Spatial Distribution And Accumulation Characteristics Of Heavy Metals In Constructed Wetlands Plants

With the rapid development of China’s industry, the content of heavy metals in sewageis increasingly growing and is difficult to be removed from the environment, which worsensthe pollution of heavy metals in soil and water environments. As a new kind of sewagetreatment process, constructed wetland has a better effect on heavy metal removal. Manywetland plants also have better capacity of heavy metal enrichment. Therefore, it is necessaryto explore the distribution characteristics and enrichment of heavy metals within constructedwetland plants.Based on the research regions including Zao River Estuary Wei Constructed Wetlandand Zao River Constructed Wetland Demonstration Base, this paper takes plants-bulrush andcattail as well as their rhizosphere matrix of both wetlands as the research objects, and studiesthe spatial distribution characteristics of heavy metals in these plants and their capacity ofheavy metal enrichment by measuring the concentrations of the six heavy metals includingCd, Cr, Cu, Ni, Pb and Zn within plants and their rhizosphere matrix. In addition, this paperalso analyzes the distribution characteristics of heavy metals within both plants, significancetesting results on heavy metal content difference, the correlation between heavy metalconcentration in plant root systems and heavy metal type in matrix, as well as the correlationbetween heavy metal concentration within plants and that within rhizosphere matrix usingStatistical Software SPASS18.0and Data Analysis Software Excel2007. Meanwhile, it alsotakes into account the heavy metal concentration within both plants, the enrichment capacityof plants as to each heavy metal element and the transfer ability of above-ground plant organsabout heavy metals so as to provide reference data for the plant selection during constructedwetland construction of Xi’an City in the future.The conclusions of the study are as follows:1. There is no significant regularity in the horizontal distribution of heavy metalsbetween bulrush and cattail in Zao River Estuary Constructed Wetland and DemonstrationBase of Zao River Constructed Wetland. In addition, compared with check samples, bulrushand cattail can better absorb Cd, Cr and Pb. The content of Zn is the highest within all theplants, followed by that of Cu and that of Cd is the lowest; The basically vertical distribution of heavy metals in bulrush and cattail is root＞stem＞leaf.2. There is a significant difference in heavy metal content between bulrush and cattail ofthe same constructed wetland, namely, the heavy metal absorption amounts of this twowetland plants in the same constructed wetland are different.3. There is a significant difference in the contents of Cu, Ni and Pb within bulrush inboth constructed wetlands. There is a markedly significant difference in Cu content in bulrush.Meanwhile, the contents of Cr, Cu and Zn within cattail are also with markedly significantdifferences. Since Bulrush and cattail grow in different constructed wetlands, the absorptionand accumulation amounts of heavy metals within bulrush are also different due to differentoperation times, process compositions, water inflows, matrix types and other factors inwetlands, namely, the absorption and accumulation amounts of heavy metals within the samewetland plant are also different in different constructed wetlands.4. Heavy metals within bulrushes in the two wetlands mainly concentrate in the rootsystems. Cd is the heavy metal most likely to be concentrated, while Ni, Pb and Cr are hardto be gathered; in addition, the stem and leaf transfer coefficients of Cu and Zn within bulrushare much higher, namely, Cu and Zn can be transferred to the stem and leaf of bulrush and beeasily absorbed and moved from the polluted matrix. Moreover, the transfer coefficient of Cdis much lower and it mainly concentrates in bulrush root and difficult to be moved upward tothe aboveground part.5. Heavy metals within cattail of both constructed wetlands mainly concentrate in theroot systems. Cu is the most easily concentrated heavy metal and Ni, Pb, Cd and Zn are hardto be gathered; in addition, stem and leaf transfer coefficients of Cu and Zn within cattail aremuch higher, namely, part of Cu and Zn can be transferred to the stem and leaf of cattail andbe easily absorbed and moved from the polluted matrix. Moreover, the transfer coefficient ofPb is much lower and it mainly exists in insoluble state and sediments on the cell wall.Besides, it is difficult to be moved upward to the aboveground part, which improves the leadresistance of plants.6. There are positive correlations between the concentrations of6heavy metals withinthe root systems of bulrush and cattail and the effective state of corresponding heavy metalelements in rhizosphere matrix, in which the correlation between Cd and Cr is significant and that between Pb and Zn is poor.7. There is no significant correlation between heavy metal concentration within bulrushand cattail and that within rhizosphere matrix. Thus, it can be seen that supply capacity ofheavy metals can not directly affect the heavy metal concentration within plants in wetlandmatrix environment.8. In the sample plots of bulrush and cattail, as heavy metals mainly concentrate inmatrix and rarely distribute in the water and plants, a large number of heavy metals still stayin the constructed wetlands. Therefore, it is ineffective to remove heavy metals by reaping theaboveground parts of plants.