Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Chemodynamics of soft Charged nanoparticles in aquatic media: Fundamental concepts
    Town, R.M. ; Buffle, J. ; Duval, J.F.L. ; Leeuwen, H.P. van - \ 2013
    The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment, & General Theory 117 (2013)33. - ISSN 1089-5639 - p. 7643 - 7654.
    metal speciation dynamics - aqueous-solutions - solvent exchange - complexes - relaxation - nmr - ion - approximation - temperature - membranes
    Dehydration and thermal inactivation of Lactobacillus plantarum WCFS1: Comparing single droplet drying to spray and freeze drying
    Perdana, J.A. ; Bereschenko, L.A. ; Fox, M.B. ; Kuperus, J.H. ; Kleerebezem, M. ; Boom, R.M. ; Schutyser, M.A.I. - \ 2013
    Food Research International 54 (2013)2. - ISSN 0963-9969 - p. 1351 - 1359.
    desiccation tolerance - phase-transitions - probiotics - temperature - membranes - vitrification - survival - bacteria - injury - cells
    We demonstrated that viability loss during single droplet drying can be explained by the sum of dehydration and thermal inactivation. For Lactobacillus plantarum WCFS1, dehydration inactivation predominantly occurred at drying temperatures below 45 °C and only depended on the moisture content. Above 45 °C the inactivation was due to a combination of dehydration and thermal inactivation, which depended on the moisture content, temperature, and drying time. A Weibull model was successfully applied to describe the thermal and dehydration inactivation and enabled the prediction of residual viability of L. plantarum WCFS1 after single droplet drying. Subsequently, the model was evaluated to predict the viability loss during laboratory scale spray drying, showing a remarkable agreement if assumed that only thermal inactivation occurred. This indicated that very high drying rates in laboratory scale spray drying could induce instant fixation of the cell suspensions in a vitrified matrix and thereby preventing dehydration inactivation. Finally, the influence of drying rate on remaining viability was evaluated by comparing single droplet drying, freeze drying and laboratory scale spray drying of the same bacterial suspension. It was shown that slow drying leads to large dehydration inactivation, which diminished in fast drying processes such as laboratory scale spray drying where thermal inactivation appears to be the predominant mechanism of inactivation.
    Theory and operation of capacitive deionization systems
    Zhao, R. - \ 2013
    Wageningen University. Promotor(en): Bert van der Wal, co-promotor(en): Huub Rijnaarts; Maarten Biesheuvel. - S.l. : s.n. - ISBN 9789461736390 - 155
    waterzuivering - drinkwater - ontzilting - elektrodes - membranen - ionenuitwisseling - water treatment - drinking water - desalination - electrodes - membranes - ion exchange
    Bioelectrochemical production of caproate and caprylate from acetate by mixed cultures
    Eerten-Jansen, M.C.A.A. van; Heijne, A. ter; Grootscholten, T.I.M. ; Steinbusch, K.J.J. ; Sleutels, T.H.J.A. ; Hamelers, H.V.M. ; Buisman, C.J.N. - \ 2013
    ACS sustainable chemistry & engineering 1 (2013)5. - ISSN 2168-0485 - p. 513 - 518.
    microbial electrolysis cells - fuel-cells - hydrogen - biomass - conversion - ethanol - reduction - transport - membranes - butyrate
    The use of mixed cultures to convert waste biomass into medium chain fatty acids, precursors for renewable fuels or chemicals, is a promising route. To convert waste biomass into medium chain fatty acids, an external electron donor in the form of hydrogen or ethanol needs to be added. This study investigated whether the cathode of a bioelectrochemical system can be used as the electron donor for the conversion of acetate into medium chain fatty acids. We show that medium chain fatty acids were produced in a bioelectrochemical system at -0.9 V vs. NHE cathode potential, without addition of an external mediator. Caproate, butyrate and smaller fractions of caprylate were the main products formed from acetate. In-situ produced hydrogen was likely involved as an electron donor for the reduction of acetate. Electron and carbon balances revealed that 45% of the electrons in electric current and acetate, and 31% of the carbon from acetate were recovered in the formed products. This study showed for the first time production of medium chain fatty acids caproate and caprylate from acetate at the cathode of bioelectrochemical systems, and offers new opportunities for application of bioelectrochemical systems.
    Optimization of salt adsorption rate in membrane capacitive deionization
    Zhao, R. ; Satpradit, O.A. ; Rijnaarts, H. ; Biesheuvel, P.M. ; Wal, A. van der - \ 2013
    Water Research 47 (2013)5. - ISSN 0043-1354 - p. 1941 - 1952.
    waterkwaliteit - water - terugwinning - ontzilting - ionisatie - ionenuitwisselingsbehandeling - membranen - water quality - water - recovery - desalination - ionization - ion exchange treatment - membranes - ion-exchange membranes - porous-electrodes - water desalination - brackish-water - transport-properties - carbon - electrochemistry - performance - efficiency - anions
    Membrane capacitive deionization (MCDI) is a water desalination technique based on applying a cell voltage between two oppositely placed porous electrodes sandwiching a spacer channel that transports the water to be desalinated. In MCDI, ion-exchange membranes are positioned in front of each porous electrode to prevent co-ions from leaving the electrode region during ion adsorption, thereby enhancing the salt adsorption capacity. MCDI can be operated at constant cell voltage (CV), or at a constant electrical current (CC). In this paper, we present both experimental and theoretical results for desalination capacity and rate in MCDI (both in the CV- and the CC-mode) as function of adsorption/desorption time, salt feed concentration, electrical current, and cell voltage. We demonstrate how by varying each parameter individually, it is possible to systematically optimize the parameter settings of a given system to achieve the highest average salt adsorption rate and water recovery.
