June 2013 Membrane Technology9

FEATURE

Ultrafiltration (UF) membranes are central to filtration systems used extensively in water-based industrial processes and in desalination pretreatment. Manufacturing processes currently used to make UF membranes can be difficult to control – yielding products that give low separation efficiencies and high running costs for users, say the researchers at the Department of Energy and Materials Technology, Arcada.

Fundamental research at the university, using advanced polymer films, has led to the development of a new method of membrane production.

Nano-sized poresArcada’s track-etch membrane research group has succeeded in producing nano-sized pores using a method that previously was considered to be scientifically impossible. The most significant advantage of this method is its ability to substantially reduce production costs.

Dr Mikael Paronen, who is the head of the Department of Energy and Materials Technology, claimed on World Water Day, held on 22 March 2013, that this new technology could be important globally in increasing the availability of clean water. According to the

United Nations, over one billion people are affected by lack of access to clean water.

Dirty or salty water can be made drinkable using technology already available. However, current cleaning methods are either too expensive or unreasonably energy intensive.

Nano-porous membranes

Of the various alternative water-treatment methods those based on nano-porous membranes are technically the most promising, but also cost the most. Such membranes can cost thousands of euros per square metre, and so their otherwise excellent technology attracts only niche applications.

‘Our starting point in the development of a new manufacturing technology for nano-porous membranes has been to significantly reduce production costs. Our research group has been exceptionally open to exploring alternative manufacturing methods, and in this context we have benefited from combining our experiences and the scientific literature,’ commented Dr Paronen.

‘The group has been driven by an exceptionally strong desire to find a solution.’

Track-etched membrane productionAfter about five years of intensive research the group has finally made a breakthrough. The technology newly developed at Arcada is based on an unexpectedly successful combination of existing technologies and their optimisation.

It enables membranes to be produced that can filter substances from water, based on their size or chemical properties. The best known way of producing membranes with this capability is the track-etch method. Arcada’s newly developed technology is based partly on the same principle, but using a minor modification has transformed production economics to make it cost-effective.

Arcada says this new production process enables membrane prices to be significantly reduced and it subsequently broadens future applications of membrane technology.

The largest volume will most likely be seen in clean water production and in the treatment of industrial process water where, for example, the aim is to separate out valuable or hazardous substances.

Contact:

Dr Mikael Paronen, Arcada University of Applied

Sciences, Department of Energy and Materials

Technology, Arcada Jan-Magnus Janssons Plats 1,

00550 Helsinki, Finland.

Tel: +358 41 443 2109,

Email: mikael.paronen@arcada.fi, www.arcada.fi

Scientists at Arcada University of Applied Sciences, Helsinki, Finland, carrying out research into track-etched membrane production technology, claim they have made a revolutionary breakthrough. They have created a low-cost fabrication process which has the potential to create affordable ultrafiltration membranes that possess excellent filtration selectivity, are efficient, and also can be tailored to exact industrial separation processes.

RESEARCH TRENDSFabrication of nano-structured hydrophobic polyazole membranes for low-energy water recovery

In this study, aromatic fluorinated polyoxadia-zoles (F-POD) and polytriazoles (F-PT) were synthesised and manufactured into porous membranes by phase inversion and electro-spinning. The F-POD and F-PT membranes produced by phase inversion had a mean flow pore size (MFP) of 51 nm and 74 nm, respec-tively. The electro-spun membranes had a much larger pore size – the MFP for the F-POD and F-PT versions was around 1.7 μm and 2.7 μm, respectively. The membranes were tested for the

desalination of water taken from the Red Sea, using direct contact membrane distillation. By combining the high hydrophobic properties of the polymers and high porosity, apparent con-tact angles up to 162° were obtained, assuring operation with practically no liquid water leak-age under pressure up to 0.9 bar. Salt selectivity as high as 99.95% and water fluxes as high as 85 lm-2h-1 were demonstrated, operating at 80°C feed and 22°C permeate temperatures.H. Maab, L. Francis, A. Al-saadi, C. Aubry,

Modified track-etching method produces membranes cost-effectively

RESEARCH TRENDS

Membrane Technology June 201310

N. Ghaffour, G. Amy and S.P. Nunes: J. of Membrane Science 423–424 11–19 (15 December 2012).http://dx.doi.org/10.1016/j.memsci.2012.07.009

