Study On Migration Characteristics And Bioavailability Of Cd In Municipal Sludge Amended Soil And Broad Beans

With the progress of urbanization, the content municipal sludge is in the sharp increase in every year, as a result, the outlets of municipal sludge is an important hot topic in the field of domestic and foreign scholars. In recent years, land utilization of municipal sludge has more and more attention by people, and the composition of municipal sludge is complicated, not only containing large amounts of nitrogen、phosphorus、 potassium and organic matter, but also containing toxic and harmful substances such as heavy metals、pathogenic bacteria, thus, the heavy metals is an important question to consider as municipal sludge used in land. For this study, the tested soil is municipal sludge amended soil that applied municipal sludge into loess, and broad bean as the selected plants, finally through pot experiment and laboratory analysis, this study explores systematically the migration characteristics and bioavailability of cadmium in municipal sludge amended soil and broad beans. The main results are as follows:1. The basic physical and chemical properties of loess amended with compost of 0.5% and 2.0% content changed. Amended with compost of 0.5% and 2.0% content, the pH of soils reduced to 7.78 and 7.71 from 7.88 respectively. The EC of soils increased to 1315?s/cm and 1508?s/cm from 1217?s/cm respectively, increased by 8.05% and 23.91%. The soil organic matter increased by 13.01% and 31.26% respectively. The soil carbonate content showed a trend of decline as municipal sludge plowed. In general, municipal sludge can effectively improve soil physical and chemical properties, supply nutrients required for crop and mineral elements. Although municipal sludge could take heavy metal into soils, found in the study, application of municipal sludge were 0.5% and 2.0%, the content of heavy metals in soil improvement basically is lower than the national standards.2. As municipal sludge into soils, broad bean biomass indexes have been improved. In not under cadmium stress, compared with the blank group, 0.5% and 2.0% content of municipal sludge group of broad bean root dry weight increased by 87.1% and 266.48% respectively, the stem weight increased by 170.68% and 347.54% respectively, the pod dry weight increased by 185.87% and 238.33% respectively. Hence it may be concluded that: within the scope of this study, low content of municipal sludge made a promoting role on the growth of broad bean. The dry weight of broad bean’s root、stem and pod showed similar discipline with the adding of exogenous Cd, namely with the increase of concentration of Cd in soil, the biomass indexes showed a general trend of decrease after increase first, it showed that the optimal concentration of exogenous added in before and after, the heavy metals promoted and hindered the growth of broad bean, respectively.3. The content of municipal sludge and exogenous cadmium would have an influence on the broad bean organs in root、stem、pod and beans. In the same organs, different municipal sludge into the amount of organs, no obvious difference between cadmium content. In the same organs, the cadmium content in different organs of adding exogenous cadmium concentrations showed a significant difference between. With adding municipal sludge, the heavy metal content of broad bean in different organs were increased, though there are differences. And with adding exogenous Cd, the heavy metal content of broad bean in different organs showed a linear relation. In municipal sludge amended soil, the size of Cd content in the organs of broad beans: root>stem> pods>bean.4. Municipal sludge affected the enrichment and migration of Cd in soils in the broad beans’ organs. The content of 0.5% and 2.0% of municipal sludge make the broad bean root of Cd enrichment coefficient increased by 0.46 and 0.40 respectively, the broad bean of Cd enrichment coefficient increased by 0.020 and 0.031 respectively. The enrichment coefficient of stem and pod showed a trend of increase after decreases first with adding municipal sludge. On the whole, the size of adsorptive ability of Cd are root、stem、pod and bean. For the migration coefficient of Cd in the broad bean system, without affected by exogenous cadmium, 0.5% municipal sludge applied make it easier for Cd in the bean from underground to the ground part, 2.0% municipal sludge applied made it harder for Cd in the bean transfer from underground to the ground part.5. Municipal sludge plowing caused soil cadmium form change. Content of exchangeable cadmium averagely increase 0.029 mg/kg and 0.308 mg/kg. Content of cadmium carbonate combination patterns averagely decrease 0.168 mg/kg and 0.343 mg/kg. Content of Fe-Mn oxides bound form averagely increase 0.0785 mg/kg and 0.0957 mg/kg. Content of organic combined cadmium averagely decrease 0.0110 mg/kg and 0.0208 mg/kg. Residue averagely increase 0.3086 mg/kg and 0.8025 mg/kg. Without exogenous cadmium stress, municipal sludge lower the content of soil effective state, and increases inert form. It may conclude that adding municipal sludge could decrease the activity of cadmium in soil.6. The content of Cd present a very significant positive correlation between the organs of broad bean and soil, namely the broad bean cadmium content in organs has a direct relationship between the degree of soil pollution. Municipal sludge not only make effect on the correlation between cadmium in broad bean and soil cadmium forms, also have an impact on the major contribution form in the form of soil cadmium. In general, municipal sludge can affect bioavailability of soil cadmium, so agricultural sludge need to pay attention to the poisonous and harmful substances, especially the agricultural risk of heavy metals.