Dual-Functional Ultrafiltration Membrane for Simultaneous Removal of

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Dual-Functional Ultrafiltration Membrane for Simultaneous Removal of Multiple Pollutants with High Performance Shunlong Pan,† Jiansheng Li,*,† Owen Noonan,‡ Xiaofeng Fang,† Gaojie Wan,† Chengzhong Yu,*,‡ and Lianjun Wang*,† †

Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, P.R. China ‡ Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, QLD 4072, Australia S Supporting Information *

ABSTRACT: Simultaneous removal of multiple pollutants from aqueous solution with less energy consumption is crucial in water purification. Here, a novel concept of dual-functional ultrafiltration (DFUF) membrane is demonstrated by entrapment of nanostructured adsorbents into the fingerlike pores of ultrafiltration (UF) membrane rather than in the membrane matrix in previous reports of blend membranes, resulting in an exceptionally high active content and simultaneous removal of multiple pollutants from water due to the dual functions of rejection and adsorption. As a demonstration, hollow porous Zr(OH)x nanospheres (HPZNs) were immobilized in poly(ether sulfone) (PES) UF membranes through polydopamine coating with a high content of 68.9 wt %. The decontamination capacity of DFUF membranes toward multiple model pollutants (colloidal gold, polyethylene glycol (PEG), Pb(II)) was evaluated against a blend membrane. Compared to the blend membrane, the DFUF membranes showed 2.1-fold increase in the effective treatment volume for the treatment of Pb(II) contaminated water from 100 ppb to below 10 ppb (WHO drinking water standard). Simultaneously, the DFUF membranes effectively removed the colloidal gold and PEG below instrument detection limit, however the blend membrane only achieved 97.6% and 96.8% rejection for colloidal gold and PEG, respectively. Moreover, the DFUF membranes showed negligible leakage of nanoadsorbents during testing; and the membrane can be easily regenerated and reused. This study sheds new light on the design of high performance multifunction membranes for drinking water purification.

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INTRODUCTION

Nanoadsorbents are promising materials for removing pollutants with low molecular weights due to their high specific surface area, abundant sorption sites and fast adsorption kinetics.22−24 However, nanoadsorbents are often prepared in the form of fine powders which cause problems in separation/ regeneration processes25,26 and potential safety concerns due to leaching into water bodies.27−29 Moreover, it is difficult to use nanoparticles directly to retain large molecules and particulates. A strategy that can successfully combine the advantages of nanoadsorbents and UF membranes together and overcome their respective drawbacks in water decontamination remains challenging. Recently, blend membranes possessing both UF functionality and adsorption performance have garnered considerable attention.30 As schematically shown in Figure 1A, I, selected nanoadsorbents are embedded in a UF membrane matrix, forming a blend membrane with dual functions. Up to now, a variety of nanoadsorbents have been introduced to modify polymeric membranes, such as MWCNT,31 PANI/Fe3O4,32

Access to clean and affordable water free from chemical and biological contaminants remains one of the most critical global challenges of the 21st century,1−3 which is difficult to address adequately by conventional water treatment processes.4−6 The rapid development of nanotechnology has led to numerous advances in the field of water decontamination,7−11 among which various advances in membrane material design have proven particularly promising.12−14 Microfiltration (MF) membranes are available for suspended solids, protozoa, and bacteria removal.15 Ultrafiltration (UF) membranes are able to remove viruses and colloidal contaminants under low pressure with high water flux.16 Nanofiltration (NF) and reverse osmosis (RO) processes are particularly effective for removing both inorganic (e.g., heavy metal ions, fluorides) and organic (e.g., pesticides) contaminants with low molecular weights,17−20 however these normally require high working pressure and high energy consumption.21 Although traditional UF membranes have the advantage of low operation costs, they are unable to efficiently remove low molecule or ionic contaminants. Therefore, there is a high demand to develop next-generation UF membrane technology with both rejection and adsorption functions for cost-effective water decontamination. © 2017 American Chemical Society

Received: Revised: Accepted: Published: 5098

October 19, 2016 March 31, 2017 April 13, 2017 April 13, 2017 DOI: 10.1021/acs.est.6b05295 Environ. Sci. Technol. 2017, 51, 5098−5107

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functional nanoadsorbents for various water purification applications.