Separation of ethylene/ethane and propylene/propaneThe separation of olefin–paraffin mixtures is one of the most important and expensive processes used by the petrochemical industry. The perfor-mance of cellulose-acetate–silica nano-composite membranes for the separation of ethylene/ethane and propylene/propane is the subject of this research. According to the authors, silica nano-particles were prepared via hydrolysis of tetraeth-oxysilane (TEOS). Membranes were prepared by using the solution-casting method. The prepared membranes were characterised using FT-IR, SEM and TGA analyses. Pure gas permeation experiments were performed using the constant volume/variable pressure method at 2 bar feed absolute pressure, at 35°C. The results revealed that the permeability of ethylene and propylene increased from 0.052 barrer and 0.046 bar-rer, in pure cellulose acetate, to 0.11 barrer and 0.098 barrer in the composite membrane con-taining 30 wt% silica particles, respectively. The comparison of the selectivities of C2H4/C2H6 and C3H6/C3H8 indicates an increase from 2.16 and 2.55 for pure cellulose acetate to 4.07 and 6.12 for a composite membrane containing 30 wt% silica particles. The diffusion coefficients of the prepared hybrid membranes were deter-mined using the time-lag method. The solubility coefficient was calculated indirectly using the per-meability and diffusion coefficients. The results showed an increase in the solubility coefficient and a decrease in the diffusion coefficient of gases whilst increasing the silica mass fraction. To investigate the possible plasticisation phenomena, the effect of feed pressure on gas permeability and O2/N2 selectivity – before and after exposure of membranes with propylene – was studied. The results revealed no plasticisation effects up to a feed pressure of 8 bar.M. Naghsh, M. Sadeghi, A. Moheb, M.P. Chenar and M. Mohagheghianb: J. of Membrane Science 423–424 97–106 (15 December 2012).http://dx.doi.org/10.1016/j.memsci.2012.07.032

Cross-linking of sulfonated aromatic polymers: a reviewSulfonated aromatic polymers (SAPs) have the potential to create inexpensive polymer electrolyte membranes for use in fuel cells. However, chal-lenges associated with using these materials still exist, including excessive swelling in water, poor

mechanical strength, low dimensional stability – especially for highly sulfonated SAPs. Cross-linking is widely proposed as an efficient strategy to deal with these challenges. The authors of this paper provide a review of cross-linked SAP and outline some prospects for further development. The following points are addressed: cross-linked SAP via covalent bonds, including esterification, addition, Friedel–Crafts reactions and formation of –SO2– bridges; cross-linked SAPs by ionic bonds; and combined covalent and ionic cross-linking.H. Hou, M.L. Di Vona and P. Knauth: J. of Membrane Science 423–424 113–127 (15 December 2012).http://dx.doi.org/10.1016/j.memsci.2012.07.038

Sodium chloride sorption in sulfonated polymers for membrane applicationsIn this research, ion-sorption measurements were conducted to characterise sodium and chloride sorption in sulfonated polymers of interest for membrane applications. An uncharged poly-mer, based on cross-linked poly(ethylene glycol diacrylate), was included for comparison. The charged polymers included a sulfonated polysul-fone random copolymer and two phase-separated sulfonated styrenic pentablock copolymers. The sodium and chloride ion concentrations sorbed in these polymers were measured after the polymer films had been equilibrated with 0.01–1.0 mol l-1 aqueous NaCl solutions at a neutral pH. For the sulfonated polymers, cation sorption was meas-ured using a polymer ashing technique and flame atomic absorption spectrophotometry. Anion sorption was measured using a desorption tech-nique and ion chromatography. Polymer charge, that is, sulfonation, influenced ion sorption. In general, in all polymers considered, salt sorption increased as water sorption increased. As expect-ed, in the uncharged polymer the molar concen-trations of sorbed sodium and chloride were equal to one another. However, the sulfonated polymers sorbed much more sodium than chloride ions because of ion exchange. The experimental mobile salt (that is, anion) sorption data were fit to a model containing contributions from both Donnan exclusion and simple salt partitioning to the overall salt uptake by the polymer. In general, the influence of Donnan exclusion on anion (that is, mobile salt) sorption in these cation exchange materials is reduced as water uptake increases – presumably, in part, because of the dilution of the hydrated polymer’s sulfonate groups.G.M. Geise, L.P. Falcon, B.D. Freeman and D.R. Paul: J. of Membrane Science 423–424 195–208 (15 December 2012).http://dx.doi.org/10.1016/j.memsci.2012.08.014

Effect of temperature fluctuations on raw domestic wastewater composition and MBR sludge filterability

