Understanding the Structure-Property Relationships in Selective Membranes for Desalination and Ion Separation

Membrane-based desalination and ion separation processes have been developed to mitigate the stress on global water supply and to satisfy the needs for the emerging clean-energy production and storage field. To continue to meet the increasing demand, advanced and highly selective membranes are required to separate water and ions from seawater in cost-effective and energy-efficient manners. A general lack of fundamental structure-property relationships frustrates the development of these membranes. Advances in membrane synthesis enables precise functionalization of the membranes with task-specific functional groups (e.g., charged group for enhanced salt rejection) or fillers (e.g., size-selective fillers for ion separation), yet more research efforts are required to fully elucidate the role of these functional groups/fillers on the membrane water/salt transport properties. Several problems, important but often overlooked, naturally arise from the membrane functionalization. First, the mismatch of introduced functional group/filler hydrophilicity and the pristine membrane hydrophilicity would cause changes in the hydrated membrane water fraction, which eventually affects the membrane water/salt transport. Next, the introduced functional groups/fillers aimed to tailor the membrane water/salt sorption could possibly change the membrane water/salt diffusion as well, thus making the overall effects of the functional groups/ fillers on the membrane water/salt transport hard to predict. Finally, any functionalization on the membrane is associated with environmental and economic costs, so the membrane functionalization is only justified if the benefits of functionalization overcome such costs.
In this thesis, fundamental theories and basic models related to membrane small molecule transport, e.g., the free volume theory and the solution-diffusion model, were first introduced, to establish the theoretical frameworks for further discussion on desalination and ion separations. The role of two types of functional groups, i.e., interactive and non-interactive functional groups, were then investigated, to study their influences on the membrane water/salt transport properties. Our results suggest that non-interactive functional groups, e.g., triptycene groups, could enhance membrane desalination performances via free-volume rearrangement. As for interactive groups, e.g., hydroxyl groups, configuring them in an even-distribution manner could promote membrane desalination performances as water-clustering in the hydrated membrane is minimized. Pioneer works related to the cleaner production of MOFs containing MMMs were conducted to investigate the feasibility of producing/using such MMMs for ion separation. Life-cycle assessment and techno-economic analysis confirmed the largely environmental and economic favorability producing UiO-66-NH2 from an aqueous-solution based system, while membrane transport studies and material characterizations suggest further research efforts are necessary to fabricate MMMs containing UiO-66-NH2 from aqueous-solution based systems for selective ion separation.