• <tr id="yyy80"></tr>
  • <sup id="yyy80"></sup>
  • <tfoot id="yyy80"><noscript id="yyy80"></noscript></tfoot>
  • 99热精品在线国产_美女午夜性视频免费_国产精品国产高清国产av_av欧美777_自拍偷自拍亚洲精品老妇_亚洲熟女精品中文字幕_www日本黄色视频网_国产精品野战在线观看 ?

    Increasing the·OH radical concentration synergistically with plasma electrolysis and ultrasound in aqueous DMSO solution

    2022-04-12 03:45:10ChaoLi李超DeLongXu徐德龍WenQuanXie謝文泉XianHuiZhang張先徽andSiZeYang楊思澤
    Chinese Physics B 2022年4期
    關(guān)鍵詞:李超

    Chao Li(李超) De-Long Xu(徐德龍) Wen-Quan Xie(謝文泉)Xian-Hui Zhang(張先徽) and Si-Ze Yang(楊思澤)

    1State Key Laboratory of Acoustics,Institute of Acoustics,Chinese Academy of Sciences,Beijing 100190,China

    2University of Chinese Academy of Sciences,Beijing 100049,China

    3Fujian Provincial Key Laboratory of Plasma and Magnetic Resonance,Fujian Engineering Research Center for EDA,Fujian Provincial Key Laboratory of Electromagnetic Wave Science and Detection Technology,Xiamen Key Laboratory of Multiphysics Electronic Information,Institute of Electromagnetics and Acoustics,Xiamen University,Xiamen 361005,China

    Keywords: plasma electrolysis,ultrasound,reactive species,OH radical

    1. Introduction

    Plasma electrolysis is a process in which a discharge resulting from high voltage is applied between the anode and cathode in an electrolyte solution. Strong plasma-liquid interactions lead to the production of active species.[1]These active species can be transported into the solution[2]and initiate chemical reactions that are unlikely to occur under mild conditions.[3]Plasma electrolysis has been widely used in many fields including polymerization to produce polymeric materials,[4]modification of the surface of a material,[5]organic wastewater treatment,[6]organic synthesis,[7]and biomass liquefaction.[8]Ultrasonic cavitation is a dynamical process in which microbubbles vibrate, grow, and collapse under the action of sound waves. A large number of active species can also be produced in this process,[9]which is widely used for polymerization to produce polymeric materials,[10]organic wastewater treatment,[11]and organic synthesis.[12]Although these two individual technologies, plasma electrolysis and ultrasonic cavitation, have been widely investigated separately for similar applications based on free-radical production,their ability to act synergistically to enhance the physiochemical effects of the combined process has received relatively little attention.

    Generally speaking, plasma electrolysis relies on high voltage to break down the gas near the electrode to produce active species. Ultrasonic cavitation produces a large number of active species at the gas-liquid interface. The common denominator between plasma electrolysis and ultrasonic cavitation is that species are generated in solution without requiring external disturbance. When the components of the solution are the same, the species produced by the two methods are essentially the same. For example, the species produced by the plasma electrolysis of aqueous solutions mainly include·OH,·O,·HO2,O2,·Hα,and H2O2.[13]The main species produced by ultrasonic cavitation are·OH,·O,·HO2, O2,·Hα,and H2O2.[14]Among them,·OH radicals are highly oxidizing(2.80 eV),affect the formation of other secondary oxygencontaining species, and participate in many related chemical and biochemical reactions.[15,16]

    Various physical and chemical methods,such as emission spectroscopy, laser induced fluorescence (LIF), electron spin resonance(ESR)spin trap,and high-performance liquid chromatography(HPLC)have been developed for the detection of hydroxyl radicals in the aqueous phase.[17]These previous experimental studies indicated that the determination of the hydroxyl radical concentration is largely dependent on the types of spin traps because of the different rate constants of the reaction of the hydroxyl radical with trapping agents.[18]Among these methods,emission spectroscopy is a valuable analytical method for quantifying the plasma-generated OH in solution and is expected to yield more precise results. However, this technique is not always straightforward quantitatively,because chemical modifications may not have the requisite selectivity toward various reactive oxygen and nitrogen species(RONS).In addition, these modifications usually require complicated optimization of the conditions for analysis apart from careful comparison and consideration of the kinetic rates.

    In this study, dimethyl sulfoxide (DMSO) was chosen and used as the probe compound for the quantification of hydroxyl radicals by using spectroscopy. DMSO was selected on the basis of its high solubility, low volatility, strong polarity, and large rate constant of reaction with hydroxyl radicals (k=4.5~7.1×109mol·L-1·s-1). More details about the measurement principle can be found elsewhere,[19,20]and the quantitative analysis of hydroxyl radicals can be performed by determining the concentration of formaldehyde and formic acid according to the following reactions:

    Based on the obtained standard curve of formaldehyde solutions, the·OH concentrations resulting from plasma electrolysis, ultrasonic cavitation, and a combination of these treatments were determined. The mass transfer of the ROS based on the reaction coefficients was further analyzed,and the pathways of·OH production were investigated to deduce the mechanism of synergy between plasma electrolysis and ultrasonic cavitation. A new discharge method was proposed by introducing the mechanism underlying the synergy between the two techniques,which can be used for further applications.

