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Advances in Colloid and Interface Science (v.164, #1-2)

Editorial Board (pp. ifc).

Special Contents (pp. vi).

Foreword by Professor Nidal Hilal Guest Editor; Professor Victor M. Starov Guest Editor (pp. 1-1).

Colloidal surface interactions and membrane fouling: Investigations at pore scale by P. Bacchin; A. Marty; P. Duru; M. Meireles; P. Aimar (pp. 2-11).

In this paper, we examine the contributions of colloidal surface interaction in filtration processes. In a first part, we describe the way surface interactions affect the transport of colloidal particles or macromolecules towards a membrane, and its theoretical description. The concept of critical flux is introduced and linked to particle–membrane wall and particle–particle surface interactions. From this review, it seems important to consider how surface interactions occur at pore scale and control the development of fouling layers. In this context, we report in a second part experiments where the capture of micron-sized particles is observed in a poly-dimethylsiloxane (PDMS) microfluidic filtration device. Direct observations of the filtering part by video-microscopy allow to investigate the way the fouling of the microchannels by the particles is taking place. The experimental results underline the important role played by the particle–wall interactions on the way particles are captured during filtration. A small change in surface properties of the PDMS has important consequences in the way pore clogging occurs: in more hydrophobic conditions the particles first form arches at the microchannels entrance, then leading to the growth of a filtration cake, whereas in more hydrophilic conditions the particles are captured on the walls between the microchannels, then leading to the progressive formation of dendrites. To conclude, both experimental and theoretical approaches show the important role played by surface interactions in filtration processes. The complex interplay between multi-body surface interactions and hydrodynamics at nanometric scale leads to clogging phenomena observed experimentally in microfluidic systems that have not been predicted by numerical simulations. In the future, the two way coupling between simulation and experimental approaches at the pore scale have to progress in order to reach a full understanding of the contribution of colloid science in membrane processes.

Keywords: Fouling; Particle capture; Separation; Colloid; Microfluidic

Positively charged nanofiltration membranes: Review of current fabrication methods and introduction of a novel approach by Shuying Cheng; Darren L. Oatley; Paul M. Williams; Chris J. Wright (pp. 12-20).

A review of the fabrication processes currently available to produce positively charged nanofiltration membranes has been conducted. The review highlights that there are few membranes and studies currently available. The preparation of a novel positively charged nanofiltration membrane is also described. This membrane was fabricated by surface modification of a prepared base membrane using polyethyleneimine followed by cross linking with butanedioldiglycidylether. The fabrication process uses standard organic solvents and avoids the need for hazardous materials, such as concentrated sulphuric acid, which significantly benefits the scale up potential of any future commercial manufacturing process.The new membrane was characterised using a number of state-of-the-art techniques, including a novel use of atomic force microscopy to determine pore size. Streaming potential measurements confirmed that this new membrane is indeed positively charged in the pH range below pH 9, which covers the majority of normal operating conditions. The performance characteristics for the new membrane were very favourable, with a pure water flux determined to be 20 LMH bar⁻¹ and a rejection of MgCl of 96%. Thus, this new membrane both adds to and complements the existing short supply of positively charged NF membranes and is suitable for applications such as the recovery of valuable cationic macromolecules in the bioprocess and pharmaceutical industries or removal of multi-valent cations such as dyes and heavy metals in the paper and pulp, textiles, nuclear, and automotive industries.

Keywords: Nanofiltration; Positive charge; Surface modification; Atomic force microscopy

Hydrodynamic permeability of aggregates of porous particles with an impermeable core by Satya Deo; Anatoly Filippov; Ashish Tiwari; Sergey Vasin; Victor Starov (pp. 21-37).

A hydrodynamic permeability of membranes built up by porous cylindrical or spherical particles with impermeable core is investigated. Different versions of a cell method are used to calculate the hydrodynamic permeability of the membranes. Four known boundary conditions, namely, Happel's, Kuwabara's, Kvashnin's and Cunningham/Mehta-Morse's, are considered on the outer surface of the cell. Comparison of the resulting hydrodynamic permeability is undertaken. A possible jump of a shear stress at the fluid-membrane interface, its impact on the hydrodynamic permeability is also investigated. New results related to the calculated hydrodynamic permeability and the theoretical values of Kozeny constant are reported. Both transversal and normal flows of liquid with respect to the cylindrical fibers that compose the membrane are studied. The deduced theoretical results can be applied for the investigation of the hydrodynamic permeability of colloidal cake layers on the membrane surface, the hydrodynamic permeability of woven materials.