    Bioconjugation of Protein-Repellent Zwitterionic Polymer Brushes Grafted from Silicone Nitride
    Nguyen, A.T. ; Baggerman, J. ; Paulusse, J.M.J. ; Zuilhof, H. ; Rijn, C.J.M. van - \ 2012
    Langmuir 28 (2012)1. - ISSN 0743-7463 - p. 604 - 610.
    biosensor applications - poly(ethylene glycol) - antibody microarrays - surface - adsorption - well - membranes - plasma - immobilization - strategies
    A new method for attaching antibodies to protein-repellent zwitterionic polymer brushes aimed at recognizing microorganisms while preventing the nonspecific adsorption of proteins is presented. The poly(sulfobetaine methacrylate) (SBMA) brushes were grafted from a-bromo isobutyryl initiator-functionalized silicon nitride (SixN4, x = 3) surfaces via controlled atom-transfer radical polymerization (ATRP). A trifunctional tris(2-aminoethyl)amine linker was reacted with the terminal alkylbromide of polySBMA chains. N-Hydroxysuccinimide (NHS) functionalization was achieved by reacting the resultant amine-terminated polySBMA brush with bifunctional suberic acid bis(N-hydroxysuccinimide ester). Anti-Salmonella antibodies were subsequently immobilized onto polySBMA-grafted SixN4 surfaces through these NHS linkers. The protein-repellent properties of the polySBMA-grafted surface after antibody attachment were evaluated by exposing the surfaces to Alexa Fluor 488-labeled fibrinogen (FIB) solution (0.1 g·L–1) for 1 h at room temperature. Confocal laser scanning microscopy (CLSM) images revealed the minimal adsorption of FIB onto the antibody-coated polySBMA in comparison with that of antibody-coated epoxide monolayers and also bare SixN4 surfaces. Subsequently, the interaction of antibodies immobilized onto polySBMA with SYTO9-stained Salmonella solution without using blocking solution was examined by CLSM. The fluorescent images showed that antibody-coated polySBMA efficiently captured Salmonella with only low background noise as compared to antibody-coated monolayers lacking the polymer brush. Finally, the antibody-coated polySBMA surfaces were exposed to a mixture of Alexa Fluor 647-labeled FIB and Salmonella without the prior use of a blocking solution to evaluate the ability of the surfaces to capture bacteria while simultaneously repelling proteins. The fluorescent images showed the capture of Salmonella with no adsorption of FIB as compared to antibody-coated epoxide surfaces, demonstrating the potential of the zwitterionic layer in preventing the nonspecific adsorption of the proteins during the detection of bacteria in complex matrices.
    Mechanical Properties of Re-constituted Actin Networks at an Oil/Water Interface Determined by Microrheology
    Ershov, D.S. ; Cohen Stuart, M.A. ; Gucht, J. van der - \ 2012
    Soft Matter 8 (2012). - ISSN 1744-683X - p. 5896 - 5903.
    particle-tracking microrheology - cytoskeletal protein networks - micropipette aspiration - myosin-filaments - molecular motors - polymer networks - living cells - cortex - microscopy - membranes
    There have been various attempts to investigate the mechanical properties of the actin cortex in cells, but the factors that control them remain poorly understood. To make progress, we develop a reconstituted model of the actin cortex that mimics its structure. We attach actin filaments to lipids lining the surface of an oil droplet using biotin–streptavidin bonds. In this way we can form a thin actin network that can be visualized and studied by confocal microscopy. Our approach allows incorporation of different actin-binding and motor proteins into this 2D network and characterization of their effect on its mechanical properties in a quantitative way. To study the viscoelasticity of the network, we use passive particle tracking microrheology, which allows storage and loss moduli to be extracted from the mean square displacement of tracer particles. We show that adding cross-linkers to the cortex increases its elasticity by several orders of magnitude and addition of myosin in the presence of ATP results in a strong and rapid stiffening of the network. This approach opens up a variety of possibilities to study viscoelastic properties of the actin cortex in vitro, allowing incorporation of any protein of interest into the system.
    Effect of low dosages of powdered activated carbon on membrane bioreactor performance
    Remy, M.J.J. ; Temmink, H. ; Rulkens, W.H. - \ 2012
    Water Science and Technology 65 (2012)5. - ISSN 0273-1223 - p. 954 - 961.
    afvalwaterbehandeling - bioreactoren - membranen - actieve kool - poeders - dosering - vervuiling door afzetting - filtreerbaarheid - energiegehalte - waste water treatment - bioreactors - membranes - activated carbon - powders - dosage - fouling - filterability - energy content - polymeric substances eps - sludge - removal - water - mbrs - dewaterability
    Previous research has demonstrated that powdered activated carbon (PAC), when applied at very low dosages and long SRTs, reduces membrane fouling in membrane bioreactors (MBRs). This effect was related to the formation of stronger sludge flocs, which are less sensitive to shear. In this contribution the long-term effect of PAC addition was studied by running two parallel MBRs on sewage. To one of these, PAC was dosed and a lower fouling tendency of the sludge was verified, with a 70% longer sustainable filtration time. Low PAC dosages showed additional advantages with regard to oxygen transfer and dewaterability, which may provide savings on operational costs.
    Understanding flow-induced particle migration for improved microfiltration
    Dinther, A.M.C. van - \ 2012
    Wageningen University. Promotor(en): Remko Boom, co-promotor(en): Karin Schroen. - S.l. : s.n. - ISBN 9789461733498 - 207
    microfluidics - filtratie - migratie - deeltjes - stroming - suspensies - emulsies - membranen - microfluidics - filtration - migration - particles - flow - suspensions - emulsions - membranes