    2. Experiment

    2.1. Experimental setup for plasma electrolysis and ultrasound

    A diagram of the equipment for the plasma electrolysis and ultrasonic treatment of the DMSO solution is shown in Fig. 1. The high-voltage electrode and the ground electrode were inserted through the two outer openings of a three-necked flask such that they extended to the bottom of the solution.The distance between the two electrodes was 10 mm, and the inclination angle was 30°,[21,22]which is conducive to the production of active species. The ultrasonic probe was inserted into the middle opening of the three-necked flask such that it extended to 5 mm below the liquid surface. The ultrasonic power supply was adjusted to form bubbles produced by ultrasonic cavitation between the anode and cathode to generate gas and active species for electrolysis. The voltage, duty cycle, and frequency of the plasma power supply were set at 350 V, 10%, and 500 Hz, respectively. The frequency and power of the ultrasonic power supply were set to 19.71 kHz and 350 W,respectively. Oxygen was passed into the solution at the rate of 1 L/min to serve as the catalyst without participating in the reaction. The DMSO solution was treated separately with plasma electrolysis,ultrasound and both of these two treatments combined. The three-necked flask was cooled in ice water to ensure that the temperature of the solution did not exceed 50°C. The concentrations of formaldehyde and formic acid of the solutions subjected to treatment by each of the three techniques were measured separately.

    Fig.1. Diagram of the experimental setup for plasma electrolysis with ultrasound.

    2.2. Preparation of DMSO solution and measurement of formaldehyde(HCHO)

    2.2.1. Formulation of DMSO solution

    The concentration of the DMSO solution affects the ability to accurately measure·OH radicals. If the DMSO concentration is too low, there is insufficient DMSO to participate in the reaction with·OH radicals. If the concentration of DMSO is too high, the concentration of HCHO, CH3OH,and CHOOH would obviously increase,which would prevent the·OH radical concentration from being accurately calculated. From[·OH]=2[HCHO]+4[HCOOH],[19]it can be seen that the concentration of OH radicals has nothing to do with methanol. Studies[19]have shown that a 50 mmol/L DMSO solution is the upper limit of the linear increase in HCHO and CHOOH.Therefore, a 50 mmol/L DMSO solution was used,and NaCl was added to adjust the conductivity to 14.5 mS/cm.We placed 80 mL of the configured DMSO solution in each of three three-necked flasks.

    2.2.2. Calibration and measurement of formaldehyde

    We used 0.1 mol/L formaldehyde standard samples to configure solutions with concentrations of 1 μmol/L,2μmol/L,5μmol/L,10μmol/L,20μmol/L,and 50μmol/L.A spectrophotometer was used to calibrate the standard samples after treatment with a formaldehyde detection kit(Shanghai Enzyme-linked Biotechnology Co., Ltd., resolution: 50 nmol/L to 0.5 mmol/L). Details of the measurement were presented in the literature.[23]

    2.2.3. Calibration and measurement of formic acid

    The determination of the formic acid(HCOOH)concentration in the solution was similar to that for the formaldehyde quantification. Firstly, we calibrated with standard solutions to obtain a specific standard curve,and then quantified its concentration in the treated solution. The measurement method was reported in detail in the literature.[24,25]

    2.3. Measuring the concentration of hydrogen peroxide,ozone,and oxygen

    In the experiment,plasma and ultrasound were combined to treat deionized water, and then the hydrogen peroxide,ozone, and oxygen concentrations in the solution were measured. The amounts of these long-lived species were used to verify the mass transfer and conversion path of the·OH radicals and to analyze the relationship between the hydrogen peroxide and·OH radicals.

    We divided the treated aqueous solution into three parts,each containing 50 mL, and used them to measure the hydrogen peroxide concentration,ozone concentration,and oxygen concentration. The respective methods for measuring the hydrogen peroxide, ozone, and oxygen concentrations were taken from the literature.[26-28]

    3. Results and discussion

    3.1. Concentration of OH radicals in solution

    The absorbance curves of 1μmol/L,2μmol/L,5μmol/L,10 μmol/L, 20 μmol/L, and 50 μmol/L formaldehyde standard solutions at 630 nm are shown in Fig. 2(a). The linear relationship between the intensity(630 nm)and formaldehyde concentration is shown in Fig.2(b). The fitting degreeR2was 0.99262,and the linear equation after fitting was

    whereyis the absorbance optical density (OD) andxis the concentration of formaldehyde (μmol/L). This linear equation can be used to calibrate the concentration of formaldehyde produced by plasma electrolysis, ultrasonic cavitation,and combined treatments. Samples of the concentration of formaldehyde obtained from different treatments,higher than 50μmol/L,were diluted before measurement.

    Fig.2. Absorbance curves of formaldehyde standard solutions and the linear relationship between the intensity of transmitted light and the formaldehyde concentration. (a) Standard formaldehyde solution absorbance curve. (b)Linear relationship between the intensity of transmitted light and formaldehyde concentration.

    Standard concentrations of formaldehyde (1 μmol/L,2 μmol/L, 5 μmol/L, 10 μmol/L, 20 μmol/L, 50 μmol/L,80μmol/L,and 100μmol/L)were added to detection reagents 1 and 2,as shown in Fig.3,which shows that the color of the standard solution changed from light to dark as the concentration increased. This served as an indicator of the solution process. It should be pointed out that at formaldehyde concentrations of 80μmol/L and 100μmol/L,the fitting coefficientsR2of the linear equation were 0.94163 and 0.90246,respectively.These values reflect a significant increase in the nonlinearity of the relationship.

    Fig.3. Photographic image of standard formaldehyde solution to which indicator was added.

    The three 50.0 mmol/L DMSO solutions were treated by plasma electrolysis, ultrasound, and both of these two treatments were combined, after which detection reagents 1 and 2 were added to each solution. Photographic images of the solutions after different treatment times are shown in Fig. 4.By comparing these samples with the samples prepared by using standard concentrations, we found that the formaldehyde concentration increased with time. After 4 min, the color of the solution changed to dark gray, the absorbance value at 630 nm decreased, and the formaldehyde concentration decreased rapidly, probably because the two treatments caused the temperature of the solution to rise rapidly. At temperatures above 50°C, hydrogen peroxide, formaldehyde, methanol,and formic acid evaporate quickly.This affects the experimental results. Therefore,the treatment time was set at 4 min. The color of the solution was darker than that of the 50μmol/L solution after exposure to the two treatments for 4 min,as shown in Fig. 4. The gray solution had to be diluted with an equal amount of distilled water before it could be measured with a spectrophotometer.

