Effects Of Aquatic Vegetation Restoration On Migration And Transformation Of Heavy Metals In Sediments

Urban water environment pollution is more and more serious nowadays because of rapid urbanization process, heavy metal pollution has caused a series of public hazard incidents which raises concerns. As the process and internal mechanism of water pollution are studied, current treatment of contaminated water bodies have been developed to the stage of ecological restoration. The primary measures are aquatic vegetation restoration. In this thesis, field survey sampling methods are combined with indoor simulated experiments to study how aquatic vegetation restoration effect on migration and transformation of heavy metals, we want to know the purification mechanism of pollutants in aquatic vegetation restoration to further improve the water pollution restoration and provide scientific base for ecological restoration technology. The main findings are as follows:(1) Characteristics of the lead fractions are:reducible> residual> oxidisable> acid solube. The proportion of reducible lead accounts for 60% to 70%. The concentration of acid soluble lead in the upper sediment is lower than in the bottom, reducible and oxidisable lead is higher in the bottom sediments.(2) Restoration time has significant impact on distribution of heavy metals in sediments.The longer the restoration time is, the lead in sediments will transform more significantly to be oxidisable, and this trend is more obvious in upper sediments where the roots effect directly. Vegetation growth also helps increase the activity of lead in sediments. As a result, residual lead transforms to acid extractable, reducible with less activity. Different forms of hydrophytes effects on sediments differently. For example, floating plant such as pond lily has strong roots and is more capable of radial oxygen, the proportion of acid soluble and reducible forms are significantly lower than other kind of hydrophytes. The impact on sediments from submerged plants such as Potamogeton crispus with smaller root is less.(3) Isotherm tests are finished and the sediments in urban rivers are found to be of strong adsorption capacity of lead and zinc, that means the composition and adsorption mechanism of sediments is complex, the adsorption is the result of a variety of forces.(4) Migration and transformation of zinc and iron in sediments are different under different redox treatment. Anaerobic treatment decrease the pH in sediments supernatant and promote release of acid soluble iron and zinc. If the sediments have been in anaerobic conditions for a long time, the zinc will be released to water numerously and brings risk once the environment becomes aerobic.