Amino-Functionalized Porous Nanofibrous Membranes for

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Surfaces, Interfaces, and Applications

Amino-Functionalized Porous Nanofibrous Membranes for Simultaneous Removal of Oil and Heavy Metal Ions from Wastewater Yang Wang, Baixian Wang, Qifei Wang, Jiancheng Di, Shiding Miao, and Jihong Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18066 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 16, 2018

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ACS Applied Materials & Interfaces

Amino-Functionalized Porous Nanofibrous Membranes for Simultaneous Removal of Oil and Heavy Metal Ions from Wastewater Yang Wang,† Baixian Wang,† Qifei Wang,† Jiancheng Di,*† Shiding Miao,§ and Jihong Yu*†‡ †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,

Jilin University, Changchun 130012, China ‡

International Center of Future Science, Jilin University, Changchun 130012, China

§

Key Laboratory of Automobile Materials of Ministry of Education, School of Materials Science

and Engineering, Jilin University, Changchun 130012, China KEYWORDS: Amino-silanization reaction, Electrospun porous nanofibers, Superwettability, Wastewater treatment, Oil/water separation, Heavy metal ions

ACS Paragon Plus Environment

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ABSTRACT

Both oil spill and heavy metal ions in the industrial wastewater cause severe problems for aquatic ecosystem and human health. In the present work, the electrospun superamphiphilic SiO2– TiO2 porous nanofibrous membranes (STPNMs) comprised of intrafiber mesopores and interfiber macropores are modified by an amino-silanization reaction, which affords the membrane (ASTPNMs) the ability to simultaneously remove the oil contaminants and the water-soluble heavy metal ions from wastewater. The under-water superoleophobicity of ASTPNMs facilitates the highly efficient separation of water and various oils, even emulsifier stabilized emulsion. Meanwhile, an optimal modification time (15 min, ASTPNM-15) is important for maintaining the under-oil superhydrophilicity of the membrane, based on which the oil contaminant in membrane can be easily cleaned by water alone, showing excellent self-cleaning performance. The adsorption of Pb2+ over ASTPNM-15 reaches equilibrium at around 20 min, and the monolayer adsorption capacity is 142.86 mg g−1 at pH = 5 at 20 oC. In the breakthrough processes, the permeation volume of ASTPNM-15 for the purification of Pb2+ (5 ppm, pH = 5) reaches to 160 mL when the concentration of Pb2+ in the filtrate increases to 0.05 ppm. The separation efficiencies of ASTPNM-15 for simulated wastewater containing both oil spill and various heavy metal ions (Pb2+, Cr3+, Ni2+) are larger than 99.5%. In addition, the separation capacity keeps stable over five purification-regeneration cycles without obvious decrease, proving excellent recyclability and reusability of ASTPNM-15 for practical applications.


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INTRODUCTION The frequent marine oil-spill accidents as well as the oil pollution along with the rapid industrialization have caused severe problems for aquatic ecosystem that threaten the human health.1-2 The membrane separation methods based on the surface superwettabilities are developed to separate and harvest oils from oil-polluted water.3-6 Among them, the underwater superoleophobic materials, which are inspired by the subaqueous repellence to oil of fish scales,7 have garnered considerable attention for their excellent oil-repellent ability, low oil adhesion and high separation efficiency. For instance, hydrogels, 8 polyelectrolyte9 and zeolite10 have been successfully employed to fabricate under-water superoleophobic surface. Comparing with these rigid membranes, the fibrous membranes with excellent flexibility, ductility and formability have been evidenced to be the eligible candidates for the separation oil/water mixtures.11-13 Recently, a polarity-based strategy was demonstrated to modulate the wettability of the surface with superamphiphilicity. Water preferred to interact with the high-polarity SiO2–TiO2 porous nanofibrous membranes (STPNMs), and formed a stable liquid-solid composite interface that could repel the immiscible oils with lower polarity.14 Furthermore, the self-cleaning performance was achieved because the infused oil in membrane could be directly substituted by water. However, the materials mentioned above can barely remove the water-soluble heavy metal ions during the oil/water separation process, which often coexist with oils in wastewater. Heavy metal ions in water bodies have brought potential hazards and global concerns owning to their high toxicity, non-degradability and bioaccumulation in organisms.15-18 Several approaches have been utilized for eliminating heavy metal ions from wastewater, including adsorption, chemical precipitation, coagulation-flocculation, ion exchange,