    Membrane microfiltration processes are used in for example the food, biotechnology, chemical and pharmaceutical industry, and more generally in e.g. wastewater treatment. Microfiltration is mostly used to separate components that are greatly different in size, e.g. micro-organisms from water, but rarely to fractionate components that are of similar size. This latter option would be interesting for many applications, since it would lead to enriched starting materials and possibly new products, but is hampered by accumulation of components in and on the membrane due to size exclusion by the pores. This leads to flux reduction and increased retention of components in time, basically the accumulated layer determines which components can pass the membrane (see Figure 1).


    Figure 1. Schematic representation of a cross-flow microfiltration process with a decrease in permeate flux over the length of the membrane due to pore blocking and particle adsorption in the pores and on the membrane walls.
    Most research focusses on accumulation mechanisms (concentration polarization, cake formation and adsorption) and concepts targeted at controlling particle accumulation. One example is back-pulsing, but this only gives a short term solution leading to extensive cleaning procedures given the way membranes are currently operated in practice. Clearly it would be beneficial if accumulation could be prevented, and through that, more stable operation could be achieved.

    This thesis presents how flow-induced particle migration can be used for stable membrane flux and retention of components in time. The particle migration mechanisms that are considered in this thesis, shear-induced diffusion, inertial lift, and fluid skimming, act on particles that are typically between 0.1 and 10 micron. They induce separation of components in the fluid moving (larger) particles away from the membrane, therewith facilitating separation; basically pore size no longer determines particle permeation. In the thesis it will be shown that these effects improve processing of dilute suspensions and make processing of highly concentrated systems possible, which is beyond the scope of current microfiltration processes.

    Before the design of these processes, methods to measure velocity and concentration profiles in microfluidic devices are described, compared and evaluated. The small dimensions of these devices will cause particles to migrate; as is used later in the thesis to facilitate segregation and separation. A drawback of the small dimensions is that they make measurement of velocity and concentration gradients difficult. Based on our evaluation, Nuclear Magnetic Resonance (NMR) and Confocal Scanning Laser Microscopy (CSLM), although expensive, are the most promising techniques to investigate flowing suspensions in microfluidic devices, where one may be preferred over the other depending on the size, concentration and nature of the suspension, the dimensions of the channel, and the information that has to be obtained.

    CSLM is used to study the behaviour of suspensions, between 9 and 38 volume%, at the particle level. Under Poiseuille flow in a closed microchannel, shear-induced diffusion causes migration in these suspensions. Under all measured process conditions, particles segregate on size within an entrance length of around 1000 times the channel height. Mostly, the larger particles migrate to the middle of the channel, while the small particles have high concentrations near the walls. This indicates that the small particles could be collected from their position close to the wall and that this principle can be applied to microfiltration (see Figure 2).


    Figure 2. Schematic representation of a cross-flow microfiltration process with a constant permeate flux over the length of the membrane due to shear-induced particle migration in combination with the use of a closed entrance length and large pores.
    Microfiltration of emulsions proves that the small particles can be removed without accumulation of particles in and on the membrane, as long as the process conditions are chosen appropriate. The membrane cross-flow module consists of a closed channel to allow particles to migrate due to shear-induced diffusion followed by a membrane with 20 micron pores, being much larger than the particles, where fractions of these emulsions can be removed. The emulsions consist of small droplets (~2.0 micron) and large droplets (~5.5 micron), with total concentrations between 10 and 47% and different ratios between small and large particles. As expected, the size of the emulsion droplets in the permeate is a function of trans-membrane pressure, membrane design and oil volume fraction (i.e., of the total, the small and the large particles). The guidelines for appropriate process conditions are described and application of the right process conditions leads to very high selectivity. This means that the permeate only consists of small droplets, and on top of that their concentration is higher than in the original emulsion. Especially at high droplet concentration (which is known to cause severe fouling in regular membrane filtration), these effects are occurring as a result of shear-induced diffusion. If only small particles are targeted in the permeate, the module can be operated at fluxes of 40 L/(h•m²); if fractionation is targeted the fluxes can be maintained considerably higher (2-10 fold higher).

    Separation of concentrated suspensions is currently done by dilution and since the process based on shear-induced diffusion works well at low velocities and high concentrations, industrial application could have major benefits in terms of energy and water use. An outlook is given on how current industrial processes can be designed and improved in terms of energy consumption by making use of particle migration. It is shown that return of investment of installation of these new membrane modules is short compared to the membrane life time, due to high energy savings. In order to reach this, it will be necessary to take unconventional process conditions that target particle migration and membrane designs as a starting point.

