Self-cleaning Piezoelectric Membrane for Oil-in-water Separation

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Functional Inorganic Materials and Devices

Self-cleaning Piezoelectric Membrane for Oil-in-water Separation Hengyang Mao, Minghui Qiu, Jiawei Bu, Xianfu Chen, Henk Verweij, and Yiqun Fan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03951 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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ACS Applied Materials & Interfaces

Self-cleaning Piezoelectric Membrane for Oil-in-water Separation Hengyang Maoa, Minghui Qiua, Jiawei Bua, Xianfu Chena, Henk Verweijb, Yiqun Fana,* a

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, No. 5 Xin Mofan Road, Nanjing 210009, PR China b Department of Materials Science and Engineering, College of Engineering, Ohio State University, 2041 N College Road, Columbus, Ohio 43210, United States

ABSTRACT: Ultrasound treatment coupled with membrane filtration has been utilized for membrane fouling control in water treatment; however, large-scale implementation of ultrasonic cleaning equipment appeared to be cost-prohibitive. In this study, a porous lead zirconate titanate (PZT) membrane is presented that enables in situ ultrasound (US) generation by application of an alternating voltage (AV), to mitigate fouling during oil-in-water (O/W) emulsion separation. We expect that this method is much more cost effective since it is more direct, avoiding build-up of fouling and the need to take the membrane off-line. Since the PZT membrane is hydrophilic, its underwater surface is oleophobic so that accumulated oil droplets will have little affinity and hence can be removed easily by in situ generated US. The effect of the in situ US generation on membrane fouling was investigated through variation of the excitation AV and its frequency, O/W emulsion pH, emulsified oil concentration, crossflow velocity and trans-membrane pressure. The results indicated that the in situ US generation resulted in a substantial decrease of fouling during the filtration process of O/W emulsions while the membrane flux was maintained closely at its initial value. KEYWORDS: piezoelectric membranes, O/W emulsion separation, membrane fouling, in situ ultrasound, PZT (lead zirconate titanate)

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INTRODUCTION Large amounts of oily wastewater are being produced in industrial and residential applications, leading to a global risk for environment and human health1. Conventional O/W emulsion treatment techniques such as gravity selling2, adsorption3 and flotation4 have limitations for efficient separation of emulsions with oil droplet less than 20 μm5-6. Hence, membrane separation technology7-10 comes into sight and is playing a very prominent role in the treatment of O/W emulsion due to its clear advantages11 such as continuous membrane process operation with simultaneous production of pure water and oil resources12-13. However, the performance of O/W emulsion separation membranes is adversely affected by fouling: the semi-permanent deposition of oil droplets and adsorption of oil on the membrane surface and/or in membrane pores14-15. In addition, these phenomena are generally enhanced by other contaminations, present in practical sources. Accordingly, mechanical and chemical cleaning or even complete replacement of membranes are ultimately required. Ultrasound (US) is an effective method for surface cleaning16-17, and indeed has been used widely in membrane cleaning and fouling control. It is confirmed that, when intense US waves propagate through a liquid, gas bubbles form in the negative pressure waves when the local tensile strength of the liquid is exceeded. These bubbles rapidly grow and subsequently collapse in the positive waves, resulting in a strong localized energy release. The application of US could not only increase the permeance for the ultrafiltration of nanoparticles, natural clay and skim milk with the permeance enhancement factor in range of 1.6-13.518-19, but had a significant effect on flux recovery after treatment at optimal ultrasound frequency, power density and irradiation direction20-21. Recently, the concept of piezoelectric separation membranes was introduced to sidestep the requirement of large-scale external equipment. The separation membranes based on piezoelectric materials have capacity of generating in situ vibration when AV signals were applied to the two sides of the membranes. The antifouling performance of these piezoelectric membranes is ascribed to vibrations and cavitation, inhibiting the cake layer formation and pollutant accumulation (Figure


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1a). Polyvinylidene fluoride (PVDF), a polymer piezoelectric material commonly used in micro- and ultrafiltration membranes, has been introduced to fabricate piezoelectric membrane22-24. Alternatively, lead zirconate titanate (PZT), an inorganic piezoelectric material, was introduced to fabricate porous PZT ceramic membrane for its superior piezoelectric property and stable porous structure25. It was found that application of 100 V resonant frequency (approximately 70 kHz) AV signals to a poled PZT membrane during particle rejection resulted in a nearly full recovery of permeance, which showed a strong antifouling performance. PZT grains, right after synthesis, contain ferroelectric domains with random orientations. Hence, to obtain permanent piezoelectric properties, porous PZT membranes must be oriented, or “poled” by subjecting them to a strong static electric field, in a direction that is perpendicular to the membrane’s surface, see Figure 1b. The poling must occur at temperatures, well below the ferro-electric ordering temperature of 180°C. Increasing the poling temperature may accelerate the poling process but also reduces the dielectric strength and hence the maximum poling field. The equilibrium extent of poling is determined primarily by the electric field and reaches a saturation value at high fields where all domains are oriented in one direction. In a published article25, porous PZT membranes were poled in air and oil respectively. It was found that PZT membranes in oil could be poled at a higher voltage and hence had a larger piezoelectric effect. However, oil-poled membranes were not chosen for further study since they became virtually water impermeable. The latter was ascribed to pore-blocking and a change of the surface chemistry from hydrophilic to hydrophobic. While a strong piezoelectric effect is beneficial for in-situ membrane fouling mitigation, it is also affected by its interface chemistry between the membrane and the waste water feed26-27. Hence several studies were done on the effect of membrane wettability on fouling28-29. All this led us to conclude that incorporating into a filtration unit of