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electrochemical treatment and so on.19 The adsorption method with high efficiency and low cost is considered to be the most promising technique to remove the heavy metal ions from wastewater.20-23 The affinity towards target heavy metal ions can be further improved by modifying suitable functional groups (-NH2, -SH, -S-, etc.) on the porous adsorbents.24-26 However, in a powder form, the adsorbents suffer from the disadvantage of poor recyclability, complex operational process and secondary pollution due to their suspended dispersive properties in water. In addition, the low oil-repellent ability of these adsorbents gives rise to the simultaneous adsorption of water and oil in wastewater, resulting no capability of oil/water separation. Thus, the traditional methods for the purification of wastewater typically involve two steps, oil/water separation and removal of heavy metal ions, which is complex and ineffective. Therefore, it is necessary to design membranebased materials with both superwettability and active adsorption sites that can simultaneously remove oil and heavy metal ions from wastewater. Thus far, only two kinds of separation membranes, the konjac glucomannan coated fabrics27 and the glass powder painted stainless steel meshes28, have been fabricated to one-step purify the wastewater containing oil and heavy metal ions. However, the low removal efficiency for heavy metal ions (69.5% and 93%, respectively) restricts the further applications of these membranes in practice. Moreover, the self-cleaning performance of the oil-fouled membranes were not concerned in these works. Herein, STPNMs fabricated by electrospinning technique are modified by amino groups to achieve the simultaneous removal of oil and heavy metal ions from wastewater. The under-water superoleophobicity of the amino-modified STPNMs (ASTPNMs) facilitates the effective separation of oil slick or emulsified oil from water. The under-oil


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superhydrophilicity endows ASTPNMs excellent self-cleaning ability once contaminated by oil. Meanwhile, the modified amino groups, together with the hierarchical porous structure, afford the ASTPNMs high adsorption capacity for various heavy metal ions in wastewater, and the absorbed heavy metal ions can be simply desorbed by washing with HCl. The purification capacity of the membrane for the simulated wastewater containing both oil spill and various heavy metal ions (Pb2+, Cr3+, Ni2+) remains stable over five purificationregeneration cycles. Therefore, ASTPNMs hold great promise as exceptional candidates for the one-step purification of wastewater in practical applications. EXPERIMENTAL SECTION Materials. Polyethylene oxide (PEO, Mw = 1,000,000) was from Alfa Aesar. Titanium (III) chloride (TiCl3, 20 wt% in 2N HCl solution) was from Acros Organics. 3-[(Trimethoxysilyl) propyl] diethylenetriamine was from Sigma-Aldrich. Absolute ethanol (EtOH, A.R.), hexadecyltrimethylammonium bromide (CTAB), tetraethoxysilane (TEOS, 98%, A.R.), ethane dichloride (A.R.), cyclohexane (A.R.) and sodium bicarbonate were from Beijing Fine Chemical Co. Ltd. N-hexane (A.R.), petroleum ether (A.R.), tetradecane (A.R.), lead (II) nitrate (Pb(NO3)2), concentrated hydrochloric acid (HCl (aq.), 37.0 wt%, A.R.), nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O) and chromium (III) nitrate nonahydrate (Cr(NO3)3·9H2O) were obtained from Shanghai Chemical Reagent Co. Ltd. All the materials were used without further purification. Fabrication of ASTPNMs. The fabrication of STPNMs was carried out via the electrospinning technique.29 Typically, 2.0 g of hexadecyltrimethylammonium bromide (CTAB) and 0.3g of polyethylene oxide were added to a mixture containing 2.5 g of TiCl3, 4.0 g of ethanol, and then the solution was stirred until the solid was completely dissolved. Subsequently, 3.8 g of tetraethoxysilane was added to the solution, which was further stirred for another hour. The as-


