Combined CaCO3 scaling and organic fouling in a pressure-retarded osmosis process using simulated hydroxide-based CO2-captured solutions

Integration of hydroxide-based direct air capture with pressure retarded osmosis (DAC-PRO) has been considered a promising approach for sustainable DAC deployment. However, the combined effects of scaling (i.e., residual calcium carbonate, CaCO 3 from DAC) and organic fouling under long-term non-sterile operation may compromise process performance. This study investigated the osmosis membrane performance in the presence of CaCO 3 -alginate fouling at different applied hydraulic pressures, and CaCO 3 /alginate ratios in NaHCO 3 -based draw solutions (CO 2 -captured solutions). The results indicated that (1) increasing hydraulic pressure (0–4 bar) increased CaCO 3 -alginate fouling resistance by ~4–48% due to co-deposition of more amorphous CaCO 3 (ACC; ~65–710%) and alginate (15–30%) with reduced foulant size; (2) higher alginate/CaCO 3 ratio (organic fouling-dominated scenario) in the draw solution reduced fouling resistance by ~20%, because more negatively-charged alginate were adsorbed onto CaCO 3 , leading to form loose-attached porous cake layer on the membrane; (3) increasing CaCO 3 /alginate ratio (scaling-dominated scenario) in the draw solution increased fouling resistance by ~29%, likely due to greater deposition of non-alginate-coated ACC on the membrane. This study provides insight into the composition of CaCO 3 -alginate combined foulants and their influences on CaCO 3 crystal surface properties, membrane-foulant interactions, and water/solute transport behaviors in the PRO process.