Effect of microwave irradiation on vacuum membrane distillation

Effect of microwave irradiation on vacuum membrane distillation Zhongguang Ji a , Jun Wang a, n , Deyin Hou a , Zifei Yin b , Zhaokun Luan a a State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China b School of Chemical & Environmental Engineering, China University of Mining & Technology, Beijing, Beijing 100083, China article info Article history: Received 5 March 2012 Received in revised form 13 November 2012 Accepted 17 November 2012 Available online 29 November 2012 Keywords: Microwave Vacuum membrane distillation Mechanical properties Hydrophobicity Membrane fouling abstract In this paper, a vacuum membrane distillation (VMD) system equipped with a microwave source was designed. And the polyvinylidene fluoride (PVDF) hollow fiber hydrophobic membranes were selected to carry out microwave vacuum membrane distillation (MWVMD). The effects of feed temperature, vacuum degree and feed velocity on microwave strengthening mass transfer process were preliminarily investigated. The results showed that microwave irradiation could effectively induce uniform heating in the radial direction of the membrane module, and significantly improve the mass transfer process of vacuum membrane distillation. The maximum membrane distillation mass transfer coefficient increasing rate in this study was obtained as about 27.7% in the conditions of the temperature of 60 1C, the feed velocity of 0.14 m/s and the vacuum degree of À96 kPa. And the increment was largened with the decrease of the feed temperature and feed velocity as well as the increase of the vacuum degree. The influences of microwave irradiation on the membrane properties were also investigated. The results indicated that microwave irradiation had no significant effect on the mechanical properties and hydrophobicity of membrane materials which were situated in microwave field for more than 120 h. The influence of microwave irradiation on the membrane fouling was also analyzed. For the solution which contains 30 mg/L Ca 2þ , microwave irradiation aggravated the deposition of calcium carbonate to some extent. & 2012 Elsevier B.V. All rights reserved. Contents 1. Introduction 474 2. Theory . . 474 2.1. Microwave irradiation destroys water molecular clusters, which accelerat es the escape of molecules from the bulk solution 474 2.2. Microwave irradiation increases the activity of the polar structure in the membrane material, which can driv e molecule to move more quickly . . 474 2.3. Polar molecules move more quickly after absorbin g microwave energy. . 475 2.4. Microwave thermal effect compensates temperature decrease 475 3. Materials and methods 475 3.1. Membrane module 475 3.2. MWVMD setup. 475 3.3. Analysis methods and instruments . . . 475 3.3.1. Mechanical properties analysis 475 3.3.2. Contact angle (CA) test . 476 3.3.3. Liquid entry pressure (LEP) test 476 3.3.4. Scanning electron microscopy (SEM) analysis. . 476 4. Results and discussion 476 4.1. The influence of the feed temperature on microwave strengthening 476 4.2. The influence of the feed velocity on microwave strengthening 477 4.3. The influence of the vacuum degree on microwave strengthening 477 4.4. Mechanical properties analysis 477 4.5. Hydrophobicity analysis . . 478 Contents lists available at SciVerse ScienceDirect journal hom epage: www.elsevier.com/locate/memsci Journal of Membrane Science 0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.11.041 n Corresponding author. Tel.: þ 86 10 62849198; fax: þ86 10 62859198. E-mail addresses: junwang@rcees.ac.cn, jizhongguang@gmail.com (J. Wang). Journal of Membrane Science 429 (2013) 473–479 4.6. Crystal morphology analysis 478 5. Conclusions 479 Acknowledgments 479 References . . 479 1. Introduction In recent years, membrane distillation (MD) displays a very good application prospect. Compared with other processes, such as nanofiltration (NF), reverse osmosis (RO) and traditional evaporation, the MD could utilize waste heat of low quality, treat waste water containing higher salt concentration [1–3], and even remove some organics that used to be difficult to remove [4–6]. Before any practical application of MD, there are three major issues need to be addressed: (1) the preparation of a hydrophobic membrane with high flux and intensity; (2) avoidance of mem- brane wetting; (3) optimization and development of MD process system. It is well known that the membrane flux is influenced significantly by temperature polarization, concentration polariza- tion and channeling effect [7–10], especially for amplified reac- tors. In