Effect And Mechanism Of Cd Immobilization In Cd-contaminated Farmland

In China,nearly 1/5 of the total farmland is contaminated with heavy metals;and Cd-polluted farmland exceeding standard ranks first among all pollutants.Cadmium is a non-essential metal element that is believed to cause damage even at very low concentrations and can be taken up by crops easily.Thus high accumulation of Cd in crops would pose a potential hazard to human health via food chain.In situ immobilization has been considered a promising restoration method for safe production of crops in Cd-contaminated soils according to the fundamental conditions of our country.However,there are some technological difficulties on remediation of Cd-contaminated soil by amendments,and the mechanism of that is not clear,which limits its widespread application.In this study,the effects and mechanism of different types of amendments on reducing the phytoavailability of Cd were discussed,in order to provide theoretical reference for the evaluation and utilization planning of land resources in China.The main results are as follows:?1?The effectiveness and mechanism of clay mineral,base mineral,humus and biochar on reducing Cd availability and increasing Cd stability in soil were studied.Results indicated that the maximum adsorption capacity of four amendments on Cd ranged from 7.47 to 17.67 mg g^-1;and that the desorption percentages of Cd from amendments were lower than 12%and 20%respectively when the ion strengths was below 0.1 M or the pH was above 3.0.In addition,the characterizations of test amendments before and after Cd loading provided the evidence that surface precipitation and ion exchange were the main reasons for clay mineral and base mineral to adsorb Cd,and complexation was for humus and biochar.?2?This study investigated how applications of various soil amendments affected soil aggregation and Cd phytotoxicity.Results showed that five amendments applied significantly decreased Cd concentration in wheat and rice plants cultivated on Cd polluted soil;and the reduction rate of Cd content in wheat and rice grains ranged from 27.5%-56.9%and 18.9%-62%respectively.And adding amendments reduced the proportions of<0.002 mm fraction by8.9%-81.5%and 6.3%-23.5%respectively,with less mass loading of Cd.Active substances in the amendments likely attached to soil Cd and formed clay-poluvalent metal-organic matter complexes.These complexes was effective in promoting the formation of large aggregates with greater mass loading of Cd.The results suggest that the redistribution of Cd among soil aggregates was the likely factor that controlled the quantity of plant available Cd in the soil-plant system.?3?In this study,the remediation effects of the amendments on the soil and plants were investigated and the potential immobilization mechanisms were analyzed.The application of amendments was found to decrease the DTPA-CaCl₂ extractable Cd concentration by10.3%-50.6%and 18.0%-65.0%in soils where wheat and rice were cultivated.These amendments have the ability to bond with Cd due to their large specific surface area,greater contents of organic matter,microorganism,adsorption sites,and the many functional groups attached on the surface.The mechanisms of immobilizing Cd by these amendments primarily contain ion exchange,complexation,precipitation and surface adsorption.?4?The pot experiment was performed to investigate the effects and mechanism of three foliar treatments?Se/Si,Fe and Zn?on Cd uptake by wheat and rice.The results showed that the Cd contents in wheat and rice grains were reduced by up to 26.7%-49.8%and 17.8%-58.4%respectively.Active ingredients of foliar fertilizers could enter plants through stoma,and react with Cd to form complexes or compete with Cd for transport sites.Thus the application of foliar fertilizers could inhibit Cd transport from leaves to grains,thereby reducing the concentration of Cd in crop grains.?5?In this study,the remediation efficiency of Cd polluted soil by biochar and humic substances in pot and field experiments was simultaneously evaluated.For the field experiment,as compared with control?CK?,the Cd reduction rate?%?of crop grain or tobacco leaves was16.2%-39.8%;amendment application resulted in the increases of crop productivity by12.3%-38.6%,and the remediation marginal efficiency for the test soil ranged from 3.99%to15.4%.The remediation effect for different amendments in pot experiment was better than that in field validation experiment.