    Figure 5 shows the absorption curve of formaldehyde,as recorded by the spectrophotometer after plasma electrolysis,ultrasound,and both of these treatments. The absorbance value at 630 nm was substituted into the linear equation(y=0.01733+0.00813x) resulting from calibration to calculate the corresponding formaldehyde concentration, as shown in Fig.6.

    Fig.4. Photographic image of the results of the synergistic treatment of DMSO solution.

    Fig.5. Absorbance of DMSO solution treated by the three different methods: (a)plasma electrolysis,(b)ultrasound,(c)both of these together.

    As shown in Fig.6,the formaldehyde concentration was the highest for the DMSO solution treated by using the two methods,whereas ultrasound produced the smallest amount of formaldehyde. When plasma electrolysis and ultrasound were used separately to treat the DMSO solution, the formaldehyde concentration increased nearly linearly with time. Figure 6 also shows that the rate of increase in formaldehyde with plasma electrolysis differed from that with ultrasound.In 2 min, the formaldehyde concentration of the combined system was approximately equal to the sum of that produced by plasma electrolysis and ultrasonic treatment separately.From 2.5 to 4 minutes, the formaldehyde concentration of the two together was higher than the sum. For example, the formaldehyde concentration of the two together at 4 minutes was 64.16 μmol/L, and that of plasma electrolysis and ultrasound was, respectively, 30.11 μmol/L and 24.11 μmol/L,thus 64.16 μmol/L>30.11 μmol/L+24.11 μmol/L. Finally,as shown in Fig.6,the formaldehyde concentration in the later reaction stage of the synergy between the two combined techniques was significantly higher than the sum of the amounts of formaldehyde produced by using the techniques individually.

    Fig. 6. Formaldehyde concentration of DMSO solution treated by plasma electrolysis,ultrasound,and both of the two techniques together.

    The concentration of formic acid in the solution was analyzed by using a microplate reader,as shown in Fig.7. These results show that the formic acid concentration was significantly lower than that of formaldehyde. Initially, the formic acid concentration increased slowly and then increased more rapidly after 2 minutes.Figure 7 also shows that the increasing trend of formic acid was similar to that of formaldehyde.More specifically, the formic acid concentration increased linearly during the first 2 minutes,and the formic acid concentration as a result of the synergetic action of the combined methods was higher than the sum of the individual methods after 2-4 minutes. The concentration of the formic acid formed by using the methods synergistically for 4 minutes was 6.08 μmol/L,whereas that after plasma electrolysis and ultrasound was 2.42 and 2.02μmol/L,respectively. This trend was the same as for the formaldehyde.

    Fig.7. Linear fit of absorbance and concentration of formic acid standard solution(a)and change in the formic acid concentration over time for the three different techniques(b).

    Based on the relationship between·OH radicals and formaldehyde and formic acid([·OH]=2[HCHO]+4[HCOOH]),the concentration of·OH radicals formed during the different treatments was calculated and the results are shown in Fig.8.

    Figure 8 shows that the increase in the concentration of·OH radicals is the same as that of formaldehyde or formic acid,that is,the·OH radicals generated in 2 minutes increased linearly with time. The concentration of·OH radicals generated synergistically by the two methods was approximately equal to the sum of the·OH radicals produced by the two separately. After 2 minutes, the·OH radical concentration was greater than the sum of the concentrations resulting from that using the two techniques separately. For example, the·OH radical concentration at 2.5 minutes was 60.14 μmol/L,which is greater than the sum of that obtained by plasma electrolysis (31.76 μmol/L) and ultrasound (23.74 μmol/L) separately. This shows that the concentration of the·OH radicals increased linearly in the initial stage during which the two techniques were used together. After 2 minutes, the concentration of·OH radicals in the solution increased nonlinearly.This may have been caused by changes in the composition of the DMSO solution after the two treatments. Therefore,it was necessary to analyze the main components of the solution.

    Fig.8. Change in the·OH radical concentration during treatment by the three different techniques.

    The·OH radicals react with DMSO to generate methanol,formic acid, and formaldehyde. The·OH radicals in solution were converted to hydrogen peroxide,which changes the conductivity and pH of the solution, as shown in Figs. 9(a) and 9(b),respectively. Figure 9(a)shows that,after the DMSO solution was treated by the two methods synergistically,the pH value was the lowest. In addition,the concentration of hydrogen peroxide and formic acid in the solution was the highest,which is consistent with the results shown in Fig.8. The conductivity as a function of time is shown in Fig. 9(b), which shows that the conductivity initially increased linearly,and after 2.5 minutes,the increase became nonlinear. This is similar to the increase in the·OH concentration in Fig. 8, and indicates that an increase in conductivity is conducive to plasma electrolysis and the generation of·OH radicals, which corresponds to that reported by Chenet al.[29]The reverse also holds true, that is, an increase in the active species increases the conductivity of the solution.

    Increases in conductivity led to changes in the discharge current. During the use of plasma electrolysis with ultrasound to treat a DMSO solution, the voltage and current were measured,as shown in Fig.10,which shows that,at constant voltage,the current changed significantly after 0.015 s(measurements started at 138 s). The results show that the intensity of the current did not change, but the density increased significantly, which indicates the transition of the discharge mode from the filament mode[30]to the spark discharge mode. The discharge current gradually stabilized after 0.04 s. The increased intensity between the two poles indicated that more gas participated in the discharge. The bubbles formed as a result of the occurrence of ultrasonic cavitation between the two poles and probably break down, thereby increasing the current. This seems to be the reason for the high concentration of·OH free radicals produced because of the synergy between the two techniques.