membranes with a strong piezoelectric effect and an oleophobic

interface30 is very promising to obtain O/W emulsion purification that is stable over prolonged time.



In this work, porous PZT membranes were poled by immersion in mineral oil, followed by post-treatment with ethanol and water to restore the porous structure and hydrophilicity of the membrane. The performance of these membranes for O/W emulsions separation was studied to determine and optimize the impact of various operational factors. These factors included the effects of cross-flow velocity (CFV), trans-membrane pressure (TMP), amplitude, VAV, and frequency, fAV, of the AV. EXPERIMENTAL SECTION Fabrication of the PZT piezoelectric membrane. Porous PZT ceramic membranes were prepared by dry pressing and sintering commercially available powder, S42 (0.5 μm, Sunnytec, China). Circular membranes with 30 mm diameters and 2 mm thicknesses were synthesized by dry pressing the S42 powder at 10 MPa, followed by sintering at 950 °C for 2 h (Figure S1-4). Lead evaporation was suppressed by embedding the disk in loosely packed S42 powder. The furnace heating and cooling rate were 2 °C/min and 3 °C/min, respectively. The average pore size and porosity, p, were ~300 μm and 32.9%, respectively, as determined by mercury intrusion porosimetry, see Figure S5. Poling of the PZT membranes. The membrane samples were sandwiched between two copper electrodes, placed inside a mineral oil (BioReagent, M5904, Sigma) bath at 120 °C, and poled for 1 h, as shown in Figure 1c. The electrodes were connected to a high voltage power supply (D-N102, Dong Wen, China) and the poling voltage was increased gradually from zero to 8 kV (corresponding to a field strength of 4×106 V/m across the membrane). The poled membranes were immersed in ethanol at a temperature of 60 °C for 3 h to dissolve the oil that filled the membranes pores. Then, the membranes were washed by deionized water to completely remove the oil. The effectiveness of this treatment was due to the fact that any remaining oil was present diluted in the adherent ethanol while the latter is water-soluble. For the sake of comparison some membranes were poled in air without further treatment. O/W emulsion preparation. Synthetic O/W emulsion with concentrations of 200-5000 ppm by weight were prepared using soybean oil (soybean oil, Jin Long Yu,


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China), and span80 surfactants (CP, Sinopharm, China). 0.4, 1, and 10 g oil, respectively, and 0.04, 0.1, and 1 g span80, respectively were added to 2 L water followed by high shear mixing. (FM3000, Fluko, China) at 5000 r/min for 15 min. The droplet size distribution of the emulsion was determined with Dynamic Light Scattering (DLS). Sonic resonance detection. The resonant mechanical frequency and ultrasonic emission of the PZT membranes were measured in a membrane module, as shown in Figure 1d. During the testing process, the membranes were placed between two stainless mesh electrodes and submerged into an oil emulsion. Both electrodes were wired to a waveform generator (DG1022, Rigol, China), which could provide AV signals with frequencies of 1 Hz to 15 MHz and amplitudes of up to 20 V. The mechanical pressure waves generated by the membranes were detected by a hydrophone (RHSM-10, HAARI, China). The hydrophone output signal was monitored with a digital oscilloscope (DS1052E, Rigol, China). Electrical impedance spectroscopy.

The electrical properties of the

cross-follow module were analyzed by electrical impedance spectrometry (EIS; Reference 3000, Gamry, America). In this method AVs are applied to two porous electrodes in the membrane module. The resulting alternating currents through the cell are then used to obtain a frequency-dependent complex impedance, Z. The AV amplitude was 1 V with frequencies of 1 Hz to 1 MHz. Two kinds of EIS measurement were performed •

Impedance of the practical filtration system, Z1, with the O/W emulsion at the feed side, the PZT membrane, and purified water at the permeate side.

