081 and 21611130175), and the Fundamental Research Funds for the Central Universities (1500219107). We sincerely thank Dr. Yao Zhao at Institute of Chemistry, Chinese Academy of Sciences, for performing ICP-MS analysis. REFERENCES (1) Montgomery, M. A.; Elimelech, M. Water and Sanitation in Developing Countries: Including Health in the Equation. Environ. Sci. Technol. 2007, 41, 17-24.


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Efficient Removal of Polycyclic Aromatic Hydrocarbons, Dyes, and

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