渗透蒸发脱盐的研究

将选择性透过膜和汽化相结合的分离过程是当前的研究热点,其中采用低热导率、疏水性多孔膜的过程称膜蒸馏,而采用非多孔性半透膜的过程则称渗透蒸发。将该分离过程用于盐水脱盐已有很多报道,并已建成中试设备,但采用的大多是膜蒸馏,所用膜的成本很高。本文研究制了一种价廉并带有致密层的非对称性含钠离子聚乙烯醇(PVA)-聚乙二醇(PEG)分离膜,并用该膜对渗透蒸发脱盐进行了初步研究。 1 实验 1.1 膜制备 将一定配比的PVA与PEG及钠盐水溶液在适当温度下混溶,加入适量醛类交联剂,数分钟后在选定的基材上流涎成膜,室温下自然晾干,再经35℃～50℃控温红外线干燥。 1.2 膜结构及脱盐流程 扫描电镜拍摄的膜断面形貌图像(图1)表明,该膜为非对称膜,表层厚约13μm,其中厚约2～3/μm的上表层为疏松结构,下表层为致密结构。将该类膜用于3.5％NaCl水溶液脱盐,运行4h后再漂洗、干燥作断面Na元素波谱线分析(图2)发现,在膜下层(右侧)厚约30μm层内检测不到Na元素。脱盐流程见文献。     

电容去离子除氯电极的构建及其脱盐性能研究进展

电容去离子技术(Capacitive deionization,CDI)是一种新兴的脱盐技术,通过在电极两端施加较低的外加电场除去水中的带电离子和分子,由于其较低的能耗和可持续性而备受关注。基于储能电池领域近年来的迅猛发展,CDI电极材料实现了从以双电层作用机理为代表的碳材料到法拉第电极材料的跨越,使得脱盐性能有了大幅度提升。Na⁺的去除与Cl^-的去除同等重要,然而,CDI中针对氯离子高效去除的电极材料研究关注较少。本文从CDI装置的构型演变发展出发,系统地归纳与梳理了CDI中关于脱氯电极材料的分类,对比了不同类型脱氯电极材料的特点,并总结了Cl^-去除的机理,分别为基于双电层的电吸附、转化反应、离子插层和氧化还原反应。本文是首篇关于CDI阳极材料的进展综述和展望,为CDI除氯电极的后续研究提供理论基础和研究思路。     

Recent advances in application of the graphene-based membrane for water purification

Graphene is an atomic layer thick carbon-based material with unique two-dimensional architecture and extraordinary physiochemical, optical, electrical, and mechanical properties. Graphene and its derivatives show significant promises for the development of nanoporous ultrathin filtration membranes capable of molecular separation properties. Graphene-based nanofiltration membranes featuring distinct laminar structures can offer various novel mass-transport phenomena for purifying water, energy storage and separation, gas separation, and proton conductors. The latest developments in water purification techniques through graphene-based membranes including engineering, design, and fabrication of diverse graphene, graphene-oxide, and graphene-composite membranes are provided here in relation to their application paradigm for purifying water. The critical views on pollutant removal mechanisms for water purification along with optimization measures are specially highlighted. In addition, the challenges, shortcomings, and future prospects are pointed out. The green and large-scale synthesis technology of graphene coupling with advanced membrane fabrication techniques can promote these state-of-the-art nanofiltration membranes for a wide range of applications.     

Electrode Materials for Desalination of Water via Capacitive Deionization

Recent years have seen the emergence of capacitive deionization (CDI) as a promising desalination technique for converting sea and wastewater into potable water, due to its energy efficiency and eco-friendly nature. However, its low salt removal capacity and parasitic reactions have limited its effectiveness. As a result, the development of porous carbon nanomaterials as electrode materials have been explored, while taking into account of material characteristics such as morphology, wettability, high conductivity, chemical robustness, cyclic stability, specific surface area, and ease of production. To tackle the parasitic reaction issue, membrane capacitive deionization (mCDI) was proposed which utilizes ion-exchange membranes coupled to the electrode. Fabrication techniques along with the experimental parameters used to evaluate the desalination performance of different materials are discussed in this review to provide an overview of improvements made for CDI and mCDI desalination purposes     

Enhanced transport properties and thermal stability of agro-based RO-membrane for desalination of brackish water

This present work focused on preparation of economic and high performance reverse osmosis membranes, characterized by high transport properties (salt rejection and flux) towards desalination of brackish water. In this respect cellulose acetate from sugar-cane bagasse (BCA) and polymethyl methacrylate (PMMA) wastes were used as the substrates of membrane. The function of PMMA for enhancing the performance of bagasse-based cellulose acetate RO-membranes was investigated at operating pressure 35.85 bar and feed temperature 25 °C. The effects of casting solution, percentage of polymer and treatment of polymer by alkali (HPMMA) on the performance of RO-membrane were discussed. The preferable composition (wt.%) of the 90% BCA and 10% HPMMA was achieved salt rejection 92.18% and flux 325.9 l h⁻¹ m⁻². High water purity was obtained by pre-passing the salted water through membrane made from dissolved bagasse (methylol cellulose) together with PMMA, instead of ion exchanger, followed by passing the accepted water through BCA–HPMMA membrane, whereas the salt rejection increased to 98%. Also, by this approach we obtained high thermal stability membrane compared to CA-RO-membrane. This data gives highlight on possibility of application such type of membrane with high temperature operation conditions.     

