Determination of equilibrium distribution constants of phenol between surfactant micelles and water using ultrafiltering centrifuge tubes

Equilibrium distribution constants, K_s, of phenol between surfactant micelles and water have been determined by micellar enhanced ultrafiltration (MEUF) using commercial ultrafiltering centrifuge tubes. Three surfactants: sodium dodecyl sulphate (SDS), polyoxyethylene 20 cetyl ether (C16E20) and cetylpiridinium chloride (CPC) were tested with a 10 000 molecular weight cut off (MWCO) membrane. Additionally, membranes of 5000 and 30 000 MWCO were used for CPC. A phenomenological mathematical model has been proposed for the batch MEUF process and checked with the experimental permeate or retentate composition. The model is based on two assumptions: monomeric molecules are not rejected by the membrane and the rejection of micelles is independent of the retentate concentration. The measured micelles rejections for different surfactants and the equivalent molecular weight of the micelles are correlated and they are not significantly affected by the addition of phenol. The estimates of K_s for SDS and CPC agree with previously reported values determined by other methods. K_s values for CPC, calculated using 5000, 10 000 and 30 000 MWCO membranes, have not been significantly different. K_s estimate has allowed to predict the phenol permeate concentration measured in continuous tangential MEUF experiments.     

Pore size distribution of ceramic UF membranes by liquid–liquid displacement porosimetry

Commercial ceramic tubular membranes made by Tami^® have been characterized by several techniques. Their pore size distributions (PSD) have been obtained by liquid–liquid displacement porosimetry (LLDP).

Computerized image analysis (CIA) of SEM pictures has been used to get information on the width of the active layer of the studied membranes. These values of thickness have helped to evaluate the porosity of the membranes and to get representative radii from measurements of the permeability to several gases and liquids. A fully automated porosimeter designed by us has been used in the determination of pore size distributions. Results show a good accuracy and reproducibility of LLDP measurements.

Binary and ternary liquid mixtures have been used to wet and penetrate into the membrane pores when performing LLDP leading to quite similar results when an effective surface tension is assigned for the ternary mixture. This procedure can be used to calibrate the technique to be extended to thick ultrafiltration and even to nanofiltration membranes.     

Crossflow filtration of washed and unwashed yeast suspensions at constant shear under nominally sub-critical conditions

Operation at sub-critical fluxes can be used to control membrane fouling. The original definition of the critical flux stated that operation was sub-critical if no or negligible fouling occurred. Over time there has been a relaxing of the criteria and many now consider a low rather than zero rate of fouling to be indicative of sub-critical operation especially when dealing with complex feeds. Here the region of low fouling is termed “nominally sub-critical”. Unwashed yeast, washed yeast and extra polymeric substance (EPS) suspensions were filtered at controlled fluxes to investigate the role of cells and soluble components in nominally sub-critical conditions using ultrafiltration (UF) and microfiltration (MF) membranes. As the UF membrane could not be effectively cleaned it was not used in the later part of the study. Tracking of membrane resistance, of the 0.2 μm membranes was continued through the whole study. After the initial increase, rises very slowly, increasing on average only 0.4% after each experiment and cleaning cycle. For the MF membranes, the rate of fouling increased with increasing feed concentration, increasing membrane pore size and decreasing shear stress. The effect of increasing shear stress was to reduce the amount of reversible fouling but the irreversible component was invariant with shear stress for the range studied. Also the rate and reversibility of fouling were found to be sensitive to changes in pH. The sum of the rates of transmembrane pressure (TMP) rise for washed yeast cells and EPS suspensions were in all cases found to be lower than that for unwashed yeast. The origin of the additional resistance is discussed and other relevant literature reviewed.     

Distribution of Heavy Metals in Water and Soil Solutions Based on Colloid-Size Fractionation

The distribution of heavy metals Cr, Ni, Zn, Cu, Cd, and Pb among the truly dissolved (molecular weight cutoff <1 kD), colloidal (1 kD-0.20 µm and 0.20-0.45 µm) and particulate (>0.45 µm) fractions was investigated in the soil solution and surface water. In 15 soil solutions heavy metals were mainly present in the truly dissolved fraction (<1 kD) with occasional exceptions. And a good correlation was also found between the concentrations of metals in the truly dissolved fractions and the cation exchange capacity of soils. As for the surface water samples, the distribution pattern was more complicated one depending on metal properties and sampling sites.     