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prepared solution was electrospun with a flow rate of 1 mL h-1. The working distance and the applied voltage were kept at 20 cm and 25 kV, respectively. The collected product was in turn heated at 110 oC in air for 12 h and calcined at 500 oC for 4 h. The amino-silanization reactions were performed in a closed chamber (2 L), in which 2.65 g 3[(trimethoxysilyl) propyl] diethylenetriamine was dropped to the bottom (the concentration was about 5.0 × 10-3 M). The as-prepared STPNM was placed on a ceramic tray that dangling fixed in the middle of the chamber. Then the chamber was evacuated to a pressure of about 0.2 atm and put into an oven at 150 oC for a certain time. The amino-functionalized STPNM were named as ASTPNMs-x (x = 15, 30 and 45) according to the chemical modification time (min). Water purification experiments. The oil and water mixtures were prepared by mixing water and oil (cyclohexane, n-hexane, petroleum ether, tetradecane and ethane dichloride, respectively) with volume ratio of 1:1 and stirred for 1 h. The oil/water emulsion was prepared by adding a certain amount of CTAB (4 mg/mL) to the mixture of water and cyclohexane (100:1 v:v) and the mixture was stirred for 3 h. The simulated wastewater was composed of 20 mL water, heavy metal ions (Pb2+, Cr3+ and Ni2+, the concentration of each ion was 5 ppm in water) and 10 mL oil (cyclohexane). The pH values of the solutions were adjusted to 5 by using HCl or NaOH. The simulated waste seawater was prepared by adding 0.72 g NaCl (about 3.5 wt% in aqueous solution) to the simulated wastewater and the pH value was adjusted to 8 by adding NaOH. All the water purification experiments were carried out at 20 oC. The ASTPNM-15 was fixed between two stainless steel flanges, and then pre-wetted with a small amount of water. 30 mL of wastewater was passed through the water-prewetted membrane and the filtrate was collected. The driving force during the purification process was its own gravity. The flux of ASTPNM-15 was


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measured by calculating the volume of the filtrate after 10 min separation. During the measurement, the height of solutions above the membrane was maintained at 15 cm to afford a constant pressure. The removal efficiency of oil and heavy metal ion was calculated according to the rejection coefficient (R), which can be expressed as: R = (1 -

Cf ) × 100% Ci

(1)

where Ci and Cf were the concentration of the contaminant in the initial solution and the filtrate, respectively. To study the adsorption kinetics, the adsorbed quantity of Pb2+ ion versus time on ASTPNM-15 was investigated and the initial Pb2+ ion concentration was selected as 100 mg L-1. Briefly, 20 mg ASTPNM-15 was mixed with 200 mL Pb2+ solution and subsequently stirred for 90 min. 5 mL suspension liquid was taken out and filtered through a filter head every 5 min. The adsorption isotherms of Pb2+ ions were studied by adding 20 mg ASTPNM-15 to 200 mL Pb2+ solution, and the adsorption equilibrium was reached by stirring the solution for 2 h. The initial Pb2+ ion concentration was varied ranging from 5 mg L-1 to 300 mg L-1, and the adsorbent was separated by a filter head. The Qt (mg g-1) and Qe (mg g-1), which correspond to the adsorption quantity of adsorbent at time t and the adsorption equilibrium state, respectively, were calculated by using the following equations: Qt =

C0 - Ct ×V msorbent solution

Qe =

Co - Ce ×V msorbent solution

(2)

(3)

where C0, Ct, and Ce are the concentration of Pb2+ (mg L-1) in the initial solution, the solution after t min adsorption, and the solution at adsorption equilibrium state, respectively; msorbent (g) and Vsolution (L) represent the mass of sorbent and the volume of initial solution, respectively.


