Membrane performance of pressure-retarded osmosis using simulated hydroxide-based CO2-captured draw solutions

Abstract

This study proposes a novel hybrid process that integrates direct air capture (DAC) and pressure-retarded osmosis (PRO) to address high water and energy demands of the DAC process and achieve simultaneous decarbonization and energy production. To explore the PRO performance in this hybrid process, membrane fouling behaviors were investigated by employing forward osmosis membranes in the PRO mode with various Na- and K-based draw solutions that simulated hydroxide-based CO2-captured solutions. Especially calcium (Ca2+) and alginate were dosed into the draw solutions as representatives of scaling precursor and organic foulants during long-term DAC-PRO operation respectively. The results revealed that (1) the presence of calcium in the NaHCO3 and KHCO3 draw solutions formed CaCO3 precipitates, whose amounts, morphologies, and particle size were associated with the dosed Ca2+ concentration and alkali solution type. (2) Higher calcium amounts in the NaHCO3 and KHCO3 draw solutions promoted more deposition of rhombohedral CaCO3 crystals on the membrane, leading to a lower water flux (Jw) and higher reverse salt flux (Js) due to increased hydraulic resistance and cake-layer-enhanced concentration polarization. (3) Dosing calcium (40 mM) into a mixture of HCO3- and CO32- draw solution resulted in accumulation of irregular-shape CaCO3 crystals on the membrane and an increase of Jw, possibly relating to the increased membrane hydrophilicity due to decreased solubility of the solute at a higher pH level. (4) The addition of alginate into calcium-dosed (40 mM) draw solutions did not influence Jw and Js but increased deposited foulants. This suggests that deposited alginate may loosely attach to CaCO3 crystals, instead of creating a dense fouling layer by bridging with Ca2+.