Synthesis of organically bridged trialkoxysilanes bearing acetoxymethyl groups and applications to reverse osmosis membranes

New bridged trialkoxysilanes bearing acetoxymethyl groups were synthesized by double hydrosilylation of 1,6‐diacetoxy‐2,4‐butadiyne, using two equivalents of triethoxysilane and a metal catalyst. With a Ru catalyst, the reaction proceeded via anti‐addition to provide BTES‐Ac‐a as a single isomer, while a similar reaction with a Pt or Rh catalyst provided an isomeric mixture of syn ‐adducts BTES‐Ac‐b. Reverse osmosis (RO) silica membranes were prepared by the sol–gel process with BTES‐Ac‐a and BTES‐Ac‐b and the membranes were examined with respect to water desalination using a 2000 ppm NaCl aqueous solution. NaCl rejection of the membranes increased to reach 96% at the early stage of the RO experiments. However, the rejection decreased gradually to 85% after 70 and 200 h for BTES‐Ac‐a and BTES‐Ac‐b, respectively, due to hydrolytic decomposition of the silica network during the experiments. In contrast, a membrane prepared from copolymerization of BTES‐Ac‐a with ethane‐bridged bistrialkoxysilane (BTES‐E1) showed improved stability towards hydrolysis with stable NaCl rejection of 96% with higher water permeance (3.5 × 10⁻¹³ m³ m⁻² s⁻¹ Pa⁻¹) than that of a membrane prepared by homopolymerization of BTES‐E1 (2.7 × 10⁻¹⁴ m³ m⁻² s⁻¹ Pa⁻¹) reported previously.     

Gas and liquid permeation properties of modified interfacial composite reverse osmosis membranes

Gas permeation tests using nitrogen, oxygen, hydrogen, helium and carbon dioxide were performed to assess how membrane modification procedures affect the separating layer morphology of thin-film composite reverse osmosis membranes. Gas selectivity data provided evidence for the presence of nanoscale separating layer defects in dry samples of six commercial membrane types. These defects were eliminated when the membrane surface was coated with a polyether–polyamide block copolymer (PEBAX 1657), as indicated by a 25-fold decrease in gas permeance and at least a 2-fold increase in most selectivity values. Treatment with n-butanol followed by drying reduced water flux and gas flux by 30% and 75%, respectively, suggesting that using n-butanol as a solvent for applying coatings negatively affects membrane performance. The results of this study demonstrate that gas permeation measurements can be used to detect morphological features that impact gas and water membrane flux.     

Innovative beneficial reuse of reverse osmosis concentrate using bipolar membrane electrodialysis and electrochlorination processes

Reverse osmosis (RO) is a widely used and rapidly growing desalination technology. A major disadvantage of this process is that the concentrate from the RO process, which could be as much as 25% of the feed stream, represents a polluting stream. This waste stream could pose a significant challenge to the implementation of this process, particularly for inland communities which do not have the option of ocean disposal. An excellent environmentally benign approach to disposal could be beneficial reuse of the waste stream. This study presents two innovative beneficial reuse strategies for RO concentrate produced by an integrated membrane system (IMS) from a wastewater reclamation facility. The technologies evaluated in this study included bipolar membrane electrodialysis (BMED) for conversion of RO concentrate into mixed acid and mixed base streams, and electrochlorination (EC) for onsite chlorine generation. Bench-scale studies conducted with BMED demonstrated that RO concentrate could be desalted while producing mixed acids and mixed bases with concentrations as high as 0.2N. Similarly, the EC process was capable of producing a 0.6% hypochlorite solution from RO concentrate. The acids and bases as well as the hypochlorite produced could be directly applied to the RO process as well as upstream pre-treatment processes. A preliminary economic evaluation of the viability of these two approaches was conducted by conducting rough order of magnitude cost estimates based on the bench-scale performance of these processes on RO concentrate. A comparison of the overall costs of an Integrated Membrane System utilizing these innovative reuse strategies with conventional disposal options and thermal zero liquid discharge treatment is presented. This comparison indicates that a reuse approach might be economically viable for inland wastewater reuse facilities that utilize RO membranes and have limited options for concentrate disposal.     

