Mechanisms of biofoulant-membrane interactions for ultrafiltration and microfiltration membranes

Since the majority of biofouling occurs on the surface of the polymeric membranes, key interfacial interactions between a membrane surface and a biofoulant play a pivotal role in biofouling phenomena. In thermodynamic perspective, biofouling can be interpreted as adhesion of two different entities, a biofoulant and a membrane, through a media, (herein, water), to be a combined column. Therefore, adhesion accompanies a structural change of interfaces existing in the aqueous system. This interface-transitive nature of biofouling phenomena leads that surface tension-based theories are adopted in the fundamental understanding of the membrane biofouling. Among those theories, the van Oss approach has been considered prominent because of its satisfactory prediction for experimental results of biofouling tests. According to van Oss approach, in aqueous media, Lewis acid-base interaction occurs between the electron-acceptor in hydrogen atoms of water and lone pairs of atoms of a solid, basically having an analogous concept with hydrogen-bonding. The membrane surface with polar properties is able to hold water molecules and form a hydration cell layer on its surface, which may impede the access of biofoulant to the membrane surface. Because the biofoulant also has the electron-donating and accepting potentials, other interfacial interactions also occur in biofoulant-water and biofoulant–membrane surface. Therefore, better thermodynamic understanding on the biofouling phenomena need a perspective that ponders the three different but deeply interconnected interfacial interactions existing in membrane-water, water-biofoulant and biofoulant-membrane. The purpose of this dissertation is to investigate the biofouling phenomena in the molecular basis that can qualitatively and quantitatively evaluate the roles of physicochemical properties of water, membranes and biofoulants in biofoulant-membrane interaction for aqueous system. First, this study investigated the water-membrane interaction in terms of surface tension properties of a membrane and their discrete interactions with water in Chapter 2. This chapter simulated the available surface tension parameters of polymeric porous membranes using different presumptions on polar properties of water and evaluates the effect of each surface tension parameter of a membrane on water-membrane interactions. The simulated libraries of surface tension parameters revealed that polymeric porous membranes highly tend to fall into a category of electron-donor monopolar or semi-monopolar solids. The high electron-accepticity of water emphasized the role of electron-doicity of a membrane in the interfacial interaction between a membrane and water and caused the polar adhesion between electron-donor sites of a membrane and electron-acceptor sites of water to be a major interaction component than other components. The high Lewis acidity of water highlighted the role of electron-donicity of a membrane in its wettability as well. A higher electron-donicity of a membrane has little impact on the total surface tension of the membrane surface, but successfully favors the electron-acceptor sites of water forming a significantly lower surface tension of the water-membrane interface and thus a greater affinity between the membrane and water.Chapter 3 investigated the effects of water-solids interactions on biofouling phenomena and explored fining key physicochemical properties of a membrane for biofouling resistance. The high electron-accepticity of water emphasized the importance of electron-donicity of a membrane and a biofoulant in their interfacial interaction through water and drove acid-base interaction to be a major interaction component in the interfacial interactions between membranes and a biofoulant. The surfaces of various biofoulants were characterized with highly variable electron-donicities and the biofoulant with lower electron donicity can be more vulnerable to biofouling due to the inferior hydration energy formed on the surface of the biofoulant. The inferior hydration energy by a low electron-donicity of a biofoulant can be compensated by an enhanced electron-donicity of a membrane; this fact leads to propose a standard of an electron-donicity demand for a membrane to have a positive energy barrier against a given biofoulant. Experimental results of a microbial adhesion test and biofouling tests qualitatively correlated well with tendencies of electron-donicities of the test membranes, proving an electron-donicity of a membrane to be a reasonable indicator for the anti-biofouling property of the membrane.Chapter 4 attempted to impart membranes with hydrophilic and anti-biofouling properties by grafting hydrophilic polymers to the surface. Photoactive perfluorophenyl azides (PFPAs) were utilized to generate highly reactive nitrenes (when exposed to UV light) that can covalently bind to the membranes’ surfaces. Three different types of small molecule PFPA derivatives were applied and the experimental data directed us to modify the UF membrane surfaces with water-soluble small molecules to maintain the membranes’ high permeability. The effects of small molecule PFPA derivatives on surface tension characteristics of a polymeric membrane were analyzed by van Oss method. Modeling data indicated that the electron-donicity of membranes plays an essential role in foulant adhesion forces. The analyzed physicochemical properties of the modified membranes were compared to the experimental results of a biofouling test with alginate solution. The modified membranes with higher electron-donicity exhibit outstanding foul-resistance against sodium alginate, a model foulant, during operation.