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

• CO 2 nanobubbles improved dissolution and gas-liquid mass transfer efficiency • Generation modes affected nanobubble density and CO₂ transfer behavior in water • Nanobubbles enabled stable CO₂ supply and pH control in algal cultures • CO 2 nanobubbles promote algal growth by increasing carbon availability and utilization efficiency 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 CO 2 utilization. This study investigated the aqueous properties of CO₂ nanobubbles and impacts on the CO 2 mass transfer, utilization, and microalgal growth. Results revealed that direct injection of CO 2 nanobubbles in DI water achieved rapid CO 2 saturation (1.48 ± 0.08 g·L -1 ) and nanobubble density (1.5 × 10 8 particles·mL -1 ) within 1 minute. By contrast, the circulation mode produced a higher nanobubbles concentration (2.6 × 10 8 particles·mL -1 ) after 20 min with a similar dissolved CO 2 concentration. Accordingly, the volumetric mass transfer coefficient ( K L ·a ) of CO 2 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 CO 2 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 CO 2 mass transfer or carbon transfer efficiency. These findings highlight the potential of nanobubble technology in algal biotechnology applications and global CO 2 emission mitigation.