Use of green solvents in nanofiltration membrane preparation via phase inversion

Membrane technology has attained an important place in separation processes and is gradually substituting conventional separation processes, such as distillation and evaporation, which are highly energy-intensive or create waste streams. Membrane-based separation processes are considered green and sustainable, but the membrane preparation process itself is far from green. More than 50 billion liters of wastewater are being generated every year contaminated with toxic solvents. This critical challenge is often ignored, and only a few attempts have been made so far to improve the sustainability of membrane fabrication by replacing toxic solvents with 'greener' alternatives. This PhD-research aims at replacing solvents like N, N-dimethylacetamide, N, N-dimethylformamide and tetrahydrofuran, which are currently used in membrane manufacturing, with 'green' solvents. For this purpose, several classes of solvents were considered: (1) bio-based solvents (more specific γ-valerolactone (GVL), as well as a set of glycerol derivatives (diacetin, α,α′-diglycerol, monoacetin, triacetin, methyl lactate, 1,2-propanediol, 1,3-propanediol, glycerol carbonate, glycerol, glycerol formal and glyceric acid)), (2) a set of organo-carbonates (dimethyl carbonate, diethyl carbonate, propylene carbonate, glycerol 1,2-carbonate, 1,2-butylene carbonate, styrene carbonate and 1,2-hexylene carbonate) obtainable through carbon dioxide fixation, and (3) environmentally safer, commercially available solvents, like Tamisolve® and triethyl phosphate. They were screened for their potential use as a solvent for membrane preparation by determining the solubility of a variety of common membrane polymers, including, CA, PI, CTA, PES, PSU, PAN, PVA, PVDF and chitosan, and by verifying their applicability in the process of phase inversion to create useful membranes with appropriate pore structure, permeance, and selectivity in the nanofiltration (NF) range. The latter creates an extra challenge as very small pores need to be created, which can only be realised under strict conditions of phase inversion, i.e., requiring a really high solubility of the membrane polymers in the casting solvents. Hansen solubility parameters were used to rationalize this polymer solubility. The green solvents were blent with a conventional solvent whenever the polymer was experimentally found insoluble in the screened green solvents. By this dilution approach, the use of toxic solvents in the overall process for membrane preparation is already at least significantly reduced. Membrane morphology was characterized using scanning electron microscopy (SEM), while membrane performance was investigated using rose bengal (RB, 1017 Da) or MgSO4 (120 Da) in water as feed to screen the potential to tune these polymer/solvent systems toward membranes with properties in the NF range. CA as a bio-based polymer was selected to prepare membranes using methyl lactate as a green solvent and 2-MeTHF as a green co-solvent to develop full bio-based technology for membrane preparation. In this system, with increasing CA-concentration in the casting solution from 8 to 20wt%, RB-rejection increased from 31.1% to 99.5%, while permeance decreased from 32.0 to 2.4 L/m2.h.bar. However, with increased co-solvent concentration (30-50wt%), both rejection and permeance surprisingly decreased, regardless of evaporation time. For membranes cast from a 20wt% polymer solution, RB-rejection remained around 99%, and permeance decreased with increasing co-solvent concentration in the casting solution. MgSO4 rejection was around 93.0% for the best membranes cast from a 20wt% polymer solution with 10wt% co-solvent without evaporation (at a flux of 1.0 L/m2.h.bar) and 96.5% with evaporation at a flux of 1.3 L/m2.h.bar. Similarly, CA/glycerol derivative (i.e., glycerol derivatives, namely triacetin, diacetin, monoacetin, and glycerol-formal) systems were selected as alternative full bio-based systems, using 2-MeTHF as co-solvent for preparation of NF membranes via NIPS. The best membranes were obtained using diacetin as a solvent and 2-MeTHF as co-solvent with permeances ranging from 12.8 to 5.5 L/m2.h.bar with RB rejections higher than 90%. When implementing a 90s evaporation step prior to coagulation, rejection increased with increasing 2-MeTHF concentration in the casting solution in the case of diacetin and triacetin. Among these glycerol derivative systems, the diacetin/co-solvent system is recommended for NF application due to the combined high permeance and high RB rejection of the resulting CA membranes. GVL was also investigated as a renewable green solvent based on the good solubility of many polymers in it. CA, PI, CTA, PES, and PSU membranes were fine-tuned for NF either by increasing the polymer concentration in the casting solution or selecting the suitable non-solvent for phase inversion. The best membranes were prepared with CTA in GVL using water as non-solvent. With increasing CTA concentration (10wt% to 17.5wt%) in the casting solution, permeance decreased from 15.9 to 5.5 L/m2.h.bar with RB rejection higher than 94%. Hansen solubility parameters were studied to rationalize the polymer solubility. In the last part of this thesis, sustainable technology was developed for PVDF-based solvent-resistant NF membranes preparation. Tamisolve® NxG was selected as a more sustainable solvent to dissolve 6-15wt% PVDF. A 10wt% alkaline solution containing polyethylenimine (PEI) was applied to crosslink the PVDF membranes. For the characterization of pristine PVDF and crosslinked XL-PVDF membranes, FTIR, SEM-EDX, TEM, TGA, and elemental analysis were used. The best NF membrane was prepared using 10wt% PVDF. After crosslinking, that membrane had a remarkable permeance of 148.8 L/m2.h.bar combined with a 95.8% RB-rejection. The molecular weight cut-off (MWCO) of the crosslinked membrane cast from a 15wt% PVDF solution reached a MO rejection above 90% with a high permeance of 12.6 L/m2.h.bar. Solvent resistance and pH stability of XL-PVDF membranes were confirmed experimentally. Although not all prepared membranes yet qualified for NF, several polymer/green solvent systems still have the potential to lead to denser membranes upon further research, e.g. by adding volatile solvents and introducing longer evaporation steps before phase inversion.

Publication year:2021

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