Molecular Mechanisms of Membrane Fouling

The membrane fouling problem is still the main obstacle that faces the application of membrane technology at the industrial and environmental application. So, the main motivation for this work is to develop an enhanced performance of commercial desalination membranes with polyamide barrier layer. In this study, we will use Layer-by-Layer (LbL) modification with tailored macromolecular surface modifiers in order to coat the membranes so that stable zwitterionic surface properties (for reduced fouling) and minimal loss in permeability are achieved. In order to study in detail this novel modification, we will use a model oligoamide system on surfaces which allow using analytical methods which cannot be used on real membranes. The deposition conditions for model surface preparation, the composition of tailored zwitterionic/cationic copolymers used for LbL modification as well as the LbL modification conditions will be optimized. The characterization tools are x-ray photoelectron spectroscopy (XPS; also known as electron spectroscopy for chemical analysis, ESCA) for determination the elemental composition of the deposited layer while scanning electron microscopy (SEM) is used to show the topography of the formed layers. Ellipsometry can be a useful tool in identification the thickness of the deposited layers at nano-scale. In addition, the surface plasmon resonance (SPR) will be used for testing the protein resistance of the deposited layers. Other physical and chemical properties will be detected such as the wettability of the layers using contact angle measurement, and the kind of surface charge and their quantity via zetapotential measurements. After model investigation steps, the same LbL sequence (with the optimum conditions) will be applied for a selected range of commercial nanofiltration (NF) and reverse osmosis (RO) membranes with polyamide barrier layers. The permeability and salt rejection will be measured using dead-end and cross flow mode. The formation potential of biofilm will be also detected.

Keywords: Desalination, Fouling, LbL, Protein resistance and Oligoamide.

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A major problem in the membrane technology for purification applications is membrane fouling, which is the accumulation and adherence of colloidal organic matter [1,2] inorganic salts (scaling), or bacteria that form biofilms (biofouling) [3]. Engineering strategies for mitigating fouling depend on the accurate characterization of the fouling mechanism on reverse osmosis (RO) and nanofiltration (NF) membranes using flux decline measurements [4] or studies of the physicochemical properties of the membranes, such as hydrophobicity, charge density, surface roughness, and porosity [5].

An extensive research has been devoted to understand the molecular mechanisms of fouling using a variety of techniques. For example, atomic force microscopy (AFM) was used to relate the surface chemical character to protein adsorptions or organic fouling intermolecular forces [6] , adsorption of proteins and detergents to surfaces, measured by SPR, was correlated with surface wettability [7], quartz crystal microbalances were used to study organic fouling mechanisms [8] and novel fluorimetric assays were used to characterize protein adsorption [9].

Recently, the effects of surface-exposed chemical groups on scaling were assessed by surface pressure–area (Langmuir) isotherm measurements [10] where aromatic polyamide films are an integral component of RO&NF membranes and they cannot be isolated from their supports for physicochemical studies. In addition, the supporting porous polymer layer prevents incorporation of polyamide into analytical devices and interferes with measurements. But this problem can be simplified by modeling RO&NF membranes using surfaces with well-defined and homogeneous chemistry.

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

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

Another work carried out by Zhao et al [15] in which zwitterionic hydrogel thin films anchored as antifouling surface layers of polyethersulfone ultrafiltration membranes via reactive copolymer additive. The main advantage of these hydrogels are their excellent durability in long term tests and hemocompatability. In another work, the Polyelectrolyte multilayers as anti-adhesive membrane coatings for virus concentration and recovery.

In our suggested modelling work to develop an oligoamide coating system as a surface mimetic for the polyamide barrier of the commercial desalination membranes, there is a need to neglect the effect of supporting layer so we choosed silicon and gold wafers which does not exhibit any selectivity by itself, the separation function for the composite membrane can be exclusively ascribed to the deposited polyelectrolyte multilayer [17] which will give the accurate modeling data for the surface that will be used in our work .

