Comparative study of CO₂ nanobubbles and macrobubbles: Effects on water chemistry, microalgal growth, and carbon utilization

Algal biotechnology presents a cost-effective approach for simultaneous carbon dioxide (CO₂) capture and bioproduct generation. However, conventional gas delivery approaches (e.g., macro and micro-bubbles) suffer from low gas-liquid mass transfer efficiency (K_L·a) and CO₂ utilization. This study investigated the aqueous properties of CO₂ nanobubbles and impacts on the CO₂ mass transfer, utilization, and microalgal growth. Results revealed that direct injection of CO₂ nanobubbles in DI water achieved rapid CO₂ saturation (1.48 ± 0.08 g·L^-1) and nanobubble density (1.5 × 10⁸ particles·mL^-1) within 1 minute. By contrast, the circulation mode produced a higher nanobubbles concentration (2.6 × 10⁸ particles·mL^-1) after 20 min with a similar dissolved CO₂ concentration. Accordingly, the volumetric mass transfer coefficient (K_L·a) of CO₂ nanobubbles in DI water reached 12.41 ± 3.49 h^-1 (circulation mode) and 18.91 ± 7.68 h^-1 (direct mode), exceeding that of macrobubbles (10.18 ± 2.38 h^-1). Compared to macrobubbles, the use of CO₂ nanobubbles in Scenedesmus obliquus cultivation increased biomass by 10.11 ± 0.01% over 14 days and garnered carbon utilization efficiency (CUE) to 27.86 ± 0.63%, supported by the enhanced CO₂ mass transfer or carbon transfer efficiency. These findings highlight the potential of nanobubble technology in algal biotechnology applications and global CO₂ emission mitigation.