view of this, it is desirable to prepare new membrane materials and develop novel MD devices that can overcome these constraints. Recently, massive work has been done on the designing of novel membrane modules and devices. Some attempts were made to introduce turbulent-inducing components such as spacers and baffles to fill in the flow channels of membrane modules, which could enhance the mixing of solutions, thus reducing the tempera- ture polarization and concentration polarization [7,8,11–13]. How- ever, the addition of turbulent-inducing components increases the power consumption greatly, and certain grids may cause mem- brane surface damage. There are also some mature techniques, such as ultrasonic technique, being coupled with membrane distillation to develop novel devices. Zhu and Liu [14] applied ultrasonic to the process of MD, which significantly improved mass transfer process. But the mechanical vibration of ultrasonic can damage the membrane structure. Microwave technique is widely used in many industrial productions and social life. Compared to above techniques, microwave has its own advantages. In 1991, Canadian scholar Gedye et al. [15] investigated microwave-assisted organic synth- esis, and put forward that microwave special effect (or non- thermal effect) existed in the process, as well as thermal effect. In 2005, microwave was applied to the gas separation through a cellulose acetate (CA) membrane [16], and it was proved that microwave irradiation could enhance the gas transfer process in membrane pores. However, as far as we know, there is little report about implementing microwave technique in MD. There- fore, in this study, the microwave frequency of 2450 MHz was chosen to couple with MD. For the operation convenience of the system, the vacuum membrane distillation (VMD) was selected to combine with the microwave process, and the new system was named microwave vacuum membrane distillation (MWVMD). 2. Theory The essence of microwave is the electromagnetic field. When dielectric is positioned in microwave electric field, the molecules of the dielectric are induced to form dipoles. Simultaneously, a large number of random dipoles form a regular arrangement. This process is called orientation polarization of dielectric mole- cule. Then, these dipoles change directions at a very high frequency with the alternating electric field generated by micro- wave. If the dipole orientation time lags behind the electric field, it will produce dielectric loss, which leads to dielectric heat. In addition, if charged particles exist in solution, the forced vibration and the displacement current of the charged particles also can cause a dielectric loss. So, the dielectric loss is related to micro- wave frequency, temperature and ion concentration of solution. The dielectric loss has a peak value for a given dielectric. Water reaches the maximum dielectric loss at the microwave frequency of 2.45 GHz. Based on the theory above, when microwave technology is applied to the process of membrane distillation, four mechanisms must be considered. 2.1. Microwave irradiation destroys water molecular clusters, which accelerates the escape of molecules from the bulk solution It is well known that liquid water exists as molecule clusters of (H 2 O) n because of the existence of hydrogen bonds [17]. In the evaporation process, the cluster structures disintegrate to single molecules first, and the single molecules diffuse into air. In the process above, the disintegration of the cluster structure is the speed control step. Manju Lata Rao [18] analyzed Raman spectra of microwave- treated water, and found that the O–H stretch band of the microwave-treated water changed significantly compared to the untreated water. The changes in the structure stay stable for at least 7 h. Li Guixia et al. [19] investigated the influence of external electric field on water molecule cluster structures. The results showed that the external electric field could destroy hydrogen bonds, and changed the molecule cluster structures. So, micro- wave cannot only heat water dramatically, but also produce non- thermal effect to change water molecule cluster structures. 