Metal desorption from copper(II)/nickel(II)-spiked kaolin as a soil component using plant-derived saponin biosurfactant

This study reports experimental measurements on the washing ability of quillaja saponin biosurfactant in kaolin suspensions, which were presorbed with the binary heavy metals of Cu(II) and Ni(II). A synthetic surfactant (SDS) and chelating agent (EDTA) were also evaluated for comparison studies. The sorption capacity for Cu(II) and Ni(II) onto kaolin were measured through the variation in pH and initial metal ion concentration. Much attention has been paid to understanding the micelle-forming properties of biosurfactant solution and its interaction with metals at the clay–water interface so washing strategies can then seek to achieve the ideal capacities of saponin biosurfactant for desorbing heavy metals from kaolin clays. The desorption of Cu(II) and Ni(II) from kaolin were examined as a function of pH, saponin concentration, temperature, time and mass/volume ratio of kaolin suspension. Because the saponin successfully competes with the kaolin clay for complexing with the metal ions by the Lewis acid–base interaction induced by pH, structure size, or charge of surfactant solution, the highest desorption improvements by 2000 mg/l of saponin were gained at 20 g/l kaolin suspension and pH 5–8, in which a single washing at room temperature could remove ∼83% of the copper and ∼85% of the nickel from kaolin containing 0.45 mg copper/g kaolin and 0.14 mg nickel/g kaolin. These results were interpreted as showing the action of a three-step micellar washing mechanism on the desorption efficiencies for Cu(II) and Ni(II). It is then pointed out quillaja saponin may be applied as a useful biosurfactant for the removal of heavy metals in contaminated clays or soils.