Rhizospheric Mechanism Of Phytoremediation In Heavy Metals Polluted Soil

heavily industrialized areas, numerous soil sites are contaminated with prohibitively high concentrations of heavy metals, which affect normal agricultural practices. A potential strategy to remediate these soils is the use of plants to remove pollutants from the habitat or to render them harmless. Successful applications of plants for remediation of contaminated soils not only depend on the phenotype and genotype of plants, but interactions between rhizospheric composition and heavy metals are also of great importance. In this paper, the rhizospheric mechanisms of phytoremediation in heavy metals polluted soil were discussed.(1) The hydroponic experiment showed that root activities i.e. the release of proton, exudate of sugar, were related to the contents of Pb, Cd. The pH in the growth medium increased with the increasing concentration of Pb, and decreased with the increasing concentration of Cd in solution as the control. The quantity of sugar exudate increased with Pb, Cd increase and was affected distinctively by Pb-Cd interaction. Capillary electrophoretic analysis showed the considerable differences in composition of root exudates from wheat as stressed under Pb, Cd.Study on the accumulation mechanism of Cd and Pb in radish roots showed that in the first 12 hours the accumulation process of Pb in roots was probably due to rapid adsorption, later, to slow chemical precipitation and cell absorption. Especially, at high Pb content, chemical precipitation played a progressively more important role in Pb removal. The accumulation of Cd in roots was mainly due to adsorption in the first 24 hours, late^ absorption process maybe was dominant. In Pb-Cd treatment, the competitive adsorption and antagonistic absorption of Pb and Cd in roots had certain effect on the accumulation mechanism of Pb or Cd. Moreover, changes in the distribution patterns of Cd and Pb were also studied using an electron probe X-ray microanalyzer. The content of Cd was lower in the stele than that in the cortex. Effect of Pb-Cd interaction was somewhat different from that of Pb or Cd.Study on the interaction between root exudates and heavy metals showed that root181exudates bound metals to an extent that depended on the metal involved. As for wheat exudates, the importance of the binding followed the order Pb > Cd. But the solubility of Pb and Cd in red soil was not affected by root exudates of wheat. The adsorption of Pb and Cd in red soil increased in the presence of root exudates. pH in equilibrium adsorption solution increased in either exudates-soil-Pb /Cd system or in exudates-soil-Pb-Cd systemThe effect of root on the surface chemistry of the iron oxide was also observed. As for amorphous ferric oxyhydroxide, the effect of root induced the production of iron hydroxide hydrate (Fe(OH)3(H2O)0.25). And for goethite, the degree of cystallinity was intensified. Thus the adsorption of Cu was modified due to the change on the surface chemistry characteristic of the iron oxide. It was showed that the adsorption of Cu was increased on the surface of amorphous ferric oxyhydroxide and decreased on that of goethite after incubation with rye grass.P.chrysosporium have a great effect on the behavior of Cu on the surface of amorphous ferric oxyhydroxide, goethite and hemitate. It indicated that P.chrysosporium could act to release surface-bound Cu into solution. The growth of P.chrysosporium made Cu release as metalloid. Besides, experiment of hemitate, showed that exudates of H+ during the growth of P.chrysosporium greatly decrease the pH in medium. Such pH modifications lead to a great release of strongly bound Cu into solution.The chemical behavior of Pb, Cd and Cu in soil rhizosphere was also investigated using a rhizobox or rhizobag. The contents of Pb extractable in both rice and wheat rhizosphere of the red soils, were higher than those in the bulk soils, which meant that the activation processes in the rhizosphere was strong and bioavailable Pb greatly increased due to the activity of roots and the action of microorganis...