•

Impedance of the oil emulsion, Z2, without membrane. The motivation for this study arose from the importance of optimizing the

piezoelectric vibration frequency for fouling control. Using EIS for non-destructive monitoring of membrane fouling is a relatively new technique that has been used for different types of membranes. Here, the impedance between two porous electrodes in the cross-flow membrane module was detected by EIS during the filtration process. Membrane filtration experiments and fouling resistance analysis. To



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determine the filtration performance of the PZT membranes, the electrically poled membranes were mounted in a cross-flow module. As shown in Figure 3, the cross-flow membrane module was fitted with two porous steel electrodes that were used to apply the AV signals to the membrane. Each electrode was kept 1 mm above/below the membrane surface by using an O-ring. The permeance was collected in a reservoir placed on an electronic balance. The cross flow was adjusted using a diaphragm pump. The oil rejection RTOC (%) was calculated according to Eq. (1): RTOC 

C f  Cp Cf

100%

(1)

where Cf and Cp are the total original carbon (TOC) contents of the feed side and permeate side, respectively. The TOC contents of the samples were determined by total original carbon analyzer (TOC-V, SHIMADZU, Japan). After the membrane filtration experiments, regarding the resistance analysis, the initial membrane resistance (Rm), membrane resistance at the end of the fouling step (Rt1 and Rt2) and membrane fouling resistance (R1 and R2) were calculated according to Darcy’s law by using Eqs. (2-6)31-32:

Rm 

P μ  Jm

(2)

Rt1 

P μ  J t1

(3)

R1  Rt1  Rm

(4)

P μ  Jt 2

(5)

R2  Rt 2  Rm

(6)

Rt 2 

where, Rt1 and R1 are the resistances without in situ ultrasound, Rt2 and R2 are the resistances with in situ ultrasound, ∆P is the TMP pressure, μ is the pure water viscosity, Jm is the initial membrane flux, Jt1 is the eventual stationary flux of the membrane without in situ ultrasound and Jt2 is the eventual stationary flux of the


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membrane with in situ ultrasound. The ultrasound efficiency (UE) of the filtration experiment with in situ ultrasound was calculated by using Eq. (7):

UE (%) 

Rt1  Rt 2 100 Rt1  Rm

(7)

The UE reflects the efficiency of the in situ ultrasound in fouling removal. RESULTS AND DISCUSSION Membrane characterization. All poled porous PZT piezoelectric membranes produced a detectable signal at ultrasonic frequencies. The maximum signal, recorded for each membrane are provided in Table 1. For all membranes, the resonant frequency was found to be approximately 190 kHz. The PZT membranes poled at the higher temperature and higher voltage, possessed a more intense signal. When oil was used as the poling media, fields of up to 4 kV/mm33 could be used. However, the highest attainable poling field in air was 2.5 kV/mm, limited by the dielectric strength of air (approximately 3 kV/mm34). The higher poling field of 4 kV/mm led clearly to a higher degree of poling and hence a stronger piezoelectric effect and acoustic emission. Based on the observations, shown in Table 1, the optimum poling temperature was concluded to be 120°C. In earlier work air poling was chosen over oil poling because of difficulties with oil removal. These difficulties were overcome in the work presented here so that poling fields as high as 4 kV/mm can now be applied. SEM images of the membrane surface and underwater oil contact angles (OCAs) of the as-sintered PZT, poled PZT and ethanol-treated PZT are shown in Figure 2a-c. The SEM image, shown in Figure 2b, revealed that the PZT pores were nearly completely blocked after poling in mineral oil. But, inspection of Figure 2c, revealed that, after treatment with ethanol and water, described in experimental section, the original pore structure was fully restored. The wettability of the membrane by the feed, is important for its application in O/W emulsion separation35-36. It is determined by the detailed chemistry of the membrane surface, exposed to air and



the feed composition. Therefore, we examined the surface wettability of the porous PZT membranes at different conditions with optical contact angle (OCA) measurements. As shown in Figure 2a and c, when a 3 μL droplet of tetrachloromethane contacts the underwater surface of the as-sintered and the ethanol-treated PZT, it retains a nearly spherical shape with an OCA of 142.3° and 139.5°, respectively. In contrast, the underwater OCA of an untreated poled PZT membrane is only 36.9°. Additional characteristics of as-sintered and ethanol-treated PZT membranes are provided in Table S1. The underwater oleophobic behavior of as-sintered and ethanol/water treated PZT is ascribed to the presence of an uneven charge distribution in the oxide surface, and associated selective chemisorption of H+ or OH- groups and H2O monolayers. All these cause near-surface dipoles and multi-poles that stabilize the PZT/water interface w.r.t. the oil/water interface. On the other hand PZT, right after poling in oil, is thought to be covered with oil molecules with a polar part attached to the polar PZT surface while the aliphatic part is present at the exposed surface. The non-polar aliphatic groups make the external surface hydrophobic. The polar part of the molecules may have been present as-received or formed by (electrochemical) oxidation during poling. Their attachment may have been promoted also by the strong electrostatic poling field. The ethanol treatment apparently replaces the oil molecules by chemisorbed C2H5OH groups, possibly promoted by the presence of a free OH group. The much smaller C2H5OH groups are already polar which makes that they can easily be replaced by H2O or simply burnt off. As was derived from Figure 2d, the average surface pore size, ~300 nm, of the washed PZT membrane was the same as the pore size of untreated sintered PZT, see Figure S5. Electrical impedance analysis. Before membrane filtration experiments, the electrical characteristics of the system were studied using electrical impedance spectroscopy (EIS). As shown in Figure 3a-b, the impedance of the system between electrodes mainly consists of the oil emulsion impedance on the feed side (Zf), the membrane impedance (Zm) and the filtrate impedance on the permeate side (Zp). The actual AV applied to the PZT membrane was lower than the voltage generated by the