    Besides concentrated suspensions, also dilute suspensions benefit from particle migration. Migration phenomena can induce fractionation of yeast cells from water in dilute suspensions, using micro-engineered membranes having pores that are typically five times larger than the cells. The observed effects are similar to fluid skimming (in combination with inertial lift), and the separation performance can be linked to the ratio between cross-flow and trans-membrane flux, which is captured in a dimensionless number that can predict size of transmitted cells. For sufficiently high cross-flow velocity, the particles pass the pore and become part of the retentate; the separation factor can simply be changed by changing the ratio between cross-flow velocity and trans-membrane flux. Since the membranes have very large pores, fouling does not play a role and constant high trans-membrane flux values of 200–2200 L/(h•m2) are reached for trans-membrane pressures ranging from 0.02 to 0.4 bar.

    In conclusion, particle migration can improve (membrane) separation processes and even has the potential to lead to totally new separation processes. Particle migration can be advantageous in both dilute as well as concentrated systems, leading to reduced fouling, reduced energy and water consumption and a reduction in waste. This can all be achieved at production capacity similar or better than currently available in microfiltration processes.

    Enzyme-catalyzed modification of poly(ethersulfone) membranes
    Nady, N. - \ 2012
    Wageningen University. Promotor(en): Remko Boom; Han Zuilhof, co-promotor(en): Karin Schroen; Maurice Franssen. - S.l. : s.n. - ISBN 9789461731456 - 172
    membranen - oppervlakteverandering - laccase - enzymen - kunststoffen - membranes - surface modification - laccase - enzymes - plastics

    The robustness of a membrane is determined by the properties of the base polymer and the functionality of its surface. One of the most popular polymers used for membrane preparation is polyethersulfone (PES), which has excellent thermo-physical properties, but the surface properties are in need of improvement to reduce membrane fouling by adsorption of e.g. protein and live cells, which cause sever flux decline during filtration. Therefore, it is not strange that a wide range of modification methods has been published to reduce surface hydrophobicity of PES membranes. However, the methods that are currently suggested are all rather offer random control over the resulting surface structure and may be environmentally adverse

    This study presents enzyme-initiated grafting of PES membranes as the first successful example of an environmentally friendly modification of PES membranes. Various phenolic acids, such as 4-hydroxybenzoic acid and gallic acid (3,4,5-trihydroxybenzoic acid), were coupled to the membrane in aqueous medium at room temperature using laccase from Trametes versicolor as catalyst. This enzyme is able to oxidize phenolic compounds to their corresponding free radicals that are subsequently grafted onto PES membranes, introducing polar groups (OH, COOH) on the membrane surface by formation of a covalent C-O linkage as was proven by spin density calculations and IRRAS.

    We succeed in altering the surface properties of PES membranes using laccase-catalyzed modification method. It was found that the surface structure or shape that can be tuned through both the modification conditions and the modifier structure, has a significant role in prevention of adsorption rather than surface hydrophilicity as is often assumed. Membrane flux is hardly influenced (10% reduction), and foulant (e.g., bovine serum albumin, dextrin, tannin, and pathogenic bacterium Listeriamonocytogenes) repellence is greatly increased.

    In conclusion, the enzyme-catalyzed modification method shows a remarkable flexibility, and allows careful tuning of the membrane properties in such a way that membrane fouling can be suppressed. Besides, the modification method does not influence the bulk properties of the membrane adversely, the modification layer is resistant to a wide range of pH, and the costs of this modification on industrial scale are reasonable, which makes this modification method an interesting eco-friendly alternative to currently used methods.

    Low concentration of powdered activated carbon decreases fouling in membrane bioreactors
    Remy, M.J.J. - \ 2012
    Wageningen University. Promotor(en): Wim Rulkens, co-promotor(en): Hardy Temmink. - S.l. : s.n. - ISBN 9789461732309 - 163
    afvalwaterbehandeling - geactiveerd slib - membranen - bioreactoren - filtratie - waste water treatment - activated sludge - membranes - bioreactors - filtration
    Het doel van deze studie was te onderzoeken welke slibeigenschappen verantwoordelijk zijn voor de membraanvervuiling in MBR systemen, en om een methode te vinden om deze eigenschappen dusdanig te manipuleren dat de membraanvervuiling drastisch kan worden gereduceerd.
    A Multi-Platform Flow Device for Microbial (Co-) Cultivation and Microscopic Analysis
    Hesselman, M.C. ; Odoni, D.I. ; Ryback, B.M. ; Groot, S. de; Heck, R.G.A. van; Keijsers, J. ; Kolkman, P. ; Nieuwenhuijse, D. ; Nuland, Y.M. ; Sebus, E. ; Spee, R. ; Vries, H. de; Wapenaar, M.T. ; Ingham, C.J. ; Schroen, K. ; Martins Dos Santos, V.A.P. ; Spaans, S.K. ; Hugenholtz, F. ; Passel, M.W.J. van - \ 2012
    PLoS ONE 7 (2012)5. - ISSN 1932-6203
    culture - microorganisms - population - resistance - membranes - bacteria - chamber - support - arrays
    Novel microbial cultivation platforms are of increasing interest to researchers in academia and industry. The development of materials with specialized chemical and geometric properties has opened up new possibilities in the study of previously unculturable microorganisms and has facilitated the design of elegant, high-throughput experimental set-ups. Within the context of the international Genetically Engineered Machine (iGEM) competition, we set out to design, manufacture, and implement a flow device that can accommodate multiple growth platforms, that is, a silicon nitride based microsieve and a porous aluminium oxide based microdish. It provides control over (co-)culturing conditions similar to a chemostat, while allowing organisms to be observed microscopically. The device was designed to be affordable, reusable, and above all, versatile. To test its functionality and general utility, we performed multiple experiments with Escherichia coli cells harboring synthetic gene circuits and were able to quantitatively study emerging expression dynamics in real-time via fluorescence microscopy. Furthermore, we demonstrated that the device provides a unique environment for the cultivation of nematodes, suggesting that the device could also prove useful in microscopy studies of multicellular microorganisms
    Dormancy cycling in seeds: mechanisms and regulation
    Claessens, S.M.C. - \ 2012
    Wageningen University. Promotor(en): Linus van der Plas, co-promotor(en): Henk Hilhorst; P.E. Toorop. - S.l. : s.n. - ISBN 9789461731906 - 161
    sisymbrium officinale - arabidopsis thaliana - kiemrust - zaden - genen - levenscyclus - slaaptoestand - membranen - metabolisme - sisymbrium officinale - arabidopsis thaliana - seed dormancy - seeds - genes - life cycle - dormancy - membranes - metabolism

