Effect of surfactant frequently used in soil flushing on oxygen mass transfer in micro-nano-bubble aeration system

Abstract

In-site soil flushing and aeration are the typical synergetic remediation technology for contaminated sites. The surfactant presented in flushing solutions is bound to affect the aeration efficiency. The purpose of this study is to evaluate the effect of surfactant frequently used in soil flushing on the oxygen mass transfer in micro-nano-bubble (MNB) aeration system. Firstly, bio-surfactants and chemical surfactants were used to investigate their effects on Sauter mean diameter of bubble (dBS), gas holdup (ε), volumetric mass-transfer coefficient (kLa) and liquid-side mass-transfer coefficient (kL) in the MNB aeration system. Then, based upon the experimental results, the Sardeing’s and Frössling’s models were modified to describe the effect of surfactant on kL in the MNB aeration. The results showed that, for the twenty aqueous surfactant solutions, with the increase in surfactant concentration, the value of dBS, kLa and kL decreased, while the value of ε and gas–liquid interfacial area (a) increased. These phenomena were mainly attributed to the synergistic effects of immobile bubble surface and the suppression of coalescence in the surfactant solutions. In addition, with the presence of electric charge, MNBs in anionic surfactant solutions were smaller and higher in number than in non-ionic surfactant solutions. Furthermore, the accumulation of surfactant on the gas–liquid interface was more conspicuous for small MNB, so the reduction of kL in anionic surfactant solutions was larger than that in non-ionic surfactant solutions. Besides, the modified Frössling’s model predicted the effect of surfactant on kL in MNB aeration system with reasonable accuracy.