2.2. Microwave irradiation increases the activity of the polar structure in the membrane material, which can drive molecule to move more quickly Nakai et al. [16] applied microwave to gas separation through cellulose acetate (CA) membrane and polystyrene (PS) membrane. The results indicated that the microwave irradiation had no influence on the permeability of the PS membrane, but for the CA membrane, the permeability was strengthened. That is because PS is a non-polar polymer, but CA is a polar polymer. And the polar structures can absorb microwave energy to make activity increase, which drives gas molecules to move quickly. In this study, a PVDF membrane was chosen. Although the PVDF material has good hydrophobic characteristics, its polarity is stronger than CA. The dielectric performances of some common polymers are presented in Table 1. The loss tangent is a parameter that measures the material’s capability of absorbing microwave energy. It can be seen from Table 1 that the dielectric constant and loss tangent of PVDF are both larger than those of CA. Therefore, it can be concluded that in terms of absorbing microwave energy, the performance of PVDF is better than CA. So, if a strong polar polymer (hydrophobic material) is used in the microwave environment, the polar structures of the material Z. Ji et al. / Journal of Membrane Science 429 (2013) 473–479474 will absorb microwave energy to make the activity of the polar structure increase, which drives gas molecules to move quickly. However, compared with CA, PVDF has no polar side group as –OH for CA. The driving effect may be not significant. 2.3. Polar molecules move more quickly after absorbing microwave energy The dielectric constant of most gas molecules is about 1.2, but polar gas molecules, such as water molecules and ammonia molecules can absorb a small amount of microwave energy. For rarefied gas, where the collision of interior molecules can be ignored, microwave only contributes to the molecule rotation of polar molecule itself. 2.4. Microwave thermal effect compensates temperature decrease All of those mechanisms above belong to microwave special effect. And microwave thermal effect is also an important mechanism. In membrane distillation, temperature polarization is a usual phenomenon. Microwave irradiation can provide a local temperature increase, which compensates the temperature decrease caused by evaporation and temperature polarization. Because microwave heating is rapid and uniform, temperature compensation could be achieved more efficiently, compared with conventional heating. For VMD, the molecule concentration in the membrane pores is very low, and the transfer velocity of the molecule is very high. So, of the four mechanisms discussed above, the first and the fourth are critical, while the second and the third one can be ignored. 3. Materials and methods 3.1. Membrane module The self-made PVDF hollow fibers were adopted to fabricate membrane modules, of which the shape can be seen from Fig. 1. And the characteristics of the fibers are given in Table 2. The total membrane length and the effective membrane length were 390 and 200 mm, respectively. The effective membrane area of the module was about 0.0107 m 2 . The module heads of the two sides were made with PTFE materials. 3.2. MWVMD setup The MWVMD experimental setup is schematically shown in Fig. 2. The low pressure condition is beneficial to membrane distillation coupled with microwave. The module was positioned in the central location of the microwave cavity so that the membrane module could absorb the microwave energy equally from all directions. The feed solution was heated to a certain temperature by a constant temperature water bath, and then circulated through the membrane module using a cycle water pump. In the process of circulation, the feed solution was radiated continually by microwave (2.45 GHz, 1000 W). The low pressure state was provided by a water cycling vacuum pump. The condensate was collected by a glass vessel. The inlet and the outlet temperatures of the solution were monitored by a digital thermometer, and the vacuum degree was controlled by the vacuum meter and the vacuum pump. The conductivity of the condensation water was detected by a conductivity meter. The membrane flux was obtained by measuring the weight of the feed solution before and after the operation. The sodium chloride solution, which contains Na þ of 10 mg/L, was used as the feed solution. A series of experiments were conducted to investigate the effects of feed temperature, feed velocity and vacuum degree on microwave irradiation strengthening the mass transfer of VMD. Two modules were chosen to implement VMD and MWVMD processes, respectively. The overall testing time at steady state of both processes is over 120 hours. 3.3. Analysis methods and instruments The influence of microwave irradiation on membrane char- acteristics should be investigated in this work. The mechanical properties and the hydrophobicity of the self-made membrane materials were tested. The crystal morphology on the surface of the membrane was also observed. 