Midway evaluation in Environmental Engineering -Lingxue Guan

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Title of thesis: Integrating Pressure-Retarded Osmosis with the Direct Air Capture process for electricity production and decarbonization: Fouling behavior

Student: Lingxue Guan

Doctoral committee:
Dr. Bing Wu, Professor at the Faculty of Civil and Environmental Engineering, University of Iceland
Dr. Christiaan Petrus Richter, Professor at the Faculty of Industrial Engineering, Mechanical Engineering and Computer Science, University of Iceland
Dr. Xia Huang, Professor at the School of Environment, Tsinghua University, China

Abstract
Direct air capture (DAC), a location-independent carbon dioxide removal technology, has been considered as a critical step toward achieving a carbon-neutral and negative-emission future. Among DAC methods, systems using aqueous hydroxide (e.g., sodium hydroxide) sorbents are one of the most advanced commercially, but remain water- and energy-intensive, requiring a locational flexible, stable, low-carbon, and low-footprint energy source. Pressure-retarded osmosis (PRO), a membrane-based osmosis energy extraction technology, meets these criteria and offers a promising solution. Integrating DAC with PRO could support DAC operation by generating renewable electricity from the carbon-captured DAC solution and freshwater (a fraction of slaker feedwater). However, residual calcium carbonate (CaCO3) in carbon-captured solution and potential organic growth during long-term operation could cause membrane fouling, reducing PRO performance and energy efficiency. Understanding fouling behavior is therefore essential for process optimization. This study investigates the effect of CaCO3 crystal morphology on PRO performance. Results show that (1) increasing the calcite proportion in scalants promotes cake layers with higher porosity and lower resistance, while increasing amorphous calcium carbonate (ACC) produces denser, more resistant layers; (2) CaCO3 polymorph properties, including surface charge, specific surface area, and hydration level, influence scalant–scalant and scalant–membrane interactions, affecting water and reverse solute fluxes, as ACC-dominant layers result in greater water flux decline and increased reverse solute flux; (3) polymorph composition dynamically changes during PRO operation, leading to variable layer properties and fluxes. Future work will examine individual and combined effects of scaling and organic fouling at different applied pressures and concentrations, develop fouling mitigation strategies, and perform process simulations and life cycle assessments for the DAC-PRO hybrid system.