Research On The Absorption And Accumulation Of Heavy Metals In Trees

Absorption and accumulation of heavy m etals Cd, Zn and Cu in poplar( populus simonii Carr. var. przewalskii(Maxim.) H. L. Yang) and paulownia(Paulownia tomentosa(Thunb.) Steud) were studied though sampling the soil and trees in a mining region in Tongling city, Anhui province, China. Additi onal pot planting experiment was also conducted for that purpose, but mainly on poplar tree and Cd. Th e heavy metals were analyzed by ICP-OES method.The analysis of mining region so il showed that the content of the three heavy metals were all higher than the local background values. For the refilling area in the mining region, the concentration decreased with depth. In the po t planting experiment, low contaminated soil was used and dif ferent amount of cadm ium nitrate was added to the pots so that the soil had a concentration gradient from blank to100mg/kg.The analysis of different parts of the trees demonstrated great vari ation of heavy m etal content in different parts of the or gans of a tree, in difference trees, as well as in d ifferent elements. For the architecture of a tree, vertically in paulownia, the contents of both Zn and Cu were distributed with leaves > root > branch > trunk, while for Cd it was leaves > root > branch > trunk. Vertically in poplar, Zn was distributed with leaves > root > branch > trunk, Cu was root > trunk > branch > leaves and Cd was leaves > root > branch > trunk with Cd content decreasing in the trunk when the height increased. The same results were obtained for Cd content distribution in poplar tree in the pot planting experiment. For trees planted in different Cd added soils, leaves had the highest Cd content and the low est was in the top trunk. So, for poplar the most important rule of Cd distribution was leaves > root > branch > low trunk> trunk in m iddle height >top trunk. For the trunk of both trees, the contents among the heavy metals with Zn>Cu>Cd for pop lar and Zn>Cu>Cd for paulownia, in the same organ of trees for heavy metal enrichment ability is quite different due to the kinds of heavy metals. Radially in the trunk of poplar from mining region, Cd was distributed with bark >heartwood>sapwood, but for that from pot planting, it was bark >sapwood>heartwood. So, both showed that Cd mainly existed radially in bark. Accumulation capability, which was rep resented by Accumulation Factor(AF) calculated as the ratio of heavy metal content in tree organs to that in the soil, was also different between the trees, among the organs of a tree and among the heavy metals. For poplar, the AF values of Cd for all or gans were greater than 0.12, with the highest in leaves attaining over 0.5. The Cd concentration could reach as much as 878.26mg/kg. While for paulownia, the AF values of Cd for all organs were less than 0.04, with the lowest in branches which was 0.03 and where the Cd concentration was 0.092mg/kg. For poplar, the AF values of Zn for all organs were greater than 0.8, for paulownia, the AF values of Zn for all organs were greater than 0.1. For poplar, the AF values of Cu for all organs were greater than 0.1, for paulownia, the AF values of Cu fo r all or gans were greater than 0.4. So,poplar demonstrated stronger accumulation capability for Cd and Zn, while weaker accumulation capability for Cu than paulownia. The pot pl anting experiment results also showed that increasing soil Cd content would decrease enri chment capability of Cd in poplar. The AF values of trees grown in blank pot could reach over 10. With the increase of Cd content in soil the AF values of related trees decrease d. This trend became obvious when the soil Cd content reached 50mg/kg and the AF values were lower than 0.5 when t he soil Cd content was 100mg/kg.Translocation ability, which was represented by Translocation Factor(TF) calculated as the ratio of heavy m etal content in tree above gr ound organs to that in the root, was also different for different trees and for dif ferent elements. For Cu, the TF values of different organs of paulownia were all greater than 1, while they were lower than 0.35 for poplar showing that paulownia had much stronger ability to translocate Cu from root to the above ground parts than poplar. For Zn, poplar had gr eater translocation ability than paulownia with TF values being greater than 0.9 a nd lower than 0.3, respectively. For Cd, the translocation ability of poplar was about double as that of paulownia, with TF values being greater than 0.5 for poplar and lower than 0.25 for paulownia. The translocation ability was different for a specific or gan among the heav y metals. For leaves of paulownia it was Zn>Cu>Cd, while for leaves of poplar it wa s Zn>Cd>Cu. So, Zn demonstrated strongest translocation ability in the l eaves of both trees. In general, Cd had stronger translocation ability in poplar than in paulownia and Cu transported m ore easily in paulownia than in poplar. The pot planting experiment results did not show obvious negative influence on TF values with the increase of Cd content in soil. When the soil Cd was 5mg/kg, the TF values of poplar were between 0.7-1.5, and when so il Cd content was 100mg/kg, they were between 0.7-1.0.Above results demonstrated rather complicated characters of absorption and accumulation of heavy m etals in poplar and paulownia. In general, poplar is m ore suitable for remediation of Cd polluted soil tha n paulownia. Hopefully, this research will help in the practice of dendroremediation of polluted soil.