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EXPERIMENTAL SECTION Materials. Poly(ether sulfone) (PES Ultrason E6020P with Mw = 58 000 g mol−1) was purchased from BASF Company and was dried at 105 °C for 12 h before use. Polyvinylpyrrolidone (PVP; K-30), anhydrous ethanol, concentrated ammonia aqueous solution (25 wt %), tetraethoxysilane (TEOS), ethylene glycol, trisodium citrate, sodium acetate, polyethylene glycol monododecyl ether (Brij35), HNO3, NaOH, Pb(NO3)2 and N,N-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd. Zirconium(IV) (80% in butanol) (ZBOT) and Polyethylenimine (PEI, 99%, Mw = 10 kDa) were purchased from Aladdin. Dopamine hydrochloride was purchased from Sigma-Aldrich. Polydopamine coatings were prepared by addition of dopamine hydrochloride to a 10 mM Tris-HCl buffer solution (pH 8.5). BSA (Mw = 67 000 g mol−1), Polyethylene glycol (PEG, Mw = 20 kDa) and PEGs (Mw = 100, 300, 600, and 900 kDa) were purchased from Fluka Chemika Co. and Sigma-Aldrich, respectively. Deionized water purified with a Millipore Elix Water Purification system was used in all the experiment. All the chemicals were of commercially analytical grade and used without further purification. PES Membrane Synthesis. PES flat membranes were prepared via the nonsolvent induced phase separation (NIPS) process. Sixteen wt % PES, 0.6 wt % PEI and 8 wt % PVP were dissolved in DMF and stirred constantly at 60 °C for 8 h to obtain a uniform and homogeneous casting solution. Remaining air bubbles that appeared due to the stirring of the casting solutions were removed by leaving the prepared solutions overnight without stirring. Afterward, the solution was spread with 100 μm of casting knife onto a clear glass plate at 90 °C. After 5s evaporation time in air, the glass plate was immediately immersed in deionized water to induce phase separation. Then the prepared membranes, denoted as M0, were kept in pure water for more than 24 h to remove the residual solvent before testing. DFUF Membranes Synthesis. The HPZNs were synthesized by a reported method.46 In this work, the organic templates of HPZNs were extracted in an acetone solution for 24 h to obtain hydrous metallic oxide. The HPZNs immobilization, dopamine coating were performed in a stirred cell (Amicon 8050, Millipore). The M0 were first soaked in 50% ethanol solution for 5 min and then washed with deionized water. After that, they were placed in the stirred cell in “sandwich” mode (own support layer facing feed with an extra nonwoven support beneath the skin layer). As illustrated in Figure 1A, II, 10 mg L−1 HPZNs solutions were added into the cell with the washed M0 for immobilization operations at 0.1 MPa and 200 rpm agitation. When 4 L of liquid had passed through the membranes, the filtration was stopped and the HPZNs-loaded membranes, denoted as M1, were washed with 100 mL of pure water at 0.05 MPa. Then, 20 mL dopamine (2 g L−1) solution was poured into the open cell and the coating was carried out at 150 rpm agitation and room temperature for 0.5h. The coating was accomplished by maintaining the system at pH 8.5 to allow the polymerization of the dopamine except where otherwise stated. After coating, the DFUF membranes were cleaned using pure water at 0.05 MPa for 30 min. Subsequently, the DFUF membranes, denoted as M2, were

Figure 1. Schematic representation of UF membrane designs: (A) Virgin polymer UF membrane and a traditional blend membrane are both synthesized through a one-step casting method (I). A new 2-step method to form DFUF membrane (II); (B) the purification process for multiple pollutants polluted water by DFUF membrane.

graphene oxide nanoplates33 and activated carbon.34 It is generally accepted that the introduction of nanoadsorbents into polymeric casting solution will inevitably decrease the selectivity of the membranes, which is due to the formation of leaky interfacial voids and nonselective defects.35−37 In order to preserve the UF performance, the content of nanoadsorbents in the membrane matrix was normally less than 6 wt %,31−34 while the adsorption capacity is unsatisfactory. Although the improved adsorption capacity can be achieved by dramatically increasing the blending content of nanoadsorbents in the membrane, the ideal UF performance of resultant membranes is rarely exhibited.38−44 Another concern for blend membrane is that the rigidified polymer layer covering the surface of nanoadsorbents will reduce the number of effective adsorption active sites and thus decrease the adsorption performance.45 The preparation of a dual-functional membrane with stable UF property and enhanced adsorption capacity is of great significance, but yet very challenging. To address the challenge, in this work a novel design of dualfunctional ultrafiltration (DFUF) membrane has been developed by entrapment of nanoadsorbents into the fingerlike pores of poly(ether sulfone) (PES) UF membrane through a two-step process (Figure 1A, II). Hollow porous Zr(OH)x nanospheres (HPZNs) were immobilized in the finger-like pores of PES UF membrane during reverse filtration in the first step (Figure 1A, IIa). In the second step, polydopamine (PDA) coating was used to “seal” the HPZNs within the finger-like cavities of the PES UF membrane. The advantage of this strategy is that the UF membranes can be endowed with adsorption ability, while the inherent ultrafiltration properties were well maintained. As shown in Figure 1B, the DFUF membrane allows for the removal of multiple pollutants from water (colloidal gold and polyethylene glycol as models) as conventional UF membrane does, moreover, our novel design leads to a remarkable adsorption performance for the removal of toxic pollutants (Pb(II) as one example). Our design of DFUF membrane has the potential to be applied to other 5099

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Figure 2. FESEM images of top surface, cross section, and bottom surface of different membranes: the virgin PES membrane (M0, a−c), the HPZNs-loaded membrane (M1, d−f), and DFUF membrane (M2, g−i).

