THE USE OF ARABIC GUM AS AN ADDITIVE TO POLYSULFONE MEMBRANES. Abstract

Transcription

1 THE USE OF ARABIC GUM AS AN ADDITIVE TO POLYSULFONE MEMBRANES Yehia Manawi, Viktor Kochkodan, Muataz Atieh* Qatar Environment and Energy Research Institute (QEERI), Hamad bin Khalifa University (HBKU), Qatar Foundation, Doha, Qatar *corresponding author: Presenting author: Yehia Mohammad Manawi Phone number: ( ) Postal code: 5825 Organization: Qatar Environment and Energy Research Institute (QEERI), Hamad bin Khalifa University (HBKU), Qatar Foundation, Doha, Qatar Abstract Arabic gum (AG) as an additive to the casting solutions of polysulfone (PS) was investigated in this work to increase the hydrophilicity of PS ultrafiltration membranes. Various AG loading (.-3. wt.%) in PS/dimethylacetamide (DMAc) dope solutions were prepared to fabricate PS membranes using phase inversion technique. The hydrophilicity and surface charge density of the PS membranes surfaces with/without AG were characterized using contact angle measurement and surface charge analyzer. The PS/AG membranes were also tested in dead-end filtration (Sterlitech) experiments. The effect of adding AG to PS membranes was found to increase the permeate flux in addition to lowering the contact angle of the membranes (by up to 43 ) and increasing the membrane s surface charge. Keywords: polysulfone membrane, Arabic gum, membrane casting, hydrophilicity, surface charge, antifouling properties

2 . Introduction PS is a thermoplastic which is widely used in fabricating microfiltration and ultrafiltration membranes due to its high thermal and chemical resistance in a wide range of ph []. Unfortunately, PS membranes are quite hydrophobic and this in turn enhances the interaction between the hydrophobic membrane surface and the hydrophobic foulants [2]. This will increase the membrane fouling and reduce the lifetime of filtration membranes. Using an additive that will increase both the surface charge and the hydrophilicity of the membrane surfaces will results in repelling out the foulants and reducing their susceptibility to attach to the membrane surface []. Arabic gum is a natural gum prepared from the exudate of the trunks and branches of some acacia trees. AG is a mixture of polysaccharides and glycoproteins and is widely used as an edible stabilizer in food industry [3]. AG has both hydrophilic and hydrophobic parts and is thought to align itself in a way that the hydrophobic part of the AG will be attached to the membrane surface leaving the hydrophilic part to form hydrogen bonds with water and create an insulative layer which prevents hydrophobic foulants from adsorption. In this work, the effect of AG as an additive on the hydrophilicity/hydrophobicity and surface charge of PS membranes will be investigated. 2. Materials and Methods 2.. Materials Arabic gum (MW: 25,), DMAc (purity: 99%), Polysulfone (MW: 35,) were acquired from Sigma-Aldrich. The nonsolvent used in coagulation bath was Millipore deionized water (DW) Membrane Preparation PS/AG membranes were fabricated using phase inversion technique via the membrane casting system (flat sheet) from Philos (South Korea). The prepared membranes have various loading of AG in the dope solution (.,.5,.,.5, 2. and 3. wt.%) and 6 wt.% PS. All of the membranes had a DMAc concentration of 84% in the casting solution (Table ). In synthesizing the casting solutions of PS/AG, the three components (DMAc, PS and AG) were mixed via IKA Ultra Turrax T25 digital mechanical stirrer (IKA, USA) and sonicated via probe ultra-sonicator (Cole Parmer, USA) for two hours at a temperature of 6 C to make sure that PS and AG dissolved well in DMAc. After that, the mixture was degassed for six hours and cast on top of a glass plate using 2 mm gap casting system at a casting speed of 2.5 m/min and at room temperature. After that, the cast membrane (along with the glass plate) was dipped in DW and left for some time until the PS/AG membrane peeled off the glass plate. The membrane was then washed and stored in DW. 2

