Comparative study of CO₂ nanobubbles and macrobubbles: Effects on water chemistry, microalgal growth, and carbon utilization
Xia Li, Jingru Wei, Yi‐Ying Lee, Yihan Zhang, Shan Xue, Sowmya Atukuri, Y.T. Li, Taha F. Marhaba, Xuezhi Zhang, Wen Zhang
Abstract
• 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.