Dynamically formed polyelectrolyte membranes on partially cured cellulose acetate

The mechanism of dynamic formation of polyelectrolyte membranes was investigated. Quaternized 2-polyvinylpyridine (2-PVP) and polyvinylamine hydrochloride (PVA) were deposited on partially cured asymmetric cellulose acetate (CA) membranes. The average pore size distribution in (CA) membranes was controlled by changing their annealing temperature within the range of 70 to 77°C. Hyperfiltration measurements revealed that a prolonged exposure of such CA-membranes to diluted polyelectrolyte solutions suffices to cause a substantial increase in their salt rejection properties (R). The increase in R depends on the initial salt rejection (R₀) of the CA membrane. ΔR_max were observed forR_o = 50 and R₀= 43 for 2-PVP and PVA respectively.Streaming potential measurements indicate the presence of acidic groups on the surface of untreated CA membranes and their interaction with the quaternary amino groups after treatment with 2-PVP. Spectrophotometric measurements do not, however, indicate that a significant amount of 2-PVP may be adsorbed on a non-porous surface of powdered acetyl cellulose. The experimental findings have been rationalized in terms of a snake-cage mechanism of formation of the dynamic membrane. Matching of the molecular weight distribution of the polyelectrolyte and the pore size distribution in the support is thus an essential condition for the formation of such membranes.     

Preparation and properties of poly(acrylic acid) composite membranes

A new type of membrane has been prepared for hyperfiltration (reverse osmosis) desalination that is essentially a very thin polyelectrolyte membrane. It is prepared by casting an aqueous solution of a polyelectrolyte, specifically poly(acrylic acid) (PAA), directly on one surface of a finely porous support membrane. In hyperfiltration tests, these composite membranes exhibit desalination performance comparable in dilute solutions to that observed with cellulose acetate membranes of the Loeb-Sourirajan type. The water flux through these membranes is linear in the pressure up to 100 atm. Salt rejection is a function of pressure; it is also a function of the concentration of the feed solution and the charge of the counterion, in qualitative agreement with the Donnan ion-exclusion mechanism. Typical long-term results range from water fluxes of 2 × 10^?3 g/cm²-sec (50 gal/ft²-day) and 80% salt rejection to 0.2 × 10^?3 g/cm²-sec (5 gal/ft²-day) and >99.5% salt rejection at 1500 psi with 0.3 wt-% NaCl. These membranes appear to be useful for brackish water desalination.     

Permeation properties of glycol chitosan–mucopolysaccharide complex membranes

The permeation properties of glycol chitosan–chondroitin sulfate C and glycol chitosan–heparin complex membranes have been studied. Discussion is devoted primarily to the desalination properties in reverse osmosis. Salt rejection and flux were dependent both on the composition of the membranes and the pH of the feed solutions. The results indicate that these complexes behave like polyampholytes. Minimum salt rejection occurs around the isoelectric point. Another characteristic of these membranes is a large pressure dependence of the salt rejection. This could be interpreted by assuming the compaction of these highly swollen membranes at high pressure.     

Control of organic fouling at two seawater reverse osmosis plants

Organic fouling of seawater Reverse Osmosis (RC) membranes is a phenomenon not well understood; it can result in a loss of membrane productivity and salt rejection properties. Two seawater RO plants using DuPont B-10 hollow fiber permeators had experienced organic fouling and were studied.The two plants used different sources of feedwater; one RO plant at Culebra, Puerto Rico, used open seawater; while the other RO plant at Grand Cayman Island, British West Indies, used a sea well. Both feed water sources possessed high concentrations of soluble organics (40–80mg/1) which were mainly humic acids. In an attempt to remove these organics with in-line cationic polyelectrolyte coagulation, the plants experienced organic fouling which caused excessive loss of productivity and salt rejection; both plants initially failed their acceptance tests.It was discovered that the fouling was actually caused by interactions between the humic acids and in-line cationic, polyelectrolyte coagulants which were not removed by in-dedth and cartridge filtration. Rather than remove the humic acid material, acid addition was initiated and in-line cationic coagulants use discontinued to keep the humic acids soluble. It should be noted that with the open seawater intake ferrous sulfate was still used to remove colloidal material and reduce the SDI. Both plants subsequently have passed their 720-hour acceptance test.Culebra and Grand Cayman plants have now exceeded design specification for both productivity and salt rejection. The aramid hollow fiber permeators on acidified feed have shown 100% rejection of these organics at both 25% and 50% conversion and organic fouling has not been evident.     