Keywords: Hydrodynamic permeability; Navier boundary condition; Stress jump boundary condition

Critical, sustainable and threshold fluxes for membrane filtration with water industry applications by Robert W. Field; Graeme K. Pearce (pp. 38-44).

Critical flux theory evolved as a description of the upper bound in the operating envelope for controlled steady state environments such as cross-flow systems. However, in the application of UF membranes in the water industry, dead-end (direct-flow) designs are used. Direct-flow is a pseudo steady state operation with different fouling characteristics to cross-flow, and thus the critical flux concept has limited applicability. After a review of recent usage of the critical flux theory, an alternative concept for providing design guidelines for direct-flow systems namely that of the threshold flux is introduced. The concept of threshold flux can also be applicable to cross-flow systems. In more general terms the threshold flux can be taken to be the flux that divides a low fouling region from a high fouling region. This may be linked both to the critical flux concept and to the concept of a sustainable flux. The sustainable flux is the one at which a modest degree of fouling occurs, providing a compromise between capital expenditure (which is reduced by using high flux) and operating costs (which are reduced by restricting the fouling rate). Whilst the threshold flux can potentially be linked to physical phenomena alone, the sustainable flux also depends upon economic factors and is thus of a different nature to the critical and threshold fluxes. This distinction will be illustrated using some MBR data. Additionally the utility of the concept of a threshold flux will be illustrated using pilot plant data obtained for UF treatment of four sources of water.►Reviewed briefly the recent usage of the critical flux theory. ►Threshold flux concept introduced. ►Threshold flux illustrated using pilot plant data for four different waters. ►Nature of sustainable flux analysed.

Keywords: Critical flux; Sustainable flux; Limiting flux; Membrane filtration; Threshold flux

Theoretical and experimental correlations of gas dissolution, diffusion, and thermodynamic properties in determination of gas permeability and selectivity in supported ionic liquid membranes by Quan Gan; Yiran Zou; David Rooney; Paul Nancarrow; Jillian Thompson; Lizhe Liang; Moira Lewis (pp. 45-55).

Supported ionic liquid membranes (SILMs) has the potential to be a new technological platform for gas/organic vapour separation because of the unique non-volatile nature and discriminating gas dissolution properties of room temperature ionic liquids (ILs). This work starts with an examination of gas dissolution and transport properties in bulk imidazulium cation based ionic liquids [C_nmim][NTf2] (n=2.4, 6, 8.10) from simple gas H₂, N₂, to polar CO₂, and C₂H₆, leading to a further analysis of how gas dissolution and diffusion are influenced by molecular specific gas-SILMs interactions, reflected by differences in gas dissolution enthalpy and entropy. These effects were elucidated again during gas permeation studies by examining how changes in these properties and molecular specific interactions work together to cause deviations from conventional solution–diffusion theory and their impact on some remarkably contrasting gas perm-selectivity performance. The experimental perm-selectivity for all tested gases showed varied and contrasting deviation from the solution–diffusion, depending on specific gas-IL combinations. It transpires permeation for simpler non-polar gases (H₂, N₂) is diffusion controlled, but strong molecular specific gas-ILs interactions led to a different permeation and selectivity performance for C₂H₆ and CO₂. With exothermic dissolution enthalpy and large order disruptive entropy, C₂H₆ displayed the fastest permeation rate at increased gas phase pressure in spite of its smallest diffusivity among the tested gases. The C₂H₆ gas molecules “peg” on the side alkyl chain on the imidazulium cation at low concentration, and are well dispersed in the ionic liquids phase at high concentration. On the other hand strong CO₂-ILs affinity resulted in a more prolonged “residence time” for the gas molecule, typified by reversed CO₂/N₂ selectivity and slowest CO₂ transport despite CO₂ possess the highest solubility and comparable diffusivity in the ionic liquids. The unique transport and dissolution behaviour of CO₂ are further exploited by examining the residing state of CO₂ molecules in the ionic liquid phase, which leads to a hypothesis of a condensing and holding capacity of ILs towards CO₂, which provide an explanation to slower CO₂ transport through the SILMs. The pressure related exponential increase in permeations rate is also analysed which suggests a typical concentration dependent diffusion rate at high gas concentration under increased gas feed pressure. Finally the strong influence of discriminating and molecular specific gas-ILs interactions on gas perm-selectivity performance points to future specific design of ionic liquids for targeted gas separations.

