Ultrasound-enhanced membrane-cleaning processes applied water treatments: influence of sonic frequency on filtration treatments

Ultrasound (US) cleaning technique was applied to remove fouling of ultrafiltration (UF) and microfiltration (MF) membranes which were used to treat peptone and milk aqueous solutions, respectively. Membrane operations were performed by cross-flow filtration with 60 kPa operating pressure in an US field. The US employed had 28, 45 and 100 kHz frequency with 23 W/cm(2) output power. For each polymeric membrane made of polysulfone UF and cellulose MF, cleaning experiments were carried out with and without US after fouling. The fouled UF and MF membranes showed volume flux decline, but the membrane property was recovered by US irradiation. It was found in 28 kHz frequency that water cleaning was effective for recovery of declined condition due to fouling. Also, US-enhanced permeability of membranes was discussed in both membrane systems. We observed that US decreased the fouling condition in both membrane systems when US was irradiated before fouling. It was found that 28 kHz frequency US could enhance formation of the fouled layer in both filtration systems of peptone and milk solution.     

Kinetics of Membrane Flux Decline: The Role of Natural Colloids and Mitigation via Membrane Surface Modification

Applications of membrane technologies for potable water production have been expanding significantly, leading to increased efforts to control membrane fouling, which can significantly reduce membrane performance, increase operating costs, and shorten membrane life. Natural organic matter is ubiquitous in all water supplies and has been implicated as a major contributor to fouling during filtration of natural water. In this review, we discuss factors that influence NOM fouling, including hydrodynamics; properties of the feed constituents such as size, hydrophobicity, charge density and isoelectric point; properties of the membrane including hydrophobicity, charge density, surface roughness, and porosity; and properties of the solution phase such as pH, ionic strength and concentration of metals. We review approaches to identify and mathematically describe fouling kinetics, including effects of pore blockage, cake formation, and osmotic pressure. Finally, we discuss strategies to mitigate fouling, with a focus on strategies that involve a modification of the nanostructure of membrane surfaces, via UV-assisted graft polymerization of hydrophilic monomers to increase surface wettability and reduce interactions between NOM and the membrane surface.     

Determination of the Maier-Saupe strength parameter from dielectric relaxation experiments: a molecular dynamics simulation study

Molecular dynamics simulations have been performed to investigate the rotational motion in the nematic and isotropic phases of a model mesogenic system in which the interactions between the molecules are represented by the Gay-Berne potential. First-rank end-over-end rotational relaxation times, analogous to those measured using dielectric relaxation spectroscopy for real mesogens with a longitudinal electric dipole, have been determined as a function of temperature and density. The relaxation times at temperatures throughout the nematic region are found to be larger than the values extrapolated from the isotropic phase to the same temperature. The simulation results are compared with the extended Debye theory for dielectric relaxation in the nematic phase. This relates the reduction in the relaxation rate to the retardation factor which depends on the Maier-Saupe strength parameter, and in turn is defined uniquely by the second-rank orientational order parameter. The simulations indicate that the retardation factor at constant strength parameter is density dependent, a feature neglected in the relaxation theory. We compare the simulation results where possible with experiment.     