Structural and performance studies of poly(vinyl chloride) hollow fiber membranes prepared at different air gap lengths

Poly(vinyl chloride) hollow fiber membranes were prepared by the dry/wet and wet/wet spinning technique at different air gap lengths keeping all other spinning parameters constants. Mean pore size, pore size distribution and mean roughness of both the internal and external surfaces of the hollow fibers were determined by atomic force microscopy. Cross-sectional structure was studied by scanning electron microscopy. Ultrafiltration experiments of pure water and aqueous solutions of different solutes having different molecular weights (bovine serum albumin, polyethylene glycol and polyvinyl pyrrolidone) were carried out. It was found that the inner and outer diameters of the PVC fiber membranes decreased with the increase of the air gap distance due to the gravitational force effect. The hollow fiber membranes prepared without and with air gap distances up to 7 cm exhibited a quite symmetric cross-structure consisting of four layers, two small finger-like structure layers at both edges of the hollow fibers and two larger finger-like voids mixed with macrovoids layers in the middle of the cross-section. The outer-middle layer thickness decreased when the air gap distance was increased to 10 cm and disappeared from the cross-section of the hollow membranes prepared with higher air gap lengths than 15 cm. For all dry/wet spun PVC hollow fibers, the outer pore size and the pure water permeation flux both increased with the increase of the air gap distance. In contrast, the solute separation factor decreased with the air gap distance. This was related to the pore size of the external surface of the PVC hollow fibers.     

Synthesis and characterization of nanoporous polycaprolactone membranes via thermally- and nonsolvent-induced phase separations for biomedical device application

A family of crosslinked poly(ethylene glycol) diacrylate (XLPEGDA) materials was synthesized via free-radical photopolymerization of poly(ethylene glycol) diacrylate (PEGDA) solutions in water. These materials are potential fouling-resistant coatings for ultrafiltration (UF) membranes. PEGDA chain length (n = 10–45, where n is the average number of ethylene oxide units in the PEGDA molecule) and water content in the prepolymerization mixture (0–80 wt.%) were varied, resulting in XLPEGDA materials with water permeability values ranging from 0.5 to 150 L μm/(m² h bar). Generally, water permeability increased with increasing prepolymerization water content and with increasing PEGDA chain length. Moreover, water permeability exhibits a strong correlation with equilibrium water uptake. However, solute rejection, probed using poly(ethylene glycol)s of well defined molar mass, decreased with increasing prepolymerization water content and increasing PEGDA chain length. That is, there is a tradeoff between water permeability and separation properties. Finally, the fouling resistance of XLPEGDA materials was characterized via contact angle measurements and static protein adhesion experiments. From these results, XLPEGDA surfaces are more hydrophilic in samples prepared at higher prepolymerization water content or with longer PEGDA chains, and the more hydrophilic surfaces generally exhibit less BSA accumulation.     

Determination of ultrafiltrable and exchangeable copper in plasma: stability and reference values in healthy subjects

The knowledge of copper (Cu) distribution in blood contributes to a better understanding of copper metabolism and to a better approach and follow up of related diseases such as Wilson’s disease (WD). Many tests can be used to investigate patients who may have WD but they show many drawbacks and do not allow real patient monitoring. Knowing that the Cu overload can result from the free and easily exchangeable form of copper in plasma, a two-step method (ultrafiltration–determination by ETAAS) was carried out to determine these two fractions. The ultrafiltration procedure and the instrumental determination showed good repeatability, and a very low limit of detection was obtained (0.7 nmol/L). In vitro stability of both ultrafiltrable copper (CuUF) and exchangeable copper (CuEXC) was studied. Plasma was ultrafiltered in 44 presumably healthy subjects to determine CuUF and CuEXC and to set reference values ranges. The method was applied on a few patients showing good correlation between both parameters and the clinical and biological features of the patients. Figure Ultrafiltrable fraction of plasma copper is very instable in vitro. A 50% loss occurs within 6 hours after blood sampling     

