Study Of The Effect Of Tea Saponin On Leaching Remediation Of Heavy Metal Contaminated Soil

Tea saponin is a kind of biosurfactant and has the characteristic of biodegradation and no secondary pollution. In recent years, it is a new research field that tea saponin is applied to restoring heavy metal contaminated soil. This paper used soil saples of artificial pollution such as brown soil, cinnamon soil and alluvial soil, and made heavy metal lead (Pb), cadmium (Cd), copper (Cu) and znic (Zn) as removing objects. Tea saponin removed heavy metal in soil by oscillation method and the experiment makes sure of its best concentration range, the proper pH range and other influencing factors. By soil leaching experiment, the study probed the best conditions that leaching removes heavy metal pollutants with tea saponin in three different types of soil. Meanwhile, it studied the influence of tea saponin on heavy metal formation in soil and main nutritional compositions. It was expected to provide theoretical basis for choosing appropriate leaching restoration material for heavy metal contaminated soil and provided theoretical foundation for restoration application. The main research results were as follows.(1) In three types of experimental soil, tea saponin had a certain removing effect on four kinds of heavy metal. The concentrations of tea saponin greatly affected the removal efficiency of heavy metals. The best removing concentrations of tea saponin for four kinds of heavy metal was between 3% and 7%. In three types of soil, for Pb and Cd ,the order of removal efficiency was brown soil>alluvial soil>cinnamon soil. For Zn and Cu, the order was cinnamon soil>brown soil>alluvial soil.(2) In three types of experimental soil, with the increase of soil environment pH, the removal efficiency of heavy metal tended to decline in general. The tea saponin solution had more evident effect on removing heavy metal within the range of the pH between 3.0 and 5.0 than between 5.0 and 7.0. In the practical application, the appropriate pH was about 5.0.(3) While Ca²⁺ entered into soil, it could combine with tea saponin and form competitive complexation with other heavy metals. In brown soil, the competitive complexation between Ca²⁺ and Cd²⁺, Zn²⁺ was stronger than between Ca²⁺ and Pb²⁺,Cu²⁺. In alluvial soil, the competitive complexation between Ca²⁺ and Cd²⁺ was the strongest, the second were between Ca²⁺ and Pb²⁺, between Ca²⁺ and Zn²⁺. But Ca²⁺ had little influence on the complexation between Cu²⁺ and tea saponin. In cinnamon soil, Ca²⁺ had a strong competitive complexation with four types of heavy metals. With the increase of the concentration of Ca²⁺, the competitive complexation greatly weakened.(4) In brown soil leaching experiment, use 5% tea-saponin solution as flushing agents, the fitting restoring conditions were as follows: pH value was 5.0±0.1, the ratio of soil to solution was 1:5. Then the removal efficiency could reach 7.88%, 40.09%, 46.92% and 4.17% for Pb, Cd, Zn and Cu respectively. The removal efficiency of heavy metals was in a sequence of Zn > Cd >Pb>Cu.(5) In alluvial soil leaching experiment, use 7% tea-saponin solution as flushing agents, the fitting restoring conditions were as follows: pH value was 5.0±0.1, the ratio of soil to solution was 1:4. Then the removal efficiency could reach 6.74%, 42.38%, 13.07% and 8.75% for Pb, Cd, Zn and Cu respectively. The removal efficiency of heavy metals was in a sequence of Cd > Zn >Cu> Pb. (6) In cinnamon soil leaching experiment, use 7% tea-saponin solution as flushing agents, the fitting restoring conditions were as follows: pH value was 5.0±0.1, the ratio of soil to solution was 1:3. Then the removal efficiency could reach 2.28%, 26.30%, 26.33% and 1.91% for Pb, Cd, Zn and Cu respectively. The removal efficiency of heavy metals was in a sequence of Zn>Cd >Pb>Cu.(7) In three types of experimental soil, the tea saponin had the best removal effect on the acid-soluble species of heavy metal. The second was oxide species. The organically- bound species and the residual species of heavy metal was relatively difficult to be removed. With the heavy metal of oxide species and acid-soluble species decreased, the environmental risk of heavy metals greatly reduced.(8) Compared with deionized water, tea saponin leaching caused less loss of nutritional composition such as, TN, TP, TK and organic matter in soil. With the increase of tea saponin concentrations, the loss rate of nutritional composition decreased. The phenomenon of the loss of nutritional element should be paid more attention when soil leaching technique is applied to restoring heavy metal contaminated soil. Fertilizer should be properly used in leached soil according to the proportion of the loss.