Micro-sized carbon dioxide gas bubbles can be applied as alternatives to air bubbles in flotation separation processes, which can reduce CO2 emissions as well as decrease energy consumption. CO2 and air bubbles were used as particle collectors in the flotation process to investigate and compare the collision efficiency and particle separation characteristics. This study measured bubble size distributions and simulated the single-collector collision (SCC) model that has been applied extensively to flotation processes. The diameter of the air bubbles was found to decrease as the saturator pressure increased, but the size of the CO2 bubbles increased when the pressure exceeded 151.99-202.65 kPa. When using CO2 bubbles as collectors to separate particles in water, the simulation results from the SCC model expressed in terms of bubble volume concentration (BVC) was related to the lower collision efficiency of CO2 bubbles, and the bubble number concentration (BNC) rather than the BVC to the effects of macrobubbles. The findings of this study suggest that the advantages of CO2 bubbles used as collectors in particle separations would be lower operating costs and reduced greenhouse gas emissions from water and wastewater treatment plants. (C) 2014 Elsevier B.V. All rights reserved.