Saponin Versus Rhamnolipids for Remediation of Cd Contaminated Soils

Cadmium should be effectively removed from soil because it is one of the most toxic soil pollutants. Thus, the present study compared a plant (saponin, SAP) and a microbial (rhamnolipids, RAM) biosurfactant for Cd removal from three different types of soil. Batch washing was tested under different operational conditions: Biosurfactant concentration, 0.1-10% SAP, 0.01-1% RAM; washing time, 0.1-6h; soil dosage, 12.5-100 g L-1; biosurfactant pH, 2-9 for SAP, 5.5-12 for RAM; washing number, three with SAP, four with RAM. SAP removed Cd from the soils more effectively than RAM because it can be used at a higher concentration and a more acidic pH. Soil washing with SAP also has a shorter equilibrium time (10-90 min.) than washing with RAM (50-300 min), and higher kinetic rate constants (0.23-4.9 kg . mg(-1)h(-1) for SAP, 0.12-0.25 kg . mg(-1)h(-1) for RAM). With an increase in soil dosage, Cd removal from all soils decreased significantly with both SAP and RAM. Multiple soil washings improved Cd removal. The predicted optimum number of washes depended on the criteria for soil quality assessment, that is, total Cd concentration, Cd mobility or ecological risk. To meet all of these criteria, SAP needed fewer washes than RAM. SAP is more suitable than RAM for treatment of soils highly contaminated with Cd and containing various amounts of clay.