of 13.4 cm2 DFUF membrane and a weight of 0.062 g was first placed in the DI water. It was then placed on the membrane holder, which was sealed with an O-ring. In this work, multiple pollutants, including 100 ppb Pb(II) as a model ionic pollutant, 5 mg L−1 25 nm colloidal gold (the preparation method was provided in Supporting Information (SI)) as a model virus or bacteria, 50 mg L−1 PEG (Mw = 60 kDa) as a model colloid pollutants were investigated. In order to determine the pollutant saturation of the membranes, the solution was filtrated and analyzed in 200 mL intervals. After filtration of 2 L combined contaminants solution, the membranes were back washed with deionized water for 20 min and cleaned membrane was filtered multiple pollutants solution again. The supernatant was analyzed for Pb(II) concentration with atomic absorption spectrometer (AAS, PinAAcle 900T, PerkinElmer). The concentration of Au3+ was determined after dissolving the suspension in 5 mL of aqua regia (3 M HCl:1 M HNO3). The solution was evaporated to exchange the aqua regia with 5 mL of water. Concentrations of gold in the feed and permeate solutions were established with ICP-OES. The PEG concentrations were measured by the total organic carbon (TOC) analyzer. The HPZNs leaching from the DFUF membrane with continuous filtration mode was analyzed by ICP-OES. For the multiple pollutants removal experiments with various applied pressures, the same membrane was used throughout the whole series of experiments. Samples of 50 mL were filtrated at specific pressures, starting with the experimental series at the highest pressure. For mechanism analysis, a piece of membrane was dissolved in DMF after filtration of multiple pollutants solution. The insoluble parts (inorganic particles and reaction products) were washed three times with DMF and dried prior to phase analysis by using a PHI Quantera II ESCA System with Al Kα radiation at 1486.8 V. For comparison, an unused piece of membrane (no filtration) was treated in the same way. After reaching the breakthrough point, the exhausted membrane was in situ regenerated by filtering deionized water for 20 min under 0.1 MPa. Subsequently, HNO3 solution with pH 2 was used to filter the membrane at the flux of 5 L m−2 h−1 for 300 min, which led to the desorption of Pb(II). The regenerated M2 was filtered by deionized water until the neutral pH prior to next cycle. A filtration experiment was

mounted in the pure water. M0 membrane with coating dopamine was prepared in the above way. Characterizations. The morphologies of different membranes were examined with a FEI Quanta 250F field emission scanning electron microscope (FESEM). The cross sections of membranes were acquired by breaking the samples after immersion in liquid nitrogen. All the samples were sputtercoated with a thin layer of platinum to enhance their conductivity prior to FSEM analysis. FESEM-EDS mapping was applied to monitor the presence and dispersion of HPZNs in the pores of membrane matrix. The pore size analysis of the prepared membranes was determined by using a capillary flow porometer (CFP, Porolux1000, IB-FT GmbH Berlin, German). The membranes were prewetted by commercial low surface tension liquid Porefil (surface tension of 16 dyncm−1). The measurements include a wet-run program and a dry run program. The mean pore radius rm (nm) and maximum pore size rmax (nm) were determined with the aid of the computer software coupled to CFP. Thermal gravimetric analyses (TGA) were carried out by a thermal analyzer (SDT Q600 Simultaneous DSC-TGA, TA) to study the behavior of the individual degradation steps of the HPZNs, pure and DFUF membrane. TGA at a heating rate of 10 °C/min under a flow of 100 mLmin−1 of air was used. Measurements were operated in triplicate for every type of samples with standard deviation below ±0.2% of weight loss. The actual wt % of HPZNs in DFUF membrane could be calculated as Z/(S − Z) × 100%, where Z = residual wt % of DFUF membranes in TGA curve, and S = residual wt % of HPZNs in TGA curve. Moreover, the stability of DFUF membrane was measured by a dissolution experiment. 0.02 g of the DFUF membrane was immersed into 100 mL water with pH 0.1−6 separately, for 96 h. Then, the mixture was filtered. The filtrates were analyzed by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Optima 7000DV, PerkinElmer). The zeta potential of HPZNs was determined by a Zeta meter (ZetaPALS, Brookhaven Instruments), using 0.01 M KCl solution as a background electrolyte at various pH from 2 to 10. Continuous Filtration Study. Filtration experiments were performed by using a cylindrical stirred ultrafiltration cell (Model 8050, Millipore). The membrane with an effective area 5100