3.3.1. Mechanical properties analysis PVDF material is a polymer with strong polarity and has a high value of loss tangent. Therefore, it must be considered that the self-made fibers may absorb microwave energy and the mechan- ical properties of the fibers may be affected. The influence of microwave on the mechanical properties of membrane materials Table 2 Membrane characteristics. Membrane Properties Membrane material PVDF Inner diameter (mm) 0.9 Outer diameter (mm) 1.2 Nominal pore size ( m m) 0.25 Porosity (%) 79 1 2 P 8 7 9 4 10 5 6 3 11 12 Fig. 2. Schematic diagram of the MWVMD experiment. (1) Solution container, (2) cycle water pump, (3) constant temperature water bath, (4) microwave source, (5) membrane module, (6) microwave cavity, (7) temperature meter, (8) vacuum meter, (9)fluid velocity meter, (10) low temperature water cycle condensation, (11) condensation water collection and (12) vacuum pump. Fig. 1. The shape of the membrane module. Table 1 The dielectric performances of some familiar polymer (20 1C, 50 Hz). Polymer Dielectric constant ( e ) Loss tangent (tan d) PVDF 9.5 0.08 PP 2.0$2.6 0.0003 PTFE 2.0$2.2 0.0002 CA 3.7$7.5 0.06 ABS 3.04 0.007 PS 3.0 0.0003 Z. Ji et al. / Journal of Membrane Science 429 (2013) 473–479 475 was measured by a strength tester (Laizhou electron instrument, YG065). The fiber was cut into 20 cm, and two ends of the segment were clipped in the fixtures of the instrument. The segment was drawn at a rate of 20 mm/min to measure the mechanical properties of the fiber. 3.3.2. Contact angle (CA) test The effect of the microwave irradiation on the contact angle of the fibers was also investigated. A dynamic contact angle meter (Dataphysics, DCAT 11) was employed to determine the dynamic contact angle of the fibers. The process could be briefly described as following. First, the sample fiber was cut into approximately 10 cm. Then, the segment was clipped by a fixture, held on the arm of an electro balance and immersed 5 mm long into pure water and successively emerged at an interfacial moving rate of 0.2 mm/min to complete a cycle. The contact angle was calculated by the Wilhemy method. The dynamic contact angle consists of advancing angle and receding angle. In this work, the advancing angle was chosen to analyze the changes of hydrophobicity. 3.3.3. Liquid entry pressure (LEP) test For measurement, one end of the fiber was filled and fixed with adhesive in a plastic pipe. On the other end, the pore of the fiber tube was plugged with adhesive. Then, the naked portion of the fiber was placed in a beaker and completely immersed into pure water, while the plastic pipe side of the module was connected with a specially made glass bottle which was filled with 20% NaCl solution. Pressure was applied to the 20% NaCl solution through the interface of the glass bottle using nitrogen. The pressure was increased gradually through regulating valve until the solution penetrated through the membrane pore and was mixed with the pure water. A conductivity meter was adopted to measure the change of the conductivity in the pure water. When the conductivity changed obviously, the pressure was recorded [7]. 3.3.4. Scanning electron microscopy (SEM) analysis The crystal morphology on the surface of the membrane after the operation of VMD and MWVMD were examined by a scanning electron microscopy (SEM) and an electron spectroscopy (EDS) analysis (HITACHI, S-3000N). 4. Results and discussion 4.1. The influence of the feed temperature on microwave strengthening The temperature is directly related to the evaporation rate and average kinetic energy of water molecules. The influence of feed temperature on microwave strengthening mass transfer was investigated first. With the feed velocity set at 0.55 m/s and vacuum degree at À96 kPa, the flux of VMD and MWVMD were measured at five different temperature levels. The results are presented in Fig. 3. It can be seen that the flux of MWVMD is higher than that of VMD at the same temperature. For instance, at the feed temperature of 52 1C, the VMD flux is 8.96 kg m À2 h À1 , while the MWVMD flux is 13.43 kg m À2 h À1 . As is known that, the feed solution will be heated dramatically while flowing through microwave cavity. Therefore, in order to analyze the special effect of microwave irradiation on strengthen- ing the VMD process, the thermal effect should be eliminated first. This can be achieved by calculating the mass transfer coefficient of two processes. The routine membrane distillation equation (Eq. (1)) is used to obtain the mass transfer coefficient. J ¼ K m D P ð1Þ where J is transmembrane flux. k m , is t he mass transfer coefficien t. D P is t he saturated vapor p ressure difference. In this study, trans- membrane flux J is obtaine d throug h experimentatio n, and the saturated vapor pressure difference D P is obtained from the following equation: D P ¼ P f ÀP 0 ð2Þ where P 0 is the vapor pressure of t he permeate side, which can be obtained directly from the vacuum meter installed on the surface of microwave c avity. P f is the saturated vapor p ressure of the feed solution located in the liquid–gas interface. And this parameter is corresponding with the temperature of the membrane surface, which is ha rdly achieved directly from experiment. The avera ge tempera- ture of the inlet and outlet is chosen as the membrane temperature to calculate the value of P f through Antoine equation [20] (Eq. (3)) P f ¼ exp 23:1964À 3816:44 TÀ46:13  ð3Þ Actually, K m calculated depending on this method is affected by not only the molecule motion but also the temperature polarization, both of which can simultaneously be improved by microwave irradiation theoretically. To simplify the investigation, the decrease in temperature polarization and the enhancement of molecule motion are also to be included in the increasing of mass transfer coefficient. The increasing rate e is defined and calculated from the following equation: e ¼ K m2 ÀK m1 K m1 ð4Þ where K m2 and K m1 are the mass transfer coefficients of the MWVMD and the VMD process, respectively. The influence of feed temperature on microwave strengthen- ing is also presented in Fig. 3. e T is the mass transfer coefficient increasing rate. It is found that the value of e T decreases when the feed temperature increases. At the feed temperature of 52 1C, e T is up to 22.6%. However, at 68 1C, e T is only 2.4%. A possible explanation is that the dielectric constant of water is decreased with the increase of temperature, and the water polarity is weakened. Therefore, the water at lower temperature can absorb more microwave energy. It also can be seen that the difference between the outlet temperature of two heating modes decreases with the increasing of the feed temperature. This experiment phenomenon demonstrates the theory analysis above. Fig. 3. The influence of the feed temperature on microwave strengthening. Z. Ji et al. / Journal of Membrane Science 429 (2013) 473–479476 Furthermore, because the existence of the microwave irradia- tion can ensure a high membrane flux at lower feed temperature, the poor quality waste heat could be used as heat source for heating feed solution in microwave vacuum membrane distillation. 4.2. The influence of the feed velocity on microwave strengthening The feed velocity determines the residence time of feed solution per unit mass in microwave cavity. And the residence time directly influences the feed solution absorbing microwave energy. Thus, the feed velocity was chosen for investigation. At the feed temperature of 60.0 1C and vacuum degree of À96 kPa, the flux of VMD and MWVMD were measured, respec- tively, at five different velocity levels. The mass transfer coeffi- cient increasing rate e V was calculated based on Eqs. (1)–(4). The results are presented in Fig. 4. It can be seen from Fig. 4 that the flux of MWVMD is higher than the flux of VMD at the same feed velocity. For instance, at the feed velocity of 0.14 m s À1 , the flux of VMD is 9.00 kg m À2 h À1 , while the flux of MWVMD is up to 17.17 kg m À2 h À1 . The flux curve of MWVMD is flatter than that of VMD, which demonstrates that the feed velocity has little influence on the flux of MWVMD. Fig. 4 also shows that the mass transfer coefficient increasing rate e V is decreased first, then it tends to become stable. At the lowest feed velocity, e V is the highest À 27.7%. That is because the longer the residence time, the greater the absorbance of micro- wave energy. On one hand, the temperature polarization decreased more obviously; on the other hand, the escaping ability of water molecules in the feed solution is enhanced. With the increasing feed velocity, the relieving effect of temperature polarization from microwave irradiation becomes smaller, so the value of e V tends to be stable. Moreover, two other advantages of microwave irradiation on membrane distillation can also be confirmed: (1) microwave irradiation can maintain the membrane distillation flux at a high level even at lower feed velocity, so the driving force can be saved. (2) Due to the homogeneous heating from microwave irradiation, the temperature difference between the inlet and outlet in a large volume reactor is no longer a thorny problem. 