基于红外光谱聚类分析的纳滤膜污染动态发展行为研究

污水再生利用是解决水资源短缺问题的有效对策。纳滤技术由于能够生产高质量的再生水,成为污水深度处理、再生利用的有效方法之一。然而,在纳滤过程中存在复杂的、动态的膜污染现象,会导致产水通量、产水质量下降等问题。研究膜污染动态发展的行为,对于膜污染的分阶段针对性控制具有重要意义。有机物是污染层动态发展过程的重要指示性成分,红外光谱是表征污染层发展过程中表面有机物官能团变化情况的重要手段。但由于红外光谱中峰的数量多,系列样品之间峰强度的差别较小（尤其是当膜污染过程中的采样间隔较小时）,利用直观观察不易甄别不同样品间的谱图差异及其变化趋势,在此水平上难以对膜污染阶段进行准确识别、对各阶段特征进行有说服力的分类概括。为探索膜污染的动态发展过程,本研究将傅里叶变换红外光谱与统计学聚类分析相结合,对膜污染过程中不同时间点的膜样本进行红外光谱分析,再对红外光谱数据进行一系列预处理和系统聚类分析,从而客观解读膜污染动态发展过程中系列样品红外光谱分阶段变化规律。考虑到类别间距离度量方法、红外吸收峰强度标准化、峰之间自相关性、峰与样本之间交互作用等因素的影响,研究采用对应分析对红外数据进行预处理,提取各样本在主要维度上的得分,随后基于标准化欧式距离对各样本进行聚类。在为期一个月的城市污水深度处理纳滤试验过程中,由于污染物在膜表面累积,纳滤膜发生了较为严重的污染。通过对13个不同时间点的膜样本进行红外光谱聚类发现,膜污染可清晰划分为如下阶段：空白膜、阶段Ⅰ（3 h～8 d）、阶段Ⅱ（10～15 d）和阶段Ⅲ（20～30 d）。采用红外聚类,得到膜表面X射线光电子能谱（XPS）和三磷酸腺苷（ATP）含量分析等方法的交互验证。结果表明,随着膜污染的发展,膜表面有机物成分与共存微生物量发生协同变化,各阶段大致特征为：阶段Ⅰ各类有机污染物初步覆盖,微生物开始富集;阶段Ⅱ多糖类污染物比例上升,微生物的富集趋于稳定;阶段Ⅲ整体污染趋于成熟,有机污染物氢键特征更加明显。该研究通过对红外数据进行聚类分析,能够灵敏地探测各红外图谱之间的差别,有助于对红外光谱规律的深度挖掘,为膜污染阶段的识别和划分提供了一种客观、自动、可量化的辅助性方法,并且有助于归纳出不同阶段的污染层特征,可作为膜污染时序特征的侦查手段。此外,除了膜污染的研究,在材料、吸附等领域,只要有一系列变化的红外光谱,均可尝试采用红外光谱聚类分析方法,获取基于红外特征的定类信息或分阶段规律。     

Effect of dielectric layer on ferroelectric responses of P(VDF-TrFE) thin films

The dielectric layer in the sandwich structural device plays a very important role in determining the electrical properties of the ferroelectric film. In this paper, we investigate the effect of the dielectric layers with different thicknesses on switching performance of ferroelectric P(VDF-TrFE) thin films. The hysteresis loops become slanting with increasing thickness of the dielectric layer. A negative slope of the ‘real’ hysteresis loop is apparently observed which demonstrates negative capacitance effect caused by the dielectric layer. This behavior is simulated qualitatively by the Weiss mean field model considering an interfacial dielectric layer in series with a ferroelectric layer. The agreement between experiments and simulations supports that negative capacitance results from the positive feedback among electric dipoles. Furthermore, the switching time of the ferroelectric film increases with the increase of dielectric layer thickness. This study shows that the ferroelectric sandwich structure provides great potential towards low power negative capacitance devices.     

石英中Al3+-空穴取向变化弛豫过程中的红外发散响应

基于α石英的晶体结构，将红外发散响应模型和双势阱模型应用到含Ａｌ杂质的α石英中Ａｌ³⁺－空穴的取向变化弛豫过程，研究其低温介电损耗特性，结果表明Ｔ＜６．５Ｋ时，介电损耗的主要贡献来自于单声子助隧道弛豫过程；Ｔ＞１０Ｋ时，主要贡献来自于热跃迁弛豫过程；而在中间温区，介电损耗是两种过程的迭加，同一弛豫体不同的弛豫过程对应于不同的红外发散响应，还讨论了同一弛豫体引起的超声弛豫损耗。 关键词：     

改善多层电光聚合物调制器直流弛豫的一种简单方法的分析与模拟

电场分布和直流漂移对多层电光聚合物调制器的器件稳定性和极化效率有着重要的影响.本文由电磁场理论建立出等效电路模型,分析结果表明多层聚合物调制器的稳态直流场分布取决于材料的电特性;材料的弛豫时间常数τ(τ=∈/σ₂,其中σ是电导率,∈是介电常数)是最大程度上消除直流场弛豫的关键.当外加直流电压时,波导层的场分布由初态到稳态指数变化,这一过程由波导层和覆盖层的弛豫时间差决定.当两者相差为零时,弛豫消失.但在实际工艺中很难作到这一点,为此,本文经过分析、模拟,提出一种简单的“自抑制”机制,能够很好地改善多层电光聚合物调制器设计中的直流漂移问题.     