Dynamic modelling of milk ultrafiltration by artificial neural network

Artificial neural networks (ANNs) have been used to dynamically model crossflow ultrafiltration of milk. It aims to predict permeate flux, total hydraulic resistance and the milk components rejection (protein, fat, lactose, ash and total solids) as a function of transmembrane pressure and processing time. Dynamic modelling of ultrafiltration performance of colloidal systems (such as milk) is very important for designing of a new process and better understanding of the present process. Such processes show complex non-linear behaviour due to unknown interactions between compounds of a colloidal system, thus the theoretical approaches were not being able to successfully model the process. In this work, emphasis has been focused on intelligent selection of training data, using few training data points and small network. Also it has been tried to test the ANN ability to predict new data that may not be originally available. Two neural network models were constructed to predict the flux/total resistance and rejection during ultrafiltration of milk. The results showed that there is an excellent agreement between the validation data (not used in training) and modelled data, with average errors less than 1%. Also the trained networks are able to accurately capture the non-linear dynamics of milk ultrafiltration even for a new condition that has not been used in the training process.     

Fouling reduction in poly(acrylonitrile-co-acrylamide) ultrafiltration membranes

Ultrafiltration membranes with similar pore sizes were prepared from acrylonitrile homopolymer and copolymers with increasing acrylamide content. The membranes containing acrylamide were more hydrophilic, had a smaller dispersion force component of the surface energy, and a smaller negative zeta potential than those prepared from the homopolymer. The effect of the differing surface chemistry of these membranes with similar pore sizes was examined by studying the ultrafiltration of bovine serum albumin (BSA) as a function of feed pH. The hydrophilic membranes showed higher permeate fluxes and flux recoveries than the hydrophobic membrane, in spite of their reduced repulsive electrostatic interaction. With increasing pH, protein transmission increased markedly for the acrylamide containing membranes whereas the transmission through the hydrophobic membrane remained low. These rejection data are explained by the combined effects of the increased hydrophilicity, decreased dispersive surface energy and reduced electrostatic repulsion of the acrylamide containing membranes.     

Interactions of thorium isotopes with colloidal organic matter in oceanic environments

The phase speciation of thorium and consequences for the residence times of colloids have been examined in seawater of the Middle Atlantic Bight (MAB) and the Gulf of Mexico. Two fractions of colloidal organic matter (COM), 0.2 μm > COM₁ > 1 kD and 0.2 μm > COM₁₀ > 10 kD, were sampled using cross-flow ultrafiltration techniques and measured for their ²³⁴Th activity and organic carbon concentration. The ratios of mass concentrations of COM₁ to those of suspended particulate matter were as high as 10 in the MAB and 6–34 in the Gulf of Mexico. Higher concentrations of colloids may be of great importance in the biogeochemical cycling of many particle-reactive nuclides or trace elements owing to their high specific surface area and complexation capacity. A significant fraction of ²³⁴Th in the traditionally defined “dissolved” pool was found to be associated with colloids. On average, about 10% of “dissolved” ²³⁴Th was in the colloidal fraction of sizes between 10 kDa and 0.2 μm, and 50% was in the 1 kDa-0.2 μm fraction. Values of the partition coefficients [K_c: (0.5−4) × 10⁶ ml g⁻¹ for K_c1 and (0.5−7) × 10⁶ ml g⁻¹ for K_c10] of ²³⁴Th between truly dissolved (<1 kDa) and colloidal fractions approximated those for Th-particle interactions [K_p: (0.3−10) × 10⁶ ml g⁻¹], indicating that colloid and suspended particle surface sites are similar. The distribution of ²³⁴Th between dissolved, colloidal, and particulate phases was broadly similar to that of organic carbon in these oceanic environments. Thus, thorium isotopes might be used as tracers of marine organic carbon cycling. Residence times of colloids derived from ²³⁴Th:²³⁸U disequilibria were consistently short, ranging from 1 to 14 days for COM₁₀ and from 5 to 65 days for COM₁, suggesting that marine colloids are highly reactive in marine biogeochemical processes. The discrepancy between apparent turnover times of colloids (1 kDa) derived from Th scavenging and ¹⁴C measurements suggest that ²³⁴Th and ¹⁴C may trace different geochemical pathways of colloids in the ocean.