RESEARCH TRENDS Na+, Mg2+, K+ and Ca2+ were obtained by ultrafiltration pretreatment followed by ED. Nanofiltration with UTC-20, N30F, Desal 51 HL, UTC-60 and Desal 5 DL membranes resulted in an insufficient removal level, especially for the monovalent ions. B. van der Bruggen, R. Milis, C. Vandecasteele, P. Bielen, E. Van San and K. Huysman: Water Research 37(16) 3867–3874 (September 2003).
Alumina and aluminate ultrafiltration membranes The fabrication of alumina ultrafiltration membranes using acetic acid surface stabilized alumina nanoparticles (A-alumoxanes) has been investigated. The pore size, pore-size distribution, and molecular weight cut-off (MWCO) parameters of the resulting membranes are highly dependent on the uniformity of the nanoparticle precursor, which is a function of the reaction time and reaction pH during their synthesis. By controlling the alumina nanoparticles, a significant improvement of the membrane performance is observed, compared with the authors’ previous results. The new alumoxane-derived membranes have a molecular weight cut-off in the range of less than 1000 g mol–1, and show good selectivity to a range of synthetic dyes. Further control over selectivity and flux of these ceramic membranes may be obtained by using doped alumina nanoparticles (Fe, Mn and La) that result in the formation of the appropriate aluminate membranes. Of these, LaAlO3 shows the most promising results. Retention coefficients and flux values also may be altered by the chemical functionalization of the interior surface of the membranes by reacting the alumina surface with carboxylic acids. K.A. DeFriend, M.R. Wiesner and A.R. Barron: J. of Membrane Science 224(1–2) 11–28 (15 October 2003).
Gas separation performance of Matrimid membranes At room temperature a chemical cross-linking modification was performed on Matrimid 5218, a material that has received pervasive attention in membrane separation. The cross-linking reaction was conducted by simply immersing the membrane in a p-xylenediamine solution at 16
ambient temperature for a stipulated period of time. The influence of cross-linking modification on thermal and gas transport properties of Matrimid membrane were studied. The gas permeability and selectivity of pure gases He, O2, N2, CH4 and CO2, as well as mixed gas of CO2/CH4 were measured. The gas permeabilities show their maximum values for one-day cross-linked membranes, but then decrease gradually with immersion time. It is found that the Matrimid membrane undergoes the plasticization phenomenon at 15 atm, however it is effectively suppressed by the proposed cross-linking modification. The ideal selectivities for O2/N2, CO2/CH4 and CO2/N2 remain almost constant and decreased slightly with the cross-linking reaction, whereas the He/N2 selectivity increased with immersion time. Thus, it appears that the crosslinking modification on Matrimid is mainly useful for separation of gas He/N2. The mixed gas permeabilities for both CO2 and CH4 are lower than their pure gas permeabilities at respective partial pressure. While the mixed gas selectivity is found to be lower for untreated membranes it is higher for cross-linked membranes, if a comparison is made with the ideal selectivity of CO2/CH4. P.S. Tin, T.S. Chung, Y. Liu, R. Wang, S.L. Liu and K.P. Pramoda: J. of Membrane Science 225(1–2) 77–90 (1 November 2003).
Mesoporous silica membranes on porous ceramic supports The authors of this study present a new approach for rapid formation of mesoporous, surfactant-templated silica membranes on coarse-pore a αAl2O3 ceramic supports. A surfactant-silica sol is dispersed in the gas phase in the form of small droplets and delivered to the surface of the planar support by a N2 carrier stream. Coalescence of deposited sol droplets combined with solvent evaporation-induced self-assembly of liquid crystalline meso-phases results in the formation of continuous, meso-structured silica-surfactant layers covering the surface of the support. These meso-structured silica membranes are impermeable right after synthesis and exhibit N2 permeance in the range 10–7–10–6 mol m–2 s–1 Pa–1 after surfactant removal. SEM studies revealed the presence of relatively smooth layers of thickness
about 1 µm on the surface of the ceramic supports, while SAXS and TEM investigations revealed that these membranes possess cubicordered mesopores of size 20 Å, without preferential orientation with respect to the substrate. Such membranes may find application in ultrafiltration separation processes, since surfactant-templating can be used for accurate control of the pore size/distribution in the proper range for a desired separation. G. Xomeritakis, C.M. Braunbarth, B. Smarsly, N. Liu, R. Köhn, Z. Klipowicz and C.J. Brinker: Microporous and Mesoporous Materials 66(1) 91–101 (18 November 2003).
Analysis of polarized layer resistance during UF A mathematical model based on filtration theory, coupled with the resistance in series model and gel polarization/film model was developed in this study. Unlike the cake filtration equipment where cake deposition continues until the wash cycle occurs, in case of continuous stirred ultrafiltration (UF) the deposited solutes are allowed to build up over the membrane indefinitely, along with continuous back transport of deposited solutes into bulk by the turbulence created by stirring action. To account for this back transport, a coefficient called ‘back transport coefficient’ has been defined which is found to be independent of any operating variables. Variation of polarized layer resistance was also studied with different operating variables like bulk concentration, stirrer speed and pressure differential. A correlation was also developed relating polarized layer resistance with those operating variables. A comparative study has been made between the experimentally found polarized layer resistance value, using ultrafiltration data of Bhattacharjee and Bhattacharya, with those found from correlation and this developed model based on filtration theory. The model has been found to predict the polarized layer resistance reasonably well once the three parameters describing the model – permeability coefficient, back transport coefficient and membrane hydraulic resistance – are known, along with the operating condition. Using the above-mentioned parameters, it is also possible to predict flux and/or total permeate
volume at any time under a given operating condition. C. Bhattacharjee and S. Datta: Separation and Purification Technology 33(2) 115–126 (1 October 2003).
Removal of chromates from drinking water by anion exchangers Strong-base anion-exchange resins were used for selective removal of Cr(VI) from tap water. The influence on the process efficiency of parameters, such as the type of anionexchange resin, the concentrations of Cl– and SO42– in the feed water, and the flow rate and pH of the feed water, was investigated. Regeneration was carried out by the reduction of the chromate with bisulphite under acidic conditions and the subsequent removal of the Cr from the column as the Cr3+ cation. The pH of the regenerant solution was then increased, causing Cr(OH)3 to precipitate, and the precipitate was removed by filtration. E. Korngold, N. Belayev and L. Aronov: Separation and Purification Technology 33(2) 179–187 (1 October 2003).
Next month News Business activities Industry trends Commercial news New products
Features Developments and applications around the world
Research Trends The latest membrane research published in the key journals
Patents Recently published US and WO patents
Events Calendar Conferences and shows relevant to the membranes industry
Membrane Technology December 2003