    Fig. 9. Change in the pH and conductivity with time: (a) pH value,(b)conductivity.

    When the discharge mode conversion occurs,the solution properties were observed with considerable changes, particularly concerning the temperature(increased by 25°C)and conductivity (increased to 15.4 mS/cm), compared to the initial ones. However,the solution pH only decreased slightly(from 7.0 to 6.52), indicating that the pH only has a limited role on affecting the mode conversion. To further verify the effect of temperature,the following investigation was conducted.

    The solution was firstly heated to 45°C to confirm the above conclusions, and the DMSO solution was treated with plasma electrolysis and ultrasound. The results showed that the filament discharge was converted to spark discharge within 52 seconds,with the solution temperature of 58°C,the OH radical content of 48.6 μmol/L, and the conductivity of 14.84 mS/cm. It can be seen that the discharge mode is determined by both the solution temperature and conductivity.

    The changes in the solution properties were caused by the mass transfer and diffusion of active species of plasma into the solution.The diffusion process of active species intensified the motion of water molecules, which increased the temperature of the solution. At the same time, the mass transfer of active species into the solution was also the generation of long-lived species in the solution. These long-lived species increased the conductivity of the solution greatly. Thus, the solution temperature and conductivity determined the discharge mode.

    Fig.10. Voltage and current measurements during the combined treatment.

    The emission spectra under different discharge modes are shown in Fig. 11. It can be seen that the species produced with the two discharge modes are almost the same, but the intensity of the spectrum is different. The spectrum contains emission lines of N2,OH,H,and W.The emission spectrum of nitrogen is caused by the dissolution of nitrogen in the water.The line of tungsten is emitted by the ionization of tungsten in the electrode. The OH radicals originate from the interaction of electrons and water molecules, and the H radicals originate from the interaction of H+and electrons near the anode (H++eaq→H). The stronger emission intensity of spark discharge indicates the higher production of active species.

    Fig.11. Emission spectra of filament and spark discharge.

    In addition,it can be found that a wave packet appears at 460-900 nm from the spectrogram of the spark discharge.The spark discharge increases the background temperature,which will cause heat radiation. It can be seen that the breakdown discharge(spark discharge)can generate a large number of active species and heat, and rapidly increase the temperature of the solution and the content of active species. It is consistent with the experimental results.

    3.2. Source of·OH radicals

    Figure 12 also indicates that H2O2and·OH radicals are interconvertible(H2O2UV--→2OH).Since hydrogen peroxide is a long-lived particle,it can diffuse into the solution.Therefore,the·OH radical content was estimated by measuring the H2O2content. Since the discharge occurs at the cathode, the electrons and OH radicals generated by the discharge are also near the cathode(the cathode has the largest potential drop).[46,47]As the content of hydrogen peroxide near the cathode increases,it appears to be attributable to H2O2?H++HO-2and·HO2?·O2+H+in the advanced oxidation system. The experiment also showed that the pH value of the solution after 4 min of exposure to both of the two techniques simultaneously was 6.58 owing to the synergistic effect,that is,the solution had become weakly acidic. This is consistent with the experimental results.

    Figure 12 also shows that the·OH radicals in the solution were mainly derived from H2O2aqand O3aq. Measuring the amount of the two species is very important for estimating the concentration of·OH radicals. Here,it was also considered to be useful to analyze the synergetic mechanism of plasma electrolysis with ultrasound. In the experiment,the concentrations of hydrogen peroxide, ozone, and oxygen in deionized water resulting from the synergy between the two techniques were measured,as shown in Table 1.

    Fig.12. Chemical pathways for the short-lived species in aqueous solution.

    Table 1. Concentration of hydrogen peroxide, ozone, and oxygen resulting from the synergy between the two techniques(plasma electrolysis and ultrasound).

    The results in Table 1 indicate that the oxygen content of the solution was mainly in the form of H2O2aq. Ozone was not detected in the experiment in which the concentrations of·OH radicals(0.152 mmol/L)and H2O2aq(0.087 mmol/L)were compared.Thus,there were less than twice as many·OH radicals as hydrogen peroxide molecules, which may be due to the low concentration of DMSO. This also shows that the·OH radicals were mainly derived from hydrogen peroxide, a finding that supports the results in the literature,[47]which,in turn,provides evidence for the rationality of our experimental results.

    Without using an external gas supply and at a lower voltage, bubble(produced by ultrasonic cavitation)discharge between the two poles was realized. The synergy between the two techniques significantly increased the current density,which was conducive to the production of active species. Discovering the parameters of the discharge resulting from the synergism of the combined technique during the initial stage will be the focus of future research.

