Experimental and theoretical study on dissolution of a single mixed gas bubble in a microalgae suspension

Abstract

Aiming at technology for biofixation of carbon dioxide by microalgae in photobioreactors, a basic phenomenon, that is, the dissolution and consumption of a mixed gas bubble consisting of CO₂ and air in a microalgae suspension, was investigated by visualization experiments using the promoted bubble grafting method. Furthermore, a theoretical model based on non-equilibrium theory at the gas–liquid interface was also proposed to predict the CO₂ dissolution and fixation characteristics of bubbles in a microalgae suspension. The effects of the initial CO₂ volume fraction, initial bubble size and microalgae concentration were discussed respectively. It was found that the bubble radius gradually decreased with time and trended towards a constant thereafter. The dimensionless Biot number in the promoted dissolution model was determined as 0.65 for the microalgae suspension. The bubble with a larger initial CO₂ volume fraction experienced faster shrinkage and had a higher dissolution rate and CO₂ fixation efficiency, while slight photosynthesis inhibition emerged at the beginning of dissolution when the initial CO₂ volume fraction in the bubble was larger than 15%. A smaller initial bubble size resulted in a lower dissolution rate but greater CO₂ fixation efficiency by photosynthesis. Higher microalgae concentration facilitated bubble dissolution and CO₂ fixation especially when OD_{680 nm} of the microalgae suspension was less than 1.0. These findings can be a guide to the design of a photobioreactor and aerator.