A Frugal, Decentralized Direct Air Capture Socio-Technical System for Community-Led CO₂ Mineralization in Brackish Water

Current marine carbon dioxide removal (mCDR) strategies face a trade-off between scalability and verifiability. Enhanced rock weathering (ERW), while theoretically scalable through the addition of alkaline minerals to soil, sequesters CO₂ on multi-year timescales and relies on complex modeling and indirect proxies for verification. In contrast, industrial direct air capture (DAC) is highly verifiable but extremely costly (on the order of hundreds to thousands of dollars per tonne of CO₂) and energy-intensive, limiting its use in developing regions. We present a novel, low-cost DAC system integrated into a modular floating platform for coastal communities. The system uses a synergistic mix of locally sourced waste alkaline materials (lime, wood ash, glycerine) added to brackish water, with air aeration in a first chamber to absorb CO₂, and a second chamber using waste mussel shells as nucleation sites to precipitate carbonate minerals. A KFRI-validated bench test confirmed the efficacy of our two-stage process. First (Absorption): Aerating ambient air through the alkaline slurry for 5 hours resulted in a significant pH drop (10.2 → 8.66) and an increase in total alkalinity (TA), confirming robust CO₂ uptake. Second (Mineralization): During an 18-hour settling period, the TA decreased by 61.6 mg/L (as CaCO₃). This drop, supported by a corresponding decrease in dissolved calcium of 6.41 mg/L, provides direct mass-balance evidence of permanent sequestration via CaCO₃ precipitation. From these results, we calculate a net sequestration rate of approximately 27 mg CO₂ per liter of treated water. This process, powered by solar energy and coupled with aquaculture, yields a carbon-negative, self-sustaining system. Our approach provides a rapid, locally verifiable alternative to other mCDR methods, directly addressing ERW’s slow kinetics and uncertain MRV.