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And , learning from previous works, we decided to make model studies to identify the best system with respect to well-defined and stable building units of synthesized nanolayers. These nanolayers will be optimized in terms of the number and thickness of building units, the concentration of the used zwitter ionic copolymers, charge polarity and density, roughness and swelling can be determined via various techniques while these parameters can be easily controlled by varying polyelectrolyte types or/and other deposition conditions [18]. Finally,The fouling resistivity of the model system will be followed via surface plasmon resonance (SPR) measurements using bovine serum albumin as model foulants. Additional foulants may be also used.

2. Experimental Part

2.1. Materials and Chemicals

1. Commercial reverse 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 gold nanoparticles. Some chemicals will be added according to the applied procedures and optimization processes.

2.2. Synthesis of the Support Substrate and the Zwitterionic Polymer

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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 modification)

The LbL method, which typically involves the alternating adsorption of poly-cations and poly-anions, with water rinsing between each adsorption. So, here in this work we will synthesize the zwitterionic polymer starting with free radical polymerization step of (N,N-dimethylamino-N-propanesulfonate) ethyl methacrylate (DMAEMA). The obtained polymer then undergoes partial conversion to zwitterionic side groups in presence of THF. And at last step, methylation of quaternization side groups to get the desired zwitterionic polymer.

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

Oligoamide is synthesized according to Kasher et al. 2011 applying LbL methodology. In this work we will make preparation of the gold surfaces 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 a wide range of physical methods.

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

1. Surface cleaned gold coated silicon wafers will immersed in 1 mM cysteamine/ethanol for 24 hours 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 washing with water for 10 minute.

2.2.3. Studies with the Model Surfaces

2.2.3.1. Coating and Characterization the Synthesized Oligoamide with the Synthesized Zwitterionic Polymer Applying LbL Methodology.

Coating the synthesized oligoamide with the synthesized zwitterionic copolymers. The formed thin films will be characterized using ellipsometry technique. Other characterization methods will be used such as FTIR, XPS and SEM. The reaction conditions will be also tested such as (type of anionic building block, coating conditions, thickness as function of anionic building unit plus the coating condition). Depending on the obtained characterization data, the number of layers will be optimized. The optimum conditions will be applied to modify the surfaces of commercial RO/NF membranes. Ellipsometry, which is a nondestructive and sensitive optical measuring method mostly used for the analysis of thin films, where here in our work we suggest using gold wafers as supporting substrate for this methods. Via these mechanistic technique we will optimize the number of applied layers on the model oligoamide layer. And, SPR will be used to measure the fouling resistivity of the model oligoamide layer.

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2.2.3.2. Evaluation of the Synthesized Oligoamide System

Two strategies will be used to evaluate the synthsized system, first one is depending on the characters that gathered from the different characterization techniques. While, The second strategy is by doing a complete assessment the antifouling properties of the synthetic moeites via:

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

2.3.3. Modification and Evaluation the Commercial NF/RO Membranes

Based on the best characters that grasped from the above sections, the best condition will be used for modification of some commercial NF/RO membranes using the synthetic zwitterionic polymer applying LbL assembly. The modified membranes will be characterized as mentioned in the above sections. The evaluation also will be done as mentioned.

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This work mainly aims to fulfill the following SIX goals

1. Synthesize model surfaces for desalination membranes (oligoamide system) on silicon or gold substrates
2. Synthesize novel cationic and zwitterionic copolymers as building block for layer-by-layer (LbL) modification
3. Study in detail LbL modification on model surfaces (layer thickness and stability as function of novel building blocks, respective anionic building block and coating conditions) with focus on nanoscale analysis with ellipsometry
4. Study in detail the resulting surface and anti-fouling properties, with focus on contact angle, zetapotential and foulant deposition measured with surface plasmon resonance
5. Transfer the best modifications to commercial membranes with polyamide barrier layer
6. Evaluate the performance of those modified membranes vs. state-of-the-art with focus on permeability, salt rejection and long-term fouling behavior.

Benefits that will be expected from 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 experiences to the National Research Center to help in establishing the membrane technology as a successful technology in many applicable fields.