Rejection spectra of reverse osmosis membranes degraded by hydrolysis or chlorine attack

Cellulose acetate (CA) and polyamide (PA) membranes were subjected to accelerated degradation for up to 24 h to determine the effects of curing temperature and degradation time on transmembrane flux and on the rejection by the membranes of salts (NaCl and CaCl₂), hexose sugars and polysaccharides. Commercial PA membranes, and one set of CA membranes made in our laboratory lost all salt rejection ability after 24 h of degradation but maintained 100% rejection of polysaccharides, and thus still functioned as ultrafilters.A second set of CA membranes was cured at different temperatures. CA membranes annealed at 45°C and 55°C degraded faster than those annealed at 75°C, based on their faster loss of salt rejection ability. This rapid degradation of the membranes annealed at lower temperatures, coupled with their inherent poor initial salt rejection enforces the conclusion that membranes to be used in reverse osmosis application should not be cured below about 55°C.Based on the study, a simple test procedure is proposed to distinguish between chemical degradation and gross failure in malfunctioning reverse osmosis units.     

Temperature Effects on Concentration Polarization Thickness in Thin‐Film Composite Reverse Osmosis Membranes

Continuous research and development of reverse osmosis (RO) technologies has led to the production of membranes that are very effective with high salt rejection abilities. As temperature is one of the factors that affects salt rejection capabilities in membranes, this paper investigates the effect of temperature on the thickness of the concentration polarization layer (CPL) deposited on thin‐film composite seawater RO membranes. Two types of membranes were studied: those with ex situ macromolecules and those with in situ macromolecules. FilmTec's reverse osmosis system analysis design software was used to predict the variation of salt rejection and permeate flow rate with temperature. The impact of these variations on the thickness of the CPL was analyzed for different polyamide concentrations in the membrane.     

Preparation and properties of a composite charged membrane

In order to develop a low pressure desalination membrane with fixed ionic charges, we made use of the normally unwanted crosslinking tendency in preparing the polyelectrolyte poly(styrenesulfonate) by sulfonation of polystyrene. After dipcoating a poly(sulfone) or poly(phenylene oxide) UF membrane with a dilute soluton of this polyelectrolyte in water in the presence of some free sulfuric acid and silversulfate, fixation and cross-linking of the coating polymer took place by a heat treatment.Different membrane properties could be obtained by varying the pore size of the supporting UF membranes, and by variation in the coating polymer (M.W., concentration) and sulfuric acid concentration. We found that fluxes sometimes decline drastically in comparison with the original pure water flux; salt rejection values (at 1.5 g/l NaCl concentration and 0.5 MPa) never are very high (≤60% for monovalent anions). The most important potential application for these membranes lies in their non-fouling properties.     

聚电解质涂层/纳米纤维膜复合滤膜的制备

以静电纺丝法制备的PAN纳米纤维多孔膜为基膜,以层层自组装成膜技术制备的壳聚糖-海藻酸钠聚电解质涂层为表面选择性涂层,成功制备了聚电解质涂层/纳米纤维膜复合滤膜。用扫描电镜(SEM)对复合膜的微观形态进行表征。在操作压力为0.7MPa的条件下,分别过滤纯水以及质量浓度为1000mg/L的NaCl和500mg/L的MgSO4溶液,测试结果表明:盐离子截留率随聚电解质涂层层数的增加而增加,但同时复合膜的水通量随之明显降低。当聚电解质涂层层数为10时,水通量均在18L/m·2h左右,对MgSO4截留率为64.22%,对NaCl截留达到52.45%。     

Performance of cellulose acetate butyrate membranes in hyperfiltration of sodium chloride and urea feed solution