    4. Conclusion

    Plasma electrolysis, ultrasound, and plasma electrolysis combined with ultrasound were used to treat a dimethyl sulfoxide (DMSO) solution. By measuring the products (the formaldehyde and formic acid concentrations)of the reaction of·OH radicals with DMSO, the concentration of·OH radicals in the solution produced by using these three methods was obtained indirectly. The concentration of·OH radicals in the three solutions treated for 4 min by plasma electrolysis,ultrasound,and the combination of the two were 57.04μmol/L,73.12μmol/L,and 152.66μmol/L,respectively. The concentration of·OH radicals produced by the synergy between the two techniques exceeded the sum of the·OH radicals produced in separate plasma electrolysis and ultrasound treatments. After treatment by the three techniques, the pH and conductivity of the solution decreased. The conductivity increased from 14.5 before treatment to 15.8 mS/cm,an increasing trend similar to that of the·OH radicals. Experimental evidence showed that the current density increased as the conductivity increased. This increase in current density indicates that the discharge modes changed from filament discharge mode to spark discharge mode. The bubbles between the two electrodes(generated by ultrasonic cavitation)were broken down and discharged, generating additional species. To verify the rationality of the measurement results, we analyzed the mass transfer path of oxygen-containing free radicals based on the reaction coefficients and found that the·OH free radicals in the aqueous solution were mainly derived from hydrogen peroxide. The experimental results showed that the concentration of hydrogen peroxide was 0.087 mmol/L, approximately 1/2 of the·OH radical concentration, which demonstrates the rationality of the experiment. The new method can greatly increase ROS production and has potential for practical application in materials engineering and pollutant processing.

    Acknowledgements

    Project supported by the National Natural Science Foundation of China (Grant Nos. 51877184 and 11474305) and the National Science and Technology Major Project of China(Grant No. 2011ZX05032-003-003). Rhoda E. and Edmund F.Perozzi,PhDs,greatly assisted with the content and English editing.

