Preliminary study on asymmetric PSF membranes for UF processes application into industrial grade wastewaters purification field.

Short Introduction The last years the use of Ultrafiltration (UF) membranes for water treatment has been increased because of UF membranes present the advantage to work in low-pressure conditions. In this context, polysulphone PSF) based membranes are among the most commonly used materials due to the high hydrolytic and chemical and thermal stability, mechanical strength, wide pH tolerance (1-13), low cost of the polymer and application flexibility. However, the membrane hydrophobicity and poor resistance to "fouling" reduce the water permeability, molecular selectivity and process duration and represent an obstacle to their use. In this work, a preliminary investigation on development of asymmetric PSF membranes using Pluronic P-123 as additive has been investigated. The phase inversion (method by non-solvent coagulation-bath-technique) was used to obtain asymmetric membranes with different characteristics of porosity, hydrophilic properties, macro-voids size and shape. Promising results in terms of compromise morphology/hydrophilic properties/stiffness were obtained. Some samples supplied a good approximation for requested morphology. The results have to be further investigated. IR and zeta-potential measurements are actually in progress. The future work will be devoted to standardize the preparation procedure and individuate the most performing samples for final application. Material and Methods Polysulphone Bisphenol A (MF=(C27H22O4S)n by Sigma-Aldrich - Average Mw?35,000 by LS) was selected as starting polymer in two different concentration (5-18wt.%). DMAc (Carlo Erba) as polymer solvent was used. Different non-solvents for membrane formation through NS-CB method using two demixing times (20 - 600s) for immersion were investigated (EtOH, H2O and their 1:1 mixture) as well as the surfactant to increase the hydrophilic properties: Pluronic P-123 (Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) - Average Mn = 5,800) by Sigma-Aldrich was tested. Inversion phase (PI) process by non-solvent coagulation bath (NS-CB) as membrane preparation procedure and doctor-blade casting technique (Elcometer® Blade Knife-Model 3580) as casting method were selected [2-4]. NS-CB is able to supply asymmetric membranes with adequate finger-like macro-voids morphology determining water flux (Jw). In-fact, the solvent/non-solvent/polymer system and coagulation medium control the porosity, pore structure and pore distribution. The membranes were characterised in terms of XRD, TGA-DSC, water retention (@rT, 60°C), contact-angle measurements, DMA, N2 adsorption-desorption isotherm measurements. Preliminary water permeability tests (0-30 psi pressure range) were carried out to determine the hydrodynamic properties of membranes. Results and Discussion The process steps for membranes preparation were defined and a preliminary standardization was performed. Eleven membrane samples (PSF2-1 to PSF2-11) were synthesised using different parameters and preparation procedure. The characterisation supplied the following main results: XRD and TG-DSC demonstrated the reliability of the prepared samples and no significant variation was recorded for samples with and without surfactant (XRD), indicating that the structure is not affected by surfactant introduction. Regarding water retention behaviour (@rT-60°C), the obtained values demonstrated the surfactant introduction enhanced the membranes hydrophilic properties. In terms N2 adsorption-desorption isotherms (BET), membranes showed not particularly high Surface Area (10-60 m2/g) with pores size in mesopores-range and a little micropores percentage (about 1 nm). PSF2-3 sample supplies the most promising surface area and differential pores volume distribution. The last BET measurements are in progress (samples PSF2-8 up to PSF2-11). Regarding the contact angles (CA) measurements, CA values << 90°C were recorded for all samples without surfactant. Instead, for samples with surfactant, CA values were not low as well as those expected, probably due to a not correct P-123 distribution inside the polymer matrix. Hydrophilic properties have to be improved through the introduction of different P-123 concentrations or new surfactants/additives. Permeability tests were performed into an Amicon cell (0-30 psi pressure range). Only PSF2-1 up to PSF2-4 samples overcame permeability tests. The other samples are little brittle and suffer the pressure operation. PSF2-3 records a correct linear trend following Darcy's law. PSF2-4 is almost linear with a gap at about 15-20psi. Conclusions The characterisation results supplied the first interesting conclusions: miscibility and fast solvent/non-solvent exchange guarantee the correct "finger-like morphology"; the most performing asymmetric membranes were obtained when lower de-mixing times (Dt) and water as a non-solvent are used (PSF2-3 and PSF2-11); mesoporous membranes obtained with CA<90°C (hydrophilic character); asymmetry with "finger-like morphology" obtained and determining parameters identified; surface porosity (500nm-2?m) obtained for some samples with channels of about 1-2?; a good permeability trend obtained in terms of Darcy's law for some samples (PSF2-3 and PSF2-4); two membranes identified as the best prototypes to be further optimized. Crossing permeabilty, CA together to BET and SEM results, PSF2-3 sample (without surfactant) and PSF 2-11 (with surfactant) seem good candidates for final application. It's surely necessary to improve the membranes manufacturing and reduce the pores and channels size as well as to better connect the synthesis parameters to the manufacturing quality in order to select the optimal membrane.