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RESULTS AND DISCUSSION Morphology and Structure of DFUF Membranes. Figure 2 shows the representative field emission scanning electron microscope (FESEM) images of the virgin PES membrane (M0, a−c), HPZNs-loaded membrane without PDA coating (M1, d−f), and DFUF membrane (M2, g−i). From the FESEM images of the top surface, no distinct difference between M0, M1, and M2 can be visualized (Figure 2a, d, g). The different membranes exhibit a similar pore size of approximately 20 nm on the top surface, indicating the immobilization of HPZNs and introduction of PDA coating had negligible effect on the upper surface structure of the PES membrane. From the cross-section and bottom surface FESEM images of M0, straight finger-like cavities in the cross-section can be clearly observed (Figure 2b) while the average pore diameter on the bottom surface are estimated to be 10−20 μm (Figure 2c). In order to evaluate the effect of adsorbent particles on the pore structure of membranes, the pore size distribution of M0 and M2 membranes was measured and the relevant results were presented in SI Figure S1. It was observed that M0 and M2 possessed very similar pore size distribution. The membrane M2 showed the mean pore size at 17.5 nm and largest pore size at 22.3 nm, compared to the pure membrane M0 which has a mean pore size of 17.9 nm and largest pore size of 22.9 nm. These results further confirm that adsorbent particles have negligible effect on the top surface structure which determines the basic membrane properties. The highly porous finger-like cavities acted as a “warehouse” for nanoadsorbent immobilization (Figure 1A, IIa). As depicted in Figure 2e and f, a large number of nanospheres with a uniform diameter of about 350 nm (inset in Figure 2f and Figures S2a, b) are located in the finger-like pores by reverse filtration. The BET surface area and pore volume of HPZNs are 582 m2g−1 and 0.38 cm−3 g−1, respectively (SI Figure S2c). After the PDA coating step (Figure 1A, IIb), the cross-section and bottom surface of M2 retain a similar morphology (Figure 2h and i) to that of M1. A smooth continuous coating is observed on the surface of HPZNs in M2 compared with those in M1 (Figure 2i inset), which is attributed to the adhesion of the PDA.47 In order to confirm the PDA coating on the membrane surface, the bottom surface of M0, M1, and M2 were analyzed by FTIR. The results are presented in SI Figure S3. The adsorption bands at 2850, 2925, and 3400 cm−1 are characteristic of PDA which confirms the successful deposition of PDA on M2.48 As shown in the digital images in SI Figure S4, the M0, M1 and M2 all present a visibly continuous and defect-free morphology. The color of M2 was obviously darker than that of M1 and M0, providing further evidence of the PDA coating.49 The energy dispersive spectrometer (EDS) mapping was used to demonstrate the distribution of HPZNs in the M2 membrane. As shown, HPZNs were homogeneously distributed in the cross section (Figure 3a) and bottom surface (Figure 3b) of M2, consistent findings with FESEM results. To compare our design of DFUF membrane with traditional blend membrane (Figure 1A), a blend membrane (M3) was synthesized by a reported method (see the SI for experimental details).50 SEM images of M3 (SI Figure S5a−d) indicate an

Figure 3. EDS mapping images of cross section (a) and bottom surface (b) of DFUF membrane (M2). The purple, red, green, and blue refer to the elements of Zr, O, S, and C, respectively.

apparently different morphology compared with M1 and M2. The leaky interfacial voids are observed on the top surface of M3 (inset in SI Figure S5a), resulting in the nonselective defects that subsequently deteriorate the ultrafiltration performance. Images of the exposed membrane cross section (SI Figure S5b) reveal that HPZN are distributed throughout the PES matrix, including on the top (SI Figure S5a) and bottom surface (SI Figure S5c, d). Images obtained by EDS mapping reveal very few HPZNs occupy the cavity spaces of the fingerlike pores of M3 (SI Figure S5b and S5i, j, k). Physical and Chemical Properties of DFUF Membranes. In order to optimize the loading amount of HPZNs in M2 during the reverse filtration step (Figure 1A, IIa), a constant pressure filtration at 0.1 MPa using a HPZNs solution (10 mg/L) was conducted. The permeate flux over filtrate volume is presented in Figure 4a. A linear decrease in flux is observed over the first 3 L of flow, indicating a continuous loading of HPZNs into the finger-like pores of the membrane which causes increased filtration resistance. From 3 to 5 L, the flux drop is slowed down until reaching a steady flow, indicating HPZNs loading is completed. Thermogravimetric analysis (TGA) was used to quantify the content of HPZNs in the membranes at different filtering volumes (Figure 4b). The weight fraction of HPZNs in the membrane reached a maximum of 68.9 wt % when the volume is above 4 L, indicating that this volume was sufficient to completely load HPZNs in the available pore space. Although the flux decreased when the permeate volume increased from 4 to 5 L, the content of HPZNs in the membrane matrix was not increased, suggesting the formation of a cake layer of HPZNs on the bottom surface of M1. As shown in SI Figure S6, the HPZNs wt % value in M3 was 62.0 wt %, showing the advantage of our DFUF membrane design in increasing the active loading content compared to conventional blend membranes. Figure 4c shows the pure water flux (column) and BSA rejection (circle) of different membranes. The blend membrane (M3) exhibited the highest water flux of 625.5 L m−2 h−1 and lowest BSA rejection of 72.5%. This is due to the existence of larger pores in M3. The bare PES membrane after PDA modification (M4) demonstrated an enhanced BSA rejection from 90.4% to 93.1% and a slight decrease in water flux as compared to pristine PES membrane (M0). The morphology of M4 is similar to M0 (SI Figure S5e−h). After entrapping HPZNs into the finger-like pores, a further reduction in water flux is observed (M1 and M2). Notably, coating with PDA further enhances BSA separation performance, as evidenced by the increase in BSA rejection of from 93.4% to 95.3% in the case of M2 compared to M1 before PDA coating. To further probe the ultrafiltration performance of DFUF membrane (M2), a PEG molecular weight cutoff (MWCO) curve was compared to that of M0 (Figure 4d). The PEG 5101