Cellulose acetate butyrate (CAB) membranes gave high salt and urea rejection with a water flux of about 3 gfd (gallons/ft² · day) during hyperfiltration at 600 psig. Evidence was obtained which indicated that the CAB membranes used in this work were asymmetric. Membrane heat treatment increased urea rejection significantly while salt rejection was invariant, and water flux decreased. An increase in feed solution temperature caused a significant increase in water flux and a small decrease in urea and salt rejection. Increasing the pressure increased water flux and urea and salt rejection. During a 400-hr life test, the water flux decreased by about 25% while urea rejection increased and salt rejection was invariant. The influence of pressure, membrane heat treatment, and compaction during CAB membranes life testing on urea and salt rejection provided evidence that these two solutes were rejected by somewhat different mechanisms. Salt rejection was consistent with a solution–diffusion mechanism for membrane transport and uncoupled flow while changes in urea rejection with pressure, membrane heat treatment, and compaction during life testing suggested that urea was at least partially rejected by membrane exclusion resulting from geometric factors.     

Influence of colloidal fouling and feed water recovery on salt rejection of RO and NF membranes

The influence of colloidal fouling and feed water recovery (or concentration factor, CF) on salt rejection of thin-film composite reverse osmosis (RO) and nanofiltration (NF) membranes was investigated. Fouling experiments were carried out using a laboratory-scale crossflow test unit with continuous permeate disposal to simulate the CF and recovery as commonly observed in full-scale RO/NF systems. For feed waters containing only salt (NaCl), permeate flux declined linearly as CF was increased and salt rejection was nearly constant for both RO and NF membranes. On the other hand, a sharp decrease in permeate flux and significant decline in salt rejection with increasing CF were observed under conditions where colloidal fouling takes place. For both RO and NF membranes, the marked permeate flux decline was attributed to the so-called “cake-enhanced osmotic pressure”. The decline in salt rejection when colloidal fouling predominated was much more substantial for NF than for RO membranes. In all cases, the decline in salt rejection was higher under conditions of more severe colloidal fouling, namely at higher ionic strength and initial permeate flux.     

Swelling behavior of chitosan/pectin polyelectrolyte complex membranes. Effect of thermal cross-linking

Summary Membranes of the polyelectrolyte complex between chitosan and pectin were prepared by precipitating the complex from a mixture of both polysaccharides. It was shown that the swelling kinetics of these membranes follows a Fickean behavior. The membranes were heated at 120 °C in order to convert the –NH₃^+-OOC- salt bonds into amide bonds. The thermally treated membranes were stable in strongly acid and basic media. The extent of amide bond formation was followed by FTIR spectroscopy. It was found that as the reaction time increased, both the absorbance ratio A₁₇₄₄/A₁₀₈₂ and the maximum swelling of the membrane decreased. The surface morphology of the membranes did not vary appreciably with the thermal treatment.     

低渗油田压裂控水用聚电解质复合溶液性能研究

将阴离子聚丙烯酰胺(HPAM)与阳离子聚合物(DP-1)应用溶液共混法,制备了低渗油田压裂控水用聚电解质复合溶液。研究了阴阳离子聚合物稳定共存的条件,评价了复合溶液的耐温抗盐性能、抗剪切耐冲刷性能以及控水性能。结果表明,在一定量外加盐(KCl)的作用下,阴阳离子聚合物可形成稳定的低粘度聚电解质复合溶液,且具有良好的耐温性和抗盐性;随剪切速率的增加,复合溶液粘度先降低后又有增加;在60℃下岩心流动实验中,盐水驱替100 PV时残余阻力系数为2.66,具有良好的耐冲刷性;水相渗透率降低90.53%,油相渗透率降低14.60%,具有明显的不等比例降低油水相渗透率特征,可用于近水或高含水低渗油层控水压裂改造,降低现场施工风险。     

Reverse osmosis performance of organosilica membranes and comparison with the pervaporation and gas permeation properties