    The life cycle of most plants starts, and ends, at the seed stage. In most species mature seeds are shed and dispersed on the ground. At this stage of its life cycle the seed may be dormant and will, by definition, not germinate under favourable conditions (Bewley, 1997).

    Seasonal dormancy cycling is a characteristic found in plant seeds. Being able to cycle in and out of dormancy allows the seed to survive decades or even centuries, allowing germination to be spread over time, but only when optimal conditions are available, not only for germination but especially for seedling establishment. In this thesis we have attempted to further elucidate the mechanisms behind dormancy, germination and dormancy cycling.

    Sisymbrium officinale seeds need nitrate and light to start germination (Chapter 2, 3, 4, 6). Nitrate acts in part by reducing the abscisic acid (ABA) levels (a plant hormone that elevates dormancy levels). The action of light and nitrate can also be reached by applying gibberellins (GAs) to the seeds (Chapter 2, 3, 4, 6). GAs are capable of inducing enzymes that hydrolyze the ensdosperm walls (Debeaujon and Koornneef, 2000; Chen and Bradford, 2000; Nonogaki et al., 2000; Manz et al., 2005) In this way GAs could be involved in lowering the physical restrictions imposed by the resistance of the seed coat and the endosperm. On the other hand, GAs may also increase the embryo growth potential.

    For successful survival of the dormant seed, metabolic activity is reduced to avoid rapid depletion of reserves. The metabolic activity of the seed was measured using electron paramagnetic resonance (EPR), with TEMPONE as a spin probe, and the respiratory activity was measured with the Q2-test (Chapter 2).We showed that primary dormancy was accompanied by hardly any metabolic or respiratory activity, and this increased considerably when dormancy was broken by nitrate. However, when the light pulse was not given and the seeds had become secondary dormant the metabolic activity slowed down.

    Regulation of dormancy is tightly linked with abiotic stress factors from the environment. The regulation and survival of the seed under stress conditions is largely dependent on the composition of the cytoplasm. We tested this by EPR, using carboxyl-proxyl (CP) spin probe (Chapter 4). The primary dormant and sub-dormant seeds possessed a higher viscosity than the germinating seeds. The viscosity of secondary dormant seeds appeared intermediate; however, the ease at which the vitrified water melted was similar to that of primary dormant seeds. As a result of the differences in viscosity, the temperature of vitrified water melting differed between the different dormancy states. The changes in cytoplasmic viscosity and vitrified water melting may be linked to changes in metabolism and the content of high molecular weight compounds.

    As membranes are the primary target for temperature perception, they are often implicated in regulating dormancy. Therefore, Hilhorst (1998) put forward a hypothesis in which changes in responsiveness to dormancy breaking factors like nitrate and light was a function of cellular membrane fluidity. In Chapter 3 we indeed showed that dormancy is a function of membrane fluidity. Primary dormant seeds of Sisymbrium officinale appeared to have very rigid membranes, whereas breaking dormancy increased membrane fluidity considerably. However, when sub-dormant seeds became secondary dormant membrane fluidity decreased again, but not to the rigidity seen in primary dormant seeds. One of the most common ways in which cells control membrane fluidity is by homeoviscous adaptation with the help of desaturases. Desaturase involvement in changes in membrane fluidity due to changes in dormancy was tested in Chapter 3 (using Sisymbrium officinale) and Chapter 5 (using Arabidopsis thaliana). Here we found that although desaturase activity may change the membrane fluidity or influence the germination/dormancy phenotype, the two are not linked, unless the effects of these enzymes are very local within the seed. Finally, in Chapter 7, we presented a new model in which a membrane anchored dormancy related protein/transcription factor is activated by changes in membrane fluidity. The activated form is transported to the nucleus, where it starts the germination process, which includes changes in metabolism and mobilization of storage reserves.

