Impact Study On The Desorption Of Pb And Zn From Soils And Minerals By Saponin

The purpose of this study is to explore the effects of surface charge properties on desorption and provide some preliminary data on the mechanism of saponin desorption heavy metals from soil. In this study, laboratory incubation of Pb/Zn contaminated on red soil and yellow-cinnamon soil was conducted to investigate the effect of saponin desorption. Meanwhile adsorption and desorption characteristics of Pb/Zn on kaolinite, montmorilonite and goethite were investigated to explore the affecting mechanisms of desorption with saponin. As a way to achieve that, we tested under different concentration, p H, shaking time and washing number, and the changes of Zeta potential during desorption processes were observed and discussed. Infrared spectrum analysis(FTIR) was also adopted to understand the combing ways between saponin and heavy metal. In order to evaluate the remediation effect of saponin in different soils, BCR continuous extraction method was used to investigate the changes of heavy metal speciation before and after treatment. Results showed that:The Langmuir equation fitting results showed that the maximum adsorption capacity of montmorillonite, kaolinite, goethite for Pb were 30.09, 11.60, 13.17 g/kg, respectively, and for Zn they were 20.01, 5.17, 9.82 g/kg(25℃、p H 5). Saponin adsorption could increase zeta potential of both kaolinite and montmorillonite, which showed that the decline of surface net negative charge, while decrease the zeta potential of goethite significantly.The desorption amounts of Pb and Zn from soils and clay minerals were enhanced with the increase of saponin concentration(0-20 g/L), and were negatively related with the acidity of aqueous saponin. The results showed that the velocity of the desorption of heavy metals was so fast that the equilibration can be basically got in 3 hours.Under the same condition, saponin could desorb more Zn than Pb from both soils, and more Pb/Zn were desorbed from red soil than yellow-cinnamon soil. While in the case of clay minerals, results changed: saponin desorbed more Zn than Pb from kaolinite and goethite, and from montmorilonite they were approximately the same. More Pb/Zn were desorbed from kaolinite than montmorilonite and goethite.After washing for 3 times, the cumulative desorption percent of Pb from kaolinite, montmorillonite and goethite were 73.48%, 27.07%, 53.22%, respectively, and they were 52.11%、30.22%、27.01% for Zn(saponin concentration: 5 g/L, p H 3). Obviously, the desorption percents of Pb were higher than Zn for clay minerals.A sequential extraction technique was used in this study to extract the Pb/Zn forms in red soil and yellow-cinnamon soil after washing with saponin for 1, 2, 3 times. After single desorption, acid soluble Pb and Zn were desorbed most from red soil, lower Zn in oxidation and residue forms were desorbed. Most reducible Pb were desorbed from yellow-cinnamon soil, and the desorption amounts of all forms of Pb/Zn from yellow-cinnamon soil were significantly lower than those of red soil. After multiple desorption, the desorption amouts of all forms of Pb/Zn increased markedly, and the second desorption showed more importance than the third.FTIR results showed that Pb ions coordinated with carboxyl groups of saponin, while for Zn ions the coordination reaction was not complete, showing a weaker complexing ability with saponin than Pb.