Metal (Cu, Cd and Zn) removal and stabilization during multiple soil washing by saponin

The influence of multiple saponin washing on copper, cadmium and zinc removal and stability in three types of soils (loamy sand, loam, silty clay) was investigated. Distribution of metals and their mobility measured as the ratio of exchangeable form to the sum of all fractions in soils was differential. After single washing the highest efficiency of metal removal was obtained in loamy sand (82–90%) and loam (67–88%), whereas the lowest in silty clay (39–62%). In loamy sand and loam metals had higher mobility factors (44–61% Cu, 60–76% Cd, and 68–84% Zn) compared to silty clay (9% Cu, 28% Cd and 36% Zn). Triplicate washing led to increase both efficiency of metal removal and percentage content of their stable forms.In consequence, fractional patterns for metals before and after treatment changed visibly as a result of their redistribution. Based on the redistribution index, the most stable metal (mainly in residual and organic fractions) after triplicate washing was Cu in loamy sand and loam. For silty clay contaminated with Cd, effective metal removal and its stabilization required a higher number of washings.

► Saponin is effective biosurfactant to remove Cu, Cd and Zn from soils.
► Soil washing was highly dependent on metals distribution and soil type.
► Triplicate washing led to increase of metal stability due to their redistribution.
► Clay soils need more washings than sandy soils for effective metal stabilization.