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power supply for the voltage division of Zf and Zp. The impedance of practical filtration system (Z1) and 500 ppm emulsion (Z2) are listed in Figure 3c, and the voltage distribution in cross-flow module can be calculated from Z1 and Z2 as shown in Supporting Information (Effect of the AV frequency section). The AV which applied to the PZT membrane was calculated as 60.2% with oil content of 500 ppm and AV frequency 190 kHz. Effect of AV frequency on sonic emission intensity and performance. Acoustic emission signals in response to 10 V AVs are shown in Figure 4a-c. The frequency of these vibration curves was similar to the frequency of the AV signals, while their amplitudes were considerably different. The amplitude of the vibration curve is proportional to the vibration intensity of the PZT membrane. The resonant frequency of the PZT membrane is 190 kHz; it can be seen that the PZT membrane with an excitation AV signal at 190 kHz possessed the strongest vibration of 29 mV. The effect of excitation frequency on normalized volumetric flux is shown in Figure 4d. It can be found that the piezoceramic membrane with an excitation frequency of 190 kHz preserved the maximum steady permeance of approximately 71% of its initial value, while this ratio was approximately 62-64% for the membranes vibrated at 5 kHz and 600 kHz. This led us to conclude that use of the 190 kHz frequency results in large vibration and best continuous antifouling performance. Effect of the signal amplitude on performance. The effect of the amplitude of the AV signal applied to the filtration of the piezoceramic membranes was investigated over applied signal voltages ranging from 0 V to 20 V at the resonant frequency (190 kHz). Figure 5a-b shows the resonant curves of the membranes excited by AV signals with different voltages. It is obvious that the vibration of the membrane is enhanced with the increase in the signal amplitude. The effect of AV amplitude on normalized volumetric flux is shown in Figure 5c, demonstrating a positive correlation with the amplitude and the stationary relative flux during filtration. The physical mechanism of antifouling is considered to be mechanical waves which caused by vibration. We assume that the concentration polarization and membrane fouling on the feed side can be alleviated more effectively with higher ultrasonic



vibration. It must be noted, however, that the application of large electrical signals, particularly at very low frequencies and higher salt concentrations can result in adverse effects of water electrolysis. But no such phenomena were observed at the largest voltage of 20 V that was used in present study. Additional contrast experiments were conducted on porous PZT ceramic membranes (no piezoelectric property) to investigate the effect of AV on the membrane filtration performance and in situ ultrasound on membrane pure water permeance (Figure S6-8). During filtration process, AV of 20 V and frequency of 190 kHz were applied to two sides of unpoled PZT membranes. No ultrasound signal was detected by hydrophone because the unpoled PZT membrane has no piezoelectric property (Figure S6). The experimental results showed that the filtration performance of the unpoled PZT membranes had no obvious improvement under the work of alternating current (Figure S7), indicating that the antifouling performance of PZT piezoelectric membrane is ascribing to the in situ ultrasound which caused by membrane piezoelectric function. Simultaneously, it can be seen from Figure S8 that the pure water permeance of membrane has no obvious change in the presence of ultrasound. This further demonstrates that the antifouling performance of ultrasound is ascribed to the pollution reduction. O/W emulsion properties vs electrical impedance and filtration performance. The effect of in situ US on filtration performance of oil emulsions with various pH values is shown in Figure 6a. It was found that the eventual stationary volumetric fluxes increased with pH: With, Jt2, and without AV, Jt1, application. Jt2 increased by 11% for the emulsion with pH of 3.7; it increased by ~20% for emulsions with pH values of 7.5 and 9.1. The field strength, due to the AVs are very small in comparison to local fields in the electric double layer (EDL) near the membrane and oil surfaces. The ζ potential is primarily affected by chemisorption of ions in the surface in combination with the detailed molecular structure within 1 nm from the surface. Consequently the AVs will not impact the detailed structure of the EDL and the ζ-potential. As shown in Figure 6b, the isoelectric point of the porous PZT membrane


