Environmental Fate Of Heavy Metals In Mudflat Soils Amended By Sewage Sludge

With the rapid development of modernization and urbanization,the shrinking of agricultural lands has become a major threat to food safety in China. The mudflats in East Coast of China are important alternative sources for arable lands but can not be used for agricultural production before amendment with massive addition of organic fertilizers. Rich in organic matter and other nutrients required for crop growth, sewage sludge is considered as one of the economic choices in mudflat amendment. However, sewage sludge amendment has been greatly restricted due to the concern of heavy metal contamination in soils. Using municipal sewage sludge in compliance with the national standards for agricultural use could reduce the risk of the accumulation of heavy metals in mudflats. Nevertheless, it is not clear whether massive input of sewage sludge would increase accumulation and availability of heavy metals in mudflat soil. In this study, field and lab experiments were conducted, in combination with analytical and biological methods to investigate the direct effects of sewage sludge amendment (SSA) with the rates of 0,30,75,150, and 300 t ha-1 on the behaviors of major heavy metals (Cd, Cu, Pb and Zn) in mudflat soil, including their existing forms and transformation, accumulation in pant body, and migration along soil profile. The results obtained were showed as following.The application of sewage sludge increased the biomass of green manure plants (ryegrass, Sesbania and maize) in mudflat soil, which indicated that mudflat soil fertility could be effectively enhanced by SSA. However, SSA increased heavy metal accumulation (Cd, Cu, Pb and Zn) in these plants, mainly due to the increase of heavy metal contents in plant body.SSA significantly increased total and available Cd, Cu, Pb and Zn contents in surface layer of mudflat soil, but had little effect on those in soil sub-layers. The existing forms and speciation of heavy metals were also affected by SSA with the trend of the increase in acid-extractable proportion and reduction in residual proportion of heavy metals in mudflat soil.Lab simulated soil column experiment showed that the Cd, Cu, Pb and Zn concentrations of soil solution in soil column increased with increasing SSA rates. Correlation analysis indicated that the metal concentrations were significantly and positively correlated with DOC concentration in soil solution, and negatively correlated with pH value in soil solution.In this study, although SSA increased heavy metal accumulation in plant body and mudflat soil, heavy metal contents in mudflat soil with SSA treatments were lower than the secondary standard for soil quality standards in China, heavy metal concentrations in ryegrass, Sesbania and maize with SSA were lower than the standard for forage products in China, and heavy metal concentrations in soil solution at SSA were lower than groundwater quality standard(IV) in China, which indicated that applying sewage sludge in mudflat soils might be feasible both for municipal sewage sludge disposal and soil amendment.Based on our study, to meet the requirements for soil fertility development and environmental safety, the recommended optimum sewage sludge amendment rate was 150t ha-1.