    Effect of temperature shocks on membrane fouling in membrane bioreactors
    Brink, P. van den; Satpradit, O.A. ; Bentem, A. van; Zwijnenburg, A. ; Temmink, B.G. ; Loosdrecht, M.C.M. - \ 2011
    Water Research 45 (2011)15. - ISSN 0043-1354 - p. 4491 - 4500.
    afvalwaterbehandeling - bioreactoren - membranen - geactiveerd slib - temperatuur - vervuiling door afzetting - viscositeit - waste water treatment - bioreactors - membranes - activated sludge - temperature - fouling - viscosity - waste-water treatment - cross-flow microfiltration - flux-step method - activated-sludge - particle deposition - size distribution - light-scattering - bubble-size - performance - ultrafiltration
    Temperature is known to influence the biological performance of conventional activated sludge systems. In membrane bioreactors (MBRs), temperature not only affects the bioconversion process but is also shown to have an effect on the membrane performance. Four phenomena are generally reported to explain the higher resistance for membrane filtration found at lower temperatures: (1) increased mixed liquor viscosity, reducing the shear stress generated by coarse bubbles, (2) intensified deflocculation, reducing biomass floc size and releasing EPS into the mixed liquor, (3) lower backtransport velocity and (4) reduced biodegradation of COD. Although the higher resistance at low temperatures has been reported in several papers, the relation with supernatant composition has not been investigated before. In this paper, the composition of the soluble fraction of the mixed liquor is related to membrane performance after exposing the sludge to temperature shocks. Flux step experiments were performed in an experimental system at 7, 15, and 25° Celsius with sludge that was continuously recirculated from a pilot-scale MBR. After correcting the permeate viscosity for temperature, higher membrane fouling rates were obtained for the lower temperature in combination with low fouling reversibility. The soluble fraction of the MBR mixed liquor was analysed for polysaccharides, proteins and submicron particle size distribution. At low temperature, a high polysaccharide concentration was found in the experimental system as compared to the MBR pilot. Upon decreasing the temperature of the mixed liquor, a shift was found in particle size towards smaller particles. These results show that the release of polysaccharides and/or submicron particles from sludge flocs could explain the increased membrane fouling at low temperatures
    Electrospun Polyurethane Fibers for Absorption of Volatile Organic Compounds from Air
    Scholten, E. ; Bromberg, L. ; Rutledge, G.C. ; Hatton, T.A. - \ 2011
    ACS Applied Materials and Interfaces 3 (2011). - ISSN 1944-8244 - p. 3902 - 3909.
    shape-memory polyurethane - activated carbon - block-copolymers - polymer nanofibers - hard segment - adsorption - membranes - vapors - desorption - sorption
    Electrospun polyurethane fibers for removal of volatile organic compounds (VOC) from air with rapid VOC absorption and desorption have been developed. Polyurethanes based on 4,4-methylenebis(phenylisocyanate) (MDI) and aliphatic isophorone diisocyanate as the hard segments and butanediol and tetramethylene glycol as the soft segments were electrospun from their solutions in N,N-dimethylformamide to form micrometer-sized fibers. Although activated carbon possessed a many-fold higher surface area than the polyurethane fiber meshes, the sorption capacity of the polyurethane fibers was found to be similar to that of activated carbon specifically designed for vapor adsorption. Furthermore, in contrast to VOC sorption on activated carbon, where complete regeneration of the adsorbent was not possible, the polyurethane fibers demonstrated a completely reversible absorption and desorption, with desorption obtained by a simple purging with nitrogen at room temperature. The fibers possessed a high affinity toward toluene and chloroform, but aliphatic hexane lacked the necessary strong attractive interactions with the polyurethane chains and therefore was less strongly absorbed. The selectivity of the polyurethane fibers toward different vapors, along with the ease of regeneration, makes them attractive materials for VOC filtration.
    Structuring microspheres
    Wagdare, N.A. - \ 2011
    Wageningen University. Promotor(en): Cees van Rijn; Remko Boom, co-promotor(en): Ton Marcelis. - [S.l.] : S.n. - ISBN 9789085859239 - 111
    emulgering - membranen - inkapseling in microcapsules - structuur - emulsification - membranes - microencapsulation - structure

    Encapsulation and use of capsules for controlled release has several applications in pharmaceuticals, foods, cosmetics, detergents and many other products for consumers. It can contribute to sustainability, since it allows an efficient use of active materials, delivery at the required site and possibly a longer shelf life of the products. Many encapsulation systems are basically very thin shells (10 nm – 10 µm) around microscopic reservoirs (100 nm – 100 µm), in which active ingredients are trapped. The release properties are strongly dependent on the material properties of the shell, but also on their size and uniformity.

    The overall objective of this research is to understand the formation process of microcapsules and microspheres by using phase separation in well-defined droplets of a polymeric solution. The primary droplets were produced with microsieve emulsification. The polymer used was Eudragit FS 30D (a commercial copolymer of poly(methyl acrylate-co-methyl methacrylate-co-methacrylic acid) 7:3:1), which contains charged carboxylate groups that make the polymer water-soluble at higher pH (>7), allowing for release by a change in pH.

    Chapter 2 presents results that give more insight into microsieve emulsification with high porosity micro-engineered membranes. The droplet formation was strongly influenced by the dynamics of surfactant adsorption. The presence of suitable surfactants in both phases prevents the coalescence of droplets and wetting of the microsieve membranes by the dispersed phase during oil droplet formation. This resulted in the formation of stable emulsions of droplets with a narrow size distribution. The flux of the dispersed phase could be increased an order of magnitude compared to previous methods, without loss of size-distribution of the droplets. Thus, use of a high-porosity membrane, in combination with suitable surfactants in both the dispersed and continuous phases resulted in a much more effective and efficient emulsification process.

