Efficiency of rhamnolipid colloidal gas aphrons to remove cadmium and zinc from metal-contaminated soil samples

Soil contamination by metal wastes represents a threat to the environment and to human health. Rhamnolipids have been recognized for its metal removal properties and presenting high biodegradability, however their application is sometimes hampered due to downstream cost production. In contrast, colloidal gas aphrons (CGAs) can serve as a cost-effective alternative to reduce the consumption of biosurfactants while also being significantly efficient in flushing off heavy metals in remediation processes. However, data concerning rhamnolipid CGAs, such as bubble diameter, stability, and percolation through porous media under water-wet conditions, are scarce. Until now, only a few studies have demonstrated the possibility of using biosurfactant CGAs to remediate metal-contaminated soils, and even fewer studies have compared its efficiency to previous methods where regular foam is applied. Therefore, a rotational central composite design was developed considering several variables and stability and bubble diameter as response parameters. Rotation speed and agitation time had a significant effect on the stability of aphrons, reaching over 600 s of stable CGA. Alkaline pH has no significant effect on the stability of rhamnolipid CGA´s. Flow rate experiments showed a significant pressure drop when rhamnolipid CGAs were pumped through the remediation column. Soil column experiments demonstrated that rhamnolipid CGA exhibited a 38 % removal rate for cadmium and a 53 % removal rate for zinc after a percolation of 10 pore volumes at pH 9.0. Extraction of cadmium and zinc was highly influenced by the number of pore volumes percolated. Based on the present results, it can be demonstrated that colloidal gas aphrons can display a similar soil remediation efficiency when compared to previous studies that applied regular foam as a metal extraction agent. The application of rhamnolipid CGAs in metal-contaminated soils is imperative for a more efficient and environmentally friendly metal remediation process.