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Molecular Mechanisms of Membrane Fouling

The membrane fouling problem is still the primary obstacle that faces the application of membrane technology at the professional and environmental software. So, the key motivation because of this work is to build up an enhanced performance of commercial desalination membranes with polyamide hurdle layer. In this particular study, we use Layer-by-Layer (LbL) changes with tailored macromolecular surface modifiers to be able to layer the membranes so that secure zwitterionic surface properties (for reduced fouling) and minimal loss in permeability are achieved. To be able to study at length this novel changes, we use a model oligoamide system on floors which allow using analytical methods which cannot be used on real membranes. The deposition conditions for model surface preparation, the structure of designed zwitterionic/cationic copolymers used for LbL changes as well as the LbL adjustment conditions will be optimized. The characterization tools are x-ray photoelectron spectroscopy (XPS; also called electron spectroscopy for chemical analysis, ESCA) for willpower the elemental composition of the transferred layer while checking electron microscopy (SEM) is used showing the topography of the shaped layers. Ellipsometry can be a useful tool in recognition the width of the transferred tiers at nano-scale. Furthermore, the top plasmon resonance (SPR) will be utilized for evaluating the protein amount of resistance of the deposited tiers. Other physical and chemical substance properties will be found like the wettability of the tiers using contact angle measurement, and the sort of surface fee and their number via zetapotential measurements. After model exploration steps, the same LbL series (with the maximum conditions) will be employed for a preferred range of commercial nanofiltration (NF) and reverse osmosis (RO) membranes with polyamide barrier layers. The permeability and sodium rejection will be measured using dead-end and cross movement mode. The creation potential of biofilm will be detected.

Keywords: Desalination, Fouling, LbL, Necessary protein resistance and Oligoamide.

A significant problem in the membrane technology for purification applications is membrane fouling, which is the build up and adherence of colloidal organic and natural matter [1, 2] inorganic salts (scaling), or bacterias that form biofilms (biofouling) [3]. Engineering approaches for mitigating fouling rely upon the accurate characterization of the fouling system on opposite osmosis (RO) and nanofiltration (NF) membranes using flux decrease measurements [4] or studies of the physicochemical properties of the membranes, such as hydrophobicity, charge thickness, surface roughness, and porosity [5].

An comprehensive research has been devoted to understand the molecular mechanisms of fouling utilizing a variety of techniques. For instance, atomic power microscopy (AFM) was used to connect the surface chemical character to protein adsorptions or organic and natural fouling intermolecular causes [6], adsorption of proteins and detergents to floors, measured by SPR, was correlated with surface wettability [7], quartz crystal microbalances were used to study organic and natural fouling mechanisms [8] and novel fluorimetric assays were used to characterize necessary protein adsorption [9].

Recently, the consequences of surface-exposed chemical substance organizations on scaling were assessed by surface pressure-area (Langmuir) isotherm measurements [10] where aromatic polyamide films are an intrinsic component of RO&NF membranes and they cannot be isolated off their supports for physicochemical studies. In addition, the aiding porous polymer part stops incorporation of polyamide into analytical devices and inhibits measurements. But this issue can be simplified by modeling RO&NF membranes using surfaces with well-defined and homogeneous chemistry.

There is a history for using model materials of polyamide from twenty century that model polyamide. Among these studies is utilizing a benzanilide derivatives, to check the resistivity toward lively chlorine [11]. But, there is unsuitability for the surface adsorption studies for these small substances. In addition to the above mentioned fact, trials were done to prepare analytical receptors using spin-coating techniques that obtained different surface chemistry from that obtained from polyamide RO membranes [12].

So applying the LbL method, which typically involves the alternating adsorption of polycations and polyanions, with normal water rinsing between each adsorption, can help in adsorption of polymer part on any substrate (silicon or silver wafers for example) [13]. In recent research done by Wang et al [14], they ready low-pressure water softening hollow fiber content membranes by polyelectrolyte deposition with two bilayers. Where they used PES UF as aiding layer which modified with the polycation and polyanion LbL deposition to split up the divalent ions from monovalent ions.

Another work carried out by Zhao et al [15] where zwitterionic hydrogel slim videos anchored as antifouling surface tiers of polyethersulfone ultrafiltration membranes via reactive copolymer additive. The primary good thing about these hydrogels are their excellent sturdiness in long term checks and hemocompatability. In another work, the Polyelectrolyte multilayers as anti-adhesive membrane coatings for computer virus concentration and restoration.

In our advised modelling work to build up an oligoamide layer system as a surface mimetic for the polyamide barrier of the commercial desalination membranes, there is a need to disregard the effect of supporting coating so we choosed silicon and gold wafers which does not show any selectivity alone, the parting function for the amalgamated membrane can be solely ascribed to the deposited polyelectrolyte multilayer [17] that may give the appropriate modeling data for the surface which will be found in our work.

And, learning from prior works, we made a decision to make model studies to recognize the best system with respect to well-defined and stable building systems of synthesized nanolayers. These nanolayers will be optimized in terms of the quantity and thickness of creating units, the focus of the used zwitter ionic copolymers, charge polarity and thickness, roughness and bloating can be motivated via various techniques while these variables can be easily handled by differing polyelectrolyte types or/and other deposition conditions [18]. Finally, The fouling resistivity of the model system will be used via surface plasmon resonance (SPR) measurements using bovine serum albumin as model foulants. Additional foulants may be used.

