Characterization of Heavy Metal Release, Transformation and Ecological Risk in Composted Sludge under Simulated Rainfall Leaching

<p indent="0mm"> Land use is the main way of sludge resource utilization and reduction disposal. However, rainfall, irrigation and other leaching processes will lead to the release of heavy metals from composted sludge into the environment, resulting in soil and groundwater contamination. Therefore, the analysis of heavy metal precipitation under different rainfall intensities before land use of composted sludge can help to determine the application rate and risk regulation. The results showed that (1) heavy metal leaching concentrated in the early stage of leaching and stabilized with the increase of rainfall duration, in which the concentrations of heavy metals Ni, Cu, Zn, As and Pb in the leachate of the short-term intense precipitation were lower than those of the light rainfall. (2) The vertical migration of heavy metals is mainly affected by rainfall intensity, rainfall duration and leaching amount. Heavy metals were mainly transported to the depth of <sc>20-25 cm</sc> under long-term light rainfall leaching, and the unstable elements Zn, Cd and Cu were more likely to be leached out. The vertical migration of heavy metals under short-term heavy precipitation leaching was not obvious. (3) The ecological risk indices <italic>CF</italic>, <italic>I</italic>_geo and <italic>PERI</italic> showed that Ni, Pb and As in composted sludge were of low ecological risk, but the ecological risk of Cu was the highest under long-term light rainfall leaching, and the ecological risk of Cd and Cu was the highest under short-duration heavy precipitation leaching. Therefore, the land use of composted sludge needs to strictly control the applied amount to reduce the environmental ecological risk of Cd and Cu, and pay attention to the heavy metal pollution from rainfall leaching.