反渗透膜前预处理工艺去除渗滤液中DOM的光谱分析

考查反渗透(RO)膜处理渗滤液前所采用的预处理工艺对致膜有机污染物的去除效果,为RO膜预处理工艺的选择提供参考,采用同步荧光光谱、三维荧光光谱、紫外-可见全光谱对某垃圾焚烧厂渗滤液原液及其经“生化(UASB+A/O)+超滤”预处理工艺各级出水进行不同分子量分布区间的分析。同步荧光光谱表明,预处理工艺总体能有效去除波长在250～320 nm各分子量区间的以及波长>320 nm、分子量>1KDa的DOM;三维荧光光谱表明,预处理工艺总体能有效去除低激发波长类酪氨酸、低激发波长类色氨酸及高激发波长类色氨酸及分子量>1KDa的类富里酸和高激发波长类酪氨酸;紫外-可见全光谱表明,预处理工艺总体能有效去除分子量>1KDa带π—π*跃迁的DOM和各分子量区间的带多个共轭体系苯环结构的DOM。结果显示,将“生化(UASB+A/O)+超滤”组合工艺作为RO膜法处理焚烧厂渗滤液的预处理工艺时,还需加强同步荧光范围>320 nm、分子量<1 KDa、带π—π*跃迁的类富里酸及高激发波长类酪氨酸的去除。     

Polymer translocation in the presence of excluded volume and explicit hydrodynamic interactions

Molecular Dynamics simulations of polymer translocation are hereby reported. No external force was applied to the polymer during translocation, and the dynamics was dominated by polymer–pore interactions. It was found that hydrodynamic interactions play an important role in the relaxation of the polymer on each side of the membrane but have a negligible impact on the translocation process itself. Also, the scaling laws obtained for the relaxation and translocation times indicate that long translocating polymers may be considered to be following a quasi-equilibrium anomalous diffusion process in the absence of external forces.     

环氧树脂高温分子链松弛与玻璃化转变特性

环氧树脂是电力设备中广泛应用的一种绝缘材料, 其介电性能受到分子链运动特性的影响. 本文制备了直径为50 mm、厚度为1 mm的环氧树脂试样, 采用差示扫描量热仪和宽频介电谱仪测试了环氧树脂的玻璃化转变温度和介电特性. 实验结果表明, 环氧树脂的玻璃化转变温度为105 ℃, 在玻璃化转变温度以上, 高频段出现了由分子链段运动造成的松弛过程, 低频段出现了由载流子在材料中迁移造成的直流电导过程. 发现环氧树脂不同尺寸分子链段的松弛时间不同, 其松弛时间分布较宽, 计算得到了分子链段在不同温度下的松弛时间分布特性. 分子链松弛峰频率和直流电导随温度的变化关系服从Vogel-Tammann-Fulcher公式. 拟合实验结果得到分子链松弛峰频率和直流电导的Vogel温度和强度系数. 由Vogel温度计算得到了与差示扫描量热测试结果一致的玻璃化转变温度, 约为102 ℃. 结果表明玻璃化转变温度以上环氧树脂的自由体积增大, 分子链段有足够的空间来响应外电场从而产生分子链松弛极化, 载流子有足够的能量在材料中迁移形成电导.     

铁电体中偶极子的滞后对剩余极化的影响

铁电体的剩余极化强度随温度降低而下降的特性引起了人们对铁电体存储数据失效的担心.运用铁电体的唯象理论和偶极子对交变电场的响应,提出了在电滞回线测量中偶极子的滞后冷冻效应模型,对极化的低温退化现象做了合理解释:温度下降导致吉布斯自由能势垒增大,致使偶极子对交变电场的响应时间延长.引入响应的滞后因子发现,极化强度随温度降低会出现峰值,在低温下降直至为零,可用偶极子的滞后与冻结效应描述.详细研究结果表明:因材料组份变化导致热力学参量的变化是重要因素:铁电-顺电相变中软模系数的增大会导致剩余极化峰移向高温;铁电性的增强,温度极化系数的增大和耐压强度或饱和电场的增强均会抑制滞后效应,从而使低温滞后效应移向低温.运用导出的公式数值模拟Ba Ti O_3/Bi Sc O_3复合陶瓷剩余极化强度的实验结果发现,Bi Sc O3含量的增加,使居里温度略有减小,但导致了软模系数较大幅度的增加,其结果是使偶极子的滞后效应发生在较高的温度.软模系数与铁电体的极化特性、铁电性、介电性和力学性均密切相关.研究结论表明:在低温下铁电体的铁电性没有失效,偶极子的低温冻结效应更有利于铁电体长久地保存数据.     

