Rates of oxygen transfer from air bubbles to aqueous NaCl solutions at various temperatures

Experiments have been conducted to investigate the effects of temperature on the interfacial surface area and on the rate of oxygen transfer from air bubbles dispersed in aqueous NaCl solutions. Tests were also conducted to estimate the effects of salt concentration on the size of the bubbles. In addition to NaCl solutions, seawater was used in some tests. The temperature effects were investigated at 5, 10, 15, 20, 25, and 30°C. The results showed a pronounced effect of the salt on the size of the bubbles, which first decreased sharply with increasing concentration, but showed no further drop when the concentration was increased beyond 0.6 M. Both in seawater and in the 0.6 M solution, the mass transfer rate, K_LA, increased almost linearly when temperature was increased within the range from 5 to 25°C. The salt solution, as well as the seawater, showed an increase of K_LA of 60–70% over that in pure water at the same temperatures. This effect was the result of increased surface area of bubbles because of decreased coalescence. The increase in surface area was strongly temperature dependent, especially between 15 and 20°C. Contrary to this behavior the surface area in pure water showed, practically, no temperature dependence. The results are explained and discussed on the basis of ion-water interactions.