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is approximately 6.5, which implies that it will be negatively charged in solutions with pH > 6.5. The soybean oil droplets have a zeta potential of about -63 mV. Consequently, at pH values well over 6.5, the membrane and oil droplets repel each other when their surfaces are within 100 nm. At that distance the Coulomb force between the EDLs at the membrane and oil surfaces becomes noticeable. The repulsion makes that permanent blocking of the membrane pores by oil droplets is less likely to occur and that any adhering droplets are easily removed with US. This, in turn, results in higher stationary fluxes as shown in Figure 6a. On the other hand, at pH values well below 6.5, the membrane surface acquires a positive charge and will exhibit an EDL attraction at distances of 20 mV. At the pH of 7.5 little electrostatic effects are expected but the oleophobic nature of the membrane makes additional, attractive chemical, interactions less likely. All this apparently results in stationary fluxes in between those for the emulsions with pH = 3.7 and pH = 9.1. The effect of in situ ultrasound generation on filtration performance of oil emulsions with various contents is investigated. Before experiments, the system impedances were tested by EIS, as shown in Figure 7a-b. It can be found that the impedance of the emulsion and membrane increased with the oil content, and the impedance in the membrane module decreased with the increase in the AV frequency. The PZT impedance distribution in module were calculated, as shown in Figure 7c. The membrane impedance distribution increased from 60% to 80% as the oil content increased from 200 ppm to 5000 ppm, indicating that majority of the AV was applied to the piezoelectric membrane. The results of filtration experiments vs oil concentration are shown in Figure 7d. The data show clearly the favorable effects of in-situ ultrasound generation on the stationary flux, Jt2. The rejection of oil droplets by size exclusion is virtually complete (>95%). As a result, nearly no oil enters the porous membrane structure and the viscosity that is contained in volumetric permeation expressions is that of pure water. Regardless, the viscosity of the O/W emulsions deviates very little from the pure



water viscosity since the oil droplet volume fraction is 0.05% or less. Assuming absence of network formation (gelation), the Stokes’-Einstein law then shows very little deviation from the pure water viscosity. In fact we measured that it was 1.04±0.0410-3 Pa·s for all compositions at 25°C. Also the surfactant concentration is in the regime where only isolated micelles form that, likewise, will have little effect on the overall viscosity. Another possible effect of US on viscosity is through heating of the liquid inside the membrane by sonic energy dissipation. But even at the lowest cross-flow velocity we never observed any significant temperature increase during operations. Temperature increases on the order of 5°C were reported before25 but for a membrane operation which a much higher power dissipation. Effect of the cross-flow velocity and trans-membrane pressure. The fact that the cross-flow velocity (CFV) has a major influence on membrane fouling is well established37-39. Increasing the CFV results in a thinner laminar boundary layer with a larger shear stress. Both effects promote fouling removal and diminish fouling deposition40. Higher shear stresses caused by moving, rotating or mechanically vibrating the membrane have also been shown to have similar effects. In fact, the in-situ generation of US, presented in the study, can be considered a special case of generating shear stress at the surface. The effect of the CFV on the normalized volumetric flux, J/J0, is shown in Figure 8a. The results clearly indicate that increasing the CFV results in larger stationary fluxes for operation with and without US. In addition, it was found that the application of US resulted in stationary flux increases by 15-25%. The effect of the trans-membrane pressure (TMP) on J/J0 is shown in Figure 8b. From these measurements it is apparent that the stationary flux decreases with increasing TMP. This observation is in agreement with reports on fouling of non-piezoelectric membranes. The stationary J/J0 was found to increase by 15-20% with the application of US. But it was also found that the beneficial effect of US diminishes with increasing TMP. This is in agreement with the general experience that fouling is more difficult to remove and avoid at higher TMP. Fouling resistances analysis and ultrasound efficiency. To investigate the


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effects of in situ US, membrane fouling resistances, R, and ultrasound efficiencies, UE, were calculated, as explained in the “Experimental” section. The data follow the trends discussed in the previous sections. Figure 9a-b shows R2 (with US) and UE values of membranes, operated with various AVs. As expected, the numbers confirm a best performance at an AV frequency fAV = 190 kHz and an AV amplitude VAV = 20 V. The highest VAV results in more intense US and hence a more efficient removal of “deep fouling”. Deep fouling occurs when the oil droplets are forced into the membranes, loose their spherical shape and become chemically attached to the pore surface. “Light fouling” on the other hand, refers to an accumulation of oil droplets at the surface but remain intact. As shown in Figure 9c-d R1 (without AV) and R2 decreased with pH while UE increased. Referring to earlier discussion, the electrostatic double layer (EDL) attraction at a low pH of 3.7 enhances deposition of oil droplets on the membrane surface and hence deep fouling. At pH=7.5 this effect is no longer present and a high pH of 9.1 the membrane surface charge becomes negative and EDL repulsion diminishes deposition of oil droplets41. The effect of US, ΔR = R1 – R2, is clearly larger at low pH where the tendency of deep fouling formation is more outspoken. Consequently, the UE at pH = 3.7 was much less, only 39%, than the UE at pH = 7.5 and 9.1: 62% and 66%, respectively. The data in Figure 9e-f confirm that R1 and R2 increase, and UE decreases with oil content. ∆R also increased with oil content, indicating that US becomes more effective at higher oil contents. However, the UE for 5000 ppm O/W emulsions was still only 46%, much less than the UE for 200 and 500 ppm O/W emulsions. Once again, this is related to the fact that at higher concentrations more deep fouling will occur that is more difficult to remove by any method. The effects of the CFV and TMP are shown in Figure 9g-j. With an increase of the CFV from 0.5 to 1 m/s, R1 decreased by 44% from 2.6×1012 to 1.5×1012 m-1. However, at a further increase to 1.5 m/s, R1 decreased only slightly to 1.3×1012 m-1. This indicates that light fouling, mainly caused by concentration build up and loose oil droplet accumulation, was already effectively removed at a CFV of 1 m/s. The ∆R