Keywords: Gas separation; Ionic liquids; Gas membrane separation; Supported ionic liquid membrane; Solution diffusion mechanism

Membranes and theoretical modeling of membrane distillation: A review by Mohamed Khayet (pp. 56-88).

Membrane distillation ( MD) is one of the non-isothermal membrane separation processes used in various applications such desalination, environmental/waste cleanup, food, etc. It is known since 1963 and is still being developed at laboratory stage for different purposes and not fully implemented in industry. An abrupt increase in the number of papers on MD membrane engineering (i.e. design, fabrication and testing in MD) is seen since only 6years ago. The present paper offers a comprehensive MD state-of-the-art review covering a wide range of commercial membranes, MD membrane engineering, their MD performance, transport mechanisms, experimental and theoretical modeling of different MD configurations as well as recent developments in MD. Improved MD membranes with specific morphology, micro- and nano-structures are highly demanded. Membranes with different pore sizes, porosities, thicknesses and materials as well as novel structures are required in order to carry out systematic MD studies for better understanding mass transport in different MD configurations, thereby improving the MD performance and looking for MD industrialization.

Keywords: Membrane distillation; Membranes; Theoretical models

Polymeric membrane materials: New aspects of empirical approaches to prediction of gas permeability parameters in relation to permanent gases, linear lower hydrocarbons and some toxic gases by O.V. Malykh; A.Yu. Golub; V.V. Teplyakov (pp. 89-99).

Membrane gas separation technologies (air separation, hydrogen recovery from dehydrogenation processes, etc.) use traditionally the glassy polymer membranes with dominating permeability of “small” gas molecules. For this purposes the membranes based on the low free volume glassy polymers (e.g., polysulfone, tetrabromopolycarbonate and polyimides) are used. On the other hand, an application of membrane methods for VOCs and some toxic gas recovery from air, separation of the lower hydrocarbons containing mixtures (in petrochemistry and oil refining) needs the membranes with preferable penetration of components with relatively larger molecular sizes. In general, this kind of permeability is characterized for rubbers and for the high free volume glassy polymers. Data files accumulated (more than 1500 polymeric materials) represent the region of parameters “inside” of these “boundaries.” Two main approaches to the prediction of gas permeability of polymers are considered in this paper: (1) the statistical treatment of published transport parameters of polymers and (2) the prediction using model of ≪diffusion jump≫ with consideration of the key properties of the diffusing molecule and polymeric matrix. In the frames of (1) the paper presents N-dimensional methods of the gas permeability estimation of polymers using the correlations “selectivity/permeability.” It is found that the optimal accuracy of prediction is provided at n=4. In the frames of the solution–diffusion mechanism (2) the key properties include the effective molecular cross-section of penetrating species to be responsible for molecular transportation in polymeric matrix and the well known force constant (ε/k)_{eff i} of {6–12} potential for gas–gas interaction. Set of corrected effective molecular cross-section of penetrant including noble gases (He, Ne, Ar, Kr, Xe), permanent gases (H₂, O₂, N₂, CO), ballast and toxic gases (CO₂, NO_, NO₂, SO₂, H₂S) and linear lower hydrocarbons (CH₄, C₂H₆, C₃H₈, C₄H₁₀, C₂H₄, C₃H₆, C₄H₈ - 1, C₂H₂, C₃H₄–m (methylacetylene) and C₃H₄–a (allen) is determined by using two above mentioned approaches. All of this allows calculating preliminary the permeability parameters of above mentioned gases for most part of known polymers based on limited experimental data.The new correlations suggested demonstrate that the available free volume of polymeric matrix plays an important role in providing of rate and selectivity of gas diffusion for glassy-like polymers; the rate and selectivity of gas diffusion in rubbers is affected mainly by cohesion energy density (CED) the both polymer parameters being calculated by traditional additive group contributions technique.Results of present study are demonstrated by calculation of expected permeability parameters in relation to lower hydrocarbons and some toxic gases for polynorbornene based polymers, PIM and PTMSP outlining potential of practical application for new membrane polymers.General equationslg(Pi)=K1+K2(d2i)+K3(εi/k),…lgPn=K1+K2d2n+K3(εn/k),Display Omitted►New N-dimensional approach for prediction of gas permeability parameters of polymers ►Corrected Stuart's diameters of penetrant's molecules can be used for calculation of permanent gases, linear lower hydrocarbons and some toxic gas permeability for polymers when these data are even unavailable. ►Expected gas permeability for set of modern polymers is calculated.

