Study On The Enrichment Of Lead And Cadmium In Elodea Densa (Planch.) Casp. And Their Tolerance Mechanism To Lead And Cadmium

The rapid development of China’s industrialization has brought into improving the lives of human beings but also poses a serious problem of heavy metal pollution. High pollution problems of heavy metal which brought from non-ferrous smelting industry is particularly prominent. The composition of non-ferrous smelting waste water is complex and the content of heavy metals is higher. According to the preliminary investigations, the lead and cadmium content of non-ferrous smelting waste water are about 10mg/L-300mg/L, a high concentration of heavy metal pollution has brought severe test to the enterprises in pollution control. At present, the main method of non-ferrous metal smelting waste water treatment are neutralization method, curing method, iron arsenic removal, adsorption, membrane, etc., but owing to economic conditions and its own technology, each method has some limitations. Therefore, finding suitable plants are the key problems of phytoremediation and have the theory and practice significance.A submerged plant, Elodea densa (Planch.) Casp., was cultivated in Pb、Cd solution under lab condition. The enrichment and tolerance mechanisms involved in Pb and Cd of Elodea densa (Planch.) Casp. were systemically studied in order to provide a theoretical basis for treatment of non-ferrous smelting wastewater and a scientific basis and reference value for Pb, Cd tolerance mechanisms of submersed plants. Through previous research, the following research results were got.1. Semi-static experimental method was used to study the enrichment of Pb, Cd in Elodea densa (Planch.) Casp and the conten of Pb, Cd by atomic absorption spectrophotometer under different concentrations treatment of Pb (0,5,10,20,40,80, 100,120mg/L) and Cd (0,0.5,1,5,10,20,40mg/L) for 9 days.The results showed that Elodea densa (Planch.) Casp.could accumulate of Pb with a maximum of 103.55mg/g (dry weight) and Cd with a maximum of 30.15 mg/g (dry weight) for 9 days, which reached the Pb, Cd critical content standards of hyperaccumulator (Pb> 1000mg/kg =1mg/g, Cd>100mg/kg=0.1mg/g) and could tolerate higher concentrations of Pb, Cd polluted water. Therefore, Elodea densa (Planch.) Casp. can be used to treat higher concentration of Pb, Cd in the non-ferrous smelting wastewater.2. The effect on biochemical indices of Eodea densa (Planch.) Casp. and biochemical indexes were measured by UV-Vis spectrophotometer. The results showed that the content of chlorophyll a, chlorophyll b and carotenoid were decreased, free proline increased and MDA rised When Eodea densa (Planch.) Casp. exposed to different concentrations treatment of Pb and Cd. These indicators are adaptation mechanisms when Eodea densa (Planch.) Casp. exposed to Pb and Cd stress.3. According to the distribution of Pb, Cd in subcellular fractions with the differential centrifugation and the contents measured by atomic absorption spectrophotometer. The results showed the distribution of Pb in Eodea densa (Planch.) Casp. decreased as the following pattern:cell wall>cell organelle and membrane>soluble fraction and the distribution of Cd in Eodea densa (Planch.) Casp. decreased as the following pattern:cell wall>soluble fraction>cell organelle and membrane, among which cell wall was apparently the major storage of Pb and Cd in the plant. So, solidification of cell wall may be one of Pb, Cd tolerance mechanism in Eodea densa (Planch.) Casp.4. The sequential chemical extraction method were used and Pb, Cd content in different chemical forms were measured by atomic absorption spectrophotometer. The distribution decreased as the following pattern:for Pb, HCl-extraction>HAc-extraction >NaCl-extraction>H20-extraction>Ethanol-extractio>residue; for Cd, NaCl-extraction >HAc-extraction>H20-extraction>Ethanol-extraction>HCl-extraction>residue. From the above, the chemical forms of Pb were chiefly HCl-extractable, HAc-extractable and NaCl-extractable forms in Eodea densa (Planch.) Casp., while Cd were chiefly NaCl-extractable, HAc-extractable and H2O-extraction forms in Eodea densa (Planch.) Casp.5. Pb, Cd chemical forms of cell wall, cell organelle and membrane in Eodea densa (Planch.) Casp. were measured by atomic absorption spectrophotometer. The results showed that the chemical forms of Pb were chiefly HCl-and HAc-extractable forms in the cell wall, and NaCl-and HAc-extractable forms in the cell organelles and membranes, while the chemical forms of Cd were chiefly NaCl-and HAc-extractable forms in the cell wall, cell organelle and membrane.6. From the above, combined with these compounds including oxalate and phosphate, adsorbed or bound protein, Pb and Cd of Elodea densa (Planck) Casp. was prevented from moving within the cell, but bound to the cell wall, which, as a result, weakened the Pb toxication and strengthened Pb tolerance of the plant. Meanwhile, the phenomenon that the content of proline increased were the mechanisms involved in Pb and Cd tolerance of Elodea densa (Planch.) Casp. to resist heavy metal stress also to some extent.