A membrane - foulant interaction study.

In this thesis the problem of membrane fouling has been considered from different points of view. The aim of the study is to gather information on the physical and chemical mechanisms of membrane fouling, so that selective solutions can be introduced to overcome or to diminish the problem.Anti-foaming agents (AFA) are by necessity widely used in the process industry. The severe problems during membrane filtration have been shown to be caused by AFA used in upstream process steps. AFA are amphiphilic compounds that exhibit strong adsorption on polysulfone (PSf) membranes and thus cause the first, strong, flux decline before the usual, less severe, flux decrease during permeation (chapter 1).Polypropylene glycol (PPG) as a representative of these AFA was selected as the model foulant and the DDS GR61 PSf membrane as the model membrane.The molecular mass of the solutes appears to affect the hydraulic resistance after adsorption (R _a ) as do the chemical natures of solute and membrane (chapter 2). Experiments with ethylene oxide / proylene oxide (EO/PO) block copolymers indicate hydrophobicity to be of major importance. Expressing this parameter in terms of the hydrophilic - lipophilic balance (HLB) does not provide a very useful parameter, although it illustrates the qualitative tendency of R _a to decrease with decreasing hydrophobicity of the polymer.Specific surface area determination (chapter 2) has illustrated that these membranes are highly porous materials. As compared to the flux through the support layer of the GR61 membrane (large pores), the flux through the PSf layer plays a dominant role due to the smaller pores. In view of this knowledge the characterization of the porosity (pore size distribution (PSD)) of the membrane was concluded to be important.The macroscopic consideration of the porosity of the PSf membrane has prompted the development of a technique for measuring PSDs (chapter 3). Herefore, dextrans were selected as calibration compounds. The retention measurements were standardized such that adsorption, configuration and concentration polarization effects were minimized. The method is applicable for pristine and fouled membranes.It has been established that the PSD within one type and even within one batch of PSf membranes shows differences. To enable the interpretation of the adsorption mechanism on PSf membranes the PSD of the fouled membrane always should be compared to that of the corresponding pristine membrane.A schematic model for the physical adsorption mechanism of PPG on PSf has been proposed. Due to adsorption small pores are blocked and do not contribute to the flux anymore. Pores intermediate in size remain unaltered after adsorption. Large pores are narrowed so that the flux through these pores after adsorption has decreased. The applicability of this model to other combinations of membranes and solutes has not been investigated.To characterize the membrane on a molecular scale, surface -sensitive spectroscopic techniques have been selected (chapter 4). Only a combination of data obtained from photoelectron spectroscopy (XPS), fast atom bombardment mass spectrometry (FAB MS) and attenuated total reflection infrared spectroscopy (ATR IR) appeared feasible in revealing the surface properties of the PSf. Similarities as well as differences between DDS GR61 and Dorr Oliver S10, both PSf membranes, were established. Criteria to distinguish between both samples have been developed.Exploiting the depth profiling properties of the three spectroscopic techniques a gradient in hydrophobicity of the membranes was established. The S02 chemical functions of the PSf are mainly oriented towards the bulk, whereas the hydrophobic CH ₃ and Aryl-C-Aryl structures are more oriented towards the surface.Application of the surface -sensitive spectroscopic techniques to the PPG fouled PSf membrane (chapter 5) revealed indeed an additional layer of PPG. The question if particular chemical groups could be indicated as being responsible for the interaction in, membrane and adsorbing compound could be answered affirmatively. The entities CH ₂ and CH ₃ in PPG and Aryl-C(CH ₃ ) ₂ -Aryl in PSf form a hydrophobic interaction.In general, more insight in and knowledge on the system DDS GR61 PSf membrane and PPG as a foulant has been gathered. A PSD determination method also applicable to the fouled membrane and a molecular characterization method consisting of a combination of three spectroscopic techniques have been developed. These methods are now available for application to other membrane and foulant combinations.A general discussion (chapter 6) combines the different points of view on the PPG foulant - PSf membrane system and the obtained results. The factor hydrophobicity influences the membrane fouling. R _a is for the major part determined by hydrophobicity and the interaction between PPG and PSf has been indicated as a hydrophobic one. Also the pore size alteration after PPG fouling has been established. The knowledge gathered in this study leads to a discussion about the possibilities to abolish and to avoid the fouling on membranes