2. Experimental Part

2. 1. Materials and Chemicals

  1. Commercial opposite osmosis (RO) and nanofiltration (NF) membranes.
  2. Polystyrene sulfonic acid.
  3. bovine serum albumin (BSA), sodium chloride (NaCl), humic acid (HA).
  4. Silicon / Gold wafers/quartz.
  5. m-phenylene diamine(mPD), dimethyl formamide (DMF), triethyl amine (ET3N), trimesoyl chloride (TMC), dichloromethane (DCM), Cysteamine and Ethanol.
  6. Cuprous chloride (CuCl2), Tetrahydro furan(THF), Methanol (MeOH), 10 nm titanium nanoparticles and 30 nm yellow metal nanoparticles. Some chemicals will be added according to the applied types of procedures and optimization operations.

2. 2. Synthesis of the Support Substrate and the Zwitterionic Polymer

2. 2. 1. Synthesis of the Zwitterionic/Cationicpoly (2-(N, N, N-trimethylamino)ethyl methacrylate)-co-(2-(N, N-dimethylamino-N-propanesulfonate)ethylmethacrylate) PTMAEMA-co-PSPE (cationic building block for LbL adjustment)

The LbL method, which typically includes the alternating adsorption of poly-cations and poly-anions, with drinking water rinsing between each adsorption. So, here in this work we will synthesize the zwitterionic polymer you start with free radical polymerization step of (N, N-dimethylamino-N-propanesulfonate) ethyl methacrylate (DMAEMA). The obtained polymer then goes through partial alteration to zwitterionic area groups in presence of THF. And at last step, methylation of quaternization aspect groups to get the required zwitterionic polymer.

2. 2. 2. In-situ Synthesis of Oligoamide Regarding to Kasher 2011 [19] (model studies)

Oligoamide is synthesized relating to Kasher et al. 2011 applying LbL methodology. In this work we will make prep of the yellow metal floors with an oligoamide layer that resembles the surface chemistry of RO/NF aromatic polyamide films and that can be tested in fouling and adsorption studies using an array of physical methods.

The synthesis process can be ascribed as follow (steps starting from 1 to 4 symbolizes one cycle that may be repeted):

  1. Surface cleaned rare metal covered silicon wafers will immersed in 1 mM cysteamine/ethanol every day and night then in 2) 1% triethylamine/dimethylformamide (ET3/DMF).
  2. Immersion in trimesoyl chloride/dichloromethane (TMC/DCM), ET3N for 15 minute.
  3. Then in mPD/DMF for 15 minute and then cleansing with water for 10 minute.

2. 2. 3. Studies with the Model Surfaces

2. 2. 3. 1. Layer and Characterization the Synthesized Oligoamide with the Synthesized Zwitterionic Polymer Applying LbL Methodology.

Coating the synthesized oligoamide with the synthesized zwitterionic copolymers. The made thin films will be characterized using ellipsometry technique. Other characterization methods will be utilized such as FTIR, XPS and SEM. The response conditions will be tested such as (kind of anionic foundation, coating conditions, width as function of anionic building product in addition to the coating condition). Depending on the obtained characterization data, the amount of layers will be optimized. The most effective conditions will be employed to modify the areas of commercial RO/NF membranes. Ellipsometry, which really is a nondestructive and hypersensitive optical measuring method generally used for the analysis of thin films, where within our work we suggest using platinum wafers as helping substrate for this methods. Via these mechanistic strategy we will optimise the amount of applied layers on the model oligoamide covering. And, SPR will be utilized to gauge the fouling resistivity of the model oligoamide covering.

2. 2. 3. 2. Evaluation of the Synthesized Oligoamide System

Two strategies will be used to judge the synthsized system, first one is with respect to the characters that accumulated from the various characterization techniques. While, The next strategy is by doing a complete evaluation the antifouling properties of the synthetic moeites via:

  1. Flux measurements via dead-end method and cross circulation mode.
  2. Measuring MWCO of the synthetic moites via GPC (Gel permeation chromatography)
  3. Rejection of some organic contaminants such as BSA (bovine serum albumin)

2. 3. 3. Modification and Analysis the Commercial NF/RO Membranes

Based on the best character types that grasped from the above portions, the best condition will be utilized for adjustment of some commercial NF/RO membranes using the synthetic zwitterionic polymer applying LbL assembly. The customized membranes will be characterized as mentioned in these sections. The evaluation also will be achieved as mentioned.

This work mainly aspires to fulfill the next SIX goals

  1. Synthesize model areas for desalination membranes (oligoamide system) on silicon or platinum substrates
  2. Synthesize novel cationic and zwitterionic copolymers as foundation for layer-by-layer (LbL) modification
  3. Study in detail LbL adjustment on model floors (layer thickness and balance as function of novel blocks, respective anionic foundation and covering conditions) with give attention to nanoscale analysis with ellipsometry
  4. Study in detail the producing surface and anti-fouling properties, with give attention to contact viewpoint, zetapotential and foulant deposition measured with surface plasmon resonance
  5. Transfer the best alterations to commercial membranes with polyamide hurdle layer
  6. Evaluate the performance of these improved membranes vs. state-of-the-art with focus on permeability, salt rejection and long-term fouling habit.

Benefits which will be expected out of this work can be summarized as follow:

  1. Increasing the fouling tolerance of the commercial membranes
  2. increasing the life time of applied membranes in addition to low maintenance periods.
  3. transfere the gained encounters to the Country wide Research Center to assist in establishing the membrane technology as a successful technology in many appropriate fields.
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