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Figure 4. (a) Flux variation of membranes with different HPZNs content during the continuous loading process. (b) TGA curves of HPZNs, DFUF membranes with different filtration volume (2, 3, 4, 6 L) of 10 mg L−1 HPZNs solutions. (c) The pure water flux (column) and BSA rejection (circle) of M0, M1, M2, M3, and M4. (d) MWCO curves with PEG for DFUF membrane (M2) and virgin PES membrane (M0).

which is 46.7% of M2. The decreased adsorption performance of M3 should be attributed to the wrapped nanoadsorbents by the polymer matrix in the blend membrane, resulting in the reduced adsorbent sites.45 This also can be confirmed by BET surface area analysis. As seen in SI Figure S2c, the BET surface area of M3 is 198.1 m2/g, which is lower than that of the M2 (370.9 m2/g). The M2 can also remove the colloidal gold (the size was shown in SI Figure S9) and PEG effectively with the final effluent concentrations below instrument detection limit (Figure 5b, c). In other words, M2 can completely reject the colloidal gold and PEG. This result is similar to that of M0, indicating that the entrapment of HPZNs into the finger-like pores of UF membrane has negligible effect on the UF performance of the PES membrane. For comparison, M3 only achieved 97.6% rejection for colloid gold and 96.8% rejection for PEG (Figure 5b, c). The decreased rejection performance of colloidal gold and PEG on M3 is due to the different approaches of HPZNs loading in the membrane. In contrast to the in-pore entrapment route (Figure 1A, II), the conventional method (Figure 1A, I) introduces nanoadsorbents in the polymer casting step, leading to leaky interfacial voids as well as nonselective defects that subsequently deteriorate the ultrafiltration performance.35 Previous studies have shown that PDA can be used as an adsorbent for heavy metal ions.55,56 In order to consider the contribution of PDA coating on the performance of M2, the adsorption performance for control membranes (M1 and M4) was investigated. It should be pointed that the bottom surface

rejection of M2 was much higher than that of M0 for the MW range below 300 kDa. Notably, the PEG rejection of M2 was 2.95 times higher than that of M0 at 20 kDa, indicative of a high removal performance of PEG with low moleclar weight. As can been see in the SI Figure S7, the HPZNs exhibited certain of adsorption capacities on PEG with different moleclar weights due to the hydrogen bonding.51 Considering the similar surface pore size of M0 and M2, the enhanced rejection of low moleclar weight PEG on M2 can be attributed to the PEG adsorption of HPZNs. The membrane antifouling performance was evaluated using BSA as model foulant. The antifouling data is shown in SI Figure S8 and Table S1. It could be inferred that membrane retains at high pure water flux and the flux recovery ratios (FRR) of 74.7%. And the membrane has a 68.3% of reversible fouling ratio (Rr) and 25.3% of irreversible fouling ratio (Rir). These results are similar to previous reports for PES membrane.52,53 Decontamination Performance of DFUF Membranes. In order to demonstrate the potential of M2 membrane for water purification, the simultaneous removal of multiple pollutants was investigated by using Pb(II), colloidal gold and PEG as model multiple pollutants. As illustrated in Figure 5a, when M2 was used to treat Pb(II) containing water with an initial concentration of 100 ppb, in the first run the effective treatment volume was 4477.0 L/m2 when the breakthrough point was set as 10 ppb, which is the drinking water standard of Pb(II) recommended by WHO.54 For comparison, only 2088.8 L/m2 of effective treatment volume was accomplished for M3, 5102

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concentration in the permeate was above 10 ppb, the filtration was stopped. A diluted nitric acid solution with pH of 2 was used to remove adsorbed contaminants and regenerate the membrane. In order to evaluate the stability of HPZNs and M2 membrane in acidic environment, the HPZNs and M2 membrane were immersed into aqueous solutions with different pH for 96 h, and the leaching of Zr ions was analyzed by ICPOES. As can be seen from SI Figure S10, HPZNs and M2 membrane are stable at a solution pH higher than 2. The reusability of M2 was investigated by multiple runs. As demonstrated in Figure 5a, the effective treatment volume of the regenerated M2 membrane is 90.2% with respect to the fresh membrane after 3 runs, indicating that the M2 membrane can be effectively regenerated for iterative removal of Pb(II) from aqueous streams. In situ regeneration of the exhausted M2 was performed by using the HNO3 (0.01 M) solution. The preloaded Pb(II) by M2 could be effectively desorbed with the regeneration efficiency which is 95.6% (SI Figure S11). The leaching of nanomaterial during filtration is a significant obstacle to the widespread application of functional membranes.57,58 To further verify the concentration of zirconium in the permeate of M1, M2, and M3, ICP-OES was used to monitor the HPZNs leakage under a continuous filtration mode (see details in the Experimental Section: Continuous Filtration Study) before and after PDA coating. As shown in Figure 6, M1