Effects of polyether–polyamide block copolymer coating on performance and fouling of reverse osmosis membranes

The effects of a water-permeable polymer coating on the performance and fouling of high-flux (ESPA1 and ESPA3) and low-flux (SWC4) polyamide reverse osmosis (RO) membranes were investigated. It was anticipated that the coating would create a smoother hydrophilic surface that would be less susceptible to fouling when challenged with a motor-oil/surfactant/water feed emulsion (used as a model foulant). AFM and FT-IR analyses confirm that a 1 wt.% polyether–polyamide (PEBAX^® 1657) solution applied to ESPA and SWC4 membranes produces a continuous polymer coating layer and, thereby, provides smoother membrane surfaces. However, pure-water permeation data combined with a series-resistance model analysis reveal that the coating does not only cover the surface of the polyamide membrane, but also penetrates into its porous ridge-and-valley structure. During a long-term (106-day) fouling test with an oil/surfactant/water emulsion, the rate of flux decline was slower for coated than for uncoated membranes. This improvement in fouling resistance compensated for the decrease in permeate flux for SWC4 over a period of approximately 40 days. However, the coating material is believed to penetrate more deeply into the polyamide surface layer of the high flux, high surface area ESPA membranes relative to the low-flux SWC4, resulting in significant water flux reduction.     

Removal of organics from produced water by reverse osmosis using MFI-type zeolite membranes

Separation of an organics/water mixture was carried out by reverse osmosis using an α-alumina-supported MFI-type zeolite membrane. The organic rejection performance is strongly dependent on the ionic species and dynamic size of dissolved organics. The membrane showed high rejection efficiency for electrolytes such as pentanoic acid. An organic rejection of 96.5% with a water flux of 0.33 kg m⁻² h⁻¹ was obtained for 100 ppm pentanoic acid solution at an operation pressure of 2.76 MPa. For non-electrolyte organics, separation efficiency is governed by the molecular dynamic size; the organics with larger molecular dynamic size show higher separation efficiency. The zeolite membrane gives an organic rejection of 99.5% and 17% for 100 ppm toluene and 100 ppm ethanol, respectively, with a water flux of 0.03 kg m⁻² h⁻¹, 0.31 kg m⁻² h⁻¹ at an operation pressure of 2.76 MPa. It was observed that organic rejection and water flux were affected by the organic concentration. As pentanoic acid concentration increased from 100 ppm to 500 ppm, both organic rejection and water flux decreased slightly.     

Novel stand-alone PVA mixed matrix membranes conjugated with graphene oxide for highly improved reverse osmosis performance

In this research, an innovative Poly (vinyl alcohol) (PVA) reverse osmosis (RO) membrane with exceptional attributes was fabricated. Graphene Oxide (GO) nanosheets and Pluronic F-127 were infused within crosslinked PVA to fabricate thin film mixed matrix membranes. The newly synthesized membranes were evaluated in terms of several parameters like surface roughness, hydrophilicity, salt rejection, water permeability, Chlorine tolerance and anti-biofouling property, utilizing a dead-end RO filtration unit. Typical characterization techniques were used to assess the characteristics of the membranes. These include SEM, AFM, contact angle measurements and mechanical strength analysis. The conjugation of Pluronic F-127 and GO enhanced the overall performance of the membranes. The modified membranes surfaces had less roughness and higher hydrophilicity in comparison with the unmodified ones. This research showed that membranes that contained 0.08 wt% and 0.1 wt% GO exhibited superior selectivity, mechanical strength, Chlorine tolerance and anti-biofouling property. The truly significant outcome to evolve from this investigation is that improvements have been accomplished while PVA was used as a stand-alone RO layer without the use of any substrate. This study showed that crosslinking of PVA and modifying it with proper fillers overcame the common PVA downsides, primarily swelling and rupture under exceptionally high pressure.     

