Removal of divalent cations reduces fouling of ultrafiltration membranes

The effect of divalent ions on hydraulic irreversible fouling of ultrafiltration membranes was studied. Not only the effect of removing divalent ions by pretreatment of raw water with ion exchange is quantitatively studied, but also the effects of different types of backwash water are considered. By replacing divalent ions with sodium in cation exchange, the amount of hydraulic irreversible fouling (remaining fouling after backwashing) is reduced by at least 60%. When adding either calcium or magnesium to water treated with cation exchange, a linear relation is found between the ion concentration and the irreversible fouling rate. The effects of calcium and magnesium are identical when the concentrations are expressed in mol/L. Removing divalent ions from the backwash water does not affect irreversible fouling, but when using MilliQ water as backwash water, irreversible fouling can (almost) completely be prevented.     

CELLULOSE ACETATE AND EPOXY RESIN BLEND ULTRAFILTRATION MEMBRANES: REPARATION,CHARACTERIZATION, AND APPLICATION

ABSTRACT

Membranes based on cellulose acetate used in ultrafiltration applications lack good, chemical, mechanical and thermal resistance. In order to prepare membranes with improved properties, modification of cellulose acetate with epoxy resin through solution blending was attempted. In the present work, the membrane casting solutions with different polymer blend compositions of cellulose acetate and diglycidyl ether of bisphenol-A (DGEBA) were prepared at 30±2°C. The maximum percent compatibility of the two polymers, cellulose acetate and diglycidyl ether of bisphenol-A, was estimated to be 60/40%. Ultrafiltration blend membranes based on various blend compositions were prepared, characterized in terms of compaction, pure water flux, water content, membrane hydraulic resistance and molecular weight cut-off. The application of these membranes, in rejection of proteins of various molecular weights, are discussed.     

Aspects of ionic diffusion through thick matrices of charged particles

A review of the literature on ionic diffusion through matrices of charged particles shows that many workers have reported higher diffusivity of co-ions than of counterions. If these observations are correct then the requirement of electroneutrality of bulk solutions is violated. In those experiments other, not looked for, ions must have been taking part. It therefore appears that complete chemical analyses of solutions of both up- and downstream sides should be carried out, otherwise misleading inferences may be drawn, with practical consequences. Some researchers have studied transport of water under osmotic pressure differentials across clay membranes. However, a diffusing ion, during its transport from higher concentration to lower concentration, always carries its water of hydration. This second water transport process has not received attention. This suggests that at a critical concentration two transport processes will cancel each other. Attention has been drawn to this second type of water flow and its consequences. In ionic diffusion, each side of the membrane-outside solution interface is subject to the effects of the Nernst layer and Donnan zone. However, these layers and zones are seldom treated quantitatively. This has been carried out in this paper by two different methods. Both methods give identical thicknesses of the Nernst layer and Donnan zone. Finally, the effects of the presence of soluble bivalent salts on the diffusivity of different types of ions have been commented up on. In these cases some of the co-ions diffuse to the upstream side.     

A comparison between polysulfone,zirconia and organo-mineral membranes for use in ultrafiltration

In this paper, representative polymeric (a PSf/PVP membrane), ceramic (a ZrO₂ membrane) and organo-mineral (a ZrO₂/PSf membrane) ultrafiltration membranes, all in the tubular configuration, are being compared for their basic membrane properties, and for the typical ultrafiltration application of protein recovery of cheese whey. These three different membranes with a quite similar pore size (the cut-off values for each of the three membranes were comprised between 25 000 and 50 000 Dalton) showed pure water permeability coefficients between 135 and 1250 l/h m² bar. The highest pure water flux was found for the organo-mineral membrane, the lowest for the polymeric membrane. By FESEM analysis of the top-surfaces (skin) of both the PSf/PVP and the ZrO₂/PSf membrane a strong difference in surface-porosity was found. These results were claimed to partially explain the difference in pure water flux. From SEM pictures of the cross-section of the ZrO₂/PSf membrane it could also be seen that the skin layer thickness is smaller, at these places where particles are present near the skin-surface, compared to the rest of the membrane as well as to the skin of the PSf/PVP membrane. These latter observations were also used to further explain the flux difference between the PSf/PVP and the ZrO₂/PSf membrane.     

Preparation of ultrafiltration membranes of HCEC and CTA blend, and studies of resistance to microbiological degradation and other properties

Blends of high cyanoethylated cellulose (HCEC) (D_s = 2.5) and cellulose triacetate (CTA) (D_s = 2.8) were prepared for evaluation as ultrafiltration membranes. The rejection R_a and water permeability (P_w) of these membranes with respect to bovine serum albumin (BSA), were measured with the variations in composition and conditions of preparation. It was possible to define the reaction conditions and compositions that offered optimum performance with respect to R_a and J_w, and resistance to microbial degradation as well as acid and alkali hydrolysis. The morphology of membrane thereby obtained was characterized by scanning electron microscopy (SEM). The UF membrane cast from a solution of 1,4-dioxane: 12.5 g, acetone: 12.5 g, HCEC: 1.5 g, CTA: 3 g and PEG (MW = 1000): 10.5 g showed the optimum performance of P_w = 10 × 10⁻¹⁰ m³/(m² s Pa), BSA Rejection (R_a) = 99%, (J_w = 17.3 m³/(m² day) at ΔP = 0.2 MPa) and a molecular weight cut off at 4.9 × 10⁴ Da. After immersion in mud for 170 days, reduction of R_a was only 8% for UF membrane containing 10 wt% HCEC blend, and was not observed to be obviously different from the membrane containing more than 30 wt% HCEC blend. The higher permeability and rejection of blend membrane may be attributable to the network structure formed via CTA and HCEC penetration.     

