Functionalization of Lignocellulosic Materials for Novel Membranes in Oil-Water Separation Technology

Abstract

With a steady growth in world population and increasing pollution, drinking water is becoming a scarce resource. The main pollutant in water is oil, mostly due to anthropogenic causes. This creates a need for novel oil/water separation devices. In order to decrease the need for oil production and transport, in this work, we proposed the use of bio-based lignocellulosic materials with anisotropic hierarchical porous structure, such as wood, for oil/water separation. Throughout its evolution, the wood structure has been optimized by nature for water and nutrient transport, and mechanical stability of the tree. This makes it an eligible bio-based material for oil/water separation. We first tested spruce wood cross sections and found out that they could be used in their native state for free oil/water separation. The separation is based on capillary forces, and it operates with high efficiencies, high fluxes, and low fouling. However, in order to separate oil/water emulsions, modification of the wood structure was shown to be necessary, since for oil repellency, low surface energy of the repelling material is needed. In the next step, we functionalized wood cross sections with a hydrophilic polyelectrolyte domain and a hydrophobic fluorinated domain, to allow for oil/water emulsions separation. These well-defined domains with opposite wettabilities were obtained through the grafting of block copolymers with amphiphilic properties via AGET-SI-ATRP. We investigated the solvent influence on the spatial distribution of the polymer-modification in the wood scaffold. We showed that by using dichloromethane (a poor wood-swelling solvent) we could limit the reaction of the initiator to the lumen/cell wall interface, in the first micrometers of the samples. Contrarily, pyridine (a good wood-swelling solvent) transported the initiator inside the cell wall, and deep inside the wood sample (confirmed by Raman and FTIR spectroscopy). These results were reflected on the homopolymerization of the superhydrophilic polyelectrolyte ([2-(Methacryloyloxy)ethyl]trimethyl ammonium chloride - METAC) and the superhydrophobic (2,2,2-Trifluoroethyl methacrylate – TFEMA) monomers, which showed polymer distributions similar to the macroinitiator. To produce two distinct polymer domains, we grafted a block copolymer consisting of a charged superhydrophilic polymer (poly([2-(Methacryloyloxy) ethyl]trimethyl ammonium chloride) - PMETAC) and a superhydrophopic and oleophobic polymer (poly(2,2,3,3,4,4,5,5-Octafluoro pentyl methacrylate) - POFPMA). The successful incorporation of the two monomers was confirmed by Raman and FTIR spectroscopy. While the grafting of PMETAC and POFPMA provide wood with either superhydrophilic or superhydrophobic properties, we showed that by combining these two monomers in different ratios to form a block copolymer, we could get intermediate wettability properties, ranging from superhydrophilic to highly hydrophobic. Based on these results, first attempts on the separation of oil/water emulsions were conducted. A limitation for the separation of oil/water emulsions was found to be the size of the prepared oil droplets in water. The oil droplets produced were much smaller than the largest lumen size, and the modified wood could not be used for this challenging separation. The preparation of emulsions with larger droplet sizes and the determination of feasible separation parameters has to be addressed in future works. Finally, we started to investigate the required settings for a continuous separation set-up. The study revealed the pressure constrains and design specifications needed for a possible scale-up. Given the low breakthrough pressure of our membrane a precise pressure control is required. The work carried out within the frame of this thesis resulted in significant advances in the field of polymeric modification on bio-based lignocellulosic materials and bio-based oil/water separation devices, which can be a step forward towards a more sustainable world.