Fouling of reverse osmosis and nanofiltration membranes by humic acid—Effects of solution composition and hydrodynamic conditions

Fouling of reverse osmosis (RO) and nanofiltration (NF) membranes by humic acid, a recalcitrant natural organic matter (NOM), was systematically investigated. The membrane flux performance depended on both hydrodynamic conditions (flux and cross-flow velocity) and solution composition (humic acid concentration, pH, ionic strength, and calcium concentration), and was largely independent of virgin membrane properties. While increasing humic acid concentration and ionic strength, and lowering cross-flow velocity affected flux performance moderately, severe flux reduction occurred at high initial flux, low pH, and high calcium concentration. At a calcium concentration of 1 mM, all the membranes exhibited an identical stable flux, independent of their respective intrinsic membrane permeabilities. The effect of solution composition was more significant at higher fluxes. Improved salt rejection was observed as a result of humic acid fouling, which was likely due to Donnan exclusion by humic material close to membrane surfaces. Greater rejection improvement was observed for membranes with rougher surfaces.     

Modeling permeation of volatile organic molecules through reverse osmosis spiral-wound membranes

Reverse osmosis is an interesting process to eliminate organic solutes from distillery condensates before recycling them into the fermentation step. However, organic solutes transport phenomena through reverse osmosis membranes are specific. Rejection and sorption of five compounds were studied on a brackish water membrane. Acetic acid and 2,3-butanediol were not sorbed on the membrane while furfural and 2-phenylethanol presented strong sorption following the Langmuir pattern. These sorption effects coupled with solute molecular weight (MW) led to low rejections of acetic acid and furfural (30–60%) and high rejections of 2,3-butanediol and 2-phenylethanol (80–98%). With intermediate sorption and MW, butyric acid showed rejections between 70 and 80%. A modified solution-diffusion model was developed to take into account the sorption pattern and predict the concentration profile along the membrane on the retentate and permeate sides. Equilibrium properties were determined experimentally while transport properties were identified with data obtained from a synthetic condensate. This model was validated for various operating conditions with the synthetic and the industrial condensates. It was then used to simulate the influence of the recovery rate on the retentate and permeate concentrations. It showed the behavior differences between solutes with a linear sorption and solutes with a saturating sorption.     

Development of novel backwash cleaning technique for reverse osmosis in reclamation of secondary effluent

The objective of the study was to further develop a novel cleaning technique for reverse osmosis in reclamation of municipal secondary effluent. This technique is a new backwash method via direct osmosis (DO) by intermittent injection of the high salinity (HS) solution without stopping of high pressure pump and it is environment and membrane friendly technique. In the study, DO-HS trials were carried out with a UF-RO pilot system which was operated on site with the secondary treated effluent as the raw feed. Different operating conditions for DO-HS treatment in the actual process were investigated. It was found that the operation for implementation of the DO-HS cleaning technique developed was easy. For the first time, the actual profiles of HS concentration, DO backwash flow rate, brine flow rate and permeate pressure during DO-HS treatment have been demonstrated. It was observed that turbidity of the brine stream during DO-HS treatment at 3 NTU was 5 times higher than that before DO-HS treatment. The results from this study have confirmed the previous hypothesis with DO-HS treatment that there would be a strong driving force for DO backwash to lift and sweep the foulants from the membrane surface which would be carried over to the brine. The optimal plant operating conditions in terms of RO feed flow rate, HS concentration and HS injection time are ready for the DO-HS method to be adopted and validated in a long-term continuous plant operation.     

Development of robust organosilica membranes for reverse osmosis

Hybrid organically bridged silica membranes have attracted considerable attention because of their high performances in a variety of applications. Development of robust reverse osmosis (RO) membranes to withstand aggressive operating conditions is still a major challenge. Here, a new type of microporous organosilica membrane has been developed and applied in reverse osmosis. Sol-gel derived organosilica RO membranes reject isopropanol with rejection higher than 95%, demonstrating superior molecular sieving ability for neutral solutes of low molecular weight. Due to the introduction of an inherently stable hybrid network structure, the membrane withstands higher temperatures in comparison with commercial polyamide RO membranes, and is resistant to water to at least 90 °C with no obvious changes in filtration performance. Furthermore, both an accelerated chlorine-resistance test and Fourier transform infrared analysis confirm excellent chlorine stability in this material, which demonstrates promise for a new generation of chlorine-resistant RO membrane materials.     