    猜你喜歡
    李超
    武技
    李超:人口大國面臨人口變局
    Angular control of multi-mode resonance frequencies in obliquely deposited CoZr thin films with rotatable stripe domains?
    Hom-Jordan李超代數(shù)的交換擴張
    Post李超代數(shù)結(jié)構(gòu)的性質(zhì)
    Rota-Baxter 3-超代數(shù)的構(gòu)造
    Hom-李超代數(shù)的同調(diào)和非交換張量積
    《漢語口語美學(xué)》評介
    李超代數(shù)的擬理想
    Cartan型模李超代數(shù)H作為osp—模的分解與零維上同調(diào)
    侵犯人妻中文字幕一二三四区| 久久久久精品性色| 少妇熟女欧美另类| 999精品在线视频| 欧美 日韩 精品 国产| 亚洲图色成人| 久久精品国产a三级三级三级| 黄色毛片三级朝国网站| 免费人妻精品一区二区三区视频| 亚洲av中文av极速乱| 99热国产这里只有精品6| av免费在线看不卡| 久久精品久久精品一区二区三区| 日韩 亚洲 欧美在线| 精品亚洲乱码少妇综合久久| 国产精品久久久av美女十八| 高清在线视频一区二区三区| 日本91视频免费播放| 美女中出高潮动态图| 赤兔流量卡办理| 99国产精品免费福利视频| 伦理电影大哥的女人| 免费观看av网站的网址| 伊人亚洲综合成人网| 蜜臀久久99精品久久宅男| 大香蕉久久成人网| 婷婷色综合大香蕉| 国产一区二区在线观看av| 国产成人免费无遮挡视频| 黄色 视频免费看| 免费在线观看完整版高清| 视频在线观看一区二区三区| 午夜免费鲁丝| 日韩制服丝袜自拍偷拍| 国产精品久久久久久久电影| 九色成人免费人妻av| 大香蕉97超碰在线| 久热这里只有精品99| 国产女主播在线喷水免费视频网站| 免费不卡的大黄色大毛片视频在线观看| 王馨瑶露胸无遮挡在线观看| 高清在线视频一区二区三区| 亚洲精品aⅴ在线观看| 精品久久蜜臀av无| 中文字幕精品免费在线观看视频 | 国产精品无大码| 中国国产av一级| 97精品久久久久久久久久精品| 男女无遮挡免费网站观看| 成人毛片60女人毛片免费| 国产精品人妻久久久影院| 一区二区三区乱码不卡18| av黄色大香蕉| 成人毛片60女人毛片免费| 2018国产大陆天天弄谢| 免费黄色在线免费观看| 欧美激情 高清一区二区三区| 亚洲av国产av综合av卡| 精品少妇久久久久久888优播| 在线观看www视频免费| 亚洲,欧美精品.| 国产极品天堂在线| 久久97久久精品| 女的被弄到高潮叫床怎么办| 九草在线视频观看| 中国三级夫妇交换| 国产精品99久久99久久久不卡 | av在线app专区| 久久久久久久久久人人人人人人| 七月丁香在线播放| 下体分泌物呈黄色| 欧美激情 高清一区二区三区| 国产精品成人在线| 精品卡一卡二卡四卡免费| 国产深夜福利视频在线观看| 亚洲人成77777在线视频| 亚洲国产av影院在线观看| 久久午夜福利片| 伊人久久国产一区二区| 男女下面插进去视频免费观看 | 国产一区二区三区综合在线观看 | 欧美97在线视频| 在线看a的网站| 欧美精品国产亚洲| 另类精品久久| 色网站视频免费| 最近最新中文字幕免费大全7| 夜夜骑夜夜射夜夜干| 国产精品麻豆人妻色哟哟久久| 日本黄大片高清| 国产精品欧美亚洲77777| 在线免费观看不下载黄p国产| 日韩成人伦理影院| 精品国产一区二区久久| 国产精品一区www在线观看| 久热这里只有精品99| 欧美日韩成人在线一区二区| xxxhd国产人妻xxx| 久久久国产欧美日韩av| 亚洲精品自拍成人| 一二三四中文在线观看免费高清| av在线app专区| 高清黄色对白视频在线免费看| 国产亚洲欧美精品永久| 妹子高潮喷水视频| 成人亚洲欧美一区二区av| 五月玫瑰六月丁香| 欧美精品亚洲一区二区| 日本黄色日本黄色录像| 免费不卡的大黄色大毛片视频在线观看| 中文乱码字字幕精品一区二区三区| 80岁老熟妇乱子伦牲交| av福利片在线| 热99国产精品久久久久久7| 两个人免费观看高清视频| 国产极品天堂在线| 王馨瑶露胸无遮挡在线观看| av视频免费观看在线观看| 国产成人91sexporn| 久久精品国产综合久久久 | 青春草亚洲视频在线观看| 蜜桃国产av成人99| 你懂的网址亚洲精品在线观看| 国产亚洲欧美精品永久| 亚洲精品456在线播放app| 国产在线免费精品| 2018国产大陆天天弄谢| 久久久久久人人人人人| 久久人妻熟女aⅴ| 丰满乱子伦码专区| 中文字幕亚洲精品专区| 满18在线观看网站| 久久97久久精品| 狠狠精品人妻久久久久久综合| 人体艺术视频欧美日本| 精品第一国产精品| 国产亚洲精品第一综合不卡 | 美女视频免费永久观看网站| 免费女性裸体啪啪无遮挡网站| 欧美国产精品一级二级三级| 亚洲精品国产av蜜桃| 美女脱内裤让男人舔精品视频| 免费久久久久久久精品成人欧美视频 | 亚洲欧美日韩另类电影网站| 亚洲成色77777| 黑人高潮一二区| 日韩 亚洲 欧美在线| 精品人妻熟女毛片av久久网站| 极品人妻少妇av视频| 久久久久网色| 黄色配什么色好看| 精品久久国产蜜桃| 久久久久久人妻| 免费观看在线日韩| 最近中文字幕高清免费大全6| 极品少妇高潮喷水抽搐| 免费高清在线观看视频在线观看| 最近手机中文字幕大全| 深夜精品福利| 大码成人一级视频| 如何舔出高潮| 欧美精品一区二区大全| 卡戴珊不雅视频在线播放| 欧美精品高潮呻吟av久久| 亚洲一区二区三区欧美精品| 亚洲高清免费不卡视频| h视频一区二区三区| 香蕉国产在线看| 久久精品国产综合久久久 | 国精品久久久久久国模美| av黄色大香蕉| 午夜av观看不卡| 一本久久精品| 亚洲,欧美,日韩| 极品人妻少妇av视频| 国产精品一区www在线观看| 日本黄色日本黄色录像| 国产男女内射视频| 亚洲国产精品国产精品| 日本与韩国留学比较| 美女内射精品一级片tv| 国产成人一区二区在线| 