Effects Of Cadmium, Lead And Organic Acids On Speciation Of Cadmium, Lead In Rhizosphere And Their Phytotoxicity

A batch method and a sequential extraction technique were adopted to study the adsorption/desorption characteristics of cadmium (Cd) and lead (Pb) in wheat rhizosphere and non-rhizosphere, and the speciation of Cd and Pb in rhizosphere and their effects on wheat in meadow cinnamon soil using a rhizobag pot experiment.The effect of organic acids (citric acid and EDTA) on the speciation of Cd/Pb and phytotoxicity was investigated.The results showed that (1)The adsorption amount of Cd or Pb in rhizosphere was more than that in non-rhizosphere.But at low concentration of Cd (0-0.6 mmol / L) and Pb (0-1 mmol / L), the difference between adsorption amount of Cd and Pb was not remarkable in both rhizosphere and non-rhizosphere. The desorption amount of Cd and Pb increased with Cd and Pb amount adsorbed. The desorption percentage of Pb (<0.7%) was much lower than that of Cd (5%~10%). The desorption amount of Cd in non-rhizosphere was more than that in rhizosphere.(2)Most of Cd was existed in residual species at slight pollution of Cd(≤l mg / kg).The exchangeable Cd and cabonate-bound Cd became the predominant species, and the residual Cd significantly decreased at the higher of Cd concentration (3≥5 mg / kg).These results demonstrated that the residual species of Cd transformed to exchangeable and cabonate-bound species in rhizosphere with Cd pollution enhancing.Therefore, the phytotoxicity of Cd was stronger in rhizosphere.The predominant species of Pb were the organic-bound and Fe-Mn oxide-bound.There are a little amount of exchangeable Pb.The predominant species of Pb did not change with Pb pollution increasing.(3)The interaction between Cd and Pb in rhizosphere showed that the content of exchangeable and cabonate-bound Pb enhanced and the content of organic-bound and Fe-Mn oxide-bound Pb decreased with the increase of Cd concentration. Such changes became much obvious, especially at low content of Pb (125 mg / kg). However, the exchangeable Cd significantly increased in the presence of Pb in non-rhizosphere,especially when the ratio of Pb to Cd was 25. (4)The absorption of Cd/Pb by wheat increased with the total amount and concentration of species of Cd/Pb,whereas the fresh weight of wheat decreased.The toxic effect of Cd on the growthof wheat was more significant than that of Pb.As the added amount of Cd 5 mg / kg the fresh weight of wheat significantly decreased,whereas the low concentration of Pb (≤250 mg / kg) appeared protective effect on wheat growth.As the concentration of Pb was 500 mg / kg, the fresh weight of wheat decreased.The interaction between Cd and Pb was complicated. The interaction between Cd and Pb caused a toxic effect on wheat at low rate of Pb (125 mg / kg),and caused a protective effect at high rate of Pb (250 mg / kg). The content of Cd in wheat became higher in the presence of Pb, whereas the content of Pb in wheat became lower in the presence of Cd. (5)Organic acids (citric acid and EDTA) significantly mobilized Cd and Pb in rhizosphere.especially at high dose of citric acid (3 mmol / L).But EDTA cound significantly activate Cd and Pb in low dose treatment (0.5 mmol / L). EDTA had the most significantly activation for Pb. (6)Addition of organic acids (citric acid and EDTA) could increase Pb concentration and decrease Cd concentration in root of wheat. But application of organic acids promoted transportation of Cd and Pb from root to shoot. The concentration of Cd and Pb in leaf and stem of wheat increased obviously.