Structures of cellulose acetate membranes for reverse osmosis

Conclusions Studies under the scanning electron microscope have shown that the cellulose acetate membranes used for reverse osmosis are high-molecular-weight condensation structures of the cellular type resulting from the dropwise separation of a new liquid phase under diffusional enrichment of the polymer solution by water, the solvent. The pore diameter, and the total pore volume, both diminish on approaching the membrane surface; the diffuse character of the active layer traces back to the concentration distribution resulting from vaporization of acetone, the volatile component, from the acetone- formamide cellulose acetate solution.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 111–115, January, 1977.     

Grafting of polyaniline by a dynamic inverse emulsion polymerization technique onto reverse osmosis membranes as an antibiofouling agent

This study reports on the benefits of an in situ interfacial dynamic inverse emulsion polymerization process under sonication of aniline in the presence of a commercial reverse osmosis (RO) membrane. This polymerization method is simple and much faster (5‐15 min) than systems reported in the literature. During polymerization, the membranes are coated with polyaniline (PANI) as verified by high‐resolution scanning electron microscopy (HRSEM) images and Fourier‐transform‐infrared (FTIR) measurements. A colony‐counting antimicrobial activity test showed that whereas the reference RO membrane developed a large bacterial colony, the polyaniline‐coated RO membrane had no colonies at all. Surface resistivity was the lowest when the pH levels were below 6, which corresponded to the polyaniline‐grafted conductive layer. The membrane flow properties were only modified slightly as a result of the polyaniline grafting, compared with a pristine reference membrane.     

Adhesion of waste water bacteria to reverse osmosis membranes

A stirred cell was used to study initial adhesion of three sewage bacteria belonging to the genus Pseudomonas to the three reverse osmosis (RO) membranes BW30, PVD and CAB2, and the nanofiltration membrane NF45. Membranes were immersed in suspensions containing 10⁸ bacteria/ml for 10 min. All three strains were capable of rapidly colonising the four membranes, but to different extents. It was found that bacteria would sometimes aggregate upon adhering to particular RO membranes. The effects of solution ionic strength and pH, and conditioning of membranes (by prior exposure to filtrates of treated and untreated sewage) on the number of adherent bacteria were investigated. Minimal bacterial attachment occurred in a very low ionic strength milieu (deionised water). Salt concentrations corresponding to waste water and to twice that concentration resulted in significantly higher but statistically similar numbers of attached microbes. Adhesion of the three isolates was not affected by pH in the range of 4–8. The number of bacteria attaching to the membranes could be increased or reduced by conditioning films of sewage origin, conditioning films could also trigger or inhibit aggregation of adherent cells. Some surface properties of the membranes (roughness, hydrophobicity) and bacterial cells (electrophoretic mobility, functional groups by affinity chromatography) were also investigated.     

Surface modification of commercial composite polyamide reverse osmosis membranes

Radical grafting of two monomers, methacrylic acid and polyethylene glycolmethacrylate, onto commercial composite polyamide reverse osmosis membranes was performed. A redox system was used for initiation, and grafting was performed in an aqueous medium at room temperature. Surface grafting was characterized by ATR-FTIR, ESCA and streaming potential measurements. It was found that the membranes were surface modified without damage to their transport properties.     

Produced water treatment by nanofiltration and reverse osmosis membranes

Produced water, water that is co-produced during oil and gas manufacturing, represents the largest source of oily wastewaters. Given high oil and gas prices, oil and gas production from non-conventional sources such as tar sands, oil shale and coal bed methane will continue to expand resulting in large quantities of impaired produced water. Treatment of this produced water could improve the economic viability of these oil and gas fields and lead to a new source of water for beneficial use.Two nanofiltration and one low-pressure reverse osmosis membrane have been tested using three produced waters from Colorado, USA. The membranes were analyzed before and after produced water filtration using field emission scanning electron microscopy (FESEM), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). In addition, membrane–water contact angles have been measured. XPS data indicate adsorption of organic and inorganic species during filtration. FESEM and ATR-FTIR data support theses findings. Water contact angles indicate the effect of membrane hydrophilicity on fouling. Our results highlight the value of using multiple surface characterization methods with different depths of penetration in order to determine membrane fouling. Depending on the quality of the produced water and the water quality requirements for the beneficial uses being considered, nanofiltration may be a viable process for produced water treatment.     

