Effective extraction and recovery of rare earth elements (REEs) in contaminated soils using a reusable biosurfactant

Tea saponin (TS), a plant derived biosurfactant, was used to investigate on its effectiveness on the extraction of three typically selected rare earth elements (REEs, light lanthanum (La), medium arrowhead (Dy) and heavy erbium (Er)) from contaminated soils, in the presence of important toxic heavy metals (lead (Pb) and cadmium (Cd)). A complete procedure, involving the extraction of REEs in soils, the recovery of REEs and TS in the extraction leachates and the reuse of the recovered TS, was established. Experimental results showed that the optimal extraction parameters were consumption of 1.2 g/g (TS/soil), pH of 5 and the extraction time of 24 h. The recovery efficiencies of La, Dy, Er, Cd and Pb achieved 96.9%, 88%, 84.3%, 88% and 91.1% using 0.3 g/g (Ca(OH)₂/soil). The overall extraction efficiencies of La, Dy, Er, Cd and Pb were 53.9%, 73.2%, 71.7%, 95.9% and 38.8% by three times using the recovered TS solution. The extractable fractions of La, Dy, Er, Cd and Pb in soil were found to be highest in their acid soluble and reducible forms. Mechanisms studies indicated the increased binding strength (I_R) and the decreased mobility (M_F) of REEs and metals after the flushing with TS. Carboxyl groups in TS were attributed to the formation of complexation and agglomeration between TS and studied REEs and other metals, confirmed by the analysis of both the X-ray photoelectron spectroscopy (XPS) and the dynamic light scattering (DLS). This study established an environmentally-friendly contaminated soil remediation and the recovery of valuable REEs by the combination use of TS and calcium hydroxide.