values at different CFVs are similar, approximately 1012 m-1; therefore, the UE increased with the CFV. This demonstrates the synergy between CFV and US. The first causes removal of light fouling by parallel shear forces, while the latter removes deep fouling. Figure 9i-j shows that R1 and R2 increased with TMP while the UE decreased at higher TMP. The ∆R values at 30, 50 and 100 kPa were 8×1011, 9×1011 and 1012 m-1, respectively, indicating that application of US became more effective at higher TMPs. Both light and deep fouling increase with TMP42-43; the increased light fouling can be removed by in situ ultrasound generation more easily, resulting in an increase of ∆R with TMP. However, the largely increased deep fouling cannot be effectively counteracted with US, resulting in a decrease of UE with increasing TMP. Separation performances of the piezoceramic membranes for O/W emulsions. The photographs of the O/W emulsions and the collected filtrates are shown in Figure 10a-c. The as-prepared O/W emulsions were milky white due to the light scattering by oil spheres. The collected filtrates were clear and transparent. As shown in the optical microscopy images, no obvious droplets could be observed in the filtrates, indicating that the large oil droplets were effectively cut off. The size of the oil droplets in O/W emulsions and filtrates were determined by dynamic light scattering (DLS), as shown in Figure 10d-e. The size distribution of the oil droplets in the O/W emulsion was bimodal, with one small peak at 0.1 to 1 μm and a major peak at approximately 4 μm. The size distribution in the filtrates had a maximum at ~30 nm; such small diameters can be ascribed to by micelles, formed by the surfactant. These distribution indicated that the PZT membranes could separate O/W emulsions effectively through sieving. CONCLUSIONS In summary, a self-cleaning PZT membrane was fabricated and applied in O/W emulsion separation successfully. The effect of the in situ ultrasound generation on membrane fouling was investigated through variation of the excitation AV and its frequency, O/W emulsion pH, emulsified oil concentration, crossflow velocity and


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trans-membrane pressure. The membrane resonant frequency of 190 kHz was chosen as the optimum operation frequency. The results indicated that the in situ ultrasound generation resulted in a substantial decrease of fouling during the filtration process while the membrane flux was maintained closely at its initial value. The extent of fouling decreased with increasing AV and crossflow velocity and decreasing trans-membrane pressure. ASSOCIATED CONTENT Supporting Information Additional details on the optimization of porous PZT ceramic membranes preparing condition, and effect of the alternating voltage on the permeance of unpoled PZT ceramic membranes. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] Author Contributions The manuscript was written with contributions from all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This study was financially supported by the National Natural Science Foundation of China (21506093, 91534108), the Natural Science Foundation of Jiangsu Province (BK20150947), the Project for Priority Academic Program Development of Jiangsu Higher

Education

Institutions

(PAPD),

the

National

key

R&D

plan

(2016YFC0205700) and the National High Technology Research and Development Program of China (2012AA03A606). REFERENCES (1) Birkhad, K., Stormy Outlook for Long-Term Ecology Studies. Nature 2014, 514, 405-405.