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The breakthrough experiment was performed by permeating Pb2+ aqueous solution through ASTPNM-15 (1 mL min−1) at 20 oC. The ASTPNM-15 was fixed between two stainless steel flanges and the initial Pb2+ ion concentration was 5 ppm at pH = 5. The residual Pb2+ ion concentration in the filtrate was measured every 10 min. To assess the reusability of the ASTPNM-15, the heavy metal ions (Cr3+, Ni2+ and Pb2+) treated membrane was orderly immersed in 0.1 M HCl aqueous solution for 1 h and in 0.1 M NaHCO3 aqueous solution for 1 h. Then the membrane was washed with distilled water and dried at 100 oC for 2 h. Characterization. The morphologies of samples were recorded by using the scanning electron microscopy (SEM) (JSM-6510) and the transmission electron microscopy (TEM) (JEOL JEM2100F). Nitrogen adsorption-desorption isotherms were obtained on ASAP2020 at -196 OC, and the testing samples were degassed at 150 °C under vacuum for 10 h before the measurement. Fourier transform infrared spectrum (FT-IR) was recorded from 400 to 4000 cm-1 on a Nicolet Impact 410 FTIR spectrometer. The contact angles were measured on the Data-Physics OCA20 machine at 20 oC and the value was measured on five different positions of each sample. The element analysis of C, H and N was performed on an ELEMENTER VARIO MICRO. The concentration of residual oil in the filtrate was measured by OIL480 infrared spectrometer oil content analyser. Optical microscopy images were obtained by a CMM-55E (Leica, Germany). The concentrations of heavy metal ions were determined with Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) analysis performed on an iCAP 7000 SERIES. RESULTS AND DISCUSSION Nanofibrous structures and textural properties of ASTPNMs. ASTPNMs were fabricated by electrospinning technique followed by an amino-functionalized process. Figure 1a reveals the


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scanning electron microscopy (SEM) image of the ASTPNMs with modification time of 15 min (ASTPNM-15), displaying the uniform fibers with diameter ranging from 200 to 300 nm. The fiber diameter of ASTPNM-15 is similar to those of membranes without (STPNMs) or with modification time of 30 and 45 min (Figure S1), confirming that the modification process has less affected on the morphology of the membranes. The macropores formed by the intertwined nanofibers are beneficial to the passage of liquid through the membrane during the purification process. The transmission electron microscopy (TEM) image of ASTPNM-15 in Figure 1b exhibits the existence of worm-like mesoporous structure with random orientations in the fibers.

Figure 1. Nanofibrous structure of ASTPNM-15. (a) SEM image of ASTPNM-15, which is composed of intertwined fibers with diameter of about 200–300 nm. (b) TEM image of ASTPNM15, showing a worm-like mesoporous structure.

N2 adsorption–desorption isotherms of ASTPNMs and their corresponding textural properties are shown in Figure 2a and Table S1, respectively. All the membranes exhibit typical IV-like isotherms, but the textural properties of which are influenced by the aminomodification time. For instance, the Brunauer–Emmett–Teller (BET) surface areas of ASTPNMs ranging from 457 m2 g-1 for ASTPNM-15 to 376 m2 g-1 for ASTPNM-45 are


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lower than that of STPNMs (653 m2 g-1), and the pore volumes dramatically decline from 0.33 cm3 g-1 for STPNMs to 0.19 cm3 g-1 for ASTPNM-45. The pore size distributions of ASTPNMs calculated by nonlocal density functional theory (NLDFT) on the adsorption branch are shown in Figure 2b, which are relatively decreased comparing with that of STPNMs. The changes on both BET surface areas and pore size distributions of ASTPNMs may be attributed to the reduction of accessibility to the support porosity by the grafted amino groups.25

Figure 2. (a) Nitrogen adsorption–desorption isotherms of ASTPNMs in comparison with STPNMs, the curves are in turn shifted along the y axis for 150 cm3 g-1; (b) The corresponding pore size distribution, which have been shifted along the y axis for 0.07 cm3 g-1 in sequence.