Keywords: Prediction of gas/hydrocarbons membrane separation; Toxic gas permeability; Structure/diffusivity relations

Micropollutant sorption to membrane polymers: A review of mechanisms for estrogens by Schafer Andrea I. Schäfer; Ime Akanyeti; Semiao Andrea J.C. Semião (pp. 100-117).

Organic micropollutants such as estrogens occur in water in increasing quantities from predominantly anthropogenic sources. In water such micropollutants partition not only to surfaces such as membrane polymers but also to any other natural or treatment related surfaces. Such interactions are often observed as sorption in treatment processes and this phenomenon is exploited in activated carbon filtration, for example. Sorption is important for polymeric materials and this is used for the concentration of such micropollutants for analytical purposes in solid phase extraction. In membrane filtration the mechanism of micropollutant sorption is a relatively new discovery that was facilitated through new analytical techniques. This sorption plays an important role in micropollutant retention by membranes although mechanisms of interaction are to date not understood. This review is focused on sorption of estrogens on polymeric surfaces, specifically membrane polymers. Such sorption has been observed to a large extent with values of up to 1.2ng/cm² measured. Sorption is dependent on the type of polymer, micropollutant characteristics, solution chemistry, membrane operating conditions as well as membrane morphology. Likely contributors to sorption are the surface roughness as well as the microporosity of such polymers. While retention—and/or reflection coefficient as well as solute to effective pore size ratio—controls the access of such micropollutants to the inner surface, pore size, porosity and thickness as well as morphology or shape of inner voids determines the available area for sorption. The interaction mechanisms are governed, most likely, by hydrophobic as well as solvation effects and interplay of molecular and supramolecular interactions such as hydrogen bonding, π-cation/anion interactions, π–π stacking, ion–dipole and dipole–dipole interactions, the extent of which is naturally dependent on micropollutant and polymer characteristics. Systematic investigations are required to identify and quantify both relative contributions and strength of such interactions and develop suitable surface characterisation tools. This is a difficult endeavour given the complexity of systems, the possibility of several interactions taking place simultaneously and the generally weaker forces involved.Display Omitted► Sorption plays an important role in micropollutant retention by membranes. ► Measured sorption was up to 1.2ng/cm² for estrogens. ► The internally accessible surface area of polymers is important. ► Material differences are attributed to a combination of hydrophobic and supramolecular interactions. ► Methodologies need to be developed to measure such interactions qualitatively and quantitatively.

Keywords: Sorption; Micropollutant; Membrane polymer; Hydrogen bonding; Supramolecular interactions; Nanofiltration

Amino acid rejection behaviour as a function of concentration by Jason Shirley; Stephen Mandale; Paul M. Williams (pp. 118-125).

The solute rejection versus concentration behaviour of five different amino acids has been investigated using a Nitto Denko NTR7450 nanofiltration membrane. The experimental data for amino acid rejection was also compared against a combined steric and charge rejection model. At its isoelectric point, lysine was effectively neutral and its behaviour was well described by the model incorporating a steric function only. For phenylalanine, the combined model was found to fit the data well. In contrast there was poor agreement between the model and rejection data for glutamine, glutamic acid and glycine whose rejection values at first increased with concentration. This result implied that another governing process was in operation. Dimerisation as an explanation for the observed phenomena was also investigated. Size analysis of amino acid molecules as a function of the prevailing concentration using dynamic light scattering was limited but showed no evidence of dimerisation. This data was supported by osmotic pressure measurements which demonstrated no evidence of non-linearity in the relation between osmotic pressure and concentration.

