Development of Thermo-Responsive Materials for Osmotic Control and Their Application to Water Treatment

Abstract

Signal-responsive materials are defined as materials which show an abrupt change in physical or chemical properties by external stimuli such as temperature, light, pH, magnetic field, electric field and so on. They have gained a lot of attention since their application area is broad. In order to exhibit the signal-responsive character it can be achieved by designing the molecular structure. Amongst a variety of signal-responsive materials thermo-responsive materials have been extensively investigated due to their easy treatment of stimuli. Thermo-responsive materials are categorised as two different materials, lower critical solution temperature (LCST) and upper critical solution temperature (UCST) materials. LCST materials are miscible with a certain solvent below a certain temperature but immiscible above the certain temperature. Whereas UCST materials are insoluble with a certain solvent below a certain temperature, while they are soluble above the temperature. A sudden change in solubility can derive the change in osmotic pressure in a solvent. It can be further applied for water treatment system using water as a solvent. The phase transition phenomenon can be explained by the Gibbs free energy equation when water and thermo-responsive materials are mixed together. Gmix = Hmix – TSmix (Gmix : the Gibbs free energy of mixing, Hmix : the enthalpy of mixing, Smix : the entropy of mixing, T : temperature) When the Gibbs free energy of mixing (Gmix) becomes zero at a certain concentration, the temperature can be defined as the phase transition temperature, either LCST or UCST. T =HmixSmix (Gmix = 0, at a certain concentration) The enthalpy term (Hmix) and the entropy term (Smix) are closely related to the phase transition temperature and dependent on the balance between hydrophobicity and hydrophilicity of the molecule. By controlling the hydrophobicity and the hydrophilicity of the molecule the thermo-responsive character is decided. Various kinds of thermo-responsive materials are synthesised for water treatment system through the systematic design of the thermo-responsive materials. Amongst water treatment systems including thermal distillation, reverse osmosis and so on, forward osmosis (FO) system has been intensively studied. Since FO system has advantages such as less membrane fouling and energy conversion efficiency, FO is recently obtaining a significant amount of attention. FO consists of two different solutions, a draw solution and a feed solution, and a semi-permeable membrane and utilises the difference in osmotic pressures between the two different solutions by osmosis. In a drawing process, the draw solution which has a higher osmosis pressure can draw water from the feed solution through the membrane. Subsequently, draw solutes should be separated in the diluted drawn solution so that water can be obtained. Thus, draw solutes are significantly important in FO system. As an ideal draw solute, it is necessary to satisfy high osmotic pressure, easy separation, regeneration, reusability and so on. One of the most extensively investigated FO system is ammonia/ammonium bicarbonate (NH3/NH4HCO¬3) FO system which meets some of the criteria. However, the NH3/NH4HCO3 system consumes a high thermal energy, causes a membrane damage and involves a complex regeneration process. Hence, it is necessary to develop a new draw solute which can exhibit high osmotic pressure, uses a small amount energy for the separation process and involves negligible membrane damage. As an alternative, FO system with thermo-responsive materials can be established. Thermo-responsive draw solutes should generate high osmotic pressure, and be separated from water with mild temperature change. In addition, the draw solution should be neutral in order not to damage the membrane. A previously studied thermo-responsive material, branched polyethylenimine was acquired by a simple acylation. Since acylated branched polyethylenimine (acyl-b-PEI) is a polymer with high molecular weight, it is unable to show high osmotic pressure based on the vant Hoff equation. In order to overcome such limitation carbon dioxide (CO2) gas was injected into the polymeric solution because tertiary ammine moieties in the polymer can absorb the hydrated form of CO2, bicarbonate ion, HCO3-. Bicarbonate-containing polymeric draw solution shows high enough osmotic pressure to draw water from seawater. After the drawing process, thermal energy was applied and the phase separation occurred and the absorbed CO2 was released. Even though CO2-combined acyl-b-PEI FO system can be applied for desalination, it is still required to develop a draw solute which can generate high enough osmotic pressure to draw water from highly concentrated wastewater. Hence, ammonium iodide salts with lower molecular weight are further investigated. Ammonium iodide salts are UCST materials and can generate high osmotic pressure due to the ionic character as well as low molecular weight. By controlling the molecular structure a series of ammonium iodide salts can be synthesised. A various ammonium iodide salts are investigated and some of the suitable salts are selected and subsequent FO process is conducted with the extracted wastewater from a factory. In conclusion, this dissertation can give a guideline to develop thermo-responsive materials with the required criteria by fine tuning the molecular structure. In addition, with the implementation of desalination and wastewater treatment, I hope that the introduced thermo-responsive draws solutes have high potential and can be applied for the feasible water treatment system.