4.3. The influence of the vacuum degree on microwave strengthening At the feed velocity of 0.55 m/s and the feed temperature of 60.0 1C, the flux of VMD and MWVMD were measured at six different vacuum degree levels. And mass transfer coefficient increasing rate e P was calculated based on Eqs. (1)–(4). The results are presented in Fig. 5. It shows in Fig. 5 that the flux difference between MWVMD and VMD becomes larger with the increase of vacuum degree. But the difference is very small at the lower vacuum degree. Fig. 5 also shows that the mass transfer coefficient increasing rate e P tends to become smaller with the decrease in vacuum degree. At near À90 kPa or lower vacuum degree, the e P is negative. It can be concluded that microwave irradiation will restrain the mass transfer of the membrane distillation process at certain low vacuum degree. This is because, at lower vacuum degree, abundant vapor residing in the microwave cavity absorbs microwave energy to make the cavity temperature increase obviously. The mass transfer resistance becomes large enough to form a negative effect. Once the negative effect exceeds the positive effect, the mass transfer coefficient becomes small. The vacuum degree experiments show that MWVMD process should work at a vacuum degree as high as possible. 4.4. Mechanical properties analysis It is mentioned that the PVDF material can absorb microwave energy. And there is research showing that the microwave irradiation changes the crystalline and the dielectric properties of PVDF [21,22]. For investigating the effect of microwave on the strength of the membrane material, the mechanical properties of the membrane material was tested. Considering the non- uniformity of the membrane material’s properties, four parallel tests were carried out, and the mean values were calculated, which can be seen from Table 3. The results show that the breaking strength and the correspond- ing tensile strength of the membrane after VMD and MWVMD both Fig. 5. The influence of the vacuum degree on microwave strengthening. Table 3 The mechanical properties of the membrane material. Membrane Breaking strength (N) Elongation at break (%) Tensile strength (MPa) Young modulus (MPa) Co-efficient of variation (%) New membrane 1.58 142 3.53 2.48 4.22 Membrane after VMD 1.50 128 3.36 2.61 18.67 Membrane after MWVMD 1.48 133 3.31 2.49 16.99 Fig. 4. The influence of the feed velocity on microwave strengthening. Z. Ji et al. / Journal of Membrane Science 429 (2013) 473–479 477 decrease slightly compared with the new membrane. But because the elongations at break decreased with a near degree too, the young modulus was relatively steady. In addition, it also can be seen in the table that the coefficient of variation (CV) of the new membrane was 4.22%, which was less than that of the membrane after VMD of 18.67% and the membrane after MWVMD of 16.99%. It can be concluded that the mechanical properties of some membrane fibers changed in the experiments. However, the changes in the two processes were nearly the same. This reveals that the microwave irradiation has no special influence on the mechanical properties of the PVDF material. Actually, the feed solution is a better receptor of the microwave energy compared with PVDF materials, so most microwave energy from microwave source is absorbed by water selectively, which indirectly restrains the effect of microwave irradiation on PVDF materials. 4.5. Hydrophobicity analysis For analyzing the effect of microwave irradiation on the hydrophobicity of the membrane fiber, the changes of the contact angle and the liquid entry pressure should be both investigated. For contact angle analysis, pure water was chosen as wetting fluid. The results are displayed in Table 4. The dynamic contact angle contains an advancing angle and a receding angle. The contact angle represents the advancing angle in Table 4. The results show that the contact angles and the liquid entry pressures of the membranes after VMD and MWVMD are both decreased slightly, compared with the new membranes. This is related to the trace sodium chloride crystals adhered to the inner surface of the fibers. If this factor is removed, it can be concluded that microwave irradiation has no significant effect on the hydrophobicity of the membrane material for the dilute solution of sodium chloride. 