    In Chapter 3 crossflow microsieve emulsification was used to prepare porous microcapsules with an average size of about 30 µm. A mixture of Eudragit and hexadecane in dichloromethane (DCM) was emulsified in water.Being a poor solvent for this polymer, demixing of the droplet into a polymeric shell and a hexadecane-rich core occurred upon extraction of the DCM into the water phase. At a low ratio of polymer to hexadecane, the resulting shells were found to be porous. Increasing this ratio resulted in a reduction of the porosity and pore size of the shell. The Eudragit has a pH-dependent solubility. It is insoluble at acidic conditions and rapidly dissolves at alkaline conditions. The capsules were found to be stable at a pH lower than 7.0, whereas the oil core was released within half an hour at pH 7.1 and within a minute at pH 8.0. The morphology of the microcapsules can be adapted with a careful choice of the concentrations of polymer, hexadecane and solvent. At higher concentrations of polymer, the tiny oil droplets that were captured in the forming Eudragit shell were unable to coalesce completely and small, isolated pores were formed within the shell matrix.

    The potential for new microcapsule morphologies was further explored in Chapter 4 where the formation of Eudragit capsules with other oils instead of hexadecane was studied, and in Chapter 5 where a blend of poly(methyl methacrylate) (PMMA) and Eudragit was used.

    In Chapter 4 the effects of chain lengths of vegetable oils on the formation of porous microcapsules with hollow and multi-compartment structures is discussed. The encapsulation of oil and the morphology of the resulting microcapsules depends on the interaction between the Eudragit polymer and the type of oil that was used. Microcapsule formation using long chain length oils such as sunflower oil, olive oil and coconut oil resulted in well-defined microcapsules with a single encapsulated oil droplet, covered with a Eudragit-rich shell. On the other hand, capsules prepared with relatively short chain length oils, such as medium chain triglyceride oil, resulted in capsules with many individual small oil droplets encapsulated in an Eudragit matrix. Extraction of the oil from the microcapsules with hexane results in the formation of hollow porous shells as was investigated with optical microscopy and SEM. These structures are formed during microcapsule formation due to the complex phase separation processes in the Eudragit-water-oil-DCM quaternary system.

    In Chapter 5 the formation of microcapsules is further explored by using a blend of PMMA and Eudragit. Microspheres formed with this blend were found to consist of a PMMA core inside an Eudragit-rich shell, which tends to be porous. As the amount of Eudragit is increased, a thicker and more porous outer shell is formed due to the enhanced interaction of water with Eudragit. After dissolution of the Eudragit at high pH, different core surface structures resulted, from irregular surfaces to microspheres with a fiber-like, swollen corona around it, and to a surface covered with small nodular structures, dependent on the concentrations of PMMA and Eudragit in the initial mixture. As already indicated above, these structures are formed as a result of complex phase separation processes between polymers and (non)solvents, and between the two polymers.

    In Chapter 6 the results described in this thesis were compared with existing literature, yielding an outlook on the field of microencapsulation through phase separation. A general concept is discussed on how to obtain various interesting complex structures with phase separation combined with microsieve emulsification. Finally, a conceptual process design is discussed for industrial scale production of microcapsules and microspheres with use of microsieve emulsification.

    This thesis has yielded insight in the formation of a range of microcapsule morphologies by investigating a range of new production methods (microsieves and demixing conditions) and formulations (different concentrations, oils and using one polymer or a blend), and through this provides better insight into the mechanisms of microcapsule formation. While some of the structures may be directly used for microcapsule formation, some other structures may well have potential for other applications.

    Figure. Examples of structured microcapsules and microspheres developed in this thesis.