The main objective of this study was to inves-tigate the influence of seasonal temperature fluctuations on raw domestic wastewater com-position and membrane bioreactor (MBR) sludge filterability. The influent and sludge samples were collected from a full-scale MBR in Heenvliet, The Netherlands. All samples were analysed in terms of filterability, particle size distribution, respirometry and fractionation, and were further characterised using an extensive set of physico-chemical measurements. Typical seasonal fluctuations and deterioration of acti-vated sludge filterability during low-temperature periods were observed. A deterioration in filter-ability at low temperatures was linked to the incoming organic load, reduced MBR biomass concentrations, increased VSS/TSS ratio and reduced biodegradation of wastewater in the mixed liquor. Biological activity of the biomass was concomitantly decreased with temperature, to reach a minimum during the winter period. The results showed that the colloidal and soluble fraction (less than 1 μm) represents the majority of the constituents, contains most of the COD and plays an important role in the increase in membrane resistance.P. Krzeminski, A. Iglesias-Obelleiro, G. Madebo, J.M. Garrido, J.H.J.M. van der Graaf and J.B. van Lier: J. of Membrane Science 423–424 348–361 (15 December 2012).http://dx.doi.org/10.1016/j.memsci.2012.08.032

Effects of cross-flow velocity and trans-membrane pressure on MF of oil-in-water emulsionsThis study addresses the issue of oil removal from water using a hydrophilic porous membrane. The effective separation of oil-in-water dispersions involves a high flux of water through the mem-brane and, at the same time, a high rejection rate of the oil phase. The effects of trans-membrane pressure and cross-flow velocity on the rejection of oil droplets and thin oil-films by pores with differ-ent cross-sections are investigated numerically by solving the Navier–Stokes equation. The authors found that in the absence of cross-flow, the critical trans-membrane pressure – which is required for the oil droplet to enter a circular pore with given surface hydrophilic characteristics – agrees well with analytical predictions based on the Young–Laplace equation. With increasing cross-flow velocity, the shape of the oil droplet is strongly deformed near the pore entrance and the critical pressure of permeation increases. The authors

June 2013 Membrane Technology11

RESEARCH TRENDS

determined numerically the phase diagram for droplet rejection, permeation and break-up, depending on the trans-membrane pressure, and shear rate. Finally, an analytical expression for the critical pressure, in terms of geometric parameters of the cross-section of the pore, was validated using numerical simulation of a continuous oil film on elliptical and rectangular pores.T. Darvishzadeh and N.V. Priezjev: J. of Membrane Science 423–424 468–476 (15 December 2012).http://dx.doi.org/10.1016/j.memsci.2012.08.043

High-flux and fouling-resistant membranes for brackish water desalinationNovel high-flux and fouling-resistant reverse osmosis (RO) membranes were synthesised and characterised under brackish water desalination conditions using 2000 ppm NaCl solution at 225 psi (1.55 MPa) and at a temperature of 25°C. The o-aminobenzoic acid-triethylamine salt was added into m-phenylenediamine (MPD) solution to react with trimesoyl chloride (TMC) during the interfacial polymerisation between MPD and TMC. The membrane synthesis conditions, including MPD and TMC concentration, and interfacial polymerisation time were optimised. The membrane synthesised under the optimal conditions was post-treated with aqueous solu-tions containing glycerol, sodium lauryl sulfate, and camphorsulfonic acid–triethylamine salt to further increase the water flux. The resulting membrane showed a flux of 2.22 m3/m2/day (54.4 gallons/ft2/day) and a salt rejection of 98.6%. The fouling-resistant property of the synthesised membrane was enhanced by physi-cally coating a cross-linked polyethylene glycol (PEG-200) layer on top of the thin film. The membrane coated with 10 wt% cross-linked PEG demonstrated a high flux of 2.46 m3/m2/day (60.4 gallons/ft2/day). Using dodecyltrimethylam-monium bromide, a cationic foulant, and tannic acid, an anionic foulant – as model foulants – the coated membrane exhibited a much reduced decline in flux.L. Zhao, P.C.-Y. Chang, C. Yen and W.S.W. Ho: J. of Membrane Science 425–426 1–10 (1 January 2013).http://dx.doi.org/10.1016/j.memsci.2012.09.018