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(21) Maskooki, A.; Kobayashi, T.; Mortazavi, S. A.; Maskooki, A., Effect of Low Frequencies and Mixed Wave of Ultrasound and EDTA on Flux Recovery and Cleaning of Microfiltration Membranes. Sep. Purif. Technol. 2008, 59, 67-73. (22) Darestani, M. T.; Coster, H. G. L.; Chilcott, T. C., Piezoelectric Membranes for Separation Processes: Operating Conditions and Filtration Performance. J. Membr. Sci. 2013, 435, 226-232. (23) Darestani, M. T.; Coster, H. G. L.; Chilcott, T. C.; Fleming, S.; Nagarajan, V.; An, H., Piezoelectric Membranes for Separation Processes: Fabrication and Piezoelectric Properties. J. Membr. Sci. 2013, 434, 184-192. (24) Coster, H. G. L.; Farahani, T. D.; Chilcott, T. C., Production and Characterization of Piezo-Electric Membranes. Desalination 2011, 283, 52-57. (25) Krinks, J. K.; Qiu, M.; Mergos, I. A.; Weavers, L. K.; Mouser, P. J.; Verweij, H., Piezoceramic Membrane with Built-in Ultrasonic Defouling. J. Membr. Sci. 2015, 494, 130-135. (26) Pi, J.; Yang, H.; Wan, L.; Wu, J.; Xu, Z., Polypropylene Microfiltration Membranes Modified with TiO2 Nanoparticles for Surface Wettability and Antifouling Property. J. Membr. Sci. 2016, 500, 8-15. (27) Yang, Y.; Li, Y.; Li, Q.; Wan, L.; Xu, Z., Surface Hydrophilization of Microporous Polypropylene Membrane by Grafting Zwitterionic Polymer for Anti-Biofouling. J. Membr. Sci. 2010, 362, 255-264. (28) Yang, H.; Liao, K.; Huang H.; W, Q.; Wan, L.; Xu, Z., Mussel-Inspired Modification of a Polymer Membrane for Ultra-high Water Permeability and Oil-in-Water Emulsion Separation. J. Mater. Chem. A 2014, 2, 10225-10230. (29) Luo, C.; Liu, Q., Oxidant-Induced High-Efficient Mussel-Inspired Modification on PVDF Membrane with Superhydrophilicity and Underwater Superoleophobicity Characteristics for Oil/Water Separation. ACS Appl. Mater. Interfaces 2017, 9, 8297-8307. (30) Yong, J.; Chen, F.; Yang, Q.; Zhang, D.; Farooq, U.; Du, G.; Hou, X., Bioinspired Underwater Superoleophobic Surface with Ultralow Oil-Adhesion Achieved by Femtosecond Laser Microfabrication. J. Mater. Chem. A 2014, 2, 8790-8795.


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(31)Alventosa-deLara, E.; Barredo-Damas, S.; Alcaina-Miranda, M. I.; Iborra-Clar, M. I., Study and Optimization of the Ultrasound-Enhanced Cleaning of an Ultrafiltration Ceramic Membrane through a Combined Experimental-Statistical Approach. Ultrason. Sonochem. 2014, 21, 1222-1234. (32) Lujan-Facundo, M.-J.; Mendoza-Roca, J.-A.; Cuartas-Uribe, B.; Alvarez-Blanco, S., Membrane Fouling in Whey Processing and Subsequent Cleaning with Ultrasounds for a More Sustainable Process. J. Clean. Prod. 2017, 143, 804-813. (33) Rybyanets, A. N., Porous Piezoceramics: Theory, Technology, and Properties. Ieee T. Ultrason. Ferr. 2011, 58, 1492-1507. (34) Pedersen, A., 1989 Whitehead Memorial Lecture-on the Electrical Breakdown of Gaseous Dielectrics-an Engineering Approach. Ieee T. El. In. 1989, 24, 721-739. (35) Yuan, T.; Meng, J.; Hao, T.; Wang, Z.; Zhang, Y., A Scalable Method toward Superhydrophilic and Underwater Superoleophobic PVDF Membranes for Effective Oil/Water Emulsion Separation. ACS Appl. Mater. Interfaces 2015, 7, 14896-14904. (36) Fang,

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Figure 1. Schematic diagram of (a) antifouling performance of piezoelectric membrane, (b) electrical poling, (c) poling setup and (d) resonant frequency response testing equipment. Table 1. Poled PZT membrane samples: poling conditions (temperature T and electric field E), detected ultrasound (resonant frequency fR and corresponding oscilloscope signal VR) and water permeance (Q). Method

T (oC)

E (kV/mm)

fR (kHz)

VR (mV)

Q (L∙m-2∙h-1∙bar-1)

3 4 3 4 2 3

192 186 191 190 183 196

5.0 12.7 10.8 37.6 7.2 19.3

81.7 86.3 86.7 84.1 82.5 83.4

100

2.5

190

2.3

88.6

120

2.5

193

3.5

84.3

140

2.5

186

6.1

85.7

100 A

120 140

B



Figure 2. Surface microphotographs and underwater oil contact angle photographs of the (a) as-sintered PZT ceramic, (b) poled PZT ceramic, (c) ethanol-treated PZT ceramic and (d) pore size distribution of the porous PZT piezoceramic membrane.

Figure 3. (a) Schematic of membrane module used in the filtration experiments. (b) Equivalent electrical circuit, Cf: capacitance of the feed side, Rf: resistance of the feed side, Zf: impedance of the feed side, Cm: capacitance of the membrane, Rm: resistance of the membrane, Zm: impedance of the membrane, Cp: capacitance of the permeate side, Rp: resistance of the permeate side, and Zp: impedance of the permeate side. (c) Effect of frequency on the impedance of the system with oil content of 500 ppm.