Figure 6. Long-term stability of the M1, M2, and M3 under the continuous filtration mode.

exhibited significant zirconium leaching. After filtrating of 6 L water containing multiple pollutants, approximately 32.2 wt % of the HPZNs were removed, indicating that the HPZNs cannot stably exist in the finger-like pores of membrane without the PDA coating. Even using the blend membrane, M3, approximately 1.8 wt % of HPZNs were still observed in the permeate, similar to previous reports for blend membrane.59,60 In contrast, the concentration of Zr was below the limit of detection of ICP-OES for M2. The result confirmed the amount of leached HPZNs for M2 was negligible over the same permeate volume, indicating excellent long-term stability of the M2 membrane under continuous filtration. These data confirm that the PDA coating acts as a functional “locking” layer which can retain the HPZNs in the finger-like pores of the UF membrane. To verify the adsorption sites of Pb(II) on M2, a sample of spent M2 was dissolved in DMF, the insoluble parts (inorganic particles and reaction products) were collected and analyzed by X-ray photoelectron spectroscopy (XPS). The high resolution

Figure 5. Filtration of multiple pollutants ((a) Pb(II), (b) Colloidal gold, (c) PEG) containing water by membrane. Conditions: continuous filtration mode, [Colloidal gold] 0 = 5 mg L −1, [PEG600 kDa]0 = 50 mg L−1, [Pb(II)]0 = 0.1 mg L−1 (100 ppb), membrane area = 13.4 cm2, pH 7, water flux = 20 L m−2 h−1.

of M1 was faced feed during the filtration of Pb(II) solution, in order to avoid the leaching of HPZNs. As shown in Figure 5a, the effective treatment volume of M0, M1, M2, M3, and M4 were 149.2, 4177.6, 4477.0, 2088.8, and 447.6 L/m 2, respectively. Based the analysis, although dopamine presents the adsorption capacity to Pb(II), the removal of Pb(II) was dominantly determined by HPZNs which is inside the fingerlike pores through our unique design (refer to Figure 1B). We further examined the regeneration potential for the functional membranes after adsorption. When the Pb(II) 5103

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Environmental Science & Technology scan of the Pb 4f peak from this sample was compared to the corresponding peak of HPZNs before Pb(II) exposure (SI Figure S12). The new peaks observed in the spent sample (143 and 138 eV) can be assigned to Pb 4f 5/2 and Pb 4f 7/2, respectively, confirming the presence of Pb(II) in the adsorbed samples. Notably, the Pb(II) is adsorbed on HPZNs without valence variation, which is consistent with previous findings.61 The flow rates can influence the mass transfer of the adsorbate to the site of absorption on the dynamic adsorption process. The results of the Pb(II) removal with M2 as a function of applied pressure are shown in Figure 7. Higher

Figure 8. Filtration of Pb(II) containing water by DFUF membrane (M2). Conditions: continuous filtration mode, membrane area = 13.4 cm2, pH 7, water flux =20 L m−2 h−1.

decrease in the effective treatment volume can be observed. Given that the weak fouling on HPZNs and the positive effect on Pb(II) adsorption of HA (SI Figure S13, S15), the decrease of effective treatment volume is caused by the competition of Ca(II) and Mg(II). Moreover, the effective treatment volume of the regenerated M2 membrane for simulated top surface water can still keep 91.5% after 3 runs, indicating a good reusability. Therefore, the M2 membranes show great flexibility in the practical application. The Removal Mechanism of Multiple Pollutants on DFUF Membrane. For the removal of three model contaminants, two removal mechanisms exist in the DFUF membrane: rejection and adsorption. Since the pore size on skin layer of PES membrane is about 20 nm, the pollutant with large size, such as PEG (Mw = 600 kDa) and colloidal gold (d = 25 nm), can be rejected on the interface of skin layer.13 Due to the small size, Pb(II) ions penetrate the skin layer and flow though the finger-like pores. During this process, Pb(II) ions can be adsorbed by HPZNs. To further reveal the adsorption mechanism of HPZNs, the effect of pH on Pb(II) uptake by HPZNs was carried out. As can be seen from SI Figure S16, Pb(II) uptake increased when the solution pH was increased from 2.0 to 7.0, and the highest capacity for Pb(II) uptake at pH 7.0. As calculated with Visual MINTEQ 3.1 software, the existence of dominated Pb(II) species are Pb2+, Pb(OH)+ under the solution pH from 2.0 to 7.0. The hydroxyl groups of HPZNs becomes protonated when the pH was below the pHpzc (4.2) of HPZNs. The positively charged HPZNs are unfavorable for capturing Pb2+ or Pb(OH)+ (eq 1, s: HPZNs surface). Similarly, it would cause the deprotonation of HPZNs when the pH was above pHpzc, the Pb2+ or Pb(OH)+ ions is expected to be effectively adsorbed onto the negatively charged HPZNs through electrostatic interaction (eq 2):65