Oxygen-vacancy-related high-temperature dielectric relaxation and electrical conduction in 0.95K0.5Na0.5NbO3-0.05BaZrO3 ceramic

The crystal structure and dielectric properties of 0.95K_0.5Na_0.5NbO₃-0.05BaZrO₃ (KNN-BZ) ceramic have been investigated by X-ray diffraction and dielectric measurement. A rhombohedral distortion was caused and the dielectric permittivity near Curie temperature was significantly enhanced by introducing BZ into KNN. The dielectric and conductivity properties of the sample were studied by using AC impedance spectroscopy and universal dielectric relaxation law in detail. The typical high-temperature dielectric relaxation process was confirmed to be related to the oxygen vacancies inside the ceramic. The effect of lattice distortion on the activation energy for oxygen vacancy migration in KNN-BZ was discussed by comparing with KNN and KNN-BaTiO₃.     

The optimisation of ultrasonic cleaning procedures for dairy fouled ultrafiltration membranes

Ultrafiltration (UF) of whey is a major membrane based process in the dairy industry. However, commercialization of this application has been limited by membrane fouling, which has a detrimental influence on the permeation rate. There are a number of different chemical and physical cleaning methods currently used for cleaning a fouled membrane. It has been suggested that the cleaning frequency and the severity of such cleaning procedures control the membrane lifetime. The development of an optimal cleaning strategy should therefore have a direct implication on the process economics. Recently, the use of ultrasound has attracted considerable interest as an alternative approach to the conventional methods. In the present study, we have studied the ultrasonic cleaning of polysulfone ultrafiltration membranes fouled with dairy whey solutions. The effects of a number of cleaning process parameters have been examined in the presence of ultrasound and results compared with the conventional operation. Experiments were conducted using a small single sheet membrane unit that was immersed totally within an ultrasonic bath. Results show that ultrasonic cleaning improves the cleaning efficiency under all experimental conditions. The ultrasonic effect is more significant in the absence of surfactant, but is less influenced by temperature and transmembrane pressure. Our results suggest that the ultrasonic energy acts primarily by increasing the turbulence within the cleaning solution.     

The conductivity and dielectric studies of solid polymer electrolytes based on poly (acrylamide-co-acrylic acid) doped with sodium iodide

Solid polymer electrolyte thin films based on polyacrylamide-co-acrylic acid (PAAC) doped with sodium iodide (NaI) with different ratios of polymer and salt added with fixed amount of additive of propylene carbonate (PC) were prepared by using solution casting method. The PC was added to the mixture of the solution to provide more flexibility to the polymer film by increasing the plasticity of the thin film membrane. The conductivity and dielectric studies were carried out on these thin films to understand the ion transport properties of the polymer electrolytes. The highest conductivity obtained was 1.88?×?10^?5 S cm^?1 for the 30% NaI salt-doped polymer electrolyte system at room temperature. The temperature-dependent conductivity agrees with Arrhenius relationship which shows that hopping mechanism of ions in the polymer matrix. The dielectric properties especially the loss tangent used to analyze the segmental relaxation of the polymer chain as more concentration of salt was incorporated. The electric modulus was studied to understand the electrical relaxation processes to overcome electrode polarization effect.     

Confinement-induced relaxation process in thin films of cis-polyisoprene

Broadband dielectric spectroscopy is employed to investigate the molecular dynamics of cis-1,4-polyisoprene (PI) in thin layers down to thicknesses comparable with the chain extension. The segmental and the normal mode dynamics of the PI chains is found not to depend on the layer thickness. Additionally, a novel confinement-induced relaxation process is detected. It is assigned to fluctuations of terminal subchains which are formed due to the immobilization of chain segments at an interface. Most of the experimental findings can be well described by simulations of the chains as ideal random walks.     