The desalination of a mixed solution of an amino acid and an inorganic salt by means of electrodialysis with charge-mosaic membranes

A new electrodialysis with charge-mosaic membranes was proposed to achieve efficient desalination of a mixed solution of an amino acid and an inorganic salt. For such a mixed solution, the conventional electrodialytic desalination with both cation-and anion-exchange membranes had resulted in a considerable loss of the amino acid through the membranes. In this method, however, the amino acid in the desalination channel of the electrodialyzer migrates away from the membranes so that the permeation loss of the amino acid through the membrane can be prevented.

Batchwise desalination experiments by this method were carried out with a glutamic acid or arginine solution including NaCl under the condition of constant electric current density. Similar experiments by the conventional method were also carried out. As a result of comparing both methods, the amino acid loss in this method became much smaller than that in the conventional one. It was confirmed that this method was very useful for the desalination of an amino acid solution. The effects of operating conditions on the desalination process are also discussed.     

The use of ultrasound to reduce internal concentration polarization in forward osmosis

Unlike reverse osmosis (RO) that is dominated by the hydraulic pressure differential, forward osmosis (FO) uses the osmotic pressure gradient as the driving force between a dilute feed solution and a concentrated draw solution across a membrane. High pressure is not required in FO, which means that FO can be used as an alternative to RO as an energy-saving separation process in desalination technology. However, a major limiting factor of the FO process is the internal concentration polarization (ICP). Because of the stagnant environment inside the porous supporting layer of a FO membrane, it is difficult to mitigate the ICP by simply increasing the shear stress or promoting turbulence. In this study, the ICP is reduced by ultrasound. The effect of the ultrasound frequency and output power on the ICP coefficient is investigated in a flat-sheet FO membrane module with counter-current flow. The ultrasound frequency and output power are varied between 25, 45, and 72 kHz and over the range of 10–70 W, respectively. NaCl solution is used as both the feed and draw solution. The results illustrate that moderate ultrasonic irradiation is effective for reducing the ICP in a FO process. A modified solution–diffusion model based on film theory is used to assess the effect of ultrasound on the ICP in a FO process. The ICP coefficient is estimated using this model.     

On-line coupling of counter-current chromatography and macroporous resin chromatography for continuous isolation of arctiin from the fruit of Arctium lappa L.

In this work, we have developed a novel hybrid two-dimensional counter-current chromatography and liquid chromatography (2D CCC × LC) system for the continuous purification of arctiin from crude extract of Arctium lappa. The first dimensional CCC column has been designed to fractionalize crude complex extract into pure arctiin effluent using a one-component organic/salt-containing system, and the second dimensional LC column has been packed with macroporous resin for on-line adsorption, desalination and desorption of arctiin which was effluent purified from the first CCC dimension. Thus, the crude arctiin mixture has been purified efficiently and conveniently by on-line CCC × LC in spite of the use of a salt-containing solvent system in CCC separation. As a result, high purity (more than 97%) of arctiin has been isolated by repeated injections both using the ethyl acetate–8% sodium chloride aqueous solution and butanol–1% sodium chloride aqueous solution. By contrast with the traditional CCC processes using multi-component organic/aqueous solvent systems, the present on-line CCC × LC process only used a one-component organic solvent and thus the solvent is easier to recover and regenerate. All of used solvents such as ethyl acetate, n-butanol and NaCl aqueous solution are low toxicity and environment-friendly. Moreover, the lower phase of salt-containing aqueous solution used as mobile phase, only contained minor organic solvent, which will save much organic solvent in continuous separation. In summary, our results indicated that the on-line hybrid 2D CCC × LC system using one-component organic/salt-containing aqueous solution is very promising and powerful tool for high-throughput purification of arctiin from fruits of A. lappa.     

Solar driven membrane pervaporation for desalination processes

We describe details of a solar driven pervaporation process for the production of desalinated water from highly contaminated waters. The membrane material is a polyetheramide-based polymer film of 40 μm thickness. This Solar Dew^® membrane is used in a tubular configuration in a direct solar membrane pervaporation process. The feed waters used in this study are untreated seawater and waste water that is simultaneously produced with the mineral oil extraction. In all cases retention of typical ions as sodium, chloride and calcium as well as specific problematic ions (arsenic, boron and fluoride) was higher than data reported for pressure driven membrane processes like NF and RO. The condensate quality was well within WHO limits for drinking water. A reduction of almost five orders of magnitude in conductivity between brine and condensate could be realized, producing condensate with conductivities of 5 μS/cm or lower. Laboratory experiments show that the measured fluxes are independent of severe fouling and virtually independent of concentration up to 100 g/l total solids.     

Preparation of thin-film-composite polyamide membranes for desalination using novel hydrophilic surface modifying macromolecules

A new concept for the preparation of thin-film-composite (TFC) reverse osmosis (RO) membrane by interfacial polymerization on porous polysulfone (PS) support using novel additives is reported. Hydrophilic surface modifying macromolecules (LSMM) were synthesized both ex situ by conventional method (cLSMM), and in situ within the organic solvent of the TFC system (iLSMM). The effects of these LSMMs on the fouling of the TFC RO membranes used in the desalination processes were studied. FTIR results indicated that both cLSMM and iLSMM were present in the active layer of the TFC membranes. SEM micrographs depicted that heterogeneity of the surface increases for TFC membranes compared to the control PS membrane, and that higher concentrations of LSMM provided smoother surface. AFM characteristic data presented that the surface roughness of the skin surface increases for TFC membranes compared to the control. The RO performance results showed that the addition of the cLSMM significantly decreased the salt rejection of the membrane and slightly reduced the flux, while in the case of the iLSMM, salt rejection was improved but the flux declined at different rates for different iLSMM concentrations. The membrane prepared by the iLSMM exhibited less flux decay over an extended operational period.