Figure 7. Pb(II) removal in DFUF membrane (M2) as a function of applied pressure.

pressure can lead to higher flux, but the shorter residence time, which is unfavorable for enhancing adsorption efficiency. At high flow rates, the remaining Pb(II) content increased linearly with the applied pressure. Below an applied pressure of 0.04 MPa, Pb(II) was completely removed because a low pressure provides sufficient residence time for the diffusion and adsorption of Pb(II) on active sites of HPZNs particles in the M2. Therefore, in order to acquire the long residence time, the operation flux of our experiment was set as 20 L m−2 h−1. In order to investigate the practicability of our DFUF membrane, the effect of Ca(II), Mg(II), as model of competing cations, and humic acid (HA), as model of natural organic matters, on Pb(II) removal for HPZNs was studied. The presence of Ca(II) and Mg(II) ions had a slight negative effect on the Pb(II) adsorption (SI Figure S13) due to the preference of HPZNs toward Pb(II) with larger ionic radius.62,63 However, the adsorption of Pb(II) on HPZNs was promoted with HA due to the formation of HA-Pb complexes on the accessible surface adsorption sites of the HPZNs (SI Figure S13).64 To demonstrate the reusability and stability of the HPZNs, an adsorption−desorption cycle of Pb(II) was repeated five times. As seen in SI Figure S14, the adsorbed Pb(II) by HPZNs could be effectively desorbed with the regeneration effectiveness of 95.2% after being desorbed for five times. The concentration of Zr in desorbed solution was below the detection limit of ICP-OES, indicating no Zr leaching occurred during regeneration and confirming the HPZNs are stable in the solution of pH 2. To examine the response of M2 membrane to the real environmental process, the simulated top surface water containing NOM (HA) and competing anions (Ca(II) and Mg(II)) were employed (Figure 8). As shown, the effective treatment volume of M2 membrane was 4326.8 L/m2 in the first run. Compared with the data in Figure 5a, only 3.3%

s − ZrOH + H+ ⇌ s − ZrOH 2+

(1)

s − ZrOH ⇌ s − ZrO− + H+

(2)

Moreover, the competing adsorption of Ca(II) and Mg(II) ions were further revealed the selective uptake of Pb(II) on HPZNs (SI Figure S13), which is ascribed to inner-sphere complexation.66 Based on the above analysis, the adsorption mechanism is supposed that ion exchange and surface complexation are the main contributors to Pb(II) adsorption on HPZNs.67,68 The adsorption process may be described as eqs 3−6): 5104