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Figure 4. Vibration curves of the PZT membranes excited by 10 V AV signals with different frequencies: (a) 5 kHz, (b) 190 kHz, (c) 600 kHz and (d) normalized membrane permeance vs. time for the piezoelectric membranes excited by AV signals. Experiments were performed at a constant pressure of 50 kPa with 1 m/s cross-flow velocity. The concentration of O/W emulsion was 500 ppm, and the pH value was 7.5.

Figure 5. Vibration curves of PZT membranes excited by (a) 5 V and (b) 20 V AV signals at



frequency of 190 kHz. (c) Normalized membrane permeance vs. time for the piezoelectric membranes excited by AV signals. Experiments were performed at a constant pressure of 50 kPa using 1 m/s crossflow velocity. The concentration of O/W emulsion was 500 ppm and pH value was 7.5.

Figure 6. (a) Normalized membrane permeance vs. time for the piezoelectric membranes excited by a 20 V AV signal at a frequency of 190 kHz. Experiments were performed at a constant pressure of 50 kPa using a 1 m/s cross-flow velocity. The concentration of O/W emulsion was 500 ppm, and the pH values were 3.7 to 9.1. (b) Zeta potential of the piezoceramic membrane in the pH range of 3 to 11.

Figure 7. Effect of frequency on the impedance of a system with a (a) 200 ppm emulsion, (b) 5k ppm emulsion and (c) impedance distribution of PZT membrane in different emulsions at 190 kHz. (d) Normalized stable permeance and oil retention of the membranes. Experiments were performed at a constant pressure of 50 kPa using a 1 m/s cross-flow velocity, and the piezoelectric membranes were excited by a 20 V AV signal at a frequency of 190 kHz. The concentrations of the O/W emulsions were 200 ppm, 500 ppm, and 5k ppm, and the pH of the O/W emulsions was 7.5.


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Figure 8. Normalized membrane permeance vs. time for the piezoelectric membranes excited by a 20 V AV signal at a frequency of 190 kHz. (a) Experiments performed at a constant pressure of 50 kPa using cross-flow velocities of 0.5 m/s to 1.5 m/s. (b) Experiments performed at a constant 1 m/s cross-flow velocity under transmembrane pressures of 30 kPa to 100 kPa. The concentration of the O/W emulsion was 500 ppm, and the pH value was 7.5.



Figure 9. R and UE of the PZT membranes with various (a) AV frequencies and (b) AV voltages. (c) R and (d) UE of the PZT membranes with various O/W emulsion pH values, (e) R and (f) UE of the PZT membranes with various O/W emulsion oil contents, (g) R and (h) UE of the PZT


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membranes with various CFVs and (i) R and (j) UE of the PZT membranes with various TMPs.

Figure 10. Emulsions separated by piezoceramic membranes: before and after filtration. Photographs and optical microscopy images of emulsions with oil contents of (a) 5000 ppm, (b) 500 ppm and (c) 200 ppm. Oil droplet size distribution of the (d) emulsions and (e) filtrates.



Graphic abstract


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Abbreviations and symbols AV: CFV: DLS: EDL: EIS: US: O/W: OCA(s): PVDF: PZT: SEM: TMP: TOC(s):

Alternating voltage. Cross-flow velocity. Dynamic Light Scattering. Electrostatic double layer. Electrical impedance spectroscopy. Ultrasound. Oil-in-water Underwater contact angle(s). Polyvinylidene fluoride. Lead zirconate titanate. Scanning electron microscopy. Transmembrane pressure. Total organic content(s).

ΔP: ε0: εr: p: μ: ρion: τp : ζ: Aeff: fAV: fR : J: J0: Jt1: Jt2:

TMP. Dielectric constant of vacuum (8.854187817×10−12 F/m). Relative dielectric constant. Porosity. Pure water dynamic viscosity. Ionic resistivity. Pore tortuosity. Zeta potential. Effective membrane area. Alternating voltage frequency. Resonant frequency. Volumetric flux. Initial J (at the start of a filtration). Eventual stationary volumetric flux without US. Eventual stationary volumetric flux with US.

QH2O :

Volumetric permeance of water.

QN2 :

Volumetric permeance of N2.

R1: R2: Rm: RTOC: Rt1: Rt2: T:

Rt1 - Rm Rt2 - Rm Initial membrane resistance. Total organic content rejection. Eventual stationary membrane resistance without US. Eventual stationary membrane resistance with US. Temperature.



UE:

Ultrasound efficiency:

Rt1  Rt 2 100 . Rt1  Rm


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Self-cleaning Piezoelectric Membrane for Oil-in-water Separation

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