777米奇影视久久| 啦啦啦啦在线视频资源| 亚洲av日韩在线播放| 黑人欧美特级aaaaaa片| 精品国产一区二区三区四区第35| 99热国产这里只有精品6| 色婷婷久久久亚洲欧美| 成人免费观看视频高清| av天堂久久9| 欧美成人午夜免费资源| 久久99精品国语久久久| 国产精品熟女久久久久浪| 亚洲,欧美,日韩| 爱豆传媒免费全集在线观看| 女的被弄到高潮叫床怎么办| 美女大奶头黄色视频| 黄色毛片三级朝国网站| 久热这里只有精品99| 亚洲欧洲国产日韩| 建设人人有责人人尽责人人享有的| 国产成人精品在线电影| 青青草视频在线视频观看| 精品亚洲成a人片在线观看| 欧美亚洲 丝袜 人妻 在线| 美女内射精品一级片tv| 免费看不卡的av| 国产麻豆69| 久久久久国产精品人妻一区二区| 你懂的网址亚洲精品在线观看| 亚洲情色 制服丝袜| 国产日韩欧美亚洲二区| 69精品国产乱码久久久| 久久人人爽人人片av| 国产高清不卡午夜福利| 日韩成人av中文字幕在线观看| 午夜影院在线不卡| av福利片在线| 欧美3d第一页| 亚洲精品视频女| videossex国产| 欧美激情 高清一区二区三区| 成人国产av品久久久| 免费观看无遮挡的男女| 美女中出高潮动态图| 最新中文字幕久久久久| a级毛色黄片| 肉色欧美久久久久久久蜜桃| 卡戴珊不雅视频在线播放| 久久久欧美国产精品| av片东京热男人的天堂| 在线亚洲精品国产二区图片欧美| 久久免费观看电影| 最后的刺客免费高清国语| 久久av网站| 亚洲精品色激情综合| 国产无遮挡羞羞视频在线观看| 永久网站在线| 人妻人人澡人人爽人人| 亚洲精品美女久久久久99蜜臀 | 免费大片黄手机在线观看| 少妇的逼好多水| 韩国精品一区二区三区 | 乱人伦中国视频| 大香蕉久久成人网| 亚洲国产色片| 成年美女黄网站色视频大全免费| 久久精品国产综合久久久 | 亚洲精品国产av成人精品| 亚洲性久久影院| 午夜激情久久久久久久| 免费黄频网站在线观看国产| 男人操女人黄网站| 国产精品一二三区在线看| 亚洲第一区二区三区不卡| 黄色怎么调成土黄色| 精品一品国产午夜福利视频| 搡女人真爽免费视频火全软件| 99热6这里只有精品| 免费女性裸体啪啪无遮挡网站| av国产精品久久久久影院| 日韩欧美一区视频在线观看| 午夜激情久久久久久久| 国产精品久久久av美女十八| 女人精品久久久久毛片| 丝瓜视频免费看黄片| 日本与韩国留学比较| 亚洲国产色片| 满18在线观看网站| 亚洲av免费高清在线观看| 亚洲婷婷狠狠爱综合网| 伦精品一区二区三区| 亚洲欧洲日产国产| 日韩精品免费视频一区二区三区 | 欧美 日韩 精品 国产| 91午夜精品亚洲一区二区三区| 一区二区三区乱码不卡18| 在线天堂中文资源库| 搡老乐熟女国产| 男人添女人高潮全过程视频| av免费在线看不卡| 天天躁夜夜躁狠狠躁躁| 中文字幕人妻熟女乱码| 97人妻天天添夜夜摸| 国产av码专区亚洲av| 亚洲色图综合在线观看| 亚洲国产日韩一区二区| 天美传媒精品一区二区| 人人妻人人爽人人添夜夜欢视频| 欧美少妇被猛烈插入视频| 国产精品久久久av美女十八| 一本—道久久a久久精品蜜桃钙片| 中文字幕免费在线视频6| 大香蕉久久成人网| 黄网站色视频无遮挡免费观看| 天天操日日干夜夜撸| 春色校园在线视频观看| 日本免费在线观看一区| 一级毛片我不卡| 国产在线免费精品| 日日摸夜夜添夜夜爱| 国产毛片在线视频| 久久影院123| 日韩欧美一区视频在线观看| 国产精品三级大全| 免费人成在线观看视频色| 亚洲综合色惰| 乱码一卡2卡4卡精品| av福利片在线| 久久久久久久国产电影| 精品国产乱码久久久久久小说| 欧美3d第一页| 国产一区二区三区av在线| 秋霞在线观看毛片| 日产精品乱码卡一卡2卡三| 亚洲国产色片| 国产永久视频网站| 久久久久国产精品人妻一区二区| 边亲边吃奶的免费视频| 黄色视频在线播放观看不卡| 久久久久久久久久久久大奶| 熟女人妻精品中文字幕| 亚洲激情五月婷婷啪啪| 秋霞在线观看毛片| 欧美日韩成人在线一区二区| 激情视频va一区二区三区| 国产黄色免费在线视频| 亚洲精品成人av观看孕妇| 亚洲国产精品一区二区三区在线| 春色校园在线视频观看| 国产免费福利视频在线观看| 热99国产精品久久久久久7| 九九在线视频观看精品| 日韩免费高清中文字幕av| 国产高清不卡午夜福利| 国语对白做爰xxxⅹ性视频网站| 五月开心婷婷网| 精品少妇久久久久久888优播| 色5月婷婷丁香| 久久久久人妻精品一区果冻| 亚洲精品一二三| 一级毛片电影观看| 18+在线观看网站| www.熟女人妻精品国产 | av国产精品久久久久影院| 高清视频免费观看一区二区| 亚洲情色 制服丝袜| 国产精品不卡视频一区二区| 在线观看美女被高潮喷水网站| 国产伦理片在线播放av一区| 在线观看美女被高潮喷水网站| 国产成人一区二区在线| 看免费av毛片| av有码第一页| 美女内射精品一级片tv| 九九在线视频观看精品| 国产精品熟女久久久久浪| 成年美女黄网站色视频大全免费| 在线 av 中文字幕| 日韩人妻精品一区2区三区| 久久久久精品性色| 亚洲精品日韩在线中文字幕| 国产日韩欧美视频二区| 91aial.com中文字幕在线观看| 韩国高清视频一区二区三区| 日本午夜av视频| 999精品在线视频| 亚洲天堂av无毛| 亚洲精品456在线播放app| 亚洲熟女精品中文字幕| 天天躁夜夜躁狠狠躁躁| 亚洲av男天堂| 夜夜爽夜夜爽视频| 夫妻午夜视频| 亚洲国产精品999| 美女xxoo啪啪120秒动态图| 久久亚洲国产成人精品v| 欧美性感艳星| 九草在线视频观看| 午夜91福利影院| 精品国产一区二区三区久久久樱花| 一区二区三区乱码不卡18| 熟女电影av网| 国产老妇伦熟女老妇高清| 满18在线观看网站| 亚洲精品第二区| 大香蕉久久成人网| 纯流量卡能插随身wifi吗| 人人妻人人澡人人看| 自线自在国产av| 精品一区二区免费观看| 国产一区有黄有色的免费视频| 黑人高潮一二区| 久久狼人影院| 男女边吃奶边做爰视频| 尾随美女入室| 国产免费视频播放在线视频| 香蕉丝袜av| 