Open Resonator System for Automatic and Precise Dielectric Measurement at Millimeter Wavelengths

As low loss dielectric materials, ZnS, MgF₂, MgAl₂O₄ and quartz ceramic have a very important application at millimeter frequencies. However, there is little information about their dielectric properties in the millimeter wavelength band. To obtain their dielectric properties, an automatic open resonator measurement system at Ka-band is designed and constructed. The importance on the precision determination of cavity length over a broad band and the checking of a measurement system are emphasized and their solutions are put forward in this paper for the first time so far as we know. The solutions of above problems ensure the credibility and high accuracy of our measurement system. The certified measurement system after a series of checking is used to measure the above materials. Lots of measurement results show that the standard deviation of measurement error is less than 0.154% in permittivity and 20.42% in loss tangent. Meanwhile, some experimental summaries on the open resonator technique are provided. Software that controls the measurement system is developed and it improves the testing efficiency greatly.     

Cleaning efficiency enhancement by ultrasounds for membranes used in dairy industries

Membrane cleaning is a key point for the implementation of membrane technologies in the dairy industry for proteins concentration. In this study, four ultrafiltration (UF) membranes with different molecular weight cut-offs (MWCOs) (5, 15, 30 and 50 kDa) and materials (polyethersulfone and ceramics) were fouled with three different whey model solutions: bovine serum albumin (BSA), BSA plus CaCl₂ and whey protein concentrate solution (Renylat 45). The purpose of the study was to evaluate the effect of ultrasounds (US) on the membrane cleaning efficiency. The influence of ultrasonic frequency and the US application modes (submerging the membrane module inside the US bath or applying US to the cleaning solution) were also evaluated. The experiments were performed in a laboratory plant which included the US equipment and the possibility of using two membrane modules (flat sheet and tubular). The fouling solution that caused the highest fouling degree for all the membranes was Renylat 45. Results demonstrated that membrane cleaning with US was effective and this effectiveness increased at lower frequencies. Although no significant differences were observed between the two different US applications modes tested, slightly higher cleaning efficiencies values placing the membrane module at the bottom of the tank were achieved.     

Glass transition and relaxation processes in supercooled water

Many of water's peculiar physical properties are still not well understood, and one of the most important unresolved questions is its glass transition related dynamics. The consensus has been to accept a glass transition temperature (T(g)) around 136 K, but this value has been questioned and reassigned to about 165 K. We find evidence that the dielectric relaxation process of confined water that has been associated with the long accepted T(g) of water (130-140 K) must be a local process which is not related to the actual glass transition. Rather, our data indicate a glass transition at 160-165 K for bulk water and about 175 K for confined water (depending on the confining system).     

Strong-coupling electrostatics in the presence of dielectric inhomogeneities

We study the strong-coupling (SC) interaction between two like-charged membranes of finite thickness embedded in a medium of higher dielectric constant. A generalized SC theory is applied along with extensive Monte Carlo simulations to study the image charge effects induced by multiple dielectric discontinuities in this system. These effects lead to strong counterion crowding in the central region of the intersurface space upon increasing the solvent-membrane dielectric mismatch and change the membrane interactions from attractive to repulsive at small separations. These features agree quantitatively with the SC theory at elevated couplings or dielectric mismatch where the correlation hole around counterions is larger than the thickness of the central counterion layer.     

Dielectric relaxation in liquid water: two fractions or two dynamics?

Dielectric relaxation in liquid water is studied using molecular dynamics (MD) simulations in the temperature range of 240 to 340?K at atmospheric pressure. The main dielectric and fast relaxation mode are identified in the spectra of dipole moment autocorrelation functions. The microscopic origin of the fast dielectric relaxation process, which takes place on a time scale of subpicoseconds at room temperature, is discussed. A new hypothesis for the fast dielectric mode is presented. It is based on the assumption of the intrawell rotational relaxation taking place during the waiting period between thermally activated large angle jumps occurring in the course of changing H-bond partners.