    Excitation energy transfer and trapping in higher plant photosystem II complexes with different antenna sizes
    Caffarri, S. ; Broess, K. ; Croce, R. ; Amerongen, H. van - \ 2011
    Biophysical Journal 100 (2011)9. - ISSN 0006-3495 - p. 2094 - 2103.
    light-harvesting complex - green plants - chlorophyll fluorescence - charge separation - arabidopsis-thaliana - core complex - kinetics - organization - membranes - lhcii
    We performed picosecond fluorescence measurements on well-defined Photosystem II (PSII) supercomplexes from Arabidopsis with largely varying antenna sizes. The average excited-state lifetime ranged from 109 ps for PSII core to 158 ps for the largest C2S2M2 complex in 0.01% a-DM. Excitation energy transfer and trapping were investigated by coarse-grained modeling of the fluorescence kinetics. The results reveal a large drop in free energy upon charge separation (>700 cm-1) and a slow relaxation of the radical pair to an irreversible state (150 ps). Somewhat unexpectedly, we had to reduce the energy-transfer and charge-separation rates in complexes with decreasing size to obtain optimal fits. This strongly suggests that the antenna system is important for plant PSII integrity and functionality, which is supported by biochemical results. Furthermore, we used the coarse-grained model to investigate several aspects of PSII functioning. The excitation trapping time appears to be independent of the presence/absence of most of the individual contacts between light-harvesting complexes in PSII supercomplexes, demonstrating the robustness of the light-harvesting process. We conclude that the efficiency of the nonphotochemical quenching process is hardly dependent on the exact location of a quencher within the supercomplexes
    Biofilm development on new and cleaned membrane surfaces
    Bereschenko, L.A. - \ 2010
    Wageningen University. Promotor(en): Fons Stams; M.C.M. Loosdrecht, co-promotor(en): G.J.W. Euverink. - [S.l. : S.n. - ISBN 9789085858065 - 161
    biofilms - membranen - ongewenste aangroei van levende (micro)organismen - kunstmatige membranen - zuiveringsinstallaties - biofilms - membranes - biofouling - artificial membranes - purification plants
    This thesis presents a comprehensive research report on microbiological aspects of biofouling occurrence in full-scale reverse osmosis (RO) systems. Biofouling is a process in which microorganisms attach to membranes and develop into a thick film that can choke the entire RO system. Management of this problem requires basic understanding of the mechanism of this phenomenon. The basic questions of this PhD research project therefore addressed the origin, succession and spatiotemporal development of biofilms in full-scale RO systems, in particular in relation to operational aspects of RO systems. The multifaceted research strategy involving acquisitions of representative samples and use of many molecular and microscopic analysis techniques in parallel was employed. The investigation showed that biofilms are able to grow on any surface in a full-scale RO plant. This gives local niches for detachment of biomass, either as single cells or cell clumps, and results in a spreading of bacteria to the further stages of the plant. In the RO membrane modules, the enriched bacteria might more easily colonise the surfaces since they will be better adapted to growth in the system than bacteria present in the feed water. Initially, the single cell colonizers (sphingomonads) form a number of flat and abundantly EPS-embedded cell monolayers over the entire membrane surface. The clumps-associated pioneers (mainly Beta- and Gammaproteobacteria) appear to be trapped mainly in the first part of the module, most likely due to a filtering action of the spacer. In time, these bacteria develop in pillar-like structures and slowly spread throughout the whole membrane module on top of the established sphingomonads biofilm. The secondary colonisers (bacteria and eukaryotes) occur in the resulting biofilm formations. Although composition of the biofilm microbial community undergoes a succession in time, the architecture of an established biofilm appears to be rather stable. Conventional treatment of RO membrane modules with chemicals did not lead to cleaning: the sphingomonads cells can be detected under the collapsed but obviously not removed biofilm EPS matrix. After cleaning, the biofouling layer seemed to grow faster (within 6 days) than a fresh biofilm (16 days). To conclude, biofouling is a complex phenomenon with two appearances: a fouling layer on the membrane limiting the water flux and a fouling layer on the spacer limiting the water flow through the spacer channel and resulting in an increased pressure drop. It became clear that cleaning strategies should focus more on the removal of accumulated biomass and not only on the killing of cells. Moreover, the basal Sphingomonas layer requires further research to appropriately control biofouling in RO systems. It might also be possible to design the RO - membrane module in a different manner, leading to a different biofilm morphology which gives less rise to operational problems.
    Why low powdered activated carbon addition reduces membrane fouling in MBRs
    Remy, M.J.J. ; Potier, V. ; Temmink, B.G. ; Rulkens, W.H. - \ 2010
    Water Research 44 (2010)3. - ISSN 0043-1354 - p. 861 - 867.
    afvalwaterbehandeling - waterzuivering - actieve kool - adsorptie - membranen - biologische filtratie - filtreerbaarheid - uitvlokking - biodegradatie - zuiveringsinstallaties - waste water treatment - water treatment - activated carbon - adsorption - membranes - biological filtration - filterability - flocculation - biodegradation - purification plants - waste-water treatment - bioreactor mbr - sludge - flux - performance - filtration - bioflocculation - operation
    Previous research had demonstrated that powdered activated carbon (PAC), when applied at very low dosages and long SRTs, reduces membrane fouling in membrane bioreactor (MBRs). In this contribution several mechanisms to explain this beneficial effect of PAC were investigated, including enhanced scouring of the membrane surface by PAC particles, adsorption of membrane foulants by PAC and subsequent biodegradation and a positive effect of PAC on the strength of the sludge flocs. It was concluded that the latter mechanism best explains why low dosages of PAC significantly reduce membrane fouling. Cheaper alternatives for PAC may have a similar effect
    Water Replacement Hypothesis in Atomic Detail - Factors Determining the Structure of Dehydrated Bilayer Stacks
    Golovina, E.A. ; Golovin, A. ; Hoekstra, F.A. ; Faller, R. - \ 2009
    Biophysical Journal 97 (2009)2. - ISSN 0006-3495 - p. 490 - 499.
    molecular-dynamics simulation - x-ray-diffraction - solid-state nmr - lipid-bilayers - phospholipid-bilayers - na+ counterions - full hydration - phase-behavior - membranes - phosphatidylcholine
    According to the water replacement hypothesis, trehalose stabilizes dry membranes by preventing the decrease of spacing between membrane lipids under dehydration. In this study, we use molecular-dynamics simulations to investigate the influence of trehalose on the area per lipid (APL) and related structural properties of dehydrated bilayers in atomic detail. The starting conformation of a palmitoyloleolylphosphatidylcholine lipid bilayer in excess water was been obtained by self-assembly. A series of molecular-dynamics simulations of palmitoyloleolylphosphatidylcholine with different degrees of dehydration (28.5, 11.7, and 5.4 waters per lipid) and different molar trehalose/lipid ratios (1:1) were carried out in the NPT ensemble. Water removal causes the formation of multilamellar stacks through periodic boundary conditions. The headgroups reorient from pointing outward to inward with dehydration. This causes changes in the electrostatic interactions between interfaces, resulting in interface interpenetration. Interpenetration creates self-spacing of the bilayers and prevents gel-phase formation. At lower concentrations, trehalose does not separate the interfaces, and acting together with self-spacing, it causes a considerable increase of APL. APL decreases at higher trehalose concentrations when the layer of sugar physically separates the interfaces. When interfaces are separated, the model confirms the water replacement hypothesis
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