Hybrid organosilica membranes were successfully prepared using bis(triethoxysilyl)ethane (BTESE) and applied to reverse osmosis (RO) desalination. The organosilica membrane calcined at 300^°C almost completely rejected salts and neutral solutes with low‐molecular‐weight. Increasing the operating pressure led to an increase in water flux and salt rejection, while the flux and rejection decreased as salt concentration increased. The water permeation mechanism differed from the viscous flow mechanism. Observed activation energies for permeation were larger for membranes with a smaller pore size, and were considerably larger than the activation energy for water viscosity. The organosilica membranes exhibited exceptional hydrothermal stability in temperature cycles up to 90^°C. The applicability of the generalized solution‐diffusion (SD) model to RO and pervaporation (PV) desalination processes were examined, and the quantitative differences in water permeance were accurately predicted by the application of generalized transport equations. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1298–1307, 2013     

Properties of polyethersulfone ultrafiltration membranes modified by polyelectrolytes

This study reports on the surface modification of ultrafiltration membranes using the layer-by-layer (LbL) technique. The novelty of this work resides in the LbL assembly of charged polyelectrolytes by electrostatic adsorption directly onto the ultrafiltration membranes without any prior treatment of the surface. Polyethersulfone ultrafiltration membranes have been employed for the deposition of branched poly(ethyleneimine) and poly(sodium 4-styrene sulfonate) to create a thin polyelectrolyte film on their surface. The modified membranes are characterized by their permeability and molecular weight cut-off (MWCO) value. Experiments show that the deposited polyelectrolyte layer causes a decrease in the permeability due to the additional resistance of the layers. However, the MWCO value is shifted meaning a better rejection of the dextran solution is achieved. Thus, the LbL assembly of polyelectrolyte multilayers on the surface of the membrane makes it possible to convert a membrane with open structure to a membrane with denser active layer.     

Electrolyte removal by mixed matrix membranes based on polyurethane

The membrane processes play a significant role in the water and wastewater treatment to remove dissolved solids, especially electrolytes. In this study, the asymmetric mixed matrix membranes based on polyurethane and SAPO-5 zeolite were used on electrolyte (NaCl) removal from water. Using a low operating pressure, the membrane performances (i.e., pure water permeation, flux and salt rejection) were measured. All membranes were showing an increase in water flux when the pressure was increased. This situation shows that the produced membranes were stable in producing flux and were suitable to be used to proceed for membrane testing process. Based on the results obtained, rejection of salt water increased as the pressure given increased for each membrane. The mixed matrix membranes showed the high rejection for the salt water (NaCl 0.02 M). This shows the good performance in both flux and rejection, and even achieves 98% rejection for the NaCl 0.02 M. Based on the experimental results, it is believed that these mixed matrix membranes are suitable for the electrolytes removal applications.     

Preparations and properties of sodium alginate formed-in-place membranes

Sodium alginate formed-in-place membranes were formed on a macroporous titanium dioxide membrane substrate at pH 3.3, 6.5, and 10.5. To investigate the rate and the mechanism of the membrane formation, the dependence of the pressure-to-flux ratio, P/J, on time, t, during the formation was evaluated using diagnostic graphs; (P/J)², (P/J)^1/2 and —In(P/J) vs. t. The microfiltration properties of the membranes were investigated by determining the permeability, J/P, and the rejection of a protein, bovine serum albumin (BSA). in 1 g/L solutions as a function of the concentration of added KCl. The stability of the membranes was evaluated by comparing the ratio of the resistances of the membranes at the end of the formation, R, after crossflow rinsing, R_m, and after crossflow rinsing following the BSA microfiltration experiment, R_a. The linearity of the graphs of (P/J)² vs. t of the membranes formed in neutral or basic conditions indicated that the membranes were formed by deposition of a layer, or cake, of the polyelectrolyte on the substrate, while the membrane formed at lower pH was initially deposited as a layer followed by a more complex mechanism. Only the membranes formed pH 3.3 were stable to the crossflow water rinse and retained high BSA rejection at high ionic strength. Their permeabilities were about 50% lower than the permeabilities obtained with the membranes formed at higher pH. The BSA rejection results imply that a continuous sodium alginate membrane is present for the membranes formed at pH 3.3 and that membranes retaining a macroporous structure are present for the membranes formed at pH 6.5 and 10.5. © 1996 John Wiley & Sons, Inc.     