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(4) Romao, J.; Mul, G. Substrate Specificity in Photocatalytic Degradation of Mixtures of Organic Contaminants in Water. ACS Catal. 2016, 6 (2), 1254−1262. (5) Li, R. Y.; Zhang, L. B.; Wang, P. Rational design of nanomaterials for water treatment. Nanoscale 2015, 7 (41), 17167−17194. (6) He, J. J.; Wang, W.; Sun, F. L.; Shi, W. X.; Qi, D. P.; Wang, K.; Shi, R. S.; Cui, F. Y.; Wang, C.; Chen, X. D. Highly Efficient Phosphate Scavenger Based on Well-Dispersed La(OH)3 Nanorods in Polyacrylonitrile Nanofibers for Nutrient-Starvation Antibacteria. ACS Nano 2015, 9 (9), 9292−9302. (7) Khin, M. M.; Nair, A. S.; Babu, V. J.; Murugan, R.; Ramakrishna, S. A review on nanomaterials for environmental remediation. Energy Environ. Sci. 2012, 5 (8), 8075−8109. (8) Wang, Z.; Feng, Y.; Hao, X.; Huang, W.; Feng, X. A novel potential-responsive ion exchange film system for heavy metal removal. J. Mater. Chem. A 2014, 2 (26), 10263−10272. (9) Han, X. X.; Schmidt, A. M.; Marten, G.; Fischer, A.; Weidinger, I. M.; Hildebrandt, P. Magnetic silver hybrid nanoparticles for surfaceenhanced resonance Raman spectroscopic detection and decontamination of small toxic molecules. ACS Nano 2013, 7 (4), 3212−3220. (10) Ai, K.; Ruan, C.; Shen, M.; Lu, L. MoS2 Nanosheets with Widened Interlayer Spacing for High-Efficiency Removal of Mercury in Aquatic Systems. Adv. Funct. Mater. 2016, 26 (30), 5542−5549. (11) Leong, S.; Li, D.; Hapgood, K.; Zhang, X.; Wang, H. Ni(OH)2 decorated rutile TiO2 for efficient removal of tetracycline from wastewater. Appl. Catal., B 2016, 198, 224−233. (12) Qu, X.; Alvarez, P. J.; Li, Q. Applications of nanotechnology in water and wastewater treatment. Water Res. 2013, 47 (12), 3931− 3946. (13) Pendergast, M. M.; Hoek, E. M. A review of water treatment membrane nanotechnologies. Energy Environ. Sci. 2011, 4 (6), 1946− 1971. (14) Kohler, S. J.; Lavonen, E.; Keucken, A.; Schmitt-Kopplin, P.; Spanjer, T.; Persson, K. Upgrading coagulation with hollow-fibre nanofiltration for improved organic matter removal during surface water treatment. Water Res. 2016, 89, 232−240. (15) Huang, H.; Schwab, K.; Jacangelo, J. G. Pretreatment for low pressure membranes in water treatment: a review. Environ. Sci. Technol. 2009, 43 (9), 3011−3019. (16) Adout, A.; Kang, S.; Asatekin, A.; Mayes, A. M.; Elimelech, M. Ultrafiltration membranes incorporating amphiphilic comb copolymer additives prevent irreversible adhesion of bacteria. Environ. Sci. Technol. 2010, 44 (7), 2406−2411. (17) Doederer, K.; Farré, M. J.; Pidou, M.; Weinberg, H. S.; Gernjak, W. Rejection of disinfection by-products by RO and NF membranes: Influence of solute properties and operational parameters. J. Membr. Sci. 2014, 467, 195−205. (18) Gao, J.; Sun, S.-P.; Zhu, W.-P.; Chung, T.-S. Chelating polymer modified P84 nanofiltration (NF) hollow fiber membranes for high efficient heavy metal removal. Water Res. 2014, 63, 252−261. (19) Lin, J.; Ye, W.; Zeng, H.; Yang, H.; Shen, J.; Darvishmanesh, S.; Luis, P.; Sotto, A.; Van der Bruggen, B. Fractionation of direct dyes and salts in aqueous solution using loose nanofiltration membranes. J. Membr. Sci. 2015, 477, 183−193. (20) Schulte-Herbruggen, H. M. A.; Semiao, A. J. C.; Chaurand, P.; Graham, M. C. Effect of pH and Pressure on Uranium Removal from Drinking Water Using NF/RO Membranes. Environ. Sci. Technol. 2016, 50 (11), 5817−5824. (21) Zaviska, F.; Chun, Y.; Heran, M.; Zou, L. Using FO as pretreatment of RO for high scaling potential brackish water: Energy and performance optimization. J. Membr. Sci. 2015, 492, 430−438. (22) Peng, Q.; Guo, J.; Zhang, Q.; Xiang, J.; Liu, B.; Zhou, A.; Liu, R.; Tian, Y. Unique lead adsorption behavior of activated hydroxyl group in two-dimensional titanium carbide. J. Am. Chem. Soc. 2014, 136 (11), 4113−4116. (23) Cao, C.-Y.; Qu, J.; Yan, W.-S.; Zhu, J.-F.; Wu, Z.-Y.; Song, W.-G. Low-cost synthesis of flowerlike α-Fe2O3 nanostructures for heavy metal ion removal: adsorption property and mechanism. Langmuir 2012, 28 (9), 4573−4579.

(I) Complex formation with deprotonated sites: s − ZrO− + Pb2 + ⇌ s − ZrO − Pb+

(3)

s − ZrO− + Pb(OH)+ ⇌ s − ZrOPbOH

(4)

(II) Exchange with H+ ions: s − ZrOH + Pb2 + ⇌ (s − ZrO − Pb)+ + H+ (5)

2(s − ZrOH) + Pb2 + ⇌ (s − ZrO − )2 Pb + 2H+ (6)

The main interaction between the HPZNs and Pb(II) was the inner-sphere surface complexes (eqs 3 and 4). However, the inner-sphere surface complexes potential is not the same for both acidic and neutral (or alkaline) pH values, given the transformation to strongly protonated hydroxy groups in acidic media, where inner-sphere surface complexes was not favored. Thus, the HPZNs would be protonated in acidic pH (eqs 1 and 2), and the protonated HPZNs could not effectively adsorb Pb(II) ions because of the electrostatic repulsion. Therefore, the desorption of Pb(II) from HPZNs can be achieved in acidic pH.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b05295. Sections S1−S5, Figures S1−S16 and Tables S1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(J.L.) Phone: +86-25-84315351; e-mail: [email protected]. cn. *(C.Y.) Phone: +61733463283; e-mail: [email protected]. *(L.W.) Phone: +86-25-84315941; e-mail: [email protected]. edu.cn. ORCID

Jiansheng Li: 0000-0002-3708-3677 Chengzhong Yu: 0000-0003-3707-0785 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant no. 51678307, 51278247) and the priority academic program development of Jiangsu higher education institutions.

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