Demonstration of direct air capture of CO2 using microalgae raceway reactors

The continuous increase in atmospheric CO₂ concentration underscores the urgent need for scalable and energy-efficient carbon removal technologies. This study demonstrates, for the first time, the implementation of a tailored Direct Air Capture (DAC) concept integrated within large-scale microalgae raceway reactors, enabling direct CO₂ uptake from ambient air without external gas supply. A 600 m² reactor operated continuously with Scenedesmus sp. maintained stable productivity (12 g m⁻² day⁻¹) under extreme carbon limitation (TIC ≈ 20 mg L⁻¹, pH ≈ 10). Fine-bubble aeration in the sump achieved nearly complete CO₂ removal from air streams, while passive absorption across the raceway and paddlewheel sections provided almost all the carbon required for biomass growth. The overall CO₂ removal efficiency reached 95%, confirming the reactor’s operation as a functional bio-DAC system. The estimated energy demand for air bubbling (≈ 2.9 kWh kg⁻¹ CO₂) is comparable to or below that of current engineered DAC technologies. This approach establishes a low-cost, renewable-compatible, and scalable pathway for atmospheric CO₂ capture that couples negative-emission performance with biomass production, laying the groundwork for decentralized, carbon-negative biotechnological systems contributing to global greenhouse-gas mitigation. • Large-scale microalgae raceways directly capture CO₂ from ambient air. • Sump-based aeration achieves up to 95% CO₂ removal efficiency. • pH-driven carbonate balance governs atmospheric CO₂ absorption and fixation. • Bio-DAC integrates carbon capture with renewable biomass production. • Demonstrated low-energy (≈ 2.9 kWh kg⁻¹ CO₂) pathway for negative emissions.