4.6. Crystal morphology analysis Theoretically, the solution absorbs microwave energy and the activity of water molecule is enhanced, which cannot only alleviate the temperature polarization, but also reduce the con- centration polarization at the interface of the membrane and the bulk solution through the slight agitation from activated water molecules. In order to investigate the influence of microwave on membrane fouling in the VMD process, the solution which contained 30 mg/L Ca 2 þ and 10 mg/L Na þ , and a certain opera- tion condition on feed temperature of 54 1C, feed velocity of 0.14 m/s as well as vacuum degree of À 94 kPa was chosen to carry out experiments of VMD and MWVMD for 100 h. And two new modules were adopted for these experiments. After that, the crystal morphology on the surface of the membranes was observed by SEM-EDS. The results are given in Figs. 6 and 7. The feed solutions were always concentrated throughout experiments. It can be seen from Fig. 6 that the membrane Fig. 6. The SEM morphology of the crystal on membrane surface. (a) VMD (b) MWVMD Element Wt% At% C 10.66 19.77 O 34.95 48.67 F 01.41 01.65 Na 01.11 01.07 Ca 51.88 28.84 Fig. 7. The EDS spectra of the crystal on membrane surface after the process of MWVMD. Table 4 The hydrophobicity of the membrane material (20 1C). Membrane Contact angle (1) Liquid entry pressure (kPa) New membrane 89.0 187 Membrane after VMD 83.5 184 Membrane after MWVMD 86.4 185 Z. Ji et al. / Journal of Membrane Science 429 (2013) 473–479478 surfaces after two processes of VMD and MWVMD were both covered by acicular crystal with a length of about 0.2 mm. However, the crystal diameters of the MWVMD were three times larger than that of the VMD. The crystal on the membrane surface was mostly calcium carbonate (aragonite), as in Fig. 7, while the sodium salt crystal constituted approximately 2% (wt%). This fact shows that the existence of the microwave irradiation strengthened the sedi- mentation of the calcium carbonate crystal to a certain degree. Rodriguez and GomezMorales [23] found in his study that when carrying out calcium carbonate crystal deposition from a homo- geneous solution containing Ca 2 þ and HCO 3 À /CO 3 2À , a faster nucleation speed and a more uniform crystal size could be obtained by microwave heating, compared with that by conven- tional heating. Combined with this study, in the microwave field, molecule agitation breaks the hydrated cation clusters and hydrogen bonds, which increases the ionization degree and strengthens the diffusion ability of ions in the membrane inter- face. Therefore, the probability of collision crystallization between ions and the crystal on the membrane surface is greatly increased. 5. Conclusions Microwave technique was introduced and coupled with mem- brane distillation in this st udy. The influences of t he feed tempera- ture, feed vel ocity and vacuum degree on mic rowave i rradiation strength of the mass transfer of VMD were preliminarily investigated. The mechanisms of microwave irradiation strengthening mass trans- ferwerealsoanalyzed.Atthesametime,theinfluencesofthe microwave irradiation on the membrane fouling, mechanical proper- ties and hydrophobicity of membrane materials were measu red. Several preliminary conclusions are listed below (1) Homogeneous heating within a large volume reactor could be achieved by implementing microwave irradiation. This may have a potential application in the future, when scaling up the system is needed. (2) Microwave i rradiati on could s trengthen the mass transfer pro- cess of VMD. The better strengthening effect was obtained at lower feed velocity, feed t emperat ure and higher vacuum degree. (3) Microwave irradiation had no significant effect on the mechanical properties and hydrophobicity of the membrane materials. For the solution containing Ca 2 þ , microwave irra- diation aggravated the deposition of calcium carbonate to some extent. 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GomezMorales, Microwave precipitation of CaCO 3 from homogeneous solutions, J. Cryst. Growth 169 (1996) 339–346. Z. Ji et al. / Journal of Membrane Science 429 (2013) 473–479 479 . stable. Moreover, two other advantages of microwave irradiation on membrane distillation can also be confirmed: (1) microwave irradiation can maintain the membrane distillation flux at a high level even at. the effect of microwave irradiation on PVDF materials. 4.5. Hydrophobicity analysis For analyzing the effect of microwave irradiation on the hydrophobicity of the membrane fiber, the changes of. Effect of microwave irradiation on vacuum membrane distillation Zhongguang Ji a , Jun Wang a, n , Deyin Hou a , Zifei Yin b , Zhaokun Luan a a State Key Laboratory of Environmental Aquatic