FT-IR spectroscopy was further carried out to confirm the successful graft of amino groups onto ASTPNM-15 (Figure S2). The adsorption peak at 961 cm−1 is assigned to SiOH. The broad adsorption peak around 3420 cm-1 is resulted from the stretching of various hydroxyl groups, including Si-OH, Ti-OH and hydroxyl group of water occluded in the sample.30 Compared with STPNMs, these adsorption peaks significantly decrease in the spectrum of ASTPNM-15, which are caused by the reaction between hydroxyl groups and silyl reagent during the modification process.31 In addition, two new adsorption peaks


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appeared at 1487 and 2947 cm−1 correspond to the vibrations of C-N bonds and C-H bonds, respectively,31 proving the amino groups have been grafted onto the surface of ASTPNM15. C, H and N analysis gives the N content of 0.98 mmol g-1 in ASTPNM-15. Wettability

of

ASTPNMs.

The

under-water

superoleophobicity

and

under-oil

superhydrophilicity of the membrane surface are highly desired during the oil/water separation process, because it is beneficial for the passage of water through the membrane and the resistance to the contamination of oil.14,

32-33

In Figure 3, both STPNMs and ASTPNMs exhibit

superhydrophilicity in air and under-water superoleophobicity (black line and red line, respectively). The ASTPNM-15 is still superhydrophilic when immersed in oil, whereas, the under-oil hydrophobicity of ASTPNMs sharply increases with the increase of modification time (blue line). The time-dependent under-oil water contact angles (WCAs) clearly reveal that a water droplet quickly permeates into the cyclohexane-infused ASTPNM-15 with a WCA of almost 0° (Figure S3a), which indicates that the oil contaminant in ASTPNM-15 can be effectively substituted by water, showing an excellent self-cleaning performance of the membrane. Prolonging the modification time up to 30 min, an opposite wettability is observed on the cyclohexane-infused ASTPNM-30. Water droplet is repelled by the membrane and keeps the spherical morphology with a WCA of 149.0 ± 1.6°after 5 min (Figure S3b). This is because the modified amino groups not only serve as the active sites to capture the heavy metal ions, but also decrease the affinity between the ASTPNMs and water due to their lower polarity than the original hydroxyl groups in STPNM.34 Meanwhile, the under-oil hydrophobicity of the membrane will be magnified by the rough structure, leading to the sharply increase of the under-oil WCA on ASTPNM-30.12 In order to maintain the under-oil superhydrophilicity of the membrane, the modification time of 15 min is


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considered to be an optimal experimental condition, and the purification of wastewater is carried out by using ASTPNM-15.

Figure 3. Wettability of ASTPNMs and STPNMs. The insides are corresponding contact angle photos.

Separation of oil and water. The separation process of oil and water mixture is illustrated in Figure 4a and b. The mixture of water and cyclohexane (stained red by Sudan III) was poured onto the fixed ASTPNM-15, which was pre-wetted by a small amount of water. The red cyclohexane is blocked in the glass tube, while water selectively permeates through the membrane and is collected in the jar. No visible red liquid is observed in the filtrate, indicating the high efficiency of the membrane for the separation of oil/water mixture. During the separation process, the flux of the ASTPNM-15 reaches up to 1517 ±53 L m-2 h-1. The separation of oil/water mixtures with oils density lower (n-hexane, petroleum ether and tetradecane) or higher (ethane dichloride, Figure S4) than water was also investigated, and then the oil content in the filtrate was measured. Experimental data in Figure 4c reveal that the oil removal efficiency are all larger than 99.99% (the corresponding oil concentrations in the filtrate are all less than 28 ppm), suggesting that the oil/water mixtures have been effectively separated by ASTPNM-15. To further evaluate the


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separation capability of ASTPNM-15, the separation of CTAB stabilized cyclohexane-in-water emulsion was carried out. The emulsion is milk white with densely-packed droplets observed under the microscope (Figure 4d). After the filtration process, the droplets are completely removed and the emulsion changes to be transparent, proving the strong separation ability of ASTPNM-15 for emulsion. The flux of ASTPNM-15 for the emulsion separation is 622 ± 46 L m-2 h-1, and the residual oil concentration in the filtrate decreases to 57.3 ±0.12 ppm with the separation efficiency of about 99.26%.

