Effects Of Slaked Lime,Sodium Silicate And Hydroxyapatite On Cadmium And Lead Accumulation In Tobacco Plant

A pot experiment using a Pb-Cd contaminated soil and three amendments(slaked lime,sodium silicate and hydroxyapatite)was carried out in order to study the effects of the amendments on the uptake and accumulation of Cadmium(Cd)and lead(Pb)in tobacco.The results were as follows:1.Soil pH increased while DTPA-extractable Cd and Pb of the soils decreased with increasing rate of the amendments.The maximum decrease in DTPA-extractable soil Cd was 52.4%(32g·kg-1 hydroxyapatite),37.0%(16 g·kg-1 slaked lime),and 14.1%(12.5 g·kg-1 sodium silicate),and the maximum reduction in DTPA-extractable soil Pb was 58.5%(32 g·kg-1 hydroxyapatite),35.7%(16 g·kg-1 slaked lime),and 18.1%(12.5 g·kg-1 sodium silicate),respectively.2.The growth of tobacco plant was improved by applying the amendments.The amendments increased the leaf number,the leaf area,the plant height,the dry weight,and the biomass of tobacco.Slaked lime and hydroxyapatite showed better effects than sodium silicate in promoting tobacco growth.3.In addition to the treatment with 2.5 g·kg-1 sodium silicate,the three amendments all inhibited the uptake and accumulation of Cd in various parts of tobacco.The inhibitory degree was increased with the increase in the rate of amendments.When higher rates of the amendments(8～6 g·kg-1 lime or 7.5～12.5 g·kg-1 sodium silicate or 8～32 g·kg-1 hydroxyapatite)were applied,slaked lime and hydroxyapatite showed better effects on reducing Cd concentration of tobacco leaves than sodium silicate.4.Low rates of slaked lime(2 or 4 g·kg-1)or hydroxyapatite(2 or 4 g-kg’1)slightly increased Pb content in tobacco leaves whereas higher rates of slaked lime(12 or 16 g·kg-1)or hydroxyapatite(16 or 32 g-kg’1)significantly decreased Pb content in tobacco leaves.The Pb content of tobacco leaves can be reduced below 15mg～kg-1 by applying 16 g·kg-1 slaked lime or 32 g·kg-1 hydroxyapatite.Application of sodium silicate did not significantly decrease Pb content in tobacco leaves.5.The transfer of Cd from roots to shoots is a limiting step for Cd accumulation in the shoots.Slaked lime and hydroxyapatite had no significant effect on root-to-stem transfer of Cd whereas sodium silicate reduced the root-to-stern transfer of Cd.The application of lime and high rate of sodium silicate and hydroxyapatite(7.5,10,12.5 g·kg-1)significantly inhibited the soil-to-root transfer and the root-to-shoot transfer of Pb.The application of the three amendments all promoted the stem-to-leaf transfer of Pb.6.The reduction in DTPA-extractable Cd and Pb by the application of lime and hydroxyapatite was the main cause for the decrease of Cd and Pb contents in tobacco root.The reduction in Cd concentration of tobacco leaves was largely resulted from the application of slaked lime and hydroxyapatite.The application of sodium silicate did not showed clear effects on Cd content in tobacco leaves.The uptake of Pb by tobacco root was mainly caused by the addition of slaked lime and hydroxyapatite.The application of lime and hydroxyapatite to soil available Cd content reduction is an important reason,sodium silicate is more complex.For the plant body,the main mechanism for the application of lime and hydroxyapatite to inhibit the enrichment of Pb in tobacco leaves is to inhibit the absorption of Pb by tobacco root from the soil.