Dynamics of water sorbed in reverse osmosis polyamide membrane

Dynamical motion of water sorbed in reverse osmosis polyamide membrane (ROPM) material is reported as studied by quasielastic neutron scattering (QENS) technique. The ROPM studied here has pore size of 4.4 Å as determined by positron annihilation lifetime spectroscopy. Analysis of the QENS data showed that diffusion behavior of the water within the membrane is describable by random jump diffusion model. A much longer residence time is found as compared to bulk water. Positive shift of the freezing point as observed in differential scanning calorimetry indicates presence of strong attractive interaction corroborating the slower diffusivity as observed in QENS.     

PEG-coated reverse osmosis membranes: Desalination properties and fouling resistance

This study focuses on the use of surface-coated reverse osmosis (RO) membranes to reduce membrane fouling in produced water purification. A series of crosslinked PEG-based hydrogels were synthesized using poly(ethylene glycol) diacrylate as the crosslinker and poly(ethylene glycol) acrylate, 2-hydroxyethyl acrylate, or acrylic acid as comonomers. The hydrogels were highly water permeable, with water permeabilities ranging from 10.0 to 17.8 (L μm)/(m² h bar). The hydrogels were applied to a commercial RO membrane (AG brackish water RO membrane from GE Water and Process Technologies). The water flux of coated membranes and a series-resistance model were used to estimate coating thickness; the coatings were approximately 2 μm thick. NaCl rejection for both uncoated and coated membranes was 99.0% or greater, and coating the membranes appeared to increase salt rejection, in contrast to predictions from the series-resistance model. Zeta potential measurements showed a small reduction in the negative charge of coated membranes relative to uncoated RO membranes. Model oil/water emulsions were used to probe membrane fouling. Emulsions were prepared with either a cationic or an anionic surfactant. Surfactant charge played a significant role in membrane fouling even in the absence of oil. A cationic surfactant, dodecyltrimethyl ammonium bromide (DTAB), caused a strong decline in water flux while an anionic surfactant, sodium dodecyl sulfate (SDS), resulted in little or no flux decline. In the presence of DTAB, the AG RO membrane water flux immediately dropped to 30% of its initial value, but in the presence of SDS, its water flux gradually decreased to 74% of its initial value after 24 h. DTAB-fouled membranes had lower salt rejection than membranes not exposed to DTAB. In contrast, SDS-fouled membranes had higher salt rejection than membranes not exposed to SDS, with rejection values increasing, in some cases, from 99.0 to 99.8% or higher. In both surfactant tests, coated membranes exhibited less flux decline than uncoated AG RO membranes. Additionally, coated membranes experienced little fouling in the presence of an oil/water emulsion prepared from DTAB and n-decane. For example, after 24 h the water flux of the AG RO membrane fell to 26% of its initial value, while the water flux of a PEGDA-coated AG RO membrane was 73% of its initial value.     

二维材料用于渗透能转换的研究进展

在河水与海水的交界处实现渗透能提取与捕获是解决未来能源危机的重要方式之一. 渗透能因为储量大, 容易获取以及绿色可持续的优势受到广泛关注. 反向电渗析技术是一种能够有效捕获渗透能的方法之一, 目前已经得到了深入的研究与发展. 离子交换膜是反向电渗析技术转换渗透能的关键组件, 其性能的优异程度决定能量转换效率的高低. 常见的膜材料主要是高分子聚合物及其改性化合物, 最近一些二维材料如石墨烯、 氧化石墨烯、 二硫化钼、 各种框架材料及其改性复合物因优异的选择性离子传输、 纳米级通道、 丰富的表面功能基团以及可修饰性成为捕获渗透能的重要膜材料. 本文综合评述了二维材料作为离子传输通道的类型以及相应的传输机理; 例举了二维材料及其复合物的设计方案和在渗透能转换方面的具体应用; 最后提出了目前二维材料在渗透能转换领域中面临的挑战以及未来的发展方向.