Figure 4. (a, b) Demonstration of the separation process of cyclohexane (stained red by Sudan III) and water (colourless) mixture by ASTPNM-15. (c) The removal efficiency of ASTPNM-15 for a selection of oils. (d) The separation of CTAB stabilized cyclohexane/water emulsion by ASTPNM-15. Heavy metal ion (Pb2+) adsorption property of ASTPNM-15. The influence of adsorption time to the adsorbed quantity of Pb2+ on ASTPNM-15 was studied. The pH value of the solution


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was adjusted and maintained at 5 during the entire experiment. It is obviously that Pb2+ can be quickly absorbed onto ASTPNM-15, wherein 90% of the total capacity of the adsorbent is reached in the first 10 min of the experiment (Figure 5a). The high adsorption rate is favourable for the membrane adsorption method because of the short adsorption time during filtration process. The ASTPNM-15 reaches the adsorption equilibrium within 20 min as the uptake of the Pb2+ ion remains almost constant with the increase of adsorption time. The adsorption isotherms are obtained by measuring Pb2+ adsorption capacity on ASTPNM-15 with different Pb2+ initial concentrations (5−300 mg L−1). According to the Giles classification, the absorption curve in Figure 5b exhibits an L-shape.35 The equilibrium adsorption data are analysed by employing the Langmuir (Equation 4) and Freundlich (Equation 5) isotherm models, which are respectively expressed by: Ce 1 Ce = + Qe Qm KL Qm

(4)

log Qe = log KF + n log Ce

(5)

where Ce (mg L-1) is the concentration of Pb2+ when the adsorption process reaches to the equilibrium state; Qe (mg g-1) and Qm (mg g−1) represent the equilibrium adsorption capacity and the monolayer adsorption capacity of the adsorbent, respectively; KL (L mg−1) is related to the energy of adsorption, which is the Langmuir constant; KF (mg g−1)(L mg−1)1/n and n are the Freundlich constants that represent the adsorption capacity and the adsorption intensity of the adsorbent, respectively. The linearized Langmuir and Freundlich plots in Figure 5c and d are used to calculate the adsorption constants. In Table 1, the correlation coefficient of the data calculated by using the Langmuir model is higher than that by the Freundlich model, which indicates that


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the adsorbate is adsorbed as monolayer onto adsorbent surface and does not interact with the adjacent adsorbates.36 The monolayer adsorption capacity of ASTPNM-15 for Pb2+ calculated by Langmuir model reaches to 142.86 mg g −1.

Figure 5. (a) The influence of the adsorption time on the adsorbed quantity of Pb2+ ions to ASTPNM-15. (b) The adsorption isotherm of ASTPNM-15 towards Pb2+ ions. (c) The Langmuir isotherm and (d) the Freundlich isotherm for Pb2+ adsorption on ASTPNM-15. Conditions: the adsorption process was conducted with the mads/Vsol of 0.1 g L−1 at pH = 5 at 20 °C. Table 1. Langmuir and Freundlich isotherm constants for the adsorption Pb2+ on ASTPNM-15. Langmuir constants

Freundlich constants

Qm (mg g−1)

KL (L mg−1)

R2

KF (mg g−1)( L mg−1)1/n

N

R2

142.86

0.26

0.999

78.00

0.119

0.972


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The breakthrough experiment was performed by permeating Pb2+ aqueous solution (5 ppm at pH = 5) through ASTPNM-15 (1 mL h−1-). When the concentration of Pb2+ in the filtrate reaches to 0.05 ppm and the corresponding removal efficiency decreases to 99%, the breakthrough volume is measured to be 160 mL (Figure. 6).