Keywords: Nanofiltration; Increased rejection; Amino acids; Dimerisation; Osmotic pressure

Colloidal interactions and fouling of NF and RO membranes: A review by Chuyang Y. Tang; T.H. Chong; Anthony G. Fane (pp. 126-143).

Colloids are fine particles whose characteristic size falls within the rough size range of 1–1000nm. In pressure-driven membrane systems, these fine particles have a strong tendency to foul the membranes, causing a significant loss in water permeability and often a deteriorated product water quality. There have been a large number of systematic studies on colloidal fouling of reverse osmosis (RO) and nanofiltration (NF) membranes in the last three decades, and the understanding of colloidal fouling has been significantly advanced. The current paper reviews the mechanisms and factors controlling colloidal fouling of both RO and NF membranes. Major colloidal foulants (including both rigid inorganic colloids and organic macromolecules) and their properties are summarized. The deposition of such colloidal particles on an RO or NF membrane forms a cake layer, which can adversely affect the membrane flux due to 1) the cake layer hydraulic resistance and/or 2) the cake-enhanced osmotic pressure. The effects of feedwater compositions, membrane properties, and hydrodynamic conditions are discussed in detail for inorganic colloids, natural organic matter, polysaccharides, and proteins. In general, these effects can be readily explained by considering the mass transfer near the membrane surface and the colloid–membrane (or colloid–colloid) interaction. The critical flux and limiting flux concepts, originally developed for colloidal fouling of porous membranes, are also applicable to RO and NF membranes. For small colloids (diameter≪100nm), the limiting flux can result from two different mechanisms: 1) the diffusion–solubility (gel formation) controlled mechanism and 2) the surface interaction controlled mechanism. The former mechanism probably dominates for concentrated solutions, while the latter mechanism may be more important for dilute solutions. Future research needs on RO and NF colloidal fouling are also identified in the current paper.Display Omitted► Classification and properties of colloids and their interactions are discussed. ► The mechanisms and factors controlling colloidal fouling are reviewed. ► Permeability loss may be caused by cake resistance and cake enhanced concentration polarization. ► Critical flux and limiting flux theories are reviewed. ► Future research needs on RO and NF colloidal fouling are discussed.

Keywords: Colloidal fouling; Reverse osmosis (RO); Nanofiltration (NF); Critical flux; Limiting flux; Concentration polarization

Adlayers of palladium particles and their aggregates on porous polypropylene hollow fiber membranes as hydrogenization contractors/reactors by V.V. Volkov; V.I. Lebedeva; I.V. Petrova; A.V. Bobyl; S.G. Konnikov; V.I. Roldughin; J. van Erkel; G.F. Tereshchenko (pp. 144-155).

Principal approaches for the preparation of catalytic membrane reactors based on polymer membranes containing palladium nanoparticles and for the description of their characteristics are presented. The method for the development of adlayers composed of palladium nanoparticles and their aggregates on the surface of hydrophobic polypropylene porous hollow fiber membranes is proposed, and their comprehensive study is performed. Various regimes of the deposition of palladium on individual fibers and on membrane surface inside membrane modulus are considered. The sizes of primary Pd particles range from 10 to 500nm, and dimensions of their aggregates vary from 200nm to tens of microns. The sizes of primary particles in a free state and in their aggregates are estimated by the methods of X-ray analysis and scanning electron microscopy. The proposed approach is used for the preparation of catalytic membrane contactors/reactors for the removal of dissolved oxygen from water. In the systems under study, the limiting stage of dissolved oxygen removal is concerned with diffusion-controlled delivery of oxygen to the surface of catalytic particles.Display Omitted► Catalytic Pd nanoparticles was deposited onto the surface of hydrophobic porous membranes. ► Catalytic membrane reactors was successfully applied for the removal of dissolved oxygen from water. ► The kinetics of dissolved oxygen removal is limited by oxygen delivery to the surface of catalytic particles.

Keywords: Palladium nanoparticles; Polymeric catalytic membrane; Membrane reactor; Membrane contactor; Dissolved oxygen; Hydrogenation

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Advances in Colloid and Interface Science (v.164, #1-2)

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Advances in Colloid and Interface Science (v.164, #1-2)