UC Berkeley

Water purification membranes can provide freshwater from the treatment of non-traditional water resources like seawater, brackish groundwater, and wastewater. However, these membranes are limited by shortcomings including fouling, the “Achilles heel” of membrane processes, and lack of selectivity towards key contaminants. These weaknesses lead to increases in energy requirements, complexity of operation, and overall costs. The concept of sacrificial membranes can address these challenges by enabling the removal and regeneration of fouled or spent surface layers. Meanwhile, the emerging field of nanomaterial-based membranes has shown great potential to improve upon the performance of conventional membranes. Molybdenum disulfide (MoS2) nanosheets in particular have shown exceptional capabilities in membranes, yielding ultrafast water transport, solute selectivity, and specific adsorption. In this dissertation, we target synthesizing sacrificial membranes using MoS2 nanosheets for the control of fouling in high-pressure membrane processes. These novel membranes combine MoS2 nanosheets with polyelectrolytes in layer-by-layer assembled sacrificial layers on commercial nanofiltration membranes. In long-term organic fouling and inorganic scaling studies, the MoS2-polyelectrolyte sacrificial membranes showed excellent fouling resistance and cleaning efficiency. The findings suggest the potential for membrane regeneration and sustained fouling resistance that may be achieved by utilizing disruptive interactions with the sacrificial layer materials. A focused study on removal mechanisms of the MoS2-polyelectrolyte sacrificial layers demonstrated that the most successful removal was achieved by oxidizing MoS2 nanosheets or physically disrupting layer interactions. Given the results from the MoS2-polyelectrolyte sacrificial layers, we designed regenerable MoS2 multi-functional membranes for the sustained long-term removal of hexavalent chromium, a toxic groundwater contaminant. By tailoring nanomaterial loading on the multi-functional membrane, the chromium removal capacity can be improved. Optimization of other key factors like membrane substrate and cleaning agent dosage will be crucial to making these multi-functional membranes truly regenerable. These insights into the complexities of deposition, removal, and regeneration have important implications for the future of nano-enabled multi-functional membranes. In this dissertation, we first introduce the field of sacrificial membranes and the potential for use of nanomaterials. Then, we go into detail on the specific materials and methods that are used in the research chapters. In Chapter 3, we discuss the synthesis and characterization of the novel MoS2-polyelectrolyte layer-by-layer assembled membranes. In Chapter 4, we evaluate and discuss the capabilities of these membranes to control fouling. In Chapter 5, the sacrificial layer removal mechanisms are tested and evaluated for MoS2-polyelectrolyte membranes. The findings from these tests are used to design sacrificial MoS2-based multi-functional membranes for targeted removal of hexavalent chromium, a toxic groundwater contaminant. Finally, we conclude in Chapter 6 with a discussion of the advantages, limitations, and potential future research directions of sacrificial membranes.