Fabrication of PVDF nano-fibre membranes for direct contact membrane distillationThe main criteria for a membrane used in mem-brane distillation (MD) are a well-designed porous structure and hydrophobic surface properties. This study covers the fabrication and optimisation

of polyvinylidene fluoride (PVDF) nano-fibre membranes for use in MD applications. Scanning electron microscopy (SEM), a capillary flow porometer, a geniometer, a home-made set-up for liquid entry pressure (LEP) measurement and a direct contact MD (DCMD) set-up were used to characterise the resultant nano-fibre PVDF mem-branes. Polymer dope compositions and spinning parameters, such as the sprayer moving speed and chamber moisture, were optimised so that their effects on the pore size distribution of the mem-branes could be analysed. Inorganic additives were also added to the dope to improve spin-ning characteristics of diluted polymer dopes in order to further decrease pore size. Meanwhile, heat-press post-treatment was considered as a necessary step to improve fresh nano-fibre membrane integrity, enhance water permeation flux and help prevent membrane pores from wetting during DCMD operation. The experi-ment confirmed that all of the electro-spun membranes exhibited a rough surface with high hydrophobicity (a water-contact angle in excess of 135°). The post-treated PVDF nano-fibre membranes provided a steady water permea-tion flux of about 21 kgm-2h-1 throughout the entire testing period of 15 h, using a 3.5 wt% NaCl solution as the feed, under feed and per-meate inlet temperatures of 323 K and 293 K, respectively. This result was better than that achieved for commercial PVDF membranes and a nano-fibre PVDF–clay membrane, reported in literature, suggesting that PVDF nano-fibre membranes have the potential to be used in DCMD applications.Y. Liao, R. Wang, M. Tian, C. Qiu and A.G. Fane: J. of Membrane Science 425–426 30–39 (1 January 2013).http://dx.doi.org/10.1016/j.memsci.2012.09.023

Preparation of electro-spun nano-fibre membranes for water treatmentNano-scale materials can be designed to exhibit novel and significantly improved physical and chemical properties. Polymer nano-fibres – an important class of nano-materials – have attracted increasing attention in the last 10 years because of their high surface-to-mass (or volume) ratio and special characteristics that are attractive for advanced applications. In particular, electro-spun nano-fibre membranes (ENMs) have high porosity, an interconnected open pore structure and a thickness that can be tailored. Moreover, their high, surface hydrophobic characterisics makes them suitable for membrane distillation (MD). This paper provides a review of recent progress made in preparing different types of nano-fibres with different secondary nano-

structures, including hollow nano-fibres. The use of hollow nano-fibres in membrane separa-tion processes are also discussed – based on an imaginary design of a hollow nano-fibre mod-ule. Experiments covering the application of ENMs in MD is reviewed. In particular, experi-mental results obtained in the authors’ labora-tory covering the desalination of saline water and gas-stripping VOCs are also highlighted.C. Feng, K.C. Khulbe, T. Matsuura, S. Tabe and A.F. Ismail: Separation and Purification Technology 102 118–135 (4 January 2013).http://dx.doi.org/10.1016/j.seppur.2012.09.037

Use of zeolite filler to mitigate fouling of PSF UF membranesIn this work, SAPO-44 zeolite was blended with polysulfone (PSF), used to make membranes, to mitigate fouling by humic acid. A casting solution was prepared by blending 5–20 wt% of SAPO-44 zeolite (average particle size of 138 nm) with a mixture of PSF, 1-methyl-2-pyrrolidinone and 1 wt% of PVA at 75°C, with a stirring speed of 400 rpm. The weight ratio of polymer to solvent was maintained at 1:4.84. Mixed matrix mem-branes were then prepared using the phase inver-sion method. SAPO-44 zeolite weakly adhered to the macroporous layer in the asymmetric membranes. Growth of finger-like macrovoids was promoted by blending SAPO-44 zeolite with PSF/PVA membrane material. A larger pore size was also induced in P-1 and P-2 samples, caus-ing a lower water-contact angle on these mixed matrix membranes. The addition of more zeolite filler resulted in smaller finger-like pores as the exchange between solvent and non-solvent had been delayed in the viscous solution. Membrane pore-size and surface energy were slightly enlarged by using a blend of 15 wt% or 20 wt% of zeolite. Membrane P-3, with 15 wt% of SAPO-44, exhibited the highest water flux – with an increment of nearly 164%, compared with the control membrane. Fouling mitigation could be achieved with minimal modification of surface energy, depending on the fouling mechanisms. Embedded zeolite may reduce fouling by humic acid initiated from pore blocking. About 80% permeate flux of the P-3 membrane was main-tained during the filtration of humic acid solu-tion. However, agglomerated filler in sample P-4, with 20 wt% of SAPO-44, caused slightly lower water permeability and relative flux during the fouling test, compared with the P-3 membrane.C.P. Leo, N.H. Ahmad Kamil, M.U.M. Junaidi, S.N.M. Kamal and A.L. Ahmad: Separation and Purification Technology 103 84–91 (15 January 2013).http://dx.doi.org/10.1016/j.seppur.2012.10.019