Using nanocomposite materials technology to understand and control reverse osmosis membrane compaction

Composite reverse osmosis (RO) membranes were formed by interfacial polymerization of polyamide thin films over pure polysulfone and nanocomposite-polysulfone support membranes. Nanocomposite support membranes were formed from amorphous non-porous silica and crystalline microporous zeolite nanoparticles. For each hand-cast membrane, water flux and NaCl rejection were monitored over time at two different applied pressures. Nanocomposite-polysulfone supported RO membranes generally had higher initial permeability and experienced less flux decline due to compaction than pure polysulfone supported membranes. In addition, observed salt rejection tended to increase as flux declined from compaction. Cross-sectional SEM images verified significant reduction in thickness of pure polysulfone supports, whereas nanocomposites better resisted compaction due to enhanced mechanical stability imparted by the nanoparticles. A conceptual model was proposed to explain the mechanistic relationship between support membrane compaction and observed changes in water flux and salt rejection. As the support membrane compacts, skin layer pore constriction increased the effective path length for diffusion through the composite membranes, which reduced both water and salt permeability identically. However, experimental salt permeability tended to decline to a greater extent than water permeability; hence, the observed changes in flux and rejection might also be related to structural changes in the polyamide thin film.     

Acrylic acid γ-ray irradiation-grafted nylon 4 membranes

This study was to improve the performances of nylon 4 membranes for washing waste-water treatment of nuclear power plants, e.g., removal of detergent and salt by membranes. The effects of the degree of grafting and ionization on the reverse osmosis performances of acrylic acid (AA)-grafted nylon 4 membranes by γ-ray irradiation modification were investigated. The relationships of operating conditions, such as feed concentrations of salt and detergent, operating temperature, and pressure, and the performances of water flux and solute rejection of the prepared membranes were obtained. Water flux of the prepared membranes was highly sensitive with the operating temperature. It was found that an increase in the operating pressure could increase the water flux and the impaction effect directly. Water flux and salt rejection were significantly improved by both ionized and nonionized AA-grafted nylon 4 membranes compared to ungrafted nylon 4 membranes. Water flux increased rapidly and solute rejection dropped off slightly as the grafted membranes were ionized. The 100% detergent rejection could be obtained by the nonionized AA-grafted nylon 4 membranes with 38.6 and 69.6% degrees of grafting under various operating conditions. Results obtained showed that these modified nylon 4 membranes were usable for washing waste-water treatment of nuclear power plants. © 1993 John Wiley & Sons, Inc.     

四种纳滤膜对高盐废水分盐效果分析

为了使煤化工零排放高盐废水分盐产出高质量NaCl和Na2SO4结晶盐,提高废水处理过程的分盐效率,以宁东某煤化工零排放高盐废水的水质情况模拟配制了无机盐溶液,选取了膜1、膜2、膜3和膜4等4种商用纳滤膜,探讨了其对模拟高盐废水中的常规离子(Mg2+、Ca2+、Na+、K+、SO42-、Cl-、NO3-)的截留率,并考察...     

Preparation of sulfonated polysulfone/polysulfone and aminated polysulfone/polysulfone blend membranes

Sulfonation and amination of polysulfone (PSf) were performed in this study to improve the hydrophilicity of PSf membranes. The sulfonated polysulfone (SPSf) and aminated polysulfone (APSf) membranes with a higher degree of reaction exhibited a higher water flux and worse mechanical strength than that of the original PSf membranes. Therefore, SPSf/PSf and APSf/PSf blended membranes were prepared in this study to improve their individual properties. By altering the formulations of casting solutions and forming conditions of the membranes (e.g., blending ratios of both polymers, additives, evaporation time, and gelation temperature), different SPSf/PSf and APSf/PSf blending membranes were prepared; and their performance in water flux and salt rejection were measured and are discussed. A difference in salt rejection was also observed between both SPSf/PSf and APSf/PSf blending membranes that rejected the various salts. Experimental results indicated that water flux increased and salt rejection decreased with an increase of the SPSf/PSf blending ratio from 1: 9 to 2: 1. The order of salt rejection, in which the SPSf/PSf blended membranes rejected four varieties of salts, was Na₂SO₄ > MgSO₄ > NaCl > MgCl₂. Furthermore, the opposite order was obtained by the APSf/PSf blended membranes. © 1996 John Wiley & Sons, Inc.