Effect of CaCO₃ crystal morphology on osmosis membrane performance during integration of direct air capture with pressure retarded osmosis process

The direct air capture-pressure retarded osmosis (DAC-PRO) hybrid process was proposed as a feasible modification to mitigate the high energy and water consumption of the hydroxide-based DAC process. However, residual calcium carbonate (i.e., CaCO₃) with various polymorph compositions may be present after sorbent regeneration in the DAC process, potentially causing scaling in the PRO process using the CO₂-captured hydroxide solution as the draw solution. This study investigated the osmosis membrane performance in the presence of different polymorphs of CaCO₃ scalants (formed at 20 °C, 40 °C, and 60 °C) in NaHCO₃- or NaHCO₃/Na₂CO₃-based draw solutions under PRO mode. The results revealed that (1) increasing the calcite proportion in the scalants (a mixture of calcite, vaterite, aragonite) promoted the formation of cake layers with higher porosity and lower resistance, while increasing amorphous calcium carbonate (ACC) in the scalants (a mixture of ACC, vaterite, aragonite) caused denser and more resistant scaling layers; (2) the properties of CaCO₃ polymorphs, such as surface charge, specific surface area, and hydration level, were associated with scalant-scalant and scalant-membrane interactions and further influenced on water and reverse solute fluxes. The ACC-dominant scaling layer resulted in more water flux decline and reverse flux increase compared to those deposited with other CaCO₃ polymorphs; (3) the composition ratios of CaCO₃ polymorphs in the scaling layer dynamically changed with PRO operation, leading to variable scaling layer properties (porosity, resistance) and water/reverse solute fluxes. This study reveals the potential influences of operating conditions of the DAC process on the PRO performance.