Figure 6. Breakthrough curves of ASTPNM-15 for Pb2+ aqueous solution (5 ppm, pH = 5). The breakthrough volume reaches to 160 mL when the removal efficiency for Pb2+ decreases to 99%. Purification of simulated wastewater and recyclability of ASTPNM-15. Simulated wastewater containing multiple heavy metal ions (Pb2+, Cr3+ and Ni2+) and oil (cyclohexane) was prepared to evaluate the purification capability of ASTPNM-15. The simulated wastewater was poured onto the water pre-wetted ASTPNM-15 and the purification process was driven only by gravity. The concentration of each heavy metal ion in the filtrate reduces to less than 10 ppb and the corresponding removal efficiencies are larger than 99.5% (Figure 7a). Meanwhile, the low oil concentration in the collected water (< 25 ppm) further confirms the high oil removal ability of ASTPNM-15 (> 99.99%, Figure 7b). The recyclability of ASTPNM-15 was investigated over five purification-regeneration cycles. The regeneration step was performed by orderly immersing ASTPNM-15 into HCl and


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NaHCO3 solution. Figure 7 gives the evolution of the removal efficiencies of ASTPNM-15 on heavy metal ions and oil during five successive purification cycles. The ASTPNM-15 shows a stable performance on the purification of wastewater with the almost unchanged removal efficiency for the oil spill. The slight decrease on the removal efficiency for heavy metal ions is attribute to the structural instability of the ASTPNM-15, leading to the loss of part of adsorption sites during the regeneration process,20 but all of them are still above 99.5%. Figure S5 shows the SEM image of ASTPNM-15 after five purificationregeneration cycles. The membrane keeps the fibrous morphology and no significant change can be observed. These results clearly demonstrate that the ASTPNM-15 possesses excellent recyclability and reusability, which is important for its practical application in the purification of industrial wastewater.

Figure 7. Purification of simulated wastewater by ASTPNM-15 and the removal efficiencies of (a) heavy metal ions and (b) oil during five purification-regeneration cycles. The purfication capability of ASTPNM-15 for simulated waste seawater was also proceeded. The removal efficiency of ASTPNM-15 in simulated waste seawater for heavy metal ions is much lower than that in simulated wastewater, especially for Ni2+, the removal efficiency decreases from 99.5% to about 88% (Figure S6). This is because the addition of NaCl significiantly increases the


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ionic strength of the solution, which will weaken the affinity of the adsorbent to heavy metal ions.37 The removal efficiencies of the membrane for Pb2+and Cr3+ are still larger than 95%, and the repellence to the oil is basically unaffected by the adding of NaCl with the removal efficiency larger than 99.99%. These results demonstrate that ASTPNM-15 also possesses a certain purification capability for the simultaneous removal of heavy metal ions and oil from waste seawater.

CONCLUSION Amino-functionalized porous nanofibrous membranes (ASTPNMs) have been fabricated by electrospinning technique followed by further amino-silanization reaction. These membranes can simultaneously realize the highly efficient oil/water separation and the removal of heavy metal ions from wastewater. Owning to the under-water superoleophobicity and under-oil superhydrophilicity, the ASTPNM-15 with an optimal modification time exhibits not only high separation efficiency for various oil/water mixtures, even surfactant-stabilized emulsion, but also possesses excellent self-cleaning performance when contaminated by oil. Meanwhile, the hierarchical porous structure endows the ASTPNM-15 high adsorption capacity for various heavy metal ions (e.g., Pb 2+, Cr3+, Ni2+) in wastewater. The purification capacity of ASTPNM-15 for simulated wastewater remains stable after five purification-regeneration cycles, and the removal efficiencies of both oil and heavy metal ions are larger than 99.5%, exhibiting good recyclability and reusability. The excellent properties of ASTPNMs promise them as ideal candidates for the practical applications in the purification of industrial wastewater.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge. Additional experimental procedures, material characterization methods, SEM images, texture properties, FT-IR spectra and under-oil water contact angles of STPNMs and ASTPNMs. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. Di); [email protected] (J. Yu) ORCID Yang Wang: 0000-0002-7464-9916 Baixian Wang: 0000-0002-4266-0790 Qifei Wang: 0000-0002-3244-1367 Jiancheng Di: 0000-0002-7778-5892 Shiding Miao: 0000-0002-6446-8409 Jihong Yu: 0000-0003-1615-5034 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is financial supported by the National Key Research and Development Program of China (Grant No. 2016YFB0701100), the National Natural Science Foundation of China (Grant


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