Development for next generation of polymeric membranes by using new classes of materials with novel preparation protocols is crucial to address the shortcomings of the existing commercial membranes such as antifouling resistance, chlorine intolerance and solvent stability. Polyaniline (PANI) membranes were foreseen a promising alternative material to existing membrane polymers, mainly due to their affordability and intrinsic antifouling behaviour, but more widespread applications were constrained by chlorine intolerance, and the need for prolonged post-treatment crosslinking to improve solute retention and solvent stability. This study presents a simplified technique to overcome these limitations by modifying PANI synthesis and tailoring the membrane preparation protocols using non-solvent induced phase separation (NIPS) technique. PANI was modified by introducing sulfonic acid groups (−SO3H) resulting in sulfonated polyaniline (S-PANI) that was used for the fabrication of S-PANI membranes at the ultrafiltration (UF) separation range (molecular weight cut off (MWCO) ≈ 25 kDa and pure water permeance 69.7 L m-2 h-1 bar-1). The prepared S-PANI membranes exhibited physical and chemical stability after soaking in 250 ppm sodium hypochlorite aqueous solution for 3 consecutive days whereas PANI membranes suffered from chemical degradation and complete structural damage as confirmed by visual observation, dynamic filtration tests (permeance and rejection), Fourier transform infra-red (FT-IR) spectra, scanning electron microscopy (SEM) imaging and atomic force microscopy (AFM) analysis. The dynamic filtration test using three model foulants showed enhanced antifouling behaviour of S-PANI membranes with flux recovery rate (Fr) 84%, 89% and 92% compared to 65%, 68%, 77% for the PANI membrane with alginic acid, humic acid and bovine serum albumin, respectively. However, the developed UF membranes would have limited application unless otherwise the solute retention is narrowed to the nanofiltration (NF) range and beyond. As such, a novel facile preparation technique was adopted to produce S-PANI NF membranes (MWCO ≈ 680 Da and pure water permeance ≈ 5 L m-2 h-1 bar-1). The presence of crosslinking sites (sulfonic and amide groups) at the polymer chains and the changes in the chemistry of the coagulation bath (1-3 M HCl) facilitated simultaneous coagulation and crosslinking. The instant stabilisation of the selective layer for the crosslinked S-PANI hindered the solvent/non-solvent exchange rate (from 2 h to ca. 24 h), enabled the production of a tailored membrane morphology with a dense skin layer (1.1-2.3 times), suppressed macro-voids (50-70%), reduced porosity (26-37%), enhanced tensile strength (2.3 times), increased hydrophilicity (21% drop in contact angle), and improved solvent stability (mass swelling degree and gel content in tetrahydrofuran was 88% and 90% (pristine S-PANI) compared to 3% and 100% (crosslinked S-PANI), respectively). In a water treatment scenario, the newly developed S-PANI NF membranes were compared with a commercial membrane and conventional adsorption-coagulation-flocculation, optimised for natural organic matter (NOM) removal. Artificially prepared surface water and seawater and a stabilised landfill leachate served as test solutions. S-PANI NF membranes showed best NOM separation performance for both surface water and seawater (higher by ~5-8% and ~24-41% for commercial membrane and conventional treatment, respectively). In contrast, the conventional treatment achieved higher leachate NOM removal by ~7% (S-PANI) and ~17% (commercial membrane). The S-PANI performed slightly better (~6%) in removal of chemical oxygen demand (COD) compared to the commercial membrane and conventional treatment. During long-term fouling S-PANI exhibited slower growth in transmembrane pressure (TMP) by ~52-52%, less affinity towards organic matter (~26-43% surface water, ~35-42% seawater and ~48-55% leachate) and higher flux recovery (~4-10% artificial surface and seawaters and ~12% in leachate) compared to the commercial membrane, particularly at high NOM concentration. The facile crosslinking technique was adopted for the preparation of in-situ tuneable high performance organic solvent NF S-PANI membranes (Tetrahydrofuran (THF) permeance was 2.1 to 16.4 L m-2 h-1 bar-1 with apparent MWCO between 250 to 1000 g mol-1). Different molecular weight organic acids (173 g mol-1 metanilic acid versus 800,000 g mol-1 poly(2-acrylamido-2-methyl-1-propanesulfonic acid)) were incorporated and self-doped PANI membranes were fabricated at different aqueous coagulation bath acidic strength (0.1-3 M HCl). The transport properties of the OSN membranes were stable in dynamic ageing filtration test with sequential feed of dye solutions (MW range from 327 to 1470 g mol-1) in Methanol, acetonitrile, and tetrahydrofuran over 250 h. The cast membranes in higher coagulation bath acidic strength showed increased crosslinking degree as confirmed by the mass swelling degree and gel content measurements. The higher crosslinking degree was associated with an order decline in permeance (52-92%) and an increase in solute rejection due to the contractions in the polymer intersegmental spaces. PAMPSA doped membranes were only in the tight UF separation range (MWCO ≈ 1.5 kDa) due to the dopant high MW and membrane post-treatment by wet annealing was required to narrow the membrane’s solute rejection to NF range without substantial compromise to the membrane permeance. The OSN filtration results were reproducible without any irreversible structural ageing. The characterisation results (SEM, AFM, mechanical properties, and hydrophilicity) indicated that no membrane ageing occurred over the static test period of 30 days with THF. These results indicate that the membranes exhibit good stability over a long-term period whilst still maintaining the excellent separation performances. This work has significant novelty and believed to attract research communities and engineers as it is the first study to exploit the PANI polymer synthesis and membrane preparation protocols using simplified approach to produce S-PANI membranes with noticeable improvement in antifouling behaviour, chlorine intolerance, and substantial improvement in permselective properties and solvent stability. The developed membranes have a scalability potential to suit numerous applications in water and wastewater treatment, organic solvent nanofiltration (OSN) related industries and potentially high fouling solutions in food processing applications.
|Date of Award||12 Oct 2022|
|Supervisor||Jannis Wenk (Supervisor), Salman Shahid (Supervisor) & Emma Emanuelsson Patterson (Supervisor)|