PVDF membranes for filtrations in aqueous and solvent media
Membrane technology has become a well-established technology for the treatment of various waste water streams. However, membrane fouling often remains a problem, while some waste water streams contain harsh solvents, impurities, an extreme pH or an elevated temperature and thus demand for a more stable membrane type. To overcome these fouling and stability problems, two approaches were investigated in this research: (i) fouling mitigation by applying membrane vibration via the magnetically induced membrane vibration (MMV) system, and (ii) optimization of the stability and applicability range (microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF)) of a poly(vinylidene difluoride) (PVDF) membrane.
In the first part of this thesis, PVDF membranes were tuned to be specifically operated in combination with the MMV system. PVDF membranes were prepared through phase inversion by adding poly(ethylene glycol) (PEG) as a pore-forming agent in different concentrations and with different molecular weight (MW) and by adjusting the polymer concentration in the casting solution. The overall performance of the modified membranes was evaluated using filtration indices (FIs) and a modified critical flux (CF) filtration method. The state of the art flux for anaerobic membrane bioreactors (AnMBRs) of 6 L/m².h could thus be improved by more than 60%. Parameters such as surface pore size and membrane bulk porosity were identified as the two important parameters to optimise membranes for vibration systems. Secondly, the MMV system was redesigned to an even more dynamic and energy efficient system, with alternating movements and tuneable frequency, amplitude and intermodule distance. This 2nd generation system was validated in an aerobic membrane bioreactor (AeMBR) and then further explored for AnMBRs. For both membrane bioreactors (MBRs), the MMV system showed to be highly capable in drastically lowering the fouling rate, with up to a factor of 10 for AnMBRs. Combining the MMV system further with a backwash cycle, proved the AnMBR to be able of operating continuously for 10 days at 20 L/m².h.
In the second part, PVDF membranes were further adapted to withstand extreme conditions. Current solvent resistant nanofiltration (SRNF) membranes still lack (i) stability towards extreme pH or certain solvent types (e.g. harsh polar aprotic solvents), (ii) lack tuning possibilities towards the desired molecular weight cut-off (MWCO), or (iii) have a relatively low solvent permeance. PVDF is a membrane material that has been extensively used for treating wastewater streams as a MF or UF membrane and has remarkable mechanical, chemical and thermal stability properties. This material is, however, not easy to tune towards MWCO within the NF range via phase inversion, nor has it been used for extreme conditions.
By accurately tuning phase inversion parameters, such as polymer concentration and evaporation time before coagulation, an integrally skinned asymmetric (ISA) NF membrane was prepared. While the produced membrane was capable of retaining small solutes (327 Da) by more than 80%, the stability towards solvents needed to be increased. This was realized through the reaction of the polymer with a crosslinker (i.e. a diamine). Crosslinking the PVDF membrane was studied through a one-pot reaction combining dehydrofluorination and crosslinking, using p-xylenediamine (XDA). In contrast to pristine PVDF, the optimal membrane exerted a very high stability towards extreme pH (5M HCl or NaOH) and various types of solvents (polar protic, polar aprotic and apolar). A remarkable solvent-activation effect was observed, boosting the permeance by a factor of 11 by freeing-up pore volume by removing reactants and loosely crosslinked polymer, without affecting the membrane retention. On its turn, retention could also be further increased (by 10-50%) by drying the membranes. During this drying process, the polymer matrix was further crosslinked through free amine groups of XDA molecules that had only reacted once during the initial crosslinking reaction. Both effects, solvent activation and drying, have thereby significantly increased the SRNF applicability of PVDF. The versatility of the approach was proven by further tuning the XL-PVDF membrane to operate in the UF range in which it was successfully used for the purification of a metal organic framework synthesized in a very challenging organic solvent medium containing strong acids.