Foam-enhanced removal of adsorbed metal ions from packed sands with biosurfactant solution flushing

• Removal of adsorbed metal ions by biosurfactant solution flushing was carried out.
• Channeling effects in the flushing operation were compared for surfactant solutions.
• Metal ion removal was enhanced by the presence of foam in the flushing operation.
• Metal ion adsorption in different pore regions affected the removal efficiency.
• The removal efficiency for metal ions was correlated with the dynamic foam capacity.

This study demonstrated the abilities of negatively charged biosurfactants, surfactin and rhamnolipid, to remove adsorbed copper and cadmium ions from sand surfaces with the foam-enhanced solution flushing technique. A popular anionic surfactant, sodium dodecylsulfate (SDS), was used for the purpose of removal efficiency comparison. The role of surfactant foaming ability in the flushing approach was then identified. It was found that the surfactant solution flushing could only result in limited removal efficiency of 3–10% and 13–36% for copper ion and cadmium ion, respectively, after 24-pore volume (PV) flushing due to the channeling effect. As compared to the surfactin solution, a less pronounced channeling effect was detected for the rhamnolipid or SDS solution. With the presence of foam in the flushing approach, the channeling effect could be inhibited, and one could obtain improved removal efficiency of 10–30% and 20–46% for copper ion and cadmium ion, respectively, after 24-PV flushing. The removal efficiency was higher for cadmium ions than for copper ions, which could be explained by the significant adsorption of the cadmium ions in the inter-particle pore regions. Moreover, the cumulative removal efficiency variations with the foam-enhanced solution flushing could be correlated with the dynamic foam capacity of the surfactant solutions.