Surfactant adsorption rather than “shuttle effect”?

In the absence of chemical reaction, mass transfer enhancement by suspended particles (mostly activated carbon) at the gas/liquid interface has been frequently reported and is usually explained by a “shuttle mechanism” exerted by particles with a high adsorption capacity for the transfer component. A major problem of this model is that unrealistic enrichment of the solids at the interface as compared to the bulk concentration has to be assumed. A comprehensive study has been carried out in a stirred tank in a wide range of the stirring speed (0–700min-1) with 9 different powdered solids suspended in water. With a flat gas/liquid interface, moderately hydrophobic solids significantly increased the mass transfer rates at low solid loadings (0.1–1kg/m3). However, the effect is not limited to particles with a high adsorption capacity for the gas (e.g. activated carbon) but it is observed also for non-porous particles (e.g. graphite or sulphur). When the particles are removed by rinsing, the absorption rates remain high. When the system is kept very clean (surfactant free), the enhancement effect is not observed. Based on these findings, it is concluded that adsorption of surfactants on hydrophobic solids cleans the interface resulting in higher mass transfer coefficients kLkL.