Experimental study of ultrafiltration membrane fouling by sodium alginate and flux recovery by backwashing

Membrane fouling is the major limitation for a broader application of membrane technology. One of the main causes of membrane fouling in advanced wastewater reclamation and in membrane bioreactors (MBR) are the extracellular polymeric substances (EPS). Among the main constituents in EPS, polysaccharides are the most ubiquitous. This study aims at a better understanding of the fouling mechanisms of EPS and the efficiency of backwashing technique, which is applied in practice to restore membrane flux. For that purpose, the evolution of fouling by sodium alginate, a microbial polysaccharide, is studied in ultrafiltration. Fouling experiments are carried out in a single fiber apparatus, aiming at identifying the significance of distinct fouling mechanisms and their degree of reversibility by backwashing. An important parameter considered in the study is the concentration of calcium ions, which promote sodium alginate aggregation and influence the rate of flux decline, the reversibility of fouling and rejection. A rapid irreversible fouling takes place due to internal pore constriction, at the beginning of filtration, followed by cake development on the membrane surface. With increased calcium addition, cake development becomes the dominant mechanism throughout the filtration step. Furthermore, fouling reversibility is increased with the increase of calcium concentration. A unique behavior of sodium alginate solution in the absence of calcium is also noted, i.e. the formation of a labile layer on the membrane surface, which is affected by the small cross-flow that exists inside hollow fibers, even in the nominally dead-end mode of operation.     

Fractionation of whey-derived peptides using a combination of ultrafiltration and nanofiltration

This paper describes the fractionation and further isolation and characterisation of peptides and proteins present in sweet whey by means of ultrafiltration using a regenerated cellulose membrane with a nominal molar mass cut-off value of 10 kg/mol and nanofiltration through sulphonated polyether sulphone membrane with a cut-off of 1 kg/mol. The concentration of whey proteins was done below the critical flux. The sieving coefficients for the whey components (proteins, lactose and salts) were estimated. Whey proteins were completely rejected by the ultrafiltration membrane. Size exclusion chromatography (SEC) and matrix-assisted laser desorption ionisation time-of-flight (MALDI-TOF) mass spectrometry were used to evaluate the molar masses of the peptide fractions that were present in the whey permeates. Nanofiltration of whey permeates obtained after ultrafiltration was conducted at two pH values (9.5 and 3.0) that corresponded to the different charged states of the membrane and of the peptides. The transmission of peptides, amino acids and lactose was found to be mainly affected by the permeability of the fouling layer. The selectivity of the nanofiltration membranes toward peptides compared to lactose was calculated as 0.82 and 6.81 at pH 9.5 and 3.0, respectively.     

Preparation of NOx modified PMMA-EGDM composite membrane for the recovery of chromium (VI)

A non-interpenetrating cross-linked poly (methyl methacrylate-ethylene glycol dimethacrylate) copolymer ultrafiltration membrane on a microporous ceramic support has been prepared from the monomers in two stages. The polymer membranes thus obtained have been nitrated using NO_x (a mixture of NO and NO₂) by the gas phase reaction at 80 °C. Separation experiments on the chromium (VI) salt solution have been carried out using unmodified (giving 68% rejection per pass) and nitrated membranes (giving as much as 67% rejection per pass). For nitrated membranes, the water flux and the solute flux increased with time of nitration about hundred folds because of the increase in the hydrophilicity as well as the pore size.     

膜亲和色谱用于胰蛋白酶抑制剂的分离

分别采用酰化-胺化和氯甲基化-胺化对聚砜进行化学修饰,用相转化法制成平均孔径为450nm的超滤膜,经重氮化反应,共价键合上具有活性的胰蛋白酶。其相对活力可达10200 u/g。每克化学改性聚砜膜上可固载化上15 mg胰蛋白酶。用此酶膜对胰蛋白酶抑制剂进行了亲和分离,一次可得6.5mg纯胰蛋白酶抑制剂。     

A comparison of liquid–liquid porosimetry equations for evaluation of pore size distribution

This paper reviews and numerically tests many of the methods for the determination of pore size distribution of liquid membranes by liquid–liquid porosimetry. The flux through membranes was defined for flow of two immiscible liquids when drops or a liquid jet is formed, as well as the case when the interface is forced out by sufficient pressure. Several methods from literature for the determination of pore size distribution, some variations of these, and one new method are presented with a consistent theoretical basis. Using numerical tests it was found that all methods were very sensitive to measurement noise as low as ±0.1%, and that some form of data smoothing, such as a smoothing spline, was required to obtain a satisfactory distribution. The effects of elastic and permanent membrane compression were tested and a method was proposed to reduce the resulting error. A method based on the ratio of flux liquids with and without a liquid–liquid interface was recommended as it was less sensitive to the effects of compression in some cases and it provides a check when compression is not repeatable.