99久国产av精品国产电影| 久久99一区二区三区| 美女国产高潮福利片在线看| freevideosex欧美| 欧美成人精品欧美一级黄| 日韩中字成人| 亚洲精品一区蜜桃| 精品久久蜜臀av无| 久久毛片免费看一区二区三区| 搡老乐熟女国产| 久久久久久久久久成人| 丝袜美足系列| 大话2 男鬼变身卡| 最近2019中文字幕mv第一页| 一区二区三区精品91| 国产日韩欧美视频二区| 99香蕉大伊视频| 国产在线一区二区三区精| 国产一区二区在线观看av| 国产色爽女视频免费观看| 一边亲一边摸免费视频| 久久av网站| 亚洲,一卡二卡三卡| 90打野战视频偷拍视频| 免费日韩欧美在线观看| 国产黄色免费在线视频| 久久久久久久大尺度免费视频| 亚洲欧美成人综合另类久久久| 亚洲伊人色综图| 亚洲精品乱久久久久久| 久久毛片免费看一区二区三区| 免费大片18禁| 亚洲成色77777| 国产精品久久久久成人av| 22中文网久久字幕| 另类精品久久| 少妇人妻久久综合中文| 成年女人在线观看亚洲视频| 成人18禁高潮啪啪吃奶动态图| 日本黄色日本黄色录像| 亚洲精品国产av成人精品| 边亲边吃奶的免费视频| 免费看av在线观看网站| 草草在线视频免费看| 丝袜喷水一区| www.熟女人妻精品国产 | 久久国内精品自在自线图片| 日韩人妻精品一区2区三区| 妹子高潮喷水视频| 精品久久久久久电影网| 久久狼人影院| 午夜免费鲁丝| 熟女电影av网| 在线免费观看不下载黄p国产| 日本色播在线视频| 午夜福利视频在线观看免费| 免费av中文字幕在线| 国产毛片在线视频| 久久精品国产综合久久久 | 欧美 日韩 精品 国产| 99热网站在线观看| 人妻 亚洲 视频| 在线观看美女被高潮喷水网站| 性高湖久久久久久久久免费观看| 欧美精品一区二区免费开放| 最近手机中文字幕大全| 人成视频在线观看免费观看| 999精品在线视频| 美女国产视频在线观看| 午夜免费观看性视频| 久久综合国产亚洲精品| 久久国产精品大桥未久av| 亚洲国产精品成人久久小说| 制服诱惑二区| tube8黄色片| 精品一区在线观看国产| 看十八女毛片水多多多| 国产精品久久久久久精品电影小说| 日本与韩国留学比较| 丝瓜视频免费看黄片| 亚洲综合色网址| a级片在线免费高清观看视频| 免费观看无遮挡的男女| 国产福利在线免费观看视频| 激情视频va一区二区三区| 中文字幕人妻熟女乱码| 少妇精品久久久久久久| 制服人妻中文乱码| 黑丝袜美女国产一区| 欧美精品亚洲一区二区| 国产欧美日韩综合在线一区二区| 欧美97在线视频| 天堂8中文在线网| 国产男女超爽视频在线观看| 97精品久久久久久久久久精品| 五月伊人婷婷丁香| 在线观看一区二区三区激情| 免费大片18禁| 中文字幕人妻丝袜制服| 韩国av在线不卡| 国产精品秋霞免费鲁丝片| 久久精品熟女亚洲av麻豆精品| 日本av手机在线免费观看| 一级毛片黄色毛片免费观看视频| 久久人人爽人人片av| 搡女人真爽免费视频火全软件| 日韩电影二区| 中文字幕av电影在线播放| 免费女性裸体啪啪无遮挡网站| a级毛色黄片| 国产成人91sexporn| 欧美变态另类bdsm刘玥| 99久久综合免费| 国产永久视频网站| 自线自在国产av| 国产精品三级大全| 久久久国产精品麻豆| 亚洲国产精品成人久久小说| av福利片在线| 伊人久久国产一区二区| 日本vs欧美在线观看视频| 飞空精品影院首页| h视频一区二区三区| 日韩三级伦理在线观看| 国产xxxxx性猛交| 一级片'在线观看视频| 亚洲精品久久成人aⅴ小说| 亚洲国产欧美在线一区| 国产亚洲av片在线观看秒播厂| 在线观看三级黄色| 亚洲精华国产精华液的使用体验| 亚洲精品久久午夜乱码| 伦理电影大哥的女人| 美国免费a级毛片| 亚洲精品aⅴ在线观看| 三级国产精品片| 男人操女人黄网站| 亚洲伊人色综图| 亚洲欧洲国产日韩| 国产亚洲av片在线观看秒播厂| 国产精品人妻久久久影院| 精品一品国产午夜福利视频| 男女边吃奶边做爰视频| 宅男免费午夜| 午夜免费鲁丝| 两个人免费观看高清视频| 久久久国产一区二区| 两个人免费观看高清视频| 欧美精品av麻豆av| 国产成人午夜福利电影在线观看| 制服人妻中文乱码| 亚洲在久久综合| 91精品国产国语对白视频| 久久久国产精品麻豆| 最后的刺客免费高清国语| av.在线天堂| 色婷婷久久久亚洲欧美| 男的添女的下面高潮视频| 欧美xxxx性猛交bbbb| 91国产中文字幕| 女人被躁到高潮嗷嗷叫费观| 草草在线视频免费看| tube8黄色片| 国产精品三级大全| 中国美白少妇内射xxxbb| 日韩成人av中文字幕在线观看| 女性生殖器流出的白浆| 人妻人人澡人人爽人人| 男女下面插进去视频免费观看 | 九草在线视频观看| videossex国产| 只有这里有精品99| 国产免费又黄又爽又色| 久久国产亚洲av麻豆专区| 日韩av不卡免费在线播放| 十八禁高潮呻吟视频| 久久影院123| 亚洲欧美一区二区三区黑人 | 久久久久久久久久久免费av| 韩国av在线不卡| 日韩三级伦理在线观看| 黄色一级大片看看| 欧美xxxx性猛交bbbb| 我的女老师完整版在线观看| 99国产精品免费福利视频| 人妻少妇偷人精品九色| 日本vs欧美在线观看视频| 99国产精品免费福利视频| 人妻少妇偷人精品九色| 国产成人免费观看mmmm| 在线精品无人区一区二区三| 男人爽女人下面视频在线观看| 国产精品秋霞免费鲁丝片| 国产成人一区二区在线| 国产精品久久久久久精品电影小说| 精品一区二区三区四区五区乱码 | 国产精品欧美亚洲77777| 最近中文字幕2019免费版| 蜜桃国产av成人99| 免费av中文字幕在线| 欧美日韩一区二区视频在线观看视频在线| 国产无遮挡羞羞视频在线观看| 一级a做视频免费观看| 亚洲人成77777在线视频| 亚洲成人一二三区av| 午夜久久久在线观看| 香蕉丝袜av| 国产欧美日韩一区二区三区在线| 中文字幕最新亚洲高清| 久久99一区二区三区| 久久精品国产鲁丝片午夜精品| 色视频在线一区二区三区| 22中文网久久字幕| 国产色爽女视频免费观看| 少妇猛男粗大的猛烈进出视频| 91国产中文字幕|