3 Table : Composition of the casting solution of the studied PS/AG membranes Membrane Number PS (wt.%) DMAc (wt.%) AG (wt. %) Characterization and testing of the membrane 2.3. Hydrophilicity of the membrane surface Ramé-hart Model 2 standard contact angle goniometer was used to measure the water contact angle between the water droplets and membrane surface. The size of the water droplet was two micro-liter Surface charge of the membrane Electrokinetic analyser SurPass 3 (Anton Paar KG, Austria) were used to measure the zeta potential values of the membranes. Helmholz Smoluchowsky equation was used to figure out the value of the zeta potential by calculating the slope of equation (): Eµ k ζ = Pε oε r () where ε r is the dielectric constant of water at 25 C, ε is the vacuum permittivity, ΔP is the pressure drop across the membrane, k is the conductivity of the electrolyte, μ is the solution viscosity and ΔE is the streaming potential Membrane filtration tests Filtration tests were performed using a dead-end Sterlitech HP475X stirred cell (USA). The stainless steel cell had a volume of 3 ml. The tested membranes were cut into a flat circular membrane (diameter: 48 mm and active membrane area: 4.6 cm 2 ).The pressurization of the stirred cell was performed by nitrogen gas. Equation (2) was used to figure out the amount of flux collected: J= Q (2) A. T Where A is the membrane cross- sectional area (in m 2 ), T is the permeation time (in hours), Q is the permeate volume (in Liters). 3

4 3. Results and discussion 3. Contact angle measurement Figure () shows the water contact angle values at various loadings of AG in doping solutions. As shown in Figure (), the use of AG as an additive to PS membranes was found to lower the contact angle of the membrane surface by up to 43.3 when compared with pure PS membranes. As explained above, the reduction in the contact angle of the membrane was attributed to the alignment of hydrophilic/hydrophobic branches of AG to form a protective layer that will prevent hydrophobic foulant from adhesion to the membrane surface. Contact Angle AG (%) Figure : Contact angle of PS/AG membranes at different loading of AG in casting solutions. 3.2 Zeta potential measurement As a function of ph, figure (2) depicts the zeta potentials for pure PS membrane and PS/AG (3 wt.%). As can be shown in the figure, pure PS membrane in direct contact with KCl solution exhibits small negative charge values (between -4.8 and -8.5 mv). These values have increased along with the increase in the ph (from 5.3 till 7.8). On the other hand, when AG was added to PS membranes, the surface negative charge was increased and that is thought due to the fact that hydrophobic branches of AG attach themselves to the membrane surface leaving polysaccharide branches cover the membrane surface with carboxylic groups which increases the surface negative charge of the membrane surface. 4

5 Zeta potential. mv ph 2-3 Figure 2: Zeta potential at different ph values for PS/AG membrane; () Pure PS membrane and (2) PS with 3wt.% AG in the casting solution 3.3 Water flux of the membranes Figure (3) shows the water flux values versus the operating pressures of PS membranes fabricated at different concentration of AG in the casting solutions. As depicted, the addition of AG increases the permeate flux due to the increase in the hydrophilicity and surface charge Flux (LMH) 5 5 No Additive.5% AG % AG 2% AG 3% AG Pressure (Bars) Figure 3: Water flux values at different operating pressure for PS/AG membranes at different AG loading in the casting solution (AG) 5

6 Conclusion In this work, the effect of using AG as an additive to PS membranes was investigated. The surface charge and hydrophilicity of the membrane surface were found to increase and results in an increase in the permeate flux. This will result in reducing the membrane fouling rate and extend the membrane lifetime. References. V. Kochkodan and N. Hilal, A comprehensive review on surface modified polymer membranes for biofouling mitigation. Desalination, : p V. Kochkodan, D.J. Johnson and N. Hilal, Polymeric membranes: Surface modification for minimizing (bio)colloidal fouling. Advances in Colloid and Interface Science, : p W. Falath, A. Sabir and K.I. Jacob, Novel reverse osmosis membranes composed of modified PVA/Gum Arabic conjugates: Biofouling mitigation and chlorine resistance enhancement. Carbohydrate Polymers, : p